Solvolysis rates of 2-(dimethylphenylsilyl)-1-(Y-phenyl)ethyl 3,5-dinitrobenzoates were determined conductimetrically in 60% (v/v) aqueous ethanol. In order to clarify the nature of the β-Si participation quantitatively, the effects of α-aryl substituents on the rates were analyzed by means of the Yukawa–Tsuno Eq. The α-aryl (Y)-substituent effect at 25 °C was correlated with r 1.0 and ρ = −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 logkY/kH of silylated and non-silylated substrates: log(kY/kH)Si = 0.52log(kY/kH)non-Si. This is the same form as the extended Brønsted relationship. The Brønsted coefficient α = 0.52 appeared to be consistent with the neighboring silyl-participation in the silyl-bridged transition state.
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.
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