Connecting Organometallic Ni(III) and Ni(IV): Reactions of Carbon-Centered Radicals with High-Valent Organonickel Complexes

Article abstract of DOI:10.1021/jacs.9b02411

This paper describes the one-electron interconversions of isolable Ni^III and Ni^IV complexes through their reactions with carbon-centered radicals (R?). First, model Ni^III complexes are shown to react with alkyl and aryl radicals to afford Ni^IV products. Preliminary mechanistic studies implicate a pathway involving direct addition of a carbon-centered radical to the Ni^III center. This is directly analogous to the known reactivity of Ni^II complexes with R?, a step that is commonly implicated in catalysis. Second, a Ni^IV-CH₃ complex is shown to react with aryl and alkyl radicals to afford C-C bonds via a proposed S_H2-type mechanism. This pathway is leveraged to enable challenging H₃C-CF₃ bond formation under mild conditions. Overall, these investigations suggest that Ni^II/III/IV sequences may be viable redox pathways in high-oxidation-state nickel catalysis.

Full text of DOI:10.1021/jacs.9b02411

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Connecting Organometallic Ni(III) and Ni(IV): Reactions of Carbon-
Centered Radicals with High-Valent Organonickel Complexes
James R. Bour, Devin M. Ferguson, Edward J. McClain, Jeff W. Kampf, and Melanie S. Sanford*
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
S
* Supporting Information
ABSTRACT: This paper describes the one-electron intercon-
versions of isolable NiIII and NiIV complexes through their
reactions with carbon-centered radicals (R•). First, model NiIII
complexes are shown to react with alkyl and aryl radicals to afford
NiIV products. Preliminary mechanistic studies implicate a pathway involving direct addition of a carbon-centered radical to the
NiIII center. This is directly analogous to the known reactivity of NiII complexes with R•, a step that is commonly implicated in
catalysis. Second, a NiIV−CH3 complex is shown to react with aryl and alkyl radicals to afford C−C bonds via a proposed SH2-
type mechanism. This pathway is leveraged to enable challenging H3C−CF3 bond formation under mild conditions. Overall,
these investigations suggest that NiII/III/IV sequences may be viable redox pathways in high-oxidation-state nickel catalysis.
Scheme 1. (a) Previous Work, Two-Electron Redox
Processes to Form/Consume Organometallic NiIV
Complexes; (b) This Work, One-Electron Redox Processes
Involving Carbon-Centered Radicals That Form/Consume
Organometallic NiIV Complexes
INTRODUCTION
■
Nickel-catalyzed cross-coupling reactions have emerged as
powerful synthetic methods for the mild and selective
construction of carbon−carbon and carbon−heteroatom
bonds.1 The vast majority of these transformations are proposed
to involve organometallic NiII and/or NiIII intermediates.1b−f,2
Higher oxidation state NiIV species were historically believed to
be inaccessible in these types of catalytic transformations.1,2
However, recent studies have shown that organonickel(IV)
complexes can be formed at or below room temperature using
common oxidants.3 As such, there is increasing interest in
understanding the generation and reactivity of such NiIV
complexes in order to interrogate their potential role(s) in
catalysis.4
To date, studies of organometallic NiIV complexes have largely
focused on two-electron redox processes that form and/or
consume these species.3 As one example, our group has
demonstrated that the net two-electron oxidative addition of
diaryliodonium salts to NiII complex I affords NiIV intermediates
of general structure II (Scheme 1a, i).3d Furthermore, II was
shown to undergo two-electron C−C bond-forming reductive
elimination to afford PhCF3 and NiII product III (Scheme 1a, ii).
However, a hallmark of nickel catalysis is the accessibility of one-
electron redox events, particularly those involving carbon-
centered radicals.1b−f,2 For instance, one of the most common
elementary steps in Ni-catalyzed cross-coupling reactions
involves the formation of a NiIII−alkyl intermediate via the
addition of a carbon-centered radical to a NiII center.1c,2 Thus,
an important outstanding question for the field is whether NiIV
intermediates can be formed and/or consumed via analogous
single-electron reactions with carbon-centered radicals.
feasibility of both of these transformations using tris(pyrazolyl)-
borate-ligated Ni model complexes in combination with
thermally generated carbon-centered radicals. Furthermore,
we show that the latter pathway enables challenging carbon−
carbon coupling reactions that are not feasible via conventional
two-electron inner-sphere C−C bond-forming reductive elim-
ination pathways at Ni.
