Photoactivation of Thianthrenium Salts: An Electron-Donor–Acceptor (EDA)-Complex Approach

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Thianthrenium salts have emerged as one of the most versatile reagents, gaining significant popularity within the synthetic community for their utility in the construction of C–C and C–X (X = N, O, S, P, halogens) bonds. The use of photoredox and transition metal catalysis with thianthrenium salts for C–C and C–heteroatom bond formation is well established. However, most of these methods require elevated temperatures, expensive catalysts, and ligands under stringent conditions for effective execution. In contrast, the photocatalysis- and transition-metal-free approaches for constructing C–C and C–X bonds using thianthrenium salt derivatives have become increasingly sought after. In this regard, electron-donor–acceptor (EDA)-complex reactions have emerged as a powerful strategy in organic synthesis, eliminating the need for photocatalysts under visible light irradiation. EDA-complex photochemistry exploits the electron-acceptor properties of thianthrenium salts, facilitating the rapid generation of radical intermediates via the C–S bond cleavage. These radical intermediates play a pivotal role in enabling a variety of valuable C–C and C–X formations. In this Perspective, we highlight significant advances in the EDA-complex-mediated reactions involving thianthrenium salts with mechanisms, substrate scope, and limitations for constructing C–C and C–heteroatom bonds. For the sake of brevity, the article is organized into five main sections: (1) Nitrogen-based donor reactions, (2) Oxygen-based donor reactions, (3) Sulfur-based donor reactions, (4) Phosphorus-based donor reactions, and (5) π-based donor reactions, with a focus on C–C, C–S, C–B and C–P bond formations.