Mechanistic aspects of samarium-mediated σ-bond activations of arene C-H and arylsilane Si-C bonds

Article abstract of DOI:10.1021/ja011472w

To investigate the potential role of Sm-Ph species as intermediates in the samarium-catalyzed redistribution of PhSiH₃ to Ph₂SiH₂ and SiH₄, the samarium phenyl complex [Cp*₂SmPh]₂ (1) was prepared by oxidation of Cp*₂Sm (2) with HgPh₂. Compound 1 thermally decomposes to yield benzene and the phenylene-bridged disamarium complex Cp*₂Sm(μ-1,4-C₆H₄)SmCp* ₂ (3). This decomposition reaction appears to proceed through dissociation of 1 into monomeric Cp*₂SmPh species which then react via unimolecular and bimolecular pathways, involving rate-limiting Cp* metalation and direct C-H activation, respectively. The observed rate law for this process is of the form: rate = k₁[1] + k₂[1]². Complex 1 efficiently transfers its phenyl group to PhSiH₃, with formation of Ph₂SiH₂ and [Cp*₂Sm(μ-H)]₂ (4). Quantitative Si-C bond cleavage of C₆F₅SiH₃ is effected by the samarium hydride complex 4, yielding silane and [Cp*₂Sm(μ-C₆F₅)]₂ (5). In contrast, Si-H activation takes place upon reaction of 4 with o-MeOC₆H₄SiH₃, affording the samarium silyl species I Cp*₂SmSiH₂(o-MeOC₆H₄) (7). Complex 7 rapidly decomposes to [Cp*₂Sm(μ-o-MeOC₆H₄)]₂ (6) and other samarium-containing products. Compounds 5 and 6 were prepared independently by oxidation of 2 with Hg(C₆F₅)₂ and Hg(o-MeOC₆H₄)₂, respectively. The mechanism of samarium-mediated redistribution at silicon, and chemoselectivity in σ-bond metathesis reactions, are discussed.