Microwave‐assisted Cadogan reaction for the synthesis of 2‐aryl‐2H‐indazoles, 2‐aryl‐1H‐benzimidazoles, 2‐carbonylindoles, carbazole, and phenazine

Abstract

Abstract magnified image 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).