Breslow Intermediates (Amino Enols) and Their Keto Tautomers: First Gas-Phase Characterization by IR Ion Spectroscopy |
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Authors: | Dr. Mathias Paul Dr. Katrin Peckelsen Thomas Thomulka Dr. Jonathan Martens Dr. Giel Berden Prof. Dr. Jos Oomens Dr. Jörg-M. Neudörfl Dr. Martin Breugst Prof. Dr. Anthony J. H. M. Meijer Prof. Dr. Mathias Schäfer Prof. Dr. Albrecht Berkessel |
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Affiliation: | 1. Department of Chemistry, Cologne University, Greinstrasse 4, 50939 Cologne, Germany;2. Institute for Molecules and Materials, FELIX Laboratory, Radboud University, Toernooiveld 7, 6525 ED, Nijmegen, The Netherlands;3. Department of Chemistry, University of Sheffield, Sheffield, S3 7HF UK |
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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. 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. |
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Keywords: | Breslow intermediate density functional calculations IR spectroscopy mass spectrometry umpolung |
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