A new three-component reaction catalyzed by silica sulfuric acid: Synthesis of tetrasubstituted pyrroles

Article abstract of DOI:10.1055/s-0031-1290955

A novel, convenient, and efficient three-component reaction of benzoin derivatives, 1,3-dicarbonyls, and ammonium acetate in the presence of silica sulfuric acid (SSA) is described for the synthesis of 2,3,4,5-tetrasubstituted pyrroles under solvent-free

Full text of DOI:10.1055/s-0031-1290955

LETTER
▌1379
lAetteNr ew Three-Component Reaction Catalyzed by Silica Sulfuric Acid:
Synthesis of Tetrasubstituted Pyrroles
Synthesis of Tetrasubstituted Pyrroles
Fatemeh Tamaddon,* Mahnaz Farahi
Department of Chemistry, Yazd University, Yazd, 89195-741, Iran
Fax +98(351)8210664; E-mail: ftamaddon@yazduni.ac.ir
Received: 19.11.2011; Accepted after revision: 23.03.2012
derivatives were prepared from commercially available
benzaldehydes via a modified benzoin condensation in re-
fluxing EtOH–PEG-400 (50:50) as outlined in Table 1.
Abstract: A novel, convenient, and efficient three-component re-
action of benzoin derivatives, 1,3-dicarbonyls, and ammonium ace-
tate in the presence of silica sulfuric acid (SSA) is described for the
synthesis of 2,3,4,5-tetrasubstituted pyrroles under solvent-free
conditions.
O
R2
R1
Ar
Key words: three-component reaction, silica sulfuric acid (SSA),
benzoin, 2,3,4,5-tetrasubstituted pyrroles, 1,3-dicarbonyls
Ar
Ar
O
O
silica sulfuric acid
80 °C, solvent-free
+
NH4OAc
+
R2
Ar
O
OH
N
R1
H
One-pot processes bringing together three or more com-
ponents with high atom economy and selectivity are
known as multicomponent reactions (MCR).1 Up to now,
MCR is one of the most interesting subclasses of modern
chemistry and a vital tool for the preparation of multifunc-
tional molecules from simple materials. Many MCR-
based strategies that make use of various energy sources
and catalysts have been reported for the synthesis of or-
ganic compounds and heterocycles such as pyrroles.2–4
Development of new MCR will offer new entries into
these interesting molecules.
Scheme 1
Table 1 Synthesis of α-Hydroxyketones (Benzoins) via Benzoin
Condensation
OH
O
R
H
KCN, PEG-400
EtOH, reflux
2
O
R
R
1a–c
Yield (%)a
Entry
R
Time (h)
The use of eco- and environmentally benign heteroge-
neous catalysts is one of the most attractive approaches to
the synthesis of organic compounds.5 Heterogeneous sol-
id acids represent a promising entry as reusable catalysts.6
One efficient reusable solid acid is silica sulfuric acid
(SSA),7 which efficiently catalyzes a variety of organic
transformations.8–12
1
2
3
H
5
10
4
85
70
90
Me
Cl
a Isolated yield.
The pyrrole ring is a structural subunit of natural products,
pharmaceuticals, and chemicals such as porphyrines, glo-
bins, cytochromes, and vitamins.13 Functionalized pyr-
roles are also bioactive compounds and attractive
synthetic intermediates in materials science and organic
chemistry.13–15 Among the numerous procedures devel-
oped for the synthesis of pyrroles, cyclocondensations of
two or more components are attractive,2,16–23 when every
component provides the carbons or nitrogen members of
the pyrrole ring.
In the second set of experiments, solvent-free condensa-
tion of benzoin, acetylacetone, and ammonium acetate as
a model reaction was performed in the absence of catalyst
at room temperature. No pyrrole product was obtained
even after 20 hours. Solvent and catalyst are fundamental
factors for MCR. Thus the model reaction was investigat-
ed in the presence of a variety of acid catalysts under dif-
ferent conditions (Table 2).
As illustrated in Table 2, SSA was a superior catalyst, and
the pyrrole product was obtained in excellent yield at
80 °C under solvent-free conditions. Optimization of the
catalyst loading was found to be 0.02 g equal to 0.05
mmol9a of H+ (Table 2, entry 8).
In continuation of our investigations into heterogeneous
reactions,24–27 we report a new SSA-catalyzed three-com-
ponent reaction of 1,3-dicarbonyls, benzoin derivatives,
and NH4OAc, in which tetrasubstituted pyrroles were ob-
tained in excellent yields under solvent-free conditions
(Scheme 1). In a first set of experiments, various benzoin
To establish the scope and generality of this three-compo-
nent reaction, various 1,3-dicarbonyls and benzoin deriv-
atives were reacted under optimized reaction conditions,
and the desired substituted pyrroles were obtained in ex-
cellent yields (Table 3).
