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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An ionic liquid supported ruthenium carbene complex can be used for ring-closing metathesis in the construction of five-to-eight-membered rings with high activity and good recyclability.
H. Wakamatsu, Y. Saito, M. Masubuchi, R. Fujita, Synlett, 2008, 1805-1808.

The room temperature ionic liquid, 1-butyl-3-methylimidazoliumhexafluorophosphate ([bmim]PF₆) is an environmentally benign solvent for the preparation of activated 3,4-dihydroisoquinolines derivatives through Bischler-Napieralski cyclization under mild conditions with excellent purity and yields.
Z. M. A. Judeh, C. B. Ching, J. Bu, A. McCluskey, Tetrahedron Lett., 2002, 43, 5089-5091.

A mild and efficient copper-catalyzed cross-coupling reaction of arylboronic acids with sulfinic acid salts affords a wide range of alkylaryl and diaryl sulfones in good yields under ambient conditions. The use of an ionic liquid allows a convenient separation of the product and recycling of copper catalyst.
M. L. Kantam, B. Neelima, B. Sreedhar, R. Chakravarti, Synlett, 2008, 1455-1458.

A basic functionalized ionic liquid, 1-butyl-3-methylimidazolium hydroxide ([bmim]OH), catalyzed the three-component condensation reaction of acid chlorides, amino acids, and dialkyl acetylenedicarboxylates in water to afford functionalized pyrroles in high yields.
I. Yavari, E. Kowsari, Synlett, 2008, 897-899.

A synergistic effect in nucleophilic fluorination has been demonstrated for the molecular combination of an ionic liquid and a tert-alcohol. Consequently, these functionalized ILs not only increase the nucleophilic reactivities of the fluoride anion but also remarkably reduce the olefin byproduct.
S. S. Shinde, B. S. Lee, D. Y. Chi, Org. Lett., 2008, 10, 733-735.

Reaction of [ArN₂][BF₄] salts immobilized in [BMIM][PF₆] ionic liquid (IL) with TMSI and TMSN₃ represents an efficient method for the preparation of iodo- and azido-derivatives via dediazoniation. Using TMSBr, competing fluorodediazoniation (ArF formation) and hydrodediazoniation (ArH formation) were observed depending on the substituents on the benzenediazonium cation.
A. Hubbard, T. Okazaki, K. K. Laali, J. Org. Chem., 2008, 73, 316-319.

Hydrophobic ionic liquids such as [Bmim]PF₆ are powerful media for bis(oxazoline)-copper-catalyzed asymmetric hetero-Diels-Alder reactions, that allow a convenient catalyst recycling. The reactivities and stereoselectivities were comparable to those of the corresponding homogeneous reactions. Furthermore, the reaction was remarkably accelerated in [Bmim]PF₆ compared to dichloromethane.
Y. J. Shin, C.-E. Yeom, M. Jeong, B. M. Kim, Synlett, 2008, 89-93.

Phosphonium bromide ionic liquids are superior media in the carbonylation of aryl and vinyl halides. Formation of acid bromide intermediates was detected in control experiments providing an extended view on the overall catalytic cycle involved. Solvent-free product isolation and recycling of the ionic liquid containing active Pd-catalyst are also demonstrated.
J. McNulty, J. J. Nair, A. Robertson, A. Lei, Org. Lett., 2007, 9, 4575-4578.

Optimized selective aerobic oxidations in ionic liquids convert various activated primary alcohols into their corresponding acids or aldehydes in good to excellent yields. The newly developed catalytic systems could also be recycled and reused for three runs without any significant loss of catalytic activity.
N. Jiang, A. J. Ragauskas, J. Org. Chem., 2007, 72, 7030-7033.

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.

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

Tuesday, 16-Mar-2010 23:23:41 CET