Silica-supported bismuth(III) chloride as a new recyclable heterogeneous catalyst for the Paal-Knorr pyrrole synthesis

Article abstract of DOI:10.1016/j.jorganchem.2012.02.008

Silica-supported bismuth(III) chloride (BiCl₃/SiO₂) acts as as a highly efficient heterogeneous Lewis acid catalyst for the Paal-Knorr pyrrole synthesis in hexane at room temperature. The catalyst exhibited remarkable reusable activity and higher catalytic performance than homogeneous BiCl₃. A plausible mechanism for the catalytic action of BiCl₃/SiO₂ has been introduced.

Full text of DOI:10.1016/j.jorganchem.2012.02.008

Journal of Organometallic Chemistry 708-709 (2012) 25e30
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Silica-supported bismuth(III) chloride as a new recyclable heterogeneous catalyst
for the PaaleKnorr pyrrole synthesis
1
1
1
1
1
Kioumars Aghapoor , Leila Ebadi-Nia , Farshid Mohsenzadeh , Mina Mohebi Morad , Yadollah Balavar ,
*
Hossein Reza Darabi
Nano & Organic Synthesis Lab., Chemistry & Chemical Engineering Research Center of Iran, Pajoohesh Blvd., km 17, Karaj Hwy, Tehran 14968-13151, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 September 2011
Received in revised form
Silica-supported bismuth(III) chloride (BiCl /SiO ) acts as as a highly efficient heterogeneous Lewis acid
3
2
catalyst for the PaaleKnorr pyrrole synthesis in hexane at room temperature. The catalyst exhibited
remarkable reusable activity and higher catalytic performance than homogeneous BiCl . A plausible
mechanism for the catalytic action of BiCl /SiO has been introduced.
Ó 2012 Elsevier B.V. All rights reserved.
3
7
January 2012
3
2
Accepted 8 February 2012
Keywords:
BiCl /SiO
3 2
Heterogeneous Lewis acid catalyst
PaaleKnorr reaction
Pyrrole
1
. Introduction
remains the most useful preparative method for generating
pyrroles [18e28]. In this method, 1,4-dicarbonyl compounds are
converted to pyrrole via acid-mediated dehydrative cyclization in
the presence of a primary amine [29,30]. The main limitations to
intensive use of this reaction are the strong reaction conditions
required for cyclization (use of boiling acetic acid for extended
times).
Heterogeneous supported catalysts have been gained much
attention in recent years, as they possess a number of advantages
[
1,2]. Immobilization of catalysts on solid support provides an ideal
method for combining the advantages of homogeneous catalysts
high activity and selectivity, etc.) with the engineering advantages
(
of heterogeneous catalysts (available active site, easy catalyst
separation, long catalytic life, thermal stability, low hygroscopic
properties, easy handling and reusability of catalysts). Therefore,
use of supported and recoverable catalysts in organic trans-
formations has economical and environmental benefits [3e8].
Pyrroles are important heterocyclic compounds displaying
remarkable pharmacological properties such as antibacterial,
antiviral, anti-inflammatory, antitumoral, and antioxidant activities
Recently, a range of catalysts and protocols have been developed
for the preparation of these compounds, e.g. alumina [31], mont-
morillonite KSF [32,33], iodine [33], montmorillonite K-
3þ
10 ꢀ microwave irradiation [34,35], Fe emontmorillonite [36],
layered -Zr(KPO [37], [bmim]I ionic liquid [38], Sc(OTf) [39],
microwave irradiation [40], ZrCl /ultrasonic irradiation [41], InCl
[42] and Ga(OTf) [43].
a
4
)
2
3
4
3
3
The importance of bismuth as a homogeneous catalyst has
already been cited in the literature for the PaaleKnorr synthesis
[44,45]. The aim of the present work is the incorporation of
bismuth into the framework of silica, combining the properties
such as catalyst selectivity and activity with the ease of separation
and catalyst reuse.
