1-Butyl-2,3-trimethyleneimidazolium bis(trifluoromethylsulfonyl)imide ([b-3C-im][NTf2]): A new, stable ionic liquid

Article abstract of DOI:10.1016/j.tetlet.2005.12.125

Using imidazole as the starting material, the synthesis of a new bicyclic ionic liquid [b-3C-im][NTf₂] is described. Except for the alkylation reaction in the second step (40% yield) of this four-step synthesis of [b-3C-im][NTf₂], others were all high yielding reactions (85-94% isolated yields). We investigated intrinsic reactivity of this and other imidazolium-based ionic liquids and found that, under strongly basic conditions (KOD in CD₃OD/D₂O (1:1) solution), the new ionic liquid was stable to solvent deuterium isotope exchange while the previously reported [bdmim][NTf₂] and [bdmim][PF₆] ionic liquids were 50% deuterium exchanged at its C-2 methyl in 30 min at ambient temperature. At the same experimental condition, the most commonly employed [bmim][PF₆] ionic liquid was deuterium exchanged instantaneously at its C-2 hydrogen. In the absence of bases (CD₃OD/D₂O = 1:1), only [bmim][PF ₆] was deuterium exchanged (50% within 1 h) and other ionic liquids gave no detectable exchanges even after one week at ambient temperature. It is therefore concluded that the new [b-3C-im][NTf₂] ionic liquid is far more chemically stable than previously reported [bmim][PF₆], [bdmim][NTf₂], and [bdmim][PF₆].

Full text of DOI:10.1016/j.tetlet.2005.12.125

Tetrahedron
Letters
Tetrahedron Letters 47 (2006) 1575–1579
1-Butyl-2,3-trimethyleneimidazolium bis(trifluoromethyl-
sulfonyl)imide ([b-3C-im][NTf ]): a new, stable ionic liquid
2
Jen-Yen Cheng and Yen-Ho Chu*
Department of Chemistry and Biochemistry, National Chung Cheng University, Chia-Yi, Taiwan 621, ROC
Received 24 November 2005; accepted 28 December 2005
Available online 19 January 2006
Abstract—Using imidazole as the starting material, the synthesis of a new bicyclic ionic liquid [b-3C-im][NTf ] is described. Except
2
for the alkylation reaction in the second step (40% yield) of this four-step synthesis of [b-3C-im][NTf
reactions (85–94% isolated yields). We investigated intrinsic reactivity of this and other imidazolium-based ionic liquids and found
that, under strongly basic conditions (KOD in CD OD/D O (1:1) solution), the new ionic liquid was stable to solvent deuterium
isotope exchange while the previously reported [bdmim][NTf ] and [bdmim][PF ] ionic liquids were 50% deuterium exchanged at
2
], others were all high yielding
3
2
2
6
its C-2 methyl in 30 min at ambient temperature. At the same experimental condition, the most commonly employed [bmim][PF
ionic liquid was deuterium exchanged instantaneously at its C-2 hydrogen. In the absence of bases (CD OD/D O = 1:1), only
bmim][PF ] was deuterium exchanged (50% within 1 h) and other ionic liquids gave no detectable exchanges even after one week
6
]
3
2
[
6
at ambient temperature. It is therefore concluded that the new [b-3C-im][NTf ] ionic liquid is far more chemically stable than pre-
2
viously reported [bmim][PF
ꢀ
6
], [bdmim][NTf
2006 Elsevier Ltd. All rights reserved.
2
], and [bdmim][PF
6
].
1
We recently demonstrated that [bdmim][PF ] and
thermal stability, non-flammability, and high ionic con-
ductivity that readily accelerates rates of reactions in,
for example, microwave-mediated organic synthesis.
