Article pubs.acs.org/IECR
Bromination of Deactivated Aromatic Compounds with Sodium Bromide/Sodium Periodate under Mild Acidic Conditions Lalit Kumar,* Tanu Mahajan, and D. D. Agarwal Department of Industrial Chemistry, Jiwaji University, Gwalior-474011, Madhya Pradesh, India S Supporting Information *
ABSTRACT: A new, simple, and practical aromatic bromination process is disclosed wherein NaBr/NaIO4 combination in acidic medium was efficiently utilized for the first time in the bromination of deactivated aromatic compounds, thus, affording the corresponding bromo-derivatives of deactivated aromatics in good yields and purity.
1. INTRODUCTION The bromination of aromatic compounds is a fundamental transformation in organic chemistry as the brominated aromatics are compounds of high practical utility. The synthetic utility of bromoarenes as flame retardants, herbicides, and biocides in agriculture and as reactive intermediates in synthetic reactions necessitate their convenient synthesis. The bromination of aromatic rings containing electron-withdrawing groups has long been area of concern.1−7 There are many known methods for the preparation of bromoarenes from activated aromatics; however, severe experimental conditions are required for the bromination of deactivated aromatics. Most aromatic bromination processes are carried out in the presence of various Lewis acids serving as catalysts; however, their susceptibility to any adventitious water and frequently needed large amounts make their use less desirable. Highly activated aromatics can be brominated with molecular Br2 even without catalysts. However, for direct bromination of deactivated aromatics with molecular Br2; the reagents reported are Br2/ silver trifluoromethanesulfonate in conc. H2SO4,7 Br2/BrF3,8 Br2/AgNO3/H2SO4,9 Br2/mercuric oxide/H2SO4,10 Br2/mercurous oxide, or HF and SbF511 and Br2 with a mixture of benzoyl peroxide and lithium bromide.12 During the past decade, Olah et al. have carried out extensive experimental as well as theoretical studies on superelectrophilic activation of electrophiles by protosolvation with superacidic systems, allowing electrophilic reactions to take place with weakly nucleophilic substrates.13−17 Recently, Rajesh et al.16 reported the bromination of deactivated aromatics by using NBS in conc. H2SO4. The reaction was performed at 60 °C and the yields were good to marginal. Since NBS has been used for long in the bromination of deactivated aromatics (e.g., NBS/ TFA/H2SO4,17 NBS/BF3−H2O,18 NBS/aqueous H2SO4,19 etc.), the disadvantage associated is that NBS is prepared using molecular Br2 at temperatures below 5 °C in highly alkaline solution. Cooling of the reaction mixture below 5 °C makes the process cost-extensive and also limits its industrial utility. Very recently, de Almeida et al.20 reported superelectrophilic bromination of deactivated aromatics with tribromoisocyanuric acid (TBCA) in 98% H2SO4. However, the method suffers from disadvantages such as the fact that a large volume of © 2012 American Chemical Society
H2SO4 used (4 mL of H2SO4 for just 2 mmol of substrate) and the additional cost of production of TBCA from isocyanuric acid and NaBr in the presence of oxone make the overall reaction cost-extensive. Several other reagents are well-reported for the synthesis of deactivated bromoarenes such as sodium monobromoisocyanurate (SMBI)/H2SO4,21 dibromoisocyanuric acid/H2SO4,5 BrNO 3 /H 2 SO 4 , KBrO 3 /H 2 SO 4 , 2 2 −2 4 NaBr−NaBrO 3 / H2SO4,25 and NaBrO3/H2SO4.26 However, most of these methods involve the use of large volumes of conc. H 2SO420−24,26 and toxic reagents, 7,8,10,11 long reaction time, 1 7 , 1 8 , 2 1 , 2 5 high temperature, 1 9 , 2 2 − 2 4 and poor yields.8,10,20,22−24 Therefore, despite the broad choice of options, no reliable, mild, high-yielding and selective method exists for the bromination of deactivated aromatics. The development of more economic and widely applicable bromination system is, therefore, still an active area of research. Sodium periodate (NaIO4) has attracted much interest owing to its potential as an oxidant.27−30 In recent times, it has been widely employed in numerous organic reactions such as halohydrin formation,27 benzylic bromination,28 and oxidation reactions.30 This reagent has several advantages: easy commercial availability, cost-effectiveness, low toxicity to humans, and the exceedingly simple and clean workup of products. Dewkar et al.27 reported NaIO4-mediated oxidative halogenation of alkenes and aromatics using alkali metal halides. This paper generally emphases the oxidative bromination of olefins and asymmetric oxidative bromohydroxylation of β-CD complexes of styrenes and conversion to their epoxides. Oxidative bromination of anisole has also been described at 80 °C for 3 h in CH3CN/H2O (2:1) medium using catalytic amounts of NaIO4 (25 mol % wrt 10 mmol of substrate), affording 4-bromoanisole in low yields (69%, NaBr; 74%, LiBr). Continuing our interest in the development of new reagent systems31−33 for the bromination of industrially important activated aromatic compounds, in the present work, we have explored the conditions for the oxidative bromination of Received: Revised: Accepted: Published: 11593
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836, 751, 706, 671, 650 cm−1. MS m/z calculated for C7H5BrO2 201.02, found 200.
