A Metal-Free and Ionic Liquid-Catalyzed Aerobic Oxidative Bromination in Water

Article abstract of DOI:10.1071/CH14161

A metal-free aerobic oxidative bromination of aromatic compounds in water has been developed. Hydrobromic acid is used as a bromine source and 2-methylpyridinium nitrate ionic liquid is used as a recyclable catalyst. Water is used as the reaction mediate. This is the first report of aerobic oxidative bromination using only catalytic amount of metal-free catalyst. This system shows not only high bromine atom economy, but also high bromination selectivity. The possible mechanism and the role of the catalyst in this system have also been discussed.

Full text of DOI:10.1071/CH14161

CSIRO PUBLISHING
Aust. J. Chem. 2015, 68, 513–517
Full Paper
http://dx.doi.org/10.1071/CH14161
A Metal-Free and Ionic Liquid-Catalyzed Aerobic
Oxidative Bromination in Water
A
A
A
A B
,
Jian Wang, Shu-Bin Chen, Shu-Guang Wang, and Jing-Hua Li
A
College of Pharmaceutical Science, Zhejiang University of Technology, No. 18
Chaowang Road, Hangzhou, China.
B
Corresponding author. Email: lijh@zjut.edu.cn
A metal-free aerobic oxidative bromination of aromatic compounds in water has been developed. Hydrobromic acid is
used as a bromine source and 2-methylpyridinium nitrate ionic liquid is used as a recyclable catalyst. Water is used as
the reaction mediate. This is the first report of aerobic oxidative bromination using only catalytic amount of metal-free
catalyst. This system shows not only high bromine atom economy, but also high bromination selectivity. The possible
mechanism and the role of the catalyst in this system have also been discussed.
Manuscript received: 26 March 2014.
Manuscript accepted: 12 July 2014.
Published online: 6 October 2014.
Introduction
Oxidative bromination has become more and more green in
recent years since hydrogen peroxide was replaced by molecular
oxygen, the most abundant and cheapest oxidant.[11] Several
examples of aerobic oxidative bromination have been presented,
involving the use of volatile organic compounds (VOCs) as
solvents and metal salts as catalysts, both of which are hazardous
to the environment. Recently, the aerobic oxidative halogenation
of aromatic compounds has been examined in ionic liquids (ILs)
to overcome these problems.[12] Only a few papers refer to the
investigation of this field so far. All of them have lower bro-
mine atom economy because of the excessive amounts of HBr
employed (up to 200 mol-%).[12a,12d] Another problem is the use
of more than stoichiometric amount ILs as a promoter and a co-
solvent. According to the proposed mechanism therein,[12–14] the
nitrate ionic liquids can be recovered by O2 or air in these
systems. Thus, it motivates us to find an efficient catalyst to
catalyze the oxidative bromination with high bromine atom
economy.
Bromination is widely used in the manufacture of drugs, flame
retardants and many other products.[1] The formed C–Br bond
can be easily transformed into C–C bond and C–hetero atom
bond via various important reactions.[2] Conventional methods
for the syntheses of aromatic bromo-substituted compounds
usually involve various brominating agents such as bromine[3]
and its complexes,[4] inorganic substances that contain positive-
valence bromine,[5] N-bromosuccinimide,[6] and other organic
bromides.[7] Recently, tribromide organic salts have been
developed as bromine-vapour-pressure-free and highly selective
brominating agents.[8] However, the Br atom economy is still
lower (less than 50 %).
An alternative solution is oxidative bromination. Typically,
the oxidative bromination reactions are carried out with a
combination of bromide anion and a variety of oxidants
(Scheme 1). In many cases, these systems require stoichiometric
oxidizing agents,[9] which lead to environmental problems and
low atom efficiency due to their undesirable by-products.
Hydrogen peroxide-based oxidative bromination is a mild and
environmentally friendly system, in which the main by-product
is water.[10] However, the instability and explosive property of
hydrogen peroxide during reaction at elevated temperature are
dangerous.
