Elucidation of a Parasitic Proton-Coupled Electron Transfer in a Photocatalytic Benzylic C-H Sulfonamidation

Strategies for C-H amination with sulfonamides have been dominated by stoichiometric strategies or Rh/Cu catalysis using strong oxidants, elevated temperatures, or excess equivalents of C-H partner. We developed a photocatalytic C-H amination with a bench-stable N-methoxypyridinium salt that is amenable to late-stage functionalization efforts. We discovered a competitive proton-coupled electron transfer pathway between sulfonamide and a pyridine byproduct that inhibits the reaction. Catalysis was re-established with the introduction of ZnBr₂.