γ-Silicon Stabilization of Carbonium Ions in Solvolysis. 3. Solvolysis of 4-(Trimethylsilyl)-3-methyl-2-butyl p-Bromobenzenesulfonates

Article abstract of DOI:10.1021/ja00200a045

It has been found that the rates of limiting solvolysis of 4-(trimethylsilyl)-3-methyl-2-butyl brosylate are accelerated relative to those of the corresponding 3,3-dimethyl-2-butyl and 4-(trimethylsilyl)-2-butyl esters.The relatively low α- and β-deuterium effects on the rates suggest a mechanism involving silicon-promoted γ-carbon participation.In order to determine the preferred stereochemical course of solvolytic substitution and of cyclopropane formation the threo and erythro alcohols were prepared separately, and the structure of the p-nitrobenzoate ester ofthe threo alcohol was established by X-ray crystallographic examination.Solvolysis of the brosylate esters of each diastereoisomeric alcohol produced mainly substitution products having exclusively retained configuration; the minor yields of 1,2-dimethylcyclopropane formed from the threo ester were predominantly of the cis configuration, and those from the erythro isomer were predominantly of the trans configuration.It is concluded that the mechanism involves the formation of an intermediate carbocation stabilized by the γ-silicon substituent through a "percaudal" interaction.The transition states for the formation and destruction of the intermediate ion are predominantly of the "W" conformation, but the "endo-sickle" also contributes and in each case nucleophilic attack apparently occurs from the front side of the bridged ion.