Why Does the Rhodium-Catalyzed Hydrosilylation of Alkenes Take Place through a Modified Chalk−Harrod Mechanism? A Theoretical Study
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Abstract
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 more easily inserted into the Rh−SiMe3 bond with a moderate Ea value than that into the Pt−SiR3 bond (Ea = 41−60 kcal/mol) and the Si−C reductive elimination of RhCl(CH3)(SiMe3)(PH3)2(C2H4) needs a very large Ea value. This difference in the ethylene insertion between Pt and Rh catalysts is reasonably interpreted in terms that an alkyl group is formed at a position trans to hydride in the Pt catalyst but formed at a position trans to PH3 in the Rh catalyst. This is because ethylene can take a position trans to PH3 in the pseudo-octahedral six-coordinate Rh(III) complex, but ethylene must take a position trans to hydride in the four-coordinate planar Pt(II) complex (remember that Rh(III) and Pt(II) have d6 and d8 electron configurations, respectively). The large Ea value of the Si−C reductive elimination results from the fact that both sp3 valence orbitals of SiMe3 and CH3 must change their directions from the Rh center toward CH3 and SiMe3, respectively, in the transition state. The present theoretical calculations also show that β-H abstraction by the Rh center easily occurs in RhClH(CH2CH2SiMe3)(PH3)2 to yield a Rh(III) vinylsilane complex, with a low activation barrier.
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