Antimony (III) sulfate catalyzed one-pot synthesis of 2,3- disubstitutedindoles

Article abstract of DOI:10.1080/10426500802388250

A novel one-pot Fischer indole synthesis approach has been developed by using antimony (III) sulfate as the catalyst. Good yields were obtained after reacting phenylhydrazines hydrochlorides and ketones in refluxing methanol. The exclusive formation of 2,3- disubstituted indoles was observed in the reaction of ethyl methyl ketone with phenylhydrazines. One-pot synthesis of indole-3-propanol using dihydropyran has also been described. The use of reusable antimony (III) sulfate as a catalyst makes this method both economically and environmentally friendly.

Full text of DOI:10.1080/10426500802388250

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Antimony (III) Sulfate Catalyzed
One-Pot Synthesis of 2,3-
Disubstitutedindoles
A. Srinivasa a , K. M. Mahadevan a , P. Prabhakara
Varma a & A. Sudhakara a
a Department of Post Graduate Studies and Research
in Chemistry, School of Chemical Sciences, Kuvempu
University, Shankaraghatta, Karnataka, India
Published online: 22 Jun 2009.
To cite this article: A. Srinivasa , K. M. Mahadevan , P. Prabhakara Varma & A.
Sudhakara (2009): Antimony (III) Sulfate Catalyzed One-Pot Synthesis of 2,3-
Disubstitutedindoles, Phosphorus, Sulfur, and Silicon and the Related Elements, 184:7,
1843-1853
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Phosphorus, Sulfur, and Silicon, 184:1843–1853, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802388250
Antimony (III) Sulfate Catalyzed One-Pot Synthesis
of 2,3-Disubstitutedindoles
A. Srinivasa, K. M. Mahadevan, P. Prabhakara Varma,
and A. Sudhakara
Department of Post Graduate Studies and Research in Chemistry,
School of Chemical Sciences, Kuvempu University, Shankaraghatta,
Karnataka, India
A novel one-pot Fischer indole synthesis approach has been developed by using
antimony (III) sulfate as the catalyst. Good yields were obtained after reacting
phenylhydrazines hydrochlorides and ketones in refluxing methanol. The exclusive
formation of 2,3- disubstituted indoles was observed in the reaction of ethyl methyl
ketone with phenylhydrazines. One-pot synthesis of indole-3-propanol using dihy-
dropyran has also been described. The use of reusable antimony (III) sulfate as a
catalyst makes this method both economically and environmentally friendly.
Keywords Antimony (III) sulfate; cyclohexanone; Fischer indole synthesis; one pot;
phenylhydrazine hydrochlorides
INTRODUCTION
Indoles are probably the most widely distributed heterocyclic com-
pounds in nature. Tryptophan-derived substances in the plant kingdom
include indole-3-yl-acetic acid, a plant growth regulator harmone, and
a huge number and structural variety of secondary metabolites are the
indole alkaloids.1 Although many methods have been developed for the
synthesis of indoles,2 Fischer indole synthesis is still one of the most
versatile and widely employed methodologies for the preparation of in-
doles and related biologically active indole derivatives.3 The Fischer
Received 7 January 2008; accepted 5 August 2008.
We are thankful to the Department of Post Graduate Studies and Research in Chem-
istry and Industrial Chemistry, Kuvempu University, Shankaraghatta, for providing
laboratory facilities. The authors also thank the Indian Institute of Science, Bangalore,
for spectral data.
Address correspondence to K. M. Mahadevan, Deptartment of Post Graduate Stud-
ies and Research in Chemistry, School of Chemical Sciences, Kuvempu University,
Shankaraghatta, Karnataka 577 451, India. E-mail: mady kmm@yahoo.co.uk
1843

