H-imidazolium Acidic Ionic Liquids as Efficient Catalysts in the Synthesis of Xanthenes under Solvent-Free Conditions

Bifunctional acidic ionic liquids, having both H-imidazolium and -SO3H groups as cation moieties (H-BFAILs) and CF3SO3- as anion, were synthesized in high yields. These H-BFAILs showed significant hydrophilic properties, lower acidity and higher thermal stability relative to common ionic liquids, due to their unique structures. The (propyl or butyl-3-sulfonic) imidazolium trifluoromethane sulfonates ([IMPS][TfO] and [IMBS][TfO]) were employed as efficient catalysts for the synthesis of 14-substituted-14H-dibenzo[a.j]xanthene derivatives via the condensation of β-naphthol with aromatic aldehydes at 80◦C under solvent-free conditions. The workup procedure is green, involving only washing the reaction mixture with water to remove the ionic liquid. The acidic ionic liquids were easily recovered and reused for at least seven successive runs without significant loss of catalytic activity.