Enantioselection mechanism in Rh-catalyzed asymmetric hydrogenation

Abstract

Combined experimental and computational study of the Rh-BenzP*-catalyzed asymmetric hydrogenation (BenzP* is 1,2-bis(tert-butylmethylphosphino)benzene) of five structurally different enamides showed significant diversity in the mechanism of hydrogen activation. In neither case, the mode of the coordination of the C=C double bond or the structure of the most reactive species corresponded to the stereochemical outcome of the reaction product. This fact proved the loss of the chiral information acquired in the resting state and/or in the oxidative addition step and determination of the stereoselection on a later stage of the catalytic cycle. Quantum chemical computations showed that in all cases the stereoselection takes place on the stage of the coordination of the C=C double bond to Rh in a Rh^III non-chelating octahedral complex. The sense of enantioselection is determined via the relative easiness of the chelate cycle formation in a less hindered quadrant of the catalyst. In the case of phenyl-, 4-nitrophenyl-, and 3,5-di(trifluoromethyl)-substituted enamides, the formation of the α-dihydride intermediate in the less hindered quadrant is directed by C—H...π interaction of the tert-butyl group of the catalyst and the aryl substituent of the enamide leading to high R-enantioselectivity. In the case of the tert-butyl-substituted enamide, the formation of α-dihydride is strongly disfavored by steric hindrance and the formation of β-dihydride in the less hindered quadrant results in high S-enantioselectivity. Comparative quantum chemical computations of the asymmetric hydrogenation of the classic system DIPAMP-Rh—methyl (Z)-α-acetamidocinnamate (DIPAMP is 1,2-bis[(2-methoxyphenyl)(phenylphosphino)]ethane) revealed essentially the same mechanism of enantioselection. Early experimental results received alternative rationalization according to our computational data.