Synthesis of bis(indolyl)methanes using a catalytic amount of ZnO under solvent-free conditions

Article abstract of DOI:10.1080/00397910701845274

Efficient electrophilic substitution reactions of indoles with various aromatic aldehydes were carried out using a catalytic amount of ZnO under solvent-free conditions to afford the corresponding bis(indolyl)methanes in excellent yields. Among the variou

Full text of DOI:10.1080/00397910701845274

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Synthesis of Bis(indolyl)methanes using a
Catalytic Amount of ZnO under Solvent‐Free
Conditions
Mona Hosseini‐Sarvari a
a Department of Chemistry, College of Science, Shiraz University, Shiraz,
Iran
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Amount of ZnO under Solvent‐Free Conditions, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 38:6, 832-840
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Synthetic Communicationsw, 38: 832–840, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701845274
Synthesis of Bis(indolyl)methanes using a
Catalytic Amount of ZnO under
Solvent-Free Conditions
Mona Hosseini-Sarvari
Department of Chemistry, College of Science, Shiraz University,
Shiraz, Iran
Abstract: Efficient electrophilic substitution reactions of indoles with various aromatic
aldehydes were carried out using a catalytic amount of ZnO under solvent-free con-
ditions to afford the corresponding bis(indolyl)methanes in excellent yields. Among
the various methods published for the synthesis of bis(indolyl)methane, ZnO is a
very cheap, environmentally friendly, and reusable catalyst that can be used for
synthesis of a new range of bis(indolyl)methane derivatives.
Keywords: aldehyde, bis(indolyl)methane, indole, solvent-free conditions, ZnO
INTRODUCTION
With the development of industrialization, organic chemists have been con-
fronted with a new challenge of finding novel methods in organic synthesis
that can reduce and finally eliminate the impact of volatile organic solvents
and hazardous toxic chemicals on the environment. So, use of nontoxic, envir-
onmentally friendly, and inexpensive solid catalysts under solvent-free con-
ditions to perform organic reactions has attracted considerable interest
recently. Undoubtedly, one of the most important solid catalysts is zinc
oxide (ZnO). ZnO is a versatile material. It is a dry, nonvolatile, nonhydro-
scopic, odorless, and white crystalline solid with outstanding physical
Received in Poland May 30, 2007
Address correspondence to Mona Hosseini-Sarvari, Department of Chemistry,
College of Science, Shiraz University, Shiraz 71454, Iran. E-mail: hossaini@.susc.
ac.ir
832

Synthesis of Bis(indolyl)methanes
833
properties and stability. It is a commercially available, very inexpensive
chemical that has key applications as a catalyst[1] and in sensors, piezoelectric
transducers,[2] transparent conductors,[3] surface acoustic wave devices,[4] and
so on.
Indoles and their derivatives are important intermediates in organic
synthesis because of their pharmacological activities.[5,6] Bis(indolyl)alkanes
and their derivatives have received more attention because of their presence in
bioactive metabolites and marine materials.[7] Hence, the synthesis of these
moieties has become interesting to synthetic organic chemists and biologists.
The simple method for the synthesis of this class of compounds involves the
electrophilic substitution of indoles with carbonyl compounds in the presence
of protic[8] or Lewis acids.[9,10] Recently many methods have been published
in the literature for the synthesis of this class of compounds.[11] However,
many of these reagents are expensive, need longer reaction time, and are
not environmentally friendly, which warrants the development of a new,
practical, economical, and environmentally friendly protocol for the
synthesis of bis(indolyl)methanes.
RESULTS AND DISCUSSION
We herein report the design and development of a truly heterogeneous catalyst
for the synthesis of a variety of biologically important bis(indolyl)methanes
with good yields under solvent-free conditions (Scheme 1).
Initially the effects of different metal oxides as catalysts of the reaction of
benzaldehyde (1a) with indole (2) were examined. Formation of bis(indolyl)-
methane 3a could be observed in all cases but the best value was achieved in
the case of ZnO (Table 1). The solvent effect was also studied, and among
those examined, under solvent-free conditions ZnO led to the best result.
The inexpensive and commercially available catalyst ZnO could be now
applied under optimized conditions to variously substituted aromatic and het-
eroaromatic aldehydes (Table 2). The methodology is found to be general as
the reactions of a variety of substituted aromatic aldehydes 1a–k as well as
alicyclic aldehydes 1l–o with indole 2 have furnished the corresponding
bis(indolyl)methanes in excellent yields. Many of the pharmacologically
relevant substitution patterns on the aromatic ring could be introduced with
high efficiency using this procedure. The electron deficiency and the nature
of the substituents on the aromatic ring affect the conversion rate; aromatic
aldehydes having electron-withdrawing groups on the aromatic ring
Scheme 1.

