Aqueous Bromination Method for the Synthesis of Industrially

Article pubs.acs.org/IECR

Aqueous Bromination Method for the Synthesis of IndustriallyImportant Intermediates Catalyzed by Micellar Solution of Sodium Dodecyl Sulfate (SDS) Lalit Kumar,*,† Tanu Mahajan,† and Dau Dayal Agarwal† †

Department of Industrial Chemistry, Jiwaji University, Gwalior-474011, Madhya Pradesh, India ABSTRACT: The reaction of molecular bromine in water catalyzed by micellar solution of sodium dodecyl sulfate (SDS) at room temperature with several aromatics instantaneously resulted in high-yield conversion to the industrially important mono-, di-, and tribrominated aromatic intermediates in excellent yields (91−99%) and purity (>99%). A simple workup procedure and the recovery of surfactant have been described. The entire process uses no organic solvent and generates absolute zero effluent and is, therefore, a ‘green’ alternative for the industrial bromination.

1. INTRODUCTION Surfactants are widely used and find a large number of applications because of their remarkable ability to influence the properties of surfaces and interfaces. The formation of micelles is generally understood in terms of hydrophobic effect, which is the main driving force behind the formation of micelles in solution.1 The proper choice of surfactant can lead to rate increases of 5- to 1000-fold compared to the same reaction in the absence of surfactant. In the face of demands for sustainable and ecologically friendly organic syntheses, clean organic reaction processes which do not use harmful organic solvents are encouraged and are in great demand today.2 Taking advantage of the hydrophobic interactions between the micellar interior and the substrate, one has the opportunity to carry out reactions in an essentially aqueous environment, the micellar surface, employing water-insoluble substrates.3−5 Brominated aromatic compounds are widely used as building blocks for pharmaceuticals, fine chemicals, agrochemicals, and other specialty chemicals.6−8 Most of the aromatic compounds are poorly soluble in water, and this has been a major limitation in the preparation of industrially important brominated compounds under aqueous conditions. Very interesting for practical purposes is that aqueous micellar solutions can replace in some cases the more dangerous, expensive, and toxic organic solvents, allowing to perform the reactions under mild aqueous conditions. The bromination reaction can take advantage of aqueous micelles as a convenient reaction media for solubility reasons since aromatic substrates are highly hydrophobic, whereas the electrophile, the bromonium ion, is hydrophilic. Cerichelli et al.9 studied the bromination of anilines in aqueous suspension of 1-hexadecylpyridinium tribromide (CPyBr3). The drawbacks include an additional step for the formation of tribromide reagent prior to bromination, complex workup procedure in which brominated product was extracted using diethyl ether and that molecular bromine is required for the preparation of tribromide. Currie et al.10 have performed the bromination of phenols and anilines in a dodecyltrimethylammonium bromide (DTAB) based microemulsion. The process uses an excess amount of hazardous HNO3 and volatile halogenated organic solvent (CH2Cl2). Firouzabadi et al.11 have © 2012 American Chemical Society

disclosed a double catalytic system for the bromination of phenol derivatives using Br2/cetyltrimethylammonium bromide (CTAB)/tungstophosphoric acid cesium salt (Cs2.5H0.5PW12O40) reagent system. The drawbacks are the use of excess amount of reagent (Br2: substrate, 1.1:1 for monoand 2.2:1 for dibromination) and expensive tungstophoric acid cesium salt. Also, filtration and evaporation of the excess amount of halogenated volatile organic solvent is cumbersome during large scale operations. The reported methods on bromination of aromatic compounds in water are rare and limited to a only few examples such as NaBrH2O2 in H2O/scCO2 biphasic system12a and H2O2−HBr/“on water” system,12b albeit low conversions, high temperature (40 °C), and a very long reaction time (from 8 to 28 h) are some of the concomitant shortcomings. There are also some other reagents that have been developed as a substitute for Br2, including, but not limited to, N-bromosuccinimide/1-butyl-3-methylimidazolium bromide,13a ZrBr4/diazene,13b [K.18-crown-6]Br3,13c 1-butyl-3methylpyridinium tribromide [BMPy]Br3,13d 3-methylimidazolium tribromide [Hmim]Br3,13e 1-butyl-3-methylimidazolium tribromide [Bmim]Br3,13f pentylpyridinium tribromide,13g ethylene bis(N-methylimidazolium)ditribromide.13h However, no such reagent is commercialized to date, because of their expensive nature, poor recovery and recycling of spent reagent, disposal of large amounts of HBr waste and that the reagents are also not so stable and weaken during long periods of storage, hence they are meant only for laboratory-scale preparations with limited applications. Preparation of all these reagents involve liquid bromine at some stage, thereby, increasing the cost of the endproduct. All the above-reported methods suffer from using not easily available compounds and others use highly corrosive or expensive reagents and toxic organic solvents. Examples are as follows: Br2/Ag2SO4,14 Br2/SbF3/HF,15 Br2/SO2Cl2/Zeolite,16 Br2/Zeolite,17 Br2/H2O2,18 Br2/H2O2/layered double hydroxideWO4,19 Br2/tetrabutylammonium peroxydisulfate,20 etc. Therefore, Received: Revised: Accepted: Published: 2227

