Ind. Eng. Chem. Res. 2005, 44, 8611-8615
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Regioselective Mononitration of Simple Aromatic Compounds under Mild Conditions in Ionic Liquids† Keith Smith,* Shifang Liu, and Gamal A. El-Hiti‡ Centre for Clean Chemistry, Department of Chemistry, University of Wales Swansea, Singleton Park, Swansea, SA2 8PP, United Kingdom
Regioselective mononitration of simple aromatic compounds has been investigated in several different air- and moisture-stable ionic liquids. Use of a mixture of nitric acid and acetic anhydride as the nitrating reagent gave rise to enhanced reactivitites and improved para-selectivities for halogenobenzenes compared to those in a molecular solvent, CCl4. In addition, the ionic liquid could be recovered easily and reused, which opens up the possibility of a more economic process. 1. Introduction Nitration of aromatic compounds is an immensely important and widely studied chemical reaction owing to its useful products, which are used as organic intermediates or energetic materials. Unfortunately, the usual commercial process is not environmentally benign since it results in disposal problems, necessitates regeneration of used acids, and often provides poor selectivity for the desired products. Various nitration approaches have therefore been explored in order to avoid the problems of the traditional mixed acid method, which uses nitric and sulfuric acids.1 The new approaches particularly involve the use of recyclable catalysts such as lanthanide triflates2 or solid acid catalysts such as a perfluorinated resin sulfonic acid,3 claycop,4 and zeolites.5 Although much success has been achieved, some problems still exist. For example, the use of lanthanide triflates or polymeric sulfonic acid resins as catalysts does not improve the selectivity, and furthermore, chlorinated solvents are required in the former process. In the processes employing zeolites as catalysts, the byproducts, water, or other small molecules formed during the reaction can block the pores of the zeolite and deactivate the catalyst. Over the past few years room-temperature ionic liquids (ILs) have received attention for their promise as alternative reaction media because of their convenient physical properties (nonvolatile, nonflammable, large liquid range) and favorable solvation behavior, which make them useful as “clean and green” solvents. Such liquids have been explored as solvents and/or catalysts in a number of reactions.6 In most of these reactions, the reaction rates were enhanced in the IL solvent even though the reasons are not always clear. The nitration of aromatics in ionic liquids should therefore be of interest to both academic and industrial researchers. As part of our continuing interest in the development of clean chemical processes,7 in particular, regioselective nitration of aromatic compounds,8-13 we have investi† Dedicated to Professor David C. Sherrington on the occasion of his 60th birthday. * To whom correspondence should be addressed. Tel.: +44(1792)295266. Fax: +44(1792)295261. E-mail: k.smith@ swansea.ac.uk. ‡ Permanent address: Department of Chemistry, Faculty of Science, Tanta University, Tanta 31527, Egypt.
gated the use of ILs in an attempt to develop a “clean and green” nitration method.14 At the time the work was initiated there were few reports of the use of ionic liquids in aromatic nitration reactions. However, during the course of the work or since its completion, several reports have appeared.15-17 Rajagopal and Srinivasan investigated nitration of phenol using iron nitrate.15 Lancaster and Liopis-Mestre16 studied several nitration systems and they demonstrated that HNO3/Ac2O was their best system. Laali and Gettwert investigated the nitration of various substrates, also using a variety of nitrating reagents, though not including HNO3/Ac2O, and they looked at nitrating efficiency and recycling of the ionic liquids.17 In the present work we report our own investigation of nitration of simple aromatic compounds in ILs, which provides insight into these interesting reactions. We also investigated several nitrating systems, but eventually chose HNO3/Ac2O as used in some of our previous nitrations,12,13 which was found to be best, as in the study by Lancaster and LiopisMestre.16 The three ILs used were 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF4, IL 1), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6, IL 2) and 1-butyl-2,3-dimethylimidazolium tetrafluoroborate ([bdmim]BF4, IL 3), chosen to allow investigation of the effect of variation in both anion and cation parts. ILs with PF6- anions are more hydrophobic than those with BF4- anions, while imidazolium cations bearing a methyl group at the 2-position are not able to take part in hydrogen bonding to electron-rich centers. With our optimum system we have been able to achieve increases in both reaction rates and paraselectivity, as well as high yields, for a number of different substrates, and have been able to recover and reuse the IL solvents. 2. Experimental Section 2.1. Materials. 1-Methylimidazole, 1,2-dimethylimidazole, bromobenzene, ammonium tetrafluoroborate, sodium hexafluorophosphate, tetradecane, tetrachloromethane, acetic anhydride, nitric acid (90%), cupric nitrate, ytterbium triflate, and all substrates employed here were purchased from Aldrich Chemical Co. or Lancaster Research Chemicals. All were used as received, except for acetic anhydride, which was distilled from phosphorus pentoxide, and tetrachloromethane, which was dried over 4 Å molecular sieves prior to use.
