New chiral imidazolium ionic liquids: 3D-network of hydrogen bonding

Article abstract of DOI:10.1016/j.tetlet.2004.04.063

New hydrophobic chiral ionic liquids bearing an imidazolium core have been stereospecifically prepared from the chiral pool; their enantiomeric purity and 3D-network of hydrogen bonding were analysed by NMR and X-ray diffraction, respectively.

Full text of DOI:10.1016/j.tetlet.2004.04.063

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 4429–4431
New chiral imidazolium ionic liquids: 3D-network of
hydrogen bonding
Jonathan J. Jodry and Koichi Mikami*
Department of Applied Chemistry, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8552, Japan
Received 5 March 2004; revised 10 April 2004; accepted 14 April 2004
Abstract—New hydrophobic chiral ionic liquids bearing an imidazolium core have been stereospecifically prepared from the chiral
pool; their enantiomeric purity and 3D-network of hydrogen bonding were analysed by NMR and X-ray diffraction, respectively.
Ó 2004 Elsevier Ltd. All rights reserved.
1
9
In the last years, intensive research has transformed
ionic liquids from the curiosity they used to be to a new
type of medium with interesting properties: they cannot
evaporate nor inflame, can be used at a large range of
temperature, easily recycled and reused, making them
solvents of choice for green chemistry; Furthermore,
they can enhance the rate and increase the selectivity of
shift reagent in F NMR, on kilogram scale from (R)-2-
3
amino-butan-1-ol. Imidazolium salts with planar chi-
4
rality (3, resolved by NMR) and chiral lateral chains
5
6
(4), as well as thiazolinium salt 5, were also prepared.
Herein, we wish to report the straightforward synthesis
of new chiral ionic liquids bearing an imidazolium core,
an easy and efficient procedure to determine their
enantiomeric purity and the 3D-network of hydrogen
bonding obtained in the solid state.
1
a number of valuable reactions. The possibility to use
chiral ionic liquids (CILs) as inducers for asymmetric
reactions has recently prompted researchers to synthes-
ise such chiral solvents, in particular. Seddon prepared
the chiral lactate salt of 1-butyl-3-methylimidazolium
Imidazolium-based cations usually lead to ionic liquids
with favourable properties, namely low melting point
and viscosity, which explain that they represent the
biggest class of ionic liquids. We therefore were inter-
ested in the introduction of chirality on such derivatives.
The following properties were also targeted: (i)
straightforward and easy preparation, applicable on
larger scale; (ii) presence of functional groups that could
lead to favourable interactions with substrates. Previ-
ously synthesised CILs have shown to form diastereo-
meric interactions with a chiral shift reagent in NMR;
however, either the magnitude of the splitting of the
signals is small, preventing a precise measurement of the
(
Alder reactions therein (however <5% ee were
bmim) 1 (Fig. 1), and investigated asymmetric Diels–
2
obtained). Wasserscheid synthesised CIL 2 as chiral
-
BETI-
OH
Tf N
2
N
O-
N
N
OH
N
N
Bu
O
1
2
3
Br-
CH OH
Et
-
4
3;7
Tf N
enantiopurity, or affording only a qualitative result.
2
N
N
S
N
Therefore, there is a need for a new procedure allowing
the direct measurement of the enantiomeric purity by
NMR.
2
Et
Bu
R
i
i
4
(R = CH , Pr, Bu)
5
3
Figure 1. Examples of chiral ionic liquids.
Commercially available and inexpensive (S)-ethyl lac-
tate was transformed onto its triflate derivative, which
upon reaction with 1-methylimidazole afforded the tri-
flate salt 6 as a solid (melting point 73 °C), in excellent
yield (Scheme 1). The enantiomeric purity of this salt
can easily be measured by using Lacour’s TRISPHAT
Keywords: Ionic liquids; Chirality; Green chemistry.
