5
382
V. V. Namboodiri, R. S. Varma / Tetrahedron Letters 43 (2002) 5381–5383
anions using microwave (MW) irradiation under solvent-
free conditions. Herein, we report an efficient method for
the preparation of RTILs that involves exposing a
mixture of imidazolium halides and ammonium tetra-
fluoroborate to microwaves in an unmodified household
MW oven. This solvent-free approach requires only a few
minutes of reaction time, in contrast to the several hours
needed using conventional methods that require a large
excess of organic solvent or expensive silver salts or
aqueous tetrafluoroboric acid, which is difficult to han-
The present method is general in nature and is suitable
for the preparation of ionic liquids bearing alkyl chains
of varying lengths (C ) or different halides. The MW-
n
assisted syntheses of a series of ionic liquids and compari-
son of these protocols with the corresponding
preparations using conventional heating (oil bath at
90°C) are summarized in Table 2. Most of the halides
used in this study are stable enough and are converted
efficiently to the corresponding tetrafluoroborates under
MW irradiation conditions. The ionic liquids comprised
of longer carbon chains such as the octyl group required
extended reaction time. The tetrafluoroborates obtained
from the corresponding iodides are slightly colored and
resisted complete decoloration even after repeated wash-
ings. The main advantages of this high yield method are
use of a kitchen MW oven, faster generation of products,
higher conversion and easier work up procedure com-
pared to methods employing aqueous tetrafluoroboric
acid.
1
2
dle. A general schematic representation and the prepa-
ration of 1,3-dialkylimidazolium tetrafluoroborates are
depicted below in Tables 1 and 2.
For these studies, a newer household MW oven (Pana-
sonic NN-S740WA-1200) equipped with inverter tech-
nology that provides a better control of the MW power
to a desired level was used. Initially, the effect of MW
power level on the formation of 1-butyl-3-methylimida-
zolium tetrafluoroborate ([C MIM][BF ]) from 1-butyl-
4
4
3
-methylimidazolium chloride ([C MIM]Cl) and
4
In a representative reaction procedure, ammonium tetra-
fluoroborate (1.05 mmol) and butylimidazolium chloride
ammonium tetrafluoroborate as reactants was examined
Table 1). The formation of ionic liquid could be mon-
(
(
1.0 mmol) were placed in a test tube, mixed thoroughly
itored visibly in the reaction when the ammonium
tetrafluoroborate crystals reacted and formed fine precip-
itates of ammonium halide. At elevated power levels, a
partial decomposition or charring of the ionic liquid
occurred possibly due to the localized overheating of
ionic liquid, resulting in lower yields. To circumvent this
problem, the reactions were conducted with intermittent
heating at a moderate power level with mixing to obtain
better yields and cleaner product formation. After the
initial exposure for 30 s to MWs at power level P3 (360
W, bulk temperature ꢀ80–110°C) the reaction mixture
was taken out, mixed for 10 s and then heated at the same
power level for an additional 30 s. This sequence was
repeated until the formation of fine precipitate of ammo-
nium halide ensued. At this stage, dry acetone was added,
the ammonium salts were simply removed by filtration
and the product was isolated by removal of acetone from
the filtrate and then drying under vacuum at 60°C. The
ionic liquids with alkyl chain length hexyl or higher are
insoluble in water and can be purified very easily by
washing with water followed by vacuum drying. The
TGA and DSC analyses show that these ionic liquids are
thermally stable up to 350°C and the spectral character-
using a vortex mixer and the contents were heated
intermittently in the MW oven at power P3 correspond-
ing to 360 W (30 s irradiation with 10 s mixing) until the
dissolution of ammonium tetrafluoroborate crystals was
completed and the formation of fine white precipitate of
ammonium halide occurred. The bulk temperature
recorded was in the range 80–110°C. The resulting ionic
liquid [C MIM][BF ] was then cooled to room tempera-
4
4
ture, dry acetone was added and ammonium chloride and
the unreacted starting materials were removed by filtra-
tion. Additional purification, if required, can be accom-
1
2
plished using the procedures described by Seddon et al.
The product was then dried under vacuum at 80°C to
afford 1-butyl-3-methylimidazolium tetrafluoroborate
1
(
90%), H NMR (250 MHz; acetone-d ), l : 0.98 (t,
6
H
CH ), 1.40 (m, CH ), 1.95 (m, CH ), 4.04 (s, N-CH ), 4.30
3
2
2
3
(m, N-CH
N(H)CN); C NMR l
31.51 (m, CH ), 35.86 (N-CH
(NCH), 123.83 (NCH), 136.61 (N(H)CN); F NMR had
a singlet at l −152.80. The same experiment conducted
via conventional heating (oil bath at 90°C for 3 h)
afforded 28% of [C MIM][BF ]. An experiment, on a
), 7.61 (s, NCH), 7.66 (s, NCH), 8.81 (s,
2
1
3
12.89 (t, CH
), 19.02 (m, CH
),
), 122.40
C
2
3
), 54.11 (N-CH
2
2
3
19
12,13
istics are in accordance with the literature reports.
4
4
a
Table 2. Preparation of 1,3-dialkyl imidazolium tetrafluoroborates using microwaves
Substrate (1 mmol)
MW-power (W)
MW-time (s)
Yield% MW (oil bath)
[
[
[
[
[
[
[
[
[
C MIM]Cl
360
360
240
360
360
360
240
360
360
30+30+30+30+30
90 (28)
92 (36)
92
4
C MIM]Br
30+30+30+30+30
30+30+30+30+30+30
30+30+30+30+30
30+30+30+30+30
30+30+30+30+30
30+30+30+30+30+30
60+60+30+30+30
60+60+30+30+30
4
b
C MIM]I
4
i-C MIM]Br
91
89
88
4
C MIM]Cl
6
C MIM]Br
6
b
C MIM]I
88
7
C MIM]Cl
89 (35)
88
8
C MIM]Br
8
a
NH BF4 (1.05 mmol).
4
b
Colored even after several washings but the NMR spectrum is in accord with the structure.
Namboodiri, Vasudevan V
