(metallo)photoredox catalysis

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Asymmetric Photoredox Catalysis with Chiral-at-Rhodium Complexes

Asymmetric Photoredox Catalysis with Chiral-at-Rhodium Complexes

an electron on the non-bonding metal-centered orbital (HOMO) could be excited to the π system of the ligand framework (LUMO), defined as metal to ligand charge transfer (MLCT) (Figure 1). Subsequent intersystem crossing affords a long-lived triplet state photocatalyst which constitutes an ideal source of electrochemical potential to promote single electron transfer (SET) events with organic substrates or other reaction partners. Namely, this excited photocatalyst could either donate a high-energy electron out (termed as oxidative quenching) or accept a single electron (termed as reductive quenching). Nevertheless, the resulted catalyst at the reduced or oxidized state features the strong thermodynamic driving force back to the original ground state, thereby promoting a second reverse-path SET event. The overall process would provide radical cations or anions which could directly undergo the chemical bond formations. Alternatively, these intermediates, upon subsequent transformation, afford thermodynamically relatively stable species that engage into diverse synthetic processes. One of the most interesting aspects of the visible-light-induced photoredox catalysis is the combination with asymmetric catalysis. Namely, the forementioned photogenerated intermediates could undergo the formation of carbon-carbon or carbon-heteroatom bonds under the stereocontrol of a chiral catalyst. The photoredox catalyst and asymmetric catalyst could derive either from the identical or a separated source.
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Combining Rhodium and Photoredox Catalysis for C[BOND]H Functionalizations of Arenes : Oxidative Heck Reactions with Visible Light

Combining Rhodium and Photoredox Catalysis for C[BOND]H Functionalizations of Arenes : Oxidative Heck Reactions with Visible Light

Nevertheless, the necessity of directing groups and relatively harsh reaction conditions still limit the sustainability of this methodology and the dependency on stoichiometric amounts of an external oxidant in particular poses a major drawback of these olefination reactions. In these procedures, the intermediary metal-hydride complex that this is formed after -H-elimination, is reoxidized an oxidant. Even though improved protocols using internal oxidants have been reported in the last years they are still depended on specific functional directing moieties which limit the general applicability to synthesize complex substrate classes. [10] Based on our work in the field of photoredox catalysis [11] we questioned to what extend the recyclization of the metal catalyst in the oxidative olefination reaction can be successfully accomplished by a photoredox-based process (Scheme 1). [12-13]
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Combining Rhodium and Photoredox Catalysis for C[BOND]H Functionalizations of Arenes : Oxidative Heck Reactions with Visible Light

Combining Rhodium and Photoredox Catalysis for C[BOND]H Functionalizations of Arenes : Oxidative Heck Reactions with Visible Light

"This is the peer reviewed version of the following article: Fabry, D. C., Zoller, J., Raja, S. and Rueping, M. (2014), Combining Rhodium- and Photoredox-Catalysis for C[BOND]H- Functionalizations of Arenes: Oxidative Heck-Reactions with Visible Light. Angew. Chem. Int. Ed. 53: 10228-10231, which has been published in final form at DOI: 10.1002/anie.201400560. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.

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C-H Functionalization of Phenols Using Combined Ruthenium and Photoredox Catalysis : In Situ Generation of the Oxidant

C-H Functionalization of Phenols Using Combined Ruthenium and Photoredox Catalysis : In Situ Generation of the Oxidant

Mechanistic studies revealed that a maximization of the regeneration reaction could be obtained when small amounts of superoxide anions, formed by photoredox-generated processes, were generated which could also work as oxidant. Since only small amount of oxidants are generated, side reactions of substrate and product could not be observed that allows the use of oxidant-sensible molecules. Based on mechanistic studies we could show that using acid additives or adjusting the catalyst ratios, respectively, allow the selective synthesis of olefinated phenol derivatives in good yields. As the yields are comparable with stoichiometric, copper-based reactions, an extension to other C-H functionalizations could be possible and feasible. Besides, the combination of metal and photoredox catalysis should be transferable to other C-H functionalizations.
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Synthesis of Indoles Using Visible Light : Photoredox Catalysis for Palladium-Catalyzed C[BOND]H Activation

Synthesis of Indoles Using Visible Light : Photoredox Catalysis for Palladium-Catalyzed C[BOND]H Activation

