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From salts to ionic liquids by systematic structural modifications: A rational approach towards the efficient modular synthesis of enantiopure imidazolium salts

Author:
Ríos Lombardía, NicolásUniovi authority; Busto García, Benjamín EduardoUniovi authority; Gotor Fernández, VicenteUniovi authority; Gotor Santamaría, Vicente MiguelUniovi authority; Porcar García, Raúl; García Verdugo, Eduardo; Luis Lafuente, Santiago Vicente; Alfonso Rodríguez, IgnacioUniovi authority; García-Granda, SantiagoUniovi authority; Menéndez Velázquez, AmadorUniovi authority
Subject:

Chirality; Green Chemistry; Imidazolium Salts; Ionic Liquids; Lipases.

Publication date:
2010
Publisher version:
http://dx.doi.org/10.1002/chem.200901623
Citación:
Chemistry - A European Journal, 16(3), p. 836-847 (2010); doi:10.1002/chem.200901623
Descripción física:
p. 836-847
Abstract:

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.

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.

URI:
http://hdl.handle.net/10651/6796
ISSN:
1521-3765
Identificador local:

20100398

DOI:
10.1002/chem.200901623
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