Theoretical studies on the mechanism and stereoselectivity of Rh(Phebox)-catalyzed asymmetric reductive aldol reaction

Density functional theory calculations (B3LYP) have been carried out to understand the mechanism and stereochemistry of an asymmetric reductive aldol reaction of benzaldehyde and tert-butyl acrylate with hydrosilanes catalyzed by Rh(Phebox-ip)(OAc)(2)(OH2). According to the calculations, the reaction proceeds via five steps: (1) oxidative addition of hydrosilane, (2) hydride migration to carbon-carbon double bond of tert-butyl acrylate, which determines the chirality at C2, (3) tautomerization from rhodium bound C-enolate to rhodium bound O-enolate, (4) intramolecular aldol reaction, which determines the chirality at C3 and consequently the anti/syn-selectivity, and (5) reductive elimination to release aldol product. The hydride migration is the rate-determining step with a calculated activation energy of 23.3 kcal mol(-1). In good agreement with experimental results, the formation of anti-aldolates is found to be the most favorable pathway. The observed Si-facial selectivity in both hydride migration and aldol reaction are well-rationalized by analyzing crucial transition structures. The Re-facial attack transition state is disfavored because of steric hindrance between the isopropyl group of the catalyst and the tert-butyl acrylate.