Ceric ammonium nitrate (CAN) as oxidizing or nitrating reagent for organic reactions in ionic liquids

Article abstract of DOI:10.1016/j.tetlet.2009.05.093

The reaction of ceric ammonium nitrate, (NH₄)₂[Ce(NO₃)₆] or CAN, with naphthalene and 2-methylnaphthalene in the ionic liquid 1-ethyl-3-methylimidazolium triflate showed that the reaction products are strongly d

Full text of DOI:10.1016/j.tetlet.2009.05.093

Tetrahedron Letters 50 (2009) 4582–4586
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Ceric ammonium nitrate (CAN) as oxidizing or nitrating reagent for organic
reactions in ionic liquids
a
b
b
a
a,*
Karen Deleersnyder , Stijn Schaltin , Jan Fransaer , Koen Binnemans , Tatjana N. Parac-Vogt
a
Katholieke Universiteit Leuven, Department of Chemistry, Celestijnenlaan 200F—bus 2404, B-3001 Heverlee, Belgium
Katholieke Universiteit Leuven, Department MTM, Kasteelpark Arenberg 44—bus 2450, B-3001 Heverlee, Belgium
b
a r t i c l e i n f o
a b s t r a c t
Article history:
4 2 3 6
The reaction of ceric ammonium nitrate, (NH ) [Ce(NO ) ] or CAN, with naphthalene and 2-methylnaph-
Received 14 April 2009
Revised 18 May 2009
Accepted 25 May 2009
Available online 29 May 2009
thalene in the ionic liquid 1-ethyl-3-methylimidazolium triflate showed that the reaction products are
strongly dependent on the water content of the ionic liquid and that cerium(IV) in the ionic liquid can
electrochemically be regenerated.
Ó 2009 Published by Elsevier Ltd.
Ammonium hexanitratocerate(IV) is without doubt the most
important cerium(IV) reagent for organic synthesis.1 This com-
pound is known to most organic chemists as ceric ammonium ni-
tions in apolar solvents, CAN is used as a suspension; thus under
heterogeneous reaction conditions.
–7
Although ionic liquids have been used as an alternative solvent
2
6–31
trate, or as CAN for short. The formula (NH
4
)
2
[Ce(NO
3
)
6
] reflects
for a plethora of organic reactions,
only very few studies of
that the cerium(IV) ion is surrounded by six nitrate groups and that
the ammonium ions are counter ions to compensate for the nega-
tive charge of the hexanitratocerate(IV) coordinating unit. CAN is a
one-electron oxidation reagent. Typical examples are the oxidation
cerium(IV)-mediated reactions in ionic liquids have been de-
3
2
scribed. Bar et al. have studied CAN-mediated oxidative free rad-
ical reaction in the ionic liquid 1-butyl-3-methylimidazolium
3
3
tetrafluoroborate. Mehdi et al. screened different cerium(IV) salts
8
9
34
of benzyl alcohol to benzaldehyde, of toluene to benzaldehyde, of
and imidazolium ionic liquids for oxidation reactions. They found
10
naphthalene to 1,4-naphthoquinone, and of 1,4-hydroquinones
that ceric ammonium nitrate and cerium(IV) triflate are the most
suitable cerium(IV) salts and that 1-alkyl-3-methylimidazolium
triflates are the preferred ionic liquids. In present Letter, the reac-
tion of CAN with naphthalene and 2-methylnaphthalene in the io-
1
1,12
to the corresponding 1,4-quinones.
nitrating agent,
CAN can also be used as
1
3,14
15
initiator for radical polymerization reactions,
or reagent to remove protecting groups.
16,17
Cerium(IV) reagents
are relatively mild oxidation reagents in comparison with other
metal-based oxidation reagents such as potassium permanganate
or chromium(VI) salts. Moreover, cerium(IV) salts are much less
toxic than chromium(VI) salts. Because cerium(IV) salts are one-
electron oxidizing agents and because they have a relatively high
molecular mass, large quantities of cerium(IV) salts are required
2
nic liquid 1-ethyl-3-methylimidazolium triflate, [C mim][OTf] is
described. The purpose of this study was to show how the
chemoselectivity of CAN-mediated reactions can be altered by
changing the water content of the ionic liquid. Moreover, we give
a
proof-of-principle for the electrochemical regeneration of
cerium(IV) in the ionic liquid.