RESULTS AND DISCUSSION
■
Accessing NiIV via Carbon-Centered Radical Addition
to NiIII Complexes. Recent reports have shown that organo-
metallic NiIV complexes can be prepared via the net two-electron
oxidation of NiII precursors with oxidants including diary-
As shown in Scheme 1b, we identified two such reactions for
investigation: (i) the addition of carbon-centered radicals to
NiIII intermediates to form NiIV products and (ii) the reaction of
organometallic NiIV complexes with carbon-centered radicals to
afford C−C coupling products. Herein, we demonstrate the
Received: March 4, 2019
© XXXX American Chemical Society
A
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liodonium salts, aryl diazonium salts, and CF3 , O+, Cl+, and F+
+
reagents (for example, Scheme 1a, i).3,5 These transformations
have close analogies to sequences that form high-valent
palladium and platinum species.6 In contrast, the generation of
NiIV intermediates via the reaction of NiIII precursors with
carbon-centered radicals has not been demonstrated. This is of
particular interest because both NiIII complexes and carbon-
centered radicals are common intermediates in Ni-catalyzed
cross-coupling. Hence, this pathway, if feasible, could potentially
be leveraged in catalytic transformations.
To probe the feasibility of this transformation, we chose the
NiIII complex TpNiIII(CF3)2(MeCN) (1, eq 1) as a model
system because our previous work has shown that it is stable and
isolable from the 1e− oxidation of [TpNiII(CF3)2]− with
AgBF4.7,8 Additionally, the relevant NiIV product
TpNiIV(CF3)2(Ph) (a close analogue of 2, eq 1) has been
independently formed from the reaction of [TpNiII(CF3)2]−
with diaryliodonium salts.3d We targeted a method for
generating carbon-centered radicals that is compatible with
both the organometallic NiIII starting material and the
organometallic NiIV product. The most common carbon-
centered radical-forming reactions involve thermolysis, photol-
ysis, reduction, or oxidation of an appropriate precursor.9
However, high-valent Ni complexes like 1 and 2 generally
decompose rapidly at high temperatures, as well as in the
presence of light and/or reductants.3d,5 On the basis of these
considerations, we selected diacyl peroxides [(RCOO)2] as the
radical source. These reagents are known to generate carbon-
centered radicals under relatively mild conditions (heating at
≤95 °C),10 without the requirement for light or reductants.11
Figure 1. (a) Synthesis of complex 3. (b) Experimental (blue) and
simulated (red) EPR spectra of 3. Gx = 2.28; Gy = 2.22; Gz = 2.01; AN =
22 G. (c) X-ray crystal structure of 3. Selected bond lengths (Å): Ni1−
N3, 2.0314(14); Ni1−N1, 1.9917(14); Ni1−N5, 1.9692(14); Ni1−
C13, 1.976; Ni1−C13, 1.9420(17). Thermal ellipsoids are drawn at
50% probability.
analysis (at 100 K in a toluene glass), which shows hyperfine
coupling to a single nitrogen.
As predicted, 3 exhibits significantly enhanced thermal
stability relative to 1. Minimal decomposition was observed
upon heating a CD3NO2 solution of 3 for 15 min at 95 °C.14
This suggests that complex 3 should be more compatible with
the thermolytic conditions required for R• generation from A.
Indeed, the treatment of 3 with 18 equiv of A at 95 °C for 19 min
produced TpNi(C4F8)(4-F-C6H4) (4) in 61% yield, as
determined by 19F NMR spectroscopy (eq 2).15 Product 4
was purified by column chromatography on silica gel and was
isolated in 31% yield as a light orange solid. This octahedral NiIV
complex was characterized by X-ray crystallography (Figure 2a),
elemental analysis, and 1H, 19F, and 13C NMR spectroscopy.
