Imidazolines metal complexes

The most practical approach is the direct treatment of azolium salts with metal complexes under neutral or basic conditions [39,154-159]. Alternatively, the free carbenes can be generated in the presence of a suitable metal complex by reduction of a carbene precursor, e.g. a thiourea [160]. Stable, uncomplexed imidazoline-2-ylidenes, isolated for the first time in 1991 by Arduengo [161] (for further examples see [162-166]), are also convenient starting materials for the preparation of carbene complexes [167,168]. The corresponding diaminocarbene complexes can be obtained by treatment of the stable diaminocarbenes with transition metal complexes. Finally, at high temperatures many transition metal complexes catalyze the carbon-carbon bond scission of tetraaminoethylenes, forming carbene complexes [169-171]. Examples of such preparations are given in Table 2.8. 

The fact that NHCs form stable compounds with beryllium, one of the hardest Lewis acids known and without p-electrons to back donate, shows the nu-cleophilicity of these ligands. Reaction of l,3-dimethylimidazolin-2-ylidene with polymeric BeCl2 results in the formation of the neutral 2 1 adduct 23 or the cationic 3 1 adduct 24. The first NHC-alkaline earth metal complex to be isolated was the 1 1 adduct 25 with MgEt2- Whereas l,3-dimesitylimidazolin-2-ylidene results in the formation of a dimeric compound, the application of sterically more demanding l,3-(l-adamantyl)imidazolin-2-ylidene gives a monomeric adduct. ... 

In C-NMR spectra, the signals for the carbene carbon are usually shifted upheld by about 20-30 ppm upon complexation of the free NHC to a transition metal. Cr-NMR data of [LCr(CO)s] complexes underline that NHC are a special case of carbene ligands because of their lack of tt-acceptor ability. Photoreactions of metal complexes containing NHCs by laser flash and continuous photolysis show that NHCs are quite inert ligands in photolysis reactions. He I and He II photoelectron spectra of platinum(O)- and palladium(O) bis(imidazolin-2-ylidene)... 

The MCR toward 2//-2-imidazolines (65) has found apphcation in the construction of A(-heterocyclic carbene (NHC) complexes (74). Alkylation of the sp Af-atom with an alkyl halide followed by abstraction of the proton at C2 with a strong base (NaH, KOtBu) resulted in the formation of the free carbene species, which could be trapped and isolated as the corresponding metal complexes (Ir or Rh) [160]. The corresponding Ru-complexes were shown to be active and selective catalysts for the transfer hydrogenaticm of furfural to furfurol using iPrOH as hydrogen source [161]. 

It is known from previous work that diazoalkanes can form carbene(alkylidene)-metal complexes [17, 18], cf. eq. (5). It is thus reasonable to assume that a metal-carbene 9 is formed from the (phosphine oxide-stabilized) species CH3Re 02) (eq. (5)). High oxidation-state metal carbene complexes have ample precedent, especially through the work of Schrock et al. [19], Isolation of type-8 species may be facilitated by sterically more-demanding auxiliary groups (e. g., C5H5 in place of CH3) or, by using heterocyclic carbenes of pronounced Lewis basicity (e. g., 1.3-imidazolin-2-ylidene [20]). 

Controlled tuning of the electronic properties of the NHC ligand can result in drastic consequences on the catalytic efficiency of the corresponding metal complexes and thus work has been done to classify and quantify this parameter. The key structural handles that can be modified to tune the electronic properties in classical five-membered NHCs are again the C4-C5 bridge of the imidazol or imidazoline skeleton, the substituents on the C4 and C5 carbon atoms, and the... 

While up to 1990 all attempts to isolate a stable N-heterocyclic carbene failed, metal complexes of unsaturated imidazol-2-ylidenes were known as early as 1968. The first complexes of this type were obtained by in situ deprotonation of imidazolium salts using mercury(ll) acetate or dimethylimidazolium hydridopentacarbonylchromate(-II) followed by coordination of the carbene to the metal center (Scheme 1.3). Shortly thereafter, the stabilization of the saturated imidazolin-2-ylidene in a metal complex was described by Lappert who treated electron-rich entetraamines of type 6=6 with coordinatively unsaturated transition metal complexes to obtain complexes with imidazolin-2-ylidene ligands (Scheme 1.3). ... 

That was only 4 years after the preparation of the first Fischer-iyp carbene complex 6 years before the first Schrock-iyp carbene complex 4 was reported, and more than 20 years before the isolation of stable imidazolin-2-ylidenes by Arduengo in 1991 [Eq. (1)]. Once attached to a metal, these Wanzlick- or Arduengo-csabenes have shown a reaction pattern completely different from that... 

In cases where the free NHC cannot be synthesized the complex formation has to be accomplished in situ from a ligand precursor, e.g., the imidazolium salt in the case of imidazolin-2-ylidenes. By this method, it is often possible to prepare complexes which do not have the maximum number of NHC ligands attached to the metal center. 

For example, complex 37 with an imidazolin-2-ylidene and a methyl ligand in cis-position to each other decomposes to yield the 1,2,3-trimethylimidazolium salt 38, Pd°, and cod (Fig. 13) [124], Additional examples for the reductive elimination of 2-alkyl and 2-aryl substituted azohum salts from palladium or nickel NHC complexes have been reported [125, 126]. Today, reductive elimination reactions have been established as one important reaction pathway for the deactivation of catalytically active metal NHC complexes [126, 127]. 

Several complexes of bivalent cobalt and zinc with imidazoline-(l//,3i/)-2-thione (34 R = H) of the forms ML2X2 (X = halide ion) and ML4X2 (X = N03 or C104) have been isolated and characterized by physicochemical techniques.247 In contrast with the S monodentate behaviour concluded for the earlier-reported Ni11 complexes, the ligands are N—S bidentate in these metal salts to form four-membered chelate rings. Similar coordination behaviour has been observed for the... 

The discovery of stable and isolable free phosphinocarbenes by Bertrand was quickly followed by the isolation of stable imidazolin-2-ylidenes by Arduengo (Figure 5.7). Imidazolylidene complexes of most of the transition metals have since been prepared directly from free carbenes. 

Direct C-H activation of 2-imidazolines in the addition to alkenes has been observed under rhodium catalysis as shown for the formation of 586 (Scheme 139) <20040L1685>. The proposed intermediate was thought to be similar to that involved in metal-N-heterocyclic carbene (NHC) complexes <2002AGE1290>. 

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