Over the past decades ionic liquids have attracted enormous attention as solvents for organic synthesis. All attempts to use ILs as solvents for organic synthesis spark from the following unique combination of their properties:
- Neglible vapor pressure
- Non-flammability
- Thermal, chemical and electrochemical stability
- Solubilizing and solvating ability (also “synthesis in a pure ligand”)
- Tuneable polarity (which mostly lies in the range of alcoholic solvents)
- Tuneable viscosity
- Tuneable miscibility with organic solvents and/or water.
In addition, ILs can be designed to be non-toxic or at least to have a low toxicity and their structure can be largely varied by introducing substituents bearing necessary functionalities.
The highlighted properties of ILs result in many advantages that use of ionic liquids as the solvents brings forward for the organic synthesis:
- Easy isolation of the product due to phase separation or extraction with immiscible solvent
- Easy reuse of precious catalyst, as it stays in the ionic liquid phase, whereas starting material and/or product form a separate phase. It is also possible to introduce groups that act either as catalyst or to anchor catalyst to the IL.
- Possibility to conduct the reactions with compounds, that are insoluble or very badly soluble in other solvents
- Possible increase in (enantio)selectivity for some reactions. It is comparatively easy to synthetize chiral ionic liquids from the natural pool of chiral molecules, which can act as catalysts for enantioselective transformations.
- Easier handling of sensitive or aggressive reagents (e.g. PCl3 is less sensitive to atmospheric humidity when dissolved in IL)[i]
- Possibility to model and design the solvent for specific application
- Possibility to reduce emissions of VOCs (Volatile Organic Compounds) in processes to a minimum
- Possibility to use not just enzymes, but even full cells as the catalysts for the organic transformations
- Readily available microwave and ultrasound assisted synthesis
[i] E. Amigues, C. Hardacre, G. Keane, M. Migaud, M. O’Neill, Chem. Commun. 2006, 72.