Solvent-free aza-Markovnikov and aza-Michael additions promoted by a catalytic amount of imidazolide basic ionic liquids

Article abstract of DOI:10.1016/j.tetlet.2011.04.117

A family of imidazolide ionic liquids were synthesized and characterized. These ionic liquids combined the virtues of strong basicity and relatively good thermal stability. Catalytic properties of these imidazolide ionic liquids were investigated and satisfactory yield was achieved when 2.0 mol % of [Bmim]Im was used as catalyst for aza-Markovnikov addition under solvent-free condition at room temperature in one hour. Experimental results show that a hydrogen bond is not formed between [Bmim]Im/imidazole and vinyl ester, and its existence is not necessary for the [Bmim]Im catalyzed aza-Markovnikov addition either. A possible mechanism for [Bmim]Im-catalyzed aza-Markovnikov addition was proposed. The use of imidazolide ionic liquids in aza-Michael addition was investigated as well.

Full text of DOI:10.1016/j.tetlet.2011.04.117

Tetrahedron Letters 52 (2011) 3588–3591
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Solvent-free aza-Markovnikov and aza-Michael additions promoted
by a catalytic amount of imidazolide basic ionic liquids
⇑
Xuewei Chen, Xuehui Li , Hongbing Song, Yu Qian, Furong Wang
School of Chemistry and Chemical Engineering, Pulp & Paper Engineering State Key Laboratory of China, South China University of Technology, Guangzhou 510640, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A family of imidazolide ionic liquids were synthesized and characterized. These ionic liquids combined
the virtues of strong basicity and relatively good thermal stability. Catalytic properties of these imidazo-
lide ionic liquids were investigated and satisfactory yield was achieved when 2.0 mol % of [Bmim]Im was
used as catalyst for aza-Markovnikov addition under solvent-free condition at room temperature in one
hour. Experimental results show that a hydrogen bond is not formed between [Bmim]Im/imidazole and
vinyl ester, and its existence is not necessary for the [Bmim]Im catalyzed aza-Markovnikov addition
either. A possible mechanism for [Bmim]Im-catalyzed aza-Markovnikov addition was proposed. The
use of imidazolide ionic liquids in aza-Michael addition was investigated as well.
Received 3 March 2011
Revised 21 April 2011
Accepted 29 April 2011
Available online 6 May 2011
Keywords:
Ionic liquids
Imidazolide
Solvent-free reactions
Aza-Markovnikov addition
Aza-Michael addition
Ó 2011 Elsevier Ltd. All rights reserved.
The Markovnikov addition is among the most useful carbon-
carbon, oxygen-carbon, or nitrogen-carbon bond-forming reac-
tions and it is widely used in organic chemistry and pharmaceuti-
ionic liquids with good quality is difficult due to their unsatisfied
stability. Therefore, development of strongly basic ionic liquids
with good stability is a practical and significative issue in basic io-
nic liquids.
1
cal synthesis. This reaction is traditionally performed under harsh
2
reaction conditions. The aza-Markovnikov addition of N-heterocy-
We have reported imidazolide ionic liquid by introducing the
imidazolide anion into the ionic liquid for the first time,15 super-
base-based protic imidazolide ionic liquids were developed after-
cles to vinyl esters has recently received much attention with some
effective approaches, for example, using acylases as biocatalysts,
16
ionic liquid as both catalyst and reaction medium, and anhydrous
2
ward and applied for the rapid and reversible capture of CO .
as catalyst in DMF.3 However, such processes still need po-
–5
Herein, the synthesis and characterization of imidazolide basic io-
nic liquids were extended, their acid–base property and thermal
stability were also discussed. Furthermore, a mild and efficient
aza-Markovnikov addition of imidazole to vinyl acetate catalyzed
by these imidazolide ionic liquids under solvent-free condition
was reported and the possible mechanism was proposed. In addi-
tion, the applicability of imidazolide ionic liquids was also ex-
tended to aza-Michael addition.
