Water-mediated green synthesis of coumarin derivatives using a recyclable heteropoly acid-niobium(v) oxide hybrid catalyst.

A highly efficient, environmentally friendly, water-mediated greener approach has been developed for the one-pot, three-component catalytic synthesis of biologically and pharmaceutically active coumarin derivatives using niobium oxide (Nb2O5) supported silico-tungstic acid (STA) as a hybrid heterogeneous catalyst, STA@Nb2O5, with the reaction of 4-hydroxycoumarin, malononitrile and various aryl aldehydes, including cinnamaldehydes. This recoverable solid catalyst was prepared via impregnation and extensively characterised to confirm its successful preparation. This efficient protocol enables the isolation of novel cinnamyl pyranochromenes from less reactive cinnamaldehydes. The catalytic reaction revealed that the substrate structures of both aryl and cinnamaldehydes strongly influence the 3-component synthesis of chromenes. A detailed mechanistic study revealed two possible pathways responsible for the formation of a variety of coumarin derivatives from different aryl aldehyde and cinnamaldehyde substrates in a one-pot, multicomponent trilogy. The formation of 3-component chromenes followed the type-A pathway, while the unexpected formation of 2-component biscoumarins from aryl aldehydes and pyrans from cinnamaldehydes followed the type-B pathway. The mechanistic pathway is completely controlled by the nature of the substituent or functional group on the aldehyde. Also, this methodology offers a straightforward process for catalyst recovery and reuse, achieving high conversion and selectivity across multiple runs with minimal loss of activity.