Unexpected Carbene Behavior

With only six valence elections, the carbene carbon at the 2-position of N-heterocyclic carbenes (NHCs) typically sees the most action, binding with metals and coupling with electrophiles. So researchers at the University of British Columbia were surprised to discover the NHC 1,3-dimesitylimidazol-2-ylidene reacts with a phosphaalkene exclusively at the 4-position, leaving the carbene carbon sitting idle (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200905401). This abnormal behavior is extremely rare between NHCs and p-block atoms, particularly when the carbene carbon isn’t bound to a metal or protecting group. The product, a 4-phosphino-2-carbene, is the first reported NHC bearing a phosphine at the 4-position, according to Derek P. Gates, Joshua I. Bates, and Pierre Kennepohl, who did the research. Furthermore, calculations reveal that the carbene lone pair of electrons and the phosphorus lone pair of electrons are close in energy, suggesting it might be possible to tune the donor properties of the bifunctional ligand. “This work opens the door to abnormal reactions of NHCs with other unsaturated molecules and to the development of novel bifunctional ligands for use in catalysis,” the researchers say.