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one pot synthesis of lewis acidic ionic liquids for friedel crafts

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									Chinese Chemical Letters Vol. 17, No. 3, pp 321-324, 2006 http://www.imm.ac.cn/journal/ccl.html

321

One-pot Synthesis of Lewis Acidic Ionic Liquids for Friedel-Crafts Alkylation

Ling HE, Guo Hong TAO, Wei Shan LIU, Wei XIONG, Tao WANG, Yuan KOU*
Green Chemistry Centre, College of Chemistry and Molecular Engineering, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Peking University, Beijing 100871 Abstract: Novel Lewis acidic ionic liquids containing thionyl cations and chloroaluminate anions were obtained by one-pot synthesis for the first time. Their acidities were determined by acetonitrile probe on IR spectrography. The ionic liquids were used as catalyst for Friedel-Crafts alkylation of benzene and 1-dodecene. The turnovers of 1-dodecene were higher than 99%. Monoalkylbenzene selectivity was 98%, while the 2-substituent product selectivity was 45%. Keywords: Ionic liquids, green chemistry, one-pot synthesis, Lewis acidity, alkylation.

Lewis acidic ionic liquids have been receiving extensive interest as green substitute for H2SO4, HF and AlCl3 catalysts in chemical processes1. In view of the principles of green chemistry2, ionic liquids themselves are highly expected to be obtained via a green route3. The quaternization, which is the most familiar way in the synthesis of ionic liquids, is unsuitable with this consideration for its commonly incomplete reaction4. New green route for preparation of ionic liquids is desired based on not only academic interesting but also commercial value5. Green chemists hope to get product via an atom-economic reaction, and addition reaction is the most ideal reaction. In this contribution, we used sulfonation of thionyl chloride as a potential green reaction to prepare Lewis acidic ionic liquids for the first time. Functional ionic liquids containing thionyl cations and chloroaluminate anions have been synthesized from cheap raw materials through this easy-preparation route, i. e., one-pot synthetic route. The ionic liquids can be used as efficient catalyst for Friedel-Crafts alkylation of benzene and 1-dodecene. The ionic liquids were obtained through direct addition reaction (Scheme 1). The thionyl chloride was dropped into the 1-alkylimidazole with vigorous stirring. Then certain amount of AlCl3 was added into the system in many small batches. Ionic liquid 3,3′thionyl-bis-1,1′-alkylimidazolium chloroaluminate ([tbaim]Cl2/AlCl3; a: alkyl, respectively are methyl, ethyl, butyl) was thus obtained. It is easy to see that the sulfonation of thionyl
*

E-mail: yuankou@pku.edu.cn

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Scheme 1 One-pot synthesis of Lewis acidic ionic liquids
O R 2 O N N Cl S Cl 2Cl2Cl-/AlCl3 R S N N N N R
AlCl3

O R S N N N N R

1a: [tbmim]Cl2/AlCl3, R=CH3; 1b: [tbeim]Cl2/AlCl3, R=C2H5; 1c: [tbbim]Cl2/AlCl3, R=C4H9

chloride is a solvent-free atom-economic reaction. The products obtained own good fluidity. The [tbaim]Cl2/AlCl3 showed the same Lewis acidity as that of 1-butyl-3-methylimidazolium chloroaluminate ([bmim]Cl/AlCl3). The acidity of these Lewis acidic ionic liquids was determined by IR spectroscopy using acetonitrile (ACN) as probe (Figure 1). In Figure 1, the presence of bands higher than 2300 cm-1 is an indication of acetonitrile coordinated to Lewis acidic sites6,7. Pure acetonitrile shows a well-resolved band at 2253 cm-1 (Figure 1A). When acetonitrile was added to [bmim]Cl/AlCl3 (x (AlCl3) = 0.67), it is clear from Figure 1B that two bands at 2335 cm-1 and 2305 cm-1 indicate strong Lewis acidity. In the [tbmim]Cl2/AlCl3 (x (AlCl3) = 0.67) (1a, Figure 1C), [tbeim]Cl2/AlCl3 (x (AlCl3) = 0.67) (1b, Figure 1D) and [tbbim]Cl2/AlCl3 (x (AlCl3) = 0.67) (1c, Figure 1E). Both the bands at 2335 cm-1 and 2305 cm-1 indicating strong Lewis acidity. This result proves that [tbaim]Cl2/AlCl3 are also Lewis acidic ionic liquids and may own the same acidity like the common Lewis acidic ionic liquids.
Figure 1 Determination of acidity of the Lewis acidic ionic liquids by FT-IR using acetonitrile (ACN) as probe
2253 A:Pure ACN 2335 2305

