The pKa values of N-aryl imidazolinium salts, their higher homologues, and formamidinium salts in dimethyl sulfoxide.

A series of imidazolinium salts, their six-, seven- and eight-membered homologues, and the related formamidinium salts were prepared, and their pKa values were determined in DMSO at 25 °C using the bracketing indicator method. The effect of each type of structural variation on the acidity of each salt was considered, particularly noting the importance of ring size and the effect of the steric and electronic nature of the N-aryl substituents. The effect of a cyclic structure was also probed through comparing the cyclic systems with the corresponding formamidinium salts, noting the importance of conformational flexibility in the latter cases. Along with allowing choice of appropriate bases for deprotonation of these species, it is anticipated that the data presented will aid in the understanding of the nucleophilicity, and potentially catalytic efficacy, of the corresponding carbenes.

The impact of cation structure upon the acidity of triazolium salts in dimethyl sulfoxide.

A series of triazolium salts, selected for their varying electronic and steric properties, were prepared and their pKa values were determined in DMSO at 25 °C using the bracketing indicator method. The effect of each systematic structural variation upon the acidity of the triazolium cation has been considered, in particular examining the effects of systematically altering electronic properties, quantified through the use of Hammett σ parameters. The first pKa value for an azolium salt that generates a mesionic carbene is also reported. These new data allow for the selection of appropriate bases for the deprotonation of such triazolium salts and the potential to correlate the pKa values determined herein with the nucleophilicity of the corresponding carbenes.

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.