Efficient synthesis of bis- and tris-indolylalkanes catalyzed by a Bronsted acid-surfactant catalyst in water

Article abstract of DOI:10.1080/00397910801979387

A simple and efficient synthesis of bis- and tris-indolylalkanes from carbonyls (aldehydes/ketones) and indoles with excellent yields in the presence of dodecylsulphonic acid (DSA) in water at room temperature is described. The catalyst is also applicable

Full text of DOI:10.1080/00397910801979387

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Efficient Synthesis of bis- and
tris-Indolylalkanes Catalyzed
by a Brønsted Acid–Surfactant
Catalyst in Water
Parasa Hazarika a , Saikat Das Sharma a & Dilip
Konwar a
a Synthetic Organic Chemistry Division, Northeast
Institute of Science and Technology (formerly RRL),
Jorhat, Assam, India
Version of record first published: 28 Aug 2008.
To cite this article: Parasa Hazarika , Saikat Das Sharma & Dilip Konwar (2008):
Efficient Synthesis of bis- and tris-Indolylalkanes Catalyzed by a Brønsted
Acid–Surfactant Catalyst in Water, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 38:17, 2870-2880
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Synthetic Communications1, 38: 2870–2880, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910801979387
Efficient Synthesis of bis- and tris-Indolylalkanes
Catalyzed by a Brønsted Acid–Surfactant
Catalyst in Water
Parasa Hazarika, Saikat Das Sharma, and Dilip Konwar
Synthetic Organic Chemistry Division, Northeast Institute of Science and
Technology (formerly RRL), Jorhat, Assam, India
Abstract: A simple and efficient synthesis of bis- and tris-indolylalkanes from
carbonyls (aldehydes=ketones) and indoles with excellent yields in the presence
of dodecylsulphonic acid (DSA) in water at room temperature is described. The
catalyst is also applicable for the synthesis of 3,3-di(3-indolyl)oxindoles. The
dodecylsulphonic acid acts as both Brønsted acid and surface-active agent in
the reaction mixture.
Keywords: bis-indolylalkane, Brønsted acid–surfactant catalyst, 3-substituted
indoles, water media
Development of nonhazardous synthetic methodologies for organic
reactions is one of the latest challenges to organic chemists. To reach this
goal, green reaction media, reactants, and catalysts are being used. Use of
water as a reaction media instead of volatile organic solvent is of para-
mount interest, especially in relation to today’s environmental concerns,
because it is a nontoxic, cheap, abundantly available, and green solvent.[1]
The catalytic dehydration reaction is one of the most common and essen-
tial steps in organic syntheses. Water formed during the dehydration pro-
cess sometimes deactivates the catalyst and may take part in co-reactions
for side products. Therefore, it is necessary to remove water by means of
azeotropic distillation or other means. In this connection, surfactant-
combined Brønsted acids play an appreciable role in overcoming this
Received December 24, 2007.
Address correspondence to Dilip Konwar, Synthetic Organic Chemistry
Division, Northeast Institute of Science and Technology, Jornat 785006, Assam,
India. E-mail: dkonwar@yahoo.co.uk
2870

