Insights Into the Photocatalytic Arene Bromination Enabled by Cs₂AgBiBr₆ Microparticles

The development of sustainable halogenation strategies remains a central challenge in synthetic chemistry, particularly for the preparation of aryl bromides, which are key intermediates in pharmaceuticals, materials, and cross-coupling transformations. Here, we report an atom-efficient photocatalytic bromination of electron-rich arenes enabled by a low-toxicity Ag-Bi double perovskite microparticles. Under mild conditions and using hydrobromic acid as a bromine source, the photocatalyst promotes highly selective mono-bromination of 1,3,5-trimethoxybenzene with excellent yields (up to 95.4%) and superior performance compared to the benchmark perovskite-like materials, such as CsPbBr₃ and Cs₃Bi₂Br₉. Mechanistic investigations reveal an unprecedented activation mode of the transformation, in which bromine radicals originate from the Cs₂AgBiBr₆ surface, as demonstrated by radical-trapping studies and catalyst degradation observed in the absence of HBr. Control experiments further rule out the involvement of oxidant species, confirming that surface-derived bromine radicals are regenerated by external bromide. The photocatalyst maintains activity over multiple cycles, retaining its structural integrity with only minor AgBr formation upon five consecutive runs. The method displays broad substrate scope, encompassing anisole derivatives, polymethoxylated arenes, and heteroaromatics, with regioselectivity governed by substrate properties. Overall, these findings highlight the unique reactivity of double-perovskite surfaces and open new avenues for sustainable photocatalytic halogenation chemistry.