The general procedure for the synthesis of 4-azabenzimidazole from 2,3-diaminopyridine and triethyl orthoformate was as follows: to evaluate the catalytic performance of different γ-Fe2O3@SiO2-HA catalysts, a catalytic amount of γ-Fe2O3@SiO2-HA (2.5 mol%) was added to a glass flask containing 1.2 mmol of triethyl orthoformate and stirred at room temperature for 15 minutes. Subsequently, 2 mmol of 2,3-diaminopyridine was added to the reaction mixture and the reaction was continued until a predetermined time was reached. The progress of the reaction was monitored by thin layer chromatography (TLC) and the unfolding agent was a solvent mixture of hexane and ethyl acetate (ratio 4:1 or 7:3 for diamines). Upon completion of the reaction, 5 mL of dichloromethane (ethyl acetate was used for entries 8 and 9) was added to the mixture, followed by separation of the catalyst by an external magnet for subsequent use. Finally, the resulting solution was concentrated on a rotary evaporator to give the purified 4-azabenzimidazole solid product.