6
2
[
bdmim][NTf ] ionic liquids are much more chemically
stable than the popular [bmim]-based ionic liquids, and
thus avoid some of the competing reactions such as
nucleophilic additions or substitutions that are evident
in [bmim] solvents. We now have extended our research
to a new, more stable and bicyclic imidazolium
ionic liquid, 1-butyl-2,3-trimethyleneimidazolium bis(tri-
2
1
b
Because of these intriguing properties, ionic liquids out-
perform conventional solvents in many organic reactions
and applications such as separation technology and bat-
1
2
3
tery electrochemistry.
fluoromethylsulfonyl)imide ([b-3C-im][NTf ]). Ionic liq-
A large number of ionic liquids have been reported in
the literature. Up to now, the alkylimidazolium salts
are still the most studied and best characterized ionic liq-
2
3
uids are a class of highly polar solvents that are
3
entirely constituted of ions. They are liquid at low tem-
perature and are often considered as recyclable and envi-
ronmentally friendly substitutes for conventional
organic solvents, primarily because of their negligible
vapor pressure, ability to dissolve a large array of com-
pounds in relatively small volumes, good chemical and
uids. 1-n-Butyl-3-methylimidazolium hexafluorophos-
phate ([bmim][PF ]) ionic liquid, for example, has been
6
widely studied in many areas of research for reasons
of its ease of preparation, lack of vapor pressure, and,
most significantly, hydrophobicity that presents an
Me
Me
N
Me
N
N
nBu
PF6
PF6
N(CF SO )
N(CF SO )
3 2 2
3
2 2
Me
N
N
n
Bu
Me
N
N
n
Bu
N
n
Bu
[
bmim][PF6]
[bdmim][PF6]
[bdmim][Tf
2
N]
[b-3C-im][NTf2]
Keywords: Ionic liquid; Organic synthesis.
*
Corresponding author. Tel.: +886 5 242 8148; fax: +886 5 272 1965; e-mail: cheyhc@ccu.edu.tw
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.12.125

1576
J.-Y. Cheng, Y.-H. Chu / Tetrahedron Letters 47 (2006) 1575–1579
obvious advantage as a potential replacement for vola-
tile organic solvents in developing green processes.
yield (85%). For the second step, substituents could be
introduced via in situ lithiation by n-butyllithium at
the most acidic C-2 of 1-butylimidazole to produce
However, it is known that the [PF ] anion in ionic liq-
6
8
uids has the propensity to decompose to release HF
and related species, and the [bmim] cation is not at all
1,2-disubstituted imidazoles. Initially, we investigated
the C-2 substitution using excessive 1,3-dibromopro-
pane with a hope to prepare the expected 1-butyl-2-(3-
bromopropyl)imidazole, but soon encountered prob-
lems of side products such as the dialkylated 2:1 adduct
as well as the unwanted quaternized ionic liquids of var-
ious forms contaminated with the desired 1:1 product.
During the course of synthesis of this new ionic liquid,
we became aware that the nature of the halo group to
be displaced in alkyl halides can dramatically determine
1
–3
compatible with the highly basic reaction conditions.
These disadvantages obviously limited the application
of ionic liquids to synthetic reactions. The development
of ionic liquids with new formulation is therefore of an
urgent need.
In our continuous quest aimed toward developing chem-
ically stable and hydrophobic ionic liquids with utilities
as green solvents for organic synthesis, we describe in
this report the facile synthesis of a new bicyclic ionic
8
b
the nature of the product. Although simple alkyl iod-
ides such as n-butyl iodide behaved as anticipated for
lithiation–alkylations, there was no guarantee that other
alkyl iodides such as 3-chloro-1-iodopropane would fol-
low suit. After several trials, we were pleased to find
that, though the reaction was low-yielding (40%) and
its experimental condition remained preliminary, the
reaction of the lithiated N-butyllimidazole with 3-
chloro-1-iodopropane cleanly furnished the 1-butyl-2-
liquid [b-3C-im][NTf ]. The bicyclic [b-3C-im] cation
2
was employed in this new formulation for reasons that
the proton at the C-2 in [bmim] as well as the C-2 methyl
in [bdmim] cations are known to be relatively acidic and
4
can be readily exchanged with D O, and under basic
2
conditions the [bmim] cation, in particular, readily
1
reacts with various electrophiles such as aldehydes. In
5
addition, because of its electron richness at C-2 (C2–
CH R in [b-3C-im] vs C2–CH in [bdmim] and C2–H
(3-chloropropyl)imidazole (Scheme 1). Upon standing
at ambient temperature for 8 days, this 1-butyl-2-(3-
chloropropyl)imidazole slowly but smoothly cyclized
to afford the ionic liquid [b-3C-im][Cl] with quantitative
yield. If the condition of the Finkelstein cyclization (NaI
in refluxed methanol) was employed, we found that the
reaction was accelerated to completion in 5 h with excel-
lent 94% isolated yield. The [b-3C-im][Cl] could readily
2
3
in [bmim]) of this constrained dihydropyrrol[1,2-a]imi-
dazolium ring, it is expected that the resulting higher
pK at C-2 makes [b-3C-im] cation more chemically sta-
a
ble. The hydrophobic [NTf ] anion was chosen in our
2
new ionic liquid formulation not only because it is
known to be hydrolytically stable, but also due to the
fact that the strong delocalization of the negative charge
in the fluoroanion weakens its interaction with the cat-
ion, therefore leading to lower melting points of ionic
be converted to the final [b-3C-im][NTf ] ionic liquid
2
via a simple ion exchange in water (Scheme 1).
