On the Feasibility of Nickel-Catalyzed Trifluoromethylation of Aryl Halides

A computational screening of 42 bidentate
phosphines (PP) has yielded
promising candidates for Ph–CF3 reductive elimination
from Ni­(II) complexes of the type [(PP)­Ni­(Ph)­(CF3)]. The
computed barriers and synthetic accessibility considerations have
identified two PP ligands, dippf and dcypf (ΔG⧧ = 22.6 and 23.2 kcal/mol, respectively), for
experimental studies with 1-Np (1-naphthyl) in place of Ph. Ligand
exchange of [(Ph3P)2Ni­(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 CF3SiMe3/F–, AgCF3/MeCN or [(Ph3P)3Cu­(CF3)] does not furnish isolable
or 19F NMR-detectable [(PP)­Ni­(1-Np)­(CF3)] (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(CF3)], a rare example of well-defined CF3Cu­(I) species. With CF3SiMe3/F–, indirect evidence has been obtained for intermediacy of [(PP)­Ni­(1-Np)­(CF3)] (PP = dippf, dcypf) and instantaneous decomposition via
pathways other than C–CF3 reductive elimination.
The first Ni­(II) complexes with fluoride trans to a non-electron-deficient
aryl, [(Cy3P)2Ni­(1-Np)­F] and [(i-PrXantphos)­Ni­(1-Np)­F], have been prepared and fully characterized.
Surprisingly, [(Cy3P)2Ni­(1-Np)­F] can be produced
from [(Cy3P)2Ni­(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 [(Cy3P)2Ni­(1-Np)­F] is unreactive toward CF3SiMe3, [(i-PrXantphos)­Ni­(1-Np)­F] is readily trifluoromethylated to
produce robust [(i-PrXantphos)­Ni­(1-Np)­(CF3)], a rare example of complexes of the type [(PP)­Ni­(Ar)­(CF3)] with PP other than dippe.