Article pubs.acs.org/IECR
Isopropylation of m‑Cresol Catalyzed by Recoverable Acidic Ionic Liquids Shijuan Liu,*,† Xiumei Liu,‡ and Chang Wang‡ †
College of Chemistry, Jilin Normal University, Siping 136000, People’s Republic of China China Ionic Liquid Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences Zhongshan Road 457, Dalian 116023, People’s Republic of China
‡
ABSTRACT: Several SO3H-functionalized ionic liquids were used as the acid catalyst for the isopropylation of m-cresol with isopropyl alcohol. The results indicated that the sequence of the catalytic activity was in good agreement with the acidity order. A 77.7% conversion of m-cresol and 48.7% selectivity for 2-isopropyl-5-methylphenol were observed in the presence of [TMBSA][HSO4] (TMBSA = N,N,N-trimethyl ammonium-N-butanesulfonic acid). The catalysts could be recovered by extraction and reused, and after eight times of recycling, a loss of catalytic activity was not evident.
1. INTRODUCTION Alkylphenols are useful chemicals, especially 2-isopropyl-5methylphenol (2I-5MP), that exhibit activity in protecting lowdensity lipoproteins and show high antioxidant effectiveness1 as well as playing an important role in perfumery. Usually 2I-5MP is prepared from m-cresol by Friedel−Crafts alkylation over acid catalysts, and the liquid-phase isopropylation of m-cresol with isopropyl alcohol (IPA) is always applied in the presence of solid acid catalysts.2−5 Rapid deactivation of these catalysts and high cost of the catalyst regeneration are two problems, though the alkylation performance of these catalysts is remarkable. SO3H-functionalized ionic liquids (ILs) combine the advantages of liquid acids and solid acids, and offer a new possibility for developing environmentally friendly acid catalysts. Davis and Cole et al. designed and prepared a series of sulfonic acid-functionalized Brønsted acidic ILs for the first time and applied them in several acid-promoted reactions, and the IL was recycled 5 times without loss of activity.6 After that, this type of IL was prepared and successfully applied in many reactions, such as esterification,7 alkylation,8 nitration,9 dehydration,10 amination11 and one-pot multicomponent condensation.12−15 Recently, silica-based sulfonic acid functionalized ionic liquids have been prepared and applied as catalysts, and ILs could be recycled and reused 5 or 6 times without an appreciable loss of activity.16,17 Moreover, the stable SO3Hfunctionalized ILs have been successfully applied to catalyze the butylation of cresol with high selectivity toward ortho tert-butyl phenol,18,19 but the isopropylation of m-cresol in this kind of IL has not been reported yet. In this paper, the isopropylation of m-cresol with IPA catalyzed by SO3H-functionalized ILs was investigated, and the recycling of the catalyst was also tested.
Scheme 1. Investigated Ionic Liquids
at around 353K, whereas other ILs were viscous liquids at room temperature. The ILs were identified by NMR (Varian INOVA 400 MHz) and ESI-MS (Q-TOF Micro Mecromous, VK). The obtained data were in agreement with the literature.6−8,20 2.2. Catalytic Experiments. In a typical reaction, 10 mmol of m-cresol, 10 mmol of IPA, and 5 mmol of the IL were added into an autoclave with magnetic stirring and reacted at 190 °C for 6 h. Then, the reaction mixture was cooled and separated into two layers spontaneously. The upper organic layer was decanted and analyzed quantitatively by gas chromatography (Agilent 7890) equipped with an HP-5 column (30 m × 0.25 mm). The recovered IL was extracted by washing with ethyl acetate and then was dried at 100 °C under 0.01 Torr vacuum, and then the IL was reused in the isopropylation of m-cresol with IPA according to the above procedure. UV−vis spectra were obtained by UV (SHIMADZU 2500).
3. RESULTS AND DISCUSSION It was observed that in the isopropylation of m-cresol with IPA, the products formed were mono-C-alkylated products (2I-5MP and 4I-3MP), dialkylated products (2,6-2I-5MP and 2,4-2I5MP), and an O-alkylated product (I-MPE), as illustrated in Scheme 2.
