Hypervalent iodine-catalyzed oxidative functionalizations including stereoselective reactions.

FOCUS REVIEW DOI: 10.1002/asia.201301582

Hypervalent Iodine-Catalyzed Oxidative Functionalizations Including Stereoselective Reactions Fateh V. Singh and Thomas Wirth*[a]

Chem. Asian J. 2014, 9, 950 – 971

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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Thomas Wirth and Fateh V. Singh

www.chemasianj.org

Abstract: Hypervalent iodine chemistry is now a well-established area of organic chemistry. Novel hypervalent iodine reagents have been introduced in many different transformations owing to their mild reaction conditions and environmentally friendly nature. Recently, these reagents have received particular attention because of their applications in catalysis. Numerous hypervalent iodinecatalyzed oxidative functionalizations such as oxidations of various alcohols and phenols, a-functionalizations of carbonyl compounds, cyclizations, and rearrangements have been developed successfully. In these catalytic reactions stoichiometric oxidants such as mCPBA or oxone play a crucial role to generate the iodineACHTUNGRE(III) or iodine(V) species in situ. In this Focus Review, recent developments of hypervalent iodine-catalyzed reactions are described including some asymmetric variants. Catalytic reactions using recyclable hypervalent iodine catalysts are also covered. Keywords: catalytic oxidations · hypervalent iodine · oxidants · rearrangement · stereoselective reactions

1. Introduction

the electrochemical gem-difluorination of dithioacetals as one of the first hypervalent iodine-catalyzed reactions.[9] The active hypervalent iodine catalyst was generated by anodic oxidation of an iodoarene using electrochemical reaction conditions. The same research group reported a modified catalytic approach in 1996.[10] According to these reports, an appropriate relationship of the oxidation potentials between the iodoarene catalyst and the substrates is required to develop catalytic reactions under electrochemical reaction conditions. A successful catalytic reaction can be achieved only if the oxidation potential of the catalysts is much lower than those of the substrates and products. Because of such limitations, these catalytic reactions are problematic in the electrochemical reoxidation in the presence of more oxidizable substrates such as phenols, and even polyalkylated arenes.[11] This fact was largely responsible for the slow development of hypervalent iodine-catalyzed reactions despite Fuchigamis report in 1994. Several research groups emphasized the need to generate hypervalent iodine species in situ by the oxidation of iodoarenes. Initially, some common inorganic oxidants and peracetic acid were used to regenerate an active hypervalent iodine species but these reactions were not facile and selective enough to develop a successful catalytic reaction.[12] Finally, m-chloroperbenzoic acid (mCPBA) was successfully employed for the in situ generation of hypervalent iodine reagents by the oxidation of iodoarenes.[13] In 2005, the first hypervalent iodine-catalyzed reactions, without electrochemical reaction conditions, were reported independently by Ochiai and Kita using mCPBA as terminal oxidant.[14, 15] After these reports, numerous iodoarene-catalyzed oxidative transformations have been investigated using various terminal oxidants.

In the past decades, hypervalent iodine reagents have been developed as highly valuable reagents in synthetic chemistry.[1] These reagents are the key replacements of toxic heavy metals, owing to their mild reaction conditions and environmentally friendly behavior.[2] Various hypervalent reagents have been developed as oxidants,[3] but their applications are not limited to only oxidative functionalization. Many other synthetic transformations such as cyclizations,[4] a-functionalizations of carbonyl compounds,[5] atom-transfer reactions,[6] and oxidative rearrangements[7] have been achieved successfully. In recent years, these reagents have received particular attention owing to their applications in catalysis.[3b, 8] Various synthetic transformations have been successfully achieved using both iodineACHTUNGRE(III) or iodine(V) species as catalytic intermediates. This review covers the recent developments of hypervalent iodine reagents in catalysis including some stereoselective variants.

2. Hypervalent Iodine-Catalyzed Reactions To develop iodoarene-catalyzed reactions, it is essential to reoxidize the released iodoarenes in situ to form active hypervalent iodine species. To achieve a successful catalytic reaction, the oxidant should only oxidize the iodoarene and not interact with the starting material and products or at least the regeneration of hypervalent iodine species should be faster than the rates of undesired side reactions. The discovery of hypervalent iodine-catalyzed transformations is a remarkable achievement in the area of hypervalent iodine chemistry. In 1994, Fuchigami and Fujita reported

2.1. Hypervalent Iodine-Catalyzed Oxidations [a] Dr. F. V. Singh, Prof. Dr. T. Wirth School of Chemistry, Cardiff University Park Place, Main Building, Cardiff CF10 3AT (UK) Fax: (+ 44) 29-2087-6968 E-mail: [email protected] Homepage: http://www.cf.ac.uk/chemy/wirth

Chem. Asian J. 2014, 9, 950 – 971

2.1.1. Oxidation of Alcohols Although a large number of hypervalent iodine-mediated approaches for the oxidation of alcohols is available, the first hypervalent iodine-catalyzed oxidation of alcohols was

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reported in 2005 by Vinod and co-workers.[16] After this discovery, various successful catalytic oxidations of alcohols have been accomplished using iodoarenes as precatalysts to generate in situ active hypervalent iodine species in the presence of terminal oxidants.[3b, 8b,c]


mCPBA, followed by treatment with p-toluenesulfonic acid (Scheme 1). In addition, a catalytic oxidation was achieved using poly(4-iodostyrene) as a recyclable catalyst under similar reaction conditions.[17]

2.1.1.1. IodineACHTUNGRE(III)-Catalyzed Oxidation of Alcohols The majority of hypervalent iodine-catalyzed oxidations of alcohols have been realized by using iodine(V) reagents such as IBX or its analogues, and only a few iodineACHTUNGRE(III)-catalyzed approaches have been reported. In 2007, Togo and co-workers[17] described the oxidation of secondary alcohols 1 to a-tosyloxyketones 2 in moderate yields using catalytic amounts of iodobenzene and potassium bromide or (2,2,6,6tetramethylpiperidin-1-yl)oxyl (TEMPO) in the presence of

Scheme 1. IodineACHTUNGRE(III)-catalyzed oxidation of secondary alcohols 1 to the corresponding a-tosyloxyketones 2 using mCPBA as an oxidant.

Another iodineACHTUNGRE(III)-catalyzed approach for the oxidation of alcohols 3 to ketones 4 was developed using iodobenzene and TEMPO as catalysts with potassium peroxodisulfate as the stoichiometric oxidant in the presence of trifluoroacetic acid (TFA) in aqueous acetonitrile (Scheme 2).[18] In addition, this approach was used for selective oxidation of primary alcohols in the presence of secondary ones.

Fateh Veer Singh was born in Ravani Katiry, Bulandshahar, Uttar Pradesh, India, in 1976. After completing a M.Sc. in organic chemistry from S. S. V. College, Hapur, Choudhary Charan Singh University, Meerut, India, in 1998, he completed his PhD in 2007 under the supervision of Dr. A. Goel in the field of synthetic organic and medicinal chemistry at CDRI, Lucknow, India. During his doctoral studies, he was involved in Molecular Target Oriented Synthesis of Potential Antidiabetic Agents. After the completion of his doctoral studies, he joined Prof. Dr. H. A. Stefanis research group at the University of Sao Paulo, Brazil in October, 2007. In his research group he was working on the synthesis of various potassium organotrifluoroborates and their applications in metalcatalyzed C C bond formation reactions. In the beginning of 2010, he was awarded the prestigious Marie Curie postdoctoral fellowship by the European Union. In May 2010, he started his postdoctoral studies in Prof. Wirths research group where he was involved in oxidative functionalizations using hypervalent iodine reagents. He was also involved in investigations of selenium catalysts for green chemistry. Currently, he is working as DS Kothari Postdoctoral Fellow in Prof. G. Mugeshs research group at IISc Bangalore, India.

Scheme 2. IodineACHTUNGRE(III)-catalyzed oxidation of alcohols 3 to ketones 4.

The catalytic cycle for the oxidation of alcohols 3 to ketones 4 is shown in Scheme 3. The catalytic cycle is initiated with the in situ oxidation of iodobenzene to [bis(trifluoroacetoxy)iodo]benzene 5, which oxidizes 6 to the oxammonium salt intermediate 7. Finally, intermediate 7 oxidizes 3 to ketones 4 with the regeneration of the hydroxylamine derivative of TEMPO 6. Regenerated [bis(trifluoroacetoxy)iodo]benzene 5 and 6 can restart the catalytic cycle.

