Kinetics and Mechanism of the Pyridinolyses of O-Methyl and OEthyl Phenyl Phosphonochloridothioates in Acetonitrile

摘要

Kinetic studies on the reactions of Omethyl (2) and O-ethyl (3) phenyl phosphonochloridothioates with X-pyridines have been carried out in acetonitrile at 35.0°C. The pyridinolysis rates of 2 are slightly faster than those of 3 The substituent effects of X on the pyridinolysis rates of 2 and 3 are similar. The Hammett and Bronsted plots with X in the nucleophiles are biphasic concave upward with a break point at X = 3-Ph for both substrates. The stepwise mechanism with rate-limiting leaving group departure from the intermediate is proposed based on the β_x values and biphasic concave upward free energy relationship for both substrates. The biphasic concave upward free energy relationships are rationalized by a frontside nucleophilic attack TSf with more basic pyridines and backside attack TSb with less basic pyridines for both substrates.