In situ acidic carbon dioxide/water system for selective oxybromination of electron-rich aromatics catalyzed by copper bromide

Article abstract of DOI:10.1016/j.cattod.2012.05.012

Carbon dioxide, being one of the major greenhouse gases responsible for global warming, its atmospheric level grows ever faster since the beginning of industrial era. Great efforts have been devoted to developing versatile technologies/processes to adjust and manipulate the rapid growth of CO ₂ emission. Besides CO₂ capture and storage/sequestration (CCS) to control its emission, chemical utilization of the captured CO ₂ (CCU) emerges to be a rational technique for economical benefits as well as environmental concerns. As for the aim of CO₂ utilization, an environmentally benign CO₂/water reversible acidic system was developed for the copper (II)-catalyzed selective oxybromination of electron-rich aromatics without the need of any conventional acid additive and organic solvent. Notably, up to 95% yields of the bromination products were attained with good regio-selectivity when cupric bromide was used as the catalyst and lithium bromide as a cheap and easy handling bromine source under supercritical CO₂. The catalytic system worked well for electron-rich aromatics including ethers, sulfides and mesitylene. Carbonic acid in situ formed from CO₂ and water is supposed to act as the proton donator in the Bronsted acid-promoted reaction. Notably, CO₂ in this study serves as a reaction medium and a promoter in conjunction with water and also provides safe environment for aerobic reactions. Given with excellent reaction efficiency as well as no need of neutralization disposal, this process thus represents an environmentally friendly approach for aerobic bromination of aromatics with essential reduction of CO₂ emission as well as an economically beneficial way for application of captured CO₂.

Full text of DOI:10.1016/j.cattod.2012.05.012

Catalysis Today 194 (2012) 38–43
Contents lists available at SciVerse ScienceDirect
Catalysis Today
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In situ acidic carbon dioxide/water system for selective oxybromination of
electron-rich aromatics catalyzed by copper bromide
An-Hua Liu, Ran Ma, Meng Zhang, Liang-Nian He∗
State Key Laboratory and Institute of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Carbon dioxide, being one of the major greenhouse gases responsible for global warming, its atmospheric
level grows ever faster since the beginning of industrial era. Great efforts have been devoted to developing
versatile technologies/processes to adjust and manipulate the rapid growth of CO2 emission. Besides CO2
capture and storage/sequestration (CCS) to control its emission, chemical utilization of the captured CO2
(CCU) emerges to be a rational technique for economical benefits as well as environmental concerns.
As for the aim of CO2 utilization, an environmentally benign CO2/water reversible acidic system was
developed for the copper (II)-catalyzed selective oxybromination of electron-rich aromatics without the
need of any conventional acid additive and organic solvent. Notably, up to 95% yields of the bromination
products were attained with good regio-selectivity when cupric bromide was used as the catalyst and
lithium bromide as a cheap and easy handling bromine source under supercritical CO2. The catalytic
system worked well for electron-rich aromatics including ethers, sulfides and mesitylene. Carbonic acid
in situ formed from CO2 and water is supposed to act as the proton donator in the Brønsted acid-promoted
reaction. Notably, CO2 in this study serves as a reaction medium and a promoter in conjunction with water
and also provides safe environment for aerobic reactions. Given with excellent reaction efficiency as well
as no need of neutralization disposal, this process thus represents an environmentally friendly approach
for aerobic bromination of aromatics with essential reduction of CO2 emission as well as an economically
beneficial way for application of captured CO2.
Received 25 February 2012
Received in revised form 27 April 2012
Accepted 14 May 2012
Available online 4 June 2012
Keywords:
Carbon dioxide
In situ carbonic acid
Copper
Aerobic oxidation
Catalysis
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
of CO2 from industrial and household gaseous emission [14,15].
