Solvent-free sonochemical preparation of ionic liquids.

Article abstract of DOI:10.1021/ol026608p

[reaction: see text] An ultrasound-assisted preparation of a series of ambient-temperature ionic liquids, 1-alkyl-3-methylimidazolium (AMIM) halides, which proceeds via efficient reaction of 1-methyl imidazole with alkyl halides/terminal dihalides under solvent-free conditions, is described.

Full text of DOI:10.1021/ol026608p

ORGANIC
LETTERS
2
002
Vol. 4, No. 18
161-3163
Solvent-Free Sonochemical Preparation
of Ionic Liquids
3
Vasudevan V. Namboodiri and Rajender S. Varma*
Clean Processes Branch, National Risk Management Research Laboratory,
U.S. EnVironmental Protection Agency, MS 443, 26 W. Martin Luther King DriVe,
Cincinnati, Ohio 45268
Varma.rajender@epa.goV
Received July 25, 2002
ABSTRACT
An ultrasound-assisted preparation of a series of ambient-temperature ionic liquids, 1-alkyl-3-methylimidazolium (AMIM) halides, which proceeds
via efficient reaction of 1-methyl imidazole with alkyl halides/terminal dihalides under solvent-free conditions, is described.
The use of a large excess of conventional volatile solvents
required to conduct a chemical reaction is of ecological and
economic concern. Ambient-temperature ionic liquids en-
compassing 1,3-dialkylimidazolium salts (I) have shown
great promise as an attractive alternative to conventional
The conventional preparation of ionic liquids using excess
solvents leaves much to be desired, and improvements are
sought. An efficient microwave-assisted preparation of 1,3-
dialkylimidazolium halides has reduced the reaction time
from several hours to a few minutes in a process that avoids
the use of a large excess of alkyl halides/organic solvents as
1
solvents. Their potential for recyclability, ability to dissolve
9
a variety of materials, and importantly their nonvolatile nature
with barely measurable vapor pressure are some of their
unique attributes responsible for newly found popularity.
the reaction medium. However, as a result of the exothermic
nature of the reaction, continuous microwave heating may
result in overheating that leads to the formation of colored
products. In view of the emerging importance of the ionic
liquids as reaction media and our general interest in the
2
Although originally studied in electrochemistry, ionic liquids
are currently being explored as environmentally benign
solvent substitutes for conventional volatile solvents in a
1
0
development of clean chemical processes, we decided to
address the main problem in dealing with these highly
viscous materials wherein efficient mixing is a challenge.
Ultrasound-accelerated chemical reactions are well-known
and proceed via the formation and adiabatic collapse of the
1
variety of applications such as chemical synthesis, liquid/
liquid separations, extractions,4 dissolution,5 catalysis,
3
6
7
8
biocatalysis, and polymerization.
(
1) (a) Welton, T. Chem. ReV. 1999, 99, 2071. (b) Wilkes, J. S. Green
Chem. 2002, 4, 73. (c) Holbrey, J. D.; Seddon, K. R. Clean Prod. Proc.
999, 1, 223.
2) (a) Hussey, C. L. Molten Salt Chem. 1983, 5, 185. (b) Wilkes, J. S.;
Levisky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg. Chem. 1982, 21, 1263.
3) Swatloski, R. P.; Visser, A. E.; Reichert, W. M.; Broker, G. A.; Farina,
L. M. Holbrey, J. D.; Rogers, R. D. Green Chem. 2002, 4, 81.
4) (a) Blanchard, L. A.; Hancu, D.; Beckmann, E. J.; Brennecke, J. F.
(7) (a) Cull, S. G.; Holbrey, J. D.; Vargas-Mora, V.; Seddon, K. R.; Lye,
G. J. Biotechnol. Bioeng. 2000, 69, 227. (b) Sheldon, R. A.; Maderia Lau,
R.; Sorgedrager, M. J.; van Rantwijk, F.; Seddon, K. R. Green Chem. 2002,
4, 147.
(8) (a) Hardacre, C.; Holbrey, J. D.; Katdare S. P.; Seddon, K. R. Green
Chem. 2002, 4, 143. (b) Carmichael, A. J.; Haddleton, D. M.; Bon S. A.
F.; Seddon, K. R. Chem. Commun. 2000, 1237.
1
(
(
(
Nature 1999, 399, 28. (b) Huddleston, J. G.; Willauer, H. D.; Swatloski,
R. P.; Visser A. E.; Rogers, R. D. Chem. Commun. 1998, 1765.
(9) (a) Varma, R. S.; Namboodiri, V. V. Chem. Commun. 2001, 643.
(b) Varma, R. S.; Namboodiri, V. V. Pure Appl. Chem. 2001, 73, 1309.
(10) (a) Varma, R. S. Green Chem. 1999, 1, 43. (b) Namboodiri, V. V.;
Varma, R. S.; Sahle-Demessie, E.; Pillai, U. R. Green Chem. 2002, 4, 170.
(b) Namboodiri, V. V.; Varma, R. S. Tetrahedron Lett. 2002, 43, 1143. (c)
Varma, R. S. In Green Chemistry: Challenging PerspectiVes; Tundo, P.,
Anastas, P. T., Eds.; Oxford University Press: Oxford, 2000; pp 221-
244.
(
5) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. J. Am.
Chem. Soc. 2002, 124, 4974.
6) (a) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39,
772. (b) Cole, A. C.; Jensen, J. L.; Ntai, I.; Loan, K.; Tran, K. L.; Weaver,
(
3
T. K. J.; Forbes, D. C.; Davis, J. H., Jr. J. Am. Chem. Soc. 2002, 5962. (c)
Zhao, D.; Wu, M.; Kon, Y.; Min, E. Catal. Today 2002, 74, 157.
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0.1021/ol026608p CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/15/2002

