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 α‐hydroxy ketones to β‐arylnitroolefins with enantioselectivities up to 98% ee. The formation of an internal hydrogen bond between the OH group of the α‐hydroxy 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.