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Acyl Donor Intermediates in N‐Heterocyclic Carbene Catalysis: Acyl Azolium or Azolium Enolate?

Azolium enolates and acyl azolium cations have been proposed as intermediates in numerous N‐heterocyclic carbene (NHC) catalyzed transformations. Acetyl azolium enolates were generated from the reaction of 2‐propenyl acetate with both saturated (SIPr) and aromatic (IPr) NHCs, isolated, and character...

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Autores principales: Biswas, Animesh, Neudörfl, Jörg‐M., Schlörer, Nils E., Berkessel, Albrecht
Formato: Online Artículo Texto
Lenguaje:English
Publicado: John Wiley and Sons Inc. 2021
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7986403/
https://www.ncbi.nlm.nih.gov/pubmed/33140529
http://dx.doi.org/10.1002/anie.202010348
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author Biswas, Animesh
Neudörfl, Jörg‐M.
Schlörer, Nils E.
Berkessel, Albrecht
author_facet Biswas, Animesh
Neudörfl, Jörg‐M.
Schlörer, Nils E.
Berkessel, Albrecht
author_sort Biswas, Animesh
collection PubMed
description Azolium enolates and acyl azolium cations have been proposed as intermediates in numerous N‐heterocyclic carbene (NHC) catalyzed transformations. Acetyl azolium enolates were generated from the reaction of 2‐propenyl acetate with both saturated (SIPr) and aromatic (IPr) NHCs, isolated, and characterized (NMR, XRD). Protonation with triflic acid gave the corresponding acetyl azolium triflates which were isolated and characterized (NMR, XRD). Acyl azolium cations have been proposed as immediate precursors of the ester product, for example, in the redox esterification of α,β‐enals. Studies with d (3)‐acetyl azolium triflate suggest that ester formation originates instead from an azolium enolate intermediate. Furthermore, the acetyl azolium enolate selectively reacted with alcohol nucleophiles in the presence of amines. While the acetyl azolium cation did not react with alcohols, an ester‐selective reaction was induced by addition of base, by intermediate formation of the acetyl azolium enolate.
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spelling pubmed-79864032021-03-25 Acyl Donor Intermediates in N‐Heterocyclic Carbene Catalysis: Acyl Azolium or Azolium Enolate? Biswas, Animesh Neudörfl, Jörg‐M. Schlörer, Nils E. Berkessel, Albrecht Angew Chem Int Ed Engl Communications Azolium enolates and acyl azolium cations have been proposed as intermediates in numerous N‐heterocyclic carbene (NHC) catalyzed transformations. Acetyl azolium enolates were generated from the reaction of 2‐propenyl acetate with both saturated (SIPr) and aromatic (IPr) NHCs, isolated, and characterized (NMR, XRD). Protonation with triflic acid gave the corresponding acetyl azolium triflates which were isolated and characterized (NMR, XRD). Acyl azolium cations have been proposed as immediate precursors of the ester product, for example, in the redox esterification of α,β‐enals. Studies with d (3)‐acetyl azolium triflate suggest that ester formation originates instead from an azolium enolate intermediate. Furthermore, the acetyl azolium enolate selectively reacted with alcohol nucleophiles in the presence of amines. While the acetyl azolium cation did not react with alcohols, an ester‐selective reaction was induced by addition of base, by intermediate formation of the acetyl azolium enolate. John Wiley and Sons Inc. 2021-01-18 2021-02-23 /pmc/articles/PMC7986403/ /pubmed/33140529 http://dx.doi.org/10.1002/anie.202010348 Text en © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH This is an open access article under the terms of the http://creativecommons.org/licenses/by/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
spellingShingle Communications
Biswas, Animesh
Neudörfl, Jörg‐M.
Schlörer, Nils E.
Berkessel, Albrecht
Acyl Donor Intermediates in N‐Heterocyclic Carbene Catalysis: Acyl Azolium or Azolium Enolate?
title Acyl Donor Intermediates in N‐Heterocyclic Carbene Catalysis: Acyl Azolium or Azolium Enolate?
title_full Acyl Donor Intermediates in N‐Heterocyclic Carbene Catalysis: Acyl Azolium or Azolium Enolate?
title_fullStr Acyl Donor Intermediates in N‐Heterocyclic Carbene Catalysis: Acyl Azolium or Azolium Enolate?
title_full_unstemmed Acyl Donor Intermediates in N‐Heterocyclic Carbene Catalysis: Acyl Azolium or Azolium Enolate?
title_short Acyl Donor Intermediates in N‐Heterocyclic Carbene Catalysis: Acyl Azolium or Azolium Enolate?
title_sort acyl donor intermediates in n‐heterocyclic carbene catalysis: acyl azolium or azolium enolate?
topic Communications
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7986403/
https://www.ncbi.nlm.nih.gov/pubmed/33140529
http://dx.doi.org/10.1002/anie.202010348
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