Solvolysis reactions of alkoxysilicon compounds.

The methods of preparation and the mechanisms of the replacement reactions of triphenylsilyl alkoxides are reviewed, with particular reference to substitution at silicon generally. A radiochemical technique has been used to make a kinetic study of the solvolysis reactions of triphenylsilyl alkoxides. The compounds were prepared with the alkoxy group labelled with tritium. The rates of replacement of the alkoxy group, and thus the rates of solvolysis, were determined by withdrawing aliquots at intervals from the reaction mixtures, extracting the silicon compounds and measuring the radioactivity with a liquid photoscintillation counter. The hydrolysis of triphenylsilyl methoxides and isopropoxides in acetone/water and the methanolysis of the methoxides, a symmetrical exchange reaction, have been studied, with acidic and basic catalysts and in the absence of catalyst. Satisfactory results were obtained except in two cases, the base catalysed hydrolysis of the methoxides and the acid catalysed methanolysis of triphenylsilyl methoxide, which were greatly affected by impurities. The results show that in the absence of a catalyst the compounds undergo solvolysis only very slowly, but there is a rapid reaction in the presence of very small quantities of acid or base. In general, acid is a more effective catalyst than base. The relative rates of reaction of the trisubstituted triphenylsilyl alkoxides do not satisfy the show Hammett relationship, but the results show there is a tendency for the electron supplying substituents to retard the reaction in all cases. Mechanisms, involving SN2 attack on silicon in the rate determining step are proposed for the reactions.