Stronger Brønsted Acids: Recent Progress.

Review pubs.acs.org/CR

Stronger Brønsted Acids: Recent Progress Takahiko Akiyama*,† and Keiji Mori†,‡ †

Department of Chemistry, Faculty of Science, Gakushuin University, 1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588, Japan Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho Koganei, Tokyo 184-8588, Japan

‡

References

1. INTRODUCTION Lewis-acid-catalyzed C−C, C−O, and C−N bond formation reactions have long been acknowledged as important strategies for the construction of organic molecules.1 Lewis acids activate such functional groups as carbonyl, imine, alkene, and alkyne moieties electrophilically, thereby lowering the LUMO level and accelerating the nucleophilic attack on the activated multiple bonds. Although Lewis acids are, in general, sensitive to moisture, water-tolerant Lewis acids such as lanthanide triflates were developed.2 The combination of metal salt and chiral ligand generates a chiral Lewis acid, and enantioselective versions of the chiral Lewis-acid-catalyzed reaction have been developed and extensively studied.3,4 In contrast to Lewis-acid catalysts, Brønsted acids have been employed primarily as catalysts for the formation and cleavage of C−O bonds, such as hydrolysis and formation of esters and acetals. The use of Brønsted acids as catalyst for carbon−carbon bond formation had been underestimated in the 20th century. At the dawn of the 21st century, however, Brønsted acids have emerged as efficient catalysts for a range of carbon−carbon bond formation reactions.5 Brønsted acids activate carbonyl, imine, alkene, alkyne, and hydroxy groups to form oxonium salt, iminium salt, carbocation, and vinylic carbocation, all of which promote the nucleophilic addition, Figure 1.

CONTENTS 1. Introduction 2. Scope of the Current Review 3. Achiral Brønsted Acids 3.1. Reactions with Carbonyl CompoundsNucleophilic Addition 3.2. Reactions with Carbonyl CompoundsCycloaddition Reactions 3.3. Reactions with Alkenes and Alkynes 3.4. Reactions with Alcohols 3.5. C−H Bond Functionalization 3.6. Carbon Acids 4. Chiral Brønsted Acids 4.1. Sulfonic Acid and Derivatives 4.1.1. Preparation of Chiral BINSA Derivatives 4.1.2. Asymmetric Reactions by Chiral BINSA Ammonium Salts 4.1.3. Asymmetric Reactions Catalyzed by Chiral BINSA-Derived Disulfonimide 4.1.4. Asymmetric Reactions Catalyzed by Chiral BINSA-Derived Disulfonimide as Chiral Stronger Brønsted Acid 4.2. Chiral Dicarboxylic Acids 4.2.1. Preparation of Axially Chiral Dicarboxylic Acid 4.2.2. Asymmetric Reactions Catalyzed by Axially Chiral Dicarboxylic Acid 4.3. Lewis (Brønsted)-Acid-Assisted Brønsted Acids 5. Summary and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments © XXXX American Chemical Society

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Figure 1. Activation of various organic molecules by Brønsted acids. Special Issue: 2015 Frontiers in Organic Synthesis Received: January 22, 2015

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DOI: 10.1021/acs.chemrev.5b00041 Chem. Rev. XXXX, XXX, XXX−XXX

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In addition to common Brønsted acids such as HCl, H2SO4, and acetic acid, trifluoromethanesulfonic acid (TfOH) was developed as a super strong Brønsted acid, which is 100 times stronger than sulfuric acid.6 Furthermore, bis(triflyl)imide (Tf2NH), a related nitrogen acid, was also developed,7 and its catalytic activity was studied. In addition to the oxygen acid and the nitrogen acid, carbon acid such as Tf2CH2 was also developed as a super strong Brønsted acid. The pKa values are shown in Figure 2.8

Figure 3. Two activation modes of Brønsted-acid catalysis and hydrogen-bond catalysis.

Figure 2. Achiral Brønsted acids and acidities.

Regarding the chiral version of Brønsted acid,9−12Yamamoto and co-workers developed Lewis-acid-assisted Brønsted-acid catalysts, which were generated in situ by combining Lewis acid and binaphthol derivative in solution.13−15 Akiyama and Terada independently demonstrated the utility of a chiral phosphoric acid derived from BINOL as a chiral Brønsted acid in 2004.16,17 After their reports, a large number of chiral phosphoric-acid-catalyzed reactions were developed.18−27 Furthermore, a range of Brønsted-acid catalysts, such as dicarboxylic acids, disulfonic acids, and disulfonimides, were reported. Furthermore, binaphthols and dicarboxylic acids were also developed. Neutral small organic molecules such as chiral thiourea derivatives,28−32 TADDOL derivatives,33,34 and squaramide derivatives35 were reported to catalyze a range of transformations as hydrogen-bond donors. Because thiourea, squaramide, and TADDOL are neutral compounds, they are preferably classified as a hydrogen-bond catalyst, although they may be classified as a Brønsted acid in a broader sense.36−40 The differentiation between Brønsted-acid catalysis and hydrogen-bond catalysis, however, is not always a simple issue.41 The transition state of the hydrogen-bond catalysis is a hydrogen-bond complex, and that of the Brønsted-acid catalysis is a protonated ion pair in a narrow sense (Figure 3). Theoretical studies of the transition states are required to fully differentiate the two activation modes. Recently, the acidity of the Brønsted acids has been determined by measurement42−44 and by computation,45−48 and the pKa values are shown in parentheses (Figure 4).

Figure 4. Chiral Brønsted-acid derivatives and acidities.

2007,10 the synthetic utility of Brønsted-acid catalysis was still in its infancy. This review covers synthetic reactions catalyzed by relatively stronger Brønsted acids from 2007 to 2014. Most of the reactions in this review are catalyzed by Brønsted acid (H+), but sometimes a Brønsted acid is employed as a precatalyst and transformed into a Lewis acid in situ to catalyze a reaction. Those Brønsted-acid precatalyst systems are also included. For achiral Brønsted acids, we mainly focused on trifluoromethanesulfonic acid (TfOH) and bis(triflyl)imide (Tf2NH) and include carbon acids as well. Furthermore, several interesting synthetic reactions promoted by relatively weaker acids have been included. Regarding chiral Brønsted-acid catalyst, which is one of the most important areas in modern organic synthesis, hydrogenbond catalysts such as thiourea and TADDOL are not included. We also focus on relatively stronger Brønsted acids. In addition, due to the recent publication of an excellent review on phosphoric acid catalysts by Rueping,27 phosphoric-acidcatalyzed reactions are not included in this review.

3. ACHIRAL BRøNSTED ACIDS

2. SCOPE OF THE CURRENT REVIEW In discussing the synthetic topic of “Stronger Brønsted Acid”, we tried to demonstrate the utility of Brønsted acids from the standpoint of organic synthesis. When we published a review entitled “Stronger Brønsted Acids” in Chemical Reviews in

3.1. Reactions with Carbonyl CompoundsNucleophilic Addition

Polyketides constitute one of the most important classes of natural products. and the aldol reaction is the most reliable B


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cyclohexanone gave β,ε-siloxy ketones 11 with high diastereoselectivity. Super silyl enolate 13 derived from acetone also participated in the sequential reaction successfully. For example, the addition of PhMgBr to the ketone 14, generated in situ by the Mukaiyama aldol reaction, resulted in the formation of tertiary carbinol 15 in a good yield and with high diastereoselectivity (Scheme 3).56

method for the construction of β-hydroxy carbonyls and/or 1,3diols, which are common motifs in the polyketides. Of the several aldol reactions that have been extensively studied, the Mukaiyama aldol reaction49−51 is the most efficient. Nonetheless, the Mukaiyama cross-aldol reaction of acetaldehyde silyl enol ethers remains a daunting challenge. Yamamoto and coworkers achieved the Mukaiyama cross-aldol reaction of silyl enolate 6 derived from acetaldehyde or propanal by using a tris(trimethylsilyl)silyl (TTMSS = super silyl) group (Schemes 1).52−54 Interestingly, the use of 2.2 equiv of silyl enolate 7 led to

