Gram-Scalable One-Pot Synthesis of Dihydro-Pyrano[3,2-<i>c</i>]Chromenes Using Magnetic Salicylaldehyde-Melamine Copper Complex

The present study reports the synthesis of a nanomagnetic heterogeneous catalyst, namely, Fe₃O₄@SiO₂@Si(CH₂)₃@melamine@salicylicaldehyde@Cu. This synthesis occurred through intermolecular forces, covalent and dative bonds. After synthesis of the catalyst, its structure was confirmed through different characterization techniques, including FTIR, FESEM, TEM, EDX, VSM, and TGA. In sequence, the activity of the as-synthesized catalyst was evaluated using 4-hydroxycoumarin, malononitrile, and various benzaldehydes as a multi-component model reaction. Then, the reaction condition was optimized to find the solvent type, catalyst amount, and temperature corresponding to ethanol, 0.03 g of catalyst, and reflux conditions. Finally, various derivatives of dihydro-pyrano[3,2-<i>c</i>]chromenes were synthesized using aryl and heterocyclic aldehydes. To evaluate the reuse of the as-synthesized catalyst, it was recovered using an external magnet and used in five cycles. The results showed that the catalyst could be recovered and reused without significant leaching and loss in efficiency. Various characterization techniques confirmed the stability of the used catalyst. Since the magnetic properties of the catalyst allow for easy separation and recycling, maintaining considerable yield for at least five cycles, probably due to the presence of melamine, which plays a vital role in stability and reusability. The present procedure can be done in gram-scale amounts, which can be a candidate for further scaling up.