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-C 6 H 4 (CH 3 ) 2 Si-) were analyzed by means of the Yukawa-Tsuno Equation. The X-phenyl-Si substituent effects (at 25 °C) were correlated against a set of σx parameters of r + ≅0.1 to give a [(ρ X )Si] value of -1.06 fora-Ph (Y = H) and [(ρ X )Si] = -1.18 for α-(m-CF 3 )phenyl (Y = m-CF 3 ) subsets, which are all distinctly lower than that of -1.75 for the 2-[dimethyl(X-phenyl)-silyl]ethyl solvolysis. The δ Y [(ρ X )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(k Y )X=H] of 2-[dimethyl(X-phenyl)silyl]-l-(Y-phenyl)ethyl system was correlated against σ Y scale characteristic of the benzylic cation reactions (r + ≅ 1.0) to give (ρ Y ) α-Ar ≅-3.0. The [(ρ X )Si]Y values changed in parallel with the reactivities δ Y [log(k Y ) X=H ] of the α-Ar substrate.