UPDATED: 1st March 2006
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There is a dramatic need to search for alternatives to damaging solvents which are used in huge quantities and are usually volatile liquids that are difficult to contain.
One of these alternatives is ionic liquids that are liquid at room temperature and that have physical properties that make them very interesting as potential solvents for synthesis:
The high solubility of ionic liquids implies that only small reactor volumes are required thus reducing waste from synthetic processes. Also since they are often composed of poorly co-ordinating ions there is a great potential for very high recovery and hence recycling of the solvent. The fact that they have no effective vapour pressure and a large liquid range means that ionic liquids, even if used at high temperatures, do not release harmful vapours thus reducing the amount of volatile organic compounds released into the atmosphere.
Ionic liquids have been shown to have applications in the field of catalysis, metal deposition, extraction and batteries. Their cost and sensitivity to oxygen and moisture have however, limited their applications to these areas. We have developed a range of ionic liquids that are inert to air and water and are similar in cost to common organic solvents. These fluids have negligible vapour pressures, large liquid ranges and are environmentally benign.
We are currently studying the physical and chemical characteristics of these liquids together with preliminary investigations to assess their suitability for a number of chemical and electrochemical applications. We have set up a joint venture company called Scionix between Leicester University and Whyte Chemicals in order to explore the potential applications of these liquids. If you would like to discuss the use of these neoteric solvents then please contact us here at Leicester.
We have carried out a number of organic reactions in ionic liquids e.g.Friedel-Craft acylations, Diels-Alder reactions, aldol reactions, Fischer indole synthesis (see below), in these cases the ionic liquid acts as both the solvent and as a catalyst
We have studied several examples of the Fischer indole synthesis in the ionic liquid choline chloride:2ZnCl 2 and find that after heating to 90 o for two hours the yields are uniformly good. These conditions are milder than the typical cyclisation which involves heating in polyphosphoric acid at 100 o . Thus we have established that we can reduce the amount of acid from use of mineral acid as a solvent to use of a Lewis acid in just over stoichiometric amounts (1.5 equivalents currently) using milder conditions giving yields that are at least as good or better than the conventional process. The work-up procedure just involves adding water which dissolves the ionic liquid which is removed by filtration. Hence, no organic solvent is necessary for the reaction or work-up.
The cyclisation of the unsymmetrical substrate 1 is of particular interest. Reaction of 1 gave only 2,3-dimethylindole 2 and there was no evidence for the isomeric product 2-ethylindole
This is in contrast to the conventional reaction conditions which give a 1:1 mixture of 2 and 3.