Design, synthesis and characterization of chiral ionic liquids with tetrafluoroborate and hexafluorophosphate anions

New chiral imidazolium-based ionic liquids, having symmetrical and asymmetrical structure of the salts, have been synthesized and characterized. Discussed salts contain optically active monoterpene moiety as follow:... [ view full abstract ]

New chiral imidazolium-based ionic liquids, having symmetrical and asymmetrical structure of the salts, have been synthesized and characterized. Discussed salts contain optically active monoterpene moiety as follow: (1R,2S,5R)-(–)-menthol, or (1S)-endo-(–)-borneol, or (1R)-endo-(+)-fenchol, which were derived from the renewable natural plant resources (Figure 1). As anions tetrafluoroborate and hexafluorophosphate were used. Synthesized symmetrical (Figure 2) and asymmetrical imidazolium salts (Figure 3) belong to Chiral Ionic Liquids (CILs) where the chirality resides in the cation and is associated with the presence of optically active monoterpene substituent.

Monocyclic and bicyclic monoterpene alcohols: (1R,2S,5R)-(–)-menthol, (1S)-endo-(–)-borneol, and (1R)-endo-(+)-fenchol, were used as a substrate in several different reactions to obtain quaternary symmetrical and asymmetrical imidazolium chlorides, which are the precursors for chiral ionic liquids. The synthesis of the chlorides precursors have been previously reported in detail by our group [1,3]. After that, metathesis of these chiral imidazolium chlorides with tetrafluoroborate sodium and hexafluorophosphate potassium in water/menthol solution were carried out. The ion exchange reactions go smoothly, with the satisfactory exceed (from 93,5 to 99,0%). All of the obtained symmetrical and asymmetrical tetrafluoroborates and hexafluorophosphates are hydrophobic, stable in the air, in contact with water, and in commonly used organic solvents. Moreover, they are non-volatile and non-flammable.

¹H, ¹³C and ¹⁹F NMR spectra of all synthesized quaternary imidazolium salts were investigated in detail. ¹H NMR and ¹³C NMR spectra indicated notable differences in the chemical shifts depending on anion used. Comparing the differences between values of the chemical shifts, the anions were ordered according to their increasing shielding capacities.

This research was financed by the National Science Centre (Poland) grant no. 2013/09/D/ST5/03904.

References

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[2] J. Feder-Kubis, B. Szefczyk, M. Kubicki, J. Org. Chem, 80 (1), 237-246 (2015).

[3] J. Flieger, J. Feder-Kubis, M. Tatarczak-Michalewska, A. Płazińska, A. Madejska; M. Swatko-Ossor, J. Sep. Sci., in press; Doi: 10.1002/jssc.201700197.