Scheme 3. Aldol Reaction/Nucleophilic Addition Sequence

Scheme 1. Mukaiyama Cross-Aldol Reaction of Super Silyl Enolate Derived from Acetaldehyde and Propanal Highly stereoselective 1,5-induction was achieved with the super silyl group. The aldol reaction of the lithium enolate of βsuper siloxy methyl ketones 16 with aldehydes in DMF led to the formation of 1,5-syn-selective aldol adducts 17 (Scheme 4). In Scheme 4. 1,5-Stereoinduction of Super Silyl Group

the formation of the cascade Mukaiyama aldol adduct 8 as a single diastereomer when pivalaldehyde was employed in 75% yield. High diastereoselectivity was observed in the reaction with chiral aldehyde 9. Because the Mukaiyama aldol reaction of acetaldehydederived super silyl enolate 7 with pivalaldehyde generates isolable aldehyde 10 using as little as 0.05 mol % of Tf2NH, a sequential, one-pot addition reaction to furnish aldehyde 10 was realized.55 Both acidic and basic conditions are viable for the sequential reaction (Scheme 2). Treatment of the aldehyde 10 with allyl magnesium bromide furnished homoallylic alcohol 12 with high 1,3-induction. Use of the super silyl enolate of

striking contrast, 1,5-anti selectivity was observed in the aldol reaction of the trimethylsilyl enol ethers of methyl ketones 18 with aldehydes in the presence of Tf2NH in toluene.57 Yuan and co-workers developed a Friedel−Crafts-type coupling reaction of indoles 21 with quinoxalin-2-one derivatives 20 in the presence of 10 mol % of TfOH to provide 3-(indol-3yl)quinoxalin-2-one derivatives 22 (Scheme 5).58 Scheme 5. Friedel−Crafts Reaction of Indoles 21 with Quinoxalin-2-one Derivatives 20

Scheme 2. One-Pot Mukaiyama Cross-Aldol Reaction and Nucleophilic Addition Reaction

AcOH promoted the benzylic C−H functionalization of 2methylazaarenes 23 and nucleophilic addition to aldehydes 24 to furnish corresponding adducts 25 in good to high yields (Scheme 6). A six-membered hydrogen-bonding transition state is proposed.59 3-Substituted-3-hydroxy-2-oxindoles are important motifs which were found in alkaloids and natural products bearing a range of biological activities such as anticancer and anti-HIV.60,61 On treating 2-methylpyridine 26 with isatin 27 in the presence of TfOH, functionalization of the C(sp3)−H bond proceeded C


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Scheme 6. Acetic-Acid-Promoted Benzylic C−H Functionalization of 2-Methylazaarenes 23

Scheme 9. Formal [3 + 3] Cycloaddition Reaction

Scheme 10. [2 + 2] Cycloaddition Reaction with Allylsilane smoothly to afford 3-substituted-3-hydroxy-2-oxindoles 28 in modest to high yields (Scheme 7).62 Scheme 7. Synthesis of 3-Substituted-3-hydroxy-2-oxindoles by Brønsted-Acid-Promoted Benzylic C−H Functionalization provide cyclopentane derivatives 38 in moderate to good yields (Scheme 11).73 Scheme 11. [3 + 2] Cycloaddition Reaction with Allylsilane

3.2. Reactions with Carbonyl CompoundsCycloaddition Reactions

Four-membered carbocyclic systems are an important structural unit.63,64 Takasu and co-workers developed a novel method for the preparation of cyclobutane unit 31, which involved the [2 + 2] cycloaddition reaction of silyl enolate 29 with methyl acrylate (30) in the presence of a catalytic amount of Tf2NH (Scheme 8).65,66 Tf2NH acted as a precatalyst, and Tf2NTBS (TBS = SiMe2(t-Bu)) generated in situ actually promoted the reaction as reported by Ishihara and Yamamoto.67

Takasu also reported a Tf2NH-catalyzed domino reaction. Upon treatment of 3-oxymethyl-2-siloxy-1,3-butadienes 40, which are prepared from Baylis−Hillman adducts, and α,βunsaturated ketones 39 with Tf2NH, [4 + 2] cycloaddition and elimination took place smoothly to furnish substituted 2alkylidenecyclohexanones 41 in good yields (Scheme 12).74 Scheme 12. [4 + 2] Cycloaddition Reaction with Siloxy Butadiene

Scheme 8. [2 + 2] Cycloaddition Reaction of Silyl Enolate 29 with Methyl Acrylate (30)

Tf2NH also catalyzed the inverse electron-demand heteroDiels−Alder reaction (Povarov reaction) of N-benzylidene aniline 42 with electron-rich alkene 43 to afford a mixture of 2,4-disubstituted tetrahydroquinoline 44 and 2,4-disubstituted quinoline 45 in good yields.75 Use of allylsilane bearing a bulky silyl group is required to attain high yields (Scheme 13). Use of α,α-dimethylallylsilane 46 bearing a TBS moiety changed the reaction course to afford 3-silylpyrrolidines 47 in moderate to high yields.76 Pyrrolidines 47 are formal [3 + 2] cycloaddition products formed by the 1,2-silyl migration of the βsilyl carbocation intermediate (Scheme 14). The aza Diels−Alder reaction of 2-siloxydienes 48 with aldimines was catalyzed by Tf2NH to furnish substituted piperidin-4-ones 49 in favor of the trans isomer (Scheme 15).77,78 Takemoto and Takasu reported a Brønsted-acid-catalyzed synthesis of α,β-unsaturated amidines that involved the [2 + 2]

They also reported a formal [3 + 3] cycloaddition of silyl enol ethers 32, which involved the domino Michael addition−Claisen condensation accompanied by the isomerization of silyl enol ether (Scheme 9). The desired reaction occurred to afford tricyclic skeleton 33 in moderate chemical yield (45%), which is the same as the core skeleton of clovane.68 Allylsilane bearing a bulky silyl moiety also participated in the formation of cyclobutane in the presence of Tf2NH.69−72 Allylsilanes 34 bearing a triisopropylsilyl or a t-BuMe2Si moiety efficiently underwent the [2 + 2] cycloaddition to furnish corresponding cyclobutane 35 in good yield accompanied by the formation of cyclopentane 36 (Scheme 10). The [3 + 2] cycloaddition reaction of trans-ethyl 2butoxycyclopropanecarboxylate (37) with silyl enolate 29 also proceeded smoothly under the influence of 1 mol % Tf2NH to D


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Scheme 13. Povarov Reaction Catalyzed by Tf2NH

Scheme 14. Formal [3 + 2] Cycloaddition Reaction with α,αDimethylallylsilane

Scheme 17. Formal Total Synthesis of Tetracyclic Steroid Core of Rhodexin A by Brønsted-Acid-Catalyzed InverseElectron-Demand Diels−Alder Reaction

Scheme 15. Aza Diels−Alder Reaction of 2-Siloxydienes 48 with Aldimines mol % of Tf2NH in dichloroethane under microwave irradiation conditions (Scheme 18).83 Scheme 18. Nazarov Reaction of Pyrrole Derivatives 56

cycloaddition of ynamides 50 to aldimines and the subsequent 4π electrocyclic ring-opening reactions of the 2-azetines 51 (Scheme 16). The torquoselectivity of the 4π electrocyclic ring

Lin and Shi reported the [3 + 2] cycloaddition reaction of vinylidenecyclopropanes 57 with electron-deficient alkenes 58, such as methyl vinyl ketone, and acrylaldehyde by means of 4 mol % of Tf2NH to give the corresponding functionalized cyclopentanes 59 in good to high yields (Scheme 19).84 The present reaction proceeded by the 1,4-addition of allene to enone followed by the ring opening of cyclopropane by the attack of the generated enol.

Scheme 16. Brønsted-Acid-Catalyzed Synthesis of α,βUnsaturated Amidines 52 by the [2 + 2] Cycloaddition of Ynamides 50 to Aldimines

Scheme 19. [3 + 2] Cycloaddition Reaction of Vinylidenecyclopropanes 57 with Electron-Deficient Alkenes 58

opening was controlled by the Brønsted acid: whereas Tf2NH afforded the anti isomer of α,β-unsaturated amidines 52 highly stereoselectively, CSA furnished syn isomer 52 preferentially.79 DFT calculation was carried out to rationalize the selectivity. For the formal total synthesis of the tetracyclic steroid core of rhodexin A, Jung and co-workers employed the inverse-electrondemand Diels−Alder reaction of acyldiene 53 with silyl enolate 54 in the presence of a catalytic amount of Tf2NH to furnish the cycloadduct 55 with the required four contiguous stereocenters in one step (Scheme 17).80 The Nazarov reaction is a versatile process for the preparation of cyclopentanones.81,82 Akiyama and co-workers reported the Nazarov reaction of pyrrole derivatives 56 in the presence of 30

Shi and co-workers developed the acid-promoted SN2-type ring opening reaction of N-(aziridin-2-ylethylidene)hydrazine 60 to provide 2-pyrazoline 61 and α,β-diamino ketones 62 (Scheme 20).85 Interestingly, whereas In(OTf)3 provided 1H-pyrazol-1ium salt intermediate 63 to give 2-pyrazoline 61 by the intramolecular nucleophilic attack of the nitrogen atom of the hydrazone moiety, TfOH promoted the formation of 2,3dihydroazetium salt intermediate 64 to furnish diamino ketones 62 selectively. Thomson reported an elegant [3,3]-sigmatropic rearrangement induced traceless bond construction under Tf2NH catalysis. The reaction of aldehydes with N-allyl hydrazones 65 E


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Scheme 20. SN2-Type Ring Opening of N-(Aziridin-2ylethylidene)hydrazine 60