3
K PO
4
lar solvent as reaction medium or require a large amount of ionic
liquid acting as catalyst and reaction medium as well. Here, we
are interested in solvent-free reactions catalyzed by task-specific
ionic liquids in order to decrease the amount of organic solvent
and ionic liquid consumed.
Ionic liquids have attracted extensive research interest in recent
years as environmentally benign solvents and catalysts due to their
6
favorable properties. They can be designed as task-specific ionic
Nitrogen-containing heterocyclic compounds, such as imidaz-
liquids by careful development and choice of novel component
ions to endow them some special physical or chemical properties.
ole, triazole, and tetrazole, have been widely used to form ener-
7
17
getic azolate anions in energy conservation studies.
Azole
A typical progress is the appearance of the air, water stable, basic
compounds are also a potential resource for the development of
basic azolate anions though little attention had been paid to the
acid–base property of azolate ionic liquids. The imidazolide basic
ionic liquids were developed here by anion exchange of bromide
ionic liquids with imidazolide salt (Scheme 1).
8
9
ionic liquids. Well known examples including lactate, dicyanam-
1
0
11
12
ide, carboxylates, amino acid anions, and amine-functional-
1
3
ized cations have been reported, but the basicity of these ionic
liquids is far weaker from the point of view of basic catalyst. The
strongly basic hydroxide ionic liquids were the most investigated
Imidazolium bromide precursors 1a–1d were prepared accord-
1
4
18
basic ionic liquids. However, the preparation of these hydroxide
ing to standard preparative procedures. And low reaction tem-
peratures were selected as it was reported that imidazolium
ionic liquids with higher qualities can be achieved under mild
temperature.
⇑
Corresponding author. Tel./fax: +86 20 8711 4707.
E-mail address: cexhli@scut.edu.cn (X. Li).
19
0
040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.04.117

X. Chen et al. / Tetrahedron Letters 52 (2011) 3588–3591
3589
basicity comparable to hydroxide ionic liquids, but also have rela-
tively good thermal stability with respect to acetate ionic liquids.
1
As shown in the H NMR spectra of [Emim]Im and [Bmim]Im,
the peaks of imidazolium C(2)-protons disappeared, which proba-
bly resulted from the hydrogen deuterium (H/D) exchange26 owing
to the interaction between the stronger acidity (pK
a
ꢀ21–23) of
2
7
imidazolium C(2)-protons and the strong Lewis basicity of the
1
imidazolide anion. Further investigation about the H NMR spec-
trum of equal molar of [Bmim]Im and ethanol mixture showed that
proton peaks of the hydroxyl and imidazolium C(2)-proton also
disappeared as expected, which verifies the above interpretation
about the H/D exchange. In addition, the notable H/D exchange
of imidazolium C(4)-proton and C(5)-proton had not been seen un-
der our condition.
Scheme 1. Synthesis of imidazolide ionic liquids.
The imidazolide anion was introduced to the desired salts 2a–
d by metathetic reaction. Equal molar of NaOH and imidazole
We employed the imidazolide ionic liquids in the Markovnikov
addition of imidazole to vinyl acetate. The progress of the reaction
was monitored by thin-layer chromatography (TLC) and the prod-
uct yield was obtained after flash chromatography. Firstly, the fea-
sibility of aza-Markovnikov addition of imidazole (10 mmol) to
vinyl acetate (60 mmol) in the presence of [Bmim]Im (0.02 mmol)
under solvent-free condition at 50 °C for 4 h was evaluated and the
expected product was formed with 92% yield (Table 1, entry 1).
Then, aza-Markovnikov addition of imidazole and vinyl acetate
was carried out and the effect of reaction parameters to the yield
of products were all investigated in detail.