3
9
liquids. Furthermore, the organofluoro compounds
[b-3C-im][NTf ] is a liquid at ambient temperature. We
2
often possess unique resistance to extremes of tempera-
ture and pressure, to corrosive acids and bases, and to
oxidizing agents.
were interested in the [NTf ]-based ionic liquids because
2
of their reported minimal water association with the
3
10,11
liquid and unusual low melting points.
In addition,
the conformationally constrained and non-planar struc-
ture of this new ionic liquid may somewhat contribute to
low melting point. For physical properties, [b-3C-
Our synthesis of [b-3C-im][NTf ] is outlined in Scheme
2
5
,6
1
.
In the first step, the needed 1-butylimidazole was
prepared from inexpensive imidazole and 1-butylbro-
im][NTf ] is miscible with polar organic solvents—meth-
2
5
,7
mide following a modified literature procedure. Under
refluxed condition (KOH in acetonitrile), this N-alkyl-
ation reaction was readily achieved with high isolated
anol, acetone, dichloromethane, ethyl acetate, and ace-
tonitrile—and insoluble in less polar solvents such as
diethyl ether, n-hexane, and toluene. Like ionic liquids
N
N
N
(
a)
(b)
Cl
+
I
Cl
N
N
Bu
N
H
n
Bu
n
(c)
N
(d)
N
I
N(CF SO )
3 2 2
N
N
n
Bu
nBu
[b-3C-im][NTf2]
Scheme 1. Preparation of a new bicyclic room temperature ionic liquid, [b-3C-im][NTf
bromobutane (1 equiv), KOH (2 equiv), acetonitrile, refluxed, 4 h, 85%; (step b) 1-butylimidazole (1 equiv), BuLi (1.2 equiv), 1-chloro-3-iodopropane
1 equiv), THF, ꢀ50 to 60 ꢁC (1 h) ! rt (7 h), 40%; (step c) 1-butyl-2-(3-chloropropyl)imidazole (1 equiv), NaI (1.1 equiv), MeOH, refluxed, 5 h,
4%; (step d) [b-3C-im][I] (1 equiv), LiNTf (1 equiv), H O, 12 h, rt, 92%.
2
]. Reagents and conditions: (step a) imidazole (1 equiv), 1-
(
9
2
2

J.-Y. Cheng, Y.-H. Chu / Tetrahedron Letters 47 (2006) 1575–1579
1577
imidazolium cation likely undergoes base-catalyzed
abstraction of the C-2 proton to form the ylide inter-
mediate stabilized by a carbene-like resonance structure.
