Molybdate sulfuric acid as a reusable solid catalyst in the synthesis of 2,3,4,5-tetrasubstituted pyrroles via a new one-pot [2+2+1] strategy

Article abstract of DOI:10.1016/j.molcata.2012.01.003

Molybdate sulfuric acid (MSA) has been found as an efficient and reusable solid acid catalyst for the synthesis of new 2,3,4,5-tetrasubstituted pyrroles via a novel [2+2+1] strategy. Thus, one-pot three-component reaction of benzoin derivatives, 1,3-dicar

Full text of DOI:10.1016/j.molcata.2012.01.003

Journal of Molecular Catalysis A: Chemical 356 (2012) 85–89
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Journal of Molecular Catalysis A: Chemical
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Molybdate sulfuric acid as a reusable solid catalyst in the synthesis of
2,3,4,5-tetrasubstituted pyrroles via a new one-pot [2+2+1] strategy
Fatemeh Tamaddona,∗, Mahnaz Farahia, Bahador Karamib
a Department of Chemistry, Faculty of Science, Yazd University, Yazd 89195-741, Iran
b Department of Chemistry, Yasouj University, Yasouj 75918-74831, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Molybdate sulfuric acid (MSA) has been found as an efficient and reusable solid acid catalyst for the
synthesis of new 2,3,4,5-tetrasubstituted pyrroles via a novel [2+2+1] strategy. Thus, one-pot three-
component reaction of benzoin derivatives, 1,3-dicarbonyls, and ammonium acetate afforded the desired
products in high yield under solvent-free conditions.
Received 17 September 2011
Received in revised form
25 December 2011
Accepted 1 January 2012
Available online 9 January 2012
© 2012 Elsevier B.V. All rights reserved.
Keywords:
2,3,4,5-Tetrasubstituted pyrroles
Benzoin
1,3-Dicarbonyls
Ammonium acetate
Molybdate sulfuric acid (MSA)
1. Introduction
in their fundamental structures [7]. Substituted pyrroles have
also many potential uses as attractive synthetic intermedi-
Recently, solid acids have been used as eco- and environ-
mentally friendly catalysts in various organic transformations [1].
The option of reusability of the solid acid catalyst, non toxi-
city, and easier work up of the reactions catalyzed by these
catalysts make them much favorite. Molybdate sulfuric acid
(MSA) is a heterogeneous solid acid which has been used as
catalyst in organic transformations [2,3]. This Brönsted acid as
an efficient proton source is insoluble in most of organic sol-
vents. It is a solid heterogeneous alternative to sulfuric acid.
This inexpensive and reusable catalyst can be readily handled
and easily separated from the reaction mixture, which these
advantages make reactions cleaner and faster with higher yield-
ing.
Pyrrole derivatives are certainly one of the most important
nitrogen containing heterocyclic compounds which constitute
the backbone of many biologically active compounds and nat-
ural products such as chlorophyll, hemoglobin, myoglobin,
cytochromes, and vitamins [4–6]. A number of clinically used
medicines with antibacterial, antiviral, anti-inflammatory, anti-
tumoral or antioxidant properties carry out the pyrrole moiety
ates in material science and biosynthesis [8–16]. Among the
various synthetic approaches to pyrroles, Knorr [10], Paal-
Knorr [11], and Hantzsch [12] methods are well-known, while
pyrrole derivatives can be also prepared via 1,3-dipolar cycload-
dition [14], reductive coupling [15], and aza-Wittig reaction
[16].
One-pot multi-component reactions (MCR) play an impor-
tant role in combinatorial chemistry, so this field remained
one of the most interesting areas of researches in recent
years [17–23]. During MCR, target compounds are formed with
greater efficiency by generating structural complexity in a sin-
gle step from three or more reactants. Due to the advantages,
many four and three-component strategies have been devel-
oped for the preparation of pyrrole derivatives which are varied
in kind of starting materials [24–29]. Based on the synthetic
design and MCR, each component provides the five members
of the pyrrole ring with the possible substitutes. This has
stimulated interest in further synthetic designs and more sim-
plified catalytic methods for the synthesis of new substituted
pyrroles.