SYNLETT 2012, 23, 1379–1383
Advanced online publication: 10.05.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0031-1290955; Art ID: ST-2011-D0687-L
© Georg Thieme Verlag Stuttgart · New York

1380
F. Tamaddon, M. Farahi
LETTER
6). This led us to speculate that the positive charge density
on carbon attached to the hydroxy group is an important
factor for the promotion of the reaction. Chemoselective
formation of the chlorophenyl-substituted pyrrole by the
competitive reaction of acetyl acetone and ammonium ac-
etate with equal amounts of 1,2-bis(4-chlorophenyl)-2-
hydroxyethanone (1c) and 1a or 2-hydroxy-1,2-di-p-tol-
ylethanone (1b) supports this speculation.
Table 2 Three-Component Condensation of Benzoin, Acetylace-
tone, and Ammonium Acetate under Various Conditions
Me
Ph
Ph
O
Ph
Ph
COMe
Me
O
O
catalyst
NH4OAc
+
+
N
OH
H
Me
Entry Catalyst (g)
Solvent
Temp
(°C)
Time
(min)
Yield
(%)a
However, 1,3-diphenylpropane-1,3-dione, cyclic 1,3-di-
carbonyls, ethyl 3-oxohexanoate, ethyl 4-chloro-3-oxobu-
tanoate, and 3-oxo-N-phenylbutanamide did not lead to
the desired products, possibly by reason of the more steric
hindrance and/or less electrophilicity of the carbonyl
group. These factors are important in the formation of en-
aminones from unsymmetrical dicarbonyls. Due to these
correlations, a probable formation of the enaminone inter-
mediate and differences in electrophilicity and hindrance
of carbonyl groups, the condensation of an unsymmetrical
1,3-dicarbonyl such as benzoylacetone with ammonium
acetate and 1a or 1c was investigated under similar reac-
tion conditions. Among the two possible regioisomers,
exclusively a single regioisomer was formed in excellent
yield either for 1a or 1c (Table 3, entries 4 and 10). Since
methyl ketone is more electrophilic and less hindered than
phenyl ketone, the formation of the enaminone from
methyl ketone and its subsequent conversion to pyrrole
was highly regioselective, evidenced by the formation of
single regioisomers for entries 4 and 10 (Table 3). The
1
2
–
–
100
100
80
80
80
80
80
80
80
80
80
20 h
360
360
120
360
30
–
b
H2SO4 (0.1)
H2SO4 (0.05)
–
–
b
3
–
–
4
PEG–SO3H (0.1)
SiO2 (0.1)
–
70
25
90
90
96
52
55
63
5
–
6
SSA (0.1)
–
7
SSA (0.05)
SSA (0.02)
SSA (0.02)
SSA (0.02)
SSA (0.02)
–
30
8
–
35
9
H2O
120
360
120
10
11
EtOH
MeCN
a Isolated yields.
b A dark and viscous mixture was obtained.
1
structures of these regioisomers were verified by H
NMR, 13C NMR, and FT-IR spectroscopy and microanal-
ysis.
No pyrrole product was obtained through the condensa-
tion of acetyl acetone and anisoin or 4-methyl-substituted
benzoin with ammonium acetate (Table 3, entries 5 and
Table 3 SSA-Catalyzed Condensation of Benzoin, 1,3-Dicarbonyl Compounds and Ammonium Acetate28–30
O
O
COR2
O
Ar
SSA
Ar
1
Ar
2 + NH4OAc
R
+
R
Ar
R1
80 °C
N
OH
H
R1
R2
Producta
Yield (%)b
96
Entry
1
Ar
Ph
Time (min)
35
Mp (°C)
170–171
Me
Me
O
O
O
Ph
Me
Me
Ph
N
H
2
3
4
Ph
Ph
Ph
Me
Me
Ph
OEt
35
45
60
205–206
177–179
221–223
94
90
87c
Ph
OEt
Me
Ph
N
H
OMe
Me
Ph
OMe
Me
Ph
N
H
O
Ph
Ph
Ph
N
Me
H
Synlett 2012, 23, 1379–1383
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Tetrasubstituted Pyrroles
1381
Table 3 SSA-Catalyzed Condensation of Benzoin, 1,3-Dicarbonyl Compounds and Ammonium Acetate28–30 (continued)
O
O
COR2
O
Ar
SSA
Ar
1
Ar
2 + NH4OAc
R
+
R
Ar
R1
80 °C
N
OH
H
R1
R2
Producta
Yield (%)b
Entry
Ar
4-MeC6H4
Time (min)
Mp (°C)
d
d
d
5
6
7
Me
Me
Me
Me
Me
Me
60
60
50
–
–
–
d
d
d
4-MeOC6H4
4-ClC6H4
–
–
–
O
Cl
234–235
89
Me
Me
N
H
Cl
Cl
O
8
9
4-ClC6H4
4-ClC6H4
4-ClC6H4
Me
Me
Ph
OEt
50
60
10
155–156
93
OEt
Me
N
H
Cl
Cl
OMe
212–213
247–249
91
O
OMe
Me
N
H
Cl
Cl
95c
10
Me
O
Ph
Me
N
H
Cl
a All the products were fully characterized by IR, 1H NMR, 13C NMR spectroscopy, and CHN analysis.
