One-pot synthesis of highly substituted pyrroles using nano copper oxide as an effective heterogeneous nanocatalyst

Article abstract of DOI:10.1016/j.crci.2013.02.008

A convenient one-pot four-component reaction of aromatic aldehydes, β-keto esters and nitromethane in the presence an amine and 10 mol% CuO nanoparticles for the synthesis of highly substituted pyrroles is described. Excellent yields, easily available and less expensive catalyst and easy work-up are the key features of the present method. The NMR spectrum was coupled with quantum chemical calculations in DFT approach using the hybrid B3LYP exchange-correlation functional to confirm the structures of (1-benzyl-4-(4-chlorophenyl)-2-methyl-1H-pyrrol-3-yl)(phenyl)methanone 14 and 1-(1-benzyl-4-(4-chlorophenyl)-2-phenyl-1H-pyrrol-3-yl)ethanone 14′.

Full text of DOI:10.1016/j.crci.2013.02.008

C. R. Chimie 16 (2013) 1063–1070
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Full paper/Memoire
One-pot synthesis of highly substituted pyrroles using nano copper oxide
as an effective heterogeneous nanocatalyst
*
Hamid Saeidian , Morteza Abdoli, Robab Salimi
Department of Science, Payame Noor University (PNU), PO Box 19395-4697, Tehran, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
A convenient one-pot four-component reaction of aromatic aldehydes,
b-keto esters and
Received 1 December 2012
Accepted after revision 7 February 2013
Available online 15 March 2013
nitromethane in the presence an amine and 10 mol% CuO nanoparticles for the synthesis of
highly substituted pyrroles is described. Excellent yields, easily available and less
expensive catalyst and easy work-up are the key features of the present method. The NMR
spectrum was coupled with quantum chemical calculations in DFT approach using the
hybrid B3LYP exchange-correlation functional to confirm the structures of (1-benzyl-4-(4-
chlorophenyl)-2-methyl-1H-pyrrol-3-yl)(phenyl)methanone 14 and 1-(1-benzyl-4-(4-
chlorophenyl)-2-phenyl-1H-pyrrol-3-yl)ethanone 140.
Keywords:
Nano copper oxide
Highly substituted pyrroles
Multicomponent reactions
Nanocatalyst
ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
DFT calculations
1. Introduction
The known used routes for synthesizing these com-
pounds are Hantzsch [3], Knorr [4], and Paal–Knorr [5]
Multicomponent reactions (MCRs) have emerged as a
vital field of chemistry and a powerful tool in modern
synthetic organic chemistry, allowing the facile synthesis
of complicated compounds in a one-pot reaction without
the isolation of any intermediates. Clearly, for multi-step
synthetic procedures the number of reaction and purifica-
tion steps is among the most important criteria for the
efficiency and practicability of the process and should be as
low as possible. Therefore, it has drawn the attraction of
organic chemists to develop novel MCRs for a broad range
of products [1].
On the other hand, pyrrole nucleus is a versatile and
valuable building block for a number of biologically and
pharmaceutically active compounds (Fig. 1). A range of
pharmacological properties, such as antibacterial, antitu-
mor, anti-inflammatory, antioxidant, anti-anginal and
antifungal activities of this important class of heterocycles
are found in the literature [2].
reactions. However, most of these methods employ expen-
sive catalysts, multi-steps synthetic operations, long reac-
tion times or harsh reaction conditions. Thus, it is clearly
evident that the need for the development of new and
flexible methods is required. Recently, literature has
highlightedtheimportanceofnanosizedmaterialsinseveral
scientific and technological areas, and many research
councils have intensified investments in nanotechnology
for the coming years [6]. The surface of metal oxides, such as
TiO2, Al2O3, ZnO, CuO and MgO, exhibits both Lewis acid and
Lewis base character [7]. They are excellent adsorbents for a
wide variety of organic compounds and increase the
reactivity of the reactants. In any metal oxide, surface atoms
make a distinct contribution to its catalyst activity. In
powder particles, the number of surface atoms is a large
fraction of the total. The high surface area to volume ratio of
metal oxide nanoparticles is mainly responsible for their
catalytic properties [8]. High yield, selectivity and recycla-
bility have been reported for a variety of nanocatalyst-based
organic reactions [9]. Nano copper oxide is certainly one of
the most interesting metal oxides, because it has surface
properties, which suggest that a very rich organic chemistry
may occur there [10]. Althougha fewnumber of new pyrrole
* Corresponding author.
E-mail address: h_porkaleh@yahoo.com (H. Saeidian).
1631-0748/$ – see front matter ß 2013 Acade´mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.02.008

