Preparation of silica-bonded S-sulfonic acid: A recyclable catalyst for the synthesis of bis-indolylmethanes

Article abstract of DOI:10.1080/10426500903019127

Silica bonded S-sulfonic acid (SBSSA) was prepared by the reaction of 3-mercaptopropylsilica (MPS) and chlorosulfonic acid in chloroform. Silica-bonded S-sulfonic acid, as a reusable solid acid catalyst, was used for the condensation reactions of indole w

Full text of DOI:10.1080/10426500903019127

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Preparation of Silica-Bonded S-Sulfonic
Acid: A Recyclable Catalyst for the
Synthesis of Bis-Indolylmethanes
Khodabakhsh Niknam a , Dariush Saberi a & Mojtaba Baghernejad b
a Department of Chemistry, Faculty of Sciences, Persian Gulf
University, Bushehr, Iran
b Islamic Azad University, Gachsaran Branch, Gachsaran, Iran
Version of record first published: 23 Mar 2010.
To cite this article: Khodabakhsh Niknam , Dariush Saberi & Mojtaba Baghernejad (2010): Preparation
of Silica-Bonded S-Sulfonic Acid: A Recyclable Catalyst for the Synthesis of Bis-Indolylmethanes,
Phosphorus, Sulfur, and Silicon and the Related Elements, 185:4, 875-882
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Phosphorus, Sulfur, and Silicon, 185:875–882, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500903019127
PREPARATION OF SILICA-BONDED S-SULFONIC ACID:
A RECYCLABLE CATALYST FOR THE SYNTHESIS
OF BIS-INDOLYLMETHANES
Khodabakhsh Niknam,1 Dariush Saberi,1
and Mojtaba Baghernejad2
1Department of Chemistry, Faculty of Sciences, Persian Gulf University,
Bushehr, Iran
2Islamic Azad University, Gachsaran Branch, Gachsaran, Iran
Silica bonded S-sulfonic acid (SBSSA) was prepared by the reaction of 3-
mercaptopropylsilica (MPS) and chlorosulfonic acid in chloroform. Silica-bonded
S-sulfonic acid, as a reusable solid acid catalyst, was used for the condensation reactions
of indole with carbonyl compounds at ambient temperature. Tetra-(indolyl)methanes were
also obtained by the condensation reaction of indole with dialdehyde compounds. Silica
bonded S-sulfonic acid was recovered and reused.
Supplemental materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental
file.
Keywords Bis(indolyl)methanes; carbonyl compounds; indole; silica-bonded S-sulfonic acid;
tetra(indolyl)methanes
INTRODUCTION
The development of heterogeneous catalysts for fine chemical synthesis has become
a major area of research, as the potential advantages of these materials, such as simplified
recovery and reusability, the potential for incorporation in continuous reactors and microre-
actors, etc., over homogeneous systems can lead to novel, environmentally benign chemical
procedures for academia and industry.1 Application of solid acids in organic transformation
have an important role because solid acids have many advantages such as simplicity in han-
dling, decreased reactor and plant corrosion problems, and increased environmentally safe
disposal.2–12 It is clear that green chemistry not only requires the use of environmentally
benign reagents and solvents, but also it is very crucial to recover and reuse the catalyst.
One way to overcome the problem of recyclability of traditional acid catalysts is to chemi-
cally anchor their reactive center onto a large surface area inorganic solid carrier to create
Received 5 February 2009; accepted 1 May 2009.
We are thankful to the Persian Gulf University Research Council for the partial support of this work.
Address correspondence to Dr. Khodabakhsh Niknam, Department of Chemistry, Faculty of Sciences,
Persian Gulf University, Bushehr 75169, Iran. E-mail: niknam@pgu.ac.ir; khniknam@gmail.com
875

