Metal-Involving Synthesis and Reactions of Oximes.

Review pubs.acs.org/CR

Metal-Involving Synthesis and Reactions of Oximes Dmitrii S. Bolotin,* Nadezhda A. Bokach, Marina Ya. Demakova, and Vadim Yu. Kukushkin* Institute of Chemistry, Saint Petersburg State University, Universitetskaya Nab., 7/9, Saint Petersburg, Russian Federation ABSTRACT: This review classifies and summarizes the past 10−15 years of advancements in the field of metal-involving (i.e., metal-mediated and metal-catalyzed) reactions of oximes. These reactions are diverse in nature and have been employed for syntheses of oxime-based metal complexes and cage-compounds, oxime functionalizations, and the preparation of new classes of organic species, in particular, a wide variety of heterocyclic systems spanning small 3-membered ring systems to macroheterocycles. This consideration gives a general outlook of reaction routes, mechanisms, and driving forces and underlines the potential of metal-involving conversions of oxime species for application in various fields of chemistry and draws attention to the emerging putative targets.

CONTENTS 1. Introduction 2. Metal-Mediated Generation of Oximes 2.1. Oximation of Carbonyl Species 2.2. Nitrosation of Organometallic Species 2.2.1. General Approaches and a Comparison of Metal-Free and Metal-Involving Nitrosation 2.2.2. Metal-Mediated Nitrosation 2.3. Reactions of Metal-Activated Nitro Compounds and Their Derivatives 2.4. Electrophilic Substitution in Hydroxamic Acid Esters 3. Metal-Mediated Reactions of the Oxime Group 3.1. O-Functionalization of Oximes 3.1.1. O-Alkylation of Oximes 3.1.2. O-Arylation of Oximes 3.1.3. O-Vinylation of Oximes 3.1.4. O-Heteroacylation of Oximes 3.2. N-Functionalization of Oximes 3.2.1. Generation of Acyclic Nitrones 3.2.2. Generation of Hydroxamic Acids 3.3. C-Functionalization of Oximes 3.3.1. C-Functionalization of Chloroximes 3.3.2. C-Functionalization of Sulfonyl Oximes 3.3.3. C-Functionalization of Aldoximes 3.4. Metal-Involving Dehydration of Aldoximes 3.4.1. Metal-Involving Dehydration of Oximes Yielding Nitriles 3.4.2. Beckmann Rearrangement Leading to Carboxamides 3.5. Reduction of Oxime Ethers and Esters 3.5.1. Reduction of Oxime Ethers and Esters with Preservation of the N−O Bond 3.5.2. Reduction of Oxime Ethers and Esters Accompanied by the N−O Bond Splitting © XXXX American Chemical Society

3.6. Oxidation of Oximes Leading to Carbonyl Compounds 4. Metal-Involving Reactions Directed to Side-Chain of Oxime Species 4.1. Functionalization at α-Position to Carboxime Moiety 4.2. Functionalization at β-Position to Carboxime Moiety 4.2.1. Functionalization of Aliphatic Chains 4.2.2. Functionalization of Vinyl Chains 4.2.3. Functionalization of Aromatic Moieties 4.3. Functionalization of γ-Position to Carboxime Moiety 5. Metal-Involving Reactions of Oximes Leading to Carbo- and Heterocycles 5.1. Generation of 5-Membered Carbocycles 5.2. Generation of Heterocycles 5.2.1. Generation of 3-Membered Heterocycles 5.2.2. Generation of 4-Membered Heterocycles 5.2.3. Generation of 5-Membered Heterocycles 5.2.4. Generation of 6-Membered Heterocycles 5.2.5. Generation of 7-Membered Heterocycles 5.2.6. Generation of Macroheterocycles 6. Conclusions and Outlook Author Information Corresponding Authors ORCID Notes Biographies Acknowledgments Abbreviations References

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DOI: 10.1021/acs.chemrev.7b00264 Chem. Rev. XXXX, XXX, XXX−XXX

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1. INTRODUCTION If oxime research needed any justification, the mere synthesis of these species (Meyer and Janny1,2) followed by the discovery of their acid-catalyzed conversion to carboxamides (Beckmann3) should suffice. Caprolactam, a precursor to nylon-6,4 whose global production exceeds several million metric tons per year,5 is derived from cyclohexanone oxime in what is perhaps the most important application of oximes. Significance of oxime chemistry, however, is not limited to Beckmann rearrangement in connection to the synthesis of caprolactam. Oximes are widely used in diverse laboratory and industrial applications. In particular, they have medical importance as active components of such drugs as Pralidoxime and Obidoxime (acetylcholinesterase reactivators6−11), and also Milbemycin oxime (efficient antiparasitic substance12,13). In addition to commercial drugs, oximes are also known to exhibit antiviral,14,15 anticancer,14−16 anticoagulant,15 antimicrobial,14,15 antihelminthicide,15 antihistaminic,15 antidepressant,15 hearth antiarrhythmic,15 antihypertensive,15 and analgesic14,15 activities. Oxime derivatives are also known as peptide modifiers17−21 and herbicides.22 In food chemistry, these species have applications as sweeteners (e.g., Perillartine23,24) that is 2000 times sweeter than sugar and is particularly popular in Japan. In materials science, various oxime derivatives serve as polymer modifiers25−27 and heavy metal sorbents.28−30 In organic and organometallic syntheses, oximes function as useful synthons for preparation of a wide range of compounds.31−47 Historically, application of metals in oxime systems was defined by several landmark discoveries. The first was the reaction of 1,2-naphthoquinone monoxime with iron and cobalt species, which resulted in the first synthetically generated chelated complex ever (Ilinski and von Knorre48,49), as well as the development of gravimetric determination of nickel(II) by its reaction with dimethylglyoxime producing a “salmon-redcolored precipitate” of the Ni(glyoximato)2 complex (Tschugaeff50). Ligation of oximes to metal centers, giving oxime and oximato metal complexes, has been under intense investigation ever since (for reviews on coordination chemistry of oximes within the past decade see refs 34 and 37). The discovery of useful catalytic51−59 and magnetic properties60−65 of oxime metal species and oxime-based clathrochelates66−70 further elevated the impact of these studies. It is worth specifically emphasizing the crucial role played by vic-dioximato cobalt complexes in the elucidation of activity of vitamin B12.71,72 For over a century after the first synthesis of oximes, they were studied almost independently within the frameworks of organic chemistry and coordination chemistry, and indeed few crossreferences between the two fields were found in relevant reviews. In the final decade of the past century, one of us (jointly with Pombeiro73−75 and Tudela75) summarized available works on oximes at the borderline between the fields, namely studies of metal-induced transformations of oxime ligands. Today, a survey of state-of-the-art research on the same topic reveals an avalanche-like increase in publications utilizing metals in organic oxime systems. In an additional paradigm shift, while 10−15 years ago most of the studied metal-involving oxime reactions were metal-mediated, currently studies on metal-catalyzed transformations are prevailing. As a result of this substantial increase in publications on metalinvolving (i.e., metal-mediated and metal-catalyzed) conversions of oximes in various fields of chemistry during the past decade, the number of investigations has increased to the point where the

subject cannot be fully addressed within a single review article, and various excellent surveys and books76,77 considering specific types of organic substrate transformations31−36,38−45,78−80 have been published, viz., reactions of aldoximes,41,42 amidoximes,34 oxime ethers,80 and esters,35,44 or to a specific type of oximeinvolving reaction, viz., metal-catalyzed Beckmann rearrangement,41 C−O,78 and C−Hal33 bond formation, palladiumcatalyzed C−H bond functionalization,32,39,79 and heterocyclization via the N−O bond cleavage.35,38,40,43−45 It is clear from the inspection of available literature data that no single review article has comprehensively covered the entirety of oxime reactions induced by metal centers. Taking into account the emerging significance of metal-involving reactions of oxime species for organometallic, organic, medicinal chemistry, materials science, and catalysis, a comprehensive critical review in this area would be timely. In this review, we summarize recent advances in the area of metal-involving reactions of oximes and discuss synthetic routes leading to oxime derivatives (section 2), functionalization of oxime moiety (section 3), ligation of the oxime group to a metal center that results in transformations of a side-chain of oxime species (section 4), and reactions leading to great varieties of carbocyclic and heterocyclic systems (section 5). Taking into account the recent reviews highlighting metalinvolving reactions of oximes, we will primarily focus on recent research papers not previously referenced in any review article. However, a limited number of papers considered in the previous reviews will also be discussed for the sake of completeness. In this review, we aim to accomplish two major goals. First, we aim to systematize and explain, based on recent advances, the abundance of diverse data on metal-involving transformations of oximes and to give a general outlook of reaction routes, mechanisms, and driving forces. Second, we will underline the potential of metal-involved conversions of oxime species for application in various fields of chemistry and draw attention to the emerging putative targets. Thus, in our consideration, we will emphasize an analytical approach to the advances in the field, rather than focus on simple documentation of the reported data.

2. METAL-MEDIATED GENERATION OF OXIMES Since Viktor Mayer’s discovery of oximes approximately one and a half centuries ago, synthetic routes leading to these species have been widely developed. In metal-free organic chemistry, the main types of high-yield reactions leading to oximes include the condensation of RR′CO (R′ = alkyl, aryl, or H) with hydroxylamine (for recent works, see refs 81−83) or Osubstituted hydroxylamines (giving oxime ethers; for recent works, see refs 84 and 85), nitrosation of aliphatic compounds (for recent work, see ref 86), or reduction of nitro species (for recent works, see refs 87−89). In this chapter, still-uncommon metal-mediated methods for the preparation of oximes are discussed. Although the synthetic potential of many of these approaches is not high, all the reactions reviewed in section 2 should be considered during the analysis of ligand reactivity. In section 2.1, we consider reactions where the formation of the oxime moiety proceeds via the treatment of carbonyl compounds with hydroxylamine in the presence of a metal center (scheme 1, A−B−E). Another type of reaction, described in section 2.2, involves the intermediate formation of C, with its subsequent reaction with NO+ followed by isomerization of aliphatic nitroso compound D to oxime E. Finally, cobalt(II)-mediated allylic substitutions in bis(oxy)enamines leading to α-nitroxy oximes are surveyed in section 2.3. B


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ing this reaction in neat PhCHO for 7−10 d followed by the addition of excess Me2SO yielded 4b with (Z)-configured benzaldoximate (c and d).91 Intermediate 5 with the ligated benzylic alcohol, which is likely generated in the reaction mixture via Cannizzaro reaction,92−94 was identified. The differences in the configurations of the coordinated benzaldoximates in 4a and 5 were rationalized in terms of the steric hindrance of the PhCH2OH ligand, which forces the aldoximate to adopt another configuration. In metal-free organic chemistry, the condensation between a carbonyl compound and a hydroxylamine is a well-known reaction, and it is generally assumed to start from the nucleophilic attack of the N center to the electrophilically activated C atom of R1R2CO. The situation is less straightforward when hydroxylamine is bound to the metal center insofar as the coordination blocks the electron pair of the N center. We assume that the oximation of carbonyls might proceed when the [UVI](η2ONH2) moiety is subject to solvolysis, giving [UVI](solvent) species and uncomplexed hydroxylaminate anion, which then reacts with R1R2CO. Alternatively, the carbonyl compounds could be coordinated to the [UVI](η2-ONH2) group; this reaction would be accompanied by the chelate cycle opening (Scheme 3). The Pearson’s hardness95 of oxygen favors both the

Scheme 1. Two General Metal-Mediated Routes to Oximes

2.1. Oximation of Carbonyl Species

Scheme 3. Reaction Pathway of Generation of Oximes from Carbonyls and a Hydroxylamine Ligand

Beirakhov et al. reported the generation of oximate ligands at a UVI center by the reaction of carbonyl compounds with coordinated hydroxylaminate. The complex trans[UO2(H2NO)2(H2O)2] (1) reacted with the carbonyl species R1R2CO (2) used as solvents to grant the oximato complexes trans-[UO2(R1R2CNO)2(R1R2CO)2] (3, Scheme 2, a; RT, 12−48 h in air; yields not reported).90 When the reaction of 1 with a 4-fold excess of PhCHO was performed in Me2SO (RT, 12−48 h, under Ar), complex 4a featuring ligated (E)-benzaldoximate was formed (b). ConductScheme 2. Uranium(VI)-Mediated Synthesis of Oximate Ligands from Carbonyl Compounds90,91a

a

ligation of R1R2CO and O-coordination of hydroxylamine and subsequent intramolecular nucleophilic attack leads to the condensation. Although the metal-mediated condensation of NH2OH and carbonyl compounds has been illustrated by only two examples, relevant metal-involving reactions between NH3 and R2CO leading to imines96−100 and between N2H4 and R2CO leading to hydrazones101,102 are more abundant. The metal-mediated oximation of carbonyl compounds was demonstrated when PhRCO species reacted with an imido ligand at a titanium(IV) center. tert-Butoximido titanium complex 6 reacted with PhC(O)R (1 equiv.; 7) to give oxime ethers 9 as a mixture of E/Z-isomers (in benzene, RT, 0.5−2 h, under dry N2 or Ar; 70−93% total yields) and dinuclear complex 8 (Scheme 4, a).103 The authors103 successfully identified intermediate 10, and they also calculated the energy profiles of these reactions (R = H, Me) by DFT. On the basis of these data, they argued that a plausible mechanism includes the nucleophilic attack of the N atom on the carbonyl C atom (b), with subsequent splitting of the C−O bond (c) giving oxime ether 9 and complex 8. It was further established that complex 6 also reacts with aromatic nitriles to give an equilibrium mixture of the amidoximate ether

2: R1/R2 = nBu/Me, tBu/H, C4H8, Ph/Me. C


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Scheme 4. Titanium(IV)-Mediated Preparation of Oxime Ethers103a

a

Scheme 6. Metal-Mediated Generation of Oximes via Nitrosation of Alkanes

7: R = H, Me, Ph.

complexes [TiCp*{PhC(NiPr) 2}{NC(NOtBu)Ar}] and [TiCp*{PhC(NiPr)2}{NC(Ar)NC(Ar)NC(NOtBu)Ar}].103 2.2. Nitrosation of Organometallic Species

2.2.1. General Approaches and a Comparison of MetalFree and Metal-Involving Nitrosation. In metal-free organic chemistry, the nitrosation of alkanes or alkenes typically proceeds via a radical pathway to give C-nitroso species (Scheme 5).104,105

nucleophilic attack of the R ligand to give the N-coordinated nitroso compound (e),109 which is subject to tautomerization leading to the oxime (f). Finally, the oxime formed leaves the metal center (g), thereby regenerating the catalyst (h). Owing to the lack of uncomplexed NO in the reaction mixture, the oximes are generally prepared in high yields because of the absence of undesirable surplus nitrosation of the oximes formed.110 The nitrosation could also proceed via another route (see Scheme 11). In those reactions, metal centers act as C−H bond activators for alkanes or as stabilizers of reactive radical intermediates for alkenes. In both cases, the involvement of a metal center prevents the dimerization of the intermediate nitroso compound because of the known coupling of the nitroso N atom before tautomerization (e and f) for metal-free species. 2.2.2. Metal-Mediated Nitrosation. In this section, the metal-mediated nitrosation of aliphatic and vinylic chains and aromatic rings will be discussed. The RMe species (11) are subject to nitrosation in the presence of Cu(OAc)2 (10 mol %), tBuONO (3 equiv), and 2hydroxyindoline-1,3-dione, C6H4(CO)2NOH (30 mol %). These syntheses were conducted in MeCN at 80 °C for 24 h under dinitrogen to give the corresponding aldoximes RC(H) NOH (12) (63−86% isolated yields Scheme 7, a).111 The reaction did not proceed in the absence of copper(II) and 2-hydroxyindoline-1,3-dione and proceeded poorly for disubstituted methylene species; diphenylmethylene, ethylbenzene, and fluorene transformed to the corresponding benzophenone-, acetophenone-, and fluoren-9-one oximes in only 28, 10, and 35% yields.111 The copper(II) center is believed to act as a C−H bond activator, and a plausible mechanism includes the 2hydroxyindoline-1,3-dione-mediated oxidation of 11 by the copper(II) center generating aryl- and heteroaryl methylene radicals (b−e). After that, the oxidative addition of the formed radical to the copper(I) center (d) followed by the nitrosation of the organometallic compound (f) gives 13, which then tautomerizes to aldoxime 12 (g). Indole derivatives 14 were transformed into oximes 15 in the presence of CuCl (1 equiv), [CF3CH2NH3]Cl (3 equiv), NaNO2

Scheme 5. Metal-Free Generation of Oximes via Nitrosation of Alkanes or Alkenes

If the thus formed C-nitroso species feature an H atom at the geminal C atom (R1 = H), they transform to oximes. This radical nitrosation is usually accompanied by the oxidation of CH−CH bonds to CC groups, the dimerization of the nitroso compounds RNO to RN+(O−)N+(O−)R, or the formation of diazeniumdiolates.104,105 For more complex compounds (e.g., alkynes, alcohols, amines, or thiols), O-, N-, or S-nitrosation might lead to azaheterocycles, ketones, or even nitriles.105 Metal-mediated nitrosations of alkenes were reviewed in 2016,106 and here we focus on only the recent works on such nitrosation that were not considered in the published survey. It is generally believed that the mechanism of metal-mediated nitrosation of alkanes (Scheme 6, a) or alkenes includes the initial generation of an organometallic compound (b). This intermediate ligates the nitrosation agent XNO by the N atom (c), and then, the nitroso ligand reversibly dissociates to the NO+ and X− ligands (d).107,108 The nitrosyl is the subject of the D


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for 24 h in air, resulting in the precipitation of complexes 17 from the reaction mixture (65−70%; Scheme 9).113

Scheme 7. Copper-Catalyzed Nitrosation of Methylarenes and Heteroarenes 11 Giving Aldoximes 12111a

Scheme 9. Nickel(II)-mediated nitrosation of oximes and representative molecular structure of 17.113 16: R1 = R2 = Me; R1/R2 = (CH2)4, (CH2)5, (CH2)6

a

11: R = Ph, 1-naphthyl, 9-anthracyl, 2-furyl, 2-(5-methylfuryl), 2thienyl, 2-pyridyl, 4-pyridyl, 2-quinolyl, 3-pyrazolyl, 4-imidazolyl; R′C6H(2−4): R′ = 4-Me2N, 2-HO, 4-HO, 4-MeO, 2,3,4-(MeO)3, 4-iPr, 4-F, 2-Cl, 4-Cl, 2,4-Cl2, 4-Br, 3-O2N, 4-O2N.

The reaction is nickel(II)-mediated and did not proceed in the absence of the metal source. The reaction mechanism was suggested to include nickel(II)-mediated hydrolysis of 0.5 equiv of 16 to give R1CH2C(R2)Odetected in the reaction mixture by LC−MS, and H2NOH. Hydroxylamine is oxidized, giving the nitrosation agent. This agent oxidizes another 0.5 equiv of 16 to furnish the vic-dioxime. However, an alternative mechanism could also be suggested. The nickel(II) center could serve as a C−H bond activator promoting the oxidation of the αCH2 moiety to give R1C(O)C(R2)NOH, similar to the reaction observed at the MnII center (see Schemes 87 and 88). The α-carbonyl oxime then reacts with H2NOH, leading to R1C(NOH)C(R2)NOH, and the path of this reaction probably relates to route A in Scheme 1. Nitrosation of the styrenes trans-R1C6H(3−4)CHCHR2 (18) was realized in the presence of iron(II). In the presence of an iron(II) catalyst (1 mol %; Fe(BH4)2/pyridyl-2,6-dicarboxylic acid mixture and iron(II) phthalocyanine [FePc] were tested), NaBH4 (1.5−2.0 equiv) and tBuONO (1.5−2.5 equiv), styrenes 18 transformed to aromatic ketoximes 19 (32−92% isolated yields) in a MeOH/H2O mixture or EtOH at RT for 3−6 h under H2 (10 atm) (Scheme 10, a).114,115 The iron(II) center acts as a stabilizer of an intermediate carbanion, and the mechanism of this nitrosation was proposed to include the generation of an organometallic intermediate (b), which is subject to nitrosation by in situ-generated NO (c) and final isomerization to the oxime (d). The reaction did not proceed for aliphatic alkenes, which gave the corresponding alkanes under the same conditions.114 Yang et al. reported an extension of this reaction to the preparation of α-sulfonyl oximes. The styrenes transR1C6H(3−4)CHCHR2 (18) reacted with excess R3C6H4SO2Na (20) and tBuONO in the presence of CuBr (10 mol %) in toluene at 60 °C for 12 h to give the functionalized aromatic oximes R1C6H(3−4)C(NOH)CH(R2)SO2C6H4R3 (21, 41− 72%; Scheme 11, a).116 Insofar as this reaction was substantially inhibited by radical scavengers (TEMPO and 2,6-di-tert-butyl-4-methylphenol were tested), it was concluded that it had a radical nature. Moreover, when the experiment was conducted in the presence of H218O, 18 O-labeled oxime 21 was formed. The obtained data indicated that the oxime HO moiety originates from water. From these

(6 equiv), and HCl (3 equiv) in ClCH2CH2Cl at 80 °C for 6 h (23−82%; Scheme 8, a).112 Scheme 8. Copper(I)-Mediated Preparation of Fluorinated Oximes 15112a

a

14: R1 = 5-MeO, 7-Et, 2-Me, 4-Me, H, 5-F, 5-Cl, 6-Cl, 5-Br, 5MeO2C, 5-NC; R2 = Ph, H, Me, Et, iPr, nBu, PhCH2.

In this reaction, the copper(I) center also behaves as a C−H bond activator. Experiments were also conducted with isotopically enriched substrates,112 and from the results, it was concluded that the carbene copper(I) complex generated from CuCl and [CF3CH2NH3]Cl (c) first electrophilically attacks the indole derivative to substitute H+ (b). In the next step, the organic ligand is subject to nitrosation with HONO (d) formed from NaNO2 and HCl (e)to give the nitroso intermediate, which then isomerizes to the oxime (f). The neat aliphatic oximes R1CH2C(R2)NOH (16; 10-fold excess) underwent nitrosation in the presence of NiCl2 at 100 °C E


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Scheme 10. Iron(II)-Catalyzed Nitrosation of Styrenes 18114,115a

Scheme 12. Aluminum(III)/Silicone(IV)-Mediated Nitrosation of Silyl Enol Ethers by Nitrite118a

a

a

18: R1 = 3-MeO, 4-MeO, 4-H2N, 2-Me, 3-Me, 4-Me, 4-ClH2C, H, benzo[c], 3-F, 4-F, 2-Cl, 3-Cl, 4-Cl, 2-Br, 4-Br, 3-OHC, 4-NC, 3-O2N, 2-pyridyl; R2 = Ph, H, Me, CH2OAc.

22: R1/R2 = (CH2)n, where n = 3−6, 10, and CH2CH(tBu)CH2CH2, Bu/nPr, tBu/H, Ph/H, o-(CH2CH2)C6H4, HCC(H)CH2CH2, HCC(H)CMe2CH2, 2-furylvinyl, 2-phenylvinyl, H/Me, H/ nPr, H/nC6H13, H/PhCH2, iPrO/H, EtO/Me, CH2CH2O, EtO2CC(O)/ CO2Et, Ac/CO2Et; R3 = Me, iPr, nBu, SiMe3. n

observations, the following mechanism was postulated. First, 1e-̅ oxidation of sulfinate (b) proceeds in the presence of CuII generated from the oxidation of CuI in air (c). This process regenerates the copper(I) (d), and an organocopper intermediate is formed in step (e). Thus, the copper(I) center stabilizes the radical species formed as intermediates. Finally, substitution of CuI by NO gives 21 (f). The NO appears in the reaction mixture from two equiv of tBuONO in the presence of H2O (g). Recently, this reaction was performed in the absence of the copper(I), utilizing the sulfinic acid/pyridine system instead of sodium sulfinates to give α-sulfonyl oximes in high yields (typically 75−88%).117 The ethers R1C(OSiR33)C(H)R2 (22) reacted with a mixture of AlCl3 and [nBu4N](NO2) in stoichiometric amounts (MeCN, temperatures below −20 °C, 2 h), and this reaction led to the corresponding α-carbonyloximes 23 (60−90%; Scheme 12, b).118 Certain other metal chlorides (e.g., TiCl4, FeCl3, and InCl3) and the nitrite salts [Na{15-crown-5}](NO2) and AgNO2 could also be used in this reaction. Under the same reaction conditions, the double nitrosation (c) leading to 2-carbonyl-1,3-dioximes 24 was not achieved. It proceeded only with an excess of any one of the nitrites and with 23 featuring a CH2 group at the CO

moiety (R1/R2 = (CH2)n, where n = 3−6, nBu/nPr) and led to 24 (30−83%; c). A conversion of 23 to 24 was not attempted. Reaction pathway involves the formation of an organometallic intermediate via allylic substitution of the SiR33+ by the aluminum(III) center (a). Thus, the aluminum(III) center behaves as a reactive electrophile promoting the reaction. However, another route to α-carbonyloximes 23 includes the reaction of α-nitroketones R1C(O)CH(NO2)R2 (25) with benzyl hydroxycarbamate PhCH2O2CNHOH (26) via the Henry reaction.119 This reaction proceeded in the presence of MnO2 (5-fold excess) and quinidine (10 mol %), and oximes 23 were isolated in 52−80% yields (in THF, RT, 48 h; Scheme 13, a).120 Upon the basis of the suggested mechanism120 and considering that manganese forms enolate complexes (which rapidly react with various types of electrophiles),121−123 a plausible mechanism for this reaction is as follows. The manganese(IV) center oxidizes 26 to a nitroso intermediate (b) and generates manganese(II) species that form the enolate complex in basic media (c). The enolate complex is subject to electrophilic attack by the nitroso compound (d). The formed intermediate eliminates the alkyl nitroso carbonate to provide the

Scheme 11. CuI-Catalyzed Nitrosation of Styrenes 18 in the Presence of Arylsulfinates116a

a

18: R1 = 2-MeO, 3-MeO, 4-MeO, 4-Me, 4-ClCH2, H, benzo[c], 3-Cl, 4-Cl, 4-Br, 4-NC, 4-O2N; R2 = H, Me, ClCH2; 20: R3 = 4-MeO, 4-Me, H, 4Cl, 2-Br, 4-Br, 4-O2N. F


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Scheme 13. Manganese-Mediated Synthesis of αCarbonyloximes120a

reacted with Ph3CSNO at RT overnight to give oximate complex 30 (93%). This complex was formed via cleavage of the S−N bond in the preliminary ligated Ph3CSNO followed by insertion of the NO ligand into the BiIII−C bond (Scheme 14, a).124 The role of the metal center is clear from Scheme 6. Complex 27 unselectively reacted with gaseous NO (1 atm, temperature range from −30 °C to RT), leading to a broad spectrum of products, from which only several crystals of 28 and 29 were separated and characterized by single-crystal XRD. However, complex 28 was selectively prepared by the two-step method. A solution of 27 in MeCN was treated with gaseous CO2 (RT, 1 h), selectively giving the carboxylate complex [Bi(C6H3(CH2NMe2)2){O2CC6H2(tBu)2O}],125 whereupon the system was then treated with gaseous NO (1 atm, THF, RT, overnight). This method led to dinuclear oximate complex 28 (66% relatively to 27; b). The addition of excess [Et3NH]Cl to the THF solution of 28 generated mononuclear bisoximate complex 29 (45% isolated yield; c). In the C−N−O moieties of 28−30, the C−N distances lay in the range 1.315(4)−1.331(4) Å, which is longer than that in oximes [1.281(13) Å],126 whereas the N−O bond is shorter [1.317(4)−1.349(3) Å] than that in oximes [1.394(18) Å].126 This observation demonstrates the significant contribution of the nitroso form in the newly formed ligands. A relevant example of the metal-mediated nitrosation of aromatic rings is the Baudisch reaction, which was developed in the middle of the 20th century and leads to orthoquinonemooxime complexes.127−130 In all nitrosation reactions (section 2.2.2), a metal center plays a triple role. It promotes the generation and stabilization of a nucleophilic carbanion, it activates the NO+ ligand toward nucleophilic addition of the carbanion, and, finally, it prevents the dimerization of the formed nitroso compound before tautomerization to the oxime.

a

15-1: R1 = 2-naphthyl, 2-furyl, 2-benzofuryl, 3-thienyl, R3C6H(3−4): R3 = 4-MeO, 4-H2CCHCH2O, 3,4-CH2O2, 4-Me, H, 2-F, 3-F, 4-F, 4Cl, 3-Br, 4-NC; R2 = Me, Et.

oximate manganese(II) complex (e and f), and the eventual protonation of the latter gives oxime 23 (g). The formation of oximate ligands was achieved via NO insertion into a BiIII−C bond. Organobismuth(III) complex 27

Scheme 14. Nitrosation of Organobismuth(III) Compounds Leading to Oximate Ligands and Representative Molecular Structures of Oximate Complexes Obtained124

G


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Scheme 16. Aluminum(III)-Mediated Generation of αPhenyl-α-Aryl Chloroximes 33134a

2.3. Reactions of Metal-Activated Nitro Compounds and Their Derivatives

The metal-free generation of oxime species from nitro compounds is typically based on their reduction with SnII or active hydrogen (for recent works, see refs 131 and 132 and ref 133, respectively). These agents reduce R1R2CHNO2 and provide expedient access to nonfunctionalized oximes R1R2CNOH. In addition to these methods, metal-mediated approaches allow the preparation of gem- and α-functionalized oximes (Scheme 15, a−b−c and a−d−e−f, respectively). The Scheme 15. Synthetic Routes to Oximes Based upon Nitro Compounds

a

33: Ar = RC6H(2−4): R = 4-MeO, 4-Me, 2,4-Me2, 2,5-Me2, 2,4,6-Me3, 4-iPr, H, benzo[b]; 2-pyrrolyl, 3-indolyl, 5-(2-methyl)thienyl, 3benzo[b]thienyl.

Notably, chlorooximes have previously been generated from the appropriate nitro compounds in the presence of TiCl4135 or TeCl4,136 but the reaction accompanied by arylation134 was performed for the first time. The generation of nonfunctionalized chlorooximes RC(Cl)NOH and oximidoyl thioates RC(SR1)NOH is also known in metal-free chemistry (for recent works, see ref 137 and ref 87, respectively). Synthetic routes to cyclic oxime ethers, particularly from bis(oxy)enamines (which are typically prepared from nitro compounds;138,139 Scheme 15, c and f) were comprehensively reviewed in 2011;80 below, we focus on reactions leading to the acyclic oxime species. These reactions are typically promoted by the R3Si+ species,140−142 or they proceed even in the absence of a promoter,140 and lead to oxime ethers XCR1R2C(R3)NOE (g) functionalized at the α-position or to appropriate oximes (X = amino,140,142 azido,140,141 hetaryl,142 oxy,142,143 chloride;142 Scheme 15). The application of cobalt(II) species as catalysts is reported to provide high-yield syntheses of acyclic α-halo and α-nitroxy oxime ethers. Ioffe and co-workers reported metal-mediated nucleophilic substitution of the trimethylsilyloxy moiety in H2CC(R)N(OSiMe3)2 (35) by the halides Cl−, Br−, and I− or by NO3− leading to the α-halo oxime ethers HalCH2C(R)NOSiMe3 (36; 47−86%; Scheme 17, a)144 or the α-nitroxy oximes O2NOCH2C(R)NOH (37, 25−32%; b),145 respectively. The reaction proceeded in the presence of 1−2 equiv of anhydrous CoX2 in THF/CH2Cl2 (2:1, v/v) or DMF/CH2Cl2 (1:1, v/v) mixtures at RT for 2.5−24 h under Ar followed by treatment of the mixture with an aqueous solution of NaHSO4 at RT in air (for O-desilylation).145 A plausible mechanism includes ligation of the sp3-hybridized N atom to cobalt(II) (c),146 which leads to activation of the vinyl C atom toward the nucleophilic attack of X− (d), which substitutes the OSiMe3− moiety (e). The preparation of oxime ethers 36 (R = H, Me, nPr, 1cyclohexenyl, Ph, PhS, EtO2C) from enamines 35 was also performed via a radical path (Scheme 17). In that instance, 35

first route (a−b−c) includes electrophilic activation of the gem-C atom of a nitro compound by E+, which, in particular, can be a metal center (a). Subsequent nucleophilic substitution of EO− by Nu− (b) gives a nitroso compound, which, in the case of R3 = H, tautomerizes to an oxime (c). By the action of an excess of electrophile E+, the nitro compound can be transformed to bis(oxy)enamines (a−d), which are nucleophilically attacked by Nu− on the α-C atom accompanied by elimination of EO− (e). The latter process gives an oxime ether, which can be hydrolyzed to the oxime (f). Yao and co-workers reported the preparation of α-phenyl-αaryl chloroximes (33) from β-nitrostyrene (31) via its reaction with various ArH (32; 1.25 equiv) in the presence of AlCl3 (2 equiv) in CH2Cl2 (RT, 3 h, 43−99%; Scheme 16, a).134 Sequence of possible stages includes an initial AlIII-mediated Friedel− Crafts alkylation of ArH by β-nitrostyrene (b; arylated intermediates 34 were isolated at −78 °C and characterized) followed by aluminum(III)-mediated tautomerization of the nitro compounds to their aci-nitro forms, where the C atom is electrophilically activated (c). This atom is then attacked by the chloride, giving gem-chloronitroso species (d). Tautomerization of these species to oxime 33 terminates the reaction, providing αphenyl-α-aryl chloroximes (e). H


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Scheme 17. CoII-Mediated Synthesis of α-Halo Oxime Ethers 36 and α-Nitroxy Oximes 37144,145a

Scheme 18. Silver(I)-Catalyzed Preparation of Oxyoxime Ethers 39148a

a

38: R1 = Me, PhCH2; R2 = MeOCH2, Me3Si, Ph; R3 = H, O2N.

Scheme 19. Boron(III)/Iodine(III)-Mediated Preparation of O-Substituted Oxyoximes 41150a a

35: R = H, Me, PhCH2, MeO2CCH2CH2, Ph; 36: Hal = Cl, Br, I.

