Castro Enrique A, Aguayo Raul, Bessolo Jorge, Santos José G
Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22, Chile.
J Org Chem. 2005 Apr 29;70(9):3530-6. doi: 10.1021/jo050119w.
[reaction: see text] The reactions of S-2,4-dinitrophenyl 4-methyl (1), S-2,4-dinitrophenyl 4-H (2), S-2,4-dinitrophenyl 4-chloro (3), and S-2,4-dinitrophenyl 4-nitro (4) thiobenzoates with a structurally homogeneous series of pyridines are subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M (KCl). The reactions are studied spectrophotometrically (420 nm) by monitoring the appearance of 2,4-dinitrobenzenethiolate anion. Pseudo-first-order rate coefficients (k(obsd)) are obtained for all the reactions, employing excess of amine. The plots of k(obsd) vs [free pyridine] at constant pH are linear with the slopes (k(N)) independent of pH. The Brønsted-type plots (log k(N) vs pK(a) of the conjugate acid of the pyridines) are curved for all the reactions. The Brønsted curves are in accordance with stepwise mechanisms, through a zwitterionic tetrahedral intermediate (T(+/-)), and a change in the rate-limiting step. An equation based on this hypothesis accounts well for the experimental points. The Brønsted lines were calculated with the following parameters: Reactions of thiolbenzoate 1: beta(1) 0.33 (slope at high pK(a)), beta(2) 0.95 (slope at low pK(a)), and pK(a)(0) = 8.5 (pK(a) at the curvature center); thiolbenzoate 2: beta(1) 0.30, beta(2) 0.88, and pK(a)(0) = 8.9; thiolbenzoate 3: beta(1) 0.33, beta(2) 0.89, and pK(a)(0) = 9.5; thiolbenzoate 4: beta(1) 0.21, beta(2) 0.97, and pK(a)(0) = 9.9. The increase of the pK(a)(0) value with the increase of the electron-withdrawing effect of the acyl substituent is explained by the argument that the rate of pyridine expulsion from T(+/-) (k(-)(1)) is favored over that of 2,4-dinitrobenzenethiolate leaving (k(2)), i.e., k(-)(1)/k(2) increases, as the acyl group becomes more electron withdrawing. The pK(a)(0) values for the title reactions are smaller than those for the reactions of the corresponding 4-nitrophenyl 4-substituted thiolbenzoates with the same pyridine series. This is explained by the larger k(2) value for 2,4-dinitrobenzenethiolate leaving from T(+/-) compared with 4-nitrobenzenethiolate, which results in lower k(-)(1)/k(2) ratios for the dinitro derivatives. The pK(a)(0) value obtained for the pyridinolysis of thiolbenzoate 2 (pK(a)(0) = 8.9) is smaller than that found for the same aminolysis of 2,4-dinitrophenyl benzoate (pK(a)(0) = 9.5). This is attributed to the greater nucleofugality from T(+/-) of 2,4-dinitrobenzenethiolate (pK(a) of conjugate acid 3.4) relative to 2,4-dinitrophenoxide (pK(a) of conjugate acid 4.1). The title reactions are also compared with the aminolysis of similar esters to assess the effect of the amine nature and leaving and acyl groups on the kinetics and mechanism.
[反应：见正文] 对S-2,4-二硝基苯基4-甲基硫代苯甲酸酯（1）、S-2,4-二硝基苯基4-H硫代苯甲酸酯（2）、S-2,4-二硝基苯基4-氯硫代苯甲酸酯（3）和S-2,4-二硝基苯基4-硝基硫代苯甲酸酯（4）与一系列结构均一的吡啶的反应，在44 wt%乙醇-水体系中、25.0℃且离子强度为0.2 M（KCl）的条件下进行动力学研究。通过监测2,4-二硝基苯硫酚阴离子的出现，采用分光光度法（420 nm）研究这些反应。使用过量的胺，得到了所有反应的准一级速率系数（k(obsd)）。在恒定pH下，k(obsd)对[游离吡啶]的作图呈线性，斜率（k(N)）与pH无关。对于所有反应，布朗斯特型作图（log k(N)对吡啶共轭酸的pK(a)）都是弯曲的。这些布朗斯特曲线符合分步机理，通过两性离子四面体中间体（T(+/-)）以及限速步骤的变化。基于该假设的一个方程很好地解释了实验点。计算布朗斯特线时使用了以下参数：硫代苯甲酸酯1的反应：β(1) 0.33（高pK(a)时的斜率），β(2) 0.95（低pK(a)时的斜率），pK(a)(0) = 8.5（曲率中心处的pK(a)）；硫代苯甲酸酯2：β(1) 0.30，β(2) 0.88，pK(a)(0) = 8.9；硫代苯甲酸酯3：β(1) 0.33，β(2) 0.89，pK(a)(0) = 9.5；硫代苯甲酸酯4：β(1) 0.21，β(2) 0.97，pK(a)(0) = 9.9。pK(a)(0)值随着酰基取代基吸电子效应的增加而增大，这是因为从T(+/-)中排出吡啶的速率（k(-)(1)）相对于2,4-二硝基苯硫酚离去的速率（k(2)）更有利，即随着酰基吸电子性增强，k(-)(1)/k(2)增大。本标题反应的pK(a)(0)值小于相应的4-硝基苯基4-取代硫代苯甲酸酯与相同吡啶系列反应的pK(a)(0)值。这是因为与4-硝基苯硫酚相比，2,4-二硝基苯硫酚从T(+/-)离去的k(2)值更大，导致二硝基衍生物的k(-)(1)/k(2)比值更低。硫代苯甲酸酯2吡啶解反应得到的pK(a)(0)值（pK(a)(0) = 8.9）小于2,4-二硝基苯基苯甲酸酯相同氨解反应的pK(a)(0)值（pK(a)(0) = 9.5）。这归因于相对于2,4-二硝基苯酚盐（共轭酸的pK(a)为4.1），2,4-二硝基苯硫酚（共轭酸的pK(a)为3.4）从T(+/-)的亲核离去能力更强。还将本标题反应与类似酯的氨解反应进行比较，以评估胺的性质以及离去基团和酰基对动力学和机理的影响。