834
M. Hosseini-Sarvari
Table 1. Screening of different metal oxides for the synthesis of 3a
Entry
Catalyst (%)
Solvent
Time
Yield (%)a
1
2
20 ZnO
10 ZnO
5 ZnO
None
None
None
45 min
45 min
1.5 h
3 h
98
98
3
90
4
10 TiO2
10 MgO
10 Fe2O3
10 Al2O3
10 ZnO
10 ZnO
10 ZnO
10 ZnO
None
None
95[15]
87
5
5 h
8 h
6
None
None
62
93
7
3 h
8
CH3CN
CH2Cl2
EtOAc
EtOH
12 h
12 h
12 h
12 h
Trace
Trace
Trace
Trace
9
10
11
aYields are the isolated compounds.
(i.e., NO2) react faster than benzaldehyde, and an electron-donating substitu-
tent (i.e., CH3) remarkably deactivated aryl aldehydes.
Furthermore, the heterocyclic aldehydes such as furfural and 2, and/or
3-thiophenecarboxaldehyde and 4-pyridylcaboxaldehyde also worked well
without the formation of any side products. The reaction condition are mild
Table 2. ZnO-catalyzed synthesis of bis(indolyl)methanea by
condensation reaction of indole with aldehydes
Time
(min)
Yield
(%)b
Entry
Ar
Product
1
2
ph
3a
3b
3c
3d
3e
3f
45
60
98
99
96
98
95
97
95
90
95
84
93
80
81
75
63
4-MePh
4-MeOPh
3-MeOPh
4-ClPh
3
90
60
4
5
30
60
6
3-ClPh
2-ClPh
4-BrPh
7
3g
3h
3i
45
45
8
9
4-NO2Ph
2-NO2Ph
2-HOPh
2-Thiophene
3-Thiophene
4-Pyridine
2-Furyl
20
45
10
11
12
13
14
15
3j
3k
3l
120
120
90
3m
3n
3o
120
90
aIR and 1H NMR spectra were found to be identical with the one
described in Refs. [11–13].
bIsolated yield.

