om500938u_si_002.cif (6.3 MB)
Ortho Derivatization of Phenols through C–H Nickelation: Synthesis, Characterization, and Reactivities of Ortho-Nickelated Phosphinite Complexes
dataset
posted on 2014-11-24, 00:00 authored by Boris Vabre, Félix Deschamps, Davit ZargarianReported
here are the synthesis and characterization of ortho-nickelated complexes
derived from phosphinite ligands and investigated as model compounds
in the development of C–H functionalization strategies for
arenol substrates. Reaction of i-Pr2POPh
with 0.6 equiv of [(i-PrCN)NiBr2]n and 0.8 equiv of NEt3 in toluene
(100 °C, 36 h) gave the yellow, monomeric cyclometalated complex trans-{κ2P,C-C6H4OP(i-Pr)2}Ni(i-Pr2POPh)Br (3a) in 93% yield. The closely related
yellow-orange dimeric species [{κ2P,C-C6H4OP(i-Pr)2}Ni(μ-Br)]2 (4a) was
obtained in 70% yield when i-Pr2POPh was
treated with 2 equiv each of the Ni precursor and NEt3.
These complexes have been characterized fully and shown to interconvert
in the presence of excess ligand (4a → 3a) or excess Ni precursor (3a → 4a). Treatment of 3a or 4a with benzyl bromide
at 90 °C over extended periods led to benzylation of the Ni–aryl
moiety in these complexes. Examination of the cyclometalation pathway
for i-Pr2POPh has shown that the first
species formed from its ambient-temperature reaction with [(i-PrCN)NiBr2]n is trans-(i-Pr2POPh)2NiBr2 (2a). NMR studies showed that 2a undergoes a rapid ligand exchange at room temperature,
which can be slowed down at −68 °C; this fluxional process
shifts in the presence of NEt3, implying the partial formation
of an amine adduct. Heating toluene mixtures of 2a and
NEt3 at 90 °C for 38 h led to the formation of 3a via C–H nickelation. That phosphinite dissociation
from 2a precedes the C–H nickelation step is implied
by the observation that the formation of 3a is hindered
in the presence of excess i-Pr2POPh. The
impact of phenol ring substituents on the C–H nickelation rate
was probed by preparing substituted derivatives of 2a, trans-(4-R-C6H4OP(i-Pr2)}2NiBr2 (R = OMe
(2b), Me (2c), COOMe (2d)),
and measuring their relative rates of C–H nickelation. These
studies showed that the formation of cyclonickelated products is favored
in the order COOMe < Me < OMe, which is consistent with an electrophilic
nickelation mechanism. Studying the C–H nickelation of 3-F-C6H4OP(i-Pr2) allowed
us to establish that metalation is favored at the para position with
respect to F (85:15).