for stoichiometric reactions. Therefore, indirect and catalytic reac-
The ionic liquid selected for this work was 1-ethyl-3-methyl-
imidazolium triflate, which will be abbreviated throughout this
tions have been developed.1
8–21
Of special interest is cerium-med-
iated electro-synthesis, where the redox active cerium(IV) species
2
study as [C mim][TfO] (Fig. 1). The selection was based on the fact
are electrochemically regenerated.2
2–25
The main advantage of
that previous screening of imidazolium ionic liquids in our labora-
tory has revealed that ceric ammonium nitrate is well soluble in
CAN over other cerium(IV) reagents is its higher solubility in or-
ganic solvents, but the reagent suffers sometimes from unwanted
side reactions. Different solvents can be selected for reactions with
ammonium hexanitratocerate(IV) as reagent. The most popular
solvents are (in decreasing order of importance) water, acetonitrile,
3
4
this ionic liquid. [C
2
mim][TfO] is a hydrophilic ionic liquid and
mim][TfO] is a room temper-
is in all ratios miscible with water. [C
2
ature ionic liquid (melting point = ꢀ9 °C) and its viscosity is 45 cP
3
5
at 25 °C. As the organic substrates, naphthalene and 2-methyl-
6
dichloromethane, THF and methanol. Often mixtures of these sol-
naphthalene were selected. It is known that cerium(IV) salts can
vents are used. Other solvents have found only marginal use for
this type of reactions. It should be mentioned that in many reac-
-
CF SO
3
3
+
N
N
*
Corresponding author. Tel.: +32 16 7612; fax: + 32 16 32 79 92.
E-mail address: Tatjana.Vogt@chem.kuleuven.be (T.N. Parac-Vogt).
Figure 1. Structure of the ionic liquid 1-ethyl-3-methylimidazolium triflate,
[C mim][TfO].
2
0
040-4039/$ - see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2009.05.093

K. Deleersnyder et al. / Tetrahedron Letters 50 (2009) 4582–4586
4583
oxidize naphthalene to 1,4-naphthoquinone in aqueous sol-
NO2
3
6,10
vents.
Nitration of naphthalene derivatives by CAN has been
NO
2
CAN
reported for CAN suspended in dichloromethane in the presence
of 2 equiv of sulfuric acid,37 and for CAN dissolved in acetonitrile.14
The oxidation product of 2-methylnaphthalene is 2-methyl-1,4-
naphthoquinone, which is better known as menadione or vitamin
[
C mim][OTf]
2
1
2
3
K
3
. It is being used as a supplement for the vitamins K
animal feed and it is also a synthetic intermediate of vitamin K.
Vitamin K is produced on an industrial scale by oxidation of 2-
methylnaphthalene with stoichiometric amounts of chromium(VI)
1 2
and K in
O
O
O
O
OH
OH
3
3
8
oxide in sulfuric acid. Other procedures to obtain menadione use
catalytic amounts of metal derivatives in combination with strong
oxidizing agents such as hydrogen peroxide and periodic acid.
5
3
9,40
4
Non-catalytic oxidation of 2-methylnaphthalene has also been re-
ported, but such procedures require potent mixtures of strong
acids. Earlier work has shown that cerium(IV) salts can also oxidize
2
Scheme 1. Reaction of CAN with naphthalene (1) in the ionic liquid [C mim][OTf],
leading to the formation of 1-nitronaphthalene (2), 2-nitronaphthalene (3), 1,4-
naphthoquinone (4) and phthalic acid (5).
2
2
2
-methylnaphthalene to 2-methyl-1,4-naphthoquinone.
In Table 1, the experiments involving the reaction between CAN
and naphthalene in [C
mim][TfO] are summarized.41 In dry ionic
water required) can efficiently compete with the oxidation reac-
tion (water required).
2
liquid, nitration is the predominant reaction. The nitration of naph-
thalene results in the formation of two isomers: 2-nitronaphtha-
lene and 1-nitronaphthalene, with 1-nitronaphthalene being the
major component. Notice that for nitration of naphthalene, a 1:1
molar ratio CAN: naphthalene is sufficient. Although at higher
CAN:naphthalene ratios (up to 6:1) the conversion of naphthalene
is higher, the amount of side products increases. These side prod-
ucts are 1,4-naphthoquinone and phthalic acid (identified by GC–
MS) (Scheme 1). The formation of 1,4-naphthoquinone and phtha-
lic acid requires an oxygen source. However, the oxygen atoms are
provided by the traces of water left in the ionic liquids. It was
In addition, complete conversion was already obtained after 1 h
using the higher reaction temperatures. This clearly illustrates the
advantage of performing reactions in ionic liquids at elevated
temperatures.
