Abstract
Background: Exploiting the twofold nature of ionic liquids as catalyst and reaction media, the synthesis of a library of 5H-benzo[a]phenothiazines has been reported. Firstly, we prepared zinc mercaptides of 2-aminobenzenethiols and 2,3-disubstituted 1,4-napthaquinones which undergo condensation in ionic liquid to generate 5H -benzo[a]phenothiazines. The ionic liquid used was [BMIM][[HSO4] which was recovered and reused for several rounds without appreciable loss in its activity. The structures of all the synthesized compounds were established by IR, 1H-NMR, 13C-NMR spectroscopy and elemental analysis.
Methods: We prepared three imidazolium based ionic liquids namely, [BMIM]Br, [BMIM]BF4 and [BMIM][HSO4] and using these ILs synthesized 6,9/6,10-disubstituted 5Hbenzo[ a]phenothiazine-5-one from zinc mercaptides of substituted 2-aminobenzenethiols and 2,3- disubstituted-1,4-napthaquinones.
Results: In this paper, 15 different derivatives of 5H-benzo[a]phenothiazine-5-one have been synthesized by reacting zinc mercaptide of 2-amino/2-amino-4-chloro/2-amino-5-nitro/2-amino-5-ethoxy benzenethiols with 2,3-dichloro/3-chloro-2-(2/3/4-substituted anilino)-1,4-napthaquinone in [BMIM]Br, [BMIM]BF4 and [BMIM][HSO4]. Out of the three ILs used [BMIM][HSO4] showed enhanced catalytic activity in terms of product yield and reaction time. This indicates that the catalytic activity of the ionic liquids on the condensation reaction was dependent on the Brønsted acidity of the counter anion. Thus, due to the high Brønsted acidity of hydrogen sulphate counter anion, [BMIM]HSO4 proved to be the most efficient solvent and catalyst for this reaction. The ionic liquid was further recycled and reused for five cycles without any appreciable loss in product yield.
Conclusion: In conclusion, an efficient and eco-compatible methodology has been developed for the synthesis of a series of nitrogen and sulphur containing benzo[a]phenothiazines. The significant advantages of this procedure include one-pot, high yields of the products, broad substrate scope, easy work-up procedure and recyclability of the ionic liquid.
Keywords: Ionic liquid, [BMIM][HSO4], 5H-Benzo[a]phenothiazine, 2-aminobenzenethiols, 1, 4-naphthoquinone.
Graphical Abstract
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