Facile synthesis of bis(indolyl)methanes using iron(III) phosphate

Article abstract of DOI:10.2298/JSC110727203B

A new, convenient and high yielding procedure for the preparation of bis(indolyl)methanes in glycerol by the electrophilic substitution reaction of indole with aldehydes in the presence of catalytic amount of FePO₄ (5.0 mol %) as a highly stabl

Full text of DOI:10.2298/JSC110727203B

J. Serb. Chem. Soc. 77 (9) 1157–1163 (2012)
JSCS–4340
UDC 542.913+546.185–034.1:
547.752+547.426+544.478
Original scientific paper
Facile synthesis of bis(indolyl)methanes
using iron(III) phosphate
*
FARAHNAZ KARGAR BEHBAHANI and MASOUMEH SASANI
Department of Chemistry, Karaj Branch, Islamic Azad University, Karaj, Iran
(Received 27 July, revised 10 November 2011)
Abstract: A new, convenient and high yielding procedure for the preparation of
bis(indolyl)methanes in glycerol by the electrophilic substitution reaction of
indole with aldehydes in the presence of catalytic amount of FePO4 (5.0 mol
%) as a highly stable and reusable catalyst is described.
Keywords: FePO4; synthesis; bis(indolyl)alkanes; aldehyde; glycerol; catalyst.
INTRODUCTION
Lewis acids accelerate a wide range of organic reactions by binding to and
thereby activating reactants. This activation often results in rate increases of
many orders of magnitude compared with the thermal reaction. Transition metal
Lewis acids are promising and interesting because the precursor is often struc-
turally well-defined and steric and electronic tuning of the ligand to a particular
reaction is facilitated. Iron(III) phosphate as a Lewis acid has been employed as a
catalyst for the transformation of various organic functional group under hetero-
1
geneous conditions, such as the selective oxidation of CH to CH OH, benzene
4
3
2
3
to phenol, one-pot synthesis of dihydropyrimidinones and thiones, synthesis of
triarylated imidazoles, acetylation of alcohols and phenols, tetrahydropyra-
nylation and tetrahydrofuranylation of alcohols and phenols, synthesis of 2-sub-
stituted benzimidazoles, and synthesis of polyhydroquinoline derivatives.
4
5
6
7
8
Furthermore, the structural motifs of indole derivatives are found in nume-
rous biologically active compounds, which are used as anti-oxidants and pharma-
9–13
ceuticals.
Bis(indolyl)alkanes are present in many biologically active natural
products and are known to have applications in research areas, such as pharma-
14–20
ceuticals and materials science.
Consequently, there is an increased interest
in the synthesis of compounds containing bis(indolyl)alkanes moieties due to
their importance and a number of synthetic methods for the synthesis of bis(in-
*Corresponding author. E-mail: Farahnazkargar@yahoo.com
doi: 10.2298/JSC110727203B

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BEHBAHANI and SASANI
21–24
dolyl)alkanes have been described in the literature.
However, the procedures
described in the literature to access bis(indolyl)alkanes are mainly focused on the
condensation of indole with carbonyl compounds in the presence of a protic acid
or a Lewis acid. Also, The synthesis of bis(indolyl)alkanes by the reaction of
25
indoles with alkynes catalyzed using Cu(OTf) has also been reported.
2
Lately, glycerine, which is readily available as a co-product in the produc-
tion of biodiesel, has attracted attention as a versatile, cheap and renewable feed-
26,27
stock in synthetic organic chemistry.
In view of current interest in catalytic
processes, the development of a method for the synthesis of bis(indolyl)methanes
using inexpensive, mild and non-polluting reagents is highly desirable. The intro-
duction of reusable catalysts is preferred in order to minimize environmental
pollution due to the usage of hazardous solvents and their disposal. The limita-
tions of the current methods, together with the demand for greener alternatives
prompted us to develop a new method for the synthesis of bis(indolyl)methanes.
Due to our interest in developing new methodologies in catalytic organic
syntheses and to show the versatility of iron(III) phosphate in organic synthe-
1–8
sis,
the preparation of bis(indolyl)methanes by reaction of indole with alde-
hydes in the presence of a catalytic amount of FePO (5.0 mol %), as an inex-
4
pensive, eco-friendly and reusable catalyst, in glycerol at 75 °C (Scheme 1) is
described herein.
Scheme 1. The preparation of bis(indolyl)methanes by reaction of indole with aldehydes
in the presence of a catalytic amount of FePO4.
EXPERIMENTAL
Melting points were measured by the capillary tube method using an Thermo Scientific
electrothermal 9200 apparatus. The IR spectra were recorded on a Perkin Elmer FT-IR spec-
trometer in the wavenumber range 4000–400 cm-1. 1H-NMR spectra were obtained on Bruker
DRX-300MHZ NMR instrument. Chemicals shifts are reported in parts per million (δ) rela-
tive to tetramethylsilane (δ = 0.0 ppm) as internal standard. GC/mass spectra were recorded on
an Agilent 6890 GC Hp-5 capillary 30 m×530 μm×1.5 μm nominal operating at 70 eV. Anal-
ytical TLC of all reactions was performed on Merck pre-coated plates (silica gel 60F-254 on
aluminium). All starting materials purchased from Merck Co. All products were characterized
by 1H-NMR, FT-IR, and comparison of their melting points with authentic samples.
Synthesis of 3,3’-bis(indolyl)phenylmethane (1)
The reaction of benzaldehyde (1 mmol) and indole (2 mmol) was used as a model reac-
tion to determine the optimal conditions for the preparation of bis(indolyl)methanes. The