1
00
[b-3C-im][NTf2]
1
2
8
0
0
0
0
In neutral conditions (CD OD/D O = 1:1), only
3
2
[
bmim][PF ] was found to be deuterium exchanged rap-
6
[
bdmim][NTf2]
6
4
2
idly (50% in 1 h) and [b-3C-im][NTf ] gave no detectable
2
exchanges even after one week at ambient tempera-
1
3
ture. Another two ionic liquids, [bdmim][NTf ] and
2
[
bdmim][PF ], produced less than 10% deuterium
6
exchange at C-2 methyl group at room temperature
for one week time. From the results of solvent deuterium
exchange experiments, we therefore concluded that the
[bdmim][PF6]
[
bmim][PF6]
new [b-3C-im][NTf ] ionic liquid is far more chemically
2
0
stable than previously reported [bmim][PF ], [bdmim]-
6
[
NTf ], and [bdmim][PF ] (i.e., [b-3C-im][NTf ] ꢁ
2
6
2
0
40
80
120
160
200
240
280
320
[bdmim][NTf ] ꢂ [bmim][PF ] ꢁ [bdmim][PF ]).
2 6 6
Time (min)
The results presented in this research clearly indicate
that C2-unsubstituted ionic liquids such as [bmim][PF6]
are chemically reactive; the replacement of the C-2
hydrogen with a methyl group to produce 2-methyl-
Figure 1. Solvent deuterium isotope exchanges at C-2 of [b-3C-
im][NTf ], [bdmim][NTf ], [bdmim][PF ], and [bmim][PF room
temperature ionic liquids (0.1 M each) in CD OD/D O (1:1, v/v;
.5 mL) containing 0.1 M KOD. The progress of deuterium exchange
2
2
6
6
]
3
2
0
1
could be readily monitored by H NMR.
substituted ionic liquids (e.g., [bdmim][NTf ]) adds
2
additional stability but is not totally inert under basic
reaction conditions. Our new [b-3C-im][NTf ] ionic
2
incorporating PF , the new ionic liquid is immiscible
with water.
liquid appears to provide sufficient chemical tolerance
to bases so that many potential side reactions that are
common in imidazolium-based ionic liquids may there-
fore be avoided.
6
Handy et al. recently investigated chemical reactivities
of C2-unsubstituted as well as C2-methyl-substituted
fructose-derived imidazolium ionic liquids and found
that, in the absence of base, only C2-unsubstituted ionic
In summary, we report a new ionic liquid [b-3C-
im][NTf ] that fulfills the requirement as an inert solvent
2
4
a
liquid underwent deuterium isotope exchange rapidly.
and this ionic liquid opens exciting perspectives of use
for synthetic applications of many natural and non-nat-
ural products of biological significance. In addition, this
They further established that the C2-methyl-substituted
ionic liquid was not totally inert and could proceed deu-
4
a
terium exchange under basic conditions. Their results
prompted us to investigate the intrinsic chemical reactiv-
conformationally constrained [b-3C-im][NTf ] ionic
2
liquid may enable new applications that are not possible
with conventional solvents.
ity of our new [b-3C-im][NTf ]. Also, we envisaged that
2
the constrained ring system and high pK at C-2 of
a
[
b-3C-im][NTf ] may exhibit resistance to solvent deute-
2
rium exchanges. For these reasons, we decided to set up
deuterium isotope exchange experiments at ambient
Acknowledgments
temperature for the new [b-3C-im][NTf ] and three
previously reported ionic liquids ([bmim][PF6],
We gratefully acknowledge support of this work
through grants from the National Science Council of
Taiwan, Republic of China (NSC94-2113-M-194-012,
NSC94-2213-E-194-046, and NSC94-2213-E-194-047).
We thank Ping-Yu Lin of Academia Sinica (Taiwan,
ROC) for mass analyses and Yang-Min Liang for his
contribution at the early stage of this work.
2
[
bdmim][PF ], and [bdmim][NTf ]).
6 2
Exchange rates of the C-2 hydrogens of imidazolium
ions were measured in two (neutral and strongly basic)
experimental conditions by observing the change in area
1
of the signal due to the C-2 hydrogens in the H NMR
spectra. We found that, under strongly basic conditions
(
0.1 M KOD in the 1:1 mixture of CD OD and D O),
3 2
Supplementary data
the new [b-3C-im][NTf ] ionic liquid (0.1 M) underwent
2
essentially no deuterium isotope exchanges at C-2 meth-
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2005.
ylene even after 1 day while the [bdmim][NTf ] ionic
2
liquid was 50% deuterium exchanged at C-2 methyl
position in 20 min at ambient temperature (Fig. 1).
