Acidic ionic liquid immobilized on cellulose: An efficient and recyclable heterogeneous catalyst for the solvent-free synthesis of hydroxylated trisubstituted pyridines

Article abstract of DOI:10.1039/c3ra23052j

The synthesis of a novel cellulose supported acidic ionic liquid (Cell-IL) for the solvent-free synthesis of hydroxylated trisubstituted pyridines is reported. Cell-IL was prepared by immobilization of an acidic ionic liquid [1-butyl-3-(3-trimethoxypropyl

Full text of DOI:10.1039/c3ra23052j

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Acidic ionic liquid immobilized on cellulose: an efficient
and recyclable heterogeneous catalyst for the solvent-
free synthesis of hydroxylated trisubstituted pyridines3
Cite this: RSC Advances, 2013, 3, 3184
Received 24th November 2012,
Accepted 8th January 2013
Shailesh P. Satasia, Piyush N. Kalaria and Dipak K. Raval*
DOI: 10.1039/c3ra23052j
www.rsc.org/advances
The synthesis of a novel cellulose supported acidic ionic liquid (Cell-
IL) for the solvent-free synthesis of hydroxylated trisubstituted
pyridines is reported. Cell-IL was prepared by immobilization of an
acidic ionic liquid [1-butyl-3-(3-trimethoxypropyl)-1H-imidazol-3-
ium hydrogen sulfate] on cellulose. The viability of this concept has
been confirmed by FT-IR, TGA-DTG, 1H NMR and elemental analysis.
The Cell-IL showed good thermal stability and exhibited a high
catalytic activity in the synthesis of a series of hydroxylated
trisubstituted pyridines. It was recovered and reused for the model
reaction three times without an appreciable change in its activity.
derivatives have been utilized as the support for catalytic applica-
tions. Cellulose, one of the most abundant natural biopolymers, has
unique properties such as being biodegradable, inexpensive,
extremely inert, non-toxic and environmentally benign. These
properties make it an attractive alternative to conventional organic
or inorganic supports for catalytic applications,16 offering a
relatively low contamination risk to the environment.
Substituted pyridines are an important class of compounds
that feature profoundly in a number of pharmaceutical targets and
are a common motif in many natural products.17,18 They have
attracted continuous interest to explore newer methods for their
construction.19–23 These compounds have also been synthesized
through the reaction of N-phenacylpyridinium salts with a,b-un-
1. Introduction
saturated ketones in the presence of ammonium acetate
24,25
In recent years, green chemistry has focused mainly on
environmentally friendly reactions, sustainable resources and
reusable catalysts. To design a catalyst with high selectivity and
efficiency, which is kind to the environment and can be easily
recovered, is an attractive and rapidly developing area in
chemistry. Ionic liquids (ILs) have attracted extensive research
interest as environmentally benign solvents due to their excellent
properties such as negligible vapour pressure, solvating ability,
non-flammability, high thermal stability and reusability.1–3
Compared to pure acidic ILs, IL-based heterogeneous catalysts
offer more advantages. They are required in lower amounts, are
easy to separate and have a competent catalyst recovery.4–6 The
concept of ‘‘immobilized’’ or ‘‘heterogenised’’ liquids is well
known for supported liquid phase (SLP) catalysts. The immobi-
lization process aims to transfer the desired catalytic properties
of the liquids to a solid catalyst.7
¨
(Krohnke’s synthesis).
The pyridinium salts and the unsatu-
rated ketones have to be synthesized first, which makes the
¨
method relatively expensive. Krohnke pyridines bearing a hydroxyl
group are usually prepared by SPOT-synthesis,26 or multi-step
protection/deprotection strategies.27,28 Many improved methods
for the preparation of 2,4,6-trisubstituted pyridines have also been
reported.29–34 Recently, Yin et al. reported the synthesis of
hydroxylated trisubstituted pyridines under microwave irradiation
and have investigated their photophysical properties.35
As part of a program directed toward the synthesis of various
heterocyclic compounds,36–41 we envisage the use of a catalytic
system based on cellulose functionalized with an acidic IL for the
synthesis of hydroxylated trisubstituted pyridines. The present
protocol, operating without the need for protection of the hydroxyl
group, is a novelty. An acidic ionic liquid [1-butyl-3-(3-trimethox-
ypropyl)-1H-imidazol-3-ium hydrogen sulfate] was immobilized on
cellulose to prepare Cell-IL. This is the first ever report describing
the synthesis of hydroxylated trisubstituted pyridines using Cell-IL
as the catalyst under solvent free conditions.
