Graphene Oxide as Metal-Free Catalyst in Oxidative Dehydrogenative C-N Coupling Leading to α-Ketoamides: Importance of Dual Catalytic Activity

Abstract

A heterogeneous, inexpensive, and environmentally friendly carbocatalyst, graphene oxide (GO) enables the formation of α-ketoamides from activated aldehydes and amines through a cross-dehydrogenative coupling pathway. The oxygenated functionalities (e.g., carboxyl, hydroxyl, ketonic, and epoxides) on the surface of graphene oxide impart acidic as well as oxidizing properties to the material. This dual catalytic property of graphene oxide is explored toward the generation of α-ketoamides where surface acidity of graphene oxide favors the initial formation of hemiaminal intermediate followed by oxidation leading to the desired final product. The hemiaminal intermediate could be isolated and confirmed by NMR and mass analysis. A few control experiments confirmed that both acidic and oxidizing catalytic activities of graphene oxide were instrumental in the coupling reaction. Use of benzoic acid and p-toluene sulfonic acid as catalysts resulted in the formation of only hemiaminal intermediate as the major product along with a trace amount of α-ketoamide. As these catalysts do not possess oxidative catalytic capability, the formation of ketoamide was not favorable. However, use of GO as the catalyst could generate the ketoamide product from the hemiaminal intermediate as initial acid-catalyzed hemiaminal formation was followed by oxidation to α-ketoamide. Graphene oxide annealed at different temperatures demonstrates the role of oxygenated functional groups in the catalytic reaction. Further investigation of catalytic activity with modified GO surfaces using various conditions, such as base, acid, or NaBH4, showed carboxylic acid groups on the surface to be the active site for the catalytic reaction. The method is also effective toward the synthesis of biologically important α-ketoamide. © 2017 American Chemical Society.