Synthesis of imidazolium and pyridinium-based ionic liquids and application of 1-alkyl-3-methylimidazolium salts as pre-catalysts for the benzoin condensation using solvent-free and microwave activation

Article abstract of DOI:10.1016/j.tet.2009.11.110

An efficient one-pot procedure for the synthesis of ionic liquids based on nitrogen-containing heterocycles, imidazolium or pyridinium under 'green chemistry' conditions has been described. Imidazolium salts and DBU have been found to catalyze efficiently the benzoin condensation giving good yields within very short reaction time using solvent-free microwave activation conditions.

Full text of DOI:10.1016/j.tet.2009.11.110

Tetrahedron 66 (2010) 1352–1356
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis of imidazolium and pyridinium-based ionic liquids and application of
1-alkyl-3-methylimidazolium salts as pre-catalysts for the benzoin condensation
using solvent-free and microwave activation
a
b
a,
*
Audrey Aupoix , Bruce P e´ got , Giang Vo-Thanh
a
Laboratoire de Catalyse Mol e´ culaire, Institut de Chimie Mol e´ culaire et des Mat e´ riaux d’Orsay, UMR 8182, Universit e´ Paris-Sud 11, 91405 Orsay Cedex, France
UMR 8086, Instut Lavoisier-Franklin, Universit e´ de Versailles, 45, avenue des Etats-Unis, 78035 Versailles Cedex, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient one-pot procedure for the synthesis of ionic liquids based on nitrogen-containing hetero-
cycles, imidazolium or pyridinium under ‘green chemistry’ conditions has been described. Imidazolium
salts and DBU have been found to catalyze efficiently the benzoin condensation giving good yields within
very short reaction time using solvent-free microwave activation conditions.
Received 2 November 2009
Received in revised form 24 November 2009
Accepted 30 November 2009
Available online 4 December 2009
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
this step was carried out at reflux in a large excess of acetone as
solvent for several hours or even some days.
4
Over the past decade, ionic liquids (ILs), salts with melting
points near room temperature, have been a subject of intense re-
search as a customizable replacement for the conventional organic
solvents used in industry and academia. The market for ionic liq-
uids is burgeoning; these compounds have been used as effective
reaction media for a wide range of organic reactions and other
Microwave (MW) activation, a non-conventional energy source,
has emerged as a powerful technique for promoting a variety of
chemical reactions and has become a useful technology in organic
5
chemistry. The combination of solvent-free conditions and MW
irradiation considerably reduces reaction time, enhances conver-
sions as well as selectivity and presents certain environmental
1
6
applications in chemistry. By modifying the structure of the cat-
advantages. This method has been successfully applied to the
7
ions or anions of ionic liquids, it has been shown that their prop-
erties can be altered in order to influence the outcomes of reactions.
The details of several types of ionic liquids have been published.
synthesis of several imidazolium-based ILs.
In 2004, we reported the first synthesis of chiral ionic liquids
possessing a chiral ephedrinium cation using solvent-free micro-
2
8
9
However, very few pyridinium salts were described. As far as we
wave irradiation conditions. Recently, Cravotto et al. described an
effective and rapid one-pot procedure to synthesize a second-
generation ionic liquid using combined microwave/ultrasound
irradiation.
know, no synthesis of alkyldimethylimidazolium-based ionic
liquids using solvent-free microwave activation has been reported.
The general syntheses of ionic liquids based on nitrogen-con-
taining heterocycles, imidazoilum or pyridinium, involve a consec-
utive quaternisation-anion metathesis procedure. The first step
affords an alkylated halide precursor, which upon metathesis with
a metal salt gives an ionic liquid with a different anion. Using con-
ventional heating methods, the first step in ionic liquid synthesis has
In view of the emerging importance of the ILs as reaction media
in organic synthesis and our general interests in MW-assisted
chemical processes, we report here the one-pot procedure for the
synthesis of imidazolium and pyridinium-based ionic liquids under
‘green chemistry’ conditions. Alkylmethylimidazolium and DBU are
then used as catalysts for the benzoin condensation using solvent-
free microwave activation conditions.
3
been performed in the presence of organic solvents, such as CH CN,
3
DMF or acetone with a large molar excess of haloalkane (10–400%)
and the reactions always take a long time (around two days) to fully
consume the substrate. The second step involves an anion exchange
of azolium halides with an alkaline earth metal or ammonium salts
2
2
. Results and discussion
ꢀ
ꢀ
ꢀ
ꢀ
of charge-delocalized soft anion (BF
4
, PF
6
, TfO , NTf ). Generally,
2
.1. Solvent-free microwave assisted synthesis
of imidazolium and pyridinium-based ionic liquids
The aim of our study is to underline the potential of coupling
solvent-free reaction with focused MW activation. The synthesis of
the ILs studied is outlined in Scheme 1.
