Photoredox catalysis with visible light.

The increasing need for more efficient synthetic methods and sustainable processes can be seen as a major driving force for new inventions; it also stimulates the creative rethinking of known concepts, which, in turn, will lead to the development of innovative chemistry. For example, starting from the challenge to mimic and understand enzymatic transformations, organocatalysis has now become an important tool in modern organic synthesis with new reactivity that complements that of enzyme and metal catalysis. As early as the beginning of the 20th century, photochemistry had already attracted the attention of (organic) chemists. But recent years have seen broader interest in photochemical transformations because of the generally mild conditions required for substrate activation—ideally light alone—and their suitability for “green reactions”. While classical photochemical steps have found notable applications in synthesis and the direct transformation of light into electric energy (photovoltaics) can already be considered a highly developed research field, efforts in photocatalysis have mainly targeted the development of artificial photosynthesis systems for the conversion of solar energy into storable chemical fuels. However, despite the obvious, practical advantages of visible light as an “infinitely available” promoter for chemical synthesis, the simple inability of most organic molecules to absorb light in the visible range of the spectrum has greatly limited the potential applications of photochemical reactions. One major strategy to address this drawback and to develop new efficient processes using visible light is the use of photosensitizers and photocatalysts. Upon irradiation, molecules are converted into their photoexcited states, which are chemically more reactive because of the significantly altered electronic distribution. Apart from following various common physical decay pathways, these photoexcited states can undergo chemical “deactivation” processes, which are either unimolecular and correspond to classical photochemical transformations (isomerizations, rearrangements, etc.) or proceed in a bimolecular manner. The interactions with other species range from bimolecular reactions such as photocycloadditions to quenching processes. Here, the most important pathways are energy-transfer reactions and electron-transfer reactions; both play a crucial role as indirect initiators for all types of photocatalytic reactions. Photoredox catalysis relies on the general property of excited states to be both more easily reduced as well as more easily oxidized than their corresponding ground states, and so the photocatalyst can serve either as an electron donor or an electron acceptor to be regenerated in the catalytic cycle (Scheme 1). The photocatalyst undergoes two distinct electron-transfer steps; both the “quenching” and the “regenerative” electron transfer can be productive with respect to a desired chemical transformation. Ideally, the two electrontransfer processes are connected by the substrates or intermediates of the catalyzed reaction and therefore do not require any sacrificial electron donor or acceptor.