Preparation Of Imidazolium Ionic Liquids And Their Reactivity In Synthesis Of β-enaminones

Over the past decade,ionic liquids have been generating increasing interest as environmentally benign solvents and catalysts due to their advantageous properties,such as low viscosities,high thermal stability,low vapor pressure and good solubility in a broad range of organic and inorganic compounds,etc.It has been known that these unique features of ionic liquids can be modified over a wide range by changes in the detailed structure of the cation or by the identity of the anion.The designability of the ionic liquids gives us more chances to manipulate the structures of them to achieve ionic liquids of special properties according to the given reactions.With their unique character,the ionic liquids have been successfully applied to many areas of chemistry.However,few studies have been completed to illustrate the relationship between the structures of the ionic liquids and their activities in organic synthesis.In this article, fifteen of imidazolium ionic liquids was synthesized and used as environmentally benign catalysts for the synthesis ofβ-enaminones under smooth reaction conditions.The synthesis ofβ-enaminone of aniline and acetyl acetone in the imidazolium ionic liquids were investigated.Experiments show that the order of the catalytic performance of the imidazolium cation ionic liquids:[Hmim]⁺＞[emim]⁺,[bmim]⁺＞[bmmim]⁺.These results suggest that the activities of these ionic liquids are dependent upon the substituent effects of the imidazolium ring.The effect of 1-n-butyl-3-methylimidazolium with different anions as catalysts was alsoinvestigated under the same reaction conditions.The order of the activity for the condensation reaction was found to be:Br^-＞PF₆^-＞BF₄^-,I^-＞Cl^-＞H₂PO₄^-＞HSO₄^-.To evaluate the synthetic utility of the procedure,various amines were chosen to be condensed with different 1,3-dicarbonyl compounds in the ionic liquid[Hmim]BF₄.Compared with the traditional methods,this protocol offers advantages in terms of its simple procedure and work up,mild reaction conditions and excellent yields.The ionic liquid[Hmim]BF₄ can be recycled for four times without a significant loss in activity.The efficacy of the supported ionic liquid[bmim]BF₄/silica in the same model reaction was also investigated.The use of[bmim]BF₄/silica and[bmim]BF₄ as the catalysts for this reaction again saw to better performance by the[bmim]BF₄/silica,with[bmim]BF₄/silica leading to a yield of 93%compared to the[bmim]BF₄ with a yield of 86%and required a long reaction time. The supported ionic liquid[bmim]BF₄/silica was reused four times without considerable loss of activity.