Bismuth nitrate promoted fischer indole synthesis: A simple and convenient approach for the synthesis of alkyl indoles

Article abstract of DOI:10.2174/157017809787582735

A novel one-pot fisher indole synthesis approach has been developed by using bismuth nitrate as a catalyst. Yields around 90-95% were obtained after reaction in methanol at reflux temperature in 20-40 min. Apart from the mild reaction conditions of the process and its excellent results, the simplicity of product isolation and the possibility to recycle the bismuth nitrate offers a significant advantage.

Full text of DOI:10.2174/157017809787582735

Letters in Organic Chemistry, 2009, 6, 159-164
159
Bismuth Nitrate Promoted Fischer Indole Synthesis: A Simple and
Convenient Approach for the Synthesis of Alkyl Indoles
Aralihalli Sudhakaraa, Honnali Jayadevappab, Hosanagara N.H. Kumarc and Kittappa M.
Mahadevan*,c
aDept of Post Graduate Studies and Research in Ind-Chemistry, School of Chemical Sciences, Kuvempu University,
Shankaraghatta, Karnataka-577 451, India
bDept of Chemistry, Sahyadri Science College (Autonomous), Shimoga-577203, Karnataka, India
cDept of Post Graduate Studies and Research in Chemistry, School of Chemical Sciences, Kuvempu University,
Shankaraghatta, Karnataka- 577 451, India
Received November 12, 2007: Revised November 12, 2008: Accepted November 12, 2008
Abstract: A novel one-pot fisher indole synthesis approach has been developed by using bismuth nitrate as a catalyst.
Yields around 90-95% were obtained after reaction in methanol at reflux temperature in 20-40 min. Apart from the mild
reaction conditions of the process and its excellent results, the simplicity of product isolation and the possibility to recycle
the bismuth nitrate offers a significant advantage.
Keywords: One pot, Fischer indole synthesis, bismuth nitrate, phenyl hydrazine hydrochloride, cyclohexanone, ethyl methyl
ketone.
1. INTRODUCTION
ceutical and biological importance of tetrahydrocarbazoles
and 2, 3-disubstituted alkyl indoles, we were intrigued with
the possibility of using bismuth nitrate as a catalyst because
of its well documented success in many organic reactions
[18]. So far to the best of our knowledge bismuth nitrate has
not been used to accomplish Fischer indole synthesis. In this
context, while tracing the potential general methods for the
synthesis of carbazoles and 2, 3-dialkyl indoles, bismuth
nitrate was found to be an efficient catalyst for such synthe-
sis, as a mild, inexpensive, and moisture-stable Lewis acid.
The use of bismuth nitrate (Bi (NO3)3 5H2O) offers several
significant advantages such as low cost, greater selectivity,
and easy isolation of products.
Indoles are probably the most widely distributed hetero-
cyclic compounds in nature. The Tryptophans which are
derived from 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 indole [2], Fischer indole synthesis is
still one of the most versatile and widely employed method-
ologies for the preparation of indole intermediates and bio-
logically active compounds [3]. Fischer indole synthesis has
been used to construct numerous carbazoles which include
simple carbozolesalkaloids [4] rutaecarpinelogs [5, 6] bis-
carbazolealkloids [7] Benzoindoloquinnes [8] thiazolocarba-
zoles [9] and thieno carbazolesoles [10] respectively. Fischer
indole sequence has been used on an industrial scale in the
manufacture of a pharmaceutical intermediate to prepare
pyrrolo [2,3-d] Pyrimidines as potential new thymidylate
synthase inhibitors [11-14]. Few practical and mild proce-
dures are available for construction of 2,3-disubstituted in-
doles [15]. Although Fischer indole synthesis is a clear vehi-
cle for the synthesis of indoles, there has never been an effi-
cient, general method for this reaction, because of the use of
corrosive Brønsted [16] and moisture sensitive Lewis acids
[17], these acids are hazardous and difficult for reuse. In
spite of diverse synthetic approaches developed so far, we
thought it is perhaps hoped that environmental friendly one-
pot options seems possible with the use of soft Lewis acid
which is stable in alcohols. Hence, keeping in view pharma-
2. EXPERIMENTAL SECTION
Products were identified by their physical and spectro-
scopic data all the melting points were recorded in open cap-
illaries. The purity of the compounds was checked by TLC
1
on silica gel. H NMR spectra were recorded on a Bruker-
400Hz spectrometer using DMSO-d6 and CDCl3 as an inter-
nal standard. IR spectra were obtained using a FTS-135
spectrometer instrument. Mass spectra were recorded on a
JEOL SX 102=DA-6000 (10kV) FAB mass spectrometer.
Solvents, Chemicals and reagents were purchased from
Merck chemical company in high-grade quality. The com-
pounds (entries a,b,f,g,h Table 2) and (a, e, g, h in Table 3)
are known, their identities was proven by means of melting
point, IR, 1H NMR, and MASS spectra [20].
2.1. General Procedure for the Synthesis of Tetrahydro-
carbazoles and 2,3-disustituted Indoles
*Address correspondence to this author at the Dept of Post Graduate Studies
and Research in Chemistry, School of Chemical Science, Kuvempu Univer-
sity, Shankaraghatta, Karnataka-577 451, India;
Tel: +91-9448207909; Fax: 91-08282 256255;
E-mail: mady_kmm@yahoo.co.uk
The phenyl hydrazine hydrochloride [1gm (6.9156
mmol)] and ethyl methyl ketone [0.49gm, (6.9065 mmol)] or
1570-1786/09 $55.00+.00
© 2009 Bentham Science Publishers Ltd.