876
K. NIKNAM ET AL.
O
O
OH
OH
OH
SH
(MeO)3Si
SH
Si
Toluene, reflux, 18 h
O
1
O
O
1) ClSO3H/ CHCl3 /4 h
S-SO3H
Si
2) Washing with MeOH
O
2
Scheme 1 Preparation of silica-bonded S-sulfonic acid (SBSSA).
a new organic–inorganic hybrid catalyst.4–6 In these types of solids, the reactive centers
are highly mobile, similar to homogeneous catalysts, and at the same time they have the
advantage of recyclability like heterogeneous catalysts. In this view, several types of solid
sulfonic acid functionalized silica (both amorphous and ordered) have been synthesized
and applied as an alternative to traditional sulfonic acid resins and homogeneous acids
in catalyzing chemical transformations.5–7 Along the line of our studies in application of
solid acid catalysts in chemical transformations,8–12 in this article we wish to describe the
preparation of silica bonded S-sulfonic acid (SBSSA), as illustrated in Scheme 1 and its use
as a catalyst in the condensation reaction of indole with carbonyl compounds under mild
conditions.
Indole and its derivatives are important intermediates in organic synthesis and exhibit
various physiological properties and pharmacological activities.13 Over the past decade,
a number of natural products containing bis(indolyl)methanes or bis(indolyl)ethanes have
been isolated from marine sources.14 Bis(indolyl)methanes, which are found in cruciferous
plants and are known to promote beneficial estrogen metabolism,15 induce apoptosis in hu-
man cancer cells. Therefore the preparation of these intermediates has received increased at-
tention from synthetic organic chemists and biologists. Generally, this reaction is promoted
using protic acids,16 Lewis acids,17 heteropolyacid,18 and heterogeneous catalysts (clay,
HY-zeolits, and supported reagents).19,20 Recently, sulfamic acid,21 silica sulfuric acid,22
ZrOCl2.8H2O/silica gel,23 ionic liquids in conjunction with In(OTf)3 or FeCl3.6H2O,24
HClO4-SiO2,25 M(HSO4)n,26 ammonium chloride,27 SbCl3,28 silica chloride,29 heteropoly
acid in water,30 P2O5/SiO2,31 Zr(DS)4,32 ZrCl4,33 iodine,34 PEG-supported sulfonic acid,35
and ZnO36 were employed for this transformation. Although the methods mentioned above
are suitable for the reaction described, there sometimes exist some drawbacks such as
long reaction time, expensive reagents, low yield of products in some cases, high catalyst
loading, corrosive reagents, and large amounts of solid supports, which would eventually
result in the generation of a large amount of toxic waste. For this reason, superior catalysts
that are cheap, less toxic, easily available, air-stable, recyclable, and water-tolerant, are
desirable.
In connection with our interest in the use of solid acids as efficient catalysts
in various transformations, and in continuation of our studies on the preparation of
indolylmethanes,22a,26,31 in this article we report the preparation of bis(indolyl)- and
tetra(indolyl)methanes from indole and carbonyl compounds in the presence of silica
bonded SBSSA at room temperature (Scheme 2).

