Ni(II) catalyzed bromination of aryl C-H bonds

Article abstract of DOI:10.1016/j.ica.2014.08.012

Bromination of unactivated aromatic C-H bonds without directing and/or chelating groups was achieved by employing an air stable N-heterocyclic Ni(II) complex. PhI(OAC)₂ and N-bromosuccinimide have been used as the oxidizing agent and the bromin

Full text of DOI:10.1016/j.ica.2014.08.012

Inorganica Chimica Acta 423 (2014) 238–241
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Inorganica Chimica Acta
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Ni(II) catalyzed bromination of aryl C–H bonds
1
2
⇑
Moumita Bhattacharya , David B. Cluff , Siddhartha Das
Department of Chemistry and Biochemistry, Utah State University, Logan, UT, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 June 2014
Received in revised form 9 August 2014
Accepted 11 August 2014
Available online 19 August 2014
Bromination of unactivated aromatic C–H bonds without directing and/or chelating groups was achieved
by employing an air stable N-heterocyclic Ni(II) complex. PhI(OAC)2 and N-bromosuccinimide have been
used as the oxidizing agent and the bromine source, respectively. Yields for bromination are as high as
>99%, especially in presence of electron-withdrawing groups like –NO2 and –CF3. This is a rare report
on C–H bond activation with Ni(II) where, instead of homo C–C coupling, reductive elimination to form
C-halogen could be achieved.
Keywords:
Ni(II)
Ó 2014 Elsevier B.V. All rights reserved.
C–H activation
Functionalization
1. Introduction
leads to C–H functionalization (insertion of acetate) [11], but for
Ni(II), the major product is C–C coupling between two molecules
Efficient conversion of aromatic C–H to C–halogen bond pro-
vides access to organic halides, which are valuable starting
materials in numerous C–C coupling reactions [1]. Presently,
halogenation of organic molecules is carried out using X2/light
[2a,b], Lewis acids (FeCl2)/X2 [3a,b], hypervalent iodine
reagents/MX (M: Li/Na/K/TMS) [4], X2/M (M: Fe) [5] methods.
Achieving halogenation of aromatic rings bearing electron-
withdrawing groups (EWG) like CF3 is especially important
since incorporation of a CF3 group in an aromatic ring is widely
used to synthesize pharmaceuticals [6]. Selective halogenation
of aromatic systems bearing EWG like CF3 or NO2 would pro-
vide a facile step for subsequent C–C couplings and, therefore,
alternative routes for drug synthesis [7a,b].
of 2-phenylpyridine (PhPy) [10]. With Ni(II), even when N-bromo-
succinimide or N-chlorosuccinimide is used, C–H halogenation is
detected only in trace amounts (in these cases a stoichiometric
amount of Ni(II) was used).
In Ni(PhPy)2 system [10], Ni(II) has a near-perfect square planar
geometry. However, in a strained system like Ni(II)CNC (See Figure
S1(a) and (b) for qualitative MO diagrams), Ni(II) cannot adopt a
square planar geometry. The geometry of 1 allows higher orbital
overlap between ligand group orbitals (LGOs) and dxy than a square
planar geometry; in Ni(II)CNC, LGOs approach Ni(II) along a direc-
tion that is in between the x and y axes, not along the axes them-
selves. This makes the metal centered dxy more destabilized than in
a square planar geometry. Also, strong
r-donation from the two
Among first-row transition metals, Ni(II) has become an appeal-
ing alternative for C–C cross coupling. Several groups, most notably
those of Chatani, Garg, Shi, and Snieckus, have extensively explored
the use of aryl systems bearing various functional groups and
ArB(OH)2/ArZnX/ArMgX (X: halide) reagents [8a–h]. In addition,
there has been a report of a Suzuki cross coupling between Ar–X
and Ar0B(OH)2, with Ni(II)CNC (1) (Scheme 1) as the catalyst [9].
