Acridinium Photocatalyzed Monofluoromethyl Radical Cascade Reaction of Alkenes with Iodine(III) Reagent: A Mechanistic Study

Acridinium salts are well‐known and highly effective organic photoredox catalysts, particularly recognized for their strong oxidizing properties. While they are widely used as photo‐oxidants, their potential as photoreductants, formed easily through interactions with electron donors, has been largely overlooked and underexplored in photocatalysis. In this study, the application of an acridinium salt as a photocatalyst for the reductive activation of iodine(III) reagent containing monofluoroacetoxy ligands is described. This process allows for the synthesis of oxindoles and related N ‐heterocycles with monofluoromethyl groups, starting from alkenyl N ‐arylamides. Detailed mechanistic studies are conducted to gain insight into the in situ formation of the active acridine radical species, catalyst species interconversion, and the photocatalytic mechanism underlying the monofluoromethyl radical cascade reaction of alkenyl N ‐arylamides. A wide range of mechanistic tools is employed, including radical trapping experiments, UV–Vis absorption spectroscopy, time‐resolved photoluminescence quenching experiments, electron paramagnetic resonance spectroscopy, and density functional theory calculations. The synthetic utility of this protocol is demonstrated through a substrate scope study, which highlights the efficient access to oxindoles and other polycyclic heterocycles featuring monofluoromethyl units which are widely recognized for their biological significance.