Origin of 1,4-Regiochemistry in the Dicouplings of Ketones with 6,6-Dimethylcyclohexadienyl Complexes of Titanium and Zirconium: A Mechanism Arising from Five-Electron Donation by Alkoxide Ligands

In an earlier study of the coupling reactions of Ti(C5H5)(6,6-dmch)(PMe3) (dmch = dimethylcyclohexadienyl) with ketones, the initially formed metal complex could not be isolated, but after hydrolysis an unprecedented 1,4-dicoupling product was isolated. To gain further insight into the origin of this regiochemistry, the reaction of a zirconium analogue, Zr(C5H5)(6,6-dmch)(PMe3)2, with acetone was investigated. A solid dimeric product could be isolated and structurally characterized, which revealed that both acetones had coupled, with the unusual 1,4-regiochemistry. This then necessitated the proposal of a new mechanism for these reactions, in which an alkoxide ligand, generated from a coupling of the initially incorporated ketone, serves as a formal five-electron donor, thereby altering the mode of coordination of the diene fragment. This proposal has been supported by the characterization of a mono(benzophenone) coupling product of Ti(C5H5)(6,6-dmch)(PMe3). This coupling product possessed both a large Ti...