Metal-Free Carbonylations by Photoredox Catalysis

Key words

carbonylation - photoredox catalysis - benzoates

Significance

Majek and Jacobi von Wangelin report a photoredox-catalyzed alkoxycarbonylation of arenediazonium salts. The reaction proceeds upon irradiation with green light, in the presence of the corresponding alcohol as solvent, an elevated pressure of CO, and the fluorescein-based dye eosin Y as photosensitizer. A variety of electron-rich and -poor arenediazonium salts served as suitable substrates; this allows the generation of the corresponding alkyl benzoates in good to excellent yields. In contrast to palladium-based carbonylation procedures, this method efficiently gave access to challenging tert-butyl benzoates.

Comment

Due to its abundant availability, carbon monoxide is an attractive C₁ building block. While established carbonylation procedures rely on transition metals as catalysts, the authors herein demonstrate the power of photoredox catalysis. Based on experimental data and DFT calculations, a mechanism was proposed that involves a single electron transfer (SET) from the photoexcited dye (EY*) to the arenediazonium ion (I), furnishing aryl radical II after denitrogenation and acyl radical III after subsequent binding to CO. An SET back to EY^+· gives rise to the highly electrophilic acylium ion IV, which generates benzoate V by addition of the alcohol.