Table 1. Ultrasound-Promoted Preparation of 1-Alkyl-3-Methylimidazolium Halides
a
c
entry
alkyl halide
RX, mmol
MIM, mmol
time, h
yield, %
yield, % (time, h)
1
2
1-bromopropane
1-chlorobutane
11
11
10
10
200
10
10
10
10
200
10
10
10
10
200
10
10
11
11
11
11
220
11
2
6
2
2
0.5
4
6
0.5
2
0.5
0.5
6
0.25
2
0.5
3
2
3
95
24
86b
94
94
93
31
89b
91
93
91
42
93b
93
92
93
94
94
93
92
92
94
98
0(2)
0(6)
0(2)
0(6)
30(0.5)
0(6)
0(6)
0(2)
0(6)
32(0.5)
28(0.5
0(6)
0(2)
0(6)
26(0.5)
0(6)
28(3)
0(3)
33(2)
0(2)
0(3)
2
2
2
20
11
11
11
11
20
11
11
11
11
20
11
11
5
3
4
5
6
1-bromobutane
1-iodobutane
2-bromobutane
1-chlorohexane
7
8
9
1-bromohexane
1-iodohexane
1-iodoheptane
1-chlorooctane
1
0
1
1
1
1
1
1
1
1
1
2
1
2
3
4
5
6
7
8
9
0
1-bromooctane
1-iodooctane
1,4-dibromobutane
1,4-diiodobutane
1,6-dibromohexane
1,6-diiodohexane
1,8-dichlorooctane
1,8-dibromooctane
1,8-diiodooctane
ethylchloroacetate
5
5
5
100
5
2
0.25b
3
2
5
11
11
10
25(2)
34(3)
0.5
a
b
Ultrasonic bath, Fisher Scientific (FS 220). Probe system, Sonic & Materials Inc., model VCX 750, power 750 W, frequency 20 kHz, tip diameter 19
c
mm, amplitude 97%). Reactions in oil bath at temperature reached under sonication conditions.
transient cavitation bubbles.11 We now report an efficient
method for the preparation of ionic liquids using ultrasound
as the energy source by simple exposure of neat reactants in
a closed container to irradiation using a sonication bath. The
current solvent-free approach requires shorter reaction time
and low reaction temperature in contrast to several hours
needed under conventional heating conditions using an excess
of reactants.
halide under vacuum or by washing with ethyl acetate/ether
and drying.
1
0,12
To establish the generality of the reaction a series of ionic
liquids were prepared via this sonication protocol and then
compared with the similar preparation using conventional
heating in an oil bath (Table 1). Most of the halides used in
this study were efficiently converted to the corresponding
ionic liquids. The reactivity trend of halides was found to
-
-
-
be in the order I > Br > Cl . The alkyl chlorides were
relatively less reactive and required longer irradiation time
and often some heating except for reactive entities such as
ethyl chloroacetate (entry 20). The higher reactivity of
bromides and iodides provided excellent yields with mini-
mum sonication time, and the reactions were completed
nearly at room temperature. To prepare chloride salts we
explored the use of a probe system (Sonic & Materials Inc.)
wherein an ultrasonic field is produced directly in the reaction
vessel. A number of chloride salts could be easily prepared
following this modified approach (entries 2, 6 and 10, Table
In the present study a laboratory ultrasonic cleaning bath
was used for the preparation of 1,3-dialkylimidazolium
halides. Initially, the effect of ultrasound on a series of
reactions comprising alkyl halides and 1-methylimidazole
(MIM) was examined (Table 1). During sonication, the
formation of the ionic liquid could be visibly monitored as
the reaction contents turned from a clear solution to opaque
(emulsification) and finally a clear viscous phase or separa-
tion of solids occurred. Upon continuous irradiation for 2 h
the temperature of the bath rose from 22 to 40 °C. At this
stage, the workup only involved the removal of residual
1
). During sonication of 1-chlorobutane and MIM, the
temperature of the reaction mixture rose from 22 to 80 °C
and became constant at 80 °C. Upon formation of the ionic
liquid due to exothermicity of the reaction, the temperature
(
11) (a) Synthetic Organic Chemistry; Luche, J. L., Ed.; Plenum Press:
New York, 1998. (b) Gaplovsky, A.; Gaplovsky, M.; Toma S.; Luche, J.
L. J. Org. Chem. 2000, 65, 8444. (c) Ultrasound, Its Chemical, Physical
and Biological Effects; Suslick, K. S., Ed.; VCH: Weinheim, 1988. (d)
Rajagopal, R.; Jarikote, V.; Srinivasan, K. V. Chem. Commun. 2002, 616.
(
2
e) Deshmukh, R. R.; Rajagopal, R.; Srinivasan, K. V. Chem. Commun.
001, 1544.
(12) Huddleston, J. G.; Visser, A. E.; Reichert, W. M.; Willauer, H. D.;
Broker, G. A.; Rogers, R. D. Green Chem. 2001, 3, 156.
3162
Org. Lett., Vol. 4, No. 18, 2002

