1-Ethyl-3-methylimidazolium halogenoaluminate ionic liquids as reaction media for the acylative cleavage of ethers

Article abstract of DOI:10.1016/S0040-4039(99)02289-3

Ionic liquids derived from 1-ethyl-3-methylimidazolium iodide (emimI), and aluminium chloride have been shown to be suitable solvents for the acylative cleavage of a series of cyclic and acyclic ethers. Difunctionalised ω-iodobenzoate adducts have been isolated from the cleavage of cyclic ethers performed in Lewis acidic ionic liquids (i.e. AlCl₃:emimI ratio is greater than 1:1) in the presence of benzoyl chloride. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(99)02289-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1343–1346
1-Ethyl-3-methylimidazolium halogenoaluminate ionic liquids as
reaction media for the acylative cleavage of ethers
Laine Green, Ivan Hemeon and Robert D. Singer ∗
Department of Chemistry, Saint Mary’s University, Halifax, Nova Scotia, B3H 3C3, Canada
Received 2 November 1999; accepted 29 November 1999
Abstract
Ionic liquids derived from 1-ethyl-3-methylimidazolium iodide (emimI), and aluminium chloride have been
shown to be suitable solvents for the acylative cleavage of a series of cyclic and acyclic ethers. Difunctionalised
ω-iodobenzoate adducts have been isolated from the cleavage of cyclic ethers performed in Lewis acidic ionic
liquids (i.e. AlCl3:emimI ratio is greater than 1:1) in the presence of benzoyl chloride. © 2000 Elsevier Science
Ltd. All rights reserved.
There has been ample recent interest in the use of ionic liquids as solvents for a variety of organic
and organometallic reactions. Ionic liquids derived from 1,3-dialkylimidazolium cations and anions
such as tetrafluoroborate, hexafluorophosphate, and heptachlorodialuminate(III) have been shown to
possess several advantages over more conventional molecular solvents.1 For example, these ionic liquids
possess a wide liquidous range (i.e. ∼−96°C to 200°C for 1-ethyl-3-methylimidazolium heptachloro-
dialuminate(III), [emim]Al2Cl7), making them suitable for studying kinetics of reactions at a variety of
temperatures. These solvents also have a negligible vapour pressure even at elevated temperatures and, in
the case of the −halogenoaluminate ionic liquids, can possess variable Lewis acidity. The ionic liquids with
−
BF4 and PF6 anions are water stable whereas the halogenoaluminate ionic liquids decompose in the
presence of water. It has been demonstrated that an array of organic and organometallic compounds such
as aromatics, metallocenes, rhodium, nickel(II), and ruthenium(IV) complexes, titanium (IV) chloride,
Scheme 1. Acylative ether cleavage in 1-ethyl-3-methylimidazolium halogenoaluminate ionic liquids2
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02289-3
tetl 16217

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Table 1
Acylative cleavage of ethers in [emimI]·AlCl3 derived ionic liquids
zinc(II) chloride, dialkylethers, and others can be dissolved readily in 1,3-dialkylimidazolium based ionic
liquids. Thus, it is likely that many chemical reactions can be conducted in these novel solvent systems
with potential enhancement in both yield and selectivity.
Despite the availability of several methods with which to cleave ethers,3 the literature lacks a general,
reliable method to selectively cleave ether linkages in complex and otherwise fragile molecules. Since