Synthesis of Bis(indolyl)methanes
835
enough not to damage moieties such as methoxy (1c, d) and acid sensitives
such as furfural (1o), which often undergoes cleavage under strong acidic
reaction conditions. In the absence of ZnO, the reaction did not yield any
product even after a long reaction time. The efficacy of various Lewis acids
was tested for this conversion, and the results are shown in Table 3. Among
these catalysts, ZnO was found to be superior in term of yield and reaction
rate, a result of the very mild Lewic acidity of zinc ion, which activates the
carbonyl group to promote the reaction.
This reaction was further explored for the synthesis of tetraindolyl
compound 3p by the condensation of terephtalaldehyde 1p with 4 equivalents
of indole under similar conditions in excellent yields (Scheme 2). Also, to
show the generality and novelty of the present protocol, the reaction of a
new aldehyde 1q was shown to have good results without any significant
influence of structure on the product yields, such this compound 3q. This
may be applied as a key step in a total synthesis of some natural or other
biologically important compound(s) (Scheme 2).
Structurally, the ZnO crystal is described schematically as a number of
alternating planes composed of four-fold coordinated O22 and Znþ2 ions.
We think that ZnO catalyzes the reaction as a mild Lewis acid even under
solvent free conditions.[14] In Scheme 3, we give the likely mechanism for
the reaction. First, ZnO activates the carbonyl group of the aromatic
aldehydes with its Lewis acid site Zn2þ to give intermediate I and is
followed by indole attack to I to give II and loss of H2O from II to afford
III, which is activated by ZnO. The other indole is added to III in the
following step to give the product IV and ZnO.
To check the reusability of the catalyst, ethyl acetate was added and the
catalyst was filtered off when the reaction of indole with benzaldehyde was
over and the product had formed. It was washed with water repeatedly, dried,
and reused for the reaction of indole with the same or different aldehydes. The
catalyst can be recycled for at least five cycles without any changes in activity.
Table 3. Effect of different Lewis acids on the reaction of
indole with benzaldehydea
Entry
Catalyst
Time
Yield (%)b
1
2
3
4
5
6
None
ZnCl2
FeCl3
AlCl3
CuCl2
ZnO
24 h
10 h
10 h
4 h
0
25
45
83
84
98
2 h
45 min
aThe reaction was carried out according to the typical
experimental procedure.
bYields are the isolated compounds.

836
M. Hosseini-Sarvari
Scheme 2.
Scheme 3.
In conclusion, we have developed an efficient electrophilic substi-
tution reaction of indoles with various aromatic aldehydes using ZnO as
a recyclable catalyst. The method offers several advantages including
high yields, short reaction times, simple experimental workup
procedure, solvent-free conditions, and use of a very inexpensive and
commercially available catalyst; hence, it is a useful addition to the
existing methods.
EXPERIMENTAL
Starting materials were obtained from Fluka Company. Melting points were
determined by Buchi 510 apparatus and are uncorrected. IR spectra