b Isolated yield.
c As a single regioisomer.
d No pyrrole product was obtained.
According to the electronic effects and chemoselectivity
of the reaction, we postulated that at first ammonium ace-
tate reacts with the less hindered ketone group of the 1,3-
dicarbonyl component and results in the formation of an
enaminone intermediate. This intermediate then condens-
es with protonated benzoin by the catalyst to give the pyr-
role product (Scheme 2).
R1
R1
R2
O
SSA
+
NH4OAc
Ar
O
O
O
HN
Ar
H
R2
Ar
O
SSA
Ar
Ar
OH
+OH2
O
O
O
To support this mechanism we prepared the 4-aminopent-
3-en-2-one by reacting acetylacetone and ammonia. The
preformed enaminone was then reacted with benzoin un-
der our experimental conditions, and the corresponding
pyrrole product was isolated again in high yield.
Ar
H
H
Ar
H
R2
R2
R2
R1
Ar
R1
R1
HO
Ar
O
N
N
H
HN
Ar
H
Scheme 2
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1379–1383

1382
F. Tamaddon, M. Farahi
LETTER
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deserving a significant decrease in reaction yield.
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In conclusion, we have developed a new three-component
reaction for the synthesis of 2,3,4,5-tetrasubstituted pyr-
roles from benzoin derivatives, 1,3-dicarbonyls, and am-
monium acetate in the presence of silica sulfuric acid
(SSA). The advantages of our work are simplicity of pro-
cedure, high reaction yields, recyclability of catalyst, and
availability of starting materials which is capable to de-
sign substituted pyrroles. These advantages make the
present protocol an interesting alternative to the previous-
ly reported methods for the synthesis of pyrrole deriva-
tives.
Acknowledgment
We acknowledge the research council of Yazd University.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.onmorinftIangoimnuSpinorttfoIangiStupor
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(6) Wilson, K.; James, H.; Clark, P. Appl. Chem. 2000, 72, 1313.
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D.; Zolfigol, M. A.; Safaiee, M.; Shamsian, A.; Ghorbani-
Choghamarani, A. Catal. Commun. 2009, 10, 1257.
(9) (a) Dabiri, M.; Salehi, P.; Baghbanzadeh, M.; Zolfigol, M.
A.; Agheb, M.; Heydari, S. Catal. Commun. 2008, 9, 785.
(b) Shaterian, H. R.; Ghashang, M.; Feyzi, M. Appl. Catal.,
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Baltork, I.; Mirjalili, B. B. F.; Bamoniri, A. Synlett 2003,
1877.
(27) Tamaddon, F.; Bistgani, J. M. Synlett 2011, 2947.
(28) Chemicals were purchased from Aldrich, Fluka, and Merck
chemical companies and freshly used without purification.
The products were isolated and identified by their physical
and spectral data. IR spectra were recorded on FT-IR
JASCO-680 using KBr disks. The 1H NMR spectra were
recorded on a Bruker instrument 400 MHz ultrashield model
as CDCl3 and DMSO-d6 solutions.
Preparation of Silica Sulfuric Acid21
A 500 mL suction flask equipped with a constant-pressure
dropping funnel and a gas inlet tube for conducting HCl gas
over an adsorbing solution (i.e., H2O) was used. It was
charged with silica gel (60.0 g). Chlorosulfonic acid (23.3 g,
0.2 mol) was added dropwise over a period of 30 min at r.t.
HCl gas evolved from the reaction vessel immediately. After
the addition was completed the mixture was shaken for 30
min. A white solid (SSA) 76.0 g was obtained.