Results and Discussion
Screening of the Catalyst
According to the existing reports of oxidative bromination,
NOꢀ3 anion plays a key role in the transformation from Brꢀ to Brþ
in the presence of O2.[12–14] Thus, following these previous
accounts (reaction was carried out in an open system where air is
an available source of oxygen), several organic nitrate ILs
were scanned using anisole as a model substrate to determine the
best catalyst (Table 1). These ILs can be easily synthesized
by adding nitric acid to an ethanol solution of amines. Among
them, 2-methylpyridinium nitrate (2-MePyꢁHNO3; 1k) and 2,6-
dimethylpyridinium nitrate (2,6-DiMePyꢁHNO3; 1n) showed the
bestcatalytic efficiency(Table 1, Entries 11and15, respectively).
Considering that 2-methylpyridine is much cheaper and more
available than 2,6-lutidine, we chose 2-methylpyridinium nitrate
(1k) as the catalyst for the subsequent experiments.
Oxidants (Oxone, m-CPBA, etc.)
H2O2, Catalyst
O2, metal
R
R
BrϪ
ϩ
Br
O2
Stoichiometric amount of ILs
Scheme 1. Summary of existing oxidative bromination methods. m-CPBA
refers to m-chloroperoxybenzoic acid.
Journal compilation Ó CSIRO 2015
www.publish.csiro.au/journals/ajc

514
J. Wang et al.
Table 1. Screening of nitrate ILs catalysts for the current aerobic
oxidative bromination systemA
Optimization of the Reaction Conditions
The conditional optimization of 2-methylpyridinium nitrate-
catalyzed oxidative bromination is presented in Table 2
(Scheme 2). The anisole could not be completely consumed until
the amount of catalyst was up to 10 % (Table 2, Entries 1–5).
However, the effect of the temperature on the yield is not crucial
(Table 2, Entries 6–8). The bromination could also proceed
smoothly with good yields even at room temperature through
prolonging the reaction time to 48 h (Table 2, Entry 8).
Cy, cyclo; Me, methyl; Py, pyridine
Entry
Catalyst
Time [h]
ConversionB [%]
1
CH3NH2ꢁHNO3 (1a)
C2H5NH2ꢁHNO3 (1b)
(CH3)2NHꢁHNO3 (1c)
(C2H5)2NHꢁHNO3 (1d)
Cy-C6H11NH2ꢁHNO3 (1e)
(C2H5)3NꢁHNO3 (1f)
(C4H9)3NꢁHNO3 (1g)
C6H5NH2ꢁHNO3 (1h)
C6H5CH2NH2ꢁHNO3 (1i)
PyꢁHNO3 (1j)
9
12
12
12
12
12
12
12
12
5
99 (95)
30
2
3
17
4
7
5
29
6
23
7
30
Application of this System to Various Aromatic
Compounds
8
0
9
24
The optimum reaction conditions (10 mol-% catalyst, 608C;
Table 2, Entry 6) were then used for the oxidative bromination of
various aromatic compounds (Table 3). Anisole and its deriva-
tives could be easily converted into their corresponding bromo-
anisoles even at room temperature (Table 3, Entries 1–10). But
for phenols, 30 % molar amount of the catalyst is preferred
(Table 3, Entries 11–20). The arenes with electron donating
groups could also be brominated with good results (Table 3,
Entries 21–23). However, the aromatic compounds with elec-
tron withdrawing groups failed (Table 3, Entries 24–25). The
results reveal that the group of substrates has an important
influence on the bromination reactivity. The higher the electron
donating capacity of the group, the easier the reaction will
proceed (Table 3, Entries 1–10, 21–25). The only exception is
phenol (Table 3, Entries 11–20); the reason for its lower reac-
tivity in this system is unclear.
10
11
12
13
14
15
16
17
18
19
20
99 (95)
100 (98)
100 (98)
10
2-MePyꢁHNO3 (1k)
2-MePyꢁHNOC3
2
2
3-MePyꢁHNO3 (1l)
12
2
4-MePyꢁHNO3 (1m)
2,6-DiMePyꢁHNO3 (1n)
2,6-DiMePyꢁHNOC3
99 (96)
100 (98)
100 (98)
99 (98)
67
2
2
2-CarboxyPyꢁHNO3 (1o)
2-CarboxyPyꢁHNO3C
2,6-DicarboxyPyꢁHNO3 (1p)
2,6-DicarboxyPyꢁHNO3C
2
8
2
100 (98)
61
6
ABromination conditions were as follows: anisole (5 mmol), catalyst
(1.5 mmol), HBr (5.5 mmol, 40 % aqueous solution); 808C; open system.
