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

Solvolysis rates of 2-(dimethylphenylsilyl)-l-(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 p = -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 a = 0.52 appeared to be consistent with the neighboring silyl-participation in the silyl-bridged transition state.

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