Silica tungstic acid and sulphated silica tungstic acid as highly efficient solid acid catalysts for the synthesis of pyrrole derivatives

Article abstract of DOI:10.1134/S1070363216120616

In the present study silica supported tungstic acid (STA) and sulphated silica tungstic acid (SSTA) were applied as efficient and cost-effective solid acid catalysts in the synthesis of N-substituted pyrrole derivatives via the Paal–Knorr reaction of 2,5-hexadione with aromatic and aliphatic amines at room temperature. The reaction completed in short time under mild conditions with high yield. The catalysts could be easily recovered upon reaction completion. Structures of all products were confirmed by elemental analysis, FT-IR, ¹H and ¹³C NMR spectra.

Full text of DOI:10.1134/S1070363216120616

ISSN 1070-3632, Russian Journal of General Chemistry, 2016, Vol. 86, No. 12, pp. 2924–2927. © Pleiades Publishing, Ltd., 2016.
Silica Tungstic Acid and Sulphated Silica Tungstic Acid
as Highly Efficient Solid Acid Catalysts for the Synthesis
of Pyrrole Derivatives1
F. Moradgholi*, J. Lari, and Y. Baratian
Chemistry Department, Payame Noor University, Tehran, 19395-4697 Iran
*e-mail: fateme.moradgholi@gmail.com
Received August 27, 2016
Abstract—In the present study silica supported tungstic acid (STA) and sulphated silica tungstic acid (SSTA)
were applied as efficient and cost-effective solid acid catalysts in the synthesis of N-substituted pyrrole
derivatives via the Paal–Knorr reaction of 2,5-hexadione with aromatic and aliphatic amines at room
temperature. The reaction completed in short time under mild conditions with high yield. The catalysts could be
easily recovered upon reaction completion. Structures of all products were confirmed by elemental analysis,
FT-IR, 1H and 13C NMR spectra.
Keywords: Paal–Knorr, pyrrole, sulphated silica tungstic acid, 2,5-hexadione, silica tungstic acid
DOI: 10.1134/S1070363216120616
INTRODUCTION
catalyzed efficiently numerous organic reactions
[18–20]. The clear advantages of those catalysts
initiated our application of STA and SSTA as
heterogeneous acid catalysts in the synthesis of N-
substituted pyrrole derivatives.
Various methods have been worked out for synthesis
of pyrroles, including the Knorr [1], Hantzsch [2] and
Paal–Knorr [3, 4] reactions. Pyrrole structures were
found in the molecules of some natural compounds
such as chlorophyll, vitamin B12 and many others [5].
EXPERIMENTAL
The widely used Paal–Knorr reaction was usually
catalyzed by various Lewis or Brønsted acids,
montmorillonite KSF-clay and iodine [6], aluminum
oxide [7], ionic liquids [8], zeolites [9, 10], sulfamic
acid [11], Bi(NO3)3·5H2O [12], Sc(OTf)3 [13], zircon-
nium phosphate and zirconium sulfophenylphospho-
nate [14], Ti (OPri)4 [15], Al2O3 [16], and p-TSA [16].
However, they are characterized by certain drawbacks
including long reaction time, harsh reaction conditions,
toxic solvents used, and complicated products separa-
tion. Some catalysts are corrosive and often cause
environmental problems. Therefore, development of
simple, efficient and environmentally friendly methods
under the action of new catalysts for the Paal–Knorr
reaction was highly desirable.
Chemicals were purchased from Merck and Fluka.
IR spectra were recorded on a Shimadzu 8300 spectro-
photometer. NMR spectra were measured on a
BrukerAvance DPX-400 spectrometer in CDCl3.
Purity of products and progress of the reactions were
monitored by TLC (silica-gel polygram SILG/UV254
plates). Elemental analysis was performed on a
Thermo Finnigan Flash EA micro analyzer. X-ray
powder diffraction studies were carried out on a
Rigaku Geigerflex-3034 diffractometer in the 2θ range
of 10–90 degrees using CuKα radiation (λ = 1.54 Å).
Preparation of silica tungstic acid (STA) [18,
19]. Oven dried silica-gel 60 (10 g) was added to
thionyl chloride (40 mL) in a round bottomed flask
(250 mL) equipped with a condenser with a CaCl2
drying tube. The mixture was refluxed upon stirring
for 48 h. Silica chloride was filtered off and dried.
Silica chloride 5 (6 g) and 7.03 g of sodium tungstate
were added to 20 mL of n-hexane and the mixture was
refluxed for 6 h. The reaction mixture was filtered,
In the recent studies silica supported tungstic acid
(STA) and sulphated silica tungstic acids (SSTA)
1 The text was submitted by the authors in English.
2924

