Visible-Light-Triggered Quantitative Oxidation of 9,10-Dihydroanthracene to Anthraquinone by O₂ under Mild Conditions

The development of mild and efficient processes for the selective oxygenation of organic compounds by molecular oxygen (O₂ ) is key for the synthesis of oxygenates. This paper discloses an atom-efficient synthesis protocol for the photo-oxygenation of 9,10-dihydroanthracene (DHA) by O₂ to anthraquinone (AQ), which could achieve quantitative AQ yield (100 %) without any extra catalysts or additives under ambient temperature and pressure. A yield of 86.4 % AQ was obtained even in an air atmosphere. Furthermore, this protocol showed good compatibility for the photo-oxidation of several other compounds with similar structures to DHA. From a series of control experiments, free-radical quenching, and electron paramagnetic resonance spin-trapping results, the photo-oxygenation of DHA was probably initiated by its photoexcited state DHA*, and the latter could activate O₂ to a superoxide anion radical (O₂ ^.- ) through the transfer of its electron. Subsequently, this photo-oxidation was gradually dominated by the oxygenated product AQ as an active photocatalyst obtained from the oxidation of DHA by O₂ ^.- , and was accelerated with the rapid accumulation of AQ. The present photo-oxidation protocol is a good example of selective oxygenation based on the photoexcited substrate self-activated O₂ , which complies well with green chemistry ideals.