Kinetics and Mechanism of the Pyridinolysis of Diethyl Isothiocyanophosphate in Acetonitrile

초록

The kinetics and mechanism of the pyridinolysis (XC5H4N) of diethyl isothiocyanophosphate are investigated in acetonitrile at 55.0℃. The Hammett and Bronsted 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