Two Carbenes Better Than One

The first N-heterocyclic dicarbene, which has carbene centers at two positions in the imidazole ring instead of the usual one, has been synthesized by Yuzhong Wang, Gregory H. Robinson, and coworkers at the University of Georgia (J. Am. Chem. Soc., DOI: 10.1021/ja106631r). N-Heterocyclic carbenes (NHCs) are popular ligands for transition-metal homogeneous catalysts and for stabilizing highly reactive main-group compounds. Normal NHCs have the carbene center located between the two nitrogen atoms of an imidazole. But the carbene center can also be located at one of the other ring carbon atoms, resulting in a slightly less stable version dubbed an abnormal NHC. Robinson and colleagues began to wonder if there was a way to install carbene centers at both locations in the same imidazole ring, which they accomplished by using a lithium reagent to reduce a normal NHC containing bulky diisopropylphenyl groups. The researchers showed that treating the dicarbene with trimethylsilylchloride results in a trimethysilyl-substituted normal NHC. The ability to tune the electronic and steric properties of NHCs by swapping out different substituents could increase the utility of NHCs, Robinson suggests.