1844
A. Srinivasa et al.
indole synthesis involves the acid or Lewis acid catalyzed rearrange-
ment of aryl hydrazone with elimination of ammonia via [3, 3] sigma
tropic rearrangement.4 Various catalysts have been used to achieve the
cyclization of aryl hydrazones derived from ketones, including Bronsted
acids (H2SO4, HCl, PPA, AcOH).5 Lewis acids (ZnCl2, TiCl4, PCl5)6 and
solid acids (zeolite, montmorillonite clay)7 have been reported for the
synthesis of the indole nucleus. However, search for a new catalyst is
still being actively pursued, because the existing methods suffer from
at least one drawback. Reported Bronsted and Lewis acids are not
ecofriendly, they are hazardous or difficult to reuse, and they are used
either in equimolar quantity or in large excess. PPA is used in eight- to
ninefold excess by weight, whereas ZnCl2 is used in threefold excess.
Although utilization of solid acids may solve the above problems, solid
acids have their own shortcomings, which limit their application to
some degree, such as restricted accessibility of the matrix bound acidic
sites, rapid deactivation, and decrease of active site per area. Non-
catalytic Fischer indole synthesis has also been studied in different
solvents, such as ethylene glycol, diethyl glycol, and tetralin. However
high temperatures above 200◦C are required for such cyclization.8
The development of a one-pot approach to the Fischer indole synthe-
sis is attractive mostly because of its significance from both commercial
and ecological point of view. In these methods, ketones and phenylhy-
drazines are reacted in the presence of an acid catalyst, resulting in
the formation of hydrazone and subsequent [3 + 3] sigma tropic rear-
rangement, which results in the formation of the indoles. This one-pot
procedure obviates the preparation or isolation of the unstable aryl hy-
drazone. Recently there are reports concerning one-pot Fischer indole
synthesis using Lewis acid ionic liquids such as 1-butyl-pyridinium
chloride.3AlCl3 or choline chloride.2ZnCl2, respectively.9 In spite of di-
verse synthetic approaches developed so far for the indole synthesis,
there is still a need for the development of an environmentally and
economically friendly catalyst. We thought that one-pot Fisher indole
synthesis promoted by an environmentally friendly catalyst seems to
address this issue. This can be achieved by the use of insoluble soft
Lewis acid that is stable to water and alcohols. Recently, we have de-
scribed the synthetic utility of Sb2 (SO4)3 as a catalyst in the synthesis
of bis-indoles.10 The Sb2(SO4)3 acid is an inexpensive, stable solid, and
it is easily available. Sb2 (SO4)3is easier to handle than halides, such
as ZnCl2,TiCl4, PCl5, and protic acids such as PPAand HCl. In this
communication, we report the synthesis of 2,3-disustituted indoles via
one-pot Fischer indole reaction using Sb2(SO4)3 as a reusable catalyst.
So far, to the best of our knowledge, Sb2(SO4)3 has not been used in
Fischer indole synthesis.

One-Pot Synthesis of 2,3-Disubstitutedindoles
1845
O
Sb2(SO4)3
+
MeOH, reflux
N
H
NHNH .HCl
2
3a
1a
2a
SCHEME 1 Screening of the reaction conditions for the synthesis of 3a.
RESULTS AND DISCUSSION
To begin laying the foundation for exploring the catalytic activity of Sb2
(SO4)3 in one-pot Fischer indole synthesis, we chose phenylhydrazine
hydrochloride 1a and cyclohexanone 2a. When the mixture of phenyl-
hydrazine hydrochloride and cyclohexanone was refluxed for 30 min in
MeOH in the presence of a catalytic amount of antimony (III) sulfate
(Scheme 1), tetrahydrocarbazole was formed. Encouraged by this re-
sult, we undertook the detailed study of this reaction. Thus initially, the
one-pot reaction of phenylhydrazine hydrochloride with cyclohexanone
was carried out as a model reaction in various solvents to investigate
the solvent effect. The results are summarized in Table I. Methanol,
ethanol, and acetonitrile were found to be better solvents for this trans-
formation. However, the best results were achieved by carrying out the
reaction in MeOH at reflux temperature afforded tetrahydrocarbazoles
TABLE I Effect of Solvents in the Synthesis of Tetrahydrocarbazolesa
b
Entry
Solvent
Sb (SO ) (mol%)
Time (min)
Yields (%)
2
4 3
1
2
3
4
5
6
7
8
9
MeOH
MeOH
MeOH
EtOH
CH3CN
THF
CH2Cl2
EtOAc
Toluene
MeOH
MeOH
5
10
15
10
10
10
10
10
10
10
0
90
40
40
55
65
80
95
91
92
82
60
48
52
70
240
125
75
40
240
55
10
11
95, 88, 85
15
aAll reactions were carried out at reflux temperature.
bIsolated yields.

