咪唑N-烷基化相转移催化反应动力学研究

本文报道在冠醚和季铵盐等相转移催化剂存在下咪唑N-正丁基化反应的动力学研究结果。在一定的催化剂浓度下,反应速度与烷基卤的浓度呈一级反应动力学关系,同时与催化剂浓度亦呈一级反应比例关系在60±0.05℃,以四丁基溴化铵为催化剂,咪唑N-正丁基化反应速率常数κ=(1.90±0.02)×10~(-2),活化能E_a=11.7±0.5 kcal/mol,此外还研究了不同类型冠醚和季铵盐对反应的影响。反应机理可设想为:在相转移催化反应条件下,由催化剂中的阳离子R_4N_ 和亲核试剂Im-形成的离子对转移到有机相中与烷基卤发生反应,因此反应速率对烷基卤浓度变化极为敏感,也与催化剂在两相间的分配系数α值有关。一般以α值为1左右的季铵盐的催化效果最好,这可能是选择有效相转移催化剂的一个标志。

Kinetic studies on N-alkylation of imidazole in the presence of phase transfer catalysis

The kinetic study of N-alkylation of imidazole with n-butyl bromide in the presence of phase transfer catalysts has been investigated. The reaction rates determined either by chemical analysis or by ^1H NMR technique are linearly dependent on concentration of n-butyl bromide and the catalyst. The rate constant(k) of the reaction of imidazole and n-butyl bromide with tetrabutylammonium bromide (1 mol%) as phase transfer catalyst at temperature 60 +/- 0,05`C is (1.90 +/- 0.02) X 10^-^2 min^-^1 and its activation energy (Ea) is 11.7 +/- 0.5 kcal/mol. An ion pair mechanism and a reaction course are proposed and discussed.