Herein, we firstly report on the development of an indol synthesis using a combination of palladium and photoredox catalysis in the presence of visible light. We started our investigations with the intramolecular cyclization of aromatic enamides that had previously been described using Pd(OAc) 2 and three equivalents of Cu(OAc) 2 . [14] The replacement of the copper additive with just 1 mol% of the photoredox catalyst led indeed after reaction optimization to formation of 46% of the desired product 2a (Table 1, entry 1). These findings that the photoredox catalyst can be essentially used for the regeneration of the palladium catalyst encouraged us to test a variety of different palladium catalysts in the cyclization reaction revealing that Pd(OAc) 2 led to higher yields than other Pd(II) precursors. Additional acetonitrile ligands on the palladium with strong or weak coordinating counterions, e.g. Cl, or BF 4 (Table 1, entries 2 and 3) led only to insufficient formation of the desired product. When [Pd(PPh 3 ) 2 ](OAc) 2 was applied in the reaction, only decomposition of the starting material could be observed (Table 1, entry 4), showing that the reaction is highly depended on the electronic and steric character of the palladium(II) precursor. As rhodium was also reported being able to activate aryl C-H bonds, the commonly used [Cp*RhCl 2 ] 2 /AgSbF 6 catalyst system was also tested in the cyclization reaction. However, only decomposition of the starting material was observed in DMF or chlorobenzene, respectively, when both [Cp*RhCl 2 ] 2 /AgSbF 6 or only AgSbF 6 was present in the reaction (Table 1, entries 5-8).
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Synthesis of Indoles Using Visible Light : Photoredox Catalysis for Palladium-Catalyzed C[BOND]H Activation

Synthesis of Indoles Using Visible Light : Photoredox Catalysis for Palladium-Catalyzed C[BOND]H Activation

Scheme 1. Combination of photoredox and Pd(II) catalysis for the synthesis of indoles. Herein, we firstly report on the development of an indol synthesis using a combination of palladium and photoredox catalysis in the presence of visible light. We started our investigations with the intramolecular cyclization of aromatic enamides that had previously been described using Pd(OAc) 2 and three equivalents of Cu(OAc) 2 . [14] The replacement of the copper additive with just 1 mol% of the photoredox catalyst led indeed after reaction optimization to formation of 46% of the desired product 2a (Table 1, entry 1). These findings that the photoredox catalyst can be essentially used for the regeneration of the palladium catalyst encouraged us to test a variety of different palladium catalysts in the cyclization reaction revealing that Pd(OAc) 2 led to higher yields than other Pd(II) precursors. Additional acetonitrile ligands on the palladium with strong or weak coordinating counterions, e.g. Cl, or BF 4 (Table 1, entries 2 and 3) led only to insufficient formation of the desired product. When [Pd(PPh 3 ) 2 ](OAc) 2 was applied in the reaction, only decomposition of the starting material could be observed (Table 1, entry 4), showing that the reaction is highly depended on the electronic and steric character of the palladium(II) precursor. As rhodium was also reported being able to activate aryl C-H bonds, the commonly used [Cp*RhCl 2 ] 2 /AgSbF 6 catalyst system was also tested in the cyclization reaction. However, only decomposition of the starting material was observed in DMF or chlorobenzene, respectively, when both [Cp*RhCl 2 ] 2 /AgSbF 6 or only AgSbF 6 was present in the reaction (Table 1, entries 5-8).
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Asymmetric Organocatalysis and Photoredox Catalysis for the α‐Functionalization of Tetrahydroisoquinolines

Asymmetric Organocatalysis and Photoredox Catalysis for the α‐Functionalization of Tetrahydroisoquinolines

Figure 1. X-ray crystal structure analysis of 3g. Conclusions In summary, we have developed a combined catalytic system for the highly enantio- and diastereoselective α-alkylation of tetrahydroisoquinolines. In the present dual catalytic protocol ketones are activated by a chiral primary amine catalyst and tetrahydroisoquinolines are activated by a visible-light photoredox catalyst. The desired α-alkylation products were obtained in good yields, with high enantio- and diastereoselectivity. Studies on further challenging asymmetric reactions combining organo- and photoredox catalysis are currently underway in our laboratories.
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Brønsted Base Assisted Photoredox Catalysis : Proton Coupled Electron Transfer for Remote C−C Bond Formation via Amidyl Radicals