[
9e11]. The pyrrole moiety is also present in many naturally
occurring compounds such as heme, chlorophyll, and vitamin B12
12e17]. In view of their high significance, many methodologies
[
have been developed for the construction of the pyrrole moiety
regarded as skeleton. Among them, the PaaleKnorr synthesis
In the continuation of our studies on the design and application
of solid acid catalysts in organic transformations [46,47], herein, we
wish to introduce silica-supported bismuth(III) chloride (BiCl
SiO ) as a novel heterogeneous catalyst for the PaaleKnorr pyrrole
cyclocondensation reaction (Scheme 1).
3
/
*
Corresponding author. Tel.: þ98 21 44580720; fax: þ98 21 44580762.
E-mail addresses: darabi@ccerci.ac.ir, r_darabi@yahoo.com (H.R. Darabi).
Tel.: þ98 21 44580720; fax: þ98 21 44580762.
2
1
0
022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.02.008

26
K. Aghapoor et al. / Journal of Organometallic Chemistry 708-709 (2012) 25e30
O
Ar
N
BiCl /SiO2 (7.5 mol%)
3
Ar NH2
+
Hexane, RT
O
3 2
Scheme 1. Condensation of hexane-2,5-dione with amines catalyzed by BiCl /SiO .
2
. Experimental
2.4. Selected spectroscopic data
2.1. Materials and methods
Although pyrroles 1c and 1f are known, no spectroscopic data
were found in the literature. All of the products were characterized
1
13
The N
2
adsorption/desorption analyses were performed on
by H NMR, C NMR and MS spectral analyses.
ꢁ
BELSORP-miniII at 77 K. Silica gel was degassed at 300 C, and BiCl
3
/
ꢁ
0
SiO
2
was degassed at 100 C both for 1.5 h under inert gas flow prior
2.4.1. N-(4 -Methylphenyl)-2,5-dimethylpyrrole (1b) [CAS Registry
analysis. Specific surface area, total pore volume, and pore diameter
of samples was obtained by BrunauereEmmetteTeller (BET)
method using BELSORP analysis software. Thermogravimetric
no. 5044-26-8] [42]
1
H NMR (500 MHz, CDCl
3
)
d
7.40 (d, J ¼ 15.8 Hz, 2H), 7.24 (d,
13
J ¼ 15.8 Hz, 2H), 6.06 (s, 2H), 2.59 (s, 3H), 2.19 (s, 6H); C NMR
analysis (TGA) measurements of silica gel and BiCl
3
/SiO
2
were
(125 MHz, CDCl
3
)
d
137.3, 136.2, 129.5, 127.7, 105.4, 21.0, 12.9; MS
þ
carried out in PerkineElmer Pyris Diamond instrument from 32 to
(EI), m/z (rel. intensity %) 185 (M , 75), 184 (100), 170 (15), 154 (10),
129 (20), 91 (10), 77 (5).
ꢁ
ꢁ
6
00 C, using a ramp rate of 10 C/min under dry N
2
.
1
13
H and C NMR spectra were recorded on a Bruker-500. All
NMR samples were measured in CDCl and chemical shifts are
expressed as ppm relative to internal Me Si. Mass spectra were
3
4
obtained on a Fisons instrument. Substrates are commercially
available and used without further purification.
a
1
05
00
1
2
.2. Preparation of BiCl
3 2
/SiO catalyst
95
90
85
80
3
0 g of silica gel (300e400 mesh) were activated by refluxing
ꢂ1
with 150 mL of 6 mol L hydrochloric acid under stirring for 24 h,
then the activated silica gel was filtered and washed with doubly
distilled water to neutral and dried under vacuum at 70 C for 24 h.
Bismuth(III) chloride (1.575 g, 5 mmol) was added to a suspen-
sion of activated silica gel (17.5 g) in toluene (30 mL). The mixture
was stirred at room temperature overnight and filtered off. The
ꢁ
75
7
0
5
6
0
100
200
300
400
500
600
Temperature (°C)
solid was washed with ethanol and filtered off. The obtained solid
ꢁ
was dried at 120 C under vacuum for 5 h to furnish BiCl
3
/SiO
2
as
b
a white free-flowing powder (6.1 wt% of O-BiOCl species as deter-
mined by TGA and 5.0 mol% of Bi as determined by Atomic
Absorption Spectrophotometry).
