Bromination of aromatic compounds using an Fe2O 3/zeolite catalyst

Article abstract of DOI:10.1039/c2gc35821b

The catalytic bromination of non-activated aromatic compounds has been achieved using an Fe₂O₃/zeolite catalyst system. FeBr ₃ was identified as the catalytic species, formed in situ from HBr and Fe₂O₃. The catalyst was easy-to-handle and cost effective and could also be recycled. The reaction system was also amenable to the one-pot sequential bromination/C-C bond formation of benzene.

Full text of DOI:10.1039/c2gc35821b

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COMMUNICATION
Bromination of aromatic compounds using an Fe2O3/zeolite catalyst†‡
Yuta Nishina*a and Keishi Takamib
Received 28th May 2012, Accepted 22nd June 2012
DOI: 10.1039/c2gc35821b
The catalytic bromination of non-activated aromatic com-
pounds has been achieved using an Fe2O3/zeolite catalyst
system. FeBr3 was identified as the catalytic species, formed
in situ from HBr and Fe2O3. The catalyst was easy-to-handle
and cost effective and could also be recycled. The reaction
system was also amenable to the one-pot sequential bromina-
tion/C–C bond formation of benzene.
Fig. 1 Formation of cationic bromine species from (a) a metal halide
and (b) a metalate.
Aromatic organobromides have been widely used as starting
materials in organic synthesis1 and as functional materials.2 The
bromination of aliphatic hydrocarbons with Br2 occurs via the
radical abstraction of a hydrogen atom, whereas the bromination
of aromatic hydrocarbons proceeds via the electrophilic addition
to Br2 or a cationic bromine species. To date, a wide variety of
protocols have been reported for the bromination of aromatic
compounds. N-Bromoimides, such as N-bromosuccinimide
(NBS)3 and 1,3-dibromo-5,5-dimethylhydantoine (DBDMH),4
are well known brominating reagents, with the polarity of the N–
Br bond facilitating the electrophilic addition of aromatic com-
pounds. Unfortunately, however, these types of brominating
agents require acidic conditions, decompose under photo
irradiation or thermal conditions, and produce the corresponding
imides as unwanted byproducts. Recently, gold-5 and palladium-
catalyzed6 bromination reactions were reported using NBS under
neutral conditions. Bromine salts represent another potential
source of bromine, as they are easy to handle and can produce
cationic bromine under oxidative conditions with a vanadium
catalyst.7 Br2, however, is the most attractive of the brominating
reagents, because of its comparatively abundant industrial avail-
ability, with bromine being obtained from sea and ground water.
All bromine-containing molecules, including even HBr and
NaBr, are produced from Br2 according to the current techno-
logy.8 Br2 is a non-polar molecule and activation is therefore
required to facilitate its reaction with electron deficient aromatic
compounds. Br2 can brominate aromatic compounds in the pres-
ence of stoichiometric amounts of metal oxides or salts with
strong acids,9 catalytic amounts of metal halides,10 or a large
Table 1 Catalyst optimizationa
Entry
Catalyst
Yieldb/%
1c
2c
3c
4c
5c
6c
7c
8
Fe2O3/zeolite
ZnOx/zeolite
MnOx/zeolite
CuOx/zeolite
CoOx/zeolite
Fe2O3/SiO2
Fe2O3/Al2O3
Fe2O3
>99
77
30
27
16
1
69
0
36
95
7
9
10
11
Zeolite
FeBr3
FeBr3·6H2O
a All reactions were performed using 1 mL of benzene (1) and 0.5 mmol
of Br2. b GC yield using dodecane as an internal standard. c 1 mmol of a
metal salt was impregnated on 1 g of support prior to 1 h of calcination
at 300 °C.
amount of reusable aluminosilicate (zeolite).11 These reagents and
catalysts can generate cationic bromine species for the electrophi-
lic addition to aromatic compounds (Fig. 1). With respect to the
use of these methodologies, metal oxides are more stable, less
hazardous, and easier to handle than metals and metal halides.
Unfortunately, however, they are less reactive for the bromination
of aromatic compounds. The activity of these materials toward
bromination can be ranked in the order metal halides (AlX3,
FeX3) > metalate (aluminosilicate) > metal oxide (Al2O3, Fe2O3).
