Palladium‐Catalyzed Regioselective Halogenation of Aromatic Azo Compounds

A highly regioselective halogenation reaction of symmetrical and unsymmetrical aromatic azo compounds has been developed at room temperature or at 50 °C. In the presence of 5 mol% palladium diacetate and 0.5 equiv. of p‐toluenesulfonic acid, a range of symmetrical aromatic azo compounds smoothly undergo monobromination with N‐bromosuccinimide to give the corresponding unsymmetrical aromatic azo compounds in good to excellent yields with >99:1 ortho‐selectivity. This chemistry has been successfully extended to unsymmetrical aromatic azo compounds, whose electron‐richer aryl groups prefer to be monobrominated. Moreover, replacing N‐bromosuccinimide with N‐iodosuccinimide in the reaction allows the synthesis of monoiodinated aromatic azo compounds with >99:1 regioselectivity.