Due to the advantages of MCR and heterogeneous catalysts, in
continuation of our researches [30–35], we report herein a new
different [2+2+1] strategy for the synthesis of tetrasubstituted
pyrroles using molybdate sulfuric acid as an efficient heteroge-
neous catalyst (Scheme 1).
∗
Corresponding author. Tel.: +98 3518122666; fax: +98 3518210644.
E-mail addresses: ftamaddon@yazduni.ac.ir (F. Tamaddon),
farahimb@yahoo.com (M. Farahi), karami@mail.yu.ac.ir (B. Karami).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2012.01.003

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F. Tamaddon et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 85–89
O
R1
Ar
Ar
Ar
O
+
R1
O
Molybdate sulfuric acid
80 oC, Solvent-free
+
NH4OAc
O
Ar
Me
OH
N
H
Me
Scheme 1. MSA catalyzes the synthesis of 2,3,4,5-tetrasubstituted pyrroles.
O
O
n-Hexane
0 o
2 ClSO3H
NaO Mo ONa +
O
HO3SO
Mo OSO H
2 NaCl
+
3
C
O
Scheme 2. Preparation of MSA catalyst.
2. Experimental
Fig. 2. Reusability of catalyst.
2.1. Preparation of catalyst
MSA was prepared via a modified version of the previously
reported procedures [2,3] (Scheme 2).
the bands at 1300–1100 cm−1 might be the asymmetric and sym-
metric stretching modes of S O. A strong band at 827 cm−1 in the
FT-IR spectrum of sodium molybdate is assigned to the stretching
mode of Mo O. This band is shifted to ∼1100 cm−1 and appeared as
an overlapped band with S O stretching bands in spectrum of MSA.
Broadening of the absorbance band positioned at 3600–3000 cm−1
is due to the rapid exchanges of acidic hydrogens via H-bonding
and reveals the formation of MSA.
In order to investigate the acid capacity of MSA, a solution of
it (0.0805 g) in distilled water (100 mL) was titrated with standard
solution of NaOH (0.1 N) in the presence of phenolphthalein as indi-
cator. At the endpoint of titration 5 mL of titrant was consumed. The
capacity of MSA was determined according the following equa-
tion as 2. (m/MW) × n = N2V2, (0.0805/322) × n = 0.1 × 0.005, thus
n = 2. Therefore, MSA can be considered as a solid heterogeneous
alternative to sulfuric acid.
Thus, anhydrous sodium molybdate (20 mmol, 4.118 g) was
added to dry n-hexane (25 mL) in a 100 mL round bottom flask
equipped with ice bath and overhead stirrer. Chlorosulfonic acid
(0.266 mL, 40 mmol) was then added dropwise to the flask during
30 min and stirred for 1.5 h. The reaction mixture was gradually
poured into 25 mL of chilled distilled water with agitation. The
molybdate sulfuric acid was separated as a bluish solid by filtra-
tion, washed with cold distilled water five times until the negative
test for chloride ion for filtrate, and dried at 120 ◦C for 5 h. The yield
of the obtained bluish acid catalyst was 90%.
2.2. Characterization of catalyst
The prepared catalyst was characterized by determination of
decomposition point, FT-IR spectrum, and neutralization titration
with standard solution of NaOH.
As a result, the prepared molybdate sulfuric acid which showed
good thermal stability decomposed at 354 ◦C.The overlaid FT-IR
spectra of sodium molybdate and molybdate sulfuric acid (MSA)
are shown in Fig. 1. As the spectrum of MSA demonstrates, the char-
acteristic bands of both anhydrous sodium molybdate and OSO3
group are shifted evidently to the higher wave numbers. The well
defined bands at 3600–3000 is related to OH stretching, the band
Table 1
Optimization of the reaction. .
