Kinetics and mechanism of the aminolysis of aryl propanedithioates in acetonitrile

Abstract

The kinetics and mechanism of the aminolysis of aryl proanedithioates with benzylamines are investigated in acetonitrile at −35.0 °C. A large magnitude of the Hammett (ρ_X and ρ_Z) and Brönsted (β_X and β_Z) coefficients and exceptionally large cross-interaction constant ρ_XZ(= 3.5) are consistent with a stepwise mechanism in which leaving group expulsion from an intermediate, T^±, is the rate-determining step. The faster rates observed for dithio esters (II) than for thio esters (I), and the validity of the reactivity-selectivity principle (RSP) are also in line with the mechanism proposed. The k_H/k_D values (= 1.0–1.8) determined with deuterated benzylamines (XC₆H₄CH₂ND₂) and the activation parameters, ΔH^≠(≃ 8 kcal mol⁻¹) and ΔS^≠(=  −16 to −23 e.u.), suggest that proton transfer occurs concurrently with leaving group departure in the transition state.