Visible Light-Photoinduced and Cu-Catalyzed Reduction of Nitrobenzenes into Anilines
Mathias Bal, Wouter Van Hoey, Robine Cleirbaut, Filip Lemière, Sabine Van Doorslaer, Pegie Cool, Bert U. W. Maes
Abstract
A visible light-photoinduced and Cu-catalyzed reduction of nitrobenzenes using i PrOH as a green and renewable hydrogen source in aqueous HCl/ i PrOH at room temperature has been developed. The reduction of nitrobenzenes into anilines proceeds through nitrosobenzene and N -phenylhydroxylamine intermediates. While visible light and i PrOH allow the reduction of nitrobenzenes and nitrosobenzenes via photoexcitation and hydrogen atom abstraction from i PrOH, 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)X 2 is then reduced to Cu(I)X by oxidizing N -phenylhydroxylamines into nitrosobenzenes. Hereby, a Cu(I)–Cu(II) catalytic cycle is obtained.