Besides the profound progress that has been gained on CO2 capture
Being one of the major greenhouse gases responsible for global
warming and climate changes, carbon dioxide emission is gain-
ing increasingly concern from both academia and governmental
agencies all cross the world [1]. The accumulation of atmospheric
CO2 due to human activities, including but not limited to, com-
bustion of fossil fuels (petroleum, coal and natural gas), hydrogen
production from hydrocarbons through steam conversion and par-
tial oxidation, as well as burning of firewood and plants, has been
rapidly increasing since the beginning of the industrial era with a
ever faster growth rate of ca. 2 ppm/year in recent years after 2000
[2–4]. Therefore, controlling CO2 emission and further reducing its
accumulation emerges to be an urgent and challenging research
topic [5,6]. In this context, CO2 chemistry based on its capture
and conversion/utilization could deserve much attention and great
efforts from scientific community and industrial sectors [7–13].
Obviously, carbon capture storage/sequestration (CCS) looks
particularly effective for temporarily preserving huge volumes
and its chemical utilization as an alternative abundant and readily
accessible C1 building block in organic synthesis, various feasible
uses of CO2 have been extensively explored, such as sound solvent
for greener processes, Lewis acid catalyst and/or promoter, and tun-
able reagent for chemical selectivity, providing feature advantages
from the principle of green chemistry to the improved economics
by means of developing efficient catalysis and improving product
separation.
In recent years, CO2 as a non-toxic reaction medium has shown
significant potential and advantage in synthetic organic chemistry
[16–19] and inorganic fabrications [20–25]. Particularly, com-
pressed CO2 proves to be an ideal solvent in oxidation reactions
for its multiple facilitating roles, such as no cross contamination
of solvent into the target products, excellent mass- and heat-
transfer properties, as well as tunable phase behaviors. Hence,
compressed CO2 could afford a high efficiency reaction media for
gas–liquid/solid biphasic oxidations [26].
Meanwhile, the well-investigated reversible reaction between
CO2 and water could form carbonic acid, offering a temporarily
generated proton donating species from an in situ Brønsted acid,
which could serve as an acid catalyst (Scheme 1). Notably, such
∗
Corresponding author. Fax: +86 22 23503878.
E-mail address: heln@nankai.edu.cn (L.-N. He).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.05.012

A.-H. Liu et al. / Catalysis Today 194 (2012) 38–43
39
Scheme 1. In situ formation of carbonic acid.
self-neutralizing in situ acid catalysis from CO2 has been applied
in many approaches, with profound advantages for both green
chemistry and improved economics [27–35]. Such CO2/H2O sys-
tem provides in situ formed acid catalysis which could be inherently
neutralized by readily removal of CO2, thus resulting simple neu-
tralization and none waste disposal. As a result, the in situ acid
catalysis from captured CO2 would bring no more of carbon emis-
sion.
Scheme 3. Oxybromination of electron-rich arenes.
molecular oxygen as the attractive terminal oxidant, as shown in
Scheme 3 [77,78]. Comparable yields and regio-selectivities along
with facilitated post-processing of reaction were achieved by the
introduction of CO2 and H2O, providing a meaningful example for
beneficial utilization of green house gases in production of fine
chemicals.
Aromatic bromides are prevalent and versatile syn-
thetic intermediates which have been applied in various
carbon–carbon/carbon–heteroatom bond formation through
well established coupling reactions [36–41], as well as commer-
cialized pesticides, herbicides, antiepileptics, flame retardants
and antibacterial drugs [42–45]. Conversional methods for the
synthesis of bromo-aromatics largely rely on Sandmeyer reaction
via interim diazonium species and the electrophilic substitution
process [46–51]. Although the aromatic electrophilic halogenation
has been well developed as the primary approach for preparing
bromoarenes (Scheme 2), regio-selectivity is not good enough. In
addition, such process with harsh reaction conditions could be
either harmful and volatile, or need of corrosive Brønsted acid
as co-catalyst or auxiliary reagent, thereby leading to environ-
mental and practical operation problem, to some extent [52–65].
Oxybrominations with hydrogen peroxide as the oxidizing agent
appear to be relative mild and eco-friendly [66–73]. But oxida-
tive reactivity of hydrogen peroxide in aqueous media toward
hydrophobic reactants would be weakened, and consequently
restrained itself as a viable option for current transformation on a
pilot plant scale. In view of wide applicability and great industrial
demand of aromatic bromides, it is still desired to develop envi-
ronmentally benign and economically effective alternative system
for such oxybromination.
