Acyl Donor Intermediates in N‐Heterocyclic Carbene Catalysis: Acyl Azolium or Azolium Enolate?. (18th January 2021)

• Record Type: Journal Article
• Authors: Biswas, Animesh; Neudörfl, Jörg‐M.; Schlörer, Nils E.; Berkessel, Albrecht
• Is Part Of: Angewandte Chemie. Volume 133:Number 9(2021)
• Page Start: 4557
• Page End: 4561
• Publication Date: 2021-01-18
• Subjects: esters -- N-heterocyclic carbenes -- reaction mechanisms -- structure elucidation -- X-ray diffraction; Chemistry -- Periodicals; 540
• DOI: 10.1002/ange.202010348
• Languages: English
• ISSNs: 0044-8249

Abstract

Abstract: Azolium enolates and acyl azolium cations have been proposed as intermediates in numerous N‐heterocyclic carbene (NHC) catalyzed transformations. Acetyl azolium enolates were generated from the reaction of 2‐propenyl acetate with both saturated (SIPr) and aromatic (IPr) NHCs, isolated, and characterized (NMR, XRD). Protonation with triflic acid gave the corresponding acetyl azolium triflates which were isolated and characterized (NMR, XRD). Acyl azolium cations have been proposed as immediate precursors of the ester product, for example, in the redox esterification of α, β‐enals. Studies with d 3 ‐acetyl azolium triflate suggest that ester formation originates instead from an azolium enolate intermediate. Furthermore, the acetyl azolium enolate selectively reacted with alcohol nucleophiles in the presence of amines. While the acetyl azolium cation did not react with alcohols, an ester‐selective reaction was induced by addition of base, by intermediate formation of the acetyl azolium enolate. Abstract : Acetyl azolium enolates resulted from the treatment of the N‐heterocyclic carbenes IPr and SIPr with 2‐propenyl acetate. Protonation converts them into acetyl azolium cations. The latter react with amines faster than with alcohols, while the azolium enolates reacted readily with alcohols, but sluggishly with amines. The use of a d3 ‐acetyl azolium triflate indicated that alcohol acetylation occurs at the azolium enolate stage.