Metal-Free Synthesis of 2H-Indazole Skeletons by Photochemistry or Thermochemistry

A simple and tuned synthesis of a 2H-indazoleskeleton under metal-free conditions was developed. Under visible-lightirradiation at room temperature, 2-((aryl/alkyl/H)ethynyl))aryltriazenesreacted with arylsulfinic acids to afford 3-functionalized 2H-indazoles without extra photocatalyst via an electrondonor-acceptor complex. In the presence of arylsulfinic acid,2-(ethynyl)aryltriazenes underwent an intramolecular oxidation/cyclizationto provide 2H-indazole-3-carbaldehydes at 50 & DEG;Cin air.