Thomas Wirth is professor of organic chemistry at Cardiff University. After PhD studies (TU Berlin, with Prof. S. Blechert) and a postdoctoral stay in Japan (Kyoto University, with Prof. K. Fuji), he started his independent research at the University of Basel (Switzerland). In the group of Prof. B. Giese, he obtained his habilitation supported by various scholarships. He took up his current position at Cardiff University in 2000. He has been invited as a visiting professor to a number of places, including the University of Toronto in Canada (1999) and Chuo University in Tokyo (2000), Osaka University (2004), Osaka Prefecture University (2008), and Kyoto University (2012) in Japan. In 2000, he was awarded the Werner Prize from the New Swiss Chemical Society and a JSPS Furusato award in 2013. His main research interests concern stereoselective electrophilic reactions, oxidative transformations with hypervalent iodine reagents, including mechanistic investigations, and organic synthesis performed in microreactors.

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Scheme 3. The cycle for iodineACHTUNGRE(III)-catalyzed oxidations of alcohols 3 to carbonyl compounds 4.

In 2006, Li and co-workers[19] developed similar catalytic oxidations of alcohols 3 in high yields by using catalytic amounts of (diacetoxyiodo)benzene 8, TEMPO, and KNO2 (Scheme 4). A catalytic amount of polystyrene-supported

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Scheme 4. (Diacetoxyiodo)benzene-catalyzed oxidation of alcohols 3 to ketones 4. Scheme 5. IBX-catalyzed oxidation of alcohols 3 to carboxylic acids 12 and ketones 4 using oxone as co-oxidant.

hypervalent iodine reagent led to similar results. Polystyrene-supported reagents were advantageous over non-supported reagents because of recovering capability and the possibility to reuse them with almost similar efficiency. Unfortunately, this procedure was not found equally efficient for the aliphatic alcohols in comparison with benzylic alcohols. Primary benzylic alcohols could be oxidized selectively over secondary alcohols. Recently, Zhdankin and co-workers[20a] developed recyclable bifunctional catalysts bearing ionic liquid-supported TEMPO (Figure 1) and these catalysts were employed in

In addition, 2-iodobenzoic acid 11, 2-iodosobenzoic acid 13, and 2-iodoxybenzoic acid (IBX) 14 (Figure 2) were also tested in similar catalytic oxidations and showed almost similar catalytic activity. These catalytic oxidations were performed using commercially available 2-iodobenzoic acid 11 as precursor to generate the IBX catalyst.

Figure 2. 2-Iodobenzoic acid 11 and catalysts 13 and 14 used in the oxidation of alcohols 3 to carboxylic acids 12 and ketones 4.

Schulze and Giannis[21a] reported another high yielding IBX-catalyzed approach for the oxidation of alcohols using tetra-n-butylammonium oxone as an oxidant in an ethyl acetate/water solvent system. Tetra-n-butylammonium oxone was generated in situ using nBu4NHSO4 as a phase transfer catalyst and oxone as a primary oxidant (Scheme 6). Benzylic primary and secondary alcohols were oxidized into benzaldehydes and ketones while carboxylic acids are generated in the oxidation of aliphatic primary alcohols. Both, 2-iodobenzoic acid 11 and IBX 14 were investigated with 11 being the preferred catalyst.

Figure 1. The structure of bifunctional catalysts 9 and 10 bearing IL-supported TEMPO.

the oxidation of a variety of alcohols 3 using peracetic acid as oxidant. The ionic liquid-supported bifunctional catalysts could be simply recovered and reused. In addition, Yakura and Ozono reported bifunctional hybrid-type catalysts (bearing TEMPO and iodobenzene moieties) for similar oxidation reactions in the presence of peracetic acid under mild reaction conditions.[20b] 2.1.1.2. Iodine(V)-Catalyzed Oxidation of Alcohols 2.1.1.2.1. IBX-Catalyzed Oxidations of Alcohols In 2005, Vinod and co-workers[16] reported the first iodine(V)-catalyzed oxidation of alcohols using iodoarenes as precatalysts for the in situ generation of the active iodine(V) species in the presence of oxone as an oxidizing agent. IBXcatalyzed oxidations of alcohols 3 were achieved in excellent yields using catalytic amounts of 2-iodobenzoic acid 11 with oxone in aqueous acetonitrile. The primary alcohols were oxidized cleanly to the corresponding carboxylic acids 12 while secondary alcohols were oxidized to ketones 4 (Scheme 5). Interestingly, Baeyer–Villiger oxidation products were not observed during the oxidation of secondary alcohols.

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Scheme 6. Oxidation of alcohols 3 to carboxylic acids 12 and ketones 4 using acid 11 as a precatalyst.

In 2007, Page et al.[21b] reported another IBX-catalyzed oxidation of alcohols 3 into carbonyl compounds 4 using catalytic amounts of 2-iodobenzoic acid 11 in the presence of tetraphenylphosphonium monoperoxysulfate (TPPP). Various primary and secondary alcohols were used as substrates

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20, and 24 showed good catalytic activity but decomposed under aqueous reaction conditions. In addition, electron-deficient catalysts were also generated from 21 and 22 but showed only poor catalytic activity. Despite its use as a common oxidant, IBX is associated with some serious drawbacks such as its insolubility in common organic solvents,[24] explosive nature at high temperatures,[25] and the need that most reactions require elevated temperature.[26] The solubility issues and explosive nature are attributed to strong intermolecular interactions that hold the molecules cohesively in the crystal lattice.[27] Based on research on twisted IBX derivatives by Goddart and Su,[23a] Moorthy and co-workers[28] developed twisted IBX analogues 27 and 28 (Figure 4) to overcome the solubil-

Scheme 7. IBX-catalyzed oxidation of alcohols 3 to carbonyl compounds 4.

and the corresponding carbonyl compounds 4 were isolated in moderate to excellent yields under mild reaction conditions (Scheme 7). Even cyclic alcohols were successfully oxidized. In 2009, Ishiharas research group[22] reported a detailed study on the catalytic activity of various 2-iodobenzoic acids 15–24 with different electron-withdrawing and electron-donating substituents at the aromatic ring (Figure 3). To observe the electronic effect, precatalysts 15–24 were used in the oxidation of 5-nonanol 25 to 5-nonanone 26 and differ-

Figure 4. The structures of more soluble IBX analogues 27, 28 and 29 generated from precatalysts 30, 31 and 32 respectively.

ity issues and used them as stoichiometric oxidant for the oxidation of alcohols and sulfides in dichloromethane at room temperature. More importantly, twisted IBX analogue 28 was generated in situ from 31 and successfully used as a catalyst in oxidations of alcohols under both, aqueous and non-aqueous reaction conditions (Scheme 9). Primary benzylic alcohols 33 were oxidized into acids 34 under aqueous reaction conditions while being oxidized selectively to aldehydes in non-aqueous solvents (Scheme 9).[22] The selective oxidation to aldehydes was probably achieved owing to the poor solubility of oxone in

Figure 3. The structures of catalysts 15–24 in the oxidation of 5-nonanol 25 to 5-nonanone 26.

ent reaction conditions (a) aqueous acetonitrile[16] and (b) dry nitromethane using oxone as co-oxidant were investigated. The oxidation product 26 was obtained in 88 % and 99 % yields using 11 as precatalyst to generate IBX in aqueous acetonitrile and dry nitromethane, respectively (Scheme 8). The IBX-analogues generated from 15 and 23 were already reported as more reactive than IBX but could not catalyze the oxidations efficiently.[23] The hypervalent compound generated from 17 showed better catalytic activity than IBX. Electron-rich IBX-analogues generated from 19,

Scheme 8. Catalyzed oxidation of 5-nonanol 25 to 5-nonanone 26 in different solvents (aqueous acetonitrile and dry nitromethane).

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Scheme 9. Oxidation of alcohols 33 to carboxylic acids 34, aldehydes 35, and ketones 4 using 31 as a precatalyst with oxone.

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dry nitromethane. Interestingly, esters were not formed during the oxidation of secondary alcohols 33 under neither aqueous nor dry reaction conditions. The other twisted precatalyst 30 was also tested but 31 gave slightly better yields. Recently, the same research group[29a] achieved similar results, as described in Scheme 9, with slightly improved yields with 29 using 32 as precatalyst in the presence of oxone under aqueous and non-aqueous reaction conditions. The catalytic oxidation of diols 36 to lactones 37 was achieved in moderate yields using 32 and oxone in nitromethane (Scheme 10).

queous solvents. The catalytic activity of IBS was more than three times higher than that of IBX in dry nitromethane with both reactions leading to full conversion, however, IBX-catalyzed oxidations did not proceed well in other nonaqueous solvents such as ethyl acetate or acetonitrile (Scheme 11). Also sodium 2-iodobenzenesulfonic acid 40 could be used as a precatalyst for the oxidation of 25 in acetonitrile and 26 was isolated in 99 % yield (Scheme 12). In addition, the sub-

Scheme 10. Oxidation of diols 36 to lactones 37 using 31 and oxone in nitromethane.

