Synthesis, characterization, and catalytic activity of ionic liquids based on biosources

Article abstract of DOI:10.1016/j.tetlet.2010.07.060

New room-temperature ionic liquids were synthesized from natural and easily available feedstocks (choline hydroxide and amino acids) following an economical and green route in which the only by-product was water. They were successfully applied as catalyst

Full text of DOI:10.1016/j.tetlet.2010.07.060

Tetrahedron Letters 51 (2010) 4877–4881
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis, characterization, and catalytic activity of ionic liquids based
on biosources
P. Moriel , E. J. García-Suárez , M. Martínez , A. B. García , M. A. Montes-Morán , V. Calvino-Casilda b,*,
a
a,*
a
a
a
b
M. A. Bañares
Instituto Nacional del Carbón (CSIC), Francisco Pintado Fe 26, 33011 Oviedo, Spain
a
b
Instituto de Catálisis y Petroleoquímica (CSIC), Marie Curie 2, Cantoblanco, 28049 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
New room-temperature ionic liquids were synthesized from natural and easily available feedstocks (cho-
line hydroxide and amino acids) following an economical and green route in which the only by-product
was water. They were successfully applied as catalysts for the Knoevenagel test reaction between benz-
aldehyde and different active methylene compounds, at room temperature and under solvent-free con-
Received 1 June 2010
Revised 7 July 2010
Accepted 9 July 2010
Available online 15 July 2010
ditions, to produce
a,b-unsaturated carbonyl compounds, exhibiting good conversions and high
selectivities. The catalytic role of cholinium and aminoacetate ions in the Knoevenagel condensation is
discussed.
Keywords:
Biosources
Ionic liquids
Ó 2010 Elsevier Ltd. All rights reserved.
Knoevenagel condensation
Green process
Catalysis
Room-temperature ionic liquids (RTILs) are increasingly attract-
ing attention due to their unique characteristics: low vapor pres-
sure, non-flammability, high thermal and chemical stability,
possibility of recycling, good solubility for a wide spectrum of com-
In this work, five new RTILs were prepared by using only renew-
able, non-toxic natural products (choline hydroxide as the source
for the cation and amino acids for the different anions) by a
straightforward synthetic procedure and without expensive chem-
ical modifications (Scheme 1).
1
pounds even gases, and high suitability for modifications. Based
on these properties, ionic liquids (ILs) have emerged as a novel
class of compounds to be used in many fields, namely electrochem-
The Knoevenagel condensation is one of the most widely used
C–C bond forming reaction in the synthesis of important interme-
2,1a,b
istry, organic synthesis and catalysis,
metal recovery from
diates for fine chemicals, such as
a,b-unsaturated carbonyl com-
solutions, and carbon dioxide capture.4 Even so, concerns have
risen over the potential toxicity as well as the low biodegradability
of most of the currently employed ILs.5 To overcome these draw-
backs some ionic liquids have already been synthesized from
renewable, non-toxic biosources,6 thus meeting with one of the
3
pounds from active methylene and carbonyl compounds (Scheme
1
0
2). Particularly, the use of methylene malonic esters leads to sev-
11,12
eral important key industrial and pharmacological products.
Traditionally, ammonia, primary or secondary amines, and their
13
salts are employed as catalysts. In recent years efforts have been
7
14
main principles of green chemistry. Among these sources, natural
focused on the search for new catalysts, including ionic liq-
1
5–17
products (amino acids, vitamins, etc.) with an interesting molecu-
lar diversity have the potential to be converted into ILs by means of
green procedures, such as simple ion exchange and/or acid–base
uids.
Moreover, in the production of fine chemicals, the devel-
opment of environmentally friendly catalysts has attracted a
7
growing interest.
8
reactions. Natural amino acids were first used in 2005 by Fukum-
The catalytic properties of the RTILs prepared in this work were
tested in the Knoevenagel condensation under solvent-free condi-
tions between benzaldehyde and different methylene compounds
oto et al. for the synthesis of 1-ethyl-3-methylimidazolium-based
8
a
ILs. Since then, many ionic liquids in which the forming anion
9
comes from a natural product (amino acid) have been prepared.
such as ethyl cyanoacetate (1) (pK
pK = 10.7), diethyl malonate (3) (pK
tate (4) (pK = 16.5) in a batch reactor at room temperature
Scheme 3). The pK value determines the minimum basicity de-
manded by the reactant to run the condensation reaction.
a
= 9), ethyl acetoacetate (2)
(
a
a
= 13.3), or ethyl bromoace-
a
(
a
*
Corresponding authors. Tel.: +34 985 119 004; fax: +34 985 297 662 (E.J.G.-S.);
tel.: +34 915 854 800; fax: +34 915 854 760 (V.C.-C.).
1
13
1
According to the H and C{ H} NMR data, the synthesized
RTILs are pure compounds with no remains of the choline hydrox-
E-mail addresses: eduardo@incar.csic.es (E.J. García-Suárez), vcalvino@icp.csic.es
(
V. Calvino-Casilda).
0
040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.07.060

