Kinetic Study on Quinuclidinolysis of O‐4‐Nitrophenyl X‐substituted Thionobenzoates and O‐3,4‐Dinitrophenyl Thionobenzoate: Effect of Amine Nature on Reactivity and Reaction Mechanism

Abstract

Second-order rate constants (k quin ) have been measured spectrophotometrically for the reactions of O-3,4-dinitrophenyl thionobenzoate (1a) with a series of quinuclidine derivatives and of O-4-nitrophenyl X-substituted thionobenzoates (2a-2f) with quinuclidine in 80 mol% H 2 O/20 mol% DMSO at 25.0 ± 0.1 C. Quinuclidine is less reactive toward the thione esters than less basic benzylamine. Steric hindrance exerted by the tertiary amine is responsible for the decreased reactivity. The Hammett plot for the reactions of 2a-2f with quinuclidine consists of two intersecting straight lines, while the Yukawa-Tsuno plot for the same reactions exhibits an excellent linear correlation with ρ X = 1.10 and r = 0.71. This indicates that the nonlinear Hammett plot is not due to a change in the rate-determining step but is caused by ground-state stabilization of substrates possessing an electron-donating group in the thionobenzoyl moiety through resonance interactions. Brønsted-type plot for the reactions of 1a with quinuclidines is linear with β nuc = 0.89, which is typical for reactions reported to proceed through a stepwise mechanism. However, the reactions have been concluded to proceed through a forced concerted mechanism. Instability of a plausible intermediate T ± forces the reactions to proceed through such a mechanism.