*
Corresponding author. Tel.: þ33 1 69157891; fax: þ33 1 69154680.
E-mail address: gvothanh@icmo.u-psud.fr (G. Vo-Thanh).
0
040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.11.110

A. Aupoix et al. / Tetrahedron 66 (2010) 1352–1356
1353
bromoalkanes (RBr) and anion exchange agents (MY) were then
tested. The more significant results are given in Table 2.
Y
R
N
3
N
N
N
N
Me
Me
The data in Table 2 indicated that very good yields were obtained
within very short reaction times. In the case of butyl chain, reactions
1
R'
1. RX , MW, no solvent
R'
ꢁ
+
-
were carried out at 90 C due to the low boiling point of bromobu-
2
. M Y , MW, no solvent
ꢁ
tane (bp: 100 C). As observed, the anion nature of exchange agents
R
N
did not affect the yields. An adjustment of temperature and reaction
time depending on the alkyl chain length was necessary to complete
the reaction. All these ionic liquids are known compounds and their
Y
4 9 8 33
R = C H , C H17, C10H21, C12H25, C16H
2
R'= H, CH
X = Cl, Br
M Y = KOTf, NaPF NaBF4, LiNTf2
3
4
1
physico-chemical properties are well documented.
+
-
6,
Some experiments using a three-component one-pot procedure
were investigated. Thus, 1-methylimidazole, bromoalkane and anion
exchange agent (KOTf) were directly submitted to MW irradiation. A
drop in yield was observed in all cases (Table 2, entries 9, 14, 19, 24).
Scheme 1. Synthesis of imidazolium and pyridinium-based ionic liquids using solvent-
free microwave activation.
With these results in hand, we then proceeded to synthesize the
In our initial studies, we attempted to optimize the reaction
conditions for the N-alkylation of 1-methylimidazole (R ¼H) with
0
0
imidazoliumsalts3(R ¼CH ), possessingthe methylgroupinposition
3
2
of N-methylimidazole and the pyridinium salts 4 (Scheme 1). As
bromo-or chloroalkane. All the MWreactions were carried out in the
CEM Discover monomode system with a strict control of power and
temperature during the reaction process and in the absence of any
solvent (Scheme 1, Step 1). The optimum reaction conditions de-
termined for the synthesis of 1-alkyl-3-methylimidazolium salts
was shown in Table 1. A slight excess of haloalkane (2–5%) was used
to make the full conversion of the substrate. Various alkyl chain
lengths with bromide and chloride (as associated anions) were then
tested. Generally speaking, good yields were obtained (Table 1).
previously mentioned, the nature of anion exchange agent did not
affect the yield of product. So we limited our study to the use of po-
tassium triflate for the anion metathesis step. Using the same strategy
previously described, 1-alkyl-2,3-dimethylimidazolium and 1-alkyl-
pyridinium salts were synthesized in very good yields within very
short reaction times. The Table 3 summarized the results and the
reaction conditions for the synthesis of these ionic liquids.
To sum up, we have developed an efficient procedure for the
preparation of imidazolium and pyridinium-based ionic liquids,
using solvent-free synthesis and microwave activation. These 1-
alkyl-3-methylimidazolium salts, in the presence of base, can be
used as catalysts for the benzoin condensation.
Table 1
Solvent-free N-alkylation of 1-methylimidazole with n-haloalkane in closed vessels
under monomode microwave irradiation
ꢁ
Entry
RX
Time (min)
Temperature ( C)
Yield (%)
MW
D
2.2. Solvent-free microwave-promoted benzoin condensation
1
2
3
4
5
6
7
8
9
C
C
C
C
C
C
C
C
C
C
4
4
8
8
H
H
H
H
9
Br
Cl
17Br
17Cl
21Br
21Cl
25Br
25Cl
23Br
23Cl
8
15
8
15
8
15
8
15
15
30
90
150
120
150
130
160
130
160
140
160
89
95
91
98
97
96
98
92
98
98
73
38
67
41
68
36
73
34
51
29
using 1-alkyl-3-methylimidazolium-based ionic liquids as
pre-catalysts
9
10
10
12
12
16
16
H
The benzoin condensation provides a convenient and powerful
H
H
H
H
H
method for the synthesis of
a-hydroxy ketones from aldehyde
10
starting materials (Scheme 2). In its traditional form, only aro-
matic aldehydes or glyoxals could be coupled using toxic cyanide
10b
10
anion as catalyst.
Several nucleophilic carbenes derived from
1
1
12
heterocyclic systems including triazoles and thiazoles have also
found application as catalysts, which has increased the scope of the
reaction allowing aliphatic aldehydes to be coupled. Unfortunately,
these systems generally rely on high catalyst loadings (10–30%),
and frequently need long reaction times, often leading to catalyst
decomposition under the basic conditions required. The benzoin
condensation is traditionally carried out in protic solvents like al-
cohol, water or in aprotic ones such as DMF, acetonitrile or even
without solvent. Imidazolium-type ionic liquids have also been
As observed, relative reactivity followed the trends expected for
an S 2 reaction, i.e. higher temperatures and longer reaction times
are required with poorer leaving groups (Cl <Br ) or with longer
alkyl chain length.