At first, it was observed that the decrease in reaction tempera-
ture had no obvious effect on the reaction yield and Markovnikov
addition could proceed smoothly both at room temperature in 1 h
and at 50 °C (Table 1, entries 2 and 3). Then, the influence of feed
molar ratio of vinyl acetate to imidazole was investigated. System-
atically reducing the number of equivalents of vinyl acetate led to
somewhat higher yield, and the highest yield was obtained when
a 1.2:1 equiv of vinyl acetate to imidazole was used (Table 1, entries
2 and 4 vs. entry 5). As for the influence of the amount of [Bmim]Im
on the reaction, it was found that 2 mol % [Bmim]Im was sufficient
to promote the reaction extensively, insufficient amount of [Bmi-
m]Im led to dramatical dropping of the yield (Table 1, entries 6
and 7). Additional amount of [Bmim]Im (3 mol %) provided limited
2
were mixed and reacted in MeOH. Then the resulting clear, color-
less mixture was added to the solution of imdazolium bromide/
MeOH. Filtration followed by concentration of the filtrate provided
the desired products. Tetrabutylammonium imidazolide ([TBA]Im)
was also obtained by anion-exchange of tetrabutylammonium bro-
mide with sodium imidazolide. Imidazolide ionic liquids were
purified by addition of anhydrous Na
2 4
SO during the preparation
and washing with Et O in workup to get rid of water and salts,
2
respectively. All these imidazolide ionic liquids are pale yellow liq-
uids at room temperature, stable in air and moisture. The lower
melting points of these imidazolide ionic liquids may be explained
by the charge delocalisation and the rigid, planar structure in the
imidazolide anion. The structure of the products is verified with
1
13
H NMR, C NMR, FT-IR, and ESI-MS. Ion-exchange chromatogra-
phy and 1H NMR analysis showed that purity degree of imi-
dazolium imidazolides were no less than 95%. As to [TBA]Im, the
ion-exchange was not complete (about 81% ion-exchange rate).
The basicity of imidazolide ionic liquids was preliminarily inves-
2
0
21
tigated. At first, we compared the pK
b
values of imidazolide to those
value of imi-
dazolide is À0.5, which is intermediate between those of acetate
9.25) and hydroxide (À1.7). In other words, the basicity of imi-
dazolide ionic liquids is much stronger than that of acetate ionic liq-
uids ( pK
= À9.8) and weaker than that of hydroxide ionic liquids
pK = 1.2). In addition, the distribution of base strength of these
imidazolide ionic liquids (H ) was also intuitively manifested by
the Hammett-Deyrup H-function method.23 2,4-Dinitroaniline
= 15.0) and 4-nitroaniline (H = 18.4) were used for Hammett
of acetate and hydroxide in aqueous solution. The pK
b
2
2
(
D
b
(
D
b
Table 1
À
Markovnikov addition of imidazole to vinyl acetate
(
H
À
À
indicators after carefully screening procedure. These imidazolide
ionic liquids together with [Bmim]OH and NaOH were able to have
the color of 2,4-dinitroaniline (H
purple (its basic color), but unable to convert 4-nitroaniline
= 18.4) to its basic color (orange). Thus, the basic strength of
imidazolide could be denoted as strong bases with
À
= 15) changed from yellow to
(
H
À
Yielda
%
Entry
Catalyst 5
Ratio 4:3
Ratio 5:3
T (°C)
t (h)
1
2
3
4
5
6
7
8
[Bmim]Im
[Bmim]Im
[Bmim]Im
[Bmim]Im
[Bmim]Im
[Bmim]Im
[Bmim]Im
[Bmim]Im
[Bmim]Im
None
6.0
6.0
6.0
4.0
1.2
1.2
1.2
1.2
1.2
1.2
6.0
1.2
1.2
1.2
1.2
1.2
1.2
1.2
0.02
0.02
0.02
0.02
0.02
0.01
0.01
0.03
0.03
0
50
rt
50
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
4
1
1
1
1
1
2
1
0.5
1
7
1
1
2
1
2
1
1
92
90
89
91
92
59
71
93
81
21
21
62
57
79
28
49
93
91
2
4
1
5.0 < H
The thermal stability of [Bmim]Im was surveyed by thermo-
gravimetric analysis and compared with other two typical basic io-
nic liquids 1-butyl-3-methylimidazolium acetate ([Bmim]CH COO)
À
< 18.4 according to the reported determination method.
