In situ generation of Iron(iii) dodecyl sulfate as Lewis acid-surfactant catalyst for synthesis of bis-indolyl, tris-indolyl, Di(bis-indolyl), Tri(bis-indolyl), tetra(bis-indolyl)methanes and 3-alkylated indole compounds in water

Article abstract of DOI:10.1039/c4ra03194f

Iron(iii) dodecyl sulfate as Lewis acid-surfactant catalyst was prepared in situ and effectively used in the synthesis of bis(indolyl)methanes and Michael reactions of indoles with α,β-unsaturated carbonyl compounds in water. Also, this method was used for the synthesis of 1,1,3-tri-indolyl compounds, producing good to excellent yield at room temperature.

Full text of DOI:10.1039/c4ra03194f

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In Situ Generated of Iron(III) Dodecyl Sulfate as Lewis AcidꢀSurfactant
Catalyst for Synthesis of Bisꢀindolyl, Trisꢀindolyl, Di(bisꢀindolyl),
Tri(bisꢀindolyl), Tetra(bisꢀindolyl)methanes and 3ꢀAlkylated Indole
Compounds in Water
Hojat Veisi,a Behrooz Maleki,*b Fereshteh Hosseini Eshbala,a Hamed Veisi,c Ramin
Masti,a Samaneh Sedigh Ashrafi,b Mehdi Baghayerib
aDepartment of Chemistry, Payame Noor University, 19395ꢀ4697 Tehran, I.R. of Iran
Fax +98(838)4227463; Eꢀmail: hojatveisi@yahoo.com
bDepartment of Chemistry, Hakim Sabzevari University, Sabzevar 96179ꢀ76487, Iran
. Eꢀ
mail:b.maleki@hsu.ac.ir; malekibehrooz@gmail.com; Fax: +98ꢀ571ꢀ4410300; Tel: +98ꢀ
571ꢀ4002643
cSchool of Medicine, Shiraz University of Medical Sciences, Shiraz, Iran.
Abstract
Iron(III) dodecyl sulfate as Lewis acidꢀsurfactant catalyst was prepared in situ, and
effectively used in the synthesis of bis(indolyl)methanes and Michael reactions of
indoles with α,βꢀunsaturated carbonyl compounds in water. Also, this method was used
for the synthesis of 1,1,3ꢀtriꢀindolyl compounds in good to excellent yield at room
temperature.
Key words: Iron(III) dodecyl sulfate; Lewis acidꢀsurfactant catalyst; Indole; Water
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Introduction
The investigation performed on the chemistry of indole has led to its becoming one of the
most active research areas of heterocyclic chemistry.1,2 Various biological and
pharmaceutical activities have been reported for this class of compounds.3
Bis(indolyl)methanes are cruciferous substances useful for promoting beneficial estrogen
metabolism in men and women. They are also effective in the prevention of cancer due to
their ability to modulate certain cancer causing estrogen metabolites. Moreover, these
compounds may normalize abnormal cell growth associated with cervical dysplasia. As
bis(indolyl)methanes are important compounds in pharmaceutical chemistry, their
synthesis has received increasing attention.
3ꢀPosition of indole is the preferred site for electrophilic substitution reaction and 3ꢀ
substituted indoles are versatile intermediates for the synthesis of a wide range of indole
compounds.4 One of the simple and direct methods for the synthesis of 3ꢀsubstituted
indoles involve the conjugate addition of them to α,βꢀunsaturated carbonyl compounds or
reaction of 2 equiv. of indoles with the carbonyl group in the presence of either protic 5,6
or Lewis acids.7ꢀ9 However, many Lewis acids are deactivated or sometimes decomposed
by nitrogen containing reactants. Even when the desired reactions proceed, more than
stoichiometric amounts of the Lewis acids are required because the acids are deactivated
by the nitrogen atom of indoles.10 Developing environmentally benign and economical
syntheses is an area of research that is being vigorously pursued, and avoiding the use of
harmful organic solvents is a fundamental strategy to achieving this. One of the most
attractive alternatives to organic solvents is water, which has witnessed increasing
popularity due to being inexpensive, readily available and environmentally benign. In
2

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addition, reactions in aqueous media illustrate unique reactivities and selectivities that are
not usually observed in organic media.11 However, organic reactions in water are often
limited in scope due to poor solubility of the organic compounds. A possible new way to
improve the solubility of substrates is the use of surfaceꢀactive compounds that can form
micelles.12 Under ambient conditions, surfactant molecules can aggregate in an aqueous
phase to form micelles with a hydrophobic core and a hydrophilic corona. The use of
micelle surfactants as catalysts is widespread, and has been investigated in detail for
various reactions in aqueous solutions.13 Recently, a new type of catalyst, a Lewis acidꢀ
surfactantꢀcombined catalyst (LASC), has been developed. This acts both as a Lewis acid
to catalyze the reaction and as a surfactant to solubilize organic substrates in water.14
Result and Discussions
In our continued interest in the development of a highly expedient methodology15 for the
synthesis of fine chemicals and heterocyclic compounds of biological importance, we
reported here the in situ generation of iron(III) dodecyl sulfate as Lewis acidꢀsurfactant
catalyst (LASC) for synthesis of indole derivatives in water at room temperature.