2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization of Ionic Liquids. SO3H-functionalized ILs (Scheme 1) were synthesized according to the literature.6−8,20 [TEBSA][BF4] and [TEBSA][tos] (TEBSA = N,N,N-triethyl ammonium-N-butanesulfonic acid; tos = p-toluenesulfonate were stiff glass that was liquefied © 2013 American Chemical Society
Received: Revised: Accepted: Published: 16719
July 25, 2013 October 29, 2013 November 1, 2013 November 1, 2013 dx.doi.org/10.1021/ie402394p | Ind. Eng. Chem. Res. 2013, 52, 16719−16723
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Scheme 2. Reaction Pathway for Isopropylation of m-Cresol with IPA
IPA and water formed between the dehydration of IPA and mcresol, which led to a large amount of byproducts such as the oligomers of propylene and I-MPE, the formation of 2I-5MP was favored in strong acidic media. The proportional amount of catalysts was equivalent to the acidity in the reaction system, and the acidity increased when the amount of the catalysts increased, which favored the reaction, so when the amount of the catalysts increased, the yields of 2I-5MP and dialkylated products increased, but the yield of I-MPE decreased (entries 11−13 and 5). However, when the amount of catalysts are further increased (entry 14), mono-C-alkylated products are further alkylated with IPA, so the selectivity of 2I-5MP decreased and the selectivity for dialkylated products increased. 3.2. Comparative Studies on the Isopropylation of mCresol in Different Ionic Liquids. Different ILs, as well as H2SO4, were investigated as the catalysts in the isopropylation of m-cresol, and the results are shown in Table 2. It could be seen that with increasing the alkyl moiety of the cation, the catalytic activity of the SO3H-functionalized ILs with anions [HSO4]− was obviously enhanced (entries 1−4 and 9−12). Also, the activity of the SO3H-functionalized ILs with anions [HSO4]− and [BF4]− was higher than that of the anion [tos]− (entries 2, 5 vs entry 6 and entries 10, 13 vs entry 14). The SO3H-functionalized ILs exhibited much higher activity than the ILs bearing no alkyl sulfonic acid group on the cations (entry 4 vs entry 7). The conversion of m-cresol was higher in H2SO4 than that in the SO3H-functionalized ILs (entries 1−7 vs entry 8), but the products were easy to be separated from the ILs, so ILs could be used as a substitute for the traditional acids in the isopropylation of m-cresol. Interestingly, it was found that the sequence of the catalytic activity was in good agreement with the order of the Brönsted acidity (H0), which was determined by the Hammett method21−25 by UV−visible spectroscopy with 4-nitroaniline as the indicator in H2O (as shown in Figure 1, the maximal absorbance of the unprotonated form of 4-nitroaniline was observed at 380 nm, which decreased with the protonation by adding acid.). The H0 values of different ILs are listed in Table 2, and the acidity order was observed as follows: [H2SO4] > [TBBSA][HSO4] > [TPBSA][HSO4] > [TEBSA][BF4] ≥ [TEBSA][HSO4] > [TMBSA][HSO4] > [TEBSA][tos] >