Scheme 22. [3 + 2] Cycloaddition Reaction of Cyclopropanes with Nitriles

Yamamoto’s group reported the TfOH-catalyzed cascade cyclization of enynes 76. Tricycles 77 were obtained in good to high yields (Scheme 24). The cyclization was successfully extended to acyclic 1,7-enynes 78 by use of Tf2NH in place of TfOH (Scheme 25).95,96 This reaction is proposed to proceeds by the generation of vinylic carbocation intermediate 80 followed by C−H bond activation, although the 1,5-hydride shift in 80 followed by an addition/elimination pathway cannot be ruled out. Yamamoto’s group also reported the intramolecular carbocyclization of tethered alkynyl ketone 81 by means of TfOH in methanol to furnish five-membered cyclic enones 82 (Scheme 26).97 The [2 + 2] cycloaddition reaction pathway is proposed. Although the same authors previously reported similar alkyne− carbonyl metathesis of carbon-tethered alkynyl ketones to form highly substituted six-membered cyclic enones by means of an Au(III) catalyst,98,99 the Au(III) catalyst was not effective for the formation of the corresponding five-membered cyclic enones 82, and TfOH proved to be the catalyst of choice. TfOH also catalyzed the cascade cyclization of arenyl 1,7enynes 83 by way of alkyne−cation cyclization followed by Friedel−Crafts reaction to furnish fused polycycles 84 in good to high yields (Scheme 27).100 Tf2NH and Au(I) also catalyzed the reaction albeit with lower efficiency.101−103 Zhu and co-workers found an efficient method for the preparation of 4-alkyl-2(1H)-quinazolinones 86, which involved the cyclization of 1-(2-alkynylphenyl)ureas 85 in dichloroethane in the presence of 1.5 equiv of TfOH (Scheme 28).104 Ye and Yu reported the TfOH-catalyzed tandem cyclopropane ring-enlargement/C−C bond formation/etherification sequence. The key intermediate was the four-membered ring carbocation 90 (Scheme 29). Addition of 88, reactivation of exomethylene 91, and internal etherification afforded cyclobutanefused dihydrofuranes 89 in moderate yields.105 They also investigated the catalytic activities of other catalysts and found that CF3CO2H and CSA were not effective, whereas Au(I) complex and Tf2NH promoted the reaction with lower efficiency.

resulted in the formation of various substituted alkenes 67 via the [3,3]-sigmatropic rearrangement of 66 followed by the loss of N2, CO2, and 2-methylpropene (Scheme 21). The transformation offers a unique means for constructing a σ-bond between two unfunctionalized sp3 carbons while simultaneously generating a stereodefined alkene.86 The [3 + 2] cycloaddition reaction of cyclopropanes with nitriles is a useful method for the preparation of 1-pyrrolines. Although Lewis-acid-catalyzed reactions have been reported,87,88 Wang and co-workers developed a Brønsted-acid-catalyzed formal [3 + 2] cycloaddition reaction of cyclopropane 1,1diester 68 with benzonitriles.89 Enantiopure substrate 70 (99% ee) underwent the [3 + 2] cycloaddition to furnish adduct 71 with slight racemization (73% ee) (Scheme 22). This result suggests that the present reaction proceeded by a SN2 mechanism in which a Ritter nitrilium intermediate 72 was involved. Zhao and co-workers reported a one-step synthesis of tetrahydro-5H-indolo[3,2-c]quinolines 75 from benzyl azides 73 and indoles 74 by a formal [4 + 2] cycloaddition reaction in the presence of 1.2 equiv of TfOH (Scheme 23).90 This reaction is proposed to proceed by iminium ion formation through the protonation of azide followed by the Friedel−Crafts addition of indole and the subsequent cyclization. 3.3. Reactions with Alkenes and Alkynes

In order to activate alkynes electrophilically, coinage metal salts (CuLn, AgLn, AuLn) and Pt salts have been extensively employed due to the strong coordination ability of the π-electrophilic Lewis acid to the carbon−carbon multiple bonds.91 In contrast, scattered examples of the activation with Brønsted acids have been reported.92−94 Recently, intriguing Brønsted-acid-catalyzed transformations that include the activation of carbon−carbon multiple bonds have been reported.

Scheme 21. Traceless Bond Construction by [3,3]-Sigmatropic Rearrangement

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Scheme 23. One-Step Synthesis of Tetrahydro-5H-indolo[3,2-c]quinolines 75 by Formal [4 + 2] Cycloaddition Reaction

Scheme 24. TfOH-Catalyzed Cascade Cyclization of Enynes 76

Scheme 26. Intramolecular Carbocyclization of Tethered Alkynyl Ketone 81 by TfOH

Takasu and co-workers developed an arene−ynamide cyclization reaction leading to 3H-pyrrolo[2,3-c]quinolines 93 in the presence of Tf2NH (Scheme 30).106 This reaction is considered to proceed by the generation of a keteniminium intermediate by a Brønsted acid followed by an electrophilic aromatic substitution reaction to furnish arene-fused quinolines. It was found that carboxylic acid catalyzed the hydroboration of alkynes 94 with pinacolborane (95) (Scheme 31).107 This reaction exhibited broad functional group tolerance to furnish the corresponding alkenyl diboronates 96 in good to high yields with excellent regio- and stereoselectivities. Although the role of the carboxylic acid is to be elucidated, this reaction offers a synthetically useful procedure for hydroboration.

Scheme 27. Cyclization of 1,7-Enynes by Alkyne−Cation Cyclization and Friedel−Crafts Reaction Sequence

Scheme 28. Intramolecular Cyclization of 1-(2Alkynylphenyl)ureas 85

3.4. Reactions with Alcohols

Sanz and co-workers reported the direct alkylation of 1,3dicarbonyl compounds with benzyl alcohol 97, which was efficiently catalyzed by TfOH. The carbocation derived from alcohol 97 reacted with 1,3-dicarbonyl compounds smoothly to afford benzylated adducts 98 in good chemical yields with low catalyst loading (Scheme 32).108 Scheme 25. Tf2NH-Catalyzed Cascade Cyclization of 1,7-Enynes

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Scheme 29. Tandem Cyclopropane Ring Enlargement/C−C Bond Formation/Etherification Sequence

Scheme 31. Carboxylic-Acid-Catalyzed Hydroboration of Alkynes with Pinacolborane

Scheme 32. Direct Alkylation of 1,3-Dicarbonyl Compounds with Benzyl Alcohol

Scheme 33. Formation of Spirocycles 100 Starting from Alkynyl Tertiary Alcohols 99

Yamamoto and co-workers developed a procedure for the formation of spirocycles 100 from alkynyl tertiary alcohols 99 by means of 10 mol % of TfOH (Scheme 33).109 The reaction was characterized by the attack of the alkynyl moiety on the tertiary carbocation intermediate to generate vinylic carbocation intermediate 101. Najera and co-workers compared the catalytic activity of FeCl3·6H2O and TfOH in the allylic amination reaction. TfOH performed better than FeCl3·6H2O, requiring lower catalyst loading and milder reaction conditions (Scheme 34).110 They also compared the reactivity in the reaction with carbon nucleophiles such as allylsilane, anisole, indole, and malonate. Both catalysts exhibited similar catalytic activity, although TfOH proved to be slightly superior. Jiao and co-workers reported a direct C(sp)−C(sp3) coupling reaction. Although the combination of Fe(OTf)3 and TfOH afforded the optimal reaction conditions, TfOH alone also promoted the reaction (Scheme 35).111 Chan developed a one-pot, two-step method for the preparation of 3-halofurans 104 which involved the TfOHcatalyzed hydroxylation/halocyclization of cyclopropylmethanols 102.112 Ring opening of the cyclopropylmethanol 102 led to the selective formation of Z-alkene 103 under kinetically controlled conditions (Scheme 36). Alkene 103 underwent halocyclization with NIS to furnish tetrahydrofuran derivatives stereoselectively. Chan also reported an efficient method for the preparation of tri- and tetrasubstituted furans 106 and 107, which involved the cycloisomerizaton of but-2-yne-1,4-diols 105 by means of pTsOH·H2O (Scheme 37). Whereas the propargyl 1,4-diols 105 selectively underwent tandem alkylation/cycloisomerizaton with 1,4-dicarbonyl compounds to give tetrasubstituted furan 107, upon heating to 80 °C, preferential acid-catalyzed dehydrative rearrangement of the unsaturated alcohol took place to furnish 2,3,5-trisubstituted furans 106.113

Scheme 34. Acid-Catalyzed Allylic Amination Reaction

Scheme 35. Acid-Catalyzed sp−sp3 Coupling Reaction

Liu and co-workers developed an efficient method for the preparation of substituted arylanthracenes and heteroacenes, which involved a TfOH-catalyzed intramolecular Friedel−Crafts reaction and a subsequent aromatization reaction (Scheme 38).114 It is noted that the gray colored part of the product originated from the Ar group in the substrate. 3.5. C−H Bond Functionalization

C−H bond functionalization has become a rapidly growing area because the prefunctionalization step to form carbon−halogen

