Ion speciation driving chirality transfer in imidazolium-based camphorsulfonateionic liquid solutions

Abstract

The underlying principle of the chirality transfer in imidazolium-based camphorsulfonate ionic liquids is rationalized by linking catalytic results from the hydrogenation of [N-(3′-oxobutyl)-N-methylimidazolium] [(+)-camphorsulfonate] to [N-(3′-hydroxybutyl)-N-methylimidazolium] [(+)-camphorsulfonate] in tetrahydrofuran with electrolyte theory by the help of dielectric relaxation spectroscopy. Using this approach we are able to explain why the maximum of the enantiomeric excess of the hydrogenation reaction in tetrahydrofuran is found at a medium concentration of 0.15 mol L⁻¹, whereas it declines at both, lower and higher concentrations. Dielectric spectra in the concentration range between 0.05 and 1.0 mol L⁻¹ reveal a solute mode due to dipolar ion pairs and larger dipolar ion clusters. They verify that at very low concentrations the ionic liquid ions are fully solvated with an increasing tendency to form neutral ion pairs with increasing concentration. Already at 0.025 mol L⁻¹ the degree of dissociation reaches a minimum reflecting a maximum of neutral ion pair formation. With increasing ionic liquid concentration ordered ion clusters are formed by two and more ion pairs. At high concentrations these clusters collapse by dilution in the excess ionic liquid and the defined ion contact necessary for the chirality transfer is lost to a great extent.