160 Letters in Organic Chemistry, 2009, Vol. 6, No. 2
Sudhakara et al.
cyclohexanone [0.67gm, (6.836 mmol)] were dissolved
methanol (20 mL), Bi (NO3)3 5H2O [(20 mole %) 0.6701
gm], was added to the reaction mixture and refluxed on wa-
ter bath for an appropriate time, after the completion of the
reaction was indicated by TLC, the reaction mixture was
poured into water (100 ml) and crude product was extracted
with ethyl acetate washed with NaHCO3, brined, dried and
evaporated under reduced pressure to provide a crude solid.
The solid was further recrystallised in acetone to obtain pure
product.
quent reactions without significant decrease in the activity of
the catalyst, which was easily separated by simple extraction
and filtration. Thus the catalyst could be recycled three times
without obvious loss of activity (entry 10: 95%, 1st run; 85%,
2nd run; 78% 3rd run). Similarly by adopting optimized reac-
tion condition various carbazoles were prepared (Table 2)
with various phenyl hydrazine hydrochlorides and cyclohex-
anone in the presence of 20 mol% Bi (NO3)3 5H2O in
MeOH.
In order to study the generality of this process, it was fur-
ther demonstrated with wide variety of functionalized phenyl
hydrazine hydrochlorides with ethyl methyl ketone (Scheme
2, Table 3). An important issue of the Fischer indole synthe-
sis in the cyclization of the phenyl hydrazones derived from
unsymmetrical ketones is that the direction of cyclization is
governed by the acidity of the reaction medium [19]. The
lower acid concentration or weaker acids promote cycliza-
tion towards the more branched carbon and higher acid con-
centrations is another way to explain stronger acids enhance
the extent of cyclization at the less branched positions. As
bismuth nitrate provides a weaker acid system direction of
cyclization occurred to produce more branched carbon dur-
ing indole synthesis.
3. RESULTS AND DISCUSSION
In this letter, we report a direct Bi (NO3)3 5H2O catalysed
one pot Fischer indole synthesis by using phenyl hydrazine
hydrochloride and cyclohexanone in MeOH (Scheme 1),
which afforded tetrahydrocarbazoles without the isolation of
corresponding hydrazones in a very short interval of time.
O
R
R
MeOH, reflux
+
Bi (NO3)3 5H2O
N
NHNH2 HCl
H
Scheme 1.
It was anticipated that the Phenyl hydrazine hydrochlo-
ride (HCl) would assists in causing Fischer cyclization;
hence the reaction was carried out in the absence of catalyst
to ensure the role of the catalyst. No reaction was observed
in the absence of catalyst in which the reaction proceeds
slowly in the beginning and after the prolonged reaction, the
poor yield and undesired side products such as part of hydra-
zone and unreacted aryl hydrazine hydrochloride remains
almost 70-80%. Thus, involvement of bismuth nitrate as
catalyst in the reaction of aryl hydrazine hydrochloride with
alkyl ketone or aldehyde have been found significant in
terms of clean reaction, high inter conversion ,easy work up.
Hence the bismuth nitrate was found essential to activate
carbonyl group to react with phenyl hydrazine hydrochloride
even in the presence of HCl.
Initially the one pot reaction of phenyl hydrazine hydro-
chloride with cyclohexanone was carried out as a model re-
action in various solvents to investigate the solvent effect.
The results are summarized in Table 1. Methanol, ethanol
and acetonitrile were found to be better solvents for this
transformation. However, the best results were achieved by
carrying out the reaction in MeOH at reflux temperature to
afford tetrahydrocarbazoles in 90-95% yield respectively.
Further we set out to establish the optimal amount of Bi
(NO3)3 5H2O, finding that reaction with a 10 mol % catalyst
loading gave a 80% yield after 1.5 hr (entry 1), however the
best result (95%) corresponds to the use of 20 mol % catalyst
(entry 2) and the reaction time reduced to 40 min. The in-
creasing amount of the catalyst did not change the reaction
time whereas isolated yield was found to be less in nature
(entry 3). Another advantage of the use of Bi (NO3)3 5H2O
was that it could be easily recovered and recycled in subse-
From these results we arrived at the conclusion that the
bismuth nitrate probably helps during hydrazone formation,
and finally cyclization occurs via thermal [3, 3] sigma tropic
Table 1. Effect of Solvents in the Synthesis of Tetrahydrocarbazolesa and 2, 3-dimethyl Indolesa
Entry
Solvent
BiNO3 (mol %)
Time (min)
Yield (%)b
1
2
3
4
MeOH
MeOH
MeOH
EtOH
10
20
25
20
90
40
40
45
80
95
91
92
5
6
CH3CN
THF
20
20
20
20
20
20
55
60
-
82
60
7
CH2Cl2
EtOAc
Toluene
MeOH
-
8
-
-
9
75
30
35
10
95, 85, 78
aAll reactions were carried out at reflux temperature.
bIsolated yields.