Scheme 12. IBS-catalyzed oxidation of 5-nonanol 25 to 5-nonanone 26 using oxone as an oxidant in acetonitrile.

stituent effect of 40 on the oxidation of alcohols showed similar trends to IBX. When compound 40 was substituted with electron-donating groups, higher catalytic activity was observed while electron-withdrawing compounds showed lower activity. Konno and co-workers have been using the same IBS catalyst 40 for the oxidation of various fluoroalkyl-substituted alcohols in excellent yields using oxone as cooxidant.[31b] Importantly, Uyanik and Ishihara achieved the IBS-catalyzed oxidation of alcohols successfully on large scale.[31c] Various alcohols including diols were used as substrates and oxidized successfully to the corresponding carbonyl compounds. Primary alcohols were selectively oxidized to aldehydes or acids under similar catalytic conditions by controlling the amount of oxone added. Aliphatic alcohols led to mixtures of acid and aldehyde owing to the higher reactivity of oxone with aliphatic aldehydes. Furthermore, IBS 39 was explored to catalyze the oxidative dehydrogenation of cycloalkanols 41 to the corresponding a,b-unsaturated ketones 42; this was achieved in high yields by generating IBS in situ using 40 as precatalyst in the presence of powdered oxone in nitromethane (Scheme 13). Various electron-donating R substituents are tolerated in this catalytic transformation. The general mechanism for IBX- or IBS-catalyzed oxidations of alcohols to ketones is shown in Scheme 14. Firstly, the catalytic reaction is initiated with the oxidation of 11 or 38 to the iodineACHTUNGRE(III) species 13 or 43 using oxone as oxidant;

In 2011, Miura et al. developed a similar oxidation of alcohols to carbonyl compounds in excellent yields using fluorous IBX as efficient catalysts with oxone as a co-oxidant. Importantly, the catalyst was recovered in the form of fluorous IBA and can be reused without losing any catalytic activity.[29b] 2.1.1.2.2. IBS-Catalyzed Oxidations of Alcohols In 2009, Ishihara and co-workers[22] synthesized IBS 39 in situ and compared its catalytic activity with IBX 14. IBS 39 has a higher Lewis acidity owing to the presence of the electron-withdrawing sulfone group.[30] The synthesis of IBS 39 was already reported in 2006 but its oxidizing capability could not investigated due to stability issues.[31a] To compare the catalytic activity of IBS 39 with IBX 14, two different catalytic oxidations of 5-nonanol 25 were performed using 2-iodobenzenesulfonic acid 38 and 11 in the presence of oxone in both aqueous and nonaqueous solvents at 70 8C (Scheme 11). Interestingly, IBX 14 showed higher catalytic activity over IBS 39 in aqueous acetonitrile in which only 24 % conversion was observed with IBS 39. Surprisingly, a dramatic increase was observed in the catalytic activity of IBS in nona-

Scheme 11. IBX- or IBS-catalyzed oxidation of 5-nonanol 25 to 5-nonanone 26 using oxone as an oxidant in different solvents.

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Scheme 13. IBS-catalyzed oxidative dehydrogenation of cycloalkanols 41 using 40 as a precatalyst.

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Scheme 16. PhIO2-catalyzed aerobic oxidation of alcohols 3 to carbonyl compounds 4.

sponding aldehydes 4 under similar reaction conditions. Interestingly, benzoin and meso-hydrobenzoin led to a C C bond cleavage and benzaldehyde was isolated as the cleaved product. A possible mechanism for these catalytic oxidations is shown in Scheme 17 consisting of three redox cycles. The first catalytic cycle is initiated by the oxidation of alcohols 3 to ketones 4 with iodylbenzene 50 to generate the reduced

Scheme 14. The general mechanism for IBX or IBS-oxidation of alcohols 3 using oxone as a terminal oxidant.

these species are then further oxidized to the iodine(V) species IBX 14 or IBS 39. Then, iodine(V) species 14 or 39 react with alcohol 3 to form intermediate 44 or 45, which form ketone 4 and iodineACHTUNGRE(III) species 13 or 43 by cleavage of the I O bond. Finally, the regenerated iodineACHTUNGRE(III) species is reoxidized to the iodine(V) species by oxone to continue the catalytic cycle. Recently, Zhu and co-workers[32] reported a one-pot Ugi four-component reaction with alcohols 3 instead of the commonly employed aldehydes. IBS-catalyzed oxidation of primary alcohols 3 to aldehydes 4 was achieved by using sodium 2-iodobenzenesulfonate 40 as precatalyst in the presence of oxone, and aldehydes were directly subjected to the Ugi four-component reaction with amine 46, isocyanide 47, and carboxylic acid 48 to afford a-acetamidoamides 49 in moderate to good yields (Scheme 15). It is important to note that most of the reactions were quite slow and required long reaction times for completion.

Scheme 17. Catalytic cycle for PhIO2-catalyzed aerobic oxidation of alcohols 3 to ketones 4.

iodineACHTUNGRE(III) species 51. IodineACHTUNGRE(III) species 51 is reoxidized to iodylbenzene 50 with bromine along with the formation of HBr. Furthermore, air oxidizes NO to NO2, which could reoxidize HBr to bromine to continue the catalytic cycle. In addition, NO2 can react with water to produce HNO3 ; this could be another possibility to oxidize HBr to bromine. Furthermore, Ishihara and co-workers attempted similar oxidation reactions under similar reaction conditions, but could not reproduce the reaction and oxidation products were observed only in trace amounts. In addition, the control experiments suggested that the actual oxidant for those oxidations could be bromine instead of PhIO2.[33b] In 2010, Zhdankin and co-workers[34] reported an elegant route for the oxidation of various alcohols 3 to the corresponding aldehydes and ketones 4 using iodobenzene and rutheniumACHTUNGRE(III)chloride as co-catalysts and oxone as terminal oxidant in aqueous acetonitrile. Various functional groups were tolerated and oxidation products 4 were obtained in very high yields in most of the catalytic oxidations (Scheme 18).

Scheme 15. IBS-catalyzed oxidation of primary alcohols 3 to aldehydes 4 and their direct application in Ugi four-component reactions to form aacetamidoamides 49.

2.1.1.2.3. Iodylbenzene-Catalyzed Oxidations of Alcohols Iodylbenzene 50 is the simplest acyclic iodine(V) reagent that does not have any functional group, which has been recently used as a catalyst in oxidation reactions. Liu and coworkers[33a] reported an efficient catalytic aerobic oxidation of alcohols 3 using the PhIO2/Br2/NaNO2 catalytic system in water (Scheme 16). Various alcohols 3 were oxidized successfully to ketones 4 in excellent yields. More importantly, primary alcohols were selectively oxidized to the corre-

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Scheme 18. PhIO2-catalyzed oxidation of alcohols 3 to carbonyl compounds 4 using PhI/RuCl3 as co-catalyst and oxone as terminal oxidant.

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Zhdankin and co-workers[35] previous report indicating that iodylarenes can be synthesized directly by Ru-catalyzed oxidation of iodobenzene using peroxyacids or oxone as oxidants, confirmed that catalytic iodine(V) was generated in situ during these oxidations and not the iodineACHTUNGRE(III) species. A plausible mechanism for these catalytic oxidations includes two catalytic redox cycles and is presented in Scheme 19. The reaction proceeds by a Ru-catalyzed oxida-

2.1.2. Oxidation of Phenols 2.1.2.1. Oxidation of Phenols without Cyclization Oxidation of substituted phenols is a frequently used approach for the synthesis of various quinones and quinols, which are structural components of various naturally occurring[37] and pharmacologically active scaffolds.[38] Hypervalent iodine reagents have been used to oxidize phenols 54 to quinones 55, which can perform addition reactions with external nucleophiles. The formation of addition products 56 or 57 is dependent on the position of substitution on the phenol, that is, ortho- and para-substituted phenols could lead to compounds 56 and 57, respectively (Scheme 21).

Scheme 19. Catalytic cycle for the PhIO2-catalyzed oxidation of alcohols 3 using PhI/RuCl3 as co-catalysts and oxone as terminal oxidant. Scheme 21. Hypervalent iodine-mediated oxidation of phenols 54.

tion of iodobenzene to PhIO2 50 in the presence of oxone, which oxidizes alcohols 3 to the corresponding carbonyl compounds 4. More importantly, reactive oxoruthenium complexes 52 are playing the key role for the reoxidation of the initially formed PhIO to PhIO2. The extension of a similar catalytic approach using RuCl3/ polystyrene-supported iodosylbenzene 53 as co-catalyst in the presence of oxone as oxidant (Scheme 20) was also reported.[36a] The oxidation products were isolated in moderate

Yakura et al.[39] developed a catalytic approach for the oxidation of functionalized p-alkoxyphenols 58 to p-benzoquinones 61 using 4-iodophenoxy acetic acid 60 as precatalyst and oxone as oxidant (Scheme 22). Unfortunately, electrondeficient phenols could not be oxidized successfully under similar conditions. Various functionalized iodoarenes were tested for these oxidations, but p-alkoxy iodobenzenes showed superiority over others.