In order to obtain a reasonable yield of 1,4-naphthoquinone,
water has to be added to the ionic liquid. Moreover, oxidation of
naphthalene into 1,4-naphthoquinone requires 6 equiv of
cerium(IV) salt, because cerium(IV) is a one-electron oxidant,
whereas nitration can be accomplished with only 1 equiv of CAN
as the nitrating agent. More than the theoretically necessary
amount of water (2 equiv) is beneficial for the formation of
1,4-naphthoquinone, but the reaction in pure water is not very
selective toward the formation of 1,4-naphthoquinone. The addi-
tion of ionic liquid to pure water increases the solubility of naph-
thalene and consequently the oxidation proceeds smoother. The
CAN-mediated reaction of naphthalene was also performed with
compressed air as the oxygen source. When air was bubbled
through the reaction mixture, complete conversion was obtained,
but 78% of the naphthalene was nitrated while only 11% of 1,4-
naphthoquinone and 11% of phthalic acid were formed. Hence,
the oxidation of naphthalene toward 1,4-naphthoquinone pro-
ceeds better in the presence of water. The highest amount of 1,4-
naphthoquinone was obtained after short reaction times. The long-
er the reaction proceeded, the more competition with nitration
was observed. It should be noted that in none of the experiments
1,4-naphthoquinone could be obtained in a high yield. There was
always a competition with nitration when CAN was used as the
oxidizing agent.
found that the hydrophilic ionic liquid [C
cult to dry. Initially, the ionic liquid contained 0.05 mass% of water
determined by Karl Fischer titration). After drying the ionic liquid
at 80 °C under vacuum or freeze-drying it for several hours, still
.03 mass % was present in the ionic liquid. Consequently, we were
2
mim][OTf] is very diffi-
(
0
not able to make this hydrophilic ionic liquid completely anhy-
drous by our drying procedures. This explains why we always no-
ticed trace amounts of oxidation products under conditions where
complete nitration was expected. However, the amount of side
products could be reduced by increasing the reaction temperature.
While still 7% of oxidation products (1,4-naphthoquinone and
phthalic acid) was observed for the nitration reaction of naphtha-
lene at 100 °C, only 2% of oxidation products was detected when
performing the same reaction at 150 °C or 180 °C. It is very likely
that the high yield for the nitration reaction at elevated tempera-
tures is due to the fact that the ionic liquid has a very low water
content at these temperatures, so that the nitration reaction (no
Table 1
Reactions of CAN with naphthalene in the ionic liquid [C
2
mim][OTf]a
O added (mL) Time (min)
Entry
CAN (equiv)
amount of H
2
Temperature (°C)
Conversion (%)
Product distribution (%)c
1-Nitro
2-Nitro
NQ
PA
1
2
3
4
5
6
7
8
9
1
1
1
1
1
6
6
6
6
6
0
0
0
0
1
1
0
1
5
180
60
60
60
60
60
30
30
30
30
100
100
150
180
100
100
100
100
100
100
88
52
89
86
90
90
16
62
79
46
21
45
7
7
8
8
0
5
4
3
1
3
2
4
1
2
3
1
2
0
1
11
1
0
13
>98
>98
7
>98
>98
96
0
84
32
6
50
78
39
68
79
b
1
0
5
a
1
mmol of naphthalene in 5 g of [C
without ionic liquid, naphthalene does not dissolve in water.
-nitro: 1-nitronaphthalene; 2-nitro: 2-nitronaphthalene; NQ: 1,4-naphthoquinone; PA: phthalic acid.
2
mim][OTf].
b
c
1

4
584
K. Deleersnyder et al. / Tetrahedron Letters 50 (2009) 4582–4586
Separation of the reaction products from the ionic liquid solvent
after reaction was not easy. We found that extraction with toluene
was the best option.
regenerating cerium(IV) in the ionic liquid phase which remained
after extraction of the reaction products with toluene.