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H. Saeidian et al. / C. R. Chimie 16 (2013) 1063–1070
R6
R4
SO2R5
Cl
R1
Br
Br
OH
HN
HN
Br
O
Cl
Br
R3
N
H
O
Br
N
OH
O
R2
antianginal
Pioluteorine
Pentabromopseudiline
Cl
HO
Cl
O
OH
HN
EtO
CF3
Br
NC
Chlorfenapyr
N
N
Cl
Br
O
Cl
Marinopyrrole B
Cl
Fig. 1. Some pyrrole-containing bioactive compounds.
syntheses, includingMCRs[2c], havebeenreportedinrecent
years, no example is known for the synthesis of highly
substituted pyrroles, using CuO nanoparticles.
(St. Louis, MO, USA), Fluka (Neu-Ulm, Germany) and Merck
(Darmstadt, Germany) companies and were used as
received. Nano copper oxide, with the mean particle size
40 nm and purity 99.9%, was purchased from Nano
Pioneers (Mashhad, Iran) company. The synthesized
compounds gave all satisfactory spectroscopic data. A
Bruker (DRX-500 Avanes) NMR was used to record the 1H-
NMR and 13C-NMR spectra. All NMR spectra were
determined in CDCl3 at ambient temperature.
All geometry optimizations and frequency calculations
of all species were carried out using the Gaussian 03
program [14]. Density Functional Theory with the Becke
three parameters hybrid functional (DFT-B3LYP) calcula-
tions were performed with a 6-311++G (2d, 2p) basis set
for all atoms. Vibrational frequencies were calculated at
the same level to ensure that each stationary point is a real
minimum. Harmonic-oscillator approximation was also
used for the thermodynamic partition functions. After
geometry optimization and frequency calculations, zero-
point energies (ZPEs) and thermal corrections are obtained
at 298 K. The NMR computations of absolute shieldings
were performed using the Gauge-Independent Atomic
Orbital (GIAO) method [15].
In the context of our general interest in the synthesis of
heterocycles [11] and following our research on nanopar-
ticles applications in organic synthesis as nanocatalysts
[12], herein, we propose a facile synthesis of highly
substituted pyrroles using CuO nanoparticles as an
efficient, commercially available and non-toxic nanocata-
lyst via a four-component reaction (Scheme 1). A joint of
experimental and DFT calculations was used to confirm the
structure of (1-benzyl-4-(4-chlorophenyl)-2-methyl-1H-
pyrrol-3-yl)(phenyl)methanone 14 and 1-(1-benzyl-4-(4-
chlorophenyl)-2-phenyl-1H-pyrrol-3-yl)ethanone 140. It
was shown that DFT methods perform NMR spectra
calculations very well and give accurate results [13].
2. Experimental
2.1. General information
All the chemicals required for the synthesis of highly
substituted pyrroles were purchased from Sigma-Aldrich
2.2. General procedure for synthesis of highly substituted
pyrroles
To a stirred suspension of CuO nanoparticles (10 mol%)
in nitromethane (5 mL) were added an aryl aldehyde
(1 mmol), a b-keto ester (1 mmol) and an amine (1.2 mol).
The reaction mixture was stirred at 100–105 8C for 12 h.
The progress of the reaction was monitored by TLC. After
completion of the reaction, the catalyst was recovered by
filtration. The nano CuO was reused in subsequent reaction
without losing any significant activity. The mixture was
concentrated under vacuum. The residue was purified by
preparative TLC (eluent:petroleum ether/ethyl acetate 5:1)
Scheme 1. Synthesis route of highly substituted pyrroles via a four-
component reaction using nano CuO.

