Aldehyde Umpolung by N-Heterocyclic Carbenes: NMR Characterization of the Breslow Intermediate in its Keto Form, and a Spiro-Dioxolane as the Resting State of the Catalytic System


  • Support by BASF SE and by the Fonds der Chemischen Industrie is gratefully acknowledged. The authors thank D. Müller for providing 13C-labeled triazolylidene.


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Surprises from a classic: The Breslow intermediate of triazolylidene-catalyzed benzoin condensations was characterized for the first time by NMR spectroscopy in its keto form (K, energetic minimum). The hitherto unknown spiro-dioxolane S, generated from the carbene catalyst and two equivalents of aldehyde, is the resting state of the catalytic system.