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

Autor: Fujiyama, Ryoji, Alam, Md. Ashadul, Shiiyama, Aiko, Munechika, Toshihiro, Fujio, Mizue, Tsuno, Yuho
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Zdroj: Journal of Physical Organic Chemistry; Sep2010, Vol. 23 Issue 9, p819-827, 9p, 4 Diagrams, 5 Charts, 4 Graphs
Abstrakt: 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. [ABSTRACT FROM AUTHOR]
Databáze: Complementary Index
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