3.1. Optimization of the Reaction Conditions. [TMBSA][HSO4] (TMBSA = N,N,N-trimethyl ammoniumN-butanesulfonic acid ) was used as the catalyst to optimize the reaction parameters (Table 1). In the present study, a major advantage of using SO3H-functionalized ILs for this reaction is the high selectivity to 2I-5MP. A 77.7% conversion of m-cresol and 48.7% selectivity for 2I-5MP were obtained when the molar ratio of nIPA/nm‑cresol/nIL was 2:2:1 at 190 °C (entry 5). It should be noted that the SO3H-functionalized IL [TMBSA][HSO4] could obtain the high catalytic activity in the isopropylation of m-cresol at a relatively low temperature (190 °C) compared with the solid acid catalysts (around 250 °C).5 The high reaction temperature over the solid acid catalysts could be due to their acid properties. According to the results in Table 1, it could be seen that with an increase in the reaction temperature, both the conversion of m-cresol and the selectivity of 2I-5MP increased, and the yields of dialkylated products (2,6-2I-5MP and 2,4-2I-5MP) also increased, but the yield of I-MPE decreased (entries 1−6). The reason was that with the increase of the reaction temperature, the catalytic activity increased and I-MPE was rearranged to mono-C-alkylated products, so the selectivity for 2I-5MP increased. Excessive IPA resulted in the increase of the conversion of m-cresol and the yield of I-MPE, but the yield of mono-C-alkylated products and dialkylated products decreased (entries 8−10 and 5). Because excess IPA would decrease the catalyst concentration because of the dilution of
Table 1. Results of Isopropylation of m-Cresol with IPA in Different Reaction Conditions in [TMBSA][HSO4]a product selectivity (%)
a
entry
nIPA/nm‑cresol/nIL molar ratio
T (°C)
m-cresol conversion (%)
I-MPE
2I-5MP
4I-3MP
2,6-2I-5MP
2,4-2I-5MP
others
1 2 3 4 5 6 7 8 9 10 11 12 13 14
2:2:1 2:2:1 2:2:1 2:2:1 2:2:1 2:2:1 1:2:1 4:2:1 6:2:1 8:2:1 8:8:1 8:8:2 8:8:3 8:8:5
150 160 170 180 190 200 190 190 190 190 190 190 190 190
58.8 67.9 73.8 74.7 77.7 78.1 45.4 92.6 92.9 94.1 54.4 67.0 70.3 76.6
28.7 10.3 5.8 4.4 2.7
39.9 45.8 46.6 46.2 48.7 48.1 55.7 33.7 31.9 32.3 42.9 46.8 48.3 43.2
14.0 17.0 18.0 17.5 19.4 19.8 20.3 13.8 12.6 12.4 15.2 17.4 17.5 16.7
4.7 7.9 8.9 10.1 9.1 10.5 8.8 12.2 8.5 7.9 4.3 7.9 9.4 12.2
3.4 6.2 7.6 8.7 8.6 11.6 11.9 12.9 9.2 8.5 3.2 6.2 7.8 11.6
9.3 12.8 13.1 13.1 11.5 10.0 3.3 21.7 18.6 17.5 9.6 12.5 15.8 13.9
5.7 19.2 21.4 24.8 9.2 1.2 2.4
Reaction conditions: m-cresol, 10 mmol; 6 h. 16720
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Table 2. Isopropylation of m-Cresol with IPA in Different Ionic Liquidsa product selectivity (%) entry
catalyst
m-cresol conversion (%)
I-MPE
2I-5MP
4I-3MP
2,6-2I-5MP
2,4-2I-5MP
others
H0b
1 2 3 4 5 6 7 8 9 10 11 12 13 14
[TMBSA][HSO4] [TEBSA][HSO4] [TPBSA][HSO4] [TBBSA][HSO4] [TEBSA][BF4] [TEBSA][tos] [TBH][HSO4] H2SO4 [TMBSA][HSO4] [TEBSA][HSO4] [TPBSA][HSO4] [TBBSA][HSO4] [TEBSA][BF4] [TEBSA][tos]
30.8 37.9 44.2 47.7 40.6 28.1 10.3 56.0 77.7 78.0 80.1 86.3 79.6 45.6
62.0 50.1 44.1 33.2 41.8 61.8 91.1 36.6 2.7 0 0 0 0 3.9
25.5 30.9 32.5 37.8 34.3 25.5 8.9 34.9 48.7 41.5 38.3 33.7 39.4 53.9
7.3 9.6 9.6 11.3 10.6 6.9 0.0 12.3 19.4 13.5 9.8 8.4 16.4 15.8
1.2 2.4 4.0 5.6 3.5 1.3 0.0 4.8 9.1 15.1 18.0 20.8 22.6 6.6
0.0 1.0 1.5 2.1 1.6 0.5 0.0 1.5 8.6 5.0 4.6 4.8 2.8 4.5
3.9 6.0 8.3 10.0 8.1 4.1 0.0 9.8 11.5 24.9 29.3 32.3 18.8 15.3
1.81 1.76 1.65 1.62 1.75 1.93 2.37 1.59 1.81 1.76 1.65 1.62 1.75 1.93
Reaction conditions. Entries 1−8: m-cresol,10 mmol; nIPA/nm‑cresol/nIL ratio = 8:8:1; 4 h; 150 °C. Entries 9−14: m-cresol,10 mmol; nIPA/nm‑cresol/nIL ratio = 2:2:1; 6 h; 190 °C. bH0= pKa + log[B]/[BH]. Indicator: 4-nitroaniline (pKa = 0.99). a
group of m-cresol could be attacked by the electrophilic isopropyl cation C3H7+ (as shown in Scheme 3). Though the Scheme 3. Proposed Mechanism for the Formation of 2I5MP
Figure 1. Absorption spectra of 4-nitroaniline for investigated ionic liquids (16 mmol/L) in H2O.