101
00
1
9
9
8
9
2.3. General procedure for the acid-catalyzed PaaleKnorr pyrrole
97
cyclocondensation
9
9
9
6
5
4
Silica-supported bismuth(III) chloride (0.30 g) was well stirred
in hexane (5 mL) for a few minutes. Primary amine (1 mmol) and
hexane-2,5-dione (1.2 mmol) were then added and the mixture
was stirred at room temperature for 1 h. The progress of the
reaction was monitored by TLC or GC. Upon the completion of the
reaction, the mixture was filtered off and the solid was washed with
dichloromethane. The combined organic phase was evaporated
under reduced pressure and afforded the crude product. It was
passed through a short column of neutral alumina [eluted with
ethyl acetate/hexane (1:9)] to give the pure products.
0
100
200
300
400
500
600
Temperature (°C)
c
1
02
00
1
98
96
94
92
90
Table 1
88
Textural data of SiO
2 3 2
and BiCl /SiO .
86
Sample
BET surface
area (m /g)
Total pore
volume (cm /g)
Average pore
diameter (A)
84
2
3
ꢀ
0
100
200
300
400
500
600
Temperature (°C)
SiO
BiCl
2
493
372
0.77
0.61
63
65
3
/SiO
2
3 2 3 2
Fig. 1. TGA plots of (a) BiCl , (b) SiO activated, (c) BiCl /SiO .

K. Aghapoor et al. / Journal of Organometallic Chemistry 708-709 (2012) 25e30
27
Table 2
The physical structure of silica gel (high surface area, large pore
volume, etc.) is one of the major reasons for its effectiveness as
a support material. However, these features were altered signifi-
cantly during its chemical modification with bismuth chloride. As
shown in Table 1, the BrunauereEmmetteTeller (BET) method
determines the dispersion of the active groups and the diffusion of
reagents in the active site of silica gel. The observed reduction in
surface area and pore volume of BiCl /SiO as compared to silica gel
3 2
3
can be attributed to the surface coverage of silica gel by BiCl to
define a new modified heterogeneous catalyst.
2
Types of bismuth moieties on SiO .
Bismuth moiety
Support
Weight loss (%)
Calculated
Found
BiCl
BiCl
BiCl
Bi
3
2
SiO
SiO
SiO
SiO
2
2
2
2
8.3
7.4
6.5
5.6
e
e
a
6.1
b
5.5
a
Determined by TGA.
Determined by Atomic Absorption Spectrometry.
b
On the other hand, the thermogravimetric analysis (TGA) gives
information on the thermal stability of loaded BiCl on silica gel and
whether it is chemically bound to the silica surface.
As shown in Fig. 1, TG curves of 13.9% BiCl /SiO
show a weight loss of around 5.1% and 2.7% up to 130 C which can
be attributed to loss of surface physisorbed water and organic
3
0
2
.4.2. N-(3 -Methylphenyl)-2,5-dimethylpyrrole (1c) [CAS Registry
no. 32570-10-8] [48]
3
2
and 5.4% SiO
2
1
H NMR (500 MHz, CDCl
3
)
d
7.43 (t, J ¼ 15.2 Hz, 1H), 7.29 (d,
ꢁ
J ¼ 15.2 Hz, 1H), 7.12 (s, 1H), 7.11 (s, 1H), 6.00 (s, 2H), 2.50 (s, 3H),
2
1
(
.14 (s, 6H); 13C NMR (125 MHz, CDCl
3
) d
129.7, 129.6, 128.7, 128.6,
materials. It shows a further weight loss of 8.8% and 2.7% gradually
28.0, 127.7, 127.5, 105.4, 21.1, 12.9; MS (EI), m/z (rel. intensity %) 185
M , 65), 184 (100), 170 (15), 154 (10), 129 (20), 91 (10), 77 (5).
ꢁ
in the range of 130e600 C for BiCl
3
/SiO
presents an additional 6.1%
weight loss, mainly attributed to the loading BiCl (Fig. 1b,c).
The bismuth content of BiCl /SiO was measured by the atomic
2 2
and SiO , respectively.
þ
3 2
Different from silica gel, the BiCl /SiO
3
0
2
.4.3. N-(4 -Nitrophenyl)-2,5-dimethylpyrrole (1e) [CAS Registry
3
2
no. 5044-22-4] [42]
absorption spectrometry and found to be 5.5% weight (5 mol%).
