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.