Palladium-Catalyzed C-H Activation/C-N Coupling Route to 1-Aryl-1H-benzotriazoles

Significance

Reported is the palladium-catalyzed synthesis of 1-aryl-1H-benzotriazoles B from aryl triazene derivatives A using a C-H activation/C-N cross coupling strategy to form the ­triazole ring. After screening different palladium catalysts, Pd(OAc)₂ in combination with Cs₂CO₃ (or K₂CO₃) as a base under oxygen atmosphere were found to be the optimal conditions. Yields were then further improved by adding 4 Å MS to the reaction mixture. The scope of aryl substi­tuents R^¹ and R^² was well studied. However, R^¹ = R^² = 4-COMe, 4-CO₂Et and 4-OMe were not tolerated. The structure of one benzotriazole was confirmed by X-ray crystallographic analysis. In the case of unsymmetrical substrates, such as C and D, the cyclization afforded a 2:1 mixture of ­regioisomers E and F in 75% yield, the cyclization occurring preferentially towards the unsubstituted aryl ring. A mechanism involving C-H activation to give a six-membered palladacycle followed by C-N bond formation and reoxidation of palladium(0) by oxygen was proposed with no evidence.