Proline-catalyzed asymmetric formal α-alkylation of aldehydes via vinylogous iminium ion intermediates generated from arylsulfonyl indoles

Angewandte Chemie - International Edition

Volumn 47, Issue 45, 2008, Pages 8707-8710

  Shaikh, Rafik R ^a   Mazzanti, Andrea ^b   Petrini, Marino ^a   Bartoli, Giuseppe ^b   Melchiorre, Paolo ^b  

^a UNIVERSITY OF CAMERINO   (Italy)

^b UNIVERSITY OF BOLOGNA   (Italy)

Author keywords

Aldehydes; Alkylation; Indoles; Organocatalysis; Proline

Indexed keywords

ALDEHYDES; ALKYLATION; CATALYSIS; HYDROCARBONS; ORGANIC COMPOUNDS; REACTION KINETICS;

ALKYLATING REAGENTS; CARBOCATION; CHEMICAL EQUATIONS; ENAMINE; ENAMINE INTERMEDIATES; ENANTIOCONTROL; IMINIUM IONS; INDOLES; ORGANOCATALYSIS; PROLINE;

CHEMICAL REACTIONS;

ALDEHYDE; IMINE; INDOLE DERIVATIVE; ION; PROLINE;

ALKYLATION; ARTICLE; CATALYSIS; CHEMICAL STRUCTURE; CHEMISTRY; STEREOISOMERISM; SYNTHESIS; X RAY CRYSTALLOGRAPHY;

ALDEHYDES; ALKYLATION; CATALYSIS; CRYSTALLOGRAPHY, X-RAY; IMINES; INDOLES; IONS; MODELS, MOLECULAR; MOLECULAR STRUCTURE; PROLINE; STEREOISOMERISM;

EID: 55249108726     PISSN: 14337851     EISSN: None     Source Type: Journal    
DOI: 10.1002/anie.200803947     Document Type: Article

References (47)