Brønsted Base Assisted Photoredox Catalysis : Proton Coupled Electron Transfer for Remote C−C Bond Formation via Amidyl Radicals

This is the peer reviewed version of the following article: Jiaqi Jia, [a] Yee Ann Ho, [a] Raoul F. Bülow [a] and Magnus Rueping* [a,b] , Brønsted base assisted photoredox catalysis: Proton coupled electron transfer for remote C-C bond formations via amidyl radicals. Chem .Eur. J. 2018, 24, 14054-14058, which has been published in final form at DOI: 10.1002/chem.201802907 . This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions. 

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Heterogeneous Visible-Light Photoredox Catalysis with Graphitic Carbon Nitride for α-Aminoalkyl Radical Additions, Allylations, and Heteroarylations

Heterogeneous Visible-Light Photoredox Catalysis with Graphitic Carbon Nitride for α-Aminoalkyl Radical Additions, Allylations, and Heteroarylations

Green Chem. 2015, 17, 715–736. (e) Zheng, Y.; Lin, L. H.; Wang, B. Wang, X. Graphitic Carbon Nitride Polymers toward Sustainable Photoredox Catalysis. Angew. Chem. Int. Ed. 2015, 54, 12868–12884. (f) Cao, S. W.; Low, J. X.; Yu, J. G.; Jaroniec, M. Polymeric Photocatalysts Based on Graphitic Carbon Nitride. Adv. Mater. 2015, 27, 2150–2176. (g) Ong, W.-J.; Tan, L.-L.; Ng, Y. H.; Yong, S.-T.; Chai, S.-P. Graphitic Carbon Nitride (g-C3N4)- Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability? Chem. Rev. 2016, 116, 7159–7329. (h) Liu, J.; Wang, H.; Antonietti, M. Graphitic Carbon Nitride “reloaded”: Emerging Applications Beyond (Photo)catalysis. Chem. Soc. Rev., 2016, 45, 2308-2326.
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C-H Functionalization of Phenols Using Combined Ruthenium and Photoredox Catalysis : In Situ Generation of the Oxidant

C-H Functionalization of Phenols Using Combined Ruthenium and Photoredox Catalysis : In Situ Generation of the Oxidant

"This is the peer reviewed version of the following article: Fabry, D. C., Ronge, M. A., Zoller, J. and Rueping, M. (2015), C[BOND]H Functionalization of Phenols Using Combined Ruthenium and Photoredox Catalysis: In Situ Generation of the Oxidant. Angew. Chem. Int. Ed., 54: 2801- 2805, which has been published in final form at DOI: 10.1002/anie.201408891. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self- Archiving.

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Dehydrogenative Aromatization and Sulfonylation of Pyrrolidines : Orthogonal Reactivity in Photoredox Catalysis

Dehydrogenative Aromatization and Sulfonylation of Pyrrolidines : Orthogonal Reactivity in Photoredox Catalysis

In recent years, visible-light photoredox catalysis has evolved as an alternative to conventional multistep routes. The use of readily available photosensitizers, catalysts, light sources and the mild reaction conditions allows the efficient generation of reactive intermediates which lead to a diverse range of synthetically useful molecules from feedstock substrates. In addition, new photoredox protocols circumvented the use of stoichiometric oxidants resulting in more sustainable synthetic transformations. [7] However, most of these protocols are based on the combination of a photoredox
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Asymmetric Catalysis with Chiral-at-Metal Complexes: From Non-Photochemical Applications to Photoredox Catalysis

Asymmetric Catalysis with Chiral-at-Metal Complexes: From Non-Photochemical Applications to Photoredox Catalysis

