Carbene catalyzed umpolung of α,β-enals: a reactivity study of diamino dienols vs. azolium enolates, and the characterization of advanced reaction intermediatesElectronic supplementary information (ESI) available: Experimental procedures, compound characterization data, and X-ray crystallographic data of compounds 4b-Et and 4b-Me. CCDC 1014843 and 1014844. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5sc01027f

Since their discovery by Bode and Glorius in 2004, N-heterocyclic carbene catalyzed conjugate umpolung reactions of α,β-enals have been postulated to involve the formation of diamino dienols ("homoenolates") and/or azolium enolates ("enolates"), typically followed by addition to electrophiles, e.g. Michael-acceptors. In this article, we provide evidence, for the first time, for the postulated individual and specific reactivity patterns of diamino dienols (γ-C-C-bond formation) vs. azolium enolates (β-C-C-bond formation). Our study is based on the pre-formation of well defined diamino dienols and azolium enolates, and the in situ NMR monitoring of their reactivities towards enone electrophiles. Additionally, reaction intermediates were isolated and characterized, inter alia by X-ray crystallography. NMR/X-ray evidence is provided for hitherto postulated reactivity patterns of homoenolate vs. azolium enolate intermediates in NHC-catalyzed umpolung of enals.