Characterization of the Key Intermediates of Carbene-Catalyzed Umpolung by NMR Spectroscopy and X-Ray Diffraction: Breslow Intermediates, Homoenolates, and Azolium Enolates
in the act: Diamino enols, diamino dienols, azoliumenolates, 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 NMRspectroscopy and X‐ray crystallography.
Umpolung by N-Heterocyclic Carbenes: Generation and Reactivity of the Elusive 2,2-Diamino Enols (Breslow Intermediates)
作者:Albrecht Berkessel、Silvia Elfert、V. Reddy Yatham、Jörg-M. Neudörfl、Nils E. Schlörer、J. Henrique Teles
DOI:10.1002/anie.201205878
日期:2012.12.3
Saturated imidazolidin‐2‐ylidenes react with aldehydes to smoothly produce the elusive 2,2‐diamino enols A (“Breslowintermediates”, 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 Breslowintermediates B are accessible by deprotonation of methoxymethyl azolium salts.