An Efficient Route for the Synthesis of 2-Arylbenzothiazoles in an Ionic Liquid, Using Ultrasound Irradiation

The condensation of 2-aminothiophenol and aromatic aldehydes promoted by an ionic liquid of [n-HexylNEt3][HSO4] results 2-arylbenzothiazoles in high yields under ultrasound irradiation.2-Arylbenzothiazoles exhibit unique biological and pharmaceutical properties, including antifungal and antiviral activity, rendering them suitable as, e.g., anticancer agents and caspase inhibitors. 1 Thus, 2-arylbenzothiazoles have received attention in the fields of chemistry, biology, and medicine.Two main routes for the synthesis of 2-arylbenzothiazoles have been reported.The first synthesis route couples benzothiazole to an aromatic ring via the loss of a leaving group, while employing metal catalysts such as iron(III) nitrate nonahydrate, 2 NiBr2(DME) (DME = 1,2-dimethoxyethane), 3 palladium diacetate, 4 and nickel(II) acetate tetrahydrate. 5 This method has been reported for the synthesis of Cu2(4,4'-oxybis(4,4'-oxybis(benzoate)))2(4,4'-bipyridine) [Cu2(OBA)2(BPY)]. 6 The second synthesis route involves a cyclic reaction of 2-aminothiophenol with aromatic aldehydes, catalyzed by a Lewis acid such as scandium tris(trifluoromethanesulfonate), 7 vanadium(IV)-salen complex, 8 ceric(IV) ammonium nitrate, 9 or nano-crystalline sulfated zirconia. 10The first route involves the use of expensive metals such as palladium and nickel, which potentially remain in the product and cannot be recycled, thus causing considerable environmental pollution.The second route presents similar environmental dangers due to the use of metal-containing catalysts.Environmentally friendly chemicals and less harmful synthesis routes have therefore become attractive alternatives to the abovementioned routes in synthetic organic chemistry.In this paper, we describe an efficient and harmless synthesis of 2-arylbenzothiazoles, involving the direct reaction of aryl aldehydes with 2-aminothiophenol in an ionic liquid upon ultrasonic irradiation.