Evidence for a Carbon−Carbon Coupling Reaction To Proceed through a Planar-Tetracoordinate Carbon Intermediate

Treatment of bis(propynyl)zirconocene with B(C6F5)3 results in a linear C−C coupling of the alkynyl ligands to form the zwitterionic complex 3. Its treatment with excess RC⋮N yields an organometallic methylenecyclopropene derivative (6). This reaction topologically requires a very endothermic substituted butenyne to methylenecyclopropene cyclization to become energetically feasible by suitable stabilization effects. A DFT study has revealed that the 3 → 6 conversion is probably triggered by nitrile addition to the metal with formation of a planar-tetracoordinate carbon intermediate, featuring coordination of the three-membered carbocycle through one of its carbon−carbon σ bonds.