(1.5 equiv) reacted with the iodides XI (1 equiv.; X = EtO2CCH2, PhO2SCH2) in the presence of Me3SnSnMe3 (1 equiv) under UV (300 nm) irradiation in an inert atmosphere in benzene for 9 h to give 36 in 73−93% yields after workup. The postulated mechanism includes the UV-induced generation of Me3Sn•, which determines the formation of X• that reacts in turn with 35.147 2.4. Electrophilic Substitution in Hydroxamic Acid Esters

The α-alkynyl benzohydroxamic esters R3C6H3(CCR2)C( O)NHOR1 (38) underwent intramolecular electrophilic substitution in the presence of Ag2O (5 mol %) in EtOH at RT for 3−4 h to give oxyoxime ethers 39 (48−92% isolated yields Scheme 18, a).148 Oxime ethers 39 were also generated in the presence of [Cp*RhCl2]2 (2.5 mol %) and AgOAc (2 equiv) under argon in MeCN at 80 °C for 24 h, but the isolated yields of 39 were as low as 8−29%.149 In this reaction, the silver(I) center behaves as an electrophilic activator of the CC bond, and a postulated mechanism includes coordination of the substrate to the silver(I) center by the CC bond (b) followed by the electrophilic substitution of the H atom by the C atom (c). The reaction is terminated by the substitution of the Ag+ (e) by the H+ to give oxyoxime esters 39. An additional report is devoted to the asymmetric oxidative preparation of oxyoxime ethers 41 (Scheme 19, a) from the Osubstituted vinyl benzohydroxamic acids 2-(R1CHCH)C6H4C(O)NHOR2 (40). The reactions proceeded in the presence of the enantiopure (R)-2-(MeO2CC(H)MeO)C6H4I(OAc)2 (1.5 equiv), BF3·OEt2 (8 equiv), and AcOH (2 equiv) in CH2Cl2 at −80 °C for 1−2 h150 and after workup gave 50−78% isolated yields of the products. The boron(III) serves as an electrophilic activator of the iodine(III) center, initially abstracting AcO− from Ar*I(OAc)2 and furnishing the reactive Ar*I+OAc species (b), which then integrates with the CC bond (c).151 After that,

a

40: R1 = H, nPr, n-pentyl; R2 = Me, Ac.

electrophilic substitution of H+ (d) followed by the elimination of the AcOH in acidic media (e) gives the cationic transintermediate. Finally, Ar*I is substituted by the AcO− via the SN2 mechanism, thereby forming functionalized oxyoxime 41. I


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Scheme 21. Metal-Mediated O-Alkylation (a), Arylation (b), Vinylation (c), and Heteroacylation (d) of Oximes

3. METAL-MEDIATED REACTIONS OF THE OXIME GROUP The first group of metal-involving reactions of oximes include those with the oxime moiety NOH. Oximes are ambidentate nucleophiles, and their O- and N-functionalizations by electrophilic agents have been extensively studied in both metal-free and metal-involved chemistry. By the end of the 20th century, the alkylation of oximes by alkyl halides was known to preferentially give O-alkyl ethers, while N-alkylation was a commonly occurring side reaction. However, examples of the alkylation and arylation of oximes in the presence of metal complexes were scarce.75 As described in sections 3.1 and 3.2, in the past decade, selective Oand N-alkylation, vinylation, and arylation reactions have been widely developed, and many expedient and selective protocols are now known. In the beginning of section 3, we discuss O- and Nfunctionalization of oximes based on electrophilic attack to the O- or N atoms, respectively, of the NOH group (sections 3.1 and 3.2; Scheme 20, a and b). After that, C-functionalization via

(43) in the presence of a chiral iron(II) catalyst and an acid (pBrC6H4CO2H, p-MeOC6H4CO2H, and 1,3-C6H4(CO2H)2 were tested). The initial treatment followed by in situ reduction of the aldehyde moiety to the alcohol group with NaBH4 led to the corresponding O-alkylated p-methoxyacetophenone oximes pMeOC6H4C(Me)NOC(R)H(CH2)2OH (44). These Oalkylated oximes were isolated in 30−62% yields and 72−76% ee (Scheme 22).160

Scheme 20. Metal-Mediated O-, N-, and C-Functionalization of Oximes (Routes a, b, and c, Correspondingly)

Scheme 22. Asymmetric O-Alkylation of Oxime 42160a

palladium-catalyzed arylation of aldoximes and silver- or cobaltcatalyzed functionalization of sulfonyl oximes yielding ketoximes (section 3.3, c), metal-mediated dehydration of aldoximes (section 3.4, d), and reduction of oximes (section 3.5, e) will be considered. 3.1. O-Functionalization of Oximes

a

Metal-involving O-functionalizations of oximes are divided into four types depending on the product obtained: alkylation (Scheme 21, a; for discussion, see section 3.1.1), arylation (b; section 3.1.2), vinylation (c; section 3.1.3), and heteroacylation (d; section 3.1.4). 3.1.1. O-Alkylation of Oximes. In the absence of metals, Oalkylation of oximes proceeds in the presence of reactive alkyl halides (for recent works, see refs 152−158) or sulfates (for recent works, see refs 158 and 159) as alkylation agents. These reactions are typically conducted in basic media and allow the preparation of oximes featuring alkyl,152,153,158,159 allyl,156−158 or propargyl154,155 substituents at the O atom. In the presence of a metal center, the scope of alkylating reagents extends to alkenes and ethers. These reactions and metal-mediated reactions involving allyl esters are surveyed in sections 3.1.1.1−2. 3.1.1.1. Reactions with Alkenes. Chang et al. reported the asymmetric oxa-Michael addition of p-methoxyacetophenone oxime 42 to the α,β-unsaturated aldehydes RC(H)C(H)CHO

43: R = Me, Et, nPr, iPr, nBu, nC7H15.

The yield of 44 was reduced when the aldehydes possessed sterically hindered substituents R, whereas these substituents did not significantly affect the enantiomeric composition of 44.160 The addition of any one of the acids was necessary to increase the yields of 44. The iron(II) center is believed to enhance the nucleophilicity of the oxime, and a postulated mechanism of this reaction includes the O-coordination of 42 to the FeII(L) center, with the subsequent insertion of the CC bond of 43 into the Fe−O bond generating ether 44 after decoordination. Another example of metal-mediated nucleophilic addition of oximes to a CC bond is the reaction between acetoximato rhenium(I) complex 47 (prepared from fac-[Re(OTf)(CO)3(phen)] (45) and acetoxime 46; Scheme 23, a) with tetracyanoethene 48 to give 49 (88%; b).161 This reaction represents an unconventional example of (NC)2CC(CN)2 reactivity. In metal-free chemistry, tetracyanoethene is known to J


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to give an intermediate palladium(II) allyl complex, and the reaction then proceeds via nucleophilic attack of the oximate on the allyl ligand, which is electrophilically activated by the metal center. When the catalytic system was replaced with [Ir2Cl2(cod)2]/ Ba(OH)2/54 (Scheme 25), the regioselectivity of the reaction

Scheme 23. Rhenium(I)-Mediated Addition of Acetoximate to Tetracyanoethene161

Scheme 25. Iridium(I)-Catalyzed O-Allylation of Benzaldoxime by Allylacetate177

between 50 (R1/R2 = Ph/H) and 51 (Ar = Ph) shifted, and PhC(H)NOC(Ph)HC(H)CH2 (53) was isolated from the reaction mixture in 85% yield and 42% ee of the (S)enantiomer.177 The iridium(I)-catalyzed O-allylation of oximes was extended to a series of allylphosphates.177 This reaction proceeded with higher enantioselectivity but lower regioselectivity than the reaction with 51. Benzaldoxime 50 reacted with trans-ArC(H) C(H)CH2OP(O)(OEt)2 (55) in the presence of 4−8 mol % of [Ir2Cl2(cod)2] and 1 equiv of Ba(OH)2·H2O and 54 to give PhC(H)NOCH2C(H)CHAr (52) and PhC(H)NOC(Ar)HC(H)CH2 (53) after workup in 35−89% combined yields and 52:53 molar ratios ranging from 5:95 to 31:69 (Scheme 26). Compounds 53 were prepared in 70−95% ee of the (S)-enantiomer.

be involved in [4 + 2]-,162−164 [3 + 2]-,165,166 [2 + 2]-,167−169 and [2 + 1]-cycloadditions,170 and only a few examples of the nucleophilic addition of C-nucleophiles171−173 have been reported. The mechanism discussed for this reaction, depicted in Scheme 23, includes the initial reversible exchange of the CO ligand to the alkene (b) followed by the insertion of the alkene into the Re−O bond (c) to give the coupling product. Similar reactions have also been performed for the rhenium(I)174 and platinum(II)175 alkoxides. The O-alkylation of the aldoximes and ketoximes R1R2C NOH (50) was also performed via the nucleophilic substitution protocol. Oximes 50 reacted with 1-acetyl-1-arylprop-2-ene, H2CCHCH(OAc)Ar (51), furnishing O-(aryl)allyl oximes R1R2CNOCH2C(H)CHAr (52; Scheme 24), which were

Scheme 26. Iridium(I)-Catalyzed O-Allylation of Benzaldoxime by Allylphosphates177a

Scheme 24. Palladium-Catalyzed O-Allylation of Oximes176a

a

55: Ar = m-ClC6H4, p-FC6H4, p-CH3C6H4, p-CH3OC6H4, 1naphthyl, 2-naphthyl.

a

50: R1/R2 = p-CF3C6H4/H, Ph/H, p-MeOC6H4/H, 2-py/H, MeO2C/H, Ph/Ph, (CH2)5; 51: Ar = p-CF3C6H4, p-ClC6H4, mClC6H4, p-FC6H4, Ph, p-CH3C6H4, p-CH3OC6H4, 1-naphthyl, 2naphthyl.

In contrast to the metal-free reactions of oximes with allenes giving O-vinyl oximes,178,179 the gold(I)-catalyzed process leads to O-allyl oximes. The aldoximes and ketoximes R1R2CNOH (56) reacted with the aminoallenes H2CCCHN(Ts)R3 (57) in the presence of gold(I) complex 59 (5 mol %) and AgOTf (15 mol %) in CH2Cl2 at RT for 5 h under dinitrogen to give O-allyl oximes 58 (52−96%; Scheme 27, a).180 The reaction did not proceed in the presence of 59 or AgOTf alone. Therefore, one could conclude that Ag+ is required to abstract the chloride from 59 to create the coordination vacancy and transform the AuI species into its catalytically active form.

isolated in 37−95% yields.176 The reaction proceeded at 20 °C in the presence of either ZnEt2 (1 equiv) and [Pd(PPh3)4] (8 mol %) in THF or K2CO3 (1 equiv) and [Pd(PPh3)4] (6 mol %) in CH2Cl2. The reaction in the presence of ZnEt2 generally gave higher yields. It is suggested176 that a base (ZnEt2 or K2CO3) is required to deprotonate the oxime moiety, thereby increasing its nucleophilicity. The palladium(0) center reacts with allyl acetate K


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Scheme 27. Gold(I)-Catalyzed O-Allylation of Oximes by Aminoallenes180a

The metal center acts as a C−H bond activator. The ytterbium(III) center provides the ligation of THF (b), followed by oxidation of the coordinated THF to the corresponding oxonium cation (c). Eventually, the nucleophilic addition of the HO moiety of 56 to the CO double bond leads to 60 (d). From the reported data,181 it is clear that the yields of 60 increased linearly with increasing amounts of Yb(OTf)3. However, no efforts were made to utilize 2 equiv of Yb(OTf)3 in the two-electron redox reaction. The necessity of a 2-fold excess of Yb(OTf)3 is indirectly confirmed by the reported yields of 60. When 1 equiv of Yb(OTf)3 was employed, compounds 60 were isolated in yields typically lower than 50%. Noticeably, no oxime alkylation product was detected when using cyclohexanone oxime, but no explanation of this phenomenon was provided. However, another work describes the O-alkylation of the aromatic oximes R1R2CNOH (56) with THF in the presence of 10 mol % of CuI, 2 equiv of BrCH2CHCH2, and 4 equiv of t BuOOtBu at 120 °C under Ar to give ethers 60 (Scheme 29, a; Scheme 29. Copper(I)/Peroxide-Mediated O-Alkylation of Oximes 56 by THF182a

a

56: R1 = Ph, 2-thienyl, 2-pyrrolyl, 2-furyl, PhCH2CH2; 4-R4C6H4: R4 = MeO, F, Br, O2N; R2 = H, Me; R1/R2 = (CH2)4; 57: R3 = R5C6H(3−4): R5 = 3,5-(MeO)2, 4-Me, H, 4-F, 3-F, 4-Br; 4MeOC6H4CH2, PhCH2, 4-FC6H4CH2, PhCH2CH2, nBu.

The gold(I) center behaves as an electrophilic activator of the CC bond of 57 (b); reaction pathway includes the nucleophilic attack of the oxime O atom on the terminal C atom of the vinyl ligand (c) followed by substitution of the gold(I) center by the H+ (d) to give O-allyl oxime 58 and regenerate the catalyst (e). 3.1.1.2. Reactions with Ethers. Some other reports have been devoted to the O-alkylation of a series of aromatic oximes R1R2CNOH (56) with THF. Shafi181 reported that the aldoximes 56 reacted with THF in the presence of 1 equiv of Yb(OTf)3 at RT to form aldoxime ethers 60 (38−52%; Scheme 28, a).

a

56: R1 = 4-MeOC6H4, 3,4-CH2O2C6H3, 4-MeC6H4, Ph, 4-FC6H4, 4ClC6H4, 4-BrC6H4, 2-naphthyl, 6-(1,2,3,4-4H-naphthyl), 2-furyl; R2 = Ph, H, Me, Et, iPr, tBu, PhCH2; R1/R2 = C6H4(CH2)2, 4MeOC6H3(CH2)3, C6H4(CH2)3.

Scheme 28. Ytterbium-Mediated O-Alkylation of Aldoximes 56 by THF181a

51−88% isolated yields).182 The completeness of these reactions was established by TLC monitoring, but the reaction time was not provided. A plausible mechanism consists of the following steps.182 A THF molecule reacts with tBuO• (generated from t BuOOtBu) to create radical (b), which consequently reacts with allyl bromide to give 2-bromofuran (c). Additionally, the oxime reacts with a CuI center to form the copper(I) complex (d), which then oxidatively inserts into the Br−C bond of (c), giving the copper(III) intermediate (e), which then undergoes reductive elimination to ether 60 accompanied by regeneration of the CuI species (g). 3.1.2. O-Arylation of Oximes. The O-arylation of oximes proceeds in metal-free chemistry by the nucleophilic substitution of ArI/X− in diaryl iodonium salts [Ar2I]X (for recent works, see refs 183−185) or by the substitution of either F− or NO2− in electrophilically activated arenes (for recent works, see refs 186−188); these arylations were usually conducted in basic media. In the presence of a metal center, the scope of these reactions is extended to cross-coupling reactions, and these metal-involving transformations are described below. A series of ketoximes and aldoximes was O-arylated with arylboronic acids in the presence of homogeneous189 and heterogeneous190 copper(II) catalysts. The oximes RR′C NOH (61) reacted with R3C6H4B(OH)2 (62; 2-fold excess) in

a

56: R1 = 4-MeOC6H4, 2-MeC6H4, 3-MeC6H4, 4-MeC6H4, Ph, 4ClC6H4, 2-O2NC6H4, 4-O2NC6H4. L


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Scheme 31. Copper(I)-Catalyzed O-Arylation of the Aromatic Oximes 61 with Aryl Iodides 65191a

the presence of pyridine (2−3-fold excess) and Cu(OAc)2 (1 equiv or 10 mol % in the form of heterogeneous catalyst 63) in 1,2-dichloroethane at RT−50 °C for 48−72 h (Scheme 30).189,190 The author190 suggested that copper(IV) catalyst is Scheme 30. Copper-Catalyzed O-Arylation of Oximes 61 with R3C6H4B(OH)2189,190a

a

61: R1 = 4-MeO, 4-Me, H, 4-Cl; R2 = Me, PhCH2, H, Ph, 4-pyridyl; R1/R2 = (CH)2, C6H4); 65: R3 = H, 4-Cl, 4-O2N.

a

61: R1 = C6H4R4: R4 = 4-MeO, 4-Me, H, 4-Cl, 4-O2N; R2 = H, Me, Ph; R1/R2 = C5H10; 62: R3 = 3-MeO, 2-Me, 3-Me, 4-Me, H, 2-Cl, 3-Cl, 4-Cl, 4-CF3, 4-NO2.

the reductive elimination to give 66 and regenerate the catalytically active copper(I) center (e). The reaction of Oarylation of benzaldoxime under copper(I) or palladium(II) catalysis was applied for transformation of aryl bromides and -iodides to the corresponding phenols under mild conditions.192,193 3.1.3. O-Vinylation of Oximes. A metal-free route for Ovinylation of oximes includes the nucleophilic addition of oximate species to the alkyne CC bond in superbasic media (for recent works, see refs 194−196) or to electrophilically activated allenes in the presence of a base (for recent works, see refs 178 and 179). In the presence of a metal center, the set of reagents is extended to vinyl boronic acids; furthermore, metalinvolving protocols allow the reactions with alkynes to proceed under significantly milder conditions than in metal-free methods. Benzophenone oxime 61 reacted with a 4-fold excess of AlkCHCHB(OH)2 (67) in the presence of copper(I) thiophene-2-carboxylate (CuTC; 1 equiv), 1,4diazabicyclo[2.2.2]octane (DABCO; 3-fold excess), AgClO4 (0.5 equiv), and Na2SO4 in 1,2-dichloroethane at 25 °C for 3 h in air to yield the O-vinyl benzophenone oximes Ph2C NOCHCHAlk (68; 56−96%; Scheme 32).197 When a 2-fold excess of 67 was used, the reaction yields decreased by 26−48%. The copper center activates the B−C

generated in the reaction media under the applied heterogeneous conditions. If this is not simply a misprint, this statement needs some additional rationale because nonredox generation of the copper(II) complex seems to be substantially more probable. The reactions with m- and p-substituted arylboronic acids granted the target products in 38−75% isolated yields after workup, whereas using o-substituted 62 gave 64 in only 10−19% yields. In addition, using 3-thienylboronic acid, the reaction provided 64 in only trace amounts.189 These differences indicate a significant dependence of the reaction yield on the steric hindrance of the applied acid. The reaction could proceed via reduction of the copper(II) center to the copper(I), with oxidative addition of the boronic acid to the formed CuI center to yield an Ar−CuIII−B(OH)2 moiety, followed by transmetalation to give Ar−CuIII−O−NCRR′ and, eventually, reductive elimination of 64. However, other reaction pathways that do not include the formation of CuIII could not be ruled out. Another protocol for the O-arylation of aldoximes and ketoximes includes the reaction of oximes 61 with the aryl iodides R3C6H4I (65) in the presence of CuI (10 mol %), Cs2CO3 (2 equiv), 1,10-phenanthroline (20 mol %), and Na,Ktartrate (40 mol %). This arylation was conducted under dinitrogen in Me2SO (30 °C; for aldoximes) for 0.6−3 h or in toluene (110 °C; for ketoximes) to give O-aryl oximes 66 (Scheme 31, a).191 This method typically gave 25−67% and 40−80% isolated yields of aldoximes and ketoximes, respectively. The reaction with PhBr gave only 30% isolated yield, and it did not proceed for donor R3. This arylation was also performed via an intramolecular protocol for the aryl halide oximes 61 (R1 = H; R2 = 2BrC6H4, 2-IC6H4), and the corresponding isoxazoles were isolated in 80−90% yields. In this reaction, the copper(I) center serves as the C−I bond activator, and a proposed mechanism includes the oxidative insertion of copper(I) into the C−I bond (c) to give a copper(III) intermediate, which ligates the deprotonated oxime (b and d). The reaction is terminated by

Scheme 32. Copper-Catalyzed O-Vinylation of Benzophenone Oxime197a

a

67: Alk = Me, PhCH2OCH2, tBuMe2SiOCH2, PhCH2, iPr, EtO2C(CH2)3, EtCH(C6H4tBu-4), NC(CH2)3, cyclopropyl, nBu, Cl(CH2)4, t Bu, n-hexyl, Cy. M


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bond, and therefore, a reaction route would be similar to the one postulated for the arylation involving ArB(OH)2 (Section 3.1.2). Cyclopalladated oximate complex 69 reacted with 1 equiv of MeO2CCCCO2Me in CH2Cl2 at RT for 1 h to give O-vinyl acetophenone oxime complex 70. This complex originates from the nucleophilic attack of the oxime O atom to the CC bond, followed by the attack of the resulting carbanion on the Pd atom leading to the chelation (Scheme 33; 71%).198 In this reaction,

Scheme 34. Electrophilic Activation Modes of Nitriles

Scheme 33. Palladium(II)-Mediated O-Vinylation198

3.1.4.1.1. Nucleophilic Addition of Oximes to Nitrile Ligands. The ketoximes and amidoximes R1R2CNOH (71) reacted with 2-nitrilium-closo-decaborates [B10H9NCR3](Ph3PCH2Ph) (72; 1.05−2 equiv) in MeCN at RT for 1−15 min to give O-iminoacylated oximes 73 (64−94% isolated yields Scheme 35, a).231 In these reactions, the closo-decaborate cluster, instead of the commonly used metal centers, serves as an electrophilic activator of the CN bond. the palladium(II) center behaves as an electrophilic activator of the CC bond. Although the oxime in 69 is stabilized in the nitrone form, no formation of 2,3-dihydroisoxazole from the known199−202 cycloaddition of the nitrone to the CC bond was observed. In metal-free chemistry, 2,3-dihydroisoxazoles are usually generated when nitrones react with alkynes under the conditions applied for the synthesis of 70.199−202 3.1.4. O-Heteroacylation of Oximes. O-Acyl oximes are easily accessible through well-developed syntheses utilizing oximes and (chloro)anhydrides or carboxylic acids esters (for recent works, see refs 203−206 and refs 207 and 208, respectively). Similarly, O-heteroacylations of oximes by isocyanates and isothiocyanates also proceed easily and under mild conditions (for recent works, see refs 209−212). When preparing O-iminoacyl oximes by the reaction of oximes with nitriles, the additional electrophilic activation of a rather inactive nitrile substrate is required. In the following sections, metalmediated reactions of O-iminoacylation of oximes (section 3.1.4.1) and the reactions of oxime ligands with isocyanates and isothiocyanates (section 3.1.4.2) are discussed. In section 3.1.4.3, the heteroacylation of oximes via metal-catalyzed cross-coupling will be considered. 3.1.4.1. Iminoacylation of Oximes by Metal-Activated Substrates. Nitriles are fairly inert substrates toward nucleophilic attack, and additional electrophilic activation is commonly required for these reactions. This activation is especially important when RCN species featuring donor groups R are treated with nucleophiles. In the absence of metals, the activation of a nitrile is usually achieved via protonation of the nitrile N atom in the first step of the Pinner synthesis213 and in alkylation,214 arylation,215 or trialkylsilylation216 (Scheme 34, a). The electrophilic activation of a nitrile CN group can also be achieved by coordination of RCN to a metal center (b), and thus, a metal center can be employed for activation followed by facile nucleophilic attack at the nitrile C atom. This route of nitrile activation has been commonly used in the past two decades due to its simplicity and efficiency in generating iminoacyl species and their derivatives (for reviews, see refs 217−230). In this section, we focus on more recent reports, which were not included in the previous reviews.

Scheme 35. O-Iminoacylation of Oximes by 2-Nitrilium-closodecaborates231a

a 71: R1 = Ph, PhCH2, 4-morpholyl; R2 = Ph, NH2; R1/R2 = (CH2)5; 72: R3 = Me, Et, nPr, iPr.

Upon the basis of the kinetic data,231 the reported reactivity order of the oxime species toward ligated nitriles was as follows: hydroxyguanidine > amidoxime ≫ ketoxime (with a two-order gap between the rate constants of the reaction for amidoximes and ketoximes). Analysis of other reports232−236 indicates that the overall reactivity of ketoximes toward the nitrile ligands is higher than those of hydrazones and chloroximes and lower than those of hydroxylamines, hydroxamic acids, and amidoximes (Scheme 36). The experimental data indicate the following order of the reactivity of the CN moiety toward oximes in X−NCR Scheme 36. Overall Reactivity Row of Nucleophiles toward Nitrile Ligands

N


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species, X: Alk or Ar237,238 > B10H9231 > PtIV239 ≫ PtII240 ≈ PdII241 ≫ ZnII242 ≈ NiII.243 Mechanistic studies revealed that the nucleophilic addition of oximes to CN and CC bonds proceeds via initial tautomerization of oximes R1R2CNOH to the corresponding aminonitrones R1R2CN+(H)O−, which nucleophilically attack the unsaturated substrates.231,244 3.1.4.1.2. Metal-Mediated O-Iminoacylation of Oximes and Concomitant Reactions. In this section, we focus on recent advances in metal-mediated O-iminoacylation of ketoximes, as we previously comprehensively reviewed metal-mediated Oiminoacylation of amidoxime species in 2016.34 Pombeiro and co-workers reported the nucleophilic addition of a series of ketoximes R1R2CNOH (74) to the nitrile ligands in the platinum(II) complexes trans-[PtCl 2 (NCR 3 ) 2 ] (75).245−247 This addition proceeded in CH2Cl2 under reflux for 15−60 min (Scheme 37) and led to the mono- and bis-

Scheme 38. Nickel(II)-Mediated O-Iminoacylation of Bifunctional Oxime 78251

Scheme 37. Platinum(II)-Mediated O-Iminoacylation of Oximes245−247a

subsequently intramolecularly reacts with the oxime moiety (Scheme 34, b). The nickel(II)-mediated iminoacylation of oximes was later extended to dialkylcyanamides, which serve as iminoacylating agents in this reaction.243 The oximes MeRCNOH (81) reacted with Me2NCN in the presence of 0.25 equiv of NiCl2 in Me2CO or MeiBuCO at 50−70 °C for 1−4 d to give [NiCln{HNC(NMe2)ONCMeR}2(H2O)m](Cl)2−n (82) (Scheme 39, a; 40−82%). Another product was obtained when the reaction was performed at RT. Acetoxime reacted with Me2NCN in the presence of 0.25 equiv of NiCl2 in Me2CO for 1 d at RT, leading to {H2N+C(NMe2)ONCMe2}2[NiCl4] (83, 70%, b).

a

74: R1/R2 = Me/Me, Me/Et, C4H8, C5H10; or R1R2CNOH is camphor oxime; 75: R3 = Ph, CH2Cl, CH2CO2Me.

Scheme 39. Nickel(II)-Mediated Dialkylcyanamide−Oxime Coupling243a

addition trans-[PtCl 2 (NCR 3 ){HNC(R 3 )ONCR 1 R 2 }] products (76; 40−62%; a) and trans-[PtCl2{HNC(R3)ONCR1R2}2] (77; 45−52%; b and c), respectively, featuring monodentate coordinated O-iminoacylated oximes. These reactions were reported245 to give a 1:1 mixture of trans- and cis-isomers of the bis-addition products [PtCl2{HNC(R3)ONCR1R2}2], but this statement requires additional confirmation, as cis/trans-isomerization has never been observed under similar conditions in similar systems.240,248−250 The unstable imine HNC(CH2CO2Me)ONCMe2 was liberated from 77 (R1/R2 = Me/Me) by treatment of the complex with 2 equiv of dppe in CDCl3.245 Bifunctional oxime 78 (1.2 equiv; Scheme 38) reacted with the nitriles RCN (R = Me, nPr, Ph) in the presence of NiCl2·6H2O (1 equiv.; 12−48 h, RT) to give the octahedral nickel(II) complexes [Ni{(HNC(R)ONC(Me)CH 2 C(Me) 2 N(H)CH 2 ) 2 }](ClO4)2 (80; 20−54%).251 The first step of this reaction includes the ligation of 78 to the nickel center to furnish the square-planar complex [Ni{(HONC(Me)CH 2 C(Me) 2 N(H)CH 2 ) 2 }](ClO4)2 (79; 51%, a). The addition of excess RCN leads to 80 (b and c). In this reaction, the nickel(II) center ligates the nitrile species and electrophilically activates the CN bond, which

a

81: R = Me, Ph; 82: n = 1, m = 0, R = Me; n = 1, m = 1, R = Ph; n = 0, m = 2, R = Me.

O


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Amidoximes RC(NH2)NOH (84; 2 equiv) reacted with dimethylcyanamide NCNMe2 (2 equiv) in the presence of Zn(OTf)2 (1 equiv) in EtOAc at RT for 3 h to give bis-chelated zinc(II) complexes featuring O-carbamidine amidoxime ligands cis-[Zn{RC(NH2)NOC(NMe2)NH}2(OTf)2] (85; 76− 82% isolated yields Scheme 40, a).252 Intermediates 86 were

Scheme 41. Nickel(II)-Mediated bis-Addition of Acetoxime to Phthalodinitrile Followed by Hydrolysis of 88253,254a

Scheme 40. Zinc(II)-Mediated Preparation of O-Carbamide Amidoxime Complexes 85 and the Amidinium Salts 87252a

a

84: R = Ph, o-ClC6H4, Et, tBu.

liberated from the reaction mixtures, and the reaction scheme includes the initial formation of trinuclear zinc(II) acetate complexes featuring amidoxime ligands stabilized in the aminonitrone form (b). The acetate ligands are suggested to be generated by the zinc(II)-mediated hydrolysis of EtOAc. After that, the aminonitrone ligand attacks the ligated Me2NCN to give 85 (c). Amidinium salts 87 were generated from 85 upon treatment with TfOH (2 equiv) in MeOH at RT for 5 min (79− 92% isolated yields, d).252 The nucleophilic addition of oximes to the nitrile CN moiety might also proceed as bis-addition to two CN groups in one complex. Reflux of 2 equiv of Me2CNOH with phthalodinitrile and Ni(NO3)2·6H2O in acetone led to in situ formation of 88 (Scheme 41, a).253 This reaction consists of two consecutive addition steps. The addition of 2 equiv of ethylenediamine to the reaction mixture furnishes 89 (77%; b). Complex 89 is formed by a substitution of the oxime ligands at the NiII center followed by hydrolysis of the ligand. Reacting the in situ generated 88 with excess RCN (90; 70 °C, 24 h) led to 1,3,5-triazapentadienate nickel(II) complexes 91 (30−62%; c).254,255 This reaction occurs via hydrolytic splitting of the O−C bonds and nucleophilic addition of the previously formed NH2 moiety to the coordinated nitrile. O-Iminoacylated oximes were hydrolytically split in ketoximecatalyzed syntheses of 1,3,5-triazapentadienate complexes. In the presence of acetoxime and butan-2-one oxime, excess R2CN (92) reacting with either NiCl2 or PdCl2 followed by addition of 2 equiv of a base (typically Et3N) at 100 °C for 12 h gave the neutral complexes 93 (Scheme 42; 47−88%, a).256−258 These reactions did not proceed under the above-mentioned conditions in the absence of the oxime, suggesting that the reaction is oximecatalyzed. The reactions are believed to start from electrophilic activation of a nitrile by its ligation to the metal center (b),

a

90: R = Me, Et, nPr, iPr, (CH2)3Cl.

Scheme 42. Ketoxime-Catalyzed Metal-Mediated Generation of 1,3,5-Triazapentadienate Complexes and Representative Molecular Structure of 93256−258a

a

92: R2 = Me, Ph, p-FC6H4, p-ClC6H4, p-BrC6H4, p-IC6H4, pNCC6H4, m-NCC6H4, 2-pyridyl, 3-pyridyl, 3-(5-methylpyridyl), 4pyridyl, 4-(2-chloropyridyl), 4-(3-chloropyridyl).

followed by nucleophilic addition of the oxime (c), subsequent hydrolysis of the coupling product generating ammonia in the reaction mixture (d), and regenerating the oxime species (e). Finally, NH3 reacts with R2CN bound to the metal center (f). P


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Scheme 44. Rhenium(I)-Mediated O-Heteroacylation of Acetoximate Ligand161

Salicylaldoxime and p-bromosalicylaldoxime 94 (Scheme 43) reacted with MeCN in the presence of CuCl2 and a base (X = Br; Scheme 43. Copper(II)-Mediated Aldoxime−Nitrile Coupling259,260

264, respectively) or RNCO.209,265 Because of the simplicity of the metal-free synthetic protocols, the metalinvolving reactions of this type are poorly explored, and only two studies have been devoted to the copper-catalyzed alkoxyacylation and palladium-catalyzed aminoacylation of oximes. Aromatic and heteroaromatic ketoximes R1R2CNOH (99) reacted with diisopropyl azodicarboxylate iPrO2CNNCO2iPr (100; 4 equiv) in the presence of CuI (10 mol %) and Na2S2O3 (1.5 equiv) to give O-alkoxyacyl oximes 101 (in THF under Ar, 120 °C, 1−8 h, 53−98%; Scheme 45, a).266

Et3N, bipy) in neat MeCN at RT to give complexes 95. These species were obtained via the addition of the HO moiety of 94 across the metal-activated CN triple bond, and the target complexes were isolated in 30% and 62% yields, respectively (a).259,260 The higher yield of the bromo derivative is explained by the promotion of the reaction with a base. When Cu2(OAc)4·2H2O was used instead of CuCl2, the reaction proceeded differently259 (b) and led to trinuclear complex 96 featuring three different ligands. The first ligand originates from the acetonitrile−oxime coupling, whereas the second ligand is formed from the doubly deprotonated salicylaldoximate, and the third ligand is obtained by the coupling of 94 and salicylic nitrile (marked in green in Scheme 43). The latter is likely obtained by copper(II)-mediated dehydration of 94 (see section 3.4). 3.1.4.2. Nucleophilic Addition to Other Unsaturated Substrates. Examples of metal-mediated O-heteroacylation of oximes via nucleophilic addition to other unsaturated substrates are fairly rare because these reactions proceed even in the absence of a metal center. Recent examples of metal-free additions include the reactions of oximes with isocyanates and isothiocyanates.209−212 To the best of our knowledge, only one metal-involving reaction of this nature is known.161 The complexes fac-[Re{ONCMe2}(CO)3(phen)] and fac-[Re{ONCMe2}(CO)3(bipy)] (47) reacted with p-tolyl isocyanide (Scheme 44, a) and p-tolyl isothiocyanide (b), respectively, to give complexes 97 and 98 featuring O-heteroacylated acetoxime ligands (THF, RT, 2 and 8 h, respectively; both 87%). The reaction was also performed with maleic anhydride as the reactivity partner to yield the corresponding ligated O-acyl acetoxime (68% isolated yield).161 3.1.4.3. Heteroacylation of Oximes via Metal-Catalyzed Cross-Coupling. O-Alkoxyacyl and O-aminoacyl oximes (oxime carbonates and carbamates, respectively) can be obtained analogously to O-acyl oximes by the reaction of oximes with aliphatic or aromatic chlorocarbonates ROC(O)Cl and R2NC(O)Cl (for recent works, see refs 261−263 and ref

Scheme 45. Copper-Mediated O-Alkoxyacylation of Oximes266a

a

99: R1 = R3C6H(3−4): R3 = 3-MeO, 4-MeO, 3,4-OCH2O, 4-Me, 3,4Me2, 3,4-(CH2)4, H, 4-Ph, 4-F, 2-Cl, 4-Cl, 4-Br, 4-I, 4-CF3, 4-O2N, benzo[c]; 2-furyl, 2-thienyl; R2 = Me, Et, iPr; R1/R2 = (CH2)3.

