Fe3+-montmorillonite as effective, recyclable catalyst for Paal-Knorr pyrrole synthesis under mild conditions

Article abstract of DOI:10.1081/SCC-200054200

Fe³⁺-montmorillonite clay catalyst was used for the first time for pyrroles preparation by Paal-Knorr condensation. The features of this catalytic system are improved yields, mild conditions, a simple procedure, and high catalytic efficiency. The catalyst could be easily recycled and reused for three times without visible losing of activity. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1081/SCC-200054200

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3+
Fe ‐Montmorillonite as Effective, Recyclable
Catalyst for Paal–Knorr Pyrrole Synthesis
Under Mild Conditions
a
a
b
a
a
Guoyong Song , Bo Wang , Guang Wang , Yuru Kang , Tao Yang
&
a
Liming Yang
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China
b
Key Laboratory of Gas Geochemistry, Lanzhou Institute of Geology, Chinese
Academy of Sciences, Lanzhou, China
Published online: 16 Aug 2006.
To cite this article: Guoyong Song , Bo Wang , Guang Wang , Yuru Kang , Tao Yang & Liming Yang (2005)
3+
Fe ‐Montmorillonite as Effective, Recyclable Catalyst for Paal–Knorr Pyrrole Synthesis Under Mild Conditions,
Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
35:8, 1051-1057, DOI: 10.1081/SCC-200054200
To link to this article: http://dx.doi.org/10.1081/SCC-200054200
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Synthetic Communications , 35: 1051–1057, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200054200
31
Fe -Montmorillonite as Effective,
Recyclable Catalyst for Paal–Knorr Pyrrole
Synthesis Under Mild Conditions
Guoyong Song and Bo Wang
State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, China
Guang Wang
Key Laboratory of Gas Geochemistry, Lanzhou Institute of Geology,
Chinese Academy of Sciences, Lanzhou, China
Yuru Kang, Tao Yang, and Liming Yang
State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou, China
3
þ
Abstract: Fe -montmorillonite clay catalyst was used for the first time for pyrroles
preparation by Paal–Knorr condensation. The features of this catalytic system are
improved yields, mild conditions, a simple procedure, and high catalytic efficiency.
The catalyst could be easily recycled and reused for three times without visible
losing of activity.
Keywords: Catalysis, diketones, heterocycles, solid support
Received in Poland October 20, 2004
Address correspondence to Liming Yang, State Key Laboratory for Oxo Synthesis
and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of
Sciences, Lanzhou 730000, China. Tel.: þ86-931-4968253; Fax: þ86-931-8277088;
E-mail: limy@ns.lzb.ac.cn

1
052
INTRODUCTION
In recent years, the use of clay, particularly montmorillonites, has received
G. Song et al.
[1]
considerable attention in chemical synthesis. Clay catalysts act as both
[2]
Brønsted and Lewis acids in their natural ion-exchanged forms. They are
inexpensive, noncorrosive, and recyclable. Thus, the montmorillonite-catalyzed
procedures have many advantages, such as environmental compatibility and
ease of handling. Various catalysts prepared by exchanged transition metal
cations on montmorillonite have been used in many typical acid-catalyzed
[
3]
[4]
reactions, such as Friedel–Crafts reactions, Beckmann rearrangement,
[6]
[5]
esterification, and nitration.
Recently, Paal–Knorr condensation has experienced a rekindled interest
in the synthetic chemistry for preparation of pyrrole, pyrazoles, and their
derivatives, as naturally occurring organic compounds and intermediates for
[
7]
[8]
pharmaceuticals. Various acidic materials, such as zeolite,
[10]
p-TSA,
KSF ), layered zirconium phosphate,
[9]
Al O , montmorillonite (K10,
and zirconium sulfophenyl phosphonate, and the microwave technology
[9a]
2
3
[11]
[12]
have been used to promote these condensations. Although some of these
processes offer distinct advantages over conventional methods, they suffer
from certain drawbacks, such as hazardous organic solvents, high cost, long
reaction time, and large amounts of solid catalysts. We previously reported
ionic liquids promoted Paal–Knorr condensation between amines and 2,5-
[
13]
hexandione, but the expensive cost of ionic liquids makes this methodology
far from practical. In continuation of our work on the synthesis of pyrroles, we
3
þ
report Fe -montmorillonite catalyzed Paal–Knorr condensation under mild
conditions.
RESULTS AND DISCUSSION
In the reaction of aniline and 2,5-hexandione, various metal-exchanged,
montmorillonite K10 catalysts have been used to promote the condensation
at room temperature (208C). See Scheme 1.
From Table 1, the catalytic activities of metal-exchanged montmorillonite
3
K10 catalysts in decreasing order are Fe . Zn . Co . Cu -
þ
2þ
2þ
2þ
Scheme 1.