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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 (KBr, 3500−440 cm−1). The yields were calculated by weight. 2.2. Typical Procedure for the Synthesis of 3,5Dibromosalicylic Acid (1). To a mixture of salicylic acid (1.38 g, 10 mmol) in 10 mL of SDS micellar solution at its CMC (8.1 × 10−3 M) was added bromine (3.2 g, 20 mmol) utilizing a pressure-equalizing funnel, and the resulting mixture was stirred at room temperature. The bromine color disappeared at once, and white thick precipitates of 3,5dibromosalicylic acid were obtained within 5 min (monitored by TLC) of reaction time at 25 °C. After 15 min, the precipitated reaction mass was separated from mother liquor by vacuum filtration and then washed with Na2S2O5 solution (10%, 10 mL × 3) and dried in oven at 100 °C to get a white crystalline powder of 3,5-dibromosalicylic acid. The total isolated yield was 2.902 g (98.06%) with an HPLC purity of 99.3%. The characteristic data recorded for the isolated product were mp 226−229 °C (lit.41 225−229 °C); 1H NMR (400 MHz, DMSO-d6): δ 7.79 (1H, d, J = 2.4 Hz, ArH), 7.94 (1H, d, J = 2.4 Hz, ArH), 10.36 (1H, s, OH), 12.04 (1H, s, COOH); IR (KBr): 3215, 3092, 3057, 2839, 2583, 2519, 1663, 1595, 1452, 1425, 1385, 1300, 1229, 1180, 1130, 876, 789, 714, 681, 658, 600, 552, 471 cm−1; MS (APCI) m/z calcd. for C7H4Br2O3: 295.8434, found 295. 2.3. Recycling of HBr and Recovery of Surfactant. The aqueous filtrate obtained after the separation of brominated product was neutralized by adding Ca(OH)2 (0.7409 g, 10 mmol). Initially, the pH of the aqueous filtrate was 225 99 minimum 98 maximum

Reaction conditions: salicylic acid 10 mmol, Br2 20 mmol, SDS 23 mg (CMC: 8.0 × 10−3 M), CTAB 3.35 mg (CMC: 9.2 × 10−4 M), TX-100 15 mg (CMC: 2.4 × 10−4 M), water 10 mL, temp 25 ± 1 °C, time 15 min. a

Figure 1. Effect of amount of SDS on the yield and melting point of 3,5-dibromosalicylic acid in the bromination of salicylic acid using molecular Br2. Reaction conditions: salicylic acid 10 mmol, Br2 20 mmol, water 10 mL, temp 25 ± 1 °C, time 15 min. 2230

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Table 3. Bromination of Various Aromatics with Molecular Br2 in SDS Micellar Solution at Its Critical Micellar Concentration (CMC) at Room Tempa

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a Confirmed by comparison with authentic samples. All reactions were carried out on 10 mmol scale, Br2 10 mmol (for mono-), 20 mmol (for di-), and 30 mmol (for tribromination), sodium dodecyl sulfate 23 mg, water 10 mL, temp 25 ± 1 °C. bYield of isolated pure product.

8 and oxine 9 could also be instantaneously dibrominated affording 3,5-dibromosulfanilamide and 5,7-dibromooxine (a potent antifungal and antiamoebic32) in yields of 97 and 99%, respectively. Using this micellar effect, pharmaceutically important33−35 aromatic aldehydes were instantaneously brominated at room temperature in excellent yields (Table 3, entries 10, 11, and 12). Another anthelmintic or antibacterial,36 2,4-dibromo6-nitrophenol, was obtained in excellent yield within 15 min from 2-nitrophenol (Table 3, entry 13). The bromination of 2-nitrophenol is difficult using a binary catalytic system (Br2/CTAB/ Cs2.5H0.5PW12O40).11 The strong feature of this micellar catalyzed bromination is an efficient, facile, rapid, and regioselective bromination of deactivated anilines (Table 3, entries 14 to 18) in excellent yields at room temperature. The regioselective bromination of anilines containing deactivated groups is not an easy task, and in most of the methods, it proceeded under harsh reaction conditions with low yields.37 Afterward, a desirable “green” approach to the present method is followed by an environmentally friendly procedure that comprised of recycling of HBr and the recovery of surfactant. The absence of organic waste and organic solvent in the reaction enabled a simple isolation procedure comprised of filtration of solid brominated product, and the aqueous liquid mixture thus obtained containing HBr byproduct was neutralized by adding powdered Ca(OH)2. Initially, the pH of the aqueous filtrate was 99% HPLC purity. Hence the rate of bromination is independent of the solubilization site of substrate in the micellar aggregate. Acetanilide 2 and benzanilide 3 were efficiently brominated to their corresponding para-brominated products in excellent yields. The exclusive formation of para-isomer obtained in the micelle-based reaction may be useful from a preparative point of view. This indicates that the position of the electrophilic attack as well as the number of entering bromine atoms can be regulated by controlling the ratio of Br2: substrate, i.e., 1:1 for mono-, 2:1 for di-, and 3:1 for tribromination of aromatic compounds. Conventional bromination using molecular bromine in organic solvent or concentrated HBr is not very selective and often results in a complex mixture of mono-, di-, tri-, and even tetrabrominated products.30 2,4,6-Tribromoaniline (Table 3, entry 4), an intermediate for agrochemicals and pharmaceuticals, and 2,4,6-tribromophenol (Table 3, entry 5), a reactive flame retardant,31 were obtained in good yields utilizing 3 mol equiv of molecular Br2. 1-Naphthol 6 and 2-naphthol 7 proceeded with good reactivity affording clean synthesis of 2,4dibromo-1-naphthol (95%) and 1,6-dibromo-2-naphthol (93%) after 15 min, respectively. It has been found that sulfanilamide 2232