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it had been previously demonstrated by us and others that 1-ethyl-3-methylimidazolium halogenoalumi-
nates possess ‘tunable’ Lewis acidity4 we proceeded to show that these ionic liquids could be used for
the acylative cleavage of cyclic and acyclic ethers (Scheme 1, Table 1). We report herein the acylative
cleavage of dialkylethers conducted in Lewis acidic 1-ethyl-3-methylimidazolium halogenoaluminate
ionic liquids (Scheme 2) and that these cleavages are sensitive to the ‘bulk’ Lewis acidity of the ionic
solvent system.
Scheme 2. Formation of ‘strictly acidic’ 1-ethyl-3-methylimidazolium heptahalodialuminate(III) ionic liquid
The cyclic ether tetrahydrofuran undergoes acylative cleavage to afford an excellent yield of 4-
iodobutylbenzoate when the strictly acidic ionic liquid [emim]Al2X7 is used (entry 1), but suffers a
decrease in yield of difunctionalised product when the mildly acidic halogenoaluminate is used as solvent
(entry 6). Interestingly, the other cyclic ethers investigated in this study, 1,5-dimethyltetrahydrofuran
and tetrahydropyran display inverse behaviour in that they afford good yield of cleavage products only
when the mildly acidic halogenoaluminate is used as solvent (entries 7 and 8). Diethyl ether, a primary
ether, gave relatively poor yields of ethylbenzoate under all conditions attempted in this study. However,
diisopropyl ether, a secondary ether, gave good yields of isopropylbenzoate in the mildly Lewis acidic
solvent system (entry 10). Anisole underwent acylative cleavage to afford phenylbenzoate, but afforded
the Friedel–Crafts adduct,1d,5 methoxybenzophenone, as the major product (entry 11). 1,4-Dioxane nor
2,3-dihydro-2H-pyran afforded any isolable cleavage products under any of the conditions utilized in this
study.
That these cleavage reactions were not the result of HCl presence during aqueous work-up was shown
by performing the aqueous work-up using distilled water and adding it to the reaction mixtures at 0°C.
The cleavage due to HCl could therefore not occur at this lowered temperature. Furthermore, if HCl
induced the cleavage reactions, chlorine would be expected to be present in the products; spectroscopic
data indicates that only iodine has been incorporated into the isolated products.
These results support a mechanism in which there is initial formation of a polarized complex between
Lewis acidic halogenoaluminate or aluminium chloride species present in solution and benzoyl chloride
or the formation of benzoyl cation upon addition of benzoyl chloride to the system. O-Acylation of the
ether then occurs to give an oxonium ion species that cleaves to give the most stable carbonium ion or
which undergoes nucleophilic attack by iodine present in these systems (Scheme 3). The lack of chlorine
in any of the products isolated in this study indicates the latter of these two mechanistic possibilities is
more likely. The differential results observed using ionic solvents of variable Lewis acidity suggest that
this mechanism may be hindered by occupation of the etherial oxygen by some Lewis acidic species
(i.e. Al2X7−) other than acylinium ion when the strictly acidic solvent system is used. We are currently
conducting studies utilizing more complex ethers and a wider range of Lewis acidities to elucidate the
full scope, selectivity, and limitations of the ether cleavage reactions conducted in these ionic solvents.

