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, economic and environmentally friendly reagent, is potential to act as the hydrogen source in reduction reactions. Since water is difficult to act as a reductant itself, photocatalysis has been explored to provide milder conditions for electron donation. This review concentrates on the advances in photoredox-catalyzed H₂O-incorporated reductive addition reaction of unsaturated bonds, with H₂O acting as a hydrogen source. Typical mechanism on water activation and utilization would be discussed by categorizing strategies into coordination activation, oxidative addition activation, and indirect activation of water by substrate activation. The mechanistic view provides descriptions on current challenges and prospective on future explorations.