Identifying Radical Pathways for Cu(I)/Cu(II) Relay Catalyzed Oxygenation via Online Coupled EPR/UV–Vis/Near‐IR Monitoring.

Copper‐catalyzed C─H oxygenation has drawn considerable attention in mechanistic studies. However, a comprehensive investigation combining radical pathways with a metal‐catalytic cycle is challenged by the intricate organic radicals and metallic intermediates. Herein, an online coupled EPR/UV–vis/near‐IR detecting method is developed to simultaneously monitor both reactive radical species and copper complex intermediates during the reaction. Focusing on copper‐catalyzed phenol oxygenation with cumene hydroperoxide, the short‐lived alkylperoxyl radical (EPR signal at g = 2.0143) as well as the unexpected square planar Cu(II)‐alkoxyl radical complex (near‐IR signal at 833 nm) are unveiled during the reaction, in addition to the observable phenoxyl radical in EPR, quinone product in UV–vis, and Cu(II) center in EPR. With a comprehensive picture of diverse intermediates evolving over the same timeline, a novel Cu(I)/Cu(II) proposed relay‐catalyzed sequential radical pathway. In this sequence, Cu(II) activates hydroperoxide through Cu(II)‐OOR into the alkylperoxide radical, while the reaction between Cu(I) and hydroperoxide leads to Cu(II)(•OR)OH with high H‐atom abstracting activity. These results provide a thorough understanding of the Cu(I)/Cu(II) relay catalysis for phenol oxygenation, setting the stage for mechanistic investigations into intricate radical reactions promoted by metallic complexes. [ABSTRACT FROM AUTHOR]