Visible Light-Photoinduced and Cu-Catalyzed Reduction of Nitrobenzenes into Anilines

A visible light-photoinduced and Cu-catalyzed reduction of nitrobenzenes using iPrOH as a green and renewable hydrogen source in aqueous HCl/iPrOH at room temperature has been developed. The reduction of nitrobenzenes into anilines proceeds through nitrosobenzene and N-phenylhydroxylamine intermediates. While visible light and iPrOH allow the reduction of nitrobenzenes and nitrosobenzenes via photoexcitation and hydrogen atom abstraction from iPrOH, a Cu salt is a crucial additive to transform N-phenylhydroxylamines into the target anilines. Mechanistic studies revealed that Cu(I)X is required to reduce N-phenylhydroxylamines into anilines. Moreover, it avoids undesired arene chlorination, delivering chloroanilines. The reduction step does not require light irradiation and is accelerated by aq. HCl. The concomitantly produced Cu(II)X2 is then reduced to Cu(I)X by oxidizing N-phenylhydroxylamines into nitrosobenzenes. Hereby, a Cu(I)–Cu(II) catalytic cycle is obtained.