The Cadogan reaction, a widely accepted route for the synthesis of nitrogen containing heterocycles, is modified by using microwave radiation as the source of heat instead of the conventional heating by reflux in a nitrogen atmosphere for several hours. Appropriate starting materials were mixed with triethyl phosphite or triphenylphosphine and irradiated with microwaves for several minutes at a specific power to give the desired products. The indazoles were prepared by irradiating N‐(2‐nitrobenzylidene) anilines with triethyl phosphite at 200 W for 12–14 min to give 85–92% product yields. Irradiation of the mixture of N‐benzylidene‐2‐nitroanilines and triphenylphosphine at 200 W for 3–5 min yielded 93–96% of the benzimidazoles. The carbonylindoles were obtained in 61–68% yields by irradiating 2‐nitrochalcone or alkyl 2‐nitrocinnamates and triphenylphosphine with microwaves at 80–200 W for 8–11 min. The mixture of 2‐nitrobiphenyl and triphenylphosphine yielded 96% of carbazole when irradiated with microwaves at 200 W for 2 min while 75% of phenazine was obtained by irradiating the mixture of 2‐nitrodiphenylamine and triphenylphosphine with microwaves at 200 W for 3.5 min. These results show that microwave‐assisted Cadogan reactions gave better product yields at shorter reaction times. J. Heterocyclic Chem., (2009).

Microwave-Assisted Cadogan Reaction for the Synthesis of 2-Aryl-2H-indazoles, 2-Aryl-1H-benzimidazoles, 2-Carbonylindoles, Carbazole, and Phenazine. -The Cadogan reaction is modified by using microwave irradiation as the source of heat instead of conventional heating. Appropriate starting materials are mixed with triethyl phosphite or triphenylphosphine and irradiated for several minutes at a specific power to provide the title products. The microwave-assisted Cadogan reaction greatly enhances the yields of the products and reduces the reaction time essentially. In the case of decomposition during the procedure, triphenylphosphine is shown to be an alternative, because the reaction can be performed at lower power and still gives good yields.