NHCs with unsaturated backbone

Precursors for NHCs with Unsaturated Backbone (Imidazolin-2-ylidenes... 

The reaction of triazolium and benzimidazolium salts with sodium methoxide yields the corresponding methoxy-triazoles and benzimidazoles [30,31], which can be also used as triazolilydene and benzimidazo-lilydene precursors. Notably, adduct formation does not occur for certain unsaturated imidazolium salts with a C = C backbone. For the latter, reaction with KOBu results in direct deprotonation to the free NHC (Scheme 8, also shows the reaction of a dihydroimidazolium salt with KOBu ) [32],... 

Another possibility to anchor the NHC unit to a polymeric support is to attach the linker unit to the imidazole backbone rather than using the wingtip group for it. This approach was realised by Connon and Blechert with a saturated functionalised NHC ligand (see Figure 4.76) [245]. There is no real advantage to this approach other than having a saturated NHC as opposed to an unsaturated one. 

When the free carbenes are stable enough, as is the case with most unsaturated imidazolylidene derivatives, they can be isolated and engaged in substitution reactions. The first such preparation using this method was reported in 1999 by Nolan [16], who prepared RuCl2(indenylidene)(imidazolylidene)(PR3) complexes 7—10 from Af,A/ -bis((mesityl)imidazol-2-ylidene) (IMes) and A5A/ -bis((2,6-diisopropylphenyl)imidazolyl-2-ylidene) (IPr) carbenes in toluene at room temperature (Scheme 14.5). Other NHCs substituted on the NHC backbone [48, 49] and various Af-aryl groups [50] have also been prepared and coordinated onto ruthenium indenylidene moieties using the free or in sif -generated carbenes with complex 6. 

During the last decade, organoboron compounds have opened up new perspectives in chemistry. Such intermediates can promote several transformations. When boranes are employed, interesting regioselectivity can be induced. Fernandez and coworkers were the first to investigate the p-boration of unsaturated carbonyl compounds (Scheme 8.12) [33]. Different NHC systems were used, with chirality either on the Af-moiety or on the backbone of the imidazole ring. Cationic and neutral systems [Cu(MeCN)(NHC)]BF4 and [Cu(Cl)(NHC)] were compared in the presence of B2pin2. The nature of the anion did not affect the enantioselectivity or the conversion, which can be explained by the formation of... 

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