Competitive Substitution Reactions at Extracoordinate Silicon during Asymmetric Hydrosilylation

Proline-derived small molecules were utilized as organic catalysts in the asymmetric hydrosilylation of ketones. Extracoordinate chiral hydrosilicates were generated in situ as shown by 29Si NMR. These active species reduce prochiral ketones enantioselectively in good yield but only moderate enantiomeric excess (up to 40%). The parameters affecting stereoselectivity have been studied in detail. By isolating some of the intermediates and performing a series of control reactions, the relative efficiency of alkoxy group exchange reactions and the ability of ketones to complex to extracoordinate hydrosilicates have been discovered to be critical factors. The proline catalyst is a poorer leaving group from the initial pentacoordinate product than the product of reduction, PhMeCHO-, which leads subsequently to several competitive hydrosilylation pathways.