Characterization of the Key Intermediates of Carbene‐Catalyzed Umpolung by NMR Spectroscopy and X‐Ray Diffraction: Breslow Intermediates, Homoenolates, and Azolium Enolates

Caught in the act: Diamino enols, diamino dienols, azolium enolates, and azolium enols are postulated intermediates of the N-heterocyclic carbene catalyzed umpolung of aldehydes and enals. Several of these elusive reaction intermediates were generated with the saturated imidazolidin-2-ylidene SIPr (R=2,6-bis(2-propyl)phenyl) and characterized by NMR spectroscopy and X-ray crystallography.