Sulfonated rice husk ash (RHA-SO3H) as a highly efficient and reusable catalyst for the synthesis of some bis-heterocyclic compounds

Article abstract of DOI:10.1039/c3ra44053b

Sulfonated rice husk ash (RHA-SO₃H), a newly reported solid acid catalyst, was efficiently used for the synthesis of 4,4′-(arylmethylene)- bis-(3-methyl-1-phenyl-1H-pyrazol-5-ols), 3,3′-(arylmethylene)-bis-(4- hydroxycoumarins) and bis(indolyl)

Full text of DOI:10.1039/c3ra44053b

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Sulfonated rice husk ash (RHA-SO3H) as a highly
efficient and reusable catalyst for the synthesis of
some bis-heterocyclic compounds†
Cite this: RSC Adv., 2013, 3, 24046
Received 1st August 2013
*
Mohadeseh Seddighi, Farhad Shirini and Manouchehr Mamaghani
Accepted 12th September 2013
DOI: 10.1039/c3ra44053b
www.rsc.org/advances
Sulfonated rice husk ash (RHA-SO3H), a newly reported solid acid coumarin were also used as anti-inammatories, antipy-
catalyst, was efficiently used for the synthesis of 4,40-(arylmethylene)- retics, antifungals, antiseptics,26 anticoagulants,27 and as
bis-(3-methyl-1-phenyl-1H-pyrazol-5-ols), 3,30-(arylmethylene)-bis- urease inhibitors.28 The preparation of 3,30-(arylmethylene)-
(4-hydroxycoumarins) and bis(indolyl)methanes. The procedure bis-(4-hydroxycoumarins), a class of biscoumarins, is based
gave the products in excellent yields within very short reaction times on the condensation of aldehydes with 4-hydroxycoumarin.
under mild and green conditions. Also this catalyst can be reused For this purpose, a variety of catalysts including piperidine,28
several times without appreciable loss of its catalytic activity.
I2,29 tetrabutylammonium bromide (TBAB),30 sodium dodecyl
sulfate (SDS),31 [MIM(CH2)4SO3H] HSO4,32 and RuCl3$nH2O33
have been reported.
Indoles are also an important group of heterocyclic
compounds with a broad scope of biological activities.34 Among
them, bis(indolyl)alkane derivatives have found a great deal of
interest because they have many applications in pharmaceuticals.
For example, they have showed antitumor,35 antileishmanial,36
antihyperlipidemic,37 and anticancer38 activities. Among various
methods that have been reported for the synthesis of bis(indolyl)
methanes,39–41 the most simple and straightforward method that
has been developed is based on the condensation of aldehydes
with indoles. For this purpose, a variety of catalysts including I2,42
sulfamic acid,43 benzyltriphenylphosphonium tribromide,44 ceric
ammonium nitrate,45 [hmim]HSO4 acidic ionic liquid,46
ZrOCl2$8H2O, camphor sulphonic acid,47 and PEG-SO3H48 have
been reported.
Although these procedures provide an improvement in the
synthesis of the above mentioned bis-heterocyclic compounds,
many of them suffer from disadvantages such as long reaction
times, harsh reaction conditions, a need for excess amounts of
the reagent, use of organic solvents, use of toxic reagents and
non-recoverability of the catalyst. Therefore, there is still a
demand for introducing simple, efficient and mild procedures
with easily separable and reusable solid catalysts to overcome
these problems.
Introduction
Heterocyclic compounds are widely distributed in nature and
are essential to life. Pyrazoles are an important class of
heterocyclic compounds that occurs widely in the pharma-
ceutical industry. For example, compounds containing the
2,4-dihydro-3H-pyrazol-3-one structural motif, including 4,40-
(arylmethylene)-bis-(1H-pyrazol-5-ols), have attracted interest
because they exhibit a wide range of biological activities such as
antimalarial,1 antifungal,2 anti-inammatory,3 antimicrobial,4
antinociceptive,5 analgesic,6 fungicide,7 and antitumor8 activities.
