Novel basic ionic liquid based on alkylammonium as efficient catalyst for Knoevenagel reaction

Article abstract of DOI:10.1080/00397911.2010.540692

The typical Knoevenagel condensation was carried out smoothly in the presence of a basic ionic liquid of N,N,N′,N′-tetramethyl-N′- hexyl-ethylenediammonium tetrafluoroborate ([TMHEDA]BF₄), and 99% of yield was obtained using ethyl cyanoacetate and benzaldehyde as substrates at 60 °C for 1 h. Four reuses of the ionic liquid without dramatic decrease in catalytic activity for Knoevenagel condensation demonstrated the good stability and operability of the ionic liquid. Moreover, the typical nucleophilic addition reactions were also accomplished by the same ionic liquid to check its feasibility. The dual function of the basic ionic liquid both as solvent and catalyst, combined with simple product separation and recycling, is expected to contribute to the development of a green and environmentally friendly strategy. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397911.2010.540692

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Novel Basic Ionic Liquid Based on
Alkylammonium as Efficient Catalyst for
Knoevenagel Reaction
a
a
a
a
a
Shuai Zhou , Lu Liu , Bo Wang , Mingguo Ma , Feng Xu &
a b
Runcang Sun
a
Institute of Biomass Chemistry and Technology, College of Material
Science and Technology, Beijing Forestry University, Beijing, China
b
State Key Laboratory of Pulp and Paper Engineering, South China
University of Technology, Guangzhou, China
Accepted author version posted online: 21 Oct 2011.Published
online: 11 Jan 2012.
To cite this article: Shuai Zhou , Lu Liu , Bo Wang , Mingguo Ma , Feng Xu & Runcang Sun (2012):
Novel Basic Ionic Liquid Based on Alkylammonium as Efficient Catalyst for Knoevenagel Reaction,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 42:9, 1384-1391
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1
Synthetic Communications , 42: 1384–1391, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.540692
NOVEL BASIC IONIC LIQUID BASED ON
ALKYLAMMONIUM AS EFFICIENT CATALYST
FOR KNOEVENAGEL REACTION
1
1
1
Shuai Zhou, Lu Liu, Bo Wang, Mingguo Ma, Feng Xu,
1
1
1,2
and Runcang Sun
1
Institute of Biomass Chemistry and Technology, College of Material Science
and Technology, Beijing Forestry University, Beijing, China
State Key Laboratory of Pulp and Paper Engineering, South China
University of Technology, Guangzhou, China
2
GRAPHICAL ABSTRACT
Abstract The typical Knoevenagel condensation was carried out smoothly in the presence of
0
0
0
a basic ionic liquid of N,N,N ,N -tetramethyl-N -hexyl-ethylenediammonium tetrafluorobo-
rate ([TMHEDA]BF ), and 99% of yield was obtained using ethyl cyanoacetate and ben-
4
ꢀ
zaldehyde as substrates at 60 C for 1 h. Four reuses of the ionic liquid without dramatic
decrease in catalytic activity for Knoevenagel condensation demonstrated the good stability
and operability of the ionic liquid. Moreover, the typical nucleophilic addition reactions
were also accomplished by the same ionic liquid to check its feasibility. The dual function
of the basic ionic liquid both as solvent and catalyst, combined with simple product separ-
ation and recycling, is expected to contribute to the development of a green and environmen-
tally friendly strategy.
Keywords Basic ionic liquid; dual function; Knoevenagel condensation; tetramethyl-
ethylenediamine
INTRODUCTION
Over the past few years, room-temperature ionic liquids (salts with a melting
point at or below ambient temperature) have attracted renewed attention for a var-
iety of reactions in the area of green chemistry because of their adjustable physical
Received June 6, 2010.
Address correspondence to Bo Wang, Institute of Biomass Chemistry and Technology, College of
Material Science and Technology, Beijing Forestry University, Beijing 100083, China. E-mail: mzlwb@
bjfu.edu.cn
1
384

