The pyridinium-based ionic liquid is an ionic liquid containing a pyridinium cation. The stability, reactivity and catalysis in organic synthesis make them widely available. Pyridinium ionic liquids can be applied in Diels-Alder reaction, Friedel-Crafts reaction, Suzuki reaction, Grignard Reaction and the synthesis of some pharmaceutical agents.
Figure 1. The structure of pyridinium ionic liquids
Applications
Diels-Alder reaction: Diels-Alder reaction is one of the important C-C bond formation reactions for the formation of six-membered rings, which could precisely control the stereoselectivity of the product. Diels Alder reaction of anthracene 9-carbinol with N-ethyl maleimide can react in pyridinium-based ionic liquids, including 1-butyl-pyridinium tetrafluoroborate ionic liquid, 1-butyl-3-pyridinium tetrafluoroborate ionic liquid, and 1-butyl-4-methyl pyridinium tetrafluoroborate ionic liquid.
- Friedel-Crafts reaction: The pyridinium ionic liquid is an effective solvent for Friedel-Crafts type acetylene addition to benzoyl chloride, and can be used as a solvent and a Lewis acid catalyst. In the reaction of several aromatic and aliphatic acid chlorides, using pyridinium ionic liquid can give the corresponding chlorovinyl ketone in high yield and purity. The pyridinium ionic liquid solvent inhibits the polymerization of acetylene, which results in improved atom economy and simpler extraction purification procedures, and provides an alternative to chlorinated organic solvents.
Suzuki reaction: In the palladium-catalyzed Suzuki reactions, the nitrile functionalized pyridinium ionic liquid is an effective immobilization solvent. ICP analysis showed that pyridinium ionic liquids significantly reduced palladium leaching compared to other non-functionalized ionic liquids. This pyridinium ionic liquid can be prepared from relatively inexpensive precursors, which provides a further impetus for manufacturing on a larger scale.
Grignard Reaction: An alkylpyridine ionic liquid having an ether functional group can be used as a solvent for the Grignard reaction. When the reaction is carried out in pyridinium ionic liquid, the reactant aldehyde can be smoothly reduced to the corresponding primary alcohol.
Pharmaceutical agent synthesis: The catalytic action of pyridinium-based ionic liquids has remarkable contribution in the synthesis of some pharmaceutical agents such as 1,4-dihydropyridine, dihydropyrimidinones and 3,5-bis(dodecyloxycarbonyl)-1,4-dihydropyridine derivatives. In the n-butylpyridinium tetrafluoroborate ionic liquid, the aldehyde, acetoacetate and ammonium acetate can be used to synthesize the 1,4-dihydropyridine derivative by one-pot method. Compared with the traditional method, this new method has the advantages of high yield, short reaction time and easy post-treatment. The recovered ionic liquid can be circulated at least five times without losing its activity.
Figure 2. Diels-Alder reaction between anthracene-9-carbinol and N-ethylmaleimide
Figure 3. The Friedel-Crafts type acetylene addition
Figure 4. The synthesis of the 1,4-dihydropyridine derivative
References
- Snelders, D. J. M.; Dyson, P. J. (2011). “Efficient Synthesis of β-Chlorovinylketones from Acetylene in Chloroaluminate Ionic Liquids.” Organic Letters 13(15), 4048–4051.
- Ford, L.; et al. (2010). “Grignard Reactions in Pyridinium and Phosphonium Ionic Liquids.” European Journal of Organic Chemistry 2011(5), 942–950.
