Synergistic visible light photoredox catalysis

Within the last decade the combination of photoredox catalysis and other catalytic modes of activation has become a powerful tool for organic synthesis to enable transformations that are not possible using single catalyst systems and hence are complementary to traditional methodology. Especially reactions proceeding via synergistic catalysis where co-catalyst and photocatalyst simultaneously and separately activate different reaction partners greatly benefit from the special properties of molecules and transition metal complexes in their excited state being oxidizing and reducing in nature at the same time. Apart from allowing for the generation of radical (open-shell) reactive intermediates by SET under mild conditions from bench-stable, abundant precursors, the photocatalyst often acts to interweave the distinct catalytic cycles by interaction at multiple points of the reaction mechanism to provide overall redox-neutral processes by shuttling electrons within in this complex network of elementary reaction steps. Synergistic strategies moreover may allow to performing such reactions with enantioselectivity, while mostly the selectivity is achieved by the chiral co-catalyst. The merger of photocatalysis has been achieved with a broad range of alternative modes of catalysis including organocatalysis, Brønstedt and Lewis acid and base catalysis, enzyme catalysis as well as in the context of cross-coupling transition metal catalysis overcoming challenging steps in this methodology and therefore has contributed to considerably expand the repertoire of suitable coupling partners. While only selected examples will be discussed, this chapter will highlight various dual catalytic platforms focusing on the photocatalytically generated intermediates, but also illustrating the diverse roles of photocatalysts in the context of such synergistic multicatalysis reactions.