Synthesis of 4- and 4,5-functionalized imidazol-2-ylidenes from a single 4,5-unsubstituted imidazol-2-ylidene.

Using the nucleophilicity of NHCs and aNHCs, as well as the leaving group ability of the former, the carbon-carbon double bond of imidazol-2-ylidenes can be readily mono- and difunctionalized. These results provide also a new light on the formation of abnormal carbene adducts from classical unsaturated NHCs.