Origin of the enantioselectivity in the hydrogen transfer reduction of carbonyls by a rhodium(I) complex: A theoretical study

Concerted mechanisms for the catalytic cycle of the carbonyl reduction by a rhodium(I) hydride complex were studied on the basis of DFT theoretical calculations. The first assumed mechanism consists of the direct transfer of a metal-bound hydride to the carbon of the carbonyl in concert with the coordination of the oxygen to the metallic center. In the second mechanism, the hydride of the complex and a proton of a nitrogen-containing ligand are transferred simultaneously to the carbonyl. Several substrates such as formaldehyde, acetone, and the;experimentally used acetophenone were investigated as starting materials, and several nitrogen-containing ligands were considered. Each postulated intermediate was confirmed to be a stationary point on the potential energy surface, and transition states were characterized, except in the case of the first mechanism, where no transition state was found. All the activation barriers were calculated to be within the range of 3.9-18.7 kcal mol-l and are consistent with experimental reaction rates. In the case of acetophenone, the calculations for chiral diamine ligands explain the origin of the observed enantioselectivity with a complete characterization of the diastereoisomer transition states.