Humic acid: A Biodegradable Organocatalyst for Solvent-free Synthesis of Bis(indolyl)methanes, Bis(pyrazolyl)methanes, Bis-coumarins and Bis-lawsones

Humic acid, a biodegradable, non-toxic and easily accessible high molecular weight organocatalyst has been explored for the straightforward and environmentally benign approach towards the preparation of diverse array of functionalized bis(indolyl)methanes, bis(pyrazolyl)methanes, bis-coumarins and bis-lawsones by the reaction of a vast range of aldehydes with indole, 3-methyl-1-phenyl pyrazolone, 4-hydroxycoumarin and 2-hydroxynaphthalene-1,4-dione respectively under solvent-free condition. These protocols proceed without any hazardous solvents, toxic metal catalysts and harsh reaction conditions. In addition, low catalyst loading, good yield and excellent functional group tolerance are the key advantages of these protocols. The used catalyst and solvent-free approach make this strategy safe to our mother earth. For the first time reusability of humic acid was investigated and found that it can be recycled up to fifth run without any significant loss of its activity at the end of the reaction. Some bioactive molecules such as arundine, trisindoline and vibribdole A were easily synthesized in our lab by utilizing this designed strategy.