deactivated aromatics taking into account all the aforesaid literature reports, and reported for the first time, the use of an NaBr−NaIO 4 /H + system as an efficient and selective brominating reagent for the oxidative bromination of deactivated aromatics (Scheme 1).
3. RESULTS AND DISCUSSION Our present study shows that the NaBr−NaIO4/H+ system is a very convenient and mild reagent for aromatic bromination, providing monobrominated products from the deactivated aromatics in high yields. The reactions were fast and completed in less than 4 h, giving deactivated bromoarenes in 81−93% yields. The reactions were carried out by mixing the substrate molecule, NaBr, NaIO4, and conc. H2SO4. The acid was added dropwise into the stirred slurry of substrate, NaBr, and NaIO4 in water. Optimization of Ideal Reaction Conditions. Concomitant tests were carried out for the screening of best reaction conditions for the bromination of deactivated aromatic compounds with respect to (wrt) the concentration of NaBr, NaIO4, and conc. H2SO4. Using benzoic acid as a test substrate, the reaction conditions were optimized to govern the ideal conditions for bromination (Table 1). When benzoic acid was
Scheme 1. Oxidative Bromination of Deactivated Aromatics Using the NaBr−NaIO4/H+ System
2. EXPERIMENTAL SECTION 2.1. Reagents and Analytics. Sodium bromide (NaBr), sodium periodate (NaIO4), and all of the aromatic substrates were obtained from commercial suppliers and were used without further purification. Doubly distilled water was used all through the study. GC/MS analyses were carried out using Agilent GC (Model 5893) with Chemstation software; columnHP5-MS, 30 m × 0.25 mm × 0.25 μm; detector mass; mass range 14−650 amu; flow 2 mL/min (constant flow); injector temp 270 °C; detector temp 300 °C; injection volume 1 μL of 5% solution in methanol. 1H and 13C NMR spectra were obtained in DMSO-d6 and CDCl3 solutions on a Bruker Avance II 400 NMR spectrometer; the chemical shifts were reported in δ ppm, relative to tetramethylsilane (TMS) as an internal standard. Mass spectra were recorded on Micromass Quattro Micro API triple quadrupole MS equipped with a standard APCI ion source. IR spectra were recorded on a Shimadzu Prestize 21 FT-IR Spectrometer. The melting points were determined by using Buchi apparatus, and the yields were calculated by weight. The structures of the products were confirmed by comparison of their spectral data and physical constants with those of the authentic samples. 2.2. General Experimental Procedure for the Synthesis of 3-Bromobenzoic Acid (1). A 100 mL three-necked round-bottom flask equipped with a mechanical stirrer, a reflux condenser, a thermometer, and a pressure-equalizing funnel was charged with benzoic acid (10 mmol, 1.2212 g), NaBr (10 mmol, 1.0289 g), and a solution of NaIO4 (5 mmol, 1.0694 g) in water (5 mL). The reaction mixture was heated to 50 °C and stirred vigorously; then 4 mL of concentrated H2SO4 was added dropwise over a period of 30 