Scheme 30. Synthesis of 3H-Pyrrolo[2,3-c]quinolines 93 by Arene−Ynamide Cyclization Reaction

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Scheme 36. Synthesis of 3-Halofurans 104 by TfOH-Catalyzed Hydroxylation/Halocyclization Sequence

Scheme 37. Synthesis of Multisubstituted Furans by AcidCatalyzed Cycloisomerizaton

Scheme 40. Synthesis of Quinazoline Derivatives 112 by Brønsted-Acid-Catalyzed C−H Bond Functionalization

Scheme 38. Synthesis of Arylanthracenes by Intramolecular Friedel−Crafts Reactions

enantioselective version of the internal redox reaction leading to tetrahydroquinolines has been also reported.128−140 On treatment of 2-aminobenzaldehyde 113 with indole in the presence of diphenyl phosphate (DPP) in toluene at 150 °C, a cascade reaction that included condensation/[1,5]-H shift/ cyclization proceeded to furnish polycyclic azepinoindoles 114 in moderate yields (Scheme 41).141 Akiyama and co-workers described an efficient method for the preparation of 3-aryl-1-trifluoromethyltetrahydroisoquinolines, which involved a benzylic [1,5]-H shift-mediated C−H bond functionalization. The [1,5]-H shift of the benzylic C(sp3)−H bond of trifluoromethyl ketimine 115 derived from p-anisidine proceeded smoothly to stereoselectively give the cis isomer of 1trifluoromethyl-3-aryltetrahydroisoquinolines 116. In contrast, use of the corresponding N-H ketimine 117 furnished Nunprotected 3-aryl-1-trifluoromethyltetrahydroisoquinolines 118 in favor of the trans isomer (Scheme 42).142 Tunge and co-workers took advantage of the reducing power of 3-pyrroline for the formation of N-alkylpyrroles by intermolecular redox amination (Scheme 43).143 On treatment

bond can be sidestepped. Although transition-metal-catalyzed C−H bond functionalization has been extensively studied,115−118 thermal or Lewis-acid-catalyzed C−H bond functionalization, which involves a [1,5]-H shift followed by a cyclization process, has also attracted much attention because no external oxidant is required.119−124 Bai and co-workers reported an internal redox reaction leading to 7,8,9-trisubstituted dihydropurine derivatives 109 from 5-amino-4-(N,N-disubstituted)aminopyrimidines 108 and aromatic aldehydes, promoted by an excess amount of trifluoroacetic acid (Scheme 39).125 A Brønsted-acid-catalyzed version of the internal redox reaction leading to quinazoline derivatives 112 was reported by Scheme 39. Synthesis of Dihydropurine Derivatives by AcidPromoted C−H Bond Functionalization

Scheme 41. Synthesis of Azepinoindole Derivatives 114 by Condensation/[1,5]-H Shift/Cyclization Sequence

Seidel’s group126 and Akiyama’s group in 2009.127 Treatment of 2-dialkylaminobenzaldehyde 110 with amine gave iminium intermediate 111, which underwent a [1,5]-H shift and a subsequent cyclization to result in the formation of quinazolines in good yields (Scheme 40). A chiral phosphoric-acid-catalyzed I


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Scheme 42. Diastereodivergent Synthesis of 3-Aryl-1trifluoromethyltetrahydroisoquinolines

Scheme 45. Intermolecular Redox Amination of Indolines with Aldehydes

Scheme 43. Formation of N-Alkylpyrroles by Intermolecular Redox Amination alkylated indoles 124 were generated by the intramolecular [1,3]-hydride transfer reaction when a simple benzaldehyde was used as the starting material, the intramolecular hydride transfer was suppressed by the intramolecular hydrogen bond, thereby promoting the intermolecular hydride shift to afford N-alkylated indolines 126. Zu and co-workers reported that the [3 + 2] cycloaddition/ skeletal rearrangement/redox isomerization pathway is efficient for the preparation of 3-allylpyrroles 127 (Scheme 46).146 Although the exact reaction mechanism has to be elucidated, the authors proposed to proceed by [3 + 2] cycloaddition and subsequent elimination, iminium formation, and [1,5]-hydride shift redox isomerization. Chen and Seidel developed a redox Mannich reaction that used pyrrolidine (128) or tetrahydroisoquinolines (129) and by means of a substoichiometric amount of benzoic acid (Scheme 47).147 The reaction of cyclic secondary amine 130 with salicylaldehyde 125 or related ketones in the presence of 1 equiv of acetic acid proceeded to furnish benzo[e][1,3]oxazine derivatives 131 (Scheme 48).148 According to the DFT calculation of the uncatalyzed reaction, the reaction is proposed to proceed by way of hemiaminal 132 and then zwitterionic intermediate 133 followed by proton transfer to generate azomethine ylide intermediate 134.149 It is proposed that acetic acid acts as a proton shuttle within the transition states TS1−3, thereby lowering the activation energy for each step. Seidel and co-workers also reported that secondary amines 130 react with thiosalicylaldehyde 135 in the presence of 10 mol % acetic acid to provide ring-fused N,S-acetals 136 in good to high yields (Scheme 49).150 Das and Seidel reported C−P bond formation at the α position of nitrogen by a three-component reaction of pyrrolidine, aldehyde, and diphenylphosphine oxide 137 in the presence of 20 mol % benzoic acid (Scheme 50).151 This redox-neutral reaction affords an efficient route to α-amino phosphine oxides 138, which are not readily available by the classic Kabachnik− Fields reactions.152,153 Seidel and co-workers developed an intramolecular [3 + 2] cycloaddition reaction of azomethine ylides derived from tetrahydroisoquinoline (129) with benzaldehyde 139 bearing a pendant electrophile at the ortho position by redox-neutral C−H functionalization (Scheme 51).154

of 3-pyrroline 119 with aldehyde or ketone in the presence of 10 mol % benzoic acid in toluene at 110 °C, redox isomerization of iminium salt intermediate 121 proceeded to furnish N-allylated pyrroles 120 in good to high yields. Yu and co-workers reinvestigated Tunge’s work by DFT calculation and found that the one-step direct 1,3-hydrogen shift process proposed by Tunge is not viable due to the very high activation energy, even though it has been proposed in some redox isomerization reactions. Instead, they proposed the formation of acetic-acid-assisted azomethine ylide intermediate 122 (Scheme 44).144 Scheme 44. DFT Calculation of Tunge’s Work

Pan and co-workers reported a benzoic-acid-catalyzed intermolecular redox amination of indolines 123 with aldehydes, which involved an intramolecular hydride transfer reaction leading to N-alkylated indoles (Scheme 45).145 Whereas conventional aldehydes gave N-alkylated indoles 124, use of salicylaldehydes 125 changed the reaction course to result in the exclusive formation of N-alkylated indolines 126. Although NJ


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Scheme 46. Synthesis of 3-Allylpyrroles by [3 + 2] Cycloaddition/Skeletal Rearrangement/Redox Isomerization

Scheme 47. Synthesis of 3-Allylpyrroles by the [3 + 2] Cycloaddition/skeletal Rearrangement/Redox Isomerization

Scheme 48. Synthesis of Benzo[e][1,3]oxazine Derivatives 131 from Cyclic Secondary Amine 130 and Salicylaldehyde 125

extensively studied.155−158 Tayama and co-workers found that the aromatic substitution of N,N-dialkylanilines 141 with αdiazoesters 142 proceeded in the presence of a catalytic amount of Cu(II) salt, and the addition of TfOH as a cocatalyst enhanced the catalytic activity (Scheme 52). They also found that TfOH alone could catalyze the C−H substitution albeit at a lower efficiency than the combination of Cu(II) and TfOH.159 Hu’s group reported that the TfOH-catalyzed Friedel−Crafts alkylation of electron-rich alkene 145 with 3-diazooxindoles 144 furnished 3-aryloxindoles 146 in good to high yields (Scheme 53).160 A deuterium-labeling study elucidated that this aromatic

Scheme 49. Synthesis of Ring-Fused N,S-Acetals 136 from Secondary Amines 130 and Thiosalicylaldehyde 135

The transition-metal-catalyzed C−H insertion of α-diazo carbonyl compounds in the presence of arene has been K


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Scheme 50. C−P Bond Formation at the α Position of Nitrogen by the Three-Component Reaction of Pyrrolidine, Aldehyde, and Diphenylphosphine Oxide

Scheme 52. Acid-Catalyzed C−H Insertion of α-Diazo Carbonyl Compounds

C−H functionalization proceeded through the acid-catalyzed protonation of the diazonium ion followed by the Friedel−Crafts alkylation of arenes with simultaneous loss of nitrogen.