12.125.
1
Rates of exchange were based on the decrease in H
NMR signals relative to those for the non-exchanging
groups. We were totally surprised to find that, under
the strongly basic condition, the widely used
References and notes
. (a) Yen, Y.-H.; Chu, Y.-H. Tetrahedron Lett. 2004, 45,
1
[
bmim][PF ] ionic liquid was deuterium exchanged
8137–8140; (b) Hsu, J.-C.; Yen, Y.-H.; Chu, Y.-H.
6
instantaneously at C-2 (Fig. 1). This 1-butyl-3-methyl-
Tetrahedron Lett. 2004, 45, 4673–4676.

1578
J.-Y. Cheng, Y.-H. Chu / Tetrahedron Letters 47 (2006) 1575–1579
2
3
. Tseng, M.-C.; Liang, Y.-M.; Chu, Y.-H. Tetrahedron Lett.
005, 46, 6131–6136.
. For most recent reviews, see: (a) Chiappe, C.; Pieraccini,
D. J. Phys. Org. Chem. 2005, 18, 275–297; (b) Jain, N.;
Kumar, A.; Chauhan, S.; Chauhan, S. M. S. Tetrahedron
refluxed for 5 h, cooled to room temperature and was
2
filtered to remove sodium chloride. The filtrate was
concentrated in vacuo to give a slight yellow solid, which
was dissolved with dichloromethane (4 · 4 mL) and fil-
tered again. After removal of the solvent, the [b-3C-im][I]
1
2005, 61, 1015–1060; (c) Sheldon, R. A. Green Chem. 2005,
was isolated as a brownish oil (3.3 g, 94%); H NMR
7, 267–278.
(400 MHz, DMSO-d ) d 0.90 (t, 3H, J = 7.4 Hz, CH ),
6
3
4
. (a) Handy, S. T.; Okello, M. J. Org. Chem. 2005, 70, 1915–
1.28 (m, 2H, CH
2
), 1.74 (m, 2H, CH
2
), 2.62 (m, 2H, CH
2
),
1
918; (b) Deng, Y.; Hlasta, D. J. Tetrahedron Lett. 2002,
3.16 (t, 2H, J = 7.8 Hz, N@CCH
2
), 4.06 (t, 2H,
J = 7.2 Hz, C@NCH2), 4.20 (t, 2H, J = 7.4 Hz,
43, 189–192.
5
. Synthesis and characterization of 1-butyl-2,3-trimethylene-
imidazolium bis(trifluoromethylsulfonyl)imide, [b-3C-im]-
C@NCH ), 7.64 (d, 1H, J = 2.0 Hz, C@CH), 7.67 (d,
2
1H, J = 2.0 Hz, C@CH).
[NTf
2
], ionic liquid:
[b-3C-im][Cl]: To a round-bottomed flask containing-
1
-butyl-2-(3-chloropropyl)imidazole (0.94 g, 4.7 mmol)
was placed at ambient temperature for 8 days. The
spontaneously cyclized product was washed by ethyl
acetate and the resulting amber oil proved to be 1-butyl-
N
1
N
-butylimidazole
2
(
(
,3-trimethyleneimidazolium
chloride
0.95 g, 100%); H NMR (400 MHz, DMSO-d
t, 3H, J = 7.4 Hz, CH ), 1.28 (m, 2H, CH ), 1.73 (m, 2H,
[b-3C-im][Cl]
1
) d 0.90
To a round-bottomed flask containing imidazole (6.0 g,
8 mmol) in acetonitrile (50 mL) were added potassium
hydroxide (9.9 g, 177 mmol) and 1-bromobutane (9.9 mL,
0 mmol). The reaction mixture was refluxed for 4 h and
then cooled to room temperature. After a flash column
chromatography (ethyl acetate/methanol = 25:1), the
6
8
3
2
CH ), 2.66 (m, 2H, CH ), 3.14 (t, 2H, J = 7.6 Hz,
2
2
N@CCH ), 4.05 (t, 2H, J = 7.2 Hz, C@NCH ), 4.19 (t,
9
2
2
2
H, J = 7.2 Hz, C@NCH
2
), 7.64 (d, 1H, J = 1.8 Hz,
C@CH), 7.67 (d, 1H, J = 1.8 Hz, C@CH).
