IMes ligand

Efforts aimed at fully understanding the mechanism of these oxidation reactions are still needed but new insights regularly appear in the literature [203-205]. The isolation and characterization of a dioxygen-derived palladium(II)-hydroperoxide complex—species generally postulated as intermediates in this reaction—has been achieved for the first time by Stahl et al. [206] (Scheme 27). The capability of IMes ligands to undergo cis-trans isomerization has been pointed out as essential for the formation of this complex. 

Complexes containing the less bulky IMes ligand or bulky phosphine ligands (PPhj, PCyj) were found to be less active, especially at lower catalyst loadings. 

It is worth noting that the IMes ligands have also been employed in the design and synthesis of the interesting class of arene-Ru complexes, e.g. 64-66 (Scheme 36). Due to their easy accessibility from the commercially available Ru dimer [(p-cymene)RuCl2], the last two complexes are of special importance for both radical and metathesis reactions. 

In 2005, Saino et al. [38] reported an Fe-catalyzed [2-I-2-I-2] cyclotrimerization using the 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) Al-heterocyclic carbene (NHC) ligand (see Chapters 1 and 2) (Scheme 9.17). The IPr ligand was shown to be more efficient when compared with the IMes ligand. 

The C H activation of IMes ligands is relatively common however, activation of the isopropyl substituent in IPr occurs more rarely. For example, the reaction of [Ir(COE)2Cl]2 (COE = cyclooctene) with IPr at room temperature, produced complex 13 (Scheme 3.8). Upon chloride abstraction, C H activation of a methyl hydrogen from the second IPr ligand was observed to give 14. Sigman and co-workers have observed a similar decomposition reaction with [(IPr)Ni] complexes. ... 

The use of diazo compounds has also attracted some attention. NHC-Au catalysts, and especially those bearing the IPr and IMes ligands, mediated the decomposition of diazo species, leading to organogold carbenes, which could be observed in the gas phase. Once formed, the NHC-Au carbene could participate in a variety of reactions that include insertions into X-H bonds (X = C, N, Buchner reactions, ° and olefin cyclopropanation. ... 

The C-H activation of IMes ligands is relatively common Nolan and Aldridge reported independently that [M(p-C1)(C0E)2]2 (M = Rh or Ir, COE = c/s-cyclooctene) reacted with IMes to form cyclometalated rhodium(iii) and iridium(iii) complexes (51 and 52) (Scheme 2.18). Activation of the isopropyl substituent in IPr occurs more rarely. The reaction of [Ir(p-Cl)(COE)2]2 with IPr at room temperature produced complex 53. Upon chloride... 

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