Electronic nature of substituent X governs reaction mechanism in aminolysis of 4-pyridyl X-substituted-benzoates in acetonitrile

Abstract

A kinetic study is reported for aminolysis of 4-pyridyl X-substituted-benzoates 5a-i. Plots of pseudo-first-order rate constants (k obsd) vs [amine] curve upward for the reactions of substrates possessing a strong electron-withdrawing group in the benzoyl moiety (5a-d) but are linear for the reactions of those bearing an electron-donating group (5e-i), indicating that the electronic nature of substituent X governs the reaction mechanism. The k 1k 2/k -1 and k 1k 3/k -1 values were calculated from the intercept and slope of the linear plots of k obsd/[amine] vs [amine], respectively. The Hammett plot for k 1k 2/k -1 consists of two intersecting straight lines, while the Yukawa-Tsuno plot exhibits an excellent linear correlation with ρ X = 0.41 and r = 1.58, implying that the nonlinear Hammett plot is not due to a change in rate-determining step but is caused by stabilization of substrates possessing an electron-donating group through resonance interactions. The small ρ X suggests that the k 2/k -1 ratio is little influenced by the nature of substituent X. The Brønsted-type plots for aminolysis of 4-pyridyl 3,5-dinitrobenzoate 5a are linear with β nuc = 0.98 and 0.79 for k 1k 2/k -1 and k 1k 3/k -1, respectively. The effect of amine basicity on the microscopic rate constants is also discussed. © 2012 American Chemical Society.