It is likely that renewable bio-polymeric supports will play a key
role in the future for the development of clean processes. In this
regard, biopolymers are attractive candidates to explore their
application in supported catalysis.8,9 Several interesting biopoly-
mers such as alginate,10 gelatin,11,12 starch13 and chitosan14,15
2. Experimental section
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar-388 120,
Gujarat, India. E-mail: dipanalka@yahoo.com; Fax: +91-02692-236475;
Tel: +91-02692-226856 – Ext. - 211
2.1 Materials and instrumentation
All the reactions were performed with commercially available
reagents. They were used without further purification. Organic
Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra23052j
3
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solvents were purified by standard methods.42 Reactions were
carried out on an Equitron round oil bath (model no. 8481ISDWO).
All the reactions were monitored by thin-layer chromatography
carried out on fluorescent coated plates (aluminium plates coated
with silica gel 60 F254, 0.25 mm thickness, Merck) and detection
of the components was made by exposure to UV light. Melting
points were measured in a mThermoCal10 (Analab Scientific pvt
Ltd., India) and the reported values are uncorrected. The
synthesized compounds were identified by 1H and 13C NMR
spectra recorded in MeOD on a Bruker Avance 400 MHz
spectrometer (Bruker Scientific Corporation Ltd., Switzerland)
using the residual solvent signal as an internal standard at 400
MHz and 100 MHz respectively. IR spectra were recorded on a
Bruker alpha E- FTIR spectrophotometer in the range 4000–400
cm21 and frequencies of only characteristic peaks are expressed.
Elemental analyses were performed on a PerkinElmer 2400 series
II elemental analyzer (PerkinElmer, USA) and all results are found
within ¡0.4% of the theoretical compositions. TGA data were
obtained with a heating rate of 10 uC min21 on a TGA-DTG (TA
instruments model 5000/2960 thermo gravimetric analyzer, USA).
Scheme 1 Synthesis of the cellulose supported IL catalyst.
water. The solid was separated by filtration through a sintered
funnel under suction. The residue was treated with methanol to
recover the insoluble catalyst from the product. The solvent was
then evaporated under vacuum and the desired product was
obtained as a yellow solid. The structures of the products were
confirmed by 1H NMR, 13C NMR, IR spectroscopy and also
supported by elemental analysis. The filtered catalyst was dried at
60 uC under vacuum (144 mm Hg) for 4 h and was recycled thrice
for the model reaction to check its catalytic efficiency.
2.2 Synthesis of 1-butyl-3-(3-trimethoxysilylpropyl)-
1H-imidazol-3-ium hydrogen sulfate (IL-HSO4)
2.5 Spectral data of synthesized compounds
4,49,499-(Pyridine-2,4,6-triyl)triphenol (3a). Yield: 88%, mp:
281–282 uC, 1H NMR (400 MHz, MeOD): d (ppm) 8.032 (d, J = 8.8,
4H), 7.797 (s, 2H, Py–H), 7.727 (d, J = 8.8, 2H), 6.973–6.921 (m, 6H);
13C NMR (100 MHz, MeOD): d (ppm) 158.5, 158.11, 156.6, 150.09,
131.10, 129.78, 128.26, 128.12, 115.55, 115.06, 114.49. IR (cm21):
3312, 1713, 1598, 1521, 1389, 1242, 1186, 826. Anal. calcd C23H17N
(355.12): C, 77.73; H, 4.82; N, 3.94. Found: C, 77.56; H, 4.96; N,
3.76.
