Redox-Neutral Photocatalytic C−H Carboxylation of Arenes and Styrenes with CO2

Schmalzbauer, Matthias; Svejstrup, Thomas D.; Fricke, Florian; Brandt, Peter; Johansson, Magnus J.; Bergonzini, Giulia; König, Burkhard

doi:10.1016/j.chempr.2020.08.022

Cited by 84 publications

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References 82 publications

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“… 2020 6 2658 2672 . 3 A highly reducing photocatalyst can activate suitable aromatic compounds to generate the corresponding radical-anion. This way, nucleophilic C(sp 2 )- instead of C(sp 3 )-centers can be generated in absence of a stoichiometric reductant.…”

Section: Key Referencesmentioning

confidence: 99%

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Strategies for the Photocatalytic Generation of Carbanion Equivalents for Reductant-Free C–C Bond Formations

Donabauer

König

2020

Acc. Chem. Res.

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Conspectus The use of photocatalysis in organic chemistry has encountered a surge of novel transformations since the start of the 21st century. The majority of these transformations are driven by the generation and subsequent reaction of radicals, owing to the intrinsic property of common photocatalysts to transfer single electrons from their excited state. While this is a powerful and elegant method to develop novel transformations, several research groups recently sought to further extend the toolbox of photocatalysis into the realm of polar ionic reactivity by the formation of cationic as well as anionic key reaction intermediates to furnish a desired product. Our group became especially interested in the photocatalytic formation of anionic carbon nucleophiles, as the overall transformation resembles classical organometallic reactions like Grignard, Barbier, and Reformatsky reactions, which are ubiquitous in organic synthesis with broad applications especially in the formation of valuable C–C bonds. Although these classical reactions are frequently applied, their use still bears certain disadvantages; one is the necessity of an (over)stoichiometric amount of a reducing metal. The reducing, low-valent, metal is solely applied to activate the starting material to form the organometallic carbanion synthon, while the final reaction product does generally not contain a metal species. Hence, a stoichiometric amount of metal salt is bound to be generated at the end of each reaction, diminishing the atom economy. The use of visible light as mild and traceless activation agent to drive chemical reactions can be a means to arrive at a more atom economic transformation, as a reducing metal source is avoided. Beyond this, the vast pool of photocatalytic activation methods offers the potential to employ easily available starting materials, as simple as unfunctionalized alkanes, to open novel and more facile retrosynthetic pathways. However, as mentioned above, photocatalysis is dominated by open-shell radical reactivity. With neutral radicals showing an intrinsically different reactivity than ionic species, novel strategies to form intermediates expressing a polar behavior need to be developed in order to achieve this goal. In the last couple of years, several methods toward this aim have been reported by our group and others. This Account aims to give an overview of the different existing strategies to photocatalytically form carbon centered anions or equivalents of those in order to form C–C bonds. As the main concept is to omit a stoichiometric reductant source (like a low-valent metal in classical organometallic reactions), only redox-neutral and reductant-free transformations were taken into closer consideration. We present selected examples of important strategies and try to illustrate the intentions and concepts behind the methods developed by our group and others.

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Section: Key Referencesmentioning

confidence: 99%

“…Gratifyingly, the presence of CO 2 as electrophile led to the formation of the desired carboxylic acid ( Figure 11 ). 3 …”

Section: Radical Anions As Nucleophilesmentioning

confidence: 99%

Strategies for the Photocatalytic Generation of Carbanion Equivalents for Reductant-Free C–C Bond Formations

Donabauer

König

2020

Acc. Chem. Res.

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“…König et al . have recently reported the redox‐neutral photocatalytic carboxylation of a range of arenes and styrenes using the photoreductant 2,3,6,7‐tetramethoxyanthracen‐9(10H)‐one (TMAH) [46] . Demonstrated examples included several benzothiophenes and benzofurans (Scheme 63).…”

Section: Other Functionalisation (Borylation Phosporylation Carbamomentioning

confidence: 99%

“…Under the standard conditions, hydroxybenzothiphene 263 was carboxylated in the 7‐position to give 264 in 99 % yield (Scheme 64). [46] …”

Section: Other Functionalisation (Borylation Phosporylation Carbamomentioning

confidence: 99%

Recent Developments in C−H Functionalisation of Benzofurans and Benzothiophenes

2020

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Benzofurans and benzothiophenes are important pharmaceutical motifs, appearing in a broad range of small molecule therapeutic classes. Often overlooked by synthetic methodologists in favour reactions of the analogous indole bicyclic system, there is nevertheless a plurality of approaches to effecting benzofuran and benzothiophene C−H functionalisations. In this review, we summarise progress in this area over the past five years, including 1) alkylations, 2) arylations and heteroarylations, 3) carboxylations, carbamoylations, and C‐heteroatom bond formations and 4) cyclisations.

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“…Erst kürzlich wurde gezeigt, dass das stark reduzierende und im sichtbaren Bereich absorbierende Tetramethoxyanthrolat TMA − (

E_{o x}^{*}

=−2.92 V vs. SCE) genutzt werden kann, um (Hetero‐)Arene und Styrole mit blauem Licht direkt in die entsprechenden Radikalanionen umzusetzen [86] . Der mit solchen elektronenreichen Spezies verbundene nukleophile Charakter wurde für C‐H‐Carboxylierungen mit CO 2 ‐Gas genutzt, um die entsprechenden aromatischen Carbon‐ bzw.…”

Section: Anionen Als Photokatalysatorenunclassified

Photoangeregte Anionen in organischen Reaktionen

2020

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Die Durchführung chemischer Reaktionen mit Licht erlaubt es, die eingestrahlte Energie selektiv auf bestimmte Moleküle zu fokussieren. Viele Reaktionen, an denen elektronisch angeregte Spezies beteiligt sind, verlaufen über radikalische Zwischenstufen, die neue, einzigartige Reaktionswege ermöglichen und somit bekannte Syntheserouten zur Umwandlung organischer Moleküle erweitern. Der direkte Einsatz nicht funktionalisierter bzw. reaktionsträger Verbindungen ist ein erstrebenswertes Ziel, um chemische Prozesse nachhaltiger und atomökonomischer gestalten zu können. Es wurde gezeigt, dass angeregte Anionen mit gepaarten Elektronen enorme Potentiale in Elektronentransferreaktionen überwinden können und dass häufig ungewöhnliche Reaktivitäten im angeregten Zustand beobachtbar sind. In diesem Aufsatz fassen wir kurz die Eigenschaften der anionischen Photochemie zusammen, heben die wegweisenden Arbeiten auf diesem Gebiet hervor und diskutieren aktuelle Entwicklungen, die durch photoangeregte anionische Spezies in der organischen Synthese erzielt wurden.

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Redox-Neutral Photocatalytic C−H Carboxylation of Arenes and Styrenes with CO2

Cited by 84 publications

References 82 publications

Strategies for the Photocatalytic Generation of Carbanion Equivalents for Reductant-Free C–C Bond Formations

Strategies for the Photocatalytic Generation of Carbanion Equivalents for Reductant-Free C–C Bond Formations

Recent Developments in C−H Functionalisation of Benzofurans and Benzothiophenes

Photoangeregte Anionen in organischen Reaktionen

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