Graphene oxide substituted pyrimidinium hydrogen sulfate as an efficient and sustainable catalyst for the synthesis of bis-coumarins under solvent-free conditions

Graphene oxide functionalized by pyrimidinium hydrogen sulfate (GO@Pyrim.H-HSO4) is an efficient and environmentally benign catalyst for the synthesis of bis-coumarins. At first, graphene oxide was prepared according to a modified Hummers method from the oxidation of graphite using violent oxidants such as concentrated sulfuric acid, sodium nitrate, potassium permanganate, and Hydrogen peroxide. It was subsequently modified by 2-aminopyrimidine through amide formation and then treated with sulfuric acid to reach the catalyst as pyrimidinium hydrogen sulfate (GO@Pyrim.H-HSO4). The catalyst was characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), and FTIR spectroscopy. The synthesis of bis-coumarins was implemented through a one-pot process using no solvent by the reaction of 4-hydroxycoumarin and functionalized benzaldehydes. The scope and limitations of the procedure were examined using aromatic aldehydes with various functional groups. The functional groups had no significant effect on the product yields. The catalyst exhibited efficient properties such as simple work-up, no byproducts, sustainability, nonmetal and safe components, stability under reaction conditions, and recoverability. The synthetic route shows significant properties including sustainable, simple work-up, solvent-free condition, short reaction times, and no by-product.