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

Abstract

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.