Rhodium-catalyzed cyclocarbonylation of azobenzene

Carbonylation of azobenzene derivatives catalyzed by rhodium carbonyls in the presence of nitrobenzene as a hydrogen acceptor gave a four-ring heterocyclic product, indazolo[2,1-a]indazole-6,12-dione, in a good yield, which is derived from a novel cyclocarbonylation with C–H bond activation and CO insertion at each benzene nucleus of azobenzene.