Reactivity of rhodium-triflate complexes with diphenylsilane: evidence for silylene intermediacy in stoichiometric and catalytic reactions

Addition of Ph2SiH2 to [Rh(iPr3P)2(OTf)] (1) yielded the thermally unstable RhIII adduct [Rh(iPr3P)2(OTf)(H)(SiPh2H)] (2), which decomposed to [Rh(iPr3P)2(H)2(OTf)] (3), liberating (unobserved) silylene. The silylene was trapped by 1, resulting in the RhI-silyl complex [Rh(iPr3P)2(SiPh2OTf)]. Complex 3 was converted to 2 by addition of diphenylsilane, providing a basis for a possible catalytic cycle. The last reaction did not involve a RhI intermediate, as shown by a labeling study. Complex 1 catalyzed the dehydrogenative coupling of Ph2SiH2 to Ph2HSi--SiHPh2. A mechanism involving a silylene intermediate in this catalytic cycle is proposed. The mechanism is supported by complete lack of catalysis in the case of the tertiary silanes Ph2MeSiH and PhMe2SiH, and by a study of individual steps of the catalytic cycle. The outcome of the reaction of Ph2SiH2 with styrene in the presence of 1 depends on the complex/substrate ratio; under stoichiometric conditions olefin hydrogenation prevailed over hydrosilylation, whereas with excess of substrates hydrosilylation prevailed. Catalytic hydrosilylation resulted in double addition giving Ph2Si(CH2CH2Ph)2. Mechanistic aspects of the reported processes are discussed, and a new hydrosilylation mechanism based on silylene intermediacy is proposed.