10.1021/ie050047z CCC: $30.25 © 2005 American Chemical Society Published on Web 04/19/2005
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Figure 2. Formation of acetyl nitrate.
Figure 1. Ionic liquids 1-3. Figure 3. Nitration of aromatic compounds.
Zeolite Hβ was a gift from Zeolyst International and was calcined at 400 °C prior to use. 2.2. Ionic Liquids Preparation and Characterization. Three ILs (Figure 1) were synthesized according to procedures in the literature18 and were dried in vacuo at 70 °C for 5 h before use. Their identities were established by comparison of their 1H and 13C NMR spectra with those reported18 and were confirmed by mass spectral analysis. 1H and 13C NMR spectra were recorded on an AV400 Bruker spectrometer operating at 400 MHz for 1H and 100 MHz for 13C measurement and mass spectra were recorded on a Quattro II spectrometer. 2.3. Quantitative Gas Chromatography Analysis. The analysis of product mixtures was carried out using a Phillips PU 4400 Gas Chromatograph using an ALTECH ECONO-CAP Carbowax column (15 m × 530 µm × 1.2 µm). The GC conditions used for analysis were as follows: 40 °C for 0.2 min, ramped to 150 °C at 15 °C/min and held for 2 min and then to 180 °C at 10 °C/ min and held for 5 min. Both the injection and the detection temperatures were 300 °C. The flow rates of gases were as follows: helium, 2 mL/min; nitrogen, 28 mL/min; hydrogen, 25 mL/min, and air, 400 mL/min. Tetradecane, hexadecane, or dodecane was used as the internal standard, depending on the substrate. Tetradecane was used for fluorobenzene and toluene; hexadecane was used in the cases of chlorobenzene, bromobenzene, tert-butylbenzene, benzene, and anisole; and dodecane was used for iodobenzene. 2.4. Nitration of Aromatic Compounds with HNO3/Ac2O in Ionic Liquids; General Procedure. All reactions were carried out using a similar procedure under a nitrogen atmosphere and the reaction is illustrated here by a general procedure. To a cooled (0 °C, ice-water bath), vigorously stirred mixture of the IL (volumes indicated in the tables) and nitric acid (90%, 2.1 mmol, 0.10 mL), acetic anhydride (2.9 mmol, 0.28 mL) was added, followed by addition of the substrate (2.1 mmol) dropwise under a flow of nitrogen. The icewater bath was removed and the reaction mixture was left to warm to 25 °C. After a certain time (see tables), the reaction was quenched by addition of water (ca. 10 mL), and then the appropriate internal standard (dodecane, tetradecane, or hexadecane, known amount, ca. 100-200 mg) was added, followed by hexane (10 mL). Three phases formed once the stirring was discontinued. The bottom one was IL, the middle one was an aqueous phase, and the top one was an organic phase. The upper two phases were removed by simple decantation from the IL, and the organic and aqueous phases were separated. The IL phase was washed twice more with a mixture of hexane (10 mL) and water (10 mL) and each time the aqueous and organic extracts were separated. The organic layers were combined, washed with additional water (5 × 10 mL), dried (MgSO4), and then analyzed by GC. The recovered IL was dried at 70 °C under reduced pressure for 5 h in order to remove
Table 1. Nitration of Fluorobenzene in Various Solvents According to Figure 3; R ) Fa yield of isomers (%)b solvent
yield (%)b
ortho
meta
para
para/orthoc
CCl4 IL 1 IL 2 IL 3d
9 94 96 55
0.6 5.3 5.0 3.0
0 0 0 0
8.3 89 91 52
14 17 18 17
a Solvent (0.5 mL), HNO (90%, 2.1 mmol), Ac O (2.9 mmol), 3 2 and PhF (2.1 mmol) at 25 °C for 15 min. b By quantitative GC. c Ratio of para/ortho calculated from original GC data. d 30 min reaction.
