Rapid and Metal-Free Green Synthesis of Coumarins Catalyzed by Humic Acid

In recent years, numerous methods have been developed for the synthesis of coumarins via Pechmann reaction catalyzed by various catalysts. Although each of the synthetic strategies previously reported has its own merit, most of these methods are associated with certain disadvantages, including the use of commercially unavailable metal catalysts and organic solvents, low yields, and long reaction times. Therefore, the development of a highly efficient, green, and sustainable catalytic methodology for the synthesis of coumarins is still desirable. Humic acid has been found to be an efficient catalyst for the synthesis of coumarins via the Pechmann reaction at 80°C under solvent-free conditions. The methodology enables the synthesis of structurally diverse coumarins from phenols and β-keto esters within 5-8 min in high yields (91-99%). The green, commercially available, inexpensive organocatalyst can be effectively recycled and reused five times with no significant dropdown in the yield of the product. The method reported has the advantages of high yield, no need for metal catalysts, short reaction time, simple operation, and green and reusable catalyst.