Synthesis of Carbene Ligands and Their Metal Complexes

Synthesis of Carbene Ligands and Their Metal Complexes 

Other routes construct the heterocycle in a one-pot reaction (routes B, C). 1-Substituted imidazoles are formed when an aqueous solution of glyoxal is treated with alkyl amine, formaldehyde, and ammonium salt (route B). The reaction proceeds in acidic media [26]. When 2equiv of the primary amine 

N-HetemcycHc Carbenes (NHCs) as Ligands in Transition-Metal-Catalyzed Hydrofornrylation 237 

Another approach, particularly valuable for the synthesis of imidazolium salts containing sterically hindered substituents, for example, multiple-substituted phenyl groups at the nitrogen atoms, consists of a two-step protocol (route D) [28]. Under mild conditions, lequiv of glyoxal is treated with 2equiv of arylamine in -propanol to form the corresponding diimine as an intermediate. Subsequent cyclization by condensation with chloromethylethyl ether leads to the desired diarylimidazolium salt in moderate yields (40 -47%) without the need of a purification step. 

Imidazolium salts with less hindered groups are relatively stable for some hours at room temperature. More sterically demanding substituents enhance the stability of the imidazolium salts by preventing the formation of enetetramines 

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