Highly selective one-pot coupling reaction of indole, aromatic aldehydes, and 4-hydroxycoumarin using copper octoate as a homogeneous catalyst

Article abstract of DOI:10.1080/00397911.2013.862723

A facile protocol for the one-pot, multicomponent reaction of indole, 4-hydroxycoumarin, and aromatic aldehydes was developed using copper octoate as an inexpensive, commercially available, and efficient catalyst. This highly selective reaction eliminates

Full text of DOI:10.1080/00397911.2013.862723

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Highly Selective One-Pot Coupling
Reaction of Indole, Aromatic Aldehydes,
and 4-hydroxycoumarin Using Copper
Octoate as a Homogeneous Catalyst
Farnoush Mousavizadeh a , Rahim Hekmatshoar a , Seyed Yahya
Shirazi Beheshtiha a & Reyhaneh Rahnamafar a
a Department of Chemistry, School of Science , Alzahra University ,
Vanak , Tehran , Iran
Accepted author version posted online: 10 Feb 2014.Published
online: 29 Apr 2014.
To cite this article: Farnoush Mousavizadeh , Rahim Hekmatshoar , Seyed Yahya Shirazi Beheshtiha
& Reyhaneh Rahnamafar (2014) Highly Selective One-Pot Coupling Reaction of Indole, Aromatic
Aldehydes, and 4-hydroxycoumarin Using Copper Octoate as a Homogeneous Catalyst, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
44:10, 1483-1491, DOI: 10.1080/00397911.2013.862723
To link to this article: http://dx.doi.org/10.1080/00397911.2013.862723
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Synthetic Communications1, 44: 1483–1491, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2013.862723
HIGHLY SELECTIVE ONE-POT COUPLING REACTION
OF INDOLE, AROMATIC ALDEHYDES, AND
4-HYDROXYCOUMARIN USING COPPER
OCTOATE AS A HOMOGENEOUS CATALYST
Farnoush Mousavizadeh, Rahim Hekmatshoar,
Seyed Yahya Shirazi Beheshtiha, and Reyhaneh Rahnamafar
Department of Chemistry, School of Science, Alzahra University, Vanak,
Tehran, Iran
GRAPHICAL ABSTRACT
Abstract
A facile protocol for the one-pot, multicomponent reaction of indole,
4-hydroxycoumarin, and aromatic aldehydes was developed using copper octoate as an
inexpensive, commercially available, and efficient catalyst. This highly selective reaction
eliminates the formation of homodimeric by-products (bisindoles and biscoumarins) and
selectively results in the formation of heterodimeric adducts containing both indole and
coumarin heterocycles.
Keywords Copper octoate; 4-hydroxycoumarin; indole; multicomponent reaction
INTRODUCTION
In the present era of organic synthetic chemistry, convenient and practical
preparation of complex molecules from simple and available starting materials is
one of the most challenging areas. In this context, one-pot, multicomponent coup-
ling reactions are efficient alternatives that introduce several elements of diversity
into a molecule in a single step.[1–6] The resulting reduced number of synthetic and
purification steps, with much less time and cost, are the desired features of an ideal
synthesis.
Indole and coumarin are the most predominant heterocycles in nature.[7–13]
The homodimeric forms of these compounds, bisindoles and biscoumarins, have
been well identified as natural products with various biological activities. For
instance, bis(3-indolyl)-3,4-dihydroxyphenyl methane 1 acts as a HIV-1 integrase
Received June 13, 2013.
Address correspondence to Rahim Hekmatshoar, Department of Chemistry, School of Science,
Alzahra University, Vanak, Tehran, Iran. E-mail: rhekmatus@yahoo.com
1483

