Gold-coated iron oxide core-shell nanostructures for the oxidation of indoles and the synthesis of uracil-derived spirooxindoles

Encapsulation of iron oxide (Fe3O4)-based nanoparticles (NPs) with Au NPs holds a promising scope for catalysis, which overcomes the hindrance of the inherent hydrophilic surface of iron species and facilitates the easy separation of colloidal Au NPs. As such, iron oxide-based NPs were encapsulated by Au NPs to form core-shell structures (Fe3O4/Au NPs) followed by further immobilization on in situ synthesized reduced graphene oxide (rGO) to yield Fe3O4/Au NPs-rGO for increased stability. Fe3O4/Au NPs-rGO was evaluated for the oxidation of NH-free indoles to synthesize isatins, potential precursors of SARS-CoV-2 protease inhibitors. Furthermore, the application of isatin was explored with the Fe3O4/Au NPs-rGO catalysed synthesis of uracil-based spirooxindoles (potential precursors of anti-cancer compounds) via mild reaction conditions and short reaction times with high yields.