Accelerating visible-light photoredox catalysis in continuous-flow reactors

This chapter gives an overview of the most important examples of visible-light photoredox catalysis in continuous-flow reactors based on the heterogeneity of the reaction mixture (homogeneous, gas-liquid, and solid-liquid). Photocatalysis in a homogeneous single phase can be straightforwardly converted to a continuous-flow protocol. In a continuous-flow system, the mixing efficiency is well controlled by the large and well-defined surface-to-volume ratios and the reaction times are defined by the flow rates. The efficient generation of reactive radical species in a gas-liquid continuous-flow system was demonstrated by Noel et al. for the trifluoromethylation of five-membered heterocycles. For gas-liquid reactions, it is important to maximize the interfacial area to avoid mass transfer limitations. Immobilization of photocatalysts in continuous-flow reactors provides a number of advantages with regard to catalyst recuperation and reuse. Transferring the chemistry to continuous flow resulted in a substantial acceleration of photocatalytic oxidation protocol.