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
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.
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.
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, 23-May-2013 02:12:59 CEST
