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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 100C, 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 on Ionic Liquids

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

Books on Ionic Liquids

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 1,4-diazobicyclo[2.2.2]octane (DABCO) based ionic liquids were used as recyclable catalysts for the aza-Michael addition at room temperature without any organic solvent. A low loading of the very efficient [DABCO-PDO][OAc] catalyzed reactions of various amines with a wide range of α,β-unsaturated amides to afford products in very good yields within hours. Moreover, the catalyst could be reused up to eight times.
A. Ying, Z. Li, J. Yang, S. Liu, S. Xu, H. Yan, C. Wu, J. Org. Chem., 2014, 79, 6510-6516.

An efficient cross-coupling reaction of aryl/het-aryl/benzyl halides with stable and easily workable sulfonyl hydrazides as thiol substitutes delivers unsymmetrical sulfides in the presence [DBU][HOAc] and CuI under microwave irradiation.
N. Singh, R. Singh, D. S. Raghuvanshi, K. N. Singh, Org. Lett., 2013, 15, 5874-5877.

A convenient and practical method for the one-pot, three-component synthesis of terminal vinylphosphonates from aldehydes, nitromethane and trialkylphosphites through a tandem Henry-Michael reaction followed by nitro elimination in the presence of 5-hydroxypentylammonium acetate (5-HPAA) as a task-specific ionic liquid offers good yields of the products under mild reaction conditions.
S. Sobhani, M. Honarmand, Synlett, 2013, 24, 236-240.

Ionic liquids [bmim][X] (X = Cl, Br, I, OAc, SCN) are highly efficient reagents for nucleophilic substitution reactions of sulfonate esters derived from primary and secondary alcohols. The newly developed protocol is very environmentally attractive because the reactions use stoichiometric amounts of ionic liquids as sole reagents without additional solvents and activating reagents. Moreover, these ionic liquids can be readily recycled.
Y. Liu, Y. Xu, S. H. Jung, J. Chae, Synlett, 2012, 23, 2663-2666.

Triflyl nitrate (TfONO2) and trifluoroacetyl nitrate (CF3CO2NO2), generated via metathesis in the readily available ethylammonium nitrate (EAN) ionic liquid as solvent, are powerful electrophilic nitrating reagents for a wide variety of aromatic and heteroaromatic compounds. Comparative nitration experiments indicate that EAN/Tf2O is superior to EAN/TFAA for nitration of strongly deactivated systems.
G. Aridoss, K. K. Laali, J. Org. Chem., 2011, 76, 8088-8094.

A straightforward and efficient Yb(OTf)3 catalyzed three-component reaction of aldehydes, alkynes, and amines under microwave irradiation in an ionic liquid provides 2,4-disubstituted quinolines in excellent yield under mild reaction condition. The catalyst can be recycled up to four times.
A. Kumar, V. K. Rao, Synlett, 2011, 2157-2162.

1-Alkyl-3-methylimidazolium cation based ionic liquids efficiently catalyze N-tert-butyloxycarbonylation of amines with excellent chemoselectivity. The catalytic role of the ionic liquid is envisaged as electrophilic activation of di-tert-butyl dicarbonate (Boc2O) through bifurcated hydrogen bond formation with the C-2 hydrogen of the 1-alkyl-3-methylimidazolium cation.
A. Sarkar, S. R. Roy, N. Parikh, A. K. Chakraborti, J. Org. Chem., 2011, 76, 7132-7140.

In a method for the Friedel-Crafts-type insertion reaction of acetylene with acid chlorides in chloroaluminate ionic liquids, the use of ionic liquids not only serves to avoid the use of carbon tetrachloride or 1,2-dichloroethane but also suppresses side reactions, and enables a simpler purification procedure, giving a range of aromatic and aliphatic β-chlorovinyl ketones in high yield and purity.
D. J. M. Snelders, P. J. Dyson, Org. Lett., 2011, 13, 4048-4051.

A strategy for the asymmetric Michael addition of aldehydes to nitroolefins with a catalytic system of an organocatalyst in combination with ionic-liquid-supported benzoic acid gives excellent diastereo- and enantioselectivities. A notable feature of this organocatalytic system is that the catalyst can be recycled more than 12 times without significant loss of enantioselectivity.
D. Sarkar, R. Bhattarai, A. D. Headley, B. Ni, Synthesis, 2011, 1993-1997.