O
Me
Me
O
Ph
O
O
Heat
Me
Me
+
+
NH4OAc
OH
at 1635 cm−1 is the H
O H bending mode of the lattice water, and
Ph
N
H
Entry Solvent Catalyst loading/temp. (mol%/◦C) Time (min) Yield (%)a
1
2
3
4
5
6
7
8
None
None
None
None
None
None
None
None
None
None
None
None
None
None
None
H2O
None/90
ZrOCl2 (5)/90
ZnCl2 (5)/90
300
360
360
360
400
60
60
60
30
30
30
30
25
55
25
60
40
40
15
81
75
85
68
66
37
57
80
85
90
96
96
93
85
50
80
80
71
75
MgBr2 (5)/80–90
CuCl2 (5)/80–90
H2SO4 (5)/80–90
HCl (5)/80–90
Glacial HOAc (5)/80–90
MSA (20)/80–90
MSA (15)/80–90
MSA (10)/80–90
MSA (5)/80–90
MSA (5)/80
9
10
11
12
13
14
15
16
17
18
19
20
MSA (5)/70
MSA (5b)/80
MSA (5)/80
MSA (5)/80
MSA (5)/80
MSA (5)/80
EtOH
EtOH
CH3CN
CH2Cl2
60
60
MSA (5)/80
a
Isolated yields.
The catalyst was reused for run 3.
b
Fig. 1. Overlaid FT-IR spectra of Na2MoO4 and H3OSO(MoO2)OSO3H (MSA).

F. Tamaddon et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 85–89
87
Table 2
Me
H
MSA-catalyzed condensation of benzoin, 1,3-dicarbonyls and ammonium acetate.
COR1
O
O
MSA
MSA
R1
Ar
O
O
Me
O
Ar
+
OH
+
NH4OAc
Ar
1)
MSA
80 o
Me
R1
NH4OAc
+
O
N
H
C
Ar
R1
Ar
Me
N
H
.
+
O
Entry Ar
R1
Time (min) Producta
Yield (%)b
OH
Ar
OH
O
2)
Ar
+
Ph
Ar
Me
Me
OH
O
3)
Ph
N
H
1
2
3
4
Ph
Ph
Ph
Ph
Me
25
96
94
90
87
O
OH2
O
O
O
H
Ar
Ar
R1
OH
Ph
R1
MSA
OEt
Me
HO
Ar
- H2O
HO
Ar
Ph
N
Me
N
H
Me
N
H
H
OEt
30
4)
Ph
Ph
OMe
Me
O
H
O
H
Ar
R1
Ar
Ar
N
H
OMe 35
R1
5)
MSA
HO
Me
Ph
Ph
Me
- H2O
N
H
Me
N
Me
Ph
Ar
H
N
H
Scheme 3. Proposed mechanism for MSA-catalyzed synthesis of tetra-substituted
pyrroles.
Me
45
45
30
45
O
Me
Me
2.3. General procedure for the synthesis of tetrasubstituted
pyrroles
N
H
Me
Cl
5
6
4-MeC6H5 Me
87
92
O
To a mixture of benzoin derivative (2 mmol), 1,3-dicarbonyl
compound (2 mmol), and NH4OAc (3 mmol) was added MSA
(5 mol%). The resulting mixture was stirred at 80 ◦C for the given
times (Table 2). After completion of the reaction (as indicated by
thin layer chromatography), EtOAc (10 mL) was added and the
catalyst was separated by filtration. Evaporation of the solvent
under reduced pressure gave the product. Further purification was
achieved by recrystallization from EtOH/H2O (70:30).
