Pt-catalyzed hydrosilylation of ethylene. A theoretical study of the reaction mechanism

Abstract

All the elementary steps involved in platinum(0)-catalyzed hydrosilylation of ethylene were theoretically investigated in detail with ab initio MO/MP2-MP4(SDQ) and CCD methods. Several important results are summarized as follows: (1) the Si–H oxidative addition of silane to Pt(PH 3) 2 occurs with a very low barrier. (2) Ethylene is more easily inserted into Pt–H than into Pt–SiR 3 (R=H, Cl, or Me). (3) The Si–C reductive elimination from Pt(CH 3)(SiR 3)(PH 3)(C 2H 4) and the C–H reductive elimination from PtH(CH 3)(PH 3)(C 2H 4) occur more easily than those from Pt(CH 3)(SiR 3)(PH 3) 2 and PtH(CH 3)(PH 3) 2, respectively. (4) The transition state of the Si–C reductive elimination is non-planar, while that of the C–H reductive elimination is planar. From those results, the reaction mechanism of Pt(PH 3) 2-catalyzed hydrosilylation of ethylene was discussed. The rate-determining step of the Chalk–Harrod mechanism is the isomerization of ethylene insertion product whose barrier is estimated to be about 22 kcal mol −1 for R=H and Me, and 26 kcal mol −1 for R=Cl (MP4SDQ values are given here), while that of the modified Chalk–Harrod mechanism is the ethylene insertion into Pt–SiR 3 whose barrier is 44 kcal mol −1 for R=H, 41 kcal mol −1 for R=Me, and 60 kcal mol −1 for R=CI. Thus, the Chalk–Harrod mechanism is more favorable than the modified Chalk–Harrod mechanism in the Pt(PH 3) 2-catalyzed hydrosilylation of ethylene. Though cis-PtH(SiH 3)(PH 3) 2 is directly produced by the SiH 4 oxidative addition to Pt(PH 3) 2, the cis-complex might isomerize to the trans-form through Berry’s pseudo-rotation mechanism. Ethylene is much more easily inserted into Pt–H and Pt–SiH 3 in trans-PtH(SiH 3)(PH 3)(C 2H 4) than in the cis-form. Even in the trans-form, ethylene is more easily inserted into Pt–H than into Pt–SiH 3. In the Chalk–Harrod and modified Chalk–Harrod mechanisms including the cis– trans isomerization, the rate-determining step is the cis– trans isomerization whose barrier is about 22 kcal mol −1 in ethylene-promoted isomerization and 29 kcal mol −1 in the PH 3-promoted one. Thus, this Chalk–Harrod mechanism is more favorable than the modified Chalk–Harrod mechanism even if a cis– trans isomerization is involved in the reaction, but the barrier of the rate-determining step in the modified Chalk–Harrod mechanism is significantly lowered by the cis– trans isomerization; part of the Pt(PH 3) 2-catalyzed hydrosilylation of ethylene might occur through the modified Chalk–Harrod mechanism including the cis– trans isomerization.