Chiral Ionic Liquids: Synthesis, Properties, and Enantiomeric Recognition

Abstract

We have synthesized a series of structurally novel chiral ionic liquids which have a either chiral cation, chiral anion, or both. Cations are an imidazolium group, while anions are based on a borate ion with spiral structure and chiral substituents. Both (or all) stereoisomeric forms of each compound in the series can be readily synthesized in optically pure form by a simple one-step process from commercially available reagents. In addition to the ease of preparation, most of the chiral ILs in this series are liquid at room temperature with a solid to liquid transformation temperature as low as -70 degrees C and have relatively high thermal stability (up to at least 300 degrees C). Circular dichroism and X-ray crystallographic results confirm that the reaction to form the chiral spiral borate anion is stereospecific, namely, only one of two possible spiral stereoisomers was formed. Results of NMR studies including 1H{15N} heteronuclear single quantum coherence (HSQC) show that these chiral ILs exhibit intramolecular as well as intermolecular enantiomeric recognition. Intramolecularly, the chiral anion of an IL was found to exhibit chiral recognition toward the cation. Specifically, for a chiral IL composing with a chiral anion and a racemic cation, enantiomeric recognition of the chiral anion toward both enantiomers of the cation lead to pronounced differences in the NMR bands of the cation enantiomers. The chiral recognition was found to be dependent on solvent dielectric constant, concentration, and structure of the ILs. Stronger enantiomeric recognition was found in solvent with relatively lower dielectric constants (CDCl3 compared to CD3CN) and at higher concentration of ILs. Also, stronger chiral recognition was found for anions with a relatively larger substituent group (e.g., chiral anion with a phenylmethyl group exhibits stronger chiral recognition compared to that with a phenyl group, and an anion with an isobutyl group has the weakest chiral recognition). Chiral anions were also found to exhibit intermolecular chiral recognition. Enantiomeric discrimination was found for a chiral IL composed of a chiral anion and achiral cation toward another chiral molecule such as a quinine derivative.