Catalytic cleavage of the SiSi bond of methylchlorodisilanes with nucleophiles: evidences for a stabilised silylene reaction intermediate

The methylchlorodisilanes MeCl2SiSiCl2Me, MeCl2SiSiClMe2 and MeCl2SiSiMe3 disproportionate in the presence of nucleophilic catalysts into the methylchlorosilanes MeSiCl3, Me2SiCl2, Me3SiCl and the polysilanes SinMenCln+2, SinMen+1Cln+1 and SinMen+2Cln (n=3–5). Qualitative kinetic studies of the disproportionation reveal that the catalytic activities of the nucleophiles are ranked according to: phosphine oxides (OPnBu3, OP(NMe2)3)>chlorides (nBu4NCl, nBu4PCl)>phosphines (PnBu3, PiPr3, PMe2Ph, P(NMe2)3, PCy3, PPh3)>phosphites (P(OPh)3). For the PR3 compounds, a correlation is observed between the π acceptor character the activity for the disproportionation, the lower π acceptor the nucleophile, the higher the rate. A two step mechanism is proposed which involves the formation of an intermediate followed by condensation with the substrate disilane to afford the polysilanes. Investigation of the reaction of the intermediate with hydrogen as a test reaction provides evidences for the condensation step to be rate limiting in the disproportionation. That result, combined with the fact that poor π acceptor nucleophiles (phosphine oxides, chlorides and trialkylphosphines) are more active disproportionation catalysts, supports the description of the intermediate as a stabilised silylene.