(S)-2-Aryl-4,4-diphenyl-3,1,2-oxazaboro[3.3.0]octanes: Efficient catalysts for the asymmetric borane reduction of electron-deficient ketones

Abstract

The obvious influence of electronic effects of ketones on the enantioselectivities was observed previously in the oxazaborolidine-catalyzed asymmetric borane reduction of ketones. On the basis of the catalytic reduction mechanism, the electronic effect of organocatalysts, B-aryl-substituted oxazaborolidines, was tuned rationally to improve the enantioselectivities of the electron-deficient ketones in the reduction. The results indicate that all B-aryloxazaborolidines show excellent enantioselectivities for the electron-deficient ketones. This indicates that B-aryloxazaborolidines show better enantioselectivities than B-unsubstituted and B-methoxy-substituted oxazaborolidines for the electron-deficient ketones. © 2005 Elsevier B.V. All rights reserved.