Aliphatic and aromatic carbon-fluorine bond activation with Cp*2ZrH2: Mechanisms of hydrodefluorination

Article abstract of DOI:10.1021/ja016087l

Cp*₂ZrH₂ (1) (Cp* = pentamethylcyclopentadienyl) reacts with primary, secondary, and tertiary monofluorinated aliphatic hydrocarbons to give Cp*₂ZrHF (2) and/or Cp*₂ZrF₂ and alkane quantitatively through a radical chain mechanism. The reactivity of monofluorinated aliphatic C-F bonds decreases in the order 1° > 2° > 3°. The rate of hydrodefluorination was also greatly reduced with -CF₂H and -CF₃ groups attached to the hydrocarbon. An atmosphere of H₂ is required to stabilize 1 against C-H activation of the Cp*-methyl groups and subsequent dimerization under the thermal conditions employed in these reactions. Reaction of 1 with fluorobenzene cleanly forms a mixture of Cp*₂ZrHF, benzene, and Cp*₂Zr(C₆H₅)F. Detailed studies indicate that radicals are not involved in this aromatic C-F activation reaction and that dual hydrodefluorination pathways are operative. In one mechanism, hydridic attack by Cp*₂ZrH₂ on the aromatic ring and fluoride abstraction is involved. In the second mechanism, an initial ortho C-H activation occurs, followed by β-fluoride elimination to generate a benzyne complex, which then inserts into the zirconium-hydride bond.