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

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.