General procedure for the synthesis of 1,7'-dimethyl-2'-propyl-1H,1'H-2,5'-dibenzo[d]imidazole from N-methyl-o-phenylenediamine hydrochloride and 2-n-propyl-4-methyl-6-carboxybenzimidazole: Suspension of 4-methyl-2-n-propyl-1H-benzimidazole-6-carboxylic acid (50 g) in poly(phosphoric acid) (300 g) was heated to 70-75 °C and maintained for 30 minutes. Subsequently, N-methylphthalimide dihydrochloride (45 g) was added in batches for 2 hours and the reaction was continued at 70-75°C for 1 hour. Next, the temperature of the reaction mixture was raised to 130-135°C and maintained for 10 hours. Upon completion of the reaction, the mixture was cooled to 70°C and water (600 mL) was slowly added to maintain the temperature at 60-90°C. Cooling was continued to 30 °C and the pH was adjusted to 8.0-8.5 with an aqueous ammonia solution.Subsequently, the reaction mixture was heated to 50-55 °C and held for 1 h. The solids were collected by filtration and the wet filter cake was washed with hot water (200 ml). The wet filter cake was suspended in water (900 mL), heated to 50-55 °C, stirred for 1 h and filtered, then washed with hot water (100 mL), and the wet filter cake was dried at 70-75 °C to constant weight. The dried product was suspended in methanol (260 mL), heated to 45-50 °C, activated carbon (6.5 g) was added and stirred for 30 min. Insoluble material was removed by filtration through a Hyflo bed, the bed was washed with hot methanol (60 mL), the filtrates were combined and cooled to 25 °C. Water (160 mL) was slowly added to the filtrate at 25-35°C, heated to reflux temperature and held for 1 hour. The reaction mixture was cooled to 0-5 °C and maintained for 2 h. The resulting solid was filtered, washed with methanol (60 mL) and the wet filter cake was dried at 70-75 °C to constant weight to give 54 g of 4-methyl-6-(1-methylbenzimidazol-2-yl)-2- n-propyl-1H-benzimidazole (77.4% yield), with a water content of 5.85% as determined by KF.