First, encouraged by the effect of a protic solvent and to reduce the reaction time in
the synthesis of bis(indolyl)methane, we studied the reaction using water as the solvent.
In a simple experimental procedure, 2 equiv. of indole with 1 equiv. of benzaldehyde in
water were allowed to react with stirring at room temperature. Unfortunately, it was
observed that indoles were insoluble in water and no reaction occurred even after stirring
the reaction mixture for a long time. Presuming the insolubility of the reactant was the
problem, we considered the use of SDS (sodium dodecylsulfate) as a surfactant, since it
forms micelles in water and can solubilize organic compounds, which are otherwise
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insoluble in water. Then, we repeated the reaction in the presence of SDS as surfactant,
but we could not observe any product after 3 hours.
In recent times, iron(III) chloride has been used as a Lewis acid catalyst in many
important organic transformations.16 In addition, We have also shown that FeCl3.6H2O is
a good Lewis acid catalyst in the synthesis of bis(indolyl)methanes under organic solvent
conditions. Although Lewis acidꢀcatalyzed reactions are of great current interest because
of their unique reactivities, selectivities, and mild conditions used, Lewis acids have been
believed to be unstable in water and therefore not to be used in aqueous media. Some of
these problems were solved with the discovery of waterꢀtolerant Lewis acids by
Kobayashi et al.14 However, it is necessary to synthesize new types of cheap and
powerful water active Lewis acids.
In continuation of our studies in this area, we decided to combine two effects together:
Lewis acidity of FeCl3.6H2O and surfactant effect of SDS. Iron tris(dodecyl sulfate)
Fe(DS)3.nH2O was synthesized by adding a solution of sodium dodecyl sulfate to an
aqueous solution of FeCl3.6H2O (Scheme 1).17
ꢀ
C12H25OSO3
Fe3+
=
=
H2O
FeCl3.6H2O
+
C12H25OSO3Na
Fe(DS)3. nH2O
Scheme 1 In situ generated of iron(III) dodecyl sulfate.
4

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The iron dodecyl sulfate was precipitated immediately. we believe that this new type of
“combined Lewis acidꢀsurfactant catalyst (LASC)’’, would act in dual role both as a
catalyst to activate the substrate molecules and as a surfactant to increase the
concentration of organic reactants to form micelle particles.
We described here the first example of the in situ generated of iron(III) dodecyl
sulfate as Lewis acidꢀsurfactant catalyst (LASC) for synthesis of bis(indolyl)methanes in
water at room temperature. The optimum conditions were indole (2 mmol) and aldehyde
(1 mmol), in the presence of SDS (0.3 mmol) and FeCl3.6H2O (0.1 mmol) (SDS and
FeCl3.6H2O should be added together at same time) in water at 25 °C (Fig. 1) (Scheme
2).
Fig. 1 Photographs of the reaction of indole with benzaldehyde in the presence of SDS
and FeCl3.6H2O in H2O at 25 °C: (left side) at the start of the reaction; (middle side) after
10 second; and (right side) at the end of the reaction (after 5 min., 98% yield).
R2
R1
O
In situ generated of Fe(DS)3
H2O, r.t.
+
N
R2
R1
N
N
H
H
H
R1: alkyl, aryl
R2: H, alkyl
Scheme 2 Synthesis of bis(indolyl)methanes in water.
