Cases of pronounced extended π,n‐participation in solvolysis of tert‐butyl and benzyl chloride derivatives

Abstract

AbstractSolvolysis rates of chlorides that share the same side‐chain comprising two neighboring groups [tertiary chloride 10, 2‐chloro‐2,6‐dimethyl‐9‐methoxy‐(6E)‐nonene, and benzyl chlorides 11, 1‐chloro‐1‐aryl‐5‐methyl‐8‐methoxy‐(5E)‐octene, with various phenyl substituents (Y = p‐OCH3, p‐CH3, H, p‐Br and m‐Br)], were measured in 80% (v/v) aqueous ethanol. Both the tertiary substrate 10 and the benzyl substrates 11 solvolyze with smaller entropy and enthalpy of activation than the corresponding reference analogs with one neighboring group, 6 and 8, respectively (ΔΔH≠ = −34 ± 6 kJ mol−1, ΔΔS≠ = −122 ± 19 J K−1 mol−1 with 10; ΔΔH≠ = −33 ± 6 kJ mol−1, ΔΔS≠ = −95 ± 17 J K−1 mol−1 with 11), indicating that in addition to the double bond, the methoxy group also participates in the rate‐determining step. Chloride 10 has a significantly reduced secondary β‐deuterium kinetic isotope effect (kH/kD = 1.07 ± 0.01 in 80E; kH/kD = 1.05 ± 0.1 in 97T) in comparison with the typical value for the tertiary chlorides (kH/kD = 1.80), as a consequence of the less positive charge on the reaction center in the transition state. The slope of the Hammett plot ρ+ value obtained with the series of 11 is considerably smaller than that obtained with the reference chlorides 8‐Y (ρ+ = −1.29 ± 0.11 vs −3.93 ± 0.10), confirming that benzyl substrates also solvolyze with extended π,n‐participation. On both types of substrates, 10 and 11, the kinetic parameters indicate that very pronounced assistance of both neighboring groups occurs in the rate‐determining step. Copyright © 2004 John Wiley & Sons, Ltd.