N
ꢀ
ꢀ
Experiments using a thermostat-controlled oil bath (conven-
tional heating,
D) were executed under identical reaction condi-
tions (time, temperature, vessel, profile of rise in temperature,
without solvent). A drop in yield was observed in all cases.
The next step in the synthesis involved an anion exchange ha-
lides for non-coordinating anions. The common methods are based
on either double-displacement anion metathesis (e.g., using Ag(I),
13
employed with success as solvents and catalysts for this reaction.
Recently, an ultrasound or microwave assisted-benzoin condensa-
tion of benzaldehyde has been reported using ionic liquids as pre-
1
14
alkali, or ammonium salts) or acid-base neutralization reactions.
catalysts. Goods yields within short reaction times were observed.
The halide-containing by-product salts resulting from these re-
actions are subsequently removed by extraction or precipitation
followed by filtration.
After achieving the synthesis of different 1-alkyl-3-methyl-
0
imidazolium salt 3 (R ¼H), we were interested in testing their
catalytic potential for the benzoin condensation reaction of alde-
hyde 5 using no solvent and microwave activation (Scheme 2).
In our initial work, the benzoin condensation reaction of benzal-
dehyde 5 (Z¼H) was irradiated by means of an 1-butyl-3-methyl-
As already reported by Varma et al.,7g an anion exchange me-
tathesis is easily performed under MW activation using a domestic
oven. In this way,1,3-dialkylimidazolium tetrafluoroborate salts were
prepared in good yields after only a few minutes of reaction time. In
ordertosimplify theoverallprocedure, we carried outthe synthesis of
0
imidazolium triflate 3 (R ¼H) in the presence of NaOMe as basic
1
4
catalyst as reported in lit. However, very low yields were obtained
along with the formation of benzoic acid as by-product. Optimization
of the reaction conditions was carried out by varying several param-
eters, in particular the temperature, the reaction time, the base used,
etc. DBU showed to be the most effective base for this reaction. The
best experimental conditions found are presented in Table 4.
1
-alkyl-3-methylimidazolium-based ionic liquids using a two-step
8
one-pot sequence reaction as developed previously by our group. All
products (not isolated) resulting from the quaternisation step were
directly submitted to an anion exchange step (Scheme 1). All the MW
reactions were conducted in the absence of any solvent. Some

1354
A. Aupoix et al. / Tetrahedron 66 (2010) 1352–1356
Table 2
Solvent-free synthesis of 1-alkyl-3-methylimidazolium salts using two-step one-pot sequence reaction under monomode microwave irradiation
Entry
RBr
MY
N-Alkylation (first step)
Time (min); Temperature ( C)
Anion metathesis (second step)
Time (min), Temperature ( C)
Yield (%)
ꢁ
ꢁ
1
2
3
4
5
6
7
8
C
4
8
8
H
H
H
9
Br
KOTf
NaPF
NaBF
LiNTf
KOTf
8; 90
15; 100
98
96
99
98
99
97
99
95
80
98
98
95
91
80
98
98
96
97
82
94
96
97
98
77
6
4
2
C
17Br
8; 120
10; 120
NaPF
NaBF
6
4
LiNTf
KOTf
KOTf
2
a
20; 120b
8; 130
9
C
C
17Br
21Br
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
10
H
10; 130
10; 140
15; 140
NaPF
NaBF
6
4
LiNTf
KOTf
KOTf
2
a
a
a
20; 130b
8; 140
C
C
10
H
H
21Br
25Br
12
NaPF
NaBF
6
4
LiNTf
KOTf
KOTf
2
C
C
12
H
H
25Br
33Br
20; 140b
15; 140
16
NaPF
NaBF
6
4
LiNTf
KOTf
2
C
16
H
33Br
30; 140b
a
Three-component one-pot procedure.
