The beta-Silicon Effect.II.Substituent Effects on the Solvolysis of 1-Aryl-2-(aryldimethylsilyl)ethyl 3,5-Dinitrobenzoates

摘要

Solvolysis rates of 2-(dimethylphenylsilyl)-1-(Y-phenyl)ethyl 3,5-dinitrobenzoates were determined conductimet-rically in 60% (v/v) aqueous ethanol.In order to clarify the nature of the beta-Si participation quantitatively,the effects of alpha-aryl substituents on the rates were analyzed by means of the Yukawa-Tsuno Eq.The alpha-aryl (Y)-substituent effect at 25 deg C was correlated with r approx= 1.0 and rho = -3.0,which is significantly reduced compared with that of -5.45 for the non-silylated 1-arylethyl system.There is a linear relationship between log k_Y/k_H of silylated and non-silylated substrates:log(k_Y/k_H)_(si) = 0.52 log(k_Y/k_H)_(non-Si).This is the same form as the extended Bronsted relationship.The Bronsted coefficient alpha = 0.52 appeared to be consistent with the neighboring silyl-participation in the silyl-bridged transition state.