Comparison of aminolysis of 2-pyridyl and 4-pyridyl x-substituted benzoates in acetonitrile: evidence for a concerted mechanism involving a cyclic transition state.

A kinetic study on reactions of 2-pyridyl X-substituted benzoates (6a-i) with a series of cyclic secondary amines in MeCN is reported. The Hammett plot for the reaction of 6a-i with piperidine consists of two intersecting straight lines while the Yukawa-Tsuno plot exhibits an excellent linear correlation with ρX = 1.28 and r = 0.63, indicating that the nonlinear Hammett plot is not caused by a change in the rate-determining step but rather by resonance stabilization of substrates possessing an electron-donating group (EDG) in the benzoyl moiety. The Brønsted-type plots are linear with βnuc = 0.59 ± 0.02, which is typical of reactions reported to proceed through a concerted mechanism. A cyclic transition state (TS), which forces the reaction to proceed through a concerted mechanism, is proposed. The deuterium kinetic isotope effect of 1.3 ± 0.1 is consistent with the proposed mechanism. Analysis of activation parameters reveals that ΔH(‡) increases linearly as the substituent X changes from an electron-withdrawing group (EWG) to an EDG, while TΔS(‡) remains nearly constant with a large negative value. The constant TΔS(‡) value further supports the proposal that the reaction proceeds through a concerted mechanism with a cyclic TS.