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These results showed that the combination of a Lewis acid moiety (cationic part) and
a surfactant (anionic part) in Fe(DS)3.nH2O is important for the reactions to proceed in
water. The Lewis acid part of the catalyst activates the substrate molecules and the
surfactant part, affecting the solubility of the substrates in water at the same time. When
these two factors are in play together, a drastic enhancement of the rates and the yields of
the products are observed. We have also tried to use prepared Fe(DS)3 instead of in situ
generated Fe(DS)3 in the same conditions, but we could not get better results than in situ
in situ generated Fe(DS)3 for the synthesis of bis(indolyl)methanes.
These results promoted us to investigate the scope and the generality of this new
protocol for various aldehydes and ketones under optimized conditions. As shown in
Table 1, a series of aromatic, aliphatic and heterocyclic aldehydes underwent
electrophilic substitution reaction with indole smoothly to afford a wide range of
substituted bis(indolyl)methanes in good to excellent yields (Table 1, entries 1ꢀ22).
Ketones required longer reaction times, which is most probably due to the electronꢀ
donating and steric effects of the methyl group.
Table 1
Synthesis of bis(indolyl)methanes in the presence of iron(III) dodecyl sulfate as Lewis
acidꢀsurfactant catalyst (LASC) in water
Entry
R1
R2
Time/min
Yield
(%)a
98
96
98
98
96
98
96
Ref.
1
2
3
4
5
6
7
8
9
C6H5
H
H
5
5
5
5
5
18
19
20
20
20
19
21
22
19
23
4ꢀMeC6H5
4ꢀMeOC6H5
2ꢀMeOC6H5
4ꢀClC6H5
2ꢀClC6H5
4ꢀBrC6H5
4ꢀOHC6H5
4ꢀNO2C6H5
3ꢀNO2C6H5
H
H
H
H
H
H
H
5
10
10
15
15
95
90
90
10
6

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11
12
13
14
15
16
17
18
19
20
21
22
4ꢀ(Me2N)C6H5
C6H5ꢀCH=CH
2ꢀFuryl
2ꢀThienyl
CH3(CH2)
CH3(CH2)8
H
H
H
H
H
H
ꢀ
15
10
10
10
12
15
15
10
15
20
25
20
90
96
92
95
85
85
20
19
19
19
21
21
24a
25
24b
21
21
29
Indolꢀ3ꢀcarbaldehyde
terephthaldialdehyde
isatin
90
ꢀ
ꢀ
90
85
96
90
85
C6H5
4ꢀNO2C6H5
Cyclohexanone
CH3
CH3
ꢀ
a Products were characterized from their physical properties, comparison with authentic samples and
by spectroscopic methods.
In continuation of our efforts to synthesize bis(indolyl)methanes, in a similar way
with Fe(DS)3, the reaction between indole (2 mmol) and benzaldehyde (1 mmol) was
carried out with Fe(DS)2 (0.1 mmol) in water (5 mL) at room temperature. The product of
this reaction was obtained in lower yield and longer time (yield: 50%, time: 60 minutes).
In order to examine the catalytic activity of lauric acid and nꢀhexyl alcohol in this
reaction, benzaldehyde and indole were selected as the model. It showed that nearly no
product could be detected when a mixture of benzaldehyde (1 mmol) and indole (2
mmol) were stirred at room temperature for 2 hours in presence of nꢀhexyl alcohol (1
mL). Also, the reaction between benzaldehyde (1 mmol) and indole (2 mmol) and lauric
acid in water (5 mL) at room temperature was carried out with a yield 32% and 60
minutes.
The catalytic effect of the in situ generated of iron(III) dodecyl sulfate in this reaction
can be explained as follows. This ‘Lewis acid–surfactantꢀcombined catalyst (LASC)’
form stable colloidal dispersion system with organic substrates in water and have been
used for Lewis acidꢀcatalyzed reactions. It has been suggested that LASCs generate
hydrophobic reaction fields in water while an active Lewis acidic site is close to the
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hydrophobic sites. On the other hand, This system was further explored for the synthesis
of tri(bisꢀindolyl)methane (B) and tetra(bisꢀindolyl)methanes (D) as new triꢀ and tetraꢀ
arylmethanes, by the condensation of aldehyde (A) with 6 equivalents indole and
aldehydes (C) with 8 equivalents indole under similar condition in high yields (Scheme 3
and 4).
H
H
N
N
CHO
O
6
O
NH
O
N
O
CH2
CH2
H
CH2
CH2
In situ generated of Fe(DS)3
CHO
H2O, r.t., 25 min.
