Synthesis of 2-Aryl-2H-indazoles by TiO₂-Photocatalyzed Reaction of Alkoxyazobenzenes

Abstract

3-Substituted 5-alkoxy-2-aryl-2H-indazoles were synthesized in low to moderate yields by the TiO₂-photocatalyzed reaction (λ>280 nm) of 4-alkoxyazobenzenes in alcohols. The formation of hydroxyalkyl radicals and the presence of an electron-donating alkoxy group in the starting azobenzenes are very important in this reaction. This is an unique method for the synthesis of 3-substituted 5-alkoxy-2-aryl-2H-indazoles from 4-alkoxyazobenzenes.