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

To investigate the potential role of Sm-Ph species as intermediates in the samarium-catalyzed redistribution of PhSiH3 to Ph2SiH2 and SiH4, the samari urn phenyl complex [Cp*2SmPh](2) (1) was prepared by oxidation of Cp*Sm-2 ( 2) with HgPh2. Compound 1 thermally decomposes to yield benzene and the phe nylene-bridged disamarium complex CP*Sm-2(mu -1,4-C6H4)SmCp*(2) (3). This d ecomposition reaction appears to proceed through dissociation of 1 into mon omeric CP*2SmPh species which then react via unimolecular and bimolecular p athways, involving rate-limiting Cp* metalation and direct C-H activation, respectively. The observed rate law for this process is of the form: rate = k(1)[1] + k(2)[1](2) Complex I efficiently transfers its phenyl. group to PhSiH3, with formation of Ph2SiH2 and [Cp*Sm-2(mu -H)](2) (4). Quantitative Si-C bond cleavage Of C6F5SiH3 is effected by the samarium hydride complex 4, yielding silane and [Cp*Sm-2(mu -C6F5)](2) (5). In contrast, Si-H activ ation takes place upon reaction of 4 with o-MeOC6H4SiH3, affording the sama rium. silyl species Cp*2SmSiH2(o-MeOC6H4) (7). Complex 7 rapidly decomposes to [Cp*Sm-2(mu -0-MeOC6H4)](2) (6) and other samarium-containing products. Compounds 5 and 6 were prepared independently by oxidation of 2 with Hg(C6 F5)(2), and Hg(o-MeOC6H4)(2), respectively. The mechanism of samarium-media ted redistribution at silicon, and chemoselectivity in sigma -bond metathes is reactions, are discussed.