In recent years, global environmental concerns have prompted a significant shift toward eco-friendly manufacturing processes in organic synthetic chemistry. One such advancement is the use of photoredox catalytic reactions, which leverage light to drive redox reactions via a photoredox catalyst. This method reduces the reliance on harsh and toxic reagents, utilizing visible light as a clean and renewable energy source.
A key research focus has been the development of recycling methods for photocatalysts, which can provide both economic and environmental benefits. Photocatalysts accelerate chemical reactions when exposed to light without being consumed, with photoredox catalysts specifically facilitating redox reactions. While there has been extensive development in recycling methods for heterogeneous photocatalysts like semiconductors and polymers, less attention has been given to recycling organic photocatalysts. Given the cost-effectiveness and low toxicity of organic photoredox catalysts, developing suitable recycling strategies is essential for achieving sustainable organic synthesis.
Addressing this gap, a team of researchers from Okayama University in Japan, including Assistant Professor Kenta Tanaka from the Research Institute for Interdisciplinary Science, along with then-graduate student Haru Ando, Associate Professor Hiroyoshi Takamura, and Professor Isao Kadota from the Department of Chemistry at the Graduate School of Natural Science and Technology, developed a novel phenothiazine-based organic photoredox catalyst. Their study was published in the journal Chemical Communications on March 19, 2024.
“Phenothiazines, or PTHs, are widely used as photocatalysts in organic chemistry,” explains Prof. Tanaka. “However, the high reactivity at the p-position relative to the nitrogen atom on 10-aryl phenothiazine molecules makes them prone to reacting with electrophiles, reducing their stability. The development of more stable and sustainable photocatalysts is therefore highly desirable. To address this, we developed new phenothiazine-based photocatalysts that are both stable and recyclable.”
The novel phenothiazine catalyst, named PTHS, features a spiral structure with a bulky electron-donating group, called tBu, substituted at the p-position of the nitrogen atom, providing enhanced stability. The researchers developed a series of phenothiazine photocatalysts (PTHS 1–3) and evaluated their structural and physical properties through electrochemical and spectroscopic experiments. They discovered that the new catalysts possess strong reducing abilities and can be activated using blue light.
To test their stability, the team compared the new catalysts with existing PTH catalysts by subjecting them to photochemical sulfonylation reactions. The results showed that while PTH could not be recovered and produced a monosulfonylated product in 78% yield, 95% of PTHS could be recovered, indicating greater stability.
Additionally, the researchers tested the recyclability of the catalysts in a photochemical phosphonation reaction. They found that the catalytic activity and reaction yield of PTH decreased with repeated use. In contrast, PTHS-1 could be effectively recovered multiple times without any loss of catalytic activity and yield. Furthermore, PTHS-1 demonstrated suitability for large-scale synthesis, achieving 96% recovery even at gram-scale production.
“The new phenothiazine photocatalysts have the potential to be applied to various visible-light-induced photochemical reactions, which was not possible with any of the previously reported phenothiazine photocatalysts. We believe our recyclable organic photocatalysts will be a promising tool for the efficient synthesis of various pharmaceuticals and functional materials,” remarks Ando.
Overall, these innovative photocatalysts represent a significant step toward achieving sustainable organic synthesis, paving the way for environmentally friendly chemical manufacturing. The development of stable and recyclable organic photocatalysts not only aligns with green chemistry principles but also opens new avenues for the efficient and eco-friendly production of valuable chemicals and materials.
Source: Okayama University