Chirality Transfer in Imidazolium Camphorsulfonate Ionic Liquids
through Ion Pairing Effects
ADVANCED SYNTHESIS & CATALYSIS, 351 (3):
432-440 (Februar 2009)DOI: {10.1002/adsc.200800569}
Zusammenfassung
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.