Novel Acidic Ionic Liquids: Design And Catalytic Properties

In this paper,novel SO₃H-functionalized ionic liquids were designed and synthesized.Their catalytic performance in Fischer indole synthesis and Pechmann reaction were studied.First,novel SO₃H-functionalized ionic liquids bearing two alkyl sulfonic acid groups in the imidazolium cations were designed and synthesized.Their Brφnsted acidity was evaluated from the determination of the Hammett acidity functions using UV-visible spectroscopy with 3-nitroanline as the indicator in ethanol.UV analysis results indicated that the designed ionic liquids have strong acidic properties.Second,such novel SO₃H-functionalized ionic liquids were successfully applied as catalysts for the one-pot Fisher indole synthesis in water medium.Various types of indoles from single-carbonyl ketones/aldehydes and cyclohexandiones were provided in 68-96%yields using the catalytic system of[（HSO₃-p）₂im][HSO₄]/H₂O.The indole products could be conveniently,separated from the reaction mixture by filtration and the remained catalytic system could be reused after treatment with a strongly acidic cation exchange resin,which of the whole process was performed in water without using any organic solvents.Finally,such novel acidic ionic liquids were applied as catalysts in Pechmann reaction under solvent-free condition.SO₃H-functionalized ionic liquid[（HSO₃-p）₂im][HSO₄]proved to be the most active catalyst. Ionic liquid loads as low as 5 mol%can lead tohigh yields of 88～93% within 3 hours at 80℃.