1846
A. Srinivasa et al.
in 95% yield, respectively. Furthermore, we set out to establish the
optimal amount of Sb2(SO4)3; the reaction with a 5mol% catalyst load-
ing gave a 80% yield after 90 min (entry 1), whereas the best result
(95%) was obtained with 10mol% Sb2(SO4)3 (entry 2) after 40 min. Fur-
thermore, when the reaction was carried out in absence of Sb2(SO4)3,
formation of tetrahydrocarbazoles was less (15%) after 240 minutes
(entry 11). Another advantage of the use of Sb2(SO4)3 was that it could
be easily recovered and recycled in subsequent reactions without signif-
icant decrease in the catalytic activity. Sb2(SO4)3 was easily separated
from the reaction mixture by simple extraction and filtration. The cata-
lyst could be recycled three times with out obvious loss of activity (entry
10: 95%, 1st run; 88%, 2nd run; 85% 3rd run.).
In order to study the generality of this process, it was decided to react
a wide variety of functionalized phenylhydrazine hydrochlorides with
different ketones (Scheme 2). The results are summarized in Table II.
These results demonstrate that this one-pot indole synthesis is quite
general and can accept a wide variety of substitution both on phenyl
hydrazine backbone as well as carbonyl component.
An important issue of the Fischer indole synthesis in the cycliza-
tion of the phenyl hydrazones derived from unsymmetrical ketones is
that the direction of cyclization is governed by the acidity of the reac-
tion medium.11 The lower acid concentration or weaker acid promote
cyclization towards the more branched carbon, and higher acid concen-
trations or stronger acid enhance the extent of cyclization at the less
branched positions. As Sb2(SO4)3 provides a weaker acid system, di-
rection of cyclization occurred to produce more branched carbon during
indole synthesis.
Recently Kevin et al. reported the one-pot synthesis of
indoles from cyclic enol ethers in H2SO4 and acetonitrile/
N,N-dimethylacetamide as cosolvent. Anticipating similar results, we
tried one-pot synthesis of indole-3-propanol derivatives 4 by using
R2
R
R
R2
Sb2(SO4)3
R1
+
O
MeOH, reflux
N
H
R1
NHNH2.HCl
1
3
2
SCHEME 2 Synthesis of 2,3-disubstituded indoles catalyzed by Sb2(SO4)3.

One-Pot Synthesis of 2,3-Disubstitutedindoles
1847
TABLE II Synthesis of 2,3-Disubstituted Indolesa
Entry
a
R
Ketones
Product
Time (min) Yields (%)b
4-H
40
95
b
4-F
50
90
c
d
e
f
4-CH3
4-OCH3
2-CH3
2,5-Cl
45
20
88
88
70
70
45
130
g
4-H
4-F
50
80
95
90
h
(Continued on next page)

1848
A. Srinivasa et al.
TABLE II Synthesis of 2,3-Disubstituted Indolesa (Continued)
Entry
i
R
Ketones
Product
Time (min)
45
Yields (%)b
4-CH3
88
j
4-OCH3
2,5-Cl2
20
90
70
k
140
l
2-NO2
150
45
70
80
m
CH3
aReaction was carried out at reflux temperature in MeOH in presence of 10 mol%
of Sb2(SO4)3.
bIsolated yields.
various phenyl hydrazine hydrochlorides and dihydropyran (Scheme
3); several representative results are summarized in the Table III. In all
cases, the reaction of phenylhydrazine hydrochloride with dihydropy-
ran proceeds smoothly at reflux temperature in methanol to produce
the corresponding indole-3-propanol in good yield in relatively shorter
reaction time.
In conclusion, we have demonstrated that Sb2(SO4)3 as an efficient
catalyst for the one-pot Fisher indole synthesis. The synthesis of di-
versely substituted 2,3-disubstituted indoles has shown the wide scope
of this reaction. The products are obtained in high yield and good purity.

