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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 33, Number 1
BKCSDE 33(1)
January 20, 2012 

Kinetics and Mechanism of the Pyridinolysis of 1,2-Phenylene Phosphorochloridate in Acetonitrile
Hasi Rani Barai, Hai Whang Lee *
Phosphoryl transfer reaction, Pyridinolysis, 1,2-Phenylene phosphorochloridate, Biphasic concave upward free energy correlation
The nucleophilic substitution reactions of 1,2-phenylene phosphorochloridate (1c) with X-pyridines are investigated kinetically in acetonitrile at –25.0 oC. 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 × 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.
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