Enantioselective direct aldol reactions catalyzed by l-prolinamide derivatives

摘要

l-Prolinamides 2, prepared from l-proline and simple aliphatic and aromatic amines, have been found to be active catalysts for the direct aldol reaction of 4-nitrobenzaldehyde with neat acetone at room temperature. They give moderate enantioselectivities of up to 46% enantiomeric excess (ee). The enantioselectivity increases as the amide N—H becomes a better hydrogen bond donor. l-Prolinamides 3, derived from the reaction of l-proline with α,β-hydroxyamines such that there is a terminal hydroxyl group, show more efficient catalysis and higher enantioselectivities. In particular, catalyst 3h, prepared from l-proline and (1S,2S)-diphenyl-2-aminoethanol, exhibits high enantioselectivities of up to 93% ee for aromatic aldehydes and up to >99% ee for aliphatic aldehydes under –25°C. Model reactions of benzaldehyde with three enamines derived from the condensation of prolinamides with acetone have been studied by quantum mechanics calculations. The calculations reveal that the amide N—H and the terminal hydroxyl groups form hydrogen bonds with the benzaldehyde substrate. These hydrogen bonds reduce the activation energy and cause high enantioselectivity. Our results suggest a new strategy in the design of new organic catalysts for direct asymmetric aldol reactions and related transformations.