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Breslow Intermediates from Aromatic N-Heterocyclic Carbenes (Benzimidazolin-2-ylidenes, Thiazolin-2-ylidenes)
Mathias Paul1, Panyapon Sudkaow1, Alina Wessels1
1Cologne University, Department of Chemistry, Organic Chemistry, Greinstrasse 4, 50939, Cologne, Germany.
Abstract:
We report the first generation and characterization of elusive Breslow intermediates derived from aromatic N-heterocyclic carbenes (NHCs), namely benzimidazolin-2-ylidenes (NMR, X-ray analysis) and thiazolin-2-ylidenes (NMR). In the former case, the diamino enols were generated by reaction of the free N,N-bis(2,6-diisopropylphenyl)- and N,N-bis(mesityl)-substituted benzimidazolin-2-ylidenes with aldehydes while the dimer of 3,4,5-trimethylthiazolin-2-ylidene served as the starting material in the latter case. The unambiguous NMR identification of the first thiazolin-2-ylidene-based Breslow intermediate rests on double 13 C labeling of both the NHC and the aldehyde component. The acyl anion reactivity was confirmed by benzoin formation with excess aldehyde.
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