We first studied the reaction of 1 with bis(4-fluorobenzoyl)-
peroxide (A), which has a t1/2 of ∼1 h at 90 °C (eq 1).10 Heating
the reaction mixture for 15 min at 95 °C resulted in the complete
consumption of 112 along with the formation of 2 in a maximum
yield of ∼3% (after 6 min). This result suggests the feasibility of
the proposed radical addition reaction. However, attempts to
improve the yield of 2 by changing the temperature,
concentration, or solvent were unsuccessful. A control reaction
showed that 1 is unstable at 95 °C, even in the absence of
peroxide. Heating at 95 °C for 15 min resulted in complete
consumption of 1 and the formation of a mixture of products
including Tp2NiII and HCF3. This suggests that the low yield of
2 is at least partially due to the instability of the NiIII starting
material.
We also examined the reaction of 3 with bis(4-
phenylbutyryl)peroxide (B).16 The treatment of 3 with 10
equiv of B for 6 min at 85 °C afforded the NiIV alkyl product 5 in
49% yield, as determined by 19F NMR spectroscopy (eq 3).17
Product 5 was isolated in 17% yield after purification by column
1
chromatography on silica gel, and it was characterized via H,
11B, 13C, and 19F NMR spectroscopy as well as by X-ray
crystallography (Figure 2b).18
Vicic has reported that NiIII complexes bearing a perfluor-
onickelocyclopentane ligand are significantly more stable than
their trifluoromethyl analogues.13 As such, we next targeted the
analogous reaction of 3. Complex 3 was synthesized via the
reaction of [TpNiII(C4F8)]− with 1 equiv of AgBF4 and was
isolated in 57% yield after purification by chromatography on
silica gel (Figure 1A). In contrast to 1, elemental analysis and X-
ray crystallography indicate that this is a five-coordinate NiIII
complex. This is further confirmed by EPR spectroscopic
B
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Scheme 2. Possible Pathways for Conversion of 3 to 4/5
under these conditions (eq 4). This strongly suggests that
pathway b is not operating in this system.
Figure 2. (a) X-ray crystal structure of 4. Selected bond lengths (A) and
angles of 4: Ni1−N1, 2.089(2); Ni1−N3, 1.980(2); Ni1−C1,
2.031(2); Ni1−C10, 1.978(2); Ni1−C7, 1.967(2); N1−Ni1−C1,
169.31(9)°. (b) X-ray crystal structure of 5. Selected bond lengths (Å)
and angles of 5: Ni1−N3, 2.1033(13); Ni1−N1, 1.9966(13); Ni1−N5,
1.990(2); Ni1−C13, 1.9787(15); Ni1−C10, 1.9860(16); Ni1−C14,
2.0581(15); N3−Ni1−C14, 171.76(6)°. Thermal ellipsoids are drawn
at 50% probability, and hydrogen atoms are omitted for clarity. (c) X-
ray crystal structure of Int-1. Selected bond lengths (Å) and angles of
Int-1: Ni1−N5, 1.945(4); Ni1−Ni3, 2.012(4); Ni1−N1, 2.006(4);
Ni1−O, 1.862(3); Ni1−C20, 1.999(8); Ni1−C17, 2.010(8); N5−
Ni1−O1, 178.02(14)°. Thermal ellipsoids are drawn at 50%
probability, and hydrogen atoms and disorder are omitted for clarity.
If pathway a were operating, then analogous reactivity would
be expected using a different source of R•. The combination of
aryl diazonium salts and ferrocene is well-known as an
alternative route to aryl radicals.21 As such, we next explored
the reaction of NiIII complex 3 with 2 equiv of 4-
fluorophenyldiazonium tetrafluoroborate in the presence of 2
equiv of ferrocene (eq 5). Notably, complex 3 does not react
with 4-fluorophenyldiazonium tetrafluoroborate over the course
of an hour at room temperature. However, upon the addition of
ferrocene, NiIV-aryl product 4 was formed in 72% yield after just
20 min.22
We envision at least two possible pathways for the conversion
of 3 to 4/5. The first involves the decarboxylative formation of
R• and subsequent addition of this aryl/alkyl radical to 3
(Scheme 2, a). The second involves the addition of initially
generated RCO2• to 3 to form Int-1 and subsequent
decarboxylation at this NiIV carboxylate to form 4/5 (Scheme
2, b). To interrogate this latter possibility, we independently
synthesized the NiIV carboxylate complex Int-1 (with R = p-
FC6H4).19 Int-1 was characterized by 1H and 19F NMR
spectroscopy (where it exhibits resonances that are clearly
distinct from those of 4) as well as by X-ray crystallography
(Figure 2c).
