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  Formation of Breslow Intermediates from N-Heterocyclic Carbenes and Aldehydes Involves Autocatalysis by the Breslow Intermediate, and a Hemiacetal

Wessels, A., Klussmann, M., Breugst, M., Schlörer, N. E., & Berkessel, A. (2022). Formation of Breslow Intermediates from N-Heterocyclic Carbenes and Aldehydes Involves Autocatalysis by the Breslow Intermediate, and a Hemiacetal. Angewandte Chemie International Edition, 61(23): e202117682. doi:10.1002/anie.202117682.

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 Creators:
Wessels, Alina1, Author
Klussmann, Martin2, 3, Author           
Breugst, Martin1, Author
Schlörer, Nils E.1, Author
Berkessel, Albrecht1, Author
Affiliations:
1Department of Chemistry, Organic Chemistry, University of Cologne, Greinstraße 4, 50939 Cologne, Germany, ou_persistent22              
2Research Group Klußmann, Max-Planck-Institut für Kohlenforschung, Max Planck Society, ou_1445608              
3Borchers GmbH, Berghausener Straße 100, 40764 Langenfeld, Germany, ou_persistent22              

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Free keywords: Autocatalysis; Breslow Intermediate; Carbenes; NMR; Reaction Mechanisms
 Abstract: Under aprotic conditions, the stoichiometric reaction of N-heterocyclic carbenes (NHCs) such as imidazolidin-2-ylidenes with aldehydes affords Breslow Intermediates (BIs), involving a formal 1,2-C-to-O proton shift. We herein report kinetic studies (NMR), complemented by DFT calculations, on the mechanism of this kinetically disfavored H-translocation. Variable time normalization analysis (VTNA) revealed that the kinetic orders of the reactants vary for different NHC-to-aldehyde ratios, indicating different and ratio-dependent mechanistic regimes. We propose that for high NHC-to-aldehyde ratios, the H-shift takes place in the primary, zwitterionic NHC-aldehyde adduct. With excess aldehyde, the zwitterion is in equilibrium with a hemiacetal, in which the H-shift occurs. In both regimes, the critical H-shift is auto-catalyzed by the BI. Kinetic isotope effects observed for R-CDO are in line with our proposal. Furthermore, we detected an H-bonded complex of the BI with excess NHC (NMR).

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Language(s): eng - English
 Dates: 2021-12-272022-03-032022-06-07
 Publication Status: Published in print
 Pages: 6
 Publishing info: -
 Table of Contents: -
 Rev. Type: Peer
 Identifiers: DOI: 10.1002/anie.202117682
 Degree: -

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Title: Angewandte Chemie International Edition
  Abbreviation : Angew. Chem., Int. Ed.
Source Genre: Journal
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Publ. Info: Weinheim : Wiley-VCH
Pages: - Volume / Issue: 61 (23) Sequence Number: e202117682 Start / End Page: - Identifier: ISSN: 1433-7851
CoNE: https://pure.mpg.de/cone/journals/resource/1433-7851