Walter Leitner and co-workers have for the first time applied a
CO2/aq.H2O2/NaHCO3/NaBr biphasic system for the bromination
of phenols and anilines with high mono-bromination selectiv-
ity [74]. However, anisole was proved to be unreactive, possibly
being due to its lack of solubility in water or deactivation effect of
the methoxy group. In scCO2/O2/ethanol reversible acidic media
with cupric chloride dihydrate as catalyst, aromatic ethers and
sulfides could be transformed to the corresponding bromide in
good to excellent yields as well as eximious regio-selectivity [75].
A similar type of in situ acidic CO2/H2O catalytic system was
also employed for recyclable TEMPO functionalized imidazolium
salt/NaNO2 catalyzed oxidation of benzylic alcohols [76]. On the
basis of advances in the catalytic oxybromination and reversible
acidic catalysis with mechanistic understanding, we envision that
in situ carbonic acid formed from CO2 and H2O could also promote
the bromination reaction as proton donator and process accel-
erant, and thus offers an alternative chemical methodology for
the synthesis of fine chemicals. In addition, H2O as the terminal
proton source is relatively more available and benign compared
with ethanol. In this article, we would like to present a copper
bromide-catalyzed oxybromination of electron-rich aromatics in
scCO2/H2O media with lithium bromide as bromine source and
2. Experimental
2.1. Cations
Experiments using compressed gases CO2 and O2 are
potentially dangerous and must only be carried out by
using the appropriate equipment and under rigorous safety
precautions.
2.2. Chemicals
The aromatic ethers, sulfides and metal catalysts were pur-
chased from Alfa Aesar China Co. Ltd and Aladdin Reagent Inc. The
rest of reagents and distilled water were obtained commercially
from Tianjin Guangfu Fine Chemical Research Institute and used
without further purification.
2.3. Equipments for characterization
1H NMR spectra were recorded at Brucker 400 spectrometer in
CDCl3 and CDCl3 (7.26 ppm) was used as internal reference, 13C
NMR were recorded at 100.6 MHz in CDCl3 and CDCl3 (77.0 ppm)
was used as internal reference. GC–MS were recorded on a
Thermo Finnigan Polaris Q equipment and HP-1800A (GCD) in
EI mode. GC analyses were performed on Shimadzu GC-2014,
equipped with
a
capillary column (RTX-17, 30 m × 0.25 ␮m
and RTX-WAX, 30 m × 25 ␮m) using
a
flame ionization
detector.
2.4. Oxybromination of aromatic ethers
A mixture of substrate (1 mmol), CuBr2 (22.4 mg, 10 mol%),
LiBr (130.3 mg, 1.5 equiv.), and 0.05 mL of water was placed in a
50 mL stainless steel autoclave equipped with an inner glass tube
in room temperature. CO2 (4 MPa) and O2 (1 MPa) were subse-
quently introduced into the autoclave and the system was heated
under the predetermined reaction temperature for 15 min to reach
the equilibration. Then the final pressure was adjusted to the
desired pressure by introducing the appropriate amount of CO2.
The mixture was stirred continuously for the desired reaction time.
After cooling, products were diluted with acetone and analyzed by
gas chromatograph (Shimadzu GC-2014) equipped with a capil-
lary column (RTX-17 30 m × 25 ␮m and RTX-wax 30 m × 25 ␮m)
using a flame ionization detector by comparing the retention times
of authentic samples. The residue was purified by column chro-
matography on silica gel (200–300 mesh, eluting with petroleum
ether/ethyl acetate from petroleum ether to 50:1) to afford the
desired product. The isolated products were further identified with
NMR spectra (Bruker 400 MHz) and GC–MS or GCD, which are con-
sistent with those reported in the literature.
Scheme 2. Oxybromination of aromatics.

