London Dispersion Rather than Steric Hindrance Determines the Enantioselectivity of the Corey-Bakshi-Shibata Reduction.

The well-known Corey-Bakshi-Shibata (CBS) reduction is a powerful method for the asymmetric synthesis of alcohols from prochiral ketones, often featuring high yields and excellent selectivi-ties.  While steric repulsion has been regarded as the key director of the observed high enantioselectivity for many years, here we show that London dispersion (LD) interactions are at least as important for enantiodiscrimination.  We exemplify this through a combination of detailed computational and experimental studies for a series of modi-fied CBS catalysts equipped with dispersion energy donors (DEDs) in the catalysts and the substrates.  Our results demonstrate that attrac-tive LD interactions between the catalyst and the substrate rather than steric repulsion determine the selectivity.  As a key outcome of our study, we were able to improve the catalyst design for some challeng-ing CBS reductions.