Platinum-catalyzed hydrosilylation of ethylene. A theoretical study on the reaction mechanism involving cis– trans isomerization of PtH(SiH 3)(PH 3) 2

Abstract

Pt(PH 3) 2-catalyzed hydrosilylation of ethylene involving cis– trans isomerization of PtH(SiH 3)(PH 3) 2 was theoretically investigated with ab initio MO/MP2–MP4SDQ and CCD methods. The cis– trans isomerization via Berry's pseudo-rotation mechanism occurs with an activation barrier ( E a) of 28 kcal/mol when promoted by PH 3 and with E a of 22.1 kcal/mol when promoted by C 2H 4, where E a values calculated with the MP4SDQ method are given since both MP4SDQ and CCD methods yield similar values. Ethylene is easily inserted into Pt–H and Pt–SiH 3 bonds of trans-PtH(SiH 3)(PH 3)(C 2H 4) with E a of 3.6 kcal/mol and 15.9 kcal/mol, respectively. The Si–C reductive elimination occurs with E a of 24.3 kcal/mol in Pt(CH 3)(SiH 3)(PH 3) 2 and 11.1 kcal/mol in Pt(CH 3)(SiH 3)(PH 3)(C 2H 4), and the C–H reductive elimination with E a of 17.7 kcal/mol in PtH(CH 3)(PH 3) 2 and 9.9 kcal/mol in PtH(CH 3)(PH 3)(C 2H 4), where CH 3 is adopted as a model of C 2H 5 and CH 2CH 2SiH 3. The rate-determining step is the cis– trans isomerization in both Chalk–Harrod and modified Chalk–Harrod mechanisms involving the cis– trans isomerization, and its barrier is similar to that of the rate-determining step (isomerization of ethylene insertion product; E a=22.4 kcal/mol) in the usual Chalk–Harrod mechanism without the cis– trans isomerization. These results suggest that the reaction mechanism involving the cis– trans isomerization cannot be ruled out, when the cis– trans isomerization is facilitated by introduction of less donating silyl group and use of electron-withdrawing alkene since they would stabilize the pseudo-trigonal pyramidal transition state of the cis– trans isomerization.