论文部分内容阅读
影响取代咪唑阳离子C2-H酸性(pK_a)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δ_(C2-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pK_a可由以下方程估算: pk_a=58.10-4.208δ_(C2-H)+1.659TSEI该式的标准偏差只有0.28 pK_a单位,估算结果与文献报导的pK_a值相吻合。表明如要精确估算pK_a,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 affecting the acidity (pKa) of C2-H substituted imidazolium cation: the partial positive charges carried by H atom and the steric shielding effect of the substituent, the former can use ~ 1H NMR chemical shifts of C2-H δ_ (C2-H) The latter is measured by the Group Topology Stereo Index TSEI. The pK_a of imidazolium cation C2-H can be estimated by the following equation: pk_a = 58.10-4.208δ_ (C2-H) + 1.659TSEI The standard deviation of this equation is only 0.28 pK_a units, and the estimation results agree well with the reported pK_a values. If we want to accurately estimate the pK_a, must consider the three-dimensional effect of substituent shielding; when you want to use lower alkaline nucleophiles C2-H substituted imidazole cation to form carbenes should use aryl-substituted imidazolium ions; if used Instead of using imidazolium ionic liquids as solvents and conducting organic reactions in the more alkaline conditions, it is best to choose non-aryl-substituted imidazolium ions with large stereospectroscopic effect.