BNumbers in the parentheses refer to the selectivity for p-bromo anisole.
Conversion and selectivity were determined by GC in comparison with
authentic samples.
CAmount of catalyst used was 1 mmol.
Catalyst Recycling
Then, the recycling experiments were conducted using anisole
as the substrate (Table 4, Scheme 3). The catalyst was recycled
by extracting the organic products from the reaction mixture,
and removing water from the aqueous layer by evaporation in
the reduced pressure. The reaction proceeded with a small
decrease in yield with long reaction times, and the catalyst could
be recycled at least 4 times (Table 4, Entries 1–4). Moreover, if
10 mol-% nitric acid was added after the extraction to re-activate
the catalyst, the reaction will proceed much more efficiently, in
both yield and reaction time (Table 4, Entries 5–8). As stated in
Perosa’s previous work,[12b] the halogen anion would partly
inactivate the catalyst by replacing the NOꢀ3 . Thus, adding nitric
acid could re-activate the catalyst by the anion exchange of Brꢀ
and NOꢀ3 .
Table 2. Optimization of reaction conditionsA
Entry Catalyst [mol-%] Temperature [8C] Time [h] ConversionB [%]
1
2
3
4
5
6
7
8
30
20
10
5
80
2
2
100 (98)
100 (98)
100 (99)
70
80
80
4
80
24
24
4
1
80
5
10
10
10
60
40
100 (99)
99 (98)
99 (98)
24
48
Room temperature
AOxidative bromination conditions were as follows: anisole (5 mmol), HBr
(5.5 mmol, 40 % aqueous solution), 2-MePyꢁHNO3 in a reaction tube which
is exposed to air and equipped with a condenser.
BNumbers in the parentheses refer to the selectivity for p-bromoanisole.
Conversion and selectivity were determined by GC in comparison with
authentic samples.
Possible Mechanism
According to previous reports,[12–14] a possible mechanism for
this oxidative bromination is presented in Scheme 4, which can
be described as a sequential cascade of double-cycle oxidation–
reduction reactions. Therefore, the entire reaction uses oxygen
as oxidant and is catalyzed by nitrate (Eqn 1).[12]
O
O
6ArH þ 6HBr þ 3O2 ! 6ArBr þ 6H2O
ð1Þ
Catalyst, HBr, Air
In addition, anisole could not be consumed completely even
with stoichiometric nitric acid and hydrobromic acid unless 5 %
tetrabutylammonium bromide was added. This indicates that
2-methylpyridinium nitrate not only plays a role of oxidative
catalyst, but also plays, to some extent, a role of phase transfer
catalyst.
Br
Scheme 2. Optimization of the reaction conditions.

A Green Oxidative Bromination Method
515
Table 3. Aerobic oxidative bromination of various aromatic compoundsA
Entry
Substrate
Time [h]
Product
YieldB [%]
1
Anisole (2a)
AnisoleC
4
48
3.5
48
4
4-Bromoanisole (3a)
4-Bromoanisole
99 (97)
98
2
3
2-Methylanisole (2b)
2-MethylanisoleC
4-Methylanisole (2c)
4-MethylanisoleC
3-Methylanisole (2d)
3-MethylanisoleC
1,3-Benzodioxole (2e)
1,3-BenzodioxoleC
Phenol (2f)
4-Bromo-2-methylanisole (2b)
4-Bromo-2-methylanisole
2-Bromo-4-methylanisole (3c)
2-Bromo-4-methylanisole
4-Bromo-3-methylanisole (3d)
4-Bromo-3-methylanisole
5-Bromobenzo[d][1,3]dioxole (3e)
5-Bromobenzo[d][1,3]dioxole
4-Bromophenol (3f)
99 (95)
99
4
5
97 (93)
97
6
48
3.5
48
5
7
99 (96)
98
8
9
96 (93)
96
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
48
12
6
72
PhenolD
4-Bromophenol
95 (78)
75
o-Cresol (2g)
o-CresolD
12
5
4-Bromo-o-cresol (3g)
4-Bromo-o-cresol
97 (80)
74
m-Cresol (2h)
m-CresolD
12
4.5
12
5
4-Bromo-m-cresol (3h)
4-Bromo-m-cresol
96 (81)
69
p-Cresol (2i)
p-CresolD
2-Bromo-p-cresol (3i)
2-Bromo-p-cresol
93 (69)
70
Hydroquinone (2j)
HydroquinoneD
Toluene (2k)
TolueneE
12
4
2-Bromo-hydroquinone (3j)
2-Bromo-hydroquinone
p-Bromotoluene (3k)
p-Bromotoluene
90 (46)
81
24
12
8
93 (82)
96 (87)
5
1,3-Dimethylbenzene (2l)
Chlorobenzene
Nitrobenzene
4-Bromo-1,3-dimethylbenzene (3l)
–
24
24
–
Trace
AOxidative bromination conditions were as follows: substrate (5 mmol), HBr (5.5 mmol, 40 % aqueous solution), 2-MePyꢁHNO3
(0.5 mmol) at 608C in a reaction tube which is exposed to air and equipped with a condenser.