SILICA TUNGSTIC ACID AND SULPHATED SILICA TUNGSTIC ACID
2925
Scheme 1. Preparation of the catalyst.
ONa
OH
O=W=O
O=W=O
OH
Si
Cl
Si
Si
O
Si
O
Sodium tungstate
Dry n-hexane
SOCl2
HCl, 0.1 N, 1 h
O
O
O
O
O
O
O
O
O
O
Reflux, 48 h
Reflux, 6 h
SiO2
SiO2
SiO2
SiO2
SO3H
O
O=W=O
ClSO3H, CH2Cl2
room temperature, 2 h
Si
O
O
O
SiO2
washed with distilled water and dried. The dry mixture
was stirred in 0.1 N HCl (40 mL) for 1 h. STA was
filtered off, washed with distilled water and dried.
1-(4-Ethoxyphenyl)-2,5-dimethyl-1H-pyrrole. IR
spectrum, ν, cm–1: 2977 (C–H), 1515 and 1404 (C=C).
1H NMR spectrum, δ, ppm: 1.25 t (J = 6.8 Hz, 3H),
2.09 s (6H), 4.14 t (J = 6.8 Hz, 2H), 5.95 s (2H), 7.02 d
Preparation of sulphated silica tungstic acid
(SSTA) [20]. A suction flask was equipped with a
constant pressure dropping funnel and a gas outlet tube
for passing HCl gas through an adsorbing solution of a
water trap. 1 mL Of cholorosulfonic acid was added
dropwise to a mixture of 2.5 g STA and 75 mL
dichloromethane at room temperature. Upon comple-
tion of addition, the mixture was stirred for 90 min for
total elimination of HCl gas. The resulting sulphated-
STA was separated and washed several times with
dichloromethane and dried at 120°C for 3 h.
13
(J = 8.8 Hz, 2H), 7.18 d (J = 8.8 Hz, 2H). C NMR
spectrum, δ, ppm: 13.0, 14.9, 63.71, 105.3, 114.73,
129.2, 129.3, 131.6, 158.3. Found, %: C 78.14; H
7.93; N 6.7. C14H17NO. Calculated, %: C 78.11; H 7.95;
N 6.5.
4-(2,5-Dimethyl-1H-pyrrol-1-yl)phenol. IR spectrum,
ν, cm–1: 3412 (OH), 2953 (C–H), 1562 and 1458 (C=C).
1H NMR spectrum, δ, ppm: 2.05 s (6H), 5.32 br (1H),
5.92 s (2H), 6.91 d (J = 8.8 Hz, 2H), 7.09 d (J = 8.8 Hz,
2H). 13C NMR spectrum, δ, ppm: 12.9, 105.2, 115.8,
129.2, 129.3, 131.6, 155.3. Found, %: C 75.34; H 8.97;
N 7.3. C12H13O. Calculated, %: C 75.36; H 8.95; N 7.32.
Synthesis of 2,5-dimethyl-1-phenyl-1H-pyrrole
catalyzed by STA. Method a. 2,5-Hexadione (1.1 mmol),
aniline (1 mmol) and ethanol (0.5 mL) were mixed
with STA (0.1 g) and stirred at room temperature for
an appropriate time. Upon completion of the reaction,
as indicated by TLC (n-hexane–ethyl acetate = 8 : 2),
ethanol was added solubilizing the product. The
remaining solid catalyst was filtered off and washed
with ethanol. The solvent was evaporated under re-
duced pressure to give the product.
3-(2,5-Dimethyl-1H-pyrrol-1-yl)phenol. IR spectrum,
ν, cm–1: 3409 (OH), 2920 (C–H), 1600 and 1492
1
(C=C). H NMR spectrum, δ, ppm: 2.07 s (6H), 5.32
br (1H), 5.93 s (2H), 6.616 t (J = 2 Hz, 1H), 6.78–7.09
13
m (2H), 7.32 d (J = 8 Hz, 1H). C NMR spectrum, δ,
ppm: 12.9, 105.5, 114.8, 115.3, 120.6, 129.0, 129.9,
140.1, 156.1. Found, %: C 75.37; H 8.96; N 7.31.
C12H13O. Calculated, %: C 75.36; H 8.95; N 7.32.
Synthesis of 2,5-dimethyl-1-phenyl-1H-pyrrole
catalyzed by SSTA. Method b. The synthesis was
carried out the same way as described above for
method a but catalyzed by SSTA (0.07 g).
2,5-Dimethyl-1-p-tolyl-phenyl-1H-pyrrole. IR spec-
1
trum, ν, cm–1: 2982(C–H), 1523 and 1434 (C=C). H
NMR spectrum, δ, ppm: 2.07 s (6H), 2.78 s (3H), 5.92
s (2H), 6.92 d (J = 8.8 Hz, 2H), 7.09 d (J = 8.8 Hz,
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 12 2016

