Ionic Liquids

A solidified ionic liquid¹⁾

An ionic liquid is a salt in which the ions are poorly coordinated, which results in these solvents being liquid below 100°C, or even at room temperature (room temperature ionic liquids, RTIL's). At least one ion has a delocalized charge and one component is organic, which prevents the formation of a stable crystal lattice.

The methylimidazolium and pyridinium ions have proven to be good starting points for the development of ionic liquids:

Properties, such as melting point, viscosity, and solubility of starting materials and other solvents, are determined by the substituents on the organic component and by the counterion. Many ionic liquids have even been developed for specific synthetic problems. For this reason, ionic liquids have been termed "designer solvents".

One of the first RTILs was a mixture of [emim]Cl with AlCl₃ forming a series of equilibria between [emim][AlCl₄], [emim][Al₂Cl₇], and [emim][Al₃Cl₁₀]. This RTIL is not water stable. The discovery of water-insoluble RTILs such as [bmim][PF₆] allowed the development of new work-up methods, including the separation of water-soluble byproducts by simple extraction. Some transition metal catalysts that are soluble in ionic liquids may be recycled together with the ionic liquid, after extraction with water and the non-polar organic solvent used for product separation. The catalyst and ionic liquid may be recycled several times.

In addition, the following RTIL, derived from fructose (a renewable feedstock), is a promising solvent for implementing fully "green" chemistry methods:

S. T. Handy, M. Okello, G. Dickenson, Org. Lett., 2003, 5, 2513-2515.

This ionic liquid was proven to be suitable for Heck Reactions:

S. T. Handy, M. Okello, G. Dickenson, Org. Lett., 2003, 5, 2513-2515.

The absence of volatility is one of the most important benefits of ionic liquids, offering a much lower toxicity as compared to low-boiling-point solvents. Ionic liquids can also make for safer microwave synthesis methods, because sudden pressure surges are not possible. The dipole characteristics of ionic liquids translate into rapid excitation by microwaves, and consequently faster reactions.

Reviews

P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. Engl. 2000 , 39, 3772. DOI

Books

	Ionic Liquids in Synthesis Peter Wasserscheid, Tom Welton Hardcover, 364 Pages First Edition, November 2002 ISBN: 3-527-30515-7 - Wiley-VCH
	Chemistry in Alternative Reaction Media D. J. Adams, P. J. Dyson, S. J. Taverner Paperback, 268 Pages First Edition, November 2003 ISBN: 0-471-49849-1 - Wiley

Recent Literature

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Demethylation of N,N-dimethylanilines was carried out in various ionic liquids and acetonitrile as well as under solvent-free conditions. The reactivity dramatically depends on the employed solvent, with [bmim]Cl showing the best reactivity.
S. Imoria, H. Togo, Synlett, 2006, 2629-2632

α-Acyloxy carboxamides were easily obtained in one step by a Passerini reaction between carboxylic acids, aldehydes and isocyanides in ionic liquids or polyethyleneglycol as green reaction media.
C. K. Z. Andrade, S. C. S. Takada, P. A. Z. Suarez, M. B. Alves, Synlett, 2006, 1539-1541.

The three-component coupling of aldehyde, dimethyl acetylenedicarboxylate (DMAD) and cyclohexyl isocyanide proceeds efficiently in [bmim]BF₄ ionic medium under extremely mild conditions to afford 2-aminofurans in high yields. The recovered ionic liquid was reused for five to six times with consistent activity.
J. S. Yadav, B. V. S. Reddy, S. Shubashree, K. Sadashiv, J. J. Naidu, Synthesis, 2004, 2376-2380.

In ionic liquids [Bmim][PF₆] or [Bmim][BF₄], a highly regioselective N-substitution of pyrrole with alkyl halides, sulfonyl chlorides, and benzoyl chloride gave substituted pyrroles in excellent yields. Michael addition of pyrrole with electrophilic olefins was completed in a highly regioselective manner to afford N-alkylpyrroles.
Z.-G. Lea, Z.-C. Chen, Y. Hu, Q.-G. Zheng Synthesis, 2004, 1951-1954.

The coupling of vinyl bromides with thiols or diphenyl diselenide using copper(I) salts as catalysts in ionic liquids based on amino acids gives vinyl chalcogenides in good to excellent yields with retention of stereochemistry. The ionic liquids act as solvent, base, and excellent promoter for the copper-catalyzed coupling reactions.
Z. Wang, H. Mo, W. Bao, Synlett, 2007, 91-94.

An improved and rapid one-pot synthesis of 2,4,5-triaryl imidazoles in a room temperature ionic liquid does not need any added catalyst. This one-pot methodology offers excellent isolated yields, simple work up procedures and efficient recovery and recycling of the ionic liquid.
S. A. Siddiqui, U. C. Narkhede, S. S. Palimkar, T. Daniel, R. J. Lahoti, K. V. Srinivasan, Tetrahedron, 2005, 61, 3539-3546.

A selective cathodic reduction of carbon dioxide in CO₂-saturated room-temperature ionic liquid BMIm-BF₄ solutions containing amines, followed by addition of EtI as an alkylating agent allows an environmentally friendly, mild, and safe synthesis of organic carbamates in good yields.
M. Feroci, M. Orsini, L. Rossi, G. Sotgiu, A. Inesi, J. Org. Chem., 2007, 72, 144-149.

