N-heterocyclic carbenes as supporting ligands in homogeneous catalysis
In the last ten years, N-heterocyclic carbenes(NHCs) have gained tremendous<br/>popularity, notably as highly versatile ligands for transition metals. Their strong -donating<br/>properties, associated with high steric hindrance, often impart enhanced stability and activity to...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2008 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/9031 |
| Acceso en línea: | http://www.tdx.cat/TDX-0123109-110450 http://hdl.handle.net/10803/9031 |
| Access Level: | acceso abierto |
| Palabra clave: | N-heterocyclic carbenes homogeneous catalysis gold palladium crosscoupling cyclization 546 547 |
| Sumario: | In the last ten years, N-heterocyclic carbenes(NHCs) have gained tremendous<br/>popularity, notably as highly versatile ligands for transition metals. Their strong -donating<br/>properties, associated with high steric hindrance, often impart enhanced stability and activity to a given metallic center. Two main successes of the NHCs in homogeneous catalysis are arguably the ruthenium-mediated olefin metathesis and the palladium-promoted crosscoupling reactions.In this work, we have studied the effect of N-heterocyclic carbenes as supporting ligands in well-defined complexes of palladium(II), gallium(III), and gold(I) that we used in homogeneous catalysis.Notably, we have synthesized, in very straightforward manners, two families of palladium compounds of formulae [(NHC)Pd(L)Cl], where L, which is a R-allyl or R-acac moiety, acts as a protecting shell for the catalytically active [(NHC)Pd0] species. Hence,<br/>upon activation under the reaction conditions, these new Pd complexes were found extremely active in the Suzuki-Miyaura, the Buchwald-Hartwig, and the -ketone arylation cross-coupling reactions. More precisely, the more active "R-allyl family" allowed for reactions to be performed with as low as 10 ppm of palladium.<br/>A series of [(NHC)GaCl3] complexes wa synthesized via a simple one-step procedure. The resulting unprecedented NHC-GaIII compounds were found extremely stable but showed only moderate activity in isomerization reactions.Demonstrating further the versatile application of NHCs in metal-based catalysis, we<br/>developed several new catalytic transformations using [(NHC)AuCl] complexes. Hence,these pre-catalysts, activated in situ with a silver salt, proved to be excellent activators of<br/>alkynes, allenes, and alkenes. This led to the development of efficient synthetically useful<br/>protocols, encompassing enyne cycloisomerization, indene cyclization,formation of conjugated enone, and allylic rearrangement.<br/>KEYWORDS gallium - gold - homogeneous catalysis - N-heterocyclic carbene - palladium 8 |
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