Me
Me
N
H
Cl
Cl
4-ClC6H5
Me
O
OEt
Me
N
H
Cl
Cl
7
8
4-ClC6H5
OEt
94
93
2.4. Reusability of catalyst
O
OMe
The recovered catalyst from the model reaction was regener-
ated by washing with EtOAc and drying at 120 ◦C for 1 h. Using the
recycled catalyst for three consecutive times in the reaction of ben-
zoin, acetylacetone, and ammonium acetate furnished the product
with a gradual decreasing of reaction yield (Fig. 2).
Me
N
H
Cl
Cl
4-ClC6H5
OMe 40
O
Me
Ph
2.5. Selected spectral data
N
H
Mec 80
95
Cl
9
4-ClC6H5
2.5.1. 1-(2-Methyl-4,5-diphenyl-1H-pyrrol-3-yl)ethanone
(Table 2, entry 1)
a
All products were fully characterized by FT-IR, 1H NMR and 13C NMR spec-
troscopy.
Pale yellow solid, mp = 170–172 ◦C. FT-IR: ꢀmax (KBr) 3177 (NH
b
Isolated yield.
Benzoylacetone was used instead of acetylacetone.
c
stretching), 1635 (C O), 1455, 767, and 698 cm−1 1H NMR (DMSO-
.
d6, 400 MHz) ı: 1.72 (s, 3H, CH3), 2.46 (s, 3H, CH3), 7.10–7.36 (m,
10H), and 11.61 (br s, 1H, NH) ppm. 13C NMR (DMSO, 100 MHz) ı:
14.32, 30.96, 122.53, 122.58, 126.53, 127.03, 127.33, 128.61, 128.76,
131.22, 132.65, 135.74, 137.57, and 194.95 ppm. Anal. Calcd for
C19H17NO: C, 82.88; H, 6.22; N, 5.09. Found: C, 82.85; H, 6.01; N,
5.05.

88
F. Tamaddon et al. / Journal of Molecular Catalysis A: Chemical 356 (2012) 85–89
OH
2.5.2.
O
1-(4,5-Bis(4-chlorophenyl)-2-methyl-1H-pyrrol-3-yl)ethanone
(Table 2, entry 6)
Me
White crystals, mp = 234–235 ◦C. FT-IR (KBr): ꢀmax (KBr) 3195,
Ar
COOEt
Me
O
O
1631, 1576, 1523, 1445, 829, and 798 cm−1 1H NMR (400 MHz,
.
MSA
80 oC
Cl
Cl
OH
NH4OAc
O
+
+
DMSO-d6) ı: 1.82 (s, 3H, CH3), 3.33 (s, 3H, CH3), 7.08–7.40 (m,
8H), and 11.73 (s, 1H, NH) ppm. 13C NMR (100 MHz, DMSO-d6) ı:
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, and 194.50 ppm. Anal. Calcd for
Ar
N
H
EtO
Ar = 4-ClC6H4 94%
Ar = 4-MeC6H4 0%
Me
Me
C19H15Cl2NO: C, 66.29; H, 4.39; Cl, 20.60; N, 4.07; O, 4.65%. Found:
C, 66.05; H, 4.25; N, 4.05%.
Scheme 4. Chemoselectivity of MSA-catalyzed pyrrole synthesis.
2.5.3. Ethyl
ammonium acetate faster than those of electron-donating ones.
This may be explained according to more positive charge located
on carbonyl group of these benzoins, which make them more reac-
tive towards nucleophilic attack of nitrogen of imine intermediate.
A proposed mechanism for the formation of pyrrole according to
these electronic effects is outlined in Scheme 3. It seems that ketone
group of 1,3-dicarbonyl compound reacts initially with NH4OAc
to form an imine intermediate that subsequently condenses with
activated benzoin by MSA to produce a cyclic intermediate. Dehy-
dration of this intermediate and elimination of water produces the
corresponding tetrasubstituted pyrrole (Scheme 3).
4,5-bis(4-chlorophenyl)-2-methyl-1H-pyrrole-3-carboxylate
(Table 2, entry 7)
Pale yellow crystals, mp = 155–156 ◦C. FT-IR (KBr): ꢁmax 3286,
1672, 1498, 1481, 831, and 732 cm−1 1H NMR (400 MHz, CDCl3) ı:
.