Moreover, the chloride content of the catalyst was determined by
1
H NMR (500 MHz, CDCl
3
)
d
8.34 (dd, J ¼ 13 Hz, 2H), 7.39 (d,
13
J ¼ 15 Hz, 2H), 5.95 (s, 2H), 2.06 (s, 6H); C NMR (125 MHz, CDCl
3
)
a titration of 0.1 M AgNO
3
solution with K
2
CrO
4
as an indicator
have been
d
146.7, 144.7, 128.7, 128.5, 124.5, 107.3, 13.1; MS (EI), m/z (rel.
(Mohr method) to show that 2 chloride ions per BiCl
3
þ
intensity %) 217 (30), 216 (M , 100), 215 (70), 169 (40), 154 (50), 77
15), 30 (40).
released. These supporting evidences are very close to that
obtained from TGA data and show that the exact weight loss of
(
3 2
BiCl /SiO is attributed to the formation of eOeBiOCl species on
0
2
.4.4. N-(3 -Nitrophenyl)-2,5-dimethylpyrrole (1f) [CAS Registry
silica gel (Table 2). The catalyst was washed with ethanol and then
separated by simple filtration. The atomic absorption spectroscopy
of the filtrate shows the absence of the bismuth, indicating that
there is almost no leaching of bismuth from the catalyst.
no. 32570-23-3]
1
3
H NMR (500 MHz, CDCl ) d 8.30 (m,1H), 8.12 (s,1H), 7.69 (t,1H),
.60 (t, 1H), 5.95 (s, 2H), 2.08 (s, 6H); C NMR (125 MHz, CDCl
148.6, 140.2, 134.6, 134.1 (2C), 123.2, 123.1, 108.4 (2C), 12.9 (2C);
MS (EI), m/z (rel. intensity %) 217 (20), 216 (M , 100), 215 (80), 199
13
7
d
3
)
þ
(
10), 169 (20), 154 (20), 77 (15).
3.2. Optimization of the model reaction
3 2
To evaluate the efficiency of BiCl /SiO , the reaction of hexane-
3
. Results and discussion
2,5-dione with aniline at room temperature was carried out. In
order to determine the optimum conditions, the solvent effect, the
proportions of catalyst to substrate and the reaction time were
examined.
3 2
3.1. Preparation and characterization of BiCl /SiO catalyst
Supported bismuth(III) chloride on silica gel was prepared by
The effect of various solvents on the yield of product is given in
Table 3. Low and even poor yields of product 1a were obtained in
water, tetrahydrofuran, ethyl acetate, acetonitrile, and
ꢂ1
adding BiCl
3
to a suspension of activated silica gel (0.29 mmol g
SiO ) in toluene at room temperature overnight.
2
Table 3
3 2
Solvent effect on the synthesis of 1a in the presence of BiCl /SiO .
O
NH2
BiCl /SiO2 (0.30 g)
3
+
N
Solvent
O
room temperature
1
.2 mmol
1 mmol
1a
Entry
Solvent
Time (h)
Conversion (%)a
1
2
3
4
5
6
7
8
9
e
1
1
1
1
1
1
1
1
1
93
90
88
4
23
25
11
19
98
Methanol
Ethanol
Water
Tetrahydrofuran
Ethyl acetate
Acetonitrile
Dichloromethane
Hexane
a
Gas Chromatography assay (%).

28
K. Aghapoor et al. / Journal of Organometallic Chemistry 708-709 (2012) 25e30
dichloromethane (Table 3, entries 4e8). Both ethanol and methanol
as well as solvent-free conditions gave about 90% yield of 1a
Table 5
Synthesis of N-substituted pyrroles catalyzed by BiCl
within 1 h.
3
/SiO
2
(7.5 mol%) in hexane
(Table 3, entries 1e3), whereas hexane was found to be the most
Conversion (%)a Yield (%)b
effective solvent (Table 3, entry 9).