SILICA-BONDED S-SULFONIC ACID
877
O
O
O
R1 R2
S-SO3H
Si
O
+
R1
R2
N
H
CH3CN/rt
N
N
H
H
3
4
5a-r
Scheme 2 Silica-bonded S-sulfonic acid–catalyzed condensation of carbonyl compounds with indole.
RESULTS AND DISCUSSIONS
First, we tried to condense indole (1 mmol) with benzaldehyde (0.5 mmol) and silica-
bonded S-sulfonic acid (0.1 g) in the presence of various solvents (Table I). As shown in
Table I, the best result was obtained in acetonitrile in terms of time and product yield.
The model reaction was also examined in the presence of various amounts of SBSSA,
and as shown in Table II, the best result was obtained as the following: indole (1 mmol),
aldehyde (0.5 mmol), and SBSSA (0.1 g) in acetonitrile at room temperature.
These results prompted us to investigate the scope and the generality of this new
protocol for various aldehydes and ketones under optimized conditions (Table III). A
series of aromatic aldehydes and simple ketones smoothly underwent an electrophilic
substitution reaction with indole to afford a wide range of substituted bis(indolyl)methanes
in good to excellent yields. This method is equally effective for aldehydes bearing electron-
withdrawing or -donating substituents in the aromatic rings. Furthermore, acid-sensitive
aldehydes worked well without any decomposition or polymerization under these reaction
conditions.
In all cases, aldehydes reacted more rapidly and gave higher yields than ketones.
Most of the reactions of aromatic aldehydes proceeded nearly quantitatively, and the nature
of substituents on the aromatic ring showed some effects on this conversion. In other words,
the electron-donating substituents such as CH3 and OCH3 required a shorter reaction time
than the electron-deficient counterparts such as CN and NO2. Ketones required longer
reaction times and lower yields, which is most probably due to the steric effects of the
methyl group (Table III, entries 15–16).
The possibility of recycling the catalyst was examined using the reaction of indole
(10 mmol) with benzaldehyde (5 mmol) in acetonitrile (10 mL), in the presence of SBSSA
Table I The effect of solvents on the conversion of indole (1 mmol) and benzalde-
hyde (0.5 mmol) into phenyl-3,3ꢁ-diindolylmethane in the presence of SBSSA (0.1
g) at room temperature
Entry
Solvent
Time (min)
Yield (%)a
1
2
3
4
5
6
8
9
H2O
CH3CN
CH3COOC2H5
CH2Cl2
CH3OH
C2H5OH
THF
Chlorobenzene
120
30
120
120
60
60
90
120
—
100
75
85
95
96
95
55
aConversion.

878
K. NIKNAM ET AL.
Table II The condensation of indole with benzaldehyde catalyzed by
SBSSA in acetonitrile at room temperature
Entry
The amount of catalyst (g)
Time (min)
Yield (%)a
1
2
3
4
5
0.01
0.03
0.05
0.07
0.1
90
75
60
40
30
75
80
90
95
100
aConversion.
(0.5 g) at room temperature. Upon completion, the reaction mixture was filtered, the
remaining solid was washed with ethanol, and the catalyst was reused in the next reaction.
The recycled catalyst could be reused 19 times without any additional treatment. No
observation of any appreciable loss in the catalytic activity of SBSSA was observed (Figure
S4, Supplemental Materials).
This reaction was further explored for the synthesis of tetra-indolyl compounds 5q, 5r
by the condensation of terephthaldehyde 6 and 2,6-diformyl-4-methoxyphenol 7 with four
equivalent of indol under similar conditions in very good yields (Scheme S1, Supplemental
Materials; Table III, entries 17, 18).
To show the efficiency of the SBSSA in comparison with previously reported proce-
dures in the literature, Table IV compares some of our results with silica sulfuric acid,22bb
PEG-supported sulfonic acid,35 HY-zeolite,20 ZrOCl2·8H2O/ silica gel,23 sulfamic acid,21
and ZnO (VI)36 with respect to reaction times and yields of obtained products. Also, the
reusability of SBSSA catalyst is good (19 times) in comparison with the previously reported
literature based on silica or polymer supported catalysts.
CONCLUSION
In conclusion, SBSSA has been employed for the synthesis of bis(indolyl)-methanes
and tetra(indolyl)methanes by the electrophilic substitution reaction of indole with aldehy-
des or ketones. The attractive features of this procedure are the mild reaction conditions,
high conversions, cleaner reaction profiles, and reusable and environmentally friendly cata-
lyst, all of which lead to an interesting strategy for the preparation of bis(indolyl)methanes
and tetra(indolyl)methanes.
EXPERIMENTAL
Chemicals were purchased from Merck, Fluka, and Aldrich chemical companies,
and 2,6-diformyl-4-methoxyphenol was prepared according to our previously reported
procedure.37 3-Mercaptopropylsilica 1 (MPS) was prepared according to the previously
reported method.5 IR spectra were run on a Shimadzu Infrared Spectroscopy FT-IR 8000.
The 1H NMR and 13C NMR were run on a Bruker Avance DPX 250 MHz and 300 MHz
instruments. Melting points were recorded on a Melting Point SMP1 apparatus in open
capillary tubes and are uncorrected. The progress of reaction was followed with TLC
using silica gel SILG/UV 254 plates. All of the products are known compounds and are

