Metallkomplexe heterocyclischer Carbene ‐ ein neues Katalysator‐Strukturprinzip in der homogenen Katalyse

Abstract

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.

Synthesis and Crystal Structure of Bis(2,3-dihydro-3-methylbenzothiazole-2-ylidene)palladium(II) Diiodide: The First Palladium Complex with Benzothiazole Carbene Ligands Suitable for Homogeneous Catalysis

“…Besides the expected square-planar core geometry, a crystal structure analysis of 1 revealed that the two carbene ligands are in a trans arrangement and are coplanar with respect to the PdI 2 plane. The Pd-C bond lengths [2.01(4)/ 2.06(3) Å ] lie within the established range for Pd-C(alkyl) single bonds, [16] although the 13 C-NMR signal of the car-bene C appears further downfield (δ ϭ 210.5) than the corresponding signals of imidazole and triazole carbenes (δ ϭ 169-185); [3][4][5] this is in accordance with the values found for the stable N-diisopropylphenylthiazole carbene. [12] From an XPS analysis of 1, a binding energy of 337.2 eV was determined, which is very close to the value of 337.0 eV typical for Pd II .…”

“…[12] From an XPS analysis of 1, a binding energy of 337.2 eV was determined, which is very close to the value of 337.0 eV typical for Pd II . It may therefore be the case that benzothiazole carbenes have pronounced σ-donor properties similar to those of imidazole carbenes [3] and that π-back-bonding is probably less significant than that seen with the electronrich phosphanes.…”

“…In view of the well-known [1] phosphane degradation through C-P bond cleavage and the sensitivity of such species to air and moisture, [2] phosphanes have recently [3][4][5][6] been substituted by nucleophilic carbenes of imidazole and triazole, these carbenes being stable and easily isolated as ligands.…”

The Heck reaction in the production of fine chemicals

“…In addition, unreactive arylating agents like aryl chlorides can be used. More variations on this theme are the use of bulky phosphite (18,41) or phosphoramidite ligands (42), carbene ligands (43,44), and pincer ligands (31,45). Another interesting, cheap, and very active class of catalysts are the palladacycles based on aromatic compounds with side chains containing N (41,46), O (47), or S (48,49).…”

N-Heterocyclic carbenes as highly efficient ancillary ligands in homogeneous and immobilized metathesis ruthenium catalytic systems

“…phosphane, amine, alkoxy, Schiff base), substituent group manipulations allow fine-tuning of the steric and electronic properties of the NHC ligand thus conferring special attributes to the metal complex. 5 Most NHCs have turned out to be quite efficient ancillary ligands in transition metal complexes widely used as highly active and rather selective catalysts (or precatalysts) in numerous fundamental chemical transformations such as hydrogenation, 6 hydrogen transfer, 7 hydroformylation, 8 hydrosilylation, 9 isomerization, 10,11 telomerization, 12 Kharash addition, 13 Pauson-Khand cyclization, 14 olefin metathesis 15 and various C-C coupling reactions. [16][17][18] Of these catalytic processes, olefin metathesis has recently undergone spectacular development, becoming a powerful synthetic protocol in organic and polymer chemistry.…”