Synthetic Communicationsw, 35: 1947–1952, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200064999
Bromination of Aromatic Compounds with
Potassium Bromide in the Presence of
Poly(4-vinylpyridine)-Supported Bromate
in Nonaqueous Solution
Hassan Tajik, Farhad Shirini, Parwin Hassan-zadeh, and
Hassan Rafiee Rashtabadi
Department of Chemistry, Guilan University, Rasht, Iran
Abstract: A simple, efficient, and mild method for selective bromination of activated
aromatic compounds using potassium bromide in the presence of poly(4-vinylpyridine)supported bromate in nonaqueous solution is reported. The results obtained revealed
excellent to good selectiveity between ortho and para positions of methoxyarenes,
anilines, and phenols.
Keywords: Aromatic compounds, bromination, poly(4-vinylpyridine)-supported
bromate, potassium bromide
Bromoarenes are useful intermediates in the synthesis of organometallic
species and pharmaceutically important compounds. Therefore, different
methods have been reported for the bromination of aromatic compounds.[1 – 5]
Direct methods for bromination of aromatic compounds involve the use of
hazardous molecular bromine or expensive transition metal –based catalysts.
In addition, direct bromination of activated aromatic compounds by bromine
generates hydrogen bromide, which is corrosive, toxic, and pollutant to the
environment.[6 – 10]
Received in Poland February 28, 2005
Address correspondence to Hassan Tajik, Department of Chemistry,
Guilan University, Rasht, 41335-1914, Iran. E-mail: tajik@guilan.ac.ir; hasan_tajik@
yahoo.com
1947
1948
H. Tajik et al.
Using molecular bromine also produces mixtures of ortho and para
isomers as well as poly-substituted products. To overcome these difficulties,
various methods have been reported for bromination of aromatic
compounds in which the use of mild conditions and the control of the selectivity, reactivity, and yield have been the point of attention.[1 – 3,10]
In continuation of our research on the halogenation of aromatic
compounds,[4] we have used potassium bromide as the source of bromine
and poly(4-vinylpyridine)-supported bromate as the oxidizing agent in acetonitrile as solvent to brominate a number of activated aromatic compounds
(Scheme 1). The results are given in Table 1.
All the reactions were first performed at room temperature and then
refluxed for the appropriate time. As shown in Table 1, methoxybenzenes
were successfully reacted to afford the desired brominated products
(Table 1, entries 1– 5). Whereas 1-methoxynaphthalene yields mainly
4-bromo-1-methoxynaphthalene, 2-methoxyanaphthalene gave quantitatively
1-bromo-2-methoxynaphthalene (Table 1, entries 6, 7). N,N-dimethylaniline
gave mainly the para isomer as the chief product (Table 1, entry 8). 1-Aminonaphthalene was brominated mainly at 4-position (Table 1, entry 9).
Diphenylamine was converted to disubstituted product at 4,40 -positions in
good yield (Table 1, entry 10). 2-Nitroaniline, however, was brominated
after 7 h in refluxing acetonitrile to afford 4-bromo-2-nitroaniline (Table 1,
entry 11). Phenol, ortho-cholorophenol, and ortho-cresol were quantitatively
converted to the para brominated products with respect to the hydroxyl
groups in good yields (Table 1, entries 12 – 14). Some other aromatic
compounds such as acetanilide and 6-methoxy,1,2,3,4-tetrahydronaphthalenone were also subjected to this reaction. Whereas the former gave less
Scheme 1.
Bromination of Aromatic Compounds
1949
Table 1. Bromination of some aromatic compounds with KBr in the presence of
poly(4-vinyl pyridine)-supported bromate in acetonitrile under reflux conditions
Entry
Substrate
Product(s)
Yield
(%)
Oxidantb/
substrate/
KBr
1
95
1.2/1/1.2
0.7
92
1.2/1/1.2
3
97
1.5/1/2
5
100
1.2/1/1.2
2
95
1/1/1.2
3
82
1/1/1.2
2
95
1.2/1/1.2
3
85
1/1/1.2
2
80
1/1/1.2
Time
(h)
a
1
2
3
4
5
6
7
8
9
(continued )
1950
Table 1.
