Direct observations of the metal-ligand bifunctional addition step in an enantioselective ketone hydrogenation.

The catalytic intermediate trans-[Ru((R)-BINAP)(H)2((R,R)-dpen)] (1) reacted on mixing with acetophenone in THF at -80 degrees C under approximately 2 atm H2 to generate the alkoxide trans-Ru((R)-BINAP)(H)((Ph)(Me)CHO)((R,R)-dpen) (6). Contrary to expectations, free Ru-amide and 1-phenylethanol were not the immediate products of this addition reaction. The addition reaction was reversible in THF. 2-Propanol prevents racemization of the alcohol product in THF solvent.