3
and 1-butyl-3-methylimidazolium hydroxide ([Bmim]OH), which
were synthesized according to standard preparative proce-
1
1,14
dures.
It has been reported that the strong Lewis basicity of
9
10
11
the anion tends to induce thermal decomposition at lower temper-
1
1,25
None
NaOH
[Bmim]CH COO
3
[Bmim]CH COO
0
ature.
However, the thermal stability of [Bmim]Im is close to
COO and much better than that of [Bmim]OH.
1
1
2
3
0.02
0.02
0.02
0.02
0.02
0.02
0.02
that of [Bmim]CH
3
Further experimental results showed that all alkylimidazolium
imidazolide salts had similar thermal stability with minor distinc-
tion according to the following sequence: [Emim]Im < [Emmi-
m]Im < [Bmim]Im < [Bmmim]Im. These imidazolide ionic liquids
began to decompose at above 160 °C and decomposed completely
around 300 °C. Thus, imidazolide ionic liquids not only have strong
14
15
3
[Bmim]Br
1
1
1
6
7
8
[Bmim]Br
[Bmmim]Im
[TBA]Im
a
Isolated yields.

3
590
X. Chen et al. / Tetrahedron Letters 52 (2011) 3588–3591
improvement to this reaction under the catalytic condition investi-
gated (Table 1, entry 8), the yield decreased accordingly when the
reaction time was shortened (Table 1, entry 9). Some blank experi-
ments were also carried out. Only poor yields (Table 1, entries 10
and 11) of the desired product were obtained, especially when
the 6:1 equiv of vinyl acetate to imidazole was applied. Then, the
optimized experimental conditions were achieved when 2.0 mol %
of [Bmim]Im was used to catalyze the reaction between vinyl ace-
tate and imidazole under solvent-free condition at room tempera-
ture in one hour (Table 1, entry 5).
Table 2
Imidazolide ionic liquids catalyzed Michael addition of imidazole with alkenesa
Yieldb (%)
Entry
Catalyst
R
1
2
3
4
[Bmim]Im
[Bmim]Im
[Bmmim]Im
[Bmmim]Im
COOCH
CN
COOCH
CN
3
3
93
89
93
90
[
Bmim]Br precursor and other basic compounds, NaOH and
a
Reactions and conditions: 10 mmol imidazole, 12 mmol alkenes and 0.2 mmol
[
Bmim]CH COO, were also examined (Table 1, entries 12–16).
3
[
Bmim]Im or [Bmmim]Im at room temperature for 1 h.
b
When NaOH was used as catalyst, a solid impurity component
was present in the reaction mixture and the isolated yield was ob-
tained as low as 62%. Limited solubility and low dispersion of
NaOH in the reaction system is believed to contribute the forma-
tion of the by-product and poor yield (Table 1, entry 12).
Isolated yields.
[
Emim]Im was mixed with vinyl acetate. In addition, two other
imidazolide ionic liquids without active hydrogen were applied
in aza-Markovnikov addition in order to understand the role of
imidazolium C(2)-proton in the catalysis process. It was interesting
to see that the same high yield can also be obtained over [Bmmi-
m]Im (93%, Table 1, entry 17) and [TBA]Im (91%, Table 1, entry
[
3
Bmim]CH COO ionic liquid showed mild catalytic activity, with
a yield of 57% in 1 h and 79% in 2 h (Table 1, entries 13 and 14).
As for [Bmim]Br, it exhibited relatively low catalytic activity in
comparison to those of other ionic liquids (Table 1, entries 15
and 16). Hence, we conclude that catalytic activities of these basic
ionic liquids in the aza-Markovnikov addition reaction depend
greatly on their Lewis basicity.