Additionally, they are applied as important intermediates in
organic synthesis,9 and as bis-Schiff bases.10 4,40-(Arylmethylene)-
bis-(1H-pyrazol-5-ols) were also used as pesticides,11 antivirals12
and ligands.13 The main synthetic method for the preparation
of this type of compounds is based on the condensation of
aldehydes with 3-methyl-1-phenyl-5-pyrazolone. Therefore, a
variety of catalysts and reagents have been used to facilitate this
reaction.13–20
Coumarins are another important class of heterocycles. This
importance can be attributed to their broad scope of pharma-
ceutical and biological activities such as anti-bacterial,21 anti-
HIV,22 anti-Hepatitis C virus,23 anti-tumor,24 and anti-cancer25
activities. Biscoumarins, the bridge substituted dimers of
The replacement of conventional, toxic and polluting
¨
Bronsted and Lewis acid catalysts with eco-friendly, reusable,
heterogeneous catalysts is an area of current interest. In this
context, there has been renewed interest in the synthesis of
solid acid catalysts for organic reactions. Solid acid catalysts
have many advantages compared to traditional liquid acids
Department of Chemistry, Faculty of Sciences, University of Guilan, Rasht, Iran, 41335.
E-mail: shirini@guilan.ac.ir; Fax: +98 131 3233262; Tel: +98 131 3233262
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra44053b
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such as their efficiency, operational simplicity, easy recyclability ltration. The products were puried by recrystallization from
and recoverability, non-corrosive nature and environmentally aqueous ethanol.
friendliness, all factors which are important in industry.
Therefore, solid acid catalysts can play a signicant role in the
development of clean technologies.
General procedure for the synthesis of bis(indolyl)methane
derivatives (4)
A mixture of the aldehyde (1 mmol), indole (2 mmol) and
Experimental section
Reagents and materials
RHA-SO3H (15 mg) was heated at 80 ꢀC under solvent-free
conditions for the appropriate time. Aer completion of the
reaction (monitored by TLC), EtOH (10 mL) was added and
the catalyst was removed by ltration. Then, water was added and
the precipitated product was collected by ltration in high purity.
Chemicals were purchased from Fluka, Merck, and Aldrich
chemical companies. All yields refer to the isolated products.
Products were characterized by comparison of their physical
constants, and IR and NMR spectra with those of authentic
samples and those reported in the literature. The purity deter-
mination of the substrate and reaction monitoring were fol-
lowed by TLC on silicagel polygram SILG/UV 254 plates.
The spectral data of the new compounds is as follows:
Compound 3m:
Catalyst preparation (RHA-SO3H)
A 50 mL suction ask charged with 3.0 g of RHA and 10 mL
CHCl3 was equipped with a constant-pressure dropping funnel
containing chlorosulfonic acid (0.7 mL) and a gas outlet tube for
conducting HCl gas into water as an absorbing solution.
Chlorosulfonic acid was added dropwise over a period of 20 min
mp 258–260 ꢀC; 1H NMR (CDCl3, 400 MHz): d ¼ 2.50 (3H, s,
while the reaction mixture was stirred in an ice bath (0 ꢀC). Aer
CH3), 6.08 (1H, s, CH), 7.16 (2H, d, J ¼ 8 Hz), 7.23 (2H, d, J ¼ 8.4
addition was completed, the mixture was stirred for an addi-
Hz), 7.39–7.45 (4H, m), 7.6 (2H, td, J1 ¼ 7.6 Hz, J2 ¼ 1.6 Hz),
tional 2 h at room temperature. Then, the mixture was ltered
8.02–8.11 (2H, m), 11.34 (1H, s, OH), 11.55 (1H, s, OH) ppm; IR
and the solid residue washed with methanol (20 mL) and dried
(KBr, cmꢁ1): 760, 1558, 1603, 1655, 2920, 3080.
at 70 ꢀC for 1 h to afford RHA-SO3H (3.6 g) as an earthy powder
Compound 4s:
(Scheme 1).
General procedure for the synthesis of 4,40-(arylmethylene)-
bis-(3-methyl-1-phenyl-1H-pyrazol-5-ol) derivatives (2)
Phenylhydrazine (2 mmol) was added to a mixture of RHA-SO3H
(60 mg) and ethyl acetoacetate (2 mmol) at 80 ꢀC, and stirred for
1 min. Then the aldehyde (1 mmol) was added and the resulting
mixture was stirred for the appropriate time (Table 1). Aer
completion of the reaction (monitored by TLC), EtOH (10 mL)
was added and the catalyst was removed by ltration. Then,
water was added and the precipitated product was collected by
ltration in high purity.
mp 304–306 ꢀC; 1H NMR (DMSO-d6, 400 MHz): d ¼ 6.50 (1H, s,
CH), 7.24 (2H, s), 7.57 (2H, d, J ¼ 8.8 Hz), 7.68 (2H, d, J ¼ 8.8 Hz),
8.0 (2H, dd, J1 ¼ 9 Hz, J2 ¼ 2 Hz), 8.22 (2H, d, J ¼ 8.8 Hz), 8.40
(2H, d, J ¼ 2.4 Hz), 11.9 (s, NH) ppm; 13C NMR (DMSO-d6, 100
Mz): d ¼ 38.4, 112.6, 116.5, 117.3, 119.7, 124.3, 126.0, 128.3,
General procedure for the synthesis of 3,30-(arylmethylene)-
bis-(4-hydroxycoumarins) derivatives (3)
129.9, 140.1, 140.8, 146.6, 152.3; IR (KBr, cmꢁ1): 3253, 1612,
1502, 1460, 1317, 1090, 800, 733.
A mixture of 4-hydroxycoumarin (2 mmol), aldehyde (1 mmol),
RHA-SO3H (40 mg) and water (5 mL) in a round bottomed ask
was heated at 80 ꢀC. The progress of the reaction was monitored
by TLC. Upon completion of the reaction, the mixture was
cooled to room temperature. The solid was ltered off, EtOH or
CH2Cl2 (10 mL) was added and the catalyst was removed by
Compound 4t:
mp 317–319 ꢀC; 1H NMR (DMSO-d6, 400 MHz): d ¼ 3.73 (3H, s,
OCH3), 6.15 (1H, s, CH), 6.89 (2H, d, J ¼ 8.4 Hz), 7.12 (2H, s),
7.30 (2H, d, J ¼ 8.8 Hz), 7.55 (2H, d, J ¼ 9.2 Hz), 7.98 (2H, dd, J1 ¼
Scheme 1 Preparation of the catalyst (RHA-SO3H).
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Table 1 Preparation of 4,40-(arylmethylene)-bis-(3-methyl-1-phenyl-1H-pyrazol-5-ols) derivatives catalyzed by RHA-SO3H under optimized conditionsa
Melting point (ꢀC)
Entry
Ar
Product
Time (min)
Yield (%)
Found
Reported
1
2
3
4
5
6
7
8
C6H5–
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
2r
3
4
3
3
3
4
4
6
4
4
5
3
3
4
4
4
3
3
8
91
89
92
91
92
87
86
85
86
91
85
90
86
89
87
88
91
90
83
167–169
237–239
206–208
212–214
180–182
192–194
173–175
212–214
192–194
173–175
220–222
148–150
224–226
204–206
228–230
156–158
204–206
238–240
208–210
168–170 (ref. 50)
236–237 (ref. 14)
207–209 (ref. 14)
215 (ref. 50)
2-Cl–C6H4–
4-Cl–C6H4–
4-Br–C6H4–
4-F–C6H4–
181–183 (ref. 51)
191–194 (ref. 50)
132–134 (ref. 19)
210–213 (ref. 16)
193–194 (ref. 50)
172–174 (ref. 16)
221–222 (ref. 16)
149–150 (ref. 14)
225–227 (ref. 15)
201–203 (ref. 15)
227–229 (ref. 51)
155–157 (ref. 15)
206–208 (ref. 18)
238–240 (ref. 18)
209–212 (ref. 51)
3-CH3–C6H5–
4-iso-PrC6H4–
2-CH3O–C6H5–
3-CH3O–C6H5–
4-CH3O–C6H5–
2-NO2–C6H4–
3-NO2–C6H4–
4-NO2C6H4–
4-CH3SC6H5–
2-OHC6H4–
4-OHC6H4–
2-naphthyl–
3-pyridyl–
9
10
11
12
13
14
15
16
17
18
19
4-OHC–C6H4–
2s
a
Reaction conditions: phenylhydrazine (2 mmol), ethyl acetoacetate (2 mmol) and RHA-SO3H (60 mg) under solvent free conditions at 80 ꢀC, and
aer 1 min: aldehyde (1 mmol).
9 Hz, J2 ¼ 2 Hz), 8.32 (2H, d, J ¼ 2.4 Hz), 11.67 (s, NH) ppm;
Aer optimization of the reaction conditions and in order
13C NMR (DMSO-d6, 100 Mz): d ¼ 38.1, 55.4, 112.5, 114.2, to establish the effectiveness and the acceptability of the
116.7, 117.1, 121.4, 126.2, 127.8, 129.6, 136.1, 140.2, method, we explored the protocol with a variety of simple,
140.6, 158.1; IR (KBr, cmꢁ1):3256, 1609, 1501, 1397, 1316, readily available substrates under the optimal conditions.
1090, 795, 730.
The results were presented in Table 1. It was observed that
under similar conditions, a wide range of aromatic aldehydes
containing electron-withdrawing as well as electron-donating
groups, such as Cl, Br, F, CH3, OCH3, NO2, iso-Pr, SCH3 and
OH in the ortho, meta, and para positions of the benzene ring,
easily converted to the corresponding 4,40-(arylmethylene)-bis-
(3-methyl-1-phenyl-1H-pyrazol-5-ols) in short reaction times
with good to excellent isolated yields (Table 1, entries 1–16). As
can be seen, the effect of the nature of the substituents on the
aromatic ring did not show very obvious effects in terms of
yields and times under the selected reaction conditions,
while the steric effects showed a very small inuence on the
reaction times (Table 1, entries 2, 8 and 11). Furthermore, 2-
naphthaldehyde, a polycyclic aromatic aldehyde, also provided
the desired product in very good yields (Table 1, entry 17).
Pyridine-3-carbaldehyde, a heterocyclic aldehyde, was also
used as a substrate under these conditions and the desired
product was successfully obtained with high yield (Table 1,
entry 18). This method was also found to be useful for
the usage of dialdehydes. In this reaction, 4 equivalents of
3-methyl-1-phenyl-5-pyrazolone successfully condensed with
Results and discussion
Very recently, we have reported the preparation and character-
ization of sulfonated rice husk ash (RHA-SO3H) as a new solid
acid catalyst and its applicability in the chemoselective prepa-
ration and deprotection of 1,1-diacetates.49
On the basis of the information obtained from the studies
on RHA-SO3H, we anticipated that this reagent could be used
as an efficient solid acid catalyst for the promotion of the
reactions which need the use of an acidic catalyst to speed
them up. Therefore, we were interested in investigating the
applicability of this reagent in the promotion of the
synthesis of 4,40-(arylmethylene)-bis-(3-methyl-1-phenyl-1H-
pyrazol-5-ols), 3,30-(arylmethylene)-bis-(4-hydroxycoumarins)
and bis(indolyl)methanes.
At rst, we focused our attention on studying the synthesis of
4,40-(aryl methylene)-bis-(3-methyl-1-phenyl-1H-pyrazol-5-ols).
For optimization of the reaction conditions, the reaction
between phenylhydrazine, ethyl acetoacetate and 4-chlor-
obenzaldehyde to form the corresponding product was selected
as a model reaction and the various conditions including the
amount of catalyst, solvent and temperature were examined.
Finally, the optimal reaction conditions for this reaction were
determined as shown in Scheme 2. Any further increase in the
amount of catalyst or temperature did not improve the reaction
time or yield.
1
equivalent of terephthaldialdehyde, and di-4,40-(phenyl-
methylene)-bis-(3-methyl-1-phenyl-1H-pyrazol-5-ol) was obtained
with high yield in a short time which shows the practical synthetic
efficiency of this reaction (Table 1, entry 19).
Aer the successful application of RHA-SO3H as a solid acid
catalyst in the synthesis of 4,40-(arylmethylene)-bis-(3-methyl-1-
phenyl-1H-pyrazol-5-ols), we decided to use it in the condensation
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Scheme 2 One-pot synthesis of 4,40-(arylmethylene)-bis-(3-methyl-1-phenyl-1H-pyrazol-5-ols).
of 4-hydroxycoumarin with aldehydes leading to biscou-
Aer optimization of the reaction conditions, a broad range
marins. For this purpose and to obtain the optimum reaction of aromatic and heterocyclic aldehydes were reacted with
conditions, the reaction between 4-nitrobenzaldehyde and different indole derivatives (indole, 2-methyl indole and 5-nitro
coumarin to its corresponding biscoumarin was studied as a indole) and the corresponding bis(indolyl)methanes were
model reaction in different conditions. In order to chose the obtained in high yields and short reaction times, as shown
reaction media, different solvents such as dichloromethane, in Table 3. Various aromatic aldehydes containing electron-
diethyl ether, ethanol and water as well as solvent free withdrawing or electron-donating groups (Cl, Br, NO2, OCH3
conditions were used, and the best results were obtained in and OH), a,b-unsaturated aldehyde (cinnamaldehyde) and
water. Finally, we found that the best result was obtained polycyclic aromatic aldehyde (2-naphthaldehyde) were effi-
using 1 mmol aldehyde, 2 mmol 4-hydroxycoumarin and 40 ciently condensed with indole in very good yields and times
mg RHA-SO3H as the catalyst in 5 mL H2O 80 ꢀC, as shown in (Table 3, entries 1–12). This method was also found to be useful
Scheme 3.
for the usage of dialdehyde. In this reaction, 4 equivalents of
Subsequently, to reveal the generality of this method, the indole successfully condensed with 1 equivalent of tereph-
condensation was carried out with a variety of substrates using thaldialdehyde and di-bis(indolyl)methanes were obtained with
RHA-SO3H under the optimal conditions. For this purpose, a good yield which shows the practical synthetic efficiency of this
broad range of aromatic, aliphatic and heterocyclic aldehydes reaction (Table 3, entry 13). 2-Methyl indole and 5-nitro indole
were reacted with 4-hydroxycoumarin and the corresponding were also successfully used in the synthesis of bis(indolyl)
3,30-(arylmethylene)-bis-(4-hydroxycoumarins) were obtained in methanes under the selected conditions (Table 3, entries
high yields and short reaction times, as shown in Table 2. 14–20). It should be noted that indoles with electron-donating
Various aromatic aldehydes containing electron-withdrawing substituents (CH3) were converted to the desired products in
or electron-donating groups (Cl, Br, CH3, OCH3, NO2, N(CH3)2, shorter times than indoles with electron-withdrawing substit-
SCH3 and OH) and also a,b-unsaturated aldehydes (cinna- uents (NO2). Furthermore, under this catalytic system, the
maldehyde and 2-nitrocinnamaldehyde) were efficiently application of ketones such as acetophenone was not successful
condensed with 4-hydroxycoumarin in very good yields and no product was obtained which indicates the chemo-
and times (Table 2, entries 1–16). Furthermore, using selectivity of the protocol and that aldehydes are more reactive
this method, heterocyclic aldehydes (indole-3-carbaldehyde, than ketones due to higher electrophilicity of aldehydes in
pyridine-3-carbaldehyde and pyridine-4-carbaldehyde), and comparison with ketones.
3-phenylpropionaldehyde as an aliphatic aldehyde were
converted to the corresponding products in good to high compares our results obtained from the synthesis of 4,40-(aryl-
yields (Table 2, entries 17–20).
methylene)-bis-(3-methyl-1-phenyl-1H-pyrazol-5-ols), 3,30-(aryl-
In order to show the merit of the selected procedures, Table 4
In the next step, RHA-SO3H solid acid catalyst was used in methylene)-bis-(4-hydroxycoumarins) and bis(indolyl)methanes
the condensation of indoles with aldehydes leading to bis(in- in the presence of RHA-SO3H with the same results reported in
dolyl)methanes. For this purpose and to obtain the optimum the literature. This method avoids disadvantages of the other
reaction conditions, the reaction between 4-nitrobenzaldehyde procedures such as long reaction times, toxic reagents, high
and indole was studied as a model reaction in different condi- temperature, organic solvents, excess reagents and low yield. It
tions and the best result obtained using 1 mmol aldehyde, 2 is also important to note that the reported procedures for the
mmol indole and 15 mg RHA-SO3H at 80 ꢀC under solvent free synthesis of 4,40-(arylmethylene)-bis-(3-methyl-1-phenyl-1H-pyr-
conditions (Scheme 4).
azol-5-ol)s are based on the condensation of aldehydes with
Scheme 3 Synthesis of 3,30-(arylmethylene)-bis-(4-hydroxycoumarin) derivatives in the presence of RHA-SO3H.
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Table 2 Preparation of 3,30-(arylmethylene)-bis-(4-hydroxycoumarins) derivatives catalyzed by RHA-SO3Ha
Melting point (ꢀC)
Entry
Aldehyde
Product
Time (min)
Yield (%)
Found
Reported
1
2
3
4
5
6
7
8
C6H5–
2-Cl–C6H4–
3-Cl–C6H4–
4-Cl–C6H4–
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
15
45
25
25
20
35
15
10
60
15
15
25
20
50
20
90
89
91
93
90
92
94
95
88
94
92
94
93
89
92
232–234
205–207
230–232
257–259
269–271
211–212
236–238
233–235
218–220
256–258
249–251
221–223
258–260
250–252
192–194
230–232 (ref. 31)
201–203 (ref. 32)
228–230 (ref. 31)
257–258 (ref. 29)
268–270 (ref. 29)
200–202 (ref. 32)
234–236 (ref. 31)
232–234 (ref. 32)
218–220 (ref. 31)
238 (ref. 28)
249–250 (ref. 32)
222–224 (ref. 31)
—
254–256 (ref. 32)
196 (ref. 28)
4-Br–C6H4–
2-NO2–C6H4–
3-NO2–C6H4–
4-NO2–C6H4–
2-CH3–C6H4–
3-CH3O–C6H4–
4-CH3O–C6H4–
4-(CH3)2N–C6H4–
4-CH3S–C6H4–
2-OHC6H4–
9
10
11
12
13
14
15
C6H5–CH]CH–
16
3p
60
90
190–192
190–192 (ref. 33)
17
3q
45
87
241–243
240–244 (ref. 29)
18
19
3r
3s
40
20
88
93
271–273
269–271
274–276 (ref. 52)
261–263 (ref. 52)
20
3t
70
90
197–199
190 (ref. 28)
a
Reaction conditions: 4-hydroxycoumarin (2 mmol), aldehyde (1 mmol), RHA-SO3H (40 mg) and water (5 mL) at 80 ꢀC.
3-methyl-1-phenyl-5-pyrazolone, while our method is based on of SO3H groups in the catalyst is necessary to obtain the best
the one-pot condensation of phenylhydrazine, ethyl acetoace- performance.
tate and aldehydes. On the basis of the obtained results we
The reusability of the catalyst was also checked in three types
believe that the reaction proceeds via the in situ generation of of the above mentioned reactions. For this purpose, the model
3-methyl-1-phenyl-5-pyrazolone (1) which in reaction with reactions were studied again under the optimized reaction
aldehydes produces the desired products. This comparison also conditions. Upon completion, the catalyst was removed by
claries an important point about the catalyst. As it can be seen, ltration, washed with dichloromethane, dried and reused for
rice husk and rice husk ash are also able to catalyze this type of the same reaction. This process was carried out over three runs
reaction, but in much longer reaction times than RHA-SO3H. in each series of the reactions. The obtained results showed that
These results give clear evidence for the truth that the presence the reactions led to the desired products without signicant
Scheme 4 Synthesis of bis(indolyl)methane derivatives in the presence of RHA-SO3H.
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Table 3 Preparation of bis(indolyl)methane derivatives catalyzed by RHA-SO3Ha
Melting point (ꢀC)
Entry
Ar
R1
R2
Product
Time (min)
Yield (%)
Found
Reported
1
2
3
4
5
6
7
8
C6H5–
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
4k
4l
4m
4n
4o
4p
4q
4r
8
18
6
8
8
91
88
91
90
89
93
94
91
92
88
90
90
86
91
92
93
95
87
94
93
0
128–130
94–95
86–88
126–127 (ref. 42)
75–76 (ref. 53)
78–88 (ref. 47)
112–113 (ref. 29)
139–141 (ref. 43)
219–221 (ref. 53)
222–224 (ref. 53)
165 (ref. 55)
186–188 (ref. 48)
124–125 (ref. 54)
100–101 (ref. 33)
201–203 (ref. 53)
194 (ref. 56)
250–253 (ref. 47)
247–248 (ref. 53)
102–103 (ref. 48)
241–243 (ref. 47)
270–272 (ref. 57)
—
2-Cl–C6H4–
4-Cl–C6H4–
4-Br–C6H4–
2-NO2–C6H4–
3-NO2–C6H4–
4-NO2–C6H4–
3-CH3O–C6H4–
4-CH3O–C6H4–
4-OHC6H4–
C6H5–CH]CH–
2-Naphthyl–
–C6H4–
110–112
138–140
220–222
237–239
163–165
186–188
122–124
100–102
200–202
193–195
257–259
249–251
213–215
240–242
271–273
304–306
317–319
—
4
3
13
22
60
8
20
25
2
2
4
2
9
10
11
12
13b
14
15
16
17
18
19b
20b
21
H
C6H5–
CH3
CH3
CH3
CH3
CH3
H
4-Cl–C6H4–
4-CH3O–C6H4–
4-NO2–C6H4–
3-Indolyl–
4-NO2–C6H4–
4-CH3O–C6H4–
Acetophenone
H
H
NO2
NO2
H
12
60
60
120
4s
4t
4u
H
H
—
—
a
Reaction conditions: aldehyde (1 mmol), indole (2 mmol), RHA-SO3H (15 mg) at 80 ꢀC under solvent free conditions. b 30 mg catalyst was used.
changes in terms of yield and reaction time which clearly mmol H+ gꢁ1. This result suggests that the catalytic nature of
demonstrates practical recyclability of RHA-SO3H. Further pH RHA-SO3H remains unchanged aer each run and leaching of
analysis of the recovered catalyst showed a loading of 2.28 the acid species did not occur during the course of the reaction.
Table 4 Comparison of the results obtained from the synthesis of 4,40-(4-chlorophenylmethylene)-bis-(3-methyl-1-phenyl-1H-pyrazol-5-ol), 3,30-(4-nitrophenyl-
methylene)-bis-(4-hydroxycoumarins) and 3,30-(phenylmethylene)-bis-(1H-indole) in the presence of RHA-SO3H with those obtained using other catalysts
Product
Catalyst (loading)
Reaction conditions
Time (min)
Yield (%)
SDS (5 mol%)
Reux/H2O
1 h
50
45
30
70
2.2 h
1 h
3 h
1.5 h
3
91.5 (ref. 14)
90 (ref. 15)
90 (ref. 16)
94 (ref. 17)
90 (ref. 18)
85 (ref. 19)
90 (ref. 20)
80 (This work)
85 (This work)
92 (This work)
Silica-bonded S-sulfonic acid (100 mg)
[HMIM]HSO4 (10 mol%)
PEG-SO3H (1.5 mol%)
Silica sulfuric acid (80 mg)
SASPSPE (100 mg)
[Sipmim]HSO4 (150 mg)
Rice husk (60 mg)
Rice husk ash (60 mg)
RHA-SO3H (60 mg)
Reux/EtOH
EtOH, ))), r.t.
Reux/H2O
70 ꢀC/EtOH–H2O
Reux/EtOH
Reux/EtOH
Reux/EtOH
Reux/EtOH
80 ꢀC/solvent free
Piperidine (a few drop)
I2 (10 mol%)
TBAB (10 mol%)
SDS (20 mol%)
r.t./EtOH
4 h
28
25
3 h
10
96 (ref. 28)
95 (ref. 29)
91 (ref. 30)
98 (ref. 31)
95 (This work)
100 ꢀC/H2O
100 ꢀC/H2O
60 ꢀC/H2O
80 ꢀC/H2O
RHA-SO3H (40 mg)
Sulfamic acid (10 mol%)
(PhCH2PPh3)+Br3ꢁ/SiO2 (10 mol%)
[hmim]HSO4 (1 mol%)
ZrOCl2$8H2O (5 mol%)
FeCl3–RiH (150 mg)
r.t./solvent free
90 ꢀC/solvent free
r.t./EtOH
30
16
1 h
3 h
15
92 (ref. 43)
80 (ref. 44)
97 (ref. 46)
93 (ref. 47)
92 (ref. 53)
91 (This work)
r.t./H2O–EtOH
80 ꢀC/EtOH
RHA-SO3H (15 mg)
80 ꢀC/solvent free
8
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Communication
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