Light Driven Pd‐catalyzed Regioselective C─H Halogenation of Benzo[ b ][1,4]oxazin‐2‐ones Using DXDMH

A mild, visible‐light‐driven protocol has been developed for the regioselective ortho ‐C(sp 2 )‐H mono‐halogenation (Cl, Br, and I) of 3‐aryl‐2 H ‐benzo[ b ][1,4]oxazin‐2‐ones via a synergistic metallaphotoredox strategy. The transformation employs Pd(OAc) 2 (20 mol %) in combination with the organic photocatalyst eosin Y (WS, 10 mol %) under irradiation with a 24 W blue LED, using bench‐stable N ‐halo‐5,5‐dimethylhydantoins (DCDMH, DBDMH, and DIDMH) as sustainable halogen sources. Conducted in 1,2‐dichloroethane (DCE) with p ‐toluenesulfonic acid (1.0 equiv.) as an essential additive, the reaction proceeds under oxidant‐free and thermally mild conditions to afford ortho ‐halogenated products in high isolated yields (75%–97%) with excellent regioselectivity. A broad range of substituents on the 3‐aryl ring, including electron‐donating, electron‐withdrawing, and extended aromatic groups, are well tolerated. Mechanistic investigations‐including radical trapping experiments (TEMPO, BHT), light on/off studies, and control reactions‐support a cooperative pathway involving Pd(II)‐directed C─H activation and photoredox‐mediated generation of halogen radicals, followed by radical capture and reductive elimination from high‐valent palladium intermediates. This operationally simple dual catalytic approach provides an efficient platform for late‐stage diversification of pharmaceutically relevant 2 H ‐benzo[ b ][1,4]oxazin‐2‐one scaffolds and advances sustainable strategies in photocatalytic C─H halogenation.