Electron density shift in imidazolium derivatives upon complexation with cucurbit[6]uril

Article abstract of DOI:10.1002/chem.200900570

In this study, we have investigated the supramolecular interaction between series of l-alkyl-3-methylimidazolium guests with variable alkyl substituent lengths and cucurbit[6]uril (CB6) in the solution and the solid state. Correct interpretation of 1H NMR spectra was a key issue for determining the binding modes of the complexes in solution. Unusual chemical shifts of some protons in the ¹H NMR spectra were explained by the polarization of the imidazolium aromatic ring upon the complexation with the host. The formation of 1:1 complex between 1 -ethyl-3-methylimidazolium and CB6 is in disagreement with previously reported findings describing an inclusion of two guest molecules in the CB6 cavity.

Full text of DOI:10.1002/chem.200900570

DOI: 10.1002/chem.200900570
Electron Density Shift in Imidazolium Derivatives upon Complexation with
Cucurbit[6]uril
Viktor Kolman,[a] Radek Marek,[b] Zora Strelcova,[b] Petr Kulhanek,[b] Marek Necas,[a]
Jan Svec,[a] and Vladimir Sindelar*[a]
Abstract: In this study, we have investi-
gated the supramolecular interaction
between series of 1-alkyl-3-methylimi-
dazolium guests with variable alkyl
substituent lengths and cucurbit[6]uril
(CB6) in the solution and the solid
complexes in solution. Unusual chemi-
cal shifts of some protons in the
1H NMR spectra were explained by the
polarization of the imidazolium aro-
matic ring upon the complexation with
the host. The formation of 1:1 complex
between 1-ethyl-3-methylimidazolium
and CB6 is in disagreement with previ-
ously reported findings describing an
inclusion of two guest molecules in the
CB6 cavity.
Keywords: cucurbiturils
·
host–
state.
Correct
interpretation
of
guest systems · ionic liquids · supra-
molecular chemistry
1H NMR spectra was a key issue for
determining the binding modes of the
Introduction
Recently, imidazolium derivatives have been investigated
as a suitable guests for CBs. CB7 was found to form a
strong inclusion complex with the bis(imidazolium) dicat-
ion.[9] Hydrogen/deuterium (H/D) exchange of the imidazo-
lium acidic proton is significantly inhibited in solution for
this complex in D2O as a result of hydrogen bonding with
the carbonyl portal of the macrocycle. Bis(imidazolium) di-
cation was also used as a guest, allowing the formation of
ternary complexes.[10] In these complexes, one guest mole-
cule is included in both CB7 and b-cyclodextrin macrocycles
leading to a cooperative supramolecular interaction between
the macrocycles. Many imidazolium compounds are known
to behave as ionic liquids (ILs), a class of organic com-
pounds that have found industrial application as designer
solvents[11] and catalysts[12] in organic reactions, and media in
separation technologies.[14] It has been published recently
that imidazolium ILs are able to form inclusion complexes
with CBs.[14] As CB6 is poorly soluble in water and insoluble
in organic solvents, its solubility in water increase upon the
complexation with IL. Furthermore the presence of CBs de-
creases the IL solution viscosity.[14b]
Cucurbit[6]uril (CB6) is the oldest and the most accessible
representative of the CB family of macrocycles and its
supramolecular interactions with various guests have been
extensively investigated.[1] After pioneering work of Beh-
rend, the ability of CB6 to behave as a synthetic receptor
was described in detail by Mock and co-workers together
with the discovery of the macrocyclic structure of the mole-
cule.[2] They reported the formation of complexes between
CB6 and aliphatic amines or diamines. Guest positioning
and complex stability strongly depended on the length of
alkyl chain of the guest.[3] Since then the complexation be-
tween CB6 and many organic guests has been studied in-
cluding polyamines,[4] viologen derivatives,[5] organic dyes,[6]
polypeptides,[7] amino acids, and dipeptides.[8]
[a] V. Kolman, Prof. Dr. M. Necas, J. Svec, Dr. V. Sindelar
Department of Chemistry
Masaryk University
The complexation between both pyridinium and ammoni-
um organic guests and CB6 in water is driven by the hydro-
phobic effect and dipole-cation interactions[3d] between the
negatively charged carbonyl portal of CB6 and the nitrogen
on the guest, where the positive charge is located. As a
result, the hydrophobic part of the molecule is included
inside the CB6 cavity while the nitrogen cation sits on the
CB6 portal. The mode of binding between these guests and
Kotlarska 2, 61137 Brno (Czech Republic)
Fax : (+420)549-49-2443
E-mail: sindelar@chemi.muni.cz
[b] Prof. Dr. R. Marek, Z. Strelcova, Dr. P. Kulhanek
National Centre for Biomolecular Research
Masaryk University
Kamenice 5A4, 62500 Brno (Czech Republic)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200900570.
