SO2-Mediated 2e-, 4e-, and 6e- Reductions: Facile Access to Aryl Amines.

Traditional electron-transfer catalysis frequently requires complex procedures to separate the catalyst from the reaction mixture. In contrast, a concept of a gas-mediated redox process may offer unique advantages, including its facile removal from the reaction medium. However, such a concept has remained unknown. Addressing this gap, we report the use of the SO2 gas molecule as an electron-transfer mediator by generating the sulfur dioxide radical anion (SO2 •-) under ambient and chemical conditions. Such SO2 •- enabled chemoselective, controlled, and scalable multi-electron reductions (2e-/2H+, 4e-/4H+, 6e-/6H+) of diverse nitrogen-containing substrates. Furthermore, the method was successfully applied to the synthesis of clinically active drugs on a multi-gram scale. Therefore, this work establishes a new paradigm in redox chemistry, demonstrating that a gas molecule can act as an electron-transfer catalyst, capable of performing reactions that typically require transition metals.