Scheme 20. Catalytic oxidation of alcohols 3 using RuCl3 and polystyrene-supported iodosylbenzene 53 as co-catalysts.

Scheme 22. Catalytic oxidation of p-alkoxyphenols 58 and p-dialkoxybenzenes 59 to p-benzoquinones 61 using 4-iodophenoxy acetic acid 60 as precatalyst and oxone as oxidant.

to good yields. In addition, iodinated polystyrene was also tested but showed much lower catalytic activity compared to 53. Furthermore, Zhdankin and co-workers have also achieved the oxidation of alcohols and hydrocarbons using other iodoarenes/RuCl3 combinations as co-catalytic systems[36b,c] in the presence of oxone under mild reaction conditions.

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4-Iodophenoxy acetic acid 60 was also used as precatalyst for the development of a catalytic oxidation of p-dialkoxybenzenes 59 to benzoquinones 61 in excellent yields using similar reaction conditions in aqueous 2,2,2-trifluoroethanol.[40] In addition, the same catalytic approach was successfully applied to synthesis of naturally occurring compound blattellaquinone 63 (Scheme 23), which is known as a sex pheromone of the German cockroach, Blattella germanica.[40]

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phase-transfer catalyst, and oxone as an oxidant under mild reaction conditions (Scheme 26). Interestingly, the reaction rate was further enhanced in nonaqueous solvents in the presence of an inorganic base and a dehydrating agent.

Scheme 23. Synthesis of blattellaquinone 63 by the oxidation of p-dialkoxybenzene derivative 62.

In 2009, Yakura and Omoto[41] reported the synthesis of p-quinols 65 by the catalytic oxidation of p-arylphenols 64 using 60 as precatalyst and oxone as oxidant in aqueous 1,4dioxane. The catalytic reactions worked smoothly and p-quinols 65 were isolated in moderate yields (Scheme 24). Detailed studies on the catalytic oxidation of p-substituted phenols are also disclosed in Yakuras recent study.[42]

Scheme 26. IBS-catalyzed oxidation of various phenols 68 to o-quinones 70 using 69 as precatalyst in the presence of oxone.

The mechanism for IBS-catalyzed oxidation of phenols 68 to o-quinones 70 is depicted in Scheme 27. The reaction was initiated with the in situ oxidation of precatalyst 69 to the iodine(V) species 71 by oxone and the phase-transfer catalyst. The iodine(V) species 71 combines reversibly with phenol 68 to give iodine(V)-phenol complex 72, which is then transformed into iodineACHTUNGRE(III)-catechol complex 73 through a concerted intramolecular [2,3]-rearrangement. Finally, iodineACHTUNGRE(III)-catechol complex 73 provides o-quinones 70 and precatalyst 69.

Scheme 24. Hypervalent iodine-catalyzed oxidation of p-arylphenols 64 to p-quinols 65.

A possible mechanism for the catalytic oxidation of p-arylphenols 64 is described in Scheme 25. The catalytic cycle is initiated with the in situ oxidation of iodoarene 60 to the iodineACHTUNGRE(III) species 66 by oxone. Compound 66 reacts with 4arylphenol 64 to form a cationic intermediate 67 and iodoarene 60. Finally, the cationic intermediate 67 is trapped by water to yield p-quinol 65. Recently, Ishihara and co-workers[43] demonstrated the first example of hypervalent iodine(V)-catalyzed regioselective oxidation of phenols 68 to o-quinones 70. Various phenols including naphthols and phenanthrols were oxidized to the corresponding o-quinones in good to excellent yields using sodium 2-iodobenzenesulfonic acid 69 as precatalyst, tetra-n-butylammonium hydrogen sulfate (nBu4NHSO4) as

Scheme 27. Mechanism for the IBS-catalyzed oxidation of phenols 68 to o-quinones 70 using 69 as a precatalyst in the presence of oxone.

2.1.2.2. Oxidation of Phenols with Cyclization Various phenolic compounds with internal nucleophiles could undergo an intramolecular cyclization using hypervalent iodine reagents, for example, the oxidative cyclization of 74 and 77 to 76 and 79, respectively (Scheme 28). The spirocyclic systems are the key structural units of various bioactive natural products. In several literature reports, hypervalent iodine reagents have been extensively used to oxidize ortho- and para-substituted phenols with nucleophilic side

Scheme 25. Catalytic cycle for the oxidation of p-arylphenols 64 to p-quinols 65 using 60 as a precatalyst and oxone as an oxidant.

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Scheme 30. IodineACHTUNGRE(III)-catalyzed cyclization of amides 83 to N-fused spirolactams 84 using iodotoluene 81 as precatalyst and mCPBA as oxidant.

Scheme 28. Hypervalent iodine-mediated oxidation of phenols 74 and 77 to the corresponding spiro compounds 76 and 79.

chains to a variety of spirocyclohexadienones via the formation of C O, C N, and C C bonds.[1j, 3a,c] In recent years, various oxidative catalytic spirocyclizations of phenols have been successfully developed using hypervalent iodine reagents. In 2005, Kita and co-workers[14] reported the first catalytic spirocyclization of phenols 80 to spiro compounds 82 using catalytic amounts of iodotoluene 81 or iodineACHTUNGRE(III) derivatives in the presence of mCPBA and TFA. The reactions had short reaction times and products 82 were isolated in excellent yields (Scheme 29). TFA was probably playing the key role to make an active iodineACHTUNGRE(III) species. Other acids were also tested but could not catalyze the cyclizations efficiently.

Scheme 31. IodineACHTUNGRE(III)-catalyzed approach for spirocyclizations of phenols 85 to spirodienones 86.

key precursors of biologically important Amaryllidaceae alkaloids. In 2010, Kita and co-workers[47a] investigated a more reactive l-oxo-bridged hypervalent iodineACHTUNGRE(III) species for the spirocyclization of amides 83 to N-fused spirolactams 84 with iodoarene 87 as precatalyst and peracetic acid as oxidant in hexafluoroisopropanol (HFIP; Scheme 32). The

Scheme 29. IodineACHTUNGRE(III)-catalyzed spirolactonization of phenols 80 to spirocyclic compounds 82 using iodotoluene 81 as a precatalyst and mCPBA as an oxidant.

Scheme 32. IodineACHTUNGRE(III)-catalyzed cyclization of amides 83 to N-fused spirolactams 84 using bis(iodoarene) 87 as a precatalyst and peracetic acid as an oxidant.

In 2007, the same research group[45] reported an iodineACHTUNGRE(III)-catalyzed spirocyclization of amides 83 to N-fused spirolactams 84 through the formation of C N bonds using iodotoluene 81 as precatalyst and mCPBA as terminal oxidant in 2,2,2-trifluoroethanol. The N-fused spirolactams 84 were isolated in good to excellent yields (Scheme 30). Another iodineACHTUNGRE(III)-catalyzed approach for spirocyclizations of phenols 85 to spirodienones 86 was developed by Kita and co-workers.[46] The products were obtained in excellent yields using precatalyst 81 and urea hydrogen peroxide adduct as an oxidant in the presence of trifluoroacetic anhydride (TFAA) in 2,2,2-trifluoroethanol (Scheme 31). The same catalytic approach was used for the synthesis of

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cyclic products 84 were isolated in good to excellent yields. In addition, the same research group also reported an organoiodine-catalyzed oxidative spirocyclization of phenols under similar reaction conditions.[47b] In 2011, Yu and co-workers[48] developed an intramolecular oxidative cyclization of substituted 4-hydroxyphenyl-Nphenylbenzamides 88 to spirooxindoles 89 using catalytic amounts of iodobenzene and mCPBA as oxidant. The cyclization reactions proceeded well and spirooxindoles 89 were isolated in good to excellent yields (Scheme 33). The rate of reaction was faster with the urea·H2O2 adduct than with

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Scheme 33. IodineACHTUNGRE(III)-catalyzed cyclization of benzamides 88 to spirooxindoles 89 using iodobenzene as a precatalyst and mCPBA as terminal oxidant.