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1
CB6 in the solution is usually determined by H NMR spec-
troscopy. The interior of the CB6 cavity comprises a
1H NMR shielding region while the outside regions near to
carbonyl portals on both sides of CB6 are weakly deshield-
ing. Thus, it is generally believed that, upon complexation,
the guest protons included inside the host cavity experience
an upfield shift and those protons located outside and close
to the carbonyl portals are displaced downfield.
Results and Discussion
In this paper, we demonstrate the formation of inclusion
complexes between imidazolium guests and CB6. We de-
scribe two different binding modes between CB6 and 1-
alkyl-3-methylimidazolium ([Cnmim]+ for which n corre-
sponds to the number of carbons in the alkyl chain) depend-
ing on the guest alkyl chain length. Upon the formation of
the complexes an unusual chemical shift for some of the
guest protons located outside of the CB6 cavity was record-
Figure 1. 1H NMR spectra (300 MHz, D2O) of [C5mim]+ in the absence
(A) and in the presence of 0.4 equiv (B) and 0.9 equiv of CB6 (C).
1
ed in the H NMR spectra. This unusual observation is also
discussed.
We selected for our study series of 1-alkyl-3-methylimida-
zolium ILs where alkyl ranges from ethyl to pentyl
(Scheme 1). These guests where prepared by the alkylation
Scheme 2. Two different modes of inclusion binding between 1-alkyl-3-
methylimidazolium guests and CB6.
Scheme 1. Structure of cucurbit[6]uril and imidazolium guests.
of 1-methylimidazole with the corresponding alkylbromide
(see the Supporting Information). The prepared imidazoli-
ums were investigated as bromide salts except in the UV/vis
experiments where the bromide anion was exchanged by
for this unexpected observation? Our explanation relies on
a shift of electron density within the imidazolium ring of the
guest upon complexation with CB6. Close proximity of the
carbonyl rim to nitrogen N3 caused partial stabilization of
the positive charge on this atom. In response to the cation
stabilization, the electron density in the aromatic imidazoli-
um system shifts towards nitrogen atom N1, which bears the
methyl substituent. Proton H5 is influenced by the increase
in electron density on its vicinity, which results as an upfield
shift in the 1H NMR spectra. Also the localization of the
charge upon complexation is only partial. For improved
clarity, we can represent this mode of binding as a complex,
where the resonance structure of imidazolium has positive
charge located on N3 and a free electron pair on N1
(Scheme 2A).
À
BF4 .
The binding interactions were monitored by 1H NMR
spectroscopy. We started our investigation with [C5mim]+
which was dissolved in D2O and titrated by addition of solid
CB6 (Figure 1). Upon addition of 0.5 equivalents of the
macrocycle all the 1H NMR resonances of the guest split
into two signals, which indicate that the exchange between
free and bound guest is slow on the NMR time scale. A new
set of signals for the pentyl chain appeared at higher field,
while H2 and H4 experience downfield shifts of 0.32 and
0.37 ppm, respectively. This indicates that the aliphatic pro-
1
1
tons are included inside the H NMR shielding region of the
Similar H NMR spectra was obtained for the titration of
CB6 cavity and protons H2 and H4 are located in the de-
shielding region outside of the carbonyl portal as illustrated
in Scheme 2A. Surprisingly, upon complexation, proton H5
shifts ꢀ0.13 ppm upfield. As proton H5 is located outside
the shielding region of CB6 either a downfield shift or no
shift for this proton signal was expected. What is the reason
[C4mim]+ with CB6 where the resonance of proton H5
shows a chemical shift of 7.36 ppm, compared to its original
chemical shift of 7.46 ppm in the absence of the host. It is
important to notice that the chemical shifts of protons H4
and H5 on the free guest are very similar and it is therefore
difficult to follow their chemical shift changes when the
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V. Sindelar et al.
