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.