Equilibration of N-(2-cyanoethyl)pyridinium cations with substituted pyridines and acrylonitrile. A change in rate-determining step in an E1cb reaction
作者:John W. Bunting、Andrea Toth、Christina K. M. Heo、Rodney G. Moors
DOI:10.1021/ja00180a034
日期:1990.11
The rates of equilibration of N-(2-cyanoethyl) pyridinium cations (1) with the corresponding pyridines and acrylonitrile have been measured in aqueous solutions of ionic strength 0.1 at 25 o C. Second-order rate constants (k OH ) have been obtained for the hydroxide ion catalyzed elimination reactions of 16 ring-substituted 1 having pyridine leaving groups of pK BH in the range 1.5-9.7. Bronsted plots
N-(2-氰乙基) 吡啶鎓阳离子 (1) 与相应的吡啶和丙烯腈的平衡速率已在 25 o C 离子强度为 0.1 的水溶液中测量。已获得二阶速率常数 (k OH )对于氢氧根离子催化的 16 个环取代的 1 的消除反应,具有 1.5-9.7 范围内 pK BH 的吡啶离去基团。log k OH 与 pK BH 的布朗斯特图是“向下凹”的,具有两个不同的线性区域,β lg =-0.30(对于 pK BH 5.8)。该观察结果与 pK BH 5.8 的决定速率去质子化 1(即 (E1cb) 不可逆 )的 E1cb 反应机制中的速率决定步骤的变化一致