However, for activation of the C–H bond, Ni(II) systems have
been reported to prefer the thermodynamically favorable C–C cou-
pling during reductive elimination instead of yielding a C-halogen
product [10]. An interesting comparison is the use of the ‘chelate
control’ with Pd(II) versus Ni(II); with 2-phenylpyridine, Pd(II)
NHC ligands in the xy plane makes dxy, and especially dx2ꢀy2 (which
experiences direct linear overlap) very destabilized. In the d8 Ni(II)
system, metal centered dxy and dx2ꢀy2 have a major contribution to
the HOMO and LUMO, respectively. Therefore, we envisioned that
Ni(II)CNC, unlike other square planar Ni(II) complexes, would have
a significantly high-lying HOMO (with major contribution from
dxy) and even a higher-lying LUMO (with major contribution from
dx2 ). We presumed that (a) the high-lying HOMO (i.e. easier to
ꢀy2
oxidize) and (b) the CNC ligand’s large steric bulk and large per-
centage buried volume [12] might help Ni(II)CNC to catalyze C–H
activation in the presence of an oxidant, and to show subsequent
reductive elimination with insertion of a small functional group
(thus, avoiding C–C homo-coupling).
This inspired us to investigate Ni(II)CNC (1) in the selective bro-
mination of aryl rings and its functional group tolerance. This opens
up the potential for carrying out a ‘‘domino reaction’’, where C–H
⇑
Corresponding author.
1
Current address: Department of Chemistry, University of Utah, Salt Lake City, UT,
USA.
2
Current address: Department of Chemistry, UC-Davis, Davis, CA, USA.
http://dx.doi.org/10.1016/j.ica.2014.08.012
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

M. Bhattacharya et al. / Inorganica Chimica Acta 423 (2014) 238–241
239
respectively. Other oxidizing agents such as benzoquinone, tert-
butylhydroperoxide, di-tert-butylperoxide and benzoyl peroxide
were not efficient.
Initially, D2O was used as a solvent since bromination had pre-
viously been achieved in aqueous solutions [7]. The bromination
could be carried out with limited success (Table S1). However, no
bromination occurred regardless of whether the reactions were
carried out in CH3CN or CH3COOH as solvents. CF3COOH was found
to be the most promising solvent.
Scheme 1. Bromination by Ni(CNC)(1).
3.2. Product analysis
halogenation is followed by C–C cross coupling using a single cata-
lyst, in a one-pot reaction. Herein, we report the bromination of
C(sp2)–H bonds in biphenyl systems with electron-donating and
electron-withdrawing groups by, mainly, the air stable Ni(II)CNC
(1) and an oxidizing agent. N-bromosuccinimide (NBS) was used
as the brominating agent. In some previous studies with a Pd(II)
catalyst NBS was used as both the bromine source and the oxidizing
agent [13]. For Ni(II), it did not afford appreciable bromination in
absence of an oxidizing agent under our reaction condition. In addi-
tion, we explored the reactivity of some commercially available
Ni(II) salts that also showed promising catalytic activity.
3.2.1. Substituted and unsubstituted mono-phenyl systems
Negligible or no bromination was observed with benzene,
PhNO2, or PhCN. PhNH2 resulted in dibromination. Significant bro-
mine insertion at the para position, both in the presence and absence
of Ni(II) catalyst, was observed with PhOMe; tribromination was
observed in all these cases. However, the methyl group of PhOMe
(expected to undergo bromination if a freely diffusing bromine rad-
ical is involved) remained unbrominated in all cases (see below for
further investigation with Ph–CH2–Ph). One challenge with these
mono-phenyl units was their low boiling points, which complicated
the work-up process. Therefore, we decided to investigate biphenyl
systems, which are solid at ambient temperature.
2. Experimental
3.2.2. Bromination of unsubstituted biphenyl
2.1. General
The reaction afforded the mono-brominated product in 2.8%
and the dibrominated product in 22.5% yield from biphenyl, always
at the 4/40-position(s).
Gas chromatographic analyses were performed on Shimadzu
GC-17A GC and QP5000 MS instrument. NMR spectra were
recorded on a Jeol 300 MHz spectrometer. The catalyst 1 was
synthesized by literature procedure [9].
