PEG3400–Cu2O–Cs2CO3: an efficient and recyclable microwave-enhanced catalytic system for ligand-free Ullmann arylation of indole and benzimidazole

A mild, simple and efficient microwave-enhanced copper-catalyzed protocol for N-arylation using high molecular weight poly(ethylene glycol) (PEG3400) as a solvent is reported. Indole and benzimidazole have been N-arylated in the presence of cuprous oxide, cesium carbonate, and PEG3400, under microwave activation, with no supplementary ligands. Simple treatment by precipitation in Et2O and filtration provided the expected product after evaporation and recovery of the catalytic system as a precipitate. The recovery and one successful re-use of the catalytic system is also described. The formation of copper-based nanoparticles was demonstrated by TEM analysis.