McMurry reactions of (η5-acetylcyclopentadienyl) cobalt-(η4-tetraphenylcyclobutadiene)with benzophenone:: ketone couplings and a pinacol/pinacolone rearrangement

The reaction of (eta(4)-C4Ph4) CO[eta(5)-C5H4-C(=O)Me], 5, with benzophenone under McMurry conditions (TiCl4/Zn/THF) gives the hetero-coupled product (eta(4)-C4Ph4)Co[eta(5)-C5H4-C(Me)=CPh2], 7, together with the dicobalt species: trans-(eta(4)-C4Ph4)Co[(eta(5)-C5H4C(Me)=C(Me)-eta(5) -C5H4-] Co(eta(4)-C4Ph4), 9, and the pinacolone Me[(eta(4)-C4Ph4)Co(eta(5)-C5H4)](2)C-C(=O)Me, 10. The latter is apparently formed from the pinacol by migration of an (eta(4)-C4Ph4)Co[(eta(5)-C5H4)] group. Preferential migration of the cobalt sandwich moiety rather than a methyl group is rationalized in terms of a favored transition state involving a metal-stabilized cation. The products 7, 9 and 10, and also the ketone (eta(4)-C4Ph4)Co[eta(5)-C5H4-C(=O)Et], 6, were all characterized by X-ray crystallography. (C) 2004 Elsevier B.V. All rights reserved.