Second‐order rate constants (k quin) for the reactions of O‐Y‐substituted phenyl thionobenzoates (3a–3i) with quinuclidine have been measured spectrophotometrically. Comparison of k quin with the rate constants reported previously for the corresponding reactions with benzylamine (k BzNH2) has revealed that quinuclidine is less reactive than benzylamine toward 3a–3i although the former is 2.1 pK a units more basic than the latter. Steric hindrance exerted by quinuclidine has been suggested to be responsible for the decreased reactivity of the tertiary amine. The Brønsted‐type plot for the reactions of 3a–3i with quinuclidine is linear with βlg = −0.37. The Hammett plot correlated with σ Y o constants exhibits many scattered points ( R 2  = 0.982). In contrast, the Yukawa‐Tsuno plot results in an excellent linear correlation ( R 2  = 0.9992) with ρ Y  = 0.96 and r = 0.51, indicating that a negative charge develops partially on the O atom of the leaving group. Thus, the reactions of 3a–3i with quinuclidine have been concluded to proceed through a concerted mechanism in which expulsion of the leaving group is advanced only a little in the rate‐determining transition state.

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.