title: Mechanistic insight into the Staudinger reaction catalyzed by N-heterocyclic carbenes.
abstract: Four zwitterions were prepared by treating 1,3-dimesitylimidazolin-2-ylidene (SIMes) or 1,3-dimesitylimidazol-2-ylidene (IMes) with either N-tosyl benzaldimine or diphenylketene. They were isolated in high yields and characterized by IR and NMR spectroscopy. The molecular structures of three of them were determined by using X-ray crystallography and their thermal stability was monitored by using thermogravimetric analysis. The imidazol(in)ium-2-amides were rather labile white solids that did not show any tendency to tautomerize into the corresponding 1,2,2-triaminoethene derivatives. They displayed a mediocre catalytic activity in the Staudinger reaction of N-tosyl benzaldimine with diphenylketene. In contrast, the imidazol(in)ium-2-enolates were orange-red crystalline materials that remained stable over extended periods of time. Despite their greater stability, these zwitterions turned out to be efficient promoters for the model cycloaddition under scrutiny. As a matter of fact, their catalytic activity matched those recorded with the free carbenes. Altogether, these results provide strong experimental insight into the mechanism of the Staudinger reaction catalyzed by N-heterocyclic carbenes. They also highlight the superior catalytic activity of the imidazole-based carbene IMes compared with its saturated analogue SIMes in the reaction under consideration.
authors.affiliation: Laboratory of Organometallic Chemistry and Homogeneous Catalysis, Institut de chimie (B6a), Université de Liège, Sart-Tilman par 4000 Liège, Belgium.
authors.forename: Morgan
authors.initials: M
authors.lastname: Hans
authors.forename: Johan
authors.initials: J
authors.lastname: Wouters
authors.forename: Albert
authors.initials: A
authors.lastname: Demonceau
authors.forename: Lionel
authors.initials: L
authors.lastname: Delaude
keywords: carbenes; lactams; organocatalysis; reaction mechanisms; zwitterions