2926
MORADGHOLI et al.
Scheme 2. Synthesis of substituted pyrroles.
O
method a: STA
R N
R NH2
method b: SSTA
O
2H). Found, %: C 84.32; H 8.17; N 7.53. C13H15N.
Calculated, %: C 84.28; H 8.15; N 7.56.
XRD patterns of STA and sulphated silica tungstic
acid demonstrated the presence of a broad peak around
2θ = 22° that indicated the link of WO3 to silica gel.
RESULTS AND DISCUSSION
STA and SSTA were used in the synthesis of
pyrrole derivatives by the reaction of 2,5-hexadione
with aromatic amines (Scheme 2).
Recently, STA and SSTA have been used as silica
supported solid acids in some organic transformations
[21–23] (Scheme 1).
The reaction of aniline (1 mmol) with 2,5-hexa-
dione (1.1 mmol) was used as the model for the
process optimization (Table 1). The accumulated data
demonstrated that the process was sensitive to the
amount of a catalyst. The effect of solvents was
studied with CH2Cl2, CH3CN, EtOH, and under
solvent free conditions. Among the screened solvents,
EtOH was the most efficient. In this media the Paal–
Interaction of sulfonic groups and tungsten moieties
with silica surface was studied by FT-IR spectra. FT-
IR spectrum of sodium tungstate demonstrated the
WO42− stretching vibrations band at 1680 cm−1, STA
similar band at 1634 cm−1 and a band attributed to
SiO2 stretching vibrations. Introduction of the SO32−
groups in STA led to decrease in intensity of the WO42−
stretching vibrations. Bands at ca. 1200–1250, 1177,
and 750–1000 cm–1 were attributed to the O=S=O
asymmetric and symmetric stretching vibrations and
S–O stretching vibrations of the sulfonic groups,
respectively.
Table 2. Synthesis of pyrrole derivatives with STA and SSTA
O
+
NH2
R
O
SiO2
Table 1. Optimization of the reaction conditions
WO3H
Method a:
O
N
catalyst, solvent
R
Method b: STA
OSO3H
R NH2
+
R N
O
Method a
Method b
R
Method aa
Method ba
time, h
2
yield, % time, min
yield, %
95
Solvent
STA, time, yield, SSTA, time, yield,
H
91
94
89
86
95
97
93
90
90
91
95
75
70
180
240
65
20
90
15
20
10
5
g
h
%
89
80
92
80
87
89
91
90
g
min
%
85
80
95
90
87
90
94
–
4-Me
4-Cl
2 : 30
5
91
CH3CN
0.10 2 : 25
0.070
90
91
CH2Cl2
EtOH
0.10
0.10
3
2
0.070 120
3-Cl
4 : 40
0.8
87
0.070
0.070
75
90
4-OMe
4-OEt
3-Me
2-OH
3-OH
4-OH
PhCH2
95
Solvent-free 0.10 1 : 30
0.8
95
EtOH
EtOH
EtOH
EtOH
0.70
3
0.025 120
0.050 120
2 : 50
0.2
91
0.35 4 : 30
90
0.60
0.20
4
2
0.100
–
75
–
0.4
88
0.2
94
a
Reaction conditions: amine (1 mmol), 2,5-heptadione (1.1 mmol)
in room temperature, method a: STA and method b: SSTA.
0.2
97
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 12 2016

