The Use of N‐Alkyl‐2,2′‐bipyrrolidine Derivatives as Organocatalysts for the Asymmetric Michael Addition of Ketones and Aldehydes to Nitroolefins

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Abstract
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.

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