1.09 (t, J = 7.2 Hz, 3H, OCH2CH3), 2.58 (s, 3H, CH3), 4.09 (q, J = 7.2 Hz,
2H, OCH2CH3), 6.99–7.26 (m, 8H), and 8.66 (s, 1H, NH) ppm. 13C
NMR (100 MHz, CDCl3) ı: 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, and 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; N,
3.68%.
The proposed mechanism was supported by the chemoselec-
tivity of method for electron-deficient benzoins. So, competitive
reaction of equal amounts of 4-Me and 4-Cl substituted benzoins
with ethylacetoacetate and NH4OAc in the presence of 5 mol% of
MSA showed a high selectivity for 4-cholorobenzoin and the corre-
sponding pyrrole was isolated exclusively in 93% yield (Scheme 4).
2.5.4.
1-(4,5-Bis(4-chlorophenyl)-2-phenyl-1H-pyrrol-3-yl)ethanone
(Table 2, entry 9)
Yellow crystals, mp = 247–249 ◦C. FT-IR (KBr): ꢁmax 3304, 1617,
1595, 1498, 831, and 699 cm−1 1H NMR (400 MHz, DMSO-d6) ı:
.
2.21 (s, 3H, CH3), 7.18–7.48 (m, 13H), and 11.85 (s, 1H, NH) ppm. 13
C
4. Conclusion
NMR (100 MHz, DMSO-d6) ı: 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, and 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; N, 3.52%.
In conclusion, a new strategy has been developed for the conve-
nient synthesis of tetrasubstituted pyrroles using MSA as a highly
efficient catalyst. In the presence of this solid acid a series of tandem
condensation and dehydration reactions occurred and resulted
in the formation of tetrasubstituted pyrroles in high yields. The
advantages of this work are solvent-free conditions, recyclability
of catalyst, and availability of starting materials which is capable
to design a range of new pyrrole derivatives. The simplicity of the
present procedure than the previously reported method of pyrrole
synthesis makes this new approach as an interesting alternative to
the complex multistep approaches.
3. Results and discussion
According to our designed synthetic strategy for the preparation
of tetrasubstituted pyrroles, reaction of benzoin (1 mmol), acetyl
acetone (1 mmol) and NH4OAc (1.5 mmol) was selected as a model
reaction. Use of the higher ratio of NH4OAc is due to its hygro-
scopic properties. This reaction was optimized by screening in the
presence of various catalysts at different conditions (Table 1).
As can be seen, the best results were obtained by carrying out
the reaction in the presence of 5 mol% of molybdate sulfuric acid
(MSA) at 80 ◦C under solvent-free conditions. Molybdate sulfuric
acid is an easily prepared and moisture tolerant solid acid which
has been used as catalyst in the oxidation of thiols and nitrosation
of amines [2,3].
Acknowledgments
We acknowledge the research council of Yazd University. We
also gratefully thank Dr. Alireza Gorji for his valuable comments
about FTIR analysis of inorganic compounds.
The reusability of catalyst is an important factor for commercial
uses. Therefore, the recovery and reusability of MSA was investi-
gated. Hence, MSA was successfully regenerated from the model
reaction by washing with EtOAc and drying at 120 ◦C. Attempts
to the reusability of MSA showed that reactivity of the recovered
catalyst was efficiently depending on the solvent applied for regen-
eration of catalyst. However, washing the filtered catalyst from
the first run by warm protic solvents such as water and alcohols
resulted in the obvious decreasing of reaction yield. In the contrast,
the recycled catalyst by EtOAc was reused three times with gradual
loss of activity in the model reaction (Table 1, entry 15, and Fig. 2).
Deploying the optimized reaction conditions, the scope of the
method was demonstrated using a variety of 1,3-dicarbonyls and
benzoins (Table 2).
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