Photocatalysis in Organic Synthesis – Past, Present, and Future

Abstract

Abstract:The use of visible light as a catalyst in organic reactions has developed greatly in the last decade as a result of the increasing urge to implement energy-efficient and green processes and through the availability of light-emitting diodesThe use of visible light to promote organic transformations has developed enormously over the last decade. Several factors have come together to make this possible: An increased awareness for optimizing the energy efficiency of reactions even on the laboratory scale identifies visible light as an ideal reagent.[1]Technological progress and broad commercial availability of light-emitting diodes that are able to provide high-intensity visible light in a narrow wavelength range for all colors have made ideal cheap and energy-efficient light sources available for photocatalysis. At the same time, advances in flow chemistry [2] and the use of transparent flow reactors in photocatalysis has helped to overcome challenges in the upscaling and reproducibility of photochemical reactions, which are important aspects for any larger-scale industrial application.Classical photochemistry using ultraviolet light for the direct excitation of organic compounds is an established and highly developed field of chemistry with a historical development spanning more than a century.[3] However, the discipline has remained, in the perception of many organic chemists, a special technique that is not easy to apply. This perception has changed with the use of visible light, photoredox catalysts, and sensitizers. The reaction setup in the synthesis lab does not differ much from that of typical thermal chemistry, with the exception of the light source. As visible light has lower energy than ultraviolet irradiation usually applied in classical photochemistry, in many cases the reactions are selective, more predictable, and easier to control. Many visible-light-mediated [a]

Structure‐Catalytic Activity in a Series of Push‐Pull Dicyanopyrazine/Dicyanoimidazole Photoredox Catalysts

“…The ground state HOMO‐LUMO gaps can be tuned within a range of 2.06 to 2.39 V. Variation of the electron releasing part of push‐pull pyrazine derivatives affected mostly the reduction potential of their excited states. DPZ catalysts 1–7 showed E red * within a relatively wide range of 1.42 to 2.07 V. For instance, catalyst 7 with E red * =2.07 V belongs to one of the strongest oxidants and reaches the current limit for visible‐light driven oxidations (2.1 V) The data obtained by the DFT calculations correlates tightly with the experimental ones and further support the aforementioned outcomes.…”

“…Variation of the electron releasing part of push‐pull pyrazine derivatives affected mostly the reduction potential of their excited states. DPZ catalysts 1–7 showed E red * within a relatively wide range of 1.42 to 2.07 V. For instance, catalyst 7 with E red * =2.07 V belongs to one of the strongest oxidants and reaches the current limit for visible‐light driven oxidations (2.1 V) …”

Anthraquinones as Photoredox Catalysts for the Reductive Activation of Aryl Halides

“…[1][2][3][4][5][6][7][8][9] Ru-and Ir-based coordination complexes have received enormous attention because of their excellent visible-light-harvesting properties, modest to extremely high oxidation and reduction potentials, relatively long excited-state lifetimes, and reasonably good chemical and photostabilities under synthetic oxidative and reductive conditions. [2] In addition, considerable efforts have been made to develop metal-free photoredox catalytic methods with organic dyes [8,[10][11][12][13][14] such as eosin Y [10,11] or rhodamine derivatives [13,14] and organic heterogeneous photocatalysts [15] for synthetic transformations.…”

Photochemical Functionalization of Helicenes

“…In the past decade, visible‐light‐mediated photoredox catalysis has evolved into a valuable method for organic synthesis, and changed the way we activate chemical bonds for chemical transformations . Recently, we and others have functionalized arenes and heteroarenes by visible‐light‐mediated photoredox activation of C(sp 2 )‐halogen or C(sp 2 )‐hydrogen bonds .…”

“…[27] In the past decade, visible-light-mediated photoredox catalysis has evolved into a valuable method for organic synthesis, and changed the way we activate chemical bonds for chemical transformations. [28][29][30][31][32][33][34] Recently, we and others have functionalized arenes and heteroarenes by visible-light-mediated photoredox activation of C(sp 2 )-halogen [35][36][37][38] or C(sp 2 )-hydrogen bonds. [33,39] The former transformation, depending on the choice of the substrate and on the reaction conditions, proceed through single-electron reduction using polypyridyl transition-metal complexes, [40] simple organic dyes, [35,36,41] or using potassium tert-butoxide as a base under ultraviolet (UV, l Ex 350 nm) irradiation (S RN 1 reactions).…”