Time (min); temperature ( C).
b
ꢁ
Table 3
0
Solvent-free synthesis of 1-alkyl-2,3-dimethylimidazolium salts 3 from 1,2-dimethylimidazole 1 (R ¼Me) and 1-alkylpyridinium salts 4 from pyridine 2 using two-step one-pot
sequence reaction under microwave irradiation
Entry
Substrate
RBr
N-Alkylation (first step)
Time (min); Temperature ( C)
Anion metathesis
(second step)
Time (min), Temperature ( C)
Yield 3 (%)
Yield 4 (%)
ꢁ
ꢁ
0
1
2
3
4
5
6
7
8
9
1 (R ¼Me)
C
C
C
C
C
4
H
9
Br
15; 90
10; 130
8; 120
10; 130
8; 130
10; 140
8; 140
10; 140
15; 140
20; 140
15; 100
10; 130
10; 120
10; 130
10; 130
10; 140
10; 140
10; 140
15; 140
15; 140
94
96
98
96
92
2
95
91
92
91
90
0
1 (R ¼Me)
8
H17Br
2
0
1 (R ¼Me)
10
12
16
H
21Br
25Br
33Br
2
0
1 (R ¼Me)
H
H
2
0
1 (R ¼Me)
10
2
With the result in hand with 1-butyl-3-methylimidazolium
Table 4
triflate (Table 4, entry 1), we proceeded to examine the nature of
Solvent-free microwave-promoted benzoin condensation. Conditions: Benzalde-
hyde: IL 3 (R0¼H):DBU¼1:0.2: 0.2. Temperature: 80 ꢁC. Time: 1 h
0
the pre-catalyst 3 (R ¼H) on the reaction, using no solvent under
microwave activation. To this purpose, a series of tests was exe-
Entry
R
Y
Conv 5a (%)
Isolated yield 6 (%)
0
cuted using 1-alkyl-3-methylimidazolium salt 3 (R ¼H) possessing
1
2
3
4
5
6
7
C
C
4
H
H
9
OTf
OTf
BF
PF6
NTf
OTf
OTf
87
88
86
83
83
85
87
64
72
59
51
59
65
59
various alkyl chain lengths in the presence of DBU as basic catalysts.
In general, good conversions were obtained. The best yield of 72%
8
17
4
was obtained with R¼C
8
H
17 (Table 4, entries 1, 2, 6, and 7).
We also studied the influence of anion nature on the catalytic
activity. To that end, some common anions were tested under
similar conditions. In general, good yields were observed (Table 4,
entries 3, 4, and 5). The anion nature did not affect the reaction
2
C
C
10
H
H
21
25
12
a
Conversions estimated by GC analysis in the presence of internal standard (ethyl
benzoate).
conversion. On the other hand, the difference of yields observed is
due to the difficulty in the workup of these experiments.
O
O
Z
The reactivity of a series of similar substrates to
tones was examined under optimized conditions, using 0.2 equiv of
Omim][OTf] and DBU in the absence of any solvent and under
microwave activation. As shown in Table 5, the benzoin conden-
sation of various substituted aldehydes including electron rich
a-hydroxy ke-
catalyst , base
solvent
2
Z
Z
OH
6
[
5
Scheme 2. Benzoin condensation.
3 3
derivatives such as CH or OCH were accomplished with no

A. Aupoix et al. / Tetrahedron 66 (2010) 1352–1356
1355
Table 5
temperature as reported in Table 2. The reaction mixture was
brought to room temperature and CH Cl (5 mL) was added. After
filtration, the solvent was evaporated. The crude product was
washed with Et
afford the lightly colored liquid, which did not need further
purification.
Solvent-free microwave-promoted benzoin condensation. Conditions: Aldehyde 5:
2
2
ꢁ
[Omim][OTf]:DBU¼1: 0.2: 0.2. Temperature: 80 C. Time: 1 h
Entry
Z
Yielda 6 (%)
O (2ꢂ5 mL) and dried under reduced pressure to
2
1
2
3
4
5
6
7
8
9
H
4-CF
72
78
69
47
93
0
0
84
89
3
4-CH
4-CH
4-Cl
4-NO
3-NO
3
All these ionic liquids are known compounds and their physico-
3
O
1,15
chemical properties are well documented.
2
2
4.3. General procedure for the solvent-free two-step one-pot
preparation of 1-alkyl-2,3-dimethylimidazolium 3 from 1,2-
dimethylimidazole 1 and 1-alkylpyridinium 4 from
pyridine 2 under microwave irradiation
3-CH
3-CH
3
3
O
a
Yields determined by GC analysis in the presence of internal standard (ethyl
benzoate).
A mixture of 1,2-dimethylimidazole 1 (1.92 g, 20 mmol) or
pyridine 2 (1.58 g, 20 mmol) and 1-bromoalkane (20 mmol) was
irradiated under microwave at the temperature for the appropriate
time (see Table 3). Alkaline salt MX (20 mmol) was added and the
resulting mixture was then placed under MW irradiation for an
additional period of time and temperature as reported in Table 3.
complications, generally resulting in the desired product 6 in
moderate to excellent yields (47–93%, except in the case where
Z¼NO
2
) and in short reaction time (1 h). As it can be observed,
, OCH , Cl, and CF had
substituent on the aromatic ring, such as CH
3
3
3
a considerable effect on the yield. The best yield of 93% was
obtained with Z¼4–Cl. On the other hand, no product was observed
The reaction mixture was brought to room temperature and CH
2
Cl
2
13a
2
in the case of the Z¼NO as previously recorded by Xia et al.
