General procedure for the synthesis of 1-methyl-2-formylbenzimidazole from N-methylbenzimidazole and 2,2-diethoxyacetic acid: N-methylbenzimidazole (0.10 mmol, 1.0 eq.), ammonium persulfate (0.30 mmol, 3.0 eq.), and Cs2CO3 (0.20 mmol, 2.0 eq.) were placed in a dry glass tube. Subsequently, anhydrous DMSO (1 mL) and 2,2-diethoxyacetic acid (0.7 mmol) were added to the tube via syringe under nitrogen protection. The reaction mixture was stirred for 24 h at room temperature using 15 W blue LED strip irradiation. After completion of the reaction, the reaction was quenched by the addition of 1.2 mL of 3.0 M HCl and stirring was continued for 20 hours. Afterwards, the pH was adjusted with saturated Na2CO3 solution and extracted with CH2Cl2. The organic layers were combined, washed with brine and dried over anhydrous Na2SO4. Finally, the target product 1-methyl-2-formylbenzimidazole was purified by rapid chromatography on silica gel, using petroleum ether and ethyl acetate as eluents, and the corresponding yield was calculated.
