Aminolysis of 3,4-dinitrophenyl cinnamate and benzoate 2: Activation parameters and transition-state structures

Abstract

The apparent second-order rate constants (kN) have been dissected into the microscopic rate constants (i.e., k1 and k 2/k-1) associated with the reactions of Y-substituted phenyl cinnamates (5a-g) with piperidine and morpholine on the assumption that the reactions proceed through a stepwise mechanism with a change in the rate-determining step (RDS). The k1 value is larger for the reactions with the more basic piperidine, and increases with decreasing the basicity of the leaving aryloxides. However, the k2/k-1 ratio is almost the same for the reaction of 3,4-dinitrophenyl cinnamate (5a) with piperidine and for that with morpholine, which is not possible if the reactions proceed through a stepwise mechanism. Thus, the aminolysis of 5a-g has been proposed to proceed through a concerted mechanism. The activation parameters (ΔH‡ and ΔS‡) have been measured for the reactions of 3,4-dinitrophenyl benzoate (1a) and cinnamate (5a) with morpholine from the kinetic study performed at 5 different temperatures in 80 mol % H2O/20 mol % DMSO. The reaction of 5a results in a lager enthalpy of activation (ΔH‡) but a less negative entropy of activation (ΔS‡) than that of 1a.