min. Heating and stirring were continued for an additional 2.5 h. When the reaction was considered complete as determined by TLC analysis, the mixture was allowed to cool to room temperature. After cooling, the reaction mixture was poured onto ice−water (20 mL) to precipitate the solids. The precipitated solids were filtered under reduced pressure, washed with cold water, and dried in a vacuum at 60 °C. The product was recrystallized from petroleum ether; 1.87 g (93% yield and 98.18% GC area purity) of white crystalline powder of 3-bromobenzoic acid was obtained with mp 155−156 °C (lit.37 155−158 °C). 1H NMR (400 MHz, DMSO): δ 12.88 (s, 1H, COOH), 8.06 (t, 1H, Ar), 7.92 (dt, 1H, Ar), 7.75 (dt, 1H, Ar), 7.43 (t, 1H, Ar). 13C NMR (100 MHz, DMSO): 165.91, 135.17, 132.97, 131.84, 130.21, 128, 121.63. IR (KBr): 3089, 2606, 1686, 1559, 1438, 1273,
Table 1. Screening of Reaction Conditions in the Oxidative Bromination of Benzoic Acid Using NaBr−NaIO4/H+ System to Afford 3-Bromobenzoic Acida NaIO4 (equiv)
conc. H2SO4 (mL)
entry
NaBr (equiv)
1
1.0
1.0
2
80
2 3
1.0 1.0
1.0 1.0
3 4
85 93
4
1.0
1.0
5
92
5
1.0
0.75
4
93
6
1.0
0.5
4
93
7
1.0
0.025
4
71
8 9
1.0 1.5
0.5
4 4
no reaction 92
yield (%)b
appearance off-white granular powder off-white powder white crystalline powder white crystalline powder white crystalline powder white crystalline powder off-white granular powder white crystalline powder
a
Experiments were conducted with benzoic acid (1 equiv) as substrate; water 5 mL; temp 50 °C, time 3 h. bIsolated yields.
treated with NaBr (1 equiv), NaIO4 (1 equiv), and H2SO4 (3 mL) at 50 °C, the corresponding 3-bromobenzoic acid34 was obtained in 80% yield (Table 1, entry 1). However, the yield significantly improved to 93% when 4 mL of conc. H2SO4 was used (Table 1, entry 3). There was no increase in yield on further increasing the volume of H2SO4. Interestingly, lowering the molar ratio of NaIO4 from 1 equiv to 0.75 and then to 0.5 equiv, respectively, caused no effect on the yield of 3bromobenzoic acid (entries 5,6). However, further lowering of the concentration of NaIO4 to 0.025 equiv had a deleterious effect on the yield of product (entry 7). On increasing the concentration of NaBr from 1.0 to 1.5 equiv keeping the optimal concentration of NaIO4 (0.5 equiv) and H2SO4 (4 mL), there was no effect on the yield of product (entry 9). After several experimentation, it was finally found that a combination of substrate:NaBr:NaIO4 in 1:1:0.5 molar ratio and conc. H2SO4 (4 mL), 50 °C, 3 h turned out to be the best 11594
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Table 2. Bromination of Deactivated Aromatic Compounds with the NaBr−NaIO4/H+ Systema
a Reaction conditions: all reactions were carried out at 50 °C using 10 mmol of substrate. The ratio of substrate:NaBr:NaIO4 was 1:1:0.5, conc. H2SO4 = 4 mL, water = 5 mL. bAll products were analyzed by comparison of spectroscopic data (IR, 1H NMR, MS) with authentic samples. cRefers to isolated yields. dBoiling point of products.