Scheme 53. Acid-Catalyzed Friedel−Crafts Alkylation of Electron-Rich Alkene with 3-Diazooxindoles

3.6. Carbon Acids

In contrast to oxygen acids such as trifluoroacetic acid and trifluoromethanesulfonic acid and nitrogen acids such as triflic imide, carbon acids have not been extensively studied in synthetic organic chemistry. Ishihara and Yamamoto’s group designed and synthesized pentafluorophenylbis(triflyl)methane as a strong carbon Brønsted acid (Figure 5).161−165 They later designed and synthesized a chiral Brønsted acid bearing a bis(triflyl)methyl group and reported a Mannich-type reaction.166 The introduction of two Tf groups on the carbon atom significantly increases the acidity. On the basis of that idea, Taguchi, Yanai, and co-workers focused on 1,1,3,3-tetrakis(triflyl)propane (147, Tf2CHCH2CHTf2) (Figure 6),167 which is a strong carbon acid bearing a CHTf2 moiety. The preparation of 147 is straightforward: treatment of readily available bistriflylmethane 148 with formaldehyde 149 under reflux conditions provided 147 in good yield.168−170 Although 147 is a colorless crystalline solid that is stable in air without conceivable decomposition, its stability in organic solution is low. They therefore employed 147 as the precatalyst for a Lewis-acid-catalyzed reaction. It is known that the Brønsted acidity follows the order TfOH > Tf2NH > Tf2CHC6F5 and that the acidity of the silylated analogue is reversed as Me3Si− C(C6F5)Tf2 > Me3Si−NTf2 > Me3Si−OTf.164 It was found that 147 exhibited higher catalytic activity as a precatalyst than Tf2CHC6F5, TfOH, and Tf2NH in the vinylogous Mukaiyama− Michael reaction of siloxyfuran 150 with α,β-unsaturated ketones 151. As little as 0.05 mol % of the catalyst was required to promote the Mukaiyama−Michael reaction to furnish adducts 152 in good to high yields (Scheme 54). It is noted that even β,βdisubstituted enones 151 participated efficiently to afford adducts 152 with quaternary carbon center in good yields.169 This protocol was successfully applied to the total synthesis of (±)-Merrilactone A by Zhai’s group (Scheme 55).171 Use of other Lewis acids such as TiCl4, BF3·OEt2, and SnCl4 resulted in diminished yields (48−57%). Tf2CHCH2CHTf2 (147) proved to be one of the most effective Brønsted acid precatalyst for the vinylogous Mukaiyama

Figure 5. Strong Brønsted-acid derivatives.

Figure 6. Preparation of Tf2CHCH2CHTf2.

Scheme 54. Extraordinarily High Catalytic Activity of 147 Compared with Other Strong Brønsted Acids

aldol reaction of sterically hindered ketone 153.172 Whereas 40 mol % of TiCl4 was requisite for completion of the vinylogous Mukaiyama aldol reaction, as low as 0.5 mol % of 147 sufficed.

Scheme 51. Intramolecular [3 + 2] Cycloaddition Reaction of Azomethine Ylide Derived from Tetrahydroisoquinoline and Benzaldehyde

L


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Triple carbon acid 163 bearing bis(triflyl)methyl groups in phloroglucinol can be employed as a simple Brønsted-acid catalyst and promoted ester and acetal formation (Scheme 59).

Scheme 55. Total Synthesis of (±)-Merrilactone A by Vinylogous Mukaiyama−Michael Reaction Catalyzed by 147

Scheme 58. Regioselective Introduction of Tf2CH2 Moiety into Organic Compounds

The carbon acid is highly effective for the Mukaiyama aldol reaction of acyclic ketone with dimethylketene silyl acetal. Although even 1 equiv of TiCl4 did not promote the addition reaction of hindered undecan-6-one (155) with the ketene silyl acetal 156, 1.0 mol % of the carbon acid efficiently promoted the Mukaiyama aldol reaction to furnish the adduct 157 in 83% yield (Scheme 56).

Scheme 59. Catalytic Performance of 163

Scheme 56. Mukaiyama−Aldol Reaction Catalyzed by 147

It also promoted the addition of ketene silyl acetal 165 to lactone 164 to generate (Z)-alkenes 166 stereoselectively by the Mukaiyama aldol reaction and subsequent elimination.177,178 Interestingly, use of zwitterion 162 afforded aldol product 168 in good yield (Scheme 60).

Tf2CHCH2CHTf2 promoted the highly chemoselective 1,4addition of silicon dienolate 158 to α,β-unsaturated aldehyde 159 (Scheme 57).173 This chemoselectivity is in contrast to the

Scheme 60. Mukaiyama Aldol Reaction of Lactone with Ketene Silyl Acetal

Scheme 57. 1,4-Addition Reaction of Silyl Enolate Catalyzed by 147

BF3·OEt2-catalyzed reaction that furnished a 1,2-adduct exclusively.174 The high 1,4-selectivity is rationalized by the suppression of the 1,2-addition by the steric bulkiness of the methide anion [Tf2CR]−.175 In order to introduce the Tf2CH2 moiety regioselectively into organic compounds, Taguchi and co-workers took advantage of the instability of Tf2CHCH2CHTf2 in solution: the propensity of Tf2CH2 elimination. Treatment of 147 with a nucleophile at a higher temperature resulted in the elimination of Tf2CH2 to generate vinyl sulfone 161, and subsequent nucleophilic attack led to the formation of a range of novel carbon acids (Scheme 58).176

A three-component synthesis starting from Tf2CH2, aldehyde, and cyclohexadiene proceeded by the self-promotion of Tf2CH2 to furnish gem-bis(triflyl)cyclohexenes 169 in good yields (Scheme 61). Further treatment led to the formation of triflylarenes 170 in high yields.179 In the present reaction, Tf2CH2 played the role of Brønsted-acid promoter for the condensation with aldehyde to furnish Tf2CCHR1 as well as a reaction component. M


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Scheme 61. Diels−Alder Reaction of Tf2CH2, Aldehyde, and Diene

Scheme 62. Preparation of Enantiopure BINSA Derivative 5a

reaction of aldehyde.181 Subsequently, Hatano and Ishihara’s group reported a scalable procedure for the preparation of a chiral BINSA derivative starting from chiral-BINOL in 2008.182 The key to the synthesis was the introduction of sulfur groups to the binaphthyl backbone by the Newmann−Kwart rearrangement of O-thiocarbamoyl compounds (R)-171 by microwave irradiation. Reduction of the resulting S-thiocarbamoyl compound 172 followed by oxidation with molecular O2 afforded disulfonic acid 5a on a greater than 10 g scale without loss of optical purity (Scheme 62). In the same year, an important patent for the preparation of 5a was reported by Sumitomo Chemical.183 They developed an alternative oxychlorination of 173 by treatment with Nchlorosuccinimide (NCS) (Scheme 63). Although NCS is more expensive than O2, this reaction protocol is very useful because the reaction proceeds under mild conditions. In 2009, List and co-workers reported a method for the preparation of disulfonimides 4c from (R)-BINOL.184,185 Giernoth and co-workers also prepared chiral disulfonimide 4a.186 For the effective construction of a chiral environment, modification of the substituents at the 3,3′-positions is important as observed in BINOL derivatives. One drawback of the BINSA derivatives is the difficulty of introducing substituents at the 3,3′positions. In 2010, Lee and co-workers reported a practical

4. CHIRAL BRøNSTED ACIDS 4.1. Sulfonic Acid and Derivatives

The activity of a catalyst is considered to be related to its acidity. In this regard, 1,1′-binaphthyl-2−2′-disulfonic acid (BINSA, 5 in Scheme 63. Modified Procedure for the Preparation of 5a

Figure 4) is extremely attractive as a stronger chiral Brønstedacid catalyst. Nevertheless, the application of 5 to asymmetric reactions had been overlooked until quite recently. 4.1.1. Preparation of Chiral BINSA Derivatives. The first example of the preparation of BINSA, albeit in racemic form, from potassium 1-iodonaphthalene-2-sulfonate by Ullmann coupling was reported by Barber and Smiles in 1928.180 It was not until 80 years later that a chiral BINSA derivative was synthesized and employed as a Brønsted-acid catalyst. List’s group prepared chiral BINSA and applied for the allylation Scheme 64. Introduction of 3,3′-Substituents to 4

N


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Scheme 65. Preparation of 3,3′-Diphenyl-Substituted BINSA Derivative 5b

Scheme 66. Asymmetric Mannich Reaction Catalyzed by BINSA Ammonium Salts

Scheme 67. Asymmetric Mannich Reaction with 3-Acetoacetyl-2-oxazolidinone (184)

coupling reaction afforded desired binaphthyl disulfonimides 4 in acceptable chemical yields (over 70%) (Scheme 64). An effective method for cleavage of the N−S bond is required to produce BINSA derivatives from 3,3′-substituted binaphthyl

synthetic route to 3,3′-substituted binaphthyl disulfonimides based on the late-stage ortho-lithiation.187 Introduction of a halogen (Br or I) followed by the Suzuki−Miyaura crossO


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Scheme 68. Enantioselective Aza-Friedel−Crafts Reaction of Aldimines with Pyrroles