A convenient and efficient oxidation of hydroxylated and methoxylated benzaldehydes and acetophenones to the corresponding phenols uses hydrogen peroxide and methyltrioxorhenium as catalyst in ionic liquids [bmim]BF₄ and [bmim]PF₆.
R. Bernini, A. Coratti, G. Provenzano, G. Fabrizi, D. Tofani, Tetrahedron, 2005, 61, 1821-1825.

A convenient, efficient, and selective N-Alkylation of N-acidic heterocyclic compounds with alkyl halides is accomplished in ionic liquids in the presence of potassium hydroxide as a base. In this manner, phthalimide, indole, benzimidazole, and succinimide can be successfully alkylated.
Z.-G. Le, Z.-C. Chen, Y. Hu, Q.-G. Zheng, Synthesis, 2004, 208-212.

Aldimines (generated in situ from aldehydes and amines) react readily with ethyl diazoacetate in the presence of 2 mol% of Bi(OTf)₃ in [bmim]PF₆ to produce the corresponding aryl aziridine carboxylates in high yields with excellent cis-diastereoselectivity.
J. S. Yaday, B. V. S. Reddy, P. N. Reddy, M. Shesha Rao, Synthesis, 2003, 1387-1389.

The ionic liquid 1-butyl-3-methylimidazonium tetrafluoroborate [bmim]BF₄ was used for ethylenediammonium diacetate (EDDA)-catalyzed Knoevenagel condensation between aldehydes or ketones with active methylene compounds. Catalyst and solvent can be recycled.
C. Su, Z.-C. Chen, Q.-G. Zhen, Synthesis, 2003, 555-559.

The functionalized ionic liquid 1-(2-hydroxyethyl)-3-methylimidazolium tetrafluoroborate ([HEmim][BF₄]) is an efficient and recyclable reaction medium for palladium-catalyzed Heck reactions under phosphine-free conditions.
L. Zhou, L. Wang, Synthesis, 2006, 2649-2652.

The basic ionic liquid 1-butyl-3-methylimidazolium hydroxide, [bmIm]OH, efficiently catalyzes the Knoevenagel condensation of various aliphatic and aromatic aldehydes and ketones with active methylenes at room temperature without requirement of any organic solvent.
B. C. Ranu, R. Jana, Eur. J. Org. Chem., 2006, 3767-3770.

Hydroformylation of alkenes can be carried out in a few minutes under microwave activation at a relatively low pressure (2.7 atm) using commercially available catalysts and ligands. After 4 min of microwave irradiation, the corresponding aldehyde is formed in high yield.
E. Petricci, A. Mann, A. Schoenfelder, A. Rota, M. Taddei, Org. Lett., 2006, 8, 3725-3727.

The palladium acetate-catalyzed coupling reaction of aryl boronic acids with carboxylic anhydrides or acyl chlorides was carried out smoothly in water in the presence of PEG or [bmim][PF₆] to give high yields of ketones. The Pd(OAc)₂-H₂O-[bmim][PF₆] catalytic system can be recovered and reused.
B. Xin, Yuhong Zhang, K. Cheng, J. Org. Chem., 2006, 71, 5725-5731.

The first Corey-Chaykovsky epoxidation and cyclopropanation using trimethyl sulfonium iodide/trimethyl sulfoxonium iodide and KOH as base in the recyclable ionic liquid, (bmim)PF₆ are described.
S. Chandrasekhar, Ch. Narasihmulu, V. Jagadeshwar, K. Venkatram Reddy, Tetrahedron Lett., 2003, 44, 3629-3630.

A microwave-assisted, palladium-catalyzed catalytic transfer hydrogenation of different homo- or heteronuclear organic compounds using formate salts as a hydrogen source was performed in ([bmim][PF₆]. Essentially pure products could be isolated in moderate to excellent yields by simple liquid-liquid extraction.
H. Berthold, T. Schotten, H. Hönig, Synthesis, 2002, 1607-1610.

A new, base-stable, imidazolium room-temperature ionic liquid (RTIL) has been prepared and applied to the addition of Grignard reagents to carbonyl compounds to afford alcohol products in good to excellent yield at ambient temperature. The RTIL can be easily recycled and reused.
S. T. Handy, J. Org. Chem., 2006, 71, 4659-4662

Brønsted acid-base ionic liquids (GILs) based on guanidine and acetic acid are efficient and reusable reaction media for palladium-catalyzed Heck reactions. These solvents play multiple roles: as strong bases and as ligands to stabilize Pd species.
S. Li, Y. Lin, H. Xie, S. Zhang, J. Xu, Org. Lett., 2006, 8, 391-394.

Ammonium salts that can act as hydrogen-bond donors exert a remarkable acceleration on the rates of the regioselective arylation of electron-rich olefins by aryl halides in ionic liquids and common solvents.
J. Mo, J. Xiao, Angew. Chem. Int. Ed., 2006, 45, 4152-4157.

Please cite and link this page as follows:

Ionic Liquids ( URL: http://www.organic-chemistry.org/topics/ionic-liquids.shtm )

1) source: Prof. Gmehling, Oldenburg

Saturday, 10-May-2008 01:31:49 CEST