A green and practical method for the synthesis of novel pyrano[2,3-c]pyrazoles and bis-pyrano[2,3-c]pyrazoles using sulfonic acid-functionalized ionic liquid (original) (raw)

2017, Journal of the Iranian Chemical Society

two-component reaction between 3-methyl-1-phenylpyrazolin-5-one and tetracyanoethylene in the presence of triethylamine as a catalyst [9]. Dihydropyrano[2,3-c]-pyrazoles were prepared in 1974 via the base catalyzed cycloaddition of 4-aryliden-5-pyrazolone [10]. Recent methodologies for synthesis of pyrano[2,3-c]pyrazole derivatives have been developed by employing l-proline, γ-alumina [11], per-6-amino-β-cyclodextrin [12], piperidine [13], N-methylmorpholine [14], piperazine [15], magnesium oxide [16], imidazole [17] cetyltrimethylammonium chloride (CTACl) [18], sodium benzoate [2], p-dodecylbenzenesulfonic acid (DBSA) [19], glycine [20] and catalyst-free [21, 22]. However, some of these methods have some disadvantages such as use of reactive starting materials, prolonged reaction times, low yields, harsh reaction conditions, non-recoverability of the catalyst and use of toxic solvents. On the other hand, ionic liquids have become a green alternative to molecular organic solvents due to their unique advantages that include inflammability, non-volatility, negligible vapor pressure, high solubility of inorganic and organic substrates, high thermal stability, wide liquid range with melting point, easy recyclability and reusability [23-27]. Ionic liquids containing imidazolium moiety are efficient catalysts in increasing the reaction rate. Furthermore, these catalysts can be separated easily and reused in several consecutive runs, therefore offering a convenient solution for catalytic recycling problem. These observations enabled us to achieve the synthesis of some novel pyrano[2,3-c]pyrazoles in the presence of previously reported [28-30] SO 3 H-functionalized Brønstedacidic halogen-free ionic liquid [DMBSI]HSO 4 which bears a butanesulfonic acid group in 1,2-dimethylimidazolium cation as catalyst (Fig. 1).