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. 2018 Jan 22;47(2):278-290.
doi: 10.1039/c7cs00509a.

Iminium and enamine catalysis in enantioselective photochemical reactions

You-Quan Zou  1 , Fabian M Hörmann, Thorsten Bach
Affiliations

Iminium and enamine catalysis in enantioselective photochemical reactions

You-Quan Zou et al. Chem Soc Rev. .

Abstract

Although enantioselective catalysis under thermal conditions has been well established over the last few decades, the enantioselective catalysis of photochemical reactions is still a challenging task resulting from the complex enantiotopic face differentiation in the photoexcited state. Recently, remarkable achievements have been reported by a synergistic combination of organocatalysis and photocatalysis, which have led to the expedient construction of a diverse range of enantioenriched molecules which are generally not easily accessible under thermal conditions. In this tutorial review, we summarize and highlight the most significant advances in iminium and enamine catalysis of enantioselective photochemical reactions, with an emphasis on catalytic modes and reaction types.

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Figures

Fig. 1
Fig. 1. Structures of representative chiral amines 1–6 for iminium catalysis in enantioselective photochemical reactions. TBS = tert-butyldimethylsilyl, TMS = trimethylsilyl, TDS = tert-hexyldimethylsilyl.
Scheme 1
Scheme 1. Diastereoselective intramolecular [2+2] photocycloaddition reaction of iminium salt 7.
Scheme 2
Scheme 2. Enantioselective β-benzylation of enals 10. a d.r. > 95/5. b d.r. = 56/44, the ee refers to the major diastereoisomer. c Toluene employed as the solvent using 2 as the catalyst.
Scheme 3
Scheme 3. Enantioselective β-benzylation of enones 14. a d.r. > 95/5. b 50 mol% benzoic acid was used instead of acetic acid. c 6 was used as the chiral amine catalyst. HOAc = acetic acid.
Scheme 4
Scheme 4. Enantioselective β-benzylation of enals via visible-light excitation of iminium ions catalyzed by chiral secondary amine 3. a d.r. = 55/45, the ee refers to the major diastereoisomer. TFA = trifluoroacetic acid.
Scheme 5
Scheme 5. Formation of products 21 by enantioselective trapping of benzodioxole-derived radicals with enones catalyzed by amine 5. TBABF4 = tetrabutylammonium tetrafluoroborate.
Scheme 6
Scheme 6. Enantioselective trapping of α-amino radicals with enones catalyzed by ent-5. Ir[III] = Ir[dF(CF3)ppy]2(dtbbpy)PF6.
Fig. 2
Fig. 2. Structures of representative chiral amines 25–37 for enamine catalysis in enantioselective photochemical reactions.
Scheme 7
Scheme 7. Enantioselective α-alkylation of aldehydes catalyzed by chiral imidazolidinone 25a. Ru[II] = Ru(bpy)3Cl2, FG = functional group, HOTf = trifluoromethanesulfonic acid, DMF = N,N-dimethylformamide.
Scheme 8
Scheme 8. Enantioselective α-alkylation of aldehydes catalyzed by chiral imidazolidinone 25a using eosin Y as the photocatalyst.
Scheme 9
Scheme 9. Enantioselective α-trifluoromethylation and α-perfluoromethylation of aldehydes catalyzed by chiral imidazolidinone 25a. Ir[III] = Ir(ppy)2(dtbbpy)PF6, Boc = tert-butyloxycarbonyl.
Scheme 10
Scheme 10. Enantioselective synthesis of β-cyanoaldehydes 43 by photoredox organocatalysis. Ru[II] = Ru(bpy)3Cl2. a Chiral secondary amine 26 was used as the organocatalyst. b d.r. = 53/47. c d.r. = 55/45. The ee refers to major diastereoisomer. DMSO = dimethyl sulfoxide, Ts = para-toluenesulfonyl.
Scheme 11
Scheme 11. Enantioselective α-alkylation of aldehydes catalyzed by 31via an electron donor–acceptor (EDA) complex mechanism. a Chiral amine 2 was used as the organocatalyst. MTBE = methyl tert-butyl ether.
Scheme 12
Scheme 12. Enantioselective α-alkylation of cyclohexanone (49) via an electron donor–acceptor (EDA) complex mechanism.
Scheme 13
Scheme 13. Enantioselective α-alkylation of aldehydes via the direct excitation of enamines 52. a Reaction performed upon sunlight irradiation. TIPS = triisopropylsilyl.
Scheme 14
Scheme 14. Enantioselective α-(phenylsulfonyl)- and α-(methylsulfonyl)methylation of aldehydes catalyzed by ent-30.
Scheme 15
Scheme 15. Enantioselective α-alkylation of aldehydes catalyzed by 25b. Ir[III] = Ir(Fmppy)2(dtbbpy)PF6 [Fmppy = 2-(4-fluorophenyl)-4-(methylpyridine)]. a d.r. = 91/9. b d.r. = 91/9. c d.r. > 95/5. d d.r. > 95/5. e d.r. = 87.5/12.5. The ee refers to the major diastereoisomer. NMA = N-methyl acetamide.
Scheme 16
Scheme 16. Intermolecular enantioselective α-alkylation of aldehydes catalyzed by ent-30. Ir[III] = Ir(dmppy)2(dtbbpy)PF6. a Reaction was performed at –65 °C. DME = dimethoxyethane, 3-Py = 3-pyridine, Cbz = carboxybenzyl.
Scheme 17
Scheme 17. Enantioselective α-alkylation of β-ketocarbonyl compounds catalyzed by 32. Ru[II] = Ru(bpy)3Cl2·6H2O. a d.r. = 96/4. b d.r. = 94/6. c d.r. > 99/1.
Scheme 18
Scheme 18. Enantioselective α-benzylation of aldehydes catalyzed by 27. Ir[III] = fac-Ir(ppy)3.
Scheme 19
Scheme 19. Enantioselective α-benzylation of aldehydes catalyzed by 31via an electron donor–acceptor (EDA) complex mechanism. a Chiral amine 2 was used as the organocatalyst.
Scheme 20
Scheme 20. Enantioselective α-benzylation of cyclic ketones catalyzed by 36via an electron donor–acceptor (EDA) complex mechanism. a Reaction performed on a 1 mmol scale.
Scheme 21
Scheme 21. Enantioselective α-benzylation of cyclic ketones catalyzed by 33. Ru[II] = Ru(bpy)3Cl2·6H2O, Co[II] = Co(dmgH)2Cl2. a d.r. = 90/10. b d.r. = 89/11. c d.r. = 67/33. d d.r. = 92/8. e Reaction performed at –5 °C. f d.r. = 89/11. The ee refers to the major diastereoisomer.
Scheme 22
Scheme 22. Enantioselective α-hydroxylation of aldehydes catalyzed by 29.
Scheme 23
Scheme 23. Enantioselective α-hydroxylation of ketones catalyzed by 34. a Reaction was performed in N-methylpyrrolidinone (NMP). b 35 was used as the catalyst.
Scheme 24
Scheme 24. Enantioselective α-amination of aldehydes catalyzed by 28.
Scheme 25
Scheme 25. Enantioselective β-arylation of cyclohexanone (49) catalyzed by 37. Ir[III] = Ir(ppy)3, DABCO = 1,4-diazabicyclo[2.2.2]octane, DMPU = 1,3-dimethyltetrahydropyrimidin-2(1H)-one.
None
You-Quan Zou
None
Fabian M. Hörmann
None
Thorsten Bach
See this image and copyright information in PMC

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