85%
NH
H2C
CH2
O
O
OHC
HN
B
A
HN
Scheme 3 Synthesis of tri(bisꢀindolyl)methane (B).
HN
NH
CHO
NH
O
O
CHO
8
H
N
N
H
O
O
O
O
In situ generated of Fe(DS)3
N
H
OHC
H2O, r.t., 30 min.
85%
O
O
HN
CHO
NH
HN
C
D
Scheme 4 Synthesis of tetra(bisꢀindolyl)methanes (D).
3ꢀSubstituted indole was examined for this reaction under the above reaction
conditions with aldehydes (Scheme 5). Since the more active site (Cꢀ3) in indole was
8

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blocked in this case electrophilic substitution took place at Cꢀ2 in indole giving the
corresponding bis(indolyl)methane in high yield under same conditions.
CHO
CH3
In situ generated of Fe(DS)3
N
N
2
+
H
H
H2O, r.t., 45 min.
85%
N
H
Scheme 5 Synthesis of bis(indolyl)methanes at Cꢀ2.
We also investigated the recycling of the catalytic system under same conditions by
the condensation of benzaldehyde with 2 equivalents indole. After completion of the
reaction, water was added and the precipitated mixture was filtered off for separation of
crude products. After washing the solid products with water completely, the water
containing catalyst (Fe(DS)3) was evaporated under reduced pressure and the catalyst was
recovered and reused (Fig. 2). The recovered catalyst was reused four runs without any
º
loss of their activities. Melting point of the recycled catalyst was 210 C that
corresponded to the literatures.17 Also, the IR spectrum of recycled catalyst showed clear
peaks in 1170 cmꢀ1 and 1320 cmꢀ1for the SO2 group. All of these are evidences for the
construction of this catalyst.
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Fig. 2. The recycling of the catalyst Fe(DS)3 for synthesis of bis(indolyl)methanes.
While most Michael additions are performed in organic solvents, today’s
environmental concerns encourage the development of “greener” conditions where
possible. Then we intended to use this system in Michael reactions of indole with α,βꢀ
unsaturated compounds in water (Scheme 6). In synthesis of all products, Michael
addition (1,4 addition) prevailed on 1,2 addition. The predominant product was 1,4
addition.
R1
O
O
In situ generated of Fe(DS)3
H2O
R2
+
R1
R2
N
HN
1a
R1 , R2 : alkyl, aryl
H
Scheme 6 Michael reactions of indole with α,βꢀunsaturated compounds in water.
The generality of the procedure was evaluated by the reaction of a number of α,βꢀ
unsaturated compounds with indole under same conditions at room temperature. The
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reactions proceeded easily, and the products were isolated in comparable yields in short
reaction times (Table 2, entries 1–15).
Table 2
Michael addition of indole to Michael acceptors in the presence of iron(III) dodecyl
sulfate as Lewis acidꢀsurfactant catalyst (LASC) in water
Entry
Products
Time/min Yield
Ref.
(%)a
1
15
15
96
95
26
O
HN
HN
HN
HN
CH3
2
3
4
26
26
26
O
OCH3
10
10
98
98
O
Cl
O
5
6
7
15
20
10
96
92
96
27
26
28
O
HN
OCH3
O
HN
CH3
Cl
O
HN
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8
9
15
10
86
95
28
O
O
O
HN
HN
HN
Cl
Cl
29
29
27
Br
CH3
10
11
15
10
90
98
Br
OCH3
O
HN
OCH3
12
25
90
26
S
O
HN
H
13
14
20
20
80
90
30
30
O
HN
OCH3
H
O
HN
CH3
a Products were characterized from their physical properties, comparison with authentic samples and
by spectroscopic methods.
Based on the results of above study, we planned to synthesis of 1,1,3ꢀtriindolyl
compounds using in situ generation of iron(III) dodecyl sulfate as Lewis acidꢀsurfactant
catalyst (LASC) in water under thermal conditions through the tandem Michael addition
and FriedelꢀCrafts reaction of α,βꢀunsaturated aldehyde and indoles (Scheme 7). Three
variety of 1,1,3ꢀtri(1Hꢀindolꢀ3ꢀyl)butanes in 70ꢀ90% yield was prepared from
crotonaldehyde as starting material.
