Azobenzenes are a unique class of compounds that originally aroused interest because of their exceptional photochromic properties. Selective reactions allowing the preparation of complex polysubstituted azo-based photoswitches, in a direct and efficient manner, have received considerable attention over the past few years. Among them, regio- and chemoselective C–H bond activation in the presence of a transition-metal catalyst has emerged as the most attractive and challenging strategy in the synthesis of sterically constrained azo compounds. This synthetic approach is currently an intensive research field for introducing functional and structural diversity from azobenzenes. This short review summarizes the recent advances in both ortho-functionalization of azobenzenes and the synthesis of azo-derived heterocycles based on transition-metal-catalyzed direct C–H bond activation. The substrate scope, limitation, reaction mechanism as well as additional applications are discussed.

A straightforward palladium-catalyzed oxidative hydroxylation of azobenzenes is reported. The developed methodology tolerates various functional groups and allows the synthesis of diverse unsymmetrical azophenols under mild conditions in good to excellent yields. A complementary procedure was also investigated by in situ generation of PIFA. This study represents the first general method for the synthesis of o-hydroxyazobenzenes starting from simple azoarenes.