Light in aminocatalysis: The asymmetric intermolecular α-alkylation of aldehydes

Angewandte Chemie - International Edition

Volumn 48, Issue 8, 2009, Pages 1360-1363

  Melchiorre, Paolo ^a  

^a UNIVERSITY OF BOLOGNA   (Italy)

Author keywords

alkylation; enamines; organocatalysis; photoredox catalysis; radicals

Indexed keywords

ALDEHYDES; ALKYLATION; AMINES; CHEMICAL REACTIONS; FOURIER TRANSFORMS; HYDROCARBONS; ORGANIC COMPOUNDS; RATE CONSTANTS; REACTION KINETICS;

ASYMMETRIC TRANSFORMATIONS; ENAMINES; ORGANIC SYNTHESIS; ORGANOCATALYSIS; PHOTOREDOX CATALYSIS; RADICALS;

CATALYSIS;

EID: 60149109803     PISSN: 14337851     EISSN: None     Source Type: Journal    
DOI: 10.1002/anie.200804995     Document Type: Short Survey

References (43)

1. For recent reviews on aminocatalysis, see: a) C. F. Barbas III, Angew. Chem. 2008, 120, 44;
2. Angew. Chem. Int. Ed. 2008, 47, 42;
3. b) P. Melchiorre, M. Marigo, A. Carlone, G. Bartoli, Angew. Chem. 2008, 120, 6232;
4. Angew. Chem. Int. Ed. 2008, 47, 6138;
5. c) S. Mukherjee, J. W. Yang, S. Hoffmann, B. List, Chem. Rev. 2007, 107, 5471;
6. for a commentary on the advent of asymmetric organocatalysis, see: d) D. W. C. MacMillan, Nature 2008, 455, 304.
7. D. A. Nicewicz, D. W. C. MacMillan, Science 2008, 322, 77.
8. a) Modern Carbonyl Chemistry (Ed.: J. Otera), Wiley-WCH, Weinheim, 2000;
9. b) D. Caine in Comprehensive Organic Synthesis, Vol. 2 (Ed.: B. M. Trost), Pergamon Press, New York, 1991, Chapter 1.1, and references therein;
10. c) D. A. Evans in Asymmetric Synthesis, Vol. 3 (Ed.: J. D. Morrison), Academic Press, New York, 1983, Chapter 1;
11. d) A. Job, C. F. Janeck, W. Bettray, R. Peters, D. Enders, Tetrahedron 2002, 58, 2253.
12. For phase-transfer catalytic asymmetric α-alkylation of glycine derivatives, see: a) T. Ooi, K. Maruoka, Angew. Chem. 2007, 119, 4300;
13. Angew. Chem. Int. Ed. 2007, 46, 4222, and references therein;
14. for asymmetric alkylations of preformed lithium enolates with oligoamine catalysts, see: b) M. Imai, A. Hagihara, H. Kawasaki, K. Manabe, K. Koga, J. Am. Chem. Soc. 1994, 116, 8829;
15. for metal-catalyzed alkylations of preformed tin enolates, see: c) A. G. Doyle, E. N. Jacobsen, J. Am. Chem. Soc. 2005, 127, 62;
16. for a catalytic asymmetric alkylation of racemic a-bromo-esters, see: d) X. Dai, N. A. Strotman, G. C. Fu, J. Am. Chem. Soc. 2008, 130, 3302.
17. The classic drawbacks associated with the stoichiometric alky-lation of preformed aldehyde enolates or enamines are atendency towards self-aldol condensation, the Canizzaro or Tischenko reactions, and O- or N-alkylations with the electro-philic alkyl halides; see: a) G. Stork, A. Brizzolara, H. Landes-man, J. Szmuszkovicz, R. Terrell, J. Am. Chem. Soc. 1963, 85, 8829;
18. b) H. O. House, W. C. Liang, P. D. Weeks, J. Org. Chem. 1974, 39, 3102.
19. a) N. Vignola, B. List, J. Am. Chem. Soc. 2004, 126, 450;
20. for a mechanistic insight, see: b) A. Fu, B. List, W. Thiel, J. Org. Chem. 2006, 71, 320.
21. As validation of its reliability and effectiveness, the enamine-catalyzed asymmetric intramolecular a-alkylation of aldehydes has recently been integrated into domino sequences, thus leading to the synthesis of complex cyclic molecules with high optical purity: a) H. Xie, L. Zu, H. Li, J. Wang, W. Wang, J. Am. Chem. Soc. 2007, 129, 10886;