water accumulated during the workup, then flushed with N2, and employed for further use. 3. Results and Discussions Our initial effort aimed to use IL as solvent and nitric acid as the nitrating reagent so that water would be the only byproduct. Reactions were monitored by gas chromatography (GC) over a period of time. However, the results obtained even with 2 equiv of nitric acid (90%) were not ideal since the reactions occurred very slowly. For example, on nitration of fluorobenzene the yield of product was only 17%, with a para to ortho ratio of 13, after 20 h in IL 1, and 33%, with a para to ortho ratio of 13, after 6 h in IL 2. Although the yields were quite low, it was exciting to note that the para-selectivity was significantly higher in the ILs than in CCl4 (para/ortho ) 6.4). This inspired us to focus on the contribution to selectivity of ILs. In an attempt to improve the yield and para/ortho selectivity, our attention was turned to investigate nitration of halogenobenzenes using other nitrating systems such as HNO3/Yb(OTf)3‚3H2O, HNO3/zeolite Hβ, HNO3/Ac2O, HNO3/Ac2O/Hβ, and Cu(NO3)2‚3H2O/ Ac2O in the presence of IL 1 or IL 2. However, the results obtained indicated that most of these systems had no advantage for the reaction rate or yield, though in some cases they showed a slight improvement in para-selectivity. The only promising system was HNO3/ Ac2O, which we have used successfully before,12,13 and this was therefore selected for further study. In an initial experiment, nitric acid (90%, 2.1 mmol) was mixed with IL 1 (0.5 mL), acetic anhydride (2.9 mmol; enough to remove all water from the nitric acid and then react to form acetyl nitrate; see Figure 2), and fluorobenzene (2.1 mmol) at 0 °C. The reaction mixture was stirred for 15 min at 25 °C (Figure 3; R ) F) and then quenched by addition of distilled water. The products were extracted into hexane and analyzed by GC. Nitrofluorobenzenes were obtained in 94% yield with a para/ortho ratio of 17. For comparison, identical reactions were conducted in the other two ILs and in tetrachloromethane. The results are given in Table 1.
Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005 8613 Table 2. Nitration of Substituted Benzenes in Various Solvents According to Figure 3a
Table 3. Nitration of Chlorobenzene in Various Quantities of Ionic Liquid According to Figure 3; R ) Cla
yield of isomers (%)b R
solvent
Cl Cl Cl Cl Br Br Br Br I I I I H H Me Me Me Me But But But But OMe OMe OMe OMe
CCl4d IL 1 IL 2 IL 3 CCl4e IL 1 IL 2 IL 3 CCl4f IL 1 IL 2 IL 3 CCl4 IL 1 CCl4 IL 1 IL 2 IL 3 CCl4 IL 1 IL 2 IL 3 CCl4 IL 1 IL 2 IL 3
yield
(%)b
3 60 81 60 4 39 70 43 5 2 14 2 2 18 78 88 95 64 59 91 96 62 99 87 63 57
yield of isomers (%)b
ortho
meta
para
para/orthoc
0.6 9.8 15 11 1.1 8.3 15 9.0 1.8 0.7 4.3 0.6
0 0 0 0 0 0 0 0.4 0 0 0 0
2.1 50 66 49 3.0 31 55 33 3.6 1.6 9.4 1.2
3.5 5.2 4.4 4.5 2.8 3.7 3.7 3.7 2.0 2.3 2.2 2.0
46 53 56 39 5.2 8.3 9.8 5.7 68 60 36 40
1.9 3.2 3.1 1.7 4.4 9.5 9.1 7.6 0 0 0 0
30 32 36 23 49 72 77 49 31 27 27 17
0.65 0.60 0.64 0.59 9.4 8.7 7.9 8.6 0.46 0.45 0.75 0.43
a Solvent (0.5 mL), HNO (90%, 2.1 mmol), Ac O (2.9 mmol), 3 2 and substrate (2.1 mmol) at 25 °C for 30 min. b,c See footnotes b and c to Table 1. d 1 h reaction. e 2 h reaction. f 24 h reaction.
