The use of N-alkyl-2,2′-bipyrrolidine derivatives as organocatalysts for the asymmetric Michael addition of ketones and aldehydes to nitroolefins

The direct Michael addition of aldehydes and ketones to nitroolefins, catalyzed by N-i-Pr-2,2'-bipyrrolidine, is described. The desired 1,4-adducts are obtained in excellent yields with enantioselectivities up to 95% ee and dr up to 95:5 of the syn aldehyde addition product. An unexpected inversion of diastereoselectivity was observed for the addition of alpha-hydroxy ketones to beta-arylnitroolefins with enantioselectivities up to 98% ee. The formation of an internal hydrogen bond between the OH group of the (xhydroxy ketone and the tertiary nitrogen of the catalyst leads to the formation of a rigid cis enamine intermediate that accounts for the inversion of the expected diastereoselectivity and the very high ees.