Reactions of aryl chlorothionoformates with quinuclidines. A kinetic study

Abstract

AbstractThe reactions of quinuclidines with phenyl, 4‐chlorophenyl, 4‐cyanophenyl, and 4‐nitrophenyl chlorothionoformates (1, 2, 3, and 4, respectively) are subjected to a kinetic study in aqueous solution, at 25.0°C, and an ionic strength of 0.2 M (KCl). The reactions are studied by following spectrophotometrically the release of the corresponding phenoxide anion/phenol generated in the parallel hydrolysis of the substrates. Under amine excess, pseudo‐first‐order rate coefficients (kobs) are found. Plots of kobs versus [amine] are linear, with slope kN. The Brønsted‐type plots (log kN vs. pKa of aminium ions) are linear, with slopes β = 0.26, 0.22, 0.19, and 0.28 for the reactions with 1, 2, 3, and 4, respectively. The magnitudes of the slopes indicate that these mechanisms are stepwise, with rate‐determining formation of a zwitterionic tetrahedral intermediate (T±). A dual parametric equation with the pKa of the nucleophiles and non‐leaving groups show βN = 0.26 and βnlg = −0.16, also in accordance with the proposed mechanism. On the other hand, the reactivity of these thiocarbonyl substrates and their carbonyl derivatives was studied using their hardness index and compared with their experimental parameters, confirming the proposed mechanisms. By comparison of the title reactions with similar aminolyses, the following conclusions arise: (i) The mechanism of the reactions under investigation is stepwise with rate‐determining formation of T±. (ii) The reactivity of the substrates toward quinuclidines follows the order 4 > 3 > 2 > 1. (iii) Quinuclidines are more reactive than isobasic pyridines toward chlorothionoformates. (iv) Chlorothionoformates are less reactive than chloroformates towards quinuclidines in accordance with the HSAB principle. (v) The kN values for phenyl chloroformate and 4 can be correlated with the pKa of quinuclidines and also with the hardness values calculated by the HF/3‐21G level of theory. Copyright © 2007 John Wiley & Sons, Ltd.