A plausible mechanism in which photoredox catalysis intertwines with asymmetric catalysis is shown in Figure 48. Herein, the catalysis is initiated by the coordination of 2-acyl imidazoles (24) to the iridium catalyst in a bidentate fashion (intermediate I), followed by the formation of a nucleophilic enolate iridium(III) complex (intermediate II) upon deprotonation. The subsequent chirality generating key step constitutes the exergonic addition of a photo-reductively generated electrophilic radical to the electron rich metal-coordinated enolate double bond, thereby affording an iridium-coordinated ketyl radical (intermediate III). Oxidation of this ketyl intermediate to a ketone by single electron transfer provides the iridium-coordinated product (complex IV), which is released upon exchange against unreacted starting material, followed by a new catalytic cycle. The single electron transfer either regenerates the iridium(III) photosensitizer or leads to the reduction of another arganobromide substrate, thereby initiating a chain reaction. Proposed key intermediate which uniquely connects the asymmetric catalysis with the photoredox cycle is the iridium(III) enolate complex II, which not only provides the crucial asymmetric induction in the catalysis cycle and but at the same time serves as the
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Reductive Umpolung of Carbonyl Derivatives with Visible-Light Photoredox Catalysis: Direct Access to Vicinal Diamines and Amino Alcohols via α-Amino Radicals and Ketyl Radicals

Reductive Umpolung of Carbonyl Derivatives with Visible-Light Photoredox Catalysis: Direct Access to Vicinal Diamines and Amino Alcohols via α-Amino Radicals and Ketyl Radicals

In conclusion, we have developed a direct photocatalytic method for the synthesis of unsymmetric 1,2-diamines, [6d] with reductive SET umpolung of an aldimine as the key step. Moreover, we also applied the concept of reductive umpolung to aldehyde–aniline couplings. Ketyl radicals derived from aldehydes were thus coupled in an intermolecular fashion by photoredox catalysis for the first time. Biologically active amino alcohols and 1,2-diamines can thus be directly synthe- sized from simple starting materials in a straightforward library format. Remarkably, a wide range of functional groups as well as amino acids were tolerated under the mild conditions of our photoredox process.
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Merging Visible Light Photoredox Catalysis with Metal Catalyzed C–H Activations: On the Role of Oxygen and Superoxide Ions as Oxidants

Merging Visible Light Photoredox Catalysis with Metal Catalyzed C–H Activations: On the Role of Oxygen and Superoxide Ions as Oxidants

Account, our initial work focused on the three most representative metals (Rh, Pd, Ru) for C −H olefination reactions. These transformations were previously achieved if large amounts of Cu(II) salts were applied. However, we successfully demon- strated that either visible light homogeneous photoredox catalysts or heterogeneous semiconductor-based catalysts are perfectly suitable substitutes. Mechanistic studies revealed that the photoredox process is independent from the C −H activation reaction. Moreover, we were able to show that the photoredox catalysts generate and carefully release superoxide radicals that can independently work as oxidants as demonstrated by the use of potassium superoxide. While more detailed mechanistic studies need to be undertaken, the successful development of our dual catalysis concept, consisting of combined visible light photoredox catalysis and metal catalyzed C −H functionalization, provides many new opportunities for further explorations. ■ AUTHOR INFORMATION
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Reductive Umpolung of Carbonyl Derivatives with Visible-Light Photoredox Catalysis: Direct Access to Vicinal Diamines and Amino Alcohols via α-Amino Radicals and Ketyl Radicals

Reductive Umpolung of Carbonyl Derivatives with Visible-Light Photoredox Catalysis: Direct Access to Vicinal Diamines and Amino Alcohols via α-Amino Radicals and Ketyl Radicals

In conclusion, we have developed a direct photocatalytic method for the synthesis of unsymmetric 1,2-diamines, [6d] with reductive SET umpolung of an aldimine as the key step. Moreover, we also applied the concept of reductive umpolung to aldehyde–aniline couplings. Ketyl radicals derived from aldehydes were thus coupled in an intermolecular fashion by photoredox catalysis for the first time. Biologically active amino alcohols and 1,2-diamines can thus be directly synthe- sized from simple starting materials in a straightforward library format. Remarkably, a wide range of functional groups as well as amino acids were tolerated under the mild conditions of our photoredox process.
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Photoredox-Catalyzed Ketyl–Olefin Coupling for the Synthesis of Substituted Chromanols

Photoredox-Catalyzed Ketyl–Olefin Coupling for the Synthesis of Substituted Chromanols

