LED-Illuminated NMR Studies of Flavin-Catalyzed Photooxidations Reveal Solvent Control of the Electron-Transfer Mechanism

Mechanistic insights into chemical photocatalysis are mainly the domain of UV/Vis spectroscopy, because NMR spectroscopy has been limited by the type of illumination so far. An improved LED‐based illumination device can be used to obtain NMR reaction profiles of photocatalytic reactions under synthetic conditions and perform both photo‐CIDNP and intermediate studies. Flavin‐catalyzed photooxidations of alcohols show the potential of this setup. After identical initial photoreaction steps the stabilization of a downstream intermediate is the key to the further reaction mechanism and the reactivity. As a chemical photocatalyst flavin can act either as a one‐ or a two‐electron mediator when the stability of the zwitterionic radical pair is moldulated in different solvents. This demonstrates the importance of downstream intermediates and NMR‐accessible complementary information in photocatalytic reactions and suggests the control of photoorganic reactions by solvent effects. Lighting up: LED illumination setups make it possible to determine NMR reaction profiles and CIDNP studies of photocatalytic reactions. Combined NMR and UV/Vis studies on flavin‐catalyzed photooxidations revealed that even without a protein shell flavin can act as either a one‐ or two‐electron mediator depending on the solvent.