with two electron-withdrawing groups. Thus, the highly deactivated 1,3-dinitrobenzene shows a good reactivity resulting 1-bromo-3,5-dinitrobenzene 4 in 88% yield within 4 h of reaction time under identical reaction conditions (Table 2, entry 4). The bromination of 2-nitrobenzaldehyde using the NaBr−NaIO4/H+ system at 50 °C gave 4-bromo-2-nitrobenzaldehyde in good yield (Table 2, entry 5). The reaction was completed in 4 h, and the product was isolated by a simple workup procedure. A similar observation was noticed in the case of 3-nitrobenzaldehyde. The reaction was completed in less than 4 h furnishing 3-bromo-4-nitrobenzaldehyde in 81% yield (Table 2, entry 6). This bromination method exhibits a high tolerance for the aldehyde group. Under these conditions, starting from 2-chlorobenzoic acid 7, 5-bromo-2-chlorobenzoic acid was obtained in 90% yield within 3.5 h of reaction time. Aromatic hydrocarbons were also smoothly brominated to their corresponding bromo-derivatives. Bromobenzene 8 was obtained in 89% yield in a reaction time of 3 h. Bromobenzene is widely used as an additive in motor oils and as a heavy liquid solvent especially where mass crystallization is required.25 It is also used in the preparation of organometallic reagents such as phenyl magnesium bromide, phenyl lithium, and diphenyl zinc. Bedekar et al.25 reported the synthesis of bromobenzene using a NaBr−NaBrO3/H2SO4 system in 88% yield. However, the method suffers from disadvantages such as slow reaction time (40 h), and the reaction was carried out at 70 °C. 1Bromonaphthalene 9 was also obtained under identical reaction conditions in good yield (91%) within 3 h of reaction time. 1-
reaction conditions in achieving a good conversion of deactivated aromatics with excellent product selectivity. In the absence of either NaIO4 or conc. H2SO4, no reaction occurred. It is to be emphasized that, in this case, the bromination can be carried out smoothly only in the presence of NaIO4 as an oxidant otherwise none of the bromo-compounds were produced. Encouraged by these results, substrate scope of NaIO4 mediated bromination of deactivated aromatic compounds was next examined using the conditions optimized in Table 1 for the bromination of benzoic acid. The generality of this approach was established on a series of deactivated aromatics as given in Table 2. The strong feature of the present method is an efficient and facile bromination of nitrobenzene 2, affording pharmaceutical-intermediate35a 3-bromonitrobenzene (83% yield) within 4 h of reaction time. In contrast, when we tried the bromination of nitrobenzene under the conditions of the work of Rajesh et al.,16 a very low conversion was observed up to 7 h of reaction time. 3-Bromonitrobenzene is used in the manufacture of 3-bromoanisole,35a 3-bromoaniline, and 3bromothiophenol.35b It was earlier synthesized by some researchers under harsh reaction conditions (50%, 60 °C, 3 h10 and 70%, 90 °C, 3 h19). We found that the less reactive chlorobenzene 3 could also be effectively brominated giving 1bromo-4-chlorobenzene in 90% yield within 3 h. Similar results were obtained with other disubstituted aromatic derivatives. The bromine in the NaBr−NaIO4/H+ system is such a strong electrophile that it successfully reacts with rings substituted 11595
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Notes
Bromonaphthalene is used as a refrigerant and a solvent for large molecular weight of substances. It can also be used for the determination of refractive index and acts as heat carrier for dry substances.36 Scheme 2 shows a plausible mechanism for the formation of brominated deactivated aromatic compounds. It is believed that
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors wish to express their sincere thanks to the Deptt. of Industrial Chemistry, Jiwaji University, for providing a University Research Fellowship. We are also thankful to Kapil Kumar, SRF Ltd., and Director, Sophisticated Analytical Instrumentation Facility, Punjab University, Chandigarh, in carrying out the spectral analyses of the compounds.
Scheme 2. Plausible Mechanism for the Formation of Brominated Deactivated Aromatic Compound
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Br2 generates due to the oxidation of NaBr by NaIO4 in strong acidic conditions and reacts with the aromatic deactivated ring to give the intermediate arenium ion (Wheland complex) I, which then subsequently resulted in the bromo-substituted deactivated aromatic compound. Since the present method avoided the use of any expensive brominating agents, organic solvents, and metal catalysts, it seemed valuable to extend this system for the bromination of other industrially important compounds. Scaling-up of the reaction should not give any significant problem for the oxidative route because of the efficient and facile bromination and easy to handle workup procedure.
4. CONCLUSIONS In conclusion, we report a simple, mild, novel, and efficient method for high-yielding bromination of deactivated aromatics using a stable, commercially available NaIO4 which oxidizes alkali metal halide efficiently in acid-aqueous medium. The reagent is readily available, inexpensive, and nontoxic. Pure molecular bromine, which is hazardous and corrosive, can thus be avoided. Also, the incompatible and environmentally hazardous reagents like NaBrO3, KBrO3, as well as expensive NBS can be avoided. In general, a practical process with industrial potential was developed which may compete with existing industrial processes for the bromination of deactivated aromatic compounds.
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ASSOCIATED CONTENT
S Supporting Information *
GC/MS chromatograms, 1H NMR, 13C NMR, IR, and MS spectra of all the synthesized brominated deactivated compounds of Table 2. This material is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*Telephone: +91-9993267029. E-mail: lalitkumar0108@gmail. com. 11596
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