Nucleophilic substitution reactions of N-chloramines:: Evidence for a change in mechanism with increasing nucleophile reactivity
Nucleophilic substitution reactions of N-chloramines:: Evidence for a change in mechanism with increasing nucleophile reactivity
复制标题
DOI：
10.1021/jo062356k
复制
发表时间：
2007-04-27
影响因子：
3.6
通讯作者：
Rios, Miguel A.
中科院分区：
文献类型：
--
作者：
Calvo, Paula;Crugeiras, Juan;Rios, Miguel A.
本文报道了在25 ℃和I = 0.5（NaClO_4）条件下，一系列水合氢离子催化的亲核试剂与N-氯牛磺酸（1）在水中反应的三级速率常数（k（Nu））（H）（M ~（-2）s ~（-1））。溶剂氘同位素对水合氢离子催化溴离子和碘离子与1反应的影响分别为（k（Br））（H）/（k（Br））（D）= 0.30和（k（I））（H）/（k（I））（D）= 0.54。这些同位素效应的逆性质和一般酸催化的情况下是一致的逐步机制，涉及质子化的1在一个快速的预平衡步骤。更亲核的SO 32-和HOCH 2CH 2S-与氯胺反应的一般酸的强催化的出现表明了向协调机制的变化，其中氯胺在氮原子处的质子化和氯转移到亲核试剂中在单一步骤中发生。在硫醇盐阴离子的存在下的质子化氯胺的寿命的粗略估计表明，一致的机制是由一个显着的质子化基板与亲核试剂接触的寿命的情况下执行。理论计算提供的证据对质子化1的亲核试剂氯化的电子转移机制。
Third-order rate constants (k(Nu))(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 (k(Br))(H)/(k(Br))(D) = 0.30 and (k(I))(H)/(k(I))(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 SO32- 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.