Micellar catalysis in aqueous-ionic liquid systems

Article abstract of DOI:10.1039/c2cc31503c

We present the application of ionic liquid-aqueous micellar solutions as reaction media for Diels-Alder reaction and found that reaction rates could be significantly increased compared to the reaction in water. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc31503c

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COMMUNICATION
Micellar catalysis in aqueous–ionic liquid systemswz
a
a
a
a
¨
Katharina Bica,* Peter Gartner, Philipp J. Gritsch,y Anna K. Ressmann,
¨
Christian Schroder and Ronald Zirbs
b
c
Received 28th February 2012, Accepted 28th March 2012
DOI: 10.1039/c2cc31503c
We present the application of ionic liquid–aqueous micellar solutions
as reaction media for Diels–Alder reaction and found that reaction
rates could be significantly increased compared to the reaction
in water.
Ionic liquids have been thoroughly investigated as reaction media
in most types of catalytic reactions, and a number of critical
aspects make them interesting media for synthesis and catalysis
with distinct chemistry of their own compared to traditional
molecular solvents.1,2
Recently, there has been a tremendous increase of interest in the
behaviour of ionic liquids in the presence of water, and it has been
shown that certain ionic liquids can form aggregates in aqueous
solution.3 In the case of 1-alkyl-3-methylimidazolium chloride salts
[Cnmim]Cl, a polarizable molecular dynamics simulation of a
50 mM [C12mim]Cl solution showed that the polar head groups
of the imidazolium cation are completely covered by water (Fig. 1).
The water cage of apolar imidazolium side chains is fragmented
by contacts with other cations, and hence micelles are formed.4 The
strength of the network of these cation–cation contacts determines
the size and shape of the micelles, and alkyl chain length, counter
ion as well as temperature were found to have a vital effect on the
critical micelle concentration (CMC) values.5
Fig. 1 Structure of [C12mim]Cl in H2O and CMC values of surface-
active ILs used in this study.
at a concentration higher than their CMC.6 Most of the
studies of ionic liquid behaviour in aqueous solution deal with
1-alkyl-3-methylimidazolium salts [Cnmim]X, where the anion X is
typically Clꢀ, Brꢀ, BF4ꢀ or PF6ꢀ, and a linear relationship between
the logarithm of CMC and the number of carbon atoms n was
found (Fig. 1).7
A variety of experimental techniques, including surface
tension and conductivity measurements, potentiometry, fluores-
cence probes, NMR spectroscopy, mass spectrometry, light
scattering and small-angle X-ray and neutron scattering (SAXS
and SANS), has been applied to understand the aggregation
behaviour of ionic liquids, showing that micelles are formed
with alkyl chains of greater than eight carbon atoms (n Z 8)
Although progress in the studies of ionic liquid self-aggregation
in aqueous solution has been made by a number of authors, these
investigations are entirely restricted to physico-chemical studies
while the potential for synthesis and catalysis remains unexplored.
An amphiphilic aggregate in aqueous media will provide a reaction
environment different from bulk water, leading to a kinetic
influence on a reaction and hence will be able to catalyze or inhibit
organic reactions.8 Considering the highly polar character of ionic
liquids and their influence on organic reactions, novel reactivities
and selectivities of ionic liquid–water micellar systems can be
expected.
a Institute of Applied Synthetic Chemistry,
Vienna University of Technology, Getreidemarkt 9/163,
1060 Vienna, Austria. E-mail: kbica@ioc.tuwien.ac.at;
Fax: +43 1 58801 16360; Tel: +43 1 58801 163601
b Institute of Computational Biological Chemistry,
University of Vienna, Wahringerstr. 17, 1090 Vienna, Austria
¨
c Group for Biologically Inspired Materials, Institute of
Nanobiotechnology (DNBT), University of Natural Resources and
Life Sciences, Muthgasse 11, 1190 Vienna, Austria
w This article is part of the ChemComm ‘Ionic liquids’ web themed
issue.
Herein, we report for the first time the application of aqueous–
ionic liquid micellar systems in catalysis and provide insight into
the influence of structural features of surface active ionic liquids on
the Diels–Alder reaction of 1,3-cyclohexadiene 1 with N-benzyl-
maleimide 2 (Fig. 2).z The chosen reaction works reasonably well
in water and takes place within 60 minutes, but is substantially
z Electronic supplementary information (ESI) available: Detailed
experimental procedures, DLS measurements for [C12mim]Cl and
[C14mim]Cl, and copies of spectra of the isolated product. See DOI:
10.1039/c2cc31503c
y Current address: Institute of Organic Chemistry, Leibniz University
of Hannover, Schneiderberg 1, 30167 Hannover, Germany.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5013–5015 5013