Entry
H. Tajik et al.
Continued
Substrate
Product(s)
Time
(h)
a
Yield
(%)
Oxidantb/
substrate/
KBr
2
75
1.5/1/2
7
92
1/1/1.2
3
75
1/1/1.2
3.5
95
1/1/1.2
3
87
1/1/1.2
10
—
2/1/1.2
10
,30
2/1/1.2
No Reaction
10
—
2/1/1.2
No Reaction
10
—
2/1/1.2
10
11
12
13
14
No Reaction
15
16
17
18
a
Ioslated yield.
Mmol of bromate in PVP-BrO2
3.
b
than 30% of the para product, the latter recovered unreacted even after 10 h
refluxing in acetonitrile (Table 1, entries 15, 16). When the para-position
was blocked, no reaction was observed and the starting material
was recovered unreacted after 10 h refluxing in acetonitrile (Table 1, entries
17, 18).
Bromination of Aromatic Compounds
1951
It is worth mentioning that chemoselective conversion of methoxyaromatic compounds to their para substituted products was achieved in
excellent yield. Another noteworthy advantage of this system lies in its
ability for selective para vs ortho bromination of N,N-dimethylaniline, diphenylamine, phenol, ortho-chlorophenol, and ortho-cresol. Deactivated aromatic
compounds, however, were recovered unreacted by using this system. The
easy procedure, simple workup, good chemoselectivity, good reaction
times, and good to excellent yields of the products will make this method a
useful addition to the available methodologies.
EXPERIMENTAL
All products were identified by comparison of their spectral and physical data
with those of the known samples.[5] The purity of the products was checked by
TLC on silica-gel polygram SIL G UV 254 plates. Potassium bromide and
other chemicals were purchased from the Merck Chemical Company.
Poly(4-vinylpyridine) in the form of a cross-linked polymer was purchased
from Fluka Chemical Company.
Preparation of Poly(4-vinylpyridine)-Supported Bromate
Poly(4-vinylpyridine) (3.0 g) was placed in 1 M hydrochloric acid (100 mL)
and stirred at room temperature for 24 h. The mixture was filtered and the
precipitate was separated, washed with distilled water (3 15 mL), and
dried at 408C for 24 h. The resulting precipitate was added to a solution of
9.0 g of potassium bromide in distilled water (100 mL) and stirred for 24 h.
Then, the solid was washed, separated, and dried at 35 – 408C for 24 h to
give 4.5 –4.8 g of polymer-supported reagent. A quantity of 0.34 – 0.38 g of
the supported polymer was equal to 1.0 mmol bromate.
Bromination of Aromatic Compounds with Potasssium Bromide
in the Presence of Poly(4-vinylpyridine)-Supported Bromate;
General Procedure
To a solution of aromatic compound (1 mmol) in acetonitrile (5 mL), KBr
(1 mmol) and poly(4-vinylpyridine)-supported bromate (0.34 – 0.68 g) were
added, and the mixture was refluxed for 1 –10 h. Progress of the reactions
was monitored by TLC (eluent: carbon tetrachloride/diethyl ether, 4:2 and
carbon tetrachloride/n-hexane, 8:2) or GC (capillary column). The reaction
mixture was cooled to room temperature and filtered. The excess bromine
was removed from the filtrate by dropwise addition of sodium thiosulfate
solution (1 M). Then, dichloromethane (5 mL) was added and the solution
1952
H. Tajik et al.
was transferred to a separatory funnel. The organic layer was separated and
dried over magnesium sulfate or calcium chloride. Evaporation of the
solvent followed by recrystallization or column chromatography on silica
gel of the crude product gave the corresponding brominated compounds in
good to excellent yields (Table 1).
ACKNOWLEDGMENT
We are grateful to the Guilan University Research Council for partial support
of this work.
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