The Hydrogenation/Transfer Hydrogenation Network in Asymmetric Reduction of Ketones Catalyzed by [RuCl2(binap)(pica)] Complexes

Chiral binap/pica-Ru-II complexes (binap = (S)- or (R)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl; pica = alpha-picolylamine) catalyze both asymmetric hydrogenation (AH) of ketones using H-2 and asymmetric transfer hydrogenation (ATH) using non-tertiary alcohols under basic conditions. The AH and ATH catalytic cycles are linked by the metal-ligand bifunctional mechanism. Asymmetric reduction of pinacolone is best achieved in ethanol containing the Ru catalyst and base under an H-2 atmosphere at ambient temperature, giving the chiral alcohol in 97-98% ee. The reaction utilizes only H-2 as a hydride source with alcohol acting as a proton source. On the other hand, asymmetric reduction of acetophenone is attained with both H-2 (ambient temperature) and 2-propanol (>60 degrees C) with relatively low enantioselectivity. The degree of contribution of the AH and ATH cycles is highly dependent on the ketone substrates, solvent, and reaction parameters (H-2 pressure, temperature, basicity, substrate concentration, H/D difference, etc.).