Carbenes, metal coordinated

The development of the chemistry of carbene complexes of the Group 8a metals, Ru, Os, and Ir, parallels chemistry realized initially with transition metals from Groups 6 and 7. The pioneering studies of E. O. Fischer and co-workers have led to the characterization of many hundreds of carbene complexes in which the heteroatoms N, O, and S are bonded to the carbene carbon atoms. The first carbene ligands coordinated to Ru, Os, and Ir centers also contained substituents based on these heteroatoms, and in this section the preparation and properties of N-, O-, S-, and Se-substituted carbene complexes of these metals are detailed. 

In comparison with other catalysts, (diphenylcarbene)tungstenpenta-carbonyl is surprisingly sluggish (26). This observation is significant as it relates to diverse views regarding the carbene-to metallocyclobutane interconversion. Whereas Casey emphasizes a need to accommodate the incoming olefin within the coordination sphere of the metal prior to rearrangement to a metallocycle [Eq. (5)], Katz (28) has described the process essentially as a dipolar attack of a polarized carbene-metal (R2C+—M-) on the olefin [Eq. (6)]. The latter does not specify a need for w-complexation of the olefin as a precondition to metathesis. 

Abstract A-Heterocyclic carbenes (NHCs) have developed into an important class of ligands in transition metal coordination chemistry. They have been employed successfully as spectator ligands in various catalytically active metal complexes and as organocatalysts. In this chapter we present some important synthetic methods for the preparation of various NHCs and their metal complexes. 

After the discovery by Fischer and Maasbol of the first stable carbene complexes in 1964, i.e., [(CO)5W =C(OMe)R ] [21], generation of related metaUacumulene derivatives [M]=C(=C) =CR2 (n > 0) was obviously envisaged. Thus, it is presently well-established that stabilization of these neutral unsaturated carbenes by coordination to a transition metal center is possible by the use of the lone pair of electrons on the carbenic carbon atom, via formation of a metal-carbon a-bond (electron back-donation from the metal fragment to the carbon ligand may strengthen this bond). This has allowed the development of a rich chemistry of current intense interest due to the potential applications of the resulting metallacumulenic species in organic synthesis, as well as in the construction of molecular wires and other nanoelectronic devices [22]. 

The reaction between a lithium amide and Cr(CO)s was first noted to give attack on a coordinated carbonyl and provide entry into amino-substituted carbene-metal complexes (equation 86).228,229... 

The elimination of a-hydrogen is not general and observed only with limited numbers of metal complexes. The elimination of a-hydrogen from the methyl group in the dimethylmetal complex 68 generates the metal hydride 69 and a carbene that coordinates to the metal. Liberation of methane by the reductive elimination generates the carbene complex 70. Formation of carbene complexes of Mo and Wis a key step in alkene metathesis. The a-elimination is similar to the 1,2-hydride shift observed in organic reactions. 

The chiral center most frequently encountered is the asymmetric carbon atom, a tetrahedral C atom, bonded to four different substituents. Chiral centers of this type are known for many other elements (4). However, chiral centers are also found in other polyhedra, e.g., the metal atoms in octahedral compounds containing three bidendate chelate ligands. Chirality axes, present in the atrop isomers of ortho-substituted biaryls, occur in coordination chemistry in appropriately substituted aryl, pyridyl, and carbene metal complexes. Well known examples of planar chirality in organometallic chemistry are ferrocenes, cymantrenes, and benchrotrenes containing two different substituents in 1,2- or 1,3-positions relative to each other (5-5). 

The deprotonation of an azolium salt to form a carbene requires a base. This base can be supplied as the anion of a transition metal compound, in which case the azolium salt is deprotonated in situ and the carbene formed coordinates to the metal generating the NHC transition metal complex. This method works best if a coordinating anion, such as bromide or iodide, is suppUed with the azolium salt. 

In this case, the pyridine end dissociates but does not leave the vicinity of the metal since it is tethered to the metal-coordinated carbene end. The metal complex can now use the free coordination site for (catalytic) reactions and subsequently, the pyridine end coordinates again to the metal stabilising the catalyst. The authors used this system in the... 

A third example comes from Clyne et al. [358] and concerns the axial chiral binaphthyl backbone [359,360], itself known from phosphorus chemistry [361]. The synthesis starts from the trifluoromethylsulfonato substituted binaphthyl with a Kumada coupling reaction [291,292] with methytmagnesiumbromide. Oxidation with NBS yields the methyl brominated derivative that can be attached to the imidazole ring. Subsequent methylation results in the bis-imidazolium salt that is deprotonated to the bis-carbene and coordinated to the transition metal halide (Pd, Ni), a rather straightforward reaction sequence (see Figure 3.113). The overall yield for the four-step reaction to the bis-imidazolium salt is surprisingly good (65%). 

Reaction of the commercially available [TiCKOlVy with a lithium adduct of an amide functionalised carbene results in the substitution of an isopropyloxy substituent on titanium(lV) by the amide sidearm of the carbene. Simultaneous coordination of the carbene unit results in a five-coordinate chelate titanium(lV) complex [111] (see Figure 4.33). The choice of the lithium carbene adduct determines the number of OPi groups still present on the metal centre. 

A widely used precursor for Rh(I) and Ir(I) in coordination chemistry is the [MC1(C0D)]2 dimer (120). In the presence of either free NHC or in situ generated carbene from the alcohol adduct or the combination imidazolium salt and base, the dimer was cleaved giving rise to [(NHC)MCl(COD)] (121) (Scheme 20). The presence of labile COD allowed for further modification of the metal coordination sphere. For example, under CO atmosphere the COD ligand was rapidly removed, leaving two CO binding the metal center. The... 

The first step is likely to be a so-called Hieber base reaction (0 in Scheme 1), describing a nucleophilic attack at metal-coordinated carbon monoxide by hydroxide. While eq. (3) is the prototypical example as first reported by Walter Hieber in 1932 [5], several other variants have since become known. For example, azides and alkyl/aryl anions attack metal carbonyls according to eqs. (4) and (5), yielding metal isocyanates (via a Curtius-type azide degradation step) and metal carbenes, respectively [6, 7]. 

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