The hydrogen bond was reported to exercise a significant influ-
ence on the mechanism for the aza-Markovnikov addition cata-
lyzed by basic ionic liquid [Bmim]OH.4 Then, it is speculated
whether hydrogen bond is formed and takes effect on the [Bmi-
m]Im-catalyzed aza-Markovnikov addition. Firstly, chemical shift
of C(2)-proton in imidazolium or N-proton in imidazole was inves-
tigated. There is a downfield shift (0.028 ppm) of imidazolium
C(2)-proton in equal molar mixture of [Bmim]Br and vinyl acetate
compared to that in [Bmim]Br, while no shift of N-proton of imid-
azole in equal molar mixture of imidazole and vinyl acetate is ob-
served compared to that in imidazole. The results indicated that
hydrogen bond was formed between [Bmim]Br and vinyl acetate
while no hydrogen bond was formed between imidazole and vinyl
ester.
1
8), which indicated that the hydrogen bond is not necessary in
aza-Markovnikov addition. In addition, the existence of trace
impurities does not have an obvious effect on the catalytic proper-
ties of these imidazolide ionic liquids through comparing the re-
sults listed in Table 1 (entries 5, 15, 17, and 18).
a,28
Based on the above experimental results, discussion and with
5
reference to previous report , the mechanism for aza-Markovnikov
addition catalyzed by [Bmim]Im is proposed in Scheme 2. Imidazo-
lide anion in the [Bmim]Im reacts with vinyl acetate to afford the
1
-(1H-imidazol-1-yl)ethanone 6 and ethenolate firstly; Then
ethenolate immediately deprives the proton of imidazole to afford
aldehyde 7 and Imidazolide anion; 1-(1H-imidazol-1-yl)ethanolate
8
aldehyde 7 afterward; Finally, the Markovnikov adduct was cre-
ated by the addition reaction between 1-(1H-imidazol-1-yl)etha-
none 6 and 1-(1H-imidazol-1-yl)ethanolate 8. The formation of
can be formed through the addition reaction of imidazolide to
5
intermediates was also given in a previous report.
Basic ionic liquids were widely applied in aza-Michael addition
However, as seen from the disappearance of imidazolium C(2)-
protons peaks in the H NMR spectra of [Emim]Im and [Bmim]Im,
1
4b,c,31
1
as base catalysts.
These imidazolide ionic liquids were fur-
ther investigated in aza-Michael addition. As shown in Table 2,
the reaction proceeded smoothly between imidazole and the sub-
strates under solvent-free condition using 2 mol % [Bmim]Im or
the hydrogen bond donor ability of imidazolium may be decreased
by the strong interaction with imidazolide anion for the strong Le-
wis basicity of imidazolide anion.2
9,30
Then, hydrogen bond is not
[
Bmmim]Im as catalyst at room temperature for 1 h. Results
formed between imidazolium and vinyl ester when [Bmim]Im or
achieved here were comparable to those of the [Bmim]OH basic io-
1
4b,c
nic liquid used.
In summary, a series of novel imidazolide ionic liquids were
synthesized and characterized. These imidazolide ionic liquids be-
long to strong bases with good thermal stability, which were effi-
cient catalysts for aza-Markovnikov addition and aza-Michael
addition under solvent-free condition. As shown from the experi-
mental results that the existence of hydrogen bond is not neces-
sary in the aza-Markovnikov addition and a possible mechanism
for aza-Markovnikov addition catalyzed by [Bmim]Im is proposed
accordingly.
Acknowledgment
We gratefully acknowledge financial support of this project by
the National Natural Science Foundation of China (Grant No.
2
0876055 and 20906029).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.04.117.
Scheme 2. The proposed mechanism.

X. Chen et al. / Tetrahedron Letters 52 (2011) 3588–3591
3591
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