NOVEL BASIC IONIC LIQUID
1385
[1]
and chemical properties. Although ionic liquids were initially introduced as alter-
native green reaction media, today they have gone far beyond, showing significant
[2]
roles in controlling reactions as catalysts. The designability gave us more opportu-
nities to achieve ionic liquids with special properties. For instance, a series of
acid-functionalized ionic liquids offered a new possibility for developing environ-
mentally friendly acid catalysts because they could integrate the superiority of both
liquid acids and solid acids, such as uniformly distributed acid sites, water and air
[3]
stability, easy separation, and reusability. Similarly, basic ionic liquids also have
[
4,5]
aroused unprecedented interest
lent catalytic efficiency and reusability over the immobilized inorganic bases in ionic
because they have more advantages such as excel-
[6–9]
liquids for some base-catalyzed processes.
Knoevenagel condensation of carbonyl compounds on compounds containing
an active methylene group is one of the most important preparation methods for
substituted alkenes. Reactions are generally catalyzed using bases or Lewis acids.
Recently, use of ionic liquid as catalysts, resulting in great selectivity, milder con-
ditions, and easier workup, has rapidly increased and has been reported as a useful
[
10]
[
11,12]
condition for Knoevenagel condensation.
For example, cyclic guanidinium lac-
[14]
[
13]
þ
15]
ꢁ
tate ionic liquid,
[MMIm][MSO ] ionic liquid
aqueous conditions.
[H N –CH –CH –OH][CH COO ] ionic liquid,
and
have been reported mainly under solvent-free and
3
2
2
3
[
4
Similarly, a basic ionic liquid paired with 1-butyl-3-methylimidazolium cation and
imidazolide anion ([bmim]Im) catalyzing Knoevenagel condensation under aqueous
[
16]
ꢁ
conditions has been reported. In addition, a series of basic ionic liquids with OH
anion functionalized hydroxyapatite-encapsulated c-Fe O magnetic nanoparticles
2
3
were also used as efficient heterogeneous catalysts for the same condensation, and
17,18]
[
8
6–95% yield and 100% selectivity for ylidenemalononitriles were obtained.
More-
over, a series of H u¨ nig’s-base tethered ammonium ionic liquids have been used to cat-
alyze Knoevenagel condensation. By adjusting the distance between the ammonium
[
19]
head group and H u¨ nig’s base, the activity of the catalyst was changed. Furthermore,
it has been reported that N-(3-aminopropyl),N(3)-(3-triethoxysilylpropyl)-4,5-
dihydroimidazolium bromide hydrobromide was successively grafted onto mesoporous
silicas of MCM-41 and SBA-15, and then they were neutralized by treatment with pot-
assium bicarbonate, leading to the immobilized basic amino-containing task-specific
ionic liquids, which showed great efficiency (particularly reusability) than that of similar
pure basic task-specific ionic liquid (TSIL) of N-(3-aminopropyl),N(3)-(methyl)-imida-
zolium tetrafluoroborate or amino-functionalized mesoporous silicas in aqueous
[
20]
media.
All the features of milder conditions, greater selectivity, simpler workup,
and easier recycling of catalyst and product make this process environmentally friendly.
As part of a program aimed at expanding the chemically modified ionic liquids,
0
0
we have prepared a novel basic N,N,N ,N -tetramethyl-N’-hexyl-ethylenediammo-
nium tetrafluoroborate ([TMHEDA]BF ) ionic liquid, depicted in Scheme 1.
4
Evidently, the alkylamine group attached to the cation of ionic liquid may promote
organic reactions, which stems from the inherent feature of the organic base. In this
study, we report our findings on this ionic liquid, by which the considerable
efficiency was displayed in several base-catalyzed reactions including Knoevenagel
condensation, Micheal addition, aldol condensation, Claisen–Schmidt, and epoxida-
tion of a,b-unsaturated carbonyl compounds under solvent-free conditions.

1386
S. ZHOU ET AL.
4
Scheme 1. Structure of [TMHEDA]BF ionic liquid.
EXPERIMENTAL
Materials and Reagents
All solvents and chemicals used were commercially available and used without
further purification unless otherwise stated. They are chemical pure or analytical
pure. Tetramethylethylenediamine (99.0%), 1-bromohexane (99.5%), n-octanal,
and mesityl oxide were purchased from Aldrich. Sodium tetrafluoroborate, benzal-
dehyde, furfural, acetophenone, cyclohexanone, hydrogen peroxide, and acetone
were obtained from Shanghai Chemicals. Butyraldehyde, acrylonitrile, ethyl cyanoa-
cetate, and 2-butanone were obtained from Sinopharm Chemical Reagent Beijing.
Analysis and Characterization
The qualitative analysis of products was carried out on a Hewlett-Packard
890=5793 gas chromatograph=mass spectrometer (GC=MS) equipped with a HP
MS column (30 m) and a FID detector, with helium as carrier gas. The column tem-
perature was raised from 60 to 260 C at a heating rate of 10 C min . The concen-
tration of reactant and product was directly given by the analysis of GC chemstation
according to the area of each chromatograph peak. NMR spectra were recorded on
a Mercury-Plus 400 NMR spectrometer.
6
5
ꢀ
ꢀ
ꢁ1
[TMHEDA]BF Ionic Liquid
4
[
N,N,N ,N -Tetramethylethylenediamine (14.9 mL, 0.1 mol) and a stoichiometric
TMHEDA]BF ionic liquid was prepared by the following procedure: (1)
4
0
0
amount of 1-bromohexane (14.2 mL, 0.1 mol) were transferred in succession to
50 mL toluene in a 250-mL round-bottom flask equipped with a reflux condenser
under magnetic stirring at room temperature. Then, the mixture was heated to
ꢀ
80 C and stirred for 12 h. During this period, the content changed from a colorless
homogenous solution to a biphasic system. The lower amber phase was separated
and washed with toluene twice (10 mL ꢂ 2) followed by treatment under vacuum
ꢀ
0
0
at 80 C to get N,N,N ,N -tetramethyl-N-hexyl-ethylenediammonium bromide
([TMHEDA]Br) (98.2% of yield), (2) [TMHEDA]Br was dissolved to 50 mL acetone,
and sodium tetrafluoroborate (22.0 g, 0.2 mol) was added under vigorous stirring.
Stirring for 48 h at room temperature was done to ensure the completion of ion
exchange: (3) The white solid in the mixture was filtered out, and the filtrate was
evaporated under vacuum to get a sticky liquid, which was diluted with 50 mL
CH Cl . The solution was washed with water twice (20 mL ꢂ 2), followed by the vac-
2
2
ꢀ
uum treatment at 80 C to get the amber and viscous ionic liquid (96.7% of yield).

