Calvo Paula, Crugeiras Juan, Ríos Ana, Ríos Miguel A
Departamento de Química Física, Facultad de Química, Universidad de Santiago, 15782 Santiago de Compostela, Spain.
J Org Chem. 2007 Apr 27;72(9):3171-8. doi: 10.1021/jo062356k. Epub 2007 Mar 31.
Third-order rate constants (kNu)H (M-2 s-1) for the hydronium ion catalyzed reactions of a range of nucleophiles with N-chlorotaurine (1) in water at 25 degrees C and I=0.5 (NaClO4) are reported. The solvent deuterium isotope effects on hydronium ion catalysis of the reaction with 1 of bromide and iodide ion are (kBr)H/(kBr)D=0.30 and (kI)H/(kI)D=0.54, respectively. The inverse nature of these isotope effects and the absence of general acid catalysis are consistent with a stepwise mechanism involving protonation of 1 in a fast preequilibrium step. The appearance of strong catalysis by general acids for the reaction of the more nucleophilic SO(3)2- and HOCH2CH2S- with the chloramine indicates a change to a concerted mechanism, with protonation of the chloramine at nitrogen and chlorine transfer to the nucleophile occurring in a single step. A rough estimate of the lifetime of the protonated chloramine in the presence of the thiolate anion suggests that the concerted mechanism is enforced by the absence of a significant lifetime of the protonated substrate in contact with the nucleophile. Theoretical calculations provide evidence against an electron-transfer mechanism for chlorination of the nucleophiles by protonated 1.
报道了在25℃、离子强度I = 0.5（高氯酸钠）的水中，一系列亲核试剂与N - 氯代牛磺酸（1）发生水合氢离子催化反应的三阶速率常数(kNu)H（M-2 s-1）。溶剂氘同位素对溴离子和碘离子与1反应的水合氢离子催化作用的影响分别为(kBr)H/(kBr)D = 0.30和(kI)H/(kI)D = 0.54。这些同位素效应的反向性质以及一般酸催化作用的不存在与一个逐步机理一致，该机理涉及在快速预平衡步骤中1的质子化。对于亲核性更强的SO(3)2-和HOCH2CH2S-与氯胺的反应，一般酸表现出强烈的催化作用，这表明转变为协同机理，即氯胺在氮上质子化以及氯向亲核试剂的转移在一步中发生。在硫醇盐阴离子存在下对质子化氯胺寿命的粗略估计表明，协同机理是由于质子化底物与亲核试剂接触时不存在显著的寿命而得以实施。理论计算提供了证据，反对质子化的1对亲核试剂进行氯化的电子转移机理。