With characterization data for Int-1 in hand, we reexamined
the reaction between 3 and A. The crude 19F NMR spectrum of
this transformation shows that Int-1 is formed in approximately
9% yield after 5 min, after which point it rapidly decays.
However, this experiment does not establish whether Int-1 is an
intermediate or just a side product formed during this
transformation. To distinguish these possibilities, a CD3NO2
solution of Int-1 was heated at 95 °C for 20 min (the conditions
for the reaction between 3 and A to form 4). This resulted in
67% conversion of Int-1 to a mixture of products (predom-
To rule out nonradical pathways, we also conducted radical
scavenger experiments using the oxidatively stable radical trap β-
nitrostyrene.23 We initially confirmed that this reagent does not
react with 3 over the time scale of the experiments. When the
reaction of 3 with peroxide A was performed in the presence of
β-nitrostyrene (1 equiv relative to A), the yield of 4 dropped
dramatically, from 61 to 19%. Similar results were observed
using peroxide B, with the yield of 5 decreasing from 49 to 14%.
This is consistent with the formation (and competitive trapping)
of alkyl radicals in the conversion of 3 to 4/5. Collectively, the
experiments outlined above are most consistent with pathway a
inantly 4-fluorobenzoic acid in 55% yield, as determined by 19
NMR spectroscopy).20 However, no trace of 4 was detected
F
C
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in Scheme 2, involving thermal O−O bond scission,
decarboxylation to form R•, and finally capture of R• at 3 to
generate NiIV product 4 or 5.
25% yield (Scheme 4). However, neither methane nor ethane
was observed in either reaction.
The Ni product of the reaction between 6 and R• is expected
to be TpNiIII(CF3)2. However, TpNiIII(CF3)2 is unstable at
temperatures ≥80 °C. As such, the reactions with A (conducted
at 90 °C) and B (conducted at 80 °C) afforded complex
mixtures of Ni byproducts that proved challenging to character-
ize.29 Analysis of these crude mixtures by 11B NMR spectroscopy
revealed the presence of Tp2Ni.30
One-Electron Carbon−Carbon Coupling Reactions at
NiIV. We next sought to evaluate the reactivity of NiIV−alkyl
complexes with carbon-centered radicals to generate C−C
bonds. Such pathways have precedent in the context of alkyl−
CoIII systems24,25 but seldom are considered in high-valent Ni
catalysis.4 In particular, methyl−CoIII corrinoid cofactors are
well-known to react with exogeneous R• to afford R−CH3 via an
SH2-type pathway (Scheme 3).24
Scheme 5 shows three pathways that could account for the
formation of R−CH3 in this reaction.31 The first (Scheme 5, i)
Scheme 3. Reaction of CoIII−CH3 Adducts with R• to Form
R−CH3
Scheme 5. Potential Mechanisms for C−C Coupling
Reactions between 6 and R•
To initially probe the viability of this transformation at NiIV,
we evaluated the reactivity of several different NiIV−alkyl
complexes with R• (generated from the thermolysis of A or B).
These studies revealed that the NiIV−CH3 complex 6 exhibits
the best balance between reactivity with R•, thermal stability,
and synthetic accessibility (Scheme 4). Notably, this is similar to
Scheme 4. Reaction of Carbon-Centered Radicals with 6
(Yields Based on 6 as the Limiting Reagent)
involves preequilibrium dissociation of an arm of the Tp ligand
followed by radical addition and subsequent concerted inner-
sphere C−C coupling. Notably, this radical addition/concerted
reductive elimination sequence is frequently proposed in
reactions of carbon-centered radicals with lower oxidation
state coordinatively unsaturated Ni centers.1c,2 However, it is
highly unlikely in the current system, because it requires the
formation of a nickel intermediate with a formal +5 oxidation
state. The second pathway (Scheme 5, ii) involves outer-sphere
radical coupling via a radical substitution (SH2) pathway.32
Here, R• reacts directly with the methyl ligand via a transition
state of general structure TS-1. This pathway is analogous to that
proposed for the reaction of methyl−CoIII corrinoid cofactors
with R• (Scheme 3). Finally, this transformation could proceed
via the heterodimerization of R• and methyl radicals formed
through spontaneous NiIV−CH3 homolysis (Scheme 5, iii).