Scheme 70. Reaction with Pivalamide (189a)

Scheme 71. Asymmetric Mukaiyama Aldol Reaction Catalyzed by BINSA-Derived Disulfonimides

disulfonimides. In 2013, Ishihara and co-workers elegantly accomplished this task.188 The most straightforward method, cleavage of the N−S bond, was troublesome, that is, simple treatment with an acid or a base was ineffective, giving mono sulfonamides. Finally, they employed a stepwise route that consisted of the simultaneous transformation of sulfonamide and sulfonate moieties by a reduction and oxidation sequence. The preparation of 3,3′-Ph2-substituted BINSA 5b is shown as a representative example. N-Me disulfonimide 176 was used as the common intermediate. A suitable aryl moiety (phenyl) was then introduced at the 3,3′-positions of the binaphthyl skeleton by Suzuki−Miyaura coupling, and this was followed by cleavage of the first N−S bond of disulfonimide 177 with the use of NaOH in MeOH. Protection of the SO3Na moiety was carried out with Et3O·BF4, followed by protection of the SO2NHMe moiety with Me3O·BF4 to furnish 178. Because both SO2NMe2 and SO3Et moieties might be reduced and oxidized, 178 was hydrolyzed prior to the reduction/oxidation step. The reduction/oxidation step afforded 179. Subsequent protonation by ion exchange provided desired 5b without loss of optical purity and with >99% ee (Scheme 65). 4.1.2. Asymmetric Reactions by Chiral BINSA Ammonium Salts. The first application of BINSA to the asymmetric reaction was reported by List and co-workers in the threecomponent Hosomi−Sakurai reaction.181 Although the enantioselectivity was low (5% ee), this result strongly implied the high synthetic potential of the BINSA derivatives. Hatano and Ishihara’s group elegantly used the BINSA derivatives in the highly enantioselective direct Mannich reaction (Scheme 66).182 In the case of an organocatalyst derived from a BINOL motif, tuning of the 3,3′-substituents is critical to achieving high enantioselectivity and reactivity. The salient feature of their approach is that the amine moiety plays a critical role in the stereoselection instead of the 3,3′-substituents. In contrast to the tuning of the 3,3′-substituents of the BINOL motif, the formation of the catalyst by simply mixing disulfonic acid 5a

Scheme 72. Reaction Mechanism of Asymmetric Mukaiyama Aldol Reaction Catalyzed by 4f

and amine 180 enabled the facile modulation of the amine moiety. They demonstrated the effectiveness of the system in the

Scheme 69. Enantioselective Synthesis of Aminal Derivatives from Carbamate and Imine

P


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Scheme 73. Vinylogous Mukaiyama Aldol Reaction Catalyzed by 4f

the best to achieve high yield and high enantioselectivity, due to the dynamic structure of the catalyst. A suitable chiral ammonium salt was easily tailor-made for a keto ester equivalent such as 3-acetoacetyl-2-oxazolidinone (184). Although 180 (R= H) resulted in low selectivity, the enantioselectivity of 185 was increased to 92% ee with 183 (R = Me) (Scheme 67). In this way, the tailor-made salt approach obviated the preparation of single-molecule catalysts in advance and resulted in expeditious screening for optimum conditions. The catalytic enantioselective aza-Friedel−Crafts reaction of aldimines with pyrroles is highly useful for the synthesis of chiral building blocks of biological and pharmaceutical agents. Much attention has been devoted to methods for the catalytic enantioselective aza-Friedel−Crafts reaction with the use of chiral metal catalysts.189 Recently, organocatalytic versions catalyzed by chiral phosphoric acids were independently developed by Antilla’s group190 and Nakamura’s group.191 One drawback of those reports is the relatively long reaction time (12−41 h). Chiral BINSA ammonium salts exhibited high catalytic performance in this reaction.192 Ishihara’s group reported an enantioselective aza-Friedel−Crafts reaction of N-Cbz-phenylaldimine 181 with N-benzylpyrrole (186) catalyzed by BINSA ammonium salts. The reaction was completed within 30 min even at −78 °C, and the corresponding products were obtained with good enantioselectivities (up to 92% ee) (Scheme 68). Interestingly, less sterically hindered N,N-dimethylbutylamine was the most effective, although the bulky amine 180 was suitable for the direct Mannich reaction. Aminals are synthetically and medicinally useful compounds found in natural products and pharmaceuticals. Nevertheless, practical catalytic methods for the synthesis of optically active

Scheme 74. Asymmetric Hetero-Diels−Alder Reaction Catalyzed by 4g

direct Mannich reaction of aldimine 181 with acetyl acetone (182). Various direct Mannich adducts were obtained in good yields with excellent enantioselectivities. Interestingly, the ratio of 5a and 180 was critical; the 5a:180 ratio of 1:1.5 to 1:2.5 was

Scheme 75. Asymmetric Hosomi−Sakurai Reaction Catalyzed by 4h

Q


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Scheme 76. Three-Component Asymmetric Hosomi−Sakurai Reaction Catalyzed by 4c

Scheme 77. Enantioselective Mukaiyama−Mannich Reaction

BINSA ammonium salts exhibited excellent catalytic activity in the addition of benzyl benzylidene carbamate to N-Cbz imine 181.199 The direct aminal formation from carboxamide 189 and aromatic imine 181 proceeded smoothly at 0 °C within 2.5 h to furnish a range of aminal derivatives 190 with excellent enantioselectivities (up to 89% ee) (Scheme 69). It is noted that unstable acrylamide also participated in the addition reaction to afford the corresponding aminal 191 with high enantioselectivity (89% ee). Pivalamide (189a), a more nucleophilic nonconjugated carboxamide, gave corresponding aminal 192 with low enantioselectivity (30% ee) when (R)-5a−188 was used as the catalyst. However, the tailor-made optimization of amine suggested that more basic trioctylamine was effective, and the enantioselectivity was significantly improved to 80% ee (Scheme 70). 4.1.3. Asymmetric Reactions Catalyzed by Chiral BINSA-Derived Disulfonimide. In 2009, List and co-workers developed a highly enantioselective reaction catalyzed by BINSA-derived sulfonimide moiety 4. This novel catalyst worked well in the Mukaiyama aldol reaction to afford corresponding adduct 193 in good yield with excellent enantioselectivity (Scheme 71).184 The important feature here is that disulfonimide 4f exhibited overwhelmingly higher catalytic activity than phosphoric acid 3. Excellent chemical yield and enantioselectivity was achieved with 4f but not with phosphoric acid 3a. Disulfonimide 4f functioned as a precatalyst, and the actual catalyst was assumed to be silylated disulfonimide 194, which was generated in situ after the initial reaction of 4f with ketene silyl acetal (Scheme 72). List and co-workers extended this methodology to the vinylogous Mukaiyama aldol reaction with 4f.200 The reaction of vinylogous ketene silyl acetal 195 with aldehydes proceeded in a highly regio- and stereoselective manner. For example, vinylogous ketene silyl acetal 195 afforded the γ-adduct 196 in excellent chemical yield and excellent enantioselectivity (96%, 94% ee) (Scheme 73). ε-Adduct 198 was obtained with high regioselectivity when double vinylogous ketene silyl acetal 197 was employed (75%, 90% ee, ε/α = 5/1).

Scheme 78. Enantioselective Vinylogous Mukaiyama− Mannich Reaction

Scheme 79. Enantioselective Abramov Reaction

acyclic aminals are still limited. The Curtius rearrangement of chiral acyl azides and the Hofmann rearrangement of chiral αamino amides are conventionally used as direct methods for the preparation of chiral aminals, although a dramatic loss of optical purity sometimes occurs due to epimerization.193,194 In this context, the group of Antilla, List, and Rueping independently reported the catalytic enantioselective synthesis of aminals that involved the direct amination of aldimines under the influence of a chiral phosphoric acid.195−198 Although acidic disulfonimides and phthalimides were feasible, primary carboxamides gave unsatisfactory enantioselectivity. R


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Scheme 80. Aldol Reaction Catalyzed by Disulfonimide 216