SILICA-BONDED S-SULFONIC ACID
879
Table III SBSSA-catalyzed condensation of indole with various carbonyl compounds to produce corresponding
bis(indolyl)methane at room temperaturea
Entry
1
Carbonyl compound
Product
Time (min)
30
Yield (%)b
95
5a36
CHO
2
3
5b36
5c36
60
90
91
90
Cl
CHO
Cl
CHO
4
5
6
5d36
5e31
5f33
60
90
90
82
80
82
Br
CHO
CHO
HO
HO
CHO
7
8
5g38
5h21
5i21
5j38
25
30
90
90
93
90
89
88
MeO
Me
CHO
CHO
CHO
9
NC
10
O2N
CHO
CHO
11
12
13
5k36
5l36
90
120
120
91
90
89
O2N
N
CHO
5m21
CHO
CHO
S
14
15
16
5n35
5o38
5p21
120
360
270
87
65
79
O
O
C-CH3
O
17
18
5q22a
5r22a
210
210
89
83
OHC
CHO
OH
OHC
CHO
OMe
aReaction conditions: indole (2 mmol), carbonyl compound (1 mmol), and SBSSA (0.1 g) in 2 mL of CH3CN
at room temperature.
bIsolated yield.

880
K. NIKNAM ET AL.
Table IV Comparison some of the results of SBSSAa with silica sulfuric acidb (I),22b PEG-supported sulfonic
acidc (II),35 HY-zeolited (III),20 ZrOCl2.8H2O/ silica gele (IV),23 sulfamic acidf (V),21 and ZnOg (VI)36 in the
preparation of bis(indolyl)methanes at room temperature
Time (min) / Yield (%)
Entry
1
Aldehyde
SBSSA
I
II
III
IV
V
VI
30/95
40/92
150/95
60/85
40/84
20/88
45/98
CHO
2
3
4
25/93
90/91
90/90
15/94
—
120/90
480/86
180/85
60/78
60/85
60/82
—
15/94
—
90/96
20/95
45/95
MeO
O2N
CHO
30/94
—
CHO
—
—
Cl
CHO
5
60/91
30/90
210/90
60/65
—
60/90
30/95
Cl
CHO
c
e
The equivalent ratios of aldehyde/indole/catalyst are a1/2/0.1 g; b2/4/0.1 g; 1/2/0.45 g; d2.5/5/0.5 g; 1/2/5
mol%; f1/2/0.2 mmol; g1/2/0.1 mmol.
1
characterized by the comparison of their spectral (IR, H NMR), TLC, and physical data
with those reported in literature.
Catalyst Preparation
To a magnetically stirred mixture of 3-mercaptopropylsilica 1 (5 g) in CHCl3 (20
mL), chlorosulfonic acid (1.00 g, 9 mmol) was added dropwise at 0◦C during 2 h. After
addition was complete, the mixture was stirred for 2 h until HCl was removed from the
reaction vessel. Then, the mixture was filtered and washed with methanol (30 mL) and
dried at room temperature to obtain silica-bonded functionalized sulfonic acid 2 (SBSSA)
as cream powder (5.22 g). Sulfur content of the samples by conventional elemental analysis
was 16.12%. Typically a loading at ca. 0.35 mmol/g is obtained. On the other hand, when the
washed sulfonated product SBSSA was placed in an aqueous NaCl solution, the solution
pH dropped virtually instantaneously to pH ≈ 1.85, as ion exchange occurred between
protons and sodium ions (proton exchange capacity: 0.34 mmol/g of SBSSA), which is in
good agreement with the result obtained from TGA and titration.
Catalyst Characterization
Thermal gravimetric analysis (TGA). The weight change of catalyst precursors
was measured using a TGA simultaneous thermal analyzer apparatus of Mettler TA-4000
under a flow of dry air. The temperature was raised from room temperature to 600◦C using
a linear programmer at a heating rate of 20◦C/min. The sample weight was 9 mg. The
TGA curve for the catalyst is illustrated in Figure S1 (available online in the Supplemental