Bismuth Nitrate Promoted Fischer Indole Synthesis
Letters in Organic Chemistry, 2009, Vol. 6, No. 2
161
Table 2. Synthesis of Tetrahydrocarbazolesa
Mp oC
Entry
R (PhNHNH2 HCl)
Ketones
Product
Time (min)
Yields (%)b
Found
Report
118-119
2a
H
Cyclohexanone
40
75
50
95
115-117
N
H
158-160
S
Tetrahydro-4H-
thiopyran-4one
2b
2c
H
88
90
155-157
93-95
N
H
-
-
F
4-F
Cyclohexanone
N
H
2d
2e
4-CH3
Cyclohexanone
Cyclohexanone
45
20
88
88
98-100
88-90
N
H
-
H3OC
4-OCH3
N
H
93-95
2f
2-CH3
Cyclohexanone
Cyclohexanone
45
90
70
75
90-92
N
H
143-144
91-93
Cl
2g
4-Cl
141-142
N
H
Cl
Cl
2h
2,5-Cl
Cyclohexanone
130
70
89-91
N
H
aReaction was carried out at reflux temperature in MeOH in presence of 20 mol% of Bi(NO3)35H2O.
bIsolated yields.
R2
R
R2
O
R
MeOH, reflux
R1
+
Bi (NO3)3 5H2O
N
H
R1
NHNH2 HCl
Scheme 2.

162 Letters in Organic Chemistry, 2009, Vol. 6, No. 2
Sudhakara et al.
Table 3. Synthesis of 2, 3-dimethyl Indolesa
Mp oC
Entry
R (PhNHNH2 HCl)
Ketones
Product
Time (min)
Yields(%)b
Found
Report
3a
H
Butan-2-one
50
95
103-105
106-107
N
H
F
3b
4-F
Butan-2-one
80
90
60-62
-
N
H
3c
3d
4-CH3
4-CH3
Butan-2-one
Butan-2-one
45
20
88
90
98-99
89-90
-
-
N
H
H3OC
N
H
3e
3f
2-CH3
2-NO2
Butan-2-one
Butane-2-one
45
80
70
78-80
95-97
76-78
N
H
150
-
N
H
NO2
Cl
3g
3h
2,5-Cl
Butan-2-one
Pentan-3-one
140
120
70
65
89-91
66-67
90-91
65-66
N
H
Cl
H
N
H
aReaction was carried out at reflux temperature in MeOH in the presence of 20 mol% Of Bi (NO3)35H2O.
bIsolated yields.
R
R2
R2
R1
R
MeOH, reflux
+
O
R1
Bi (NO3)3 5H2O
N
N
NHNH2HCl
Bi (NO3)3 5H2O
H
R2
R2
R1
HN
R
R
[3+3] rearrangement
Bi (NO3)3 5H2O
R1
H
Bi (NO3)3 5H2O
N
N
H
Bi (NO3)3 5H2O
Scheme 3.

