Stabilized by “carbene” donor ligands, the Pd complex 1 catalyzes the Heck olefination of aryl halides unexpectedly efficiently and yet has long‐term stability at elevated temperatures. The active Pd0 species can be generated during the Heck reaction or deliberately prepared by reduction of 1 with, for instance, hydrazine or sodium formate. Another similar catalyst can be synthesized in situ from Pd0 complexes and 1,3‐dimethyldihydroimidazoline‐2‐ylidene.
Stable mono-and dicarbene adducts of late transition metals are readily accessible either by reaction of imidazolium salts with metal complexes bearing basic ligands, or by the cleavage of chloroand acetato-bridged dinuclear metal complexes with the free carbenes (e.g., 1,3-dimethylimidazoline-2-ylidene) . A general novel method for the deprotonation of N-substituted azolium salts in liquid ammonia-the liquid ammonia route-is described. This method yields not only the known free monomeric 1,3-dimethylirnidazoline-2-ylidene in quantitative yield, but also otherwise hardly accessible derivatives. For example, imidazoline-2-ylidenes with linear, branched, cyclic, heteroatom-substituted and even chiral hydrocarbon residues can be obtained. The nucleophilic behaviour of 1,3-dimethylimidazoline-2-ylidene is reported and compared with that of other donor ligands. Novel carbene complexes of Ru", Rh', Pd", 0s" and Ir' are presented. Reaction of the potentially chelating ligand 1,1'-(1,2-ethyIene)-3,3'-dimethyldiKeywords carbenes -Group 8-10 complexesheterocycles * structure elucidation imidazoline-2,2-diylidene with [(COD)-RhCI], yields a dinuclear complex, in which two rhodium centres are linked by the dicarbene bridge. Four single-crystal X-ray diffraction structures of new metalcarbene complexes (Rh, 0s) are reported. N-Heterocyclic carbene complexes of Group 8-10 transition metals are both thermally and chemically very stable. They do not show the typical reactivity of metal-carbon double bonds. For a number of reasons, these complexes must be regarded as donor adducts of the Lewisbasic imidazoline-2-ylidene ligand and the Lewis-acidic organometallic fragment.
Durch „Carben”︁‐Donorliganden stabilisiert ist der Pd‐Komplexe 1, der die Heck‐Olefinierung von Arylhalogeniden unerwartet effizient katalysiert. Die aktive Pd0‐Spezies kann man gezielt durch Reduktion von 1 oder auch in situ aus Pd0‐Komplexen und 1,3‐Dimethyldihydroimidazol‐2‐yliden erhalten. Der von Imidazol abgeleitete „Carben”︁ ‐Ligand, der treffender als C‐basischer Donorligand zu formulieren ist, macht den erstmals hergestellten Komplex 1 zu einem auch bei hohen Reaktionstemperaturen langzeitstabilen Katalysator für die Heck‐Olefinierung.
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