The product distribution of these reactions provides initial
evidence against the homolysis/radical heterodimerization
pathway (mechanism iii). Unstabilized carbon-centered radicals
are known to dimerize with low selectivity and at near diffusion-
limited rates.33 However, the reaction of 6 with A and B forms
the cross-coupled product H3C−R in much higher yield (65 and
78%, respectively) than R−R or H3C−CH3 (5 and 25% yield,
respectively).34,35
the Co systems, in which the methyl variants are typically
thermally stable yet highly reactive with carbon-centered
radicals.24 As such, complex 6 was selected as the model system
for more detailed studies.26
Heating a solution of 6 with 3 equiv of peroxide A at 90 °C for
1 h resulted in the formation of 4-fluorotoluene in 65% yield
along with 5% of 4,4′-difluorobiphenyl, as determined by 19F
NMR spectroscopic analysis (Scheme 4).27,28 Notably, 44% of A
(or ∼1.3 equiv relative to 6) was consumed in this reaction.
Similarly, the reaction of 6 with 3 equiv of B at 80 °C for 1 h
1
afforded n-butyl benzene in 78% yield, as determined by H
NMR spectroscopy. Notably, 67% of B (or ∼2 equiv relative to
6) was consumed in this reaction. In this case, the radical
homocoupling product 1,6-diphenylhexane was also detected in
To further interrogate the viability of mechanism iii, we
monitored the thermolysis of 6 in the presence of 5 equiv of the
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radical scavenger β-nitrostyrene. If mechanism iii were
operating, any free H3C• that is generated should be rapidly
captured by β-nitrostyrene, resulting in fast decomposition of 6.
In contrast, in the event of mechanism ii, 6 should be stable in
the presence of this reagent. As shown in Figure 3, heating a
with R•. They also reveal that NiIV−methyl complexes can
engage R• to form various types of C−C bonds. Preliminary
mechanistic studies are most consistent with a pathway
involving an SH2-type reaction of R• with the NiIV−CH3 ligand.
In contrast, inner-sphere coupling and radical heterodimeriza-
tion mechanisms are deemed unlikely. This unconventional C−
C bond-forming pathway was found to enable C(sp3)−CF3
coupling, a reaction that is highly challenging through traditional
inner-sphere reductive elimination at Ni centers. Overall, these
investigations suggest that, with the appropriate choice of ligand,
NiII/III/IV sequences involving carbon-centered radicals could be
targeted for the formation of challenging bonds. Future work in
our laboratory will seek to extend these insights to the
development of new catalytic methods.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.9b02411.
■
S
Figure 3. Thermal decomposition of 6 (0.015 M in CD3NO2) in the
presence of various additives. Blue triangles = no additive. Green
diamonds = β-nitrostyrene (0.075 M, 5 equiv relative to 6). Red squares
= A (0.045 M, 3 equiv relative to 6). Yellow rectangles = B (0.045 M, 3
equiv relative to 6).
Crystallographic Information File for compound 3 (CIF)
Crystallographic Information File for compound 4 (CIF)
Crystallographic Information File for compound 5 (CIF)
Crystallographic Information File for compound 7 (CIF)
Crystallographic Information File for compound Int-1
(CIF)
solution of 6 in the presence of β-nitrostyrene had minimal
impact on the decay of 6. In marked contrast, the addition of A
or B resulted in rapid loss of 6 (with concomitant formation of
the respective C−C coupled product). Collectively, the
experiments outlined above are most consistent with mecha-
nism ii.
