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

Abstract

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.

Alkaline Hydrolysis of 4‐Nitrophenyl X‐Substituted‐Benzoates Revisited: New Insights from Yukawa–Tsuno Equation

“…It is noted that the r ‐value in Eq. represents extent of the resonance contribution . Thus, an r ‐value of 0.50 found in this study is consistent with our proposal that resonance stabilization of the substrate possessing an EDG in the benzoyl moiety is responsible for the negative deviation from the linear Hammett plot composed with the substrates possessing an EWG in the benzoyl moiety ( e .…”

“…Although the Yukawa–Tsuno equation (Eq. ) was derived to account for the experimental results for solvolysis of benzylic system, we have shown that Eq. is highly effective in deduction of the reaction mechanism for various nucleophilic substitution reactions of various esters …”

Manifestation of the β-Silicon Effect in the Reactions of Unsaturated Systems Involving a 1,2-Silyl Shift

“…25,26 To determine transition state structures, the solvolysis kinetics of -silyl systems 22 were measured for variously substituted -aryldimethylsilyl systems (Figure 3). [27][28][29][30] The obtained data were interpreted with the Yukawa-Tsuno equation, a modification of the Hammett equation that accounts for the resonance effects on a reactive center (Equation 1). The parameter  describes the sensitivity of the transition state towards the electronic effects of its substituents.…”

Nucleophilicities of para‐substituted aniline radical cations in acetonitrile: Kinetic investigation and structure–reactivity relationships

“…This indicates that these anilines exist as a radical cation XC 6 H 4 NH 2 +• and hence justifies to a possible transition from the polar to the SET mechanism in this field. (2) As shown in Figure 6B, the Yukawa–Tsuno 44–46 plot given by Equation (8) for the reaction of thiophene 1 with anilines 2a–f is linear ( R 2 = 0.9912) with an extent of resonance contribution r ‐value of 1.39. Therefore, the present result clearly suggests that stabilization of the radical cation XC 6 H 4 NH 2 +• through resonance or hyperconjugation interaction between the electron‐donating substituent X and the −NH 2 functionality as illustrated by resonance structures I–IV (Scheme 3) is responsible for the positive deviation shown by donor substituents from the Hammett plot: …”