Synthesis and Characterization of Modified Imidazolium Salts as In Situ Carbene Precursors

Ionic liquids (ILs) based on 1,3-dialkylimidazolium salts are well-known to form in situ carbenes when combined with weakly basic anions such as acetate. This approach avoids the need for strong bases to preform the carbene for organocatalytic applications. However, the reactivity of these in situ carbenes is limited by their low concentration in solution, leading to slower reaction rates compared to the use of similar preformed carbenes. More basic anions, such as hydroxide, lead to the degradation of the corresponding imidazolium salt, demonstrating a balance between reactivity and stability for these systems. Toward identifying more effective IL-based in situ carbene precursors, we have prepared 10 imidazolium and benzimidazolium salts featuring an acetate anion, as well as examining 4 salts that contained alternative anions. These salts were structurally characterized, their thermal properties investigated, including melting points and thermal stability, and their ability to form carbenes interrogated through their reaction with elemental sulfur. These studies identified 3 low-melting imidazolium and benzylimidazolium salts that displayed similar thermal stability to the archetypical 1-butyl-3-methylimidazolium acetate IL, with enhanced carbene reactivity.