Ortho-Fluoro Effect on the C–C Bond Activation of Benzonitrile Using Zerovalent Nickel
Résumé
The effect of fluoro substitution on the C-C bond activation of aromatic nitriles has been studied by reacting a variety of fluorinated benzonitriles with the nickel(0) fragment [Ni(dippe)] and by locating the reaction intermediates and transition-state structures on the potential energy surface by using density functional theory calculations with the [Ni(dmpe)] fragment (dippe = 1,2-bis-(diisopropylphosphino)ethane, dmpe = 1,2-bis(dimethylphosphino)ethane). As in the previous reports, the reaction of fluorinated benzonitriles with the [Ni(dippe)] fragment initially formed an η 2 -nitrile complex, which then converted to the C-CN bond activation product. Thermodynamic parameters for the equilibrium between these complexes have been determined experimentally in both a polar (tetrahydrofuran) and a nonpolar (toluene) solvent for 3-fluoro-and 4-fluorobenzonitrile. The stability of the C-C bond activation products is shown to be strongly dependent on the number of ortho-F substituents (-6.6 kcal/mol per o-F) and only slightly dependent on the number of meta-F substituents (-1.8 kcal/mol per m-F).
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