(5 mL) was added. After filtration, the solvent was evaporated. The
crude product was washed with Et
reduced pressure to afford the lightly colored liquid, which did not
need further purification.
2
O (2ꢂ5 mL) and dried under
3
. Conclusion
In conclusion, we have developed an efficient two-step one-pot
procedure for the synthesis of ionic liquids based on nitrogen-
containing heterocycles, imidazolium or pyridinium, under ‘green
chemistry’ conditions. We have also described an environmentally
4
.3.1. 1-Butyl-2,3-dimethylimidazolium trifluoromethanesulfonate. IR
ꢀ1
1
(neat)
n
¼3668, 3125, 2930, 2854, 1570, 1378, 1354, 1170 cm
. H
NMR (300 MHz, CDCl ) d
3
0.90 (3H, t, J¼7.5 Hz), 1.32 (2H, m),1.81 (2H,
benign and highly efficient procedure for the preparation of a-hy-
m), 2.60 (3H, s), 3.92 (3H, s), 4.15 (2H, t, J¼7.3 Hz), 7.37 (1H, s), 7.40
droxy ketone derivatives via benzoin condensation. 1-Alkyl-3-
methylimidazolium salts and DBU have been found to catalyze
efficiently the benzoin condensation giving moderate to good
yields within very short reaction time using solvent-free micro-
wave activation conditions. The preparation of optically active
imidazolium-based ionic liquids, derived from naturally chiral
sources will be discussed in a future report. Efforts to develop an
asymmetric benzoin condensation using these chiral imidazolium-
based ionic liquids under solvent-free microwave synthesis are
currently under investigation.
13
(1H, s). C NMR (75 MHz, CDCl
3
)
d
9.9, 13.3, 19.4, 31.9, 36.3, 49.8,
þ
122.2, 123.7, 136.6. HRMS (M-OTf) m/z (%) Calcd for [C
9 17 2
H N ] :
153.1386. Found: 153.1388.
4
.3.2. 1-Octyl-2,3-dimethylimidazolium trifluoromethanesulfonate. IR
(neat)
n
¼3413, 3111, 2929, 2858, 1573, 1467, 1379, 1262, 1225, 1162,
ꢀ1
1
1031 cm . H NMR (300 MHz, CDCl
3
)
d
0,84 (3H, t, J¼7.5 Hz), 1.28
(
10H, m), 1.86 (2H, t, J¼7.5 Hz), 2.65 (3H, s), 4.0 (3H, s), 4.13 (2H, t,
13
J¼7.2 Hz), 7.34 (1H, s), 7.44 (1H, s). C NMR (75 MHz, CDCl
3
) d 9.8, 13.7,
2
2.2, 25.8, 28.7, 28.8, 29.8, 31.3, 36.1, 49.8,121.9,123.5,136.4. HRMS (M-
þ
OTf) m/z (%) Calcd for [C13
H
25
N
2
] : 209.2012. Found: 209.2018.
4
4
. Experimental section
.1. General information
4
.3.3. 1-Decyl-2,3-dimethylimidazolium trifluoromethanesulfonate. IR
(neat)
n
¼3584, 3153, 3114, 2927, 2857, 1574, 1467, 1262, 1162, 1031,
ꢀ
1 1
8
48, 756, 639 cm
.
H NMR (300 MHz, CDCl
3
)
d
0.86 (3H, t,
Melting points were measured on a Kofler bank. The NMR
J¼7.5 Hz), 1.24 (14H, m), 1.86 (2H, t, J¼7.2 Hz), 2.65 (3H, s), 3.91 (3H,
1
spectra were recorded in CDCl
3
3
. H NMR spectra were recorded at
13
s), 4.18 (2H, t, J¼7.5 Hz), 7.28 (1H, s), 7.34 (1H, s). C NMR (75 MHz,
60 or 300 MHz. The chemical shifts (d) are reported in ppm rela-
CDCl
3
)
d
9.4,13.8, 22.4, 26.0, 28.7, 29.0, 29.1, 29.2, 29.5, 31.6, 35.1, 48.5,
13
tive to TMS as internal standard. J values are given in Hertz. C NMR
spectra were recorded at 90 or 75 MHz. IR spectra were recorded on
a FTIR Perkin–Elmer instrument. TLC experiments were carried out
in 0.2 mm thick silica gel plates (GF254) and visualization was
accomplished by UV light or phosphomolybdic acid solution. The
columns were hand-packed with silica gel 60 (200–300).