■ CONCLUSIONS In conclusion, we report a ketyl −olefin coupling for the preparation of substituted 3-benzylchroman-4-ols, promoted by visible light photoredox catalysis. Importantly, we uncovered trialkylamines as a cheap and readily available alternative to the previously reported electron −proton donor system consisting of Hantzsch ester/Brønsted acid. By employing the trialkyl- amine/photocatalyst system, we have been able to develop an e fficient intramolecular ketyl−olefin and ketyl−alkyne coupling employing readily prepared substrates. The formal hydro- acylation protocol provides the chromanol derivatives in good yield under mild reaction conditions and with low catalyst Scheme 1. Photoredox-Catalyzed Reduction of Aldehydes
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Aufbau komplexer Heterocyclen mittels Photo-Tandem-Katalyse und Photoredox-induzierten Radikalkaskaden

Aufbau komplexer Heterocyclen mittels Photo-Tandem-Katalyse und Photoredox-induzierten Radikalkaskaden

55 Zur Untersuchung des Einflusses einer möglichen Stabilisierung des Carbokations 127 auf die Produktselektivität wurden mit Brommalonsäuredinitril 114b und dem Mononitril- Derivat 114c zwei modifizierte  -Brommalonsäurederivate unter den Reaktions- bedingungen der photoredox-induzierten Cyclisierung umgesetzt. Durch ihre im Vergleich zu Brommalonat 114a ähnlichen elektronischen Eigenschaften, aber schlechtere Fähigkeit zur Stabilisierung eines Kations durch Ion-Dipol-Wechselwirkung, wurde ein geringerer Anteil aromatisierten Produktes im Rohprodukt erwartet. Allerdings kam es bei ihrer Umsetzung zu einer vollständigen Defunktionalisierung, sodass hieraus keine Rückschlüsse über die Rolle der Stabilisierung von Kation 127 für die Produktselektivität der photoredox-vermittelten 6-exo-trig-Cyclisierung gezogen werden konnten (Schema 57).
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Catalysis with Palladium(I) Dimers

Catalysis with Palladium(I) Dimers

Franziska Schoenebeck received her PhD in 2008 from the University of Strathclyde (Glasgow, UK) working with Prof. J. A. Murphy. After a postdoctoral stay with Prof. K. N. Houk at UCLA, she started her independent career at the ETH Zrich in 2010. In 2013 she was appointed Professor at the RWTH Aachen University, where she was promoted to Full Professor and Chair in 2016. Her research is based at the interface of synthetic organic, mechanistic, and com- putational chemistry with an emphasis in homogeneous metal catalysis.

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Orthogonal Nanoparticle Catalysis with Organogermanes

Orthogonal Nanoparticle Catalysis with Organogermanes

However, despite the many publications on nanoparticle catalysis, to date, there is no precedence of unambiguously unique and orthogonal reactivity of nanoparticles compared to homogeneous molecular or heterogeneous bulk catalysts in organic transformations. Such insights would be of utmost importance as there is a high demand for innovative and orthogonal synthetic strategies. Especially, a modular and straightforward access to richly functionalized biaryl motifs is in considerable demand, owing to their widespread abun- dance in drugs, materials, or privileged catalysts. [10] In this

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Protein engineering for biohybrid catalysis

Protein engineering for biohybrid catalysis

In a recent report by Pellizzoni et al., the streptavidin scaffold was complemented with naturally occurring structural motifs as second coordination sphere elements 92 . Two families of variants were generated via the introduction of either extended 2D structural motifs (α-helices or β- strands, 24-60 amino acids) or short naturally occurring loops (5-12 amino acids) in streptavidin loops around the synthetic catalyst. 40% of the 2D-expanded variants and 75% of variants with short loops remained soluble and correctly folded after overexpression. To determine the effect of the structural motifs on activity and selectivity, a biotinylated Ir piano stool complex, GH-type catalyst and naphthalenediimide surface were anchored within the engineered variants for asymmetric transfer hydrogenation (ATH), ring-closing metathesis and anion-π catalysis, respectively. Among expressible variants, biotin-binding was not significantly affected by the structural motifs. Upon analysis of the catalyzed reactions, the authors identified streptavidin loop 3/4 and the C-terminus (due to the tetrameric nature of streptavidin) as having the greatest potential for impact (e.g. seven-fold higher activity of ATHase compared to wild type variant). The authors expect this study to be a good starting point for future directed evolution with promising structural motif-expanded variants 92 .
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