Divergent photoreduction of nitroaromatic compounds catalysed by dithienoquinoxaline

Abstract

Nitroaromatic compounds can be conveniently reduced to anilines with the aid of a variety of chemical reductants. However, chemodivergent and chemoselective reduction of functionalized nitroaromatics into valuable nitroso, azo(xy), (hydr)azo and (hydroxyl)amino derivatives remains rather elusive. Electro- and photochemical strategies utilizing various catalytic systems and reaction media have been tested but most have been directed towards individual reduction products. Here, we present the photoredox-enabled chemodivergent reduction of nitroaromatics to nitroso, bis-(N,O-diacetyl)-N-arylhydroxylamine, azoxy, azo and amino derivatives. The developed protocol utilizes a novel photocatalyst, 6,9-dimethoxydithieno[2,3-f:3′,2′-h]quinoxaline-2,3-dicarbonitrile, and Hantzsch ester/triethanolamine as reductants. With the single organic photocatalyst, the reaction environment, temperature, time and catalyst loading can be varied to achieve chemodivergent photoreduction of nitroaromatics bearing various functional groups and to access a library of valuable products. The application of this methodology to multigram preparations of molecules of pharmaceutical and industrial relevance highlights its practical utility.