The Yukawa–Tsuno Relationship for the β-Silicon Effect in the Solvolysis Rates of 2-(Aryldimethylsilyl)ethyl Chlorides

Abstract

Solvolysis rates of 2-(aryldimethylsilyl)ethyl chlorides were determined conductimetrically in 60% (v/v) aqueous ethanol. The effects of aryl substituents at the silyl atom on the solvolysis rates at 50 °C were correlated with essentially nonresonant parameters of r = 0.10 in terms of Yukawa–Tsuno (Y–T) Eq. 1, giving a ρ value of −1.75. Such a high ρ value may be regarded as the effect of aryl ring on the bridged Si in the rate-determining step. The Si-bridging is consistent with the fact that the solvolysis of the unsubstituted substrate with d2-labeled ethylene moiety gave substitution products with label scrambling. The arylsilyl substituent effects were likewise analyzed for several relevant sets of β-silyl systems in order to ascertain significant variations of ρ values from system to system; Y–T Eq. 1 correlated quite excellently with essentially nonresonant sigmas of negligible resonance demand (r 0.10), to exhibit significant variations of ρ from −1.75 to −0.95.

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

“…The reaction constant ρ parameter varies with structures of substrate and also depends on the intrinsic stabilities of intermediate cations. The β ‐arylsilyl substituent effect log ( k X / k H ) 2 on the solvolysis of the primary system 2 resulted in ρ 2 = −1.75 8. The ρ value of −1.5 for secondary substrates 4 and 5 is even close to ρ 2 = −1.75 for the reference primary system 2 .…”

“…The solvolysis rates of 2‐[dimethyl(X‐phenyl)silyl]‐1‐methylethyl trifluoroacetates ( 4‐OTFA ), 1‐ tert ‐butyl‐2‐[dimethyl(X‐phenyl)silyl]ethyl trifluoroacetates ( 5‐OTFA ), and chlorides ( 5‐Cl ) were measured conductimetrically in 60E at several temperatures in the same manner as reported before1, 8, 9 and were extrapolated to the rate data at 50 °C to compare with the β ‐aryl‐Si substituent effects log ( k X / k H ) in the relevant β ‐silyl substrate systems. Kinetic data are listed in Tables 1–3.…”

“…Our previous study8 reveals that the aryl substituent effect log ( k X / k H ) 2 of the silyl moiety in the solvolysis of primary ( α ‐R = H) system 2 that proceeds via the Eaborn (non‐vertical) mechanism gives ρ 2 of −1.75 against essentially non‐resonant $\bar {\sigma }_{\rm x} $ with r + = 0.10.…”

“…In previous studies,1, 8, 9 we have employed the reaction constants ρ for the substituent effects log ( k X / k H ) of the aryldimethylsilyl moiety ( β ‐ArMe 2 Si) in the substrates as a diagnostic probe of the β ‐silyl participation.…”

“…Our approach is based on how the reaction constants ρ for the substituent effect log ( k X / k H ) depend on changes in the stability of the TS (or the intermediate) of the substrate. Previous papers1, 8, 9 reported that the β ‐arylsilyl substituent effects on solvolyses of 2‐(aryldimethylsilyl)ethyl system ( 2 ) in 60% (v/v) aqueous ethanol (60E) at 50 °C, in which the non‐vertical (Si‐bridged) mechanism evidently occurs,5 resulted in ρ 2 = −1.75 against essentially non‐resonant sigmas ($\bar {\sigma }$ X ) of negligible resonance demand ( r + ≅ 0.10) 8. In contrast, the α ‐Ph derivative 3 gave a distinctly low ρ 3 value of −0.99 (in Table 5).…”