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Scheme 3. O-Acylation of tetrahydrofuran by acylinium ion followed by nucleophilic attack by iodide
Acknowledgements
We are grateful to the Natural Sciences and Engineering Research Council of Canada, the Petroleum
Research Fund, administered by the American Chemical Society, and Saint Mary’s University, Senate
Research, for financial support of this research.
References
1. (a) Guterman, L. Chemistry, A. C. S.; 12, Summer 1999; (b) Earle, M. J.; McCormac, P. B.; Seddon, K. R. Green Chemistry;
23, 1999; (c) Earle, M. J.; McCormac, P. B.; Seddon, K. R. J. Chem. Soc., Chem. Commun. 1998, 2245; (d) Boon, J. A.;
Levisky, J. L.; Pflug, J. L.; Wilkes, J. S. J. Org. Chem. 1986, 51, 480; (e) Surette, J. K. D.; Green, L.; Singer, R. D. J. Chem.
Soc., Chem. Commun. 1996, 2753; (f) Stark, A.; MacLean, B. L.; Singer, R. D. J. Chem. Soc., Dalton Trans. 1999, 63; (g)
Dyson, P. J.; Grossel, M. J.; Srinivasan, N.; Vine, T.; Welton, T.; Williams, D. J.; White, A. J. P.; Zigras, T. J. Chem. Soc.,
Dalton Trans. 1997, 3465; (h) Monteiro, A. L.; Zinn, F. K.; de Zouza, R. F.; Dupont, J. Tetrahedron: Asymmetry 1997, 8, 177;
(i) Chen, W.; Xu, L.; Chatterton, C.; Xiao, J. J. Chem. Soc., Chem. Commun. 1999, 1247; (j) Einloft, S.; Dietrich, F. K.; de
Souza, R. F.; Dupont, J. Polyhedron 1996, 15, 3257; (k) Suarez, P. A. Z.; Dullius, J. E. L.; Einloft, S.; de Souza, R. F.; Dupont,
J. Polyhedron 1996, 15, 1217.
2. Typical procedure: (Ionic liquid preparation): Aluminium chloride (3.08 g, 23.1 mmol for a X(AlCl3)=0.52, mildly acidic
ionic liquid, or 5.6 g, 42 mmol for a X(AlCl3)=0.67, strictly acidic ionic liquid) was added slowly over 30 min to anhydrous
1-ethyl-3-methylimidazolium iodide (5.0 g, 21 mmol) in a dry box to afford a clear liquid upon stirring. The r.b. flask was
sealed with a septum and removed from the dry box. (Acylative cleavage of tetrahydrofuran): Tetrahydrofuran (0.20 mL, 2.5
mmol) freshly distilled from potassium/benzophenone ketyl was added via syringe to the ionic liquid, as prepared above, and
allowed to stir for 15 min. Freshly distilled benzoyl chloride (0.58 mL, 5 mmol) is then added via syringe to the solution
and stirred under argon atmosphere for 2 h at room temperature. Aqueous work-up involved the careful addition of 20 mL of
0.1 M HCl or distilled water followed by extraction with chloroform (3×30 mL). The combined organic extracts were then
washed with 50 mL of saturated sodium bicarbonate followed by drying with anhydrous MgSO4, filtration through fluted
filter paper, and concentration in vacuo. The crude reaction mixture was then purified using flash column chromatography on
silica gel and using 10:1 hexanes/ethyl acetate eluent to afford 0.72 g, 95% yield of 4-iodobutyl benzoate: IR ν 1719 cm−1
(carbonyl); 1H NMR (CDCl3) δ 8.06–8.02, 7.60–7.41 (m, 5H, C6H5), 4.35 (t, J=6 Hz, 2H, CH2-COPh), 3.26 (t, J=6 Hz, 2H,
CH2-I), 2.03–1.87 (m, 4H, CH2CH2); 13C NMR (CDCl3) δ 166.56 (carbonyl), 133.01, 130.20, 129.58, 128.41 (C6H5), 63.78
(CH2-I), 30.12 (CH2-OAc), 29.69 (CH2), 6.03 (CH2).
3. (a) Suzuki, Y.; Matsushima, M.; Kodomari, M. Chem. Soc. Jap., Chem. Letters 1998, 319; (b) Guindon, Y.; Therien, M.;
Girard, Y.; Yoakim, C. J. Org. Chem. 1987, 52, 1680; (c) Cloke, J. B.; Pilgrim, F. J. J. Am. Chem. Soc. 1939, 61, 2667; (d)
Ganem, B.; Small Jr., V. R. J. Org. Chem. 1974, 39, 3728; (e) Alper, H.; Huang, C.-C.; J. Org. Chem. 1973, 38, 64; (f) Pri-Bar,
I.; Stille, J. K. J. Org. Chem. 1982, 47, 1215; (g) Iqbal, J.; Srivastava, R. R. Tetrahedron 1991, 47, 3155; (h) Oriyama, T.;
Kimura, M.; Oda, M.; Koga, G. Synlett 1993, 437.
4. The ‘bulk’ Lewis acidity of [emimI]·AlCl3 ionic liquids is best described by the mole fraction of aluminium chloride,
−
[X(AlCl3)], present. When X(AlCl3) <0.5, halide ion (I− or Cl−) is in excess−over Al2X7 and the ionic solvent is ‘basic’;
when X(AlCl3)=0.5, the ionic solvent is ‘neutral’; when X(AlCl3) >0.5, Al2X7 ion is in excess over halide ion and the ionic
solvent is ‘acidic’.
5. Adams, C. J.; Earle, M. J.; Roberts, G.; Seddon, K. R. J. Chem. Soc., Chem. Commun. 1998, 2097.