[TBH][HSO4]. This was consistent with the conversion of mcresol in the isopropylation reaction. The SO3H-functionalized ILs with different cations and anions exhibited different Brönsted acidity. For the ILs with the same anion [HSO4]−, the long alkyl moiety of the cation would increase the acidity of the IL (entries 1−4). It could be explained by the difficult combination of the sulfonate ion, which came from the reaction of the SO3H group with the IPA, and the hydrogen proton in a nonaqueous solvent. For the ILs with the same cation [TEBSA]+, the acidity of the IL with the anion [HSO4]− or [BF4]− was obviously stronger than that with anion [tos]− (entry 2, 5 vs entry 6). This could be due to the molecular geometries of ILs, as it was reported previously.23 Also, the SO3H-functionalized ILs have stronger acidity than nonfunctionalized ILs (entry 4 vs entry 7), which conformed to the effect of the alkyl sulfonic acid group on the acidity of the ILs.25 Therefore, it could be concluded that the Brönsted acidity of the ILs was increased by introducing functional acidic groups, and their catalytic activity also improved. 3.3. Mechanism for the Formation of 2I-5MP. As shown in Tables 1 and 2, in the isopropylation of m-cresol with IPA catalyzed by the SO3H-functionalized IL, 2I-5MP is the major product. That could be attributed to the electronic and steric effects of the reactants. The aromatic ring or the hydroxyl
hydroxyl group could be attacked to generate I-MPE, the rearrangement of I-MPE might take place easily to form 2I5MP and 4I-3MP. Because of more sterical crowd at the 2position of m-cresol and the hydrogen bonding interaction between the hydroxyl group of m-cresol and IPA, the electrophilic substitution reaction is more likely to occur at the 6-position of m-cresol to generate 2I-5MP product. SO3H-functionlized ILs have two acid sites. One is the alkyl sulfonic acid group, and the other is the HSO4− anion. The acidities of the ILs depend on both of them. The acidity of the sulfonic acid group was significantly stronger than that of HSO4− anion,17 and our experimental results (entry 2 vs entry 7) conformed to the conclusion. Thus, the proton of the alkyloxonium ion might be first obtained from the sulfonic acid group rather than HSO4− anion. The mechanism for the formation of 2I-5MP in the presence of SO3H-functionlized ILs was postulated and shown in Scheme 3. First, a proton transfers from the SO3H-functionlized IL to the hydroxyl group of IPA to produce an alkyloxonium ion (intermediate 3), and it 16721
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dehydrates to form the isopropyl cation (C3H7+). This cation attacks the 6-position of m-cresol to form an arenium ion intermediate 4, and it is less stable and subsequently generates 2I-5MP by the loss of a proton. At the same time, the SO3Hfunctionlized IL recovers. 3.4. Recycling of the Ionic Liquid. A series of recycle experiments were conducted to investigate the possibility of the reuse of the SO3H-functionalized ILs. The used [TMBSA][HSO4] was extracted by washing with ethyl acetate and then dried at 100 °C under a 0.01 Torr vacuum. This procedure was repeated for eight cycles (see Figure 2). No obvious changes in
