Kinetics and mechanism of the aminolysis of S-phenyl cyclopropanecarboxylates in acetonitrile

Abstract

The kinetics and mechanism of the aminolysis of S-phenyl cyclopropanecarboxylates [cyclo-C₃H₅C(O)SC₆H₄Z] with benzylamines (XC₆H₄CH₂NH₂) were investigated in acetonitrile at 35.0 °C. The large magnitudes of the Brönsted coefficients β_X (1.7–2.3) and β_Z (−0.9 to −1.3), together with a large positive cross-interaction constant ρ_XZ = +1.4, are interpreted to indicate a stepwise mechanism with rate-limiting breakdown of the zwitterionic tetrahedral intermediate, T^±. The proposed mechanism is supported by adherence of the rates to the reactivity-selectivity principle (RSP). The kinetic isotope effects involving deuterated benzylamines (XC₆H₄CH₂ND₂) are greater than unity (k_H/k_D > 1.0), suggesting the possibility of a hydrogen-bonded four-center transition state. The activation parameters, ΔH^≠ (≅5 kcal mol⁻¹) and ΔS^≠ (=−45 to −54 e.u.), are consistent with this transition state structure.