1484
F. MOUSAVIZADEH ET AL.
Figure 1. Samples of biologically active bisindole and biscoumarin and their corresponding heterodimer.
inhibitor[14,15] and bis(4-hydroxycoumarin-3-yl)phenylmethane 2 and its derivatives
can exhibit in vitro anti-DNA and RNA virus activity.[16] Recently, a heterodimeric
form containing both indole and coumarin motifs 3 was reported to exhibit substan-
tial antibacterial activity.[17] Interestingly, it is indicated that in some cases heterodi-
meric structures emerge more efficiency than their homodimeric analogs.[18–20]
Although homodimeric compounds can be easily synthesized via Lewis acid–
catalyzed condensation of aldehydes with electron-rich nucleophiles (indole and
4-hydroxycoumarin),[21] the preparation of heterodimers still needs more study and
exploration. One serious challenge that arises in the synthesis of heterodimeric
compounds is the formation of homodimeric by-products. In some of the reported
strategies these by-products are separated by tedious and time-consuming chromato-
graphy.[22,23] Producing a stable intermediate containing one of the nucleophilic aro-
matic moieties and formation of the heterodimeric end product with the nucleophilic
attack of the other aromatic moiety to that intermediate is another synthetic
approach, which usually comprises a multistage procedure.[24–26] Applying specific
reaction conditions such as ultrasonic irradiation,[23] nitrogen atmosphere,[27] and
catalysts with hardly available and expensive ligands[28,29] is the major drawback
observed in some other presented methods. In this article, a one-pot synthesis of com-
pound 4a as a heterodimeric entity, with aldehyde, indole, and 4-hydroxycoumarin
Figure 2.

COUPLING REACTION CATALYZED BY COPPER OCTOATE
1485
Scheme 1. One-pot coupling reaction of 4-hydroxycoumarin, indole, and aromatic aldehydes using copper
octoate as catalyst.
constituents using copper octoate as a very low-cost and readily available catalyst is
presented.
Appendino et al.[18] recommended an elegant procedure utilizing a biphasic
system containing chloroform and water (1:1) in which water was attributed to act
as a promoting agent. However, the reaction time was 48 h, and the products were
isolated by column chromatography. Our strategy for the synthesis of this compound
is via a one-pot multicomponent fashion using Lewis acidic activity of copper octoate
to accelerate the reaction and improve the yields. According to the high nucleophilic
activity of indole and 4-hydroxycoumarin, one-pot application of both nucleophiles is
often restricted by the formation of a multitude of products such as bisindole, biscou-
marin, and the desired crossed adduct. However, by optimizing reaction conditions,
a logically designed one-pot multicomponent reaction resulting selectively in the
formation of heterodimer can be obtained.
Transition metal–catalyzed processes have long been powerful tools for the
development of organic reactions.[30–32] Copper octoate or copper(II) 2-ethylhexano-
ate is the copper salt of 2-ethylhexanoic acid. This compound is a fungicide and bac-
tericide used to control a wide range of planet diseases. Copper carboxylate systems
have been studied as wood preservatives for several years.[33] An outstanding feature
that may distinguish this salt is its high solubility in organic solvents such as xylene
and oils, furnishing it to act as a homogeneous catalyst in organic reactions.[34–36] Cop-
per octoate is commercially available solvated in organic solvents in various concentra-
tions with quite low costs. Because of interest for both academic as well as the
industrial community, it is desirable to expand the application scope of copper octoate
catalyst in organic transformations because of its unique and remarkable advantages.
We report here a practical protocol for the one-pot coupling reaction of indole,
4-hydroxycoumarin, and aromatic aldehydes, using copper octoate as a homo-
geneous catalyst in n-hexane under reflux condition (Scheme 1). Various aromatic
aldehydes bearing electron-donating and electron-withdrawing components have
been applied for this reaction.
RESULTS AND DISCUSSION
The critical factor in driving the reaction specifically toward the synthesis of the
desired heterodimers 4 and minimizing the formation of homodimeric by-products,
bisindole 8 and biscoumarin 9, is the proper selection of the solvent. To study the
solvent effect, the model reaction of benzaldehyde, indole, and 4-hydroxycoumarin
was carried out in a variety of solvents from protic polar solvents such as water, acetic
acid, and ethanol to aprotic polar solvent such as acetonitrile. In addition, this
reaction was performed in nonpolar solvents such as toluene, 1,2-dichloroethane,

