Heterocyclic compounds, intricate organic molecules featuring a ring structure comprising multiple elements, are increasingly coveted in the chemical and pharmaceutical industry due to their versatility and potent physiological activities. However, conventional synthesis methods often involve high temperatures, pressures, or precious metal catalysts, contributing to economic and environmental concerns.
A groundbreaking study by a collaborative team from Japan and Bangladesh, led by Professor Yutaka Hitomi of Doshisha University, introduces a straightforward yet highly effective approach to tackle these challenges. Published in the journal Advanced Synthesis & Catalysis, the researchers showcased the synthesis of 20 sulfur-containing heterocyclic compounds using the photocatalyst titanium dioxide (TiO2) and visible light.
While TiO2 as a photocatalyst has intrigued synthetic chemists, many processes typically necessitate ultraviolet light. In this study, the team discovered that under anaerobic conditions, sulfur-containing organic compounds reacted with maleimide derivatives when exposed to blue light, facilitated by TiO2. This led to the formation of dual carbon–carbon bonds, yielding novel heterocyclic organic compounds.
Prof. Hitomi elaborates, “Our approach allows for the selective one-electron oxidation of the substrate molecules using visible light, differing from ultraviolet light's generation of highly oxidative holes. This versatility can be applied in various organic chemical reactions.”
The researchers selected specific thioanisole and maleimide derivatives, introducing TiO2 into the reaction system and irradiating it with blue light. This resulted in the synthesis of 20 thiochromenopyrroledione derivatives with moderate-to-high yield. Notably, the reaction between thioanisole and N-benzylmaleimide, under blue light exposure for 12 hours, produced a thiochromenopyrroledione derivative with a 43% yield, close to the theoretical maximum.
Analyzing substituent effects, the team proposed a mechanism involving charge transfer from thioanisole to the TiO2 conduction band. Blue light irradiation triggered one-electron oxidation of thioanisole, initiating the generation of α-thioalkyl radicals through deprotonation.
In essence, this refined approach unveils TiO2's potential for visible light photocatalysis in organic synthesis, offering vital insights into the complex chemistry of heterocyclic compound synthesis. Beyond its immediate implications, this methodology hints at a paradigm shift from resource-intensive industrial processes to a more energy-efficient system.
Prof. Hitomi emphasizes, “Our study was driven by the desire to contribute to a sustainable chemical industry, and our findings mark a positive stride in this direction.”
The team envisions the widespread adoption of this visible light-driven technology for the accessible and affordable synthesis of pharmaceuticals, potentially transforming global health outcomes. Thanks to Prof. Hitomi and his team's efforts, this study paves the way for innovative approaches in organic synthesis, poised to revolutionize diverse chemical industries.
Source: Doshisha University