40
A.-H. Liu et al. / Catalysis Today 194 (2012) 38–43
Table 2
3. Results and discussion
Influence of CO2 pressure and amount of water on the reaction outcome.a
Yield (%)b
3.1. Reactivity survey of various catalysts
Entry
CO2 (MPa)
H2O (mL)
p-Isomerc
o-Isomerd
In the preliminary exploration, anisole (1a) with two possible
reaction sites was selected as model substrate to carry out the
regio-selective oxybromination. Anisole (1a) reacted smoothly to
provide mono-brominated product in 38% yield under the tenta-
tive reaction conditions (1 MPa O2, 0.1 mL H2O, 1.1 equiv. LiBr and
0.2 equiv. CuBr2) with the aid of 1 MPa of CO2 (entry 1, Table 1).
In particular, remarkable regio-selectivity (p-:o-isomer = 9:1) can
be obtained. The absence of CO2 would obviously cause the bromi-
nation reaction inhibited (entry 2). Those results prompted us to
further elucidate the promotive effect of CO2. Then the catalyst
activity was investigated as summarized in Table 1. A variety of
copper catalysts with different anions were screened, but none
exhibited a superior reactivity compared with CuBr2 (entries 3–7).
The continuative examination of ferric salts showed no more pre-
ponderance of the iron catalyst than the copper counterparts, only
Fe(NO3)3·9H2O provided a relatively good yield because of the
strong oxidizing ability of the nitrate anion coordinated to ferric
cation (entries 8–10). Therefore, CuBr2 was chosen as the most effi-
cient catalyst with comparable regioselectivity among the metal
salts examined, which was selected for further optimization.
1
2
3
4
5
6
7
8
0.5
1
4
8
12
8
8
8
0.1
0.1
0.1
0.1
0.1
0.2
0.05
0
29
3
4
5
6
5
3
5
34
36
54
45
25
75
5
Trace
Reaction conditions: CuBr2 20 mol%, LiBr 1.1 equiv., O2 1 MPa, 100 ◦C, 10 h.
Determined by GC with biphenyl as internal standard.
Yield of p-bromo-anisole.
a
b
c
d
Yield of o-bromo-anisole.
operation. Increase of H2O amount from 0.1 mL to 0.2 mL just leads
to a negative effect possibly due to the solvation problem for the
substrate (entry 4 vs. 6). Cutting water amount to 0.05 mL could
reach an adequate point for better yield (entry 7), while further
eliminate water from reaction system would confront a failure for
optimization as no proton supplier presented in reaction system
(entry 8).
Other alkali metal bromides were also evaluated for this reac-
tion, but failed to obtain the comparable result, revealing that LiBr
was the choice for bromination (entries 1–3, Table 3). Further ele-
vating LiBr amount to 1.5 equiv. would be suitable to obtain a good
yield of p-bromo anisole (87%) and nice regio-selectivity (93:7)
(entry 4). The essential quantity of copper catalyst loading could
be reduced to 10 mol% without the significant restraint of catalytic
activity, but further decreasing the molar ratio to 5 mol% would
result in remarkable drop-off in the product yield (entries 5 vs. 6).
As a consequence, the reaction conditions were eventually settled
as CuBr2 10 mol%, LiBr 1.5 equiv., H2O 0.05 mL, O2 1 MPa, CO2 8 MPa,
100 ◦C, 10 h.
3.2. Influences of different reaction parameters
As illustrated in plenty of documents, elevating pressure of
CO2 could enhance the acidity of in situ carbonic acid in CO2/H2O
biphasic system [79–81]. For such reason, CO2 pressure was varied
and the results were presented as Table 2. Low CO2 concentra-
tion may cause the restricted formation of carbonic acid and thus
results in deficient proton source (entry 1 vs. 2, Table 2). By increas-
ing of CO2 pressure, an optimal productivity of bromo anisole was
attained at the supercritical state of 8 MPa (entries 3 and 4). But fur-
ther pumping CO2 to 12 MPa would be unfavorable, probably owing
to the dilution effect by high density CO2, thereby would cause inef-
ficiency of aerobic oxidation (entry 5). Consequently, 8 MPa of CO2
would be appropriate for the in situ carbonic acid promotion.
