Photocatalytic carbanion generation from C–H bonds – reductant free Barbier/Grignard-type reactions

Berger, Anna Lucia; Donabauer, Karsten; König, Burkhard

doi:10.1039/c9sc04987h

Cited by 41 publications

(37 citation statements)

References 45 publications

(27 reference statements)

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“…We currently exclude an alternative mechanism where the benzylic radical III gets reduced by the photo catalyst to the corresponding anion that is acylated with the azolium I in an ionic process, because the cascade does not proceed upon replacing benzoyl fluoride and F by acetone that should react via the benzylic anion if formed to the alcohol 8, that was not identified (see Scheme 4, equation 4). [25] In summary, a radical alkene acyltrifluoromethylation cascade was developed that operates by cooperative photoredox/NHC catalysis. SET-reduction of readily generated acylazolium intermediates to give ketyl-type radicals has not been well explored in synthetic radical chemistry.…”

Section: Angewandte Chemiementioning

confidence: 99%

Cooperative NHC and Photoredox Catalysis for the Synthesis of β‐Trifluoromethylated Alkyl Aryl Ketones

2020

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Despite the great potential of radical chemistry in organic synthesis, N-heterocyclic carbene (NHC)-catalyzed reactions involving radical intermediates are not well explored. This communication reports the three-component coupling of aroyl fluorides, styrenes and the Langlois reagent (CF 3 SO 2 Na) to give various b-trifluoromethylated alkyl aryl ketones with good functional group tolerance in moderate to high yields by cooperative photoredox/NHC catalysis. The alkene acyltrifluoromethylation proceeds via radical/radical cross coupling of ketyl radicals with benzylic C-radicals. The ketyl radicals are generated via SET reduction of in situ formed acylazolium ions whereas the benzylic radicals derive from trifluoromethyl radical addition onto styrenes.

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Section: Angewandte Chemiementioning

confidence: 99%

Cooperative NHC and Photoredox Catalysis for the Synthesis of β‐Trifluoromethylated Alkyl Aryl Ketones

2020

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“… 2019 10 10991 10996 . 2 This merger of photo- and HAT-catalysis allowed the activation of a C–H bond to a carbon centered nucleophile, which leads to the desired Grignard-type reaction in full atom economy. Redox-Neutral Photocatalytic C-H Carboxylation of Arenes and Styrenes with CO 2 Schmalzbauer M. Svejstrup T. D. Fricke F. Brandt P. Johansson M. J.…”

Section: Key Referencesmentioning

confidence: 99%

Section: Key Referencesmentioning

confidence: 99%

“…The merger of benzonitrile-based donor–acceptor photocatalysts 37 and i Pr 3 SiSH as the HAT-catalyst ( Figure 7 D) was found as a competent system to orchestrate such a C–H to carbanion activation in the benzylic position ( Figure 7 ). 2 , 38 The proposed reaction mechanism thus operates as follows: the i Pr 3 SiSH HAT-catalyst is deprotonated in the presence of a base opening its facile oxidation by the excited photocatalyst. This SET step renders the reduced photocatalyst (PC •– ) and simultaneously the activated HAT-catalyst species (( i Pr) 3 Si–S • ).…”

Section: Carbanion Generation Via Radical Reductionmentioning

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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“…Dieser wurde jedoch nicht detektiert (siehe Schema 4, Gleichung 4). [25] Zusammenfassend wurde eine radikalische Alken-Acyltrifluormethylierungskaskade entwickelt, die durch kooperative Photoredox/NHC-Katalyse ermçglicht wird. Die SET-Reduktion von leicht zu erzeugenden Acylazolium-Intermediaten zu Ketyl-artigen Radikalen ist in der synthetischen Radikalchemie bisher kaum erforscht worden.…”

Section: Photoredox-katalysatorunclassified

Kooperative NHC‐ und Photoredox‐Katalyse zur Synthese β‐trifluormethylierter Alkylarylketone

2020

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Trotz des großen Potentials der Radikalchemie in der organischen Synthese, sind N-heterozyklische Carben (NHC)-katalysierte Reaktionen, die radikalische Intermediate durchlaufen, kaum erforscht. In dieser Verçffentlichung wird eine Dreikomponentenkupplung von Aroylfluoriden, Styrolen und dem Langlois-Reagenz (CF 3 SO 2 Na) beschrieben. Kooperative Photoredox/NHC-Katalyse führt in mäßigen bis hohen Ausbeuten zu b-trifluormethylierten Alkylarylketonen mit guter funktioneller Gruppentoleranz. Die Alkenacyltrifluormethylierung läuft über Kreuzkupplung von Ketyl-mit benzylischen Radikalen ab. Die Ketylradikale werden dabei über SET-Reduktion von in situ erzeugten Acylazoliumionen gebildet und die benzylischen Radikale resultieren aus einer CF 3-Radikaladdition an das Alken. Aufgrund ihrer einzigartigen Lewis-Basizität und Nukleophilie wurden N-heterozyklische Carbene (NHCs) in weitreichenden Gebieten der organischen Synthesechemie als Organokatalysatoren genutzt. [1] Im Allgemeinen verlaufen NHC-katalysierte Reaktionen über die Bildung von Breslow-, [2] p-erweiterten Breslow-[3] oder Azolium-Intermediaten [4] ab, welche mit Elektrophilen oder Nukleophilen in ionischen Prozessen reagieren. Im Gegensatz dazu ist die NHC-Katalyse, die über Ein-Elektronentransfer-Prozesse (single electron transfer, SET) agiert und Kupplungen von radikalischen Intermediaten involviert, kaum erforscht. Im Jahr 2008 haben wir eine NHC-katalysierte Oxidation von Aldehyden zu TEMPO-Estern (TEMPO = 2,2,6,6-tetramethylpiperidin-N-oxyl) entwickelt und damit gezeigt, dass Breslow-Intermediate durch das TEMPO-Radikal effizient zum entsprechenden Radikalkation SET-oxidiert werden kçnnen. [5] Wird die SET-Oxidation p-erweiterter Breslow-Intermediate betrachtet, zeigt das daraus resultierende Radikalkation Reaktivität in b-Position, die erfolgreich für b-CO -und b-CC -Bindungsknüpfungen über Radikal/Radikal-Kreuz-und Homokupplungen genutzt werden konnte (Schema 1 a, Weg a). [6] Des Weiteren kçnnen Enale, die in g-Angewandte Chemie

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Photocatalytic carbanion generation from C–H bonds – reductant free Barbier/Grignard-type reactions

Abstract: We report a redox-neutral method for the generation of carbanions from benzylic C–H bonds in a photocatalytic Grignard-type reaction.

Cited by 41 publications

References 45 publications

Cooperative NHC and Photoredox Catalysis for the Synthesis of β‐Trifluoromethylated Alkyl Aryl Ketones

Cooperative NHC and Photoredox Catalysis for the Synthesis of β‐Trifluoromethylated Alkyl Aryl Ketones

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

Kooperative NHC‐ und Photoredox‐Katalyse zur Synthese β‐trifluormethylierter Alkylarylketone

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