On the Feasibility of Nickel-Catalyzed Trifluoromethylation of Aryl Halides

Abstract

A computational screening of 42 bidentate phosphines (PP) has yielded promising candidates for Ph–CF<sub>3</sub> reductive elimination from Ni­(II) complexes of the type [(PP)­Ni­(Ph)­(CF<sub>3</sub>)]. The computed barriers and synthetic accessibility considerations have identified two PP ligands, dippf and dcypf (Δ<i>G</i><sup>⧧</sup> = 22.6 and 23.2 kcal/mol, respectively), for experimental studies with 1-Np (1-naphthyl) in place of Ph. Ligand exchange of [(Ph<sub>3</sub>P)<sub>2</sub>Ni­(1-Np)­Cl] with dippf or dcypf has cleanly produced [(dippf)­Ni­(1-Np)­Cl] and [(dcypf)­Ni­(1-Np)­Cl], the first examples of trans square-planar 1,1′-ferrocenediyl backbone-based diphosphine metal complexes devoid of M···Fe dative interactions. Treatment of these chlorides with CF<sub>3</sub>SiMe<sub>3</sub>/F<sup>–</sup>, AgCF<sub>3</sub>/MeCN or [(Ph<sub>3</sub>P)<sub>3</sub>Cu­(CF<sub>3</sub>)] does not furnish isolable or <sup>19</sup>F NMR-detectable [(PP)­Ni­(1-Np)­(CF<sub>3</sub>)] (PP = dippf, dcypf). Other transformations have been observed instead, e.g., ligand exchange with the Ag and Cu complexes, the latter leading to [(dcypf)­Cu­(CF<sub>3</sub>)], a rare example of well-defined CF<sub>3</sub>Cu­(I) species. With CF<sub>3</sub>SiMe<sub>3</sub>/F<sup>–</sup>, indirect evidence has been obtained for intermediacy of [(PP)­Ni­(1-Np)­(CF<sub>3</sub>)] (PP = dippf, dcypf) and instantaneous decomposition via pathways other than C–CF<sub>3</sub> reductive elimination. The first Ni­(II) complexes with fluoride trans to a non-electron-deficient aryl, [(Cy<sub>3</sub>P)<sub>2</sub>Ni­(1-Np)­F] and [(<i>i-</i>PrXantphos)­Ni­(1-Np)­F], have been prepared and fully characterized. Surprisingly, [(Cy<sub>3</sub>P)<sub>2</sub>Ni­(1-Np)­F] can be produced from [(Cy<sub>3</sub>P)<sub>2</sub>Ni­(1-Np)­Cl] and CsF rather than AgF that is conventionally used for the synthesis of late transition metal fluorides via X/F exchange. While [(Cy<sub>3</sub>P)<sub>2</sub>Ni­(1-Np)­F] is unreactive toward CF<sub>3</sub>SiMe<sub>3</sub>, [(<i>i-</i>PrXantphos)­Ni­(1-Np)­F] is readily trifluoromethylated to produce robust [(<i>i-</i>PrXantphos)­Ni­(1-Np)­(CF<sub>3</sub>)], a rare example of complexes of the type [(PP)­Ni­(Ar)­(CF<sub>3</sub>)] with PP other than dippe

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