Kinetics and Mechanism of Pyridinolyses of Aryl Methyl and Aryl Propyl Chlorothiophosphates in Acetonitrile

Abstract

The nucleophilic substitution reactions of Y-aryl methyl (8) and Y-aryl propyl (10) chlorothiophosphates with X-pyridines are studied kinetically in acetonitrile at <TEX>$35.0^{\circ}C$</TEX>. The Hammett and Bronsted plots with X in the nucleophiles for both substrates exhibit biphasic concave upwards with a break region between X = 3-Me and H. The obtained values of the cross-interaction constants (<TEX>${\rho}_{XY}$</TEX>) are negative with 8 while positive with 10 despite the same free energy correlations with X for both substrates. A stepwise mechanism with a rate-limiting bond formation is proposed with 8, whereas a stepwise mechanism with a rate-limiting leaving group departure from the intermediate is proposed with 10 based on the sign of <TEX>${\rho}_{XY}$</TEX>, negative and positive with 8 and 10, respectively. A frontside nucleophilic attack is proposed with strongly basic pyridines based on the considerably great magnitudes of <TEX>${\rho}_X$</TEX> and <TEX>${\beta}_X$</TEX> values while a backside attack is proposed with weakly basic pyridines based on the relatively small magnitudes of <TEX>${\rho}_X$</TEX> and <TEX>${\beta}_X$</TEX> for both substrates.