Organic Syntheses, Vol. 79, p. 236
Checked by Susan L. Fulmer, David P. Richardson, Thomas E. Smith, and Steven Wolff.
1. Procedure
2. Notes
1.
N-Methylimidazole
is commercially available. The submitters used a product from Aldrich Chemical
Company, Inc. (99%) dried over
potassium hydroxide (KOH) pellets and
distilled (210-212°C).
4. The external temperature should not exceed 85°C.
6. The volatile material (
acetonitrile
and the excess of
1-chlorobutane)
is captured in a
liquid nitrogen trap. This solution (35 wt%
in
1-chlorobutane, determined
by GC) can be stored in a
dark flask and used for further synthesis.
8. The seed crystal is obtained by dissolving a sample (≈1
g) of the crude imidazolium salt in a minimum amount of
acetonitrile
(3 mL); this solution is allowed to stand at −30°C
overnight. The checkers observed spontaneous crystallization upon removal of volatile
materials.
9. A 150-rpm agitation speed is used and the rate of the addition
will determine the morphology of the imidazolium salt (from finely divided powder
to solid blocks that are difficult to powder).
10. If solid blocks are obtained they should be ground before drying.
11. Differential scanning calorimetry is performed at a heating rate
of 2°C/min from 20°C to 100°C. The checkers used a
conventional melting
point apparatus.
12. The product has the following spectral properties:
1H NMR (300 MHz, CDCl
3)
δ: 0.80 (t, 3 H,
3J
HH = 7.3), 1.23
(m, 2 H), 1.75 (m, 2 H), 3.98 (s, 3 H), 4.19
(t, 2 H,
3J
HH = 7.4), 7.46 (s, 1 H), 7.63
(s, 1 H), 9.55 (s, 1 H);
13C NMR (75 MHz, CDCl
3) δ:
13.6, 19.6, 32.3, 36.6, 49.8,
122.3, 124.0, 137.8; IR (neat film/NaCl plate) cm
−1: 3137, 3046,
2959, 2935, 2873, 1571, 1465,
1382, 1336, 1172.
13. The reaction is performed in air without any special precaution.
15. Although BMI · BF
4 is soluble in water at room temperature,
the presence of
potassium chloride
(KCI) gives a salting-out effect, affording two phases. By adding more
water a homogeneous colorless solution can be obtained.
17. A glass transition (−74°C) is obtained by differential
scanning calorimetry performed at the cooling rate of 10°C/min from 20°C to −150°C
followed by an isothermal at this temperature for 10 min and then heated to 30°C,
at the same heating rate. When the cooling rate was decreased to 2 or 1°C/min, the
crystallization transition at −73°C was barely observable.
18. The product has the following spectral properties:
1H NMR (300 MHz, acetone-d
6)
δ: 0.95 (t, 3 H,
3J
HH = 7.3), 1.37
(m, 2 H), 1.93 (m, 2 H), 4.07 (s, 3 H), 4.40
(t, 2 H,
3J
HH = 7.1), 7.79 (s, 1 H), 7.85
(s, 1 H), 9.55 (s, 1 H);
13C NMR (75 MHz, acetone-d
6) δ:
13.1, 19.3, 32.2, 35.9, 49.4,
122.7, 124.0, 138.9; IR (neat film/NaCl plate) cm
−1: 3160, 3119,
2963, 2938, 2876, 1573, 1171,
1059. For comparison with the literature data see Ref.
2.
20. A glass transition (−75°C) and two broad bands close to
0°C are obtained by differential scanning calorimetry performed at the cooling rate
of 10°C/min from 20°C to −150°C followed by an isothermal at this temperature
for 10 min and then heated to 30°C at the same rate. If the heating rate is lowered
to 2 or 1°C/min, a crystallization peak is obtained at 10°C.
21. The product has the following spectral properties:
1H NMR (300 MHz, acetone-d
6)
δ: 0.96 (t, 3 H,
3J
HH = 7.3), 1.37
(m, 2 H), 1.93 (m, 2 H), 4.05 (s, 3 H), 4.36
(t, 2 H,
3J
HH = 7.3), 7.68 (s, 1 H), 7.74
(s, 1 H), 8.95 (s, 1 H);
13C NMR (75 MHz, acetone-d
6) δ:
13.0, 19.3, 32.1, 36.0, 49.6,
122.7, 124.1, 137.0; IR (neat film/NaCl plate) cm
−1: 3171, 3125,
2965, 2939, 2878, 1571, 1167,
836. For comparison with the literature data see Ref.
2.
All toxic materials were disposed of in accordance with "Prudent Practices in the
Laboratory"; National Academy Press; Washington, DC, 1995.
3. Discussion
The primary advantage in the first step of the method described here (using
1-chlorobutane diluted in MeCN) is
that it eliminates long reaction periods
3 and allows the use of secondary alkyl halides
without competitive elimination reactions. For example, the reaction of
sec-butyl bromide with
N-methylimidazole
using the classical method (in neat alkyl halide) produces, along with the desired
product, 20-30% of butenes and
1-methylimidazole
hydrobromide. In the second step, the use of water as solvent
allows the anion metathesis reaction to be quantitative in a very short time and allows
the easy purification of the ionic liquids. Moreover, employing the potassium salt
avoids the use of corrosive and difficult to handle
hexafluorophosphoric
acid3 and the expensive
silver tetrafluoroborate.
4
The ionic liquids
1-butyl-3-methylimidazolium
tetrafluoroborate (BMI·BF4) and
1-butyl-3-methylimidazolium
hexafluorophosphate (BMI·PF6) have a broad application
as "green" solvents for organic synthesis,
5 extraction technologies,
6 electrochemistry,
7 biphasic organometallic catalysis
8 and as stationary phases for
GC.
9 In particular these room temperature
ionic liquids are highly thermal- and electrochemically stable, they possess negligible
vapor pressure, have relatively low viscosity and high density (see Table).
10 The most important advantage of the use of these ionic
liquids as solvents, in particular for biphasic organometallics, is that it allows
the facile separation of the products from the reaction (in most of the cases by simple
decanting) and the recovered ionic catalyst solution can be reused. Moreover, ionic
liquids can improve or promote reactions that occur with difficulty or do not occur
at all in classical organic solvents.
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