The β ‐silicon effect. 4: substituent effects on the solvolysis of 1‐alkyl‐2‐(aryldimethylsilyl)ethyl trifluoroacetates

Solvolysis rates of 2‐(aryldimethylsilyl)‐1‐methylethyl and 2‐(aryldimethylsilyl)‐1‐ tert ‐butylethyl trifluoroacetates were determined conductimetrically in 60% (v/v) aqueous ethanol. The effects of aryl substituents at the silicon atom on the solvolysis rates at 50 °C were correlated with $\bar {\sigma }$ parameters of r +  = 0.15 with the Yukawa–Tsuno equation, giving ρ values of −1.5 for both secondary α ‐Me and α ‐ tert ‐Bu systems. The ρ values for those secondary systems are less negative than −1.75 for the 2‐(aryldimethylsilyl)ethyl system that proceeds by the Eaborn (non‐vertical) mechanism, while they are distinctly more negative than −0.99 for 2‐(aryldimethylsilyl)‐1‐phenylethyl system that should proceed by the Lambert (vertical) mechanism. There was a fairly linear relationship between the reaction constants ( ρ ) for the β ‐silyl substituent effects and the solvolysis reactivities for a series of β ‐silyl substrates. The solvolyses of the α ‐Me and tert ‐Bu substrates proceed through the transition state (TS) with an appreciable degree of the β ‐silyl participation, close to the Eaborn (non‐vertical) TS rather than to the Lambert (vertical) TS. Copyright © 2010 John Wiley & Sons, Ltd.