KHSO4-assisted michael addition-elimination reactions of indole with 3-dimethylamino-1-phenylprop-2-en-1-ones in water: An environmentally friendly synthesis of novel 3-indolylchalcones

Article abstract of DOI:10.1080/00397911.2012.658946

KHSO₄-assisted Michael addition-elimination reactions of 2-methylindoles and 1,2-dimethylindoles with 3-dimethylamino-1-phenylprop-2-en- 1-ones (2) in water, leading to the formation of 3-(2-methyl-1H-indol-3-yl)-1- arylpropenones (3a-e) and 3-

Full text of DOI:10.1080/00397911.2012.658946

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KHSO4-Assisted Michael
Addition–Elimination Reactions of
Indole with 3-Dimethylamino-1-
phenylprop-2-en-1-ones in Water: An
Environmentally Friendly Synthesis of
Novel 3-Indolylchalcones
A. Satyapati Devi a , P. Helissey b , R. L. Nongkhlaw c & Jai N.
Vishwakarma a d
a Organic Research Lab., Department of Chemistry, St. Anthony's
College, Shillong, India
b Laboratoire de Chimie Thérapeutique, UMR CNRS No. 8638,
Université Rene Descartes, Faculte des Sciences Pharmaceutiques et
Biologiques, Paris, France
c Department of Chemistry, North Eastern Hill University, Shillong,
India
d Organic Research Lab., Department of Chemical Sciences, Don
Bosco University, Azara, Guwahati, India
Accepted author version posted online: 08 Aug 2012.Version of
record first published: 06 Mar 2013.
To cite this article: A. Satyapati Devi , P. Helissey , R. L. Nongkhlaw & Jai N. Vishwakarma
(2013): KHSO4-Assisted Michael Addition–Elimination Reactions of Indole with 3-Dimethylamino-1-
phenylprop-2-en-1-ones in Water: An Environmentally Friendly Synthesis of Novel 3-Indolylchalcones,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic
Chemistry, 43:12, 1653-1660
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Synthetic Communications1, 43: 1653–1660, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.658946
KHSO4-ASSISTED MICHAEL ADDITION–ELIMINATION
REACTIONS OF INDOLE WITH 3-DIMETHYLAMINO-
1-PHENYLPROP-2-EN-1-ONES IN WATER: AN
ENVIRONMENTALLY FRIENDLY SYNTHESIS
OF NOVEL 3-INDOLYLCHALCONES
A. Satyapati Devi,1 P. Helissey,2 R. L. Nongkhlaw,3 and
Jai N. Vishwakarma1,4
1Organic Research Lab., Department of Chemistry,
St. Anthony’s College, Shillong, India
2
´
´
Laboratoire de Chimie Therapeutique, UMR CNRS No. 8638, Universite
Rene Descartes, Faculte des Sciences Pharmaceutiques et Biologiques,
Paris, France
3Department of Chemistry, North Eastern Hill University, Shillong, India
4Organic Research Lab., Department of Chemical Sciences,
Don Bosco University, Azara, Guwahati, India
GRAPHICAL ABSTRACT
Abstract KHSO4-assisted Michael addition–elimination reactions of 2-methylindoles and
1,2-dimethylindoles with 3-dimethylamino-1-phenylprop-2-en-1-ones (2) in water, leading
to the formation of 3-(2-methyl-1H-indol-3-yl)-1-arylpropenones (3a–e) and 3-(1,2-
dimethy-1H-lindol-3-yl)-1-arylpropenones (3f–j) respectively in good to excellent yields
have been reported. However, cyclodehydration of adducts 3a–f to give carbazoles 4 failed
to take place under the reaction conditions.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords Enaminones; indoles; KHSO4; Michael addition–elimination reactions
Received December 21, 2011.
Dedicated to Rev. Fr. Dr. Stephen Mavely, SDB, on his 60th birthday.
Address correspondence to Jai N. Vishwakarma, Organic Chemistry Laboratory, Department of
Chemistry, St. Anthony’s College, Shillong 793001, India. E-mail: jnvishwakarma@rediffmail.com
1653

1654
A. S. DEVI ET AL.
INTRODUCTION
Water plays an essential role in life processes; however, its use as a solvent has
been limited in organic synthesis. Despite the fact that it is the cheapest, safest, and
most nontoxic solvent in the world, its presence is generally avoided through the
dehydrative drying of substrates and solvents. The use of water as a medium for
organic reactions is therefore one of the latest challenges for modern organic che-
mists. An excellent review about stereoselective organic reactions in water has been
recently published.[1] A number of reports on the use of water as solvent for organic
reactions have appeared in the recent past.[2–16]
The indole scaffold is a prominent and privileged structural motif found in
numerous natural products and various synthetic compounds. Recently, a number
of indole-containing compounds have revealed remarkable pharmacological activity
and their utility as therapeutic agents has attracted considerable attention from
chemists.[17,18] Libraries based on the indole scaffold have been developed to address
the need for novel drugs with increased potency.[19,20] Subsequently, the develop-
ment of efficient methods that allow rapid access to functionalized indoles with
different substitution patterns (at C-2, C-3, N-atom, and aromatic ring) constitutes
an emerging area.[21]
Michael addition of indoles to electron-deficient olefins has been promoted by
several Lewis acids such as Yb(OTf)3 ꢀ 3H2O,[22] zirconium triflate,[23] samarium
triiodide,[24] and CeCl3 ꢀ 7H2O-NaI.[25] Catalytic asymmetric Lewis acid–mediated
addition reactions with indoles have been reported.[26,27] However, the majority of
Lewis acids cannot play their desired role in water because of the basic character
of water molecules. Recently, aluminiumdodecylsulfate[28] and KHSO4
been shown to substitute for Lewis acids in water.
[29,30]
have
Our literature survey at this stage revealed that the reactions of indole with
enaminones of type 2 have not been reported in the literature to the best of our
knowledge, and hence as a part of our interest in the synthetic applications[31–33]
of enaminones 2,[34] we report herein the results of our studies on the reactions of
indoles with enaminones 2 assisted by KHSO4 in water, envisaging that addition–
elimination product 3 formed in the case of 2-methylindoles could undergo cyclo-
dehydration under the reaction conditions, leading to carbazoles 4.
RESULTS AND DISCUSSION
The reaction of unsubstituted indole (1a) was taken up as a test case to evaluate
the feasibility of addition and the optimum reaction conditions. The addition–
elimination reaction of 1a with 2a assisted by KHSO4 in water showed encouraging
signs as an oily product started floating on water soon after starting the reaction.
After the disappearance of the staring material (monitored by thin-layer chromato-
graphy, TLC), the crude product obtained was found to be a complex mixture from
which no product could be isolated. All attempts to arrive at isolable products by
varying the reaction conditions failed. We then turned our attention to the reaction
of N-methylindole 1b with 2a under similar conditions. In this case, too, the result
was equally discouraging as no product could be isolated from the complex reaction
mixture formed.