The reaction did not proceed when aliphatic cyclohexanone oxime was employed, and it proceeded only in 19% yield for the aldoxime 4-MeC6H4C(H)NOH. A plausible mechanism includes the initial generation of a dialkoxyacyl copper(III) complex (b), which would then react with 99 to give the oximate copper(III) complex (c). The latter is subject to reductive elimination leading to 101 (d) and the catalytically active copper(I) center (e).266 The heteroacylation of oximes utilizing cross-coupling reactions was further extended to the preparation of Ocarbamide oximes (oxime carbamates). Aromatic oximes R1C6H(3−4)C(R2)NOH (102) reacted with isocyanides R3NC (1 equiv.; 103) in the presence of palladium(0) catalyst Q


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3.2.1. Generation of Acyclic Nitrones. Nucleophilic addition of oximes by the N atom to a CC bond results in the formation of acyclic nitrones. A series of the aromatic and vinylic aldoximes and ketoximes R1R2CNOH (105) reacted with the acrylic acid esters H2CC(R3)CO2R4 (106) in the presence of a stoichiometric amount of the CdI2/BF3·nBu2O system.274 The reaction proceeded in a variety of solvents such as CH2Cl2, benzene, and DMF and at a broad range of temperatures (RT−153 °C) for 6−24 h, leading to nitrones 107 (48−100%; Scheme 47, a). The authors274 recognize that the catalytic cycle is

[Pd(PPh3)4] (3 mol %), NaOAc (1 equiv), and H2O (3 equiv) to give 104 (in toluene, 80 °C, 3 h, 52−77%; Scheme 46, a).267 Scheme 46. Palladium-Catalyzed Generation of O-Carbamide Oximes267a

Scheme 47. Preparation of Nitrones 107 from Oximes 106 and Alkyl Acrylates 105 and a Plausible Mechanism of the Reaction274a

a

105: R1 = H, R2 = MeC(H)C(H), PhC(H)C(H), 2-furyl, 2pyridyl, 3-pyridyl, 4-pyridyl; R1 = Me, R2 = 2-pyridyl, 4-pyridyl; 106: R3 = H, R4 = Me, Et, iPr; R3 = Me, R4 = Me, Et. a

102: R1 = 3,4-(MeO)2, H, 4-F, 2-Cl, 3-Cl, 4-Cl, 4-NC, benzo[c]; R2 = Me, Et; R1/R2 = (CH2)3, 4-MeO/(CH2)3, (CH2)4; 103: R3 = 4MeOC6H4, tBu, cyclopentyl, Cy, tBuCH2CMe2, 1-adamantyl.

not clear. However, based upon literature data, we assume that BF3 is required to block the HO-nucleophilic center and stabilize the oxime species in the nitrone form (c),275 whereas CdI2 activates the electrophile (b). The activation of the double CC bond by the cadmium(II) center is confirmed by the catalytic ability of these species in Michael reactions regardless of the nature of the nucleophile (see ref 276 for HC- and ref 277 for HN-nucleophiles, respectively). Miyabe et al. reported that depending on the nature of a catalytic system, the reaction of benzaldoxime 50 with 3substituted 3-acetyl-prop-1-enes, H2CC(H)C(H)ROAc (51), led not only to O-alkyl oximes 52 (see section 3.1.1) but also to nitrones 108 generated via N-alkylation of 50.176 Benzaldoxime 50 reacted with H2CC(H)C(H)ROAc (51) in the presence of [PdCl2(cod)] (10 mol %) under solvent-free conditions (90 °C, 6 h) to give aldonitrones 108 (32−62%) as a mixture of E/Zisomers (Scheme 48).176 Treating benzophenone oxime,

This cross-coupling reaction likely proceeds via an initial insertion of palladium(0) into the oxime O−H bond to give a hydride palladium(II) intermediate (b) (for relevant examples of the Pd0 insertion into O−H bonds, see refs 268 and 269) followed by ligation of the isocyanide (c) with its subsequent insertion into the Pd−O bond (d). Next, the proton-assisted substitution of the hydride ligand by HO− is accompanied by the elimination of H2 (e). The reductive elimination of 104 (f) regenerates the catalytically active palladium(0) species (g) to terminate the overall process.267 Experiments utilizing 18OH2 indicated the selective transfer of the labeled O atom to the carbamate moiety; these data confirm the postulated mechanism. 3.2. N-Functionalization of Oximes

Scheme 48. N-Alkylation of Benzaldoxime Leading to Aldonitrones 108176a

In the absence of metal centers, oximes can be intramolecularly N-alkylated by certain unsaturated moieties to give cyclic nitrones (for recent works, see refs 270−272), N-vinylated by allenes, and N-arylated by diaryliodonium salts to give acyclic nitrones (for recent works, see refs 178 and 179 and refs 183 and 273, respectively). Metal-involving reactions allow the preparation of N-alkyl nitrones via an intermolecular route, and the involvement of metal centers also extends the scope of the reagents for N-vinylation of oximes to vinylboronic acids. In this section, reactions leading to N-functionalization of oximes are divided into two categories depending on the products obtained: alkylation and vinylation furnishing nitrones (section 3.2.1) and formal acylation giving hydroxamic acids (section 3.2.2).

a

R

51: R = p-MeC6H4, Ph, p-ClC6H4, p-CF3C6H4, 1-naphthyl, Cy. DOI: 10.1021/acs.chemrev.7b00264 Chem. Rev. XXXX, XXX, XXX−XXX

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Scheme 50. Tin(IV)-Mediated N-Acylation of Aldoxime Ethers Followed by Cope Rearrangement280a

Ph2CNOH, with H2CC(H)C(H)PhOAc under the same conditions gave Ph2CN+(O−)CH2C(H)C(H)Ph (23%) even after heating for 15 h, with the starting oxime recovered in 42% yield. The acidity of the media plays an important role in the chemoselectivity of these reactions.176 In basic media, the deprotonated oximate attacks the electrophile by the O− center, whereas in acidic media, the oxime attacks the electrophile by the N atom (Schemes 24 and 48, respectively). The aromatic and vinylic oximes R1R2CNOH (109) reacted with allylboronic acids R4CHC(R3)B(OH)2 (110; 1.5−3.0-fold excess) in the presence of Cu(OAc)2 (0.1−2 equiv), pyridine (5−10-fold excess), cod (0−1.2 equiv), and Na2SO4 (8−9-fold excess) in ClCH2CH2Cl at 25 °C for 18 h in air to grant nitrones 111 (typical isolated yields 41−86%; 15% when R1 = R2 = Ph, R3 = R4 = Me; Scheme 49).278,279

a

Scheme 49. N-Vinylation of Oximes 109 Leading to N-Vinyl Ketonitrones 111278,279a

112: R′ = Me, R = Me, n-pentyl, Me2CCH, Ph; R′ = Et, R = Me.

Scheme 51. Metal-Free and Metal-Mediated Routes for CFunctionalization of Oximes

a

109: R1 = 4-MeOC6H4, Ph, 4-CF3C6H4; PhCHCH; R2 = PhCH CH, 2-(2-furyl)vinyl; fluorenone oxime; 110: R3 = H, Me, Et, Ph, 4FC6H4, 4-CF3C6H4, 4-O2NC6H4; R4 = Me, Et, nBu, Ph, 4-FC6H4; R3/ R4 = (CH2)3, (CH2)4, CH2O(CH2)2, CH2C((OCH2)2)(CH2)2, CH2CHtBu(CH2)2, CH2CH(Ph)(CH2)2, (CH2)5.

The metal-involving C-functionalization reactions extend the scope of oximes to sulfonyloximes and aldoximes featuring, formally, labile leaving groups such as RSO2− and H−. These reactions are described in the sections that follow. 3.3.1. C-Functionalization of Chloroximes. The metalmediated C-functionalization of chloroximes, RC(Cl)NOH, is poorly explored as these reactions proceed rather easily even under metal-free conditions and the involvement of metals is not practical. The nucleophilic substitution of the chloride in chloroxime ethers RC(Cl)NOR′ is rare, and it does not proceed with even such strong nucleophiles as nBuLi, PhMgBr, and KCN.294 Voloshin and colleagues reported numerous examples of nucleophilic substitutions for chloroglyoximate iron(II) clathrochelates (115; Scheme 52, section 3.3.1.1 and Table 1).297−328 These species can also be involved in radical substitution reactions (section 3.3.1.2). It is clear that the iron(II) center serves as an electrophilic activator of the C atom of the Cl−CN−O moiety, whereas upon the radical substitution, this metal center apparently serves as a 1e-donor ̅ to stabilize the radical ligand formed as an intermediate. 3.3.1.1. Nucleophilic Substitution in Chloroglyoximate Clathrochelates. Chloroglyoximate clathrochelates featuring both iron(II)/boron(III) centers readily react with C-, N-, O-, P-, and S-nucleophiles and with I− as a nucleophile to give ketoximes, amidoximes, oxyoximes, phosphoximes, thiooximes, and iodoximes, respectively (Table 1). These reactions have been well-studied for iron(II) species (Scheme 53), but some reactions were performed with cobalt(II, III)295−298 and ruthenium(II)296 clathrochelates. The substitution gives open-

A proposed mechanism for this reaction is similar to that suggested for the O-arylation of 61 (Scheme 30), but oxime 109 is believed to coordinate to the copper center by the oxime N atom due to the absence of steric hindrance at the metal center provided by the amino acid ligands in 63. 3.2.2. Generation of Hydroxamic Acids. Zhou et al. reported the N-acylation of the aldoxime ethers RC(H)NOR′ (112) by 3,3,4-trimethylpent-4-en-2-one (113) followed by oxy2-azonia-Cope rearrangement281−284 leading to hydroxamic acid ethers (114) in the presence of a stoichiometric amount of SnCl4 (RT, 24 h, 39−97%; Scheme 50).280 This reaction was than extended to the preparation of macroheterocycles (see section 5.2.6). Tin(IV) is required for the activation of the carbonyl moiety by coordinating with the O atom and increasing the electrophilicity of the carbonyl C atom. 3.3. C-Functionalization of Oximes

In metal-free organic chemistry, the success of C-functionalization of oxime species is mostly determined by the leaving ability of the X group (Scheme 51). In practice, however, only chloroximes (X = Cl), that are typically prepared by chlorination of aldoximes via the Piloty reaction,202,285,286 function for the conventional C-functionalization of the oxime moiety. Their reactions with C-,287,288 N-,83,289,290 O-,289,291 and S-nucleophiles289,291−293 in basic media lead to ketoximes, amidoximes, oxyoximes (or hydroxamic acids), and thiooximes (or thiohydroxamic acids), respectively. These reactions are believed to proceed via the generation of intermediate nitrile oxides. S


chain species (a and b) accompanied, in the case of nucleophiles with two or more functional groups, by cyclization (c). The obtained data are summarized in Table 1. 3.3.1.2. Radical Substitution in Chloroglyoximate Clathrochelates. Some reports by Voloshin’s group focused on the preparation of ketoximes from chloroxime complexes by using radical substitution protocols. The clathrochelate 115 (R1 = Ph; R2 = F) reacted with cyclohexane,329 EtOH, iPrOH (mixture of the alcohol with benzene used as a solvent),330 or neat 1,4dioxane331 in the presence of tBuOOH (5−20 mol %) under reflux for 1.5−14 h to give ketoxime complexes 118 after workup (25−60%; Scheme 54). In the reaction with dioxane, the clathrochelate derived from the bis-substitution was isolated as a byproduct (10%). In the reaction with the alcohols, substitution proceeded at the α-position to the O atom. 3.3.2. C-Functionalization of Sulfonyl Oximes. The αarylsulfonyl oxime ethers PhSO2C(R1)NOCH2Ph (120) reacted with the aliphatic carboxylic acids R2CO2H (119) to furnish the oxime ethers R1R2CNOCH2Ph (121), which were isolated from the reaction mixtures in 28−93% yields (Scheme 55, a).332 Kinetic measurements indicated that, in general, the reaction was promoted by electron-withdrawing R1 substituents; the reaction rate increased in the following order Me ≪ EtO2C < CF3 < H < NC (the high reactivity of aldoximes is rationalized by the small steric hindrance at the reaction center provided by the H atom332). The experimental data suggest that the reaction mechanism includes the silver(II)-mediated generation of alkyl radical R• (b and c) (potassium persulfate is required to oxidize the AgI to AgII; d), which subsequently attacks the oxime C atom (e) and substitutes the PhSO2• moiety (f). We believe that this

T

P

E4

I

O

E3

E6

N

E2

S

C

E1

E5

nucleophilic center nucleophile

open-chain: −SR3 (R 3 = Me, HO(CH2)2, HS(CH2)2, nBu, tBu, 1-adamantyl, HS(CH2)2S(CH2)2, EtS(CH2)2S(CH2)2, [O3S(CH2)3]−, 3,5-(tBu)2-4-HOC6H2, Ph, C6F5, 4-ClC6F4, 4-CF3C6F4, 2HO2CC6H4, 3-HO2CC6H4, 4-HO2CC6H4, [(CH2)2O(CH2)2B10H9]2−); ring-closure: HSCH2CH2SH, HS(CH2)2S(CH2)2SH, 1,2-(HS)2C6H4, dimercaptomaleodinitrile, 4,5-dimercapto-1,3-dithiole-2-thione, thiacalix[4]arenes I−

open-chain: HP(=X)R32 (X = O, S; R3 = EtO, Ph)

open-chain: HNR3R4 (R3 = H, Me, nBu, Cy, HCCCH2, 2-pyCH2, MeS(CH2)2, 2-py(CH2)2, HO(CH2)2, MeO(CH2)2, H2N(CH2)4, H2N(CH2)5, 4-tBuC6H4, 4-HO2CC6H4, 4-O2NC6H4; R4 = H, Me; R3/R4 = C4H8O, (CH2)5), imidazole; ring-closure: H2N(CH2)2NH2, Me2N(CH2)3NH2, 2-H2NC6H4NH2, benzamidine, DBU open-chain: HOR3 (R3 = Et, nBu, nC3F7CH2); (nBu4N)OH; ring-closure: (HO)2R4 (R4 = (CH2OCH2CH2)2, O(CH2CH2OCH2CH2)2)

open-chain: 5,6-dimethylpyrazine-2,3-dicarbonitrile; ring-closure: DBU

2 equiv CuI, 1.1 equiv NaI, in N-methyl-2-pyrrolidone:toluene (1:1, v/ v), 110 °C, 2 h, under Ar

1−8 equiv of the alcohol, 1.5−6 equiv [Cd(NR52)2] (R5 = iPr, SiMe3), THF, from −20 °C to −10 °C, 1−12 h, under Ar or 4 equiv (nBu4N) OH, 1,4-dioxane, RT, reaction time is not reported, in air or 0.33 equiv. Na in HC(OEt)3, RT, 3 h, in air (for R3 = Et) 0.8−1.2 equiv of a nucleophile, 5 mol % [(PhCH2)3NH]Cl, 5.4 equiv K2CO3 or 0.7−0.8 mol % NaOH in H2O, MeCN or CH2Cl2, RT, 1.5−2.5 h, in air 1.1−10 equiv of the thiolate; solvents: toluene, CH2Cl2, MeCN, THF, DMF or their mixtures; from −10 to 110 °C, 40 min−7 d, under Ar or in air

11−96%

32%

R1 = Ph; R2 = F

70−98%

24−69%

47−95%

R1 = Ph, Cl; R2 = n Bu, 1-adamantyl, Ph, C6F5, 3OHCC6H4, F

R1 = Ph; R2 = F

R1 = Ph, Cl; R2 = F

R1 = H, Ph, Cl; R2 = nBu, Ph, F

yields 38−60%

R1/R2 R1 = Ph; R2 = F

conditions 1 equiv. Me2C4N2(CN)2, 2 equiv [Cd{N(SiMe3)2}2], THF, 12 h, under Ar or 10.5 equiv DBU, Me2SO, RT, 5 weeks 2−11 equiv of the amine, solvents: toluene, CH2Cl2, THF, MeCN, DMF, Me2SO, or their mixtures; typically RT, 2−24 h, in air or under Ar

328

297, 298, 302, 306, 308, and 316−327

304 and 315

307, 313, and 314

299 and 300 299, 301−312

references

Scheme 52. Nucleophilic and Radical Substitution in Chloroglyoximate Clathrochelates

entry

Table 1. Reactants, Conditions, and Yields of the Nucleophilic Substitution in Chloroglyoximate Clathrochelates

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mechanism requires additional confirmation because carbon radicals, in fact, are electron-deficient species and should act as electrophiles. However, the kinetic data support just the opposite, viz., the reaction is promoted by electron-withdrawing groups R1, which means that the oxime ether should be attacked by the nucleophile. Because of that, an alternative mechanism including the generation of R2(−) should also be considered. The reactive aldoxime and cyanoxime ethers 120 (1.2 equiv) reacted with alkenes R2R3CCR4R5 (122) in the presence of cobalt(II) catalyst 123 (2 mol %) and PhSiH3 (1 equiv) in EtOH:MeCN (4:1, v/v) mixture at RT for 1.5−15 h under argon to yield oxime ethers 124 (63−99%; Scheme 56).333 The reaction followed Markovnikov’s rule, and this supports the generation of the carboradical, rather than the corresponding carbanion, from the alkene.

Scheme 53. Nucleophilic Substitution in Chloroglyoximate Clathrochelates

Scheme 56. Cobalt(II)-Catalyzed Alkylation of α-Arylsulfonyl Oxime Ethers 120333a

Scheme 54. Radical substitution in chloroglyoximate clathrochelate 115329−331a

a

120: R1 = H, CN; 122: R2 = R6OCH2: R6 = PhCH2, PhCO, 4OHCC6H4, 2-pyrrolylcarbonyl, 2-furylcarbonyl, tBuPh2Si; Ph(CH2)2, PhCO(CH2)2, PhCO2(CH2)2, Ph; R2/R3 = (CH2)2(CHPh)(CH2)2, R2/R4 = 2-CH2C6H4; R3 = H, Me; R4 = H, Me; R5 = H, Me.

Reaction mechanism includes the generation of R• by the reaction of RH with tBuOOH followed by its attack on the oxime C atom and substitution of Cl•.

a

3.3.3. C-Functionalization of Aldoximes. The β- and γaryl aldoxime ethers featuring a halide (Hal = I, Br) in the orthoposition of the aryl ring at R2(Hal)C6H3XCH2C(H)NOR1 (125) underwent intramolecular cross-coupling during reflux in dioxane in the presence of 10 mol % [Pd(PPh3)4] and 2 equiv of K2CO3. This reaction granted cyclic aromatic oxime ethers 126 in 56−89% isolated yields as mixtures of the E/Z-isomers (Scheme 57, a).334 Sequence of possible stages of the reaction includes the oxidative insertion of palladium(0) into the C−Hal bond (b), 1,2-insertion of the oxime CN bond into the Pd−C bond (c), and then, reductive elimination to regenerate the catalyst (e) and give HHal (d). The reaction was also performed with the 2-bromoindole derivative, which gave the cyclic aromatic oxime (70%).334 The 2-pyridyl carbaldoxime ethers 127 reacted with a 5-fold excess of R3C6H4I (128) in the presence of [PdCl2(PPh3)2] (10 mol %) and a 2.5-fold excess of AgOAc (dioxane, 125 °C, 24 h) to grant aromatic ketoximes 129 isolated in 48−92% yields (28% in the case of the sterically hindered R2 = tBu) (Scheme 58, a).335 The reaction did not proceed with aromatic aldoximes featuring CH groups at both ortho-positions, and this indicates the need for the chelation of the PdII center. Upon the basis of experimental data,335 the suggested plausible mechanism includes the formation of a palladium(II) chelated complex (b), oxidative insertion of the palladium(II) center into the C−I bond (c), and Ag+-supported formation of the iminoacyl

Scheme 55. Silver-Catalyzed Alkylation of α-Arylsulfonyl Oxime Ethers 120332a

a 119: R2 = isopentyl, cyclopentyl, Cy, nBuCHEt, n-nonyl, adamantyl, (CH2)5CCH(CH2)5, tert-tridecyl, PhCH2CHMe, Ph(CH2)3, Br(CH2)10, MeO(CH2)3CHnBu, 5-(adamantan-2-onyl), C 6 H 4 O 2 CH 2 CH, MeO 2 C(CH 2 ) 6 , EtO 2 C(CH 2 ) 2 CMe 2 , 4ClC6H4OCH2, 4-ClC6H4OCMe2, 4-PhC6H4C(O)(CH2)2, PhC( O)(CH2)2CHMe, Ph(CO)N(CH2CH2)2CH, isoindolinedionylmethyl; 120: R1 = Me, PhCH2CH2, H, CF3, C2F5, nC4F9, CO2Et, CN.

U


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Scheme 57. Intramolecular Palladium-Catalyzed Arylation of Aldoximes 125334a

Scheme 59. Metal-Mediated Dehydration of Aldoximes Leading to Nitriles336−341a

a 130: R = aromatic R1C6H(0−4): R1 = 4-Me2N, 2,3,4-(MeO)3, 3,4,5(MeO)3, 2,3-(MeO)2, 2-HO-3-MeO, 2-MeO, 3-MeO, 4-MeO, 2,4(HO)2, 2-HO, 4-HO, 5-Br-2-HO, 3,5-(Cl)2-2-HO, 5-Cl-2-HO, 3-Br-5Cl-2-HO, 4-HO, 2-HO, 2-Me, 3-Me, 4-Me, 2,4,6-Me3, 4-iPr, 4-tBu, 4MeS, 4-(EtO)2CH, H, 4-Ph, 4-F, 2-Cl, 3-Cl, 4-Cl, 2,4-Cl2, 2,6-Cl2, 2Cl-4-F, 2-Cl-6-F, 4-Br, (F)5, 4-CF3, 3-MeO2C, 4-NC, 2-O2N, 3-O2N, 4-O2N, 2,4-(Cl)2-5-O2N; 1-naphthyl, 2-naphthyl, 9-anthracenyl, 9phenanthrenyl, 4-pyrenyl; heteroaromatic: 5-indolyl, 5-(4-methyl)thiazolyl, 2-furyl, 3-furyl, 2-thienyl, 3-thienyl, 3-pyridyl, 4-pyridyl, 3-(2amino)pyridyl, 4-(3-chloro)pyridyl; alkynylic: PhCC; vinylic: PhCHCH, 4-ClC 6 H 4 CHCH; aliphatic: Me, PhCH 2 , Et, PhCHMe, PhCH2CH2, PhCH2CH(NHBoc), 4-iPrC6H4CH2CHMe, n Pr, CH3CH(OMe)CH2, Et2NCH2, nC5H11, nC6H13, Cy, (S)-Me2C CH(CH2)2CHMeCH2, tBuO2CN(CH2CH2)2CH, nC9H19, n-C11H23; [1,1′-binaphthalene]-2,2′-dicarbaldoxime.

a

125: R1 = Me, PhCH2; R2 = MeO, Me, H, MeO2C; X = NTs, O, CH2NTs; Hal = Br, I.

Scheme 58. Intermolecular Palladium-Catalyzed Arylation of Aldoximes 127335a

The coupling products i appear to be significantly less stable than their ketoxime analogs and undergo splitting to nitriles and ligated iminoles, which, in particular, undergo tautomerization to carboxamides upon decoordination (c). The first example of ligand-mediated dehydration of aldoximes via addition to a nitrile ligand at a PtIV center was reported in our work.342 In that work, trans-[PtCl4{HNC(Et)ONC(H)R}2] (i; R = Ph, PhCO, Me) were isolated from the reaction mixture and fully characterized, including by X-ray diffraction. Further conversions of trans-[PtCl4{HNC(Et)ONC(H)R}2] to RCN and trans[PtCl4{HNC(Et)OH}2] were also reported. 3.4.1. Metal-Involving Dehydration of Oximes Yielding Nitriles. Dehydration of Aldoximes. The dehydration of aldoximes to nitriles has been studied both under metal-free conditions with various organic dehydrating reagents343−346 and at various metal centers (Scheme 59, a), which are all more kinetically labile than platinum(IV). These reactions were typically conducted under reflux in MeCN, and intermediates i were not detected, but similarly to the reaction at the PtIV center, their formation was assumed. Recently, relevant reactions were performed for aryl, hetaryl, vinyl, and alkyl oxime species at both transition metal centers (e.g., TiIV,347 CoII,336 NiII,337 CuII,338 PdII,339−341 and OsII348) and at some p-element centers such as GaIII349 and SnII.349 The aromatic and aliphatic aldoximes RC(H)NOH (130) were transformed to the corresponding nitriles 131 in the presence of TiCl4 (1.2 equiv) in pyridine at 40 °C for 30−45 min. The nitriles were isolated in excellent (85−99%) yields.347 However, another group of aldoximes 130 was converted to the corresponding nitriles 131 in the presence of CoCl2 (3 mol %) and excess NaF in neat MeCN at 80−85 °C for 1−6.5 h (82− 99%). In this reaction, Co(acac)2 and Co(OAc)2 behaved similarly to CoCl2, whereas the reaction yields depended significantly on the inorganic base employed. Thus, when

a

127: R1 = H, 4-PhCH2O, 4-TsO, 3-Ph, 4-Ph, 4-(4-FC6H4), 4-Br, benzo[c]; X = CH,N; R2 = Me, Me3Si(CH2)2, iPr, tBu, cyclopentyl, Cy; 128: R3 = 4-MeO, 3-Me, 4-tBuMe2SiOCH2, 2-F, 4-F, 3-Cl, 4-Br, 4CF3, 4-NC, 4-MeO2C.

palladium(IV) intermediate (d) followed by reductive elimination of ketoxime 129 accompanied by the regeneration of the palladium(II) center. The reaction is not catalyzed by palladium(0) species (e.g., [Pd2dba3] or [Pd(PPh3)4]), under dinitrogen, while it proceeds in the presence of these precatalysts in air. These observations confirm the mechanism involving PdII/ PdIV rather than Pd0/PdII. 3.4. Metal-Involving Dehydration of Aldoximes

As with ketoximes (see section 3.1.4.1.1), aldoximes react with nitriles in the presence of an appropriate metal source to give coupling products derived from the nucleophilic attack of the oxime HO moiety to the nitrile CN bond (Scheme 59, b). V


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recommended for metal-mediated conversions of aldoximes to nitriles. Aromatic, heteroaromatic, vinylic, and aliphatic aldoximes 130 underwent dehydration in the presence of the guanidinate osmium(II) complex [OsCl{4-ClC6H4NC(NHiPr)NiPr}(4-iPrC6H4Me)] in MeCN at 80 °C for 1−3 h under argon to give the corresponding nitriles in 81−90% yields.348 Complexes featuring other substituents in the aryl ring of the guanidinate ligand also efficiently catalyzed the dehydration. Aromatic, heteroaromatic, alkynylic, vinylic, and aliphatic aldoximes 130 were converted to nitriles 131 in the presence of GaCl3 (10 mol %) in neat MeCN (80 °C, 2.5−36 h; 80− 99%).349 All these aldoximes were also dehydrated in the presence of SnCl2 (10 mol %) instead of GaCl3 (3−48 h; 61− 90%).349 Recently, it was reported that aromatic and aliphatic aldoximes 130 were converted to the corresponding nitriles 131 in the presence of Fe(OTf)3 (10 mol %) under argon in toluene under reflux for 6−48 h (typically 24 h), and the appropriate nitriles were formed in 47−94% 1H NMR yields and some nitriles were isolated in 69−85% yields.351 The reaction conditions were similar to those of the aldoxime dehydration studied under heterogeneous metal oxide catalysis conditions,352−354 which are all significantly harsher than the conditions of nitrile-mediated dehydration. These observations indicate a favorable effect of nitrile addition on the reaction conditions. Dehydration of oximes applied for hydration of nitriles. Metal-catalyzed aldoxime−nitrile coupling342 has been applied for the selective hydration of the nitrile moiety to give the carboxamide group. For these reactions, excess acetaldoxime MeC(H)NOH is usually employed. As depicted in Scheme 59, the nitrile, electrophilically activated by the metal center, is attacked by the aldoxime to give an unstable O-iminoacylated aldoxime ligand, which then splits to carboxamide 133 and acetonitrile (Scheme 60).

MCO3 (M = Li2, Na2, Ca, Sr, Ba) or Ba(OAc)2 were used as bases, the reported yields of the reaction were lower when using NaF.336 However, another paper dealt with the conversion of aromatic, vinylic, and aliphatic aldoximes RC(H)NOH (130) to nitriles 131 promoted by [Ni{2,6-di(pyrazol-1-yl)pyridine}(H2O)3]Cl2 (5 mol %). The reaction occurred over 4 Å-MS in MeCN under reflux for 7 h (46−100%);337 it did not proceed for 2pyridylcarbaldoxime, whereas for 3,4-dihydroxybenzaldoxime, the yield of the reaction was only 16%. We assume that the chelation to the metal center inhibited the conversion of the oximes to the corresponding nitriles. The reaction also proceeded in the case of CoCl2 or NiX2 (X = Br, NO3, AcO), but the yields of 131 were significantly lower. No conversion was observed when FeCl2 was used instead of NiCl2 or in the absence of any catalyst. Aromatic, heteroaromatic, and aliphatic aldoximes 130 also gave the corresponding nitriles 131 in the presence of Cu(OAc)2 (10 mol %) in neat MeCN under reflux for 0.25−2 h (57− 100%).338 The authors338 reported that other late 3d-transition metal species, viz., CuO, MCl2, and M(OAc)2 (M = CoII, NiII, CuII, ZnII), were also efficient in this transformation but with yields lower by 3−10%. In the presence of MnCl2, Mn(OAc)2, or FeCl2 instead of Cu(OAc)2, the dehydration was substantially less efficient, and the corresponding nitrile 131 (R = 4MeOC6H4) was generated in only 12−16% GC yields. The reaction did not proceed in the absence of catalyst and/or at RT. The immobilization of Cu(OAc)2 at Fe3O4 nanoparticles featuring external NH2 moieties was reported to increase yields of the nitriles.350 Aromatic aldoximes 130 were transformed to the corresponding nitriles in the presence of the immobilized Cu(OAc)2 (7 mol %) in MeCN under reflux for 1−4 h (50−98% isolated yields).350 Because almost half of the copper(II) was leached from the heterogeneous catalyst after six cycles, we assume that the catalysis by the copper(II) center could proceed under homogeneous conditions. Several works339−341 have demonstrated the PdII-mediated conversion of 130 to 131. For aromatic, heteroaromatic, and vinylic aldoximes 130, this transformation typically proceeded in the presence of both Pd(OAc)2 (10 mol %) and PPh3 (20 mol %) in neat MeCN at 80−85 °C for 1−12 h (81−91%).339 However, when R was 2-naphthyl, the reaction unexpectedly required 80 h to complete. The authors339 noticed that in the case of aryls featuring moderate acceptor or donor groups, 10−50 mol % of Cs2CO3 should be used to promote the reaction, but the role of the base is unclear. In addition, the reaction also proceeded in the absence of PPh3, but the addition of the phosphine accelerated the reaction. Aromatic, heteroaromatic, and aliphatic aldoximes 130 were dehydrated to 131 (44−95% 1H NMR yields) in the presence of [Pd(NO3)2(en)] (10 mol %) in neat MeCN at 60 °C for 16 h.340 Another work dealt with the transformation of aromatic, vinylic, and aliphatic aldoximes 130 to the corresponding 131 in the presence of [PdCl2{2-Ph2PC6H4C(H)NOH}] (5 mol %) in neat MeCN at 82−100 °C for 24 h, giving 82−92% isolated yields of the nitriles after workup.341 The isolated yields of 4MeOC 6 H 4 CN, 336−339,341 4-O 2 NC 6 H 4 CN, 337−339,341 and PhCHCHCN336,337,339,341 (80−90%338,339,341) are rather high for all reported procedures. An exception is 4O2NC6H4CN, for which only 54% yield of the target product was obtained.337 On the basis of these data, we believe that highyield methods that utilize cheap salts of CoII336 and CuII338 can be

Scheme 60. Metal-Catalyzed Selective Hydration of Nitriles to Carboxamides Accompanied by the Dehydration of Acetaldoxime355−358a

a

132: R = aromatic: R1C6H(3−4): 4-MeO, 3-Me, 4-Me, H, 2-F, 4-Cl, 2,6-Cl, 2-EtO2C, 4-O2N; heteroaromatic: 2-thienyl, 2-pyridyl, 3pyridyl, 2-(3-methyl)pyridyl, 3-pyridyl, 4-pyridyl, 2-furyl; vinylic: CH 2 CH, 1-(3-(3-hydroxy)indolin-2onyl)vinyl, PhCHCPh, PhCHC(CO2Me); aliphatic: PhCH2, PhCH2CH2, PhC(O)CH2, n-C7H15, n-C11H23.

The aromatic, heteroaromatic, and aliphatic nitriles RCN (132) were selectively hydrated to carboxamides 133 in the presence of a 2−4-fold excess of acetaldoxime and 10 mol % Na2MoO4 in aqueous media under reflux for 5−16 h, with 133 isolated in 73−95% yields.355 NaVO3 and Na2WO4 could also catalyze the reaction, but in those cases, the yields of 133 were substantially lower. The reaction did not proceed at RT within at least 24 h. Another group of aromatic, vinylic, and aliphatic nitriles 132 and a series of R′C(OH)HC(CN)CH2 (R′ = 4-MeOC6H4, 2W


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generally assumed that both reactions are promoted by a metal center.41 Metal-mediated Beckmann rearrangements of aldoximes were comprehensively reviewed in 2015,41 and therefore, in this section, we describe only recent relevant works. We assume that in the first cycle of the listed plausible mechanism (Scheme 62), step (a) consists of the oxidative insertion of a Mn center into the oxime N−O bond followed by β-elimination (b) and reductive elimination of water (c) to give the nitrile ligand. This type of dehydration is fundamentally different from the oxime-mediated dehydration of aldoximes described above, and it worked even for aldoxime ethers.359 The oxime ethers RC(H)NOMe (R = Ph, 2-naphthyl, 4pyridyl, nC7H15, R1C6H4: R1 = 4-Me2N, 4-MeO, 2-Me, 4-Me, 4-F, 4-NC, 4-O2N) were transformed to the corresponding nitriles (80−85%) in the presence of a [RuH2(CO)(PPh3)3]/Xantphos system (3.5−5 mol %) in toluene under reflux for 24 h under N2.359 The experimental conditions of this reaction were more drastic than those for the nitrile-mediated dehydration of aldoximes, and it can be concluded that this catalytic cycle is rate limiting in this reaction. As proposed,41 the second catalytic cycle (i.e., the metalmediated hydration of a nitrile furnishing an amide) consists of metal-mediated aldoxime−nitrile coupling (d), heterolytic splitting of the N−O bond accompanied by proton transfer (e), and elimination of amide (f) to regenerate the nitrile complex (g). Additional experiments with 18O-labeled aldoximes or 18OH2 were performed to confirm the proposed mechanism.360 On the one hand, the metal-catalyzed rearrangement of 4-MeC6H4C(H)NOH to MeC6H4CONH2 using 18OH2 gave exclusively unlabeled product. On the other hand, the rearrangement of 4MeC6H4C(H)N(18O)H led to MeC6H4C(18O)NH2. These data strongly support the mechanism depicted in Scheme 62. Aromatic, heteroaromatic, vinylic, and aliphatic aldoximes RC(H)NOH (134) are transformed to the corresponding amides 135 in the presence of cis-[RuCl2{2-Ph2PC6H4C(H) NOH}2] (5 mol %) in water at 100 °C for 2−14 h, with the amides isolated in 75−90% yields.361 The reaction proceeds for 2−12 h (24 h for 2-pyridylcarbaldoxime). The authors suggest that the reversible exchange of 2-Ph2PC6H4C(H)NOH ligands to 2-pyC(H)NOH slows down the reaction involving 2-pyridylcarbaldoxime.361 The corresponding nitriles RCN were detected in the reaction mixtures as byproducts. Aromatic, heteroaromatic, vinylic, and aliphatic aldoximes 134 underwent Beckmann rearrangement to amides 135 in the

MeOC6H4, 3-MeC6H4, Ph, 2-ClC6H4, 4-O2NC6H4, 2-naphthyl, 3-pyridyl, 4-pyridyl, n-C5H11) were selectively converted to 133 in the presence of [Pd(OAc)2(PPh3)2] (10 mol %) and a 2-fold excess of acetaldoxime in an EtOH/H2O (4/1, v/v) mixture. This reaction was conducted under reflux for 3−5 h, and the carboxamides were obtained in 70−94% yields.356,357 However, with PhCHC(Ph)CN, the reaction was not complete even after 12 h, and PhCHC(Ph)CONH2 was isolated in 76% yield, while the starting nitrile was recovered from the reaction mixture (21%). Aromatic, heteroaromatic, vinylic, and aliphatic nitriles 132 were converted to carboxamides 133 in the presence of InCl3 (5 mol %) and a 3-fold excess of acetaldoxime in toluene under reflux for 3−5 h to give 73−99% isolated yields of 133 after workup.358 Similarly to the aforementioned PdII-mediated reaction,356 the reaction of PhCHC(Ph)CN was slower than those of the other nitriles, taking 24 h. The reaction with acrylonitrile was performed at RT for 24 h to avoid polymerization of the nitrile.358 3.4.2. Beckmann Rearrangement Leading to Carboxamides. Although the metal-mediated rearrangements of aldoximes to carboxamides seem to be very similar to the classic Beckmann reactions, their mechanisms are different (Scheme 61). It is generally believed that the rearrangement of aldoximes Scheme 61. Metal-Mediated Beckmann Rearrangement of Aldoximes to Carboxamidesa

a

134: R = aromatic: R1C6H(0−4): R1 = 2-MeO, 3-MeO, 4-MeO, 3MeO-4-HO, 3-HO, 2-HO, 4-CF3O, 2-Me, 3-Me, 4-Me, 4-Et, 4-tBu, 4MeS, H, 2-F, 3-F, 4-F, (F)5, 2-Cl, 3-Cl, 4-Cl, 2-Cl-6-F, 2,4-Cl2, 2,6-Cl2, 2-Br, 3-Br, 4-Br, 2-CF3 , 4-CF3, 2-O2 N, 4-O 2N, 2,4-(O 2N)2 ; heteroaromatic: 1-naphthyl, 2-naphthyl, 9-anthracenyl, 2-pyrrolyl, 2thienyl, 2-(3-methylthienyl), 2-(5-methylthienyl), 2-(4-bromothienyl), 2-(5-bromothienyl), 2-furyl, 3-furyl, 2-(5-methylfuryl), 2-pyridyl, 3pyridyl; vinylic: R2CHCH: R2 = 4-MeOC6H4, Ph, 4-ClC6H4, 2O2NC6H4, 2-furyl; aliphatic: PhCH2CH2, nC5H11, nC6H13, Cy, (S)Me2CCHCH2CH2CH(Me)CH2.

involves (i) the initial dehydration of the oxime giving the corresponding nitrile and (ii) the subsequent hydration of the resulting nitrile by the aldoxime leading to the carboxamide. It is

Scheme 62. Mechanism of Metal-Mediated Beckmann Rearrangement of Aldoximes

X


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presence of [RuCl(PPh3)2{C19H13N5}]Cl (C19H13N5 is 2,6bis(imidazo[1,2-a]pyridin-2-yl)pyridine; 0.5 mol %) in air in H2O under reflux for 12 h (32−99% isolated yields).362 The reaction was also performed for aldoximes generated in situ from an aldehyde and [H3NOH]Cl. Aldoximes 134 were transformed to amides 135 in the presence of the ruthenium(IV) complex [{RuCl(μ-Cl)(η3:η3C10H16)}2] (C10H16 is 2,7-dimethylocta-2,6-diene-1,8-diyl; 5 mol %) under dinitrogen in a H2O:glycerol = 1:1 (v/v) mixture at 120−130 °C or under MW (300 W) irradiation for 0.5−2 h.363 Amides 135 were isolated in 25−92% (with heating) or 41−93% (with MW irradiation) yields. From the experimental data indicating that the ruthenium(II) catalyst [{RuCl(μ-Cl)(η6C6Me6)}2] also catalyzes this reaction and that the ruthenium(IV) center oxidizes alcohols to carbonyl compounds with subsequent reduction to the ruthenium(II) center,364 we assume that the reaction can be catalyzed by in situ-generated ruthenium(II) compounds. Kinetic experiments363 confirmed the intermediate formation of nitrile. However, another work reported the conversion of aromatic, vinylic, and aliphatic aldoximes 134 to 135 in the presence of [PdCl2{2-Ph2PC6H4C(H)NOH}] (5 mol %; H2O, 100 °C, 24 h under Ar, 58−91%).341 The kinetics of the rearrangement of benzaldoxime were studied, and based upon the data obtained [i.e., (i) the substantial induction period (ca. 2 h) of the reaction, (ii) the formation of a black suspension (possibly palladium black), and (iii) the inhibition of the reaction by mercury (the socalled mercury drop test365), the authors believe that this Beckmann rearrangement is provided by Pd0 nanoparticles formed under the reaction conditions.341

Scheme 63. Metal-Mediated Reduction of the CN Bond of Oximes

3.5. Reduction of Oxime Ethers and Esters

The first part (section 3.5.1) of this section is focused on the reduction of oximes with N−O bond preservation, whereas the second part (section 3.5.2) surveys the reactions with N−O bond reduction. In the latter section, the reactions are considered according to the degree of reduction of the oxime moieties. We begin with 1e-reduction of oximes leading to homoazines ̅ (section 3.5.2.1), and then, we consider 2e-reduction of oximes ̅ leading to imines and enamines (section 3.5.2.2). Finally, we analyze 4e-reductions yielding amines (section 3.5.2.3). It is ̅ notable that all reactions discussed in this section were conducted exclusively with oxime ethers or oxime esters as starting materials. 3.5.1. Reduction of Oxime Ethers and Esters with Preservation of the N−O Bond. In all reactions considered in this section, coordination of the oxime N atom to a metal is required to increase the electrophilicity of the oxime carbon, which is attacked by the hydride or a carbanion with subsequent reduction of the CN bond (Scheme 63). Typically, this coordination is sufficiently strong only when chelation occurs via both the oxime N atom and supporting X center (route A). Alternatively, esterification of the oxime moiety leads to esters that might give another type of metallacycle, viz., M{O C(R)O−NCR2} (route B), thus facilitating the reduction. The aromatic and aliphatic ketoxime ethers R1R2CNOR3 (136) were reduced to the corresponding O-substituted monoalkyl hydroxylamines 137 in the presence of H2 (100 bar) and B(C6F5)3 (5 mol %) in toluene at 25−60 °C (Scheme 64, a).366 The reaction proceeded for 18 h and gave 137 isolated in 66−99% yields. An attempted reaction for nonsubstituted oximes did not proceed because of, the authors believe, inhibition of the catalyst.366 In the cases of less sterically bulky substituents

Scheme 64. B(C6F5)3-Catalyzed Reduction of Oxime Ethers 136 to Hydroxylamine Ethers 137366a

a

136: R1 = 4-MeOC6H4, 2,4-Me2C6H3, Ph, 4-ClC6H4, 4-BrC6H4, 4CF3C6H4; R2 = Ph, Me, Et; R1/R2 = (CH2)5; R3 = tBu, iPr3Si.