CB6 is added into the solution. 2D NOESY experiments
were used to assign unequivocally the H NMR signals for
dazolium ring is shifted towards nitrogen N3, while the posi-
tive charge is partially localized on N1, owing to the ion-
dipole interactions with the portal of CB6. The localization
of positive charge on nitrogen N1 is responsible for the lack
of chemical shift on proton H5 observed upon complexation
despite of the placement of the proton inside the macrocy-
cle. This binding mode is presented in Scheme 2B. For the
sake of clarity the charge is fully localized on N1.
1
the guest inside the host cavity (see the Supporting Informa-
tion). Previously, the proposed position of the [C4mim]+
within CB6 cavity was different, owing to an incorrect as-
1
signment of H4 and H5 signals in the H NMR spectra of
the complex solution.[14a]
1
Analysis of the H NMR spectra indicates that [C3mim]+
forms an inclusion complex with CB6 in which the alkyl
chain is incorporated inside the cavity of the macrocycle,
whereas the imidazolium ring remains outside. However, in
As our interpretation of NMR spectra differs from previ-
ously published results, we decided to support out findings
about the placement of imidazolium derivatives within the
CB6 cavity using X-ray crystallography and computational
methods. We were able to obtain single crystals of the
[C4mim]+-CB6 complex suitable for X-ray diffraction analy-
sis. The crystal structure shows the superimposition of two
[C4mim]+-CB6 complexes differing in the relative turning of
[C4mim]+ around the CB6 longitudinal axis (Fig-
ure 3A,B).[16] In both cases, the butyl chain is located within
1
this case the complexation is fast on the H NMR time scale.
Fast exchange of the guest molecules inside the CB6 cavity
is probably caused by a lower equilibrium association con-
stant, owing to the shorter alkyl substituent [C3mim]+ com-
pared to [C5mim]+ and [C4mim]+. Unfortunately, we were
not able to determine the corresponding association con-
stants to support this prediction.
Different binding interactions were observed between
[C2mim]+ and CB6 (Figure 2) compared to those observed
Figure 2. 1H NMR spectra (300 MHz, D2O) of [C2mim]+ in the absence
(A) and in the presence of 0.25 equiv (B) and 0.5 equiv of CB6 (C).
for [C3mim]+, [C4mim]+ or [C5mim]+. Upon addition of an
increasing amount of CB6 into the [C2mim]+ solution an
average spectrum of the free and complexed guest was ob-
served indicating the fast exchange on the NMR time scale.
A maximum of 0.5 equiv of CB6 can be added into the solu-
tion before precipitation takes place. Upon addition of
0.5 equiv of the host, a substantial upfield shift of 0.55 ppm
and 0.31 ppm was observed for the aromatic protons H2 and
H4, respectively, and H5 does not experience any chemical
shift. The intensity of the H2 proton signal is suppressed as
a result of H/D exchange of the proton in D2O solution. The
signals for the ethyl protons shift upfield. The induced up-
field shifts correspond to the formation of an inclusion com-
plex in which the imidazolium ring, together with the ethyl
substituent, are included inside the cavity of the host. We
predict that in this complex the electron density in the imi-
Figure 3. Crystal structure of [C4mim]+-CB6 complex: A) side and
B) front views and C) comparison of crystal structure (black) and struc-
ture optimized using HF/6-31G* method (gray).
the cavity, which confirms the complex structure proposed
from our NMR spectroscopic data.
