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, β_X (ρ_X) and β_Z (ρ_Z), (ii) positive and large cross-interaction constants ρ_XZ (=2.05), and (iii) adherence to the reactivity–selectivity principle. The significant normal kinetic isotope effects (k_H/k_D = 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.