purified product 1-butylimidazole was isolated as an oil
1
(
9.5 g, 85%); H NMR (400 MHz, CDCl
3
) d 0.93 (t, 3H,
), 1.75 (m, 2H, CH ),
.92 (t, 2H, J = 7.2 Hz, NCH ), 6.89 (s, 1H, C@CH), 7.04
N
J = 7.4 Hz, CH
3
), 1.32 (m, 2H, CH
2
2
N(CF SO )
2 2
3
(
3
2
N
s, 1H, C@CH), 7.44 (s, 1H, N@CH).
n
Bu
n
Bu
[b-3C-im][NTf ]
2
N
Cl
To a solution containing 1-butyl-2,3-trimethyleneimidazo-
lium iodide (3.32 g, 11.3 mmol) and water (30 mL) was
added the bistrifluoromethanesulfonimide lithium salt
N
1
-butyl-2-(3-chloropropyl)imidazole
(
3.24 g, 11.3 mmol). The mixture was allowed to proceed
the ion exchange for 12 h at room temperature. Two
phases were formed in the mixture solution. The resulting
solution was diluted with dichloromethane (20 mL) and
then washed with water (3 · 10 mL). Removal of the
solvent under reduced pressure afforded 1-butyl-2,3-tri-
methyleneimidazolium bis(trifluoromethylsulfonyl)imide
Under dry nitrogen, a solution of butyllithium in hexanes
2.5 M, 16 mL, 40 mmol) was added dropwise in 10 min
(
time to a cold (ꢀ50 to 60 ꢁC) solution of 1-butylimidazole
(
(
3.96 mL, 30 mmol) in anhydrous tetrahydrofuran
60 mL). The yellow solution was stirred at ꢀ50 to 60 ꢁC
for 1 h, and 1-chloro-3-iodopropane (3.22 mL, 30 mmol)
was added dropwise, and the mixture was allowed to
slowly warm to room temperature (for a total of 7 h).
Ethyl acetate (100 mL) and brine (60 mL) were added and
[b-3C-im][NTf ] as a slightly brownish liquid (4.63 g, 92%);
2
1
H NMR (400 MHz, DMSO-d ) d 0.90 (t, 3H, J = 7.4 Hz,
6
CH ), 1.28 (m, 2H, CH ), 1.72 (m, 2H, CH ), 2.65 (m, 2H,
3
2
2
CH ), 3.13 (t, 2H, J = 7.6 Hz, N@CCH ), 4.04 (t, 2H,
2
2
the organic layer was separated and dried (Na
2
SO
4
). After
J = 7.2 Hz, C@NCH2), 4.18 (t, 2H, J = 7.3 Hz,
1
evaporation, the H NMR spectrum of the reaction
mixture indicated the presence of about 50% starting 1-
butylimidazole and the crude 1-butyl-2-(3-chloropropyl)-
imidazole was then purified by column chromatography of
C@NCH ), 7.62 (d, 1H, J = 2.0 Hz, C@CH), 7.64 (d,
2
1
3
1H, J = 2.0 Hz, C@CH). C NMR (100 MHz, DMSO-
d6) d 13.6, 19.2, 22.8, 25.9, 31.3, 48.1, 48.2, 118.2, 119.8 (q,
JCF = 320 Hz, CF ), 126.0, 152.6; FAB-HRMS m/z [M]
+
3
silica gel (ethyl acetate/methanol = 25:1) to afford an
calcd = 165.1392, obsd = 165.1392.
