Synthesis and Characterization of Chiral Imidazolium Salts

Abstract

Chiral imidazolium salts that can be classified as ionic liquids (ILs) were derived from the ‘chiral pool’ precursors camphor, β-pinene, and tartaric acid. ILs containing chiral imidazolium cations as well as chiral anions were synthesized. Furthermore, the anion of the IL 1-methyl-3-[(S)-2′-methylbutyl]imidazolium tosyl­ate was substituted on an ion-exchange resin for the chiral (S)-camphorsulfonate anion thus forming the first well-characterized ‘doubly chiral’ IL.

Keywords

imidazolium salts - ionic liquids - chiral anions, chiral cations

References

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8

Presumably, methylimidazolium formation is due to Hoffmann type elimination. NMR spectroscopy of crude reaction mixtures indicated concomitant alkene formation.

12

The interchange of anions of similar size, such as OTf^-, sulfonates, or BF₄ ^- in the usual CH₂Cl₂-H₂O system is not possible.

13

We found that the classical ILs BMI·BF₄ or BMI·PF₆ are obtained in a purer form by the ion-exchange method than by the standard CH₂Cl₂-H₂O extraction method.