Ru(arene)(amino alcohol)-catalyzed transfer hydrogenation of ketones: Mechanism and origin of enantioselectivity

The mechanism of the Ru(arene)(amino alcohol)-catalyzed transfer hydrogenat ion of ketones using isopropyl alcohol as the hydrogen source has been stud ied by means of hybrid density functional methods (B3PW91). Three mechanist ic alternatives were evaluated, and it:was shown that the reaction takes pl ace via a six-membered transition state, where a metal-bound hydride and a proton of a coordinated amine are transferred simultaneously to the ketone. Further calculations provided a general rationale for the rate of the reac tion by comparison of steric effects in the ground and transition states of the ruthenium hydride complex. It was found that the TS has a strong prefe rence for planarity, and this in turn is dependent on the conformational be havior of the O,N-linkage of the amino alcohol ligand. Finally, a,a general model, rationalizing the enantioselectivity of the reaction, was developed . Experimental studies of both rate and enantioselectivity were used in ord er to support the computational results.