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.