The formal addition of dissimilar carbenes across an unsymmetrically substituted alkyne has been achieved for the first time by a two-step sequence in which a cobalt alkyne complex undergoes reaction with a carbene addend (CHR(1)) to give a metallacyclobutene complex, followed by reaction with a second carbene addend (CHR(2)) to give a mixture of diene complexes. In situ treatment of the diene mixture with fluoride leads to desilylation and conversion to a single cobalt diene complex with a high degree of regio- and stereoselectivity.
