|
Ionic liquids have been thoroughly investigated as solvents in most types of catalytic reactions.1-4 Their merit lies in the ease with which their properties can be tuned by varying either the anion, the cation itself, or its substitution on the cation – in effect creating designer solvents.5 In catalysis, this feature can be favorably exploited by rendering a homogeneous reaction mixture biphasic, thus combining the advantages of homogeneous and heterogeneous catalysis.
Several researchers have found that impurities in ionic liquids exhibit an influence on the reaction outcome in catalysis.4,6-17 Nevertheless, the quantitative specification of each batch of ionic liquid used is seldom reported, and therefore the direct comparison of results from different laboratories is difficult to make.
Dr. Annegret Stark of the Friedrich-Schiller University of Jena, Germany has found that impurities contained in ionic liquids most frequently stem from their preparation (e.g. 1-methylimidazole, water, trace halides). In the metathesis of 1-octene catalyzed by Grubbs 1st generation catalyst (Scheme 1), Sigma-Aldrich’s high purity 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4, 39931) or 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim]BF4, 39736) with specified water content <200 ppm and halogen content <10 ppm were spiked with known amounts of either of these impurities to demonstrate the importance of the quality of ionic liquids employed in catalysis. Figure 1 shows that the catalyst is relatively resistant to the presence of water, and even a 100-fold excess of water over catalyst (2.04 mol % water in ionic liquid) does not significantly inhibit the reaction.
The presence of chloride exhibits a much more pronounced effect, as shown in Figure 2. A ratio of chloride:catalyst of 2.3 (0.07 mol % chloride in ionic liquid, corresponding to only 100 ppm) reduced the turnover frequency from 820 to 700 h-1.
Interestingly, the catalyst precursor is most sensitive to the presence of traces of 1 methylimidazole. Figure 3 shows that residual 1-methylimidazole in an ionic liquid fully deactivates the catalyst at a ratio of 6.3 (1-methylimidazole:catalyst), which corresponds to traces of 0.18 mol % (600 ppm) 1-methylimidazole in the ionic liquid.
Sigma-Aldrich is pleased to introduce a new set of highly purified ionic liquids for catalysis with water contents below 200 ppm and halogen content below 10 ppm.
| |
back to top |
| Product Name |
Product # |
| 1-Butyl-3-methylimidazolium hexafluorophosphate, for catalysis, ≥98.5% T |
18122 |
| 1-Butyl-3-methylimidazolium tetrafluoroborate, for catalysis, ≥98.5% T |
39931 |
| 1-Ethyl-3-methylimidazolium tetrafluoroborate, for catalysis, ≥98.5% T |
39736 |
| 1-Butyl-3-methylimidazolium chloride, dry, ≥99% AT |
55509 |
| 1-Butyl-3-methylimidazolium chloride, puriss., dry, ≥99% AT |
04129 |
For a complete listing of Ionic Liquids available from Sigma-Aldrich please click here to view ChemFiles Vol. 5 No. 6 Enabling Technologies: Ionic Liquids.
References:
1. Holbrey, J. D.; Seddon, K. R. Clean Prod. Proc. 1999 1, 223.
2. Wasserscheid, P.; Keim, W. Angew. Chem. Int. Ed. 2000 3, 3772.
3. Welton, T. Coord. Chem. Rev. 2004 248, 2459.
4. Sheldon, R. A. et al. Green Chem. 2002, 4, 147.
5. Freemantle, M. Chem. Eng. News 1998, 76, 32.
6. Chauvin, Y.; Olivier-Bourbigou, H. Chemtech 1995, 26, 26.
7. Chauvin, Y. et al. Angew. Chem. Int. Ed. 1996, 34, 2698.
8. Suarez, P. A. Z. et al. Inorg. Chim. Acta 1997, 255, 207.
9. Monteiro, A. L. et al. Tetrahedron: Asymmetry 1997, 8, 177.
10. Dyson, P. J. et al. Chem. Commun. 1999, 25.
11. Ellis, B. et al. Chem. Commun. 1999, 337.
12. Carmichael, A. J. et al. Org. Lett. 1999, 1, 997.
13. Dyson, P. J. et al. Electrochem. Soc. Proc. 2000, 99-41, 161.
14. Song, C. E.; Roh, E. J. Chem. Commun. 2000, 837.
15. Seddon, K. R.; Stark, A. Green Chem. 2002, 119.
16. de Souza, R. F. et al. Adv. Synth. Catal. 2002, 344, 153.
17. Klingshirn, M. A. et al. Chem. Commun. 2002, 1394.
|
