Umpolung by N-Heterocyclic Carbenes: Generation and Reactivity of the Elusive 2,2-Diamino Enols (Breslow Intermediates)


  • Support by the Fonds der Chemischen Industrie and by BASF SE is gratefully acknowledged. We thank Felix Klauck for providing samples of 13C-labeled 2,4-bis(trifluoromethyl)benzaldehyde.


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54 years later: Saturated imidazolidin-2-ylidenes react with aldehydes to smoothly produce the elusive 2,2-diamino enols A (“Breslow intermediates”, first postulated in 1958) of carbene-catalyzed umpolung reactions. The 2,2-diamino enols A react with additional aldehyde in a cross-benzoin reaction. The methylated Breslow intermediates B are accessible by deprotonation of methoxymethyl azolium salts.