þ
120.7, 122.4, 143.5. HRMS (M-OTf) m/z (%) Calcd for [C15
29 2
H N ] :
2
37.2325. Found: 237.2330.
4
.3.4. 1-Dodecyl-2,3-dimethylimidazolium trifluoromethanesulfonate. IR
ꢀ1
(
neat)
n
¼3641, 3156, 2927, 2856, 1573, 1467, 1351, 1191, 1137, 1058 cm
H NMR (300 MHz, CDCl
2H, t, J¼7.2 Hz), 2.60 (3H, s), 3.96 (3H, s), 4.17 (2H, t, J¼7.5 Hz), 7.26 (1H,
.
1
3
)
d
0.87 (3H, t, J¼7.5 Hz), 1.32 (18H, m), 1.86
All reagents and solvents were purchased from commercial
sources (Acros, Aldrich) and were used without further purification.
(
13
s), 7.27 (1H, s). C NMR (75 MHz, CDCl 9.5, 13.9, 22.5, 26.1, 28.9, 29.1,
3
) d
2
9.2, 29.4, 29.5, 29.6, 31.7, 35.2, 48.6, 120.7, 122.6, 143.6. HRMS (M-OTf)
þ
33 2
m/z (%) Calcd for [C17H N ] : 265.2638. Found: 265.2640.
4
.2. General procedure for the solvent-free two-step one-pot
preparation of 1-alkyl-3-methylimidazolium 3 from
1
-methylimidazole 1 under microwave irradiation
4.3.5. 1-Hexadecyl-2,3-dimethylimidazolium
trifluoromethane-
sulfonate. IR (neat)
1190, 1140, 1060 cm . H NMR (300 MHz, CDCl ) d 0.90 (3H, t,
n
¼3640, 3150, 2932, 2851, 1575, 1470, 1350,
ꢀ
1 1
A mixture of 1-methylimidazole 1 (1.64 g, 20 mmol) and 1-
3
bromoalkane (20 mmol) was irradiated under microwave at the
temperature for the appropriate time (see Table 2). Alkaline salt MX
J¼7.5 Hz), 1.38 (26H, m), 1.81 (2H, t, J¼7.2 Hz), 2.65 (3H, s), 3.92
13
(3H, s), 4.15 (2H, t, J¼7.5 Hz), 7.30 (1H, s), 7.36 (1H, s). C NMR
(75 MHz, CDCl 10.0, 14.1, 22.8, 26.0, 28.8, 28.9, 29.0, 29.1,
29.2, 29.3, 29.4, 29.5, 29.6, 29.8, 31.7, 35.2, 48.6, 120.7, 122.6,
(
20 mmol) was added and the resulting mixture was then placed
3
) d
under MW irradiation for an additional period of time and

1356
A. Aupoix et al. / Tetrahedron 66 (2010) 1352–1356
þ
1
43.6. HRMS (M-OTf) m/z (%) Calcd for [C21
H
41
N
2
] : 321.3264.
chromatography on silica gel. All products are identified by GC–MS
and their H NMR and C NMR were identical to authentic samples.
1
13
Found: 321.3268.
4
.3.6. N-butyl-pyridinium
trifluoromethanesulfonate. 1
H
NMR
Acknowledgements
(
360 MHz, CDCl
3
) d
0.95 (3H, t, J¼7.5 Hz), 1.37 (2H, m), 1.96 (2H, m),
4
8
6
.69 (2H, t, J¼7.3 Hz), 8.06 (2H, t, J¼6.0 Hz), 8.46 (1H, t, J¼6.4 Hz),
We are grateful to the Interchim Company with doctoral
fellowship for Audrey Aupoix. the CNRS (UMR 8182) and the
Universit e´ Paris-Sud 11 for financial supports.
13
.98 (2H, d, J¼4.5 Hz). C NMR (75 MHz, CDCl
3
)
d
13.4, 19.3, 33.5,
þ
2.3, 128.6, 144.8, 145.6. HRMS (M-OTf) m/z (%) Calcd for [C
36.1121. Found: 136.1128.
9
H
14N] :
1
References and notes
4
(
.3.7. N-octyl-pyridinium
360 MHz, CDCl
m), 4.67 (2H, t, J¼7.3 Hz), 8.08 (2H, t, J¼5.8 Hz), 8.48 (1H, t,
trifluoromethanesulfonate. 1
H
0.84 (3H, t, J¼7.5 Hz), 1.30 (10H, m), 1.99 (2H,
NMR
3
)
d
1. (a) For selective review, see Welton, T. Chem. Rev. 1999, 99, 2071; (b) Was-
serscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772; (c) Hagiwara, R.;
Ito, Y. J. J. Fluorine Chem. 2000, 105, 221; (d) De Souza Dupont, R. F.; Suarez, P. A.
Chem. Rev. 2002, 102, 3667; (e) Rogers, R. D.; Seddon, K. R. Ionic Liquids In-
dustrial Applications to Green Chemistry. ACS Symposium Series 2001, 818; (f)
Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis, 2nd ed.; Wiley-VCH:
Weinheim, 2008; (g) Rogers, R. D.; Seddon, K. R. Ionic Liquids as Green Solvents.
