Acyl Donor Intermediates in N‐Heterocyclic Carbene Catalysis: Acyl Azolium or Azolium Enolate?

Azolium enolates and acyl azolium cations have been proposed as intermediates in numerous N-heterocyclic carbene catalyzed transformations. Acetyl azolium enolates were generated from the reaction of 2-propenyl acetate with both saturated (SIPr) and aromatic (IPr) N-heterocyclic carbenes, isolated, and characterized by NMR and XRD. Protonation with triflic acid gave the corresponding acetyl azolium triflates which were isolated and characterized (NMR, XRD) as well. Acyl azolium cations have been proposed as immediate precursors of the ester product e.g in the redox esterification of α,β-enals. Our current studies, involving isotopically labeled d 3 -acetyl azolium triflate, suggest that ester formation instead originates from an azolium enolate intermediate. Furthermore, the acetyl azolium enolate was found to selectively react with alcohol nucleophiles in the presence of amines. While the acetyl azolium cation did not react with alcohols, an ester-selective reaction could be induced by addition of base, via intermediate formation of the acetyl azolium enolate.