R3 (Me, Et3Si, tBuEt2Si), 137 was formed in significantly lower yields. In addition, 137 (R1 = Ph; R2 = Me; R3 = iPr3Si) could be transformed to the corresponding O-unsubstituted hydroxylamine PhMeC(H)N(H)OH (77%) by treatment with excess [pyH]F (THF, 0 °C, 3 h).366 In that reaction, B(C6F5)3 acts as an activator of H2 to heterolytically split the H−H bond in the Y


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Scheme 66. ZnII-Mediated Allylation of Aromatic Aldoximes 141372a

presence of oximes as Lewis bases (b). The hydride of the generated borate nucleophilically attacks the C atom of the oximium salt (c), and then, the boron(III) center is regenerated (e) to give the hydroxylamine 137 (d).367 The oxime CN bonds were also reduced under noncatalytic conditions, leading to alkyl hydroxylamines. Reducing oximes R1R2CNOH with stoichiometric amounts of NaBH3CN in acidic media (H+ serves as an electrophilic activator of the oxime C atom; Scheme 64) provided the corresponding racemic hydroxylamines in good to moderate yields (40−78%).368,369 Oxime ethers R1R2CNOR3 were stereoselectively reduced to the corresponding hydroxylamine ethers (45−94% ee) with borane-oxazaborolidine derived from (−)-norephedrine BH3NaHCHMeCHPhOBbH(a−b).370 The reduction of the ketoxime ethers R1C(CO2R2) NOCH2Ph (138) is accompanied by alkylation or allylation of the oxime C atom (Scheme 65).371 Oxime ethers 138 reacted

a 141: R1 = 4-Me2N, 2-MeO, 4-MeO, 4-Me, H, 4-F, 4-Cl, 2-Br, 4-Br, 4CF3, 4-O2N; R2 = Me, 4-MeOC6H4, Ph, 4-CF3C6H4.

Scheme 65. ZnII/BF3-Mediated Reductive Alkylation and Allylation of Oxime Ethers 138371a

but the yields were somewhat lower. The conversion did not proceed when PdCl2 or M(OTf)3 (M = Sc3+, La3+, Sm3+, Eu3+, and Yb3+) were employed. A plausible reaction mechanism373,374 includes coordination of the tin(II) metal center to the oxime N atom (b), leading to the electrophilic activation of the oxime C N bond (Scheme 63, route A). Then, the allylmetal species nucleophilically attacks the oxime C atom to give tin(II) Obenzyl hydroxylamides (c and d), and finally, hydrolysis of the amide results in 144 (e). The R1C6H4C(R2)=NOMe species (145) reacted with internal alkynes R3CCEWG (146) in the presence of the rhodium(III) complex [Cp*Rh(MeCN)3](SbF6)2 (5 mol %) in ClCH2CH2Cl under dinitrogen at 110 °C for 6 h to produce hydroxylamine ethers 147 (36−85%; Scheme 68, a).375 The regioselectivity of the reaction significantly depended on the bulkiness of substituent R1. Thus, in the case of unsymmetrical aryls in 145, the reaction proceeded at the para-position to the R1 (3-Me, 3-Cl, 3-Br, 3-CF3), whereas in the case of R1 = 3-F, the reaction was directed to the ortho-position of the substituent. A suggested mechanism involves cyclometalation (b), whereupon the alkyne inserts into the Rh−C bond (d; other examples of such alkyne insertions are known376−380 ), and finally, nucleophilic attack of the C− to the oxime C atom occurs, forming 147.375 The salicylaldoxime ethers 2-HO-3-R2-4-R3-C6H4CH NOR1 (148) reacted with BF3·OEt2 (1.5-fold excess) in the presence of 1.5 equiv of Me3SiCH2CHCH2 (in ClCH2CH2Cl, RT, 1 h). The in situ-generated complexes 149 (Scheme 69, b; complexes with R2 = R3 = H; R1 = Me and PhCH2 were isolated in 86 and 88% yields, respectively) were then reacted with 1 equiv of (C6F5)3SiF and a 1.2-fold excess of NaOAc in MeCN under reflux for 3 h (c), with subsequent hydrolysis by H2O at RT (d) yielding O-substituted hydroxylamines 150 (74−94%; a).381 An example of the reduction of the oxime CN bond accompanied by alkylation of the N atom is known. Clathrochelate 151 reacted with Et3B·THF (5 equiv) to give complex 152 (in air in benzene, RT, 2 h, 60%; Scheme 70).382 Complex 152 is believed382 to be formed by the consecutive addition of two Et• species, which are generated by the oxidation of Et3B in air under the reaction conditions. The reaction proceeds as a trans-addition.

a

138: R1 = Me, iPr, tBu, CH2CO2Me; R2 = Me, Et; 139: R3 = Et, iPr, Bu, tBu, CH2CHCH2, PhCH2CH2.

n

with (BrMg)[ZnR33] (139; 2−4-fold excess) in the presence of 1 equiv of BF3·OEt2, and this reaction gave hydroxylamine ethers 140 (in CH2Cl2 under N2, 0−40 °C, 24 h, 67−100%). The conversion could also be performed using ZnR32 instead of (BrMg)[ZnR33], but this modification gave lower yields. The reaction could also be promoted by Ti(OiPr)4, TiCl4, and SnCl4 instead of BF3·OEt2 but also with substantially lower yields. In the absence of any Lewis acid additives to (BrMg)[ZnR33], the reaction did not proceed. It is believed that Lewis acids enhance the electrophilicity of the oxime group toward the reaction with dialkylzinc (Scheme 63, route A). The aldoxime esters R1C6H4C(H)NOC(O)R2 (141) reacted with the allyl bromides BrCH2C(H)CHR3 (R3 = H, Ph) in the presence of Zn in a mixture of THF and aqueous NH4Cl at RT for 30 min to give N-allyl hydroxylamines 142 in good to excellent yields (48−98%; Scheme 66, a).372 The reaction did not proceed in the case of the oxime and oxime ethers PhC(H)NOR (R = H, Piv, CH2Ph). Because of that, one could conclude that the chelation of the in situ-formed zinc(II) allyl bromide (b) is required (Scheme 63, route B). The intermediate depicted in Scheme 66 undergoes [3,3]-sigmatropic shift, granting allyl amine 142 (c) as the final product. Other reports have described the diastereoselective reductive allylation of O-benzyl aldoximes RC(O)CHNOCH2Ph (143; R is a chiral substituent, Scheme 67) to the hydroxylamines RC(O)CH(N(H)OCH2Ph)CH2CHCH2 (144). This reaction was performed in the presence of Sn(OTf)2 (1 equiv) and an excess of Al(CH2CHCH2)3 (CH2Cl2,−78 °C, 10 min) or 2 equiv of nBu3SnCH2CHCH2 (MeCN, −40 °C, 1 h).373,374 The reported procedure provides 144 (75−94%; de 76−99%; except G: de 10%, and D: de 37%). This reductive allylation could also be catalyzed by Zn(OTf)2 or AgOTf instead of Sn(OTf)2, Z


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Scheme 67. Tin(II)-Mediated Reductive Allylation of O-Benzyl Oximes 143373,374

Scheme 69. Boron(III)-Mediated Reductive Arylation of OSubstituted Salicylaldoximes and Representative Molecular Structure of 149381a

Scheme 68. Rhodium(III)-Catalyzed Reductive Vinylation of O-Methyl Aromatic Ketoximes 145375a

a

148: R1 = Me, PhCH2; R2 = H, OMe, H2CCHCH2; R3 = H; R2/R3 = benzo.

anion (d). This route is suggested for the rhodium(I) and palladium(0) centers. Another route includes two consecutive 1e-reductions with intermediate radical formation (b and c). This ̅ route is proposed for the iron(II), copper(I), and ruthenium(II) centers. 3.5.2.1. Reduction of Oxime Ethers and Esters Leading to Homoazines. The aromatic ketoximes R1C6H(3−4)C(R2) NOH (153) reacted with excess Boc2O to furnish the corresponding O-acylated oximes R1C6H(3−4)C(R2)NOC(O)tBu (Scheme 72, b). These species then underwent reductive homocoupling in the presence of CuI (10 mol %) and excess

a

145: R1 = 4-MeO, 4-tBu, 2-Me, 3-Me, 4-Me, H, 2-F, 3-F, 4-F, 3-Cl, 4Cl, 3-Br, 4-Br, 3-CF3, 4-CF3, 4-MeO2C, 4-MeSO2, 4-O2N; R2 = Me, Et, nPr; 146: R3 = 4-MeC6H4, Ph, 4-ClC6H4, nPr, EtO2C; EWG = Tf, PhSO2, EtO2C.

3.5.2. Reduction of Oxime Ethers and Esters Accompanied by the N−O Bond Splitting. In general, reduction of the oxime N−O bond proceeds by two routes (Scheme 71). The first route includes oxidative insertion of a metal center into the N−O bond (a) with subsequent decoordination of the imino AA


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Scheme 70. Iron(II)-Mediated Reduction of the Oxime Moiety Accompanied by Alkylation of the N Atom382

3.5.2.2. Reduction of Oxime Ethers and Esters Leading to Imines and Enamines. In this section, various reductions of oximes leading to imines and enamines are split into two parts. The first part describes reductions without functionalization of oxime side-chains (section 3.5.2.2.1), and the second part is devoted to the reactions accompanied by functionalization (section 3.5.2.2.2). 3.5.2.2.1. Reduction to Imines and Enamines without SideChain Functionalization. The ketoxime PhC(iPr)=NOC(O)tBu (155) reacted with [RhCl(PPh3)3] (1 equiv) in the presence of 1 equiv of Me3SiCl in benzene (RT, 24 h) to give imine complex 156 (96%; Scheme 73, a).387 A plausible

Scheme 71. General Routes of Metal-Mediated Reduction of the Oxime N−O Bond

Scheme 73. Rh-Mediated Reduction of Oxime 155387

Scheme 72. Copper-Mediated Reductive Homocoupling of Oxime Esters 153383a

mechanism of this reaction includes the coordination of the oxime to the rhodium(I) center (b), oxidative insertion into the oxime N−O bond (c), cyclometalation (d) and proton transfer (e). The cyclometalated imine formation proceeds even in the absence of Me3SiCl, which is required for the generation of dichloride complex 156. Cyclohex-2-enone and tetralone oxime pivalates 157 (Scheme 74) transformed to aniline and 1-aminonaphthalene derivatives 158, respectively (Scheme 74, a).388 The reaction proceeded in the presence of Pd(OAc)2 (10 mol %), P(cyclopentyl)3 (20 mol %), tBuCO2H (30 mol %), and K2CO3 (4-fold excess) in toluene at 95−120 °C for 8 h and leads to amines 158 (46−87%). The mechanism of this reaction388 includes the oxidative addition of the oxime N−O bond to the palladium(0) center generated in situ (b) followed by tautomerization (c) and migration of the metal center to the C atom (d). β-Elimination with subsequent reductive elimination (e) and tautomerization of the generated imine to the aniline (f) terminates the overall process. There have been several reports devoted to the reduction of oximes leading to imines. The formed imines were isolated as Nacetylated enamines (Scheme 75, a).389−392 These reactions are believed to proceed via acylation of the oxime 159 to give O-acyl oxime (b) followed by 1e-reduction of the N−O bond (c) with ̅

a 153: R1 = 3-MeO, 3,4-Me2, 3,4-(CH2)4, 4-Me, H, 4-F, 4-Cl, 4-Br, benzo[c]; R2 = Me, Et; R1/R2 = (CH2)2, (CH2)3.

NaHSO3 (ClCH2CH2Cl, under Ar, 140 °C, 9−24 h) to give homoazines 154 (60−90% isolated yields; a).383 This reaction includes initial reductive cleavage of the N−O bond (c) followed by homocoupling of the radical species (d). NaHSO3 is required for the reduction of the formed copper(II) to recover copper(I) (e). Homocoupling of oxime carboxylates leading to homoazines is known in metal-free chemistry.384−386 This reaction proceeds under UV irradiation, and the radicals R1R2CN• and •OCOR3 are believed to form from R1R2CNOCOR3 (R3 = Ar, Het) at the initial step of the reaction, whereupon R1R2CN• dimerizes. AB


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Scheme 75. Metal-Mediated Reduction of Oximes to N-Acetyl Imines389−392a

Scheme 74. Palladium-Catalyzed Reduction of Oxime Esters 157 Leading to Anilines 158388a

a

159: R1 = aromatic: R4C6H(3−4): R4 = 2-MeO, 3-MeO, 4-MeO, 3,4CH2O2, 4-NHAc, 2-Me, 4-Me, 2,5-Me2, 3,4-Me2, 3,4-(CH2)4, 4-nBu, H, 4-F, 4-Cl, 2-Br, 3-Br, 4-Br, 4-I, 4-CF3, 4-NC, 4-NO2, benzo[c]; heteroaromatic: 2-thienyl; aliphatic: nPr; R2 = H, Me, Et; R3 = H, Me; R 1 /R 2 = C 6 H 4 (CH 2 ), C 6 H 4 (CH 2 ) 2 , 4-MeOC 6 H 3 (CH 2 ) 2 , 3MeOC6H3(CH2)2, 3-ClC6H3(CH2)2, (CH2)3, (CH2)4, (CH2)2CH(tBu)CH2, (CH2)3CHMe, (CH2)5; R2/R3 = (CH2)5; R1/R2 = C6H4(CH2), C6H4(CH2)2, 4-MeOC6H3(CH2)2, 3,5-Me2C6H2(CH2)2.

a 158: R = 2-Ph, 2-iPr3SiCH2, 3-(4-MeOC6H4), 3-Ph, 3-(4-ClC6H4), 3(4-CF3C6H4), 3-PhSO2, 6-Ph, 3-Ph-4-EtO2C, 3-(2,4,5-(MeO)3C6H25-Ph, 3-(4-MeC6H4)-5-Ph, 3,5-(Ph)2, 3-PhSO2-5-Ph, 2,5,6-(Ph)3, 3Ph-4-MeO2C-5-Me, benzo[b], 4-Me-benzo[b], 3-Me-(2-MeO-benzo)[b], (2-F-benzo)[b], (2-O 2 N-benzo)[b], (3-MeO-benzo)[b], (3-tBuO2C-benzo)[b], (2,3-CH2O2-benzo)[b], (2,3-(MeO)2-benzo)[b], (2,4-Me2-benzo)[b].

that depicted in Scheme 75, where the RuII center acts as a 1e-̅ reducing agent in the catalytic cycle and NaHSO3 is required to regenerate the oxidized catalyst. Other works have been devoted to the reduction of aliphatic and aromatic ket- and aldoxime esters R1R2CNOR3 (161) accompanied by vinylation or arylation of the imine N atom (Scheme 76, a). These reactions probably proceed via transmetalation (b) and oxidative insertion of the copper(I) center into the oxime N−O bond (c) followed by reductive elimination of 163 (d). The O-benzoyl oximes (161) reacted with ethoxy vinyl zirconium(IV) complexes Cp2Zr(OEt)(R4CR5R6) (162), generated in situ from R4CCR5, Cp2ZrR62, and EtOH, in the presence of 1 equiv of CuCl (THF, under N2, 50 °C, 12 h), and this reaction gives N-vinyl imines R1R2CNC(R4)R5R6 (163; 38−85%).393 The reaction mechanism includes transmetalation (b), oxidative addition (c), and reductive elimination of 163 (d). Similarly, the O-acetyl or perfluorobenzoyl ketoximes 161 reacted with the aryl- or vinyl boronic acids R5R6CCR4B(OH)2 or stannanes R5R6CCR4SnnBu3 to grant N-aryl and Nvinyl imines 163 (52−98%).394 Under these conditions, the aldoximes undergo copper-mediated dehydration and cannot be used for the reduction (see section 3.4.1). As the imines 163 [R1 = Me, R2 = Ph; R1/R2 = (CH2)5] are unstable, they were isolated as the corresponding secondary amines R1R2C(H)N(H)C(R4)R5R6 (53−94%) by treatment of the reaction mixture with an excess of the reducing system NaBH3CN/CF3CO2H.394 The O-propargylic aldoximes R1CH2C(H)NOCH(R2)C CR3 (164) underwent rearrangement to the respective substituted N-methylene enamines trans-R1CHCH−N

subsequent reduction of the radical (d). The next step is the acylation of the anion to give the N-acyl imine (e), which finally tautomerizes to N-acyl enamine 160 (f).391,392 Metal centers suitable for 1e-transfer reactions promote the N−O bond ̅ cleavage followed by reduction of the N-radical to the appropriate amide (Scheme 71, b and c). The aromatic ketoximes R1C(CHR2R3)=NOH (159) were transformed into the enamines R1C(NHAc)=CR2R3 (160) in THF in the presence of Fe(OAc)2 (2 equiv), AcOH (3 equiv), and Ac2O (2 equiv) under dinitrogen at 65−67 °C for 7−15 h;389,390 the products of this reaction were isolated in 45−90% yields. The aromatic and aliphatic ketoximes 159 converted to 160 in the presence of CuI (10 mol %), NaHSO3 (3 equiv), and Ac2O (2 equiv) under argon (in ClCH2CH2Cl, 120 °C, 6−75 h, 52−95%).391 The mechanism is depicted in Schemes 75 and 71. In the FeII-mediated reaction, two equiv of iron(II) salt were used for the oxime reduction, whereas in the CuI-catalyzed reactions, NaHSO3 was employed as a reducing agent for the catalyst regeneration. Another study covered the reduction of aromatic, heteroaromatic, and aliphatic oximes 159 in the presence of [RuCl2(pMeC6H4iPr)]2 (1−5 mol %), NaHSO3 (2 equiv; 1 equiv. KI can be used instead), and Ac2O (2 equvs). When the synthesis was performed under dinitrogen in ClCH2CH2Cl under reflux for 2− 24 h,392 the N-acetyl enamines were obtained in 60−89% yields. The reduction was also successfully performed in the presence of propionic anhydride substituting for Ac2O, giving PhC{NHC(O)Et}CH2 (77%).392 A proposed mechanism is similar to AC


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Scheme 76. Copper-Mediated Generation of N-Vinyl Imines from Oximes 161393,394a

Scheme 77. Copper(I)-Catalyzed Reduction of O-Propargylic Aldoximes 164395a

a 161: R1 = 4-MeC6H4, Ph, 4-ClC6H4, 2-furyl, Me, Et, tBu; R2 = Ph, 4ClC6H4, H, Me, Et; R1/R2 = (CH2)5, R1R2C = 9-fluorenyl; R3 = Ac, PhCO, C6F5CO; 162: R4 = Ph, MeCC, Et; R5 = Ph, Me, Et; R6 = Et, H2CCHCH2, PhCH2CH2; R5R6CCR4 = 4-MeOC6H4, 3HOC 6 H 4 , 3,4-(CH 2 O 2 )C 6 H 3 , 2-MeC 6 H 4 , Ph, 2-naphthyl, 3OHCC6H4, 4-MeO-2-OHCC6H4, 2-furyl, 2-thienyl; R4 = R6 = H, R5 = Ph, 4-ClC6H4, 4-CF3C6H4, nBu.

CR3{CHOCH}R2 (165) in the presence of CuCl (10 mol %) and Cy2NMe (50 mol %) (in MeCN under Ar, 100 °C, 0.5−7 h, 57−93%; Scheme 77, a).395 A suggested mechanism for this reaction includes side-on coordination of the CC bond to the copper(I) center, leading to electrophilic activation of the alkyne fragment (b). This is followed by intramolecular nucleophilic attack of the oxime N atom (c), whereupon a base is required to deprotonate the intermediate (d), which consequently undergoes N−O bond splitting (e). Finally, the O− attacks the carbene C atom to form the oxirane cycle (f), and the intermediate is protonated to yield the product (g) and regenerate copper(I) (h). 3.5.2.2.2. Reduction to Imines and Enamines Involving Side-Chain Functionalization. Several articles have reported reductions of oximes involving functionalization of the oxime side-chain at the α-position (Scheme 78, a). This transformation starts from a 1e-reduction of the oxime ester to a radical species ̅ (b) and the parallel 1e-oxidation of additive AX [R4C( ̅4 5 O)CO2H, CF3SO2Na, R R P(H)O, R4SO2Na, or KI] (c). The next steps include the tautomerization of the N-centered radical to the C-centered species (d) and the radical coupling to obtain amine 167 (e). Aromatic ketoxime esters R1C(NOAc)CH2R2 (166) underwent transformation to acyl enamines R1C(NH2) CR2{C(O)R4} (167; in DMF in air, 90 °C, 6 h, 48−85%; a) in the presence of α-ketocarboxylic acid R4C(O)CO2H (1.2 equiv), CuI (10 mol %), and 4 Å-MS.396 No significant effects were observed when the reactions were performed either under O2 or N2 atmosphere. The authors396 indicated that there was a chance of using the acyl enamines as precursors to 1,3-diketones, pyrazoles, and isoxazoles. Ketoxime esters R1C(NOAc)CH3 (166) reacted with CF3SO2Na (1.5 equiv) and Ac2O (1.5 equiv) in the presence of CuF2 (20 mol %) to furnish the N-acetyl enamines R1C(NHAc)CHCF3 (168; in MeCN under Ar, 80 °C, 48−72 h, 54−61%; f).397 The enamines hydrolyzed to the corresponding ketones R1C(O)CH2CF3 (169; 14−60%; h) upon treatment with 2 M HCl in 1,4-dioxane at 80 °C for 2 h.397

a

164: R1 = 4-MeOC6H4, Ph, 4-CF3C6H4, Et, tBu; R2 = Ph, nPr, tBu, Cy; R3 = 4-MeOC6H4, Ph, 4-CF3C6H4, nPr, tBu, Cy.

Oxazoles and functionalized acetamides were also generated from 168. Ketoxime esters R1C(NOAc)CH2R2 (166) reacted with disubstituted phosphine oxides R4R5P(H)O in the presence of CuCl (10 mol %), PCy3 (10 mol %), and Ac2O (1 equiv) to give R1C(NAc)CH{P(O)R4R5}R2 (168; R1 = R4 = R5 = Ph; R2 = H; in dioxane under N2, 130 °C, 5 h, 71%; f). These species were hydrolyzed by aqueous HCl to the corresponding R1C( O)CH(P(O)R4R5)R2 (47−86%; h).398 In the presence of R4SO2Na instead of the phosphine oxide, aromatic and heteroaromatic oxime esters 166 were converted to the appropriate R 1 C(NH 2 )CR 2 {S(O) 2 R 4 } (167; R 1 = R3C6H(3−4): R3 = 2-Me, H, 2-Cl, 2-Br, benzo[b]; R2 = H; 72− 92%; a). The R1C(NH2)CR2{S(O)2R4} compounds were hydrolyzed in situ to the ketones R1C(O)CHR2{S(O)2R4} (169; 70−96%; g).399 Oxime esters 166, formed in situ from the corresponding oximes R1C(NOH)CH2R2 and Ac2O (2.0−2.5 equiv), reacted with KI (1.1−1.2 equiv) in the presence of CuI (5−10 mol %) to give the N-acetyl enamines R1C(NHAc) C(I)R2 (168; in ClCH2CH2Cl under Ar, 120 °C, 50−82%; f).400 The reaction time was not reported, and the completeness of the reaction was only noted as being monitored by TLC. The AD


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Scheme 78. Copper-Mediated Reduction of Oxime Esters 166 Accompanied with the α-Functionalization396−400a

Scheme 79. Stereoselective Boron-Mediated Reduction of Oxime Ethers 170 to Amines 171401−403a

a 166: R1 = R3C6H(3−4): R3 = 2-MeO, 3-MeO, 4-MeO, 2-Me, 3-Me, 4Me, 4-Et, 2,5-Me2, 3,4-Me2, 3,4-(CH2)4, NHAc, H, 2-F, 3-F, 4-F, 3-Cl, 4-Cl, 2-Br, 3-Br, 4-Br, 4-I, 4-CF3, 4-MeO2C, 4-O2N; 1-naphthyl, 2naphthyl, 2-thienyl, 2-furyl, 2-pyridyl, trans-PhCHCH, H, Me, Et, PhCH2CH2; R2 = H, Me, Et, Ph; R1/R2 = C6H4CH2CH2. 167: R4 = 2naphthyl, 2-thienyl, 2-furyl, Me, Et, Cy, R5C6H4: 4-MeO, 3-Me, 4-Me, 4-F, 2-Cl, 4-Cl, 4-Br, 4-CF3, 4-O2N. 168: R4 = R5 = Ph. 169: R4 = R5 = R6C6H(3−4): R6 = 2-Me, 4-Me, 2,4-Me2, 3,4-Me2, H, 4-F, 4-Cl, benzo[c]; R4 = R5 = OEt; R4/R5 = Ph/OEt; R4 = 2-naphthyl, Me, Et, Cy, R6C6H4: R6 = 4-MeO, Me, H, 4-F, 2-Cl, 4-Cl, 4-Br, 4-CF3.

a

170: R1 = R4C6H4: R4 = 4-MeO, 4-nBuO, H, 4-F, 4-Cl, 4-O2N; 3thienyl, 3-(2,5-dimethyl)thienyl, 3-pyridyl, 3-(6-methoxy)pyridyl, 4pyridyl, Me, Et, cyclopropyl, iPr3SiO(CH2)3; R2 = Ph, Me, Et, nPr, Cy, i Pr3SiO(CH2)3; R1/R2 = C6H4(CH2)2, C6H4(CH2)3, C6H4OCH2CH2, 3-ClC6H3OCH2CH2, C6H4SCH2CH2, 3-ClC6H3SCH2CH2; R3 = Me, n Bu, PhCH2.

reduced to amines 171 by the action of 174 (10 mol %) and BH3· THF (4 equiv) under dinitrogen in dioxane at 0 °C for 36−48 h followed by the addition of 6 M HCl. The products of the reaction were separated from the reaction mixtures as N-acetyl amines via treatment with Ac2O and Et3N.403 The chiral catalysts 172−174 are believed to react with BH3· THF to lead to an open-chain intermediate (b), which gives the oxime complex via ligand exchange (c). The hydride nucleophilically attacks the oxime C atom, whereas the boron electrophilically attacks the O atom (d). After that, the elimination of formal [R3OB(THF)H]+ residue grants the boron amide intermediate (e). Finally, protonation of the boron amide leads to amine 171 and regenerates the catalyst (f). Boron activates the CN bond of the coordinated oxime toward the nucleophilic attack of the hydride and promotes the N−O bond cleavage in the hydroxylamine intermediate. The oxime acetates R1C6H4C(CH2R2)NOAc (175) were enantioselectively reduced to the corresponding N-acylamines 176 in the presence of 10 mol % [Rh(cod)2](SbF6) and 11 mol % enantiopure ligand 177 in 1,4-dioxane under H2 (50 atm) at 40 °C for 24 h to give 37−80% 1H NMR yields and 39−91% ee of the target products (Scheme 80, a).404

reaction was also performed for (EtCO)2O instead of Ac2O, and this modification granted PhC{NHC(O)Et}C(I)H (67%). The yield of the reaction appeared to be sensitive to the nature of R2. In particular, the isolated yield of PhC(NHC(O)Et) C(I)Me was only 15%, whereas the yield of PhC(NHC(O)Et) C(I)H was 72%. 3.5.2.3. Reduction of Oxime Ethers and Esters Leading to Amines. Recent reports have been dedicated to the asymmetric reduction of O-substituted oximes, including the boronmediated reduction of oxime ethers and the rhodium-mediated reduction of oxime esters. The ketoxime ethers R1R2CNOR3 (170) were reduced by the chiral boron catalyst 172 (1 equiv) in the presence of BH3· THF (1.5 equiv) under argon in THF at 0 °C for 48 h to yield (S)-R1R2CHNH2 (171; 74−87% isolated yield; ee 75−98%) after treatment of the reaction mixture with 2 M HCl (Scheme 79, a).401 (R)-Amines 171 were obtained from the corresponding oxime ethers 170 in the presence of chiral boron catalyst 173 (1.25 equiv) and BH3·THF (1.25 equiv) in THF at RT for 24 h, followed by reflux with an additional amount of BH3·THF (4 equiv) and hydrolysis with 2 M HCl.402 The amines were isolated in 68−91% yields and 40−99% ee. The ketoxime ethers 170 were AE


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bond (b), tautomerization (c), and migration of the rhodium(III) center to the C atom (d). Intramolecular nucleophilic attack of the imine to the acyl moiety (e and f) would give N-acyl imine, and, finally, reduction of the CN double bond by external H2 and reductive elimination would furnish N-acyl amine 176 (g). For a set of the amines 176 (R1 = 4-MeO, 4-Me, H, 4-Cl, 4-Br, 4CF3; R2 = Me), the (S)-enantiomer was the major product.404 A similar reaction was also performed for 2,3-dihydro-1H-inden-1one oxime in the presence of 1 mol % of [Rh(μ−OH)(cod)]2, 0.5 equiv of Cs2CO3, H2 (130 atm), and 1 mol % of a set of enantiopure diphosphines in iPrOH at 55 °C for 17 h, but despite high yields (up to quantitative) of 2,3-dihydro-1H-inden-1amine, the ee was typically less than 32%.405

Scheme 80. Rhodium-Catalyzed Reduction of Oxime Acetates 175 to N-Acyl Amines 176404a

3.6. Oxidation of Oximes Leading to Carbonyl Compounds

The development of mild and efficient methods for the selective cleavage of oxime derivatives to yield carbonyl compounds continues to be a significant aspect of organic chemical transformation. Metal-mediated deoximation can be achieved by hydrolytic,406−408 reductive (see section 3.5), and oxidative methods (scheme 81). The latter route of deoximation is considered in this section. Oxidative deoximation of oximes leading to carbonyl compounds was comprehensively reviewed in the year 2009,409 and here, we describe only recent works that were not included in that review. Although mechanisms of oxidative deoximation depend on the reaction conditions and the catalyst employed, the overall principle is that a metal center in high oxidation state (e.g., Mn=O or Mn−1−O•) accepts two electrons, with the subsequent generation of nitroso complexes. These nitroso species were isolated as products in these reactions by the end of the 20th century,34,74 but no recent examples have been published. Aromatic, vinylic, and aliphatic oximes 178 were transformed to the corresponding aldehydes and ketones 179 in CH2Cl2 at RT for 1.5−2 h in the presence of the Jones reagent (CrO3· H2SO4; 2 equiv) absorbed on silica.410 Carbonyl compounds 179 were isolated in 84−93% yields. Sequence of possible stages includes nucleophilic attack of the O atom of chromium(VI) oxide to the oxime ligated to the CrVI center (Scheme 81, a), followed by the C−N bond splitting (b) and decoordination of the carbonyl compound 179 to give nitroso species and the chromium(IV) species (c).

a

175: R1 = 2-MeO, 3-MeO, 4-MeO, 4-Et, 3-Me, 4-Me, H, 3-F, 4-F, 2Cl, 3-Cl, 4-Cl, 3-Br, 4-Br, 3-CF3, 4-CF3, benzo[c]; R2 = H, Me, Et.

The mechanism of this reaction is reported404 to be unclear. However, we believe that it might be similar to that of the palladium-catalyzed reaction388 (Scheme 74), including oxidative insertion of the rhodium(I) center into the oxime N−O

Scheme 81. Hydrolytic, Reductive, And Oxidative Routes to Carbonyl Compounds 179 from Oximes 178a

a

Oxidation state of a metal is given for NO ligand featuring a 1− formal charge. 178: R1 = R3C6H(2−4): R3 = 2-HO, 4-MeO, 3,4-(MeO)2, 4-Me, 4-iPr, 4-tBu, 3,4-Me2, 2,4,6-Me3, H, 2-F-6-Cl, 2-Cl, 4-Cl, 2,4-Cl2, 2-Br, 4-Br, 3-CF3, 4-CF3, 2-O2N, 3-O2N, 4-O2N; 2-thienyl, PhCHCH, nBu, R2 = Ph, 4ClC6H4, H, Me, CH2Br; R1/R2 = H, (CH2)4; (CH2)5, (CH2)7. AF


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h giving aldehydes and ketones 179, which were isolated in 41− 98% yields.414 The mechanism of this reaction is likely similar to that described in Scheme 81 (a−c), and the reaction probably leads to CeS2O7 and NO.

