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:
[email protected]; 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
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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).
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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
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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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