Interplay of acidity and ionic liquid structure on the outcome of a heterocyclic rearrangement reaction

The study of suitable probe reactions is a powerful tool to investigate the properties of nonconventional solvents such ionic liquids (ILs). In particular, we studied the acid-catalyzed mononuclear rearrangement of heterocycles (MHR) of the Z-phenylhydrazone of 5-amino-3-benzoyl-1,2,4-oxadiazole into the relevant 1,2,3-triazole, in solution of ILs by means of kinetic measurements. We chose as solvents six ILs differing both in the cation and anion, in the presence of five carboxylic and sulfonic acids as catalysts. For a useful comparison, the reaction was also performed in 1,4-dioxane and methanol. In general, the reaction occurs faster in ILs, compared to conventional solvents, according to the weaker solvation interactions operating in the former media. The effect of IL anion and cation on the reactivity and on the acidic strength of the catalysts was analyzed. To this aim, we measured the acidic strength of the sulfonic acids in each IL, estimated by the equilibrium formation constant of each acid with 4-nitroaniline. We found that the trend of reactivity as a function of the IL anion mainly reflects the larger difference in acidic strengths of the catalyst. Conversely, acidic strength spans a narrower range as a function of the IL cation. As a result, other factors come into play, such as the π-π interactions involving aromatic IL cations, substrate, and transition states, leading to a more articulate trend.