31
Fe -Montmorillonite Catalyst
1053
Table 1. Paal–Knorr condensation of aniline with 2,5-
hexandione using various metal-exchange montmorillonite
clay catalysts
a
Entry
Catalyst
Time (h)
Yield (%)
3
Fe -mont.
þ
1
2
3
4
5
3
5
5
5
5
95
87
81
76
72
2
Zn -mont.
þ
2
þ
Co -mont.
2þ
Cu -mont.
K10
a
GC yield.
montmorillonite. Commercial K10 montmorillonite was also evaluated in this
reaction, but its activity was slightly inferior as compared with the others. As
3
þ
a result, Fe -montmorillonite proved to be a more effective catalyst for
the reaction of aniline and 2,5-hexandione as compared with the others;
this observation is analogous to previous investigations on
[
3]
[5]
Friedel–Crafts reactions and esterification. The high activity of the
þ
3
Fe -montmorillonite catalyst may be because of the enhanced interlayer
[
14]
acidity with increasing ratio of charge to cation radius.
3
þ
The effect of reaction temperature on the Fe -montmorillonite catalyzed
Paal–Knorr condensation of aniline with 2,5-hexandione was also inves-
tigated and the results are shown in Fig. 1. When the temperature was
3
Figure 1. The effect of temperature on the Fe -montmorillonite catalyzed
þ
Paal–Knorr condensation of aniline with 2,5-hexandione.

1
054
G. Song et al.
raised, maximum yield was reached soon. At 808C the substrates could be
completely consumed within 1 h; it is very interesting that the reaction also
can be finished at room temperature (208C) within 3 h. It should be pointed
out that the dosages of catalysts are in general more than 100% (wt) in
[7,9,10]
3þ
many reported solid catalytic systems.
In our Fe -montmorillonite
system, the dosage of catalyst was only 12% (wt), indicating the effectiveness
of this catalyst.
With these results in hand, other amines have been used to react with
2
,5-hexandione, and the results are listed in Table 2. To meet the requirement
of minimization of energy consumption, the reactions were carried out at
08C. It is evident that several amines could be converted to the corresponding
2
3þ
N-substituted pyrroles in good to excellent yields over the Fe -montmorillonite
catalyst. It is well known that the aliphatic amines are more nucleophilic
compared with aromatic amines, thus aliphatic amines give higher yields or
[7,13]
shorter reaction times than those generally observed in other systems.
Additionally, the possibility of the large-scale production of this reaction
was investigated by using aniline and 2,5-hexadione as substrates with
dosages of 50 times (0.1 mol) the initial amount, and the outcome is listed
in Table 2, entry 8. Compared to that of 2-mmol dosage, the reaction time
of the large-scale reaction was relatively reduced but the yield of product
increased to some extent. This result may be because of the increased exother-
micity of the reaction.
Table 2. Pyrrole derivatives produced by Paal–Knorr condensation
3þ
using Fe -montmorillonite catalyst
a
Entry
R
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
n-Propyl
n-Butyl
1
1
2
2
2
3
3
2
3
6
6
94
96
92
93
96
96
iso- Butyl
n-Heptyl
Benzyl
Phenyl
Phenyl
Phenyl
b
93
95
c
p-Methoxyphenyl
Cyclohexyl
2-Pyridinyl
94
74
69
1
1
0
1
a
b
Isolated yields.
Yield of N-phenyl-2,5-dimethylpyrrole obtained using the Fe
3þ
-
montmorillonite catalyst that had been reused three times.
c
Yield of N-phenyl-2,5-dimethylpyrrole using substrates with
dosage of 50 times of initial amount.

