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Kinetic Study of the Aminolysis and Pyridinolysis of O-Phenyl and O-Ethyl O-(2,4-Dinitrophenyl) Thiocarbonates. A Remarkable Leaving Group Effect

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journal contribution
posted on 02.04.2004, 00:00 by Enrique A. Castro, María Cubillos, Margarita Aliaga, Sandra Evangelisti, José G. Santos
The reactions of a series of secondary alicyclic (SA) amines with O-phenyl and O-ethyl O-(2,4-dinitrophenyl) thiocarbonates (1 and 2, respectively) and of a series of pyridines with the former substrate are subjected to a kinetic investigation in water, at 25.0 °C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, all the reactions obey pseudo-first-order kinetics and are first-order in amine. The Brönsted-type plots are biphasic, with slopes (at high pKa) of β1 = 0.20 for the reactions of SA amines with 1 and 2 and β1 = 0.10 for the pyridinolysis of 1 and with slopes (at low pKa) of β2 = 0.80 for the reactions of SA amines with 1 and 2 and β2 = 1.0 for the pyridinolysis of 1. The pKa values at the curvature center (pKa0) are 7.7, 7.0, and 7.0, respectively. These results are consistent with the existence of a zwitterionic tetrahedral intermediate (T) and a change in the rate-determining step with the variation of amine basicity. The larger pKa0 value for the pyridinolysis of 1 compared to that for 2 (pKa0 = 6.8) and the larger pKa0 value for the reactions of SA amines with 1 relative to 2 are explained by the greater inductive electron withdrawal of PhO compared to EtO. The larger pKa0 values for the reactions of SA amines with 1 and 2, relative to their corresponding pyridinolysis, are attributed to the greater nucleofugalities of SA amines compared to isobasic pyridines. The smaller pKa0 value for the reactions of SA amines with 2 than with O-ethyl S-(2,4-dinitrophenyl) dithiocarbonate (pKa0 = 9.2) is explained by the greater nucleofugality from T of 2,4-dinitrophenoxide (DNPO-) relative to the thio derivative. The stepwise reactions of SA amines with 1 and 2, in contrast to the concerted mechanisms for the reactions of the same amines with the corresponding carbonates, is attributed to stabilization of T by the change of O- to S-. The simple mechanism for the SA aminolysis of 2 (only one tetrahedral intermediate, T) is in contrast to the more complex mechanism (two tetrahedral intermediates, T and T-, the latter formed by deprotonation of T by the amine) for the same aminolysis of the analogous thionocarbonate with 4-nitrophenoxide (NPO-) as nucleofuge. To our knowledge, this is the first example of a remarkable change in the decomposition path of a tetrahedral intermediate T by replacement of NPO- with DNPO- as the leaving group of the substrate. This is explained by (i) the greater leaving ability from T of DNPO- than NPO- and (ii) the similar rates of deprotonation of both T (formed with DNPO and NPO).