Clearly, the use of all three ILs was advantageous from the points of view of the reaction yield and selectivity in comparison to the reaction in CCl4. From the results in IL 1 and IL 2 it appears that the anion of the IL has little effect on the yield or selectivity of the reaction. However, the reaction in IL 3, which has a methyl group at C-2 of the imidazolium cation structure, rendering it incapable of taking part in hydrogen bonding at that position, was much slower, giving a yield of only 55% even after 30 min. The para/ortho ratio, however, was very similar to that obtained in IL 1 and IL 2. In view of the success of the new reactions, it was decided to apply them to a range of simple substrates (Figure 3). For comparison, all reactions were conducted under similar conditions (25 °C, 30 min) in CCl4, IL 1, IL 2, and IL 3. The results are shown in Table 2. As shown in Table 2, reactions carried out in ILs, whether in hydrophilic IL 1 or IL 3 or in hydrophobic IL 2, almost invariably proceeded faster than those in CCl4. This could possibly be because of better solubility of the nitrating reagent, or to better solvation of a charged intermediate electrophilic species, NO2+, in the charged, higher polarity ILs.19 The effect was most noticeable for the less reactive substrates, which gave very low yields in tetrachloromethane. The one exception was the reaction with anisole, where all reactions were fast, even the one in CCl4. In some cases (e.g., benzene) the substrate was not miscible with the ILs, which may also have affected the reaction rate. Reactions conducted in IL 2 were generally somewhat faster than those in IL 1 or IL 3, which may reflect greater solubility of acetic anhydride in ILs having PF6- anions than in those having BF4- anions.20 Again, the only exception was the reaction of anisole. From Table 2 it is clear that the yields of nitroaromatics obtained in IL 2 are generally higher than those reported by Lancaster
solvent (mL) IL 1 (1.0) IL 1 (0.50) IL 1 (0.25) IL 2 (1.0) IL 2 (0.50) IL 2 (0.25)
yield
(%)b
25 60 70 83 81 70
ortho
meta
para
para/orthoc
3.5 9.8 13 15 15 14
0 0 0 0 0 0
21 50 57 68 66 56
6.1 5.2 4.4 4.6 4.4 4.0
a IL, HNO (90%, 2.1 mmol), Ac O (2.9 mmol), and PhCl (2.1 3 2 mmol) at 25 °C for 30 min. b,c See footnotes b and c to Table 1.
and Liopis-Mestre, who used a different IL.16 However, the results cannot realistically be compared because in the work of Lancaster and Liopis-Mestre the substrate was the most abundant material present, followed by acetic anhydride. The IL was more in the nature of an additive than a solvent and yields were based on nitric acid rather than substrate since the latter was used in large excess. It is also difficult directly to compare the results with those of Laali and Gettwert, who used different nitrating systems and different reaction conditions.17 Table 2 also shows that the reactions of fluorobenzene, chlorobenzene, and bromobenzene were more paraselective in ILs than in CCl4. However, this trend was not apparent for toluene, tert-butylbenzene, and iodobenzene. This might suggest a correlation between the degree of para-selectivity and the ability of electronwithdrawing substituents to induce an electrostatic attraction between substrate and IL, causing greater hindrance at positions ortho to such substituents. However, for anisole the effects were more subtle, perhaps because of the high reactivity of this substrate in all solvents. The fact that selectivities for this substrate were very similar in IL 1 and IL 3 suggests that specific hydrogen bonding21 between the substrate and the hydrogen atom at C-2 of the cation of IL 1 is not particularly important, while the fact that the selectivity was significantly more in favor of the paraisomer in IL 2 than in IL 1 may suggest that the anion or the hydrophobicity of the IL has some significance in this respect. Although the magnitudes of the para/ortho selectivities observed were affected as described above by the nature of the IL solvent, the broad trends in selectivity in ILs were similar to those in conventional solvents. For example, with alkylbenzenes (toluene and tertbutylbenzene) the more bulky substituent resulted in a higher para/ortho ratio, whereas for halobenzenes the trend followed the -I effect of the substituents,1c with iodobenzene having a lower para/ortho ratio and chlorobenzene having a higher para/ortho ratio despite the bigger size of iodine than chlorine. To test whether the selectivity would be affected by the concentration at which the reaction was run, reactions of chlorobenzene were performed in different amounts of IL 1 and IL 2. The results are shown in Table 3. It can be seen that the reaction became a little more selective at higher dilution. In the case of IL 1, the reaction was clearly more rapid at higher concentration, but for IL 2 the yield was high at all dilutions tried. In addition, we attempted to recycle the ionic liquid, taking nitration of chlorobenzene in IL 2 as an example. The ratio of ionic liquid to chlorobenzene was the same in each run. The results are listed in Table 4.