Bismuth Nitrate Promoted Fischer Indole Synthesis
Letters in Organic Chemistry, 2009, Vol. 6, No. 2
163
rearrangement at reflux temperature in presence of catalyst.
The possible mechanism of Fischer indole synthesis cata-
lyzed by bismuth nitrate is as shown in Scheme 3.
7.1 (dd, 2H, J = 8.9 Hz), 6.7 (d, 1H, J = 8.08 Hz), 2.3 (s,
3H), 2.2 (s, 3H), 2.1 (s, 3H); 13C NMR (75 MHz,CDCl3):
168.1, 155.4, 131.8, 131.5, 112.3, 109.3, 104.3, 97.8, 16.1,
11.8, 8.7; IR 3376; MS (EI 70 eV): m/z (%): 159 (M+). Anal.
Calcd (%): C (82.97), H (8.23), N (8.80); Found; C (81.22),
H (8.05), N (8.25).
In summary, we have developed an efficient method for
the synthesis of indoles and tetrahydrocarbazoles on a solid
support which provides excellent yields and purities under
very mild conditions. The developed method might open
new opportunity and alternatives of immense interest in the
design and synthesis of wide variety of indoles and structural
sub unit of many natural products.
5-methoxy-2,3-dimethyl-1H-indole (3d): crystalline so-
o
1
lid. Mp 89-90 C; H NMR (400 MHz, CDCl3): 10.4 (br s,
NH), 7.1 (d, 1H, J = 8.6 Hz), 6.8 (s, 1H), 6.6 (d, 1H, J = 2.24
Hz), 3.7 (s, 3H, -OCH3), 2.2 (s, 3H), 2.1 (s, 3H); 13C NMR
(75 MHz,CDCl3): 154.7, 131.7, 131.6, 108.1, 106.9,104.5,
102.5, 98.8, 20.4, 11.7, 8.9; IR 3382; MS (EI 70 eV): m/z
(%): 176 (M+). Anal. Calcd (%): C(75.40), H(7.48), N(9.13);
Found C(74.93), H(7.30), N(6.90).
ACKNOWLEDGEMENTS
The authors are grateful to Dept. of Post Graduate Stud-
ies and Research in Chemistry, School of Chemical Sci-
ences, Kuvempu University for providing Laboratory facili-
ties. We wish to special acknowledge to Indian Institute of
Science Bangalore, for Spectral data. Finally, we are thank-
ful to Vinay K.M. Lecturer Dept. of English for Proofread-
ing.
7-nitro-2,3-dimethyl-1H-indole (3f): crystalline solid.
o
1
Mp 95-96 C; H NMR (400 MHz, CDCl3): 7.6 (br s, NH),
7.5 (d, 1H, J = 7.6 Hz), 7.4 (d, 1H, J = 8.2 Hz), 7.3 (t, 1H, J
= 8.0 Hz); 13C NMR (75 MHz,CDCl3): 135.2, 131.1, 129.5,
120.9, 119.4, 117.9, 110.3, 107.8, 11.54, 8.4; IR 3368; Anal.
Calcd (%): C (62.12), H (5.11), N (13.90); MS (EI 70 eV):
m/z (%): 190 (M+). Anal. Calcd (%): C(19.40), H(1.48),
N(2.33); Found C(18.93), H(2.10), N(2.60).
SPECTRAL DATA
6-fluoro-2,3,4,9-tetrahydro-1H-carbazole (2c): crystal-
line brown solid, Mp 93-95 oC; 1H NMR (400 MHz, DMSO-
d6): 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);
13C NMR (75 MHz, CDCl3):ꢀ 137.8, 132.6, 131.9, 129.0,
123.8, 121.4, 118.4, 113.4, 110.1, 22.3, 20.4,; IR 3395; MS
(EI 70 eV): m/z (%): 190 (M+) Anal. Calcd (%): C (76.17),
H (6.39), N (7.40); found: C (74.17), H (6.10), N (7.10).
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1
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