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

Abstract

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.

Photocatalyst‐ and Transition‐Metal‐Free Light‐Induced Formation of Carbon‐Chalcogen Bonds

“…These compounds have the potential for utilization as photocatalysts, natural products, and pharmaceutically important compounds. [41] Scheme 3. Visible-light and heat assisted synthesis of diaryl ether.…”

Sustainable Photooxidation using a Subpart-per-million Heavy-Metal-Free Red-Light Photocatalyst

“…Adding to this, the relative instability of common PSs generates a large variations of kinetic profiles, which in turn engenders numerous scale-up difficulties. This stability issue is even more important when considering low energy light activation (above 600 nm), which has recently gained high interest in photocatalysis due to the higher photon throughput and improved light penetration in the reaction medium. ,− Red or near-infrared (NIR) light also offers advantages in terms of irradiation selectivity due to orthogonal absorption of the photocatalyst and the reaction medium (which is sometimes not achievable for UV or blue light activations). , In addition, the triplet to singlet transition of molecular oxygen is far lower in energy (24 kcal/mol) compared to blue light activation (90 kcal/mol). Accordingly, red or NIR sensitization is almost four times more energy-efficient compared to blue light.…”