H. Saeidian et al. / C. R. Chimie 16 (2013) 1063–1070
1065
Nano CuO
O
O
O
R3
R2
Ar
H
R1
NH2
NH
NO2
R3
R1
Ar
O
Ar
O
N
O
O
HN
R1
N
H
R2
R3
H
H
O
Nano CuO
O
R2
I
II
III
O
R2
H
Ar
O
R2
H
R3
Ar
Ar
O
N
O
a
n
o
N
C
R2
O
R3
u
N
O
N
N
HN
R1
Scheme 2. Suggested reaction mechanism for the formation of desired highly substituted pyrroles.
R3
O
R1
Nano CuO
R1
O
to afford the desired compound. The structure of the
products was confirmed by 1H-NMR, 13C-NMR and
comparison with authentic samples prepared by reported
methods [16,17]1.
3. Results and discussion
To find the optimal conditions, the synthesis of ethyl 1-
benzyl-4-(4-chlorophenyl)-2-methyl-1H-pyrrole-3-car-
boxylate (Table 1, entry 8) was used as a model reaction. A
mixture of 4-chlorobenzaldehyde (1 mmol), methyl acet-
oacatate (1 mmol), benzyl amine (1.3 mol) and nitro-
methane (5 mL) was stirred under various reaction
conditions at 100 8C. The reaction was checked without
nano CuO as a catalyst, and we did not get any product,
while good results were obtained in the presence of CuO
nanoparticles after 12 h. On using the optimized amount
of catalyst, we found that 10 mol% of CuO nanoparticles
could effectively catalyze the reaction for the synthesis of
the desired product. Commercially available CuO bulk also
was evaluated for the synthesis of the desired title
compounds. Clearly, the reaction time, using CuO nano-
particles, has been reduced by three times with higher
yield than CuO bulk (81% versus 45%). The optimum
reaction conditions used was 10% nano CuO and boiling
nitromethane as the reaction medium. The 1H-NMR
spectrum of ethyl 1-benzyl-4-(4-chlorophenyl)-2-meth-
yl-1H-pyrrole-3-carboxylate (Table 1, entry 8) showed
1
Representative spectroscopic data: Table 1, Entry 1: 1H-NMR
(400 MHz, CDCl3): d 2.51 (s, 3H), 3.71 (s, 3H), 5.10 (s, 2H), 6.63 (s,
1H), 7.11 (d, J = 6.8 Hz, 2H), 7.30–7.43 (m, 8H); 13C-NMR (100 MHz,
CDCl3): d 11.56, 50.57 (2 C), 110.76, 120.59, 126.18, 126.21, 126.57,
127.65, 127.84, 128.96, 129.11, 135.80, 136.60, 136.73, 166.33. Table 1,
Entry 2: 1H-NMR (400 MHz, CDCl3): d 2.52 (s, 3H), 3.73 (s, 3H), 5.09 (s,
2H), 6.62 (s, 1H), 7.12 (d, J = 8.4 Hz, 2H), 7.32–7.43 (m, 7H); 13C-NMR
(100 MHz, CDCl3): d 11.64, 50.61, 50.63, 110.68, 120.67, 125.09, 126.57,
127.78, 127.92, 128.98, 130.47, 132.02, 134.34, 136.61, 136.90, 166.10.
Table 1, Entry 5: 1H-NMR (400 MHz, CDCl3): d 2.52 (s, 3H), 3.64 (s, 3H),
5.12 (s, 2H), 6.60 (s, 1H), 7.11 (d, J = 8.4 Hz, 2H), 7.23–740 (m, 7H); 13C-
NMR (100 MHz, CDCl3): d 11.39, 50.57, 50.65, 112.13, 120.57, 120.93,
122.91, 126.03, 126.57, 127.64, 127.82, 128.98, 129.11, 131.61, 134.35,
136.01, 136.76, 166.01; Table 1, Entry 7: 1H-NMR (400 MHz, CDCl3): d
1.16 (t, J = 7.2 Hz, 3H), 2.51 (s, 3H), 4.20 (q, J = 7.2 Hz, 2H), 5.10 (s, 2H), 6.63
(s, 1H), 7.11 (d, J = 6.8 Hz, 2H), 7.30–7.44 (m, 8H); 13C-NMR (100 MHz,
CDCl3): d 11.51, 14.08, 50.55, 59.41, 111.07, 120.44, 126.14, 126.22,
126.58, 127.53, 127.81, 128.95, 129.30, 135.88, 136.45, 136.78, 165.91.
Table 1, Entry 8: 1H-NMR (400 MHz, CDCl3): d 1.21 (t, J = 7.2 Hz, 3H), 2.53
(s, 3H), 4.22 (q, J = 7.2 Hz, 2H), 5.10 (s, 2H), 6.63 (s, 1H), 7.12 (d, J = 7.2 Hz,
2H), 7.32–7.46 (m, 7H); 13C-NMR (100 MHz, CDCl3): d 11.61, 14.22, 50.58,
59.52, 110.96, 120.57, 125.08, 127.56, 127.66, 127.90, 128.97, 129.01,
129.34, 130.65, 131.98, 134.44, 165.68. Table 1, Entry 12: 1H-NMR
(400 MHz, CDCl3): d 1.18 (t, J = 7.2 Hz, 3H), 1.86 (d, J = 6.8 Hz, 3H), 2.48 (s,
3H), 4.19 (q, J = 7.2, 2H), 5.40 (q, J = 6.8 Hz, 1H), 6.74 (s, 1H), 7.12 (d,
J = 8.4 Hz, 2H), 7.3–7.35 (m, 7H); 13C-NMR (100 MHz, CDCl3): d 11.49,
14.16, 22.18, 55.17, 59.48, 110.83, 116.92, 124.79, 125.83, 127.63, 127.69,
128.92, 130.58, 131.91, 134.69, 136.70, 141.98, 165.81.
two signals (
group, a singlet (
pyrrole ring, a resonance for the deshielded benzylic
proton ( 5.09), a singlet ( 6.62) for the aromatic proton
d
1.21 and 4.22) for the protons of ethoxy
d
2.52) for the methyl protons of the
d
d
of the pyrrole ring. The presence of this signal in 1H-NMR
spectra of synthesized compounds is a good indication of
the formation of the desired chemicals. The 1H-decoupled