One-Pot Synthesis of 2,3-Disubstitutedindoles
1849
R
R
O
Sb2(SO4)3
HO
+
MeOH, reflux
N
H
NHNH2.HCl
4
1
SCHEME 3 Synthesis of indoles-3-propanols catalyzed by Sb2(SO4)3.
In unsymmetrical case, regiospecific formation of a single product with
more substituents in the indole ring is obtained, which indicates that
the antimony (III) sulfate provides a weaker acid system for the syn-
thesis of indoles and has a high selectivity as catalyst. The present
protocol describes simple isolation and environmentally benign pro-
cess. The developed method will open new opportunities in the design
and synthesis of wide variety of indoles, a structural sub unit of many
natural products.
EXPERIMENTAL
All the melting points were recorded in open capillary. The purity of the
compounds was checked by TLC on silica gel, and the compounds were
purified by column chromatography. 1H NMR spectra were recorded on
a Bruker-400 Hz spectrometer using TMS as an internal standard. IR
spectra were obtained using a FTS-135 spectrometer instrument. Mass
spectra were recorded on a JEOL SX 102/DA-6000 (10 kV) FAB mass
spectrometer. The compounds (entries a, e, f, g, k, and m in Table I and
entries a–e in Table II) are known, and their identities were proven by
means of melting point, IR, 1H NMR, and mass spectra. Herein we give
melting points and spectral data for compounds (entries b, c, d, h, i, j,
and l in Table I), which could not be found in the literature.
General Procedure for the Synthesis of Indoles
The phenylhydrazine hydrochloride 2.0 g (0.013 mol) and cyclohex-
anone 1.36 g (0.013 mol) were dissolved in MeOH (20 mL). 20 mol%
of Sb2(SO4)3 was added to the reaction mixture and refluxed on wa-
ter bath for the appropriate time (Table II). After the completion of
the reaction as indicated by TLC, the reaction mixture was cooled to
room temperature and filtered into water (100 mL), and the crude
product was extracted with ethyl acetate (2 × 50 mL). The combined

1850
A. Srinivasa et al.
TABLE III Synthesis of Indole-3-propanola
Entry
a
R
Product
Time (min)
40
Yields (%)b
92
H
b
c
4-F
50
45
20
45
88
92
94
88
4-CH3
4-OCH3
2-CH3
d
e
aReaction was carried out at reflux temperature in MeOH in the presence of 10
mol% of Sb2(SO4)3.
bIsolated yields.
ethyl acetate extracts were washed with water, dried over Na2SO4, and
evaporated under reduced pressure to provide a crude brown solid. The
solid thus obtained was further purified by column chromatography.
All other compounds were similarly prepared.
6-Fluoro-2,3,4,9-tetrahydro-1H-carbazole (b: C12H12FN)
◦
1
Crystalline solid, Mp: 93–95 C; IR (KBr): V = 3386 (NH) cm−1; H
NMR (DMSO):δ = 10.7 (br s, NH ), 7.2 (dt, 1H,J = 4.64), 7.07 (dt, 1H,
J = 2.32), 6.8 (s, 1H ), 2.55–2.72 (m, 4H ), 1.74–1.88 (m, 4H ); IR 3395;
13C NMR 154.3, 134.5, 132.5, 130.8, 112.5,112.6, 107, 106.6, 35.6, 35,
¯

