Photoredox-catalyzed reduction of unsaturated compounds using water as the hydrogen source

Reduction of unsaturated compounds is a practical approach for functional group modification in organic synthesis. Water, as a readily obtainable, economical, and environmentally friendly reagent, has the potential to act as a hydrogen source in reduction reactions. Since water is difficult to serve as a reductant itself, photocatalysis has been explored to provide milder conditions for electron donation. This review concentrates on advances in photoredox-catalyzed H2O-incorporated reductive addition reactions of unsaturated bonds, with water acting as a hydrogen source. Typical mechanisms of water activation and utilization are discussed by categorizing strategies into coordination activation, oxidative addition activation, and indirect activation of water by substrate activation. The mechanistic view provides descriptions of current challenges and prospects for future explorations.