(2) Umamaheswari, V.; Palanichamy, M.; Murugesanl, V. Isopropylation of m-cresol over mesoporous Al−MCM-41 molecular sieves. J. Catal. 2002, 210, 367−374. (3) Vinu, A.; Ariga, K.; Saravanamurugan, S.; Hartmann, M.; Murugesan, V. Synthesis of highly acidic and well ordered MgAlMCM-41 and its catalytic performance on the isopropylation of mcresol. Microporous Mesoporous Mater. 2004, 76, 91−98. (4) Vinu, A.; Satish Kumar, G.; Ariga, K.; Murugesan, V. Preparation of highly ordered mesoporous AlSBA-15 and its application to isopropylation of m-cresol. J. Mol. Catal. A: Chem. 2005, 235, 57−66. (5) Reddy, S. S.; Kumar, V. S.; Padmasri, A. H.; Satyanarayana, C. V. V.; Raju, B. D.; Rao, K. S. R. Beneficial role of SiO2 plugging in AlCl3 heterogenized plugged hexagonal templated silica (PHTS) for the isopropylation of m-cresol. Catal. Commun. 2007, 8, 1080−1087. (6) Cole, A. C.; Jensen, J. L.; Ntai, I.; Tran, K. L. T.; Weave, K. J.; Forbes, D. C.; Davis, J. H. Novel Brønsted acidic ionic liquids and their use as dual solvent-catalysts. J. Am. Chem. Soc. 2002, 124, 5962−5963. (7) Gui, J.; Cong, X.; Liu, D.; Zhang, X.; Hu, Z.; Sun, Z. Novel Brønsted acidic ionic liquid as efficient and reusable catalyst system for esterification. Catal. Commun. 2004, 5, 473−477. (8) Gui, J.; Ban, H.; Cong, X.; Zhang, X.; Hu, Z.; Sun, Z. Selective alkylation of phenol with tert-butyl alcohol catalyzed by Brönsted acidic imidazolium salts. J. Mol. Catal. A: Chem. 2005, 225, 27−31. (9) Zolfigol, M. A.; Khazaei, A.; Moosavi-Zare, A. R.; Zare, A.; Kruger, H, G.; Asgari, Z.; Khakyzadeh, V.; Kazem-Rostami, M. Design of Ionic Liquid 3-Methyl-1-sulfonic Acid Imidazolium Nitrate as Reagent for the Nitration of Aromatic Compounds by in Situ Generation of NO2 in Acidic Media. J. Org. Chem. 2012, 77, 3640− 3645. (10) Kotadia, D. A.; Soni, S. S. Symmetrical and unsymmetrical Brønsted acidic ionic liquids for the effective conversion of fructose to 5-hydroxymethyl furfural. Catal. Sci. Technol. 2013, 3, 469−474. (11) Han, F.; Yang, L.; Li, Z.; Xia, C. Sulfonic Acid-Functionalized Ionic Liquids as Metal-Free, Efficient and Reusable Catalysts for Direct Amination of Alcohols. Adv. Synth. Catal. 2012, 354, 1052−1060. (12) Zolfigol, M. A.; Khazaei, A.; Moosavi-Zare, A. R.; Zare, A.; Khakyzadeh, V. Rapid synthesis of 1-amidoalkyl-2-naphthols over sulfonic acid functionalized imidazolium salts. Appl. Catal., A 2011, 400, 70−81. (13) Zolfigol, M. A.; Khakyzadeh, V.; Moosavi-Zare, A. R.; Zare, A.; Azimi, S. B.; Asgari, Z.; Hasaninejad, A. Preparation of various xanthene derivatives over sulfonic acid functionalized imidazolium salts (SAFIS) as novel, highly efficient and reusable catalysts. C. R. Chim. 2012, 15, 719−736. (14) Zare, A.; Moosavi-Zare, A. R.; Merajoddin, M.; Zolfigol, M. A.; Hekmat-Zadeh, T.; Hasaninejad, A.; Khazaei, A.; Mokhlesi, M.; Khakyzadeh, V.; Derakhshan-Panah, F.; Beyzavi, M. H.; Rostami, E.; Arghoon, A.; Roohandeh, R. Ionic liquid triethylamine-bonded sulfonic acid {[Et3N−SO3H]Cl} as a novel, highly efficient and homogeneous catalyst for the synthesis of β-acetamido ketones, 1,8dioxo-octahydroxanthenes and 14-aryl-14H-dibenzo[a,j]xanthenes. J. Mol. Liq. 2012, 167, 69−77. (15) Zolfigol, M. A.; Khazaei, A.; Moosavi-Zare, A. R.; Zare, A.; Asgari, Z.; Khakyzadeh, V.; Hasaninejad, A. Design of ionic liquid 1,3disulfonic acid imidazolium hydrogen sulfate as a dual-catalyst for the one-pot multi-component synthesis of 1, 2, 4, 5- tetrasubstituted imidazoles. J. Ind. Eng. Chem. 2013, 19, 721−726. (16) Kotadia, D. A.; Soni, S. S. Silica gel supported-SO3H functionalised benzimidazolium based ionic liquid as a mild and effective catalyst for rapid synthesis of 1-amidoalkyl naphthols. J. Mol. Catal. A: Chem. 2012, 353−354, 44−49. (17) Vafaeezadeh, M.; Dizicheh, Z. B.; Hashemi, M. M. Mesoporous silica-functionalized dual Brønsted acidic ionic liquid as an efficient catalyst for thioacetalization of carbonyl compounds in water. Catal. Commun. 2013, 41, 96−100. (18) Liu, X.; Liu, M.; Guo, X.; Zhou, J. SO3H-functionalized ionic liquids for selective alkylation of m-cresol with tert-butanol. Catal. Commun. 2008, 9, 1−7.
