Photocatalytic phosphine-mediated water activation for radical hydrogenation

The chemical activation of water would allow this earth-abundant resource to be transferred into value-added compounds, and is a topic of keen interest in energy research^1,2. Here, we demonstrate water activation with a photocatalytic phosphine-mediated radical process under mild conditions. This reaction generates a metal-free PR₃-H₂O radical cation intermediate, in which both hydrogen atoms are used in the subsequent chemical transformation through sequential heterolytic (H⁺) and homolytic (H^•) cleavage of the two O-H bonds. The PR₃-OH radical intermediate provides an ideal platform that mimics the reactivity of a 'free' hydrogen atom, and which can be directly transferred to closed-shell π systems, such as activated alkenes, unactivated alkenes, naphthalenes and quinoline derivatives. The resulting H adduct C radicals are eventually reduced by a thiol co-catalyst, leading to overall transfer hydrogenation of the π system, with the two H atoms of water ending up in the product. The thermodynamic driving force is the strong P=O bond formed in the phosphine oxide by-product. Experimental mechanistic studies and density functional theory calculations support the hydrogen atom transfer of the PR₃-OH intermediate as a key step in the radical hydrogenation process.