Kinetic Study of the Aminolysis and Pyridinolysis of <i>O-</i>Phenyl and <i>O</i>-Ethyl <i>O</i>-(2,4-Dinitrophenyl) Thiocarbonates. A Remarkable Leaving Group Effect CastroEnrique A. CubillosMaría AliagaMargarita EvangelistiSandra SantosJosé G. 2004 The reactions of a series of secondary alicyclic (SA) amines with <i>O</i>-phenyl and <i>O</i>-ethyl <i>O</i>-(2,4-dinitrophenyl) thiocarbonates (<b>1</b> and <b>2</b>, 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 p<i>K</i><sub>a</sub>) of β<sub>1</sub> = 0.20 for the reactions of SA amines with <b>1</b> and <b>2</b> and β<sub>1</sub> = 0.10 for the pyridinolysis of <b>1</b> and with slopes (at low p<i>K</i><sub>a</sub>) of β<sub>2</sub> = 0.80 for the reactions of SA amines with <b>1</b> and <b>2</b> and β<sub>2</sub> = 1.0 for the pyridinolysis of <b>1</b>. The p<i>K</i><sub>a</sub> values at the curvature center (p<i>K</i><sub>a</sub><sup>0</sup>) are 7.7, 7.0, and 7.0, respectively. These results are consistent with the existence of a zwitterionic tetrahedral intermediate (T<sup>⧧</sup>) and a change in the rate-determining step with the variation of amine basicity. The larger p<i>K</i><sub>a</sub><sup>0</sup> value for the pyridinolysis of <b>1</b> compared to that for <b>2</b> (p<i>K</i><sub>a</sub><sup>0</sup> = 6.8) and the larger p<i>K</i><sub>a</sub><sup>0</sup> value for the reactions of SA amines with <b>1</b> relative to <b>2</b> are explained by the greater inductive electron withdrawal of PhO compared to EtO. The larger p<i>K</i><sub>a</sub><sup>0</sup> values for the reactions of SA amines with <b>1</b> and <b>2</b>, relative to their corresponding pyridinolysis, are attributed to the greater nucleofugalities of SA amines compared to isobasic pyridines. The smaller p<i>K</i><sub>a</sub><sup>0</sup> value for the reactions of SA amines with <b>2</b> than with <i>O</i>-ethyl <i>S</i>-(2,4-dinitrophenyl) dithiocarbonate (p<i>K</i><sub>a</sub><sup>0</sup> = 9.2) is explained by the greater nucleofugality from T<sup>⧧</sup> of 2,4-dinitrophenoxide (DNPO<sup>-</sup>) relative to the thio derivative. The stepwise reactions of SA amines with <b>1</b> and <b>2</b>, in contrast to the concerted mechanisms for the reactions of the same amines with the corresponding carbonates, is attributed to stabilization of T<sup>⧧</sup> by the change of O<sup>-</sup> to S<sup>-</sup>. The simple mechanism for the SA aminolysis of <b>2</b> (only one tetrahedral intermediate, T<sup>⧧</sup>) is in contrast to the more complex mechanism (two tetrahedral intermediates, T<sup>⧧</sup> and T<sup>-</sup>, the latter formed by deprotonation of T<sup>⧧</sup> by the amine) for the same aminolysis of the analogous thionocarbonate with 4-nitrophenoxide (NPO<sup>-</sup>) as nucleofuge. To our knowledge, this is the first example of a remarkable change in the decomposition path of a tetrahedral intermediate T<sup>⧧</sup> by replacement of NPO<sup>-</sup> with DNPO<sup>-</sup> as the leaving group of the substrate. This is explained by (i) the greater leaving ability from T<sup>⧧</sup> of DNPO<sup>-</sup> than NPO<sup>-</sup> and (ii) the similar rates of deprotonation of both T<sup>⧧</sup> (formed with DNPO and NPO).