We designed metal oxide–metalate composite materials for
use as stable and easy-to-use catalysts for the bromination of aro-
matic compounds with Br2 under neutral conditions. Following a
period of catalyst screening, a combination of iron oxide and
zeolite (Fe2O3/zeolite) was found to be the optimum catalytic
system (Table 1, entries 1–7). Although many different types of
aResearch Core for Interdisciplinary Sciences, Okayama University,
Tsushimanaka, Kita-ku, Okayama 700-8530, Japan.
E-mail: nisina-y@cc.okayama-u.ac.jp; Fax: +81-86-251-8718;
Tel: +81-86-251-8718
bFaculty of Engineering, Department of Applied Chemistry, Okayama
University, Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
†Products and yields are determined by GC comparing the standard
sample, and using dodecane as an internal standard.
‡Electronic supplementary information (ESI) available: Methods and
experimental procedure. See DOI: 10.1039/c2gc35821b
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Table 2 Substrate investigationa
Entry
1c
Substrate
Conditions
40 °C, 2 h
Products and yieldb/%
2c
3
80 °C, 2 h
40 °C, 2 h
4
40 °C, 2 h
5
6
7
40 °C, 2 h
40 °C, 2 h
85 °C, 2 h
8
9
85 °C, 2 h
85 °C, 2 h
10
11
85 °C, 2 h
85 °C, 2 h
12
13
25 °C, 2 h
40 °C, 2 h
a All reactions were performed using 1 mmol of aromatic compound and 0.5 mmol of Br2. The zeolite JRC-Z-B25 was provided by the Catalysis
Society of Japan, and 1 mmol of Fe2O3 was supported on 1 g of zeolite. b GC yield using dodecane as an internal standard. c Reactions are performed
in the dark to prevent the photoinduced formation of bromine radicals.
zeolite performed well as a component of the catalyst system,
JRC-Z-B25 was selected for application in this research.12
Crystal structure analysis of the Fe2O3/zeolite (JRC-Z-B25) com-
posite using XRD measurements revealed that α-Fe2O3 was
formed (Fig. 2b). During the initial optimization work, however,
α-Fe2O3 did not show any catalytic activity (Table 1, entry 8). In
contrast, zeolite moderately promoted the bromination process
(Table 1, entry 9).
We proceeded to investigate the scope of the transformation
with a variety of different substrates (Table 2).13 The bromination
of toluene (2) generally occurs at the benzylic position under
photolysis,14 heating, and MnO2-mediated15 reaction conditions.
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We then proceeded to consider the structural analysis of the
catalyst. Following the bromination reaction, α-Fe2O3 was not
detected and only signals corresponding to the zeolite (Fig. 2c)
were observed by XRD measurement. In contrast, the chemical
composition of the recovered catalyst, as determined by energy-
dispersive X-ray spectroscopy (EDX), was roughly Fe–Al–Si =
1.0 : 0.75 : 11 in at%, and the Fe had been homogeneously dis-
persed onto the zeolite (Fig. 3). α-Fe2O3 was regenerated by
heating the recovered catalyst for 1 h in air at 300 °C (Fig. 2a).
These results suggested the in situ formation of non-crystalline
FeBr3 which would be the actual active species in the Fe2O3/
zeolite system. This observation was consolidated by the earlier
observation when FeBr3 behaved as a catalyst during the opti-
mization process (Table 1, entry 10). Unfortunately, however,
FeBr3 is water sensitive, leading to a reduction in the
product yield when hydrated even by moisture in the air
(Table 1, entry 11).
Fig. 2 XRD analysis of (a) Fe2O3/zeolite catalyst heated at 300 °C for
1 h after recovery from the reaction mixture, (b) as prepared Fe2O3/
zeolite, and (c) as recovered catalyst. The markers (▼) indicate the
signals of α-Fe2O3 and (○) indicate the signals of zeolite.
The reaction pathway for the bromination of aromatic com-
pounds by Fe2O3/zeolite is shown in Scheme 1. Initially, the
zeolite promotes the bromination of benzene (1) to form bromo-
benzene (1′) and HBr (Table 1, entry 9), which activates Fe2O3
into the highly reactive FeBr3. The resulting H2O would then be
adsorbed by the zeolite, maintaining the high catalytic perform-
ance of FeBr3.
