Oxidatively Induced C-H Activation at High Valent Nickel

Article abstract of DOI:10.1021/jacs.7b02387

This communication describes a series of oxidatively induced intramolecular arene C-H activation reactions of Ni^II model complexes to yield Ni^IV σ-aryl products. These reactions proceed within 10 min at room temperature, which repres

Full text of DOI:10.1021/jacs.7b02387

Communication
pubs.acs.org/JACS
Oxidatively Induced C−H Activation at High Valent Nickel
†
†
,‡
,‡
,†
Eugene Chong, Jeff W. Kampf, Alireza Ariafard,* Allan J. Canty,* and Melanie S. Sanford*
†Department of Chemistry, University of Michigan, 930 N. University Avenue, Ann Arbor, Michigan 48109, United States
‡School of Physical Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia
*
S Supporting Information
IV
Scheme 1. (a) Strategy for Stabilizing Ni ; (b) Design of
Model System To Study C−H Activation at High Valent Ni
ABSTRACT: This communication describes a series of
oxidatively induced intramolecular arene C−H activation
II
IV
reactions of Ni model complexes to yield Ni σ-aryl
products. These reactions proceed within 10 min at room
temperature, which represents among the mildest
conditions reported for C−H cleavage at a Ni center. A
combination of density functional theory and preliminary
experimental mechanistic studies implicate a pathway
−
II
involving initial 2e oxidation of the Ni starting materials
+
by the F transfer reagent N-fluoro-2,4,6-trimethylpyr-
idinium triflate followed by triflate-assisted C−H cleavage
IV
at Ni to yield the products.
ransition metal-mediated C−H activation is a key
T
elementary step in the catalytic C−H functionalization of
organic molecules. The vast majority of examples of both
stoichiometric and catalytic C−H activation involve low valent
intramolecular C−H cleavage reaction to form a product of
3b
II
II
II
1
general structure 2 (Scheme 1b). We selected trifluoromethyl
ligands as the other supporting ligands in this model system, as
fluoroalkyls are well-known to stabilize high valent Ni center-
metals, such as Pt , Pd , and Ni . However, there is increasing
evidence that higher oxidation state group 10 metal centers can
2
−5
also participate in C−H cleavage reactions. For instance, our
6e,f,j,7c−e,8
s.
Finally, N-fluoro-2,4,6-trimethylpyridinium triflate
group has recently reported several examples of intramolecular
IV
3
II
(NFTPT) was selected as the oxidant because it has been used for
C−H activation at Pd centers. In these systems, the Pd
−
II
IV 9,10
the 2e oxidation of Pd to Pd ,
including for a related
precursors are unreactive toward cyclometalation. However,
upon treatment with a 2e oxidant, C−H cleavage proceeds
rapidly to afford an octahedral Pd metallacycle. These
IV 3b
−
oxidatively induced C−H activation reaction at Pd .
Wefirstturnedtodensityfunctionaltheory(DFT)toassessthe
feasibility of the transformation proposed in Scheme 1b. As
summarized in Figure 1, computational studies reveal that the
IV
11
stoichiometric studies have motivated the development of a
variety of catalytic transformations that leverage C−H activation
−
IV
IV
4
inner sphere 2e oxidation of 1a with NFTPT to form Ni −F
at Pd as a key step in the catalytic cycle.
‡
IV
intermediate B proceeds with an activation barrier (ΔG ) of just
High oxidation state organonickel complexes (particularly Ni
IV
6,7
10.5 kcal/mol. The steps involved in converting 1a to B are
species) are far rarer than their Pd analogues. Nonetheless,
IV
directly related to those previously reported for the oxidation of
growing evidence supports the feasibility of accessing Ni
II
IV
+
−
6e−g,j
(bipy)Pd (CH )(OAc) to[(bipy)Pd (CH ) (OAc)(F)] using
3 3
10
intermediates using various 2e oxidants.
