Novel cleavages of benzyl- and m-methoxybenzyltrimethylsilane by trifluoroacetic acid

Article abstract of DOI:10.1016/S0022-328X(00)90156-6

Benzyltrimethylsilane is cleaved slowly by trifluoroacetic acid at 70°C to give, in addition to the expected trimethylsilyl trifluoroacetate and toluene, mainly o- and p-trimethylsilylmethyldiphenylmethane (I and II) along with smaller amounts of dibenzyl, o- and p-methyldiphenylmethane, and benzyl trifluoroacetate. Addition of benzyl trifluoroacetate substantially increases the yields of I and II, and it is suggested that they are formed by benzyl trifluoroacetate benzylation of benzyltrimethylsilane, and that the benzyl trifluoroacetate is formed from methylenecyclohexadiene intermediates themselves produced by ring-protonation of benzyltrimethylsilane followed by loss of the Me₃Si⁺ entity. Addition of water up to about 5 wt.-% increases the rate of cleavage and diverts the reaction towards formation of toluene and hexamethyldisiloxane. m-Methoxybenzyltrimethylsilane is much more readily cleaved by trifluoroacetic acid, which is consistent with reaction via a methylenecyclohexadiene type of intermediate, and m-methylanisole is the sole aromatic product.