Photoredox-Catalyzed Transfer Hydrogenation of Unsaturated Substrates via In Situ Generated Diimide at Room Temperature

Transfer hydrogenation using in situ-generated diimide offers significant advantages over conventional metal-catalyzed hydrogenation, but existing methods require large excesses of hydrazine or harsh conditions limiting functional group compatibility. Herein, we report a practical photoredox-catalyzed protocol for the transfer hydrogenation of unsaturated substrates that operates at room temperature under neutral conditions. This transition-metal-free approach employs rose bengal as an organic photocatalyst and quinoline N-oxide as a mild oxidant to generate diimide from hydrazine hydrate under visiblelight irradiation. Mechanistic investigations reveal a photoredox catalytic cycle initiated by reductive quenching of excited-state rose bengal. The protocol demonstrates excellent chemoselectivity and functional group tolerance, successfully reducing thermally unstable substrates such as benzyl azides while leaving protecting groups intact. This operationally simple system provides a sustainable alternative for transition-metal-free transfer hydrogenation.