Self-Sensitized Photooxidation of Naphthols to Naphthoquinones and the Use of Naphthoquinones as Visible Light Photocatalysts in Batch and Continuous Flow Reactors.

Visible light photooxidation of naphthols to produce naphthoquinones, such as the natural product juglone, has been known for decades and has been widely utilized to benchmark the performances of a variety of photocatalytic systems. We discovered that these transformations can occur without the help of a photocatalyst and, even more intriguingly, that the photocatatyst-free process provides higher yields compared to control experiments utilizing state-of-the-art photocatalysts. In addition, we demonstrate that naphthoquinones and their corresponding naphthol precursors can act as alternatives to commonly used organic and organometallic photocatalysts with applications to challenging targets, such as the antimalarial drug artemisinin. This approach was finally transposed in continuous flow reactors where high photocatalyst stability and process efficiency are demonstrated with a 23× improvement in the space-time yield.