1 -Methylimidazol-2-ylidene, palladium complex

Garbene-transfer reactions are particularly valuable in the formation of Pd-carbene complexes in which the free carbene is not readily accessible, or where a functionalized imidazolium salt, which contains acidic protons other that the G2 proton, is used. Therefore, an important development in the synthesis of carbene complexes was the use of Ag-carbene compounds to transfer the carbene to other metal centers. The complex PdCl2(A,jV -diethylbenzimidazol-2-ylidene) was prepared by this route.The Ag-carbenes are prepared by the reaction of imidazolium salts with the weakly basic Ag20. Thermally stable liquid crystalline Pd(ii)-carbene complexes 67 and 68 were also prepared by this method.In this case, the Ag-carbene compounds were generated and used in situ to form the palladium complexes. The unsymmetrical complex PdCl2(l-ethyl-3-methylimidazol-2-ylidene)2 complex, with different A-alkyl substituents, has also been synthesized via the Ag transfer route and the complex has been structurally characterized. 

Crabtree prepared rhodium, iridium, ruthenium, and palladium complexes e.g. 30) bearing NHCs by heating the carboxylate compound and an appropriate metal precursor in acetonitrile [eqn (2.7)]. It was also shown that the carboxylate of l,3-dimethylimidazol-2-ylidene (IMe) could be prepared from the direct reaction of 1-methylimidazole with dimethyl carbonate. Subsequently, Delaude showed that these species can be used in lieu of free NHCs for the synthesis of alkene metathesis pre-catalysts and to form palladium cross-coupling catalysts in situ while Olszewski has used these to prepare silver(i) and eopper(i) complexes. Falvey has shown that solvent choice is important, beeause the rate of decarboxylation is higher in less polar solvents. ... 

---