Decisive Steps of the Hydrodefluorination of Fluoroaromatics using [Ni(NHC)(2)]

作者：Fischer Peter; Goetz Kathrin; Eichhorn Antonius; Radius Udo^*

来源：Organometallics, 2012, 31(4): 1374-1383.

DOI：10.1021/om2009815

The hydrodefluorination reaction of perfluorinated arenes using [Ni-2((i)Pr(2)Im)(4)(COD)] (1; (i)Pr(2)Im = 1,3-bis(isopropyl)imidazolin-2-ylidene) as a catalyst as well as stoichiometric transformations to elucidate the decisive steps for this reaction are reported. The reaction of hexafluorobenzene with 5 equiv of triphenylsilane in the presence of 5 mol % of 1 affords 1,2,4,5-tetrafluorobenzene after 48 h at 60 degrees C and 1,4-difluorobenzene after 96 h at 80 degrees C; the reaction of perfluorotoluene and 5 equiv of Et3SiH for 4 days at 80 degrees C results in the selective formation of 1-(CF3)-2,3,5,6-C6F4H. Stoichiometric transformations of the complexes cis-[Ni((i)Pr(2)Im)(2)(H)(SiPh3)] and cis-[N-i((i)Pr(2)Im)(2)(H)(SiMePh2)] with hexafluorobenzene at room temperature lead to the formation of trans-[Ni((i)Pr(2)Im)(2)(F)-(C6F5)] (2) and trans-[Ni((i)Pr(2)Im)(2)(H)(C6F5)] (4) with elimination of the corresponding silane or fluorosilane. The reactions of the C-F activation products trans-[Ni((i)Pr(2)Im)(2)(F)-(C6F5)] (2) and trans-[Ni((i)Pr(2)Im)(2)(F)(4-(CF3)C6F4)] (3) with PhSiH3 and Ph2SiH2 afford the hydride complexes trans-[Ni((i)Pr(2)Im)(2)(H)(C6F5)] (4) and trans-[Ni((i)Pr(2)Im)(2)(H)(4-(CF3)C6F4)] (5), which convert into the compounds trans-[Ni((i)Pr(2)Im)(2)(F)(2,3,5,6-C6F4H)] (7), trans-[Ni((i)Pr(2)Im)(2)(F)(3-(CF3)-2,4,5-C6F3H)] (9a), and trans-[Ni-((i)Pr(2)Im)(2)(F)(2-(CF3)-3,4,6-C6F3H)] (9b), respectively. In the case of the rearrangement of trans-[Ni((i)Pr(1)Im)(2)(H)(4-(CF3)C6F4)] (5) the intermediate [Ni((i)Pr(2)Im)(2)(eta(2)-C,G-(CF3)C6F4H)] (8) was detected. Reaction of 8 with perfluorotoluene gave the C-F activation product trans-[Ni((i)Pr(2)Im)(2)(F)(4-(CF3)C6F4)] (3). All these experimental findings point to a mechanism for the HDF by [Ni((i)Pr(2)Im)(2)] via the %26quot;fluoride route%26quot; involving C-F activation of the polyfluoroarene, H/F exchange of the resulting nickel fluoride, reductive elimination of the polyfluoroaryl nickel hydride to an intermediate with a eta(2)-C,C-coordinated arene ligand, subsequent ligand exchange with the higher fluorinated polyfluoroarene, and renewed G F activation of the polyfluoroarene. Without additional reagents, [Ni((i)Pr(2)Im)(2)(eta(2)-C,C-(CF3)C6F4H)] (8) rearranges to the isomers trans-[Ni((i)Pr(2)Im)(2)(F)(3-(CF3)-2,4,5-C6F3H)] (9a; major) and trans-[Ni((i)Pr(2)Im)(2)(F)(2-(CF3)-3,4,6-C6F3H)] (9h; minor) in a ratio of 80:20. %26lt;br%26gt;DFT calculations performed on conversion of trans-[Ni((i)Pr(2)Im)(2)(H)(4-(CF3)C6F4)] 5 into the two products trans-[Ni((i)Pr(2)Im)(2)(F)(3-(CF3)-2,4,5-C6F3H)] 9a and trans-[Ni((i)Pr(2)Im)(2)(F)(2-(CF3)-3,4,6-C6F3H)] (9b) using the commonly accepted intramolecular concerted pathway via re-C,F-a-bound transition states predict 9b to be the major product. We thus propose that this reaction mechanism is not valid for the [Ni(NHC)(2)]-mediated C-F activation of partially fluorinated arenes with special substitution patterns.

• 出版日期2012-2-27