GENERAL METHOD: A mixture of 4,5-dichloro-1,2-phenylenediamine (200 mg, 1.85 mmol) and ethyl acetoacetate (722 mg, 5.55 mmol) was added with GdCl3-6H2O (25 mg, 0.09 mmol), and the reaction mixture was stirred for 3.0 hours at 80 °C. After completion of the reaction (monitored by TLC), the reaction mixture was poured into ice-cold water and extracted with ethyl acetate. The organic layer was dried over anhydrous sodium sulfate and concentrated under reduced pressure to give 5,6-dichloro-2-methylbenzimidazole crude. The crude product was further purified by column chromatography with the eluent being a hexane solution of 10% ethyl acetate.
