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 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
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.
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.
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
Thursday, 09-Sep-2010 09:44:09 CEST