3-(2,6-Bis(4-methoxyphenyl)pyridin-4-yl)phenol (3i). Yield:
81%, mp: 138–139 uC, 1H NMR (400 MHz, MeOD): d (ppm)
8.133 (d, J = 8.8 Hz, 4H), 7.824 (s, 2H, Py–H), 7.224–7.383 (m, 3H),
7.069 (d, J = 8.8 Hz, 4H), 6.904–6.933 (m, 1H), 3.882 (s, 6H, OCH3);
13C NMR (100 MHz, MeOD): d (ppm) 160.83, 157.87, 157.14,
150.44, 140.29, 132.06, 129.85, 128.24, 128.03, 118.03, 115.44,
115.23, 113.77, 113.64, 54.47, 54.25. IR (cm21): 3412, 1614, 1504,
1409, 1248, 1163, 1026, 879, 761. Anal. calcd C25H21NO3 (383.44):
C, 78.31; H, 5.52; N, 3.65. Found: C, 78.55; H, 5.74; N, 3.42.
3-(2,6-Di([1,19-biphenyl]-4-yl)pyridin-4-yl)phenol (3l). Yield:
84%, mp: 233–234 uC, 1H NMR (400 MHz, MeOD): d (ppm)
8.305 (s, 2H, Py–H), 8.071 (d, J = 8.4 Hz, 4H), 7.928 (d, J = 8.4 Hz,
4H), 7.711 (d, J = 7.2 Hz, 4H), 7.096–7.516 (m, 10H) : 13C NMR (100
MHz, MeOD): d (ppm) 159.04, 158.64, 158.37, 153.84, 145.09,
139.16, 136.19, 130.73, 129.11, 128.85, 127.79, 126.96, 121.37,
118.97, 114.44. IR (cm21): 3421, 1607, 1523, 1401, 1239, 1180,
1123, 842, 754. Anal. calcd C35H25NO (475.19): C, 88.39; H, 5.30; N,
2.95. Found: C, 88.63; H, 5.14; N, 3.02.
IL1, 1-butyl-3-(3-trimethoxysilylpropyl)-1H-imidazol-3-ium chloride
was prepared according to the known process.43 Then it was
modified, concerning the anion of the IL. Conc. H2SO4 (1.35 mL,
25 mmol) was added drop wise into a solution of the above in
ethanol (35 mL) over 30 min. The final mixture was stirred at 50 uC
for another 8 h and evaporated under reduced pressure to give a
viscous orange liquid in 96% yield (IL2).
1H NMR (400 MHz, CDCl3): d = 0.942–0.980 (m, 2 H), 1.172 (t, J =
7.4 Hz, 3 H), 1.341 (m, 2 H), 1.907–1.938 (m, 4H), 3.501 (s, 9 H),
4.290 (m, 4 H), 7.469–0.477 (m, 2H), 9.139 (s, 1 H), 14.524 (s, 1 H),
IR:
n = ; anal. calcd for
3140, 2932, 2870, 1566 cm21
C13H27ClN2O3Si: C 48.35, H 8.43, N 8.68; found: C 48.68, H 8.58,
N 8.81.
2.3 Immobilization of IL-HSO4 on cellulose
In the preparation of Cell-IL, cellulose (5.0 gm) was first activated
by treatment with 25 mL 0.1 M aqueous sodium hydroxide
solution. The mixture of IL2 (2.0 g) and the sodium salt of
cellulose was stirred in 30 mL methanol at reflux for 24 h. The
ionic liquid was immobilized on cellulose upon etherification
(Scheme 1). The obtained Cell-IL was cooled to room temperature,
filtered and washed thrice with water, dried under vacuum at (144
mm of Hg) 60 uC for 2–3 h to give the supported acidic ionic
liquids in powder form.
IR: 3139, 2949, 2872, 1644, 1564, 1454 cm21
Elemental analysis found: C 32.56, H 4.89, N 4.26.