To investigate the role of BiCl
Product designation Product
3
2
/SiO , the same reaction was
carried out in the absence or presence of catalyst. In the absence of
BiCl /SiO , low yield of product 1a was obtained, which indicates
that catalyst is obviously necessary for the reaction (Table 4, entries
e2). A remarkable yield for 1a was observed (76%) when the
reaction was carried out under catalytic influence of BiCl (Table 4,
entry 3), indicating the prominent role of BiCl as Lewis acid cata-
lyst. In contrast, heterogeneous BiCl /SiO proved to be more
effective compared to BiCl itself (Table 4, entries 4e7). Therefore,
the remarkable efficiency of BiCl /SiO can be explained by a better
synergetic effect of BiCl with SiO , due to the existence of multiple
Lewis acid catalytic centers.
In the study of catalyst loading, the effect of the relative
amounts of BiCl /SiO (i.e., active species of BiCl ) on the outcome
of the model reaction was also studied. The optimum amount of
BiCl /SiO was found to be 0.30 g of catalyst containing 0.075 mmol
of BiCl per 1 mmol of substrate (Table 4, entry 6). The fewer
1
1
1
a
b
c
98
90
81
92
81
73
N
3
2
1
3
3
Me
Me
N
3
2
3
3
2
3
2
N
3
2
3
Me
3
2
3
1
1
d
e
53
57
81
31
46
50
71
22
N
amount gave a lower yield, and the more amounts could not cause
the obvious increase for the yield of product.
The study on the reaction time showed that BiCl /SiO gave 90%
3 2
yield of product within 15 min. However, the optimal reaction time
is 60 min leading to quantitative conversion of 1a (98%).
O N
N
2
3.3. Evaluation of the reaction scope
O N
2
To expand the generality of this novel catalytic method, various
aromatic primary amines were tested under the optimized condi-
tions and the results are presented in Table 5. In all cases, the
reactions proceeded at room temperature, although the yields were
highly dependent on the substrate used. Although the aromatic
1f
N
NO2
N
2
amines bearing para- and meta-NO are enough active substrates,
the ortho-NO isomer shows very poor activity which may be due to
2
1g
the existence of the steric hindered effect (Table 5, 1eeg).
It is noteworthy to mention that the classical reaction for the
synthesis of 1g in boiling acetic acid during 7 h gave 23% yield [49],
while the same yield was obtained under BiCl
3 2
/SiO catalyzed
reaction conditions at ambient temperature within 1 h.
1h
1i
100
81
96
72
94
Br
N
N
There are recent reports of carrying out this reaction in the
presence of other catalysts under heterogeneous conditions. The
data presented in Table 6 show the promising features of this
protocol in terms of reaction rate, product yield, low catalyst
loading, and catalyst reusability when compared with other
methods.
NC
3 2
The reason why the BiCl /SiO is so efficient is not known. The
reaction is likely to take place inside the silica-cavities, which hold
the bismuth in the appropriate conformation for Lewis acid acti-
vation. Bismuth (a heavier p-block element) has potential for both
1j
N
100
Table 4
BiCl
3
/SiO
2
catalytic effect on the synthesis of 1a.
Me
Entry
Catalyst
amount (g)
Loaded BiCl
(mmol)
3
Solvent
Time
(h)
Conversion
(%)
a
1k
N
65
92
58
86
1
2
3
4
5
6
7
e
0.20b
e
Hexane
Hexane
Hexane
Hexane
Hexane
Hexane
Hexane
1
1
1
1
1
1
1
30
47
76
87
95
98
98
Me
e
e
0.05
0.025
0.05
0.075
0.10
0.10
0.20
0.30
0.40
Cl
1l
Cl
N
a
Gas Chromatography assay (%).
Activated silica gel in the absence of BiCl
b
a
3
.
Gas Chromatography assay (%).
Yields refer to those of pure isolated products.
b

K. Aghapoor et al. / Journal of Organometallic Chemistry 708-709 (2012) 25e30
29
Table 6
Literature results for the synthesis of 1a under various heterogeneous conditions.
