Eco-friendly approaches for synthesis of indolyl 1H-pyrroles using rice-husk-derived carbonaceous sulfonation as the green catalyst

BACKGROUND

Amorphous carbon-bearing sulfonic acid groups (AC–SO₃H) are a kind of solid acid that exhibits high acidity and belongs to the new generation of solid acids. The presence of acidic functional groups, including carboxylic acid, phenolic and sulfonic acid groups, allows for several key activities such as strong Brønsted acid properties, high surface area, stability, reusability and recyclability.

RESULTS

In this work, we synthesized indolyl 1H-pyrroles from indoles, phenylglyoxal monohydrate, 1,3-diketones and ammonium acetate using rice-husk-derived carbonaceous sulfonation (AC–SO₃H) as a catalyst. We explored the impact of catalyst quantity, reaction time, solvent, mineral acid and temperature in order to determine the optimal reaction conditions. Additionally, indolyl 1H-pyrrole derivatives were also synthesized based on the optimal conditions that were completely investigated, and the structure of these synthetic compounds was characterized by ¹H- and ¹³C-nuclear magnetic resonance.

CONCLUSION

The yield of indolyl 1H-pyrroles was up to 84% in 6 h at 100 °C with the activity of 30 mg AC–SO₃H. Besides, based on the optimal conditions, 21 indolyl 1H-pyrrole derivatives were formed with the high-yield and green method. The catalyst could be reused multiple times without a significant decrease in catalytic performance. According to the findings from the experiments, a potential mechanism that included a radical process has been suggested. This eco-friendly approach provided a straightforward and efficient method to produce a variety of indole–pyrrole conjugates in a single step. © 2024 Society of Chemical Industry (SCI).