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 55.0 ºC. The Hammett and Brönsted 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 ρ Y is changed from unusual negative (ρ Y < 0) with the weaker electrophiles to positive (ρ Y > 0) with the stronger electrophiles. The stepwise mechanism is proposed on the basis of the ρ X , β X , and ρ XY 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 R 1 R 2 P(=S)Cl-type substrates are discussed.

Kinetics and Mechanism of the Pyridinolyses of O‐Methyl and O‐Ethyl Phenyl Phosphonochloridothioates in Acetonitrile

“…(i) in Y-O-aryl methyl phosphonochloridothioates [1; Me(YC 6 H 4 O)P(=S)Cl], the free energy correlations with X are discrete biphasic with a break region whereas those with Y are biphasic concave downward [1]; (ii) in Y-O-aryl phenyl phosphonochloridothioates [4; Ph(YC 6 H 4 O)P(=S)Cl], the free energy correlations for both substituent X and Y variations are linear [2]; and (iii) in Y-S-aryl phenyl phosphonochloridothioates [5; Ph(YC 6 H 4 S)P(=S)Cl], the free energy correlations with X are concave upward with a break point, whereas those with Y are biphasic concave upward with a break (minimum) point [3]. As part on going kinetic studies on the pyridinolyses of the phosphonochloridothioates, the nucleophilic substitution reactions of O-methyl (2) and O-ethyl (3) phenyl phosphonochloridothioates with X-pyridines have been carried out kinetically in acetonitrile (MeCN) at 35.0 ± 0.1…”