NOVEL BASIC IONIC LIQUID
1387
However, the NMR characterization had illustrated that the ionic liquid was not pure;
it is a mixture attributed to the nearly 20% of dialkylated ammonium compound
0
0
0
(
N,N,N ,N -tetramethyl-N,N -dihexyl-ethylenediammonium tetrafluoroborate).
1
[
TMHEDA]BF . H NMR (400 MHz, DMSO TMS) d ¼ 0.88 (m, CH CH ,
4
2
3
3
H), 1.30 (m, (CH ) , 6H), 1.61–1.67 (m, CH , 2H), 2.22 (s, NCH , 6H), 2.64 (t,
2 3 2 3
þ
þ
NCH , J ¼ 5.64 Hz, 2H), 3.05 (s, N CH , 6H), 3.29 (m, N CH , 2H), 3.38
2
þ
3
2
13
t, N CH , J ¼ 6.28 Hz, 2H). C NMR (400 MHz, DMSO TMS) d ¼ 13.82
(
(
2
1C, CH CH ), 21.69 (1C, CH ), 21.85 (1C, CH ), 25.41 (1C, CH ), 30.65 (1C,
2
3
2
2
2
þ
CH ), 44.95 (2C, NCH ), 50.36 (2C, N CH ), 52.21 (1C, NCH ), 59.68 (1C,
2
3
3
2
þ
þ
N CH ), 63.67 (1C, N CH ).
2
2
N,N,N’,N’-Tetramethyl-N,N’-dihexyl-ethylenediammonium tetrafluor-
1
oborate. H NMR (400 MHz, DMSO TMS) d ¼ 0.89 (m, CH CH , 6H), 1.32 [m,
2
3
þ þ
CH ) , 12H], 1.64–1.69 (m, CH , 4H), 3.10 (s, N CH , 12H), 3.34 (m, N CH ,
2 3 2 3 2
(
þ
13
4
H), 3.79 (s, N CH , 4H). C NMR (400 MHz, DMSO TMS) d ¼ 13.82 (2C,
2 3 2 2 2 2
2
CH CH ), 21.65 (2C, CH ), 21.91 (2C, CH ), 25.34 (2C, CH ), 30.65 (2C, CH ),
þ þ þ
0.44 (4C, N CH ), 54.91 (2C, N CH ), 64.41 (2C, N CH ).
3 2 2
5
A Representative Procedure for Knoevenagel Condensation in
TMHEDA]BF4
[
All the reactions were performed in a 50-mL round-bottomed flask equipped
with a condenser under magnetic stirring for the desired time. The basic ionic liquid
TMHEDA]BF (2.0 g) was put into the flask, and then benzaldehyde (0.1 mol) and
[
4
ethyl cyanoacetate (0.1 mol) were added sequentially with stirring. The mixture was
ꢀ
heated to 60 C and stirred for 1 h. After the mixture cooled to room temperature,
diethyl ether (20 mL) was poured into it. The upper ether phase was separated, and
the lower phase was extracted with diethyl ether (10 mL ꢂ 2). By combining the ether,
the product was obtained by removing the volatile ether. [TMHEDA]BF ionic liquid
4
can be recovered, purified under vacuum, and directly used in the next run.
RESULTS AND DISCUSSION
It is evident that the basicity of [TMHEDA]BF ionic liquid originated from
4
the alkyl amino group. That is to say, the cation of this basic ionic liquid probably
predominated the catalytic efficiency. Because the base-catalyzed reactions in
[
less-efficient activity were still inextricable.
BMim][BF ] have been described by the pioneering work, low conversion and
4
Thus, it is very suitable to check the
reactivity of [TMHEDA]BF using Knoevenagel condensation as a test reaction.
[
21]
4
Initially, the feasibility of [TMHEDA]BF used as the catalyst for Knoevenagel
4
condensation was investigated using benzaldehyde and ethyl cyanoacetate as sub-
strates as shown in Table 1, and only 43% of yield was achieved at room temperature
ꢀ
after 1 h (entry 1). As the temperature was raised to 40 C, a better result was
ꢀ
obtained (88% yield, entry 2). Then, subsequential experiments revealed that 60 C
is necessary to complete the reaction to the maximum yield (entry 3). However, when
changing benzaldehyde to aliphatic aldehyde and ketones, more effort for
Knoevenagel condensation has to be made to improve the yields. As can be seen

