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

Cp*2ZrH2 (1) (Cp* = pentamethyleyelopentadienyl) reacts with primary, secondary, and tertiary monofluorinated aliphatic hydrocarbons to give Cp*2ZrBF (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 degrees > 2' > 3'. The rate of hydrodefluorination was also greatly reduced with -CF2H and -CF3 groups attached to the hydrocarbon. An atmosphere of H-2 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 I with fluorobenzene. cleanly forms a mixture of CP*2ZrBF, benzene, and CP*Zr-2(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 beta -fluoride elimination to generate a benzyne complex, which then inserts into the zirconium-hydride bond.