Singlet Carbenes Derived from Ferrocene and Closely Related Sandwich Complexes

This microreview describes the tremendous progress that has been made during the past decade in the area of ferrocene-based singlet carbenes, the vast majority of which belong to the family of N-heterocyclic carbenes (NHCs). The ferrocene motif is present in such NHCs either as a ferrocenyl (Fc) substituent or a 1,1'-ferrocenediyl (fc) backbone. In terms of applications, the modification of popular ferrocene-based chiral P ligands by following the phosphane-NHC analogy has been an important guideline for much of the work directed towards catalysis. A particularly exciting development has occurred with NHCs that contain an fc backbone, since these compounds have proved to be capable of small-molecule activation. The recent development of singlet carbenes that are not based on ferrocene but rather on related ruthenium sandwich complexes demonstrates the wide scope and highly dynamic nature of the field.