Universität zu Köln

Wessels, Alina ORCID: 0000-0001-7915-5627, Klussmann, Martin ORCID: 0000-0002-9026-205X, Breugst, Martin, Schloerer, Nils E. and Berkessel, Albrecht (2022). Formation of Breslow Intermediates from N-Heterocyclic Carbenes and Aldehydes Involves Autocatalysis by the Breslow Intermediate, and a Hemiacetal. Angew. Chem.-Int. Edit., 61 (23). WEINHEIM: WILEY-V C H VERLAG GMBH. ISSN 1521-3773

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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).

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Wessels, Alina UNSPECIFIED orcid.org/0000-0001-7915-5627 UNSPECIFIED Klussmann, Martin UNSPECIFIED orcid.org/0000-0002-9026-205X UNSPECIFIED Breugst, Martin UNSPECIFIED UNSPECIFIED UNSPECIFIED Schloerer, Nils E. UNSPECIFIED UNSPECIFIED UNSPECIFIED Berkessel, Albrecht UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-670659
DOI:	10.1002/anie.202117682
Journal or Publication Title:	Angew. Chem.-Int. Edit.
Volume:	61
Number:	23
Date:	2022
Publisher:	WILEY-V C H VERLAG GMBH
Place of Publication:	WEINHEIM
ISSN:	1521-3773
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language THIAMINE ACTION; MECHANISM; CATALYSTS; UMPOLUNG; CONDENSATION Multiple languages Chemistry, Multidisciplinary Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/67065