Separation of mandelic acid enantiomers using solid-liquid biphasic systems with chiral ionic liquids

Abstract This work aims to take full advantage of ionic liquids’ (ILs) “designer solvent” nature in order to expand the applicability of solid-liquid biphasic systems (SLBS) as alternative chiral resolution techniques. To this purpose, twelve chiral ILs, bearing chirality on the cation or the anion, were used as chiral selectors in SLBS to selectively precipitate mandelic acid enantiomers. The precipitation studies were performed using aqueous solutions of the chiral ILs or their chiral precursors, where the impact of the chemical structure and chirality was investigated. The most efficient chiral ILs were employed to evaluate the influence of different operational conditions on the formed precipitate, namely resolution time, resolution speed, temperature, initial concentration of chiral IL, initial concentration of racemic mandelic acid and water content. When compared to their precursors, chiral ILs showed an enhanced ability to induce enantioselective precipitation. The composition of the precipitate formed, and the molecular-level mechanisms leading to enantioselective precipitation, were also uncovered. Due to cooperative interactions between the chiral IL and the target mandelic acid enantiomer, the [N4444][D-Phe]-based SLBS proposed allowed to obtain maximum enantiomeric excess of 51% in a single-step, without any further additives.