3. Results and discussion
2.4 Synthesis of hydroxylated 2,4,6-trisubstituted pyridines
3.1 Characterization of catalysts
A mixture of 4-hydroxybenzaldehyde 1 (25 mmol), 4-hydroxyace-
tophenone 2a (50 mmol) and NH4OAc (150 mmol) was heated at
110 uC for 30–35 min in the presence of Cell-IL (60 mg) under
solvent-free conditions. The progress of the reaction was
monitored by TLC. After completion of the reaction, the solid
product gradually formed upon cooling was poured into cold
The resulting biopolymer supported acidic IL catalyst was
characterized by FT-IR spectroscopy and TGA analysis. Fig. 1
shows the FT-IR spectra of Cell-IL and Cell-ONa catalysts. The FT-
IR spectrum of Cell-IL (curve 1) exhibited some characteristic
stretching vibration bands due to the imidazole ring at 1644 and
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Table 1 Optimization of amounts of Cell-IL in the formation of 3aa
Entry
Immobilized IL (mg)
Time (min)b
Yieldc (%)
1
2
3
4
5
40
50
60
70
80
80
60
52
56
54
66
77
81
80
80
a
4-Hydroxy benzaldehyde (25 mmol), 4-hydroxy acetophenone (50
b
mmol), ammonium acetate (150 mmol). All reactions were run
until completion as indicated by TLC. Isolated yield.
c
Fig. 1 FT-IR spectra comparison of Cell-ONa and immobilized IL.
Table 2 Effect of temperature on the synthesis of 3aa
Entry
Temperature (uC)
Time (min)b
Yieldc (%)
1564 cm21, which may be attributed to the CLN and CLC vibration
peaks.44 Additional bands at 3139, 2949 and 1454 cm21 are due to
C–H stretching and deformation vibrations of the imidazole
moiety and alkyl chain. These are the additional characteristic
peaks of the functionalized IL which are absent in the IR spectrum
of Cell-ONa (curve 2).
The thermal stability of the prepared catalyst was checked by
TGA. TG and DTG curves are shown in Fig. 2. The TG curve
indicates an initial weight loss of 1.9% up to 100 uC due to the
adsorbed water in Cell-IL. A major weight loss in the IL supported
onto the cellulose was recorded in the temperature range 220 uC to
400 uC. The weight loss was about 66.25%. The DTG curve shows
that decomposition of the organic structure mainly occurred in
one step from 220 uC to 360 uC, which is related to the main
weight loss of 62.05%.
1
2
3
4
5
6
30
50
80
100
110
120
180
120
95
52
36
—
23
51
81
88
86
50
a
4-Hydroxy benzaldehyde (25 mmol), 4-hydroxy acetophenone (50
b
mmol), ammonium acetate (150 mmol), Cell-IL (60 mg). All
reactions were run until completion as indicated by TLC. Isolated
yield.
c
(Table 2, entry 5). This temperature was chosen as the optimum
reaction temperature for a series of compounds (3b–3p) synthe-
sized to study the effect of various substituted building blocks on
the reaction.
A number of various catalysts were tried for the synthesis of 3a.
The model reaction was attempted without any catalyst (Table 3,
entry 1), which afforded compound 3a in poor yield. Acetic acid,
p-TSA, Amberlyst-15, montmorillonite K-10 and Dowex50WX4 also
promoted the reaction with low yields (Table 3, entries 2–6). The
results indicated that Cell-IL was found to be the most effective
catalyst (Table 3, entry 7), leading to 88% yield of 3a without any
added solvent.
With these optimized conditions in hand, different aromatic
aldehydes and ketones were attempted to prepare a series of
trisubstituted hydroxylated pyridines to explore the scope and
generality of the reaction (Table 4). In 3a–3l, the corresponding
3.2 Applications of Cell-IL as the catalyst for synthesis of
hydroxylated trisubstituted pyridines derivatives
Initial screening for the catalytic efficiency of Cell-IL in the model
three-component condensation reaction was carried out in the
presence of 40 mg immobilized IL (Cell-IL) under solvent free
conditions at 100 uC. The efficiency of the reaction was affected by
the amount of the catalyst. From the performed sets of reactions,
it was found that 60 mg of Cell-IL was the optimum amount of
catalyst required to carry out the reactions with maximum yield
(Table 1, entry 3).
The model reaction was also optimized for the reaction
temperature. At room temperature, the reaction did not proceed.
