Application Of Ionic Liquids As Efficient Phase-Transfer Catalysts In The Alkylation Reaction

The development of environmentally friendly catalysts and solvents for organic chemistry is an area of great importance. Recently, the ionic liquids (ILs) research became a new field of green chemistry. ILs possessed several properties, such as nonvolatility, noninflammability, wide liquid range, thermal and chemical stability, good solubility, recoverability. ILs, especially those 1,3-dialkylimidazolium based ones, have recently gained recognition as alternative reaction media and catalysts for organic synthesis, solvents and extractants for separation and electrolytes for electrochemistry. Based on the unique chemical and physical properties, current research in ILs is driven by the perceived opportunities for improvements in industrial processes using green chemistry principles leading to overall efficiencies. From both economic and environmental points of view, researching on the new chemical functionality and application area of ILs is very attractive. Because ionic liquids are comprised of bulky organic cations and small inorganic anions, they seem well suited as efficient phase-transfer catalysts (PTCs) in organic reaction. In this paper, we report that ILs based on 1,3-dialkylimidazolium and ammonium cations act as efficient and environmentally benign PTCs for the base-promoted alkylation reaction.Phase-transfer catalysis is an efficient methodology for the preparation of numerous classes of non chiral and chiral compounds. The phase-transfer catalyst, generally quaternary ammonium salt or crown ether, acts as a shuttle for the reactant anion between a polar phase that contains the salt reactant and a non-polar phase that contains the organic reactant. The primary benefits from PTCs which have been realized recently include: mild reaction conditions, short cycle time, increasing yield and selectivity, easy purification, pollution prevention. However, the main disadvantages of typical PTCs include the thermal instability of ammonium ones and toxicity of crown ether. In addition, the catalyst separation and recovery are significant challenges. ILs possessed attractive properties for overcoming these limitations, such as nonvolatility, innoxious, thermal stability and recoverability. In all the alkylation reaction, the application of ionic liquids as PTCs has many obvious advantages including the high yield, mild reaction condition, short reaction time, high selectivity of monoalkylation, the ease of product isolation and the potential for recycling.Ionic liquids based on 1,3-dialkylimidazolium and tetraalkylammonium cations were employed as a series of efficient, environmentally benign PTCs for the base-promoted monoalkylation of diethyl malonate. The data show that [BMIM] [PF₆] and [(n-C₄H₉)₄N][BF₄] are most efficient. The influence of various heterogeneous bases on yield was studied. Good yields were obtained. Solvent-free, mild reaction condition, shorter reaction time, and easy purification were achieved. The catalytic system (IL-Base) could also be recycled after the extraction of products with ether.Ionic liquids were employed as a series of efficient and environmentally benign PTCs for the base-promoted N-alkylation of imidazole. We also applied our green protocol to the N-alkylation of various azaheterocycles (benzimidazole, pyrrole and indole) with alkyl halides. The efficacy of the process depended on the amount of the PTC and the two ions making up the PTC. The primary benefits include: high yield and selectivity of N-monoalkylation, shorter reaction time, the ease of product isolation and reuse of the catalytic system.Ionic liquids based on 1,3-dialkylimidazolium cations were employed as efficient, environmentally benign PTCs for the N-alkylation of phthalimide. Various reaction factors affecting the yield of alkyl phthalimide have been studied. The optimal reaction conditions were: reaction system molar ratio n(Phthalimide): n(n-BuBr): n([BMIM][BF₄]):n(K₂CO₃) =1.0: 1.5: 0.1: 1.5, reaction temperature 80℃, reaction time 3.0h in 1,4-dioxane. The advantages of catalytic system include shorter reaction time, high yield, the ease of product isolation, the potential for recycling.