SYNTHESIS OF BIS(INDOLYL)METHANES
1159
effect of varying the amount of FePO4 (0–10 mol %) was studied by stirring the reactants in 4
mL glycerol for 3 h at 75 °C (Table I) After completion of the reaction (monitored by TLC),
the mixture was treated with ethyl acetate and the catalyst filtered off. After evaporation of the
solvent, the residue was collected and purified by recrystallization from ethanol. Using the
optimal concentration (5 mol %) of FePO4, the synthesis was repeated for the same time at the
same temperature but in different solvents (Table II). Finally, the synthesis was performed in
glycerol using the optimal concentration of FePO4 by stirring for 3 h at different temperatures
(25–80 °C) (Table III).
TABLE I. Effect of varying the amount of catalyst on the yield of 3,3’-bis(indolyl)phenyl-
methane; benzaldehyde, 1.0 mmol, indole, 2.0 mmol, glycerol, 4.0 mL at 75 °C
Entry
Catalyst, mol %
τ / h
Yield, %
1
2
3
4
–
3.0
3.0
3.0
3.0
10
60
90
90
2.0
5.0
10
TABLE II. Effect of the solvent on the yield of 3,3’-bis(indolyl)phenylmethane; benzalde-
hyde, 1.0 mmol, indole, 2.0 mmol, at 75 °C in 3.0 h
Entry
Solvent (4.0 mL)
Yield, %
1
2
3
4
5
Glycerol
EtOH
H2O
EtOH/H2O
–
90
70
60
65
60
TABLE III. Synthesis of 3-[(1H-indol-3-yl)phenylmethyl]-1H-indole using FePO4 in glycerol
at different temperatures
Entry
t / °C
τ / h
Yield, %
1
2
3
4
25
50
75
80
3.0
3.0
3.0
3.0
30
65
90
90
Synthesis of 3,3’-bis(indolyl)methane (2–15)
Compounds 1–14, Table IV, were prepared by reaction of indole (2 mmol) with the
required aldehyde (1 mmol) in 4 mL glycerol at 75 °C in the presence of 5 mol % FePO4.
After completion of the reaction, controlled by TLC (the reaction times are given in Table
IV), ethyl acetate was added, the catalyst filtered off and the solvent evaporated. The residue
was recrystallized from ethanol to afford the pure compound.
Reusability of the catalyst
To test its reusability, the catalyst recovered by gravity filtration at the end of the reac-
tion was washed with ethyl acetate and reused for 5 sequential preparations of compound 1.