22. b) R. Rios, H. Sundén, J. Vesely, G.-L. Zhao, P. Dziedzic, A. Córdova, Adv. Synth. Catal. 2007, 349, 1028;
23. c) D. Enders, C. Wang, J. W. Bats, Angew. Chem. 2008, 120, 7649;
24. Angew. Chem. Int. Ed. 2008, 47, 7539.
25. I. Ibrahem, A. Cordova, Angew. Chem. 2006, 118, 1986;
26. Angew. Chem. Int. Ed. 2006, 45, 1952.
27. a) T. D. Beeson, A. Mastracchio, J.-B. Hong, K. Ashton, D. W. C. MacMillan, Science 2007, 316, 582;
28. b) H.-Y. Jang, J.-B. Hong, D. W. C. MacMillan, J. Am. Chem. Soc. 2007, 129, 7004;
29. c) H. Kim, D. W. C. MacMillan, J. Am. Chem. Soc. 2008, 130, 398; a SOMO activation mechanism has also been reported for the α-oxidation of aldehydes,
30. see: d) M. P. Sibi, M. Hasegawa, J. Am. Chem. Soc. 2007, 129, 4124.
31. a) B. Giese in Radicals in Organic Synthesis: Formation of Carbon-Carbon Bonds, Pergamon Press, Oxford, 1986;
32. b) Radicals in Organic Synthesis (Eds.: P. Renaud, M. P. Sibi), Wiley-VCH, Weinheim, 2001.
33. For a perspective on this discovery, see: P. Renaud, P. Leong, Science 2008, 322, 55.
34. a) A. Juris, S. Barigelletti, S. Campagna, V. Balzani, P. Belser, A. von Zelewsky, Coord. Chem. Rev. 1988, 84, 85;
35. b) V. Balzani, A. Juris, M. Venturi, S. Campagna, S. Serroni, Chem. Rev. 1996, 96, 759.
36. In the course of luminescence-quenching studies, it was observed that preformed chiral enamines decreased the emission intensityof 6, while the presence of a-bromo carbonyl compounds did not. This result supports the mechanistic proposal that 6 is acting as an oxidant in the photoredox cycle.
37. Circumstantial evidence for the direct involvement of the enamine intermediate in the key bond-forming step has been collected. The coupling of an electron-rich enamine with electron-deficient radicals has the converse mechanism to that of the previously described SOMO-catalyzed reactions, where the susceptibility of the transient enamine for undergoing selective oxidation generates a reactive 3π-electron radical cation, see Ref. [9]. This complementary reactivity further expands the synthetic potential of SOMO catalysis.
38. As a propagation step, the direct SET from the electron-rich α-amino radical 9 to the a-bromo carbonyl compound cannot be excluded. As a matter of fact, good results are obtained in the absence of any photoredox catalyst while irradiating the reaction mixture with a high-energy UV lamp. For an example of a catalytic enantioselective reaction driven by photo-induced electron transfer, see: A. Bauer, F. Westkaemper, S. Grimme, T. Bach, Nature 2005, 436, 1139.
39. The possibility to employ other activated alkyl bromides in the a-alkylation chemistry (e.g. the asymmetric benzylation of aldehydes using benzyl bromide) has been preliminary reported by D. W. C. MacMillan et al. at a recent conference: IASOC conference, Ischia (Italy), October 1, 2008.
40. Enantioselective radical reactions catalyzed by chiral Lewis acids are nowadays a common practice in organic synthesis: a) J. Zimmerman, M. P. Sibi, Top. Curr. Chem. 2006, 263, 107; however, only isolated examples of organocatalyzed asymmetric processes involving radical intermediates are known:
41. b) D. H. Cho, D. O. Jang, Chem. Commun. 2006, 5045;
42. c) T. Aechtner, M. Dressel, T. Bach, Angew. Chem. 2004, 116, 5974;
43. Angew. Chem. Int. Ed. 2004, 43, 5849.