Reactivities of silicas with organometallic methylating agents

The surface of silica gel was modified by a variety of procedures aimed at the introduction of CH3-Si moieties. The formation of these moieties by rea ctions of either SOCl2-chlorinated silica or SiCl4-modified silica with sim ple organometallic reagents (methyllithium, methylmagnesium bromide, dimeth ylzinc, or trimethylaluminum) was examined. Solid-state Si-29 and C-13 NMR techniques were used to characterize the resulting materials, especially to determine the extent of methylation of the silica surface. It was shown th at, for a synthetic pathway that includes a chlorination step, methylmagnes ium bromide was the best reagent for simply attaching a methyl group to a s urface silicon atom. Methyllithium is very destructive to the silica framew ork, and trimethylaluminum forms a very small amount of Si-CH3 moieties whe n allowed to react with silica itself, SOCl2-chlorinated silica, or SiCl4-t reated silica. There was no obvious evidence that dimethylzinc reacted with dry silica or with either SOCl2-modified or SiCl4-modified silicas to form Si-CH3 bonds.