The reaction progressed with 88% yield within 36 min at 110 uC
Table 3 Influence of different catalysts for the synthesis of 3aa
Entry
Catalyst (amount)b
Timec
Yieldd (%)
1
2
3
4
5
6
7
Nil
Acetic acid
p-TSA
Amberlyst-15
Montmorolonite K-10
Dowex50WX4
Cell-IL
120
52
68
84
43
73
36
39
51
53
42
62
53
88
a
4-Hydroxy benzaldehyde (25 mmol), 4-hydroxy acetophenone (50
b
mmol), ammonium acetate (150 mmol). Amount of catalyst was 60
c
mg. All reactions were run until completion as indicated by TLC.
d
Fig. 2 TG-DTG analyses for the Cell-IL catalyst.
3186 | RSC Adv., 2013, 3, 3184–3188
Isolated yield.
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Table 4 Cell-IL catalyzed synthesis of 3a under solvent free conditions
m.p. (uC)
Entry
Compounds
Aldehydes (R1)
Ketones (Ar)
Time (min)b
Yieldc (%)
Found
Reported31
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
4-OH
3-OH
3-OH
3-OH
3-OH
2-OH
4-N(CH3)2
2-thienyl
H
4-OH-C6H5
-C6H5
4-Me-C6H5
4-OMe-C6H5
4-F-C6H5
36
35
32
30
33
26
30
32
40
33
38
31
43
34
39
34
88
86
89
88
81
91
88
85
81
82
80
84
67
82
78
88
281–282
179–180
275–276
243–244
275–276
262–263
138–139
224–225
138–139
162–164
124–125
233–234
224–225
244–246
232–233
197–198
281–282
209–210
274–275
243–244
—
263–264
136–137
223–225
—
163–164
124–125
—
226–227
246–247
231–232
229–230
2-furyl
2-thienyl
4-Ph-C6H5
4-OMe-C6H5
2-furyl
2-thienyl
4-Ph-C6H5
4-Ph-C6H5
4-OH-C6H5
4-OH-C6H5
4-OH-C6H5
9
10
11
12
13
14
15
16
a
b
Aldehydes (25 mmol), acetophenones (50 mmol), ammonium acetate (150 mmol), Cell-IL (60 mg). All reactions were run until completion
c
as indicated by TLC. Isolated yield.
trisubstituted hydroxylated pyridines were obtained in good to
excellent yield. However, the ortho-substituted aromatic aldehydes
4. Conclusions
A simple and efficient supported ionic liquid catalyst for the
solvent-free synthesis of hydroxylated trisubstituted pyridines is
developed. The work provides an example for the applications of
cellulose supported ionic liquid catalysts as environmentally
benign efficient alternatives in organic synthesis. The attractive
features of this approach are a simple reaction procedure, short
reaction time, easy work up and reusability of the Cell-IL. Cell-IL
has proven to be a highly efficient heterogeneous catalyst
exhibiting a high catalytic activity for a series of targeted pyridine
derivatives with easy recovery and reusability for three successive
turns.
under the above optimized conditions gave lower yields (Table 4,
entry 13).
The reaction was feasible without protecting the hydroxyl group
present in the aromatic aldehydes and ketones. All the prepared
compounds were obtained in pure form after recrystallization
from alcohol. The present method is better in terms of yield as
well as mildness of the reaction conditions compared to the
methods reported earlier.30,33,45
3.3 Recyclability of the catalyst
To investigate the catalytic efficiency of the recycled catalyst, three
successive cycles of the model reaction were run under the optimal
reaction conditions using recycled Cell-IL from the previous run
(Table 5). The activity of the catalyst did not show any significant
decrease in the yields after three successive runs for the model
reaction. It was revealed that the catalyst displayed very good
reusability.
Acknowledgements
SPS and PNK acknowledge UGC, New Delhi for providing
research fellowships. The authors also thank the Head,
Department of Chemistry, Sardar Patel University for provid-
ing research facilities and SICART, Vallabh Vidyanagar for
elemental analysis and TGA at concessional rates.
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