4
878
P. Moriel et al. / Tetrahedron Letters 51 (2010) 4877–4881
Scheme 1. RTILs synthesis.
In the condensation of benzaldehyde and ethyl acetoacetate
pK = 10.7) (Table 3), [Chol][Ala] showed the highest activity at
(
a
shorter reaction times (5 h); while [Chol][Gly] exhibited the low-
est. For longer reaction times (24 h), a maximum activity value
up to 57% was afforded by [Chol][Thr] and up to 55% by [Chol][Gly].
Predictably, the use of ethyl acetoacetate as a methylene precursor
promotes side reactions, including self condensation, that lessen
1
9
the selectivity toward the Knoevenagel product.
Selectivities to Knoevenagel product of 100% were found for the
reaction between benzaldehyde and diethylmalonate (pK = 13.3)
Table 4). However, noticeable differences in the final conversions
Scheme 2. Knoevenagel condensation between carbonyl and active methylene
compounds.
a
(
among the synthesized RTILs were observed for this reactant. Thus,
a conversion value of 60.5% was achieved after 7 h of the reaction
by using the most active [Chol][Ala], whereas that corresponding
to [Chol][His] amounted to only 19%.
The RTILs were also tested in the reaction of benzaldehyde and ethyl
a
bromoacetate (pK = 16.5) (Table 5). Results were very limited both in
terms of the overall yields and the selectivity. For these particular reac-
tants, the formation of the corresponding epoxide (Darzens reaction)
competes strongly with the Knoevenagel condensation, under the cur-
rent experimental conditions. Nevertheless, the best result was ob-
tained for [Chol][Gly] with a 36% selectivity toward the condensed
product and 6% of overall yield.
Scheme 3. Knoevenagel condensation of benzaldehyde with activated methylene
compounds in the presence of the RTILs.
20
ide or amino acids used as feedstocks. Changes in the chemical
shifts were especially significant for the aminoacetate ions as a
1
The efficiency of the RTILs presented in this work compares rea-
sonably well with the previous results obtained using task-specific
consequence of the ionic liquid formation. As an example, the
H
NMR spectrum of the [Chol][Ala] shows a doublet at 1.23 ppm
and a quadruplet at 3.32 ppm corresponding to the CH and CH
of the alaninate anion; those in the free amino acid appearing at
1
5–17
ILs.
Besides the already stressed advantage of their low toxic-
3
ity as well as biodegradability, the [Chol][Aminoacetate] liquids
exhibit significant differences to other basic ILs employed in Knoe-
venagel condensation reactions. Although specific mechanisms
have been scarcely discussed, most approaches to the design of
new task-specific ILs for this reaction are focused on the enhance-
ment of their basic strength. Thus, the best performing IL reported
1
.48 ppm and 3.78 ppm, respectively.
Results of the TGA analysis are given in Table 1. All the RTILs
show decomposition temperatures below 180 °C which are be-
tween those of choline hydroxide and the corresponding amino
acid. No relationship between the cation size and the decomposi-
tion temperature was found for these RTILs. Previous studies in
the literature on the synthesis/characterization of this type of ILs
are limited to those prepared from choline hydroxide and proline
so far for Knoevenagel condensations is, to our knowledge, a strong
À
4
base such as [C mim][OH]. The prominent relevance of the [OH]
ion in that particular system was pointed out, with the replace-
ment of the IL anion leading to the deactivation of the catalyst.1
In principle, the aminoacetate anions of the RTILs reported here
were expected to be the reaction promoters through the formation
of a stabilized enolate anion by abstraction of the proton from the
5b
(
(
[Chol][Pro]), which shows a similar decomposition temperature
1
8
159 °C).
Results of the Knoevenagel condensation between benzalde-
hyde and ethyl cyanoacetate (pK = 9) are given in Table 2. Conver-
sions ranging from 57% at 60 min up to 74% after 3 h of reaction
a
2
1
methylene group of the malonic ester (Scheme 4). However, the
RTIL synthetic route depicted in Scheme 1 might be reversed as
water is produced in the Knoevenagel reaction (Schemes 2 and 4),
thus leading to significant amounts of choline hydroxide. This latter
compound has demonstrated its catalytic activity in aldol condensa-
1
time were achieved with only 1% of catalyst loading. Based on
H
NMR data, the E-stereoisomer was found to be the sole reaction
product. Since the selectivity to the Knoevenagel product was
1
00% in all the cases, conversions are also the product yields.
2
2
tions. Therefore, it was decided to test the efficiency of [Chol][OH]
in the Knoevenagel condensation to better elucidate a plausible
mechanism for the reactions under study. Results are also included
in Tables 2–5.
Table 1
Decomposition temperature of the RTILs
RTIL
Decomposition temperature (°C)
Overall, the performance of [Chol][OH] was rather limited, its
efficiency decreasing as the basicity demand of the methylene
compound increases. The ability of [Chol][OH] in transferring effec-
[
[
[
[
[
Chol][Ala]
Chol][Gly]
Chol][Phe]
Chol][Thr]
Chol][His]
152
148
166
171
128
À
tively [OH] to the reaction medium seems much lower than that
mim][OH].15b This is somehow surprising since [Chol][OH] is
of [C
also considered to be a strong base. On the other hand, the catalytic
4