The initial electrochemical studies consisted of cyclic voltam-
In the CAN-mediated reactions of 2-methylnaphthalene in [C
2
mi-
2
metry measurements on the pure ionic liquid [C mim][OTf] and
4
2
m][OTf] only 2-methyl-1,4-naphthoquinone was observed as the oxi-
dation product, and no oxidation of the methyl group was observed.
Moreover, depending on the reaction conditions, various nitrated 2-
methylnaphthalenes were obtained. Among these isomers, 1-nitro-
on a solution of CAN in the ionic liquid. The electrochemical win-
dow of the ionic liquid was approximately 4 V, which is in agree-
4
3
ment with literature data. Cyclic voltammograms of solutions
of 0.1 M, 0.2 M and 0.3 M of CAN in [C mim][OTf] were measured
2
2
(
-methylnaphthalene was always formed as the major product
Scheme 2). This is in agreement with various other nitration proce-
dures of 2-methylnaphthalene using nitric acid, wherein 2-methyl-
-nitronaphthaleneisformedinthe largestquantity (50–60%)among
and compared with the pure ionic liquid (see Fig. 3). The peak cur-
rents for the oxidation and reduction processes increase linearly
with concentration up to 0.2 M. The peak current in the cyclic vol-
tammogram labeled 0.3 M is not three times the value of the cyclic
voltammogram with 0.1 M due to incomplete dissolution of CAN.
In order to have more detailed information on the position of the
oxidation and reduction peaks, experiments were performed using
1
the six occurring nitrated isomers. The other isomers were formed in
the following order: 7-methyl-1-nitro > 3-methyl-1-nitro > 6-
methyl-1-nitro > 2-methyl-6-nitro > 2-methyl-3-nitro (in trace
amounts). The experimentsinvolving 2-methylnaphthalene are sum-
marizedinTable2. Figure2givesanoverview of thedifferentisomers.
In order to obtain oxidation of naphthalene and 2-methylnaph-
thalene in the absence of the competing nitration reactions, reac-
tions were also performed with hydrated cerium(IV) triflate,
+
a reference electrode based on the ferrocene redox couple Fc/Fc in
[C
2
mim][OTf]. With this reference electrode, the cyclic voltammo-
mim][OTf] were measured. As can
gram of solutions of CAN in [C
2
be seen in Figure 3, the reduction of cerium(IV) to cerium(III) oc-
curs at about 0.8 V (or at 1.2 V vs SHE) and oxidation occurs at
about 1.0 V (or at 1.4 V vs SHE). However, after oxidation of
cerium(III) to cerium(IV) the electric current did not drop back to
zero. Further experiments with the controlled addition of small
amounts of water to the ionic liquid have indicated that this
remaining electric current originates from the decomposition of
traces of water in the ionic liquid. Additional experiments also
showed that under the conditions at which cerium(III) is oxidized
Ce(OTf)
are required for the oxidation of naphthalene. However, such large
amounts of Ce(OTf) O could not be dissolved in [C mim][OTf].
ꢁxH
Therefore, reactions were performed with 1 equiv of Ce(OTf)
ꢁxH
in order to check if some amount of 1,4-naphthoquinone could be
obtained. However, the reaction of naphthalene in 5 g of [C mi-
m][OTf] and 1 mL of water in the presence of 1 equiv of Ce(OTf)
ꢁx-
O did not proceed at 80 °C. At this temperature, naphthalene
was poorly soluble in the water/[C mim][OTf] mixture. Higher
4
ꢁxH
2
O, instead of CAN. Six equivalents of Ce(OTf)
4
2
ꢁxH O
4
2
2
4
2
O
2
4
H
2
4 2 3 6
to cerium(IV), the ammonium group of (NH ) [Ce(NO ) ] is electro-
2
chemically stable.
reaction temperatures led however to sublimation of naphthalene
out of the reaction mixture.
Because the reactions presented in this study require stoichi-
ometric amounts of CAN and because CAN has a relatively high
The cyclic voltammetry study of CAN in [C
that it should be possible to reoxidize cerium(III) to cerium(IV) di-
rectly in the ionic liquid [C mim][OTf]. Hence, the ionic liquid
phase obtained after the CAN-mediated oxidation reaction of ben-
zyl alcohol to benzaldehyde was investigated for reoxidation of the
cerium salt and reuse for further oxidation reactions. We have se-
lected oxidation of benzyl alcohol rather than the oxidation of
naphthalene as a model reaction for the electrochemical regenera-
tion of cerium(IV), because the oxidation of benzyl alcohol by CAN
is less prone to side reactions like nitrations. A mixture of 1 equiv
2
mim][OTf] indicated
2
r
molecular mass (M = 548.26), large quantities of CAN are required.
