Reactivity of Rhodium–Triflate Complexes with Diphenylsilane: Evidence for Silylene Intermediacy in Stoichiometric and Catalytic Reactions

Abstract

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.

Multiple Reaction Pathways in Rhodium‐Catalyzed Hydrosilylations of Ketones

“…[71] Though various attempts to trap a Rh-silylene complex have remained unsuccessful in our case, previous reports indicate that a Rh-silylene complex could indeed be generated under the conditions of catalysis. [20] In the catalytic cycle proposed in this work, the silylene complex is formed by a migration of the hydrogen atom, which is in fact a common synthetic route to silylene complexes. [72] For instance, Pt, Ir, and Os silylene complexes have been obtained by successive oxidative addition, liberation of a coordination site by methide or bromide extraction, and a migration.…”

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

“…Other researchers have contributed indirect evidence of silylene complex formation using related rhodium catalysts (Scheme 50). Reactions of silanes with bisphosphine [104] and bis-NHC [99c] supported rhodium complexes have resulted in observation of dehydrogenative silane oligomerization and olefin hydrogenation processes that are consistent with the activation of both Si-H bonds of a secondary silane at a single rhodium center. Additionally, both studies used t BuMe2SiOH to trap the putative silylene intermediates in a silane alcoholysis process that was not observed with tertiary silanes.…”

Reactions of Hydrosilanes with Transition Metal Complexes and Characterization of the Products

“…3-292 (113) Two of the metal precursors shown in Table 4 have also been used to generate trimers. Thermolysis of (Me 3 P) 2 …”