The aromatic and aliphatic oximes 178 underwent oxidative deoximation leading to 179 (in toluene, 50 °C, 2−8 h, 56−99% isolated yields) in the presence of [Mn{tetraphenylporphirinate}Cl] (0.1 mol %) and benzaldehyde (15 equiv) under dioxygen.411 The FeIII, CoIII, and RuIII porphirinates catalyzed this reaction with much lower efficiencies. In addition, the reaction did not proceed for oximes with strong electronwithdrawing substituents (e.g., R1 = 2-O2NC6H4 or 2-py). Benzaldehyde acts as an oxidation promoter of MnIII (MnIII + O2 + PhCHO → MnIV−O• + PhCOOH), and the reaction did not proceed in the absence of the aldehyde. The aromatic aldoximes 178 were transformed to carbonyl compounds 179 (70−98%) in the presence of FeCl3·TEMPO (10 mol %; generated in situ from FeCl3 and TEMPO) under O2 in toluene:H2O = 3:1 (v/v) mixture at 60−80 °C for 3−12 h (Scheme 82).412 This reaction is sensitive to steric hindrance

4. METAL-INVOLVING REACTIONS DIRECTED TO SIDE-CHAIN OF OXIME SPECIES Our consideration here focuses on metal-mediated and metalcatalyzed transformations of the side-chains of oxime species, with particular emphasis on those reactions where the oxime moiety is not simply a spectator but is involved in some steps of the reaction. The oxime group in these processes usually behaves as a directing N-coordinating functionality (Scheme 83). Scheme 83. Metal-Mediated and Metal-Catalyzed Transformations of the Side-Chain of Oxime Species

Scheme 82. Reaction Route of [FeCl3(TEMPO)]-Catalyzed Oxidative Deoximation of Oximes412

Typically, these reactions belong to C−H functionalization, an area recently receiving intense attention,39,40,415−418 where amides, carboxylates, ketones, other directing groups and oximes have been widely applied.33,78,79,419−422 Most of the studied reactions are directed to the β-position of the carboxime moiety, although reactions involving the α- and γ-positions are also known. In each subsection, alkylation and allylation reactions are first described, followed by consideration of the corresponding arylations, vinylations, and acylations. Finally, the introduction of other groups from lower to higher atomic weights is reviewed. 4.1. Functionalization at α-Position to Carboxime Moiety

Reactions at the α-position include examples of C−H and C−Cl activation leading to the introduction of Ar, = CR2, oxy-, and oxogroups. The mechanisms of these transformations can vary, involving O- or N-coordination of the oxime moiety, coordination of another group before C−X (X = H, Cl) activation, or proceeding without coordination of an oxime group. The functionalization of [60]fullerene423,424 with the Nsulfonyl-o-aminoacetophenone O-alkyl oximes R1ONC(Me)C6H(4‑n)R2(NHSO2R3-2) (182) produced [60]fullerene-fused tetrahydroazepinonimines 183 (isolated yields 15−33%).425 The reaction proceeded with any of the oximes (4-fold excess) and both Cu(OAc)2 and Cs2CO3 (2-fold excess) at 120−130 °C in a 1,2-Cl2C6H4:MeCN (7:1, v/v) solvent mixture for 2−4 h (Scheme 84, a). Sequence of possible stages for this reaction includes base-supported ligation of the oxime (b), insertion of the fullerene into the Cu−C bond (c), and reductive elimination of the copper(0) to give 183 (d). Noverges et al. reported the Suzuki cross-coupling51,426−429 of the α-chloro oxime ethers R1C(CH2Cl)NOR2 (184), which were arylated with the aryl boronic acids R3B(OH)2 (185) to

from R2. Thus, in the case of R2 = Me (R1 = Ph, 4-ClC6H4), the reaction gave 28 and 31% yields, respectively. Upon the basis of the data reported in ref 413, a plausible mechanism probably includes the coordination of the oxime to the iron(III) center (a), followed by the concerted proton-coupled electron transfer from the H−O bond to the substrate N atom to give O-ligated TEMPO−H and the radical nitroso ligand ligated to the iron(III) center (b). The latter could be represented as a cationic ligand coupled with the iron(II) (c). External H2O nucleophilically attacks the carbocation to afford the gem-hydroxy nitroso ligand (d), which, after splitting the C−N bond, eliminates the carbonyl compound, leaving the nitroso iron(II) complex (e). Finally, this iron(II) complex is oxidized by O2 to the starting iron(III) complex (f). Another report dealt with the oxidative deoximation of aromatic, vinylic, and aliphatic aldoximes and ketoximes 178 in the presence of Ce(SO4)2 (3 equiv) in CHCl3 at RT for 0.33−23 AG


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Scheme 84. Copper(II)-Mediated N-Heteroannulation of [60]fullerene425a

Scheme 86. Iridium(I)-Catalyzed CC Bond Making at the α-Position to the Oxime Moiety431

a 182: R1 = Me, PhCH2; n = 1, R2 = H, 5-Br; n = 2, R2 = 4,5-(OMe)2; R3 = 4-MeC6H4, 4-MeOC6H4, 4-O2NC6H4, 2-thienyl, Me.

give the oximes R1C(CH2Ar)NOR2 (186; 40−98%; Scheme 85).430 The reaction proceeded with a 2-fold excess of the Scheme 85. Palladium-Mediated Cross-Coupling Leading to α-Aryl Oxime Ethers 186430a

a

184: R1 = 4-MeOC6H4, Ph, 4-ClC6H4, Me, tBu; R2 = Me, PhCH2; R1/R2 = CH2CH2CH(OEt); 185: Ar = 4-MeOC6H4, 2-MeC6H4, 4MeC6H4, Ph, 4-OHCC6H4, 2-MeO2CC6H4, 4-CF3C6H4, 3-O2NC6H4, 1-naphthyl, 2-furyl, 1-thienyl, 2-thienyl, PhCHCH.

equiv of MnBr2·4H2O in MeCN at RT for 1 d to give a mixture of solid complexes 190 (ca. 50%) and 191 (ca. 20%) (a).432 Complex 190 features the α-oxo oxime ligands, whereas 191 derives from the ligation of 189. When Mn(ClO4)2 in an EtOH/ AcOEt mixture was employed (b), oxime 189 at RT for a few days yielded complex 192 (18%), which features α-oxy oxime ligands (b) at the manganese(III) center. For the mechanism of this oxidation (Scheme 88), it was suggested432 that oxime 189 coordinates to the manganese(II) center (a) and the formed intermediate A reacts by two routes, viz., it transforms to nonoxidized 191 and coordinates dioxygen to give manganese(III) species B (b). B oxidizes oxime 189 to αoxy oxime C (c), which reacts with O2 to form α-oxo oxime D (d), which, in turn, serves as the entering ligand for the generation of 190. In step c, manganese(IV) species E are formed, followed by their transformation to F (e) and then to 191. Later, the same group433 reported a similar oxidation of 189 accompanied by Beckmann rearrangement at the copper(II) center. CuBr2 reacted with 189 (2-fold excess) in MeCN at RT in air for several days, and then, crystals suitable for single-crystal Xray diffraction were obtained by slow evaporation of the reaction mixture. XRD results indicated the formation of N-picolinoylpicolinamidato copper(II) bromide [Cu{(NC5H4CO)N}Br]

boronic acid in the presence of [Pd(PPh3)4] (10 mol %) and CsF (3−4 equiv) in THF at 65 °C for 0.5−5 h. When the O-acetyl oxime 184 (R1 = Ph; R2 = Ac) was employed in the reaction, the yield of the coupling product was significantly lower (18% after 12 h vs 83 and 75% after 1 h for R2 = Me, PhCH2, respectively). Upon the basis of NMR data, it was suggested430 that the oxime moiety coordinates to the palladium(II) center through the O atom (R2 = Me, PhCH2) to give the oxidative addition intermediate. Another report was devoted to the intramolecular dehydrogenative construction of a CC double bond at the α-position to the oxime moiety. ortho-Substituted acetophenone oxime ether 187 underwent transformation to 188 in the presence of [IrCl(cod)]2 (2.5 mol %) and Ph2PCH2CH2PPh2 (5 mol %) in AcOH under reflux for 24 h (Scheme 86, a).431 This reaction is believed to start with the oxidative insertion of the iridium(I) center into the C−H bond (b), followed by abstraction of H2 in the presence of AcOH (c). Then, the reductive elimination of AcOH (d) is followed by C−C coupling (e). Iridium(I)promoted oxidation of the intermediate (f) to generate 188 terminates the reaction. Oxime 189 (Scheme 87) reacts with manganese(II) salts through two routes to form α-oxo and α-oxy oximes ligated to MnII and MnIII centers, respectively. Oxime 189 reacted with 0.5 AH


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Scheme 87. Manganese-Mediated Oxidation of Oxime 189 at the α-Position and Representative Molecular Structures of 189−191432

Scheme 88. Plausible Mechanism of the Manganese-Mediated Oxidation of Oxime 189 at the α-Position432

(30−40%).433 No mechanism was investigated for this reaction, and these observations deserve further study. 4.2. Functionalization at β-Position to Carboxime Moiety

In the vast majority of cases, functionalization at the β-position to the carboxime moiety is related to either CAlk−H, or CAr−H functionalizations, although several examples of Cvinyl−H activation have also been reported. In this section, we discuss functionalization of the aliphatic chain followed by substitution of the hydrogen at the Csp2−H bond. In the context of the discussion given in section 4.2, it is noteworthy that a recent review32 outlines palladium-catalyzed acetoxylation at the β-position of aliphatic oximes R1ON C(R2)CHR3CH2R4 (R1 = Alk, Ac, R2 = Alk, R3 = Alk, R4 = H, Alk), giving R1ONC(R2)CHR3CH(OAc)R4. This palladiumcatalyzed Csp3−H bond activation using an oxime directing group has been applied for natural product synthesis (i.e., jiadifenolide, paspaline, and oridamycin B),32 and it will not be further considered in this article. 4.2.1. Functionalization of Aliphatic Chains. Arylation. Several examples of metal-catalyzed arylation of ketoximes have been reported in the literature (Scheme 89, a).434−436 The reaction includes Csp3−H activation (α-metalation of oxime; 2 equiv of Ag2CO3 as a base are applied for the PdII-catalyzed reaction; b), oxidative addition to a metal center (c), and reductive elimination to yield arylated product 181 (d). Selective β-arylation of oxime ethers by using a palladium catalyst with diaryliodonium salts has been reported.434 The oxime esters MeCR2R3C(R1)NOMe (180) reacted with

Ar2IOTf (193) in the presence of Pd(OAc)2 (5 mol %), PivOH (0.6 equiv), and Ag2CO3 (2 equiv) in a ClCH2CH2Cl/ (CF3)2CHOH (3:1 v/v) solvent mixture at 85 °C for 5 h. After that, the reaction mixture was treated with HCHO and HCl in THF at RT for 5 h to grant ketones OC(R1)CR2R3CH2R4 (18 examples) in 56−83% total yields after two steps. Notably, the secondary carbons could be involved in the β-arylation reaction (e.g., the oxime esters MeONC(Me)R (R = Cy, adamantyl) could be arylated under similar conditions [Pd(OAc)2 (5 mol %), PivOH (0.6 equiv), Ag2CO3 (2 equiv) in ClCH2CH2Cl: (CF3)2CHOH (3:1 v/v; 95 °C, 10 h)]. The authors of ref 434 applied this arylation protocol for some complex molecules derived from natural products. Mechanistic studies were performed; the intermediate cyclometalated complex [PdCl(PPh 3 ){N(OMe)C(Me)CMe2CH2}] was trapped when MeONC(Me)tBu reacted with Pd(OAc)2 (1 equiv) in the presence of PivOH (0.6 equiv) in ClCH2CH2Cl:(CF3)2CHOH (3:1, v/v) at 80 °C for 4 h followed by the addition of PPh3 (1 equiv) at 80 °C for 2 h. The complex [PdCl(PPh3){N(OMe)C(Me)CMe2CH2}] was further conAI


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Scheme 89. Metal-Catalyzed Arylation of Ketoximes434−436a

Scheme 90. Palladium-Catalyzed Cyclization of 2Iodobenzoic Acids436a

a

180: R1 = H, Me, Et, nPr, iPr, nBu, EtO2CCH2; R2 = H, Me, Et, nPr, CO2Me); R3 = H, Me; R1/R3 = (CH2)4, (CH2)5; 193: Ar/Ar′ = R4C6H(2−4): R4 = 4-MeO, 3,4-(MeO)2, 2-Me, 3-Me, 4-Me, 2,4,6-Me3, 4-iPr, 4-tBu, H, 4-Ph, 2-F, 4-F, 2-Cl, 4-Cl, 2-Br, 3-Br, 4-Br, 4-I, 4-CF3, 4-MeO2C; 2-naphthyl, 3-naphthyl, dibenzo[b,d]furan-4-yl. a

194: R1 = Me; R2 = Me, Et; R3 = Me, Et, nBu, n-C7H15, CyCH2; R1/ CR2R3 Me = (CH2)3CMeCMe; 196: R4 = 4-MeO, 3-Me, 4-Me, 3,5Me2, 4-tBu, H, 4-Ph, 3-CF3, 4-CF3, 4-F, 4-Cl, 3-Br, 4-Ac, 4-MeO2C; 9H-fluoren-2-yl.

verted to the corresponding arylated oxime (80% by 1H NMR integration) when reacted with the iodonium (4-BrC6H4)2IOTf (1 equiv) and Ag2CO3 (2 equiv) in ClCH2CH2Cl:(CF3)2CHOH (3:1 v/v) at 85 °C for 5 h. Xia and Shi435 reported the iridium-catalyzed arylation of ketoximes with diaryliodonium salts. The ketoximes MeCR2R3C(R1)NOMe (180) were treated with the iodonium salts (Mes)(Ar)IOTf (193) in the presence of [{Cp*IrCl2}2] (2.5 mol %), and this reaction resulted in MeONC(R1)CR2R3CH2R4 (28 examples; 33−93%). The synthetic procedure included a 3-fold excess of oxime 180 in the presence of AgNTf2 (15 mol %), PivOH (3 equiv), and 4 Å-MS in cyclohexane at 100 °C for 12 h. DFT calculations revealed the mechanism of this reaction;435 it includes the coordination of an oxime with C−H cleavage to generate the {IrIII{N(OMe)C(R1)CR2R3CH2}} moiety. Then, oxidation of this intermediate with the diaryliodonium salt (Mes)(Ar)IOTf forms an IrV species {Ir(Ar){N(OMe)C(R1)CR2R3CH2}}, which then undergoes reductive elimination leading to the target product. Another work reported the arylation of the oxime ethers CH 3 CR 2 R 3 C(R 1 )NOMe (194) with the aryl iodides R4C6H(3−4)I in the presence of Pd(OAc)2 (5 mol %), phosphoric acid 195 (10 mol %), and AgOTf (2 equiv) in PhCF3 at 80 °C for 12 h. The arylation led to the oxime ethers MeON C(R1)CR2R3CH2C6H(3−4)R4 (197; 20 examples; 51−77%).436 The reaction of MeONCaCMeCMe(CH2)2CbH2(a−b) with the aryl iodides 2-HO2CC6H(2−3)(R4)I (R4 = 4,5-(MeO)2, 4-Me, 4-Cl, 4-Br) under the same conditions gave 5-membered lactones (198; 5 examples; 50−63% isolated yields) via double palladium-catalyzed C−H activation (Scheme 90).436 Amination. The oxime ethers MeR2R3CC(R1)NOR4 (199) were aminated with the organic acyl azides R5N3 (200) in the presence of [{Cp*IrCl2}2] (5 mol %), AgNTf2 (20 mol %), and Ag2CO3 (10 mol %) or AgOAc (6 mol %) in ClCH2CH2Cl (60 °C; 24 h) to furnish the substituted species R4ON C(R1)CR2R3CH2NHR5 (44 examples, 48−90%; Scheme 91, a).437,438 This reaction proceeds through C−H bond activation (b), nitrenoid insertion into the Ir−C bond [which proceeds via coordination of the azido species R5N3 and loss of N2 with subsequent transfer of the R5N fragment (c)], and liberation of 201 (d) by the action of H+. Silver(I) acts as a chloride

Scheme 91. Iridium-Catalyzed Amidation of Oximes 199437,438a

a

199: R4 = Me, Et, iBu, tBu, PhCH2; R1 = Me, nPr, nBu, MeO2CCH2; R2 = H, Me; R3 = H, Me, Et; R1/CR2R3 = (CH2)4CH, (CH2)4CMe, (CH 2 ) 4 CH, CHC(Me)C(NOH)CHC, (CH 2 ) 3 CHC, CH 2 CH( i Pr)CH 2 CHC, CH 2 CH{C(Me)CH 2 }CH 2 CHC, CH 2 CH( i Pr)CH 2 CH 2 CH, CH 2 CH{C(Me)CH 2 }CH 2 CH 2 CH, (CH2)4C(CO2Et), 2,2-dimethylcyclopentane-1,3-diyl; R−R1 = O(CH2)2O; 200: R5 = R6SO2: R6 = Ph, 4-MeC6H4, 4-C12H25C6H4, 4MeOC6H4, 4-CF3C6H4, 4-O2NC6H4, 4-ClC6H4, 2-naphthyl, PhCH2, Me, nBu, (1S,4R)-7,7-dimethyl-2-oxobicyclo[2.2.1]heptan-1-yl)methyl, PhCHCH; R7O2C: R7 = 3,5-(CF3)2C6H3, 3,5-(O2N)2C6H3, 4O2NC6H4, 3-CF3-4FC6H3; tBu, CCl3CH2, PhCH2, 4-O2NC6H4, (9Hfluoren-2-yl)CH2CO2.

abstracting agent to produce the active form of the IrIII catalyst. The corresponding primary alkylamines and anilines were AJ


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Scheme 94. Simplified Mechanism of Metal-Catalyzed orthoFunctionalization of Vinyl, Aryl, and Hetaryl Oximes

obtained upon deprotection of the carbamate groups (R4 = t BuCO2, CCl3CH2CO2, PhCH2CO2, 4-O2NC6H4CO2, (9Hfluoren-2-yl)CH2CO2) of 201.438 Acylation. sBuMeCNOMe and camphor oxime methyl ether were acetoxylated in the presence of Pd(OAc)2 (5 mol %), K2S2O8 (1 equiv), or PhI(OAc)2 (1.1−1.3 equiv) at 100 °C for 12 h in excess AcOH or AcOH/Ac2O to give products 181 in 45−75% yield (Scheme 92).439 A proposed reaction pathway439 Scheme 92. Acetoxylation of the Alkyl Group of the Oximes439

includes the initial generation of a cyclometalated palladium(II) complex, which is oxidized by PhI(OAc)2 to a palladium(IV) intermediate, and reductive elimination of 181 and regeneration of the catalytically active palladium(II) center terminate the reaction. Mei et al. reported the acylation of oxime ethers 180 via electrochemical oxidation in the presence of Pd(OAc)2 (10 mol %) and R6CO2Na (4 equiv). This treatment was performed in the corresponding R6CO2H at 50−100 °C in an H-type divided cell with two platinum electrodes and a Nafion 117 membrane (Scheme 93).440 This reaction led to acylated oxime ethers 202,

Fluorination. The vinyl group of the oxime ethers MeON C(R1)C(R2)CHR3 [R1 = Me, Et; R2 = Me, 4-MeOC6H4, Cl, Br; R3 = Ph; R1−R2 = (CH2)n, n = 3−6; R2−R3 = (CH2)4] was fluorinated with N-fluorobenzenesulfonimide (1.5 equiv) in the presence of [Pd2(dba)3] (5 mol %) or Pd(OAc)2 (10 mol %), AgNO3, or KNO3 (30 mol %) in nitromethane (RT, 36 h) to give MeONC(R1)C(R2)CFR3 (10 examples; 65−88% isolated yields Scheme 94).442 4.2.3. Functionalization of Aromatic Moieties. Examples of CAr−H activation reactions (Scheme 94), which are the most common β-functionalizations of oxime side-chains, are summarized in Table 2. These reactions include the introduction of alkyl, allyl, aryl, vinyl, (imino)acyl, carboxyl, cyano, amino, nitro, alkoxy, thio, and halogen moieties. Likewise, the reactions of vinyl oximes are listed (see section 4.2.2), where the Ncoordinated oxime moiety acts as the directing group and the reaction starts from the initial metalation of the oxime leading to C−H cleavage at the β-position to the carboxime moiety, viz., ortho-CH activation. Typically, these reactions are metal-catalyzed, and the tested catalysts include RuII (E5 and E8, Table 2), RhIII (E2−E4, E15− E19, E23, and E26), and PdII (E1, E6, E7, E9−E12, E20−E22, E24, and E25) species. An example metal-mediated reaction is PdII-mediated insertion of CNR into Pd−C bonds leading to palladium complexes bearing iminoacyl-functionalized oxime ligands (E13). The mechanism of metal-catalyzed functionalization of arylsubstituted oximes involves N-coordination of the oxime moiety followed by cyclometalation at one of the earlier steps (Scheme 94, a). Further steps could be migratory insertion/elimination (route A: e.g., E4, E5, E8, E13, and E15−E19, E26) for such substrates as olefins, isocyanides, cyanamides, azides, chloroalkylamines, nitrosobenzene, benzoisoxazoles, and PhI(OAc)2. An alternative mechanism includes (oxidative) addition/reductive elimination (route B: E6, E7, E9−E12, E14, and E20−25) for the substrates represented by boronic acids, arenes, aldehydes,

Scheme 93. Acylation of the Alkyl Group of 180440a

a

202: R1 = H; R2 = Me, Et, AcO(CH2)2; R3 = Me, Et; R4 = Me, nPr, EtO2CCH2, R7(CH2)3: R7 = tBuMe2Si, AcO, Cl, PhSO2NH, NC; R5 = Me, PhCH2, Ph(CH2)3, nC12H25; R6 = Me, CHF2, CF3, Et, CHF2CF2, n Pr, nC6F13.

which were isolated in 25−92% yields. The reaction also allowed the introduction of TsO and MeO moieties in the oxime aliphatic chain, but the yields were low (19 and 18%, respectively).440 4.2.2. Functionalization of Vinyl Chains. Vinyl oximes were functionalized with CN and F moieties, and the catalytic cycles of both reactions begin with initial metalation of the oxime (Scheme 94, a).441,442 For cyanation with Ph(Ts)NCN, the reaction plausibly proceeds via insertion (b) and elimination (c),441 whereas the fluorination with N-fluorobenzenesulfonimide involves oxidative addition (d) and subsequent reductive elimination (e).442 Cyanation. The vinylic C−H bond in the oxime ether MeONC(Me)CaCH(CH2)3CH2b(a−b) was cyanated with Ph(Ts)NCN in the presence of Ag2CO3 (20 mol %) and NaOAc (20 mol %) utilizing [RhCp*(NCMe)3](SbF6)2 (5 mol %) as the catalyst (in ClCH2CH2Cl under Ar, 120 °C, 24 h; Scheme 94). This cyanation produced MeONC(Me)C a C(CN)(CH2)3CH2b(a−b), which was isolated in 76% yield after workup.441 This procedure was further extended to the cyanation of a dehydropregnenolone oxime derivative.441 AK


Chemical Reviews

Review

Table 2. Metal-Involving CAr−H Activation of ortho-Position of Aromatic Oximes

AL


Chemical Reviews

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Table 2. continued

AM


Chemical Reviews

Review

Table 2. continued

AN


Chemical Reviews

Review

Table 2. continued

AO


Chemical Reviews

Review

Table 2. continued

α-keto acids, alcohols, oxalate monoester, NO2−, AcOH, disulfide PhSSPh, N-fluorobenzenesulfonamide, and N-bromo(iodo)succinimide. The mechanism of alkylation with oxiranes (E1) includes an oxirane ring-opening step followed by 1,3insertion into the M−C bond and elimination of the product via protonation (Scheme 95). Alkylation with diazopropionate (E3) involves metal-catalyzed N2 extrusion followed by the carbenoid

{Pd(C(CF3)CO2Me)} moiety generation terminated by a 1,1migratory insertion (Scheme 96). Intermediate complex 204 (Scheme 94, a and Scheme 97) was obtained from the catalytic bromination (or iodination) of aldoxime and ketoxime ethers R 1 C 6 H 4 C(R 2 )NOMe (203).469−471 Complex 204 was isolated from the reaction mixture (97%), and it converted to the ortho-bromo benzophenone oxime ether 205 in the presence of NBS (2 AP


Chemical Reviews

Review

(53−97%; Scheme 98).198,458 In the case of X1 = MeCN, the nitrile ligand was substituted by R3NC, thus achieving more stable isocyanide complexes.

Scheme 95. Alkylation of Oximes with Oxiranes443,444

Scheme 98. Insertion of Isocyanides into the Pd−C Bond of Cyclopalladated Complexes 206198,458a

a

206: R1 = 4,5,6-(MeO)3, H; X1 = Cl, R3NC, MeCN; X2/R2 = Cl/H, 2-pyCH2, MeSCH2; n = 0, 1.

This metal-mediated reaction sheds additional light onto the mechanism of the catalytic functionalization of aryl-substituted oximes.

Scheme 96. Alkylation of Oximes with Diazopropionate447

4.3. Functionalization of γ-Position to Carboxime Moiety

The initial functionalization step of the γ-position to the carboxime moiety includes β- or γ-metalation of N-coordinated oximes. Further steps can lead to the functionalization of the C atom at the position that is activated by the metal. These reactions are relevant to the functionalization of the β-position of oximes described above (Scheme 99).474 However, another type of reactivity includes intermolecular reaction at the activated para-carbon relative to a metal center in β-metalated oximes (Scheme 100).475 The cyclopalladated phenyl acetoxime complexes 208 reacted with the alkynes R1CCCO2Me (1−9-fold excess; R1 = Ph, CO2Me) in CH2Cl2 at RT for 6−40 h to give complexes 209 Scheme 99. Insertion of Alkynes into the Pd−C Bond of 208474a

Scheme 97. Ortho-Halogenation of Aryl Oxime Ethers 203469−471

equiv) and AgOTf (10 mol %) in ClCH2CH2Cl at 120 °C for 2.5 h.470 In the case of R2 = Ar, the reaction proceeded regioselectively, leading to exclusive halogenation of the aryl ring at the E-position to the OMe moiety.470 Vicente and co-workers reported the metal-mediated insertion of isocyanides473 R3NC (R3 = Xy, tBu) into the Pd−C bond in the cyclopalladated acetophenone oxime complexes 206. This reaction proceeded in the presence of R3NC (1−2 equiv) in CH2Cl2 or CHCl3 at RT for 1.5−20 h to give complexes 207

a

208: X1 = 4-MeC5H4N; X2 = Br; n = 0; X1/X2 = 4,4′-di-tert-butyl2,2′-bipyridine; n = 1. AQ


Chemical Reviews

Review

Scheme 100. Ruthenium-Catalyzed meta-Selective Nitration of Aromatic Oxime Ethers475,476a

5. METAL-INVOLVING REACTIONS OF OXIMES LEADING TO CARBO- AND HETEROCYCLES In organic chemistry, oximes are highly potent reagents for attaining various systems, including heterocyclic species. Their importance in heterocyclizations is accounted for by their diversity of bond-making processes and the high degrees of chemo-, regio-, and stereoselectivity reached in many reported cases. Metal-mediated processes often allow certain heterocyclizations that are not feasible without the involvement of metal centers. Reactions leading to the cyclization of oximes typically include some combination of any of the above-discussed reactions, viz., O-, N-, or C-functionalization (sections 3.1−3.3), reduction (section 3.5), and functionalization of a side-chain induced by ligation of the oxime group (section 4). In the past decade, reviews, summarized in Table 3, have been devoted to diverse heterocyclizations, including those of oximes. In Table 3, we provide the scope of these reviews, indicate the types of heterocycles derived from oxime species, and specify the number of references about metal-involving heterocyclizations of oximes. In this section, all heterocyclizations of oximes that were not discussed in refs 35, 36, 38, 40, and 42−45 will be treated comprehensively. The generation of cyclic systems already considered in the reviews in Table 3 will be inspected selectively to provide a general picture of oxime heterocyclization. We start with the metal-mediated generation of carbocycles from oxime species (section 5.1), and we then consider the generation of heterocycles (section 5.2). The oxime cyclizations are arranged based on the ring size of the target cyclic systems, from smaller to larger cycles. In each section on heterocycle generation, the discussed products proceed from a lower to a higher number of heteroatoms and from less to more hydrogenated cycles.

a

211: R1 = 2-MeO, 4-MeO, 3,4-CH2CH2O, 3,4-OCH2CH2O, 2-Me, 3-Me, 4-Me, 3,4-Me2, 4-tBu, H, 4-Ph, benzo[c], 2-F, 3-F, 2-Cl, 3-Cl, 4Cl, 4-Br, 4-MeO2C; R2 = H, Me, nBu, Ph, MeO2C; R1/R2 = OCH2CH2, (CH2)4.

5.1. Generation of 5-Membered Carbocycles

(80−92%; Scheme 99, a).474 When 208 reacted with PhCCPh (3-fold excess) in CHCl3 (60 °C; 48 h), tetranuclear complex 210 was formed, and it was isolated in 42% yield (b). Reaction b is suggested to start466 from the initial formation of a complex that is similar to 209, which then reacts with the second equiv of PhCCPh to lead to the tetraphenylcyclopentadiene ring. Zhang et al. 475 reported the nitration of O-methyl acetophenone oxime 211 leading to 3-O2NC6H4C(NOMe)Me (212; 21%; Scheme 100, a), which required the presence of Ru3(CO)12 (10 mol %), oxone (KHSO5·1/2KHSO4·1/2K2SO4, 1.5 equiv), AgOTf (1.5 equiv), and Cu(NO3)2·3H2O (3.0 equiv); the latter nitrate is the source of the nitro group. Optimization of the reaction conditions allowed the preparation of the 3-nitroaryloxime ethers in 36−76% yields. The optimized synthesis proceeded in the presence of AgNO3 (1.8 equiv), Ru3(CO)12 (7.5 mol %), and PhI(O2CCF3)2 (1 equiv) in ClCH2CH2Cl under O2 at 100 °C for 0.5−36 h (48 h for R2 = CO2Me).476 Sequence of possible stages of this meta-selective nitration includes initial generation of catalytically active RuII species (b), cyclometalation (c), attack of •NO2 to the para-carbon relative to the ruthenium(II) center (d), and oxidative proton elimination/ aromatization (e) followed by decoordination (f). This is a rare example of a CAr−H functionalization that proceeds selectively at the meta-position of the aromatic ring (for a recent review of meta-selective functionalization, see ref 477). A crucial feature of this reaction is its occurrence at the paraposition to the metal center in the metalated aryl ring rather than at the gem-position (via insertion into the M−C bond, which is the conventional route of side-chain functionalization478−480).

Zhu et al. reported the palladium-catalyzed oxidative cyclization of the (2-allylcycloxexyl)oxime esters C6H10(CH2CHCH2)C(R)NOP(O)(OEt)2 (213) leading to cyclopentene (214; Scheme 101, a) or cyclopentenone (215; b) rings.481 Aldehydes 214 were formed from oxime esters 213 in the presence of Pd(dba)2 (20 mol %) and Et3N (5 equiv) under O2:N2 (1:1, v/v) in a DMF:H2O (9:1, v/v) mixture at 110 °C for 3−6 h; these aldehydes were isolated in 16−73% yields. Reaction scheme includes the oxidative insertion of the palladium(0) into the oxime N−O bond (c) followed by endo-insertion of the C C bond into the Pd−N bond (d). Then, the palladium(II) transfers to the N atom with subsequent nucleophilic attack of the carbanion to the imine C atom (e), followed by azetidine ring-opening (f). Next, the base-supported reductive elimination (g) leads to the aldimine, which is consequently oxidized in the presence of the palladium center and dioxygen (h and i). Finally, the aldimine is hydrolyzed in the presence of water to the corresponding aldehyde 214 (j). In another experiment, oxime species 213 transformed to cyclopentenones 215 in the presence of Pd(OAc)2 (20 mol %) and Et3N (5 equiv) under O2 (DMF, 120 °C, 3−8 h, 25−59%). The proposed mechanism includes steps c−f leading to an amide intermediate, which undergoes elimination of methaneimine (k), and palladium-mediated oxidation to alcohol (l and m), which is finally oxidized to ketone 215 (n). It is suggested481 that the reduction of palladium(II) to palladium(0) proceeds spontaneously by the action of Et3N under the given reaction conditions. The use of 18O2 in the experiments confirmed that the O atoms of the OH group of 214 and the CO moiety of 215 originated from O2, whereas the O atom of the CO AR


2017

2015

7

11

2015

6

2017

2014

5

10

2014

4

2017

2012

3

9

2011

2

2016

2006

1

8

year

entry

J. Mo, L. Wang, Yu. Liu, X. Cui H. Huang, X. Ji, W. Wu, H. Jiang H. Huang, J. Cai, G.-J. Deng J. Li, Yi. Hu, D. Zhang, Q. Liu, Yu. Dong, H. Liu N. J. Race, I. R. Hazelden, A. Faulkner, J. F. Bower S. Choi, S. Ha, C.-M. Park

E. Abele, R. Abele

L. He, H. Nie, G. Qiu, Yu. Gao, J. Wu

K. Narasaka, M. Kitamura T. J. Barker, E. R. Jarvo S. Chiba

author(s)

benzopyrroles; pyrroles; bicyclic imidazoles

azete oxides

− −

azirines; oxiranes

−

pyrroles; pyrrolines; pyrrolidines; imidazoles

pyrroles; triazoles; isoxazoles; 2isoxazolines

− −

−

−

− −

−

azirines

RhIII-, RuII-, CuI/II-catalyzed coupling reaction; RhIII-, RuII-, CoIII-, AuI-, NiII-, FeIII-, IrIIIcatalyzed cyclization; PdII-, CoIII-catalyzed C−H functionalization; PdII-, CuII-catalyzed amino-Heck reaction; 66 refs

Pd0-catalyzed Narasaka−Heck cyclization; PdII-, CuII-catalyzed aza-Heck reaction; 65 refs

AuI-, CuII-catalyzed cycloaddition; AgI-, CuII-, NiII-, RhII-catalyzed cyclization and rearrangement of α-diazo oxime ethers; 52 refs

benzopyrroles; pyrroles; pyrrolines; pyrrolidines; imidazoles; pyrazoles; thiazolines benzopyrroles; pyrroles; furanes; pyrrolidines; 1H-indazoles; imidazoles; pyrazoles; isoxazoles; thiazolines

benzopyrroles; isobenzopyrroles; isobenzofuranes

Pd0- and CuI-catalyzed intramolecular cyclization; Pd0/II-, RhI/III-catalyzed C−H activation; CuI/II-catalyzed reactions for α C(sp3)−H functionalization of O-acyl oximes; 51 refs

bicyclic aziridine

−

−

AgI/CuII-catalyzed intramolecular cycloisomerization; AgI, BiIII-catalyzed tandem electrophilic cyclization−deoxygenation; AgI-catalyzed tandem cyclization−[3 + 2] cycloaddition−rearrangement; InIII-catalyzed tandem Beckmann rearrangement−cyclization; AgI/YbIII-catalyzed tandem 6-endo cyclization−[3 + 3] cycloaddition of 2alkynylbenzaldoxime; 32 refs CuI/II-catalyzed cyclization, C−H functionalization; Pd0/II-catalyzed cascade cyclizationalkenylation, amino-Heck reaction; RhII/III- catalyzed cyclization, tandem 2,3-rearrangement/heterocyclization, C−H functionalization; AuI-, Pd0-, AgI-, NiII-, YbIII-, ZrIV-, InIII-, and FeII-mediated cyclization; PtIV-, AlIII-, ZnII-, SnII-catalyzed reactions of oximes with nucleophiles; 80 refs internal-oxidant-handled Pd0-catalyzed cyclization, RhI−CuII-catalyzed reactions with alkynes; 24 refs

Pd0-catalyzed intramolecular aromatic C−H bond amination; CuI-catalyzed heterocyclization involving oximes; RhIII-catalyzed C−H activation of oximes; RuII-, Ni0-, FeIIIcatalyzed reactions of oximes with alkynes leading to heterocycles; 26 refs

tetrahydrocarbazole; benzopyrroles; pyrroles; dihydropyrroles; indazoles; tetrazoles; benzisoxazoles; isoxazolines; oxazole; 1,2,4-oxadiazoles benzopyrroles; pyrroles

−

−

RhIII/CuI-catalyzed heterocyclization upon reactions of oximes with alkynes; 17 refs

benzopyrroles; pyrroles; 3,4-dihydro-2H-pyrroles −

−

−

CuI- and Pd0-catalyzed cyclization of O-substituted oximes; 17 refs

5-membered 3.4-dihydropyrroles; pyrrolidines

−

4-membered

−

3-membered

Cu0- and CuI-mediated heterocyclization of O-substituted oximes; 15 refs

scope

Table 3. Reviews Published after the Year 2006 that Consider Metal-Mediated Heterocyclization of Oximes 6-membered

AS

benzopyridines; pyridines; pyrazines

pyridine N-oxides; benzopyridines; pyridines; tetrahydroquinolines; piperidines; tetrahydropyrans benzopyridines; pyridines

benzopyridines; pyridines



pyridine N-oxides; pyridines; 1,2-oxazines; thiazines

benzopyridines; pyridines pyridines; benzopyridines benzopyridine N-oxides; benzopyridines; 1,2dihydroisoquinolines; benzothiazines

pyridines

−

−

−

−

−

47

46

31

35

38

40

36

azepines; ε-caprolactam; azepanes; oxazepines; thiazepines; 1,2,4-oxadiazepines −

43

44

45

refs

42

7-membered

−

−

−

−

Chemical Reviews Review


Chemical Reviews

Review

Scheme 101. Palladium-Catalyzed Formation of Cyclopentene and Cyclopentenone Rings481a

a

213: R = Me, Et, nPr, nBu.

moiety of 214 derived from H2O.481 These reactions are known to proceed only as metal-mediated transformation, with no examples of relevant metal-free processes.