1486
F. MOUSAVIZADEH ET AL.
and n-hexane and also in solvent-free conditions. It was obviously indicated that the
selectivity of reaction is extremely associated with the solvent of choice and the best
results are obtained when relatively nonpolar solvents such as n-hexane, toluene, and
1,2-dichloroethane are utilized. The reluctance of nonpolar, solvents to the formation
of biscoumarins 9 can be explained by negligible solubility of 4-hydroxycoumarin in
such solvents, which reduces its high nucleophilic activity and moderates its consecu-
tive nucleophilic attacks. This subsequently prevents the formation of biscoumarins 9
(Scheme 2).
To evaluate the efficiency of the aforementioned nonpolar solvents, the model
reaction was carried out under reflux conditions, loading 10 mol % of copper octoate
as catalyst. As shown in Table 1, entries 1, 2, and 3, maximum yield is achieved in
n-hexane as reaction medium. Indeed, application of this solvent is preferred from
the environmental point of view. To optimize the amount of catalyst, the model
reaction was performed at the same conditions, in the presence of different quantities
of copper octoate. As revealed in Table 1, entries 3–7, addition of small amounts of
catalyst to the reaction mixture accelerates the reaction considerably, with improved
yields of the products. Regarding entry 5, the reaction seems to be very sluggish in the
absence of the catalyst because even after 48 h the outcome of the reaction was only 61
%. One could claim that the relatively poor yield obtained is due to longer reaction
time, which results in decomposition of the product. Nevertheless, tracking the
progress of the reaction by thin-layer chromatography (TLC) in short time intervals
indicates the presence of significant amounts of all three starting materials even after
48 h. This could somehow prove that inactivity of the starting materials, especially
4-hydroxycoumarin as the initiator of the reaction, which is intentionally caused by
chosen reaction medium, is the main reason for poor yields. Moreover, short reaction
Scheme 2. High selectivity of nonpolar solvents for the synthesis of desired heterodimers rather than
homodimeric adducts.

COUPLING REACTION CATALYZED BY COPPER OCTOATE
1487
Table 1. Effect of solvent and catalyst loading on time and yield of the reaction
Entry
Solvent
Amount of catalyst (mol%)
T (ꢀC)
Time (h)
Yield (%)
1
2
3
4
5
6
7
Toluene
10
10
10
0
111
84
68
68
68
68
68
1
1
86
78
88
23
61
59
84
1,2-Dichloroethane
n-Hexane
n-Hexane
1
1
n-Hexane
n-Hexane
n-Hexane
0
48
3
5
15
1
time did not improve the yields, revealing the necessity of applying catalyst. The
optimum amount of catalyst was 10 mol %.
A plausible mechanism of the reaction is presented in Scheme 3. Lewis acid–
catalyzed Knoevenagel condensation of 4-hydroxycoumarin 5 with aldehyde 6
generates the intermediate 10, which undergoes conjugate addition with indole 7
to give the heterodimeric adduct 4. In addition, it is postulated that according to
the nonpolar character of 2-ethylhexanoate groups in catalyst, coordination of the
catalyst to 4-hydroxycoumarin could drag this constituent to the nonpolar phase
in which other starting materials exist and consequently facilitates the progress of
the reaction this way as well.
In addition to the heterodimeric products, formation of trace amounts of bisin-
dole by-product 8 occurred. One pathway for the formation of this by-product could
simply be the coupling reaction of aldehyde with 2 mol of indole without interference
of 4-hydroxycoumarin. Another possible mechanism for this side reaction, which
was suggested by Appendino et al.,[18] is outlined in Scheme 4. Initially, the
Scheme 3. Plausible mechanism for one-pot coupling reaction of indole, 4-hydroxycoumarin, and
aromatic aldehydes utilizing copper octoate as catalyst.

