Synthesis and spectroscopic properties of symmetrical ionic liquids based on (-)-menthol

Several symmetrical imidazolium salts were obtained from the natural chiral pool of (1R,2S,5R)-(-)-menthol. First 1,3-bis[(1R,2S,5R)-(-)-menthoxymethyllimidazolium chloride, which is a prototype for ionic liquids, was prepared with the use of two different methods. Furthermore, metathesis of this symmetrical imidazolium chloride with various salts was carried out. The ion exchange reaction goes smoothly, with the satisfactory yield of 97.5 to 99.5%. Obtained 1,3-bis[(1R,2S,5R)-(-)-menthoxymethyl]imidazolium salts are stable in the air, in contact with water and in commonly used organic solvents. Moreover they are non-volatile and non-flammable. Discussed symmetrical salts belong to chiral ionic liquids (CILs) where the chirality resided in the cation and is associated with the presence of optically active (1R,2S,5R)-(-)-menthol. The diastereotopic protons in the H-1 NMR were thoroughly described. Moreover, the H-1 NMR and C-13 NMR spectra indicated notable differences in the chemical shifts depending on the anion used. Comparing the differences between values of the chemical shifts, the anions were ordered according to their increasing shielding capacities. The changing of the electron density surrounding the imidazolium ring was discussed. (C) 2016 Elsevier B.V. All rights reserved.