1
amber oil (2.4 g, 40%); H NMR (400 MHz, CDCl ) d 0.96
6. Using the same protocol, 1-methyl-2,3-trimethylene-
imidazolium bis(trifluoromethylsulfonyl)imide [m-3C-im]
3
(
t, 3H, J = 7.4 Hz, CH ), 1.36 (m, 2H, CH ), 1.72 (m, 2H,
3
2
CH
2
), 2.30 (m, 2H, CH
2
), 2.83 (t, 2H, J = 7.4 Hz,
[NTf ] was also prepared. This ionic liquid is however a
2
N@CCH
2
), 3.67 (t, 2H, J = 6.1 Hz, CH
2
Cl), 3.86 (t, 2H,
solid (mp 83–85 ꢁC) at room temperature.
J = 7.3 Hz, NCH ), 6.83 (s, 1H, C@CH), 6.94 (s, 1H,
7. Diez-Barra, E.; de la Hoz, A.; Sanchez-Migallon, A.;
Tejeda, J. Synth. Commun. 1993, 23, 1783–1786.
2
C@CH).
8
. (a) Huang, L.-F.; Bauer, L. J. Heterocycl. Chem. 1997, 34,
123–1130; (b) Davis, F. S.; Huang, L.-F.; Bauer, L. J.
1
N
N
Heterocycl. Chem. 1995, 32, 915–920; (c) Iddon, B.;
Ngochindo, R. I. Heterocycles 1994, 38, 2487–2568; (d)
Iddon, B. Heterocycles 1985, 23, 417–443.
I
Cl
N
Bu
N
or
n
n
Bu
[b-3C-im][Cl]
9
. 1-Methyl-2,3-trimethyleneimidazolium chloride ([m-3C-
im][Cl]) and its iodide ([m-3C-im][I]) were previously
[
b-3C-im][I]
8
a
prepared by Bauer and co-workers.
[
b-3C-im][I]: To a solution of 1-butyl-2-(3-chloropro-
10. Bonhote, P.; Dias, A.-P.; Papageorgiou, N.; Kalyana-
sundaram, K.; Gratzel, M. Inorg. Chem. 1996, 35, 1168–
1178.
pyl)imidazole (2.4 g, 12 mmol) in methanol (20 mL) was
added sodium iodide (1.98 g, 13.2 mmol). The mixture was

J.-Y. Cheng, Y.-H. Chu / Tetrahedron Letters 47 (2006) 1575–1579
1579
1
1. Koch, V. R.; Nanjundiah, C.; Appetecchi, G. B.; Scrosati,
B. J. Electrochem. Soc. 1995, 142, L116–L118.
2. Solvent deuterium isotope exchanges at C-4 and C-5 of
imidazoles have previously been determined to be very
much slower than at C-2. For example, the relative rates of
the C-2, C-4, and C-5 position of 1-methylimidazole in
13. Wherever possible comparisons can be made, our data
on rates of isotope exchange are in agreement with the
acidity data of corresponding C–Hs in imidazoles. For
1
1
3
a
example, using C NMR spectroscopy the pK value of
C-2 proton on N-methylimidazole in tetrahydrofuran was
reported to be 33.7: Fraser, R. R.; Mansour, T. S.; Savard,
S. Can. J. Chem. 1985, 63, 3505–3509; More recently,
Streitwieser and Kim employed UV–vis spectroscopy to
measure the acidity of a 1,3-di-tert-butylimidazolium salt
undergoing deuteration in D O are 54,500:1.6:1 at 163 ꢁC
2
or 31,578:2.7:1 at 50 ꢁC, respectively: (a) Takeuch, Y.;
Yeh, H. J. C.; Kirk, K. L.; Cohen, L. A. J. Org. Chem.
1
978, 43, 3565–3570; (b) Takeuch, Y.; Kirk, K. L.; Cohen,
and reported its C-2’s pK in DMSO as 22.7: Kim, Y.-J.;
a
L. A. J. Org. Chem. 1978, 43, 3570–3578; (c) Wong, J. L.;
Keck, J. H., Jr. J. Org. Chem. 1974, 39, 2398–2403.
Streitwieser, A. J. Am. Chem. Soc. 2002, 124, 5757–
5761.