Since crystal environment might exhibit conditions that
are far away from the situation observed in the solution, we
decided to probe the structural and dynamical features of
the complex by the means of quantum chemical and molec-
ular dynamics methods. One of two observed structures in
the crystal was extracted and used as the starting point for
further modeling. The structure was subjected to geometry
optimization in vacuum employing quantum chemical meth-
ods implemented in Gaussian 98[15] and Turbomole 5.6[17]
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Cucurbit[6]uril Complexation
FULL PAPER
program suites. Only minor changes in the resulting struc-
ture were observed using Hartree–Fock method with 6-
31G*[18] basis set, with
a root-mean-square deviation
(RMSD) of 0.46 ꢁ between the initial and optimized struc-
tures (Figure 3C). A similar difference (RMSD=0.50 ꢁ)
was observed using density functional theory (B3LYP[19]
)
and larger basis set (cc-pVDZ[20]). These results reveal very
small impact of the crystal field on the complex but still
they do not say anything about the situation in solution.
Therefore, the complex was immersed into a box containing
about 6000 water molecules. Owing to the extreme complex-
ity of such system, quantum chemical computations had to
be replaced by a faster empirical molecular mechanics ap-
proach. This change also permits to study the dynamic be-
havior of the system by means of molecular dynamics. For
this purpose, the sander program from the AMBER pack-
age[21] was used. The complex was described using the
GAFF[22] force field, which is well tuned for the simulation
of small organic molecules, whereas water molecules were
described using TIP3P[23] potential. At the beginning of the
simulation, the system was heated up to 300 K with pressure
maintained at 1 bar (for detailed description of used proto-
cols, see the Supporting Information section). After system
equilibration, the simulation was run for an additional 10 ns
with temperature and pressure maintained at 300 K and
1 bar, respectively. During this period, the complex was
stable showing the same structural features as those ob-
served by X-ray diffraction and proposed from the interpre-
tation of NMR spectra. To avoid possible computational ar-
tifacts coming from for example, interlocked structure, the
simulation was re-run using different initial complex struc-
ture. [C4mim]+ was moved along axial axis of complex in
such a way that 3-methyl group was roughly situated in the
cavity centre and the butyl chain was outside of CB6. This
complex arrangement was stable only for ꢀ600 ps. After
this period, [C4mim]+ was moved back and the resulting
complex was stable until the end of the simulation.
We were also able to obtain the single crystals of the
[C2mim]+-CB6 complex. The crystal structure contains two
different types of complexes.[16] The first type contains
[C2mim]+ located within the CB6 cavity (Figure 4A,B). The
second one is composed of two superimposed structures,
both containing [C2mim]+ within the CB6 cavity, but with
opposite head-to-tail orientations. In all cases, the imidazoli-
um moiety is located inside the CB6 cavity, contrary to our
findings with the [C4mim]+-CB6 complex, where imidazoli-
um sits on top of one of the carbonyl portals. One might
argue that the small molecule [C2mim]+ is pushed into the
cavity by packing forces dominating in the crystal. However,
structure optimization of this complex by quantum chemical
methods reveals that the structure is stable even without the
crystal field (Figure 4C). Furthermore, the calculated com-
plex formation energy is about 5 kcalmolÀ1 smaller (B3LYP/
cc-pVDZ) than that for [C4mim]+-CB6 complex, which is in
agreement with our expectations and with the lower stability
of [C2mim]+-CB6 complex detected by NMR. Lower com-
plex stability is also observed in molecular dynamics simula-
Figure 4. Crystal structure of [C2mim]+-CB6 complex: A) side and
B) front views and C) comparison of crystal structure (black) and struc-
ture optimized using HF/6-31G* method (gray).
tions. The complex dissociates after 3–5 ns (proved by three
simulations starting from different conditions).
