Efficient synthesis of N-alkyl-2,7-dihalocarbazoles by simultaneous carbazole ring closure and N-alkylation

The N-alkyl-2,7-dihalocarbazole as the main product was formed by the reaction of 4,4′-dihalo-2-nitrobiphenyl with aromatic nitro compound and trialkyl phosphite. The presence and crucial role of aromatic nitro compound causes simultaneous carbazole ring closure and N-alkylation unlike the Cadogan ring closure where non-alkylated carbazole is formed as a main product. In addition, the mixture of aromatic nitro compound and trialkyl phosphite was found to be a possible N-alkylating agent for heterocycles, such as carbazole or indole.