3.2.3. Bromination of substrates with electron-rich aromatic rings
Electron-rich substrates led to brominated iodobenzene (1-
bromo-4-iodobenzene) as the major product instead of bromination
of biphenyl rings, likely from the oxidizing agent, PhI(OAc)2.
To test this hypothesis, we added 0.25 equivalents of PhI during
catalysis. If 1 has a preference for electron-poor rings then addition
of PhI would not change the product composition. Indeed we saw
no change in the yield of brominated iodobenzene upon addition of
PhI. This suggested that the bromination of the PhI(OAc)2 could be
the first step towards the formation of brominated iodobenzene.
As C–F bonds have been shown to be less susceptible to activation
compared to C–H bonds, (C6F5)I(OCOCF3)2 was used as the oxidizing
agent, to prevent any attack on the oxidant [14]. Consequently, bro-
mination of electron-rich substrates was performed successfully
(Table S2).
2.2. Materials
All solvents and substrates were purchased from Sigma Aldrich,
Acros Organic, Alfa Aesar, Matrix Scientific and TCI Chemicals and
were used as received.
2.3. Standard procedure for catalytic bromination
To 2 mL CF3COOH (or D2O), 0.25 mmol substrate, 0.5 mmol oxi-
dizing agent, 0.375 mmol N BS and 5 mol% catalyst were added.
The reagents were taken in a specially designed tube (10 mL), were
purged with N2 gas, and sealed; the mixture was stirred for 24 h at
room temperature. Next, the volatiles were removed under
reduced pressure. The brominated products were analyzed by 1H
NMR spectroscopy and the yields were calculated by GC/MS using
dodecane as an internal standard.
3.2.4. Bromination of substrates with electron-deficient aromatic rings
PhI(OAc)2 proved to be rather efficient in supporting bromina-
tion of substrates bearing electron-withdrawing groups. The yields
varied between 53–>99% and 87–97% selectivity for the aromatic
C–H bond at the 40-position with respect to the electron-deficient
ring. Control experiments without PhI(OAc)2 indicated that NBS
in this case functions mostly as the brominating agent; its poten-
tial role as an oxidant is not apparent (Table 1). In one case, with
a CF3-substituted biphenyl system (entry V, Table 1), the uncata-
lyzed reaction also provided a high yield of bromination. However,
consistent high yield and high selectivity in bromination was
observed only with Ni(II)CNC.
3. Results
3.1. Optimization of catalytic conditions
Our hypothesis suggests that 1 is a competent catalyst for func-
tionalization of aromatic C–H bonds. Optimal reaction conditions
were determined by varying substrates, solvents, and oxidizing
agents. It was found that 5 mol% of pre-catalyst, along with an oxi-
dizing agent and NBS led to the bromination of biphenyl substrates
at ambient temperature. KBr was also tested as brominating agent
and it was found that the bromide anion was oxidized in the
presence of the oxidizing agent to molecular bromine.
Control experiments in the absence of oxidizing agent gave
lower yields compared to the reactions carried out in its presence.
PhI(OAc)2 (or, C6F5I(OCOCF3)2) carried out the bromination reac-
tions effectively with electron poor and electron rich substrates
3.2.5. Investigation for involvement of free radicals
Freely diffusing radical-driven reactions are serious concerns in
C–H functionalization, especially if regioselectivity is desired [15].
In order to investigate if freely diffusing radicals are playing a
role in our reactions, diphenylmethane, which has highly activated
benzylic C–H bonds, was used. Freely diffusing radicals would have
higher probability to attack the benzylic C–H instead of the

240
M. Bhattacharya et al. / Inorganica Chimica Acta 423 (2014) 238–241
Table 1
Bromination of substrates bearing electron-deficient aromatic rings.a
Entries
I
Substrate
Mol% of 1
PhI(OAc)2(mmol)
Product(s)
GC Yield (% age)b (o:m:p)
0
5
0
5
0.5
0
0
14.2(2:0:98)
3.4(0:0:100)
1.3(0:0:100)
53(2:1:97)
0.5
II
0
5
0
5
0.5
0
0
13.7(11:0:89)
40(15:0:85)
0.5(16:0:84)
>99(12:1:87)
0.5
III
IV
0
5
0
5
0
5
0
5
0.5
0
0
0.5
0.5
0
0.7(17:0:83)
8(23:0:77)
0.5(18:0:82)
59(13:0:87)
30.6(11:0:89)
35(5:30:65)
16.5(11:0:89)
77(9:0.02:90.8)
0
0.5
V
0
5
0
5
0.5
0
0
78(11:0:89)
16(19:0:81)
11(12:0:88)
>99(11:0:89)
0.5
a
Catalytic conditions: 0.25 mmol substrate with 0.5 mmol PhI(OAc)2 and 0.375 mmol NBS in 2 mL CF3COOH at room temperature for 24 h in a sealed tube under an N2
atmosphere.
