Oxidation of Ni(II) to Ni(IV) with Aryl Electrophiles Enables Ni-Mediated Aryl-CF3 Coupling

Article abstract of DOI:10.1021/jacs.5b04892

This communication describes the synthesis and reactivity of Ni^IV(aryl)(CF₃)₂ complexes supported by trispyrazolylborate and 4,4′-di-tert-butylbipyridine ligands. We demonstrate that isolable Ni^IV complexes can be accessed under mild conditions via the oxidation of Ni^II precursors with S-(trifluoromethyl)dibenzothiophenium triflate as well as with diaryliodonium and aryl diazonium reagents. The Ni^IV intermediates undergo high yielding aryl-CF₃ bond-forming reductive elimination. These studies support the potential viability of Ni^IV intermediates in nickel-catalyzed coupling reactions involving diaryliodonium and aryldiazonium electrophiles.

Full text of DOI:10.1021/jacs.5b04892

Communication
pubs.acs.org/JACS
Oxidation of Ni(II) to Ni(IV) with Aryl Electrophiles Enables Ni-
Mediated Aryl−CF3 Coupling
James R. Bour,† Nicole M. Camasso,† and Melanie S. Sanford*
University of Michigan, Department of Chemistry, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
S
* Supporting Information
NiII precursors to NiIV products? and (2) What is the reactivity
of the putative NiIV(aryl) complexes? To address these
ABSTRACT: This communication describes the syn-
thesis and reactivity of NiIV(aryl)(CF3)2 complexes
questions, we needed access to detectable and ideally isolable
NiIV(aryl) species. A recent report from our group has shown
that organometallic NiIV complexes can be prepared by the
oxidation of NiII starting materials with electrophilic trifluor-
supported by trispyrazolylborate and 4,4′-di-tert-butyl-
bipyridine ligands. We demonstrate that isolable NiIV
complexes can be accessed under mild conditions via the
oxidation of NiII precursors with S-(trifluoromethyl)-
dibenzothiophenium triflate as well as with diaryliodonium
and aryl diazonium reagents. The NiIV intermediates
undergo high yielding aryl−CF3 bond-forming reductive
elimination. These studies support the potential viability of
NiIV intermediates in nickel-catalyzed coupling reactions
involving diaryliodonium and aryldiazonium electrophiles.
omethylating reagents (CF3 in Figure 1a).9 Both the facial
+
ver the past decade, nickel-catalyzed cross-coupling has
Oemerged as an attractive method for a variety of carbon−
carbon and carbon−heteroatom bond-forming reactions.1 The
mechanisms of these transformations are generally proposed to
involve sequences of 1e− and 2e− redox events that interconvert
Ni0, NiI, NiII, and/or NiIII intermediates.1,2 In contrast,
organometallic NiIV intermediates are rarely invoked in cross-
coupling reactions. This is largely due to Kochi’s pioneering
mechanistic studies that implicated NiI and NiIII-aryl
intermediates in Ni-mediated carbon−carbon bond-forming
processes.2b,c
Figure 1. (a) Previous work demonstrating that Tp and CF3 ligands
stabilize NiIV complex A; (b) proposed synthetic routes to the target
NiIV(aryl)(CF3)2 complex 2.
Recently, Chatani and co-workers have suggested that Ni-
catalyzed C−H arylation reactions with aryl iodide3 and
diaryliodonium4 electrophiles proceed via NiIV(σ-alkyl)(σ-
aryl) intermediates. While these putative NiIV species were
not detected directly, radical trapping experiments provided
evidence against the involvement of single electron pathways.
This work reopens questions about the plausibility of NiIV
intermediates in catalytic transformations, particularly those
involving aryl electrophiles.5 Importantly, if such NiIV
intermediates are accessible, they have the potential to exhibit
complementary reactivity profiles compared to their lower
valent Ni counterparts.6 In this report, we describe the design
and synthesis of organometallic model systems that demon-
strate the feasibility of the 2e− oxidation of NiII to NiIV with aryl
electrophiles. Furthermore, our studies show that the NiIV
products of these oxidation reactions participate in aryl−CF3
bond-forming reductive elimination, a transformation that
remains extremely challenging to achieve at lower valent Ni
centers.7,8
tridentate ligand trispyrazolylborate (Tp) and the trifluor-
omethyl ligand were found to stabilize the NiIV product A.
