The Corey–Bakshi–Shibata Reduction: Mechanistic and Synthetic Considerations – Bifunctional Lewis Base Catalysis with Dual Activation

Abstract

The Corey-Bakshi-Shibata (CBS) reduction is unique among catalytic reactions in that the oxazaborolidine catalysts used in this reaction achieve their "catalytic power" by incorporating thses categories: placing the two stoichi-ometric reactants in proximity, activating the nucleophilic borohydride via Lewis base catalysis, and activating the electrophilic carbonyl via Lewis acid catalysis. The simultaneous use of Lewis acid and Lewis base catalysis integrates the stoichiometric reductant, carbonyl-con-taining substrate, and catalyst into a cyclic transition state whereby enantioselection can be enforced, making this reaction one of the reliable means of reducing prochiral ketones with exceptional stereocontrol. This chapter summarizes the mechanistic understanding and synthetic utility of the CBS reaction that has emerged since the review by Corey and Helal. The Houk group was among the first to recognize that frequency calculations based on transition structures optimized using density functional theory would serve as a means of using experiment to corroborate transition structure models via kinetic isotope effects (KIEs).