*
Corresponding author. Tel.: +81-3-5734-2142; fax: +81-3-5734-2776;
e-mail: kmikami@o.cc.titech.ac.jp
0
040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.04.063

4430
J. J. Jodry, K. Mikami / Tetrahedron Letters 45 (2004) 4429–4431
TfO-
HPF
OEt
(92%)
/ H O (73%)
6 2
OH
i) Tf
CH
2
O, 2,6-lutidine
O
7
8
X = PF6
X = NTf
Cl , 0°C
2
OEt
2
N
N
X-
LiNTf2 / H O (89%)
2
ii) 1-methylimidazole
Et O, -78°C
O
O
2
2
N
N
6
OEt
LiBETI / H O (94%)
2
9
1
X = N(SO C F )
2
2
5 2
Scheme 1. Preparation of ester-imidazolium salt.
(BETI)
6
X = OTf
Li(SO C F )Tf
2
4 9
0 X = N(SO C F )Tf
2
4 9
H O, 88%
2
7
8
anion as NMR chiral shift reagent. Addition of this
salt to a CDCl solution of the racemic triflate salt 6
obtained from (rac)-ethyl lactate) showed a baseline
separation between the signals of the two enantiomers
Scheme 2. Anion metathesis.
3
(
Table 1. Melting points and glass transition temperatures of CIL 6–10
(
(
Fig. 2, spectrum b, Ddmax ¼ 0:12 ppm), while only one
°C, DSC)
set of signals was observed with 6 obtained from (S)-
ethyl lactate (spectrum c, >97% ee). Control of the
reaction temperature is crucial: when the triflate deriv-
ative reacted with 1-methylimidazole at 0 °C, salt 6 was
obtained as a racemic mixture. No enantiospecificity
was obtained starting from halide, mesylate or tosylate
derivatives.
a
CIL
Anion
OTf
Melting point
Glass transition
6
73
––
)50
)58
)57
)56
)54
b
7
PF
6
c
8
NTf2
45
9
10
N(SO
N(SO
2
C
C
2
F
F
5
)
)Tf
2
––
––
2
4
9
a
b
c
Measured by heating samples cooled at )120 °C.
Viscous oil.
Anion metathesis was performed to get salts 7–10
(
Scheme 2). This synthetic pathway has a double
From solid obtained by slow recrystallisation; once melted, this CIL
only displayed a glass transition at )57 °C during repetitive cooling/
warming cycles.
advantage: salt 6 is obtained as a white solid, allowing to
get rid of the colouration, which can appear during the
quaternisation of the 1-methylimidazole; and no trace of
halide––incompatible with several asymmetric processes
and difficult to remove completely––is present in the
products. All CILs 7–10 are hydrophobic, and therefore
can be washed with water to ensure that no triflate anion
remains. They are obtained as colourless liquids at room
temperature. While 7 has an oily consistency, salts 8–10
are free-flowing, low-viscous liquids. CIL 8, whose
preparation can be easily scaled-up (50 g, overall yield
for two months, small crystals melting at 45 °C
appeared.
The structure of 8 was resolved by low temperature
9
X-ray diffraction analysis (Fig. 3). To our surprise, a
complete racemisation of the salt was observed. All
cations are surrounded closely by three NTf anions. C–
2
Hꢀ ꢀ ꢀO and C–Hꢀ ꢀ ꢀN hydrogen bonds are observed
10
8
2%), does not solidify upon cooling but rather becomes
very viscous at around )20 °C. DSC measurements
Table 1) showed that this liquid phase undergoes a glass
between hydrogens of the cations and the oxygen or
nitrogen of the anions (Table 2). Due to the negative
(
charge of the anion, very short distances are observed.
0
transition ()57 °C). All other CILs (7, 9, 10) exhibit a
similar phase transition. Despite multiple attempts, none
of those CILs could be crystallised. However, when a
flask containing neat salt 8 was left open on the bench
Typically, interactions between protons C(1 ) and
C(2)––the most acidic of the cation––with, respectively,
the nitrogen and the two oxygens of anions can be
considered as relatively strong, in regards of their
lengths (2.36–2.44 A) and angles (140°–160°).