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Table 4. Recycling of Ionic Liquid 2 According to Figure 3; R ) Cla runs 1d 2e 3f 4g
yield (%)b 82 86 88 68
yield of isomers (%)b ortho meta para 15 17 17 13
0 0 0 0
67 69 71 55
para/orthoc
IL 2 recovery (%)
4.5 4.2 4.2 4.2
fresh 81 77 70
a All reactions were carried out at 25 °C for 30 min, using identical proportions of all materials, but absolute quantities were reduced in successive runs because of losses of IL during recovery. b,c See footnotes b and c to Table 1. d IL 2 (1.6 g, fresh), HNO 3 (90%, 4.2 mmol), Ac2O (5.8 mmol), and PhCl (4.2 mmol). e IL 2 (1.3 g, recovered from run 1) and PhCl (3.4 mmol). f IL 2 (1.0 g, recovered from run 2) and PhCl (2.6 mmol). g IL 2 (0.65 g, recovered from run 3) and PhCl (1.6 mmol).
As can be seen from Table 4, the IL could be recovered in high yield with losses of about 0.3 g during each cycle. On a large scale such losses would be insignificant. After three reuses of the IL, the reaction still provided a good yield and the same selectivity as initially. To compare with the results in the recycled IL, an identical experiment was performed under the same conditions except without any solvent. After 30 min, the starting material was intact. This is further confirmation that ILs enhance the rate of these nitration reactions. 4. Conclusions The present work shows that nitration of simple aromatics using nitric acid and acetic anhydride in ILs proceeds successfully with good yields and generally with somewhat faster rates and better para-selectivites than in tetrachloromethane. Both the nature of the substituent on the aromatic ring and the structure of the IL can influence the para-selectivity. Furthermore, the ILs could be recovered easily by simple procedures and could be used successfully again. However, although the nitration system is relatively cheap and commercially viable, the improvements in yield and selectivity may not be sufficient to outweigh the economic disadvantages arising from the high cost of IL solvents and the inevitable small losses during their recovery. Acknowledgment We thank the University of Wales Swansea and the Chinese Government for financial support and the EPSRC Mass Spectrometry Centre at Swansea for mass spectra. We also thank the EPSRC, the Higher Education Funding Council for Wales (ELWa-HEFCW), and the University of Wales Swansea for grants that enabled the purchase and upgrading of NMR equipment used in the course of this work. G. A. El-Hiti thanks the Royal Society of Chemistry for an international author grant. Literature Cited (1) (a) Olah, G. A.; Malhotra, R.; Narang, S. C. Nitration, Methods and Mechanisms; VCH: New York, 1989. (b) Schofield, K. Aromatic Nitration; Cambridge University Press: Cambridge, 1980. (c) Taylor, R. Electrophilic Aromatic Substitution; John Wiley and Sons: Chichester, 1990. (2) (a) Barrett, A. G. M.; Braddock, D. C.; Ducray, R.; McKinnell, R. M.; Waller, F. J. Lanthanide triflate and triflide catalyzed atom economic nitration of fluoroarenes. Synlett 2000, 57. (b) Waller, F. J.; Barrett, A. G. M.; Braddock, D. C.; Ramprasad, D. Lanthanide(iii) triflates as recyclable catalysts for atom economic aromatic nitration. Chem. Commun. 1997, 613.
