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
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.
Figure 2. Diels-Alder reaction between anthracene-9-carbinol and N-ethylmaleimide
Figure 3. The Friedel-Crafts type acetylene addition
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 4. The synthesis of the 1,4-dihydropyridine derivative
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