To a 100 mL round-bottomed flask under nitrogen protection was added 4-chloro-5-iodobenzene-1,2-diamine (2.69 g, 10.02 mmol, 1.00 equiv), ethanol (50 mL), carbon disulfide (6 g, 78.95 mmol, 8.00 equiv), and potassium hydroxide (1.68 g, 30.00 mmol, 3.00 equiv). The reaction mixture was heated to reflux for 3 hours. After completion of the reaction, the mixture was concentrated under reduced pressure. The concentrate was diluted with 100 mL of water and subsequently extracted with ethyl acetate (3 x 100 mL). The organic layers were combined, washed with 100 mL of saturated saline and then dried over anhydrous sodium sulfate. The dried organic phase was concentrated under reduced pressure to give 2.5 g of 5-chloro-6-iodo-1H-benzo[d]imidazole-2(3H)-thione as a brown solid in 80% yield.
