Contribution of DFT Calculations to the Understanding of an Asymmetric Reaction, the Hydrogen Transfer Reduction of Ketones by a Rhodium(I) Complex

Abstract

AbstractThe density functional theory (DFT) calculations presented here allow us to understand the mechanism of an asymmetric reaction, the hydride transfer reduction of acetophenone by a RhI complex bearing a chiral diamine ligand, RhH(diam)(COD). The mechanism which accounts for the enantioselectivity is a concerted one, where both the hydride and one hydrogen carried by a nitrogen are transferred simultaneously to the ketone. The two diastereoisomeric transition states leading to the R and S alcohols have been determined, the former being the most stable. The activation free energies, the kinetic constants and the enantiomeric excesses ee have been calculated, giving values in good agreement with the experiments. Two conditions ensure the enantioselectivity. The first is the existence of a unique isomer of the starting complex, which is achieved by the presence of bulky substituents on the diamine asymmetric carbons. The second is the presence of one hydrogen on the diamine nitrogens (secondary amines) and the existence of a C2 axis for the diamine ligand, which means that only one of these hydrogens points in the same direction as the Rh−H bond, a condition necessary for a low‐energy concerted transfer. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)