Within the past decade, advancements in photochemical technology have led to powerful new transformations through the selective generation of radical species. Many of these methodologies, however, involve expensive catalysts. This work leverages the unique photoactivity of acyl azoliums to directly effect the photocatalyst-free α-acylation of protected amines. Recent developments in single-electron N-heterocyclic carbene (NHC) catalysis have shown that acyl azoliums can be irradiated to directly access a triplet excited state similar to the reactivity of benzophenone and acetophenone photosensitizers. After excitation of an acyl azolium to its triplet diradical state, abstraction of an activated hydrogen atom generates an alkyl radical in addition to the azolium-derived radical. Recombination of these species and elimination of the NHC directly affords ketones from feedstock chemicals without the need for prefunctionalization or activating groups. This methodology has been adapted to an accessible flow system that increases reaction efficiency. The design, optimization, and substrate scope of this transformation will be described.

Photocatalyst-free C–H acylation via photoexcitation of acyl azoliums

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ACS Fall 2022

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