Bromination of some aromatic compounds with potassium bromide in the presence of benzyltriphenylphosphonium peroxodisulfate

Article abstract of DOI:10.1080/00397910601033906

A simple, efficient, and mild method for selective bromination of some activated aromatic compounds using potassium bromide in the presence of benzyltriphenylphosphonium peroxodisulfate in nonaqueous solution is reported. The results obtained revealed good to excellent selectivity between ortho and para positions of phenols and methoxyarenes. Copyright Taylor & Francis Group, LLC.

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Bromination of Some Aromatic Compounds
with Potassium Bromide in the Presence of
Benzyltriphenylphosphonium Peroxodisulfate
a b
c
a b
Hassan Tajik
, Iraj Mohammadpoor‐Baltork & Jalal Albadi
a
Department of Chemistry, Guilan University, Rasht, Iran
Persian Gulf University, Bushehr, Iran
b
c
Department of Chemistry, College of Science, Isfahan University, Isfahan,
Iran
Version of record first published: 25 Jul 2007.
To cite this article: Hassan Tajik , Iraj Mohammadpoor‐Baltork & Jalal Albadi (2007): Bromination of
Some Aromatic Compounds with Potassium Bromide in the Presence of Benzyltriphenylphosphonium
Peroxodisulfate, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 37:2, 323-328
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Synthetic Communications , 37: 323–328, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910601033906
Bromination of Some Aromatic Compounds
with Potassium Bromide in the Presence of
Benzyltriphenylphosphonium
Peroxodisulfate
Hassan Tajik
Department of Chemistry, Guilan University, Rasht, Iran and Persian
Gulf University, Bushehr, Iran
Iraj Mohammadpoor-Baltork
Department of Chemistry, College of Science, Isfahan University,
Isfahan, Iran
Jalal Albadi
Department of Chemistry, Guilan University, Rasht, Iran and Persian
Gulf University, Bushehr, Iran
Abstract: A simple, efficient, and mild method for selective bromination of some
activated aromatic compounds using potassium bromide in the presence of benzyltri-
phenylphosphonium peroxodisulfate in nonaqueous solution is reported. The results
obtained revealed good to excellent selectivity between ortho and para positions of
phenols and methoxyarenes.
Keywords: benzyltriphenylphosphinium peroxodisulfate, bromination, methoxy-
arenes, phenols, potassium bromide
Haloarenes are useful versatile synthetic intermediates for a number of trans-
formations in organic chemistry. They are also found in pharmaceutically and
biologically important compounds. Therefore, various methods have been
Received in the U.K. March 30, 2006
Address correspondence to Hassan Tajik, Department of Chemistry, Guilan Univer-
sity, Rasht 41335-1914, Iran. E-mail: tajik@guilan.ac.ir; hasan_tajik@yahoo.com
3
23

3
24
H. Tajik I. Mohammadpoor-Baltork, and J. Albadi
developed for the bromination of aromatic rings using different reaction con-
ditions. Direct treatment of aromatic compounds with molecular bromine
normally results in a mixture of mono-, di-, and polysubstituted products. In
addition, direct bromination of activated aromatic compounds by bromine
generates hydrogen bromide, which is corrosive, toxic, and pollutes the
[
1–4]
environment.
To overcome these difficulties, numerous methods have been proposed to
[5–12]
increase the selectivity and also the yields of the desired para-products.
In continuation of our research on the halogenation of aromatic
[
13,14]
compounds,
this article reports the use of potassium bromide as the
source of bromine and benzyltriphenylphosphonium peroxodisulfate as the
oxidizing agent in acetonitrile as solvent for the bromination of a number of
phenols and methoxyarenes (Scheme 1). The results are shown in Table 1.
The reactions were first performed under reflux conditions for the appro-
priate time. As shown in Table 1, methoxybenzenes were successfully reacted
to afford the desired monobrominated products (Table 1, entries 1–9).
2-Methoxynaphthalene, however, produced 1-bromo-2-methoxynaphthalene
(
Table 1, entry 7). N,N-dimethylaniline gave mainly the para isomer as the
major product (Table 1, entry 10). Catechol, phenol, ortho-cholorophenol,
ortho-cresol, and ortho-methoxyphenol were quantitatively converted to the
para-brominated products with respect to the hydroxyl groups (Table 1,
entries 11–15). Para-nitro phenol and 2,4-dinitrophenol were also
converted to the monobrominated products over longer reaction times
(
Table 1, entries 16 and 17). Resorcinol and para-methoxyphenol were quan-
titatively reacted to give the corresponding monobrominated products
Table 1, entries 18 and 19). Some other aromatic compounds such as
-naphthol and 2-naphthol were also subjected to these reaction conditions,
(
1
and the corresponding products were obtained in good yields (Table 1,
Scheme 1.

