Pyridinolysis of O,O-diphenyl S-phenyl phosphorothiolates in acetonitrile

The reactions of O,O-diphenyl Z-S-phenyl phosphorothiolates with X-pyridines have been studied kinetically in acetonitrile at 35.0 oC. The Hammett plots for substituent (Z) variations in the leaving group (log k2 vs. ? Z) are biphasic concave downwards with breaks at Z = H. The large magnitudes of ρX (ρnuc), βX (ρ nuc), and the0 cross-interaction constant, ρXZ, suggest frontside nucleophilic attack toward the leaving group. The sign reversal of ρZ from positive in σZ ≤ 0 to negative in σZ ≥ 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 ρZ 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.