Gold catalysis under visible light

Visible light has been used to induce diverse organic transformations, such as isomerization, rearrangement and cycloaddition reactions. However, since many organic molecules cannot absorb in this area of the spectrum, the use of photocatalysts has been widely investigated. These compounds are excited by direct absorption of visible light and induce a single electron transfer to the substrate (photoredox catalysis), generating a radical that can further react to generate valuable compounds. Additionally, the excited photocatalyst can cause the indirect excitation of the substrate through photosensitization (energy transfer catalysis). The merger of photoredox catalysis or energy transfer catalysis with transition metal catalysis also known as metallaphotoredox catalysis has recently known intense developments. Among the transition metals employed in this dual type of catalysis, gold has received significant attention. Gold complexes are highly effective catalysts for the electrophilic activation of multiple carbon–carbon bonds towards nucleophilic addition, generating organogold intermediates that usually undergo protodemetalation. The merger of gold catalysis with photocatalysis enables the formation of carbon–carbon bonds as an alternative to the traditional protodemetalation pathway. Furthermore, the exceptional photophysical properties of dinuclear gold complexes, such as [Au2(µ-dppm)2]2+ (dppm = bis(diphenylphosphino)methane), make them suitable photoredox catalysts in diverse reactions. Finally, gold(i) complexes equipped with adequate ligands have proven to be versatile photocatalysts for energy transfer catalysis.