Photochemical reduction of acylimidazolium salts

Light-mediated methodologies for the reduction of acylazolium species generated during <i>N</i>-heterocyclic carbene (NHC)-catalyzed reactions have been developed. Employing the simple amine, DIPEA, as the terminal reductant, products resulting from overall 2-electron or 4-electron-reduction processes could be obtained using either a photocatalytic approach under blue light irradiation or directly under UV-A light irradiation without an additional photocatalyst. Moreover, under the same photocatalyst-free conditions, UV-A-light-mediated reduction could be achieved using triethylsilane as the only reductant with subsequent desilylation and NHC elimination with fluoride delivering the corresponding aldehyde product.