One-Pot Synthesis of 2,3-Disubstitutedindoles
1851
26.2, 24.5. HRMS: (m/z) = 190.2 (M+1). Anal. Calcd: C (76.17%) H
(6.39%) N (7.40%) Found C (77.19%) H (6.45%) N (7.50%).
6-Methyl-2, 3, 4, 9-tetrahydro-1H-carbazole (c: C13H15N)
◦
Crystalline solid, Mp: 98–100 C; IR (KBr): V = 3394 (NH) cm−1; 1H
NMR (DMSO):δ = 10.4 (br s, NH ), 7.1 (dt, 1H, J = 8.6 ), 6.8 (s, 1H ), 6.6
(dt, 1H, J = 2.08), 2.54–2.71 (m, 4H), 2.34 (s, 3H), 1.77–1.96 (m, 4H);
IR 3399; 13C NMR 135, 133.2, 131.1, 130.9, 121.2, 120, 112.5, 110.5,
35.3, 35.5, 26.5, 25.2, 21.2. HRMS: (m/z) = 186.2 (M+1). Anal. Calcd:
C (84.28%) H (8.16%) N (7.56%). Found C (85.30%) H (9.16) N (7.60%).
¯
6-Methoxy-2, 3, 4, 9-tetrahydro-1H-carbazole (d: C13H15ON)
◦
1
Crystalline solid, Mp: 88–90 C; IR (KBr): V = 3381 (NH) cm−1; H
NMR (DMSO):δ = 10.4 (br s, NH), 7.1 (dt, 1H, J = 8.6), 6.8 (s, 1H), 6.6
(dt, 1H, J = 2.08), 3.7 (s, 3H), 2.56–2.74 (m, 4H ), 1.74–1.94 (m, 4H);
IR 3387; 13C NMR 155.2, 135.3, 132.1, 128.2, 112.5, 112.0, 106, 105.2,
55.9, 35.5, 35.3, 26.5, 25.1. HRMS: (m/z) = 202 (M+1). Anal. Calcd: C
(77.58%) H (7.51%) N (6.96%). Found C (77.64%) H (8.55%) N (7.01%).
¯
5-Fluoro-2,3-dimethyl-1H-indole (h: C10H10FN)
◦
1
Crystalline solid, Mp: 60–61 C; IR (KBr): V = 3364 (NH) cm−1; H
NMR (CDCl3):δ = 7.6 (br s, NH), 7.1 (dd, 2H, J = 4.38), 6.8 (t, 1H, J =
2.41), 2.3 (s 3H ), 2.1(s, 3H); IR 3394; 13C NMR 155.2, 135.3, 133.1,
131.5, 112.6, 112.5, 107.4,106.5, 8.2, 6.7. HRMS: (m/z) = 164.1 (M+1).
Anal. Calcd: C (73.60%) H (6.18%) N (8.58%). Found C (74.50%) H
(6.20%) N (8.58%).
¯
2,3,5-Trimethyl-1H-indole (i:◦C11H13N)
1
Crystalline solid, Mp: 98–99 C; IR (KBr): V = 3388 (NH) cm−1; H
¯
NMR (DMSO):δ = 10.4 (br s,1H, NH), 7.1 (dd, 2H, J = 8.9 ), 6.7 (d, 1H,
J = 8.08), 2.3 (s, 3H ), 2.2 (s, 3H), 2.1 (s, 3H); IR 3376; 13C NMR 135.2,
133.1, 131.2, 130.5, 121.1, 120.1,112.5,110.6, 21.2, 8.1, 6.6. HRMS: (m/z)
= 160 (M+1). Anal. Calcd: C (82.97 %) H (8.23%) N (8.80%). Found C
(83.01%) H (8.37) N (9.10%).
5-Methoxy-2,3-dimethyl-1H-indole (j: C11H13ON)
◦
1
Crystalline solid, Mp: 60–61 C; IR (KBr): V = 3345 (NH) cm−1; H
NMR (DMSO):δ = 10.4 (br s, NH), 7.1 (d, 1H, J = 8.6), 6.8 (s, 1H), 6.6
(d, 1H, J = 2.24), 3.7(s, 3H, -OCH3), 2.2 (s, 3H), 2.1 (s, 3H); IR 3382;
13C NMR 155.2, 135.2, 132.3, 128.5, 112.5, 112.0, 106.1, 105.1, 56, 8.2,
6.5. HRMS: (m/z) = 176.2 (M+1). Anal. Calcd: C (75.40 %) H (7.48%) N
(7.99%). Found C (76.60%) H (7.64) N (8.0%).
¯

1852
A. Srinivasa et al.
7-Nitro-2,3-dimethyl-1H-indole (l: C10H10O2N2)
◦
1
Crystalline solid, Mp: 95–96 C; IR (KBr): V = 3378 (NH) cm−1; H
NMR (CDCl3):δ = 7.6 (br s,NH), 7.5 (d, 1H, J = 7.6), 7.4 (d, 1H, J = 8.2),
7.3 (t, 1H, J = 8.0 ), 2.3 (s, 3H), 2.2 (s, 3H); IR 3368; 13C NMR 135.2,
132.4, 131.2, 130.9, 126.5, 122.6, 114.5, 112.7, 8.2, 6.7. HRMS: (m/z) =
191 (M+1). Anal. Calcd: C (63.15 %) H (5.30%) N (14.73%). Found C
(64.24 %) H (6.10%) N (14.96%).
¯
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