Absorbance

B:ACN+[bmim]Cl/AlCl3 C:ACN+[tbmim]Cl/AlCl3 D:ACN+[tbeim]Cl/AlCl3 E:ACN+[tbbim]Cl/AlCl3

2400 2380 2360 2340 2320 2300 2280 2260 2240 2220 2200 Wave number / cm
-1

One-pot Synthesis of Lewis Acidic Ionic Liquids

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[Bmim]Cl/AlCl3 ionic liquids have been used as dual-functional catalyst and solvent in the Friedel-Crafts alkylation of benzene and 1-dodecene to develop an environmentally benign process8. A comparative study of the ionic liquids [tbaim]Cl2/AlCl3 catalysis system with the traditional AlCl3 or [bmim]Cl/AlCl3 systems was carried out (Table 1). Similar to the [bmim]Cl/AlCl3 (x (AlCl3) = 0.67), the [tbaim]Cl2/AlCl3 (x (AlCl3) = 0.67) had better catalytic efficiency than AlCl3. The reaction almost instantaneously completed after 1-dodecene was dropped into the benzene/ionic liquids mixture. The turnovers of 1-dodecene were higher than 99%. Varying reaction time, no obvious promotion was observed. When the mole fraction of AlCl3 (x (AlCl3)) was 0.50, which meant no Al2Cl7- in the ionic liquids, the alkylation could not occur. This result was consistent with the fact that [bmim]Cl/AlCl3 (x (AlCl3) = 0.50) had no Lewis acidity9. The effects of the molar ratio of benzene to 1-dodecene were also explored. Increasing the molar ratio of benzene to 1-dodecene, the yield of the target product was enhanced. Furthermore, use of [tbaim]Cl2/AlCl3 led to a significant simplification in product isolation. The maximum yield of the monododecylbenzene was about 98%, while the 2-product selectivity was about 45%.
Table 1 Alkylation of benzene with 1-dodecene catalyzed by [tbaim]Cl2/AlCl3 Benzene / 1-dodecene
10 / 1 10 / 1 10 / 1 10 / 1 10 / 1 10 / 1 10 / 1 10 / 1 10 / 1 10 / 1 10 / 1 10 / 1 10 / 1 10 / 1 10 / 1 2/1 4/1 6/1 8/1 12 / 1

Catalyst
AlCl3 [bmim]Cl/AlCl3 x = 0.50 [bmim]Cl/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.50 [tbmim]Cl2/AlCl3 x = 0.67 [tbeim]Cl2/AlCl3 x = 0.50 [tbeim]Cl2/AlCl3 x = 0.67 [tbbim]Cl2/AlCl3 x = 0.50 [tbbim]Cl2/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.67 [tbmim]Cl2/AlCl3 x = 0.67

Reaction time (min)
15 15 15 15 15 15 15 15 15 5 10 30 60 120 240 5 5 5 5 5

2-Product selectivity
32.2 0 43.8 0 44.8 0 42.0 0 40.9 42.7 42.0 43.9 43.9 42.5 42.8 36.1 40.5 42.0 41.3 45.0

Monoalkyl selectivity
77.6 0 97.8 0 98.2 0 94.5 0 98.5 96.3 97.8 97.8 98.0 97.2 97.6 78.3 79.8 85.2 86.0 96.4

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Acknowledgments
We are grateful to the grant from the National Natural Science Foundation of China (Key Program 20533010).

References
1. 2. 3. 4. 5. 6. 7. 8. 9. D. B. Zhao, M. Wu, Y. Kou, E. Z. Min, Catal. Today, 2002, 74, 157. P. T. Anastas, J. C. Warne, Green Chemistry: Theory and Practice, 1998. A. West, Chemistry World, 2005, (3), 32. P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, 2003. G. H. Tao, L. He, N. Sun, Y. Kou, Chem. Commun., 2005, (28), 3562. Y. L. Yang, Y. Kou, Chem. Commun., 2004, (2), 226. X. H. Wang, G. H. Tao, X. M. Wu, Y. Kou, Acta Phys. Chim. Sin., 2004, (1), 44. C. DeCastro, E. Sauvage, M. H. Valkenberg, W. F. H olderich1, J. Catal., 2000, 196, 86. L. Y. Piao, X. Fu, Y. L. Yang, G. H. Tao, Y. Kou, Catal. Today, 2004, 93-95, 301.

Received 22 August, 2005


								
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