Reactions Involving Cyclometalated Ligands

A useful application of cyclometalated complexes is seen in the smooth and highly selective chlorination reaction (14). 

A cyclometalated intermediate is implicated in the reaction of (48) with phosphines to yield (49). The reaction only occurs in the presence of nickel or copper salts, and is thought to involve M—C bonded species formed by insertion into the phenyl C—Br bond ortho to the 2,2 -bipyridine. Support for this proposal 

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Major achievements in cyclometallation processes involving NHC ligands have been achieved by Nolan and coworkers. It was observed that the solvent could play a crucial role in the formation of rhodium- and iridium-based complexes. As shown in Scheme 39, the reaction performed in pentane between [M(COE)2Cl]2 (M = Rh or Ir) and four equivalents of FBu led only to COE substitution by the NHC ligand affording rhodium-based product (243) (the iridium complexing proving difficult to isolate). On the other hand, the same reaction carried out in hexanes gave, via C H activation, the hydride complexes (244) and (245). Finally, in benzene, a unique double cyclometallation process occurred to yield the coordinatively unsaturated 16-electron... 

The solution and solid state structural features of the optically pure [Rh(TpMenth)(CO)2] have been reported, TpMenth being found in both k2- and k3-coordination modes. The optically active TpMenth and XpMementh are able to control the stereoselectivity of the C-H bond activation by their coordinated rhodium center. Irradiation of [Rh(TpMenth)(CO)2] under N2 resulted in the generation of an 85 15 mixture of diastereomeric alkyl hydrides due to intramolecular cyclometalation reaction involving the methyl substituents on the ligand isopropyl group (Fig. 3.51).240... 

The ligand 2,6-di-t-butylphenoxide has been shown to undergo cyclometallation with a number of early transition metals. The reaction involves the intramolecular activation of the CH bonds of the Bu groups to generate six-membered rings. Besides alkyl leaving groups, alkylidene and benzyne functions have been shown to be potent for the activation of these normally inert C—H bonds. 

Other examples of OA reactions are shown in Figure 19.4. The first reaction listed is an important intermediate in the Monsanto acetic acid process. In the second example, the 18-electron [Fe(CO)j] compound must first dissociate a ligand before it can undergo OA. The third reaction represents an example of C-H bond activation and the fourth reaction involves an intramolecular cyclometalation of the PPh3 ligand. 

Generally, 2,2 -bipyridines are representative -chelating ligands that are similar to ethylenediamine and 1,10-phenanthroline as inorganic chelate ligands [30]. Only recently has a new coordinating behavior called a rollover cyclometalation reaction appeared in the literature. A rollover reaction involves y-C-H bond activation and formation of a y-C-M bond, as shown in Eq. (7.7) [31]. 

In the case of alkyl leaving groups, mechanistic studies show that a-bond metathesis pathways are active, proceeding via four-centre, four-electron transition states. In the case of unsaturated functional groups, the reaction involves addition of the CH bond across the r-component, so that the substrate hydrogen atom remains (labelling studies) within the product molecule. The cyclometallation of 2,6-dimethylphenoxide by a tantalum alkylidene ligand has also been demonstrated. ... 

A unique cyclometallation process was utilized to synthesize zirconium dibenzyl complex 229 involving the reaction of 2-(2 -phenol)-6-arylpyridine with Zr(CH2Ph)4185 (Equation (16)). This complex is supported by an unsymmetric tridentate, phenolate-pyridine-carbanion [CT,N,0 ]-typs ligand. Both the solution NMR data and... 

The mechanism of the coupling was not explored in detail, but likely involves ligand exchange at the rhodium center and coordination by the pyridine substrate, directed cyclometalation, transmetalation of the organotin reagent, and reductive elimination to afford the ortho arylated products (Scheme 28). The nature of the reoxidation process is more equivocal. Trichloroethylene is generated during the reaction when 1,1,2,2-tetrachloroethane is chosen as solvent, which may be... 

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