Aminolysis of O-aryl thionobenzoates: Amine basicity combines with modulation of the nature of substituents in the leaving group and thionobenzoate moiety to control the reaction mechanism

Abstract

(Chemical Equation Presented) A kinetic study is reported for aminolysis of O-Y-substituted phenyl thionobenzoates (la-f) and O-4-nitrophenyl X-substituted thionobenzoates (2a-f) in 80 mol % H 2O/20 mol % DMSO at 25.0 ± 0.1°C. The reaction proceeds through one or two intermediates (i.e., a zwitterionic tetrahedral intermediate T ± and its deprotonated form T -) depending on the basicity difference between the nucleophile and nucleofuge, that is, the reaction proceeds through T ± when the leaving aryloxide is less basic than the attacking amine, but through T ± and T - when the leaving group is more basic than the amine. However, the reaction mechanism is not influenced by the electronic nature of the substituent X in the nonleaving group. The Hammett plot for the reactions of 2a-f with benzylamine is consisted of two intersecting straight lines, which might be interpreted as a change in the rate-determining step (RDS). However, the Yukawa-Tsuno plot for the same reactions exhibits an excellent linear correlation, indicating that the nonlinear Hammett plot is not due to a change in the RDS but caused by stabilization of the ground-state of the substrate through resonance interaction between the electron-donating substituent X and the thionocarbonyl moiety. © 2008 American Chemical Society.