Thus, in the current study we targeted the NiIV-aryl complex 2,
which is supported by stabilizing Tp and CF3 ligands (Figure
1b). We hypothesized that 2 could be accessed from two
+
complementary pathways: (i) via the oxidation of 1 with CF3
reagents (by analogy to Figure 1a)9 or (ii) via the reaction of 3
with aryl electrophiles (Aryl+).
We first sought to prepare 2 by the 2e− oxidation of
[TpNiII(Ph)(CF3)]NBu4 (1) with S-(trifluoromethyl)-
dibenzothiophenium triflate (TDTT) (Scheme 1). The NiII
precursor was synthesized in 46% isolated yield from the
reaction of (dtbpy)NiII(Ph)(CF3) (dtbpy = 4,4′-di-tert-butyl-
bipyridine) with NBu4Tp. The treatment of 1 with 1.3 equiv of
TDTT afforded the diamagnetic NiIV product TpNiIV(Ph)-
(CF3)2 (2) in 90% isolated yield. Complex 2 was characterized
by 1H, 13C, 11B, and 19F NMR spectroscopy. Elemental analysis
Our initial studies focused on designing an organometallic
model system that would enable us to answer two key
questions: (1) Can aryl electrophiles effect the 2e− oxidation of
Received: May 11, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b04892
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Journal of the American Chemical Society
Communication
time that NiII/IV manifolds are accessible under mild conditions
with aryl diazonium and diaryliodonium reagents.
Scheme 1. Synthesis of NiIV Complex 2 via the Oxidation of
1 with TDTT
We next investigated the reactivity of the NiIV product 2.
Upon heating at 55 °C for 15 h in CD3CN, 2 underwent clean
C(sp2)−CF3 bond-forming reductive elimination to afford
benzotrifluoride in 76% yield as determined by 19F NMR
spectroscopy (Figure 3).13 The NiII byproducts of the
and X-ray crystallography (Figure 2) further confirmed the
identity and structure of this octahedral NiIV complex.
We next examined whether 2 could be accessed via the 2e−
oxidation of the NiII(CF3)2 complex 3 with aryl electrophiles.
Seminal studies by Vicic10 and Mirica11 have shown that related
NiII(CF3)2 complexes react with outer-sphere 1e− oxidants to
yield NiIII products. However, the analogous 2e− oxidation of
such complexes has not been disclosed.
Figure 3. Hammett plot for reductive elimination from 2.
Figure 2. ORTEP of NiIV complex 2. Thermal ellipsoids are drawn at
50% probability. The rotational disorder in the CF3 ligands and the
hydrogen atoms have been removed for clarity.
reaction14 are NiIITp2 (26% yield) and (CD3CN)2NiII(CF3)2
(29% yield).9,15 These are presumably generated via ligand
disproportionation from the initial reductive elimination
product, TpNiIICF3. This reaction represents the first reported
example of high yielding aryl−CF3 reductive elimination from a
discrete Ni complex.16,17
The NiII starting material 3 was prepared in 94% isolated
yield by the reaction of NBu4Tp with (MeCN)2NiII(CF3)2.10
No reaction was observed upon the treatment of 3 with phenyl
iodide, phenyl bromide, or phenyl triflate, even after heating at
70 °C for 12 h (Scheme 2).12 However, 3 underwent rapid net
2e− oxidation with the more electrophilic arylating reagents
Ph2IBF4 and PhN2BF4. The reaction of 3 with 1.1 equiv of
PhN2BF4 afforded NiIV complex 2 in 42% yield after just 10
min at 23 °C. Ph2IBF4 yielded the NiIV product in 77% yield
after 10 min at −35 °C. These results demonstrate for the first
A series of NiIV complexes bearing substituted aryl ligands
were prepared to investigate electronic effects on this aryl−CF3
coupling reaction. The complexes 2-OMe, 2-Me, 2-Br, and 2-
CO2Me were synthesized via the treatment of NiII complex 3
with the appropriate Ar2IBF4 reagents (see Supporting
Information for full details). Heating the substituted NiIV
complexes at 55 °C in CD3CN for 4−18 h afforded the
corresponding benzotrifluorides in 70−95% yield as deter-
mined by 19F NMR spectroscopy. The rate constant (kobs) for
reductive elimination from each complex at 55 °C was obtained
by monitoring the reactions by 19F NMR spectroscopy. A
Hammett plot of the resulting data is shown in Figure 3. This
plot shows a ρ value of −0.91, indicating that reductive