Our results from crystallographic measurements and com-
putations strongly support a 1:1 binding mode between
[C2mim]+ and CB6. However, our findings contrast with a
recent report on the formation of a 2:1 complex between
[C2mim]+ and CB6.[14a] Therefore, to further support our re-
sults, we constructed a Job plot using UV-visible spectrosco-
py. A maximum was found at c [C2mim]+ =0.5, which also
indicates a 1:1 stoichiometry between the host and the
guest. Furthermore, a major signal at 1107 m/z correspond-
ing to 1:1 [C2mim]+-CB6 complex was observed in the
MALDI TOF MS spectrum, when the complex solution in
the presence of more than fourfold excess of the guest was
analyzed with no sign of the 2:1 complex. Similar MALDI
TOF MS spectra indicating the presence of 1:1 complex
were obtained for the remaining guests upon the complexa-
tion with CB6 (see the Supporting Information).
The complexation between [C5mim]+ and CB6 was also
investigated using 15N NMR spectroscopy. We anticipated
that the shift of electron density in the imidazolium ring pre-
sumably induced by complexation with CB6 should affect
both 15N resonances on the imidazolium skeleton. A solu-
tion containing 1 equiv of [C5mim]+ and 0.5 equiv of CB6
was used for the 15N NMR measurements.[24] Two sets of sig-
nals corresponding to the free and bound forms of the guest
were observed because of the slow exchange process. 15N
resonances were assigned unequivocally to the N1 and N3
atoms by using 1H-15N GSQMBC experiments.[25] Interac-
tions of the protons on the methyl group with N1 and pro-
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V. Sindelar et al.
tons on the CH2 groups of the pentyl chain with N3 un-
doubtedly confirmed the resonance assignments summarized
in Table 1. N1 is shielded by 4.0 ppm upon complexation of
in which only the alkyl chain is engulfed by the host, leaving
the imidazolium ring outside the host cavity. We have also
shown that the imidazolium aromatic ring is polarized upon
complexation with CB6. For both binding modes, the elec-
tron density shifts from the nitrogen located in the proximi-
ty of the carbonyl portal of CB6 toward the opposite nitro-
gen atom.
Table 1. 15N NMR chemical shifts (d in ppm) for free and bound
[C5mim]+ in D2O at 275.5 K.
AHCTUNGTRENNUNG
[C5mim]+ [ppm]
Free
Complexed
Dd
N1
N3
169.8
182.7
165.8
185.4
À4.0
+2.7
Experimental Section
Starting materials were purchased from commercial suppliers and were
used without further purification. 1D and 2D NMR spectra were record-
ed using a Bruker Avance 600 spectrometer operating at frequencies of
600.13 MHz (1H), 150.77 MHz (13C), and 60.76 MHz (15N), a Bruker
Avance 500 spectrometer operating at frequencies of 500.13 MHz (1H),
125.77 MHz (13C), and 50.76 MHz (15N), and a Bruker Avance 300 spec-
trometer operating at frequencies of 300.13 MHz (1H) and 75.77 MHz
(13C). 13C NMR chemical shifts (d in ppm) were referenced to the signal
of tetramethylsilane (TMS), which was used as an external standard to
prevent its interference with CB.[6] The 15N NMR chemical shifts were
referenced to liquid CH3NO2 (381.7 ppm) and are reported relative to
liquid NH3. The MALDI-TOF mass spectra were obtained using Ultra-
flex III spectrometer. Spectra were measured in reflection positive mode,
200 laser shots were accumulated (Nd:YAG laser - 355 nm). UV/vis ab-
sorption spectra were measured on Shimadzu UV 1602 UV-VIS spectro-
photometer with 1 cm quartz cell. Diffraction data were collected on a
KUMA KM-4 k-axis CCD diffractometer with MoKa radiation (l=
0.71073 ꢁ). The temperature during data collection was 120(2) K. The
structures were solved by direct methods and refined by full-matrix least-
squares methods using ShelXTL software.[19]
[C5mim]+ inside the cavity whereas complexation-induced
deshielding of N3 amounts to 2.7 ppm. The observed 15N
chemical shifts indicate partial shift of electron density from
nitrogen N3 to N1 upon complex formation and is consistent
with the hypothesis formulated from the results of our
1H NMR measurements.
