Biswas Animesh, Neudörfl Jörg-M, Schlörer Nils E, Berkessel Albrecht
Department of Chemistry (Organic Chemistry), University of Cologne, Greinstraße 4, 50939, Cologne, Germany.
Angew Chem Int Ed Engl. 2021 Feb 23;60(9):4507-4511. doi: 10.1002/anie.202010348. Epub 2021 Jan 18.
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 -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.
氮杂环烯醇化物和酰基氮杂环阳离子已被认为是众多氮杂环卡宾（NHC）催化转化反应中的中间体。乙酸烯丙酯与饱和（SIPr）和芳香族（IPr）氮杂环卡宾反应生成乙酰氮杂环烯醇化物，对其进行分离和表征（核磁共振、X射线衍射）。用三氟甲磺酸质子化得到相应的三氟甲磺酸乙酰氮杂环化物，并对其进行分离和表征（核磁共振、X射线衍射）。例如，在α,β-烯醛的氧化还原酯化反应中，酰基氮杂环阳离子被认为是酯产物的直接前体。对三氟甲磺酸d-乙酰氮杂环化物的研究表明，酯的形成反而源于氮杂环烯醇化物中间体。此外，乙酰氮杂环烯醇化物在胺存在下与醇亲核试剂发生选择性反应。虽然乙酰氮杂环阳离子不与醇反应，但通过加入碱，由乙酰氮杂环烯醇化物的中间体形成诱导了酯选择性反应。
