Synthesis of benzo[a]carbazole derivatives via intramolecular cyclization using Brønsted acidic carbonaceous material as the catalyst

In this work, a new procedure for the synthesis of benzo[a]carbazole from 1,3-diketones, primary amines, phenylglyoxal monohydrate, and malononitrile employing a solid acidic catalyst has been developed. The multicomponent reaction provided 3-cyanoacetamide pyrrole as an intermediate and then the formation of benzo[a]carbazole via intramolecular ring closure. The reaction was carried out for 2 h at 240 °C, resulting in the desired product with 73% yield. Acidic sites on the solid acid catalyst, made from rice husk-derived amorphous carbon with a sulfonic acid core (AC-SO(3)H), provided the best activity. Acidic sites on the surface of the catalyst, including carboxylic, phenolic, and sulfonic acids, were 4.606 mmol g(−1) of the total acidity. AC-SO(3)H demonstrated low cost, low toxicity, porosity, stability, and flexibility of tuning and reusability.