Synthesis of Bis(indolyl)methanes
837
recorded on Perkin-Elmer spectrometer. Proton NMR spectra were recorded
on Bruker Advance DPX FT 250-MHz instrument.
General Procedure for Synthesis of Bis(indolyl)methanes
A mixture of ZnO (0.008 g, 0.1 mmol), benzaldehyde (1 mmol), and indole
(0.4 g, 2 mmol) was added to a test tube and heated in an oil bath at 808C
with stirring. The progress of the reaction was monitored by thin-layer chrom-
atography (TLC) eluent: hexane–EtOAc, 80:20). After the reaction was
completed, the catalyst was filtered off, following by washing with ethyl
acetate (3 ꢀ 30 ml). The filtrate was concentrated under reduced pressure,
and the product was purified by column chromatography. The following
compounds were obtained:
1
3,30-Bisindolylphenylmethane (3a): H NMR (CDCl3): 7.82 (br s, 2H), 7.13–
7.57 (m, 13H), 6.56 (s, 2H), 6.02 (s, 1H); 13C NMR (CDCl3): (40.3, 102.3,
111.4, 119.4, 120.0, 121.9, 123.9, 124.5, 126.3, 128.4, 128.9, 136.8, 144.4.
1
3,30-Bisindolyl-4-methylphenylmethane (3b): H NMR (CDCl3): 7.68 (br s,
2H), 7.12–7.55 (m, 12H), 6.56 (s, 2H), 5.97 (s, 1H), 2.45 (s, 3H); 13C
NMR (CDCl3): (22.0, 39.9, 102.4, 111.3, 119.8, 121.9, 123.8, 127.1, 129.1,
135.6, 136.7, 141.2.
3,30-Bisindolyl-4-methoxyphenylmethane (3c): 1H NMR (CDCl3): 7.93 (br s,
2H), 6.92–7.91 (m, 12H), 6.42 (s, 2H), 5.99 (s, 1H), 3.95 (s, 3H); 13C NMR
(CDCl3): (39.5, 55.5, 102.3, 111.4, 119.8, 121.9, 123.9, 127.2, 129.8, 135.9,
136.8, 157.9.
3,30-Bisindolyl-2-methoxyphenylmethane (3d): 1H NMR (CDCl3): 7.84 (br s,
2H), 6.81–7.40 (m, 12H), 6.64 (s, 2H), 6.35 (s, 1H), 3.82 (s, 3H); 13C NMR
(CDCl3): (32.3, 56.0, 110.8, 111.1, 119.2, 119.8, 120.2, 120.6, 121.9, 123.7,
127.3, 129.9, 132.5, 136.9, 157.1.
1
3,30-Bisindolyl-4-chlorophenylmethane (3e): H NMR (CDCl3): 7.73 (br s,
2H), 7.29 (d, 2H, J ¼ 7.8), 7.03–7.13 (m, 8H), 6.88–6.95 (m, 2H), 6.34
(s, 2H), 5.73 (s, 1H); 13C NMR (CDCl3): (44.5, 111.0, 112.1, 119.0, 120.5,
121.7, 122.8, 128.8, 130.8, 131.6, 135.8, 136.5.
1
3,30-Bisindolyl-3-chlorophenylmethane (3f): H NMR (CDCl3): 7.81 (br s,
2H), 6.92–7.41 (m, 12H), 6.45 (s, 2H), 5.85 (s, 1H); 13C NMR (CDCl3): d
40.0, 11.4, 118.4, 119.4, 119.8, 120.3, 122.1, 126.5, 127.1, 129.4, 130.5,
134.1, 136.8, 146.5.
1
3,30-Bisindolyl-2-chlorophenylmethane (3g): H NMR (CDCl3): 7.89 (br s,
2H), 7.07–7.51 (m, 12H), 6.53 (s, 2H), 6.36 (s, 1H); 13C NMR (CDCl3):
(36.9, 110.1, 111.3, 119.3, 119.8, 122.0, 124.0, 126.7, 127.0, 128.3, 130.4,
130.7, 135.3, 136.7, 141.5.

