Shining Light on Copper: Unique Opportunities for Visible-Light-Catalyzed Atom Transfer Radical Addition Reactions and Related Processes

Visible-light photoredox catalysis offers exciting opportunities to achieve challenging carbon-carbon bond formations under mild and ecologically benign conditions. Desired features of photoredox catalysts are photostability, long excited-state lifetimes, strong absorption in the visible region, and high reduction or oxidation potentials to achieve electron transfer to substrates, thus generating radicals that can undergo synthetic organic transformations. These requirements are met in a convincing way by Ru-II(phenanthroline)3- and Ir-III(phenylpyridine)3-type complexes and, as a low-cost alternative, by organic dyes that offer a metal-free catalyst but suffer in general from lower photostability. Cu-I(phenanthroline)(2) complexes have been recognized for more than 30 years as photoresponsive compounds with highly negative Cu(I)* -> Cu(II) oxidation potentials, but nevertheless, they have not been widely considered as suitable photoredox catalysts, mainly because their excited lifetimes are shorter by a factor of 5 to 10 compared with Ru(II) and Ir(III) complexes, their absorption in the visible region is weak, and their low Cu(II) -> Cu(I) reduction potentials might impede the closure of a catalytic cycle for a given process. Contrasting again with (RuL3)-L-II and (IrL3)-L-III complexes, (CuL2)-L-I assemblies undergo more rapid ligand exchange in solution, thus potentially reducing the concentration of the photoactive species. Focusing on atom transfer radical addition (ATRA) reactions and related processes, we highlight recent developments that show the utility of Cu-I(phenanthroline)(2) complexes as photoredox catalysts, demonstrating that despite their short excited-state lifetimes and weak absorption such complexes are efficient at low catalyst loadings. Moreover, some of the inherent disadvantages stated above can even be turned to advantages: (1) the low Cu(II) -> Cu(I) reduction potential might efficiently promote reactions via a radical chain pathway, and (2) the tendency for ligand exchange in (CuL2)-L-I assemblies allows the efficient synthesis of heteroleptic (CuLL)-L-I' complexes to tune the steric and electronic properties and also might coordinate and thus activate substrates in the course of a reaction in addition to electron transfer. Moreover, new photoredox cycles have also been discovered beyond the visible-light-induced Cu(I)* -> Cu(II) electron transfer that is arguably best known: examples of the Cu(II)* -> Cu(I) and Cu(I)* -> Cu(0) transitions have been realized, greatly broadening the potential for copper-based photoredox-catalyzed transformations. Finally, a number of organic transformations that are unique to Cu(I) photoredox catalysts have been discovered.