Therefore, a method to regenerate cerium(IV) in the ionic liquid
solution would be highly desirable. A very promising route, which
takes advantage of the electrochemical stability and the electric
conductivity of ionic liquids is the electrochemical regeneration
of cerium(IV). In the past, the electrochemical oxidation of ceriu-
m(III) to cerium(IV) in aqueous solutions of triflic acid or methane-
sulfonic acid has been studied. We investigated the possibility of
2
of benzyl alcohol and 2 equiv of CAN in [C mim][OTf] was heated
and stirred in an oil bath at 100 °C during 6 h under nitrogen atmo-
sphere. Afterwards, the solution was extracted with toluene and
the organic phase was analyzed with gas chromatography using
dodecane as the internal standard. A nearly quantitative conver-
sion of benzyl alcohol was observed, with benzaldehyde as the
main reaction product (95%), besides traces of benzyl alcohol and
benzoic acid. Consequently, most of the cerium in the ionic liquid
phase is after the reaction present under the form of cerium(III).
The cerium-containing ionic liquid phase was heated under re-
duced pressure to remove traces of toluene. The resulting solution
was introduced in the anode compartment of a two-compartment
electrochemical cell. The cathode compartment was filled with
pure ionic liquid. The oxidation of cerium(III) was performed
NO2
O
O
CAN
[
C mim][TfO]
2
6
8
7
Scheme 2. Reaction of CAN with 2-methylnaphthalene (6) in the ionic liquid
mim][OTf], leading to the formation of 1-nitro-2-methylnaphthalene (7) and 2-
[C
2
methyl-1,4-naphthoquinone (8).
Table 2
Reactions of CAN with 2-methylnaphthalene in the ionic liquid [C
2
mim][OTf]a
Entry
CAN (equiv)
H
2
O added (mL)
Time (min)
Conversion (%)
Product distribution (%)b
NQ
1-Nitro
8-Nitro
4-Nitro
5-Nitro
1
1
1
1
2
3
1
6
6
0
1
1
60
300
60
>98
>98
>98
14
53
54
40
34
30
19
7
8
17
4
5
10
2
3
a
1
mmol of 2-methylnaphthalene in 5 g of [C
NQ = 2-methyl-1,4-naphthoquinone; 1-nitro = 2-methyl-1-nitronaphthalene; 8-nitro = 7-methyl-1-nitronaphthalene; 4-nitro = 3-methyl-1-nitronaphthalene; 5-
nitro = 6-methyl-1-nitronaphthalene.
2
mim][OTf], oil bath adjusted to 100 °C.
b

K. Deleersnyder et al. / Tetrahedron Letters 50 (2009) 4582–4586
4585
NO3
NO2
The study of the reactions of naphthalene and 2-methylnaphtha-
lene with ceric ammonium nitrate (CAN) in the ionic liquid 1-ethyl-
CH3
CH3
3
-methylimidazolium triflate shows that the outcome of the
reaction strongly depends on the water content of the ionic liquid.
Nitration is observed in dry ionic liquids, whereas an appreciable
amount of oxidation products is formed in wet ionic liquids. How-
ever, nitration cannot be avoided, whatever the water content is.
High yields of 1-nitronaphthalene and 2-nitronaphthalene (isomer
ratio 90:8) could be obtained by performing the reaction between
naphthalene and CAN at elevated temperatures (150–180 °C). This
illustrates once again that ionic liquids are a good solvent for organic
reactions at elevated temperatures, thanks to the very low vapor
2
-methyl-1-nitro
7-methyl-1-nitro
CH
3
CH
3
O N
NO
2
2
2
-methyl-6-nitro
2-methyl-3-nitro
3
4
pressure and the thermal stability of these solvents. The chemose-
lectivity of reactions in ionic liquids can thus be changed not only by
4
4
a proper choice of the anion, but also by modifying the water con-
tent. This Letter gives the proof-of-principle that the cerium in the
ionic liquid can be electrochemically oxidized after reaction to the
tetravalent state and the regenerated cerium(IV)-containing ionic li-
quid solutions can be reused for oxidation reactions. The system is
still prone to improvement, because part of the cerium is lost during
theelectrolysis by diffusion to the cathodecompartment. Furtherre-
search is focused on the development of cerium-mediated electro-
synthesis in ionic liquids.
