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Enantioselective Palladium-Catalyzed Hydrosilylation of Styrene: Detailed Reaction Mechanism from First-Principles and Hybrid QM/MM Molecular Dynamics Simulations

The mechanism of the enantioselective hydrosilylation of styrene catalyzed by Pd0 species generated in situ from dichloro[1-[(R)-1-[(S)-2(diphenylphosphino-kP)ferrocenyl]ethyl]-3-trimethylphenyl-5-1H-pyrazole-kN]palladium (1) was investigated in detail through ab initio mol. dynamics and hybrid ab initio mol. dynamics/mol. mechanics (QM/MM) calcns. Different QM/MM models were adopted in order to probe the specific steric and electronic contributions of different substituents. The catalytic cycle is initiated by the formation of a weakly bound p-complex (DE   -5.4 kcal/mol) under simultaneous dissocn. of the pyrazole ligand. In agreement with a Chalk-Harrod mechanism, the next step is the migratory insertion of the hydride, which leads to a h3-coordination mode of the benzylic fragment. The significant stabilization of the allylic intermediate (DE   -11 kcal/mol) is responsible for the high regioselectivity of the reaction (as well as for its enantioselectivity). The rate-detg. step with an activation barrier of 16 kcal/mol is the migration of the silyl ligand to the a-carbon of the substrate with concomitant closure of the ligand chelate ring. This step leads to the formation of an intermediate in which the Ph moiety of the product remains coordinated in an h2-mode to the palladium. The addn. of trichlorosilane leads to product formation and hence to the regeneration of the catalyst. An alternative reaction pathway comprising in concerted transfer of the silyl ligand to the benzylic fragment simultaneous oxidative addn. of a mol. of HSiCl3 is disfavored by high activation barrier of .apprx.30 kcal/mol. [on SciFinder (R)]

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    Record created on 2006-02-27, modified on 2016-08-08

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