Ionic Liquids for Reaction Solvent
Ionic liquids can dissolve metal organic compounds and replace volatile organic solvents, inert solvents and solid acid catalysts. The catalyst dissolved in ionic liquid has better activity and stability, and has the advantages of homogeneous and heterogeneous catalysts. The reaction with ionic liquid as the reaction solvent has the characteristics of fast rate, high yield and good selectivity, and the reaction system can be one phase, two phase or even multiple phase. When ionic liquid is used as the reaction solvent, the reaction products can be separated by static stratification or distillation, and the catalyst can be recycled with the ionic liquid. Therefore, ionic liquid as reaction solvent can eliminate the environmental pollution of organic solvents, in line with the requirements of green chemical industry.
- Polymerization: Ionic liquids as reaction solvents can be used in radical polymerization, addition polymerization and others. For example, ionic liquids can be used as a reaction solvent for the dimerization of 1-3 butadiene, and it is found that when ionic liquids are used as the solvent, the conversion of 1-3 butadiene is very high. In addition, ionic liquid can be used as the reaction solvent for the free radical polymerization of methyl methacrylate with styrene. The resulting polymer is insoluble in ionic liquid, so the reaction rate is fast. After polymerization, the monomer can be completely transformed into polymer. Filtration can separate the polymer from the ionic liquid. Furthermore, the filtrate can be vaporized to regenerate the ionic liquid.
- Friedel-Crafts reaction: Friedel-Crafts reactions are commonly used in industry to add alkyl and acyl groups to molecules containing aromatic rings to generate complex compounds. However, in industry, HF or AlCl3 are used as catalysts for Friedel-Crafts reaction, and a large amount of waste water is generated during the reaction process. Ionic liquids with Lewis acidity are catalytic solvent systems with dual functions, which can replace solvents and catalysts. For example, a research group carried out Friedel-Crafts reaction under the condition of adding metal halides to ionic liquids. Considering the price, feasibility and ease of treatment, ionic liquids are all good catalysts with high conversion rate and good selectivity.
Figure 1. An example of ionic liquids used as reaction solvent for Friedel-Crafts reaction.
- Diels-Alder reaction: Diels-alder reaction is a very useful C-C binding reaction in organic synthesis, and its essence is the [4+2] cycloaddition reaction of dienes and dienophiles through synergistic action. It is found that the type and composition of ionic liquid have a great influence on the proportion of the products. Compared with traditional reaction solvents, the reaction rate of ionic liquid system is faster and the selectivity of internal spin product is higher. In addition, the organic phase in the product is easy to separate and extract, and the ionic liquid can be regenerated.
Figure 2. An example of ionic liquids used as reaction solvent for Diels-Alder reaction.
- Others: In addition to the above applications, ionic liquids can also be used as reaction solvents for other reactions, including Heck reaction, Biginelli reaction, esterification reaction, cyclozation reaction, coupling of aromatic halides, oxidation of aromatic aldehydes, enzyme-catalyzed reaction and others.
- Amir Taheri, Bingbing Lai, Cheng Cheng et al. Brnsted acid ionic liquid-catalyzed reductive Friedel–Crafts alkylation of indoles and cyclic ketones without using an external reductant[J]. Green Chemistry, 2015, 17.
- Deepa, Yadav G D, Chaudhary P, et al. Chiral Imidazolidinne with catalytic amount of Dicationic ionic liquid act as a recoverable and reusable Organocatalyst for asymmetric Diels lder reaction[J]. Chirality, 2019, 32(30).
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