Carbon-hydrogen bond oxidative addition of partially fluorinated aromatics to a Ni(P~iPr_3)_2 synthon: The influence of steric bulk on the thermodynamics and kinetics of C-H bond activation

摘要

The reaction of (P~iPr_3)_2NiCl_2 with the anthracene adduct (THF)_3Mg(η~2-C _(14)H_(10)) in THF provides the anthracene adduct (P ~iPr_3)_2Ni(η~2-C_(14)H _(10)). In aromatic solvents (benzene, toluene, mesitylene) a thermal equilibrium exists between the bis(phosphine)nickel(0) anthracene adduct, (P~iPr_3)_2Ni(η~2-C _(14)H_(10)), and the monophosphine solvent adduct, (P ~iPr_3)Ni(η~6-solvent). The reaction of (P~iPr_3)_2Ni(η~2-C _(14)H_(10)) with 1,2,4,5-C_6F_4H _2 affords the C-H activation product trans-(P~iPr _3)_2NiH(2,3,5,6-C_6F_4H). The thermodynamic C-F activation product is not obtained even after hours of heating at 100 °C. Similar reactions with 1,2,3,5-C_6F_4H _2 and pentafluorobenzene produce the desired C-H activation products, trans-(P~iPr_3)_2NiH(2,3,4,6-C_6F _4H) and trans-(P~iPr_3)_2NiH(C _6F_5), 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(PEt _3)_2 synthons, the bulkier Ni(P~iPr _3)_2 moiety did not provide observable mononuclear or dinuclear η~2-fluoroarene adducts. Solutions of Ni(COD) _2 with 2 equiv of triisopropylphosphine and 1,2,4,5-C _6F_4H_2 reacted to give the 1,5-cyclooctadiene insertion and rearrangement product, (η~3-C_8H _(13))Ni(P~iPr_3)(2,3,5,6-C_6F _4H). The same reaction with 1,2,3,5- and 1,2,3,4-C_6F _4H_2 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 (P~iPr_3) _2NiH(Ar~F) (Ar~F = 2,3,5,6-C_6F _4H; C_6F_5) with 3-hexyne provided a mixture of the alkyne adduct (P~iPr_3)_2Ni(η~2- EtC=CEt), with the liberation of Ar~FH, and the insertion product (P~iPr_3)_2Ni(CEt=CHEt)(Ar~F), even in the presence of excess fluorinated aromatic Ar~FH. The reaction of (P~iPr_3)_2Ni(η~2-EtC=CEt) with 1,2,4,5-C_6F_4H_2 resulted in no reaction at room temperature, but heating at 50 °C provided the insertion product (P ~iPr_3)_2Ni(CEt=CHEt)(Ar~F) as the initial product, followed by the product of reductive elimination, 1,2,4,5-C_6F_4H-CEt=CHEt. In contrast, the reaction of (PEt_3)_2Ni(η~2-EtC=CEt) with 1,2,4,5-C _6F_4H_2 at 80 °C slowly produced (PEt _3)_2Ni(CEt=CHEt)(2,3,5,6-C_6F_4H), but very little 1,2,4,5-C_6F_4H-CEt=CHEt.