Bromination of Aromatic Compounds
325
Table 1. Bromination of some aromatic compounds with potassium bromide in the
presence of benzyltriphenylphosphonium peroxodisulfate in acetonitrile under reflux
conditions
Oxidant/
substrate/
KBr
Time Yield
(%)
Entry
1
Substrate
Product(s)
(h)
3.5
100
1.2/1/1.2
1.2/1/1.2
2
3.0
100
3
4
4.5
2.5
100
100
1.2/1/1.2
1.2/1/1.2
5
6
2
6
100
82
1/1/1.2
1/1/1.2
7
5
95
1.2/1/1.2
8
9
6
4
100
100
1/1/1.2
1/1/2
1
0
1
3
80
1/1/1.2
1
3.5
100
1.2/1/1.2
12
4.5
100
1/1/1.2
(
continued)

3
26
H. Tajik I. Mohammadpoor-Baltork, and J. Albadi
Table 1. Continued
Oxidant/
substrate/
KBr
Time Yield
(h)
Entry
Substrate
Product(s)
(%)
1
3
7.0
100
1/1/1.2
1/1/1.2
1
4
4.5
100
1
1
5
6
4
100
100
1/1/1.2
1// 1/1.2
8.5
1
7
9.5
100
1/1/1.2
1
1
2
8
9
0
2
100
100
70
1/1/1.2
1/1/1.2
1/1/1.2
4.5
6.5
2
2
1
2
5.5
5
90
90
1/1/1.2
1/1/1.2
2
3
6
65
1/1/1.2

Bromination of Aromatic Compounds
327
entries 21 and 22). Salicylic acid and para-hydroxysalicylic acid produced the
brominated products over longer times and with low yields compared to the
other activated phenols and methoxyarenes (Table 1, entries 20 and 23).
It is worth mentioning that chemoselective conversion of methoxyaromatics
to their para-substituted products was achieved in excellent yield. Another
noteworthy advantage of this system lies in its ability to selectively brominate
the para vs. ortho positions in N,N-dimethylaniline, catechol, phenol, ortho-
chlorophenol, 2-methoxyphenol, and ortho-cresol. Simple workup, good
chemoselectivity, good reaction times, and 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
[
15]
with those of the known samples.
The purity of the products was checked
by thin-layer chromatography (TLC) on silica-gel polygram SIL G UV 254
plates. Potassium bromide and also other chemicals were purchased from
the Merck Chemical Company. Benzyltriphenylphosphonium peroxodisulfate
[16]
was prepared according to a previously reported method.
Bromination of Aromatic Compounds with Potasssium Bromide in
the Presence of Benzyltriphenylphosphonium Peroxodisulfate;
General Procedure
To a solution of aromatic compound (1 mmol) in acetonitrile (5 mL), KBr
(1 mmol) and benzyltriphenylphosphonium peroxodisulfate (0.9 g) were
added and heated under reflux for 2–9.5 h. Progress of the reaction 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
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 thankful to the Guilan University and also the Persian Gulf University
Research Councils for their partial support of this work.

3
28
H. Tajik I. Mohammadpoor-Baltork, and J. Albadi
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