Synthesis of bis- and tris-Indolylalkanes
2871
problem. Surfactants are a class of chemicals composed of a hydrophilic
polar head group and a hydrophobic chain, and they can assemble
automatically in water or organic solvents.[2] Brønsted acid–surfactant
catalyst not only activates the substrates as a Brønsted acid but also solu-
bizes the substrates in water.[3]
Indoles are privileged heterocyclic rings, and many biologically active
and natural products are 3-substituted indoles.[4] Naturally occurring bis-
indolylalkanes consist of two indoles or substituted indole units in a mol-
ecule. Indoles react with aromatic or aliphatic aldehydes and ketones to
produce azafulvenium salt, which undergo further addition with a second
molecule of indole to afford bis-indolylalkane. Synthetically, these com-
pounds are obtained by the condensation of indoles with aldehy-
des=ketones in the presence of several Brønsted[5] and Lewis acid
[AlCl3, BF3, LnCl3, Ln(OTf)3, Dy(OTf)3, In(OTf)3, etc][6] catalysts, ami-
[7c]
nocatalysts,[7a] ionic liquids,[7b] AlPW12O40,
ceric ammonium nitrate
[7d]
[7e]
(CAN),
LiClO4,
I2,[7f] etc. However, despite the potential utility
of these catalysts, many of these methodologies for the synthesis of bis-
indolylalkanes are associated with several shortcomings such as use of
organic solvents, longer reaction time, and strong Brønsted acids; most
of the Lewis acid catalysts are moisture sensitive and are easily decom-
posed or deactivated in the presence of water. In many Lewis acid–cata-
lyzed reactions, more than a stoichiometric amount of catalyst is required
because it is trapped or deactivated by nitrogen-containing reactants.[8]
Zolfigol et al. have reported the synthesis of bis- and tris-indolylalkanes
in water=water–ethanol using metal dodecylsulfates (Lewis acid–surfac-
tant catalyst, LASC, a reverse micelle system).[9] They used a variety of
LASCs for this synthesis; among them, Zr(DS)4 gave the best result,
which is associated with transition metal zirconium. Therefore, the devel-
opment of a cost-effective, safe, and environmentally friendly reagent sys-
tem is desirable. To the best of our knowledge, there is no report of the
use of Brønsted acid–surfactant catalyst for the synthesis of bis- and tris-
indolyl alkanes in water.
In continuation of our research on the development of green meth-
odologies for the synthesis of fine chemicals and heterocyclic compounds
of biological importance,[3c,10] we report here an efficient and green
method for the synthesis of bis- and tris-indolylalkanes in water at room
temperature. bis-Indolylalkanes are obtained in good to excellent yields
when indole (2 mmol) was treated with various aldehydes or ketones
(1 mmol) in the presence of a catalytic amount (0.1 mmol) of dodecylsul-
phonic acid (DSA) (Scheme 1) in water.
Initially the catalytic activity of DSA and the influence of solvents
on the synthesis of bis-indolylalkanes as a model reaction were
attempted. In typical experiments, anisaldehyde (1 equiv), indole (2

2872
P. Hazarika, S. D. Sharma, and D. Konwar
Scheme 1. Synthesis of bis-indolylmethane.
equiv), and DSA were simply mixed in water (2 ml) and stirred at room
temperature in different solvent systems. Out of these, water was found
to be an excellent reaction media. To justify the effectiveness of the
catalyst, DSA in this reaction, the loading of the catalyst was opti-
mized (Table 1). In an experiment under the same reaction conditions,
a reaction without any catalyst showed a trace amount of product
Table 1. Synthesis of bis(indolyl)-4-methoxyphenylmethane under different
catalyst loading and solvent systems
Entry
Catalysts
Catalyst load (mol%) Solvents Time (h) Yield (%)a,b,c
1
2
3
4
5
6
7
8
9
10
11
12
13
DSA
DSA
DSA
DSA
DSA
DSA
DSA
DSA
DSA
SDSd
10
10
10
10
10
10
5
15
20
10
10
10
—
CH3CN
MeOH
CHCl3
Toluene
Neat
H2O
Neat
Neat
Neat
Neat
Neat
H2O
Neat
1.5
1
4
75
70
37
35
94
90
92
94
90
NR
90
92
trace
4
0.5
0.5
0.5
0.5
0.5
12
0.5
0.5
e
SDS þ AcOH
SDS þ HCl
No catalyst
12
aIsolated yield.
bStirring at room temperature.
1
cThe structure of the compound was determined using H NMR, FTIR, and
MS (m=z).
dSodium dodecyl sulfate.
eNo reaction.

Synthesis of bis- and tris-Indolylalkanes
2873
formed after 12 h of stirring. It was found that 10 mol% of dodecylsul-
phonic acid (DSA) was optimum amount of catalyst for the reaction.
This electrophilic substitution reaction of indoles with aldehydes as
well as ketones occurred smoothly at ambient conditions. To generalize
the reaction, we synthesized several bis-indolylalkanes under the opti-
mized reaction conditions (Table 2). Aromatic (entries 1a–1f), hetero-
cyclic (entries 3a and 3b), and aliphatic (entries 2a-2e) carbonyls
gave very good to excellent yields in the reaction mixture. Acid- or
heat- sensitive aldehydes such as furfural afforded the desired products
with excellent yields. On the other hand, the reaction of indoles with
ketones (cyclic as well as acyclic) gave the products in good yields.
The reaction of ketones with indoles took a longer reaction time in
comparison to aldehydes. Under the Brønsted acid–surfactant con-
ditions, the ether group (such as MeO) remained intact during the reac-
tion. As indicated in Table 2, both the electron-donating and the
electron-withdrawing substitutions on the phenyl ring of aromatic alde-
hydes showed slight influences on the reactions. The electron-with-
drawing groups accelerated the rate of reactions in comparison to
the electron-donating groups. This efficient and mild Brønsted acid–
surfactant catalyst is also used for the synthesis of naturally occurring
tris-indolylalkanes and 3,3-di(indolyl)-oxindoles, isolated from bac-
terium Vibrio parahaemolyticus with excellent yields (Scheme 2). These
two compounds are obtained by the reaction of indole-3-carboxalde-
hyde and isatin with indoles respectively.
Dodecylsulphonic acid (DSA) can self-assemble to form micelles
with hydrophobic substrates (Scheme 3). This condensation reaction
probably proceeds through the activation of a carbonyl group as well
as the indole moiety by Hþ at the reaction interface. The probable path-
way seems to be an addition of indole to the carbonyl group of aldehyde
or ketone, followed by the addition of the second molecule of indole
(Scheme 3).
In conclusion, this synthetic method describes a simple and highly
efficient dehydration process for the synthesis of indole derivatives in
water. Synthesis of bis-indolylalkanes through the electrophilic substi-
tution of indoles with aldehydes or ketones using dodecylsulphonic
acid (DSA) as an environmentally benign catalyst at room temperature
has been successfully performed. This green methodology can also be
´
applied for the one-step production of tris-indolylalkanes and 3, 3-
di(indolyl)oxindoles. The salient features of this procedure are short
reaction time, improved yields, and an economic and environmentally
friendly process. The dual nature of the catalyst, both Brønsted acid
as well as water separator, significantly contributes to the practice of
green chemistry.