In addition, recent
IV
NFTPT. The observation of Ni−aryl interactions in B suggests
that this Ni intermediate is highly electrophilic. Indeed, we
observe a low energy pathway for cyclometalation at Ni via an
S Ar mechanism. The interaction of the triflate counterion with
reports have shown that Ni compounds can undergo reductive
IV
6
a−c,e−j
6c,j
6g
elimination,
transmetalation,
and group transfer
IV
reactions to yield a diverse array of organic and organometallic
12
IV
products. The emerging studies of C−H activation at Pd led us
E
IV
B lowers the energy of this cation by >5 kcal/mol (to form
tohypothesize that Ni complexes might also participate in C−H
intermediate C). The TfO− then assists as a base in the
5
bond cleavage reactions. Insights into the feasibility and
IV
cyclometalation step, where D can be viewed as a Ni -arenium
mechanism of this fundamental organometallic transformation
could ultimately inform its incorporation into Ni-catalyzed
reactions.
species involving an electrophilic C −H bond cleavage. This
aryl
transformation ultimately results in the formation of the
IV
II
Ni (aryl) (fluoride) complex 2a as a TfOH adduct (structure
We sought to design a Ni model complex that could undergo
II
−
F) that is −31.9 kcal/mol lower in energy than the Ni starting
C−H cleavage upon treatment with a 2e oxidant. Previous work
material 1a + NFTPT. Overall, these calculations demonstrate
hasshown thathighvalent Niintermediates arestabilizedbyfacial
tridentate nitrogen donor ligands such as tris(2-pyridyl)methane
6e
(
py CH) (Scheme 1a). We hypothesized that replacing one of
Received: March 9, 2017
3
the pyridine arms of py CH with an aryl group might enable an
3
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.7b02387
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Figure 1. Energy profile for the oxidative transformation of 1a to 2a using NFTPT, with all computation employing the solvent used in synthetic studies
(
3
dichloromethane). Structures B and C exhibit short Ni···Cipso interactions (2.408, 2.394 Å) with no indication of C−H agostic interactions (Ni···Cortho
.010−3.101 Å, Ni···Hortho 3.288−3.383 Å). Energies ΔG (ΔH) in kcal/mol. Bond lengths in Å.
−
19
that a pathway involving the 2e oxidation of 1a with NFTPT
within 10 min, as determined by F NMR spectroscopy, and no
IV
2
IV
diamagnetic intermediates were detected. The diamagnetic Ni
followed by C(sp )−H cleavage at the resulting Ni center is
energetically feasible.
product 2a (eq 2) shows a Ni−CF resonance at −24 ppm and a
3
These initial DFT studies motivated us to explore this
II
transformation experimentally. The Ni precursor 1a was
II
prepared via ligand exchange between (MeCN) Ni (CF ) and
2
3 2
py CFAr fluorine signal at −182 ppm. Notably, this latter signal
2
shifts substantially from the starting material (−138 ppm), and
IV
this large shift is diagnostic for the formation of Ni complexes of
1
13
py CFPh (eq 1). Complex 1a was characterized by H, C, and
2
this ligand. This new complex also shows a broad resonance at
1
9
F NMR spectroscopy and elemental analysis. X-ray quality
IV
15,16
−
283 ppm, which is indicative of a M −F.
crystals of 1a were obtained by vapor diffusion of diisopropyl
ether into an acetonitrile solution of 1a, and an X-ray structure is
shown in Figure 2a. In the solid state, 1a exhibits a square planar
Complex 2a was purified via an aqueous workup followed by
trituration with diethyl ether, and was isolated as an analytically
pure yellow solid in 70% yield. Unlike many transition metal
fluorides, this complex is stable to ambient air and moisture.
Characterization by H, C, and FNMR spectroscopy as well as
17
1
13
19
IV
X-ray crystallography confirmed that 2a is the σ-aryl Ni product
of an oxidatively induced C−H activation reaction. The X-ray
IV
structureof2a(Figure3a)showsanoctahedralNi complexwith
theσ-arylligandtranstothefluoride. Toourknowledge, thisisthe
IV
first example of an organometallic Ni fluoride complex. Overall,
Figure 2. X-ray structures of (a) 1a, (b) 1b, and (c) 1d plotted with 50%
probability ellipsoids for non-H atoms.