Experimental details, compound characterization, spectra,
and safety considerations (PDF)
AUTHOR INFORMATION
Corresponding Author
*mssanfor@umich.edu
ORCID
James R. Bour: 0000-0002-0372-7323
Melanie S. Sanford: 0000-0001-9342-9436
■
Mechanism ii is fundamentally different than traditional 2e−
inner-sphere C−C bond-forming reductive elimination reac-
tions. As such, we hypothesized that it might enable the
formation of C(sp3)−CF3 bonds, which are challenging to forge
via direct inner-sphere pathways.3c,7,36 For example, heating 6
for 18 h at 100 °C in MeCN yields <5% of H3C−CF3. Instead,
other decomposition pathways outcompete H3C−CF3 cou-
pling.37 This result is consistent with the known difficulties
associated with direct inner-sphere C(sp3)−CF3 coupling.
However, in contrast, the treatment of 6 with 2.5 equiv of
bis(trifluoroacetyl)peroxide (C) at just 45 °C for 90 min
resulted in rapid decay of the NiIV starting material along with
the concomitant formation of H3C−CF3 and TpNiIV(CF3)3 7 in
74 and 63% yield, respectively (eq 6).38 Control reactions show
that H3C−CF3 is not formed at this temperature unless both 6
and C are present. This represents a rare example of metal-
mediated C(sp3)−CF3 coupling.36,39 In our system, this
transformation appears to be enabled by the accessibility of a
1e− outer-sphere pathway.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the US National Science
Foundation (CHE 1664961 to M.S.S. and CHE 0840456 for
X-ray instrumentation). We acknowledge Dr. Eugene Chong for
initial synthetic efforts demonstrating the accessibility of
complex 4 and Elizabeth Meucci for supplying
(MeCN)2Ni(CF3)2. We also thank James Shanahan and Jessica
Wilson for assistance in collecting UV−vis data.
REFERENCES
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SUMMARY AND CONCLUSIONS
■
In summary, this Article describes the first detailed study of the
reactivity of high-valent organonickel complexes with carbon-
centered radicals (R•). These studies demonstrate that NiIV
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(10) Cooper, W. The effect of structure of diacyl peroxides on the
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(11) Tp-supported high-valent organonickel complexes are light
sensitive and thus unstable under photolytic conditions.
(12) The consumption of 1 was determined by 11B NMR
spectroscopic analysis of the crude reaction mixture.
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Budnikova, Y.; Vicic, D. A. Accessing perfluoroalkyl nickel(II), (III),
and (IV) complexes bearing a readily attached [C4F8] ligand. Dalton
Trans. 2015, 44, 19443.
(14) Heating 3 at 100 °C for 12 h resulted in complete decomposition
of the starting material to form a mixture of nickel-containing products
including NiTp2.
́
́
(i) Rovira, M.; Roldan-Gomez, S.; Martin-Diaconescu, V.; Whiteoak,
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Ni(IV)-CF3 intermediate species. Chem. - Eur. J. 2017, 23, 11662.
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formation reactivity of organometallic Ni(II), Ni(III), and Ni(IV)
complexes. J. Am. Chem. Soc. 2017, 139, 35.
(15) The yield of 4 appears to be limited by the competitive
decomposition of this NiIV product at 95 °C. Analysis of the crude
reaction between A and 3 revealed that only 11% (∼1.8 equiv relative to
Ni) of the initial A was consumed at the time of maximum NiIV yield.
The remaining mass balance of the reaction was fluorobenzene and 4-
fluorobenzoic acid, presumably formed through H-atom abstraction
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H bonds in benzamides and acrylamides with functionalized alkyl
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F
DOI: 10.1021/jacs.9b02411
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
from the solvent (see Figure S3). Using fewer equivalents of A resulted
in lower yields of 4 (11% yield with 1 equiv, 40% yield with 5 equiv).
(16) Diacyl/aroylperoxides are potentially explosive materials and
should be handled with appropriate precautions (see the Supporting
Information for details).
with <10% decomposition after 1 h at 90 °C in CD3NO2. This high
thermal stability is likely due to the high barrier for CH3−CF3 coupling
via inner-sphere reductive elimination.
1
(27) A similar yield (58%) was obtained on the basis of H NMR
spectroscopic analysis of the crude reaction mixture, confirming that
the methyl is not solvent-derived.
