An unusual cobalt-mediated cleavage of a hindered alkyne

When 1-(pentaphenylphenyl)-2-phenylacetylene (1) is heated with (eta(5)-cyclopentadienyl)dicarbonylcobalt, 1,2,3,4-tetraphenylfluorene (3) and related minor products are formed rather than the expected tetraaryl-cyclobutadiene. 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. (C) 2000 Elsevier Science Ltd. All rights reserved.