Abstract
It is well known that the β-silicon substituent greatly facilitates the heterolysis of the C-Lg bond by 1012-fold, which is caused by the trimethylsilyl group in an antiperiplanar conformation. The β-silicon effect has been explained in terms of either the open form that is stabilized by σ-π-hyperconjugation, or the Si-bridged form by Si-participation. We thought that these two mechanisms can be distinguished by the position of Si atom, i. e., estimation of charge by means of substituent effects of Si-aryl ring. In this work, the substituent effects on the solvolysis rates of aryl-substituted derivatives [Ar-SiMe2-CR4R3-CR1R2L where (R4 = R3 = R2 = H), (R1 = H, t-Bu, m-CF3-Ph, Ph), L = Leaving group] in 60% aq. EtOH at 50°C were analyzed by the Yukawa-Tsuno Equation. The congested α-tBu system was found to indicate an intermediate ρ value of -1.47 between the open-cation system (ρ = ca. -1.00 for R1 = Ph, m--CF3-Ph) and the Si-bridged system (ρ = -1.75 for R1 = H). Since the ρ value is changing continuously, it is concluded that the mechanism of β-silicon effect can be described as a continuous spectrum of transition state between the open cation and the Si-bridged cation based on the continuous change in the ρ value.