2874

2875

2876
P. Hazarika, S. D. Sharma, and D. Konwar
EXPERIMENTAL
Typical Experimental Procedure for the Synthesis of
3,3-bis-(Indolyl)alkanes
In a 50-ml round-bottom flask, a mixture of aldehyde (1 mmol) and
indole (2 mmol) was stirred in the presence of dodecyl sulfonic acid
(0.1 mmol) in water (2 ml) at room temperature for the stipulated time.
After completion of the reaction, the product was extracted with EtOAc
(2 ꢀ 10 ml). The organic layer was washed with brine (2 ꢀ 15 ml) and then
water (2 ꢀ 10 ml), dried over anhydrous sodium sulfate, and concentrated
under reduced pressure to give a crude product, which was purified by
thin-layer chromatography(TLC).
Data
3,3-bis-(Indolyl)dodecane (Table 2, entry 2a)
1
Dark red gum; FTIR (CHCl3): 3406, 2923, 2852, 1456 cmꢁ1; H NMR
(CDCl3,300 MHz): d 0.87 (t, 3H, J ¼ 6.8 Hz), 1.21–1.34 (m, 18H), 2.17
(m, 2H), 4.42 (t, 1H, J ¼ 6.8 Hz), 6.88–7.59 (m, 10H), 7.76 (br, s, 2H);
13C NMR (CDCl3, 75 MHz): d 14.63, 23.17, 28.81, 29.83, 30.12, 30.15,
30.28, 32.39, 34.39, 36.30, 111.53, 119.38, 120.10, 120.91, 121.88,
122.10, 127.57, 136.96; HRMS calcd. for C28H36N2 (Mþ ) 400.2844;
found 400.2827. Anal. calcd. C, 83.82; H, 9.03; N, 7.05.
Scheme 2. Synthesis of tris-indolylmethane.

Synthesis of bis- and tris-Indolylalkanes
2877
Scheme 3. Plausible mechanism for the synthesis of bis-indolylalkanes.
3,3-bis(Indolyl)-2-phenylpropane (Table 2, entry 2b)
Pale pink solid; mp: 72–73 ꢂC; FTIR (KBr): 3418, 3051, 2922, 1455 cmꢁ1;
1HNMR (CDCl3, 300 MHz): d 1.26 (d, 3H, J ¼ 1.95 Hz), 3.73 (m, 1 H),
4.77 (d, 1 H, J ¼ 4.11 Hz), 7.05–7.13 (m, 17 H); 13C NMR (CDCl3,
75 MHz): d 22.24, 40.70, 45.00, 111.27, 111.57, 119.22, 119.36, 119.48,
119.98, 121.91, 122.07, 122.62, 122.91, 126.14, 127.92, 127.97, 128.03,
128.41, 136.36, 136.65, 147.35; HRMS calcd. for C25H22N2 (Mþ )
350.1738; found 350.1721. Anal. calcd. C, 85.71; H, 6.28; N, 8.00; found
C, 85.65; H, 6.22; N, 8.08.
ACKNOWLEDGMENT
The authors acknowledge the director, Dr. P. G. Rao, and Dr. N.
Borthakur, Head of the Division, Synthetic Organic Chemistry Division,
the Analytical Division of Regional Research Laboratory, Jorhat,
Assam, India, for their help. P. H. and S. D. S. thank Council of Scien-
tific and Industrial Research, New Delhi, for research fellowships.
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