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Keywords:

  • carbenes;
  • NMR spectroscopy;
  • reaction mechanisms;
  • umpolung;
  • X-ray diffraction
Thumbnail image of graphical abstract

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.