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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 32, Number 5
BKCSDE 32(5)
May 20, 2011 

Kinetics and Mechanism of the Benzylaminolysis of O,O-Diphenyl S-Aryl Phosphorothioates in Dimethyl Sulfoxide
Keshab Kumar Adhikary, Hai Whang Lee *
Phosphoryl transfer reaction, Benzylaminolysis, Deuterium kinetic isotope effect, Nonlinear biphasic free energy correlation
Kinetic studies of the reactions of O,O-diphenyl Z-S-aryl phosphorothioates with X-benzylamines have been carried out in dimethyl sulfoxide at 55.0 °C. The Hammett (log k2 vs σX) and Brönsted [log k2 vs pKa(X)] plots for substituent X variations in the nucleophiles are biphasic concave downwards with a maximum point at X = H, and the unusual positive ρX and negative βX values are obtained for the strongly basic benzylamines. The sign of the cross-interaction constant (ρXZ) is negative for both the strongly and weakly basic nucleophiles. Greater magnitude of ρXZ value is observed with the weakly basic nucleophiles (ρXZ = −2.35) compared to with the strongly basic nucleophiles (ρXZ = −0.03). The deuterium kinetic isotope effects (kH/kD) involving deuterated benzylamines [XC6H4CH2ND2] are primary normal (kH/kD > 1). The proposed mechanism is a concerted SN2 involving a frontside nucleophilic attack with a hydrogen bonded, four-center-type transition state for both the strongly and weakly basic nucleophiles. The unusual positive ρX and negative βX values with the strongly basic benzylamines are rationalized by through-space interaction between the π-clouds of the electron-rich phenyl ring of benzylamine and the phenyl ring of the leaving group thiophenoxide
1625 - 1629
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