Solventless selective phosgene-free N-carbonylation of N-heteroaromatics (pyrrole, indole, carbazole) under mild conditions

Abstract

N-Heteroaromatics HetNH, such as pyrrole (1), indole (2) and carbazole (3), have been selectively N-carbonylated by a direct reaction with diphenyl carbonate (DPC), used as an environmental friendly carbonyl active species in place of toxic and hazardous phosgene. The carbonylation reaction can be effectively catalyzed by 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which can act as a base catalyst by activating the HetNH substrate, and as a nucleophile catalyst by activating the organic carbonate. The influence of reaction parameters (temperature, reaction time, DBU load, DPC/HetNH molar ratio) on the productivity of the process has been also investigated. The synthetic methodology does not require severe temperature conditions, is solventless, simple (only one step), efficient and selective, and offers a new solution to the synthesis of synthetically versatile HetNCO₂Ph derivatives through a route alternative to the current traditional phosgenation methods.