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

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 foll ows: (1) the Si-H oxidative addition of silane to Pt(PH3)(2) occurs with a very low barrier. (2) Ethylene is more easily inserted into Pt-H than into Pt-SiR3 (R = N, Cl, or Me). (3) The Si-C reductive elimination from Pt(CH3) (SiR3)(PH3)(C2H4) and the C-H reductive elimination from PtN(CH3)(PH3)(C2H4 ) occur more easily than those from Pt(CH3)(SiR3)(PH3)(2) and PtH(CH3)(PH3) (2), respectively. (4) The transition state of the Si-C reductive eliminati on is non-planar, while that of the C-H reductive elimination is planar. Fr om those results, the reaction mechanism of Pt(PH3)(2)-catalyzed hydrosilyl ation of ethylene was discussed. The rate-determining step of the Chalk-Har rod mechanism is the isomerization of ethylene insertion product whose barr ier is estimated to be about 22 kcal mol(-1) for R = H and Me, and 26 kcal mol(-1) for R = CI (MP4SDQ values are given here), while that of the modifi ed Chalk-Harrod mechanism is the ethylene insertion into Pt-SiR3, whose bar rier 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-Warred mechanism in the Pt(PH3)(2)-catalyzed hydrosilyl ation of ethylene. Though cis-PtH(SiH3)(PH3)(2) is directly produced by the SiH4, oxidative addition to Pt(PH3)(2), the cia-complex might isomerize to the trans-form through Berry's pseudo-rotation mechanism. Ethylene is much more easily inserted into Pt-H and Pt-SiH3, in trans-PtH(SiH3)(PH3)(C2H4) than in the cia-form. Even in the trans-form, ethylene is more easily inser ted into Pt-H than into Pt-SiH3. In the Chalk-Harrod and modified Chalk-Har rod mechanisms including the cia-trans isomerization, the rate-determining step is the cia-trans isomerization whose barrier is about 22 kcal mol(-1) in ethylene-promoted isomerization and 29 kcal mol(-1) in the PH3-promoted one. Thus, this Chalk-Warred mechanism is more favorable than the modified Chalk-Harrod mechanism even if a cia-trans isomerization is involved in the reaction, but the barrier of the rate-determining step in the modified Cha lk-Harrod mechanism is significantly lowered by the cia-trans isomerization ; part of the Pt(PH3)(2)-catalyzed hydrosilylation of ethylene might occur through the modified Chalk-Harrod mechanism including the cis- trans isomer ization. (C) 1999 Elsevier Science S.A. All rights reserved.