(17) The yield of 5 appears to be limited by the competitive
(28) The ethyl analogue, TpNiIV(CF3)2(CH2CH3), reacts with 6
equiv of A under analogous conditions to afford 4-fluoroethylbenzene
in 7% yield. However, it is important to note that this complex is not
very stable at the reaction temperature, and it decomposes
competitively at 90 °C to form ethylene and CF3H. See the Supporting
Information, p S29, for more details.
decomposition of this NiIV product at 85 °C. Analysis of the crude
1
reaction mixture by H NMR spectroscopy and GCMS revealed the
formation of 1,6-diphenyl hexane and allylbenzene. The allylbenzene is
formed during the thermal decomposition of 5 via an apparent net β-
hydride elimination.
(18) Complex 5 was isolated in 17% yield through the reaction of 3
with 5 equiv of B at 85 °C for 6 min. Notably, diacyl peroxides are
potentially explosive compounds and should be handled with caution.
The equivalents of B were reduced to mitigate risk on the preparatory
scale.
(19) See the Supporting Information, pp S7 and S8, for the synthesis
and characterization of Int-1.
(20) The nature of the primary nickel-containing product of this
reaction is currently unclear. The 11B NMR rules out the formation of
NiTp2 or free unbound Tp.
(29) Attempts to isolate Ni-containing products from the crude
reaction mixtures with A and B (by column chromatography on silica or
recrystallization) were unsuccessful.
(30) Tp2Ni was also the major identifiable nickel-containing
compound in the thermal decomposition of 1.
(31) A reviewer suggested that an alternate pathway for the reactions
in Scheme 4 and eq 6 could involve the transmetalation of a methyl
group from 6 to reduced nickel byproducts and subsequent C−C
coupling from a lower valent Ni center. While we cannot definitively
rule out this possibility, we note that this would require a H3C−CF3
bond-forming reductive elimination reaction from a lower valent Ni
intermediate. This transformation currently does not have precedent
from group 10 metal centers. See ref 7.
(21) Galli, C. Radical reactions of arenediazonium ions: easy entry
into the chemistry of the aryl radical. Chem. Rev. 1988, 88, 765.
(22) Similarly, complex 1 reacts with 4-fluorophenyldiazonium
tetrafluoroborate and ferrocene to yield 2 in 52% yield under these
conditions.
(23) A key consideration in the choice of radical scavenger is its
stability to the highly oxidizing conditions of this reaction. For example,
the use of TEMPO as a radical scavenger resulted in violent
decomposition of the peroxides. On this basis, we selected the electron
deficient Michael acceptor β-nitrostyrene. Barton, D. H. R.; Togo, H.;
Zard, S. R. The invention of new radical chain reactions. Part X. High-
yield radical addition reactions of -nitroolefins. An expedient
construction of the 25-hydroxy-vitamind D3 side chain from bile
acids. Tetrahedron 1985, 41, 5507.
(24) (a) Mosimann, H.; Krautler, B. Methylcorrinoids methylate
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A. New insight into the mechanism of methyl transfer during the
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Li, Q.; Jia, Y.; Ji, X.; Qianzhu, H.; Zhang, Q. Emerging diversity of the
cobalamin-dependent methyltransferases involving radical-based
mechanisms. ChemBioChem 2016, 17, 1191. (d) Bridwell-Rabb, J.;
Drennan, C. L. Vitamin B 12 in the spotlight again. Curr. Opin. Chem.
Biol. 2017, 37, 63.
(25) Analogous outer-sphere radical C−C coupling mechanisms are
commonly proposed for iron(II)/(III)-catalyzed transformations. For
examples, see: (a) Hatakeyama, T.; Hashimoto, T.; Kondo, Y.;
Fujiwara, Y.; Seike, H.; Takaya, H.; Tamada, Y.; Ono, T.; Nakamura,
M. Iron-catalyzed Suzuki-Miyaura coupling of alkyl halides. J. Am.
Chem. Soc. 2010, 132, 10674. (b) Hatakeyama, T.; Okada, Y.;
Yoshimoto, Y.; Nakamura, M. Tuning chemoselectivity in iron-
catalyzed Sonogashira-type reactions using a bisphosphine ligand
with peripheral steric bulk: selective alkynylation of nonactivated alkyl
halides. Angew. Chem., Int. Ed. 2011, 50, 10973. (c) Daifuku, S. L.; Al-
Afyouni, M. H.; Snyder, B. E. R.; Kneebone, J. L.; Neidig, M. L. A
(32) Ingold, K. U.; Roberts, B. P. Free radical substitution reactions:
Bimolecular homolytic substitutions at saturated multivalent atoms; Wiley:
New York, 1971.
