Pyridinolysis of O,O-Diphenyl S-Phenyl PhosphorothioIates in Acetonitrile

摘要

The reactions of O,O-diphenyl Z-S-phenyl phosphorothiolates with X-pyridines have been studied kinetically in acetonitrile at 35.0 °C.The Hammett plots for substituent (Z) variations in the leaving group (log k2 vs.sigma_z) are biphasic concave downwards with breaks at Z=H.The large magnitudes of rho_X (rho_(nuc)),beta_x (rho_(nuc)),and the cross-interaction constant,pxz,suggest frontside nucleophilic attack toward the leaving group.The sign reversal of rho_z from positive in sigma_z <=0 to negative in sigma_z=> 0 is interpreted as the change in mechanism from concerted to stepwise with rate-limiting expulsion of the leaving group.The anomalous negative sign of rho_z for leaving groups with electron-withdrawing substituents is interpreted as the intramolecular ligand exchange process of the leaving group from the equatorial position in the intermediate to the apical position in the TS.