Kinetics and Mechanism of the Pyridinolysis of Aryl Ethyl Chlorothiophosphates in Acetonitrile

摘要

The nucleophilic substitution reactions of Y-aryl ethyl chlorothiophosphates with X-pyridines are studied kinetically in acetonitrile at 55.0 °C. The Hammett and Bronsted plots for substituent X variations in the nucleophiles exhibit biphasic concave upwards with a break point at X = 3-Me. The substituents of X = 4-CN and 4-Ac show great positive deviations from both the Hammett and Bronsted plots. The Hammett plots for substituent Y variations in the substrates exhibit biphasic concave upwards with a minimum point at Y = H. The obtained values of the cross-interaction constants (ρ_(XY)) are all null in spite of the biphasic free energy correlations for both substituent X and Y variations, since the px values with both the strongly and weakly basic pyridines are almost constant. A stepwise mechanism with a rate-limiting leaving group departure from the intermediate is proposed where the distance between X and Y does not vary from the intermediate to the second transition state. A frontside attack is proposed with the strongly basic pyridines based on the considerably great magnitudes of ρ_X and β_X values and a backside attack is proposed with the weakly basic pyridines based on the relatively small magnitudes of ρ_X and ρ_X. The positive deviations of the two strong π-acceptor para-substituents, X = 4-Ac and 4-CN, from both the Hammett and Bronsted plots are rationalized by the great extents of bond formation and breaking.