BDetermined by GC in comparison with authentic samples. The numbers in the parenthesis refer to isolated yields.
CThe reaction was carried out at room temperature.
DThe amount of 2-methylpyridinium nitrate used was 1.5 mmol.
EReaction took place under reflux conditions.
R
R
Table 4. Catalyst recyclingA
NOϪ3
BrϪ
H2O
Entry
Recycle Number
Time [h]
ConversionB [%]
Br
1
2
3
4
5
6
7
8
1
72
72
72
72
4
92
92
91
90
99
99
98
98
2
Br2
O2
NO
3
4
1C
2C
3C
4C
Scheme 4. Possible mechanism for the current reaction system.
4
4
4
Conclusion
AReaction conditions were as follows: substrate (5 mmol), HBr (5.5 mmol,
40 % aqueous solution), 2-MePyꢁHNO3 (0.5 mmol) at 608C in a reaction tube
which is exposed to air and equipped with a condenser.
BDetermined by GC.
In conclusion, we have developed a catalytic aerobic oxidative
bromination of aromatic compounds. This is the first report of
aerobic oxidative bromination using only catalytic amount
(10 %) of metal-free catalyst. Anisole and its derivatives could
be efficiently brominated even at room temperature and other
electron-rich aromatic compounds could also be brominated
under mild conditions. This procedure represents an attractive
green bromination approach for several reasons: (1) the reaction
proceeds with high product yields and high (.90 %) bromine
atom economy; (2) handling of Br2 can be avoided; (3) no metal
or VOCs, as solvent, is used in this system; (4) the components
and by-product of the reaction are both non-toxic, cheap, and
readily available; and (5) the catalyst can be easily recovered
and recycled. Therefore, this method offers an efficient system
for green bromination of aromatic compounds with high atom
efficiency and good product yields.
CNitric acid (0.05 mmol) was added after extraction.
O
O
Recycled Catalyst
HBr, Air
Br
Scheme 3. Recycled catalyst for the reaction.

516
J. Wang et al.
Experimental
with EA (3 ꢂ 20 mL). The aqueous layer was separated and
concentrated under reduced pressure until water was completely
removed, after which fresh anisole (5 mmol) and HBr (5.5 mmol,
40 % aqueous solution) were added to restart the reaction.
General Methods
Gaschromatography(GC)analysiswasperformedonaTechcomp
7890II instrument fitted with a SE-54 30 m ꢂ 0.32 mm ꢂ 0.25 mm
capillary column and a hydrogen flame ionization detector.
Identification of the constituents was based on comparison of the
retention times with those of authentic samples. 1H NMR spectra
were recorded on a Bruker Avance III 500 MHz Spectrometer.
Chemical shifts (d) are given in ppm. All substrates and authentic
samples (for GC analysis) were commercial products.
Catalyst Recovery
After completion of the reaction (time was defined by GC),
15 mL water was added to the mixture, which was then washed
with EA (3 ꢂ 20 mL). After that, 0.05 mmol nitric acid was
added to the aqueous layer and the solution was stirred at room
temperature for 1 h. Then, the solution was condensed under
reduced pressure until water was completely removed, after
which fresh anisole (5 mmol) and HBr (5.5 mmol, 40 % aqueous
solution) were added to restart the reaction.
