A study of the cleavage of alkylsilanes by strong acids

Abstract

The stepwise cleavage of methyl groups from organosilanes of the type (CH₃)₃SiCH₂X (X = H, I, Br, Cl, and CH₃) has been studied in strong acid systems. The number of methyl groups cleaved could be controlled by varying the temperature and the acid strength. The progress of the reactions and the products formed could be directly observed by nuclear magnetic resonance spectroscopy. At low temperatures (-70 to -20°) in fluorosulfonic acid-sulfur dioxide the compounds undergo exclusive cleavage of one methyl group to form methane and the corresponding trimethylsilyl fluorosulfonate. The kinetics were studied for tetramethylsilane and (chloromethyl)-trimethylsilane. The relative rates showed that the key step in the mechanism is electrophilic attack of the acid at the methyl group. The cleavage of a second silicon-carbon bond occurs at a convenient rate at about 30°. Fluorosulfonic acid was used without sulfur-dioxide as a solvent. Compared to the simplicity of the first cleavage reaction, the second cleavage is complex and a number of new reaction paths were found. The cleavage of methyl groups from trimethylsilyl fluorosulfonate and (chloromethyl)-dimethylsilyl fluorosulfonate results in the formation of methane and methanesulfonyl fluoride (CH₃SO₂F) as the cleavage products. Detailed kinetic data show that methane and methanesulfonyl fluoride are formed in a competitive manner. The methanesulfonyl fluoride is formed as an integral part of the cleavage mechanism and not from the subsequent reaction of methane with the fluorosulfonic acid..