Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 6094411, Chile.
J Org Chem. 2009 Dec 4;74(23):9173-9. doi: 10.1021/jo902005y.
The reactions of S-phenyl, S-(4-chlorophenyl), and S-(2,3,4,5,6-pentafluorophenyl) 4-nitrophenyl thiocarbonates (9, 11, and 16, respectively) with a series of secondary alicyclic (SA) amines and those of S-(4-methylphenyl) 4-nitrophenyl thiocarbonate (8) and compounds 9 and 11 with a series of phenols are subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically. Under nucleophile excess, pseudo-first-order rate coefficients (k(obsd)) were found. For all these reactions, plots of k(obsd) vs. free amine or phenoxide anion concentration at constant pH are linear, the slope (k(N)) being independent of pH. The Brønsted-type plots (log k(N) vs. pK(a) of the conjugate acids of the nucleophiles) for the aminolysis of 9, 11, and 16 are linear with slopes beta = 0.85, 0.90, and 0.67, respectively. The two former slopes are consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate, which breaking to products is rate determining. The latter beta value is consistent with a concerted mechanism. The Brønsted-type plots for the phenolysis of thiocarbonates 8, 9, and 11 are linear with slopes beta = 0.62, 0.70, and 0.69, respectively. These beta values and the absence of curvature at pK(a) = 7.5 confirm a concerted mechanism. In all these reactions, except those of 16, the main nucleofuge is 4-nitrophenoxide, being the thio benzenethiolate the minor nucleofuge. For the reactions of thiocarbonate 16 the main nucleofuge is pentafluorobenzenethiolate whereas little 4-nitrophenoxide was found. The reactions of two SA amines with S-(3-chlorophenyl) 4-nitrophenyl thiocarbonate (10) were subjected to product analysis, showing 60% 4-nitrophenoxide and 40% 3-chlorobenzenethiolate. The study is completed with a theoretical analysis based on the group electrophilicity index, a reactivity descriptor that may be taken as a measure of the ability of a group or fragment to depart from a molecule with the bonding electron pair. The theoretical analysis is in accordance with the experimental results obtained and predicts relative nucleofugalities of O-aryl vs. S-aryl groups in a series of diaryl thiocarbonates not experimentally evaluated to date.
S-苯基、S-(4-氯苯基)和 S-(2,3,4,5,6-五氟苯基) 4-硝基苯基硫代碳酸酯（分别为 9、11 和 16）与一系列脂环族仲胺以及 S-(4-甲基苯基) 4-硝基苯基硫代碳酸酯（8）和化合物 9 和 11 与一系列酚的反应在 44wt%乙醇-水中进行动力学研究，在 25.0°C 和离子强度为 0.2M 的条件下进行。反应通过分光光度法进行跟踪。在亲核试剂过量的情况下，发现了拟一级速率系数 (k(obsd))。对于所有这些反应，在恒定 pH 下，k(obsd) 与游离胺或酚氧阴离子浓度的关系图都是线性的，斜率 (k(N)) 与 pH 无关。亲核试剂的 Brønsted 型图（log k(N) 与亲核试剂共轭酸的 pK(a) 的关系）对于 9、11 和 16 的氨解反应是线性的，斜率分别为β=0.85、0.90 和 0.67。前两个斜率与逐步机制一致，通过两性离子四面体型中间体，该中间体断裂为产物是速率决定步骤。后一个β值与协同机制一致。硫代碳酸酯 8、9 和 11 的酚解 Brønsted 型图呈线性，斜率分别为β=0.62、0.70 和 0.69。这些β值和在 pK(a)=7.5 处没有曲率证实了协同机制。在所有这些反应中，除了 16 的反应外，主要亲核试剂是 4-硝基苯酚盐，而苯硫酚盐是次要亲核试剂。对于 16 的反应，主要亲核试剂是五氟苯硫酚盐，而发现的 4-硝基苯酚盐很少。两种脂环族仲胺与 S-(3-氯苯基) 4-硝基苯基硫代碳酸酯（10）的反应进行了产物分析，显示 60%的 4-硝基苯酚盐和 40%的 3-氯苯硫酚盐。研究还基于基团亲电性指数进行了理论分析，该指数是一种反应性描述符，可作为衡量基团或片段与键合电子对脱离分子的能力的指标。理论分析与实验结果一致，并预测了一系列尚未进行实验评估的二芳基硫代碳酸酯中 O-芳基与 S-芳基的相对亲核性。