31
Fe -Montmorillonite Catalyst
1055
Because the present catalyst is a solid material, it could be easily recycled
after reaction by simple filtration. After further treatments including washing
and activation at 1208C, the recycled catalyst was used in the next run. In the
3
þ
reaction between aniline and 2,5-hexandione, the Fe -montmorillonite
catalyst could be reused three times without noticeable loss of activity.
CONCLUSION
3
þ
Fe -montmorillonite has proved to be a good catalyst for Paal–Knorr
condensation between amines and 2,5-hexandione. The features of the
3
þ
Fe -montmorillonite–catalyzed procedure are mild conditions, improved
product yield, and simple workup. Furthermore, it is relatively inexpensive
and results in less chemical waste because of the lower quantity of catalyst
(12% wt) needed as compared with the reported solid catalysts. Another
advantage is the reliable reusability of the catalyst; this property makes the
3
þ
present Fe -montmorillonite–catalyzed Paal–Knorr condensation an attrac-
tive green procedure with previously mentioned features.
EXPERIMENTAL
Catalyst Preparation
Montmorillonite K10 (2 g) was added to 25 mL aqueous solution (1 M) of
FeCl , ZnCl , CuCl , and CoCl . The mixture was stirred vigorously for
3
2
2
2
24 h. The clay suspension was centrifuged and the supernatant solution was
discarded. The obtained clay catalyst was then washed with distilled water
until free of chloride ions as indicated by an AgNO test. Before catalytic
3
reaction, the catalyst was dried in an oven at 1208C overnight.
Typical Procedure
Clay catalyst (12% wt) was added to a stirred solution of an amine (2 mmol),
2
,5-hexandione (2 mmol), and a little dichloromethane ( , 1 mL) in a 25-mL
round-bottom flask. The mixture was stirred at room temperature (208C) for
the desired time. After reaction, the solid catalyst was filtered and washed
with dichloromethane (3 ꢀ 2 mL). The liquid content was analyzed with
GC [HP 6890 equipped with a HP 5MS column (30 m)] and GC/MS
(HP 6890/5793). The concentration of reactant and product was directly
given by the system of GC chemstation according to the area of each chro-
matograph peak. The pure products could be obtained by evaporated under
[
10]
vacuum.

1
056
Analytical Data for Selected Compounds
,5-Dimethyl-1-butyl-1H-pyrrole: (yellow oil); d (300 MHz, CDCL ) 0.96
G. Song et al.
2
H
3
(
CH , t, 3H), 1.37 (CH , m, 2H), 1.58, (CH , m, 2H), 2.21 (CH , s, 6H), 3.69
3 2 2 3
(
CH , s, 2H), 5.71 (pyrrolics, s, 2H).
2
[
9]
1
5
7
1
-Phenyl-2,5-dimethyl-1H-pyrrole: (white solid, mp 50–518C, reported
08C); dH (300 MHz, CDCl ,) 2.04 (CH , s, 6H), 5.91 (pyrrolics, s, 2H),
3
3
.20 (PhH, m, 2H), 7.41 (PhH, m, 3H); d (75 MHz, CDCl ) 12.97 (2CH ),
05.56 (C-3, C-4), 127.58 (C-2, C-5), 128.19, 128.76, 129.01, 138.41 (Ph).
C
3
3
[
12]
1
3
5
-Benzyl-2,5-dimethyl-1H-pyrrole: (white solid, mp 40–418C, reported
7–388C); dH (300 MHz, CDCl3), 2.11 (CH , s, 6H), 4.99 (CH2, d, 2H),
3
.86 (pyrrolics, s, 2H), 6.86 (PhH, m, 2H), 7.28 (PhH, m, 3H).
1
-(4-Methoxyphenyl)-2,5-dimethyl-1H-pyrrole: (yellow solid, mp 57–598C
[
11]
reported 59–608C); d (300MHz, CDCl ), 2.04 (CH , s, 6H), 3.86 (OCH , s,
H
3
3
3
3H), 5.88 (pyrrolics, s, 2H), 6.93 (PhH, m, 2H), 7.14 (m, PhH, 2H).
2-(2,5-Dimethyl-1H-pyrrol-1-yl) pyridine: (colorless oil); dH (300 MHz,
CDCl ), 2.12 (CH , s, 2H), 5.90 (pyrrolics, s, 2H), 7.22 (PhH, m, 2H), 7.77
3
3
(PhH, m, 1H), 8.62 (PhH, m, 1H).
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