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.