Berkessel, A., Paul, M., Peckelsen, K. et al. (8 more authors) (2021) Breslow intermediates (aminoenols) and their keto tautomers: first gas‐phase characterization by IR ion spectroscopy. Chemistry – A European Journal, 27 (8). pp. 2662-2669. ISSN 0947-6539
Abstract
Breslow intermediates (BIs) are the crucial nucleophilic amino enol intermediates formed from electrophilic aldehydes in the course of N‐heterocyclic carbene (NHC) catalyzed Umpolung reactions. Both in organocatalytic and enzymatic Umpolung, the question whether the Breslow intermediate exists as the nucleophilic enol, or in the form of its electrophilic keto‐tautomer, is of utmost importance for its reactivity and function. We herein report the preparation of charge‐tagged Breslow intermediates/keto tautomers derived from three different types of NHCs (imidazolidin‐2‐ylidenes, 1,2,4‐triazolin‐5‐ylidenes, thiazolin‐2‐ylidenes) and aldehydes. An ammonium charge‐tag is introduced by either the aldehyde unit or the NHC. ESI‐MS IR‐Ion spectroscopy allowed for the unambiguous conclusion that in the gas‐phase, the imidazolidin‐2‐ylidene derived BI indeed exists as a diamino enol, while both 1,2,4‐triazolin‐5‐ylidenes and thiazolin‐2‐ylidenes give the keto‐tautomer. This result coincides with the tautomeric states observed for the BIs in solution (NMR) and in the crystalline state (XRD), and is in line with our earlier calculations on the energetics of BI keto‐enol equilibria.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Copyright, Publisher and Additional Information: | © 2020 The Authors. Published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial License, (http://creativecommons.org/licenses/by-nc/4.0/) which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. |
Keywords: | Breslow intermediate; carbenes; density functional calculations; mass spectrometry; Umpolung |
Dates: |
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Institution: | The University of Sheffield |
Academic Units: | The University of Sheffield > Faculty of Science (Sheffield) > Department of Chemistry (Sheffield) |
Depositing User: | Symplectic Sheffield |
Date Deposited: | 27 Oct 2020 16:01 |
Last Modified: | 26 Jan 2022 14:06 |
Status: | Published |
Publisher: | Wiley |
Refereed: | Yes |
Identification Number: | 10.1002/chem.202003454 |
Related URLs: | |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:166941 |