Scheme 102. Generation of allyl Aziridines 218 from Oximes 216 and Allyl Zinc Bromides 217482a

5.2. Generation of Heterocycles

5.2.1. Generation of 3-Membered Heterocycles. The αhalogenated aliphatic and aromatic oxime ethers HalCH2C(R1)NOR2 (216) reacted with the allyl zinc(II) bromides H2CCR3CH2ZnBr (217) in THF under dinitrogen at RT for 14 h to give allyl aziridines 218 (40−93%; Scheme 102, a).482 The reaction is sensitive to steric effects in the allyl moiety. Thus, for MeCHCHCH2ZnBr and BrCH2C(Ph)NOMe, the reaction proceeded in 30% yield, whereas the reaction did not proceed for PhCHCHCH2ZnBr and Me2CCHCH2ZnBr or for alkyl (benzyl and n-octyl) zinc bromides. Upon the basis of the experimental data, it is believed482 that the reaction starts from nucleophilic attack of the vinyl C atom to the oxime C atom (b and c), followed by intramolecular nucleophilic substitution of the halogen (d).

a

216: Hal = Cl, Br; R1 = 4-MeOC6H4, 4-MeC6H4, Ph, 4-PhC6H4, 4FC6H4, 4-ClC6H4, 2-BrC6H4, 4-BrC6H4, 2-naphthyl, 1-pyrenyl, 1thienyl, Me; R2 = Me, PhCH2; 217: R3 = H, Me.

The formation of 1-alkoxyaziridines from oxime O-ethers is known in organic chemistry (e.g., see the recent example of fused AT


Chemical Reviews

Review

aziridine formation under the action of NaH483) and also in organometallic chemistry (see ref 36 for the rhodium-catalized reaction). 5.2.2. Generation of 4-Membered Heterocycles. OPropargylic aromatic aldoximes are transformed to azete oxides in the presence of a copper(I) catalyst.484,485 It is believed that the reactions include the copper(I)-catalyzed generation of Nallyl aldonitrone intermediates, which subsequently give azete oxides by a metal-free route.484,485 These works have been previously reviewed in refs 36 and 486, and the mechanism of this reaction was studied theoretically.487 5.2.3. Generation of 5-Membered Heterocycles. The αdiazo oxime ethers R1C(NOMe)C(N2)C(O)CHCR2R3 (219) were employed as substrates for the selective synthesis of substituted 1H-pyrroles 220 in the presence of nickel complexes (Scheme 103, a).488 The efficiency of the employed catalyst depended on the nickel oxidation state, and NiII species proved to be more efficient; the best results were achieved with [NiCl2(PPh3)2]. The reaction occurred in the presence of the catalyst (10 mol %) in PhCl at 130 °C for 36 h to give 44−92% isolated yields of substituted pyrroles 220 after workup.

Pathway of this reaction includes the substitution of N2 by the nickel(II) center (b) followed by rearrangement leading to ketene derivative (c), which transforms into the carboxy azirine (d and e). After that, metal-mediated N−C bond cleavage with subsequent electrophilic attack of N+ to the CC bond occurs (f), leading, after decoordination, to 1H-pyrrole 220 (g).488,489 Notably, in the case of 219 (R1 = Ph, R2 = Me, R3 = iBu and 4O2NC6H4), the reaction led to ca. 2:1 mixtures of 220 and the isomeric methyl 3-(2-phenyl-4-R2-5-R3-pyrrolyl)carboxylates. The reaction of 219 (R1 = Ph, R2/R3 = CHCH(CH2)2) resulted in spontaneous oxidation of the formed 220 to finally give an indole derivative. The transformation of α-diazo oxime ethers to substituted pyrroles is the common reactivity mode of these compounds.47 The ketoximes R1C(CH2R2)NOH (221) reacted with MeO2CCCCO2Me in the presence of Eu(OTf)3 (10 mol %) under dinitrogen in toluene under reflux for 12−18 h to furnish 1H-pyrrole-2,3-dicarboxylates (222), which were isolated in 60− 75% yields (Scheme 104, a).490 When the reaction was Scheme 104. Europium(III)-Catalyzed Generation of 1HPyrrole-2,3-dicarboxylates490a

Scheme 103. Synthesis of Pyrroles 220 from α-Diazo Oxime Ethers 219488a

a

221: R1 = Ph, 4-FC6H4, 2-(5-methylfuryl), 2-thienyl, 1-naphthyl, Et; R2 = Ph, H, Me, Et, Br; R1/R2 = (CH2)n (n = 4, 5, 6, 10), i PrCH(CH2)2CHMe.

performed at 50 °C for 5 h, the O-vinyl oxime 223 (R1 = Ph, R2 = H) was isolated in 78% yield. Oxime ether 223 was transformed to 222 in the presence of europium(III) in toluene under reflux for 7 h (82%). In the absence of the catalyst, 223 remained intact under the reaction conditions even after 24 h. The authors argue that the mechanism includes the europium(III)-catalyzed coupling of the oxime with MeO2CCCCO2Me to give 223 (b and c). The metal center could accelerate the imine−enamine tautomerization (d) for further aza-Cope rearrangement (for recent works, see refs 491−494), leading to 1-imine-4-one (e), which subsequently undergoes heterocyclization to 222 (f). The reaction proceeded in substantially lower

a

219: R1 = Ph, 2-furyl, PhCHCH, Cy; R2 = Me, R3 = 4BrC6H4CH2, 4-MeOC6H4, Ph, 4-O2NC6H4, 2-furyl, (E)−CH CHPh, CCPh, Me, iBu, tBu; R2/R3 = CHCH(CH2)2, CH CH(CH2)3, (CH2)4, (CH2)6. AU


Chemical Reviews

Review

(2.5−5.0 mol %) at RT−80 °C for 1−24 h to give N-methoxy pyrroles 229 (Scheme 106, a).84

yields in ClCH2CH2Cl or THF under reflux for 24 h and did not proceed in MeCN and MeOH. Moreover, the reaction occurred only for the strongly activated MeO2CCCCO2Me; no reaction was observed for less activated aromatic and aliphatic alkynes.490 The oxime−alkyne coupling is widely abundant via metal-free protocol, but these reactions proceeded under basic conditions (for recent works see refs 495 and 496). Next group of methods for the pyrrole ring construction involves reactions of oxime ethers and -esters with alkenes and alkynes. These reactions are known to occur under metal-free and metal-mediated conditions and they were previously discussed in reviews 31, 35, 36, 38, 40, 44, and 45 (Table 3). In paragraphs that follow, we discuss only recent examples of the known reactions. Substituted pyrroles 226 were obtained when reacting oxime esters R2CH2C(NOAc)R1 (224) with silyl enol ethers Me2SiOC(R3)CHR4 (225) in the presence of FeCl2 (5 mol %) in MeCN at 80 °C for 24 h (Scheme 105, a).497

Scheme 106. Palladium(II)-Catalyzed Generation of NOxypyrroles from Oxime Ethers 22784a

Scheme 105. Iron-Catalyzed Generation of Pyrroles 226 from 224497a

a 227: R1 = H, Me, Ph; R2 = 1-naphthyl, 2-furyl, 2-thienyl, PhCH2, R5C6H4: R5 = 3-MeO, 4-Me, 2-HOCH2, H, 4-Ph, 2-F, 4-Cl, 4-MeO2C, 4-O2N; R3 = H, Ph; R1/R3 = (CH2)3; 228: R4 = iPrO, nBuO, PhCH2O, 4-iBuC6H4CH(Me)CO2, Ts(Boc)N, EtO2C(NC)CH; 2-fold excess in ClCH2CH2Cl; R4 = MeO, AcO; neat.

The proposed mechanism includes the activation of the CC bond by the palladium(II) center (b) followed by nucleophilic attack to the oxime N atom on the CC bond to give the 3Hpyrrole cycle (c). After that, ring-opening nucleophilic attack provides 1H-pyrrole (d), which is consequently protonated (e) by H+ from the nucleophile (g), regenerating PdII (f) and giving N-methoxy pyrrole 229. The proton transfer from the nucleophile to the 3-position of the pyrrole was confirmed by experiments conducted in CD3OD. These experiments indicated 99% occupancy of the 3-position by deuterium. The reaction was also extended to O-unsubstituted oximes in neat AcOH or MeOH, and these synthetic schemes led to the N-acetyloxy (in AcOH) or N-hydroxy (in MeOH) pyrroles.84 The reaction did not proceed for the corresponding oximes featuring cyclobutane or cyclohexane rings instead of the cyclopropane moiety.84 A similar reaction was performed between the ketoxime ethers Ar3CCC(CHAr2)C(NOR1)Me (230) and the enimines Ar6CHCR5CHNAr4 (231), which reacted in the presence of [AuCl(PPh3)] (5 mol %), AgOTf (5 mol %), and 4 Å-MS at RT for 10−60 min to give N-oxypyrroles 232 (66−91% 1H NMR yields Scheme 107, a).498 Products 232 were isolated in 43−74% yields after recrystallization. The mechanism of the reaction includes gold(I)-mediated nucleophilic attack of the oxime N atom on the CC bond (b−d) with subsequent nucleophilic attack of the imine N atom of 231 on the carbocation (e) and 6membered ring-closure leading to 3H-pyrrole intermediate (f).

a

224: R1 = Me, 4-MeOC6H4, Ph, 4-BrC6H4, 3-BrC6H4, 2-thienyl; R2 = Me, PhCH2, EtO2C, Ph, R1R2 = (CH2)4, C6H4(CH2)2−2; 225: R3 = Ph(O)C, EtO2C, CHCHPh, 4-MeOC6H4, Ph, 4-ClC6H4, 4MeO2CC6H4, 4-O2NC6H4, 3-BrC6H4, 2-naphthyl, 2-furyl; R4 = H, Me, Ph; R3R4 = C6H4(CH2)-2.

Using the diphenyl-substituted silyl enol ether in this reaction gave a low yield (14%) of 226. A proposed reaction mechanism includes the FeII-mediated formation of iminyl radical via the N− O bond cleavage (Scheme 105, b), 1,3-hydride shift and formation of the stabilized radical (c), its coupling with silyl enol ethers (d), FeIII-mediated formation of ketone (e), and dehydrative cyclization (f). Notably, using ring-strained cyclobutanone-derived O-acyloximes R2CH2C(NOAc)R1 (224; R1R2 = CR5R6CHR7; R5, R6, R7 = H, Alk, Ar) in this reaction leads to 1,6-ketonitriles, NCCH2CR5R6CHR7CHR4CR3O, via selective cyclobutanone ring-opening.497 The 1-alkynyl cyclopropyl carboxime ethers (R3CHCH2)C(CCR2)C(NOMe)R1 (227) reacted with HO-, HN-, and HC-nucleophiles R4H (228) in the presence of Pd(O2CCF3)2 AV


Chemical Reviews

Review

Scheme 107. Gold(I)-Catalyzed Generation of N-Oxypyrroles from Oxime Ethers 230498a

Scheme 108. Generation of 2H-Pyrroles 235 from O-Acetyl Oximes 233499a

a

233: R = Me, Ph.

Scheme 109. Au-Catalyzed Formation of Pyrroles from the Diazo Oxime Ethers 236500,501a

a 230: R1 = Me, PhCH2; Ar2 = 4-R2C6H4: R2 = MeO, Me, H, Cl, Br; Ar3 = 4-R3C6H4: R3 = MeO, Me, H, Cl, Br; 231: Ar4 = 4-R4C6H4: R4 = MeO, Cl; R5 = Me, Ph; Ar6 = 4-R6C6H4: R6 = H, O2N.

Finally, a 1,2-alkyl shift proceeds to substitute the gold(I) center and give N-oxypyrrole 232 (g). O-Acetyl acetophenone and benzophenone oximes PhC(R)NOAc (233) reacted with the tungsten(0) carbene complex [W(CO)5{EtOCCCCPh}] (234) in toluene at 90 °C for 18 h to give substituted 2H-pyrroles 235 that were isolated in 19% (R = Me) and 31% (R = Ph) yields (Scheme 108, a).499 The authors499 suggested a plausible mechanism of this reaction that was supported by theoretical calculations. They argued that the N atom of the oxime ether nucleophilically attacks the alkyne C atom (b) followed by a 1,2-shift of the carbene ligand (c), whereupon it undergoes heterocyclization to the 2H-pyrrolium species (d). The final reduction of the ligand leading to 235 was not studied theoretically. While the authors said that the reaction was studied by quantum-chemical methods, in fact, only a part of the overall process was investigated. The diazo oxime ethers R1(O)C(NOMe)C(N2)CR1 (236; 1 equiv) reacted with enol ethers R3CHC(R4)OR5 (237; 3- or 10-fold excess) in the presence of [AuCl(PPh3)] (5 mol %) and AgOTf (5 mol %) in ClCH2CH2Cl at 90 °C for 15 min to yield N-methoxypyrroles 238 (Scheme 109; 56−92%

a

236: R1 = Me, PhCH2, 4-O2NC6H4CH2, iPr, Cy, Ph, 4-MeOC6H4, 4CF3C6H4, 4-O2NC6H4, 2-furyl, 2-tienyl; R2 = OMe, OEt, OCH2Ph, i Pr; 237: R3 = H, Et; R4 = H, Ph, 4-MeOC6H4, 4-ClC6H4; R3R4 = (CH2)4; R5 = Et, Ac, SiMe2tBu.

yields).500 In 236, replacing the ester group (R2 = OEt) with a keto group (R2 = iPr) affected the conversion, giving only a moderate (66%) yield of 238. It is assumed that the reaction proceeds via the formation of a Au-carbene complex (b), a cyclopropane intermediate (c), ring expansion (d), and elimination of R5OH (e) to give 238. N-Methoxypyrroles 238 were reduced to the corresponding 1H-pyrroles (35−91%). AW


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α-Diazo oxime esters 236 reacted with nitriles R2CN under the same conditions ([AuCl(PPh3)] (5 mol %), AgOTf (5 mol %), ClCH2CH2Cl, 90 °C, 30 min) to give the oxazolines bearing pendant oxime functional groups.500 The generation of Nmethoxypyrroles 238 (79−87%) from diazo oxime ethers 236 also proceed in the presence of [Rh2(OAc)4] (2 mol %) in ClCH2CH2Cl at 80 °C, but the latter complex exhibit lower catalytic activity than the AuI/AgI-system and it is required 8 h to be completed.501 The reaction of oxime esters with isocyanides discussed below is a unique example of a hitherto unknown reactivity. Reaction of aryl oximes R1C(R2)NOH (102) with isocyanides R3NC (103; 3 equiv) in the presence of [Pd(PPh3)4] (3 mol %), CsF (3 equiv), and H2O (1 equiv) in toluene at 100 °C for 10 h produced functionalized pyrroles (239; 30−90%; Scheme 110,

239 and regenerating the catalytically active palladium(0) species (g), terminates the overall reaction. This reaction was conducted with aryl oxime esters R1C(CH2R2)NOAc (240), which reacted with isocyanides R3NC (103; 2 equiv) in the presence of [Pd(PPh3)4] (5 mol %) and DBU (2.2 equiv) in toluene at 115 °C for 1−16 h to give pyrroles (239; 50−89%; Scheme 111, a).502 This reaction did not proceed for aliphatic oxime esters.502 Scheme 111. Palladium-Catalyzed Generation of Functionalized Pyrroles 239 from Oxime Esters 240 and Isocyanides 103502a

Scheme 110. Palladium-Catalyzed Generation of Functionalized Pyrroles 239 from Oximes 102 and Isocyanides 103267a

a 240: R1 = R4C6H4: R4 = 2-HO, 4-MeO, 2-MeO, 3-Me, 4-Me, 2AcNH, H, 3-F, 4-Cl, 4-MeO2C, 4-NC, 4-MeO2C; 3,4-R42C6H3: R4 = MeO, Cl; 2-thienyl, 2-furyl, 3-pyridyl; R2 = H, Ph; R1/R2 = C6H4(CH2)2−2, 4-MeOC6H3(CH2)2−2; 103: R3 = 4-MeOC6H4, 4MeC6H4, Xy, tBu, Cy.

The proposed mechanism of this reaction is similar to that depicted in Scheme 110. The authors demonstrated a possibility of the generation of 1H-pyrrole-2,3-diones (241; 30−70%) via hydrolysis of some of the pyrroles (R1 = R2 = Ph; R1/R2 = 4MeOC6H3(CH2)2, C6H4(CH2)2; b).502 Metal-mediated heterocyclization of oxime ethers and -esters to give pyrrolines and oligoheteronuclear cyclic analogs is widely utilized reaction, which was previously discussed in the review,35 and here we discuss only recent advances relevant to this reaction. Extending the reaction of oxime esters with silyl enol ethers Me3SiOC(R4)CH2 (5 mol % FeCl2, MeCN, 80 °C, 24 h; Scheme 105) to γ,δ-unsaturated oxime esters CH2 CR3CH2ZC(NOR1)R2 (242) allowed the synthesis of pyrrolines and thio-/aza-derivatives 243 (Scheme 112).497 The γ,δ-unsaturated ketoximes H2CCH2C(R22)C( NOH)R1 (244) were treated with HNR3R3 (3-fold excess; 245) in the presence of Cu(OAc)2 (20 mol %), bpy (25 mol %), and tBuOOtBu (3 equiv) in MeCN at 100 °C for 1.5 h to give

a 102: R1 = R4C6H4: R4 = 4-MeO, 3-MeO, 4-Me, H, 4-Ph, 4-F, 2-Cl, 3Cl, 4-Cl, 4-Br, 4-CF3, 4-NC, 4-MeO2C; 3,4-R42C6H3: R42 = Me2, (MeO)2, OCH2O; 2-naphthyl, 3-naphthyl, 2-thienyl; R2 = H; R1/R2 = C6H4CH2−2, 4-MeOC6H3CH2−2, C6H4CHMe-2, C6H4(CH2)2−2; 103: R3 = tBu, tBuCH2CMe2.

Scheme 112. Iron-Catalyzed Generation of Pyrrolines and Thio-/Aza-Derivatives 243497a a).267 A wide range of aryl oximes could be employed in this reaction, while R3NC was restricted to R3 = tBu or MeC(Me2)CH2C(Me2). The application of R3NC with R3 = Cy, 4MeOC6H4 did not lead to 239. Sequence of possible stages267 includes the generation of oxime carbamates (b; see Scheme 46 for details) followed by insertion473 of the palladium(0) center into the N−O bond (c). After that, two isocyanides 103 undergo insertion into the Pd−N bond (d), followed by the Csp3−H activation (e). The reductive elimination accompanied by tautomerization, eventually giving

a

242: R1 = Ac, Ph(O)C; Z = CH2, S, NC(O)Ph; R2 = Ph, 2-furyl; R3 = H, Me; Me3SiOC(R4)CH2: R4 = 4-MeOC6H4, Ph, 4-ClC6H4, 4MeO2CC6H4, 2-naphthyl.

AX


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Scheme 114. Preparation of Pyrrolidines 249 from O-Benzylγ-tosyl Butanaldoximes 247505a

pyrroline N-oxides 246 (42−87%; Scheme 113, a).503 This reaction could also proceed under a peroxide-free protocol in air, Scheme 113. Copper-Mediated Generation of Pyrroline NOxides from 244503a

a

248: R′ = iPr, sBu, iBu, cyclopentyl, Cy, (CH2)2CHMe.

n

C14H29, EtO2C-

obtained in 13−80% and 13−44% yields, respectively. In the absence of R′I (248), the pyrrolidine 249 (R′ = Et) was obtained in 75% yield after 3.5 h. Using R = EtO2C(CH2)2CHMe, the reaction led to a mixture of two diastereomers in a 1:1 ratio. This reaction could also be efficiently catalyzed by Mg(ClO4)2, Me3SiOTf, Sc(OTf)3, Cu(OTf)2, and Hf(OTf)4, but the yields of the pyrrolidines were lower. The reaction did not proceed at RT in the absence of any of the indicated catalysts. The postulated mechanism of this transformation505 includes the addition of Et• or R• to the C atom of the oxime moiety, 1e-̅ reduction, and nucleophilic substitution of TsO− by the N atom. Notably, this mechanism does not explain the role of the catalyst. The oxime esters R1C(CH2R2)NOAc (250) were treated with the pyridines R3C5H(3−4)N (251) in the presence of CuI (10 mol %) in N-methylpyrrolidone at 100 °C for 4−6 h in air to give imidazo[1,2-a]pyridines 252 (66−85%; Scheme 115, a).506 When using the unsymmetrical pyridines 251 (R3 = 3-F, 3-Cl, 3Br, benzo[c]), 8-substituted imidazopyridines 252 were

a

244: R1 = 2-thienyl, 2-pyridyl, PhCH2CH2, R5C6H4: R5 = 4-MeO, H, 4-Cl, 4-Br, 4-CF3; R22 = Me2, (CH2)4, (CH2)5; 245: R3 = H, Me; R4 = 2-benzothiazolyl, 2-pyridyl, Cy, R6C6H4: R6 = 4-MeO, 2,4-(MeO)2, 2Me, 4-Me, 2,4,6-Me3, 2-iPr, H, 4-Cl, 2,6-Cl2, 4-CF3, 4-NO2; R3/R4 = (CH2)5, (CH2)2O(CH2)2.

Scheme 115. Copper-Mediated Oxidative Coupling of Oxime Esters 250 and Pyridines 251 Giving Imidazo[1,2-a]pyridines 252506a

but the yields were lower (35−73%). The mechanism of this transformation is believed to include 1e-oxidation of the oxime ̅ moiety to produce radical species (b and c), which then undergo intramolecular radical cyclization (d). Concurrently, amine 245 coordinates to the copper(II) center (e), and the formed species reacts with the cyclic radical to furnish the copper(III) intermediate (f), which finally gives pyrroline N-oxide 246 via reductive elimination (g). The oxidation agent is required for oxidation of the copper(I), generated in the reaction, to the catalytically active copper(II) species (h). Similar pyrroline N-oxides were obtained from the unsaturated ketoximes R 4 HCCH(R 3 )C(R 2 2 )C(NOH)R 1 [R 1 = PhCH2CH2, Ph, CH2CH(CH2)3; R22 = H2, Me2, (CH2)4, (CH2)5; R3 = H; R4 = H, Ph; R3R4 = (CH2)2, (CH2)3] and tertbutyl nitrite (3 equiv) under argon in MeCN at RT (isolated yields 73−97%). This reaction does not require a metal catalyst, and TBN acts as both iminoxyl radical initiator and nitric oxide source.504 Miyata et al. found a novel synthetic route to pyrrolidines 249 based on the reaction of the O-benzyl-γ-tosyl butanaldoxime, TsO(CH2)3C(H)NOCH2Ph (247), and alkyl iodides R′I (248; 20-fold excess) in the presence of a 6-fold excess of Et3B and a (2−4)-fold excess of BF3·Et2O. The reaction occurred in CH2Cl2 at RT under dinitrogen (Scheme 114),505 and it was completed in 3−4 h (R′ = iPr, cyclopentyl, Cy) or 22−23 h (R′ = s Bu and iBu), leading to a mixture of 249 (R′ = iPr, sBu, iBu, cyclopentyl, Cy) and the corresponding pyrrolidine featuring R′ = Et. After separation, these components of the mixture were

a

250: R1 = PhCHCH2, R4C6H(3−4): R4 = 3-MeO, 4-MeO, 3,4(MeO)2, 3,4-(CH2OCH2), 4-Me, 2,4-Me2, 2,5-Me2, 3,4-Me2, 3,4(CH2)4, 4-tBu, H, 4-F, 4-Cl, 4-Br, 4-I, 4-SO2Me, benzo[c]; R2 = H, Me, n Pr; 251: R3 = 4-MeO, 3-Me, 4-Me, 3,5-Me2, 3-F, 3-Cl, 3-Br, benzo[c]. AY


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generated, whereas in the case of 251 (R3 = 3-Me), a 2:1 mixture of 8- and 6-substituted 252 was formed. The transformation most likely proceeds via the 1e-reduction of oxime ester 250 (b) ̅ followed by tautomerization to the C-centered radical (c), which attacks the N atom of pyridine 251 (d). Then, the imino N atom attacks the cycle (e), and finally, the radical cyclic intermediate is oxidized in the presence of the copper(II) and air to give imidazo[1,2-a]pyridines 252 (f). This reactivity mode is an extension of the known reaction,507 which was discussed previously in a review35 (Table 3). Aliphatic and aromatic oxime esters R1R2CNOAc (253) reacted with vinyl azides H2CC(R3)N3 (1.2 equiv; 254) in the presence of Fe(OAc)2 (5 mol %) to give 2H-imidazoles 255 (in ClCH2CH2Cl, 90 °C, 6 h, 53−90%; Scheme 116, a).508 Reaction

Scheme 117. Palladium(0)-Catalyzed Cyclization-Coupling of Unsaturated Oximes and Aryl Halides509,510a

Scheme 116. Iron-Catalyzed Generation of 2HImidazoles508a

a

256: R = PhCHCH, nPr, R1C6H(3−4):: 4-MeO, 2-PhCH2O, 2,4(MeO)2, 4-Me, H, 4-F, 4-Cl, 3-O2N; 257: Ar: 1-naphthyl, 2-thienyl, 2pyridyl, 3-pyridyl, R2C6H(3−4): 3-Me2N, 3-MeO, 4-MeO, 2-PhCH2O, 4-Me, 2,4-Me2, H, 4-Ph, 2-Cl, 3,4-Cl2, 4-OHC, 4-tBuO2C; Hal = Br, I.

philic addition of the O− center of the oxime to the CC double bond (d) followed by reductive elimination of 258 (e). The metal catalyst plays a crucial role in these reactions, and no examples of relevant reactions are known in metal-free chemistry. Some works have presented the preparation of functionalized isoxazolines starting from β,γ-unsaturated oximes (Scheme 118). The general mechanism of these reactions includes a 1e-̅ reduction of the oxime N−O bond by a Mn center (b), attack of the O-centered radical on the CC bond (c), and oxidative addition to the metal center to give complexes with an Mn+1 center (d), which reductively eliminate isoxazoline 260 (e) to yield Mn−1. An oxidant is required for the final oxidation of the metal center to the catalytically active form (f). The CF3-substituted isoxazolines (67−93%) were obtained from the reaction of the β,γ-unsaturated oximes R1C( NOH)CR2R3CHCH2 (259) with Me3SiCF3 (3.6 equiv) in the presence of trichloroisocyanuric acid (TCCA, 0.67 equiv), 1,10-phen (3 equiv), CuOAc (3 equiv), and CsF (3.6 equiv) under argon in MeCN at RT (Scheme 118, a).511 Copper(I) is required for the oxidation of TCCA to the corresponding radical OC{N(Cl)C(O)}2N•, which then reacts with 259 to give the active CN−O• radical. Copper acetate is also required to activate Me3SiCF3 and obtain the (CF3)CuI complex. Another preparation of the CF3-substituted isoxazolines included the reaction of 259 with 3-oxo-1-(trifluoromethyl)-1,2-benziodoxole in the presence of CuCl (20 mol %) and NaOAc (1.2 equiv) in DMF at 50 °C for 30 min under Ar. The isoxazoles 260 were isolated in 45−85% yields.515 Notably, Me3SiCF3 did not react under these conditions with oximes 259.515 The trifluororomethyl isoxazolines (45−96%) were also prepared by the action of 5-(trifluoromethyl)-5H-dibenzo[b,d]thiophen-5-ium tetrafluoroborate (1.1 equiv) on 259 in the presence of [Ru(bpy)3](PF6)2 (1 mol %) and Na2CO3 (2 equiv) in degassed MeCN at RT for 12−18 h upon exposure to 3 W blue LED irradiation.516 These reactions do not proceed in the absence of a metal source, and no examples of these reactions are known in the metal-free chemistry. The cyano-substituted isoxazolines (44−72%) were obtained from the reaction of the β,γ-unsaturated oximes R1C( NOH)CR2R3CHCH2 (259) with CuCN (2.5 equiv) in the presence of pentamethyldiethylenetriamine (2.5 equiv) and t BuOOH (3 equiv) under argon in MeCN at RT for 12 h (Scheme 118, a).522 The authors522 suceeded in conducting the

a

253: R1 = 2-naphthyl, 2-thienyl, 3-thienyl, 2-benzo[d]thienyl, Et, PhCH2CH2, nC5H11, nC6H13, R4C6H(3−4): R4 = 4-MeO, 4-MeS, 3-Me, 4-Me, 3,4-Me2, 4-Et, 4-nBu, H, 2-F, 3-F, 4-F, 2-Cl, 4-Cl, 4-Br, 4-CF3, 4O2N; R2 = Me, Et, Ph; R1/R2 = (CH2)5, C6H4(CH2)3-2, C6H4(CH2)42; 254: 2-naphthyl, Ph(CH2)3, R5C6H4: R5 = 4-AcO, 3-Me, 4-Me, 4-tBu, H, 2-F, 3-F, 3-Cl, 4-Cl, 4-Br, 4-CF3, 4-O2N.

pathway includes iron(II)-mediated reductive splitting of the N− O bond to give iminium anion (b) and the parallel thermally induced generation of the azirine (c). Aziridine, at least formally, undergoes 1,3-dipolar cycloaddition to the anion to give imidazoline anion (d). The latter is oxidized by the iron(III) species to grant the 2H-imidazole and regenerate the catalytically active iron(II) species (e). The authors508 successfully performed this reaction in gram scales in good yields. This reaction has no analogs in the oxime chemistry. Next group of reactions includes generation of functionalized isoxazolines from β,γ-unsaturated oximes; this is one of the most abundant reactions of such type of oxime species. These transformations have been reviewed in refs 31 and 36 (Table 3) and here we discussed only relevant recent advances. The β,γunsaturated oximes 4-R 1 C 6 H 4 C(NOH)CH 2 CHCH 2 (256) underwent heterocyclization to grant isoxazolines 258 in the presence of a 5-fold excess of any of the aryl iodides 3-R2-4R3C6H3I (257), K2CO3, and 10 mol % of [Pd(PPh3)4] in DMF (80 °C, 18 h, 11−78%; Scheme 117, a).509 Oximes 256 also reacted with aryl bromides (1.2 equiv) in the presence of [Pd2(dba)3] (1 mol %), Xantphos (2 mol %), and tBuONa (1.2 equiv) to yield arylated isoxazolines 258 (in toluene, 105 °C, 3 h, 46−94%).510 Although the mechanism of these reactions was not discussed,509 we assume that it includes the deprotonation of the oxime (b), oxidative addition of the aryl iodide to the palladium(0) center (c), and palladium(II)-mediated nucleoAZ


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260 (36−76%) in the presence of Pd(OAc)2 (10 mol %), phen (10 mol %), AcOH (10 equiv), and H2O (15 equiv) at 40 °C for 12−48 h in air (1 atm). In experiments utilizing 18O2 or H218O, it was found that the O atom in the introduced HO-moiety derived from air O2 rather than from water.517 Oximes 259 also underwent cyclization to hydroxyisoxazolines 260 in MeOH in the presence of [Mn(acac)3] (0.2 mol %) in air at RT for 24−48 h (44−83%).513 The reaction did not proceed in the presence of the other tested [M(acac)n] complexes [n = 3, M = TiIII, FeIII, CoIII; n = 2, M = NiII, CuII (traces of product), MoVIO2]. However, another study dealt with a cobalt-mediated synthetic approach to oxyisoxazolines 260. Oximes 259 transformed to oxyisoxazolines 260 (55−92%) in the presence of [Co(nmp)2] (10 mol %), tBuOOH (10 mol %; 70% in water) in iPrOH at 55 °C for 5 h under dioxygen.518 Metal-free protocols of this reaction are known, but they require a large excess of a peroxide526 or complex organic photocatalyst.527 Li et al. reported the metal-catalyzed heterocyclization of oximes 259 leading to fluorinated isoxazolines 260 (52−72%), which proceeded in the presence of AgOAc (10 mol %), Selectfluor (2 equiv), and AcOH (1 equiv) in a toluene:H2O (2:1, v/v) mixture at 30 °C for 20−48 h under argon.520 The conversion of oximes 259 to fluoro-substituted 260 was realized under metal-free conditions utilizing pyridinium hydrofluoride and PhI(OPiv)2.528 Liu and Zhu reported the preparation of the chloro- and bromo-functionalized isoxazolines 260 in the presence of Pd(OAc)2 (10 mol %) and CuHal2 (2 equiv; Hal = Cl and Br, respectively) in dimethyl sulfoxide at RT for 2−11 h. The isoxazolines were isolated in 60−85% and 65−86% yields, respectively.521 The chloro- and bromo-substituted isoxazolines 260 could be prepared from 259 via a metal-free protocol in the presence of excess tBuONO and AlCl3 or CBr4 as halogen atom sources, respectively.529 The β,γ-unsaturated oximes R1C(NOH)CR2R3CR4 CHR5 (259) reacted with AgSCF3 (1.5 equiv) in the presence of Cu(OAc)2 (20 mol %) to give trifluoromethylthio-functionalized isoxazolines 260 (in DMF, 80 °C, 12 h, 52−90%).519 Other metal centers such as AlIII, ScIII, FeIII, NiII, ZnII, InIII, and LaIII were not effective as catalysts for this reaction.519 In the case of reaction of 259 with KSCN (2.0 equiv) in the presence of FeCl3 (20 mol %) and K2S2O8 (2.5 equiv), the transformation gives the thiocyano-functionalized isoxazolines (40−84%).530 No direct metal-free protocols for these reactions are known, but the thio-substituted isoxazolines can be prepared from oximes 259 via two-step metal-free procedures.525,531 Wang and Li514 reported the reaction of oximes 259 with sodium sulfonates RSO2Na (3 equiv) in the presence of Cu(OAc)2 (2 equiv) and KF (2 equiv) in Me2SO at RT for 36 h to give sulfone-substituted isoxazolines 260 (25−91%). Copper(II) is required for the oxidation of the oxime moiety to the CN−O• radical and the formation of RO2S• from RSO2Na; the coupling of these two radicals gives 260. Alternatively, the reaction could proceed through generation of the sulfonylic cation RO2S+. Isoxazolines and isoxazoles 263 were prepared in the blueLED induced photoredox reaction of hydroxyimino acids R1C(NOH)CO2H (261) with the alkenes R2R3CCHR4 (262) and alkynes R2CCH (R2 = Ph, nC5H11) catalyzed by [Ru(bipy)3]Cl2 (2 mol %) in the presence of NaHCO3 (2 equiv) and oxone (2 equiv) in DMF at RT (25−95% isolated yields Scheme 119, a).532 The reaction mechanism includes deprotonation of the hydroxyimino acid (b) and light-induced and RuIIIcatalyzed formation of hydroxyminium radical via a single-

Scheme 118. Copper-Mediated Oxidative Coupling of 259 with N-Nucleophiles Giving Functionalized Isoxazolines 260503,511−521a

a 259: R1 = R8C6H(3−4): R8 = 2-MeO, 3-MeO, 4-MeO, 3,4-(MeO)2, 3,4,5-(MeO)3, 3-Me, 4-Me, 3,4-Me2, 2,4,6-Me3, H, 2-F, 4-F, 4-HCC, 2-Cl, 3-Cl, 4-Cl, 2,4-Cl2, 2-Br, 3-Br, 4-Br, 4-NC, 3-CF3, 4-CF3, 4-O2N; 1-naphthyl, 2-naphthyl, 3-(1-(tert-butoxycarbonyl)-1H-indolyl, 3-(1tosyl)-1H-indolyl, 2-furyl, 2-thienyl, 2-(3-methyl)thienyl, H, iPr, tBu, PhCH2, PhCH2CH2, nC5H11, cyclopentyl, tBuMe2SiO(CH2)4, BocHN(CH2)5, nC6H13, Cy, 3-cyclohexenyl, nC7H15; R2 = H, Me, Ph; R3 = H, Me, Ph; R4 = H, Me, 4-MeC6H4, Ph, 4-FC6H4, 4-ClC6H4, 2,4Cl2C6H3; R5 = H, Ph; R1/R2 = (CH2)4, R3/R4 = (CH2)3, (CH2)4; R3/ R5 = (CH2)2, (CH2)3; R4/R5 = (CH2)3, (CH2)4; NR6R7: R6 = nBu, Cy, n C7H15, (S)-PhC(H)Me, 2-benzothiazolyl, 2-pyridyl, 4-R9C6H4: R9 = MeO, Me, H, Cl, CF3; R7 = H, Me; R6/R7 = (CH2)5, (CH2)2O(CH2)2; S(O)2R6: R6 = Me, CF3, R7C6H4: R7 = 4-MeO, 3-Me, 4Me, H, 3-F, 4-F, 2-Cl, 4-Cl.

reaction in gram scales. No metal-free analogs for this reaction are known in the literature, but similar heterocycles can also be prepared via 1,3-dipolar cycloaddition of nitrile oxides (generated in situ from chloroximes) to allyl cyanide derivatives.523,524 Oximes 259 reacted with amines R6R7NH (3 equiv) in the presence of Cu(OAc)2 (20 mol %), bpy (25 mol %), and t BuOOtBu (3 equiv) in MeCN at 100 °C for 1.5 h to give aminoisoxazolines 260 (21−83%). 503 The reaction also proceeded under a peroxide-free protocol in air (28−71%). No examples of metal-free reactions and other introductions of azaradicals are known. Another example is the synthesis of isoxazolines from β,γunsaturated oximes R1C(NOH)CR2R3CR4CHR5 (259) that react with Me3SiN3 (1.5 equiv) in the presence of Cu(OAc)2 (20 mol %) and NaOAc (1.2 equiv) to grant azido oxazolines 260 (in DMF under Ar, RT, 24 h, 42−89%).512 Noticeably, no additional oxidant is required for this reaction. These data indicate that the Me3Si+ moiety is reduced to Me3Si− under the reaction conditions. Azidoisoxazolines can be prepared from oximes 259 by a two-step metal-free protocol by the action of NaN3 on the initially generated corresponding iodoisoxazolines.525 The β,γ-unsaturated oximes R1C(NOH)CR2R3CR4 CHR5 (259) underwent transformation to hydroxyisoxazolines BA


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Scheme 120. Synthesis of Spiro-Fused Pyrazolin-5-ones536a

Scheme 119. Ruthenium-Catalyzed Photoredox Synthesis of Isoxazolines and Isoxazoles532a

a

264: R1 = Me, Ph, 4-MeC6H4, 4-MeOC6H4, 4-ClC6H4, 2-MeC6H4; R2 = nBu, tBu, Cy, PhCH2, Ph, 4-MeC6H4, 4-MeOC6H4, 4-ClC6H4, 4BrC6H4, 4-FC6H4, 4-EtO2CC6H4, 4-CF3C6H4, 2-MeC6H4, 3-MeC6H4, 2-MeOC6H4, 3-MeOC6H4, 2-ClC6H4, 3-ClC6H4, 2,4,6-Me3C6H2, 3Cl,4-MeOC6H4.

