Bi-functionality of organic acids as acid catalysts and a hydrogen source for one-pot production of secondary amines from primary amines and aromatic aldehydes over an Au–C3N4 photocatalyst†

Carbon nitride modified with a gold (Au) cocatalyst (Au–C₃N₄) was utilized in a one-pot synthesis of N-phenylbenzylamine (PBA) from benzaldehyde (BAD) and aniline (AN) in the presence of an organic acid. In this process, the organic acid fulfilled two functions: (1) as an acid catalyst for the condensation of AN and BAD, producing benzylideneaniline (BAN) in the initial thermal step, and (2) as a hole scavenger (hydrogen source) for the hydrogenation of BAN to PBA in the subsequent photocatalytic step over Au–C₃N₄. Various organic acids were utilized, and oxalic acid was found to be the most effective due to its capacity to maintain an acidic pH environment as a divalent acid and its ability to effectively capture positive holes as a hole scavenger, resulting in the generation of carbon dioxide. Photoirradiation of the system under equilibrium conditions in the condensation process induced effective hydrogenation of BAN, resulting in the production of PBA with a yield of >99%. A series of experiments were conducted to study the action spectrum, the impact of alcohols, and reusability, and halogen-substituted PBA was synthesized. The outcomes of this study demonstrate the efficacy of oxalic acid in photocatalytic reactions and the potential of a cocatalyst-loaded C₃N₄ photocatalyst for organic synthesis.