Chirality transfer in ionic liquids via ion pairing effects
Technische Chemie
Zusammenfassung der Projektergebnisse
The multitude of possible interactions between the organic ions of an ionic liquid (IL) distinguishes ILs from uncharged organic compounds and molten salts. In organic compounds van der Waals forces and hydrogen bonds and in molten salts solely coulombic interactions are prevalent, whereas ILs combine all of them. In diluted solutions of ILs these interactions lead to the formation of aggregates. Depending on the strength of the interactions and the concentration of the IL different aggregates are formed. At low concentrations ion-pairs are the dominant species. The working thesis of our project is using the enantioselectivity of a reaction carried out on a prochiral ion in presence of a chiral counterion as a probe for the aggregation behavior. As an asymmetric induction requires structurally well defined interactions which are mostly found in ion-pairs, the induced enantioselectivity should correlate with the probability of ion pairing. The first proof of this concept was the heterogeneous (Ru/C) asymmetric hydrogenation of [N-(3’-oxobutyl)-N-methylimidazolium]-[(R)-camphorsulfonate] and -[(S)-camphorsulfonate] respectively, both keto-functionalized imidazolium ILs with a chiral anion. The enantioselectivity induced showed a strong dependence on the concentration with a maximum at medium concentrations of up to 94% ee. We could link this behavior to the probability of the formation of ion pairs, which we determined by DOSY-NMR measurements and dielectric relaxation spectroscopy. These analytical techniques allowed for a deep insight into the molecular level, and showed that at low concentrations single solvated ions are dominant and the probability of ion pairing increases with concentration. At high concentrations larger aggregates are formed in which the interactions between the ions are weaker. The highest probability of ion pairing and the highest enantioselectivity correlated well. Furthermore the influence of the solvent was investigated and it was found, that especially at low concentrations of IL, there is a connection between the polarity of the solvent and the ee induced, as it represents the solvents ability to stabilize single ions. In addition we could show experimentally and with the measurements mentioned above that the structure and especially the H-bonding ability has a strong influence on ion-pairing and therefore the effectiveness of asymmetric induction. We could also show successful asymmetric induction via ion pairing interactions between the ions of chiral ILs consisting of other chiral anions of different structure and even axial chirality. For ILs with a chiral cation ([(1S,2R)-Dimethylephedrinium]) and a prochiral anion we also found a concentration dependent asymmetric induction. Summing up we have showed, that ion pairing interactions have a big influence on the chemistry of diluted ILs, as we could also show, that they have a large influence on the regioselectivity and reaction rate of the hydroformylation of the cation or anion of several ionic liquids.
