An unusual cobalt-mediated cleavage of a hindered alkyne

When 1-(pentaphenylphenyl)-2-phenylacetylene (1) is heated with (eta(5)-cyc lopentadienyl)dicarbonylcobalt, 1,2,3,4-tetraphenylfluorene (3) and related minor products are formed rather than the expected tetraaryl-cyclobutadien e. The formation of 3 requires that a seven-carbon fragment (formally a phe nylcarbyne) must be lost. Two minor products, however, result from intramol ecular cyclization of the alkyne and these two retain all of the carbons fr om the starting material. (C) 2000 Elsevier Science Ltd. All rights reserve d.