Radical Chemistry with the Hydrogen Atom Through Water Activation

ongoing

"Water activation, which allows the transfer of universally abundant hydrogen into value added compounds, is an important research field in modern science. This task has been realized mainly by using transition-metal-based systems. Herein we will use a conceptually novel mild water activation strategy which proceeds through a photocatalytic phosphine-mediated radical process. The active species in these processes is a metal-free R3P-OH2 radical cation intermediate where both hydrogen atoms are used in the following chemical transformation through sequential heterolytic (H+) and homolytic (H•) cleavage of the two O-H bonds. The R3P-OH radical intermediate provides an ideal platform to mimic the reactivity of a ""free"" hydrogen atom that can be directly transferred to various π-systems to give H-adduct C-radicals, which are eventually reduced by a thiol cocatalyst leading to overall transfer hydrogenation of π-systems, with the two H-atoms of water ending up in the product. The driving force is the strong P-O bond formed in the phosphine oxide byproduct. Hydrogenation of alkenes, arenes and hetero(arenes) will be investigated, also addressing stereoselective reductions using chiral H-donors. Deuteration and tritiation with D2O and T2O as formal reductants will be explored. Experimental studies will be supported by DFT calculations throughout the studies. Polar effects exerted by the H-donors will be studied, which will allow the design of more complex intramolecular and intermolecular reductive cascade reactions comprising C-C bond forming steps. Reactivity of the novel R3P-OH radicals towards various functionalities will be systematically investigated and the reactive functional groups then incorporated into cascade reactions. Atom-economic radical H-transfer isomerization reactions that proceed with P-based catalysts will be developed. Finally, photoactive P-compounds will be used as mediators for water activation in hydrogen atom transfer radical chemistry. "