Kinetics and mechanism of the aminolysis of aryl thiocarbamates: effects of the non-leaving group

Abstract

The kinetics of the aminolysis of aryl thiocarbamates [ATC: H₂NC(O)SC₆H₄Z] with benzylamines (XC₆H₄CH₂NH₂) in acetonitrile at 10.0 °C have been studied. The rate order with variation of the non-leaving amino group, RNH, in RNHC(O)SC₆H₄Z is NH₂ < PhNH < EtNH indicating that the polar (σ*) and steric (E_s) effects of the RNH group are insignificant, and the strength of push to expel the leaving group in the tetrahedral transition state is the sole, important effect. The strong push provided by the NH₂ group, the negative ρ_XZ (−0.38) value, the size of β_Z (−0.54), and failure of the reactivity–selectivity principle are all consistent with the concerted mechanism. The kinetic isotope effects involving deuterated amine nucleophiles (XC₆H₄CH₂ND₂) are normal (k_H/k_D ≈ 1.40–1.73) suggesting a hydrogen-bonded cyclic transition state.