Metal-Free Carbonylations by Photoredox Catalysis

The synthesis of benzoates from aryl electrophiles and carbon monoxide is a prime example of a transition‐metal‐catalyzed carbonylation reaction which is widely applied in research and industrial processes. Such reactions proceed in the presence of Pd or Ni catalysts, suitable ligands, and stoichiometric bases. We have developed an alternative procedure that is free of any metal, ligand, and base. The method involves a redox reaction driven by visible light and catalyzed by eosin Y which affords alkyl benzoates from arene diazonium salts, carbon monoxide, and alcohols under mild conditions. Tertiary esters can also be prepared in high yields. DFT calculations and radical trapping experiments support a catalytic photoredox pathway without the requirement for sacrificial redox partners. A metal‐free and base‐free carbonylation has been developed which affords primary, secondary, and tertiary alkyl benzoates under irradiation with visible light in the presence of eosin Y as a photocatalyst. The mechanism has been studied by spectroscopic, theoretical, and preparative methods, and appears to involve intermediate aryl and aroyl radical species as well as a light‐driven one‐electron redox cycle without any sacrificial redox partner.