Chelating NHCs

Scheme 6.15 Pd-catalysed hydroarylation or norbornene using chelating NHC/P-Ugands...

Iridium NHC complexes have also been used to catalyse the coupling of alcohols 7 and 8. Complexes 12 [4] and 13 [5], containing chelating NHC ligands, are also effective for catalysing the same reaction. 

Polydentate NHC ligands have provided numerous new compounds [19]. Chelate-N-heterocyclic ligands also allow the fine-tuning of topological properties such as steric hindrance, bite angles, chirahty and fluxional behavior. In the case of Ir—chelate—NHC compounds, most of the data that have been reported were obtained as an extension of investigations into their rhodium analogues [19, 20]. 

Certain transition metal complexes can serve as templates for the synthesis of chelating NHC ligands. For example, 1-phenylphosphole complexes of pal-ladium(II) are attacked in a Diels-Alder reaction by 1-vinylimidazole. If 1,2-dichloroethane is used as the solvent the imidazole is alkylated in situ and then subjected to a spontaneous carbometallation reaction [Eq. (37)]. 

Scheme 8.10 Ruthenium allenylidene complexes with chelating NHC ligand.

Further insights into the relationships between structure and catalyst activity and selectivity have been achieved through variation of the three major components the anionic ligand the non-chelating NHC substituent and the nature of the chelating substituent. 

The chelate character of the complex was determined by X-ray crystallography.9 The ESI-MS and analytical data are also consistent with this structure. The conformation of the chelating NHC renders the two protons on the CH2 linker nonequivalent by NMR spectroscopy. 

Employing the same protocol as for the chelating NHC/phosphine system, nonchelating NHC ligands [60] gave high yields of trans coupling products for an array of aryl bromides (Scheme 15). 

The NHC were chosen as supporting ligands as a result of comparisons with previously described catalytic systems based on dicationic phosphine or sterically encumbered P-diimine. The phosphine and p-diimine systems prevent chain transfer and termination processes. By analogy, it was speculated that mutually c/s-chelating NHC, stabilized by sterically demanding aryl substituents, would help promote the copolymerization of ethylene and carbon monoxide. Unfortunately, mesityl substituents on nitrogen atoms were the only... 

Bis-chelating NHCs with the same pyridine side-arm gave a TON of only 195 under similar reaction conditions. 

The benchmark was set by Herrmarm s carbene complex 65 which, under optimized reaction conditions, showed a respectable TON of 13000. In a similar study by Baker et al., the Mizoroki-Heck arylation of iodobenzene (60b) with 63 in the presence of catalyst 66 bearing a chelating NHC ligand with a cyclophane backbone showed an outstanding TON of 7100000 [34]. In another study, Buchmeiser et al. introduced a new class of NHC based on 1,3-disubstituted tetrahydropyrimi-... 

Second-Generation Ruthenium Indenylidene Catalysts Bearing a N-Heterocyclic Carbene or a Chelating NHC-Phosphine Ligand... 

The family of ruthenium(NHC)(phosphine) complexes also includes chelating NHC/phosphine ligands. In a recent example by Hofmann [59], a ruthenium-indenylidene complex was prepared in 60% yield from 5 and the free phosphine-carbene ligand 46 in toluene at 80 °C. In this, the chelate forces the carbene and phosphine ligands into a cis position (Scheme 14.20). No evaluation ofthe catalytic activity of complex 47 in olefin metathesis was reported. 

Rg. 5 Reductive elimination of C2-methylated imidazolium salts from Pd"-NHC complexes containing chelating NHC ligands... 

The xylyl-linked chelating NHC complexes 61-64 showed higher selectivity, with the iV-pentamethylphenyl derivative 64 proving to be most selective toward bis-arylation products [83]. Good activity was also reported with the half-sandwich complex 43 [84]. 

Other chiral C2-symmetric chelating NHC ligands were prepared by Veige and coworkers [36, 63, 64] by nucleophilic substitution of a bis-triflate with 1-alkylbenzimidazol to produce corresponding bis-imidazolium salts (Scheme 2.168). The latter were treated individually with 2 equiv of KN[Si(Me)3]2. 

Scheme 2.169 Synthesis of another chiral Cj-symmetric chelating NHC-based rhodium...

In addition to alkyl and aryl groups, allyl substituents were shown to undergo facile reductive elimination, in a process that was used for the generation of catalytically active Pd species from Pd precursors (see Section 3.5.3). Although they generally showed increased stability, chelating NHC ligands also underwent reductive elimination reactions. ... 

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