Scheme 36. PhIO2-catalyzed benzylic oxidation of alkylarenes 93 and 95 to carbonyl compounds 94 and 96, respectively, using PhI/RuCl3 as catalysts in the presence of oxone.

mCPBA as an oxidant but the products were isolated in slightly lower yields. Recently, Hutt and co-workers[49] reported an iodineACHTUNGRE(III)catalyzed synthesis of spirofuran 91 by oxidative cyclization of vinylogous esters 90 using catalytic amounts of iodobenzene and mCPBA as oxidant in HFIP/TFA (Scheme 34). This catalytic method allows the rapid synthesis of spirofurans 91 in decent yields by the formation of a C O bond.

lyzed benzylic oxidation of arenes 93 in moderate yields using iodobenzene and RuCl3 as co-catalyst in the presence of oxone in aqueous acetonitrile (Conditions A, Scheme 36). A similar catalytic approach was successfully used to oxidize benzylic C H positions of cyclic arenes of type 95. In 2012, Yan and co-workers[50] reported an iodineACHTUNGRE(III)catalyzed benzylic oxidation of alkylarenes 93 and 95 to 94 and 96, respectively, using catalytic amounts of iodobenzene and potassium bromide in the presence of mCPBA. The catalytic oxidations of both cyclic and linear substrates proceeded smoothly and the corresponding arylketones 94 and 96 were isolated in good yields (Conditions B, Scheme 36). Interestingly, oxidations of alkylbenzenes with electronwithdrawing groups at the aryl ring led to lower yields. The same research group[51a] also developed an iodineACHTUNGRE(III)-catalyzed oxidation of a benzylic C H moiety of alkylarenes 93 and 95 in good to excellent yields using tert-butyl hydroperoxide (TBHP) and mCPBA in the presence of a catalytic amount of iodobenzene. The oxidation of cyclic alkylbenzenes 93 provided better yields than acyclic alkylarenes 93 (Conditions C, Scheme 36). In addition, similar types of oxidations have been achieved using cyclic iodine(V) reagents as catalysts including IBX[51b] and IBS[51b] in the presence of oxone.

Scheme 34. IodineACHTUNGRE(III)-catalyzed synthesis of spirofurans 91 by oxidative cyclization of vinylogous esters 90 using PhI and mCPBA.

The general mechanism for these catalytic oxidative cyclizations is depicted in Scheme 35. The catalytic cycle is initiated with the in situ oxidation of iodoarene 81 or 87 to the iodineACHTUNGRE(III) species 92 by the stoichiometric oxidant. Compound 92 then oxidizes phenols 77 to spirocyclic compounds 79 through formation of a cationic intermediate 78. The regenerated iodoarene 81 or 87 can then re-enter the catalytic cycle.

2.1.4. Oxidation of Alkenes Hypervalent iodine-catalyzed oxidations of alkenes have not been as well explored as earlier described oxidation reactions. In 2009, Ochiai and co-workers[52a] reported the first iodineACHTUNGRE(III)-catalyzed oxidative cleavage of C=C and C < = 3 > C bonds (97 or 98) using iodomesitylene 99 as precatalyst and mCPBA as terminal oxidant. The reaction was performed in aqueous acetonitrile under an inert atmosphere and carboxylic acids 12 were isolated in good to excellent yields (Scheme 37). Both alkenes and alkynes were cleaved smoothly under these catalytic reaction conditions. In 2010,

Scheme 35. A general mechanism for the iodineACHTUNGRE(III)-catalyzed oxidative cyclization of phenols 77 to spirocyclic compounds 79.

2.1.3. Oxidation of Alkylarenes Hypervalent iodine reagents have been used as catalysts to oxidize the benzylic positions of alkylarenes. In 2010, Zhdankin and co-workers[34] reported an iodylbenzene-cata-

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Scheme 37. IodineACHTUNGRE(III)-catalyzed oxidative cleavage of C=C and CC bonds using iodomesitylene 99 as a precatalyst and mCPBA as an oxidant.

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Thottumkara and Vinod also reported a hypervalent iodinecatalyzed approach for oxidative cleavage of olefins in high yields using 4-iodobenzoic acid as precatalyst and oxone as a co-oxidant.[52b] The mechanism for iodineACHTUNGRE(III)-catalyzed oxidative cleavage of alkenes is shown in Scheme 38. The reaction is initiated by in situ generation of a tetracoordinated square planar hydroxy-l3-iodane 100 as active species, which undergoes oxidative cleavage of the 1,2-diol 102 derived from an olefin 97, probably through formation of a cyclic intermediate dialkoxy-l3-iodane 101. Finally, the regenerated iodomesitylene 99 re-enters the catalytic cycle. Scheme 40. The mechanism for iodineACHTUNGRE(III)-catalyzed syn-diacetoxylation of alkenes 97 to syn-diacetates 103.

Very recently, Bigi and White[54] reported a palladium and hypervalent iodine-catalyzed tandem Wacker oxidation/dehydrogenation reaction of terminal olefins 106 using both (diacetoxyiodo)benzene 8 and [PdACHTUNGRE(MeCN)4]ACHTUNGRE[BF4]2 as catalysts. This catalytic approach provides selective synthesis of a broad range of linear aryl and alkyl a,b-unsaturated ketones 107 in moderate yields (Scheme 41). Zhu and coworkers reported the nBu4NI-catalyzed regioselective difunctionalization of unactivated alkenes using tert-butylperoxide as oxidant, however, a hypervalent iodine species was not generated during this transformation.[55] Scheme 38. The mechanism for the iodineACHTUNGRE(III)-catalyzed oxidative cleavage of C=C and CC bonds.

The first organocatalytic syn-diacetoxylation of alkenes 97 using a catalytic amount of iodomesitylene 99 in the presence of mCPBA or hydrogen peroxide has recently been reported by Li and co-workers.[53] Various electron-donating and electron-deficient alkenes were smoothly oxidized to the oxidation products 103 in good to excellent yields and with high diastereoselectivities (Scheme 39).

Scheme 41. Palladium/hypervalent iodine-catalyzed tandem Wacker oxidation/dehydrogenation reaction of terminal olefins 106.

2.2. a-Functionalization of Carbonyl Compounds Functionalization at the a-position of carbonyl compounds is a very important reaction in organic chemistry. Many approaches are available in the literature to achieve the synthesis of a-functionalized carbonyl compounds.[56] In past two decades, these scaffolds have been synthesized frequently using stoichiometric amounts of hypervalent iodine reagents.[5] Recently, hypervalent iodine-catalyzed a-functionalizations of carbonyl compounds have been achieved using terminal oxidants such as mCPBA, oxone, or peroxides. In 2005, Ochiai et al.[15] reported the first iodineACHTUNGRE(III)-catalyzed a-acetoxylation of ketones 108 in moderate yields using a catalytic amount of iodobenzene in the presence of mCPBA (Scheme 42). In addition, water and borontrifluoride etherate were used to suppress Baeyer–Villiger oxidations. Both electron-withdrawing and electron-donating iodoarenes were able to catalyze these reactions. A variety of

Scheme 39. IodineACHTUNGRE(III)-catalyzed syn-diacetoxylation of alkenes 97 using catalytic amounts of iodomesitylene 99 in the presence of mCPBA or hydrogen peroxide.

The mechanism for the iodineACHTUNGRE(III)-catalyzed syn-diacetoxylation of alkenes is shown in Scheme 40. The catalytic cycle is initiated with the oxidation of iodoarene 99 to a hypervalent iodineACHTUNGRE(III) species 104, which undergoes electrophilic addition to alkenes to form a three-membered iodonium intermediate 105. This intermediate 105 provides the syn-diacetate 103 through a Woodward reaction pathway.

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Scheme 42. IodineACHTUNGRE(III)-catalyzed a-acetoxylation of ketones 108 using a catalytic amount of iodobenzene in the presence of mCPBA.

Scheme 44. IodineACHTUNGRE(III)-catalyzed one pot synthesis of thiazoles 124 using IL-supported catalysts 110–113.

dialkyl and alkyl aryl ketones have been successfully used for acetoxylation at the a-position. In 2007, Huang and co-workers used a similar catalytic approach for the synthesis of a-acetoxylated ketones 109 just by replacing the oxidant mCPBA with hydrogen peroxide in acetic anhydride.[57] The oxidation also proceeded smoothly and the products were obtained in good to excellent yields. By replacing the nucleophile (acetic acid) with ptoluenesulfonic acid in aqueous acetonitrile, Yamamoto and Togo succeeded in a hypervalent iodine-catalyzed a-tosyloxylation of ketones under similar catalytic reaction conditions.[58] Importantly, the p-toluenesulfonic acid acts as a Brønsted acid and also as a ligand to generate in situ Kosers reagent as the reactive iodineACHTUNGRE(III) species. As mentioned above (Scheme 1), Togo and co-workers synthesized a-tosyloxy ketones in excellent yields by direct oxidation of secondary alcohols with catalytic amounts of iodobenzene and KBr in the presence of m-CPBA in acetonitrile followed by p-toluenesulfonic acid.[17] In addition, TEMPO can also be used as a catalyst instead of KBr during these oxidations. In 2007, Togo and co-workers[59a] developed new ionic liquid (IL)-supported catalysts 110–113 for iodineACHTUNGRE(III)-catalyzed a-tosyloxylations of ketones 108 (Scheme 43). All the IL-supported catalysts 110–113 showed good catalytic activity and could be reused 2–3 times with almost the same catalytic efficiency. A similar catalytic approach was used for the one-pot synthesis of thiazoles 114 by the addition of thioacetoamide or thiobenzamide. All four IL-supported precatalysts 110–113

were investigated in this reaction and thiazoles 114 were isolated in moderate yields (Scheme 44).[59a] In addition, Togos research group also reported an iodine-catalyzed a-tosyloxylation of ketones 108 in moderate yields using mCPBA as oxidant with p-toluenesulfonic acid in the presence of molecular iodine. The reaction was initiated by the in situ generation of a hypervalent iodoarene species in the presence of iodine and p-toluenesulfonic acid; this led to an even more active hypervalent iodine species.[59b] Recently, Yan and co-workers[60a] reported the first iodineACHTUNGRE(III)-catalyzed efficient approach for a-phosphoryloxylation of ketones 108 using iodobenzene as catalyst and mCPBA as the stoichiometric oxidant. All the catalytic reactions proceeded well and reaction products 115 were isolated in decent yields as shown in Scheme 45.

