Carbon−Hydrogen Bond Oxidative Addition of Partially Fluorinated Aromatics to a Ni(PiPr3)2 Synthon: The Influence of Steric Bulk on the Thermodynamics and Kinetics of C−H Bond Activation
journal contributionposted on 22.11.2010, 00:00 by Jillian A. Hatnean, Robert Beck, Jenna D. Borrelli, Samuel A. Johnson
The reaction of (PiPr3)2NiCl2 with the anthracene adduct (THF)3Mg(η2-C14H10) in THF provides the anthracene adduct (PiPr3)2Ni(η2-C14H10). In aromatic solvents (benzene, toluene, mesitylene) a thermal equilibrium exists between the bis(phosphine)nickel(0) anthracene adduct, (PiPr3)2Ni(η2-C14H10), and the monophosphine solvent adduct, (PiPr3)Ni(η6-solvent). The reaction of (PiPr3)2Ni(η2-C14H10) with 1,2,4,5-C6F4H2 affords the C−H activation product trans-(PiPr3)2NiH(2,3,5,6-C6F4H). The thermodynamic C−F activation product is not obtained even after hours of heating at 100 °C. Similar reactions with 1,2,3,5-C6F4H2 and pentafluorobenzene produce the desired C−H activation products, trans-(PiPr3)2NiH(2,3,4,6-C6F4H) and trans-(PiPr3)2NiH(C6F5), respectively, in >95% yield. The reaction with 1,2,3,4-tetrafluorobenzene did not produce an observable C−H activation product. Unlike previously reported analogous C−H activation products with Ni(PEt3)2 synthons, the bulkier Ni(PiPr3)2 moiety did not provide observable mononuclear or dinuclear η2-fluoroarene adducts. Solutions of Ni(COD)2 with 2 equiv of triisopropylphosphine and 1,2,4,5-C6F4H2 reacted to give the 1,5-cyclooctadiene insertion and rearrangement product, (η3-C8H13)Ni(PiPr3)(2,3,5,6-C6F4H). The same reaction with 1,2,3,5- and 1,2,3,4-C6F4H2 afforded analogous compounds, which demonstrates that C−H bond activation is kinetically accessible at room temperature with 1,2,3,4-tetrafluorobenzene despite the presence of a single ortho-fluorine substituent adjacent to the site of activation. The room-temperature reactions of the C−H activation products (PiPr3)2NiH(ArF) (ArF = 2,3,5,6-C6F4H; C6F5) with 3-hexyne provided a mixture of the alkyne adduct (PiPr3)2Ni(η2-EtCCEt), with the liberation of ArFH, and the insertion product (PiPr3)2Ni(CEtCHEt)(ArF), even in the presence of excess fluorinated aromatic ArFH. The reaction of (PiPr3)2Ni(η2-EtCCEt) with 1,2,4,5-C6F4H2 resulted in no reaction at room temperature, but heating at 50 °C provided the insertion product (PiPr3)2Ni(CEtCHEt)(ArF) as the initial product, followed by the product of reductive elimination, 1,2,4,5-C6F4H-CEtCHEt. In contrast, the reaction of (PEt3)2Ni(η2-EtCCEt) with 1,2,4,5-C6F4H2 at 80 °C slowly produced (PEt3)2Ni(CEtCHEt)(2,3,5,6-C6F4H), but very little 1,2,4,5-C6F4H-CEtCHEt.