SILICA-BONDED S-SULFONIC ACID
881
Materials). The thermo gravimetric curve seems to indicate two-stage decomposition, which
is considered to be due to removal of physical absorbed water (80–110◦C) or basic silica
sulfonic acid (120–300◦C), respectively, and is involved with a total overall weight loss of
catalyst of 50%.
FT-IR spectrum of silica-bonded S-sulfonic acid. The FT-IR spectrum of the
catalyst is shown in Figure S2 (Supplemental Materials). The catalyst is a solid, and the
solid state IR spectrum was recorded using the KBr disk technique. For silica (SiO2), the
major peaks are broad antisymmetric Si–O–Si stretching from 1200 to 1000 cm−1 and
symmetric Si–O–Si stretching near 802 cm−1, and bending modes lies near 470 cm−1. For
sulfonic acid functional group, the FT-IR absorption range of the O S O asymmetric
and symmetric stretching modes lies in 1176.5 and 1072.3 cm−1, respectively, the S–O
stretching mode lies in 550–700 cm−1, and that of the S–S stretching mode lies from 400 to
500 cm−1. FT-IR spectrum shows the overlap asymmetric and symmetric stretching bands
of SO2 with Si–O–Si stretching bands in the silica functionalized S-sulfonic acid. The
spectrum also shows a broad OH stretching absorption from 3600 to 2491.9 cm−1
.
X-ray diffraction (XRD) of silica-bonded S-sulfonic acid. Powder X-ray, a
diffraction measurement, was performed using D8 Advance Diffract Meter made by the
Bruker Axs Company in Germany. Scans were taken with a 2θ step size of 0.05 and
a counting time of 1.0s using Cu Kα radiation source generated at 40 kV and 30 mA.
Specimens for XRD were prepared by compaction into a glass-backed aluminum sample
holder. Data were collected over a 2θ range from 5◦ to 100◦, and phases were identified by
matching experimental patterns to entries in the Diffractplus version 6.0 indexing software
(Figure S3, Supplemental Materials).
General Procedure: Synthesis of Bis-(indolyl)methanes 5
To a solution of indole 1 (2 mmol) and aldehyde or ketone (1 mmol) in acetonitrile
(2 mL), SBSSA (0.1 g) was added and magnetically stirred at room temperature. The
progress of the reaction was followed by TLC. After completion of the reaction, the
suspended mixture was filtered. The remained catalyst was washed with ethanol (2 × 5
mL). The evaporation of ethanol gave the product which was purified by recrystallization
from EtOAc/ petroleum ether (1/2) or plate chromatography on silica gel eluted with EtOAc/
petroleum ether (1/2).
General procedure, synthesis of tetra-(indolyl)methanes 5q, 5r. To a so-
lution of indole 1 (4 mmol) and dialdehyde 6 or 7 (1 mmol) in acetonitrile (3 mL), SBSSA
(0.2 g) was added and magnetically stirred at room temperature. The progress of the reac-
tion was followed by TLC. After completion of the reaction, the suspended mixture was
filtered. The remaining catalyst was washed with ethanol (2 × 10 mL). The evaporation of
ethanol gave the product, which was purified by plate chromatography on silica gel eluted
with EtOAc:petroleum ether (1:2).
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