Molecular Photoelectrocatalysis for Radical Reactions.

Abstract

ConspectusMolecular photoelectrocatalysis, which combines the merits of photocatalysis and organic electrosynthesis, including their green attributes and capacity to offer novel reactivity and selectivity, represents an emerging field in organic chemistry that addresses the growing demands for environmental sustainability and synthetic efficiency. This synergistic approach permits access to a wider range of redox potentials, facilitates redox transformations under gentler electrode potentials, and decreases the use of external harsh redox reagents. Despite these potential advantages, this area did not receive significant attention until 2019, when we and others reported the first examples of modern molecular photoelectrocatalysis. These studies showcased the immense synthetic potential of this hybrid strategy, which not only inherits the strengths of its parent fields but also unlocks unprecedented reactivity and selectivity, enabling challenging transformations under mild conditions while minimizing the reliance on external stoichiometric harsh oxidants or reductants.In this Account, we present our efforts to develop photoelectrocatalytic strategies that leverage homogeneous catalysts to facilitate diverse radical reactions. By integrating electrocatalysis with key photoinduced processes such as single electron transfer (SET), ligand-to-metal charge transfer (LMCT), and hydrogen atom transfer (HAT), we have established photoelectrocatalytic methods to transform substrates such as organotrifluoroborates, arenes, carboxylic acids, and alkanes into reactive radical intermediates. These intermediates subsequently engage in heteroarene C-H functionalization reactions. Importantly, under these photoelectrochemical conditions with homogeneous catalysts, reactive radical intermediates generated in the bulk solution readily participate in efficient radical reactions without undergoing further overoxidation into carbocations, a common challenge in conventional electrochemical systems.By further integration of photoelectrocatalysis with asymmetric catalysis, we have developed photoelectrochemical asymmetric catalysis (PEAC), which proves to be efficient in the enantioselective synthesis of chiral nitriles. This approach involves two relay catalytic cycles: the initial photoelectrocatalytic process engenders benzylic radicals from precursors such as alkyl arenes, benzylic carboxylic acids, and aryl alkenes, and these C-radicals are then subjected to enantioselective cyanation in a subsequent copper-electrocatalytic cycle.Within the realm of oxidative photoelectrochemical transformations, the anode serves as a crucial component for recycling or generating the photocatalyst, while the cathode promotes proton reduction. This dual functionality enables oxidative transformations via H₂ evolution, eliminating the reliance on external chemical oxidants. Furthermore, the adaptability of electrochemical systems, achieved through precise manipulation of electric current or potential, ensures meticulous control over the generation and turnover of multiple catalytic species of diverse electrochemical properties. This unique tunability allows for exceptional control over the catalytic process. As a result, despite being a relatively nascent field, molecular photoelectrocatalysis has become instrumental in enabling numerous challenging transformations that were once difficult or required harsh conditions.