Scheme 45. IodineACHTUNGRE(III)-catalyzed a-phosphoryloxylation of ketones 108 using mCPBA as the terminal oxidant.

A general mechanism for the iodineACHTUNGRE(III)-catalyzed a-functionalization of carbonyl compounds 108 is shown in Scheme 46. After oxidation of the iodoarene to iodineACHTUNGRE(III) species 116, which undergoes ligand exchange on the iodineACHTUNGRE(III) with an enol 117, derived from a ketone 108, to another iodineACHTUNGRE(III) species 118. On subsequent SN2 displacement,

Scheme 43. IodineACHTUNGRE(III)-catalyzed a-tosyloxylation of ketones 108 using IL-supported catalysts 110–113 in the presence of mCPBA.

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Scheme 46. A general mechanism for iodineACHTUNGRE(III)-catalyzed a-functionalization of carbonyl compounds 108.

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the intermediate provides a-functionalized ketones 2, 109 or 115 with regeneration of iodobenzene. In addition, the presented mechanism was also supported by ESI-MS experiments.[60b] In 2008, Huang and co-workers[61] reported hypervalent iodine-catalyzed a-hydroxylation of arylketones 108 with iodobenzene and oxone in aqueous acetonitrile. All the catalytic reactions proceeded smoothly and a-hydroxy ketones 119 were isolated in good to excellent yields (Scheme 47).


Scheme 49. IodineACHTUNGRE(III)-catalyzed iodocyclizations of unsaturated acids 124 to 125.

Gulder and co-workers[64] reported an iodineACHTUNGRE(III)-catalyzed bromocarbocyclization of amides 126 to functionalized oxindoles 128 in good to excellent yields using catalytic amounts of o-iodobenzamide 127 and NBS (Scheme 50). A similar approach was used to synthesize the alkaloid physostigmine 129. Scheme 47. Hypervalent iodine-catalyzed a-hydroxylation of arylketones 108 using iodobenzene as catalyst and oxone as an oxidant.

2.3. Cyclization Reactions Hypervalent iodine reagents are not only known as oxidants but they can also be used as powerful electrophiles to achieve a variety of cyclization reactions.[1j, 4] In recent years, some catalytic variants of these reactions have been successfully achieved using similar stoichiometric oxidants as described earlier. In 2006, Braddock et al.[62] reported a hypervalent iodine-catalyzed bromolactonization of unsaturated carboxylic acids 120 using ortho-substituted iodobenzene 121 as catalyst to transfer the electrophilic bromine from Nbromosuccinimide (NBS) to alkenes via cyclic intermediate iodineACHTUNGRE(III) species 122. All catalytic reactions were completed in short reaction times and cyclic products 123 were isolated in excellent yields (Scheme 48).

Scheme 50. IodineACHTUNGRE(III)-catalyzed bromocarbocyclizations of amides 126 to functionalized oxindoles 128.

Yan et al.[65] investigated an efficient hypervalent iodinecatalyzed sulfonyloxylactonization of alkenoic acids 130 using (diacetoxyiodo)benzene 8 as a recyclable catalyst in combination with mCPBA as an oxidant in the presence of sulfonic acids. The sulfonyloxylactones 131 were isolated in excellent yields (Scheme 51). The reaction was probably proceeding through in situ formation of a Koser type iodineACHTUNGRE(III) species.

Scheme 48. IodineACHTUNGRE(III)-catalyzed bromolactonization of unsaturated carboxylic acids 120. Scheme 51. (diacetoxyiodo)benzene-catalyzed sulfonyloxylactonization of alkenoic acids 130 to sulfonyloxylactones 131.

An iodineACHTUNGRE(III)-catalyzed approach for iodolactonization of unsaturated acids 124 to lactones 125 using iodobenzene as a catalyst in the presence of tetra-n-butylammonium iodide (TBAI) and sodium perborate monohydrate was developed by Tan and Liu (Scheme 49).[63] Alkynoic acids can also be cyclized into the corresponding iodolactones under similar reaction conditions.

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Furthermore, Zhou and He[66] reported an iodineACHTUNGRE(III)-catalyzed phosphoryloxylactonization of functionalized pentenoic acids 132 using iodobenzene as a precatalyst and mCPBA as the terminal oxidant in CF3CH2OH as the solvent. The phosphoryloxylactones 133 were isolated in decent yields (Scheme 52).

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Scheme 55. IodineACHTUNGRE(III)-catalyzed cyclization of d-alkynyl b-ketoesters 141 to cyclopentane products 142.

Scheme 52. IodineACHTUNGRE(III)-catalyzed phosphoryloxylactonization of pentenoic acids 132.

decent yields with excellent diastereoselectivities (Scheme 55). There are two possible pathways for the mechanism of the iodineACHTUNGRE(III)-catalyzed cyclization of 141 to cyclopentanes 142 (Scheme 56). In both pathways the oxidation of iodobenzene to an iodineACHTUNGRE(III) species occurs. This coordinates either to the alkyne functionality of 141 to form 143

New ion-supported (IS) species 136-138 were generated in situ and used as a precatalyst with mCPBA in iodineACHTUNGRE(III)catalyzed cyclizations of N-methoxy-2-arylethanesulfonamides 134 to N-methoxy-3,4-dihydro-2,1-enzothiazine-2,2ioxides 135 in trifluoroethanol (Scheme 53).[67] All IS catalysts 136-138 were able to catalyze these cyclization reactions, however, 136 showed better catalytic efficiency than 137 and 138.

Scheme 53. IodineACHTUNGRE(III)-catalyzed cyclization of 2-arylethanesulfonamides 134 to 3,4-dihydro-2,1-enzothiazine-2,2-ioxides 135.

Ishihara and co-workers[68] achieved the first hypervalent iodine-catalyzed oxylactonization of aryl ketoacids 139 to ketolactones 140 through the in situ generation of Koser type iodineACHTUNGRE(III) species with catalytic amounts of iodobenzene and TsOH·H2O in the presence of mCPBA as a stoichiometric oxidant (Scheme 54). Most of cyclization reactions worked smoothly and ketolactones 140 were obtained in significant yields. Cyclic products were observed only in traces when alkyl ketoacids were used as substrates. Rodriguez and Moran[69] developed an intramolecular cyclization of d-alkynyl b-ketoesters 141 by generating hypervalent iodine species in situ under oxidative reaction conditions. The cyclopentane products 142 that contain adjacent quaternary and tertiary stereocenters were obtained in

Scheme 56. Mechanism for iodineACHTUNGRE(III)-catalyzed cyclization of d-alkynyl b-ketoesters 141 to cyclopentanes 142.

(path A) or forms a ketoester–iodineACHTUNGRE(III) species 144 (path B). Both intermediates 143 or 144 can be converted into the cyclopentane intermediate 145. On oxidative fragmentation, intermediate 145 produced carbocationic intermediate 146 and iodobenzene. The cationic intermediate 146 is trapped by water to form the final product 142. Zhdankin and co-workers[70a] developed a hypervalent iodine-catalyzed synthesis of isoxazolines 149 by the oxidation of aldoximes 147 to nitrile oxides 148, followed by cycloaddition with alkenes. Isoxazolines 149 were obtained in good yields, as aromatic alkenes showed better participation in the cycloaddition with nitrile oxides 148 than aliphatic alkenes. Alkynes could be used for the synthesis of isoxazoles 150 (Scheme 57). Recently, Lupton and co-workers have demonstrated a hypervalent iodine-catalyzed approach for the synthesis of complex polycyclic furans through cascade C O/C C formations. Although, yields were moderate in most of reactions.[70b]

Scheme 54. IodineACHTUNGRE(III)-catalyzed oxylactonization of keto acids 139 to ketolactones 140.