SILICA TUNGSTIC ACID AND SULPHATED SILICA TUNGSTIC ACID
2927
Chem. Lett., 2000, vol. 8, p. 1423.
6. Banik, B.K., Samajdar, S., and Bnik, I., J. Org. Chem.,
2004, vol. 69, p. 213.
7. Ballini, R., Barboni, L., Bosica, G., and Petrini, M.,
Synlett, 2000, p. 391.
8. Wang, B., Gu, Y., and Luo, C., Tetrahedron Lett., 2004,
vol. 45, p. 3417.
9. Sreekumar, R. and Padmakumar, R., Synth. Commun.,
Knorr reaction proceeded smoothly and efficiently
particularly under the action of SSTA.
Under the optimized conditions aliphatic and
aromatic amines gave pyrrole derivatives in high to
excellent yields. Reaction of anilines with an electron-
donating group led to the corresponding products in
high yield within short time (Table 2).
1998, vol. 28, p. 1661.
10. Texier-Boullet, F., Klein, B., and Hamelin, J., Synthesis,
1986, vol. p. 409.
11. Luo, H., Kang, Y., Li, Q., and Yang, L., Heteroatom.
Chem., 2008, vol. 19, p. 2643.
The recovered STA and SSTA were washed with
ethanol (3 × 10 mL), dried at 100°C for 1 h and reused
in a subsequent reaction. The catalysts exhibited high
catalytic activity after four consecutive runs (80% STA
and 89% SSTA).
12. Banik, B.K., Banik, I., Renteriaa, M., and Asgupta, D.S.K.,
Tetrahedron Lett., 2005, vol. 46, p. 2643.
CONCLUSIONS
13. Chen, J., Wu. H., Zheng, Z., Jin, C., Zhang, X., and
Su, W., Tetrahedron Lett., 2006, vol. 47, p. 5383.
14. Curini, M., Montanari, F., Rosati, O., Lioy, E., and
Margarita, R., Tetrahedron Lett., 2003, vol. 44, p. 3923.
15. Yu, X.S. and Quesne, P.W.L., Tetrahedron Lett., 1995,
The highly efficient one-pot synthesis of pyrrol
derivatives by the Pall-Knorr reaction of 2,5-hexadione
with aromatic aldehydes catalyzed by STA or SSTA in
ethanol is developed.
vol. 36, p. 6205.
ACKNOWLEDGMENTS
16. Ballini, R., Barboni, L., Bosica, G., and Petrini, M.,
Synlett, 2000, p. 391.
17. Raghavan, S. and Anuradha, K., Synlett, 2003, p. 711.
18. Karami, B., Hashghaee, V., and Khodabakhshi, S.,
The authors are thankful to the Research Council of
Dargaz Payame Noor University for their support.
Catal Commun., 2012, vol. 20, p. 71.
19. Karami, B., Khodabakhshi, S., and Eskandari, K.,
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RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 12 2016