P. Moriel et al. / Tetrahedron Letters 51 (2010) 4877–4881
4879
Table 2
Knoevenagel condensation between benzaldehyde and ethyl cyanoaetate (pK
a
= 9)a versus reaction time
Timeb
Yieldc (%)
[Chol][Ala]
[Chol][Phe]
[Chol][Thr]
[Chol][Gly]
[Chol][His]
[Chol][OH]
1
3
6
20
80
5
0
0
63.7
64.3
65.8
66.2
72.5
50.3
54.5
57.1
65.5
70.0
60.1
61.0
63.8
66.0
66.6
63.5
65.3
68.0
70.1
73.7
13.7
19.0
65.4
72.5
74.2
42.0
46.8
53.8
66.2
68.1
1
1
a
b
c
Benzaldehyde 14 mmol, ethyl cyanoacetate 14 mmol, 1% of RTIL, room temperature.
Minutes.
1
00% selectivity to Knoevenagel product.
Table 3
Knoevenagel condensation between benzaldehyde and ethyl acetoacetate (pK
a
= 10.7)a versus reaction time
Timeb
Conversion (%) [selectivity to Knoevenagel product (%)]
[Chol][Ala]
[Chol][Phe]
[Chol][Thr]
[Chol][Gly]
[Chol][His]
[Chol][OH]
5
7
4
45.4 [67.2]
51.0 [71.9]
52.5 [76.4]
29.7 [87.5]
38.1 [82.7]
52.8 [68.2]
29.4 [91.2]
44.0 [90.1]
57.4 [90.8]
7.2 [90.3]
17.6 [64.2]
55.1 [72.6]
34.5 [86.7]
37.4 [81.0]
48.6 [76.7]
16.8 [46.4]
17.0 [46.0]
23.1 [39.8]
2
a
Benzaldehyde 9 mmol, ethyl acetoacetate 9 mmol, 2% RTIL, room temperature.
Hours.
b
Table 4
Knoevenagel condensation between benzaldehyde and diethyl malonate (pK
a
= 13.3)a versus reaction time
Timeb
Yieldc (%)
[Chol][Ala]
[Chol][Phe]
[Chol][Thr]
[Chol][Gly]
[Chol][His]
[Chol][OH]
2
4
7
38.9
51.3
60.5
27.6
39.5
54.7
20.3
27.8
32.1
33.9
45.6
52.0
1.0
17.9
19.0
3.0
4.7
5.9
a
b
c
Benzaldehyde 7 mmol, diethyl malonate 7 mmol, 10% RTIL, room temperature.
Hours.
1
00% selectivity to Knoevenagel product.
Table 5
Knoevenagel condensation between benzaldehyde and ethyl bromoacetate (pK
a
= 16.5)a versus reaction time
Timeb
Conversion (%) [selectivity to Knoevenagel product (%)]
[
Chol][Ala]
[Chol][Phe]
13.5 [3.0]
[Chol][Thr]
3.5 [3.0]
[Chol][Gly]
6.1 [36.0]
[Chol][His]
6.3 [0.0]
[Chol][OH]
0.0 [0.0]
7
7.2 [3.0]
a
Benzaldehyde 7 mmol, ethyl bromoacetate 7 mmol, 10% RTIL, room temperature.
Hours.
b
efficiency of the [Chol][OH] IL was very similar to that reported for
Basic strength would also explain the good performance of
[Chol][Thr] in the condensation between benzaldehyde and ethyl
+
the task-specific ionic liquid [H
3 2 2
N –CH –CH –OH][Acetate] having
a weak basic character.1
5a
acetoacetate (pK
= 10.7) (Table 3). However, for the rest of RTILs
, the mechanism depicted in
a
Nevertheless, [Chol][OH] could only compete with the RTILs as
a catalyst in the reaction between benzaldehyde and ethyl cya-
bearing –NH groups of lower pK
2
a
Scheme 5 would help to understand the relatively good results ob-
tained. This plausible mechanism has been proposed to explain the
efficiency in the Knoevenagel condensation reaction of relatively
weak bases with limited proton abstraction ability, especially
noacetate (pK
The series of RTILs tested in this work allow one to get a further
insight into the possible mechanisms of the reaction. Since the pK
of the –NH group of the aminoacetates under consideration spans
an interval of approximately one pK
.1 ([Phe], [His]) < 9.6 ([Gly], [Ala]) < 10.4 [Thr], the whole series of
RTILs appears to have enough basic strength (pK P 9) to abstract
the proton from the methylene group of ethyl cyanoacetate
Scheme 4). Therefore, no significant conversion differences be-
a
= 9) (Table 2).
a
2
when high pK
a
-demanding methylene compounds are consid-
2
3
24,25
a
unit in the following order:
ered.
It envisages the formation of hemiaminal intermediates
9
which are strong electrophiles, by addition of the amine group to
the carbonyl group of the benzaldehyde. The same path should
a
be followed when the pK
this case, the efficiency of the catalysts does not correlate at all
with the pK of their –NH group.
Obviously, this mechanism would co-exist with that described
in Scheme 4 for relatively low pK -demanding reactions. The three
a
demand increases to 13.3 (Table 4). In
(
tween the RTILs tested were found after 3 h (Table 2). Even so, it
is worth noticing the differences in yields observed for [Chol][Phe]
and particularly for [Chol][His] at low reaction times. This can be
understood in terms of steric hindrance of the two bulky side
chains of these two amino acids (Scheme 1).
a
2
a
+
methyl groups attached to the nitrogen atom of the [Chol] ion
should restrict their activating ability through a mechanism similar