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Scheme 57. IodineACHTUNGRE(III)-catalyzed synthesis of isoxazolines 149 and isoxazoles 150 via formation of nitrile oxides 148.


Scheme 59. IodineACHTUNGRE(III)-catalyzed intermolecular C H amination or hydrazination of arenes 155 to 156.

2.4. C H Amination Reactions A direct approach for C H bond functionalization is an important tool in synthetic organic chemistry. In recent years, few organic chemists have been using hypervalent iodine reagents as catalysts to develop direct C H aminations of arenes. In 2011, Antonchick et al.[71] reported a hypervalent iodine-catalyzed intramolecular C H amination of arenes 151 using 2,2’-diiodo-4,4’,6,6’-tetramethylbiphenyl 87 as a precatalyst to generate a hypervalent iodine species in the presence of peracetic acid. The amination proceeded smoothly and carbazoles 152 were isolated in good yields (Scheme 58). Various electron-withdrawing and electron-donating groups were tolerated during these catalytic C H aminations. Scheme 60. The catalytic cycle for iodineACHTUNGRE(III)-catalyzed intermolecular CH amination or hydrazination of arenes 155 to 156.

157 is formed by oxidation with peracetic acid in the presence of TFA. After a ligand exchange on the iodineACHTUNGRE(III) center to 158, oxidative fragmentation forms the nitrenium ion 159 and the hypervalent species 160. The arene 155 attacks the electron-deficient nitrenium ion to give the amination product 156. Finally, the hypervalent species 160 is transformed into the m-oxo-bridged species 157 and continues the catalytic cycle. Recently, Punniyamurthy and co-workers developed an iodineACHTUNGRE(III)-catalyzed intramolecular C H amination of Nsubstituted amidines 161 by generating an iodineACHTUNGRE(III) species in situ using catalytic amounts of iodobenzene in the presence of mCPBA.[73a] The catalytic C N bond formation reactions progressed smoothly and 1,2-disubstituted benzimidazoles 162 were isolated in good yields (Scheme 61). Both electron-withdrawing and electron-donating functionalities were tolerated. The reaction proceeds through formation of an iodineACHTUNGRE(III) species and a nitrenium ion intermediate similar to the mechanism shown in Scheme 60. In addition, Zhu and co-workers also developed a similar iodineACHTUNGRE(III)-catalyzed C H amination reaction to afford nitrogencontaining heterocycles. Importantly, a more reactive hypervalent iodine species was generated in situ by the oxidation of iodobenzene using peracetic acid as a co-oxidant. The reaction proceeded smoothly and various N-heterocycles were isolated in excellent yields.[73b]

Scheme 58. Hypervalent iodine-catalyzed intramolecular C H amination of arenes 151 to carbazoles 152.

The same research group also reported an iodineACHTUNGRE(III)-catalyzed approach for the introduction of amine or hydrazine functionalities 154 into unfunctionalized arenes 155 by intermolecular C H amination or hydrazination, respectively.[72a] These C H aminations were achieved by using catalytic amounts of iodoarene 87 in the presence of peracetic acid and TFA. The amination reactions proceeded smoothly and new C N bond formation products 156 were obtained in good to excellent yields (Scheme 59). In addition, Kita and co-workers have recently demonstrated that similar 2,2’-diiodobiphenyl catalysts 153 can achieve hypervalent iodine-catalyzed cross-biaryl coupling of anilides with aromatic hydrocarbons. Most of the coupling reactions proceeded smoothly and coupling products were isolated in excellent yields.[72b] The catalytic cycle for the iodineACHTUNGRE(III)-catalyzed intermolecular C H amination of arenes 155 to 156 is shown in Scheme 60. Initially, the hypervalent m-oxo-bridged species

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Scheme 61. IodineACHTUNGRE(III)-catalyzed intramolecular arenes 161 to benzimidazoles 162.

C H

amination


of

Also an oxidative amination of heteroarenes catalyzed by nBu4NI using hydrogen peroxide and tert-butylperoxide as oxidants was reported.[74]

Scheme 63. A general catalytic cycle for iodineACHTUNGRE(III)-catalyzed halogenation of electron-rich arenes 163.

2.5. Halogenation

backs caused by stoichiometric reactions with iodosylarenes.[76b]

In recent years, hypervalent iodine reagents have been employed catalytically for the halogenation of arenes. In 2011, Zhou and He[75] developed a catalytic approach for the monobromination of electron-rich arenes 163 by using iodobenzene as a precatalyst, mCPBA as a stoichiometric oxidant, and LiBr as a source of bromine. All the catalytic reactions were completed in short reaction times and monobrominated arenes 164 were isolated in high yields (Scheme 62). Interestingly, various electron-rich arenes were tolerated but electron-poor arenes could not facilitate the reaction.

2.6. Rearrangements Despite the oxidative behavior of hypervalent iodine reagents, they have the unique ability to react as electrophiles and then behave as excellent leaving groups, thereby making them highly suitable reagents for generating cationic intermediates. These can either directly react with nucleophiles or lead to rearranged products owing to different rearrangement processes.[7] In recent years, hypervalent iodine-catalyzed oxidative rearrangements have been developed successfully. 2.6.1. IodineACHTUNGRE(III)-Catalyzed Rearrangements In 2012, Ochiai and co-workers[77] reported the first hypervalent iodine-catalyzed Hofmann rearrangement of carboxamides 168 to ammonium chloride salts 169 using iodobenzene in the presence of mCPBA and HBF4 (Scheme 64). All

Scheme 62. Koser reagent-catalyzed regioselective monobromination of electron-rich arenes 163.

A related iodineACHTUNGRE(III)-catalyzed approach for the chlorination of electron-rich arenes 163 using a similar catalyst/oxidant combination has been published recently.[76] However, 10 mol % of iodobenzene was not sufficient to catalyze the chlorination, and 40 mol % of the catalyst was required. The catalytic chlorination with LiCl as a chloride source was much slower than the bromination, but chlorinated arenes were isolated in excellent yields. In addition, the trend in electronic effects was quite similar to the catalytic bromination reaction. A general catalytic cycle for the halogenation of electronrich arenes 163 is illustrated in Scheme 63. The catalytic cycle begins with the in situ oxidation of iodobenzene to Kosers reagent 165, which can then undergo a ligand exchange with LiX to iodineACHTUNGRE(III) species 166. An electrophilic substitution with arenes 163 then produces the halogenated products 164 or 167. Recently, Kitamura et al. have developed an iodineACHTUNGRE(III)-catalyzed fluorination of 1,3-dicarbonyl compounds in decent yields by using iodoarene as a precatalyst, mCPBA as a terminal oxidant, and aqueous HF as fluorine source. The catalytic fluorination is advantageous over stoichiometric fluorination because it minimizes the draw-

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Scheme 64. IodineACHTUNGRE(III)-catalyzed Hofmann rearrangement of carboxamides 168 to 169 using mCPBA acid as an oxidant.

the reactions were carried out in aqueous dichloromethane and reaction products 169 were isolated in excellent yields. The retention of stereochemistry of the migrating groups was observed, and this is similar to the classical Hofmann rearrangement. Recently, another iodineACHTUNGRE(III)-catalyzed Hofmann rearrangement of carboxamides 170 to carbamates 171 was reported by Zhdankin and co-workers.[78] By replacing mCPBA with oxone and using a MeOH/HFIP/water solvent combination, products 171 were obtained in excellent yields (Scheme 65).

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Scheme 65. IodineACHTUNGRE(III)-catalyzed Hofmann rearrangement of carboxamides 170 to 171 using oxone as oxidant.

The catalytic cycle for the iodineACHTUNGRE(III)-catalyzed Hofmann rearrangement of amides 168 or 170 is shown in Scheme 66. The iodineACHTUNGRE(III) species 172 reacts with amides 168 or 170 to form another iodineACHTUNGRE(III) species 173. After Hofmann rearrangement to the isocyanate 174 with the generation of iodobenzene, 174 is then transformed into the corresponding ammonium chloride salts 169 or carbamates 171, while iodobenzene re-enters the catalytic cycle.

Scheme 67. Iodine(V)-catalyzed oxidative rearrangement of cyclic tertiary alcohols 175 to unsaturated ketones 176.

Scheme 68. Catalytic cycle for iodine(V)-catalyzed oxidative rearrangement of cyclic tertiary alcohols 175 to unsaturated ketones 176.

Very recently, Purohit et al.[80] developed an efficient hypervalent iodine-catalyzed approach for the oxidative 1,2shift of functionalized terminal alkenes 180 to homobenzylic ketones 181 using oxone as an oxidant. The rearranged products 181 were isolated in reasonable yields (Scheme 69). Cyclic systems could be rearranged into the ring-expanded b-benzocycloalkanones under similar conditions.