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Fe(DS)3. nH2O
HN
HN
NH
NH
H
H
NH
90%
N
20 min.
H
70%
45 min.
O
H
H2O
4
N
H
80 ºC
N
H
N
N
N
H
82%
30 min.
N
Scheme 7 Synthesis of 1,1,3ꢀtriindolyl compounds.
Experimental
Materials and Methods
All chemicals were commercially purchased and used without further purification. All
products were identified by comparison of their spectral and physical data with those
previously reported.15ꢀ27 Progress of the reactions was monitored by Thinꢀlayer
Chromatography (TLC). Melting points were recorded on a Bamstead electrothermal
type 9200 melting point apparatus. IR spectra were recorded on a Shimadzu 435ꢀUꢀ04
spectrophotometer (KBr pellets). NMR spectra were obtained using Jeol FT NMR 90
MHz and spectrometer in CDCl3 or DMSO using TMS as an internal reference.
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Synthesis of bis(indolyl)methanes
The aldehyde or ketone (1 mmol) and indole (2 mmol) were added to a solution of
FeCl3.6H2O (0.1 mmol) and SDS (0.3 mmol) in H2O (5 mL), and the mixture stirred at
room temperature for the time given (Table 1). The progress of the reaction was
monitored by TLC (eluent: 4:1 nꢀhexaneꢀacetone). After completion of the reaction, the
resulting precipitate of product filtered off. Finally, the crude product was recrystallized
in ethanolꢀwater (85ꢀ98% Yield). All of the products are known compounds, and their
identity was easily confirmed by comparison with authentic samples (1H NMR, 13C NMR,
and Mp).
Michael addition of indole to α,βꢀunsaturated carbonyl compounds
The α,βꢀunsaturated carbonyl compound (1 mmol) and indole (1 mmol) were added to a
solution of FeCl3.6H2O (0.1 mmol) and SDS (0.3 mmol) in H2O (5 mL), and the mixture
stirred at room temperature for the time given (Table 2). The progress of the reaction was
monitored by TLC (eluent: 3:1 nꢀhexaneꢀacetone). After completion of the reaction, the
resulting precipitate of product filtered off. Finally, the crude product was recrystallized
from ethanolꢀwater (70:30) afforded pure Michael adducts (80ꢀ98% Yield), which were
characterized by spectral methods.
Conclusion
In conclusion, we have developed a new type in situ generated of ‘‘combined Lewis acidꢀ
surfactant catalyst (LASC)’’, derived from FeCl3.6H2O and SDS, would act in dual role
both as a catalyst to activate the substrate molecules and as a surfactant to increase the
concentration of organic reactants to form stable colloidal dispersion system for synthesis
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of bis(indolyl)methanes and Michael reactions of indoles with α,βꢀunsaturated carbonyl
compounds in water. The operational simplicity, excellent yields of the products, and
high chemoselectivity are the main advantages of this method, and furthermore, this
procedure is cheap, safe and environmentally benign. This novel concept is expected to
contribute to the development of more benign reactions in water, and the related work is
underway in our group.
Acknowledgements
We are thankful to Payame Noor University (PNU), and Hakim Sabzevari university
for partial support of this work.
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RSC Advances
Page 18 of 18
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DOI: 10.1039/C4RA03194F
Graphical Abstract
In Situ Generated of Iron(III) Dodecyl Sulfate
as Lewis Acid-Surfactant Catalyst for
Synthesis of Bis-indolyl, Tris-indolyl, Di(bis-
Fe(DS)3. nH2O
HN
indolyl),
Tri(bis-indolyl),
Tetra(bis-
indolyl)methanes and 3-Alkylated Indole
Compounds in Water
HN
NH
H
R1
R2
O
N
H
RT
Hojat Veisi,a Behrooz Maleki,*b
Fereshteh
R1
R2
HN
Hosseini Eshbala,a Hamed Veisi,c Ramin Masti,a
Samaneh Sedigh Ashrafi,b Mehdi Baghayerib
R1, R2: H, alkyl, aryl
80 ºC
N
H
O
H2O
H
ꢀ
In situ generated of Iron(III) dodecyl
sulfate
Efficient recycling of catalyst
Water as solvent
RT
O
R1
O
R2
ꢀ
ꢀ
ꢀ
R1
R2
R1, R2: alkyl, aryl
HN
Synthesis of poly-indolyl derivatives