Figure 2. Recycling of [TMSA][HSO4] in isopropylation of m-cresol with IPA.
the conversion of m-cresol and the selectivity to 2I-5MP were observed. This indicated that SO3H-functionalized ILs as the catalyst for the isopropylation of m-cresol was stable enough to be recycled.
4. CONCLUSION In summary, we obtained the following conclusions: (1) the SO3H-functionalized ILs showed good catalytic activity in the isopropylation of m-cresol with IPA, and a 77.7% conversion of m-cresol and 48.7% selectivity for 2I-5MP were obtained in [TMBSA][HSO4] at 190 °C, (2) the sequence of the catalytic activity was in agreement with the Brönsted acidity order of the ILs, and (3) a simple reaction−separation−recycle process was realized, and the ILs were reused eight times without an obvious decrease in catalytic activity.
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AUTHOR INFORMATION
Corresponding Author
*S. Liu. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the State Key Laboratory of Fine Chemicals (KF0811), Dalian University of Technology, China.
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REFERENCES
(1) Milos, M.; Mastelic, J.; Jerkovic, I. Chemical composition and antioxidant effect of glycosidically bound volatile compounds from oregano. Food Chem. 2000, 71, 79−83. 16722
dx.doi.org/10.1021/ie402394p | Ind. Eng. Chem. Res. 2013, 52, 16719−16723
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Article
(19) Liu, X.; Zhou, J.; Guo, X.; Liu, M.; Ma, X.; Song, C.; Wang, C. SO3H-functionalized ionic liquids for selective alkylation of p-cresol with tert-butanol. Ind. Eng. Chem. Res. 2008, 47, 5298−5303. (20) Ganeshpure, P. A. G.; Das, George, J. Brønsted acidic ionic liquids derived from alkylamines as catalysts and mediums for Fischer esterification: Study of structure−activity relationship. J. Mol. Catal. A: Chem. 2008, 279, 182−186. (21) Thomazeau, C.; Olivier-Bourbigou, H.; Magna, L.; Luts, S.; Gilbert, B. Determination of an acidic scale in room temperature ionic liquids. J. Am. Chem. Soc. 2003, 125, 5264−5265. (22) Gu, Y.; Zhang, J.; Duan, Z.; Deng, Y. Pechmann reaction in nonchloroaluminate acidic ionic liquids under solvent-free conditions. Adv. Synth. Catal. 2005, 347, 512−516. (23) Xing, H.; Wang, T.; Zhou, Z.; Dai, Y. The sulfonic acidfunctionalized ionic liquids with pyridinium cations: Acidities and their acidity−catalytic activity relationships. J. Mol. Catal. A: Chem. 2007, 264, 53−59. (24) Wang, Y.; Jiang, D.; Dai, L. Novel Brønsted acidic ionic liquids based on benzimidazolium cation: Synthesis and catalyzed acetalization of aromatic aldehydes with diols. Catal. Commun. 2008, 9, 2475− 2480. (25) Xu, D.; Wu, J.; Luo, S.; Zhang, J.; Wu, J.; Du, X.; Xu, Z. Fischer indole synthesis catalyzed by novel SO3H-functionalized ionic liquids in water. Green Chem. 2009, 11, 1239−1246.
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