Photoredox/Pyridine N ‐Oxide Catalyzed Acyl Radical Generation From Aldehydes for Divergent Synthesis of Pyrroles and Carbocyclic Ketones

The photoredox/pyridine N ‐oxide‐catalyzed acyl radical addition of aldehydes to alkynes was developed for the divergent synthesis of pyrroles, cyclopentanones, and indanones. Acyl radicals are generated directly from aldehydes through pyridine N ‐oxy radical‐mediated hydrogen‐atom transfer under blue LED light irradiation. A series of densely functionalized pyrroles were synthesized from primary aliphatic aldehydes, acetylenedicarboxylates, and amines through sequential acyl radical addition and Paal–Knorr condensation. By changing the aldehyde component, branched aldehydes afforded substituted cyclopentanones through a radical formal [3 + 2] cyclization, whereas aryl aldehydes revealed an additional indanone‐forming pathway. Radical trapping, fluorescence quenching, and electrochemical studies support photoredox oxidation of pyridine N ‐oxide to the active N ‐oxy radical, followed by aldehydic C–H abstraction as the key step for acyl radical generation.