Synthesis of environmentally benign new ionic liquids for the preparation of 2-aryl/heteroaryl benzimidazoles/benzoxazoles under ultrasonication

Abstract

Ionic liquids (ILs) possess a broad spectrum and cutting-edge impact on promising and sustainable research works due to their utilization in academic as well as industrial applications. These are regarded as notable catalysts/solvents due to their distinct physical, thermal and chemical properties. Among the different ILs existing, imidazolinium salts are quite interesting because of their convenient preparation and widespread applications in organic catalysis. By keeping in mind aforesaid, we have synthesized a series of ionic liquids; symmetrical/unsymmetrical 1,3-dialkyl imidazolium/benzimidazolium salts from imidazole/benzimidazole with alkyl chlorides/bromides. Further, ILs with other counter-anions such as tetrafluoroborate (BF4−) and hexafluorophosphate (PF6−) in benzimidazolium group were developed by the reaction of N, N′-disubstituted o-phenylenediamine, triethyl orthoformate and ammonium chloride/tetrafluoroborate/ammonium hexafluorophosphate. By these ionic liquids, a series of substituted 1-alkyl-2-aryl-1H-benzo[d]imidazole/2-arylbenzo[d]oxazoles were prepared from the reaction between N-substituted-1,2-phenyldiamine/o-aminophenol and aroyl chlorides without adding any acid catalyst and organic solvents. The current work discloses, when increase the length of the alkyl chain on the imidazolinium ionic liquids correspondingly the product yields were raised, similarly, the different anions offered a different rate of reactivity as BF4− ∼ Br− > Cl− > PF6−. Further, we extended our studies to conduct the same reaction by the combination of ionic liquids as well as ultrasonication to produce superior results when compared to the simple conventional method. This methodology has the advantages of no usage of toxic catalysts and volatile organic solvents, yet a simple work-up procedure and high yields. Worthily, products can be separated from the ILs by ether and/or hexane solvent(s) washings. Convincingly, the catalyst could be recycled and reused for five cycles with no major loss in catalytic performance. Further, we confirm the stability of the recovered ionic liquids by the TGA and 1HNMR techniques. This system affords a green and improved protocol over the reporting methods.