Beyond the influence of CO2, the actual proton donator like H2O
could also play the crucial role in in situ acid formation and process
3.3. Substrate scope
The generality and feasibility of the present protocol were
then evaluated by performing the oxybromination with a broad
range of aromatics under optimal reaction conditions (Table 4).
Para-substituent group in anisole renders the reaction to occur
at ortho-position, whereas steric congestion would depress the
oxybromination (entry 2, Table 4). Furthermore, the electron-
negativity para-substituent would cause a further restraint of
reactivity (entry 4). 4-Methoxy-benzylalcohol (3a) failed to give
target product as its ease of oxidation for benzylic hydroxy to alde-
hyde (entry 3). Naphthalene methyl ethers were popular substrates
in good yields and regio-selectivities (entries 5 and 6). Mono-,
di- and tri-methoxy substituted benzenes underwent bromination
smoothly with moderate yields, but disubstitution happened in all
Table 1
Metal-promoted oxybromination of anisole with CO2/O2/H2O system.a
OMe
OMe
OMe
Br
catalyst
+
LiBr
+
O2/CO2/H2O
Br
Entry
CO2 (MPa)
Catalyst
Yield (%)b
p-Isomerc
o-Isomerd
Table 3
Further optimization of bromide reagents and the catalyst amount.a
1
2
3
4
5
6
7
8
9
1
0
1
1
1
1
1
1
1
1
CuBr2
CuBr2
Cu(OAc)2
CuCl2·2H2O
Cu(NO3)2·4H2O
CuO
34
17
21
27
24
22
30
28
26
21
4
2
2
3
3
2
3
1
2
2
Entry
CuBr2 (mol%)
Br source (equiv.)
Yield (%)b
p-Isomerc
o-Isomerd
1
2
3
4
5
6
20
LiBr (1.1)
NaBr (1.1)
KBr (1.1)
LiBr (1.5)
LiBr (1.5)
LiBr (1.5)
75
22
18
87
86
66
5
1
1
6
6
4
CuBr
20
20
20
10
5
Fe(NO3)3·9H2O
FeBr3
10
FeCl3
a
Reaction conditions: catalyst 20 mol%, LiBr 1.1 equiv., H2O 0.1 mL, O2 1 MPa,
a
100 ◦C, 10 h.
Reaction conditions: CO2 8 MPa, O2 1 MPa, 100 ◦C, 10 h.
Determined by GC with biphenyl as internal standard.
Yield of p-bromo-anisole.
b
b
c
Determined by GC with biphenyl as internal standard.
Yield of p-bromo-anisole.
Yield of o-bromo-anisole.
c
d
d
Yield of o-bromo-anisole.

A.-H. Liu et al. / Catalysis Today 194 (2012) 38–43
41
Table 4
Cu(II)-catalyzed aerobic oxybromination of electron-rich arenes.a
Entry
Substrate
OMe
Time (h)
10
Product
Yieldb (%)
Br
OMe
1a
1b
1c
92
Br
OMe
1c
Br
OMe
OMe
Br
2a
2b
2e
10
10
88
OMe
OMe
HO
HO
3a
3b
3
N.D.d
Br
MeO
O
MeO
O
OMe
OMe
4a
4b
OMe
4e
20
10
19
94
OMe
5a
5bBr
5
Br
OMe
OMe
6a
6b
6
10
12
88
73
Br
MeO
OMe
MeO
OMe
7a
7b
7f
OMe
OMe
OMe
OMe
Br
8a
8b
8
9
12
12
83
67
MeO
OMe
MeO
OMe
9a
9b Br
OMe
OMe
Br
MeO
OMe
MeO
Br
OMe
10a
10b
10f
11
10
10
59
80
O
O
O
11a
11bO
O
O
O
12a
Br
Br
O
12b
12
13
14
15
10
10
83
87
41
OEt
OEt
13a
14a
13b
Br
SMe
SMe
14b
Br
SMe
SMe
15a
15b
15
12
13

42
A.-H. Liu et al. / Catalysis Today 194 (2012) 38–43
Table 4 (Continued)
Entry
Substrate
Time (h)
Product
Yieldb (%)
NMe2
Br
Br
NMe2
OH
16a
16b
17b
16
17
10
10
N.D.d
N.D.d
OH
Me
17a
Me
Me
Me
Br
Me
Me
18a
18b
18
10
95
a
Reaction conditions: CuBr2 10 mol%, LiBr 1.5 equiv., CO2 8 MPa, O2 1 MPa, 100 ◦C.
b
c
d
e
f
Determined by GC.