increased to 115 °C. Similarly, in the case of reaction
between 1-chlorooctane and MIM the temperature reached
The ionic liquids prepared were characterized by spectral
1
13
( H and C NMR) and elemental analysis and were in good
1
2
100 °C and upon formation of the ionic liquid it further
agreement with the literature reports. Thermogravimetric
(TG) and differential scanning calorimetric (DSC) analyses
showed that all ionic liquids were free of the starting
materials and thermally stable up to 280 °C.
increased to 125 °C.
Butyl, hexyl, and octyl dicationic salts (entries 13-19,
Table 1) were also prepared efficiently by this methodology.
From the NMR data the dicationic salts generated from
chloroalkanes (entry 17, Table 1) were slightly contaminated
with the corresponding monocationic intermediate (<5%).
However, the diiodo and dibromoalkanes, being more
reactive, afforded pure products. The purity of dicationic salts
prepared via sonication was found to be superior to those
prepared via conventional heating methods, presumably
because of inefficient mixing and heat transfer in the latter,
once the solid product (II) begins to form.
Interestingly, this ultrasound-accelerated method is adapt-
able for efficiently accomplishing a one-pot palladium acetate
catalyzed Suzuki reaction of iodobenzene with 4-fluoro-
phenylboronic acid at room temperature by subsequent
addition of reactants to in situ generated ionic liquid. The
ionic liquid and catalyst can be recycled four times without
the loss of any reactivity (91%). The preparation of other
In conclusion, a solvent-free sonochemical protocol is
developed for the clean synthesis of ambient-temperature
ionic liquids at nearly room temperature. The in situ
generation of ionic liquids and their subsequent utilization
as reaction medium by sequential addition of reactants in
the same pot renders this an ideal approach for chemical
transformation.
Acknowledgment. The authors wish to thank Mr. Ballard
Mullins for the TGA and DSC analyses. V.V.N. is a
postgraduate research participant at the National Risk
Management Research Laboratory administered by the Oak
Ridge Institute for Science and Education through an
interagency agreement between the U.S. Department of
Energy and the U.S. Environmental Protection Agency.
4
ionic liquids bearing anions such as BF is being explored
via this method.
OL026608P
Org. Lett., Vol. 4, No. 18, 2002
3163