An expeditious and solvent-free synthesis of substituted pyrroles using sulfated anatase-titania as a solid acid catalyst

Article abstract of DOI:10.1246/bcsj.20120221

Sulfated anatase-titania (TiO₂S₄ 2¹) has been used as a solid acid catalyst for the synthesis of substituted pyrroles from diketone and aromatic/aliphatic (acyclic and cyclic) primary amines by simple physical grinding. This sulfated titania gives an excellent yield with less reaction time and is an inexpensive, easily recyclable nanocatalytic material for this reaction. Higher catalytic activity of TiO₂S ₄ 2¹ is due to its increased Broonsted acidity.

Full text of DOI:10.1246/bcsj.20120221

370
Bull. Chem. Soc. Jpn. Vol. 86, No. 3, 370-375 (2013)
© 2013 The Chemical Society of Japan
An Expeditious and Solvent-Free Synthesis of Substituted Pyrroles
Using Sulfated Anatase-Titania as a Solid Acid Catalyst
Krishnaswamy Ravi, Balu Krishnakumar, and Meenakshisundaram Swaminathan*
Photocatalysis Laboratory, Department of Chemistry, Annamalai University, Annamalainagar-608 002, India
Received August 15, 2012; E-mail: chemres50@gmail.com
2¹
Sulfated anatase-titania (TiO2-SO4 ) has been used as a solid acid catalyst for the synthesis of substituted pyrroles
from £-diketone and aromatic/aliphatic (acyclic and cyclic) primary amines by simple physical grinding. This sulfated
titania gives an excellent yield with less reaction time and is an inexpensive, easily recyclable nanocatalytic material for
2¹
this reaction. Higher catalytic activity of TiO2-SO4 is due to its increased Brønsted acidity.
Modified semiconductor oxides with anionic species such as
acidic sites of TiO2.21 Though sulfated metal oxides have both
Lewis and Brønsted acid sites, their super acidity has been
attributed to the Brønsted acid sites.22 Surface acidity can be
determined by spectrophotometric methods on the basis of
irreversible adsorption of pyridine,23,24 but this method esti-
mates both Lewis and Brønsted acidity. Brønsted acidic sites
can be selectively determined by temperature-programmed
desorption (TPD) study using 2,6-dimethylpyridine as a probe
and this method revealed many fold increase of Brønsted acidic
sites in TiO2 by the loading of sulfate using sulfuric acid.25
This sulfated TiO2 has been used for several organic trans-
formations in our laboratory.1,2,26-28 In the present work, we
report a simple, practical, and efficient method for the synthesis
of pyrroles from £-diketones and primary amines catalyzed by
sulfate, phosphate, molybdates, and tungstates have recently
gained an increasing interest in the field of heterogeneous
catalysis, because of their enhanced activity in acid-catalyzed
reactions,1-5 and are considered solid “superacids.” Acidity
of titania and zirconia based metal oxides has been either
attributed to the surface Lewis acid strength by inductive effect
of the surface oxoanions3-5 or to the generation of very strong
Brønsted acid sites.6 Titania containing sulfates1-3 and phos-
phates7 are reported to behave as strong acidic materials.
Heterocyclic moieties play an important role in the search
for new therapeutic and drug materials. Pyrroles form an
important class of heterocyclic compounds and are structural
units found in a vast array of natural products, synthetic
materials, and bioactive molecules, such as heme, vitamin B12,
and cytochromes.8 Classical methods for their preparation
include the Knorr9 and Paal-Knorr condensation reactions.10-20
One of the most common approaches to pyrrole synthesis is
the Paal-Knorr reaction in which 1,4-dicarbonyl compounds
are converted to pyrroles in the presence of primary amines. In
this reaction, the 1,4-dicarbonyl compounds provide the four
carbons of the pyrroles with the possible substitutes, whereas
the amine provides the nitrogen with its substituent. Many
catalysts such as montmorillonite-KSF,10 Bi(NO3)3¢5H2O,13
Sc(OTf)3,14 TolSO3H,15 layered zirconium phosphate and
zirconium sulfophenyl phosphonate,16 silica sulfuric acid
(SSA),17 titanium,18 and TiCl4/Et3N,19 have been used for this
conversion. However, methods using these catalysts require
prolonged reaction times, use of volatile organic solvents and
toxic metals. Thus, a milder, selective, nonhazardous, inex-
pensive, recyclable, and eco-friendly catalyst is still in demand.
