Kinetics and Mechanism of the Pyridinolysis of 1,2-Phenylene Phosphorochloridate in Acetonitrile

The nucleophilic substitution reactions of 1,2-phenylene phosphorochloridate (1c) with X-pyridines are investigated kinetically in acetonitrile at .25.0 .deg. C. The free energy correlations for substituent X variations in the nucleophiles exhibit biphasic concave upwards with a break point at X = 3-Ph. The pyridinolysis rate of 1c with a cyclic five-membered ring is 2.70 x 105 times faster than its acyclic counterpart (1a: phenyl ethyl chlorophosphate) because of great positive value of the entropy of activation of 1c (ΔS≠ = +26 eu) compared to negative value of 1a (ΔS≠ = -24 eu) over considerably unfavorable enthalpy of activation of 1c (ΔH≠ = 20.5 kcal mol-1) compared to 1a (ΔH≠ = 12.7 kcal mol-1). Great enthalpy and positive entropy of activation are ascribed to sterically congested transition state (TS) and solvent structure breaking in the TS. A concerted mechanism involving a change of nucleophilic attacking direction from a frontside attack with the strongly basic pyridines to a backside attack with the weakly basic pyridines is proposed on the basis of greater selectivity parameters (ρX = -1.99 and βX = 0.41) with the strongly basic pyridines compared to those (ρX = -0.42 and βX = 0.07) with the weakly basic pyridines