List and co-workers found that chiral disulfonimide is an efficient precatalyst for the enantioselective Mukaiyama− Mannich reaction between N-Boc-amino sulfones 207 and ketene silyl acetal.204 N-Boc-amino sulfones 207 are stable and easy to handle compared with the corresponding aldimines. Aldimines are usually generated in situ from the N-Boc-amino sulfone on treatment with a base. List and co-workers accomplished the reaction from 207 without a base under the low catalyst loading of 4i (2 mol %), and the desired adducts 208 were obtained in excellent chemical yields with high enantioselectivities (Scheme 77). It is noted that even an aliphatic substrate participated as a counterpart in the reaction, and adduct 209 was obtained in 90% yield with 51% ee. The Mukaiyama−Mannich reaction was also extended to the vinylogous version, as reported by List and co-workers.205 Cyclic siloxydiene 210 was selected because the resulting dioxinone is a good synthetic platform for further derivatization. The desired reaction proceeded smoothly even at −50 °C, and adducts 211 were obtained with good to excellent enantioselectivities (Scheme 78). The ease of transformation from adduct 211a (Ar = Ph) without erosion of chiral information clearly indicated the utility of 211 as a chiral building block and the importance of the present reaction. The formal synthesis of (−)-lasubin was also accomplished with this reaction. The hydrophosphonylation of aldehydes with dialkyl phosphite (Pudovik reaction) is an atom-economic approach to α-hydroxy phosphonates.206−209 It is known that dialkyl phosphite is in equilibrium with dialkyl phosphonate formed by tautomerizatioin, the latter being the predominant but unreactive form.210−212 Interestingly, silyl esters of dialkyl phosphites, which were originally introduced by Abramov,213−215 displayed excellent reactivity in the hydrophosphonylation reaction of aldehydes. However, in contrast to the analogous Mukaiyama aldol reactions, the enantioselective Abramov reactions of aldehydes are completely unknown. List and co-workers reported the first catalytic enantioselective Abramov reaction that proceeded in the presence of a chiral

Scheme 81. Asymmetric Friedel−Crafts Reaction

List and co-workers also developed a highly enantioselective hetero-Diels−Alder reaction.201 The reactions of Danishefsky’s diene 199 with aldehydes proceeded in the presence of 1 mol % 4g at −78 °C to afford multisubstituted pyranes 200 with excellent enantioselectivities (Scheme 74). In the same manner as the Danishefsky’s diene, more challenging diene 201, 1,3bis(siloxy)-1,3-dienes, participated in the reaction, and corresponding adduct 202 was obtained in excellent chemical yield with high enantioselectivity (94%, 96% ee). The catalytic enantioselective Hosomi−Sakurai reaction, a very useful carbon−carbon bond formation reaction, was also viable under the influence of disulfonimide.202 List and coworkers found that chiral trimethylsilylium ion, which was derived from chiral disulfonimide 4h and ketene silyl acetal, catalyzed the Hosomi−Sakurai reaction efficiently to provide a range of homoallylic alcohols 204 in good yields with high enantioselectivities (Scheme 75). Their work further highlights the generality of disulfonimide catalyst for enantioselective reactions involving Si nucleophiles On the basis of the above results, Gandhi and List and developed an asymmetric three-component synthesis of homoallylic amines.203 When a solution of aldehydes, carbamate 205, and allyltrimethylsilane was treated with 10 mol % sulfonimide 4c in chloroform, corresponding homoallylic amines 206 were obtained in good yields with excellent enantioselectivities (Scheme 76). This is the first application of a chiral disulfonimide based Lewis-acid catalyst in the activation of imine. S


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Scheme 82. Asymmetric Torgov Reaction and Total Synthesis of (+)-Estrone

Scheme 83. Preparation of Chiral Dicarboxylic Acid 2

Scheme 84. Asymmetric Mannich Reaction of Aromatic Aldehydes with α-Diazoester

disulfonimide catalyst.216 Chiral sulfonimide 4f with 3,5(CF3)2C6H3 and/or 4g with 3,5-(i-C3F7)2C6H3 at the 3,3′positions were the catalyst of choice, and corresponding αhydroxy phosphonates 215 were obtained in good yields with high enantioselectivities (up to 98% ee) (Scheme 79). Although

excellent selectivities were achieved with aromatic aldehydes, the use of aliphatic aldehydes resulted in only 10% ee. The development of the high-performance organocatalysts with turnover numbers exceeding 10 000 continues to pose a huge challenge. Even though chiral BINSA and disulfonimides T


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Scheme 85. Asymmetric Imino Aza−Ene Reaction

Scheme 87. Derivatization of Adduct 230a

Scheme 88. Asymmetric Aza−Ene Reaction Using Azomethine Imine

exhibited higher catalytic activity than chiral phosphoric acid, common bottlenecks include versatility of the catalyst. Quite recently, List and co-workers reported the development of highperformance chiral disulfonimide derivatives.217 The key to enhancing the catalytic activity of sulfonimide is the use of internal hydrogen bonds, and the bis[3,5-bis(trifluoromethyl)phenyl]methyl carbinol unit was introduced as a suitable hydroxy moiety. They examined the catalytic activities in the Mukaiyama aldol reaction of benzophenone (217) with ketene silyl acetal. Dramatic enhancement of the reactivity was observed in the presence of simple disulfonimide. The reaction was completed within 0.5 h at 0 °C under 0.5 mol % 216 (Scheme 80). In contrast, almost no reaction was observed with 4f even at a higher temperature and with an increased catalyst loading (1 mol %, rt, 2 h). By taking advantage of the extraordinary catalytic activity of 216, they designed an unprecedented Mukaiyama aldol reaction using 1,2-bis(trimethylsiloxy)cyclopentane 219, which, due to the absence of strong polarization in the C−C double bond, is less nucleophilic than the analogous silyl enol ether. Although the reaction at −78 °C did not proceed with 4f, good chemical yield and excellent enantioselectivity were achieved with 216. These results clearly demonstrate the advantage of the concept and underscore the possibility of developing various reactions that are difficult to achieve with conventional catalysts. 4.1.4. Asymmetric Reactions Catalyzed by Chiral BINSA-Derived Disulfonimide as Chiral Stronger Brønsted Acid. Unlike List’s concept (ACDC, asymmetric counteranion-directed catalysis), Lee and co-workers showed that chiral disulfonimide catalyst 4f itself acts as a chiral Brønstedacid catalyst.218 The catalytic asymmetric Friedel−Crafts reaction of indoles with aldimines proceeded smoothly in the presence of 4f, and corresponding adducts 221 were obtained in good yields with excellent enantioselectivities (Scheme 81). The Torgov reaction is a very useful transformation for the synthesis of steroids, such as estrone, a female sex hormone. Although much effort has been devoted to the catalytic enantioselective version of the reaction, high selectivity and turnover numbers have not been realized. List and co-workers reported a highly enantioselective Torgov reaction catalyzed by chiral disulfonimide 4j as a stronger Brønsted acid.219 The introduction of electron-withdrawing groups at the 5,5′-

Scheme 89. Asymmetric Aziridination of Diazoacetamides with N-Boc Aldimine

positions (nitro groups) as well as the 3,3′-positions (3,5(SF5)2−C6H3 groups) was critical for achieving the excellent enantioselectivity with low catalyst loading (5 mol %). This methodology realized the shortest total synthesis of (+)-estrone (Scheme 82). 4.2. Chiral Dicarboxylic Acids220

Carboxylic acids are an important class of moderately strong Brønsted acids. Nevertheless, the application of carboxylic acid to enantioselective reactions as a chiral Brønsted acid had been rarely realized except the enantioselective nitroso aldol reaction by Yamamoto and Momiyama.221 Maruoka and co-workers focused on axially chiral dicarboxylic acid bearing a BINOL backbone as a novel chiral Brønsted acid and developed a range of enantioselective reactions. 4.2.1. Preparation of Axially Chiral Dicarboxylic Acid.222 A 3,3′-disubstituted axially chiral dicarboxylic acid was prepared starting from (R)-1,1′-binaphthyl-2,2′-dicarboxylic acid using a procedure developed for the functionalization of the 3,3′-positions of binaphthyl carboxylic acid 2 (Scheme 83).223 Protection of the carboxy groups as 2-(trimethylsilyl)ethyl ester and subsequent ortho-bromination gave dibromide 224. Suzuki−Miyaura coupling and deprotection provided 3,3′disubstituted axially chiral dicarboxylic acid 2 in good yield (Scheme 83).