II
geometry at the Ni center. A relatively long distance (2.906(2)
Å) is observed between Ni and the ipso carbon of the pendant aryl
group, suggesting that there is minimal bonding interaction in the
II
13
Ni oxidation state.
The treatment of 1a with 1.3 equiv of NFTPT in CH Cl at 25
2
2
°
Cresultedinaninstantcolorchangefromamberyellowtobright
yellow. The Ni starting material was completely consumed
Figure 3. X-ray structures of (a) 2a, (b) 2b, and (c) 2c plotted with 50%
probability ellipsoids for non-H atoms.
1
4
II
B
DOI: 10.1021/jacs.7b02387
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
II
this transformation demonstrates the viability of oxidatively
the Ni cation 3a was formed in 87% yield (eq 4), along with a
23,24
induced C−H cleavage at Ni under mild conditions (<10 min, 25
mixture of CF H, CF , and C F .
Complex 3a was
3
4
2 6
1
13
19
°
C). These represent amongthe mildest conditions reported fora
cyclonickelation reaction at any oxidation state of Ni.
characterized via H, C, and F NMR spectroscopy as well as
X-ray crystallography (eq 4). Further consistent with the
importance of triflate, the C−H activation reactivity in this
5
,13,18−20
We next probed the scope and mechanism of this trans-
formation. A first set of experiments focused on determining the
isotopeeffectassociatedwiththeC−Hcleavage event(eq3). The
system could be recovered by adding 2 equiv of NBu OTf to the
4
NFTPB oxidation. Underthese conditions, 2awas formed in52%
yield.
Overall, these results are consistent with the computational
studies, which suggest that the triflate is intimately involved in the
C−H cleavage step (see TS_D/E in Figure 1). DFT also shows
that substitution of tetrafluoroborate for triflate in TS_D/E raises
the barrier for that step by approximately 8 kcal/mol. The rapid
II
formation of Ni product 3a (along with CF H, CF , and C F )
3
4
2 6
with NFTPB suggests that, when the barrier to C−H cleavage at
high valent Ni is relatively high, the rates of other decomposition
pathways become competitive and ultimately dominate the
observed reactivity. These counterion effects are particularly
noteworthy because similar trends have been seen in other Ni-
catalyzed C−H functionalization reactions. For instance, Chatani
reportedthatdiaryliodoniumtriflatesareeffectiveoxidantsforNi-
catalyzed C−H arylations, whereas the tetrafluoroborate
reaction proved too fast for rate studies, even at −50 °C. As such,
wewereunabletoobtainanintermolecularKIE.However, theuse
of substrate 1a-d afforded a 24:76 ratio of 2a:2a-d, which
corresponds to an intramolecular isotope effect of ≈3 (DFT
computed value ≈5). The observed value is significantly smaller
2
IV
3c
thanthosereportedforC(sp )−HactivationatPd (7−12) but
IV
comparable to that for related transformations at Pt centers (2.3
2
a
and 3).
We also investigated electronic effects using a series of
25
analogues afford essentially no reaction.
In summary, we describe a series of oxidatively induced arene
substituted analogues of 1 bearing OMe, F, and CF substituents
3
IV
II
C−H activation reactions that yield Ni σ-aryl products of
on the arene ring (1b−d in eq 2). X-ray structures of the Ni
general structure 2. These reactions proceed within 10 min at
room temperature, thereby representing among the mildest
conditions reported for C−H cleavage at a Ni center. Preliminar−y
studies suggest the viability of a pathway involving the 2e
starting materials 1a, 1b, and 1d are shown in Figure 2. The bond
distances and bond angles in these structures are nearly identical,
including the Ni−ipso-C distance (2.906(2), 2.923(2), and
2
.926(2) Å, respectively). This suggests that the remote
II
oxidation of the Ni starting materials by NFTPT followed by
substituents have minimal impact on the interaction of the aryl
IV
II
triflate-assisted electrophilic C−H cleavage at Ni . However, it is
ring with the Ni center. All three complexes were found to react
important to note that a recent report by Mirica implicated the
rapidly with NFTPT at 25 °C. In the case of 1b and 1c, the major
III
5
IV
possibility of C−H activation at related Ni centers.