A mixture of two regio-isomers (1b:1c = 94:6).
Not detected.
CuBr2 (20 mol%).
LiBr (1.1 equiv.).
cases (entries 7–10). Oxy-containing bicyclic ring such as benzo-
dioxole (11a) and benzodioxane (12a) could also be successfully
brominated, providing regio-specific aromatic bromide (entries 11
and 12). Ethoxyl was found to be a promotive electron-donating
group, while methylthio was less effective owing to the ease of sul-
foxidation (entries 13–15). However, two electron-rich substrates,
i.e. phenol (16a) and N,N-dimethyl-aniline (17a), were unable to
proceed the reaction (entries 16 and 17), probably being ascribed
to the strong coordination toward copper catalyst and the forma-
tion of phenylcarbonic acid from phenol and CO2. Mesitylene could
be transformed to mono-brominated product in excellent yields,
indicating the versatility of such approach (entry 18).
carbonic acid in situ formed from CO2 and H2O could significantly
enhance the catalysis (entry 1 vs. 2, Table 1). Finally, the combina-
tion of lithium cation and bicarbonate anion leads to the formation
of lithium bicarbonate (Eq. (5)).
4. Conclusions
By the investigation of acid catalysis induced by CO2, we have
developed an efficient copper-catalyzed bromination of electron-
rich aromatics with lithium bromide under aerobic conditions with
the aid of in situ acidic CO2/H2O system. This regioselective oxy-
bromination system affords mono-substituted aromatic bromide in
up to 95% yields without any corrosive acid additives. The present
protocol represents an attractive greener approach which offers
robust catalysis, simple post-reaction neutralization with no salted
waste disposal and safe operation for aerobic reaction. The reaction
showed remarkably broad scope of the substrate including aro-
matic ethers, sulfides and mesitylene. Such a protocol could open
up the possibility of turning captured CO2 from waste into potential
resource as a reaction medium and a promoter, which would ren-
der its wide applications in green chemical processes. Accordingly,
the findings disclosed here provides an ecologically safe and cost-
effective route to mono-brominated aromatics, thus shows much
potential application in green process and technological innova-
tion.
3.4. Tentative mechanism
A plausible reaction pathway was proposed for the copper-
catalyzed aerobic oxybromination on the basis of the experimental
investigations in this study and previous reports. The proposed
mechanism is depicted in Scheme 4. A reversible acid equilibrium
from CO2/H2O, is considered as a vital factor for running the reac-
tion. As shown in Eq. (1), dissociation of in situ carbonic acid could
release protons, which would be crucial for the reaction. Although
the precise route of oxybromination step is not clear at the present
stage, such process could probably be initiated through decompo-
sition of CuBr2 into CuBr and Br2, and bromine would be the actual
species for bromination [78] (Eq. (2)). The CuBr2 catalyst would
be regenerated by aerobic oxidation either in the Br2-mediated
bromination or a single oxidation process, both of which may pro-
moted by proton from the in situ formed carbonic acid (Eqs. (3)
and (4)). Such reactivity improvement induced by CO2 was clearly
displayed in the experimental results, for instance, 20 mol% CuBr2-
catalyzed bromination reaction only gave a turnover number of
less than 1 in the absence of CO2 (entry 2, Table 1). As expected,
Acknowledgments
We are grateful to the National Natural Science Foundation of
China (Grant No. 21172125), the “111” Project of Ministry of Edu-
cation of China (Project No. B06005), and the Committee of Science
and Technology of Tianjin.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.cattod.
2012.05.012.
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