Scheme 121. Synthesis of Fulleroimidazolines538a

a 261: R1 = Ph, PhCH2, nC6H13; 262: R2 = H, Me; R3 = Ph, 4MeOC6H4, 4-tBuC6H4, 4-FC6H4, 4-ClC6H4, 4-BrC6H4, 4-IC6H4, 4MeO2CC6H4, 2-naphthyl, 2-pyridyl, MeO2C, (C4H8)NC(O), MeC(O), NC, PhSO2, Ph(CH2)2, CHO; R2R3 = (CH2)2NBoc; R4 = H, MeO2C; R3R4 = C(O)NPhC(O), CH(OMe)OCH(OMe).

electron transfer (c), with the consequent RuIII-catalyzed singleelectron transfer leading to nitrile oxide (d) and [2 + 3]cycloaddition to alkene (e) achieving 263. Oxone is required to regenerate the catalytically active ruthenium(III) from ruthenium(II). This reaction is an extension of previously known533 protocol for isoxazoline generation from 261 in the presence of alkene and (NH4)2[Ce(NO3)6] and represents a complex version of more abundant reaction that starts from chloroximes (for recent works see refs 534 and 535). Spiro-fused pyrazolin-5-ones 265 were obtained via an intramolecular N−N bond formation from cyclopropyl O-acetyl oximes R1C(NOAc)C(C2H4)C(O)NHR2 (264) in the presence of CuBr (10 mol %) in toluene at 100 °C for 0.5−5 h (69−97% isolated yields Scheme 120, a).536 A plausible mechanism for this reaction includes the oxidative addition of copper(I) to the N−O bond (b) and ring formation (c). Ketones R1C(O)C(C2H4)C(O)NHR2 were obtained as byproducts in the systems using the alkyl-substituted O-acetyl oximes (R1 = Me or R2 = nBu, tBu, Cy, PhCH2); their generation was driven by the hydrolysis of the intermediate complex. Noticeably that the O-tosyl analogs of 264 featuring aromatic R2 transform to 265 even in the absence of a metal center in basic media at RT for 40− 90 min to give excellent yields (85−93%) of the product.537 Fulleroimidazolines 267 were obtained in 15−40% yields by the reaction of [60]fullerene with amidoximes R1C(NOH)NHR2 (266; 5 equiv) in the presence of FeCl3·6H2O (1 equiv) and 4-dimethylaminopyridine (DMAP; 2 equiv) (in 1,2Cl2C6H4, 130 °C, 8−12 h; Scheme 121, a).538 A postulated mechanism for the reaction includes O-coordination of the amidoxime to iron(III) (b) followed by dehydration (c) to the 1,3-dipole, which undergoes cycloaddition to [60]fullerene (d).

a

266: R1 = 4-MeC6H4, Ph, 4-FC6H4, 4-O2NC6H4, PhCH2CH2; R2 = 4MeOC6H4, 4-MeC6H4, 3-MeC6H4, Ph, 4-FC6H4, 3-FC6H4, PhCH2.

Stereodivergent intramolecular cycloaddition of the cyclopropane moiety to the oxime group in (R)-R1R2CNO(CH2)2C3H3(CO2Me)2 (268) led to isoxazolidines 269 and proceeded in the presence of Yb(OTf)3 (5 mol %) in CH2Cl2 at RT−40 °C for 16 h (75−99%; Scheme 122).539 The reaction was also performed with dioximes such as 2,6-pyridyldicarbaldoxime and glyoxaldoxime and resulted in the formation of the corresponding bis-isoxazolidines in 70 and 83% yields, respectively.539 The authors of ref 539 believe that the reaction includes nucleophilic attack of the oxime N atom to the cyclopropane C atom with consequent ring closure via the coupling of the (MeO2C)2C− and O−N−C+ moieties. In this mechanism,539 however, the role of the ytterbium(III) was not explained. Aldoxime 270 reacted with cyclopentadiene to give isoxazolidine 271 in the presence of BF3 (1 equiv) and TfOH (1 equiv) in CH2Cl2 at −12 °C for 5 h (Scheme 123, a). Other Lewis and Brønsted acids also catalyzed this reaction, but in those cases, the yields of 271 were significantly lower.540 Theoretical calculations indicated that the reaction mechanism includes the BB


Chemical Reviews

Review

Scheme 122. Ytterbium(III)-Catalyzed Synthesis of Isoxazolidines 269 from O-(2-Cyclopropyl)ethyl Oxime Species 268539a

Scheme 124. Generation of Complex 273 Bearing Pendant Isothiazole541

a

268: R1 = H, R2 = 4-MeOC6H4, Ph, 4-BrC6H4, 4-O2NC6H4, 6bromo-2-naphthyl, 2-pyridyl, PhCHCH, PhCHCMe, iBu, tBu, (R)-(EtS)2CHCH2CH(OSiiPr2tBu); R1 = CO2Me, R2 = Ph.

Scheme 123. Boron(III)-Mediated Synthesis of Isoxazolidine 271 from Aldoxime 270 and Cyclopentadiene540

The reactions discussed below represent recent advances in syntheses of tetrazoles via metal-mediated aldoxime−azide coupling; this reaction can also be conducted via metal-free protocols (for recent works see refs 545 and 546). The aldoximes RC(H)NOH (276) underwent conversion to the corresponding tetrazoles RCN4H (277) upon treatment with a 1.5-fold excess of NaN3 in the presence of InCl3 (3 mol %). The reaction was performed in DMF at 120 °C for 11−48 h (20−95%; Scheme 126, a).547 The chlorides ZnCl2, FeCl3, and SnCl4 could also catalyze this reaction, but with significantly lower tetrazole yields. The reaction did not proceed when BF3·Et2O or B(C6F5)3 were employed as the catalyst. Tetrazoles were also generated via a copper(II)-mediated protocol. 4-Bromosalicylaldoxime reacted with sodium azide in the presence of 1 equiv of [CuCl2(bipy)] and a 2-fold excess of Et3N in a Me2SO:CH2Cl2 mixture at RT for 7 d to give tetrazolate complex 278 (60%).260 The metal center is believed to stabilize the oxime species in the nitrone form, thereby increasing the electrophilicity of the C atom (b), followed by nucleophilic attack by N3− (c); finally, the N−O bond heterolytically splits, and the N+ moiety couples with the N− center of the azide group to create the new N−N bond and yield the tetrazole cycle. 5.2.4. Generation of 6-Membered Heterocycles. Aryl ketoximes R1C6H4C(R2)NOH (279) underwent self-condensation in the presence of 10 mol % of Pd(OAc)2 in MeCN at 130 °C for 24 h, leading to 3-aryl isoquinolines 280 that were isolated in 25−72% yields (Scheme 127).548 These yields do not depend significantly on the electronic effects of the substituents in the aryl ring but are strongly affected by the nature of R2. Thus, the reaction yield with R2 = Et is drastically lower than with R2 = Me (25% vs 72% for the oximes with R1 = H). Another example of the generation of a pyridine ring including the homocoupling of an oxime has been reported for O-acetyl

generation of the corresponding nitrone from the oxime upon its reaction with the boron(III) center, followed by [3 + 2]cycloaddition of the formed nitrone to a double CC bond of cyclopentadiene.275 A nickel(II) complex featuring ortho-thiabenzaldoxime derivative 272 underwent a cascade reaction in the presence of 1 equiv of H2NOH in MeOH at 65 °C, leading to complex 273 (45%; Scheme 124).541 The role of the metal center was not confirmed, but the entire process should start from the in situ formation of aldoxime intermediate (a), which then undergoes dehydration of one of the aldoxime moieties to grant the nitrile group (colored in green in Scheme 124; b). Nucleophilic attack of the S− center to the oxime N atom of the other aldoxime moiety leads to elimination of the HO− species and generation of the isothiazole ring (colored in red; b). The generation of the benzothiazole ligands from the 2-thiobenzaldoxime derivatives also proceeded at a palladium(II) center.542 Generation of benzothiazoles from 2-thiobenzaldoximes also is known in a metal-free protocol.543 The oximes R1C(CH2R2)NOH (274) reacted with acid anhydrides (R3CO)2O (3 equiv; R3 = Me, Et, nPr, iPr, tBu, Cy) and KSCN (2 equiv) in the presence of CuI (10 mol %) under dinitrogen in toluene at 120 °C to give thiazoles 275 (51−85% isolated yields Scheme 125, a). The suggested mechanism starts from acylation of the oxime (b). The formed acylated oxime then undergoes a 1e-reduction with CuI (c), forming the imine radical ̅ that isomerizes to the alkyl radical. The latter reacts with SCN− and forms another intermediate after oxidation with copper(II) (d). The next steps include N-acylation (e), hydrolysis (f), intramolecular nucleophilic attack with cyclization (g), and intramolecular dehydration (h) to eventually yield 275.544 This reaction is a unique type of reactivity of ketoximes and no more examples of this transformation have been reported. BC


Chemical Reviews

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Scheme 125. Generation of Thiazoles 275544a

Scheme 126. Indium(III)-Catalyzed and Copper(II)Mediated Generation of Tetrazole Rings260,547a

a

276: R = 4-MeOC6H4, 4-HOC6H4, 4-MeC6H4, Ph, 4-FC6H4, 2FC6H4, 3,4-Cl2C6H3, 4-BrC6H4, 4-O2NC6H4, 4-NH2NHC(O)CH2OC6H4, 2-thienyl, 2-(4-phenyl)thienyl, 2-furyl, 3-pyridyl, nPr, n Bu, iBu.

a

274: R1 = Ph, 4-MeC6H4, 4-PhC6H4, 4-FC6H4, Ph, 4-ClC6H4, 4BrC6H4, 4-IC6H4, 4-MeOC6H4, 4-PhCH2OC6H4, 4-MeO2CC6H4, 4O 2 NC 6 H 4 , 3-MeOC 6 H 4 , 2-MeOC 6 H 4 , 3,4-Me 2 C 6 H 4 , 3,4(CH2OCH2)C6H4, 2-naphthyl, 2-thienyl, nBu, R2 = H, Me, nPr; R1R2 = (CH2)5, (CH2)6.

Scheme 127. Palladium(II)-Catalyzed Self-Condensation of Aryl Ketoximes 279 Leading to 3-Aryl Isoquinazolines 280548a

oximes R1C(NOAc)Me (283). Compounds 285 reacted with N,N-dialkylanilines PhN(CH2R2)2 (281) in the presence of FeCl3 (10 mol %) and tBuOOtBu (1.5 equiv) in ClCH2CH2Cl at 120 °C for ca. 5 h (the reaction time was not reported, and the progress of the reaction was monitored by TLC) to give pyridines 282 (45−86%; Scheme 128, a).549 This reaction was also performed for N-phenyltetrahydroquinoline instead of PhN(CH2R2)2, and this modification gave the corresponding 282 (R2 = 2-(PhHN(CH2)2C6H4) in 45% yield. A suggested mechanism of this transformation included the reductive cleavage of the N−O bond giving the iminium anion (b) accompanied by the oxidation of two eqivalents of FeII to FeIII. The iron(III) species are reduced by the amine, which consequently oxidizes to the iminium cation (c). The two formed ions couple with each other to give the amino imine (d), which then undergoes tautomerization to the enamine (e). The enamine reacts with the second equiv of the oxime ester (f). After that, the elimination of H2NOAc (g) and the ring closure (h) result in the formation of dihydropyridine, whose oxidation by the peroxide to pyridine 282 (i) terminates the reaction. For oxime esters R1C(CH2R2)NOAc (283) featuring substituents at the oxime moiety that are more branched than methyl, the reaction proceeds differently. Compounds 283 were treated with N,N-dimethylaniline (0.5 equiv; 281) in the

a

280: R1 = 4-MeO, 4-iBu, 4-Et, 4-Me, H, 2-F, 4-F, 3-Cl, 4-Cl, 4-Br; R2/ R3 = Me/H, Et/Me.

presence of Fe(OTf)3 (10 mol %) and tBuOOtBu (1.5 equiv) in dichloroethane at 120 °C for 4−6 h in air to give symmetrical tetrasubstituted pyridines 284 (46−75%; Scheme 129, a).550 The choice of amine is crucial for the reaction. Different tertiary amines ArNMe2 can be used in the reaction, and better results were obtained with amines featuring electron-donating groups (e.g., Ar = 4-MeC6H4) than with those having electronwithdrawing substituents (e.g., Ar = 4-NCC6H4). The reaction is sensitive to steric hindrance in the aromatic ring of the oxime BD


Chemical Reviews

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Scheme 129. Iron-Mediated Synthesis of Pyridines 284550a

Scheme 128. Iron-Mediated Preparation of Pyridines 282 from Oxime Esters 283 and N,N-Dialkylanilines 281549a

a

283: R1 = 4-MeOC6H4, 4-MeC6H4, Ph, 4-tBuC6H4, 3-CF3C6H4, 4FC6H4, 4-ClC6H4, 3-ClC6H4, 4-BrC6H4, 3-O2NC6H4, 3,4-Me2C6H3, 3,4-(CH2)4C6H3, 2-naphthyl, 2-thienyl; R2 = Me, Et, Ph; R1/R2 = C6H4(CH2)2-2.

Scheme 130. Iron-Mediated Synthesis of Pyridines 285549a

a 283: R1 = 2-thienyl, 2-furyl, R3C6H(3−4): R3 = 3-MeO, 4-MeO, 2-Me, 4-Me, 2,5-Me2, 3,4-Me2, 3,4-(CH2)4, H, benzo[c], 4-F, 2-Cl, 4-Cl, 4-Br, 4-CF3; 281: R2 = H, Me, Ph.

ester; for instance, 2-MeC6H4C(CH2Me)NOAc failed to react with PhNMe2. Dialkyl substituted oxime esters AlkC(CH2R2) NOAc were also inert in the reaction. Sequence of possible stages includes two Fe-mediated 1e-reduction steps of oxime ester 283 ̅ accompanied by two 1e-oxidation steps of PhNMe2 (b and c), ̅ FeIII-mediated coupling of the generated charged species (d), and the nucleophilic attack of the imine group on the oxime C atom of the second equiv of the oxime ester (e). The intermediate formed undergoes intramolecular FeIII-mediated deaminative cyclization (f) and oxidative aromatization to give pyridines 284 (g).550 Oxime esters 283 reacted with N,N-diallylaniline, PhN(CH2CHCHPh)2, in the presence of FeCl3 (10 mol %) and t BuOOtBu (3 equiv) in ClCH2CH2Cl at 120 °C to give pyridines 285 (56−72%; Scheme 130, a; reaction time was not reported).549 It is believed that the mechanism includes the iron-mediated coupling of the oxime ester and the aniline (b; see Scheme 128, b−d), followed by the iron(III)-mediated abstraction of PhCHCHCH2NHPh (c). After that, a [3,3]sigmatropic shift provides the dihydropyridine (d), which is then oxidized by the peroxide to pyridine 285. The metal-mediated reactions of generation of pyridine ring incorporating C atom(s) from the amine species are novel with no previously reported analogous transformations.

a

283: R1 = 2-thienyl, R2C6H(3−4): R2 = 4-Me, 2,5-Me2, 3,4-Me2, 3,4(CH2)4, benzo[c], 4-F, 4-Cl, 4-Br.

General part of construction of pyridine ring includes functionalization of the β-Csp2 atom at the carboxime moiety, which have been repeatedly reviewed previously (Table 3), and here we represent relevant recent advances. Aromatic ketoximes R1C6H(3−4)C(R2)NOH (286) reacted with alkenones H2CCHC(O)R3 (287) in the presence of [RhCl2Cp*]2 (2 mol %), tBuCO2H (2 equiv), and Ag2CO3 (2 equiv) in MeCN typically at RT (76 °C for the donor R3 = MeO, t BuO, and ArHN) for 18 h to grant 3-acyl isoquinolines 288 (53−98% isolated yields Scheme 131, a).551 Reaction pathway includes cyclometalation of the oxime (b) and insertion of alkene BE


Chemical Reviews

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Scheme 131. Rhodium-Catalyzed Generation of 3-Acyl Isoquinolines 288551a

Scheme 132. Metal-Catalyzed Preparation of Substituted Pyridines and Isoquinolines (R1CHC(R2) = Ar) 291 from Oximes 290 and Alkynes 289552−554a

a 289: R4 = 4-MeOC6H4, 4-MeC6H4, Ph, 4-FC6H4, 4-BrC6H4, 2thienyl, H, Me, Et, nPr, iPr, nBu, tBu, cyclopentyl, Cy, (cyclohexyl)methyl; R5 = 4-MeOC6H4, 4-MeC6H4, Ph, 4-FC6H4, 4-BrC6H4, 2thienyl, H, Me, Et, Br(CH2)2, nPr, nBu, MeO2C, Me3Si; R4 = R5 = Et, n Pr, 2-thienyl, R7C6H4: R7 = 4-MeO, 3-Me, 4-Me, H, 4-F, 4-Cl, 4-Br, 4CF3; 290: R1 = PhCH2CH2, Ph, 2-furyl; R1CHCR2 = 2-thienyl, 4pyridyl, 3-indolyl, R6C6H(3−4): R6 = 4-MeO, 3,4-(MeO)2, 3,4(OCH2CH2O), 2-Me, 3-Me, 4-Me, 4-iPr, H, 4-Ph, 4-F, 3-Cl, 4-Cl, 3- Br, 4-Br, 4-I, 4-CF3, 4-O2N; R1/R2 = X−C(C6H4R6)N: X = O, S; R6 = 3-Me, 4-Me, 4-tBu, H, 4-Cl, 4-CF3; R3 = Me; R3/R6 = (CH2)3; R3 = Me, Et, iPr, PhCH2, Ph; NHR8: R8 = tBu, Cy, iPr, PhCH2.

a

286: R1 = 3-MeO, 4-PhCH2O, 3,4-(MeO)2, 4-BocHN, 4-Me, H, 4Cl, 4-Br, 4-O2N; R2 = Ph, PhCO, MeO2C, Me, Et, PhCH2, TsCH2, 4MeOC6H4CH2CH2; R1/R2 = (CH2)3; 287: R3 = H, Me, PhCH2CH2, MeO, tBuO, 4-MeOC6H4HN, PhHN, 2-thienyl, 2-furyl.

287 into the Rh−C bond (c) followed by reductive elimination of the pyrimidine N-oxide to generate the rhodium(I) (d). Rhodium(I) deoxygenates the pyrimidine N-oxide to the corresponding pyrimidine and gives the catalytically active RhIII species (e). Finally, the pyrimidine ring is oxidized by O2 to the pyridine (f). The authors551 alternatively suggest Ag2CO3 as an oxidant in the final step, but this would require further confirmation insofar as the oxidation of pyrimidines to pyridines by Ag2CO3 is as yet unknown, and moreover, the reaction proceeds even in the absence of Ag2CO3.551 Several articles have presented the preparation of substituted pyridines via the reaction of aromatic or vinylic ketoximes with alkynes in the presence of a catalyst (Scheme 132). The mechanisms of these reactions are similar to those suggested for alkenes, and they include chelation of deprotonated 290 (b), alkyne (289) insertion into the M−C bond (c), and consecutive reductive elimination to give a pyridine N-oxide (d). The latter oxidizes the metal center (f), thus regenerating the catalyst (g) as it is reduced to pyridine 291 (e). Accordingly, aromatic and heteroaromatic ketoximes R1CH C(R2)C(R3)NOH (290) reacted with internal and terminal alkynes R4CCR5 (289) in the presence of [CoCp*I2(CO)] (5 mol %) and KOAc or Mn(OAc)2 (10 mol %) in CF3CH2OH at 80−100 °C for 18 h to give isoquinolines 290 (65−95% isolated yields). When using 290 (R6 = 3,4-(OCH2CH2O)), the reaction yielded a mixture of two isomeric pyridines 291 (59% 5,6OCH2CH2O and 33% 6,7-OCH2CH2O).552 The reaction was also performed for amidoximes R1CHC(R2)C(NHR8) NOH (290) and alkynes R4CCR5 (289) in the presence of

[CoCp*I2(CO)] (10 mol %) and CsOAc (20 mol %) in CF3CH2OH at 120 °C for 24 h, allowing the synthesis of the aminosubstituted isoquinolines 290 (R3 = NHR7; 51−96%).552 α-Fluoro-α,β-unsaturated ketoximes R1CHC(F)C(Me) NOH (290) reacted with both internal and terminal alkynes 289 in the presence of [RhCl2Cp*]2 (2.5 mol %), tBuCO2Cs (20 mol %), and H2O (10 equiv) in tBuOH at 100 °C for 4−12 h, leading to substituted 3-fluoropyridines 291 (48−82% isolated yields).553 Oxazole and thiazole oximes were also employed as reactants for this reaction. Oximes 290 (1.5-fold excess) reacted with internal aliphatic and aromatic alkynes 289 in the presence of [RhCl2Cp*]2 (2.5 mol %) and NaOAc (2 equiv) in MeOH at 60−70 °C for 24 h (65−95% isolated yields), although in the case of R5 = Br(CH2)2 and MeO2C (X = S; R4 = Ph; R6 = H), the pyridines were isolated in only 41 and 19% yields, respectively.554 Aromatic ketoximes R2C6H3(I)C(R1)NOH (292) featuring an iodo substituent at the ortho-position reacted with internal and terminal alkynes R3CCR4 (293; 2-fold excess) and with ethyne in the presence of [NiBr2(PPh3)2] (3 mol %) and Zn powder (3.5-fold excess) in a MeCN:THF (1:1, v/v) mixture at 80 °C for 15−24 h to give isoquinolines 294 (41−99% isolated yields Scheme 133).555 The mechanism of this reaction is similar to that described above but includes the initial generation of Ni0 species formed from the reduction of NiII by metallic zinc. The oxidative insertion of the nickel(0) center into the C−I bond to BF


Chemical Reviews

Review

Scheme 135. Ag+-Mediated Generation of ortho-Substituted Pyridines 298557a

Scheme 133. Nickel-Catalyzed Generation of Isoquinolines 294 from Iodooximes 292 and Alkynes 293555a

a

292: R1 = Me, nBu, PhCH2, Ph, 4-MeC6H4; R2 = 3-Me, H, 3-F; 293: R3 = H, Et, nPr, Ph, 4-MeC6H4; R4 = H, Et, nPr, HOCH2, Ph, Ac, EtO2C.

give the cyclometalated intermediate leads to intermediate similar to that obtained on step b (Scheme 132). The reaction depicted in Scheme 132 can be stopped at the stage of the pyridine N-oxide. Aromatic ketoximes R2C6H4C(R1)NOH (290) reacted with alkynes R3CCR4 (293) in the presence of Pd(OAc)2 (10 mol %), ZnBr2 (1 equiv), and CF3CO2H (0.2 equiv) in a chlorobenzene:1,4-dioxane (2:1, v/v) mixture at 120 °C for 6−12 h to grant isoquinoline N-oxides 295 (41−89% isolated yields Scheme 134).556 This reaction stops at the N-oxide step because of the oxidation of the palladium(0), formed in the catalytic cycle, by dioxygen from air rather than by the N-oxide. Scheme 134. Palladium-Catalyzed Synthesis of Isoquinoline N-Oxides 295 from Oximes 290 and Alkynes 293556a

a

a 296: R1 = MeO, H; R2 = 3-thienyl, 3-pyridyl, nBu, R4C6H(3−4):R4 = 4Me2N, 4-MeO, 2,6-(MeO)2, H, 4-CF3; 297: X = H, R3 = 2,6(MeO)2C6H3O, 2,6-Me2C6H3O, PhO, 4-NCC6H4O, 3-pyO, 4MeOC6H4CO2, 4-BrC6H4CO2, PhS, 4-BrC6H4S, PhCH2S; R3X = KCN, NaN3, NaBH4 (R3 = H).

A reported modification of the reaction given in Scheme 132 allows the functionalization of pyridine N-oxides before their reduction to the appropriate pyridines. Alkynyl aldoximes 296 reacted with the nucleophilic agents R3X (297; R3H = R4OH, R4SH, R4CO2H, R4NH2, HBH3−; R3 (−) = CN−, N3−) to furnish ortho-substituted pyridines 298 (17−99% isolated yields Scheme 135, a).557 The reaction proceeded via addition of AgOTf (10 mol %) to oxime 296 in CH2Cl2 at RT for 18 h followed by treatment with [PBr(NC4H8)3](PF6) (1.5 equiv), a nucleophile R3X (1.1 equiv; 297), and NEtiPr2 (3.7 equiv) and maintaining the reaction mixture at RT for an additional 18 h. The reaction also occurred easily when 4-MeC6H4SO2Cl was used instead of [PBr(NC4H8)3](PF6), whereas in the presence of PhCOCl, it did not proceed for such nucleophiles as 4oxypyridine, MeOH, and EtOH. A plausible mechanism of this reaction includes activation the CC bond by the silver(I) center with subsequent nucleophilic attack of the oxime N atom to the electrophilically activated CC moiety to give the pyridine N-oxide ring (b; see also Schemes 106 and 107 for details in similar PdII and AuI-catalyzed processes). After that, nucleophilic attack of the O atom on the phosphonium cation gives the aromatic ring activated toward nucleophilic attack (c). In basic media, an anionic nucleophile R3(−) attacks the activated

ring (d), followed by elimination of H+ and the phosphine oxide (e). Quinazoline N-oxides 301 were prepared from ketoximes R1C(R2)NOH (299) and 1,4,2-dioxazol-5-ones (300; 1.2 equiv) via RhIII-catalyzed C−H activation−amidation of the ketoximes and subsequent ZnII-catalyzed cyclization of the formed ortho-amido-functionalized aryl oximes (50−95%; Scheme 136).558 The reaction proceeded in the presence of [Cp*RhCl2]2 (4 mol %), Zn(NTf2)2 (or Zn(OTf)2; 30 mol %), and HOAc (2 equiv) in CF3CH2OH under dinitrogen at 80 °C for 12 h. Ketoximes with a heterocyclic backbone (e.g., benzothiophene) and aldoximes (R2 = H) failed to react. A proposed reaction mechanism includes cyclometalation (b), coordination of dioxaline 300 followed by CO2 elimination to give the nitrenoid intermediate (c), migratory insertion (d), amide release (e), and zinc(II)-catalyzed cyclization-condensation (f) to realize 301. 2-Bromo-functionalized aromatic ketoxime esters R1BrC6H3C(Me)NOAc (302) reacted with the CH-acids R2CH2C(R3)O (303; 1.2 equiv) in the presence of CuI (10 mol %) and K2CO3 (2 equiv) to give isoquinolines 304 (in DMF under N2, 120 °C, 6 h, 61−85% isolated yields 42% for thienopyridine derived from thienyl derivative 302; Scheme 137, a).559 The reaction probably proceeds via the initial generation of

290: R1 = Me, Ph; R2 = 3-MeO, 4-MeO, 4-Me, H, 4-F, 3-Cl, 4-Cl; 293: R3 = 4-MeOC6H4, Ph, nPr; R4 = Ph, Et, nPr.

