Hydrazine–Halogen Exchange Strategy Toward N═N-Containing Compounds and Process Tracking for Mechanistic Insight

An anhydride-promoted traceless hydrazine-I/Br exchange strategy is reported, where hydrazine hydrate and cyclic/linear iodonium, including rarely explored cyclic bromonium, are converted to benzo[c]cinnolines/azobenzenes in one pot. The reaction proceeds through diacylation (first and second C─N formation), N,N'-diarylation (third and fourth C─N formation), and deacylation/oxidation (2 C─N cleavages and 1 N═N formation). The reaction mechanism is investigated by isolating multiple intermediates and kinetic studies. Furthermore, time-dependent electrospray ionization mass spectrometry (TD ESI-MS) was applied to track the process by detecting most intermediates. The complex [Cu^III(iodobiphenyl)(bipy)I]⁺ (Int-C) was detected for the first time, giving evidence for oxidative addition of cyclic iodonium to Cu catalyst. Another complex [Cu^I(PHA)(bipy)] (Int-B) via ligand-exchange between the hydrazide and Cu catalyst was also detected, indicating a two-path initial activation process.