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

Solvolysis rates of chlorides that share the same side‐chain comprising two neighboring groups [tertiary chloride 10 , 2‐chloro‐2,6‐dimethyl‐9‐methoxy‐(6 E )‐nonene, and benzyl chlorides 11 , 1‐chloro‐1‐aryl‐5‐methyl‐8‐methoxy‐(5 E )‐octene, with various phenyl substituents ( Y  =  p ‐OCH 3 , p ‐CH 3 , 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 (ΔΔ H ≠  = −34 ± 6 kJ mol −1 , ΔΔ S ≠  = −122 ± 19 J K −1  mol −1 with 10 ; ΔΔ H ≠  = −33 ± 6 kJ mol −1 , ΔΔ S ≠  = −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 ( k H / k D  = 1.07 ± 0.01 in 80E; k H / k D  = 1.05 ± 0.1 in 97T) in comparison with the typical value for the tertiary chlorides ( k H / k D  = 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.