Aliphatic and Aromatic Carbon−Fluorine Bond Activation with Cp*2ZrH2: Mechanisms of Hydrodefluorination

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Abstract
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.

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