Based on the results obtained from 1H and 15N NMR
measurements we proposed two different modes of binding
between 1-alkyl-3-methylimidazolium guests and CB6,
which are clearly dependent on the length of alkyl chain
(Scheme 2). Similar effect of the alkyl chains on the com-
plex structures was previously described for complexes be-
tween CB7 and dialkylviologenes.[26] [C3mim]+, [C4mim]+,
and [C5mim]+ bear a long alkyl substituent, which is favored
to bind inside the CB6 cavity as a result of hydrophobic ef-
fects. As propyl, butyl, and pentyl substituents fit well in the
cavity, the imidazolium ring remains outside the host. Posi-
tive charge is then partially localized on N3 and stabilizes
the inclusion complex by ion-dipole interaction
(Scheme 2A). On the other hand [C2mim]+ contains a short
alkyl chain, allowing the inclusion of both the alkyl chain
and the imidazolium ring. In this complex, positive charge is
partially shifted toward nitrogen N1, thus stabilizing the
complex by interaction with the host portal (Scheme 2B). In
other words, the binding position of the guest within the
cavity is determined by the hydrophobic effect, which also
dictates to which of the two nitrogens the positive charge
will be predominantly shifted in order to stabilize the com-
plex through interactions with the CB6 portal.
Acknowledgements
We are grateful to the Grant Agency of the Czech Republic (203/07/P382
to VS) and Ministry of Education of the Czech Republic
(MSM0021622413 and LC06030 to RM, and MSM0021622413 to PK).
The access to the METACentrum supercomputing facilities provided
under the research intent MSM6383917201 is highly appreciated.
[1] a) J. W. Lee, S. Samal, N. Selvapalam, H.-J. Kim, K. Kim, Acc.
Chem. Res. 2003, 36, 621–630; b) J. Lagona, P. Mukhopadhyay, S.
Chakrabarti, L. Isaacs, Angew. Chem. 2005, 117, 4922–4949; Angew.
Chem. Int. Ed. 2005, 44, 4844–4870.
[2] W. A. Freeman, W. L. Mock, N.-Y. Shih, J. Am. Chem. Soc. 1981,
103, 7367–7368.
[3] a) W. L. Mock, N.-Y. Shih, J. Org. Chem. 1983, 48, 3618–3619;
b) W. L. Mock, N.-Y. Shih, J. Org. Chem. 1986, 51, 4440–4446;
c) W. L. Mock, N.-Y. Shih, J. Am. Chem. Soc. 1988, 110, 4706–4710;
d) W. L. Mock, N.-Y. Shih, J. Am. Chem. Soc. 1989, 111, 2697–2699.
[4] a) H. Isobe, N. Tomita, J. W. Lee, H.-J. Kim, K. Kim, E. Nakamura,
Angew. Chem. 2000, 112, 4427–4430; Angew. Chem. Int. Ed. 2000,
39, 4257–4260; b) H. Isobe, S. Sota, J. W. Lee, H.-J. Kim, K. Kim, E.
Nakamura, Chem. Commun. 2005, 1549–1551.
[5] Y. Tan, S. Choi, J. W. Lee, Y. H. Ko, K. Kim, Macromolecules 2002,
35, 7161–7165.
[6] C. Mꢂrquez, R. R. Hudgins, W. M. Nau, J. Am. Chem. Soc. 2004,
126, 5806–5816.
Conclusions
In conclusion, we have investigated the formation of inclu-
sion complexes between 1-alkyl-3-methylimidazolium guests
and CB6 in the solution and the solid state, using H and
1
15N NMR spectroscopy, UV/vis spectroscopy, X-ray crystal-
lography, mass spectrometry and computational methods.
We have demonstrated that all imidazolium guests form 1:1
complexes with CB6, in which the mode of inclusion binding
depends on the length of the alkyl substituent. In the case of
the [C2mim]+ guest, the imidazolium ring together with the
alkyl substituents are pushed inside the host cavity, although
the guests with longer aliphatic chains form a CB6 complex
[7] H.-J. Buschmann, L. Mutihac, R.-C. Mutihac, E. Schollmeyer, Ther-
mochim. Acta 2005, 430, 79–82.
