The Mills reaction and cyclization of readily available 2‐aminobenzyl alcohols and nitrosobenzenes using thionyl bromide provided 2H‐indazoles in up to 88 % yields. In the metal‐free process, acetic acid played a crucial role for the both Mills reaction and cyclization. A brominated 2H‐indazole could also be obtained through the one‐pot sequence.
A catalyst- and oxidant-free electrochemical method for the halogenation of 2H-indazoles with NaCl or NaBr has been developed. Under the optimized conditions, a broad range of substrates were well tolerated, affording the desired halogenation products in 45–96 % isolated yields with good regioselectivity.
An unprecedented metal-free regioselective halogenation of 2H-indazoles is reported, and not only realizes the highly selective synthesis of mono-halogenated products, but also complete poly-halogenations.