From Salts to Ionic Liquids by Systematic Structural Modifications: A Rational Approach Towards the Efficient Modular Synthesis of Enantiopure Imidazolium Salts

• Published in: Chemistry : a European journal Vol. 16; no. 3; pp. 836 - 847
• Main Authors: Ríos-Lombardía, Nicolás, Busto, Eduardo, Gotor-Fernández, Vicente, Gotor, Vicente, Porcar, Raúl, García-Verdugo, Eduardo, Luis, Santiago V., Alfonso, Ignacio, García-Granda, Santiago, Menéndez-Velázquez, Amador
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 18.01.2010; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Subjects: Biocatalysis; Calorimetry, Differential Scanning; Chemistry; Chemistry, Multidisciplinary; chirality; Crystallography, X-Ray; green chemistry; Hydrogen Bonding; Imidazoles - chemical synthesis; Imidazoles - chemistry; imidazolium salts; ionic liquids; Ionic Liquids - chemistry; Ions; Kinetics; Lipase - metabolism; lipases; Liquids; Molecular Conformation; Physical Sciences; Science & Technology; Spectroscopy, Fourier Transform Infrared; Stereoisomerism; ROOM-TEMPERATURE; GREEN SOLVENTS; DESIGN; chirality; MOLECULAR-STRUCTURE; NUCLEAR-MAGNETIC-RESONANCE; VIBRATIONAL-SPECTRA; green chemistry; HYDROGEN-BONDS; PHYSICOCHEMICAL PROPERTIES; AMINO-ACID; SPECTROSCOPY; imidazolium salts; lipases; ionic liquids
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200901623

This paper reports a simple and robust modular synthetic strategy that leads to a large variety of configurationally and structurally diverse imidazole‐based chiral ionic liquids (CILs) by lipase‐catalyzed resolution. The intimate microscopic interactions of the supramolecular ionic network of these imidazolium chiral salts at the molecular level are investigated both spectroscopically (NMR, FT‐IR‐ATR) and theoretically, and a topological analysis of the experimental electron densities obtained by X‐ray diffraction of single crystals is performed. Our results support the key role played by the relative configuration of the ‐OR group on the hydrogen‐bonding pattern and its strong influence on the final physical properties of the imidazolium salt. We also obtained a reasonable correlation between the observed melting point and the non‐covalent interactions. The spectroscopic data and the topological analysis reflect the key role played by hydrogen bonds between the OH and imidazolium C2H groups in both cation–anion and cation–cation interactions, with the presence of an OH group leading to an additional inter‐cation interaction. This interaction significantly affects the properties of stereoisomeric salts. Even more interestingly, we also studied the effect of the chirality by comparing enantiopure CILs with their racemic mixtures and found that, with the exception of trans‐Cy6‐OH‐Im‐Bn‐Br, the melting points of the racemic mixtures are higher than those of the corresponding enantiomerically pure forms. For stereoisomeric examples, we have successfully explained the differences in melting temperatures in light of the corresponding structural data. Chirality should therefore be taken into account as a highly attractive design vector in the preparation of ILs with specifically desired properties. From salt to liquid through hydrogen bonds: The chirality of imidazolium salts is used as a design vector in the preparation of ionic liquids with specifically desired properties (see figure). More than 30 enantiopure imidazolium salts are reported enabling the systematic mapping of all the possible structural variables within the chiral molecules.