[8] H.-J. Buschmann, E. Schollmeyer, L. Mutihac, Thermochim. Acta
2003, 399, 203–208.
6930
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Cucurbit[6]uril Complexation
FULL PAPER
[9] R. Wang, L. Juan, D. H. Macartney, Chem. Commun. 2006, 2908–
2910.
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif
[10] L. Leclercq, N. Noujeim, S. H. Sanon, A. R. Schmitzer, J. Phys.
Chem. B 2008, 112, 14176–14184.
[17] R. Ahlrichs, M. Bꢃr, M. Hꢃser, H. Horn, C. Kçlmel, Chem. Phys.
Lett. 1989, 162, 165–169.
[11] Ionic Liquids in Synthesis (Eds.: T. Welton, P. Wasserscheid), Wiley
VCH, Weinheim, 2002.
[12] P. J. Dyson, T. J. Geldbach in Metal Catalysed Reactions in Ionic Liq-
uids, Springer, New York, 2006.
[13] Ionic Liquids in Polymer Systems, Vol. 913 of ACS Symposium
Series (Eds.: Brazel, C. S.; Rogers, R. D.), Oxford University Press,
New York, 2005; .
[14] a) L. Liu, N. Zhao, O. A. Scherman, Chem. Commun. 2008, 1070–
1072; b) P. Montes-Navajas, A. Corma, H. Garcia, J. Mol. Catal. A
2008, 279, 165–169.
[18] P. C. Hariharan, J. A. Pople, Theor. Chim. Acta 1973, 28, 213–222.
[19] A. D. Becke, J. Chem. Phys. 1993, 98, 5648–5652.
[20] T. H. Dunning, Jr., J. Chem. Phys. 1989, 90, 1007–1023.
[21] AMBER 9, D. A. Case, T. A. Darden, T. E. Cheatham III, C. L.
Simmerling, J. Wang, R. E. Duke, R. Luo, K. M. Merz, D. A. Pearl-
man, M. Crowley, R. C. Walker, W. Zhang, B. Wang, S. Hayik, A.
Roitberg, G. Seabra, K. F. Wong, F. Paesani, X. Wu, S. Brozell, V.
Tsui, H. Gohlke, L. Yang, C. Tan, J. Mongan, V. Hornak, G. Cui, P.
Beroza, D. H. Matthews, C. Schafmeister, W. S. Ross, P. A. Kollman,
University of California, San Francisco, 2006.
[15] Gaussian 98 (Revision A.9), M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski,
J. A. Montgomery, Jr., R. E. Stratmann, J. C. Burant, S. Dapprich,
J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas, J.
Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C. Pomelli,
C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q.
Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. Cioslowski, J. V. Ortiz, B. B. Stefanov, G. Liu, A.
Liashenko, P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
C. Gonzalez, M. Challacombe, P. M. W. Gill, B. G. Johnson, W.
Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle,
J. A. Pople, Gaussian, Inc., Pittsburgh, PA, 1998.
[22] J. Wang, R. M. Wolf, J. W. Caldwell, P. A. Kollman, D. A. Case, J.
Comput. Chem. 2004, 25, 1157–1174.
[23] W. L. Jorgensen, J. Chandrasekhar, J. Madura, M. L. Klein, J. Chem.
Phys. 1983, 79, 926–935.
[24] a) R. Marek, A. Lycka, Curr. Org. Chem. 2002, 6, 35–66; b) R.
Marek, A. Lycka, E. Kolehmainen, E. Sievanen, J. Tousek, Curr.
Org. Chem. 2007, 11, 1154–1205.
[25] R. Marek, L. Kralik, V. Sklenar, Tetrahedron Lett. 1997, 38, 665–
668.
[26] K. Moon, A. E. Kaifer, Org. Lett. 2004, 6, 185–188.
[16] CCDC 722277 and 722278 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
Received: March 3, 2009
Published online: June 9, 2009
Chem. Eur. J. 2009, 15, 6926 – 6931
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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6931