Carbene catalyzed umpolung of α,β-enals: a reactivity study of diamino dienols azolium enolates, and the characterization of advanced reaction intermediates.

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, 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) 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 NMR monitoring of their reactivities towards enone electrophiles. Additionally, reaction intermediates were isolated and characterized, by X-ray crystallography.