Curcumin-Enabled Metal-Free Photocatalytic Oxidation of Aryl- and Alkyl-methanamines to Imines Under Green Conditions.

Herein, we report a fully bio-based, metal-free photocatalytic protocol for the aerobic oxidation of aryl- and alkyl-methanamines to imines under blue-light irradiation. This transformation leverages curcumin, a naturally abundant and renewable polyphenol, as an efficient visible-light photosensitizer and 2-methyltetrahydrofuran (2-MeTHF), a biomass-derived solvent, establishing a sustainable catalytic system composed entirely of organic, bio-derived components. The reaction proceeds efficiently at room temperature using molecular oxygen as the sole terminal oxidant, strictly avoiding transition metals, stoichiometric oxidants, and hazardous additives. A broad range of benzylic and aliphatic amines is converted to the corresponding imines in good to excellent yields (up to 98%), demonstrating the generality and practicality of the protocol. Comprehensive mechanistic investigations-including light on/off experiments, oxygen-dependence studies, radical trapping, Stern-Volmer fluorescence quenching studies and Hammett analysis-support a sensitizer-controlled aerobic photocatalytic pathway involving photoinduced electron transfer and oxygen-mediated catalyst regeneration via superoxide radical anions. This work highlights the efficacy of natural pigments in driving high-efficiency, sustainable organic transformations.