(3) Olah, G. A.; Malhotra, R.; Narang, S. C. Aromatic substitution. 43. Perfluorinated resinsulfonic acid-catalyzed nitration of aromatics. J. Org. Chem. 1978, 43, 4628. (4) (a) Gigante, B.; Prazeres, A. O.; Marcelo-Curto, M. J. Mild and selective nitration by claycop. J. Org. Chem. 1995, 60, 3445. (b) Delaude, L.; Laszlo, P.; Smith, K. Heightened selectivity in aromatic nitrations and chlorinations by the use of solid supports and catalysts. Acc. Chem. Res. 1993, 26, 607. (c) Corne´lis, A.; Delaude, L.; Gerstmans, A.; Laszlo, P. A procedure for quantitative regioselective nitration of aromatic hydrocarbons in the laboratory. Tetrahedron Lett. 1988, 29, 5657. (5) See for example: (a) Dagade, S. P.; Waghmode, S. B.; Kadam, V. S.; Dongare, M. K. Vapor phase nitration of toluene using dilute nitric acid and molecular modeling studies over beta zeolite. Appl. Catal., A 2002, 226, 49. (b) Choudary, B. M.; Sateesh, M.; Lakshmi Kantam, M.; Koteswara Rao, K.; Ram Prasad, K. V.; Raghavan, K. V.; Sarma, J. A. R. P. Selective nitration of aromatic compounds by solid acid catalysts. Chem. Commun. 2000, 25. (c) Vessena, D.; Kogelbauer, A.; Prins, R. Potential routes for the nitration of toluene and nitrotoluene with solid acids. Catal. Today 2000, 60, 275. (d) Kwok, T. J.; Jayasuriya, K.; Damavarapu, R.; Brodman, B. W. Application of H-ZSM-5 zeolite for regioselective mononitration of toluene. J. Org. Chem. 1994, 59, 4939. (e) Nagy, S. M.; Yarovoy, K. A.; Shubin, V. G.; Vostrikova, L. A. Selectivity of nitration reactions of aromatic-compounds on zeolites H-Y and H-ZSM-11. J. Phys. Org. Chem. 1994, 7, 385. (f) Nagy, S. M.; Yarovoy, K. A.; Shakirov, M. M.; Shubin, V. G.; Vostrikova, L. A.; Ione, K. G. Nitration of aromatic compounds with benzoyl nitrate on zeolites. J. Mol. Catal. 1991, 64, L31. (6) See for example: (a) Brausch, N.; Metlen, A.; Wasserscheid, P. New, highly acidic ionic liquid systems and their application in the carbonylation of toluene. Chem. Commun. 2004, 1552. (b) Song, C. E. Enantioselective chemo- and bio-catalysis in ionic liquids. Chem. Commun. 2004, 1033. (c) Rorsyth, S. A.; Pringle, J. M.; MacFarlane, D. R. Ionic liquids-an overview. Aust. J. Chem. 2004, 57, 113. (d) Dupont, J.; Spencer, J. On the noninnocent nature of 1,3-dialkylimidazolium ionic liquids. Angew. Chem., Int. Ed. 2004, 43, 5296. (e) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; Wiley-VCH: Weinheim, 2003. (f) Oliver-Bourbigou, H.; Magna, L. Ionic liquids: perspectives for organic and catalytic reactions. J. Mol. Catal. A: Chem. 2002, 182-183, 419. (g) Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Ionic liquid (molten salt) phase organometallic catalysis. Chem. Rev. 2002, 102, 3667. (h) Sheldon, R. Catalytic reactions in ionic liquids. Chem. Commun. 2001, 2399. (i) Song, C. E.; Roh, E. J. Practical method to recycle a chiral (salen)Mn epoxidation catalyst by using an ionic liquid. Chem. Commun. 2000, 837. (j) Wasserscheid, P.; Keim, W. Ionic liquids - new “solutions” for transition metal catalysis. Angew. Chem., Int. Ed. 2000, 39, 3772. (7) See for example: (a) Smith, K.; Liu, S.; El-Hiti, G. A. Use of ionic liquids as solvents for epoxidation reactions catalysed by a chiral Katsuki-type salen complex: enhanced reactivity and recovery of catalyst. Catal. Lett. 2004, 98, 95. (b) Smith, K.; Ewart, G. M.; El-Hiti, G. A.; Randles, K. R. Study of regioselective methanesulfonylation of simple aromatics with methanesulfonic anhydride in the presence of reusable zeolite catalysts. Org. Biomol. Chem. 2004, 2, 3150. (c) Smith, K.; Lock, S.; El-Hiti, G. A.; Wada, M.; Miyoshi, N. A convenient procedure for bismuthmediated Barbier-type allylation of aldehydes in water containing fluoride ions. Org. Biomol. Chem. 2004, 2, 935. (d) Smith, K.; Roberts, S. D.; El-Hiti, G. A. Study of regioselective dialkylation of naphthalene in the presence of reusable zeolite catalysts. Org. Biomol. Chem. 2003, 1, 1552. (e) Smith, K.; El-Hiti, G. A.; Jayne, A. J.; Butters, M. Acetylation of aromatic ethers using acetic anhydride over solid acid catalysts in a solvent-free system. Scope of the reaction for substituted ethers. Org. Biomol. Chem. 2003, 1, 1560. (f) Smith, K.; El-Hiti, G. A.; Jayne, A. J.; Butters, M. Acylation of aromatic ethers over solid acid catalysts: scope of the reaction with more complex acylating agents. Org. Biomol. Chem. 2003, 1, 2321. (8) Smith, K.; Almeer, S.; Black, S. J.; Peters, C. Development of a system for clean and regioselective mononitration of aromatic compounds involving a microporous solid, dinitrogen tetroxide and air. J. Mater. Chem. 2002, 12, 3285. (9) Smith, K.; Almeer, S.; Peters, C. Regioselective mononitration of aromatic compounds by zeolite/dinitrogen tetroxide/air in a solvent-free system. Chem. Commun. 2001, 2748.