(33) The dimerization of localized and sterically accessible carbon-
centered radicals, such as •CH3 and •Ph, is thought to be nearly
́
barrierless. Costentin, C.; Saveant, J.-M. Origin of activation barriers in
the dimerization of neutral radicals: A “nonperfect synchronization”
effect. J. Phys. Chem. A 2005, 109, 4125.
1
(34) Ethane was not detected by H NMR spectroscopy. However,
potential loss of ethane to the headspace of the reaction vessel could not
be mitigated due to the hazards associated with eliminating reaction
vessel headspace in gas-evolving reactions. The mass balance of other
methyl-containing products sets the maximum yield as 17 and 11%.
(35) The H-atom abstraction products fluorobenzene and n-
propylbenzene were observed in 30% and <2% yield, respectively.
(36) Inner-sphere C(sp3)−CF3 coupling has recently been proposed
to occur from CuIII(CF3)3(CH3) complexes. SH2 mechanisms involving
the direct attack of •CF3 on CuIII(CH3) were not discussed. See:
(a) Tan, X.; Liu, Z.; Shen, H.; Zhang, P.; Zhang, Z.; Li, C. Silver-
catalyzed decarboxylative trifluoromethylation of aliphatic carboxylic
acids. J. Am. Chem. Soc. 2017, 139, 12430. (b) Paeth, M.; Tyndall, S. B.;
Chen, L.-Y.; Hong, J.-C.; Carson, W. P.; Liu, X.; Sun, X.; Liu, J.; Yang,
K.; Hale, E. M.; Tierney, D. L.; Liu, B.; Cao, Z.; Cheng, M.-C.;
Goddard, W. A.; Liu, W. C(sp3)-CF3 reductive elimination from well-
defined Cu(III) complexes. J. Am. Chem. Soc. 2019, 141, 3153.
(37) In marked contrast, the analogous phenyl complex
TpNiIV(CF3)2(Ph) undergoes high yielding inner-sphere Ph−CF3
coupling at 55 °C over 14 h. See ref 3d for details.
(38) 7 is likely formed through the addition of •CF3 to TpNi(CF3)2
which is more stable at these lower temperatures.
(39) Levin, M. D.; Chen, T. Q.; Neubig, M. E.; Hong, C. M.; Theulier,
C. A.; Kobylianskii, I. J.; Janabi, M.; O’Neil, J. P.; Toste, F. D. A catalytic
fluoride rebound mechanism for C(sp3)-CF3 bond formation. Science
2017, 356, 1272.
̈
combined Moossbauer, magnetic circular dichroism, and density
functional theory approach for iron cross-coupling catalysis: electronic
structure, in situ formation, and reactivity of iron mesityl-bi-
sphosphines. J. Am. Chem. Soc. 2014, 136, 9132. (d) Przyojski, J. A.;
Veggeberg, K. P.; Arman, H. D.; Tonzetich, Z. J. Mechanistic studies of
catalytic carbon-carbon cross-coupling by well-defined iron NHC
complexes. ACS Catal. 2015, 5, 5938.
(26) Complex 6 was prepared in 40% yield via the reaction of
NMe4[TpNiII(CF3)2] with excess methyl iodide and 1.3 equiv of 2,6-
difluorobenzendiazonium tetrafluoroborate (see the Supporting
Information, pp S9 and S10). This NiIV complex was characterized
1
1
via H, 13C, 19F, and H/19F HMBC NMR spectroscopy as well as
elemental analysis. Complex 6 exhibits relatively high thermal stability,
G
DOI: 10.1021/jacs.9b02411
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX