Silica gel supported -SO3H functionalised benzimidazolium based ionic liquid as a mild and effective catalyst for rapid synthesis of 1-amidoalkyl naphthols

Article abstract of DOI:10.1016/j.molcata.2011.11.003

The use of ionic liquid in catalysis is attracting more and more attention in the field of chemistry. In line with the research we have studied Supported Ionic Liquid Catalyst (SILC) which consist of benzimidazolium based ionic liquid immobilized on silic

Full text of DOI:10.1016/j.molcata.2011.11.003

Journal of Molecular Catalysis A: Chemical 353–354 (2012) 44–49
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Silica gel supported –SO3H functionalised benzimidazolium based ionic liquid as
a mild and effective catalyst for rapid synthesis of 1-amidoalkyl naphthols
Deepali A. Kotadia, Saurabh S. Soni∗
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar 388120, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 July 2011
Received in revised form 11 October 2011
Accepted 1 November 2011
Available online 7 November 2011
The use of ionic liquid in catalysis is attracting more and more attention in the field of chemistry. In line
with the research we have studied Supported Ionic Liquid Catalyst (SILC) which consist of benzimida-
zolium based ionic liquid immobilized on silica based solid support. The SILC proved to be an efficient
heterogeneous catalyst for solvent less synthesis of 1-amidoalkyl naphthols from 2-naphthol, amides
and aldehydes. The process represents a simple, ecologically safer, cost effective route to 1-amidoalkyl
naphthol with high product quality, as well as easy product recovery and catalyst recycling.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Silica supported ionic liquid
Benzimidazolium ionic liquid
1-Amidoalkyl naphthol
Multicomponent reaction
One pot solvent free synthesis
1. Introduction
thiamine hydrochloride [22], trityl chloride [23], [FemSILP] l-
prolinate [24], silica sulphuric acid [25,26], 1-hexane sulphonic
Devising reactions that achieve multi-bond formation in one-
operation is becoming one of the major challenges in step economic
process. Multicomponent reactions (MCRs) have emerged as an
important tool for building of diverse and complex organic
molecules through carbon–carbon and carbon–heteroatom bond
formations taking place in tandem manner [1–4]. o-Quinone
methides (o-QMs) have emerged as interesting molecules due to
their toxicological properties against both normal and cancerous
cells and also proposed intermediary in the formation of many bio-
logically important polymers [5]. o-QMs also act as intermediates
for the synthesis of antitumor agents [6]. One of the tandem reac-
tions which involves the in situ generation of o-QMs and its reaction
with acetamide or benzamide gives amidoalkyl naphthols [7]. 1-
Amidoalkyl naphthols can be easily hydrolysed to 1-aminoalkyl
naphthol, which shows biological activities like depressor and
brady-cardiac effect [8,9]. This 1-aminoalkyl alcohol-type ligand
has been used for asymmetric synthesis and also as a catalyst
[10,11]. Amidoalkyl naphthols can be prepared by the multicom-
ponent condensation of aldehydes, 2-naphthols and amides using
different catalysts such as Montmorillonite K10 [12], p-TSA [7],
I2 [13], K5CoW12O40·3H2O [14], HClO4–SiO2 [15], Fe(HSO4)3 [16],
FeCl3–SiO2 [17], BAIL [18,19], P2O5 [20], cyanuric chloride [21],
acid sodium salt [27], Zwitterionic-type molten salt [28]. However,
some of these reported procedures suffer from one or more draw-
backs such as long reaction times, high temperatures, low yields
of products, use of toxic organic solvents, use of expensive metal
salts as catalysts and tedious work up procedures. Therefore, there
is still an increasing interest in catalytic system for the synthesis of
1-amidoalkyl naphthols.
In recent years, ionic liquids (ILs) have become powerful alter-
natives to conventional molecular organic solvents due to their
particular properties, such as undetectable vapour pressure and the
ability to dissolve many organic and inorganic substances [29]. In
addition, ILs are readily recycled and tuneable to specific chemical
tasks. One type is Bronsted acidic ILs (BAILs). The acidic nature of
these ILs as catalyst has been exploited for many other important
organic reactions, which proceed with excellent yields and selec-
tivities and demonstrate the great potential of these ILs in catalytic
technologies for chemical production [30–34].
Recently, immobilization process involving acidic ILs on solid
supports has been designed. The heterogenization of catalysts and
reagents can offer important advantages in handling, separation
and reuse procedures. Based on economic criteria, it is desirable to
minimize the amount of IL utilized in a potential process. Immo-
bilized acidic ILs have been used as novel solid catalysts for many
acid catalysed reactions such as esterification, nitration reaction
and acetal formation [35,36]. On the other hand examples for the
synthesis of amidoalkyl naphthol using efficient heterogeneous
∗
Corresponding author. Tel.: +91 2692 226857x219.
E-mail address: soni b21@yahoo.co.in (S.S. Soni).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.11.003

D.A. Kotadia, S.S. Soni / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 44–49
45
Table 1
systems with solid acids, polymer supported peracids, zeolites or
amberlyst 15 with various groups deposited onto silica are known
[37,38].
In continuation of our recent studies to develop mild and envi-
ronmentally friendly procedures using ILs in organic synthesis,
herein, we report our results on the efficient and rapid synthesis
of 1-amidoalkyl naphthols using sulfonic acid functionalised benz-
imidazolium based Supported Ionic Liquid Catalyst (SILC) as a mild
and effective catalyst under solvent free conditions.
The effect of different amounts of SILC on the reaction of 2-naphthol, acetamide and
3-nitrobenzaldehyde.
Entry
Immobilized IL (mg)
Time (min)
Yield %
1
2
3
4
5
6
7
0
10
30
50
80
60
60
20
15
7
No reaction
32
52
78
95
95
92
100
120
7
6
2. Experimental
2.1. Materials and instrumentation
room temperature, filtered and washed several times with toluene.
This was further treated with 36% w/w concentrated hydrochloride
(3 mmol) and allowed to stand at room temperature for 24 h. The
obtained material was then washed with ether, dried under vac-
uum at 100 ◦C for 24 h to give sulfonic acid functionalised Supported
Ionic Liquid Catalyst (SILC) (3).
Silica gel 230–440 mesh (0.037–0.063 mm) was purchased
from Spectrochem, India and was used for the preparation
of the support. (3-Chloropropyl)triethoxysilane, benzimidazole,
1,3-propanesultone, were purchased from Sigma–Aldrich, India
and were used without further purification. FT-IR spectra were
recorded on ABB FTIR, Canada, using KBr pellets. Thermal gravimet-
ric analysis (TGA) data was obtained with a heating rate of 5 ◦C/min
on a TGA/DTA (TA instruments model 5000/2960 thermogravimet-
ric analyser, USA). C, H, N elemental analysis was carried out on
Perkin Elmer, USA (2400, Series II). The synthesized amidoalkyl
naphthols were identified by 1H and 13C NMR, (400 MHz Bruker
Scientific, Switzerland).
IR: 463, 795, 1088, 1158, 1566, 1643, 1859, 2962, 3472 cm−1
.
TGA: Data shows complete loss of covalently attached organic
moiety occurs at 542 ◦C and the amount of organic moiety was
about 24% against solid support.
CHN: Found C, 8.02; H, 2.133; N, 1.79%.
Calc. for SiO2·0.0458(C13H20N2–SO3Cl)·0.5H2O: C, 8.54; H, 2.29;
N, 1.86%.
Determination of acidic sites: In a typical experiment 0.05 g of
immobilized IL (vacuum dried sample to remove moisture pres-
ence) was added to 10 g aqueous solution of NaCl (2 M). The
resulting suspension was allowed to equilibrate for 48 h and then
titrated potentiometrically with 0.01 M NaOH (aq) using phenolph-
thalein as an indicator [40].
2.2. Synthesis of 3-chloropropyl silica (1)
3-Chloropropyl silica (1) was prepared by modifying the method
used by Adam et al. [39]. A mixture of silica (5.0 g) and (3-
chloropropyl)triethoxysilane (5.0 mL, 42.5 mmol) were taken in
10 mL of toluene was allowed to stir at room temperature for 15 min
and then refluxed for 24 h. After completion of reaction, reaction
mixture was cooled, and the product was filtered and repeatedly
washed with toluene (3 × 5 mL) and dried under reduced pressure
at 100 ◦C for 8 h to produce 3-chloropropyl silica (1) (4.9 g).
2.5. General synthesis for the preparation of amidoalkyl
naphthols
A mixture of aldehyde (20 mmol), 2-naphthol (20 mmol), amide
(24 mmol) and SILC (80 mg) were taken in round bottom flask and
stirred for the desired time (as indicated by TLC) at 100 ◦C in a pre-
heated oil bath. The resultant solid was then washed with hot water
to remove excess amide. Then acetone was added to it stirred well
and filtered off. The solid catalyst (residue) was washed with ace-
tone and dried. The filtrate was evaporated to remove solvent and
the crystalline material left was taken up in ethanol for recrystalli-
sation. The synthesized amidoalkyl naphthols were characterized
by 1H NMR and 13C NMR and comparison of their physical data
with the literature.
IR: 473, 805, 959, 1091, 1631, 3422 cm−1
.
TGA: Data shows that the prepared silica is stable up to 245 ◦C.
Complete loss of all the covalently attached organic structure is
seen at about 618 ◦C and the amount of organic moiety was about
15% against solid catalyst silica.
2.3. Synthesis of 3-(1-benzimidazole)propyl silica (2)
A solution of benzimidazole (1.17 g, 10 mmol) was prepared in
dry benzene to which 50% sodium hydride in mineral oil (0.479 g,
10 mmol) was added and stirred at room temperature for 3 h
under nitrogen atmosphere to give sodium benzimidazole. Then
3-chloropropyl silica (1) (5.00 g) was added and the mixture was
refluxed under a nitrogen atmosphere for 24 h. The resulting prod-
uct was filtered and washed with ethanol and dried under vacuum
at 100 ◦C for 24 h to give 4.89 g of 3-(1-benzimidazole)propyl silica
(2).
Spectral
data
of
N-[(2-hydroxynaphthalen-1-yl)(4-
nitrophenyl)methyl]acetamide (Table 3, entry 7): pale yellow
solid; M.P. 248–249 ◦C. 1H NMR (400 MHz, DMSO-d6): 2.032 (s,
3H), 7.18–7.42 (m, 6H), 7.80–8.15 (m, 5H), 8.56 (d, 1H), 10.11 (s,
H); 13C NMR (400 MHz, DMSO-d6): 23.0, 48.3, 118.3, 118.8, 123.0,
123.4, 123.7, 127.1, 127.6, 128.4, 128.9, 129.1, 129.3, 132.6, 142.1,
146.4, 151.7, 153.8, 170.1.
IR: 479, 743, 803, 1088, 1463, 1629, 3424 cm−1
.
TGA: Data shows that complete loss of the covalently attached
organic moiety occurs at 536 ◦C and the amount of organic moi-
ety was about 23% against the solid support and the loading of
benzimidazole group was found to be 0.743 mmol/g.
2.6. Reuse of the sulfonic acid functionalised solid support
The reusability of SILC was tested for the reaction of 3-
nitrobenzaldehdye, acetamide and 2-naphthol. After completion of
reaction acetone was added to the reaction mixture for dissolution
of product. The catalyst was filtered, washed with acetone and dried
under vacuum at 80 ◦C for 24 h. This recycled catalyst was used for
the synthesis of amidoalkyl naphthols using procedure Section 2.5.
The SILC was then recycled for five runs and the catalytic activity
displayed very good reusability.
2.4. Synthesis of sulfonic acid functionalised solid support (3)
3-(1-Benzimidazole)propyl silica (2) (4.00 g, 3 mmol of benzim-
idazole group) was suspended in 10 ml of toluene and 1,3-propane
sultone (0.378 g, 3.1 mmol) was added to it. The reaction mixture
was allowed to stir at 100 ◦C for 6 h. The resultant is cooled to

46
D.A. Kotadia, S.S. Soni / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 44–49
Table 2
The effect of temperature on the reaction of 2-naphthol, acetamide and 3-
nitrobenzaldehyde.
Entry
Temperature (◦C)
Time (min)
Yield %
1
2
3
4
5
6
40
60
80
100
110
120
240
150
45
7
7
6
72
77
83
95
94
92
tions of the benzimidazole ring. Moreover, two important peaks
at 1088 cm−1 and 1165 cm−1 assigned to S O stretching vibration
[41,42]. The broad peak at 3472 cm−1 belongs to Si–OH groups and
adsorbed water in silica. Additional bands at 2962, 1859 cm−1 are
due to C–H stretching deformation vibrations of the benzimidazole
moiety and alkyl chain.
Scheme 1. Synthesis of sulfonic acid functionalised benzimidazolium based Sup-
ported Ionic Liquid Catalyst (SILC).
The stability of the SILC was determined by thermogravimetric
analysis (Fig. 2). The TG curve indicates initial weight loss of 3.6% up
to 66 ◦C due to surface silanol groups and the adsorbed water in sil-
ica. Complete loss of all the covalently attached organic structure
is seen in the temperature range of 230–660 ◦C. The shouldering
observed from 357 ◦C onwards may be due to the decomposition
of alkyl-sulfonic acid groups. The amount of organic moiety was
found to be about 24% against total solid catalyst. From the ther-
mogravimetric analysis data, the empirical formula of the catalyst
can be calculated as SiO2·0.0458(C13H20N2–SO3Cl)·0.5H2O and was
also in consideration with the elemental analysis results.
The amounts of sulfonic acid groups after synthesis of SILC were
measured by means of acid–base titration. Ion-exchange capacities
of the sulfonic acid group were determined using aqueous solution
of sodium chloride as exchange agents. The acidic site loading in
SILC was found to be 0.588 mmol/g and the nitrogen analysis of
SILC indicates 0.64 mmol/g of acidic IL was grafted on to the sur-
face of the silica support. The agreement between the ion exchange
capacities measured using sodium as exchange ions as well as with
the organic group loading determined by TGA (0.546 mmol/g), is
in clear evidence that the sulfonic groups are principally located
on the porous surfaces of the silica, where they are accessible for
adsorption and catalytic reaction processes.
3. Results and discussion
3.1. Preparation and characterization of the catalyst
Supported Ionic Liquid Catalysts are prepared either by simple
physical adsorption or by covalent attachment of ILs onto the sur-
face of a solid support. We choose the later method to immobilize
the IL since this provides better stability in the catalytic reaction
and less leaching during the work up. Sulfonic acid functionalised
benzimidazolium based Supported Ionic Liquid Catalyst (SILC)
was synthesized in two steps. In the initial step (3-chloropropyl)
triethoxysilane was first treated with silica gel, where the bind-
ing between the groups occurs through covalent bonds giving
3-chloropropyl silica (1). After this, nucleophilic addition of the
chlorine with benzimidazole gave 3-(1-benzimidazole)propyl sil-
ica (2). Condensation of 3-(1-benzimidazole)propyl silica with
1,3-propane sultone produced the benzimidazole salt, which was
treated with one equivalent of hydrochloric acid to give SILC (3)
(Scheme 1). This is a simple two step synthesis of acidic IL grafted
silica when compared with the earlier approach for the preparation
of a similar catalyst [39].
SILC
was
characterized
by
FT-IR
spectroscopy,
analysis
thermogravimetric–derivative
thermogravimetric
(TG–DTA) and elemental analysis. As can be seen from Fig. 1,
the FT-IR spectra of SILC exhibits two characteristic peaks at
1566 cm−1 and 1643 cm−1 which are due to C N and C C vibra-
3.2. Synthesis of 1-amidoalkyl naphthols using SILC
To optimize the reaction conditions, reaction of 2-naphthol, 3-
nitrobenzaldehyde and acetamide in presence of SILC was selected
as a model reaction. To check the activity, initially the reaction
was stirred at 100 ◦C for 10 min, and the corresponding product
was obtained in 95% yield. With this result in hand, we gradu-
ally increased the amount of SILC from 0 to 120 mg (Table 1). The
result shows that in absence of SILC no reaction was observed
over a period of 1 h. As the amount of SILC increased, there is
gradual increase in yield and on further increase up to 120 mg
a little decrease in the yield was observed. Thus we used 80 mg
of SILC for the synthesis of 1-amidoalkyl naphthol. Then to opti-
mize the temperature of the reaction, we carried out the reaction
using 80 mg of SILC at various temperatures under solvent free
conditions (Table 2). The results clearly show that 100 ◦C is an
effective temperature in terms of reaction time and yield obtained.
Therefore we used 80 mg of SILC for the synthesis of amidoalkyl
naphthols from various aldehydes, amides or urea and 2-naphthols
under solvent free conditions at 100 ◦C. To check the reproducibil-
ity of the catalyst, the same reaction was carried out five times;
it gave the same yield of the product ( 2%). Possible mechanism
of the reaction is shown in Scheme 2. The sulfonic acid function-
alised part participate in the reaction which activate the aldehyde
followed by nucleophilic addition of 2-naphthol forming the
1859
2962
1566
795
1643
3472
1158
1088
1000
463
4000
3000
2000
Wave number/ cm-1
Fig. 1. FT-IR spectrum of SILC.

D.A. Kotadia, S.S. Soni / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 44–49
47
Fig. 2. TGA–DTA of SILC.
intermediate ortho-quinone methides (o-QMs). The o-QMs then
react with amides or urea via a Michael addition to afford the
expected amidoalkyl naphthols.
o-QM intermediate) as compared to that of alkene containing e−
donating groups may be responsible for making the present reac-
tion faster. These are in accordance with the proposed mechanism.