Scheme 66. Catalytic cycle for iodineACHTUNGRE(III)-catalyzed Hofmann rearrangement of amides 168 or 170.

2.6.2. Iodine(V)-Catalyzed Rearrangements Furthermore, oxidative rearrangements of tertiary allylic alcohols to a,b-unsaturated ketones have been successfully achieved using iodine(V) reagents. In 2009, Ishihara and coworkers reported the catalytic variant of oxidative rearrangement of tertiary allylic alcohols 175 to enones 176 using sodium 2-iodobenzenesulfonic acid 40 and oxone as an oxidant in the presence of potassium carbonate and tetrabutylammonium hydrogen sulfate.[79] The rearranged products 176 were isolated in good yields (Scheme 67). In addition, acyclic tertiary alcohols were also rearranged into the expected ketones in moderate yields under similar reaction conditions. Scheme 68 shows the catalytic cycle for this rearrangement. The catalytic cycle is initiated by the in situ formation of iodine(V) species 39, which reacts with alcohol 175 to form the alcohol-IBS(V) complex 177, and then undergoes [3,3]-migration to intermediate 178; this then gave the rearranged product 176 and iodineACHTUNGRE(III) species 179. Finally, the iodine(V) reagent 39 is regenerated by oxidation of 179.

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Scheme 69. Hypervalent iodine-catalyzed oxidative 1,2-shift of functionalized terminal olefins 180 to homobenzylic ketones 181.

2.7. Stereoselective Reactions Stereoselective transformations using chiral hypervalent iodine reagents is a comparatively new area in hypervalent iodine chemistry.[81] Several chiral iodoarenes have been used for the in situ generation of chiral hypervalent iodine catalysts to develop asymmetric oxidative functionalizations.

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Kita and co-workers[82] reported an iodineACHTUNGRE(III)-catalyzed enantioselective oxidative spirolactonization of naphthols 182 using spirobiindane-based chiral iodoarene 183 as a precatalyst to generate hypervalent iodineACHTUNGRE(III) species in the presence mCPBA. The spirolactones 184 were isolated in moderate yields with up to 69 % enantiomeric excess (Scheme 70).


iodineACHTUNGRE(III) catalysts.[84b–d] Very recently, Kita and co-workers[85] introduced chiral ortho-functionalized spirobiindanes 188 as precatalyst and spirolactonizations were achieved with up 92 % enantiomeric excess. In 2009, Quideau et al. published the iodineACHTUNGRE(III)-catalyzed enantioselective dearomatization of 2-methylnaphthol 189 to the corresponding epoxides 191 using catalytic amounts of axially chiral binaphthyl iodoarene 190 and an excess amount of mCPBA.[86a] The epoxides 191 were obtained in good yields with up to 29 % enantiomeric excess (Scheme 71). Recently, Volp and Harned have demonstrated the enantioselective synthesis of p-quinols using chiral hypervalent iodine catalysts in decent yields but they could not achieve high selectivities.[86b]

Scheme 70. IodineACHTUNGRE(III)-catalyzed enantioselective spirolactonization of naphthols 182 using iodoarene 183 as precatalyst in the presence of mCPBA.

Other research groups introduced chiral iodoarenes 185– 188 (Figure 5), which were used as precatalysts in enantioselective oxidative spirolactonizations. In 2010, Ishihara and co-workers[83b] used chiral iodoarene 185 as a precatalyst and spirolactonizations were achieved in poor yields with up

Scheme 71. IodineACHTUNGRE(III)-catalyzed enantioselective dearomatization of 2methylnaphthol 189 using axially chiral binaphthyl iodoarene (S)-190 as precatalyst with excess mCPBA.

In 2008, Wirth and co-workers[87] developed the first hypervalent iodine-catalyzed enantioselective a-tosyloxylation of ketones 108 using catalytic amounts of chiral iodoarene 192 and stoichiometric amounts of mCPBA. The prodcuts 2 were obtained in good yields with up 28 % enantiomeric excess (Scheme 72). Also other sulfonic acids were used as nucleophiles under similar reaction conditions.

Figure 5. Chiral iodoarenes 185 to 188 used as precatalysts in iodineACHTUNGRE(III)catalyzed spirolactonization of naphthols 182.

to 32 % enantiomeric excess. Interestingly, a dramatic increase in selectivity was observed by using the C2-symmetric chiral iodoarene 186 and spirolactones were obtained with 92 % enantiomeric excess in excellent yields.[83a] Recently, Ishihara and co-workers[84a] also modified the C2-symmetric chiral iodoarene 186 to another iodoarene 187 and spirolactonizations of naphthol and also phenol derivatives were achieved in excellent yields with more than 99 % enantiomeric excess. In addition, it was also demonstrated that intramolecular hydrogen-bonding interactions and additional achiral alcohols play crucial roles in the selectivity. Fujita and co-workers have achieved hypervalent iodine-catalyzed enantioselective oxylactonization with up to 91 % enantiomeric excess by using a similar type of chiral lactate-based

Chem. Asian J. 2014, 9, 950 – 971

Scheme 72. IodineACHTUNGRE(III)-catalyzed enantioselective a-tosyloxylation of ketones using catalytic amounts of 192 in the presence of mCPBA.

Zhang and co-workers achieved a similar functionalization with slightly higher enantiomeric excess of up to 50 % using spirobiindane-based chiral iodoarene 193, but this approach was associated with poor yields.[88] Recently, Legault and co-workers achieved the a-tosyloxylation in good yields

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Figure 6. Chiral iodoarenes 193 to 195 used as precatalyst in iodineACHTUNGRE(III)catalyzed stereoselective a-functionalizations of ketones 108.

with up to 49 % ee using iodoarenes 194.[89] Furthermore, the efforts of Rodrguez and Moran could provide up to 18 % ee using chiral iodoarene 195 (Figure 6).[90]

Conclusions This Focus Review summarizes various applications of hypervalent iodine reagents in catalysis. Several oxidative transformations such as oxidation of alcohols and phenols, a-functionalizations of carbonyl compounds, cyclizations, C H aminations, and rearrangements have been successfully developed using catalytic amounts of iodoarenes and stoichiometric oxidants. Recently, some asymmetric variants of oxidation of naphthols and a-functionalization of carbonyl compounds have been also developed. A number of products obtained during these catalytic transformations are important synthetic intermediates for the construction of biologically active synthetic and naturally occurring scaffolds.

Acknowledgements The authors are thankful to the European Union for an FP7 IIF Marie Curie grant as financial support and to the School of Chemistry, Cardiff University.

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Flexible stereoselective functionalizations of ketones through umpolung with hypervalent iodine reagents.

Facile oxidative rearrangements using hypervalent iodine reagents.

Structurally Defined Molecular Hypervalent Iodine Catalysts for Intermolecular Enantioselective Reactions.

Gold-catalyzed oxidative reactions of propargylic carbonates involving 1,2-carbonate migration: stereoselective synthesis of functionalized alkenes.

Cyclic Hypervalent Iodine Reagents for Atom-Transfer Reactions: Beyond Trifluoromethylation.

Mild hypervalent iodine mediated oxidative nitration of N-aryl sulfonamides.

Oxidative Heterocycle Formation Using Hypervalent Iodine(III) Reagents.

Enantioselective Oxidative Rearrangements with Chiral Hypervalent Iodine Reagents.

Oxidative reactions of hemoglobin.

Stereoselective synthesis of indolines via organocatalytic thioester enolate addition reactions.

Synthesis of (+)-discodermolide by catalytic stereoselective borylation reactions.

Oxidative C-H amination reactions.

Difluoromethanesulfonyl hypervalent iodonium ylides for electrophilic difluoromethylthiolation reactions under copper catalysis.

Hypervalent iodine-mediated selective oxidative functionalization of (thio)chromones with alkanes.

Syntheses and Functionalizations of Porphyrin Macrocycles.

Hypervalent iodine-mediated oxidative cyclisation of p-hydroxy acetanilides to 1,2-dispirodienones.

Photodermatoses, including phototoxic and photoallergic reactions (internal and external).

Oxidative reactions of hydroxylated chlorpromazine metabolites.

Stereoelectronic Model To Explain Highly Stereoselective Reactions of Seven-Membered-Ring Oxocarbenium-Ion Intermediates.

Metal and carbene organocatalytic relay activation of alkynes for stereoselective reactions.

Stereoselective synthesis of highly functionalized indanes and dibenzocycloheptadienes through complex radical cascade reactions.

borylation reactions.

Stereoselective titanium-mediated aldol reactions of a chiral lactate-derived ethyl ketone with ketones.

Brønsted acid catalyzed asymmetric diels-alder reactions: stereoselective construction of spiro[tetrahydrocarbazole-3,3'-oxindole] framework.