Bi-SO₃H-functionalized room temperature ionic liquids based on bipyridinium: Highly efficient and recyclable catalysts for the synthesis of naphthalene-condensed oxazinone derivatives

Several bi-SO₃H-functionalized room temperature ionic liquids were synthesized and their catalytic performances for the synthesis of naphthalene-condensed oxazinone derivatives were studied theoretically, as well as experimentally. Compared with traditional single-SO₃H- functionalized ionic liquids, less catalyst and higher yields are the key features of this methodology. Hammett function values and the minimum-energy geometries of bi-SO₃H-functionalized ionic liquids were calculated and the results revealed that the acidities and catalytic activities of ionic liquids in the synthesis of naphthalene-condensed oxazinone derivatives were influenced by their structures. There are strong hydrogen bond networks in these bi-SO₃H-functionalized ionic liquids. The H-O bond of the sulfonic acid groups was lengthened to different levels compared with the one in the isolated cation. The ionic liquid [(PS)₂bPy][OTf]₂ with the shortest H-O bond distance had the strongest hydrogen bond and hence showed the strongest acidity and the highest catalytic activity.