A comparative study of CH and mCH mechanisms in hydrosilylation reactions

Abstract

The cyclic process of Si-H oxidation addition, olefin insertion, and reductive elimination was discussed by density functional theory (DFT) and high-precision quantum chemical calculations. All the calculations were performed at the B3LYP-D3/def2-TZVP level. Two fundamental mechanisms of Chalk-Harrod (CH) and modified Chalk-Harrod (mCH) were explored, in the perspective of computational chemistry. Pt(PH₃)₂ was used as model for the renowned Speier's catalyst, while HSiR (R=(CH₃)₃ and CH₃(OSi(CH₃)₃)₂ and CH₂=CHR’ (R’=H, CH₂OH and CH₂OCH₂CH₂OH) were used as the reactant models. Significant findings include the identification of the olefin insertion as the rate-determining step in both mechanisms.The energy barrier of rate-determining step was 27.6 kcal/mol and 41.0 kcal/mol, respectively, according to the CH mechanism and the mCH mechanism, when HSi(CH₃)₃ and CH₂=CH₂ were used as the reactant models. But the barrier of rate-determining step become to 27.2 kcal/mol and 28.4 kcal/mol, when HSiCH₃(OSi(CH₃)₃)₂ and CH₂=CHCH₂OCH₂CH₂OH were used as the reactant models. The difference in energy barriers betweeen the CH and mCH mechanisms become closer as the reactant models increase. The reaction between polyhydrosiloxane and allyl polyether may very potentially follow the mCH mechanism rather than CH mechanism.