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

Abstract

Solvolysis rates of 2-(dimethylphenylsilyl)-1-(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 ρ = −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 logkY/kH of silylated and non-silylated substrates: log(kY/kH)Si = 0.52log(kY/kH)non-Si. This is the same form as the extended Brønsted relationship. The Brønsted coefficient α = 0.52 appeared to be consistent with the neighboring silyl-participation in the silyl-bridged transition state.

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

“…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.…”

“…Furthermore, the β ‐silyl substituent effect of the α ‐Ph system 3 (Y = H) was dealt with rate data at 25 °C in 60E based on the Y‐T Eqn (1) in the preceding papers;1, 9 ρ 3 = −0.99 was obtained based on k X values extrapolated to 50 °C in 60E, in a precision ( R = 0.998 and SD = ±0.024).…”

“…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).…”

Nucleophilic Aliphatic Substitution

“…The solvolysis rates of 2-(dimethylphenylsilyl)-1-(Y-phenyl)ethyl 3,5-dinitrobenzoates (62) in 60% aqueous ethanol were analysed using using the Yukawa-Tsuno equation. 103 The ρ value of −2.95 with r ≈ 1.04 found by changing the α-aryl substituent was much smaller than the ρ value of −5.45 found for the corresponding non-silylated system. This, the fact that the rate constant for the silylated substrate was >10 5 -fold faster than the rate of the non-silylated compound, and the small α value of 0.52 found from the log(k Y /k H ) Si = α log(k Y /k H ) non-Si plot was taken as evidence that the reaction of the silyl compound occurred via a tight transition state with significant neighbouring silyl participation (Scheme 25).…”