MICHAEL ADDITION–ELIMINATION REACTIONS
1655
Scheme 1. Reaction of indole with enaminone.
The reaction of 2-methylindole (1c) with 2a was then undertaken under similar
conditions. Thus, when an equimolar mixture of 2-methylindole (1c) and 2a was
treated with KHSO4 in water, a solid product could be isolated in 81% yield, which
was characterized as (E)-3-(2-methyl-1H-indol-3-yl)-1-phenylpropenone (3a) on the
basis of spectral and analytical data. The reaction 1c with enaminones 2b–e was
found to follow similar trend, giving adducts 3b–e in 86–88% overall yields. The
structures of 3a–e were established with the help of spectral and analytical data
(Scheme 1, Table 1). Thus, the infrared (IR) spectra showed strong peaks around
1
3200 cmꢁ1 and 1640 cmꢁ1 for NH and carbonyl groups respectively. The H NMR
spectra exhibited singlet in the vicinity of 8.6 ppm due to the N-H proton. The
methyl protons at C-2 of indole resonated between 2.45 and 2.60 ppm. The signals
due to vinylic protons appeared as doublets near 7.60 and 8.20 ppm with coupling
constants close to 15 Hz, thereby confirming the trans geometry of the product
(Fig. 1). We expected 3a to undergo intramolecular cyclodehydration giving 4a
Table 1. Synthesis of 3-(2-methyl-1H-indol-3-yl)-1-arylpropenones and 3-(1,2-dimethy-1H-lindol-3-yl)-
1-arylpropenones
Product
R
R1
Ar
C6H5
Time (h)
Yield (%)
Mp (ꢂC)
3a
3b
3c
3d
3e
3f
H
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
5
81
88
88
86
86
90
80
80
90
90
183 (lit.[35] 183–184)
H
4-MeC6H4
4-ClC6H4
4-O2NC6H4
4-MeOC6H4
C6H5
3
210
215
237
H
5
H
3
H
3.5
5
170 (lit.[36]a
115
139
)
Me
Me
Me
Me
Me
3g
3h
3i
4-MeC6H4
4-ClC6H4
4-O2NC6H4
4-MeOC6H4
4.5
3
158
180–181
158–160
4
4.5
3j
aMelting point not reported.

1656
A. S. DEVI ET AL.
Figure 1. Stereochemistry of indolylchalcone.
under the reaction conditions but it failed, probably because of the presence of the
enolizable NH proton.
At this stage, it was envisaged that if a methyl group replaces the NH proton,
the protons of the 2-CH3 group will attain a good degree of acidity and hence the
chance of cyclodehydration leading to carbazole would be increased. Thus, when
1,2-dimethylindole (1d) was reacted with 2a in the presence of KHSO4 in water,
the product isolated (90%) was found to be the usual addition–elimination product
(3f) and no trace of the envisaged carbazole was formed. The reactions of 1d with
other enaminones 2b–e were found to be equally facile under identical conditions,
giving the corresponding addition-elimination products 3g–j in 80–90% overall
Scheme 2. Plausible mechanism for the reaction of indole with enaminone.