b
Yields are calculated using dodecane as an internal standard.
aromatic C–H bonds. In addition, this substrate would give informa-
tion about the selectivity of aromatic C–H versus methylene C–H
functionalization. Bromination was observed only at the aromatic
C–H bonds and not the methylene C–H bonds (yield: 71.5%). Inter-
estingly, similar results were observed in the absence of 1.
3.2.6. Impact of bromination on substrates with a directing group
When 2-PhPy was used as the substrate, no significant effect
was observed with 1, unlike the results reported by Sanford and
coworkers [13]. Under our catalytic conditions, instead of observ-
ing a product from 2-PhPy, bromination of the C–H bonds of iodo-
benzene was observed.
The chiral outcome of bromination of a,b-unsaturated carbox-
ylic acids (e.g. trans-cinnamic acid) with NBS and PhI(OAc)2 have
been used by Jørgensen et al. and others to disprove the possibil-
ity of involvement of free radicals [16] If free radicals are
involved, upon decarboxylative bromination from trans isomer,
both the cis and trans isomers of the brominated products are
expected in appreciable amounts. This was not observed. We
performed similar studies, with trans-cinnamic acid, under
conditions that are identical to Jørgensen and coworkers, i.e. in
CH3CN:H2O (2:1) with NBS and PhI(OAc)2, (a) without and (b)
with added catalyst 1, as well as under our catalytic conditions,
i.e. in CF3COOH with NBS and PhI(OAc)2, (c) without and (d) with
added catalyst 1. In all cases, we observed trans-brominated prod-
uct as the major product (94–97% of the total product); based on
1H NMR, percentages of the trans product (i.e. (E)-(2-bromovi-
nyl)benzene) in the total product mixture for conditions
(a):94%, (b): 96.5%, (c): 96% and (d): 97%. Consistent with prior
mechanistic studies, this indicates that, under our catalytic condi-
tions, involvement and/or appreciable formation of free radicals is
unlikely.
3.3. Bromination with commercially available Ni(II) salts
Bromination of the aromatic systems with electron-withdrawing
groups by commercially available Ni(II) salts was also tested. The
motivation was to (a) compare activities (as we did not find any prior
studies with Ni(II) salts for bromination under similar conditions)
and (b) to investigate whether Ni(II)CNC (1) is generating Ni(II) salts
that, in turn, are catalyzing the reaction. Compounds NiBr2,
Ni(OCOCH3)2, and Ni(OCOCF3)2 were found to brominate the elec-
tron-poor aromatic systems with varied yield and selectivity, miss-
ing the consistency that we observed with 1 (Tables 1 and 2). For
example, for 3-nitro-1, 10-biphenyl (substrate II, Tables 1 and 2),
with 1, the conversion was 99%. NiBr2 and Ni(OCOCH3)2 catalyzed
the bromination with rather high yields (ꢁ60%) in this case; how-
ever for all other substrates the conversion was poor, unlike that
with 1. Ni(OCOCF3)2 gave a high yield (75%) only with 3-trifluoro-
methyl-1,10-biphenyl (substrate IV, Tables 1 and 2), but not with
any other substrate.
