Donor-functionalized imidazolium

Alternatively, complexes 54 and 55 with abnormally bound NHC ligands have been obtained by oxidative addition of a C-H bond to Pd° and Pt° while blocking the C2 position of the azolium salt [155] or even without any blocking of the C2 position by oxidative addition of a C5-I bond of a donor-functionalized imidazolium cation (Fig. 19) [156]. 

Reports on A,A -donor-functionalized, chiral and polydentate N-hetero-cyclic carbenes appeared almost immediately after the reports on the preparation of the first stable NHCs. Herrmann s report on the first donor-functionalized imidazolium cations 18 and 19 (Figure 1.3) from 1996 was followed by a large number of publications dealing with imidazolium... 

First attempts to isolate monocarbene-hydrido complexes by oxidative addition of A -(2-pyridyl)imidazolium cations to Pd° with utilization of the chelate effect of the donor-functionalized carbene ligand failed and only the dicarbene complexes such as 29 were isolated [112]. The iridium hydrido complex 30 was obtained in the oxidative addition of an W-(2-pyridylmethyl)imidazolium cation to iridium(I) (Fig. 11) [113]. This reaction proceeds most likely via the initial coordination of the nitrogen donor which brings the imidazolium C2-H bond in close proximity to the metal center. No reaction was observed with Rh under these conditions. 

A large number of differently bridged diazolium salts of type 24 (Figure 1.4) have been described. Compound 25, the precursor for an interesting bis-carbene containing carbene and alcoholato donor functions, was prepared by Arnold et al. by the reaction of two equivalents of an A-alkylimidazole with a functionalized epichlorhydrin (Figure 1.4). Dibenzimidazolium salts are also known. Trisimidazolium salt 26 was first described by Dias and Jin, " shortly thereafter followed by the description of trisimidazolium salts of type 27. Tripodal tris-imidazolium salts of types 26, 28 and 29 were prepared by Hu and Meyer. ... 

Macrocyclic bis(imidazolium) salt 56" possesses after C2 deprotonation both NHC and pyridine donor functions. The doubly lutidine-bridged... 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

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