The reaction mechanism depicted in Scheme 1 suggested that
Fe2O3 was not necessarily supported by the zeolite. When
α-Fe2O3 and zeolite were separately added into the mixture of
benzene (1) and Br2, however, the desired product (1′) was
obtained in 97% yield, although it took 3 h for completion of the
reaction. Furthermore, many types of iron oxide worked as a
catalyst, even rusted steel gave 1′ in 95% yield for 3 h.
Once the recovered catalyst is exposed to air, it loses its cata-
lytic activity as a consequence of hydration. Two methods were
developed for recovering the catalyst by (A) evaporation of all of
the solvents, substrates and products in the absence of air, and
(B) filtration of the reaction mixture followed by heating of the
recovered catalyst at 300 °C for 1 h to remove the adsorbed
water and transform unstable FeBr3 into stable Fe2O3. The cata-
lyst recovered by method (A) could be reused in the bromination
of benzene (1) without significant loss of catalytic activity and
selectivity. In contrast, the catalyst recovered by method (B)
suffered a decrease in the yield caused by leaching of FeBr3
from zeolite during the filtration procedure (Fig. 4).
It was envisaged that our novel Fe2O3/zeolite-catalyzed bromi-
nation of benzene could be used as the platform for making new
carbon–carbon bonds. Using the well-established C–C bond
forming transformation of aryl bromides, in conjunction with the
Fe2O3/zeolite-catalyzed bromination, would create a new route
to aromatic compounds.17 The one-pot functionalization of
benzene (1) through bromobenzene (1′) was investigated. The
Fe2O3/zeolite catalyst did not inhibit the conversion of 1′ during
either the palladium-catalyzed Suzuki coupling reaction or the
bromine–lithium exchange reaction (Scheme 2).
Fig. 3 Micrographs of Fe2O3/zeolite. (a) SEM image, and elemental
distribution of (b) Al, (c) Fe, and (d) Si.
In contrast, the Fe2O3/zeolite promoted electrophilic bromination
to give a mixture of p- and o-bromotoluenes (2′ and 2′′) in 51%
and 38% yields, respectively (entry 1). The catalyst did not
promote the bromination of the aliphatic hydrocarbon, cyclo-
hexane (3), with only a trace of the product being detected
(entry 2). This result suggests that a bromine radical was not
formed in the Fe2O3/zeolite system. Polyhalobenzenes play an
important role in the synthesis of organic functional molecules.
We investigated the catalytic effectiveness of Fe2O3/zeolite
for the bromination of halobenzenes. Fluorobenzene (4) gave
p-bromofluorobenzenes (4′) in high yield and selectivity.
In contrast, the para/ortho selectivity of the bromination process
tended to decrease when other arylhalides were used (entries
4–6).16 When the p-dihalobenzenes (8–10) were used, mono-
bromination (8′–10′) and dibromination products (8′′–10′′) were
obtained in substantial yields (entries 7–9), whereas the m- and
o-difluorobenzenes (11 and 12) gave only the monobromination
products (11′ and 12′) with excellent levels of selectivity (entries
10 and 11). The selective α-bromination of naphthalene (13) and
p-bromination of biphenyl (14) were successfully achieved with
the current catalytic system (entries 12 and 13).
Conclusions
We have successfully developed the in situ formation of an
active FeBr3 catalyst from Fe2O3, which was initiated by the
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The Circle for the Promotion of Science and Engineering, and
Hatakeyama Culture Foundation. We also received generous
support from Mr Noriyasu Kimura for SEM-EDX analysis.
JRC-Z-B25, JRC-Z5-90NA, JRC-Z-HB150, JRC-Z-HB25,
JRC-Z-HM90, JRC-Z-HY5.3, and JRC-Z-B25 were gifted from
the Catalysis Society of Japan.
Notes and references
Scheme 1 Reaction pathway for bromination of aromatic compounds
in the Fe2O3/zeolite system.
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Fig. 4 Catalyst recycling after bromination of bromobenzene by (A)
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Acknowledgements
Financial support for this study was provided by Funds for the
Development of Human Resources in Science and Technology,
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