Kinetics and Mechanism of the Benzylaminolysis of ο,ο-Diphenyl S-Aryl Phosphorothioates in Dimethyl Sulfoxide

Kinetic studies of the reactions of ο,ο-diphenyl Z-S-aryl phosphorothioates with X-benzylamines have been carried out in dimethyl sulfoxide at 55.0 .deg. C. The Hammett (log k₂ vs σ_X) and Bronsted [log k₂ vs pK_a(X)] plots for substituent X variations in the nucleophiles are biphasic concave downwards with a maximum point at X = H, and the unusual positive ρ_X and negative β_X values are obtained for the strongly basic benzylamines. The sign of the cross-interaction constant (ρ_XZ) is negative for both the strongly and weakly basic nucleophiles. Greater magnitude of ρ_XZ value is observed with the weakly basic nucleophiles (ρ_XZ = -2.35) compared to with the strongly basic nucleophiles (ρ_XZ = -0.03). The deuterium kinetic isotope effects (k_H/k_D) involving deuterated benzylamines [XC₆H₄CH₂ND₂] are primary normal (k_H/k_D > 1). The proposed mechanism is a concerted S_N2 involving a frontside nucleophilic attack with a hydrogen bonded, four-center-type transition state for both the strongly and weakly basic nucleophiles. The unusual positive ρ_X and negative β_X values with the strongly basic benzylamines are rationalized by through-space interaction between the π-clouds of the electron-rich phenyl ring of benzylamine and the phenyl ring of the leaving group thiophenoxide