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. d) G. Lelais, D.W. C. MacMillan, Aldrichimica Acta 2006, 39, 79.
7. N2 addition to alkyl halides and generally utilizes stoichiometric amounts of metal enolates.
8. a) Modern Carbonyl Chemistry (Ed.: J. Otera), Wiley-VCH, Weinheim, 2000;
9. b) S. Carrettin, J. Guzman, A. Corma, Angew. Chem. 2005, 117, 2282;
10. Angew. Chem. Int. Ed. 2005, 44, 2242.
11. For phase-transfer catalytic asymmetric α-alkylation of glycine derivatives, see: a) T. Ooi, K. Maruoka, Angew. Chem. 2007, 119, 4300;
12. Angew. Chem. Int. Ed. 2007, 46, 4222 and references therein.
13. For catalytic 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.
14. For metal-catalyzed asymmetric alkylations of preformed tin enolates, see: c) A. G. Doyle, E. N. Jacobsen, J. Am. Chem. Soc. 2005, 127, 62.
15. a) N. Vignola, B. List, J. Am. Chem. Soc. 2004, 126, 450;
16. b) A. Fu, B. List, W. Thiel, J. Org. Chem. 2006, 71, 320. This reaction was the first nucleophilic substitution proceeding under enamine catalysis and opened up unexplored routes for asymmetric aminocatalysis.
17. a) H. O. House, W. C. Liang, P. D. Weeks, J. Org. Chem. 1974, 39, 3102;
18. b) G. Stork, A. Brizzolara, H. Landesman, J. Szmuszkovicz, R. Terrell, J. Am. Chem. Soc. 1963, 85, 8829.
19. For organocatalytic asymmetric domino reactions consisting of Michael addition followed by intramolecular α-alkylation of aldehydes, leading to cyclic compounds, see: a) H. Xie, L. Zu, H. Li, J. Wang, W. Wang, J. Am. Chem. Soc. 2007, 129, 10886;
20. b) R. Rios, H. Sundén, J. Vesely, G.-L. Zhao, P. Dziedzic, A. Córdova, Adv. Synth. Catal. 2007, 349, 1028;
21. c) R. Rios, J. Vesely, H. Sundén, I. Ibrahem, G.-L. Zhao, A. Córdova, Tetrahedron Lett. 2007, 48, 5835;
22. d) D. Enders, C. Wang, J. W. Bats, Angew. Chem. 2008, 120, 7649;
23. Angew. Chem. Int. Ed. 2008, 47, 7539.
24. I. Ibrahem, A. Córdova, Angew. Chem. 2006, 118, 1986;
25. Angew. Chem. Int. Ed. 2006, 45, 1952.
26. a) T. D. Beeson, A. Mastracchio, J.-B. Hong, K. Ashton, D. W. C. MacMillan, Science 2007, 316, 582;
27. b) H.-Y. Jang, J.-B. Hong, D. W. C. MacMillan, J. Am. Chem. Soc. 2007, 129, 7004;
28. c) H. Kim, D. W. C. MacMillan, J. Am. Chem. Soc. 2008, 130, 398.
29. N1-type reaction involving the carbocation, leading to a "formal" α-alkylation, may also be possible (see Ref. [17]). Considering the unconventional nature of the electrophilic system, the presented process surely escapes classical definitions.
30. For reviews, see: a) B. List, Tetrahedron 2002, 58, 5573;
31. b) M. Movassaghi, E. N. Jacobsen, Science 2002, 298, 1904.
32. a) R. Ballini, A. Palmieri, M. Petrini, R. R. Shaikh, Adv. Synth. Catal. 2008, 350, 129;
33. b) A. Palmieri, M. Petrini, J. Org. Chem. 2007, 72, 1863;
34. c) R. Ballini, A. Palmieri, M. Petrini, E. Torregiani, Org. Lett. 2006, 8, 4093.
35. Also the use of 5-chloro-(2H)-sulfonyl indole derivative afforded poor results in terms of stereoselectivity (88% yield, d.r. 3:1, 17% ee). This result is likely due to a steric rather than an electronic effect.
36. CCDC 700758 (3) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
37. O-Methylation of the carboxylic group of proline had a deleterious effect on both the reactivity and selectivity of the process (25 mol% of hydrochloric salt of proline methyl ester, less than 30% conversion, d.r. 1.1:1 and <10% ee under the optimal reaction conditions).
38. Adding silica gel (100 mg per 0.1 mmol) to the reaction mixture had a beneficial effect on the reactivity, which supports the mechanistic requirement for the protonation of 4. Even catalysts that proved to be inactive under the reported conditions led to product formation: for example, catalyst B (25 mol%) in the reaction of propanal with 1a afforded 2a in 45%yield with high enantioselectivity (85% ee) but poor diastereoselectivity (d.r.1.1:1).
39. The formation of such a carbocation has been recently proposed by Enders; see: a) D. Enders, A. A. Narine, F. Toulgoat, T. Bisschops, Angew. Chem. 2008, 120, 5744;
40. Angew. Chem. Int. Ed. 2008, 47, 5661.
41. For related papers, see: b) F. Colombo, G. Cravotto, G. Palmisano, A. Penoni, M. Sisti, Eur. J. Org. Chem. 2008, 2801;
42. c) B. Ke, Y. Qin, Q. He, Z. Huang, F. Wang, Tetrahedron Lett. 2005, 46, 1751.
43. The proposed mechanism closely resembles the direct electrostatic activation (DEA) concept advanced by MacMillan to rationalize the mechanism of the aminocatalytic asymmetric cyclopropanation of enals; see: a) R. K. Kunz, D. W. C. MacMillan, J. Am. Chem. Soc. 2005, 127, 3240.
44. It should be also noted that proline is typically a poor catalyst for enamine-activated aldehyde additions to Michael acceptors: b) B. List, P. Pojarliev, J. Martin, Org. Lett. 2001, 3, 2423..
45. The sense of asymmetric induction is opposite to that observed in other proline-catalyzed asymmetric α-functionalization of aldehydes believed to proceed by a hydrogen-bond-directing approach of the electrophiles. The observed stereochemical outcome is consistent with the proposed electrostatic activation mode when assuming the syn-E-enamine of proline as the reactive intermediate, because of the closer proximity with the vinylogous iminium ion intermediate; see Ref. [18a] for similar considerations. Theoretical studies are underway to shed more light on the mechanistic path.
46. A stereoselective Friedel-Crafts approach to 2 should be based upon an asymmetric addition to α,β-disubstituted unsaturated aldehydes, a challenging yet elusive transformation. For a different organocatalytic entry to 2, see: Y. Chi, S. T. Scroggins, J. M. J. Fréchet, J. Am. Chem. Soc. 2008, 130, 6322.
47. Note added in proof: After acceptance of this manuscript, the asymmetric intermolecular α-alkylation of aldehydes with activated alkyl halides has been accomplished, exploiting the combination of photoredox catalysis with emamine catalysis: D. A. Nicewicz, D.W. C. MacMillan, Science 2008, DOI:10.1126/science.1161976.