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H. Saeidian et al. / C. R. Chimie 16 (2013) 1063–1070
Table 1
CuO nanoparticles catalyzed formation of highly substituted pyrroles.
Entry
1
b
-Keto esters
Amines
Aldehydes
Products
Yield (%)
75
NH2
O
H
O
O
O
H3CO
OCH3
H3C
N
Bn
2
78
Cl
NH2
O
H
O
O
O
OCH3
H3CO
H3C
N
Cl
Bn
3
69
OCH3
NH2
O
O
O
H
O
OCH3
H3CO
H3C
N
Bn
OCH3
H
4
65
NO2
O
NH2
O
O
O
OCH3
H3CO
H3C
N
Bn
NO2
5
73
O
H
NH2
O
O
O
Cl
H3CO
OCH3
Cl
H3C
N
Bn

H. Saeidian et al. / C. R. Chimie 16 (2013) 1063–1070
1067
Table 1 (Continued )
Entry
6
b
-Keto esters
Amines
Aldehydes
Products
Yield (%)
76
Cl
O
H
NH2
O
O
O
Cl
OCH3
H3CO
Cl
H3C
N
Cl
Bn
7
77
O
H
O
O
NH2
O
OC2H5
C2H5O
H3C
N
Bn
8
81
Cl
O
O
NH2
O
H
O
OC2H5
C2H5O
H3C
N
Cl
Bn
9
70
OCH3
O
O
NH2
O
H
O
OC2H5
C2H5O
H3C
N
Bn
OCH3
H
10
74
O
O
O
NH2
NO2
OC2H5
O
C2H5O
NO2
H3C
N
Bn

