Boron Radical Promoted Metal-Free Transformation of Nitroazobenzene into Benzotriazole.

Although the triazole skeleton is significant in biochemistry as a click reaction candidate, as well as in material chemistry due to its excellent absorption of UV light, the preparation of these compounds relies on multinitrogen reagents such as diazo and azido compounds. In this work, o-nitroazobenzenes are first used in a series of neat, fast, green, and efficient reactions for the synthesis of 2-aryl-2H-benzotriazoles under visible light, without RN3 and metals. It is the visible light-induced boron radical that initiates the reaction by reducing the nitro group into a nitroso group, followed by a barrierless N-N coupling and a facile further deoxygenation by diboron ester to yield benzotriazoles as potential UV absorbers in excellent yields.