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sulfate was precipitated in the aqueous solution as a Ca-salt of surfactant, thus forming insoluble calcium dodecyl sulfate (CDS).38 After the separation of CDS, the aqueous mixture containing CaBr2 was concentrated to precipitate CaBr2 as a crystalline solid. The generated CaBr2 is an industrially significant product having potential industrial uses39 (such as in drilling fluids, neuroses medication, freezing mixtures, food preservatives, photography, and fire retardants). The CDS also finds its use in the aqueous cosmetic compositions which are used as moisturizing lotions.40 Thus, the byproduct HBr has been utilized effectively, and, at the end of reaction, we discharge zero organic waste and zero effluent to the environment. Since the present method avoided the use of any expensive brominating agents, organic solvents, strong acids, hazardous oxidants, and metal catalysts and operates completely in water, 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 micellar route because of the rapid and facile bromination and easy to handle workup procedure. 3.3. UV−Vis Study. The possible brominating species43 that can be formed in aqueous bromine solutions are HOBr, BrO−, Br3−, and Br5−, respectively. Table 4 gives the UV−vis

Figure 2. UV−vis study showing disappearance of acetanilide peak and formation of p-bromoacetanilide using Br2/SDS in H2O brominating system at 25 °C.

surfactant anionic head) is brought closer to aromatic substrate and thereby results in instant bromination.

4. CONCLUSIONS In an effort to eliminate the use of toxic and expensive organic solvents used in conventional bromination techniques,15,16,19,20,30 we have exploited the aqueous solution of sodium dodecyl sulfate (SDS) at its CMC as a reaction medium for a fast synthesis of industrially important brominated compounds quantitatively and qualitatively under ambient conditions using inexpensive molecular Br2 as a brominating agent. This method proceeded purely in water providing a new procedure for the synthesis of brominated compounds of industrial importance. A comparison of the brominating ability of the present system with those of published methods9−11,16,19 shows that the present protocol is inexpensive, simpler, faster, and more efficient than other catalytic bromination systems used for this purpose. The use of water as a reaction medium and the methodology of “micellar catalysis”3 made the investigated method attractive and interesting from both the economic and environmental points of view. The generation of CaBr2 (an industrially significant product) thereby eliminating the disposal of HBr waste and the recovery of surfactant in the form of calcium dodecyl sulfate are the green features of the system.

Table 4. Maximum Absorbance Wavelengths in Reaction Media for Different Possible Brominating Species of Interest species

λmax (nm)

ref

Br2 Br3− Br5− HOBr BrO− acetanilide p-bromoacetanilide

390 265 315 284, 350 329 240 252

43a 43a 44 45 46 47 47

spectral characteristics for various brominating species of interest. The UV−vis studies were carried out to find out the active brominating species. The bromine was dissolved in aqueous solution of SDS, and a UV−vis spectrum was recorded. A weak band at 262 nm and an intense band at 390 nm were obtained which are the characteristic absorption bands of Br2 (Table 4). This validates that when Br2 is dissolved in aqueous SDS solution, there is no HOBr formation which shows its absorption maxima at 284 and 350 nm (Table 4). Afterward, the solution of acetanilide dissolved in MeCN was added to a solution of Br2/SDS in H2O, and a UV−vis spectrum was recorded. The absorptions at 260 and 390 nm disappeared instantaneously which shown that the Br2 molecule has been instantly polarized in the presence of SDS and the generated Br+ has been transferred to the acetanilide. This was confirmed by the appearance of a peak at 252 nm which corresponds to p-bromoacetanilide (Figure 2). Figure 2 shows that bathochromic shift operates in which the absorption maxima at 240 nm (λmax. of acetanilide) was shifted toward a longer wavelength at 252 nm (λmax. of p-bromoacetanilide), owing to the consumption of substrate and bromine in the reaction. Hence, in the present system, we can conclude that SDS provides kinetic acceleration for the polarization of a Br2 (Brδ+--Brδ‑) molecule and dissociating it rapidly in aqueous solution due to which the bromonium ion (as carried away via

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AUTHOR INFORMATION

Corresponding Author

*Phone: +91-9993267029. E-mail: [email protected].

ACKNOWLEDGMENTS The authors wish to express their sincere thanks to Dept. of Industrial Chemistry, Jiwaji University for providing 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 analysis of the compounds.

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REFERENCES

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