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H. Saeidian et al. / C. R. Chimie 16 (2013) 1063–1070
Table 1 (Continued )
Entry
11
b
-Keto esters
Amines
Aldehydes
Products
Yield (%)
75
Cl
O
H
O
O
NH2
Cl
O
OC2H5
C2H5O
Cl
Cl
H3C
N
Bn
12
68
O
O
Cl
O
H
H3C
NH2
OC2H5
O
C2H5O
H3C
Cl
N
Ph
13
66
O
H
O
O
H3C
NH2
O
Cl
OC2H5
C2H5O
Cl
H3C
N
Ph
13C-NMR spectrum of ethyl 1-benzyl-4-(4-chlorophenyl)-
2-methyl-1H-pyrrole-3-carboxylate showed 17 distinct
resonances in agreement with the proposed structure.
After finding the suitable conditions, we subjected a
esters. Subsequently, the enamine I reacts with nitrostyr-
ene II to provide the Michael adduct III. The intermediate
III undergoes cyclization with a loss of HNO2 to yield the
desired products. The high surface area to volume ratio of
CuO nanoparticles is mainly responsible for their catalytic
properties [8].
The applicability of the present methodology is further
extended by performing the reaction with 4-chloroben-
zaldyde, benzoylacetone, benzyl amine and nitromethane
in the presence of CuO nanoparticles (Scheme 3).
This reaction can lead to two structural isomeric
products; hence, their NMR spectra are very similar
together. In order to provide an assignment of NMR
spectra of the studied compounds, the NMR spectra of
isomers were coupled with quantum chemical calculations
series of other aromatic aldehydes, primary amines and b-
keto ester to obtain the corresponding poly-substituted
pyrroles under the optimized reaction conditions. The
results are summarized in Table 1. Generally, the reactions
were performed well and the reaction rates as well as the
yields of products are satisfactory.
The suggested mechanism for this reaction is patterned
in Scheme 2. We believe that CuO nanoparticles are
coordinated to the oxygen of the aldehyde and activate it
for nucleophilic attack [18]. On the other hand, CuO
nanoparticles facilitate the enaminezation of the
b-keto

H. Saeidian et al. / C. R. Chimie 16 (2013) 1063–1070
1069
Cl
+
Cl
O
H
NH2
O
O
Ph
O
O
H3C
CH3NO2
+
+
Ph
Nano CuO
H3C
Ph
N
N
Cl
Bn
Bn
14'
14
Scheme 3. CuO nanoparticles catalyzed reaction of 4-chlorobenzaldyde with benzoylacetone.
Fig. 2. Numbering optimized structures of 14 (140).
in the DFT approach of hybrid B3LYP exchange-correlation
functional to confirm the structures of the synthesized
compounds. The optimized structures of 14(140), are given
in Fig. 2 with atom numbers. In order to express the
chemical shifts in ppm, the geometry of the TMS molecule
had been optimized and then, its NMR spectrum was
calculated by using the same method and basis set as in the
case of the calculations on the compounds 14(140).
The calculated isotropic shielding constants were then
transformed to chemical shifts relative to TMS. The
calculated spectra are compared with the experimental
data. Theoretical and experimental chemical shifts of NMR
of 14(140) are given in Table 2.
4. Conclusions
In conclusion, we have reported an efficient procedure
for the synthesis of highly substituted pyrroles using CuO
nanoparticles as a non-toxic and inexpensive heteroge-
neous nanocatalyst. The method also offers some other
advantages, such as clean reaction, low loading of catalyst
and use of various substrates, which make it a useful and
attractive strategy for the synthesis of highly substituted
pyrroles. A joint of experimental and DFT calculations was
also used to confirm the structure of (1-benzyl-4-(4-
chlorophenyl)-2-methyl-1H-pyrrol-3-yl)(phenyl)metha-
none 14 and 1-(1-benzyl-4-(4-chlorophenyl)-2-phenyl-
1H-pyrrol-3-yl)ethanone 140.
As it can be seen from Table 2, the NMR shifts obtained
by the DFT method at the B3LYP/6-311++G (2d,2p) level
are in reasonable agreement with the experimental values
of the 1H-NMR and 13C-NMR spectra.
Acknowledgements
We would like to acknowledge the financial support
from Payame Noor University (PNU). We also are thankful
to Mr. Biglari for NMR spectral support.
Table 2
Experimental and calculated 1H-NMR and 13C-NMR chemical shifts (ppm)
for 14(140).
References
Atom
14
Calcd.
140
Calcd.
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