A team of researchers now present an eco-friendly and innovative approach for the blue light-promoted synthesis of heterocyclic thiochromenopyrroledione derivatives catalyzed by titanium dioxide. Credit: Professor Yutaka Hitomi, Doshisha University
Discover the world of heterocyclic compounds, highly sought-after organic molecules with multiple elements in their ring structure. Researchers are now proposing a simple yet effective method for synthesizing these compounds with minimal economic and environmental costs.
While several methods are available for synthesizing these compounds, most of them involve high-temperature and pressure conditions, or the use of precious metal catalysts, adding to the economic and environmental cost of producing heterocyclic organic compounds.
Now, however, a team of researchers from Japan and Bangladesh have proposed a simple yet effective method for overcoming these challenges. Their study was recently published in the journal Advanced Synthesis & Catalysis. Using the proposed strategy, the team demonstrated the synthesis of 20 sulfur-containing heterocyclic compounds in the presence of photocatalyst titanium dioxide (TiO2) and visible light.
The study was led by Professor Yutaka Hitomi from the Department of Applied Chemistry, Graduate School of Science and Engineering, Doshisha University, and co-authored by a Ph.D. candidate Pijush Kanti Roy from Doshisha University, Associate Professor Sayuri Okunaka from Tokyo City University, and Dr. Hiromasa Tokudome from Research Institute, TOTO Ltd.
TiO2 as a photocatalyst for driving organic reactions has captured the attention of synthetic chemists for a while now. However, many such processes require ultraviolet light to trigger the reaction. In this study, however, the research team found that under anaerobic conditions, sulfur-containing organic compounds like thioanisole derivatives, when hit with blue light, reacted with maleimide derivatives to form dual carbon–carbon bonds, yielding a new heterocyclic organic compound.
“We observed that while ultraviolet light generates highly oxidative holes, our approach allows for the selective one-electron oxidation of the substrate molecules using visible light. This approach can thus be employed in various organic chemical reactions,” explains Prof. Hitomi.
The researchers chose five 4-substituted thioanisoles and four N-substituted maleimides for the annulation or ring formation reactions. The team irradiated the starting material with blue light (wavelength > 420 nm) but observed no reaction. However, introducing TiO2 into the reaction system led to the synthesis of 20 different thiochromenopyrroledione derivatives with moderate-to-high yield. They found that within 12 hours of exposure to blue light, the reaction between thioanisole and N-benzylmaleimide led to the formation of a thiochromenopyrroledione derivative with 43% yield, which was close to the theoretical maximum yield of 50%.
The research team also observed substituent effect in the reactions to understand the corresponding mechanistic aspects. From the results, they postulated that the reaction proceeds through charge transfer from thioanisole to the conduction band of TiO2. Furthermore, they suggested that irradiation with blue light triggered one-electron oxidation of thioanisole,