2-Hydroxyethylammonium formate as a cost-effective and task-specific ionic liquid efficiently promotes the condensation of nitroalkanes with various aldehydes to produce β-nitrostyrenes in very good yields at room temperature. This reaction does not involve any hazardous organic solvent and toxic catalyst. The ionic liquid can be recovered and recycled for subsequent reactions.
A. Alizadeh, M. M. Khodaei, A. Eshgi, J. Org. Chem., 2010, 75, 8295-8298.

The novel 1,3-dialkyl-1,2,3-triazolium ionic liquids [bmTr][PF6] and [bmTr][NTf2] are chemically inert under basic conditions and more suitable media for reactions involving bases than common 1,3-dialkylimidazolium ionic liquids such as [bmim][PF6]. [bmTr][PF6] and [bmTr][NTf2] were efficient reaction media for the Baylis-Hillman reaction.
Y. Jeong, J.-S. Ryu, J. Org. Chem., 2010, 75, 4183-4191.

A highly efficient and selective palladium-catalyzed ligand-free cyclocarbonylation reaction of o-iodophenols with terminal acetylenes under atmospheric CO pressure affords diversified chromones in very good yields. The use of a phosphonium salt ionic liquid as the reaction medium enhances the efficiency of the cyclocarbonylation reaction.
Q. Yang, H. Alper, J. Org. Chem., 2010, 75, 948-950.

An efficient intermolecular hydroamination of unactivated alkenes with anilines catalyzed by lanthanide salts gives Markovnikov products in good yields.
P. Yin, T.-P. Loh, Org. Lett., 2009, 11, 3791-3793.

A rapid and convenient free-radical-based synthesis of functionalized quinazolines relies on microwave-promoted reactions of O-phenyl oximes with aldehydes in the presence of ZnCl2. The method worked well with alkyl, aryl, and heterocyclic aldehydes and for a variety of substituents in the benzenic part of the molecule.
F. Portela-Cubillo, J. S. Scott, J. C. Walton, J. Org. Chem., 2009, 74, 4934-4942.

Lanthanide triflates are efficient catalysts for the intramolecular Markovnikov-type hydroalkoxylation/cyclization of primary/secondary and aliphatic/aromatic hydroxyalkenes in room temperature ionic liquids (RTILs) to give five- and six-membered oxygen heterocycles in very good yields.
A. Dzudza, T. J. Marks, Org. Lett., 2009, 11, 1523-1526.

An improved one-pot van Leusen oxazole synthesis using tosylmethyl isocyanide (TosMIC), aliphatic halides and various aldehydes in ionic liquids allows the preparation of 4,5-disubstituted oxazoles in high yields. The recovered ionic liquids could be reused as solvent for six runs without significant loss of yields.
B. Wu, J. Wen, J. Zhang, J. Li, Y.-Z. Xiang, X.-Q. Yu, Synlett, 2009, 500-504.

Urea-hydrogen peroxide in the presence of a catalytic amount of magnesium bromide efficiently oxidizes primary and secondary benzylic alcohols into the corresponding aromatic aldehydes and ketones.
H. J. Park, J. C. Lee, Synlett, 2009, 79-80.

The application of the 1-butyl-3-methylimidazolium-based ionic liquid [BMIM][NTf2] as solvent enabled clean α-methylenations of carbonyl compounds in a short time and good yields. This ionic liquid was reused without affecting the reaction rates or yields over seven runs.
J. A. Vale, D. F. Zanchetta, P. J. S. Moran, J. A. R. Rodrigues, Synlett, 2009, 75-78.

An eco-friendly method allows the synthesis of 2,4-disubstituted quinolines via Meyer-Schuster rearrangement of 2-aminoaryl ketones and phenylacetylenes in the presence of a catalytic amount of zinc trifluoromethanesulfonate in the ionic liquid [hmim]PF6. The ionic liquid can be recycled.
R. Sarma, D. Prajapati, Synlett, 2008, 3001-3005.

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.

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1) source: Prof. Gmehling, Oldenburg