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

Abstract

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.).