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

Abstract

The reactions of O,O-diphenyl Z-S-phenyl phosphorothiolates with X-pyridines have been studied kinetically in acetonitrile at <TEX>$35.0{^{\circ}C}$</TEX>. The Hammett plots for substituent (Z) variations in the leaving group (log <TEX>$k_2$</TEX> vs. <TEX>$\sigma$</TEX> Z) are biphasic concave downwards with breaks at Z = H. The large magnitudes of <TEX>${\rho}X(\rho_{nuc})$</TEX>, <TEX>${\beta}X(\rho_{nuc})$</TEX>, and the cross-interaction constant, <TEX>$\rho$</TEX>XZ, suggest frontside nucleophilic attack toward the leaving group. The sign reversal of <TEX>$\rho$</TEX>Z from positive in <TEX>$\sigma$</TEX> Z <TEX>$\leq$</TEX> 0 to negative in <TEX>$\sigma$</TEX> Z <TEX>$\geq$</TEX> 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 <TEX>$\rho$</TEX> 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.