product of this reaction is the Ni C−H activation product 2b or
−
Furthermore, NFTPT has been proposed to act as a 1e oxidant
2c (formed in 89% and 84% yield, respectively). These products
26
in some systems. Thus, at this time, we cannot rule out the
possibility of sequential 1e− oxidation pathways for this
transformation. Nonetheless, this report demonstrates the first
areformedwithhigh(>20:1)selectivityforactivationoftheC−H
bondparatothesubstituent(forX-raystructuresof2band2c,see
Figure 3b,c). This selectivity likely reflects a high sensitivity to the
steric environment of the C−H bond. Similar effects have been
observed in intramolecular C−H functionalization reactions at
IV
example of accessing Ni σ-aryl products via oxidatively induced
C−H cleavage. This opens up exciting possibilities for this and
related transformations to be exploited in catalysis.
2
1
Pd.
In contrast, the CF -substituted complex 1d afforded <2% of
3
II
II
ASSOCIATED CONTENT
Supporting Information
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/jacs.7b02387.
2
d. Instead, the Ni cation [(py CFAr)Ni (CF )(2,4,6-trime-
thylpyridine)] , 3d, was formed as the major Ni product in 60%
yield (see eq 4 for the unsubstituted analogue 3a). As discussed
■
2
3
+
*
S
CIF data for 1a (CIF)
CIF data for 1b (CIF)
CIF data for 1d (CIF)
CIF data for 2a (CIF)
CIF data for 2b (CIF)
CIF data for 2c (CIF)
CIF data for 3a (CIF)
Experimental details, complete characterization data for all
new compounds, and DFT calculations (PDF)
II
below, Ni products of general structure 3 appear to form in these
22
reactions when the C−H activation step is slow. Slower C−H
activation on the electron deficient CF -substituted aryl ring is
3
consistent with the electrophilic C−H activation mechanism
AUTHOR INFORMATION
Corresponding Authors
Alireza.Ariafard@utas.edu.au
Allan.Canty@utas.edu.au
mssanfor@umich.edu
■
*
*
*
proposed in Figure 1.
The DFT studies suggest that triflate plays an important role in
the C−H cleavage step. Thus, we next probed the impact of
replacing the triflate with a tetrafluoroborate. The treatment of 1a
with NFTPB (N-fluoro-2,4,6-trimethylpyridinium tetrafluorobo-
rate) in place of NFTPT under otherwise identical conditions
ORCID
IV
resulted in <2% of the Ni C−H activation product 2a. Instead,
Allan J. Canty: 0000-0003-4091-6040
C
DOI: 10.1021/jacs.7b02387
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
Melanie S. Sanford: 0000-0001-9342-9436
M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.;
Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara,
M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.;
Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K.
N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the NSF CHE-1361542. E.C. thanks
NSERC for a postdoctoral fellowship.
■
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(
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The formation of this side product could be minimized to <2% by
conducting the reaction at −35 °C.
24) CF H (42%), CF (30%), and C F (5%) were detected by 19F
(
3 4 2 6
NMR spectroscopy when the reaction of 1a with NFTPB was carried out
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2
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2
2
analogous reaction was conducted in CD Cl , the major detectable
2
2
(
organic products were CF D (2%), CF (43%), and C F (2%). These
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latter results suggest that the H/D derives from the solvent.
(25) Iyanaga, M.; Aihara, Y.; Chatani, N. J. Org. Chem. 2014, 79, 11933.
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(26) (a) Umemoto, T.; Fukami, S.; Tomizawa, G.; Harasawa, K.;
Kawada,K.;Tomita,K.J.Am.Chem.Soc.1990,112,8563.(b)Strekowski,
L.; Kiselyov, A. S. Adv. Heterocycl. Chem. 1995, 62, 1.
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DOI: 10.1021/jacs.7b02387
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX