A study of the ionic liquid mediated microwave heating of organic solvents

Article abstract of DOI:10.1021/jo016297g

The use of ionic liquids as aids for microwave heating of nonpolar solvents has been investigated. We show that hexane and toluene together with solvents such as THF and dioxane can be heated way above their boiling point in sealed vessels using a small q

Full text of DOI:10.1021/jo016297g

J . Org. Chem. 2002, 67, 3145-3148
3145
A Stu d y of th e Ion ic Liqu id Med ia ted
Micr ow a ve Hea tin g of Or ga n ic Solven ts
Nicholas E. Leadbeater* and Hanna M. Torenius
Department of Chemistry, King’s College London,
Strand, London WC2R 2LS UK
nicholas.leadbeater@kcl.ac.uk
Received November 20, 2001
F igu r e 1. Ionic liquids used in the studies.
Abstr a ct: The use of ionic liquids as aids for microwave
heating of nonpolar solvents has been investigated. We show
that hexane and toluene together with solvents such as THF
and dioxane can be heated way above their boiling point in
sealed vessels using a small quantity of an ionic liquid,
thereby allowing them to be used as media for microwave-
assisted chemistry. Using the appropriate ionic liquid, the
heating can be performed with no contamination of the
solvent. To show the applicability of the system, two test
reactions have been successfully performed.
THF (ꢀ ) 7.58), and dioxane (ꢀ ) 2.21). However, with
their substantially lower dielectric constants these are
clearly less suitable for use in microwave-assisted syn-
thesis. In their microwave-assisted synthesis of thiocar-
bonyls using a polymer-supported thionating reagent,
Ley et al. show that addition of a small quantity of an
ionic liquid to a toluene solution can greatly increase the
rate and yields of reaction.7 Ionic liquids made of organic
cations and appropriate anions have attracted much
recent attention as solvents for chemistry because of the
fact that they have melting points close or near to room
temperature.8 It has also been shown that they can be
used as reactants.9 They have negligible vapor pressure
and are immiscible with a range of organic solvents
meaning that organic products can be easily removed and
the ionic liquid can be recycled. From the perspective of
microwave chemistry one of the points of key importance
is their high polarity and that this is variable depending
on the cation and anion so can effectively be tuned to a
particular application. Despite this, application of micro-
wave chemistry to reactions using ionic liquids has not
been exploited on many occasions.10
The use of microwave ovens as tools for synthetic
chemistry is a fast growth area.1,2 Since the first reports
of microwave-assisted synthesis in 1986,3,4 the technique
has been accepted as a method for reducing reaction
times often by orders of magnitude and for increasing
yields of product compared to conventional methods.5,6
As a result, this has opened up the possibility of optimiz-
ing new reactions in a very short time. A key advantage
of modern scientific microwave apparatus is the ability
to control reaction conditions very specifically, monitoring
temperature, pressure, and reaction times. Several meth-
ods have been developed for performing reactions using
microwaves including using solvent-free conditions or
adsorbing reactants onto inorganic supports such as
silicas or clays. If the reaction needs to be carried out in
a solvent, the medium needs to have a high dielectric
constant (ꢀ) in order to take advantage of the microwave
heating effect. To this end, solvents such as 1-methyl-2-
pyrrolidone (NMP) (ꢀ ) 32.2), DMSO (ꢀ ) 46.7), DMF (ꢀ
) 36.7), and other high boiling polar solvents are often
used. Although these are excellent solvents for perform-
ing the reaction, the subsequent workup procedure is
complicated by the need to remove the solvent at the end
of the reaction. In addition, there are situations where
it would be desirable to perform the reaction in less polar
solvents such as hexane (ꢀ ) 1.88), toluene (ꢀ ) 2.38),
In this note we report the results of our investigations
building on the idea of using ionic liquids as aids for
microwave heating of nonpolar solvents. We show that
hexane and toluene as well as with solvents such as THF
and dioxane can be heated way above their boiling point
in sealed vessels using a small quantity of an ionic liquid,
thereby allowing them to be used as media for microwave-
assisted chemistry. Our attention has focused not only
on the heating effects but also on studying the contami-
nation, if any, of the parent solvent with the ionic liquid
or any decomposition products formed as they are heated.
In such cases, there would clearly be a disadvantage to
using this route, especially if the decomposition products
interact with the reaction mixture to stop the reaction
or lead to unwanted byproducts.
* Tel: ++44 (0)20 7848 1147; fax: ++44 (0)20 7848 2810.
(1) For reviews on the area see: (a) Lindstro¨m, P.; Tierney, J .;
Wathey, B.; Westman, J . Tetrahedron 2001, 57, 9225. (b) Perreux, L.;
Loupy, A. Tetrahedron 2001, 57, 9199. (c) Deshayes, S.; Liagre, M.;
Loupy, A.; Luche, J .-L.; Petit, A. Tetrahedron 1999, 55, 10851. (d)
Strauss, C. R. Aust. J . Chem. 1999, 52, 83. (e) Varma, R. S. Green
Chem. 2001, 3, 98. (f) Galema, S. A. Chem. Soc. Rev. 1997, 26, 233.
(2) For reviews on the concepts, see: (a) Gabriel, C.; Gabriel, S.;
Grant, E. H.; Halstead, B. S.; Mingos, D. M. P. Chem. Soc. Rev. 1998,
27, 213. (b) Mingos, D. M. P. Chem. Soc. Rev. 1991, 20, 1.
(3) Gedye, R.; Smith, F.; Westaway, K.; Humera, A.; Baldisera, L.;
Laberge, L.; Rousell, L. Tetrahedron Lett. 1986, 27, 279.
(4) Giguere, R.; Bray, T. L.; Duncan, S. M.; Majetich, G. Tetrahedron
Lett. 1986, 27, 4945.
Our starting point was to prepare and screen a range
of 1,3-dialkylimidazolium halides (Figure 1 entries 1, 2,
and 3) for the heating of organic solvents. The ionic
liquids were prepared by literature methods from alkyl
halides and N-methylimidazole. 1 and 2 were prepared
(7) Ley, S. V.; Leach, A. G.; Storer, R. I. J . Chem. Soc., Perkin Trans.
1 2001, 358
(8) For recent reviews, see: (a) Welton, T. Chem. Rev. 1999, 99, 2071.
(b) Wasserscheid, P. and Keim, W. Angew. Chem., Int. Ed. 2000, 39,
3772.
(9) Ren, R. X.; Wu, J . X. Org. Lett. 2001, 3, 3727.
(10) (a) Varma, R. S.; Namboodiri, V. V. Chem. Commun. 2001, 643.
(b) Fraga-Dubreuil, J .; Bazureau, J .-P. Tetrahedron Lett. 2001, 42,
6097. (c) Fraga-Dubreuil, J .; Bazureau, J .-P. Tetrahedron Lett. 2000,
41, 7351. (d) Westman, J . Patent WO0072956, 2000.
(5) For some recent examples, see: (a) Westman, J . Org. Lett. 2001,
3, 3745. (b) Kuhnert, N.; Danks, T. N.; Green Chem. 2001, 3, 98. (c)
Loupy, A.; Regnier, S. Tetrahedron Lett. 1999, 40, 6221. (d) Danks, T.
N. Tetrahedron Lett. 1999, 40, 3957.
(6) Stadler, A.; Kappe, A. C.; Eur. J . Org. Chem. 2001, 919.
10.1021/jo016297g CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/09/2002

3146 J . Org. Chem., Vol. 67, No. 9, 2002
Notes
Ta ble 1. Th e Micr ow a ve Hea tin g Effects of Ad d in g a
Sm a ll Qu a n tity of 1 a n d 2 to Hexa n e, Tolu en e, THF , a n d
Dioxa n ea
studied, temperatures well in excess of the boiling points
being achieved rapidly. CAUTION!!15 The effects of
varying the quantity of ionic liquid used in the solvent
heating was investigated. We found that for heating 2
mL of solvent, 0.2 mmol ionic liquid was the optimal
amount, any more resulting in too rapid heating and the
heating being too slow when using any less.
ionic
liquid
added
temp
time
T without
boiling
point,
°Cc
solvent
used
attained, taken, ionic liquid,
°C
s
°Cb
46
hexane
toluene
THF
1
2
1
2
1
2
1
2
217
228
195
234
268
242
264
246
10
15
150
130
70
60
90
90
69
111
66
To determine whether there is any contamination of
the solvent by the ionic liquid, the solution was decanted
off from the ionic liquid and the solvent removed under
vacuum. The flask was then washed with a small volume
of CDCl3 and the 1H NMR of the whole concentrate
recorded. In all cases some level of contamination was
found, the level being directly correlated to the temper-
ature attained in the microwave heating experiment.
Further investigation showed that the contamination was
coming from decomposition of the ionic liquids over time
at elevated temperatures. We find that although 1 and
2 can be used for microwave heating of solvents to around
160 °C, increasing the temperature much above this leads
to the onset of some decomposition with major decom-
position occurring above 230 °C. Our studies show that
the ionic liquids decompose to give methyl iodide and
n-propylimidazole in the case of 1, and methyl bromide
and isopropylimidazole in the case of 2. This decomposi-
tion at elevated temperatures is not totally unexpected.
Vacuum pyrolysis of 1-methyl-3-alkylimidazolium halides
at between 300 and 600 °C leads to a similar decomposi-
tion, again forming the corresponding methyl halide and
alkylimidazole.16 In all these cases, the halide ion, X-,
presumably acts as a nucleophile in attacking the cation
with the subsequent elimination of MeX.
To avoid problems of contamination by decomposition
to alkyl halides and alkyl imidazoles, a range of other
ionic liquids were prepared by anion metathesis of 1, 2,
and 3. Using literature methods, treatment of 1, 2, or 3
in water with HPF6 leads readily to the corresponding
PF6-coordinated ionic liquids 4, 5, and 6, respectively.17
Treatment of 1 with NaBF4 leads to the corresponding
BF4 coordinated ionic liquid 7.18 The PF6- and BF4-
coordinated ionic liquids 4-7 would be expected to be
more thermally stable to decomposition and hence may
prove more useful in microwave heating experiments. To
assess their potential, again the microwave heating
effects of adding a small quantity of ionic liquid to
solutions of hexane, toluene, THF, and dioxane were
investigated. As with 1 and 2, dramatic heating effects
were observed with 4, 5, and 7. A summary of the data
is shown in Table 2. The results show that both 4 and 7
prove useful in microwave heating of solvents, with 4
being more effective. Indeed it was necessary to reduce
the microwave power to half that used in the other
experiments (i.e., 100 W) in order to stop the solvents
reaching temperatures well in excess of 300 °C within a
few seconds. There is no contamination19 of the solvent
109
112
76
dioxane
101
a
Experiments run using a microwave irradiation power of 200
W. b Temperature attained during the same microwave irradiation
time but without any ionic liquid added. c Boiling point of the
solvent used (for comparison purposes).
by heating a toluene solution of methyl imidazole and
alkyl halide,11 and 3 was prepared by refluxing diiodo-
methane and methylimidazole in THF for 2 days.12 When
purified, 1 and 2 are liquids at room temperature while
3 is a solid, melting at 260 °C.
Once prepared and purified, the microwave heating
properties of the ionic liquids were investigated in the
absence of any other solvent. Reactions were performed
in sealed vessels so that any pressure build-up could be
monitored. With 1 and 2, using 100 W irradiation power,
the preselected maximum temperature of 300 °C was
reached within 15 s. For such rapid heating in common
organic solvents, the term microwave flash heating has
recently been introduced by Hallberg and co-workers.13
To avoid the formation of hot-spots, the irradiation of the
ionic liquids was accompanied by rapid stirring using a
Teflon-coated stirring bar. The fact that ionic liquids have
negligible vapor pressure even at elevated temperatures
was clearly evidenced in our experiments, no significant
pressure rise being noted during the course of the
irradiation. These results indicate that ionic liquids may
offer an alternative to solvents such as NMP6 and DMF14
for flash heating organic compounds. In the case of 3,
using 300 W irradiation power, it took 4 min to reach
the preselected maximum temperature of 300 °C. This
lengthy time is because the solid first has to start to melt.
Once melting has started, the temperature rise is very
dramatic, moving from 200 °C to 300 °C within 2 s.
Using 1 and 2, the microwave heating effects of adding
a small quantity of ionic liquid to solutions of hexane,
toluene, THF, and dioxane were investigated. The tem-
peratures attained and time taken to reach this point are
tabulated in Table 1. As a comparison, temperatures
reached in the absence of ionic liquids are also shown.
The experiments were run using 0.2 mmol (between 10
and 55 mg) of ionic liquid and 2 mL of solvent. All
experiments were run using an irradiation power of 200
W. From the table it is clearly seen that the ionic liquids
have a dramatic effect on the heating of all the solvents
(15) The solvents are heated well above their boiling points so all
necessary precautions should be taken when performing such experi-
ments. Vessels designed to withhold elevated pressures must be used.
The microwave apparatus used here incorporates a protective metal
cage around the microwave vessel in case of explosion. After completion
of an experiment, the vessel must be allowed to cool to a temperature
below the boiling point of the solvent before removal from the
microwave cavity and opening to the atmosphere.
(16) Begg, C. G.; Grimmett, M. R.; Wethey, P. D. Aust. J . Chem.
1973, 26, 2435.
(17) Huddleston, J . G.; Willauer, H. D.; Swatloski, R. P.; Visser, A.
E.; Rogers, R. D. Chem. Commun. 1998, 1765.
(11) (a) Wilkes, J . S.; Levisky, J . A.; Wilson, R. A.; Hussey, C. L.
Inorg. Chem. 1982, 21, 1263. (b) Chan, B. K. M.; Chang, N.-H.;
Grimmett, R. M. Aust. J . Chem. 1977, 30, 2005.
(12) Herrmann, W. A.; Goossen, L. J .; Kocher, C.; Artus, G. R. J ..
Angew. Chem., Int. Ed. Engl. 1996, 35, 2805.
(13) (a) Olofson, K.; Kim, S. Y.; Larhed, M.; Curran, D. P.; Hallberg,
A. J . Org. Chem. 1997, 62, 5583. (b) Vallin, K. S.; Larhed, M.;
J ohansson, K.; Hallberg, A. J . Org. Chem. 2000, 65, 4537.
(14) (a) Mergler, M. Nyfeler, R.; Gosteli, J .; Tanner, R. Tetrahedron
Lett. 1989, 30, 6745. (b) Hanessian, S.; Yang, R.-Y. Tetrahedron Lett.
1996, 37, 5835.
(18) Hirao, M.; Ito, K.; Ohno, H. Electrochim. Acta 2000, 45, 1291.

Notes
Ta ble 2. Th e Micr ow a ve Hea tin g Effects of Ad d in g a
J . Org. Chem., Vol. 67, No. 9, 2002 3147
Sm a ll Qu a n tity of 4, 5, a n d 7 to Hexa n e, Tolu en e, THF ,
a n d Dioxa n ea
ionic
liquid
added
temp
attained,
°C
time
taken,
s
solvent
used
level of
contamination
hexane
4
5
7
4
5
7
4
7
4
7
279
90
192
280
79
165
231
95
149
184
20
300
60
60
120
90
60
50
100
120
none
none
none
none
toluene
none
contaminated
none
THF
contaminatedb
none
dioxane
F igu r e 2. Reactions studied for assessment of the microwave
contaminated
heating method for synthesis.
a
Experiments run using a microwave irradiation power of 100
b
W. 7 is slightly soluble in THF and so cannot totally be removed;
so contamination is due to 7 rather than decomposition.
To show that these ionic liquids can be used to heat
solvents for extended times, samples of hexane and
toluene containing 4 was heated to 300 °C and held at
this temperature for 30 min. Analysis of the solvent at
the end of the heating showed no contamination.
Ta ble 3. Th e Micr ow a ve Hea tin g Effects of Ad d in g a
Sm a ll Qu a n tity of P r em elted a n d Cooled 5 a n d 6 to
Hexa n e, Tolu en e, THF , a n d Dioxa n ea
ionic
liquid
added
temp
attained,
°C
time
taken,
s
Ley et al. have already demonstrated the potential of
the ionic liquid heating method in synthesis,7 but to
expand on this we have screened three further reactions
to show the applicability of the methodology (Figure 2).
The first reaction chosen was the Diels-Alder reaction
between 2,3-dimethylbutadiene and methyl acrylate to
give the [4 + 2] adduct 8.21 This reaction is traditionally
performed in toluene or xylene and takes between 18 and
24 h to reach completion, giving yields of 8 varying from
9 to 90% depending on the solvent used and the temper-
ature at which the reaction is run.22,23,24 Even when using
catalysts such as AlCl3, the reaction still takes 5 h to
reach 73% yield.25 Using a mixture of toluene and 5 we
were able to prepare 8 in 5 min in 80% yield, this offering
a significant rate enhancement over the conventional
methods. During the reaction, a microwave power of 100
W was used and the temperature of the mixture held at
200 °C. The NMR spectrum of the crude product shows
that the ionic liquid used does not decompose during the
reaction or contaminate the product mixture. In a control
experiment, the reaction was repeated in the absence of
5, and it was found that after the same time (5 min at
100 W microwave power) there was no product formed.
Interestingly this same reaction has been reported previ-
ously using microwave irradiation with xylene as solvent,
but the time taken to reach the same level of product
yield is well in excess of 3 h, presumably because the
reaction mixture was heated only to 95 °C.26,27
solvent
used
level of
contamination
hexane
toluene
THF
5
6
5
6
5
6
5
6
243
300
278
291
181
181
209
233
40
120
50
120
60
120
60
120
none
none
none
none
contaminatedb
none
dioxane
contaminatedb
none
a
Experiments run using a microwave irradiation power of 100
W. Contamination is due to trace of 5 in solution rather than
decomposition.
b
when using 4 with any of the solvents screened or when
7 is used with hexane. There is contamination due to
decomposition of 7 when used with toluene or dioxane,
the extent being much less in the case of the latter. 7 is
slightly soluble in THF, so in this case the only source of
contamination at the end of the heating experiments is
a trace of the parent ionic liquid rather than any
decomposition.
When using freshly prepared samples of 5 and 6 for
microwave heating experiments, poor results are ob-
tained. This is because they are higher melting ionic
liquids (mp s 80 °C and 185 °C, respectively, for 5 and 6)
and, in the presence of the solvents, do not get to a
significant temperature at which they can melt. In a
related set of experiments we found that if a freshly
prepared sample of 5 is heated to melting point on its
own in the microwave and then allowed to cool back to
room temperature, then it undergoes a phase change and
melts at a much lower temperature on heating for a
second time. A similar observation was made with 6.20
We therefore assessed the heating capabilities of samples
of 5 and 6 which had been melted and then cooled before
adding the solvent of choice. As shown in Table 3, the
effects are significant offering the best microwave heating
of all the ionic liquids screened. Again there is little or
no decomposition of the ionic liquids and hence little or
no contamination of the solvent.
The second reaction chosen for study was the base-
catalyzed Michael addition of methyl acrylate to imid-
azole to yield 9. We chose this reaction, as we thought it
would be a test for whether the ionic liquid was indeed
acting solely as a heater in the reaction rather than
(21) It should be noted that rate acceleration in other Diels-Alder
reactions via microwave irradiation has been observed before, but the
synthetic procedure is often not as convenient as presented here. For
examples, see refs 1a and 1b.
(22) Monin, J . Helv. Chim. Acta 1958, 41, 2112
(23) Vedejis, E.; Gadwood, R. C. J . Org. Chem. 1978, 43, 376
(24) Fang, X.; Wamer, B. P.; Watkin, J . G. Synth. Commun. 2000,
30, 2669.
(25) Inukai, T.; Kasai, M. J . Org. Chem. 1965, 30, 3567
(26) Berlan, J .; Giboreau, P.; Lefeuvre, S. Marchand, C. Tetrahedron
Lett. 1991, 32, 2363.
(27) The authors do not say whether this is the maximum temper-
ature obtainable because of the use of a nonpolar solvent or if this
was the temperature chosen for the study.
(19) Experiments suggest that NMR is sensitive to very low levels
of contaminants so is a useful assay tool.
(20) Such phase changes have also been noted with other higher
melting point ionic liquids. See for example: Larsen, A. S.; Holbrey,
J . D.; Tham, F. S.; Reed, C. A. J . Am. Chem. Soc. 2000, 122, 7264.

3148 J . Org. Chem., Vol. 67, No. 9, 2002
Notes
delivery system with operator selectable power output from 0
to 300 W. Reactions were performed in glass vessels (capacity
10 mL) sealed with a septum. The pressure is controlled by a
load cell connected to the vessel via a 14-gauge needle which
penetrates just below the septum surface. The temperature of
the contents of the vessel was monitored using a calibrated
infrared temperature control mounted under the reaction vessel.
Dried solvents were used in every case. All experiments were
performed using a stirring option whereby the contents of the
vessel are stirred by means of a rotating magnetic plate located
below the floor of the microwave cavity and a Teflon-coated
magnetic stir bar in the vessel. All chemicals were reagent grade
becoming chemically involved, as there would be scope
for reaction between the ionic liquid and the base or
acrylate during the course of the reaction. This reaction
has already been performed using microwave irradiation,
using basic clays (Li+ and Cs+ montmorillonites) as
catalysts.28 We replaced the clays by triethylamine and
found that using a mixture of toluene and 5 we were able
to prepare 9 in 2 min in an isolated yield of 75%. During
the reaction, a microwave power of 100 W was used and
the temperature of the mixture held at 200 °C. Again,
the NMR spectrum of the crude product mixture shows
that the ionic liquid used does not decompose during the
reaction or contaminate the product mixture. This result
is interesting in that it shows that we could not only
improve reaction times and yields over the conventional
methods (no product observed after this time) but also
over the previously reported microwave method (yields
of 40-50% being obtained after 3 min irradiation). In a
control experiment, the reaction was repeated first in the
absence of 5 and toluene and second in the absence of
toluene, and in both cases it was found that after the
same time (2 min at 100 W microwave power) there was
no product formed.
1
and used as purchased. The H and 31P{1H} NMR spectra were
recorded at 250 MHz and 293 K and referenced to TMS and
H3PO4 respectively.
Rep r esen ta tive P r oced u r e for Stu d y of th e Micr ow a ve
Hea tin g Effects of Ion ic Liqu id s. A small quantity of ionic
liquid (20-50 mg) was placed in a microwave vessel containing
a Teflon-coated magnetic stirrer bar. To this was added 2 mL of
dried solvent of choice. The vessel was then sealed with a septum
and the experiment undertaken. For safety, the maximum
pressure and temperature were set to 200 bar and 300 °C,
respectively. A microwave irradiation power of either 100 or 200
W was used for the experiments depending on the ionic liquid
used. The power used was governed by the results of experi-
ments heating the ionic liquid alone, the power being ramped
up in increments until a suitable and controllable rate of heating
was observed.
The third reaction studied was the alkylation of pyr-
azoles with alkyl halides. This again is a reaction that
has been studied previously using microwave irradia-
tion.29 We find that this reaction is not possible using
our experimental protocol, the aryl halide reacting with
the ionic liquid at the elevated temperatures used in the
reaction. Although we have not managed to characterize
the reaction products, it is clear to see that all the ionic
liquid is destroyed since the biphasic starting mixture
(solvent and ionic liquid) becomes a monophasic mixture
after just a few seconds of microwave irradiation. This
shows the limitations of our protocol; it not being possible
to undertake reactions which use or generate nucleo-
philes such as halide ions. Although this removes a
number of possible reactions from the synthetic chemist’s
portfolio, the success of the other reactions does show the
potential of the technique to preparative chemistry.
In conclusion, we have shown that hexane and toluene
together with solvents such as THF and dioxane can be
heated way above their boiling point in sealed vessels
using a small quantity of an ionic liquid, thereby allowing
them to be used as media for microwave-assisted chem-
istry. With careful choice of ionic liquid, contamination
of the solvent can be avoided. In the cases of the solid
ionic liquids studied we find that if they are melted and
allowed to cool they undergo a phase change and melt at
a much lower temperature on heating for a second time
and can be very powerful microwave heating agents. We
have shown the potential of this heating method to
synthesis and believe that it could offer significant
advantages to preparative chemists attempting to im-
prove rates and yields of reactions best run in nonpolar
solvents.
P r ep a r a tion of 8. To a microwave vessel containing 2 mL
of toluene and 5 (55 mg) was added 2,3-dimethylbutadiene (0.23
mL, 2 mmol) and methyl acrylate (0.18 mL, 2 mmol). A Teflon-
coated magnetic stirrer bar was added and the vessel sealed.
The sample was irradiated for 5 min using a power of 100 W.
For safety, the maximum pressure was set to 200 bar. After
being cooled to room temperature, the vessel was opened and
the toluene layer removed using a pipet. The ionic liquid layer
was washed with toluene, and these washings were combined
with the original toluene layer. Removal of the solvent resulted
in a light yellow oil, the NMR spectrum of which showed product
but no evidence for ionic liquid or its decomposition products. A
yield of 8 of 80% was obtained, the product being chracterized
by comparison of 1H and 13C NMR data with that in the
literature.31
P r ep a r a tion of 9. To a microwave vessel containing 2 mL
of toluene and 5 (55 mg) were added imidazole (135 mg, 2 mmol),
methyl acrylate (0.18 mL, 2 mmol), and triethylamine (0.28 mL,
2 mmol). A Teflon-coated magnetic stirrer bar was added and
the vessel sealed. The sample was irradiated for 2 min using a
power of 100 W. For safety, the maximum pressure was set to
200 bar. After cooling to room temperature, the vessel was
opened and the toluene layer removed using a pipet. The ionic
liquid layer was washed with toluene, and these washings were
combined with the original toluene layer. Removal of the solvent
resulted in a light yellow residue, the NMR of which showed a
mixture of starting materials and product but no evidence for
ionic liquid or its decomposition products. Washing the yellow
residue with acetone led to a light yellow oil, characterized as 9
by comparison of 1H and 13C NMR data with that in the
literature (150 mg, 75%).32
Ack n ow led gm en t. The Royal Society is thanked for
a University Research Fellowship (N.E.L.). King’s Col-
lege London is thanked for a studentship (H.M.T.), and
Hoffmann La Roche and the Royal Society are thanked
for funding.
Su p p or tin g In for m a tion Ava ila ble: Syntheses and spec-
troscopic data for the ionic liquids used in these studies (1-
7). This material is available free of charge via the Internet
at http://pubs.acs.org.
Exp er im en ta l Section
Gen er a l. Microwave experiments were conducted using a
CEM Discover Synthesis Unit (CEM Corp., Matthews, NC).30
The machine consists of a continuous focused microwave power
J O016297G
(28) Mart´ın-Aranda, R. M.; Vincente-Rodr´ıguez, M. A.; Lo´pez-
Pestan˜a, J . M.; Lo´pez-Peinado, A. J .; J erez, A.; Lo´pez-Gonza´lez, J . de
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