elimination is fastest with electron-donor substituents on the
aromatic ring. This effect mirrors the trend reported for aryl−
CF3 bond-forming reductive elimination from related
PdIV(aryl)(CF3) complexes.18 The electronic effect can be
rationalized in two ways: (1) the larger trans-effect of electron-
rich σ-aryl groups facilitates ligand dissociation to generate a
reactive five-coordinate NiIV intermediate from which reductive
elimination occurs, and/or (2) the electron donor substituents
accelerate a nucleophilic attack by the σ-aryl ligand onto the
electrophilic CF3 group in the transition state.19
Scheme 2. Synthesis of NiIV Complex 2 by the Reaction of 3
with PhX
In a final set of experiments, we examined analogous
reactions using a less stabilizing and thus potentially more
catalytically relevant ligand. The bidentate ligand 4,4′-di-tert-
a
b
10 equiv of ArX, 70 °C, 12 h. 1.1 equiv of PhN2BF4, 23 °C, 10 min.
c
1.1 equiv of Ph2IBF4, −35 °C, 10 min.
B
DOI: 10.1021/jacs.5b04892
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Journal of the American Chemical Society
Communication
butylbipyridine (dtbpy) was selected because it is commonly
used in Ni-catalyzed C−C and C−heteroatom coupling
reactions.20 The treatment of dtbpy-supported NiII complexes
4 and 5 with 1.5 equiv of TDTT and PhN2BF4, respectively,
afforded benzotrifluoride in 57% and 67% yield as determined
by 19F NMR spectroscopy (Figure 4a). Notably, these
of charge on the ACS Publications website at DOI: 10.1021/
jacs.5b04892.
AUTHOR INFORMATION
■
Corresponding Author
*mssanfor@umich.edu
Author Contributions
†J.R.B. and N.M.C. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
Grant CHE-1111563. We gratefully acknowledge Dr. Jeff
Kampf for X-ray crystallographic analysis of 2, as well as
funding from NSF Grant CHE-0840456 for X-ray instrumen-
tation.
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ASSOCIATED CONTENT
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* Supporting Information
Experimental and spectral details for all new compounds and all
reactions reported. The Supporting Information is available free
C
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Journal of the American Chemical Society
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(12) The addition of halide abstractors (e.g., AgBF4 or AgOAc) to
these reactions resulted in the consumption of 3 and formation of at
least one new paramagnetic species (likely a NiIII intermediate).
(13) The presence of the radical traps styrene, TEMPO, or butylated
hydroxytoluene (BHT) did not impact the yield of benzotrifluoride in
the reductive elimination from 2. This suggests that reductive
elimination does not proceed via a free radical mechanism.
(14) A mixture of [Ni(CF3)] compounds is observed at the end of
the reaction. After additional heating, these products converge to
NiTp2 and (MeCN)2Ni(CF3)2.
(15) The maximum yield of (CD3CN)2Ni(CF3)2 and NiTp2 are both
50% respectively.
(16) Vicic has reported the low yielding (11−22%) formation of
aryl−CF3 upon the treatment of (diphosphine)NiII(Ph)(CF3) with Zn
salts and/or water over extended reaction times. The nature of the
reactive Ni species is unclear in this system. See ref 7a.
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possibility, we conducted the stoichiometric 1e− chemical reduction of
2-Me with Cp2Co and then examined the resulting products.
Subsequent thermolysis of this reaction mixture yielded <5% of
aryl−CF3 along with organic products (CF3H and toluene) consistent
with the formation of free radical intermediates. This suggests strongly
against aryl−CF3 reductive elimination from NiIII in the current
system. See Supporting Information for full details.
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(21) The reaction of complex 5 with PhN2BF4 was conducted at 23
°C and then rapidly cooled down to −25 °C to resolve JFF coupling.
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DOI: 10.1021/jacs.5b04892
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX