Kinetic and Mechanistic Investigation of the Aminolysis of 3-Methoxyphenyl 3-Nitrophenyl Thionocarbonate, 3-Chlorophenyl 3-Nitrophenyl Thionocarbonate, and Bis(3-nitrophenyl) Thionocarbonate

The reactions of the title thionocarbonates (6, 7, and 8, respectively) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt % ethanol−water, 25.0 °C, ionic strength 0.2 M (KCl). Under excess amine, pseudo-first-order rate coefficients (k_obsd) are obtained for all reactions. Reactions of substrates 6 and 7 with piperidine and of thionocarbonate 8 with the same amine and piperazine, 1-(2-hydroxyethyl)piperazine, and morpholine show linear k_obsd vs [amine] plots, with slopes (k₁) independent of pH. On the other hand, these plots are nonlinear upward for the reactions of substrates 6 and 7 with all the amines, except piperidine, and also for the reactions of compound 8 with 1-formylpiperazine and piperazinium ion. For all these reactions a mechanistic scheme is proposed with the formation of a zwitterionic tetrahedral intermediate (T^±), which can transfer a proton to an amine to give an anionic intermediate (T^-). Rate and equilibrium microcoefficients of this scheme, k₁, k_-1, K₁ (= k₁/k_-1), and k₂, are obtained by fitting the nonlinear plots through an equation derived from the scheme. The Brönsted-type plots for k₁ are linear with slopes β₁ = 0.19, 0.21, and 0.26 for the aminolysis of 6, 7, and 8, respectively. This is consistent with the hypothesis that the formation of T^± (k₁ step) is the rate-determining step. The k₁ values for these reactions follow the sequence 8 > 7 > 6, consistent with the sequence of the electron-withdrawing effects from the substituents on the “nonleaving” group of the substrates. The k₁ values for the aminolysis of 6, 7, and 8 are smaller than those for the same aminolysis of 3-methoxyphenyl, 3-chlorophenyl, and 4-cyanophenyl 4-nitrophenyl thionocarbonates (2, 3, and 4, respectively). The k₂ values (expulsion of the nucleofuge from T^±) increase as the electron withdrawal from the nonleaving group increases. These values are smaller for the aminolysis of 6, 7, and 8 compared to those for the same aminolysis of 2, 3, and 4, respectively.