Kinetics and Mechanism of the Pyridinolysis of S-Aryl Phenyl Phosphonochloridothioates in Acetonitrile

Abstract

Kinetic studies on the reactions of Y-S-aryl phenyl phosphonochloridothioates with X-pyridines have been carried out in MeCN at <TEX>$55.0^{\circ}C$</TEX>. The Hammett and Bronsted plots for substituent X variations in the nucleophiles are biphasic concave upwards with a break point at X = H. The Hammett plots for substituent Y variations in the substrates are biphasic concave upwards with a break point at Y = H, and the sign of <TEX>${\rho}_Y$</TEX> is changed from unusual negative (<TEX>${\rho}_Y$</TEX> < 0) with the weaker electrophiles to positive (<TEX>${\rho}_Y$</TEX> > 0) with the stronger electrophiles. The stepwise mechanism is proposed on the basis of the <TEX>${\rho}_X$</TEX>, <TEX>${\beta}_X$</TEX>, and <TEX>${\rho}_{XY}$</TEX> values as follows: a ratelimiting leaving group departure from the intermediate involving a frontside attack and product-like TS for the stronger nucleophiles and weaker electrophiles; a rate-limiting leaving group departure from the intermediate involving a backside attack and product-like TS for the weaker nucleophiles and electrophiles; a rate-limiting bond formation involving a frontside attack for the stronger nucleophiles and electrophiles; a rate-limiting bond formation involving a backside attack for the weaker nucleophiles and stronger electrophiles. The substituent effects of X and Y on the pyridinolysis mechanisms of <TEX>$R_1R_2P$</TEX>(=S)Cl-type substrates are discussed.