On the Feasibility of Nickel-Catalyzed Trifluoromethylation
of Aryl Halides
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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