Hydride transfer reduction of carbonyls by a rhodium(I) complex: A theoretical study. 1. The two-step mechanism

The two-step mechanism for the catalytic cycle of carbonyl reduction by a rhodium(I) hydride model complex, RhH(NH3)(2)(C2H4)(2), was studied on the basis of DFT theoretical calculations. This assumed mechanism consists of the dissociation of a N-Rh bond on the hydride complex, the coordination of the carbonyl, followed by the hydride migration, the recoordination of the N-ligand, and finally the exchange of alkoxy ligands to give the desired alcohol. The cycle is terminated by the reverse reactions, acetone is eliminated, and the hydride complex is regenerated. Several substrates such as formaldehyde, acetone, and the experimentally used acetophenone were investigated as starting materials. Each postulated intermediate was confirmed to be a stationary point on the potential energy surface, and transition states were determined. The potential energy profile was found to be smooth without excessive activation barriers. The hydride migration and its reverse reaction (beta-H transfer) are found to be the rate-determining steps.