I2-Catalyzed Oxidative N–S Bond Formation: Metal-Free Regiospecific Synthesis of N-Fused and 3,4-Disubstituted 5-Imino-1,2,4-thiadiazoles

A novel and expeditious approach for the synthesis of N-fused and 3,4-disubstituted 5-imino-1,2,4-thiadiazole derivatives has been achieved through the molecular iodine-catalyzed oxidative cyclization of 2-aminopyridine/amidine and isothiocyanate via N–S bond formation at ambient temperature. The present one-pot transition-metal-free protocol provides the facile and highly efficient regiospecific synthesis of various 1,2,4-thiadiazole derivatives in a scaled-up manner with good to excellent yields using inexpensive I2 as a catalyst.