Fig. 2 Diels–Alder reaction of N-benzylmaleimide with 1,3-cyclohexa-
diene in ionic liquid–water micellar solutions.
slower with reaction times of 448 h in organic solvents, e.g. Et2O
or DCM.9,10 Ionic liquids have considerable potential for Diels–
Alder reactions, especially when hydrogen-bond donating cations
are used in combination with non-polar and weakly coordinating
anions.11 Since the first example was reported by Jager and Tucker
¨
in 1989,12 intensive studies on solvent effects of different ionic
liquids were published.13 Yet, despite the variety of cation–anion
combinations available in ionic liquids, higher reaction rates have
been reported when the reaction was performed in water.14
In order to obtain reliable and reproducible results for micellar
catalysis, solutions of 1-alkyl-3-methylimidazolium ionic liquids
([Cnmim]Cl, n = 8–14) were prepared and shaken for 24 h at
360 minꢀ1. Solutions of diene and dienophile were independently
prepared, combined, and immediately transferred to a UV-vis
spectrometer to follow the disappearance of the dienophile at
298 nm (ESIz, Fig. S1). A ratio of dienophile–diene 1:60 was
chosen to calculate pseudo-first-order rate constants.
Fig. 3 Relative reaction rate kIL/kW of a Diels–Alder reaction in aqueous
solutions of the amphiphilic ionic liquids [Cnmim]Cl, n = 8–14.
surfactants, we found that the surface-active ionic liquid
[C14mim]Cl (kIL/kW E 4.10 in a 50 mM solution) can be more
efficient in this particular Diels–Alder reaction than the cationic
surfactant CTAB (kS/kW E 3.68 in a 50 mM solution), the
anionic surfactant sodium dodecylsulfate SDS (kS/kw E 3.82
in a 50 mM solution) or the zwitterionic surfactant Empigens
(kS/kw E 3.72 in a 50 mM solution).
Initially, we determined the aqueous rate constant for the
reaction run with 0.0002 mmol dienophile and 0.012 mmol diene
in 1 ml H2O and found a reaction rate of k = 0.0091 ꢁ 0.0007.8
Subsequently, we performed the reaction in aqueous solutions of
amphiphilic 1-alkyl-3-methylimidazolium chloride ILs ([Cnmim]Cl,
n = 8–14) and compared the rate constants with the values
obtained for pure water.10
The reaction rates could be further increased via addition of
a Lewis-acid that is able to coordinate with 1-alkyl-3-methyl-
imidazolium chlorides to form Lewis acidic ionic liquids of
the type [Cnmim]MexXy. Although chloroaluminate(III) ionic
liquids have been the most prominent examples, they suffer
from limited stability and fast hydrolysis. Recently, more
benign and stable metal-containing ionic liquids, e.g. In(III)
and Fe(III)-based ionic liquids, have been successfully used for
catalysis.15,16 For example, Earle et al. could recover and
successfully reuse the ionic liquid [C4mim]Cl–InCl3 (w = 0.6)
from aqueous solution, thus indicating a substantial water
stability. Furthermore, indium(III) halide has been shown to be
an effective Lewis acid for various reactions in aqueous
medium, and thus we further investigated the Diels–Alder
reaction with surface active chloroindate ionic liquids.17 We found
that the addition of indium(III) chloride resulted in a further
increase in the reaction rate, and best results were obtained when
0.1 eq. of InCl3 (corresponding to [C12mim]Cl–InCl3 w = 0.09)
were added (Table 1, entry 4). Unfortunately, we could not
further increase the mole fractions of InCl3 since the solution
became turbid.
We did not only observe enhanced reaction rates (up to kIL
/
kw E 4.1) for ionic liquid–aqueous micellar systems compared
to the reaction in pure water, but also found that the maximum
of the reaction rate occurred above the CMC of the respective
amphiphilic ionic liquid (Fig. 3). For example, the observed rate
constants for [C10mim]Cl reach a maximum at concentrations of
B60 mM, which is above the CMC and then decrease again with
a further increase in the concentration of [C10mim]Cl. A similar
effect was reported by Rispens and Engbert for related Diels–
Alder reactions in SDS- or CTAB-aqueous solutions.10 For this,
the dilution of both reactants over increasingly more micelles has
been held responsible for the decreasing reaction rate at higher
concentrations.
Comparable results were observed for the ionic liquids
[C12mim]Cl and [C14mim]Cl, and the monitored reaction rates
reach a maximum at concentration just above the CMC. The
maxima of the relative rate constants kIL/kW were dependent on the
chain length of the ionic liquids, and we found the highest kIL/kW
values for [C14mim]Cl. In contrast to 1-alkyl-3-methylimidazolium
ionic liquids [Cnmim]Cl with n = 10–14, we were unable to repeat
the series with [C8mim]Cl since the reaction mixture became turbid
and rate constants were unreproducible when approaching the
CMC. However, we could determine a relative rate constant of
kIL/kW 0.91 in a 50 mM solution, and again the order of relative
rate constants kIL/kW C8 o C10 o C12 o C14 was confirmed.
When comparing reaction rates for ionic liquid–aqueous
micellar solutions with those obtained for conventional
In parallel to the assessment of reactivity, we performed
dynamic light scattering (DLS) experiments to get further
insight into the nature of self-organization of ionic liquids in
water and their influence on catalysis. We found that a
100 mM solution of [C10mim]Cl was composed of micelles
with an average diameter of 80 nm but also to a large extent of
single molecules. Interestingly, the addition of the dienophile
N-benzylmaleimide 2 resulted in a substantial shift towards
micelle formation, and the solution was almost completely
c
5014 Chem. Commun., 2012, 48, 5013–5015
This journal is The Royal Society of Chemistry 2012