Recently Satyanarayana and Sivakumar reported uranyl nitrate
hexahydrate as a catalyst for the synthesis of N-substituted
pyrroles under microwave and at room temperature.20 But this
catalyst is radioactive and hygroscopic in nature. Uranyl nitrate
is an oxidizing and highly toxic compound and should not be
ingested and may react explosively with cellulose and certain
organic solvents.
2¹
sulfated titania (TiO2-SO4 ) and grinding at room temperature
under solvent-free conditions.
Experimental
Materials and Methods.
Aniline, substituted anilines,
other amines, benzophenone (s.d.fine), and £-diketone (Aldrich
Chemicals) were used as received. AnalaR grade titanium
isopropoxide (Himedia 98.0%), 2-propanol (Spectrochem
99.5%), hydrazine hydrate (s.d.fine), and H2SO4 (Fischer
98%) were used as such. Benzophenone hydrazone was pre-
pared according to the literature procedure.29
2¹
Preparation of Sulfated TiO2 Catalyst. TiO2-SO4 was
prepared by sol-gel method as reported earlier.1 12.5 mL of
tetraisopropyl ortho-titanate (Himedia 98.0%) was dissolved
in 100 mL of 2-propanol and to this solution, 3.2 mL of 1 M
H2SO4 was added dropwise under vigorous stirring. The
resulting colloidal suspension was stirred for 4 h. The gel
obtained was filtered, washed, and dried in an air oven at
100 °C for 12 h. The sample was calcined at 400 °C in a muffle
furnace for 1 h and characterized. This catalyst contained
2¹
5 wt % of SO4
.
Characteristics of Sulfated Titania. Sulfated titania was
characterized by FT-IR, XRD, SEM, EDS, HR-TEM, AFM,
and BET surface area measurements and reported from our
laboratory.1,26,28 XRD peaks exactly match with the anatase
phase of TiO2, and sulfate modification does not change the
TiO2 is a reusable, nontoxic, and eco-friendly solid acid
catalyst. Loading of sulfate using sulfuric acid increases the
Published on the web March 9, 2013; doi:10.1246/bcsj.20120221

K. Ravi et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 3 (2013)
371
temperature 250 °C. Proton and carbon NMR spectra were
recorded on a BRUKER AVIII FT-NMR spectrometer operat-
ing at 400 MHz for all the samples.
Results and Discussion
General Procedure for Preparation of 2,5-Dimethyl-
pyrroles. A mixture of 1 mmol of amine, 1.2 mmol of £-
2¹
diketone (hexane-2,5-dione) with 100 mg of TiO2-SO4 were
mixed and ground in a pestle and mortar at room temperature.
The progress of the reaction was monitored by thin layer chro-
matography (TLC). After completion of the reaction, ethanol
was added to the mixture and the catalyst was filtered off.
The catalyst was washed with more ethanol and the washings
were combined. The combined organic extracts were dried over
anhydrous sodium sulfate and concentrated under vacuum to
get the product. All products were characterized by GC/mass
spectroscopy and selected products were characterized by
1H NMR, 13C NMR. Mass spectra for the compounds synthe-
sized are given in supplementary information (Figures S1-S9,
see Supporting Information). NMR data of two representative
compounds, 1-benzyl-2,5-dimethylpyrrole and 2,5-dimethyl-1-
octadecylpyrrole are given below.