ꢀ
10a;11
1
1
Figure 2. H NMR chiral shift experiments. H NMR (300 MHz,
CDCl of (a) (rac)-6; (b) (rac)-6 + [n-Bu N][D-TRISPHAT]
1.6 equiv); (c) (R)-6 + [n-Bu N][D-TRISPHAT] (3.0 equiv).
3
)
4
(
4
Figure 3. 3D-network of (rac)-8 obtained by X-ray diffraction analysis.

J. J. Jodry, K. Mikami / Tetrahedron Letters 45 (2004) 4429–4431
Table 2. Distances [A] and angles of C–Hꢀ ꢀ ꢀ(O/N) hydrogen bonds
4431
ꢀ
References and notes
Cꢀ ꢀ ꢀX
Angle C–Hꢀ ꢀ ꢀX
Angle C–Hꢀ ꢀ ꢀX
1
. For books, see: (a) Ionic Liquids in Synthesis; Wassersc-
heid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, 2002;
0
C(1 )–Hꢀ ꢀ ꢀN
2.445
2.444
2.361
2.509
3.395
3.329
3.185
3.287
160.62
156.85
146.13
140.18
C(4)–Hꢀ ꢀ ꢀO
C(2)–Hꢀ ꢀ ꢀO
C(2)–Hꢀ ꢀ ꢀO
(
b) Green Industrial Applications of Ionic Liquids; Rogers,
R. D., Seddon, K. R., Volkov, S., Eds.; Kluwer Academic:
Dordrecht, 2002; For reviews, see: (c) Welton, T. Chem.
Rev. 1999, 99, 2071; (d) Wasserscheid, P.; Keim, W.
Angew. Chem., Int. Ed. 2000, 39, 3772; (e) Sheldon, R.
Chem. Commun. 2001, 2399; (f) Zhao, D.; Wu, M.; Kou,
Y.; Min, E. Catal. Today 2002, 74, 157; (g) Dupont, J.; de
Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102, 3667;
(h) Mikami, K. Ed. New Environmentally Benign Reaction
Media; Scheffield Academic Press, in press.
Presence of this network of hydrogen bonding is in
sharp contrast with the previously reported structure of
a NTf ionic liquid, in which substitution of the C(2)
proton by a methyl group and absence of polar func-
tional group prevented formation of strong hydrogen
bonds. In the meanwhile, the mother liquor from
which the crystals rac-8 were obtained was still enan-
tiomerically pure. Separate experiments showed that in
the presence of a base, racemisation occurs readily due
to the acidity of the proton C(1 ). However, the imi-
dazolium-ester cation is configurationally stable in
neutral or acidic conditions, even upon heating.
2
2
3
4
. Earle, M. J.; McCormac, P. B.; Seddon, K. R. Green
Chem. 1999, 1, 23.
. Wasserscheid, P.; Boesmann, A.; Bolm, C. Chem. Com-
mun. 2002, 200.
. Ishida, Y.; Miyauchi, H.; Saigo, K. Chem. Commun. 2002,
12
2
240.
0
5
6
. Bao, W.; Wang, Z.; Li, Y. J. Org. Chem. 2003, 68, 591.
. Levillain, J.; Dubant, G.; Abrunhosa, I.; Gulea, M.;
Gaumont, A.-C. Chem. Commun. 2003, 2914.
7
8
9
. (a) Lacour, J.; Ginglinger, C.; Grivet, C.; Bernardinelli, G.
Angew. Chem., Int. Ed. Engl. 1997, 36, 608; (b) Lacour, J.;
Ginglinger, C.; Favarger, F. Tetrahedron Lett. 1998, 39,
4825; (c) Jodry, J. J.; Lacour, J. Chem. Eur. J. 2000, 6, 4297.