Scheme 86. Asymmetric Vinylogous Imino Aza−Ene Reaction

U


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Scheme 90. Asymmetric Ugi-Type Reaction

Scheme 91. Asymmetric Synthesis of α-Amino-β-aryl Ether

Scheme 93. Enantioselective Addition of Styrylboronic Acid 245 to Dienone

4.2.2. Asymmetric Reactions Catalyzed by Axially Chiral Dicarboxylic Acid. Hashimoto and Maruoka designed an axially chiral dicarboxylic acid bearing a (R)-BINOL backbone and demonstrated its catalytic activity in the asymmetric Mannich reaction of aromatic aldehydes with α-diazoester224 to furnish β-amino-α-diazoacetates in high yields with excellent enantioselectivities (Scheme 84).225,226 Although chiral phosphoric-acid-catalyzed enantioselective reactions with N-acyl imine were already reported,227,228 use of a weaker carboxylic acid was necessary for the N-Boc imine. α-Diazomethylphosphonates 225a were also suitable substrates, and corresponding βamino-α-diazomethyphosphonates 226 were obtained in high yields with excellent enantioselectivities (Scheme 84). Maruoka and co-workers subsequently reported the imino aza−ene reaction of N,N′-tetramethylenehydrazones of aldehyde with N-Boc imine catalyzed by dicarboxylic acid 2a (Scheme 85).229 In that reaction, N,N-dialkylhydrazone 227 functioned as an acyl anion equivalent. Because α-amino hydrazones 228 were readily transformed into α-amino ketones without racemization, this protocol provides a facile access to chiral α-amino ketones (Scheme 85). The same group extended the reaction to vinylogous aza− enamine 229 successfully by using N-Bz aldimine in place of NBoc aldimine (Scheme 86).230 Aza−enamine adduct 230a was readily transformed into acrylonitrile derivative 231 by treatment with MMPP (magnesium monoperoxyphthalate) without deterioration of the optical purity (Scheme 87). They further employed azomethine imine, which had not been extensively employed in the asymmetric reaction, as an electrophile in the aza−ene reaction.231 On treatment of aldehyde, N′-alkylacylhydrazides 232, and α-diazoester 225b with 5 mol % 2a, the addition of α-diazoester to in situ generated

Scheme 94. Enantioselective Addition of Vinylboronate 245 to N-Acyl Quinolinium Salt

azomethine imine proceeded smoothly to afford adducts 233 with high to excellent enantioselectivities (Scheme 88). Maruoka and co-workers developed an enantioselective aziridination of diazoacetamides 234 with N-Boc aldimine using chiral dicarboxylic acid 2a as the catalyst. This reaction is highly stereoselective, and trans-aziridines 235 were obtained exclusively (Scheme 89).232 The Ugi reaction is a four-component reaction of an aldehyde, a primary amine, an isocyanide, and a carboxylic acid. The realization of the enantioselective-catalyzed version of the Ugi reaction is still an unaccomplished task.233,234 Dicarboxylic acid

Scheme 92. O-Monoacyltartaric Acid 240-Catalyzed Asymmetric Addition of Boronic Acid to Enone

V


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Figure 7. Lewis-acid-assisted Brønsted-acid (LBA) and Brønsted-acid-assisted Brønsted-acid (BBA) strategies.

catalyst proceeded smoothly to furnish cyclopentanone 247 with excellent optical purity (Scheme 93).239 Schaus and co-workers found that tartaric acid catalyzed the enantioselective addition of vinylboronates 245 to N-acyl quinolinium salts, generated from 249, to afford dihydroquinolines 250 with high enantioselectivities (Scheme 94).240,241 2Ethoxy-1-ethoxycarbonyl-1,2-dihydroquinolines (249) were employed as stable N-acyl quinolinium salt precursors. The addition of an excess amount of trichloroethanol improved both chemical yield and enantioselectivity. ESI-MS experiments suggested the generation of the dimeric tartaric acid adduct 251 as the resting state of the catalyst.

Scheme 95. Internal Lewis-Acid-Assisted Benzoic Acid

4.3. Lewis (Brønsted)-Acid-Assisted Brønsted Acids

The demand for the development of novel organocatalysts with high acidity is now ofincreasing interst. As we described, much effort has been devoted to the effective design of the catalyst, such as dicarboxylic acid, sulfonic acid, sulfonimide, and so on. Novel types of enantioselective reactions, which were difficult to achieve by means of the traditional catalysts, were developed. Enhancing the Brønsted acidity of the conventional acid functionality, “combined acids system” is another promising approach. Yamamoto and co-workers introduced the concept, namely, Lewis-acid-assisted Brønsted acid (LBA) and Brønstedacid-assisted Brønsted acid (BBA) as for Brønsted-acid motif (Lewis-acid-assisted Lewis acid (LLA) and Brønsted-acidassisted Lewis acid (BLA) were also available for Lewis-acid catalysts) (Figure 7).15 Both TADDOL derivatives developed by Rawal and dicarboxylic acids with binaphthyl backbone took advantage of the BBA concept (strong acidity was secured by the internal hydrogen bonding). However, the dramatic advances in chiral BBAs, an analogous LBA concept to the strong Brønsted acid, such as carboxylic acid, have been overlooked until quite recently. In 2012, Mattson and co-workers reported that an internal boron moiety played a critical role for the enhancement of the catalytic activity of benzoic acid derivatives (Scheme 95).242 Although the decreasing of the catalyst loading and enhancement of the acidity was not dramatic, the results implied the potential of internal Lewis-acid-assisted benzoic acid derivatives 252 as a possible design of organocatalyst. Maruoka and co-workers designed a stronger Brønsted acid composed of a chiral diol and 2-boronobenzoic acid.243 The combination of 253 and 1,1,1-triphenylethanediol 254 was the most suitable, and corresponding aziridine derivatives 235 were

Scheme 96. Enantioselective Aziridination Catalyzed by Stronger Brønsted Acid Composed of a Chiral Diol

2c catalyzed the enantioselective Ugi-type reaction among an aldehyde, a hydrazine, and an isocyanide (Scheme 90).235 Dicarboxylic acid 2a efficiently activated quinone imine ketal electrophilically to furnish α-amino-β-aryl ether 239 wherein quinone imine 237 acted as a functionalized aromatic ring surrogate (Scheme 91).236 Transformation of the products 239 yielded chiral β-aryl amines and α-aryl ethers. Sugiura and co-workers reported that O-monoacyltartaric acid 240 catalyzed the asymmetric addition of boronic acid 242 to enone 241 with good enantioselectivities.237 DFT calculation was undertaken to elucidate the reaction mechanism (Scheme 92).238 The enantioselective addition of styrylboronic acid 245a to dienone 244 also proceeded in the presence of 240 to furnish monostyrylated product 246 with good enantioselectivity. The RCM of the diene products using the Hoveyda−Grubbs II W


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obtained in good chemical yields with excellent enantioselectivities (Scheme 96). An interesting feature of the catalyst is that the boron center also has central chirality. Although detailed investigation of the “real” catalytic species suggested that the (S,S)-catalyst shown as 255 is the active catalyst, the possibility of the stereomutation of the chirality at the boron center in the reaction could not be excluded.

Laboratories, Shionogi & Co., Ltd., as a research chemist, he was appointed as an assistant professor at Ehime University (1988). He was a visiting scholar at Stanford University in 1992−1993, where he worked with Professor Barry M. Trost. He joined Gakushuin University as an associate professor in 1994 and was promoted to professor in 1997. He is a recipient of the Chemical Society of Japan Award for Creative Work (2009), Daiichi-Sankyo Award for Medicinal Organic Chemistry (2009), and Nagoya Silver Medal (2012). His current research interests include the development of new and useful synthetic methodologies based on the design of novel chiral Brønsted-acid catalysts as well as the utilization of transition-metal catalysts.

5. SUMMARY AND OUTLOOK According to the Brønsted−Lowry acid−base theory, a Brønsted acid is a substance that donates a proton (H+). Its use as a catalyst for the hydrolysis and formation of esters and acetals had been recognized by synthetic organic chemists in the 20th century. The synthetic utility of Brønsted acid as a catalyst for the C−C bond formation reaction has seen significant growth in the 21st century, and a range of stronger Brønsted-acid-catalyzed reactions have been developed. Strong Brønsted acids, such as TfOH and Tf2NH, efficiently activated carbonyl groups, alkenes, alkynes, in addition to hydroxy groups. They sometimes functioned complementarily to Lewis-acid catalysts. Carbon acids are also a unique family of the stronger Brønsted acids of synthetic interest, and their development of carbon acid is expected in the future. Chiral Brønsted acid has become one of the most attractive subjects in organocatalysis in the past decade because of the versatility for a wide range of reactions. It is a chiral proton bearing a chiral counteranion. Because of its ability to catalyze a plethora of reactions, BINOL-derived chiral phosphoric acid has become mainstream in the chiral Brønsted acids. In addition to the chiral phosphoric acids, chiral dicarboxylic acids, chiral disulfonic acids, and chiral sulfonimides have emerged as stronger Brønsted acids, and their synthetic utility has gained wide acceptance. We have high hopes for the further growth of the scope of those catalysts and the development of novel chiral Brønsted acids in the next decade.

Keiji Mori graduated in 2003 and received his Ph.D. degree in 2008 from the Tokyo Institute of Technology under the supervision of Professor Keisuke Suzuki. After obtaining his Ph.D. degree, he was appointed Assistant Professor at Gakushuin University in 2008. He was promoted as a Associate Professor at the Tokyo University of Agriculture and Technology in 2015. He is a recipient of the Chemical Society of Japan Award for Young Chemists in 2015. His current research interests include the development of new synthetic transformation based on hydride shift-mediated C(sp3)−H bond functionalization.

ACKNOWLEDGMENTS The author’s work shown in this review was partially supported by a Grant-in-Aid for Scientific Research on Innovative Areas “Advanced Molecular Transformation by Organocatalysis” from MEXT, Japan, and a Grant-in-Aid for Scientific Research from JSPS.

AUTHOR INFORMATION Corresponding Author

*Fax: +81-3-5992-1029. E-mail: takahiko.akiyama@gakushuin. ac.jp. Notes

The authors declare no competing financial interest.

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