Visible Light-Photoinduced and Cu-Catalyzed Reductionof Nitrobenzenes into Anilines

A visible light-photoinduced and Cu-catalyzed reduction of nitrobenzenes using <i>i</i>PrOH as a green and renewable hydrogen source in aqueous HCl/<i>i</i>PrOH at room temperature has been developed. The reduction of nitrobenzenes into anilines proceeds through nitrosobenzene and <i>N</i>-phenylhydroxylamine intermediates. While visible light and <i>i</i>PrOH allow the reduction of nitrobenzenes and nitrosobenzenes via photoexcitation and hydrogen atom abstraction from <i>i</i>PrOH, a Cu salt is a crucial additive to transform <i>N</i>-phenylhydroxylamines into the target anilines. Mechanistic studies revealed that Cu(I)X is required to reduce <i>N</i>-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 <i>N</i>-phenylhydroxylamines into nitrosobenzenes. Hereby, a Cu(I)–Cu(II) catalytic cycle is obtained.