Organic Photoredox-Catalyzed TEMPO Disproportionation: Development of a Mild Platform for In Situ Oxoammonium Ion...

The organic photoredox-catalyzed disproportionation of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) is reported with application to the oxidation of alcohols. Although traditional methods of TEMPO disproportionation rely on strong acids, this research provides milder, sustainable conditions using visible light and a weak acid with very small loading (<0.10 mol %) of a common and affordable organic photocatalyst. This method is compatible with acid-sensitive functional groups and applicable to a broad range of primary benzylic, allylic, and aliphatic alcohols with excellent to moderate yields. Detailed mechanistic investigations were employed including Stern-Volmer luminescence quenching analysis, direct UV-vis observation of the fleeting oxoammonium ion intermediate, kinetic isotope effect analysis, and partial rate order determination of the weak acid. These results highlight the key role of the excited photocatalyst in promoting disproportionation with only a small dependence on weak acid. This novel approach expands the applications of TEMPO disproportionation using mild photoredox-catalyzed conditions to bypass the strong acid requirement for greater substrate compatibility while exploring new methodologies for in situ oxoammonium ion generation without the use of external oxidants.