Charakterisierung der Schlüsselintermediate von carbenkatalysierten Umpolungen durch Kristallstrukturanalyse/NMR‐Spektroskopie: Breslow‐Intermediate, Homoenolate und Azoliumenolate

Abstract

Schema 1. Durch N-heterocyclische Carbene katalysierte Umpolung von a) einfachen Aldehyden und b) a,b-ungesättigten Aldehyden.

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

“…The most characteristic 1 H NMR signals of 7 are a doublet at δ = 5.44 ppm [ 3 J HH = 9.0 Hz, 1H, H4 ( trans )] and a doublet at δ = 5.85 ppm [ 3 J HH = 6.6 Hz, 1H, H4 ( cis )]. In line with our earlier experience, 18 the liberated NHC SIPr reacted with benzaldehyde to cleanly afford the diamino enol 8 (see ESI † for the NMR identification of lactone 7 and diamino enol 8 ).…”

“…We had reported earlier 18 that under strictly oxygen-free conditions, the saturated imidazolidinylidene SIPr (1,3-bis[2,6-di-(2-propylphenyl)]imidazolidin-2-ylidene) reacts smoothly with E -cinnamic aldehyde in THF at room temperature to the diamino dienol 1 ( Scheme 3 ). Protonation of the latter exclusively gives the Cγ-protonation product 2 (an azolium enol), and thus nicely proves Cγ-nucleophilicity ( Scheme 3 , top).…”

“…The first successful generation of both diamino dienols I and azolium enolates II from α,β-unsaturated aldehydes and carbenes, and their characterization by NMR and X-ray, was recently reported by our group. 18 …”

On the Involvement of a Spiroepoxide Intermediate in N‐Heterocyclic Carbene (NHC)‐Catalyzed Benzoin Condensations – An Approach by Oxygenation of Deoxy‐Breslow Intermediates

“…Postulated already in 1958,1,2 the first successful generation of the diamino enols 3 from aldehydes and carbenes was reported by us in 2012 (Scheme ) 5. Saturated imidazolidinylidenes react smoothly with a variety of aldehydes, affording the 2,2‐diamino enols 3 which have been unambiguously characterized by NMR spectroscopy5a and X‐ray crystallography 5b. Most importantly, we could show by selective cross‐benzoin formation that the 2,2‐diamino enols 3 indeed react as acyl anion equivalents in the presence of additional aldehyde 5a…”

N‐Heterocyclic‐Carbene‐Catalyzed Umpolung of Imines

“…[1] Theu nderlying principle behind many of the NHC organocatalytic reactions is the umpolung of aldehydes. [2] The nucleophilic acyl anion equivalents (Breslow intermediate) [3] generated by the addition of NHCs to aldehydes can add to av ariety of carbonyl compounds (benzoin reaction), [4] Michael acceptors (Stetter reaction), [5] and even to unpolarized, electron-neutral carbon-carbon multiple bonds (hydroacylation reactions;S cheme 1a). [6] Moreover,N HCs are employed for the conjugate umpolung of enals for the generation of homoenolate equivalents.…”