General Features of Ionic Liquids in Catalysis

Ionic liquids represent a unique class of reaction media for catalytic processes, and their application in catalysis has entered a period of exploding growth. The number of catalytic reactions involving ionic liquids continues to increase rapidly. These liquids offer promising solutions to the problems associated with conventional organic solvents the potential advantages may include enhanced reaction rates, improved chemo- and regioselectivities, and facile separation of products and catalyst recovery. 

Because most transition metal complexes are soluble in ionic liquids, the ionic liquids are particularly suited for the isolation of catalysts in biphasic synthesis. 

Ionic liquids offer a broad spectrum of solvent properties that can be readily distinguished from those of the typical conventional organic solvents. The possibilities for tailoring properties of ionic liquids by the selection of their structures are particularly attractive. 

When the solubility of reactants in ionic liquids is limited, mixed organic solvents and ionic liquids can be used advantageously to achieve homogeneous systems in which the reactants and catalysts have the best solubility. The product separation can be facilitated by the removal of the organic solvent from the mixed solvent containing the ionic liquid (e.g., by extraction or distillation), because the solubility of the product in the ionic liquid is reduced upon the removal of the organic solvent. 

In catalytic reactions, ionic liquids with controllable coordinating strengths and/or reactivities toward the catalyst are particularly important. The coordinating strength of the ionic liquids depends on the nature of the anions and on the metal complex involved in the catalysis. With a large selection of available anions, it is possible to tailor the ionic liquid to suit specific chemical reactions, and variation of the structure and the composition of the cations and anions can alter the solubility of organic reactants in the ionic liquid. 

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