Preparation of the Catalyst (1a–1q)
In a 250 mL flask, amine (100 mmol) was dissolved in ethanol
(50 mL), then nitric acid (105 mmol) was dropped into the
solution. During addition, the temperature was maintained to
below 108C (an ice bath may be required). The mixture was
stirred at room temperature for an additional hour after the
addition. Evaporation of the solvents under reduced pressure
gave the nitrate catalyst.
1H NMR Data of Products
3a: dH (500 MHz, CDCl3; TMS) 7.40 (2H, d, J 9), 6.80 (2H, d,
J 9), 3.79 (3H, s).
3b: dH (500 MHz, CDCl3; TMS) 7.28–7.30 (2H, m), 6.69–
6.71 (1H, m), 3.83 (3H, s), 2.23 (3H, s).
3c: dH (500 MHz, CDCl3; TMS) 7.21 (1H, d, J 2 Hz), 6.90
(1H, dd, J 8, J 2), 6.63(1H, d, J 8), 3.69 (3H, s), 2.12 (3H, s).
3d: dH (500 MHz, CDCl3; TMS) 7.42 (1H, d, J 9), 6.81 (1H,
d, J 3), 6.64 (1H, dd, J 9, J 3), 3.79 (3H, s), 2.39 (3H, s).
3e: dH (500 MHz, CDCl3; TMS) 6.94–6.97 (2H, m), 6.69
(1H; d, J 8), 5.97 (2H, s).
Typical Procedure
The substrate (5 mmol), HBr (5.5 mmol, 40 % aqueous solution)
and catalyst were vigorously stirred in a reaction tube that is
exposed to air at a certain temperature and equipped with a
condenser. The reaction was carried out for a specific time.
3f: dH (500 MHz, CDCl3; TMS) 7.34 (2H, dd, J 6.5, J 2), 6.75
(2H, dd, J 6.5, J 2), 6.22 (1H, br).
Analytical Procedure
During the process of the reaction, a small portion of the mixture
was sampled and added to 1 mL saturated NaHSO3 aqueous
solution. After extraction by ethyl acetate (EA), a trace of the
organic layer was injected into the GC instrument (oven tem-
perature: from 708C to 1708C at a rate of 258C minꢀ1; injector
temperature: 2508C; detector temperature: 2508C) to monitor
the conversion and yield of the reaction in comparison with
authentic samples.
3g: dH (500 MHz, CDCl3; TMS) 7.25 (1H, d, J 2.5), 7.18 (1H,
dd, J 8.5, J 2.5), 6.66 (1H, d, J 8.5), 4.90 (1H, s), 2.23 (3H, s).
3h: dH (500 MHz, CDCl3; TMS) 7.36 (1H, d, J 9), 6.75 (1H,
d, J 3), 6.57 (1H, dd, J 9, J 3), 5.01 (1H, br), 2.35 (3H, s).
3i: dH (500 MHz, CDCl3; TMS) 6.94–7.30 (3H, m), 5.53 (1H;
br), 2.30 (3H; s).
3j: dH (500 MHz, CDCl3; TMS) 7.01 (1H, d, J 3), 6.91 (1H, d,
J 9), 6.74 (1H, dd, J 9, J 3), 5.15 (1H, br), 4.71 (1H, br).
3k: dH (500 MHz, CDCl3; TMS) 7.41 (2H, d, J 8), 7.08 (2H,
d, J 8), 2.34 (3H, s).
Purification and Identification of the Product (3a–3l)
For 3a–3e, 3k, 3l
3l: dH (500 MHz, CDCl3; TMS) 7.43 (1H; d, J 8), 7.08 (1H,
s), 6.89 (1H, d, J 8), 2.40 (3H, s), 2.31 (3H, s).
After completion of the reaction (time was defined by GC),
the mixture was quenched with 15 mL saturated NaHSO3
aqueous solution. Then, the mixture was extracted with EA
(3 ꢂ 20 mL). The combined organic layer was washed with
brine and dried over MgSO4, then concentrated under reduced
pressure. The product was isolated by column chromatography
(Petroleum ether (PE): EA from 100 : 1 to 10 : 1) and character-
ized by 1H NMR spectroscopy.
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