4
880
P. Moriel et al. / Tetrahedron Letters 51 (2010) 4877–4881
a
Scheme 4. Mechanism of the Knoevenagel condensation between benzaldehyde and methylene compounds [Z = CN (1)] in the presence of the RTILs, for low pK demanding
reactions.
Scheme 5. Alternative mechanism of the Knoevenagel condensation between benzaldehyde and methylene compounds [Z = COCH
RTILs, for high pK demanding reactions.
3 2
(2), CO Et (3)] in the presence of the
a
to that of Scheme 5, that is, [Chol][OH] catalytic activity in this
reaction would be limited by its capacity to abstract a proton from
the methylene group of the malonic ester. Finally, although the dif-
ferent experimental conditions limit a direct comparison, the
[Chol][Aminoacetate] RTILs render better efficiencies than those
previously reported when using amino acids as promoters for

P. Moriel et al. / Tetrahedron Letters 51 (2010) 4877–4881
4881
Knoevenagel reactions in different media, including ILs.24,26,27 This
might be explained on the basis of the stabilization of different
reaction intermediates by the cholinium counterion (Schemes 4
and 5).
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and easily available feedstocks. Their synthesis was carried out
by a green route in which the only by-product was water. All of
the synthesized RTILs were successfully tested as catalysts for
the Knoevenagel condensation between benzaldehyde and three
different active methylene compounds affording good conversions
and high selectivities, running at room temperature. This might
avoid the use of low biodegradable imidazolium or imidazolium-
derivate compounds suggested so far as catalysts for this reaction.
The aminoacetate part of the RTILs was identified as the promoter
of the condensation reactions through two different mechanisms,
8
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Acknowledgments
Financial support from the Spanish Ministry of Science and
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(
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