(29) General Experimental Procedure for the Synthesis of
Tetrasubstituted Pyrroles
A mixture of benzoin derivative (1 mmol), 1,3-dicarbonyl
compound (1 mmol), NH4OAc (2 mmol), and SSA (0.02 g)
was stirred at 80 °C under solvent-free conditions. The
progress of the reaction was monitored by TLC (hexane–
EtOAc = 8:2). After completion of the reaction, the obtained
mixture was diluted with EtOAc (10 mL) and filtered to
remove the catalyst. The filtrate was washed with aq 5%
NaHCO3, dried over Na2SO4, and evaporated. The resulting
product was purified in some cases by crystallization from
H2O–EtOH (30:70) to afford pure product.
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Lett. 2011, 40, 484. (b) Baghernejad, B. Mini-Rev. Org.
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Mohammadpoor-Baltork, I. Green. Chem. 2002, 4, 562.
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S.; Zare, A.; Foroughi, H. Mol. Diversity 2011, 15, 373.
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Chemistry; Elsevier: New York, 2009, Chap. 5.2 145–171.
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Academic Press: London, 1977. (c) Cox, M.; Lehninger, A.
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Worth Publishers: New York, 2000.
(30) Selected Characterization Data
1-(2-Methyl-4,5-diphenyl-1H-pyrrol-3-yl)ethanone
(Table 3, Entry 1)
Pale yellow needles; mp 170–171 °C. IR (KBr): νmax = 3177,
1635, 1602, 1579, 767, 698 cm–1. 1H NMR (400 MHz,
DMSO-d6): δ = 1.72 (s, 3 H, CH3), 2.46 (s, 3 H, CH3), 7.10–
(14) Fürstner, A. Angew. Chem. Int. Ed. 2003, 42, 3528.
Synlett 2012, 23, 1379–1383
© Georg Thieme Verlag Stuttgart · New York

LETTER
7.36 (m, 10 H), 11.61 (s, 1 H, NH) ppm. 13C NMR (100
Synthesis of Tetrasubstituted Pyrroles
1383
1631, 1576, 1523, 1445, 829, 798 cm–1. 1H NMR (400 MHz,
DMSO-d6): δ = 1.82 (s, 3 H, CH3), 3.33 (s, 3 H, CH3), 7.08–
7.40 (m, 8 H), 11.73 (s, 1 H, NH) ppm. 13C NMR (100 MHz,
DMSO-d6): δ = 14.35, 31.10, 121.61, 122.45, 126.21,
128.80, 128.82, 131.17, 131.39, 132.14, 132.93, 136.06,
136.16, 194.50 ppm. Anal. Calcd (%) for C19H15Cl2NO: C,
66.29; H, 4.39; Cl, 20.60; N, 4.07; O, 4.65. Found: C, 66.05;
H, 4.25.
MHz, DMSO-d6): δ = 14.32, 30.96, 79.64, 122.53, 122.58,
126.53, 127.03, 127.33, 128.61, 128.76, 131.22, 132.65,
135.74, 137.57, 194.95 ppm. Anal. Calcd (%) for C19H17NO:
C, 82.88; H, 6.22; N, 5.09. Found: C, 82.74; H, 6.19.
Ethyl 2-Methyl-4,5-diphenyl-1H-pyrrole-3-carboxylate
(Table 3, Entry 2)
Pale yellow solid; mp 205–206 °C. IR (KBr): νmax = 3310,
1676, 1599, 766, 697 cm–1. 1H NMR (400 MHz, DMSO-d6):
δ = 0.93 (t, J = 6.4 Hz, 3 H, OCH2CH3), 2.49 (s, 3 H, CH3),
3.91 (q, J = 6.4 Hz, 2 H, OCH2CH3), 7.11–7.24 (m, 10 H),
11.56 (s, 1 H, NH) ppm. 13C NMR (100 MHz, DMSO-d6):
δ = 13.69, 14.27, 58.76, 111.91, 123.02, 126.48, 127.18,
127.33, 127.88, 128.58, 131.12, 132.71, 136.26, 137.14,
165.11 ppm. Anal. Calcd (%) for C20H19NO3: C, 74.75; H,
5.96; N, 4.36; O, 14.94. Found: C, 74.50; H, 5.90.
Methyl 2-Methyl-4,5-diphenyl-1H-pyrrole-3-
carboxylate (Table 3, Entry 3)
Pale yellow needles; mp 177–179 °C. IR (KBr): νmax = 3340,
1681, 1598, 1522, 756, 696 cm–1. 1H NMR (400 MHz,
DMSO-d6): δ = 2.47 (s, 3 H, CH3), 3.45 (s, 3 H, OCH3),
7.07–7.24 (m, 10 H), 11.58 (s, 1 H, NH) ppm. 13C NMR (100
MHz, DMSO-d6): δ = 13.81, 50.48, 111.57, 123.02, 126.56,
127.32, 127.57, 127.95, 128.59, 130.08, 131.08, 132.67,
136.30, 136.89, 165.53 ppm. Anal. Calcd (%) for
C19H17NO2: C, 78.33; H, 5.88; N, 4.81; O, 10.98. Found: C,
78.25; H, 5.82.