MICHAEL ADDITION–ELIMINATION REACTIONS
1657
yields. The structures of 3f–j were also established with the help of spectral and ana-
lytical data. Our attempts, however, to cyclodehydrate 3 to yield the corresponding
carbazoles 4 under various conditions are in progress. A plausible mechanism for the
formation of 3a–j has been rationalized in Scheme 2.
In summary, we have developed a facile and environmentally friendly strategy
for the Michael addition–elimination reaction of indoles with formylated acetophe-
nones in water as solvent. This methodology eliminates the use of hazardous organic
solvents and involves a simple workup and isolation procedure.
EXPERIMENTAL
Melting points were recorded by the open capillary method and are uncor-
rected. The IR spectra were recorded on a BOMEM DA-8 FTIR instrument.
1
High-resolution H NMR (300 MHz), 13C NMR (75 MHz), and two-dimensional
(2D) NMR 1H-1H correlation spectrometry (COSY) and 1H-13C heteronuclear
single quantum coherence (HSQC) spectra were recorded on Bruker ACF-300
spectrometer. The chemical shifts (d ppm) and the coupling constants (Hz) are
reported in the standard fashion with reference to tetramethylsilane (TMS) as
internal reference. Fast atom bombardment (FAB)–mass spectra (MS) were
measured on a Jeol 3SX102=DA-6000 mass spectrometer using argon as the FAB
gas and m-nitrobenzylalcohol as the matrix. Elemental analyses were performed
on a Heraeus CHN-O-Rapid analyzer. Formylated acetophenones 2 were synthe-
sized by our previously reported procedure.[34]
Reaction of 2-Methyl/1,2-dimethylindole with 3-Dimethylamino-
1-arylpropenone
KHSO4 (2 mmol) was added to a mixture of indole 1c or 1d (1 mmol) and
enaminone 2 (1 mmol) suspended in water (4 mL), and the resulting mixture was
stirred at 50 ꢂC for 3–5 h. After the completion of the reaction (monitored by
TLC), the oily product formed was extracted with dichloromethane (3 ꢃ 2 mL).
The combined organic extract was washed with water (3 ꢃ 2 mL) and dried (Na2SO4),
and the solvent was distilled off to give a viscous mass, which on trituration with
hexane yielded a practically pure product. Further purification for analytical purposes
was achieved by column chromatography over silica gel using 10% EtOAc–hexane
for elution. Spectral and analytical data of two representative products are presented
and the rest are given in the Supplementary Information, available online.
(E)-3-(2-Methyl-1H-indol-3-yl)-1-phenylpropenone (3a)
1
Pale yellow solid, mp 183 ꢂC; IR (KBr): 3264, 1636, 1578 cmꢁ1; H NMR
(CDCl3): d 2.61 (s, 3H, CH3), 7.28 (m, 2H, 2H-indole), 7.38 (m, 1H, H-indole),
7.57 (m, 3H, 3H-phenyl), 7.62 (d, 1H, J ¼ 15 Hz, Ha), 7.99 (m, 1H, H-indole), 8.10
(m, 2H, 2H-phenyl), 8.20 (d, 1H, J ¼ 15 Hz, Hb), 8.76 (s, 1H, NH);13C NMR
(CDCl3): d 12.4 (CH3), 110.7 (Cq), 111.2 (CH-indole), 116.4 (CH-a), 120.3
(CH-indole), 121.7 (CH-indole), 122.7 (CH-indole), 126.3 (Cq), 128.3 (2 CH-phenyl),
128.5 (2 CH-phenyl), 132.2 (CH-phenyl), 135.9 (Cq), 138.3 (CH-b), 139.3 (Cq), 142.2

1658
A. S. DEVI ET AL.
(Cq), 190.9 (CO); MS: m=z 262 (MHþ). Anal. calcd. for C18H15NO: C, 82.73; H,
5.79; N, 5.36. Found: C, 82.62; H, 5.82; N, 5.34%.
(E)-3-(1,2-Dimethyl-1H-indol-3-yl)-1-phenylpropenone (3f)
Yellow solid, mp 115 ꢂC; IR (KBr): 1643, 1556 cmꢁ1; 1H NMR (CDCl3): d 2.62
(s, 3H, CH3), 3.75 (s, 3H, CH3), 7.33 (m, 3H, 3H-indole), 7.53 (m, 3H, 3H-phenyl),
7.62 (d, 1H, J ¼ 15 Hz, Ha), 8.02 (m, 1H, H-indole), 8.09 (m, 2H, 2H-phenyl), 8.25
(d, 1H, J ¼ 15 Hz, Hb); 13C NMR (CDCl3): d 10.9 (CH3), 30.1 (CH3), 109.6
(CH-indole), 110.2 (Cq), 115.8 (CH-a), 120.4 (CH-indole), 121.7 (CH-indole),
122.4 (CH-indole), 125.7 (Cq), 128.2 (2 CH-phenyl), 128.5 (2 CH-phenyl), 132.0
(CH-phenyl), 137.8 (Cq), 138.3 (CH-b), 139.43 (Cq), 143.7 (Cq), 190.7 (CO); MS:
m=z 276 (MHþ). Anal. calcd. for C19H17NO: C, 82.88; H, 6.22; N, 5.09. Found:
C, 82.71; H, 6.19; N, 5.13%.
ACKNOWLEDGMENTS
The authors thank I. Warpakma, principal of the college, for research facilities
and J. Nallanatt for encouragement during the course of this investigation. Thanks
are also due to B. Koikara for his help. The authors express their gratitude to the
University Grants Commission (UGC), New Delhi, for financial assistance and
SAIF-NEHU, Shillong, for spectral and analytical data. A.S.D. thanks the UGC
for a research fellowship.
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