Key Roles of Free Cations in Modulating the Reaction Rate in Imidazolium Ionic Liquid–Dimethyl Sulfoxide Mixtures

Imidazolium ionic liquids (ILs) are widely utilized in various fields due to their distinctive properties. However, their high viscosity limits their application in specific reactions, and mixing ILs with organic components is a way to solve this problem. While previous studies mainly focused on the structural changes of ILs after adding organic molecules, no studies elucidated the influence of their existing species on chemical reactions. In this study, aerobic α-hydroxylation of 2-methylcyclohexanone was chosen as a model reaction, and the reaction rate was found to be adjusted by varying imidazolium concentration in its mixtures with dimethyl sulfoxide. To elucidate the mechanism, the distribution of species in an IL solution and its change with concentration were studied by molecular dynamics simulations, and the results revealed the significant impact of the concentration of free cations on the reaction rate. The interaction between the ionic species and reaction intermediate, as calculated by density functional theory, highlighted the crucial role of free cations in this reaction. This study demonstrates the feasibility of tuning the concentration of free cations by varying the concentration of the IL solution, establishing the relationship between its microstructure and chemical reaction efficiency, thus providing vital information for the design and application of ILs.