BG


Chemical Reviews

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Scheme 136. RhIII/ZnII-Involving Synthesis of Quinazoline NOxides 301 from Ketoximes 299 and 1,4,2-Dioxazol-5-ones 300558a

Scheme 137. Copper-Catalyzed Generation of Isoquinolines 304 from Oximes 302 and CH Acids 303559a

a 299: R1 = Ph, 4-MeC6H4, 4-EtC6H4, 4-iPrC6H4, 4-CF3C6H4, 4MeOC6H4, 4-ClC6H4, 4-BrC6H4, 4-IC6H4, 3-MeC6H4, 3-CF3C6H4, 3MeOC6H4, 3-ClC6H4, 3-BrC6H4, 2-FC6H4, 3-MeC6H4, 3,4-Me2C6H3, 3-naphthyl; R2 = Me, Et, nPr, iPr; R1/R2 = C6H4(CH2CH2CH2−2); 300: R3 = Ph, 4-MeC6H4, 4-CF3C6H4, 4-FC6H4, 4-ClC6H4, 4-IC6H4, 3-CF3C6H4, 2-thienyl, PhCH2, Me.

a

302: R1 = H, 3-F, 4-F; 2-((3-bromo)thienyl)methylcarboxime acetate; 303: R2 = NC, R4CO: R4 = Me, iBu, MeO, EtO, H2C CHCH2; R5C6H4: R5 = 3-MeO, 4-MeO, 4-Me, H, 4-F, 4-Cl; R3 = 2thienyl, Me, Et, 4-R6C6H4: R6 = MeO, Me, H, F, Cl; R3/R4 = (CH2)3, CH2CMe2CH2, CH2CHiPrCH2.

the cyclometalated intermediate (b) and the base-promoted generation of the enolate (c). Next, the enolate inserts into the Cu−C bond (d), followed by reductive N−O bond splitting (e and f). Finally, nucleophilic attack of the imino N atom on the carbonyl C atom gives the dihydropyridine ring (g), whose aromatization to isoquinoline 304 is accompanied by the elimination of copper(II) and OH−. The copper(II) centers are reduced to the catalytically active copper(I) by DMF, viz., 2CuII + 2OH− + DMF → 2CuI + H2O + CO2 + HNMe2 (i). Under the same conditions, O-acetyl 2-bromoacetophenone oxime BrC6H4C(Me)NOAc (302) reacted with cyanoacetic esters NCCH2CO2R (303) to give 3-(2-aminoisoquinolinyl)carboxylic esters 305 (70−87%; Scheme 138).559 The reaction includes steps (b−f) (Scheme 138), but the nucleophilic attack of the imino N atom proceeds on the nitrile C atom instead of the carboxyl C atom. Ketoxime acetates R1C(Me)NOAc (306) reacted with arylmethanes R2C6H(3−4)Me (2 equiv; 307) in the presence of Cu(OTf)2 (20 mol %) and PhI(OAc)2 (2 equiv) in toluene at 100−120 °C for 8 h to give substituted pyridines 308 (38−82%; Scheme 139, a).560 Substrates bearing electron-donating and moderate electron-withdrawing groups (e.g., F, Cl, Br) could be used in the reaction, while the use of 4-O2N-substituted oxime and aryl methane did not give the desired product. The authors of ref 560 propose the following mechanism of this trans-

Scheme 138. Copper-Mediated Preparation of 3-(2Aminoisoquinolinyl)carboxylic Esters 305 from O-Acetyl 2Bromoacetophenone Oxime 302 and Cyanoacetic Esters 303559a

a

303: R = Et, iPr, nBu, iBu, tBu, H2CCHCH2, PhCH2.

formation: the oxidative addition of the acetylated oxime 306 to two CuI centers (b), tautomerization of the intermediate (c), chelation-promoted coupling with the in situ obtained aromatic aldehyde (d and e), condensation with the second molecule of the oxime acetate (f), β-elimination giving aza-hexa-1,3,5-triene (g), and eventually, electrocyclization (h) to furnish 308. Most BH


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Scheme 139. Copper-Catalyzed Syntheses of Substituted Pyridines from Acetyl Ketoxymes and Arylmethanes560a

Scheme 140. Iron-Catalyzed Syntheses of Substituted Pyridines from Acetyl Ketoximes and Aldehydes561a

a

306: R1 = Ph, 4-MeC6H4, 3-MeC6H4, 2-MeC6H4, 4-MeOC6H4, 3MeOC6H4, 4-FC6H4, 4-ClC6H4, 4-BrC6H4, 4-PhC6H4, 3-naphthyl; and also 2-(CaH2CH2CH2)C6H4CbNOAc(a−b); 307: R2 = H, 4-Me, 3Me, 2-Me, 3,5-Me2,4-MeO, 4-F, 4-Cl, 4-Br, 2-Cl, 3-Cl, 3,5-Cl2.

a 309: R1 = Ph, 4-MeC6H4, 3-MeC6H4, 4-MeOC6H4, 4-FC6H4, 4ClC6H4, 2-ClC6H4, 4-BrC6H4, 4-MeO2CC6H4, 3,4-Me2C6H3, 2,5Me2C6H3, 3,4-(CH2)4C6H3, 3,4-(OCH2O)C6H3, 3-naphthyl, 2-furyl, 2-thienyl, EtO2C; R2 = Me; R1R2 = 4-C6H4(CH2)3) and aryl aldehydes R3CHO (R3 = Ph, 4-MeC6H4, 3-MeC6H4, 4-MeOC6H4, 4-FC6H4, 4ClC6H4, 2-ClC6H4, 4-BrC6H4, 4-MeO2CC6H4, 2,4-Cl2C6H3, 2-thienyl. 310: R3 = Et, Cy.

likely, the precatalyst Cu(OTf)2 could be converted into the active CuI form via reduction. Symmetrical pyridines 311 were obtained in 60−92% yields from ketoxime acetates R1C(R2)NOAc (309; 3 equiv) and aldehydes R3CHO (310) in the presence of FeCl3 (20 mol %) in toluene at 140 °C under argon for 6−8 h (Scheme 140, a). Notably, the aliphatic aldehydes R3CHO (310) in the same reaction with PhC(Me)NOAc produced unsymmetrical 2ethyl(or cyclohexyl)-4,6-diphenylpyridines (48 and 56% yields, respectively). A postulated mechanism is similar to that suggested for the copper-catalyzed reaction (Scheme 139) and consists of the reductive cleavage of the N−O bond of the ketoxime under the action of Fe II (Scheme 140, b), tautomerization of the imine (c), reaction with the in situformed (e) benzoyl radical (d), formation of the iminoketone (f), condensation with the second molecule of 309 (g), intramolecular cyclization (h), and dehydration as the final step (i).561 Ketoximes and aldoximes 279 reacted with the vinyl azides R3C6H4C(N3)CH2 (312) to furnish 3-aryl isoquinolines 313 in the presence of 10 mol % of Pd(OAc)2 in toluene at 90 °C for 8 h (21−85%; Scheme 141, a).548 Sequence of possible stages of the reaction involves the cyclometalation (b) and thermal generation of azirine (c), whereupon the ring-opening insertion of the azirine into the Pd−O bond occurs (d). Next, the palladium(II) inserts into the N−O bond of the oxime ester (e) and eliminates the organic molecule (f), which, in particular, eliminates H2NOH to yield the final isoquinoline 313.548

An additional paper dealt with the formation of a pyridine-3-ol framework from δ-diazooxime ethers 314, which transformed to 315 (Scheme 142, a) and 316 (b) in 60−92% and 65% isolated yields, respectively.562 The authors562 suggest that the reaction starts from the electrophilic substitution of dinitrogen by the rhodium(II) center (c) followed by the insertion of the carbenoid into the oxime N−O bond (d). Finally, elimination of MeOH (e) or PhSO2H (f) occurs, leading to 315 or 316, respectively. Jiang and Park reported the rhodium(II)-catalyzed preparation of nicotinic acid ester derivatives 317 from α-diazooximes 219 (Scheme 143). The reaction proceeded in the presence of 2 mol % of [Rh2(OAc)4] in PhCl at 130 °C for 24 h and led to 317 isolated after workup in 64−80% yields.488 Notably, in the presence of [NiCl2(PPh3)2], oximes 219 underwent conversion to substituted pyrroles (Scheme 103, a). A plausible mechanism of the rhodium-involving reaction includes rhodium(II)catalyzed formation of the azirine ring (b; for relevant NiIIcatalyzed rearrangement see Scheme 103) with subsequent rhodium(II)-mediated ring-opening (c). After that, a 1,6-hydride shift yields the allylic cation (d), which electrophilically attacks the N atom to yield the dihydropyridine complex (e). The ligand liberation (f) regenerates the catalyst (g) and gives free dihydropyridine, which is oxidized by the air oxygen under the reaction conditions (h).563 BI


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two equiv of copper(I) (e). Finally, two CuI centers reduce the N-methoxy pyridinium to pyridine 319 via their oxidation to catalytically active copper(II) (f). The copper carboxylates [Cu2(RCO2)4(RCO2H)2] (R = tBu, t BuCH2) reacted with acetylacetone dioxime (320) in Me2CO to give the coordination polymers [Cu(RCO2)2(qunx)]n bearing N,N′-coordinated quinoxalone (321; 25−45% isolated yields Scheme 145).567 Currently no analogs of this reaction were reported. The authors suggested a complicated multistep mechanism of the formation of 321 that includes two Beckmann rearrangements and several pericyclic reactions, but no experimental evidence was obtained to support those considerations. The cyclopalladated phenylacetone oxime complex 208 (X1 = 4-MeC5H4N; X2 = Br; n = 0) reacted with CO (1.4 atm) in CHCl3 at RT for 5 h to give isoquinolin-1-one 322 (X3 = O), which was isolated in 84% yield (Scheme 146).474 Consequently, the structurally similar complex 208 (X1/X2 = 4,4′-di-tert-butyl2,2′-bipyridine; n = 1) was treated with a 2-fold excess of RNC (R = Xy, tBu) in CH2Cl2 at RT for 40−48 h to grant 1iminoisoquinalines 322 (X3 = NXy 74% yield, NtBu 95% yield). These reactions are accompanied by the formation of palladium black. Another report describes the palladium(II)-catalyzed enantioselective and diastereoselective formation of a tetrahydropyridine ring upon the reaction of aromatic aldoximes and activated alkenes. The substituted ortho-allyl benzaldoximes R1C6H3(CH2CHCH2)C(H)NOH (323) reacted with activated alkenes R2C(O)CHCHC(O)R2 (324) in the presence of 10 mol % Pd(acac)2, 15 mol % enantiopure phosphine ligand 325 and 20 mol % TfOH in CHCl3 at 45 °C for 4−12 h; this reaction yielded a mixture of tetrahydropyridine derivatives 326 (92−100%; ee 51−70%; Scheme 147, a).568 In general, the exo-product was dominant, and the exo/endo ratio varied from 93/7 to 97/3 for R22 = NR and was 50/50 for R2 = OMe. The reaction was sensitive to the steric effects of substituents of the oxime species. Thus, in the case of the aldoxime 2-(PhCHCHCH2)C6H4C(H)NOH and the ketoxime 2-(H2CCHCH2)C6H4C(Me)NOH, the final products were detected only in trace amounts. The authors568 suggest that the reaction starts from the ligation of the oxime to the palladium(II) center featuring enantiopure ligand 325 (b) followed by nucleophilic attack of the oxime N atom to the C atom of allyl moiety to generate the ligated aldonitrone (c). The nitrone is decoordinated upon protonation (d), and it undergoes 1,3-dipolar cycloaddition to alkene 324, giving 326 (e). Notably, the final step in this mechanism is not metal-mediated. O-Methyl β-aryloxyoximes featuring a cyclobutenone ring in the aryl group R3C6H3{CH2CO}O(CH2)nCHR2C(R1) NOMe (327) underwent rearrangement to N-methoxy isoquinolinones 330 in the presence of [Rh(cod)2](BF4) (10 mol %) and two chiral diphosphine ligands, viz., 328 (6 mol %) and 329 (6 mol %). The reaction was conducted under dinitrogen in 1,4-dioxane at 110−130 °C for 48 h (37−74% isolated yields, ee 81−95%; Scheme 148, a).569 The reaction was suggested to start from initial oxidative insertion of the RhI center into the C−C bond (b; this reaction was previously developed by Huffman et al.570,571) followed by the insertion of the NC moiety into the Rh−C bond (c). The reaction terminates upon reductive elimination of 330 (d), which regenerates the catalytically active rhodium(I) center (e). N-Oxy-2-(methylcarboxy)tetrahydropyridines were also prepared as a mixture of byproducts from the reaction between

Scheme 141. Palladium(II)-Catalyzed Generation of 3-Aryl Isoquinolines 313 from Aromatic Oximes 279 and Vinyl Azides 312548a

a

279: R1 = 4-PhCH2O, 4-nBu, 4-iBu, 4-Et, 3,4-Me2, 2-Me, 3-Me, 4-Me, H, 4-Ph, 4-MeS, 2-F, 4-F, 4-Cl, 4-Br, 4-O2N, benzo[3,4]; R2 = Ph, H, Me, Et, nPr, iPr, nBu; 312: R3 = 4-tBu, 2-Me, 3-Me, 4-Me, 2-F, 4-F, 3Cl, 4-Cl, 4-Br, 4-CF3, benzo[3,4].

An interesting example of pyridine ring generation starting from oxime ethers was reported for 2-(3-indolyl)benzaldoxime ethers 318.564 The catalytic system for this reaction includes the synergic action of two metal centers, viz., copper and palladium; this catalytic system has been previously applied in the Wacker process.565,566 Oximes 318 transformed to pyridines 319 in the presence of Cu(OTf)2 (20 mol %) and Pd(OAc)2 (10 mol %) in o-xylene at 100 °C for 2−6 h (Scheme 144, a). The reaction yields were significantly higher than those obtained in the presence of any one of the two cocatalysts, 45% when only CuII (20 mol %) was employed and 5% when only PdII (10 mol %) was used. These two yields are much lower than the 74% yield of the target product when the mixture of these two metals was used for the catalytic system. The suggested mechanism includes activation of the CC bond toward nucleophilic attack via coordination of the palladium(II) center (b), whereupon the oxime N atom attacks the activated CC double bond to give the palladium(II) complex featuring a N-methoxy dihydropyridinium ligand (c). After that, transmetalation leads to the copper(II) complex (d), where the ligand is oxidized in the presence of excess copper(II) to give N-methoxy pyridinium and BJ


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Scheme 142. Rhodium(II)-Mediated Synthetic Route to 3-Oxypyridine Derivatives 315 and 316562a

a

314: R1 = 4-MeOC6H4, 2-MeC6H4, Ph, 2-ClC6H4, 3-ClC6H4, 4-ClC6H4, 2-naphthyl, 3-furyl, 3-thienyl, tBu, cyclohex-1-enyl; R2 = H, Me; R3 = Ac, MeO2C, P(O)(OMe)2, PhSO2; 315: R3 = Ac, MeO2C, P(O)(OMe)2; 316: R1 = Ph; R2 = H; R3 = PhSO2.

Scheme 143. Rhodium(II)-Mediated Synthesis of Nicotinic Acid Ester Derivatives 317488a

Scheme 144. Pd/Cu-Mediated Preparation of Pyridines 319 from Oxime Ethers 318564a

a 219: R1 = 4-MeOC6H4, Ph, 2-furyl, PhCHCH, Cy; R2 = 4-BrC6H4, H; R3 = Me, iBu, tBu; R2/R3 = (CH2)2, (CH2)3, (CH2)4; R4 = Me, PhCH2.

a

318: R1 = MeO, H; R2 = MeO, H, NC; R3 = Me, Et, iPr; X = CH, N.

oxime 270 and cyclopentadiene (18% combined yield for the two isomers).540 BK


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Scheme 145. Copper(II)-Mediated Synthesis of Quinoxaline Ligands567

Scheme 147. Palladium(II)-Catalyzed Enantioselective and Diastereoselective Synthesis of Tetrahydropyridines 326568a

Scheme 146. Palladium(II)-Mediated Generation of Isoquinolin-1-one and 1-Iminoisoquinolines 322474

Miyata et al. reported the preparation of N-hydroxy piperidine derivatives 332 starting from the δ-tosyl aldoxime ether TsO(CH2)4CHNOCH2Ph (331). The reaction proceeded in the presence of RI (20-fold excess), Et3B (3−6 equiv), and BF3·OEt2 (2−4 equiv) in CH2Cl2 at RT for 4−23 h with subsequent treatment of the reaction mixture with silica gel. An isolated yield of 51% was achieved for R = Et (Scheme 149).505 The process was sensitive to the steric effects of R, and the yield of 332 decreased from primary to secondary alkyls. The reaction did not proceed for RI featuring tertiary alkyls. The mechanism of the reported transformation was not given. Steroidal aldoxime ethers 333 underwent rearrangement to piperidine derivatives 334 in the presence of BF3·OEt2 (1 equiv) in CH2Cl2 at 40 °C for 4−8 h (71−78% isolated yields Scheme 150, a).572 The boron(III) center was believed to coordinate to the oxime N atom, thus increasing the electrophilicity of the oxime C atom (b). A 1,5-hydride shift (c) followed by nucleophilic attack of the N atom to the formed carbocation leads to the piperidine cycle (d). O-Acetyl 2-iodoacetophenone oxime, IC6H4C(Me)NOAc (335), reacted with substituted indoles 336 in the presence of Cu(OAc)2 (10 mol %), tBuOK (1 equiv), and DBU (1 equiv) under dinitrogen in Me2SO at 120 °C for 6 h; this treatment yielded pyrimidines 337 (64−83% isolated yields Scheme 151, a).559 The reaction was also performed for pyrrole, and the corresponding pyrimidine was isolated in 74% yield. Reaction scheme includes the initial reduction of copper(II) to the catalytically active copper(I) center followed by oxidative insertion of CuI into the I−C bond, giving the copper(III) complex (b). After that, the iodide ligand is replaced to the indolide (d) [generated in the basic media (c)], and subsequent reductive elimination leads to a new N−C bond and regenerates copper(I) (e). Subsequently, the copper(I) is oxidatively inserted into the N−O bond (f), followed by insertion of the CC bond into the Cu−N bond (g). Finally, reductive elimination (h) followed by aromatization via elimination of AcOH (i) gives pyrimidine 337. Scott et al. reported the generation of a mixture of 3,5-dimethyl 2H- and 6H-1,2-oxazines from cyclopropyl oxime 338 (Scheme 152, a).573 This reaction proceeded in the presence of MgCl2 (10

a

323: R1 = 3-MeO, H, 5-F; 324: R2 = OMe; R22 = 4-MeC6H4N, PhN, 4-ClC6H4N, HN, MeN, EtN, PhCH2N.

mol %) and Me2NCH2CH2NMe2 (1 equiv) in MeOCH2CH2OMe at 120 °C for 18 h and resulted in the formation of an inseparable mixture of 339 and 340 (73% combined isolated yield and 3:1 ratio). Upon the basis of the mechanism suggested573 for the relevant hydrazones, a sequence of possible stages of this reaction could be as follows. First, the oxime coordinates by the N atom to the magnesium(II) center, which provides ring-opening and generation of the allylic cation (b). In the next step, the oxime O atom nucleophilically attacks the carbocation (c). The oxazine formed decoordinates from the magnesium(II) center (d) and isomerizes to more stable cycles featuring internal π-delocalization (e and f). α-Oxoaldoximes and cyanoximes R1C(O)C(R2)NOH (341) reacted with the vinylboronic acids R4CHC(R3)B(OH)2 (5-fold excess; 342) in the presence of Cu(OAc)2 (1 equiv), pyridine (3 equiv), and Na2SO4 (6.6 equiv) to obtain oxazine N-oxides 343 (in ClCH2CH2Cl, RT, 2 h, 27−94%; Scheme 153, a).574 It is argued that this reaction proceeds via the initial copper(II)-mediated oxidative N-vinylation of oxime 341 (b; see section 3.2.1 for details) followed by Cope rearrangement BL


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Scheme 148. Rhodium-Catalyzed Asymmetric Rearrangement of O-Methyl Oximes 327 to N-Methoxy Isoquinolinones 330569a

Scheme 151. Copper-Catalyzed Generation of Pyrimidines 337 from O-Acetyl Oxime 335 and Indoles 336559a

a

327: R1 = H, Me, Et, iPr, CH2OSiMe2tBu; R2 = H, CO2Me; R1/R2 = (CH2)4; R3 = 3-Me, 4-Me, 3-iPr, 4-iPr, 4-Cl, 3-MeO2C; n = 0, 1.

Scheme 149. Preparation of Piperidines 332 from O-Benzyl-δtosyl Pentanaldoxime 331505

a 336: X = CH, R1 = 4-MeO, 5-MeO, 7-MeO, 4-PhCH2O, 5-Me, H, 5F, 6-F, 5-NC; X = N, R1 = H; R2 = H, Me.

of the formed nitrone (c) (for recent works, see refs 491−494).574 5.2.5. Generation of 7-Membered Heterocycles. OAcetyl benzophenonoxime, Ph2CNOAc (233), reacted with the tungsten(0) carbene complexes [W(CO)5{ROCC CCPh}] (234; R = Me, Et) under UV irradiation in MeCN for 3 h to give 3-alkoxy-1,5-diphenyl-3H-benzo[c]azepines (344; R = Me, Et; 12 and 9% isolated yields, respectively; Scheme 154).575 The authors of ref 575 suggest that the reaction proceeds via initial homolytic splitting of the oxime N−O bond followed by attack of Ph2CN• on the carbene C atom and 1e-reduction of ̅ the generated species and C−H functionalization of one of the phenyl rings of the oxime. For the ethoxy derivative, the isolated yield was increased to 35% by using perdeuterated 233 in a MeCN:H2O (9:1, v/v) mixture. 5.2.6. Generation of Macroheterocycles. Metal-mediated reactions of oxime-derived macroheterocycle construction considered in this section are novel and no analogous transformations were reported. The acetophenone oxime ethers RC6H4C(Me)NO(CH2)nC6H4N3-2 (345) underwent intramolecular cyclization via C−H activation accompanied by amination of the ortho-position of the oxime aryl ring to give

Scheme 150. Boron(III)-Mediated Rearrangement of Aldoxime Ethers 333 to Piperidine Derivatives 334572a

a

333: R = Ph, 4-O2NC6H4, H2CCH.

BM


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Scheme 152. Magnesium(II)-Catalyzed Generation of Oxazines 339 and 340573

Scheme 153. Copper(II)-Mediated Generation of Oxazine NOxides 343574a

Scheme 155. Rhodium(III)-Catalyzed Intramolecular Heterocyclization Leading to 9−11-Membered Cyclic Oxime Ethers576a

a

341: R1 = 2-furyl, 2-thienyl, tBu, R5C6H(2−4): R5 = 4-MeO, 4-MeS, 4Me, 2,4,6-Me3, 3,4-(CH2)4, H, 4-F, 4-Cl, 4-CF3, 4-MeO2C; R2 = H, NC; 342: R3 = 4-R6C6H4: MeO, Me, H, CF3, MeO2C, O2N; R4 = Et; R3/R4 = C6H4(CH2)2−2.

Scheme 154. Generation of 3-Alkoxy-1,5-diphenyl-3Hbenzo[c]azepines from O-Acetyl Benzophenone Oxime575 a

345: R = 4-MeO, 3-Me, 4-Me, H, 4-Br, 4-CF3, 4-NC, 4-O2N; n = 1− 3.

the catalytically active [RhCp*]2+ species (f). When using metasubstituted oxime ethers (345), the reaction proceeded to the 6position of the aryl. This reaction did not proceed intramolecularly for 345 featuring aliphatic azide substituents (see below for intermolecular homocoupling of aliphatic azide derivatives).576 N-Acylation of the aldoxime ethers R1C(H)NOR2 (112) by cyclic α-isopropenyl ketones (347; n = 1−4, 8) followed by oxy2-azonia-Cope rearrangement led to the macrocyclic hydroxamic acid ethers 348. This conversion was conducted in the presence of a stoichiometric amount of SnCl4 or EtAlCl2 in CH2Cl2 under argon (RT, 24 h, 29−97%; Scheme 156).280 The mechanism of this reaction is described in section 3.2.2. This reaction is atomeconomic, and generally, it proceeds stereoselectively. Thus, the E/Z-ratios of the products were mainly >99:1, except for the reaction of 112 (R = 4-O2NC6H4) with 347 (n = 8), which gave a 25:75 mixture of the isomers. The dioxime ether Me2CNOCH2C(Me)NOH (349) underwent self-condensation in the presence of Fe(ClO4)2 (25 mol %) in MeCN at 60 °C for 13 h to furnish complex

9−11-membered heterocycles (346; 45−99%; Scheme 155, a).576 This reaction proceeded in the presence of [(RhCl2Cp*)2] (5 mol %), AgNTf2 (20 mol %), and NaOAc (20 mol %) in ClCH2CH2Cl at 60 °C for 12 h, and it is relevant to the amination of the side-chains of aromatic oximes (Table 2, E16). A mechanism includes initial chloride abstraction of the rhodium(III) complex by Ag+ to give the catalytically active [RhCp*]2+ species (b),576 which then forms the cyclometalated intermediate (c). Oxidative insertion of the rhodium(III) center into the N−N bond results in elimination of N2 and, at least formally, the rhodium(V) intermediate (d). Reductive elimination terminates the reaction, giving 346 (e) and regenerating BN


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Scheme 156. Tin(IV)- and Aluminum(III)-Mediated Generation of Ethers of Macrocyclic Hydroxamic Acids280a

Scheme 158. Rhodium(III)-catalyzed intermolecular heterocyclization leading to 22−36-membered cyclic oxime ethers576a

a

112: R2 = Me, R1 = Me, n-pentyl, Me2CCH, Ph, 4-O2NC6H4; R2 = Et, R1 = Me.

350 (69% isolated yield; Scheme 157, a).577 Hydrolysis of the in situ obtained 350 gave metal-free macroheterocycle 351 (90% Scheme 157. Iron(II)-Involving Formation of Macroheterocycle 351 and Its Iron(II) Complex 350577

a

352: R = 4-MeO, 3-MeH, 4-Br; n = 5−12.

oxides, quinoxalines, pyrimidines, and oxazines). Metal-mediated heterocyclizations include examples of intramolecular reactions of functionalized oximes and intermolecular two- or multicomponent processes. In the overwhelming majority of cases, these transformations of oximes are possible only in the presence of a metal center, and their metal-free analogues remain unknown. A wide range of metal centers were tested in oxime cyclizations (e.g., Mg, B, Al, Sn, In, Yb, Eu, W, Mn, Fe, Ru, Co, Rh, Ni, Pd, Cu, Ag, and Zn). Metals were taken as single components of a catalyst or in various combinations. Some multistep reactions involve a metal center acting at more than one reaction step. In some instances, a metal center activates the oxime functionality, and in other cases, it activates other functional groups. Reactions where a metal center plays the role of an oxime activator include nucleophilic addition to a CN bond, reductive N−O bond cleavage with the formation of reactive CN• or CN− moieties, cleavage with formation of CN+, or oxidative generation of CN−O•. In some cases, the oxime functionality plays the role of the directing group after the N- or O-coordination. Because of the options in choosing the activating metal center and varying the oxime-containing substrates, metal-mediated reactions of oximes allow the synthesis of a great number of cycles of practical importance. In many instances, these reactions proceed under mild conditions with good to high regio- and stereoselectivities. These reactions are useful tools for the targeted construction of cyclic systems, especially 5- and 6membered nitrogen heterocycles.

isolated yield). Route (a) could be realized in the presence of Ni(ClO4)2 or Cu(ClO4)2 instead of iron(II) perchlorate, but the yields were significantly lower (36 and 24%, respectively). The authors577 reported the coordination of 351 to the PdII and AgI centers. In the case of silver(I), two types of complexes were prepared, viz., [Ag(351)](OTf) and [Ag2(351)](OTf)2; the yields of these silver complexes were not given. Oxime ethers featuring aliphatic azide substituents at the O atom (352) underwent intermolecular amination of the oxime aryl ring in the presence of [(RhCl2Cp*)2] (5 mol %), AgNTf2 (20 mol %), and NaOAc (20 mol %) in ClCH2CH2Cl at 100 °C for 6−12 h to grant macrocyclic oxime ethers (353; 22−36membered cycles; 30−52%; Scheme 158).576 The suggested reaction scheme includes stepwise C−H functionalization similar to that given in Scheme 155. The reaction proceeded at a 0.8 M concentration of 352 and led exclusively to macrocycles with an even number of atoms.576 In this section, we inspected recent reports of metal-involving oxime-based generation of various heterocyclic and carbocyclic systems spanning 3- to 7-membered rings and even further to macroheterocycles. Within 5-membered heterocycles, obtained via metal-catalyzed reactions of oximes, the cycles included those with one heteroatom (viz., pyrroles, pyrrolines, pyrroline-Noxides, pyrrolidines, and furans), two heteroatoms (viz., imidazoles, isoxazolines, pyrazolin-5-ones, isoxazolidines, isothiazines, and thiazoles), and tetrazoles with four heteroatoms in the ring system. Six-membered systems are represented by rings featuring one heteroatom (viz., pyridines, isoquinolines, isoquinolin-1-ones, isoquinoline-N-oxides, and tetrahydropyridine-N-oxides) and two heteroatoms (viz., quinazoline-N-

6. CONCLUSIONS AND OUTLOOK We attempted to classify and summarize the past 10−15 years of advancements in the field of metal-involving reactions of oximes. These reactions are diverse in nature and have been employed for syntheses of oxime-based metal complexes and cage-compounds, oxime functionalizations, and the preparation of new classes of organic species, in particular, a wide variety of heterocyclic systems spanning small 3-membered ring systems to macroheterocycles. In all studied reactions, metal centers play diverse roles in the promotion of the oxime reactions. In nonredox chemistry, the coordination of substrates having multiple bonds (e.g., CC, CC, CN) to positively charged metal centers leads to their electrophilic activation and facilitates nucleophilic attack by oximes. In contrast, metal-bound oximes are easily deprotonated, as an effect of the metal centers, and are subject to attack by BO


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various electrophiles. In some cases, a metal center plays the dual role of activating both the oxime and substrate, thus providing their coupling. Importantly, that coordination of an oxime could lead to C−H, C−Hal, or C−N activation of remote groups, thus inducing oxime side-chain functionalization. When a redox step is involved, metals provide consequent oxidative addition−reductive elimination reactions, which are quite useful for modern organometallic catalysis, and stimulate reductive splitting of the N−O bond, oxidation, or reduction of the oxime N atom, accompanied by splitting of the CN bond. In the vast majority of cases, the reported oxime reactions are catalyzed or mediated by group 8−13 metals, with dominant contributions by copper, palladium (or their combination), rhodium, and iron species. Reactions of oximes involving boron, cobalt, nickel, zinc, ruthenium, iridium, and platinum centers are also known but are less abundant. The promotion of oxime transformations by the early transition metals is either little explored or even unknown for group 3−5 d metals. Despite the broad spectrum of the reported oxime reactions, these transformations were typically performed with conventional ketoximes and aldoximes, whereas reactions with functionalized oximes featuring heteroatomic substituents at the oxime moiety, viz., amidoximes, nitrooximes (nitrolic acids), oxyoximes, cyanooximes, sulfonyloximes, and chlorooximes, are almost unexplored. Future achievements in metal-involving oxime chemistry could be sought with the early transition metals and the utilization of less common (but easily available) oximes in addition to the commonly used ketoximes and aldoximes. We hope that this survey will stimulate additional interest in the area of organometallic and metal-involving organic chemistry of oximes.

University and graduated with distinction in 1998. She received her Ph.D. in inorganic chemistry from Saint Petersburg Technological Institute (2002), followed by postdoctoral work at Instituto Superior Técnico in Lisbon, Portugal (2003−2004; under the supervision of Prof. A.J.L. Pombeiro). Prof. Bokach simultaneously held a researcher position at Saint Petersburg University starting from 2002 and was appointed associate professor in 2007. She received her posthabilitation DSc degree in organometallic chemistry in 2012, was awarded full professorship in 2014, and become Professor of the Russian Academy of Sciences in 2016. She is a recipient of Academia Europea’s award for young Russian scientists (2002), National L’Oreal Award for Woman in Science (2007), Leonard Euler Prize from the Government of Saint Petersburg (2010), and the Presidential Award for Young Scientists (2012), the highest official honor for young Russian researchers. Prof. Bokach is an author and coauthor of more than 90 original papers and 7 reviews. Her research interests include transition metals coordination chemistry, ligand reactivity, metal-mediated synthesis, and metalinvolving reactions of carbon−heteroatom multiple bonds.

AUTHOR INFORMATION

Vadim Yurievich Kukushkin was born in 1956 in Leningrad (now Saint Petersburg), Russian Federation. He studied chemistry at Lensovet Technological Institute (Technical University), where he obtained his Diploma with distinction in 1979 and doctoral degree in 1982. Following two years at the industrially oriented Mekhanobr Institute (Leningrad), he joined the faculty at Saint Petersburg State University (1984). He obtained his posthabilitation DSc degree in 1992, was appointed full Professor in 1996, and became head of the Department of Physical Organic Chemistry in 2007. He is a corresponding member of the Russian Academy of Sciences (elected 2006), foreign member of the Academy of Sciences of Lisbon (Portugal; elected 2011), invited chair professor at the National Taiwan University of Science and Technology (since 2007). He is vice-president (elected 2016) of the Russian Chemical Society and the chairman (since 2012) of the Saint Petersburg branch of this society, member of the Councils of the Russian Foundation for Basic Research (2008−2016), Grant Commission of the Government of the Russian Federation (since 2012), and the Russian Science Foundation (since 2014; coordinator in chemistry since 2017). Prof. Kukushkin is a recipient of numerous prizes for his achievements in science and teaching. His research interests include platinum group metal chemistry, ligand reactivity, noncovalent interactions, organic synthesis involving metal complexes, and catalysis. He is an author of ca. 350 original papers, patents, reviews, as well as two books and a number of book chapters.

Marina Yakovlevna Demakova was born in 1987 in Kirov, Russian Federation. She studied chemistry at Vyatka State Humanities University and graduated with distinction in 2009. She received her Ph.D. in chemistry from Institute of Chemistry, Komi Research Center, Ural Branch of the Russian Academy of Sciences (2013). She is a recipient of award of the Government of Komi Republic for young scientists (2012). In 2013, she started her postdoctoral work at the Institute of Chemistry, Saint Petersburg State University, under the supervision of Prof. Vadim Yu. Kukushkin, and in 2016, she was appointed senior research associate at the same Institute. Her current research interests include metal-mediated and metal-catalyzed reactions, organometallic chemistry of late transition metals, and ligand reactivity. In her spare time, Marina enjoys ballet, rock climbing, and floriculture.

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Dmitrii S. Bolotin: 0000-0002-9612-3050 Nadezhda A. Bokach: 0000-0001-8692-9627 Vadim Yu. Kukushkin: 0000-0002-2253-085X Notes

The authors declare no competing financial interest. Biographies Dmitrii Sergeevich Bolotin was born in 1990 in Leningrad, USSR (now Saint Petersburg, Russian Federation). He studied chemistry at Saint Petersburg State University and graduated with distinction in 2012. He conducted his postgraduation project under a Rector’s scholarship for gifted students and obtained his Ph.D. in chemistry in 2014. He received a position of adjunct professor at the Institute of Chemistry in 2014 and then was promoted to assistant professor (2015). In 2016, he was a recipient of the highest annual award of Saint Petersburg State University for scientific achievements. His research interests include organometallic chemistry of late transition metals, activation of small molecules, and metal-mediated and metal-catalyzed reactions of oxime species. Today Dr. Bolotin is an author of 20 original papers and reviews. Outside work Dmitrii enjoys spending time with his wife Svetlana and two children Viktoria (born 2015) and Miroslav (born 2017 during preparation of this review).

ACKNOWLEDGMENTS We are indebted to our current and former co-workers, postdocs, and students who shared with us the fascination by metalinvolving chemistry and whose contributions we acknowledge by citing papers they coauthored. We are much obliged to the Russian Science Foundation for supporting of our studies

Nadezhda Arsenievna Bokach was born in Vologda, Russian Federation (1976). She studied biology and chemistry at Vologda State Pedagogical BP


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REFERENCES

devoted to metal-mediated and metal-catalyzed reactions of oximes and other small molecules (Grant 17-73-20004). The authors also thank the reviewers for the careful reading of the manuscript and valuable suggestions.

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ABBREVIATIONS Ac acetyl acac acetylacetonate Alk alkyl Ar aryl bipy 2,2′-bipyridine Boc tert-butyloxycarbonyl i Bu iso-butyl n Bu n-butyl s Bu sec-butyl t Bu tert-butyl cod cycloocta-1,5-diene Cp cyclopentadienyl Cp* 1,2,3,4,5-pentamethylcyclopentadienyl Cy cyclohexyl dba dibenzylideneacetone DBU 2,3,4,6,7,8,9,10-octahydropyrimido[1,2-a]azepine (diazabicycloundecene) DMF dimethylformamide DFT density functional theory dppe 1,2-bis(diphenylphosphino)ethane E electrophilic radical or species en ethylenediamine Et ethyl equiv equivalent EWG electron-withdrawing group GC gas chromatography Hal halogen Het heteroaryl LED light-emitting diode Me methyl Mes 2,4,6-trimethylphenyl MS molecular sieves NBS N-bromosuccinimide NHS succinimide nmp (Z)-2-hydroxy-5,5-dimethyl-1-(4-methylpiperazin1-yl)hex-2-ene-1,4-dione Pc phthalocyanine Ph phenyl Phen phenantroline Piv 2,2-dimethylpropanoyl (pivaloyl) i Pr iso-propyl n Pr n-propyl PTSA p-toluenesulfonic acid py pyridyl RT room temperature Selectfluor 1-(chloromethyl)-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane ditetrafluoroborate TEMPO 2,2,6,6-tetramethylpiperidin-1-oxyl Tf trifluoromethanesulfonyl (triflyl) TLC thin-layer chromatography Ts para-tolylsulfonyl (tosyl) THF oxolane (tetrahydrofuran) UV ultraviolet Xantphos 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene XRD X-ray diffraction Xy 2,6-dimethylphenyl (xylyl) BQ


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Chemical Reviews

Review

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CF


Efficient synthesis of eight-membered nitrogen heterocycles from o-propargylic oximes by rhodium-catalyzed cascade reactions.

Synthesis and antiproliferative activity of some androstene oximes and their O-alkylated derivatives.

[Unsaturated oximes. 13. Separation of the isomeric pentadeuterobenzylidene-acetone-oximes].

Synthesis and pharmacological evaluation of novel bisindole derivatives bearing oximes moiety: identification of novel proapoptotic agents.

Synthesis of B,O,N-Doped Adamantanes and Diamantanes by Condensation of Oximes with Boronic Acids.

Benzylidine pregnenolones and their oximes as potential anticancer agents: synthesis and biological evaluation.

Inorganic-base-mediated hydroamination of alkenyl oximes for the synthesis of cyclic nitrones.

Organic synthesis by quench reactions.

Synthesis of 3,5-isoxazolidinediones and 1H-2,3-benzoxazine-1,4(3H)-diones from aliphatic oximes and dicarboxylic acid chlorides.

Synthesis and cytotoxic evaluation of some 6-arylidene-2-(alpha-hydroxyamino-alpha-arylmethyl)cyclohexanone oximes and related compounds.

Synthesis and structure-activity relationship of cyclopentenone oximes as novel inhibitors of the production of tumor necrosis factor-α.

Iron catalysed nitrosation of olefins to oximes.

Synthesis and reactions of formylmethylcobalamin and of related compounds.

Synthesis and reactions of leucrose and its exocyclic glycal.

Correction: Pyridine synthesis from oximes and alkynes via rhodium(III) catalysis: Cp* and Cp(t) provide complementary selectivity.

Synthesis and in vitro kinetic study of novel mono-pyridinium oximes as reactivators of organophosphorus (OP) inhibited human acetylcholinesterase (hAChE).

Modulation of lipase hydrolysis and synthesis reactions using carbohydrates.

Induction of hypozincemia and hepatic metallothionein synthesis in hypersensitivity reactions.

Synthesis, reactions and biological activities of furochromones: a review.

[Reactions of pentadienones, XVII: -Synthesis of substituted thiane derivatives by double addition reactions (author's transl)].

Direct synthesis of 4-fluoroisoxazoles through gold-catalyzed cascade cyclization-fluorination of 2-alkynone O-methyl oximes.

Phosphorylated aminosugars: synthesis, properties, and reactivity in enzymatic reactions.

Regiochemistry of cyclocondensation reactions in the synthesis of polyazaheterocycles.

Synthesis of rhenabenzenes from the reactions of rhenacyclobutadienes with ethoxyethyne.