1160
BEHBAHANI and SASANI
TABLE IV. FePO4 catalyzed synthesis of bis(indolyl)methanes
M.p. / °C M.p. / °C
Entry
Aldehyde
τ / h
Yield, %
Ref.
(found)
(reported)
1
2
3
4
5
6
7
8
Benzaldehyde
2-Chlorobenzaldehyde
4-Chlorobenzaldehyde
Trans-Cinnamaldehyde
2-Methoxybenzaldehyde
4-Methoxybenzaldehyde
3,4-Dimethoxybenzaldehyde
4-Methylbenzaldehyde
4-Dimethylaminobenzaldehyde
2-Hydroxybenzaldehyde
4-Hydroxybenzaldehyde
4-Nitrobenzaldehyde
Hexanal
3
8
8
7
5
4
5
5
7
5
6
5
7
6
8
90
80
85
85
93
92
84
84
75
75
87
84
76
75
90
125
72
75
100
138
123–126
69–71
76–78
31
30
30
29
28
28
31
28
29
30
30
32
30
32
29
95–97
134–136
178–181
198–199
97–99
210–212
340–342
120–123
216–218
67–74
176
195–198
64–98
208–220
350
150
220
65
80
340–345
9
10
11
12
13
14
15
Butanal
Furfural
73–75
322–324
RESULTS AND DISCUSSION
To the best of our knowledge, the preparation of bis(indolyl)methanes cata-
lyzed by anhydrous iron(III) phosphate has not been reported previously.
The main aim of this work was to provide a new catalytic and environment-
ally benign protocol for the synthesis of bis(indolyl)methanes. Herein, a simple
and fast method for the synthesis of bis(indolyl)methanes in glycerol using
FePO is reported.
4
To establish the optimum condition for this reaction, various ratios of FePO
4
were examined using indole and bezaldehyde in glycerol at 75 °C as a model re-
action (Table I). It can be seen that very little of the desired product was obtained
in the absence of catalyst and the best yields were obtained with FePO . Thus,
4
the catalyst is an essential component for the synthesis of bis(indolyl)methanes
by the reported method.
Then the effect of the solvent was examined using different solvents in the
model reaction, whereby different yields were obtained (Table II). Obviously,
glycerol stands out as the solvent of choice with its fast reaction rate, high yield,
selectivity, cheapness and environmental acceptability.
The effect of temperature was investigated by performing the model reaction
in glycerol and at different temperatures (25, 50, 75 and 80 °C). It was observed
that the yield is a function of temperature with the yield increasing with in-
creasing reaction temperature up to 75 °C. Thereafter, no further increase in the
yield was registered. Therefore, all further reactions were performed in glycerol
at 75 °C (Table III).

SYNTHESIS OF BIS(INDOLYL)METHANES
1161
To evaluate the scope and limitations of this work, different bis(indolyl)me-
thanes were synthesized using aliphatic, heterocyclic and aromatic aldehydes
(Scheme 1, Table IV). As can be seen in Table IV, the aromatic and heterocyclic
aldehydes reacted similarly to give the corresponding bis(indolyl)methanes in
high yields and relatively short reaction times. The aliphatic aldehydes reacted
smoothly providing the corresponding products in good to moderate yields
(entries 4, 14 and 15).
This condensation reaction probably proceeds through the activation of a
carbonyl group by FePO as a Lewis acid to give intermediate I, which is fol-
4
lowed by indole attack of I to give II. After proton rearrangement to give III, the
other indole is added to III with the loss of FePO and H O to afford IV, with
4
2
the following step resulting in the product V (Scheme 2).
Scheme 2. Suggested mechanism for the synthesis of bis(indolyl)methanes
At the end of the reaction, the catalyst was recovered by gravity filtration
and recycled after washing with ethyl acetate and employed for further prepa-
rations. The results showed that the yield of product after five runs was only
slightly reduced.
CONCLUSION
In conclusion, FePO in glycerol was found to be mild and effective catalyst
4
for the electrophilic substitution reactions of indole with aldehydes, giving bis-
(indolyl)methanes in excellent yields. The use of this inexpensive and easily
available catalyst under mild conditions, the clean reaction and greater selectivity
make this protocol practical and economically attractive. The procedure was

1162
BEHBAHANI and SASANI
found to be general as a variety of aldehydes reacted with indole under mild reac-
tion conditions.
SUPPLEMENTARY MATERIAL
Physical and spectral data of the synthesized compounds are available electronically
from http://www.shd.org.rs/JSCS/, or from the corresponding author on request.
И З В О Д
ЛАКА СИНТЕЗА БИС(ИНДОЛИЛ)МЕТАНА ПОМОЋУ ГВОЖЂЕ(III)-ФОСФАТА
FARAHNAZ KARGAR BEHBAHANI и MASOUMEH SASANI
Department of Chemistry, Karaj Branch, Islamic Azad University, Karaj, Iran
Описан је нов поступак за синтезу бис(индолил)метана реакцијом индола и алде-
хида, у глицеролу у присуству каталитичких количина FePO4 (5,0 mol %). Производи се
добијају у високом приносу а катализатор је стабилан и може се користити више пута.
(Примљено 27. јула, ревидирано 10. новембра 2011)
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