In comparison with other catalyst employed for the synthesis
of amidoalkyl naphthol from 3-nitrobenzaldehdye, acetamide and
2-naphthol, SILC showed a much higher activity in terms of short
reaction time and mild conditions (Table 4).
In addition, the recyclability of SILC was investigated for the
reaction of 3-nitrobenzaldehdye, acetamide and 2-naphthol. After
completion of reaction the separated catalyst can be reused after
washing with acetone and drying at 80 ◦C. The catalyst was recov-
ered in excellent yield and was used in the mentioned reaction for
After optimizing the reaction conditions, and understanding the
mechanism, we extended the scope of present method using SILC
as a catalyst for a variety of aromatic aldehydes. Various aromatic
aldehydes with substituent’s carrying either e− donating or e−
withdrawing groups reacted successfully and gave the products in
high yields (Table 3). The aromatic aldehydes with e− withdrawing
groups reacted faster as compared with those having e− donating
groups. As per earlier report [17], the lower energy of LUMO of the
alkene containing e− withdrawing groups (a carbonyl group as in
Table 3
Synthesis of amidoalkyl naphthol using SILC
.
Entry
Aldehyde R1
Amide R2
Product
Time
Yield %a
M.P. (◦C) ( 1 ◦C)b
1
2
3
4
5
6
7
8
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
7
7
10
10
7
7
7
7
12
7
7
7
10
7
7
7
7
10
90
95
87
89
92
93
89
80
82
82
94
90
89
91
88
86
90
92
242
183
207
186
194
226
249
124
193
236
217
228
209
178
283
174
193
168
3-NO2
4-OH
4-OCH3
2-Cl
4-Cl
4-NO2
4-(CH3)N
3,4,5-(OCH3)3
H
3-NO2
4-NO2
4-OCH3
4-Cl
2-Cl
H
3-NO2
4-Cl
9
CH3
10
11
12
13
14
15
16
17
18
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
NH2
NH2
NH2
a
Isolated yields of the product.
M.P. are in accordance with the reported data [15–17].
b

48
D.A. Kotadia, S.S. Soni / Journal of Molecular Catalysis A: Chemical 353–354 (2012) 44–49
Scheme 2. Possible mechanism for the synthesis of amidoalkyl naphthol using SILC.
Table 4
Comparison of different catalyst for the reaction of 3-nitrobenzaldehyde, acetamide and 2-naphthol.
S. no.
Catalyst (catalyst amount)
Temp./◦C
Time
Yield %
TOF (min−1
)
Ref.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
SILC (80 mg)
I2 (5 mol%)
Montmorillonite K10 (0.1 g)
p-TSA
100
125
125
125
125
85
110
120
100
80
60
80
85
100
120
7 min
15 h
0.5 h
4 h
95
89
96
90
78
83
95
94
94
88
97
88
92
86
96
2.84
0.026
–
Our work
13
12
7
14
16
15
17
21
22
20
28
26
24
18
–
K5CoW12O40·3H2O (1 mol%)
Fe(HSO4)3 (5 mol%)
HClO4–SiO2 (0.6 mol%)
FeCl3–SiO2 (25 mg)
Cyanuric chloride (10 mol%)
Thiamine HCl (0.5 mmol)
P2O5 (10 mol%)
Zwitterionic salt (10 mol%)
Immobilized IL (80 mg)
Fe–SILP (50 mg)
3 h
0.433
1.7
3.87
–
0.9
–
1.92
0.097
6.59
4.29
4.8
65 min
30 min
8 min
8 min
4 h
5 min
1.5 h
5 min
–
SO3H functionalised IL (10 mol%)
3 min
five times giving good to excellent yields of the products. The effect
of catalyst was unchanged which was determined by measuring
the acidic site loading of the reused SILC and hence conforming the
purity of SILC.
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In summary, we have developed an efficient procedure for the
synthesis of amidoalkyl naphthols. This involves the use of the ben-
zimidazolium based IL covalently attached to the surface of the
support called as SILC as an inexpensive, stable and easily available
Bronsted acidic catalyst. The use of a powerful, easily accessible and
recyclablesolid catalystmakes this methodquite simpleand conve-
nient. The high yield of amidoalkyl naphthols, the use of relatively
inexpensive catalyst and the straightforward isolation of the prod-
ucts are the advantages of this method over the other procedures
reported.
Acknowledgment
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DAK thanks University Grants Commission (UGC) Delhi for
financial assistance in terms of Meritorious Fellowship.
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