Palladium-Catalyzed sp2 C-H Arylation of Azoarenes with Arylhydrazines

Transition-metal-catalyzed direct functionalization of C-H bonds is important for new compound synthesis, but usually needs special reagents and harsh conditions. With the aid of an azo directing group, the palladium-catalyzed ortho-sp(2) C-H bond activation of azoarenes with hydrazines has been explored. In the reaction, the Pd catalyst reacts with azobenzene to form a palladacyclic intermediate, which reacts with a phenyl radical generated from phenylhydrazine under heating and oxygen to produce a Pd-IV or Pd-III intermediate. This finally undergoes reductive elimination to give the ortho-arylated products with the release of the Pd-II catalyst. This reaction provides a novel access to ortho-aryl azoarenes under mild conditions in high yield.