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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 35, Number 5
BKCSDE 35(5)
May 20, 2014 

Significant Substituent Effects on Pyridinolysis of Aryl Ethyl Chlorophosphates in Acetonitrile
Keshab Kumar Adhikary, Hai Whang Lee*
Substituent effects, Cross-interaction constant, Phosphoryl transfer reaction, Pyridinolysis, Aryl ethyl chlorophosphates
The substituent effects on the pyridinolysis (XC5H4N) of Y-aryl ethyl chlorophosphates are investigated in acetonitrile at 35.0 oC. The two strong π-acceptor substituents, X = 4-Ac and 4-CN in the X-pyridines, exhibit large positive deviations from the Hammett plots but little positive deviations from the Brönsted plots. The substituent Y effects on the rates are really significant and the Hammett plots for substituent Y variations in the substrates invariably change from biphasic concave downwards via isokinetic at X = H to biphasic concave upwards with a break point at Y = 3-Me as the pyridine becomes less basic. These are interpreted to indicate a mechanistic change at the break point from a stepwise mechanism with a rate-limiting bond formation (ρXY = –6.26) for Y = (4-MeO, 4-Me, 3-Me) to with a rate-limiting leaving group expulsion from the intermediate (ρXY = +5.47) for Y = (4-Me, H, 3-MeO). The exceptionally large magnitudes of ρXY values imply frontside nucleophilic attack transition state.
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