The β-Silicon Effect. III. Substituent Effects of the β-Si-Aryl Moiety in the Solvolysis of 1-Aryl-2-(aryldimethylsilyl)ethyl 3,5-Dinitrobenzoates

Solvolysis rates of 2-[dimethyl(X-phenyl)silyl]-1-(Y-phenyl)ethyl 3,5-dinitrobenzoates were determined conductimetrically in 60% (v/v) aqueous ethanol. The effects of substituents X in the β-arylsilyl moiety (X-C6H4(CH3)2Si–) were analyzed by means of the Yukawa–Tsuno Equation. The X-phenyl-Si substituent effects (at 25 °C) were correlated against a set of X parameters of r+≅0.1 to give a [(ρX)Si] value of −1.06 for α-Ph (Y=H) and [(ρX)Si]=−1.18 for α-(m-CF3)phenyl (Y = m-CF3) subsets, which are all distinctly lower than that of −1.75 for the 2-[dimethyl(X-phenyl)silyl]ethyl solvolysis. The δY[(ρX)Si]Y can be regarded as the effect of the aryl substituents on the β-Si moiety on the benzylic cation center in the rate-determining step. The α-aryl substituent effect δY[log(kY)X=H] of 2-[dimethyl(X-phenyl)silyl]-1-(Y-phenyl)ethyl system was correlated against Y scale characteristic of the benzylic cation reactions (r+≅1.0) to give (ρY)α-Ar≅−3.0. The [(ρX)Si]Y values changed in parallel with the reactivities δY[log(kY)X=H] of the α-Ar substrate.