Binding energy/eV
Binding energy/eV
Binding energy/eV
1-Benzyl-2,5-dimethylpyrrole:
1H NMR (CDCl3, 400
MHz, ¤): ethlyenic protons at pyrrol ring 5.913 (s, 2H),
methylene proton of C6H5-CH2 attached to N- of pyrrole ring
5.059 (s, 2H), two methyl protons at pyrrole ring 2.19 (s,
6H), aromatic protons 6.928-6.941 (m, 3H), aromatic protons
7.327-7.340 (m, 2H); 13C NMR (CDCl3, 400 MHz, ¤): 12.24-
12.47 (methyl carbons at pyrrole ring), 46.74 (ethyl-benzyl),
105.32, 105.41 (ethylenic carbons at pyrrole ring), 125.67-
128.74 (aromatic carbons), 138.57 (ipso-carbon-aromatic).
GC-MS (m/z) = 185.2 (M+).
Binding energy/eV
2¹
Figure 1. X-ray photoelectron spectra of TiO2-SO4
(a) survey spectrum, (b) Ti2p peak, (c) O1s peak, and
:
(d) S2p peak.
2,5-Dimethyl-1-octadecylpyrrole: 1H NMR (CDCl3, 400
MHz, ¤): ethlyenic protons at pyrrol ring 5.96 (s, 2H), methyl-
ene proton directly attached to N- of pyrrole ring 3.68-3.72 (m,
2H), two methyl protons at pyrrole ring 2.21 (s, 6H), methyl
proton at alkyl chain 0.87 (t, 3H J = 5.2 Hz), ethyl proton
attached to methyl group of alkyl chain 1.6-1.7 (s, 2H), other
alkyl protons 1.2-1.3 (m, 30H), GC-MS (m/z) = 347.4 (M+).
To determine the optimal conditions, synthesis of 1-benzyl-
2,5-dimethylpyrrole was carried out under different condi-
tions (Table 1). When a mixture of benzyl amine (1 mmol)
and £-diketone (1.2 mmol) without catalyst, was ground for
10 min, no reaction occurred. However, grinding the mixture
2¹
phase. The FWHM of TiO2-SO4 peaks are higher than TiO2
and this broadening of peaks implies the decrease in crystalline
size of TiO2. Both size reduction and retardation of aggregation
result in increase in surface area of the catalyst. HR-TEM and
AFM analysis reveal a globular structure of the catalyst and
a slight corrosion of TiO2 by sulfate. TiO2-SO4 has a higher
surface area (142.8 m2 g¹1) than bare TiO2 (86.0 m2 g¹1). TiO2-
2¹
2¹
SO4 was further characterized by XPS analysis.
The XPS survey spectrum (Figure 1a) of the TiO2-SO4
2¹
indicates the peaks of element Ti, O, and S. Figures 1b, 1c,
and 1d show the binding energies of Ti, O, and S, respectively.
Binding energy peaks of Ti2p occur at 458.6 and 463.6 eV
(Figure 1b). Normally the Ti2p line in the case of pure TiO2
samples can be observed at 458.5 eV.30 The O1s (Figure 1c)
show two contributions at 530.4 and 532.2 eV. The main peak
at 530.4 eV could be ascribed to lattice oxygen in TiO2, while
the signal at 532.2 eV could be associated oxygen in sulfate.31
The binding energy peaks of the S2p are observed at 168.8 and
170.2 eV (Figure 1d).
Apparatus. For GC analysis, a Perkin-Elmer GC-9000
with a DB-5 capillary column and flame ionization detector
was used. GC/MS analysis was carried out using a GC model:
Varian GC-MS-Saturn 2200 Thermo, capillary column VF5MS
(5% phenyl-95% methylpolysiloxane), 30 m length, 0.25 mm
internal diameter, 0.25 ¯m film thickness, temperature of col-
umn range from 50 to 280 °C (10 °C min¹1), and injector
2¹
with 100 mg of TiO2-SO4 initiated a condensation reac-
tion producing 98.0% 1-benzyl-2,5-dimethylpyrrole in 3 min
(Scheme 1). When bare TiO2 was used under the same con-
ditions only 45.0% product yield was obtained in 10 min. It
was observed that the yield was 80% when the reaction was
carried out in ethanol for 15 min under refluxing temperature
2¹
with 100 mg of TiO2-SO4 . These results reveal that simple
grinding is more efficient than the reaction in ethanol with
2¹
TiO2-SO4 .
2¹
The effect on catalyst (TiO2-SO4 ) dosage on the formation
of 1-benzyl-2,5-dimethylpyrrole was investigated by varying
the catalyst amount from 0.05 to 0.2 g (Figure 2). When the
amount of the catalyst is increased from 0.05 to 0.1 g, forma-
tion of 1-benzyl-2,5-dimethylpyrroles increases from 55% to
98.0% in 3 min of grinding. As seen from the Table 1, even for

372
Bull. Chem. Soc. Jpn. Vol. 86, No. 3 (2013)
Synthesis of Substituted Pyrroles Using Sulfated Anatase-Titania
Table 1. Percent Yield of Product from the Reaction of
1 mmol of Benzylamine and 1.2 mmol of £-Diketone under
Different Conditions
100
Catalyst
quantity
/mg
Time Yielda)
Entry Conditions
/min
/%
80
1
2
3
4
5
6
7
8
9
No solvent
Ethanol/RT
Ethanol/TiO2-SO4 /RT
®
®
10
15
15
15
10
3
10
3
3
®
®
50
80
45
55
93
98
98
98
2¹
100
100
100
50
2¹
60
40
Ethanol/TiO2-SO4 /Reflux
Bare TiO2/Solvent free
2¹
TiO2-SO4 /Solvent free
2¹
TiO2-SO4 /Solvent free
50
2¹
TiO2-SO4 /Solvent free
100
150
200
2¹
TiO2-SO4 /Solvent free
0
0.05
0.1
0.15
0.2
0.25
2¹
10
TiO2-SO4 /Solvent free
3
Catalyst amount/g
a) Yield with respect to benzylamine.
Figure 2. Effect of catalyst loading: benzyl amine: 1 mmol,
£-diketone: 1.2 mmol, solvent: solvent free, catalyst:
sulfated titania, time: 3 min (grinding at room temperature).
O
TiO2-SO42-/R.T/Grinding
H N
R
N
+
2
O
R
R= aromatic, aliphatic groups (acyclic and cyclic)
2¹
Scheme 1. The condensation reaction of amines with £-diketone as catalyzed by TiO2-SO4
.
2¹
10 min of grinding the yield was only 93% for 0.05 g of the
catalyst. Above 0.1 g of the catalyst, no significant change
in the percentage of product formation occurred. Hence the
optimum catalyst loading was found to be 0.1 g for the con-
version of 1 mmol of benzyl amine.
In order to show the advantage of using TiO2-SO4 ,
the efficiencies of other catalysts are compared with the
TiO2-SO4 in the synthesis of 1-benzyl-2,5-dimethylpyrrole
2¹
(Table 3). Except silica sulfuric acid (SSA), other catalysts
2¹
gave lesser yield with longer reaction time. But TiO2-SO4
In order to show the generality and scope of this new
protocol, we used various aromatic and aliphatic amines and
benzophenone hydrazone for condensation with £-diketone and
the results obtained are summarized in Table 2. Condensation
with all amines proceeded very cleanly at room temperature
and no undesirable side-reactions were observed, although the
yields and grinding time were highly dependent on the nature
of amines and their substitutents.
has more efficiency even at the fifth run when compared to
SSA. The efficiencies at different runs are given in Table 3.
