Hidden Enantioselective Hydrogenation of N-Silyl Enamines and Silyl Enol Ethers in Net C═N and C═O Hydrosilylations Catalyzed by Ru–S Complexes with One Monodentate Chiral Phosphine Ligand

Abstract

Ruthenium thiolate complexes with one chiral monodentate phosphine ligand are applied to enantioselective hydrosilylation of enolizable imines and ketones. The structural features of the catalyst exclude the presence of more than one phosphine ligand at the ruthenium center in the enantioselectivity-determining step. The enantiomeric excesses obtained in these reduction reactions are moderate (up to 66% ee), but the stereochemical outcome enables an experimental analysis of the reaction pathways operative in this catalysis. A two-step sequence consisting of successive N–Si/O–Si dehydrogenative coupling and enamine/enol ether hydrogenation is the prevailing mechanism of action. Both steps involve cooperative bond activation at the Ru–S bond of the coordinatively unsaturated ruthenium complex: Si–H bond activation in the dehydrogenative coupling and heterolytic H–H splitting in the hydrogenation. Previously documented side reactions such as deprotonation/protonation equilibria as well as competing direct C═N or C═O hydrogenation have been excluded.