An unusual cobalt-mediated cleavage of a hindered alkyne

When 1-(pentaphenylphenyl)-2-phenylacetylene (1) is heated with (η5-cyclopentadienyl)dicarbonylcobalt, 1,2,3,4-tetraphenylfluorene (3) and related minor products are formed rather than the expected tetraarylcyclobutadiene. The formation of 3 requires that a seven-carbon fragment (formally a phenylcarbyne) must be lost. Two minor products, however, result from intramolecular cyclization of the alkyne and these two retain all of the carbons from the starting material.