Kinetics and mechanism of the aminolysis of aryl phenyldithioacetates in acetonitrile

Abstract

The aminolysis reactions of Z-aryl phenyldithioacetates (C6H5CH2C(S)SC6H4Z) with benzylamines (XC6H4CH2NH2) are investigated in acetonitrile at −25.0 °C. The reactions are predicted to proceed by a stepwise mechanism with rate-limiting expulsion of the thiophenolate anion from the zwitterionic tetrahedral intermediate, T±, based on (i) large magnitudes of selectivity parameters, βX (ρX) and βZ (ρZ), (ii) positive and large cross-interaction constants ρXZ (=2.05), and (iii) adherence to the reactivity–selectivity principle. The significant normal kinetic isotope effects (kH/kD = 1.14–1.81) involving deuterated amines and low activation enthalpies (ΔH‡ ≈ 3 kcal mol−1) together with large negative activation entropies (ΔS‡ = −37 to −53 e.u.) are consistent with concurrent proton transfer of amine hydrogen with the leaving group departure in the transition state.