We also performed our catalysis of C–H halogenation with
4-nitrobiphenyl (substrate of entry III in Table 1) in presence of
TEMPO as a radical trap. TEMPO has been used to probe a radi-
cal-based mechanism [17]. If radicals are involved as intermedi-
ates, TEMPO quenches the radicals, and thus affects the yield and
product ratios of the reaction. TEMPO has been reported to inter-
cept radicals with half-lives as low as 0.2 ns [17]. We did not
observe any noticeable effect from TEMPO on catalytic yield and/
or product ratio (catalyst:TEMPO = 1:1).
Complete negation of a pathway involving free radicals is
always a challenge in C–H functionalization. However, above-
mentioned experiments provide very convincing indication of a
non-radical pathway.
4. Discussion
Reductive elimination with Pt(IV), Pd(II), and Ni(II) was pro-
posed to follow different mechanisms: fast dissociation of the anio-
nic functional group for Pt(IV) [18]; concomitant elimination of the
C–heteroatom bond with Pd(II) [19]; oxidation to a Ni(III) species
with Ni(II) [20a–e]. For Ni(II), reductive elimination involves an
intramolecular reaction involving the formation of a four, six, or
seven-membered ring as the intermediate. However, when the
possibility of intramolecular reductive elimination is removed, C–
C bond formation was reported to be favored over C–heteroatom
bond formation [13].

M. Bhattacharya et al. / Inorganica Chimica Acta 423 (2014) 238–241
241
Table 2
Catalyses with selected Ni(II) salts.a
b
Pre-catalyst
NiBr2
Yield with I (%)
Yield with II (%)
Yield with III (%)
Yield with IV (%)
Yield with V (%)
2.45
60
28.5
17.2
46.4
1:0:99(o:m:p)
5.7
3:0:97(o:m:p)
34.2
11:0:89(o:m:p)
59
12:0:88(o:m:p)
7.56
16:0:84(o:m:p)
31
13:0:87(o:m:p)
3.1
13:0:87(o:m:p)
42.5
11:0:89(o:m:p)
74.8
9:0:91(o:m:p)
23.3
1:0:9(o:m:p)
11.8
Ni(OCOCH3)2
Ni(OCOCF3)2
4:0:96(o:m:p)
11:0:89(o:m:p)
14:0:86(o:m:p)
10:0:90(o:m:p)
14:0:86(o:m:p)
a
Catalytic condition:0.25 mmol substrate with 0.5 mmol PhI(OAc)2 and 0.375 mmol NBS in 2 mL CF3COOH at 25 °C for 24 h in a sealed tube under an N2 atmosphere.
Yields are calculated using dodecane as an internal standard.
b
Under our conditions, 1 yields C-halogenated products as the
Appendix A. Supplementary material
major species from aryl C–H bonds with high selectivity. For sub-
strates bearing benzylic C–H bonds, no halogenation at the ben-
zylic position was observed, even for highly activated benzylic C–
H bonds like those of diphenyl methane (Ph–CH2–Ph). Therefore,
involvement of freely diffusing halogen radicals is unlikely. In addi-
tion, it is difficult to conceive that a cyclic intermediate involving
the metal site is likely. Based on the mass balance performed for
entries in Table 1 (for entry I: 70%; II: 90%; III: 90%; IV: 78%; V:
70%), it can be inferred that the C–C coupling process is not dom-
inant. The selective C–H halogenation, specially the preference for
electron-deficient rings with 1 is intriguing. In fact, if the substrate
does not bear an electron-deficient ring (as in C6H5–C6H5, C6H5–
C6H4–OMe), the oxidant PhI(OAc)2 undergoes halogenation
instead. These observations are indicative of the involvement of
the electron-deficient ring through, perhaps, weak coordination
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2014.08.012.
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yields and selectivity towards C–halogenation with
1 were
observed especially with electron-deficient substrates. Further
studies to look into the reaction mechanism and further explora-
tion of the reactivity will be the future direction. The work presents
a new direction for C–H functionalization, instead of homo C–C
coupling, with easily accessible Ni(II) catalysts.
Acknowledgements
This work has been partly supported with start-up funding
from Utah State University. We thank Paula Diaconescu (UCLA)
for her suggestions especially during manuscript preparation.