Group 13, carbene complexes

The first group 1 carbene complex with an N-bound anionic functional group was reported in 2004.12 An alkylamino carbene is readily deprotonated using //-butyl lithium to afford 4 (Fig. 3). The solid state structure comprises a discrete dimer via bridging amido groups. Although there is severe distortion of the lithium-NCN bond (147.9° compared to the closer to linear 161.8° in 3), the lithium-NHC bond distance of 2.124(4) A is still short, suggesting that the interaction is predominantly ionic. 

Non-heteroatom-substituted carbene complexes cover a broad spectrum of different reactivities, largely dependent on the electronic properties of the metal. In Chapter 1 the division of carbene complexes into Fischer-type and Schrock-type carbenes was discussed. This way of grouping carbene complexes, although difficult to apply... 

Note The n-sulfurane in Figure 4.92 is a very early and still rare example of a main group carbene complex using a functionalised NHC ligand. 

Many other organometaUic compounds also react with carbonyl groups. Lithium alkyls and aryls add to the ester carbonyl group to give either an alcohol or an olefin. Lithium dimethyl cuprate has been used to prepare ketones from esters (41). Tebbe s reagent, Cp2TiCH2AlCl(CH2)2, where Cp = clyclopentadienyl, and other metal carbene complexes can convert the C=0 of esters to C=CR2 (42,43). 

Dimesitylimidazolium chloride with nickelocene gives the carbene complex [(T) -Cp)NiCl(L)] (L= l,3-dimesitylimidazol-2-ylidene), in which the chloride ligand can be substituted by a methyl group by reacting the product with methyl-lithium (OOJOM(596)3). 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

The Group VI organometallic chemistry is mainly characterized by the occurrence of N- and C-coordination and carbene complex-formation, as well as by some unique cases of Se- (Te-) coordination, ring opening and deselenation. The Group VII organometallic chemistry is known for the carbene and chelate structures of the derivatized thiazoles. 

The unsaturated substituent in the carbene complex 1 often is aromatic or heteroaromatic, but can also be olefinic. The reaction conditions of the Dotz procedure are mild various functional groups are tolerated. Yields are often high. The use of chromium hexacarbonyl is disadvantageous, since this compound is considered to be carcinogenic however to date it cannot be replaced by a less toxic compound. Of particular interest is the benzo-anellation procedure for the synthesis of anthra-cyclinones, which are potentially cytostatic agents. ... 

In pyridinium chloride ionic liquids and in l,2-dimethyl-3-hexylimida2olium chloride ([HMMIMjCl), where the C(2) position is protected by a methyl group, only [PdClJ was observed, whereas in [HMIMjCl, the EXAFS showed the formation of a bis-carbene complex. In the presence of triphenylphosphine, Pd-P coordination was observed in all ionic liquids except where the carbene complex was formed. During the Heck reaction, the formation of palladium was found to be quicker than in the absence of reagents. Overall, the EXAFS showed the presence of small palladium clusters of approximately 1 nm diameter formed in solution. 

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. 

The reactivity of these carbene complexes can be understood as an electron-deficient carbene carbon atom due to the electron-attracting CO groups, while... 

Schrock-type carbenes are nucleophilic alkylidene complexes formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7T-acceptor ligand to metals in high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and it has been discussed whether the structures may be considered as ylides. A tantalum Schrock-type carbene complex was synthesized by deprotonation of a metal alkyl group [38] (Scheme 7). 

Recently, Aumann et al. reported that rhodium catalysts enhance the reactivity of 3-dialkylamino-substituted Fischer carbene complexes 72 to undergo insertion with enynes 73 and subsequent formation of 4-alkenyl-substituted 5-dialkylamino-2-ethoxycyclopentadienes 75 via the transmetallated carbene intermediate 74 (Scheme 15, Table 2) [73]. It is not obvious whether this transformation is also applicable to complexes of type 72 with substituents other than phenyl in the 3-position. One alkyne 73, with a methoxymethyl group instead of the alkenyl or phenyl, i.e., propargyl methyl ether, was also successfully applied [73]. 

Cycloaddition Reactions of Group 6 Fischer Carbene Complexes... 

Electron-deficient 1,3-dienes are known to react when heated with metho-xy(aryl)- or methoxy(alkyl)carbene complexes to afford vinylcyclopropane derivatives with high regioselectivity and diastereoselectivity [8a, 24]. Cyclo-propanation of the double bond not bearing the acceptor functional group and... 

The first [3S+2C] cycloaddition reaction using a Fischer carbene complex was accomplished by Fischer et al. in 1973 when they reported the reaction of the pentacarbonyl(ethoxy)(phenylethynyl)carbene complex of tungsten and diazomethane to give a pyrazole derivative [45]. But it was 13 years later when Chan and Wulff demonstrated that in fact this was the first example of a 1,3-dipolar cycloaddition reaction [46,47a]. The introduction of a bulky trime-thylsilyl group on the diazomethane in order to prevent carbene-carbon olefi-nation leads to the corresponding pyrazole carbene complexes in better yields (Scheme 15). 

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