Humic Acid: A Promising and Green Bioorganic Catalyst in Organic Syntheses

Abstract

The growing pollution and threat to the environment demand green synthetic methods. Green chemistry mainly focuses on minimizing the waste and utilization of green solvents, energy sources, and catalysts and prefers one‐pot reactions. In this context, humic acid (HA), a natural bioorganic catalyst found in rivers, peat coal and sewage; is a high molecular weight macromolecule containing multifunctional groups, mainly contains quinone, phenolic OH, and COOH groups which make it acidic and activate carbonyl groups by protonation. Owing to its green aspect such as biodegradability, recyclability and reusability, humic acid is considered as sustainable bioorganic catalyst which meets the criteria for industrial requirements. The HA is mild acidic in nature, thus many functional groups tolerate in the HA catalyzed reactions. After few initial reports in 2001 and 2009, the HA catalyzed reactions gained much attention by the organic synthetic community from 2019 in aldol condensation, Knoevenagel condensation, Claisen‐Schmidt reaction, Michael addition, Strecker synthesis, tetrazole synthesis, Hantzsch dihydropyridine synthesis, hydrosilylation of terminal alkenes, hydrogenation, α‐aminophosphonate synthesis, cross coupling reactions such as Heck and Suzuki coupling reactions. Herein we report the comprehensive analysis on the role of HA in organic transformations providing diverse array of molecular scaffolds till date and future perspective.