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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 33, Number 3
BKCSDE 33(3)
March 20, 2012 

 
Title
Kinetics and Mechanism of the Pyridinolysis of Diethyl Isothiocyanophosphate in Acetonitrile
Author
Keshab Kumar Adhikary, Hai Whang Lee*
Keywords
Phosphoryl transfer reaction, Pyridinolysis, Diethyl isothiocyanophosphate, Discrete biphasic free energy relationship
Abstract
The kinetics and mechanism of the pyridinolysis (XC5H4N) of diethyl isothiocyanophosphate are investigated in acetonitrile at 55.0 oC. The Hammett and Brönsted plots for substituent X variations in the nucleophiles exhibit the two discrete slopes with a break region between X = 3-Ac and 4-Ac. These are interpreted to indicate a mechanistic change at the break region from a concerted to a stepwise mechanism with rate-limiting expulsion of the isothiocyanate leaving group from a trigonal bipyramidal pentacoordinated intermediate. The relatively large βX values with more basic and less basic pyridines imply much greater fraction of frontside nucleophilic attack TSf than that of backside attack TSb.
Page
1042 - 1046
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