1388
S. ZHOU ET AL.
Table 1. Knoevenagel condensation of some aldehydes=ketones with ethyl cyanoacetate in the [TMHE-
a
DA][BF ] ionic liquid
4
ꢀ
b
Entry
R
1
R
2
Temp. ( C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
H
r.t.
40
60
60
80
60
60
60
1.0
1.0
1.0
0.5
4.0
2.5
2.0
2.5
43
88
H
H
H
c
99 (95)
75
67
95
98
89
Me
Cyclohexanone
n-C
Et
7
H
15
H
Me
a
[
GC yield, conversion based on ethyl cyanoacetate.
TMHEDA]BF
4
(2.0 g), aldehyde or ketone (0.1 mol), and ethyl cyanoacetate (0.1 mol).
b
c
Ionic liquid reused for the fourth time.
from entries 5–8, extended reaction time and even greater reaction temperature
entry 5) afforded the product in 67–98% yields. It should be pointed out that these
results are generally high and compare favorably with those catalyzed by solid basic
(
[22]
catalyst in conventional organic solvents.
It is well-known that the usual ethyl cyanoacetate-benzaldehyde test reaction is
much more facile with most base catalysts. Clearly this method would be substan-
tially improved if the ionic liquid could be efficient in other reactions. With this in
mind, we have investigated some organic reactions catalyzed by [TMHEDA]BF4
ionic liquid, and the results are summarized in Table 2.
Reactions of 1–4, are typical nucleophilic addition reactions, usually carried
out in the presence of a base catalyst. Herein, [TMHEDA]BF ionic liquid could
4
exhibit potential catalytic activity for these reactions with moderate to good yields.
Importantly, no by-product was detected in these nucleophilic addition reactions
except for the unreacted substrates. However, in the epoxidation of mesityl oxide,
the inescapable ring-opening reaction contributed to the poor yield. Interestingly,
the ionic liquids mostly used for Claisen-Schmidt reactions are reported to be
[
23,24]
acidic.
However, the ionic liquid-supported pyrrolidine amide catalyst has been
developed to perform Claisen–Schmidt reaction, and the (E)-a,b-unsaturated ketone
[25]
products were obtained in good yields, which also evidenced the wide applications
of the ionic liquids based on the alkylamino groups.
CONCLUSION
0
0
A novel basic ionic liquid of N,N,N ,N -tetramethyl-N-hexyl-ethylenediammo-
nium tetrafluoroborate was synthesized and applied to Knoevenagel condensation as
basic catalyst. Moderate to excellent yields were obtained using cyanoacetate
and benzaldehyde as substrates. Four reuses without dramatic decrease in yield of

NOVEL BASIC IONIC LIQUID
1389
a
4
Table 2. Organic reactions carried out in [TMHEDA]BF ionic liquid
b
Reaction
Reactants
Product
Time (h)
8
Yield (%)
1. Micheal addition
53.7
2
3
. Aldol
4
4
77.4
60.5
. Claisen–Schmidt
4
. Claisen–Schmidt
14
2
46.0
7.9
c
5
. Epoxidation
2 2
H O
a
4
Reaction conditions: [TMHEDA]BF , 20 wt%; equivalent amount of two substrates; room
temperature.
b
GC yield.
c
2 2
Five times excess of H O to mesityl oxide.
Knoevenagel condensation demonstrated the good stability and operability of the
ionic liquid. In addition, the catalytic activity of this ionic liquid was further tested
through some other nucleophilic addition reactions including Michael condensation,
Claisen-Schmidt reaction, and epoxidation. The dual function of the basic ionic
liquid both as solvent and catalyst, combined with simple recycling and product sep-
aration, is expected to contribute to the development of green and environmentally
friendly strategies.
ACKNOWLEDGMENTS
The authors are grateful for financial support from the National Natural
Science Foundation of China (31170556), Fundamental Research Funds for the
Central Universities (YX2011-36, TD2011-11), Research Fund for the Doctoral Pro-
gram of Higher Education of China (20100014120007), China Postdoctoral Science
Foundation (20110490303), Beijing Forestry University Young Scientist Fund
(BLX2009003), Major State Basic Research Development Program of China (973
Program, No. 2010CB732204), and China Ministry of Education (No. 111 project).
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