Mechanistic insight into the Staudinger reaction catalyzed by N-heterocyclic carbenes

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. Catalysts caught in the act: The N-heterocyclic carbenes 1,3-dimesitylimidazolin-2- ylidene (SIMes) and 1,3-dimesitylimidazol-2-ylidene (IMes) react with N-tosyl benzaldimine or diphenylketene to afford the corresponding zwitterions in high yields (see scheme). The molecular structures of three of them were determined by X-ray crystallography and their thermal stability was monitored by thermogravimetric analysis. The NHC×ketene betaines were found to be key intermediates for the Staudinger reaction catalyzed by NHCs.