. (a) Lacour, J.; Goujon-Ginglinger, C.; Troche-Haldi-
mann, S.; Jodry, J. J. Angew. Chem., Int. Ed. 2000, 39,
Experimental data
3
695; (b) Lacour, J.; Jodry, J. J.; Ginglinger, C.; Torche-
Haldimann, S. Angew. Chem., Int. Ed. 1998, 37, 2379.
. Crystal dataofrac-8: C11
, M ¼ 463:38, mono-
clinic, a ¼ 13:1697ð3Þ, b ¼ 8:5394ð3Þ, c ¼ 17:4529ð4Þ A,
[
[
1-(1-(R)-Ethoxycarbonyl-ethyl)-3-methyl-imidazolium]-
trifluoromethanesulfonate] 6: To a solution of (S)-2-
15 6 3 6 2
H F N O S
trifluoromethanesulfonyloxy-propionic acid ethyl ester
30.14 g) in dry Et
O was added dropwise at )78 °C a
cooled solution of 1-methyl-imidazole (9.910 g) in Et O.
After 30 min, the mixture was warmed at rt, the white
solid was filtered and washed with Et O to give 39.28 g
ꢀ
(
2
ꢀ
3
U ¼ 1871:91ð9Þ A , T ¼ 223 K, space group P2
1
/c (#14),
ꢁ1
2
Z ¼ 4, l(MoK ) ¼ 0.377 mm , 3940 reflections measured,
a
int
3651 unique (R ¼ 0:0594), which were used in all
2
calculations. The final wRðF Þ was 0.1587 (all data).
2
CCDC 200303 contains the supplementary crystallographic
data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the CCDC, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336033; e-mail: deposit@ccdc.cam.
ac.uk).
of 6 (98%). NMR chiral shift experiments: To 5–10 mg
of 6 in CDCl (500 lL), were added 1–3 equiv of [n-
Bu N][D-TRISPHAT] salt (prepared following Ref. 7).
Ee was measured by direct integration. Similar proce-
dure was successfully used for CILs 7 and 8. CIL 7–10
were prepared by classical procedures from acid (HPF6)
3
4
1
1
0. (a) Desiraju, G. R. Acc. Chem. Res. 1996, 29, 441; (b)
Steiner, T.; Desiraju, G. R. Chem. Commun. 1998, 891; (c)
Thaimattam, R.; Reddy, D. S.; Xue, F.; Mak, T. C. W.;
Nangia, A.; Desiraju, G. R. J. Chem. Soc., Perkin Trans. 2
2 2 4 9
or lithium salts (LiNTf , LiBETI, LiN(SO C F )Tf),
washed with H
2
O and dried.
1
8
998, 1783; (d) Calhorda, M. J. Chem. Commun. 2000,
01.
1. Similar C–Hꢀ ꢀ ꢀO hydrogen bonds have already been
observed in ionic liquids: (a) Wilkes, J. S.; Zaworotko,
M. J. J. Chem. Soc., Chem. Commun. 1992, 965; (b)
Holbrey, J. D.; Reichert, W. M.; Swatloski, R. P.; Broker,
G. A.; Pitner, W. R.; Seddon, K. R.; Rogers, R. D. Green
Chem. 2002, 4, 407; (c) Holbrey, J. D.; Visser, A. E.;
Reichert, W. M.; Swatloski, R. P.; Broker, G. A.; Rogers,
R. D. Green Chem. 2003, 5, 129.
Acknowledgements
We thank Prof. M. Wakihara and Dr. H. Ikuta (Tokyo
Institute of Technology) for DSC measurements, Dr. M.
Hatano (from our group) for X-ray measurement,
Central Glass Co, Ltd for the generous gift of triflic
anhydride and lithium salts, and the Japan Society for
the Promotion of Science for funding.
12. Golding, J. J.; MacFarlane, D. R.; Spiccia, L.; Forsyth, M.;
Skelton, B. W.; White, A. H. Chem. Commun. 1998, 1593.