基团立体屏蔽效应对取代咪唑离子液体酸性的影响  

The influence of topological steric effect index of substituent on the acidity of substitutedimidazolium ionic liquid

作　　者：袁华[1] 曹晨忠[1] 曾荣今[1] 

机构地区：[1]湖南科技大学化学化工学院,理论化学与分子模拟教育部重点实验室,湖南湘潭411201

出　　处：《计算机与应用化学》2009年第5期534-538,共5页Computers and Applied Chemistry

基　　金：国家自然科学基金(20772028);湖南省自然科学基金(06JJ2002)资助项目

摘　　要：影响取代咪唑阳离子C2-H酸性（pKa）的重要因素有二：H原子所带的部分正电荷和取代基的立体屏蔽效应，前者可用C2-H的^1H NMR化学位移δC2-H度量，后者用基团拓扑立体效应指数TSEI衡量。咪唑阳离子C2-H的pKa可由以下方程估算：pKa=58．10—4．2086δC2-H＋1．659TSEI该式的标准偏差只有0．28pKa单位，估算结果与文献报导的pK值相吻合。表明如要精确估算pKa，必须考虑取代基立体屏蔽效应；当希望用碱性较低的亲核试剂脱除取代咪唑阳离子的C2-H形成卡宾时，应采用芳香基取代的咪唑离子；若使用取代咪唑离子液体作溶剂，并且在碱性较强的条件下进行有机反应，则最好选择立体屏蔽效应大的非芳香基取代的咪唑离子。There are two important factors influencing the acidity （ pKa ） of C2-H insubstitutedimidazolium ： the partial positive charge on H atom and the steric screened effect of substituents, the former can be measured by the ^1H NMR chemical shift δC2-H of C2-H, and the latter can be scaled by the topological steric effect index （TSEI）. The acidity pK, of substitutedimidazolium can be estimated with the following equation： pKa=58.10-4.2086δC2-H＋1.659TSEI The standard error of the above equation is only 0.28 pKa unit, and the estimated results are in agreement with the pKa values reported by literatures. The obtained results show that it is necessary to employ the sterie screened effect of substituent for evaluating the pKa more accurately; If one wants to get the N-Heterocyclic Carbene by deprotonation of substitutedimidazolium using the nueleophilie reagent with less basicity, the imidazolium with aromatic groups should be employed. When the 1, 3-disubstitutedimidazolium is used as solvent to carry out an organic reaction in the condition with stronger basicity, it had better to choose the imidazolium with non-aromatic groups having larger steric screened effect.

关 键 词：离子液体 取代咪唑离子 酸性 拓扑立体效应指数 

分 类 号：TQ015.9[化学工程] O6-39[理学—化学]