1488
F. MOUSAVIZADEH ET AL.
Scheme 4. Possible mechanism for the formation of bisindole by-products.
4-hydroxycoumarin moiety is eliminated from the heterodimeric adduct 4. The
remaining intermediate 11 suffers a nucleophilic attack by indole 7, giving the
bisindole by-product 8. To investigate the accuracy of the proposed mechanism
for the formation of bisindoles, equimolars amounts of heterodimer 4 and indole
7 were refluxed in n-hexane. After 3–4 h, very slight amounts of 4-hydroxycoumarin
5 and bisindole 8 were detected by thin-layer chromatography. It was observed that
protic solvents such as ethanol enhance this side reaction. This might be due to the
high solubility of 4-hydroxycoumarin 5 in such solvents, which stabilizes this
nucleus and thus makes it a better leaving group. Moreover, it is speculated that
coordination of protic solvents to the oxygen atom of hydroxyl group in
4-hydroxycoumarin enhances the elimination process.
The generality of this method was studied applying various aromatic
aldehydes. The results are collected in Table 2. Aromatic aldehydes with different
substitutions on aromatic ring underwent smooth reaction with indole and
4-hydroxycoumarin, furnishing the respective heterodimers in good yields and
considerable shortened reaction time in comparison with the previous reported
methods. To investigate the function of this method for aliphatic carbonyl
Table 2. Synthesis of (4-hydroxycoumarin-3-yl)(3-indolyl)phenylmethanes using copper octoate as
catalysta
Entry
Ar
Product
Time (h)
Yield (%)b
Mp (ꢀC)
1
2
3
4
5
6
C6H5
4a
4b
4c
4d
4e
4f
1
4
88
86
96
81
95
83
198–199[18]
169–170
207[18]
186–187
190
4-CH3C6H4
4-NO2C6H4
2-OCH3C6H4
4-ClC6H4
45c
2
3
3-BrC6H4
4
206
aReaction conditions: indole (1 mmol), 4-hydroxycoumarin (1 mmol), aldehyde (1 mmol), 6% xylene
solution of copper octoate (0.58 g, 0.1 mmol), n-hexane, reflux.
bYields refer to pure products after recrystallization.
cMin.

COUPLING REACTION CATALYZED BY COPPER OCTOATE
1489
compounds, the reaction was carried out with acetaldehyde and acetone. In the case
of acetone, even after 12 h only trace amounts of the corresponding bisindole was
formed. This result could be caused by the mildness of the reaction conditions.
The same result was obtained for acetaldehyde, while in this case very slight amounts
of other products, which were supposed to be the desired product and biscoumarin,
were detected, which discloses the lower selectivity for this aliphatic aldehyde. On the
other hand, our attempts to perform the reaction with 2-methylindole instead of
indole failed. After 2 h of refluxing the reaction mixture of equimolar amounts of
benzaldehyde, 4-hydroxycoumarin, and 2-methylindole under reaction conditions,
the corresponding bisindole was obtained as the only product. It seems that the
methyl substitution on indole ring makes it exceed the nucleophilic activity of
4-hydroxycoumarin and as a result changes the expected selectivity of the reaction
in favor of the bisindole by-product.
CONCLUSION
In summary, a practical and highly selective alternative for the one-pot multi-
component synthesis of hybridized adducts containing both indole and 4-hydroxy-
coumarin moieties is established with maximum simplicity and brevity. Taking
advantage of copper octoate’s substantial catalytic activity, reaction times could
be significantly reduced and product yields are improved. Different solvents were
studied for this reaction and a pronounced effect was observed: Nonpolar solvents
that are unable to dissolve 4-hydroxycoumarin are superior to provide a selective
transformation.
EXPERIMENTAL
All the chemicals were purchased from Merck and Sigma-Aldrich companies.
Melting points were measured using the capillary tube method with an Electrother-
1
mal 9200 apparatus. H NMR and 13C NMR spectra were recorded on a Bruker
AQS Avance-500-MHz spectrometer, using tetramethylsilane (TMS) as an internal
standard (CDCl3 solution). Infrared (IR) spectra were recorded on the FT-IR
Bruker Tensor 27. Mass spectra were documented on an Agilent Technology (HP)
mass spectrometer operating at an ionization potential of 70 eV. Elemental analysis
(% C, H, N) was carried out on a Perkin-Elmer 2400 CHN elemental analyzer.
To a mixture of 4-hydroxycoumarin (1 mmol, 0.162 g), aromatic aldehyde
(1 mmol), and indole (1 mmol, 0.117 g) in n-hexane (2 mL) was added 0.58 g of a
6% solution of copper octoate in xylene (10 mol%). The mixture was refluxed for
an appropriate time (Table 2). After completion of the reaction, as indicated by
TLC (petroleum ether–ethyl acetate–ethanol, 4:1:1), the solvent was evaporated
and 10 mL of ethanol was added to the residue. The resulting mixture was stirred
for 15 min filtered, and washed with ethanol to give the crude product, which was
further purified by recrystallization from 95% ethanol.
FUNDING
The authors are thankful to Alzahra Research Council for partial financial
support.

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F. MOUSAVIZADEH ET AL.
SUPPLEMENTAL MATERIAL
Supplemental data for this article can be accessed on the publisher’s website.
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