Platinum-catalyzed hydrosilylation of ethylene. A theoretical study on thereaction mechanism involving cis-trans isomerization of PtH(SiH3)(PH3)(3)

Pt(PH3)(2)-catalyzed hydrosilylation of ethylene involving cis-trans isomer ization of PtH(SiH3)(PH3)(2) was theoretically investigated with ab initio MO/MP2-MP4SDQ and CCD methods. The cis-trans isomerization via Berry's pseu do-rotation mechanism occurs with an activation barrier (E-a) of 28 kcal/mo l when promoted by PH3 and with E-a of 22.1 kcal/mol when promoted by C2H4, where E-a values calculated with the MP4SDQ method are given since both MP 4SDQ and CCD methods yield similar values. Ethylene is easily inserted into Pt-H and Pt-SiH3 bonds of trans-PtH(SiH3)(PH3)(C2H4) 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(CH3)(SiH3)(PH3)(2) and 11.1 kcal/mol in Pt( CH3)(SiH3)(PH3)(C2H4), and the C-H reductive elimination with E-a of 17.7 k cal/mol in PtH(CH,)(PH,), and 9.9 kcal/mol in PtH(CH3)(PH3)(C2H4), where CH 3 is adopted as a model of C2H5 and CH2CH2SiH3. The rate-determining step i s the cis-trans isomerization in both Chalk-Harrod and modified Chalk-Harro d mechanisms involving the cis-trans isomerization, and its barrier is simi lar to that of the rate-determining step (isomerization of ethylene inserti on product; E-a= 22.4 kcal/mol) in the usual Chalk-Harrod mechanism without the cis-trans isomerization. These results suggest that the reaction mecha nism involving the cis-trans isomerization cannot be ruled out, when the ci s-trans isomerization is facilitated by introduction of less donating silyl group and use of electron-withdrawing alkene since they would stabilize th e pseudo-trigonal pyramidal transition state of the cis-trans isomerization . (C) 1999 Elsevier Science B.V. All rights reserved.