838
M. Hosseini-Sarvari
1
3,30-Bisindolyl-4-bromophenylmethane (3h): H NMR (CDCl3): 7.58 (br s,
2H), 6.86–7.25 (m, 12H), 6.37 (s, 2H), 5.68 (s, 1H); 13C NMR (CDCl3):
(39.6, 111.2, 118.9, 119.4, 119.8, 119.9, 122.1, 123.7, 126.8, 130.5, 131.3,
136.6, 143.1.
1
3,30-Bisindolyl-4-nitrophenylmethane (3i): H NMR (CDCl3): 8.17 (d, 2H,
J ¼ 7.81), 7.74 (br s, 2H), 7.45–7.62 (m, 10H), 7.03 (s, 2H), 6.01 (s, 1H);
13C NMR (CDCl3): d 44.5, 110.0, 112.1, 119.6, 119.8, 120.9, 121.3, 121.9,
130.2, 133.8, 136.2, 143.1, 145.2.
3,30-Bisindolyl-2-nitrophenylmethane (3j): 1H NMR (CDCl3): 8.36 (br s, 2H),
7.10–7.87 (m, 12H), 6.55 (s, 2H), 5.24 (s, 1H); 13C NMR (CDCl3): d 34.9,
111.5, 111.6, 119.5, 120.7, 121.9, 122.2, 124.3, 126.8, 129.6, 131.2, 132.6,
134.2, 136.8, 149.7.
3,30-Bisindolyl-2-hydroxyphenylmethane (3k): 1H NMR (DMSO): 7.58 (br s,
2H), 6.69–7.24 (m, 13H), 6.67 (s, 2H), 5.85 (s, 1H); 13C NMR (DMSO): d
35.5, 111.2, 111.4, 116.5, 119.9, 120.0, 120.8, 121.9, 122.2, 124.4, 126.8,
129.4, 130.1, 154.4.
3,30-Bisindolyl-2-thienylmethane (3l): 1H NMR (CDCl3): 7.79 (br s, 2H), 7.62
(d, 2H, J ¼ 7.80), 7.14–7.33 (m, 10H), 6.65 (s, 2H), 6.09 (s, 1H); 13C NMR
(CDCl3): d 35.9, 110.3, 111.5, 119.3, 120.0, 121.8, 122.6, 123.5, 125.3, 126.3,
128.9, 136.7, 145.5.
1
3,30-Bisindolyl-3-thienylmethane (3m): H NMR (CDCl3): 7.85 (br s, 2H),
7.75 (d, 2H, J ¼ 7.80), 7.08–7.56 (m, 10H), 6.74 (s, 2H), 6.22 (s, 1H); 13C
NMR (CDCl3): d 35.3, 111.2, 111.4, 119.8, 120.7, 121.5, 122.0, 122.5,
126.5, 128.4, 135.4, 136.7, 148.9.
1
3,30-Bisindolyl-4-pyridylmethane (3n): H NMR (CDCl3): 7.43 (br s, 2H),
6.78–7.22 (m, 12H), 6.44 (s, 2H), 5.71 (s, 1H); 13C NMR (CDCl3): d 39.7,
111.1, 111.4, 119.7, 120.6, 121.8, 122.4, 124.3, 126.7, 136.7, 149.1, 154.0.
3,30-Bisindolyl-2-furylmethane (3o): 1H NMR (CDCl3): 7.74 (br s, 2H), 7.32–
7.63 (m, 9H), 7.09 (s, 2H), 6.35 (d, 1H, J ¼ 3.21), 6.21 (d, 1H, J ¼ 4.10), 5.82
(s, 1H); 13C NMR (CDCl3): d 41.2, 102.1, 110.3, 111.8, 112.6, 119.4, 120.3,
121.5, 122.6, 131.0, 136.4, 141.2, 152.0.
3,30,300,3000-Tetraindolyl(terephthalyl)dimethane (3p): 1H NMR (CDCl3): 8.12
(br s, 4H), 6.95–7.84 (m, 24H), 6.98 (s, 4H), 5.89 (s, 2H); 13C NMR (CDCl3):
d 34.9, 111.3, 112.8, 119.8, 120.3, 121.0, 122.2, 124.5, 129.8, 131.3, 134.7,
142.5.
2-[Di(1Hindol-3-yl)methyl]-4-(morphonilomethyl)phenol (3q): 1H NMR
(DMSO-d6): 1.94 (s, 4H, 2ꢀ -N-CH2-), 2.98 (s, 2H, -CH2-), 3.28 (s, 4H,
2ꢀ -O-CH2-), 5.99 (s, 1H, -CH-), 6.53 (s, 2H, 2 ꢀ NH), 6.60–7.33
(m, 13H, Ar-H), 10.42 (br s, 1H, OH); 13C NMR (DMSO-d6): d 31.8,
52.3, 59.7, 66.1, 111.2, 118.7, 119.2, 120.6, 120.8, 123.5, 124.8, 126.8,

Synthesis of Bis(indolyl)methanes
839
127.3, 130.1, 130.4, 136.6, 152.2, 153.3. GC-Ms/EI: m/z (%) 437 (Mþ,
100%). Anal. calcd. for C28H27N3O2: C, 76.86; H, 6.22; N, 9.60. Found:
C, 76.95; H, 6.30; N, 9.72.
ACKNOWLEDGMENTS
We gratefully acknowledge the support of this work by Shiraz University. We
are also grateful to Prof. H. Sharghi for synthesis of aldehyde 1q and his
helpful cooperation.
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