Mechanistic Insight into the Staudinger Reaction Catalyzed by N-Heterocyclic Carbenes

Authors

  • Morgan Hans,

    1. Laboratory of Organometallic Chemistry and Homogeneous Catalysis, Institut de chimie (B6a), Université de Liège, Sart-Tilman par 4000 Liège (Belgium), Fax: (+32) 4-366-3497
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  • Prof. Johan Wouters,

    1. Department of Chemistry, Facultés Universitaires N.-D. de la Paix, 61 Rue de Bruxelles, 5000 Namur (Belgium)
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  • Prof. Albert Demonceau,

    1. Laboratory of Organometallic Chemistry and Homogeneous Catalysis, Institut de chimie (B6a), Université de Liège, Sart-Tilman par 4000 Liège (Belgium), Fax: (+32) 4-366-3497
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  • Prof. Lionel Delaude

    Corresponding author
    1. Laboratory of Organometallic Chemistry and Homogeneous Catalysis, Institut de chimie (B6a), Université de Liège, Sart-Tilman par 4000 Liège (Belgium), Fax: (+32) 4-366-3497
    • Laboratory of Organometallic Chemistry and Homogeneous Catalysis, Institut de chimie (B6a), Université de Liège, Sart-Tilman par 4000 Liège (Belgium), Fax: (+32) 4-366-3497

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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.

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