Targeted and Systematic Approach to the Study of pKa Values of Imidazolium Salts in Dimethyl Sulfoxide

A range
of more than 25 imidazolium salts, chosen for their differing
steric and electronic features, were prepared, and their pKa values were determined using the bracketing
indicator method. Through the systematic change in the structure of
the imidazolium cation, the effect of varying substituents at each
position on the heterocyclic ring was determined; particularly, the
transmission of electronic effects was quantified using Hammett parameters.
These new data give an indication of the strength of base required
for deprotonation and the potential to correlate these data with the
nucleophilicity of the corresponding carbenes.

本研究选取了25种以上的咪唑类盐，这些盐因其独特的立体和电子特性而被精心挑选，并通过夹点指示剂法测定了它们的pKa值。通过对咪唑阳离子结构的系统性调整，研究了不同取代基在杂环环上各个位置的变化对异环结构的影响；特别是，利用Hammett参数量化了电子效应的传递。这些新的数据揭示了实现去质子化所需的碱的强度，并有望将这些数据与相应羰基的亲核性进行关联。