Table 1 Rate constants of the Diels–Alder reaction in a 50 mM
solution of chloroindate ionic liquids
Notes and references
z Reaction procedure for the Diels–Alder reaction in ionic liquid–
Entrya
Surfactant
—
—
[C12mim]Cl
[C12mim]Cl–InCl3 w = 0.09
Rate constant
kIL/kW
micellar solution on
a preparative scale: N-benzylmaleimide 2
(18.9 mg, 0.1 mmol) was dissolved in 10 ml of a 100 mM [C10mim]Cl
solution. Freshly distilled cyclohexadiene (0.5 mmol) was added and
the reaction mixture was left stirring for 60 min at 25 1C. After
complete reaction the product crystallized from the solution at 0 1C
and was collected via filtration. Yield: 24.0 mg (92%) colourless
crystals. 1H-NMR (CDCl3, 200 MHz): d (ppm) = 7.28 (m, 5H),
6.06 (dd, J1 = 4.60 Hz, J2 = 3.04 Hz, 2H), 4.55 (s, 2H), 3.14 (m, 2H),
2.84 (m, 2H), 1.58 (m, 2H), 1.36 (m, 2H).
1
2
3
4
0.0091 0.0007
0.0096 0.001
0.0284 0.002
0.0381 0.002
1.0
b
1.06
3.13
4.20
a
Reactions were carried out with 0.0002 mmol dienophile and
b
0.012 mmol diene (ratio 1 : 60) in 1 ml of solvent at 25 1C. Addition
of 0.05 mmol InCl3.
8 All experiments were repeated at least 5 times and reaction rates are
reported as mean ꢁ SD; n Z 5.
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compared to that of the reaction in water. Combining the amphi-
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active chloroindate ionic liquids could further increase the reaction
rate. We thus expect that ionic liquid–water micellar systems lead to
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This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 5013–5015 5015