Rates and Mechanism for the Solvolyses of 2,2-Dimethyl-2-sila-1-indanyl Bromide and α-Trialkylsilylbenzyl <i>p</i>-Toluenesulfonates. α-Silicon Effect on the Stability of Benzylic Cations in Solution

Abstract

Abstract α-Silicon effect on the benzylic solvolysis has been investigated. The solvolysis of 2,2-dimethyl-2-sila-1-indanyl bromide in aq acetone exhibits a linear response to the solvent ionizing power YBr with a slope m close to unity (m=0.93) and gives the corresponding alcohol without skeletal rearrangement indicative of a kc mechanism; it solvolyzes 4.98×102 times less rapidly than a carbon reference, 2,2-dimethyl-1-indanyl bromide, in 60% aq acetone at 25°C suggesting that solvolytic generation of the α-silylated benzylic cation is electronically about 4 kcal mol−1 less favorable than that of the corresponding α-alkylated benzylic cation. A more pronounced rate-retardation of 1.65×104 by an α-SiMe3 group relative to Me is observed in the solvolysis of an open benzylic system due to the additional steric effect. Mechanism for the solvolysis of α-trialkylsilylbenzyl p-toluenesulfonates including SiMe3, SiEt3, and Si(t-Bu)Me2 groups has been also examined on the basis of the solvent effect, salt effect, and ethanol/water selectivity as well as product analysis; a kc mechanism including a product forming step via preferential front side solvent attack is suggested.