C-F Activation of Fluorinated Arenes using NHC-Stabilized Nickel(0) Complexes: Selectivity and Mechanistic Investigations

摘要

The reaction of [Ni_2(~iPr_2lm)_4(COD)] 1a or [Ni(~iPr_2lm)_2(η~2-C_2H_4)] 1b with different fluorinated arenes is reported. These reactions occur with a high chemo- and regioselectivity. In the case of polyfluorinated aromatics of the type C_6F_5X such as hexafluorobenzene (X = F) octafluorotoluene (X = CF_3), trimethyl(pentafluorophe-nyl)silane (X = SiMe_3), or decafluorobiphenyl (X = C_6F_5) the C-F activation regioselectively takes place at the C-F bond in the para position to the X group to afford the complexes frans-[Ni(~iPr_2lm)_2(F)(C_6F_5)] 2, trans-[Ni(~iPr_2lm)_2(F)(4-(CF_3)C_6F_4)] 3, trans-[Ni(~iPr_2lm)_2(F)(4-(C_6F_5)C_6F_4] 4, and trans-[Ni(~iPr_2lm)_2(F)(4-(SiMe_3)C_6F_4)] 5. Complex 5 was structurally characterized by X-ray diffraction. The reaction of 1a with partially fluorinated aromatic substrates C_6H_xF_y leads to the products of a C-F activation trans-[Ni(~iPr_2lm)_2(F)(2-C_6FH_4)] 7, trans-[Ni(~iPr_2lm)_2(F)(3,5-C_6F_2H_3)] 8, trans-[Ni(~iPr_2lm)_2(F)(2,3-C_6F_2H_3)] 9a and trans-[Ni(~iPr_2lm)_2(F)(2,6-C_6F_2H_3)] 9b, trans-[Ni(~iPr_2lm)_2(F)(2,5-C_6F_2H_3)] 10, and trans-[Ni(~iPr_2lm)_2(F)(2,3,5,6- C_6F_4H)] 11. The reaction of 1a with octafluo-ronaphthalene yields exclusively trans-[Ni(~iPr_2lm)_2(F)(1,3,4,5,6,7,8-C_(10)F_7)] 6a, the product of an insertion into the C-F bond in the 2-position, whereas for the reaction of 1b with octafluoronaphthalene the two isomers trans-[Ni(~iPr_2lm)_2(F)(1,3,4,5,6,7,8-C_(10)F_7)] 6a and trans-[Ni(~iPr_2lm)_2(F)(2,3,4,5,6,7,8-C_(10)F_7)] 6b are formed in a ratio of 11:1. The reaction of 1a or of 1b with pentafluoropyridine at low temperatures affords trans-[Ni(~iPr_2lm)_2(F)(4-C_5NF_4)] 12a as the sole product, whereas the reaction of 1b performed at room temperature leads to the generation of trans-[Ni(~iPr2lm)_2(F)(4-C_5NF_4)] 12a and trans-[Ni(~iPr_2lm)_2(F)(2-C_5NF_4)] 12b in a ratio of approximately 1:2. The detection of intermediates as well as kinetic studies gives some insight into the mechanistic details for the activation of an aromatic carbon-fluorine bond at the {Ni(~iPr_2lm)_2} complex fragment. The intermediates of the reaction of 1b with hexafluorobenzene and octafluoronaphthalene, [Ni(~iPr_2lm)_2(η~2-C_6F_6)] 13 and [Ni(~iPr_2lm)_2(η~2-C_(10)F_8)] 14, have been detected in solution. They convert into the C-F activation products. Complex 14 was structurally characterized by X-ray diffraction. The rates for the loss of 14 at different temperatures for the C-F activation of the coordinated naphthalene are first order and the estimated activation enthalpy ΔH~≠ for this process was determined to be ΔH~≠ = 116 ± 8 kJ mol~(-1) (ΔS~* = 37 ± 25 J K~(-1) mol~(-1)). Furthermore, density functional theory calculations on the reaction of 1a with hexafluorobenzene, octafluoronaphthalene, octafluorotoluene, 1,2,4-trifluorobenzene, and 1,2,3-trifluorobenzene are presented.