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

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.