Enantioselective Hydrosilylation by Chiral Pd Based Homogeneous Catalysts with First-Principles and Combined QM/MM Molecular Dynamics Simulations

Abstract

The enantioselective hydrosilylation of styrene catalyzed by Pd0 species generated in situ from dichloro{1-{(R)-1-[(S) 2 (diphenylphosphino- κP) ferrocenyl]ethyl}-3-trimethylphenyl-5-1H-pyrazole-κN} palladium. 1, has been investigated in detail by ab initio molecular dynamics and combined quantum mechanics and molecular mechanics (QM/MM) simulations. The nature of the unique structural features observed in the pre-catalyst, 1, and its bis(trichlorosilyl) derivatives have been explored. Using the combined QM/MMmethod we have been able to pinpoint the steric and electronic influence of specific ligands on these geometric distortions. The whole catalytic cycle of the enantioselective styrene hydrosilylation has been examined in detail with mixed QM/MM Car-Parrinello molecular dynamics simulations. The simulations show that the reaction proceeds through the classical Chalk-Harrod mechanism of hydrosilylation. The rate-determining step was found to be the migration of the silyl ligand to the α-carbon of the substrate. The nature of the regiospecificity and enantioselectivity of the catalysis has been established. In both cases the formation of a η3-benzyllic intermediate plays a crucial role. The mechanistic detail afforded by the computational study provides a framework for rationalligand design that would improve the enantioselectivity of the catalysis.