Progress and Prospects; Oxdord University: USA, Washington, 2003; (h) Song, C.
E. Chem. Commun. 2004, 1033; (i) Jain, N.; Kumar, A.; Chauhan, S.; Chauhan, S.
M. S. Tetrahedron 2005, 61, 1015; (j) Malhotra, S. V.; Kumar, V.; Parmar, V. S. Curr.
Org. Synth. 2007, 4, 370; (k) Durand, J.; Teuma, E.; G o´ mez, M. C.R. Chim. 2007, 10,
13
J¼5.6 Hz), 8.97 (2H, d, J¼4.2 Hz). C NMR (75 MHz, CDCl
3
)
d
14.0, 22.5, 25.9, 28.9, 29.0, 31.6, 31.7, 62.4, 128.6, 144.7, 145.4.
þ
HRMS (M-OTf) m/z (%) Calcd for [C13
H
22N] : 192.1747. Found:
192.1751.
4
.3.8. N-decyl-pyridinium
360 MHz, CDCl
0.86 (3H, t, J¼7.5 Hz), 1.31 (14H, m), 2.0 (2H,
m), 4.70 (2H, t, J¼7.3 Hz), 8.06 (2H, t, J¼6.2 Hz), 8.47 (1H, t,
trifluoromethanesulfonate. 1
H
NMR
(
3
) d
152; (l) Parvulescu, V. I.; Hardacre, C. Chem. Rev. 2007, 107, 2615; (m) Plechkova,
N. V.; Seddon, K. R. Chem. Soc. Rev. 2008, 37, 123; (n) Toma, S.; Meciarov a´ , M.;
Sˇ ebesta, R. Eur. J. Org. Chem. 2009, 3, 321.
13
J¼5.4 Hz), 8.96 (2H, d, J¼5.0 Hz). C NMR (75 MHz, CDCl
3
)
2. Deetlefs, M.; Seddon, K. S. Green Chem. 2003, 5, 181.
d
14.0, 22.4, 25.8, 28.9, 29.0, 29.1, 29.4, 31.6, 31.7, 62.4, 128.6,
þ
3. Wilkes, J. S.; Levinsky, J. A.; Wilson, R. A.; Hussey, C. L. Inorg. Chem. 1982, 21,
263.
4. Suarez, P. A. Z.; Dullius, J. E. L.; Einloft, S.; Souza, R. F.; Dupont, J. Polyhedron
996, 15, 1217.
144.7, 145.4. HRMS (M-OTf) m/z (%) Calcd for [C15
H
26N] :
1
2
20.2060. Found: 220.2064.
1
.3.9. N-dodecyl-pyridinium trifluoromethanesulfonate. 1
5. For recent reviews on microwave chemistry, see: (a) De la Hoz, A.; Diaz-Ortis, A.;
Moreno, A.; Langa, F. Eur. J. Org. Chem. 2000, 3659; (b) Alterman, M.; Hallberg, A.
J. Org. Chem. 2000, 65, 7984; (c) Perreux, L.; Loupy, A. Tetrahedron 2001, 57, 9199;
4
(
H
NMR
360 MHz, CDCl
3
)
d
0.90 (3H, t, J¼7.5 Hz), 1.35 (18H, m), 2.1 (2H, m),
(d) Lidstrom, P.; Tierney, J.; Wathey, P.; Westman, J. Tetrahedron 2001, 57, 9225;
¨
4
8
2
.65 (2H, t, J¼7.3 Hz), 8.10 (2H, t, J¼6.1 Hz), 8.50 (1H, t, J¼5.2 Hz),
13
(e) Hayes, B. L. Microwave Synthesis: Chemistry at the Speed of Light; CEM:
Matthews, NC, 2002; (f) Microwaves in Organic Synthesis; Loupy, A., Ed.; Wiley-
VCH: Weinheim, 2006; (g) Kappe, C. O.; Stadler, A. Microwaves in Organic and
Medicinal Chemistry; Wiley-VCH: Weinheim, 2005; (h) Ermolat’ev, D. S.; Gime-
nez, V. N.; Babaev, E. V.; Van der Eycken, E. J. Comb. Chem. 2006, 8, 659.
. (a) Loupy, A.; Petit, A.; Hamelin, J.; Texier-Boullet, F.; Jacquault, P.; Math e´ , P.
Synthesis 1998, 1213; (b) Varma, R. S. Green Chem. 1999, 1, 43; (c) Tanaka, K.
Solvent-Free Organic Synthesis; Wiley-VCH: Weinheim, 2003; (d) Polshettiwar,
V.; Varma, R. S. Acc. Chem. Res. 2008, 41, 629.
.90 (2H, d, J¼4.8 Hz). C NMR (75 MHz, CDCl
3
) d 14.0, 22.4, 25.8,
8.9, 29.0, 29.1, 29.2, 29.4, 29.5, 31.6, 31.7, 62.4, 128.6, 144.7, 145.4.
