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

The reaction of (η4-C4Ph4) Co[η5-C5H4–C(double bond; length as m-dashO)Me], 5, with benzophenone under McMurry conditions (TiCl4/Zn/THF) gives the hetero-coupled product (η4-C4Ph4)Co[η5-C5H4–C(Me)double bond; length as m-dashCPh2], 7, together with the dicobalt species: trans-(η4-C4Ph4)Co[(η5-C5H4–C(Me)double bond; length as m-dashC(Me)-η5-C5H4−)] Co(η4-C4Ph4), 9, and the pinacolone Me[(η4-C4Ph4)Co(η5-C5H4)]2C–C(double bond; length as m-dashO)Me, 10. The latter is apparently formed from the pinacol by migration of an (η4-C4Ph4)Co[(η5-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 (η4-C4Ph4)Co[η5- C5H4–C(double bond; length as m-dashO)Et], 6, were all characterized by X-ray crystallography.