Evidence for a concerted mechanism in the solvolysis of phenyldimethylsilyl ethers

Article abstract of DOI:10.1021/ja00076a033

The trifluoroethoxide-catalyzed trifluoroethanolysis and the hydroxide-catalyzed hydrolysis of a series of phenyldimethylsilyl ethers were examined. A Bronsted plot of the logarithm of the second-order rate constant K_TFE for reaction with trifluoroethanol against the pK_LG is not linear. The nonlinear plot might be taken as evidence for a change in rate-determining step of a reaction that proceeds through a pentavalent intermediate. However, the Bronsted plot for the hydroxide-catalyzed hydrolysis, where all the leaving groups are of lower pK_a than hydroxide, has an identical shape as the Bronsted plot for the trifluoroethanolysis reaction. Therefore, the unusual shape of the Bronsted plots is not due to a change in rate-determining step. It is suggested that the results are most consistent with a one-step concerted mechanism and not with a mechanism involving a pentavalent intermediate.