Acridinium Photo-Catalyzed 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 photo-reductants, formed easily through interactions with electron donors-has been largely overlooked and underexplored in photocatalysis. In this study, we describe the application of acridinium salt as photocatalyst for the reductive activation of iodine(III) reagents containing monofluoroacetoxy ligands. This process allows for the synthesis of oxindoles and related N-heterocycles with monofluoromethyl groups, starting from alkenyl N-arylamides. We conducted detailed mechanistic studies 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 was employed, including radical trapping experiment, UV-vis absorption spectroscopy, time-resolved photoluminescence quenching experiments, EPR spectroscopy, and density functional theory (DFT) 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.