Ethyl 4,5-Bis(4-chlorophenyl)-2-methyl-1H-pyrrole-3-
carboxylate (Table 3, Entry 8)
Pale yellow needles; mp 155–156 °C. IR (KBr): νmax = 3286,
1672, 1498, 1481, 831, 732 cm–1. 1H NMR (400 MHz,
CDCl3): δ = 1.09 (t, J = 7.2 Hz, 3 H, OCH2CH3), 2.58 (s, 3
H, CH3), 4.09 (q, J = 7.2 Hz, 2 H, OCH2CH3), 6.99–7.26 (m,
8 H), 8.66 (s, 1 H, NH) ppm. 13C NMR (100 MHz, CDCl3):
δ = 13.89, 14.04, 59.45, 112.36, 122.46, 126.50, 127.92,
128.11, 128.73, 130.38, 132.08, 132.47, 134.28, 136.15,
165.50 ppm. Anal. Calcd (%) for C20H17Cl2NO2: C, 64.18;
H, 4.58; Cl, 18.95; N, 3.74; O, 8.55. Found: C, 63.96; H,
4.31.
Methyl 4,5-Bis(4-chlorophenyl)-2-methyl-1H-pyrrole-3-
carboxylate (Table 3, Entry 9)
White crystals; mp 212–213 °C. IR (KBr): νmax = 3330,
1685, 1524, 1499 cm–1. 1H NMR (400 MHz, DMSO-d6): δ =
3.33 (s, 3 H, CH3), 3.49 (s, 3 H, OCH3), 7.07–7.31 (m, 8 H),
11.80 (s, 1 H, NH) ppm. 13C NMR (100 MHz, DMSO-d6): δ
= 13.79, 50.66, 111.53, 122.04, 126.63, 128.15, 128.83,
129.01, 131.13, 131.46, 131.87, 132.81, 135.38, 136.82,
165.26 ppm. Anal. Calcd (%) for C19H15Cl2NO2: C, 63.35;
H, 4.20; Cl, 19.68; N, 3.89; O, 8.88. Found: C, 63.22; H,
4.38.
(2-Methyl-4,5-diphenyl-1H-pyrrol-3yl)(phenyl)-
methanone (Table 3, Entry 4)
Yellow needles; mp 221–223 °C. IR (KBr): νmax = 3289,
1610, 1576, 1522, 740, 696 cm–1. 1H NMR (400 MHz,
DMSO-d6): δ = 2.24 (s, 3 H, CH3), 6.94–7.48 (m, 15 H),
11.65 (s, 1 H, NH) ppm. 13C NMR (100 MHz, DMSO-d6):
δ = 11.11, 121.49, 122.52, 126.16, 126.80, 127.31, 127.65,
128.10, 128.19, 128.70, 129.37, 130.74, 131.79, 132.73,
134.55, 136.08, 140.27, 193.27 ppm. Anal. Calcd (%) for
C24H19NO: C, 85.43; H, 5.68; N, 4.15; O, 4.74. Found: C,
85.22; H, 5.59.
[4,5-Bis(4-chlorophenyl)-2-methyl-1H-pyrrol-3-
yl](phenyl)methanone (Table 3, Entry 10)
Yellow solid; mp 247–249 °C. IR (KBr): νmax = 3304, 1617,
1595, 1498, 831, 699 cm–1. 1H NMR (400 MHz, DMSO-d6):
δ = 2.21 (s, 3 H, CH3), 7.18–7.48 (m, 13 H), 11.85 (s, 1 H,
NH) ppm. 13C NMR (100 MHz, DMSO-d6): δ = 13.21,
121.67, 126.42, 128.28, 128.36, 128.92, 129.31, 131.05,
131.58, 131.97, 132.36, 134.71, 135.11, 140.15, 192.98
ppm. Anal. Calcd (%) for C24H17Cl2NO: C, 70.95; H, 4.22;
Cl, 17.45; N, 3.45; O, 3.94. Found: C, 70.78; H, 4.37.
1-[4,5-Bis(4-chlorophenyl)-2-methyl-1H-pyrrol-3-
yl]ethanone (Table 3, Entry 7)
White crystals; mp 234–235 °C. IR (KBr): νmax = 3195,
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1379–1383

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