Résumé
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.
langue originale | Anglais |
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Pages (de - à) | 9668-9676 |
Nombre de pages | 9 |
journal | Chemistry: A European Journal |
Volume | 19 |
Numéro de publication | 29 |
Les DOIs | |
Etat de la publication | Publié - 15 juil. 2013 |
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Physico-chimie et caractérisation (PC2)
Wouters, J. (!!Manager), Aprile, C. (!!Manager) & Fusaro, L. (!!Manager)
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CCDC 910981: Experimental Crystal Structure Determination
Delaude, L. (Contributeur), Demonceau, A. (Contributeur), Hans, M. (Contributeur) & Wouters, J. (Contributeur), Cambridge Crystallographic Data Centre, 1 janv. 2013
DOI: 10.5517/cczkyhy, http://www.ccdc.cam.ac.uk/services/structure_request?id=doi:10.5517/cczkyhy&sid=DataCite
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CCDC 910980: Experimental Crystal Structure Determination
Delaude, L. (Contributeur), Demonceau, A. (Contributeur), Hans, M. (Contributeur) & Wouters, J. (Contributeur), Cambridge Crystallographic Data Centre, 1 janv. 2013
DOI: 10.5517/cczkygx, http://www.ccdc.cam.ac.uk/services/structure_request?id=doi:10.5517/cczkygx&sid=DataCite
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CCDC 910982: Experimental Crystal Structure Determination
Delaude, L. (Contributeur), Demonceau, A. (Contributeur), Hans, M. (Contributeur) & Wouters, J. (Contributeur), Cambridge Crystallographic Data Centre, 1 janv. 2013
DOI: 10.5517/cczkyjz, http://www.ccdc.cam.ac.uk/services/structure_request?id=doi:10.5517/cczkyjz&sid=DataCite
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