Photochemical Insights on Acyl Azolium Salts Enable the Design of a Tandem Hydrogen Atom Transfer/Halogen Atom Transfer Acylation of Alkyl Bromides and Chlorides.

Acyl azolium salts have been recently described as good HAT mediators in their excited state-similarly to diaryl ketones-and good acylating reagents, with the beneficial property of enabling incorporation of the acyl residue into the final molecule. Nevertheless, despite significant synthetic efforts in the field, knowledge and understanding of the key reactive intermediates─especially through spectroscopic investigations─remain elusive. Herein, we present a photochemical study of the reactivity of acyl azolium salts that comprises the detection and characterization of the triplet excited state and the decisive ketyl radical intermediate. Moreover, this mechanistic insight allowed us the development of an alternative method based on a silane-mediated tandem HAT/XAT activation strategy that enables not only the acylation of alkyl bromides but also the acylation of the more challenging alkyl chlorides. The method has proven to be efficient, regardless of the electronic properties of the acyl azolium or the substituents present in the alkyl halide. Furthermore, its robustness has been proven through the functionalization of natural product derivatives either as acyl azolium or alkyl bromide derivatives.