Benzannulated NHCs

Likewise, the direct addition of a free NHC to an early transition metal precursor may lead to the corresponding NHC-M adducts. For example, Kuhn et al. reported that the reaction of a free NHC with TiCU leack to the formation of the mono-adduct (NHOTiCU (NHC = l,3-dialkyl-4,5-dimethylimidazol-2-ylidene). A careful hydrolysis of the latter afforded the corresponding NHC p-oxo titanium(IV) complex, which was X-ray characterized [29]. Direct coordination of the free carbene was also used to access various monodentate adducts such as (NHC)2TiF4 [30,31], (NHC)TiCl2(NMe2)2 [32,33], or benzannulated NHC Ti complex (NHC)TiCU [34]. 

The reductive desulfurization of cyclic thiones (Scheme 1.9) has become an alternative method for the preparation of saturated, unsaturated and benzannulated NHCs. " It is, however, less frequently used than the deprotonation of azolium salts. The choice of reducing agent depends on the starting thione. Imidazol-2-thiones were reduced to the free carbenes with potassium in boiling THF within 4h, while benzimidazol-2-thiones were reduced with a Na/K alloy in toluene requiring a reaction time of up to 3 weeks at ambient temperature. 

An alternative method for the synthesis of [(NHC)M] complexes is to bind ligands to a transition metal that can subsequently be converted into an NHC ligand. Using 2-azidophenyl isocyanide" " or 2-nitrophenyl isocyanide" as synthons for the unstable 2-amino isocyanide, diprotic benzannulated NHC-metal complexes were obtained, and easily alkylated to give A,A -disubstituted benzimidazolylidenes. This methodology was also applied to ruthenium-based catalysts (Scheme 3.6)." ... 

Scheme 3.6 Templated synthesis of a diprotic benzannulated NHC-ruthenium complex.

Density functional theory studies have shown that annulation of imid-azol-2-ylidenes generally destabilizes the NHC. This becomes apparent also when analogies between benzannulated NHCs and saturated... 

Cyclotrimerization of triynes has been reported using cobalt or iron complexes of NHCs without CO and Cp ligands.31 Benzannulated compounds have been isolated with up to quantitative yields. 

Figure 1.9 Benzannulated and heterocycle-annulated NHCs (Np = neopentyl Am = amyl).

DFT studies showed that annulation of imidazol-2-ylidenes generally destabilizes the NHC. This becomes also apparent when analogies between benzannulated N-heterocyclic carbenes and saturated imidazolin-2-ylidenes 86 (see Figure 1.8) are considered. Both types of NHC dimerize rapidly if the nitrogen atoms of the heterocycle are substituted with sterically less demanding substituents. No such dimerization is observed with imidazol-2-ylidenes 78, which possess an unsaturated heterocycle like the benzimidazol-2-ylidenes. 

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