Why does the rhodium-catalyzed hydrosilylation of alkenes take place through a modified Chalk-Harrod mechanism? A theoretical study

Rh-catalyzed hydrosilylation of ethylene was theoretically investigated with the DFT, MP4(SDQ), and CCSD(T) methods, where RhCl(PH3)3 was adopted as a model catalyst. The rate-determining step in the Chalk−Harrod mechanism is Si−C reductive elimination, the activation barrier (Ea) of which is 27.4 (28.8) kcal/mol, where the values without parenthesis and in parenthesis are calculated with the DFT and MP4(SDQ) methods, respectively. The rate-determining step in the modified Chalk−Harrod mechanism is either ethylene insertion into the Rh−SiMe3 bond (Ea = 13.5 (16.9) kcal/mol) at the MP4(SDQ) level or oxidative addition of HSiMe3 (Ea =15.7 (11.3) kcal/mol) at the DFT level. From these results, it should be clearly concluded that the Rh-catalyzed hydrosilylation of ethylene proceeds through the modified Chalk−Harrod mechanism, unlike Pt-catalyzed hydrosilylation of alkene, which takes place through the Chalk−Harrod mechanism. The difference between Rh and Pt catalysts arises from the facts that ethylene is mo...