þ
HRMS (M-OTf) m/z (%) Calcd for [C17
H
30N] : 248.2373. Found:
2
48.2370.
6
4
(
(
.3.10. N-hexadecyl-pyridinium trifluoromethanesulfonate. 1H NMR
360 MHz, CDCl
3
)
d
0.88 (3H, t, J¼7.3 Hz), 1.32 (26H, m), 2.05
7. (a) Varma, R. S.; Namboodiri, V. V. Chem. Commun. 2001, 643; (b) Varma, R. S.;
2H, m), 4.72 (2H, t, J¼7.0 Hz), 8.15 (2H, t, J¼5.9 Hz), 8.45 (1H, t,
Namboodiri, V. V. Pure Appl. Chem. 2001, 73, 1309; (c) Khadilkar, B. M.; Rebeiro,
G. L. Org. Process Res. Dev. 2002, 6, 826; (d) Law, M. C.; Wong, K. Y.; Chan, T. H.
Green Chem. 2002, 4, 328; (e) Varma, R. S.; Namboodiri, V. V. Chem. Commun.
13
J¼5.0 Hz), 8.92 (2H, d, J¼5.1 Hz).
3
C NMR (75 MHz, CDCl )
d
14.0, 22.4, 25.8, 28.8, 28.9, 29.0, 29.1, 29.2, 29.3, 29.4, 29.5,
2
2
002, 342; (f) Dubreuil, J. F.; Famelart, M. H.; Bazureau, J. P. Org. Process Res. Dev.
002, 6, 374; (g) Varma, R. S.; Namboodiri, V. V. Tetrahedron Lett. 2002, 43, 5381.
2
9.6, 29.8, 31.6, 31.7, 62.4, 128.6, 144.7, 145.4. HRMS (M-OTf) m/z
þ
(
%) Calcd for [C21
H38N] : 304.2999. Found: 304.2995.
8. Vo-Thanh, G.; P e´ got, B.; Loupy, A. Eur. J. Org. Chem. 2004, 1112.
9
. (a) L e´ v eˆ que, J. M.; Estager, J.; Draye, M.; Boffa, L.; Cravotto, G.; Bonrath, W.
Monatsh. Chem. 2007, 138, 1103; (b) Cravotto, G.; Calcio-Gaudino, E.; Boffa, L.;
L e´ v eˆ que, J. M.; Estager, J.; Bonrath, W. Molecules 2008, 13, 149.
1
0. (a) Lapworth, A. J. J. Chem. Soc. 1903, 83, 995; (b) Ide, W. S.; Buck, J. S. Org. React.
948, 4, 269; (c) Kool, E. T.; Breslow, R. J. Am. Chem. Soc. 1988, 110, 1196; (d)
Potnick, J. R.; Johnson, J. S. J. Am. Chem. Soc. 2004, 126, 3070.
1. (a) Enders, D.; Kallfass, U. Angew. Chem., Int. Ed. 2002, 41, 1743; (b) Enders, D.;
1
4
.4. General procedure for solvent-free microwave-assisted
benzoin condensation reaction
1
Breuer, K.; Kallfass, U.; Balensiefer, T. Synthesis 2003, 8, 1292.
12. (a) Knight, R. L.; Leeper, F. J. Tetrahedron Lett. 1997, 38, 3611; (b) Dvorak, C. A.;
Rawal, V. H. Tetrahedron Lett. 1998, 39, 2925; (c) Knight, R. L.; Leeper, F. J.
J. Chem. Soc., Perkin Trans. 1 1998, 1891.
A mixture of aldehyde 5 (2 mmol), DBU (0.4 mmol, 60 mg) and
IL 3 (0.4 mmol) was irradiated at 80 C for 1 h. After the completion,
the reaction was quenched with 0.1 N HCl and extracted with
2 2 2 4
CH Cl . The organic phase was washed with H O, dried over MgSO ,
filtered and then concentrated under reduced pressure. Yield was
determined by GC analysis in the presence of ethyl benzoate as
internal standard or by calculated after purified by flash
ꢁ
1
3. (a) Xu, L. W.; Gao, Y.; Yin, J. J.; Li, L.; Xia, C. G. Tetrahedron Lett. 2005, 46, 5317;
(b) Storey, J. M. D.; Williamson, C. Tetrahedron Lett. 2005, 46, 7337.
4. (a) Estager, J.; L e´ v eˆ que, J. M.; Turgis, R.; Draye, M. J. Mol. Catal. 2006, 256, 261;
b) Estager, J.; L e´ v eˆ que, J. M.; Turgis, R.; Draye, M. Tetrahedron Lett. 2007, 48,
75.
15. See web site: http://www.il-eco.uft.uni-bremen.de.
1
(
7