CH
3
CH
3
NO2
NO2
3-methyl-1-nitro
6
-methyl-1-nitro
Figure 2. Different isomers formed upon nitration of 2-methylnaphthalene.
Acknowledgments
This project was supported by the K. U. Leuven (project IDO/
0
5005 and GOA 08/05). S.S. is indebted to the IWT-Flanderen for
a PhD fellowship. The authors wish to thank IoLiTec (Denzlingen,
Germany) for support of this research. Professor D. E. De Vos is
acknowledged for giving access to GC–MS facilities. GC–MS mea-
surements were made by Sabina Accardo.
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2
2
27. Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667–3691.
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3
3
3
2. Binnemans, K. Chem. Rev. 2007, 107, 2592–2614.
gas chromatograph. The resulting sample was diluted with acetone and
analyzed by gas chromatography. Similar experiments were performed with a
CAN:substrate ratio of 1:1 instead of the above mentioned 6:1 ratio. In this
case the amount of CAN used was 1 mmol (548 mg), with the other
experimental conditions left unchanged.
3. Bar, G.; Bini, F.; Parsons, A. F. Synth. Commun. 2003, 33, 213–222.
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3
42. The electrochemical experiments were conducted using a Solartron SI 1287
Electrochemical Interface. Cyclic voltammetry experiments were performed at
3
3
6. Bhatt, M. V.; Periasamy, M. J. Chem. Soc., Perkin Trans. 2 1993, 1811–1814.
7. Mellor, J. M.; Mittoo, S.; Parkes, R.; Millar, R. W. Tetrahedron 2000, 56, 8019–
a
scan rate of 50 mV/s. The regeneration of cerium(IV) by oxidation of
8
024.
cerium(III) was performed potentiostatically. The working electrodes consisted
of platinum wire (diameter: 0.5 mm, Goodfellow, 99.99%) sealed in epoxy
resin. Before use, the working electrodes were mechanically polished with
3
3
8. Sheldon, R. A. Top. Curr. Chem. 1993, 164, 21–43.
9. Bohle, A.; Schubert, A.; Sun, Y.; Thiel, W. R. Adv. Synth. Catal. 2006, 348, 1011–
1
015.
diamond paste (3 lm), rinsed in demineralized water, and dried. All potentials
4
4
0. Yamazaki, S. Tetrahedron Lett. 2001, 42, 3355–3357.
1. To a solution of CAN (6 mmol, 3.288 g) in 1-ethyl-3 methylimidazolium triflate,
were measured versus a reference electrode based on a solution of ferrocene
þ
ꢀ
+
(Fc) and Fc PF6 in [C
2
mim][OTf] and are reported versus Fc/Fc , whose
1
[
C
2
mim][OTf] (5 g, 3.6 mL) was added naphthalene (1 mmol, 128 mg) or 2-
standard reduction potential is +400 mV versus SHE. The counter electrode
was made of platinum. The electrochemical experiments were performed at
methylnaphthalene (1 mmol, 142 mg) and eventually some water (1 mL or
5
5
mL). At room temperature 6 mmol of CAN was not completely dissolved in
g of [C mim][OTf], but a clear solution was obtained upon heating. The
room temperature in
separated by a glass frit with pore sizes in the range 10–20
electrode used during this experiment had a surface area of 0.196 mm .
43. McEwen, A. B.; Ngo, H. L.; LeCompte, K.; Goldman, J. L. J. Electrochem. Soc. 1999,
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a
two-compartment cell: both compartments were
m. The working
2
l
2
mixture was stirred and heated in an oil bath adjusted to 100 °C. After a given
period of time the solution was cooled and extracted with toluene (5 ꢂ 5 mL).
4
The organic phase was dried with MgSO , which also removed traces of ionic
liquid that would contaminate the injection system and/or the column of the
44. Earle, M. J.; Katdare, S. P.; Seddon, K. R. Org. Lett. 2004, 6, 707–710.