Ind. Eng. Chem. Res., Vol. 44, No. 23, 2005 8615 (10) Smith, K.; Almeer, S.; Black, S. J. para-Selective nitration of halogenobenzenes using a nitrogen dioxide-oxygen-zeolite system. Chem. Commun. 2000, 1571. (11) Smith, K.; Gibbins, T.; Millar, R. W.; Claridge, R. P. A novel method for the nitration of deactivated aromatic compounds. J. Chem. Soc., Perkin Trans. 1 2000, 2753. (12) Smith, K.; Musson, A.; Deboos, G. A. A novel method for the nitration of simple aromatic compounds. J. Org. Chem. 1998, 63, 8448. (13) Smith, K.; Musson, A.; Deboos, G. A. Superior methodology for the nitration of simple aromatic compounds Chem. Commun. 1996, 469. (14) This paper is based on work reported in Liu, S. Nitration of simple aromatics and epoxidation of alkenes in ionic liquids. Ph.D. Thesis, University of Wales Swansea; UK, 2003. (15) Rajagopal, R.; Srinivasan, K. V. Regio-selective mono nitration of phenols with ferric nitrate in room-temperature ionic liquid. Synth. Commun. 2003, 33, 961. (16) Lancaster, N. L.; Liopis-Mestre, V. Aromatic nitrations in ionic liquids: the importance of cation choice. Chem. Commun. 2003, 2812. (17) Laali, K. K.; Gettwert, V. J. Electrophilic nitration of aromatics in ionic liquid solvents. J. Org. Chem. 2001, 66, 35. (18) (a) Cammarata, L.; Kazarian, S. G.; Salter, P. A.; Welton, T. Molecular states of water in room-temperature ionic liquids. Phys. Chem. Chem. Phys. 2001, 3, 5192. (b) Holbrey, J. D.; Seddon, K. R. The phase behaviour of 1-alkyl-3-methylimidazolium tetrafluoroborates; ionic liquids and ionic liquid crystals. J. Chem. Soc., Dalton 1999, 2133.
(19) (a) Aki, S. N. V. K.; Brennecke, J. F.; Samanta, A. How polar are room-temperature ionic liquids? Chem. Commun. 2001, 413. (b) Bonhoˆte, P.; Dias, A. P.; Papageorgiou, N.; Kalyanasundaram, K.; Cratzel, M. Hydrophobic, highly conductive ambienttemperature molten salts. Inorg. Chem. 1996, 35, 1168. (20) Zhao, D.; Wu, M.; Kou, Y.; Min, E. Z. Ionic liquids: applications in catalysis. Catal. Today 2002, 74, 157. (21) (a) Huang, J.-F.; Chen, P.-Y.; Sun, I.-W.; Wang, S. P. NMR evidence of hydrogen bonding in 1-ethyl-3-methylimidazoliumtetrafluoroborate room-temperature ionic liquid. Inorg. Chim. Acta 2001, 320, 7. (b) Thomas, J.-L.; Howarth, J.; Hanlon, K.; McGuirk, D. Ferrocenyl imidazolium salts as a new class of anion receptors with C-H‚‚‚X- hydrogen bonding. Tetrahedron Lett. 2000, 41, 413. (c) Sato, K.; Arai, S.; Yamagishi, T. A new tripodal anion receptor with C-H‚‚‚X- hydrogen bonding. Tetrahedron Lett. 1999, 40, 5219. (d) Avent, A. G.; Chaloner, P. A.; Day, M. P.; Seddon, K. R.; Welton, T. Evidence for hydrogen-bonding in solutions of 1-ethyl3-methylimidazolium halides, and its implications for roomtemperature halogenoalumibate(III) ionic liquids. J. Chem. Soc., Dalton 1994, 3405.
Received for review January 13, 2005 Revised manuscript received March 23, 2005 Accepted March 25, 2005 IE050047Z