Method
Catalyst/Solvent
Temperature
Time
Yield (%)
Catalyst reusability
[Ref]
ꢁ
1
2
3
4
5
6
7
8
Montorillonite KSF
Montorillonite K10
25
90
25
60
60
25
25
25
C
C
C
C
C
C
C
C
10 h
3 min
3 h
2 h
24 h
3 h
96
98
96
88
56
96
96
92
e
[32,33]
[34]
[36]
[37]
[37]
[38]
[45]
This work
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
a
e
3þ
Fe emontmorillonite/CH
Zirconium sulfophenyl phosphonate (6 mol%)
-Zirconium potassium phosphate (12 mol%)
2
Cl
2
3 Runs
3 Runs
3 Runs
3 Runs
e
a
[Bmim]I ionic liquid
Bismuth nitrate (50e100 mol%)/CH
Cl
2 2
10 h
1 h
BiCl
3
/SiO
2
(7.5 mol%)/C
6
H
14
6 Runs
a
Microwave irradiation.
3 2
Scheme 2. Plausible mechanism for BiCl /SiO catalyzed PaaleKnorr pyrrole cyclocondensation.
covalent and coordinate bonding due to weak shielding of the 4f-
shell (lanthanide contraction). This lowers the activation energy of
the reaction.
primary amine on the activated carbonyls in complex A produces
the intermediates B and C. Eventually, detachment of bismuth ion
and subsequent loss of water (intermediate D) leads to the
formation of the pyrrole ring.
Accordingly, we propose a plausible mechanism for the BiCl
3
/
SiO catalyzed double condensation of hexane-2,5-dione with
2
primary amine in Scheme 2. Bismuth sites of the catalyst interact
with carbonyl groups of the dicarbonyl compound to form the
activated carbonyls of complex A. Nucleophilic attack of the
3
.4. Recycling of BiCl
3 2
/SiO
The reusability of catalyst in hexane was also examined. As
shown in Fig. 2, the catalyst was recycled for subsequent runs. It
was found that the catalyst is still active in hexane even in sixth run
without great loss of activity. The recovery of catalyst was very easy.
Product is soluble in hexane, while the catalyst remains insoluble.
After completion of the reaction, the catalyst was simply filtered
Catalyst Recyclability
1
00
9
0
0
0
0
0
0
0
8
7
6
5
4
3
from the reaction mixture, washed with dichloromethane, dried at
ꢁ
1
20 C for 2 h and reused in subsequent run. No fresh catalyst was
added.
sion(%)
er
4. Conclusion
C
o
n
v
20
10
3 2
In conclusion, BiCl /SiO is introduced as a novel heterogeneous
0
Lewis acid catalyst for the PaaleKnorr pyrrole cyclocondensation.
In most cases, the catalyst showed moderate to excellent activity
for the condensation reaction.
1
st Run
2nd Run
3rd Run
4th Run
5th Run
6th Run
3 2
Fig. 2. Recyclability of BiCl /SiO in hexane for the synthesis of 1a.

3
0
K. Aghapoor et al. / Journal of Organometallic Chemistry 708-709 (2012) 25e30
[22] V.F. Ferreira, M.C.B.V. De Souza, A.C. Cunha, L.O.R. Pereira, M.L.G. Ferreira, Org.
The procedure offers several advantages including low catalyst
Prep. Proced. Int. 33 (2001) 411e454.
loading, operational simplicity, and increased variation of substit-
uents in the product.
[
[
23] G. Balme, Angew. Chem. Int. Ed. 43 (2004) 6238e6241.
24] U. Joshi, M. Pipelier, S. Naud, D. Dubreuil, Curr. Org. Chem. 9 (2005) 261e288.
The catalyst was stable under the reaction conditions, and its
activity in terms of yields slightly decreased with increasing
number of cycles of the reaction (conversion: 98% for the first run;
[25] S. Agarwal, S. Cämmerer, S. Filali, W. Fröhner, J. Knöll, M.P. Krahl, K.R. Reddy,
H.-J. Knölker, Curr. Org. Chem. 9 (2005) 1601e1614.
[
26] M. Biava, G.C. Porretta, G. Poce, S. Supino, G. Sleiter, Curr. Org. Chem. 11 (2007)
092e1112.
[27] C. Schmuck, D. Rupprecht, Synthesis 20 (2007) 3095e3110.
1
72% for the sixth run) making the procedure a powerful tool from
[28] J. Roger, H. Doucet, Adv. Synth. Catal. 351 (2009) 1977e1990.
[29] G. Minetto, L.F. Raveglia, M. Taddei, Org. Lett. 6 (2004) 389e392.
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758e761.
an industrial vantage point.
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