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A novel and efficient methodology for the arylation of benzoxazoles with aromatic aldehydes catalyzed by deep eutectic solvent has been developed. The reaction smoothly proceeded with a wide range of substrates to give the desired products in high yields within short reaction time. Deep eutectic solvents are easily recovered and reused without significant loss of catalytic activity. Results and discussion Preparation and characterization of [ZnCl 2 ][ethylene glycol] 4DESs were synthesized by mixing zinc chloride and ethylene glycol (HBDs) at a dened molar ratio (1 : 4) and heating at 120 C for 1 h at atmospheric pressure under constant magnetic stirring until a homogeneous liquid was formed. The charge delocalization which occurs through hydrogen bonding between the halide anion and ethylene glycol is responsible for the decrease in the T m of the mixture. 52,53

A highly efficient and green strategy for the synthesis of 2-arylbenzoxazoles, 2-arylbenzimidazoles, and 2-arylbenzothiazoles catalyzed by phosphonium acidic ionic liquid has been developed via the condensation of o-aminophenol, o-phenylenediamines, and o-aminothiophenol, respectively, with aldehydes. The reaction has a good yield, the broad substrate scope, and mild condition. Triphenyl(butyl-3-sulphonyl) phosphonium toluenesulfonate catalyst was easily obtained from cheap and available starting materials through a one-pot synthesis. Its structure was identified by 1 H NMR, 13 C NMR, 31 P NMR, and FT-IR techniques. Other properties including thermal stability and acidity were determined by TGA and Hammett acidity function method. Interestingly, the catalyst can maintain its constantly outstanding performance till the fourth recovery.