Chirality Transfer in Imidazolium Camphorsulfonate Ionic Liquids through Ion Pairing Effects

The paper describes our studies on ion pair interactions in ionic liquids (IL) using an asymmetric hydrogenation reaction as probe. Three different ionic liquids carrying prochiral keto‐functionalized cations were hydrogenated in the presence of their chiral, enantiomerically pure counter‐ion using an achiral heterogeneous ruthenium catalyst. For the hydrogenation of N‐(3′‐oxobutyl)‐N‐methylimidazolium camphorsulfonate (2), N‐(3′‐oxobutyl)imidazolium camphorsulfonate (4) and N‐(5′‐oxohexyl)‐N‐methylimidazolium camphorsulfonate (6) we found a strong dependency of the enantiomeric excess (ee in the cation) on the polarity of the solvent, the concentration of the IL and the structure of the IL. The highest ee values of up to 94% were found for the hydrogenation of 2 in ethanol. Interestingly, we observed that the ee (and consequently the strength of ion pair interaction) had a pronounced maximum for a certain concentration of the IL in the solvent depending on the nature of the solvent and on the substrate. Remarkably, the concentration leading to the maximum ee could be rationalized by independent determination of the degree of dissociation which was obtained by a combination of diffusion‐ordered NMR spectroscopy and conductivity measurements.