Addition of Dissimilar Carbenes across an Unsymmetrically Substituted Alkyne: Regio- and Stereoselective Synthesis of Trisubstituted 1,3-Dienes

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 (CHR1) to give a metallacyclobutene complex, followed by reaction with a second carbene addend (CHR2) 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.