Pd-Catalyzed Inter- and Intramolecular Carbene Transfer from Group 6 Metal-Carbene Complexes
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- 作者：Miguel A. Sierra ; Juan C. del Amo ; Marí ; a J. Mancheñ ; o ; and Mar Gó ; mez-Gallego
- 刊名：Journal of the American Chemical Society
- 出版年：2001
- 出版时间：February 7, 2001
- 年：2001
- 卷：123
- 期：5
- 页码：851 - 861
- 全文大小：152K
- 年卷期：v.123,no.5(February 7, 2001)
- ISSN：1520-5126
文摘
The use of group 6 metal-carbene complexes in inter- and intramolecular carbene transfer reactionshas been studied. Thus, pentacarbonyl[(aryl)(methoxy)carbene]chromium(0) and tungsten complexes, 10,efficiently dimerize at room temperature in the presence of diverse Pd(0) and Pd(II)/Et3N catalysts. The effectof additives (PPh3, AsPh3, or SbPh3) on the nature and the isomeric ratio of the reaction products is negligible.The nature of the reaction products is more catalyst-dependent for metal carbenes 12 bearing alkyl groupsattached to the carbene carbon. In these cases, either carbene ligand dimerization or 
-hydrogen eliminationreactions are observed, depending on the catalyst. The carbene ligand dimerization reaction can be used toprepare conjugated polyenes, including those having metal moieties at both ends of the polyene system, aswell as enediyne derivatives. The intramolecular carbene ligand dimerization of chromium bis-carbene complexes28 and 30 allows the preparation of mono- and bicyclic derivatives, with ring sizes from six to nine members.For bis-carbene derivatives the -hydrogen elimination reaction is inhibited, provided that both metal centersare tethered by an o-xylylene group. Other alkyl complexes 32 form new mononuclear carbene complexes 37or decompose to complex reaction mixtures. The results obtained in these reactions may be explained bytransmetalation from Cr(0) to Pd(0) and the intermediacy of Pd-carbene complexes. Aminocarbene-chromium(0) complexes 15, need harsher reaction conditions to transfer the carbene ligand, and this transfer occurs onlyin the presence of deactivated olefins. The corresponding insertion/hydrolysis products 48 resulted in thesecases. A catalytic cycle involving transmetalation from a chromacyclobutane to a palladacyclobutane is proposedto explain these results.
-hydrogen eliminationreactions are observed, depending on the catalyst. The carbene ligand dimerization reaction can be used toprepare conjugated polyenes, including those having metal moieties at both ends of the polyene system, aswell as enediyne derivatives. The intramolecular carbene ligand dimerization of chromium bis-carbene complexes28 and 30 allows the preparation of mono- and bicyclic derivatives, with ring sizes from six to nine members.For bis-carbene derivatives the -hydrogen elimination reaction is inhibited, provided that both metal centersare tethered by an o-xylylene group. Other alkyl complexes 32 form new mononuclear carbene complexes 37or decompose to complex reaction mixtures. The results obtained in these reactions may be explained bytransmetalation from Cr(0) to Pd(0) and the intermediacy of Pd-carbene complexes. Aminocarbene-chromium(0) complexes 15, need harsher reaction conditions to transfer the carbene ligand, and this transfer occurs onlyin the presence of deactivated olefins. The corresponding insertion/hydrolysis products 48 resulted in thesecases. A catalytic cycle involving transmetalation from a chromacyclobutane to a palladacyclobutane is proposedto explain these results.