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Breslow Intermediates (Amino Enols) and Their Keto Tautomers: First Gas‐Phase Characterization by IR Ion Spectroscopy
Author(s) -
Paul Mathias,
Peckelsen Katrin,
Thomulka Thomas,
Martens Jonathan,
Berden Giel,
Oomens Jos,
Neudörfl JörgM.,
Breugst Martin,
Meijer Anthony J. H. M.,
Schäfer Mathias,
Berkessel Albrecht
Publication year - 2021
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.202003454
Subject(s) - tautomer , chemistry , nucleophile , umpolung , electrophile , enol , carbene , keto–enol tautomerism , aldehyde , reactivity (psychology) , medicinal chemistry , photochemistry , stereochemistry , organic chemistry , catalysis , medicine , alternative medicine , pathology
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. Herein, 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 is reported. An ammonium charge tag is introduced through the aldehyde unit or the NHC. ESI‐MS IR ion spectroscopy allowed 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.