As seen from Table 1, bare TiO2 produced 45% product
2¹
in 10 min whereas TiO2-SO4 produces 98% yield in 3 min.
2¹
Surface acidity of TiO2 and TiO2-SO4 are found to be 1600
and 2200 ¯g by pyridine adsorption method.23,24 When we
compare the yield of product in 3 min for TiO2 and TiO2-
2¹
SO4 , the efficiency increase is many fold. This increase can
Results in Table 2 show that electron-donating groups at
the phenyl ring of aniline favored the formation of product
(Table 2, Entry 2). Substrate bearing a strong electron-with-
drawing NO2 group gave trace of product even after 40 min
(Table 2, Entry 5). Except nitro group, other groups have less
effect on the reaction. 2-Aminobenzenethiol and 2-(1-pyrrolyl)-
aniline gave moderate yields (Table 2, Entries 6 and 7). The
negative effect with nitro substituent is due to reduced electron
density at the amine nitrogen by the electron-withdrawing
nature of NO2 group (¹I group) by delocalization of electrons,
hindering the formation of intermediate. Benzophenone hydra-
zone gave good yield with £-diketone (Table 2, Entry 8).
When compared to aniline, benzyl amine gave higher yield
(Table 2, Entries 1 and 9). Aliphatic amines react faster than
aromatic amines (Table 2, Entries 10-12) because aromatic
amines are less basic than aliphatic amines.
be explained by the increase in Brønsted acidic sites. Brønsted
acidities reported for bare TiO2 and 5 wt % TiO2-SO4 by
2¹
TPD using 2,6-dimethylpyridine as probe, are 0.77 and 5.17
respectively.25 Brønsted acidity of TiO2-SO4 is about 7 times
2¹
2¹
than bare TiO2. Hence it reveals that TiO2-SO4 with in-
creased Brønsted acidity catalyses the reaction efficiently when
compared to bare TiO2. A reaction mechanism catalyzed by
2¹
Brønsted acidity or proton acidity of TiO2-SO4 is proposed
in Scheme 2.
This mechanism involves the protonation of £-diketone with
2¹
acidic TiO2-SO4 . The amino group attacks one of the pro-
tonated carbonyl groups of diketone and this is followed by
2¹
cyclization and dehydration to give the product. TiO2-SO4
promotes both dehydration and deprotonation. The possibility
of recycling the catalyst was examined for the reaction of
benzyl amine with £-diketone. When the reaction was com-

K. Ravi et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 3 (2013)
373
Table 2. Solvent-Free Synthesis of Substituted Pyrroles Using Sulfated Anatase-Titania as a Solid Acid Catalysta)
Grinding time
/min
Molecular weight
M or M + 1
Yieldb)
/%
Entry
1
Amines
Product
H3C
N
NH
20
20
170.1
186.3
95.0
97.0
2
H3C
H3C
N
H C
NH
2
2
3
4
5
3
H3C
H3C
H3C
H3C
H3C
N
Cl
NH
2
20
15
40
206.3
191.2
216.2
92.0
94.0
trace
H3C
Cl
N
F
NH
2
H3C
F
N
O2N
NH
2
H3C
O
2N
H3
C
NH
SH
2
N
6
7
30
30
203.1
236.2
85.0
80.0
CH3
SH
H3C
N
NH2
N
N
H3C
H3C
NH2
N
N
N
8c)
3
3
275.1
185.2
99.0
98.0
CH3
CH3
NH
2
N
9
H3C
H3C
H3C
H3C
10
11
10
20
137.1
98.0
99.0
N
NH2
CH3
H3
C
H
H
C
3
CH3
N
N
347.62
2
CH3
Continued on next page:

374
Bull. Chem. Soc. Jpn. Vol. 86, No. 3 (2013)
Synthesis of Substituted Pyrroles Using Sulfated Anatase-Titania
Continued:
Entry
Grinding time
/min
Molecular weight
M or M + 1
Yieldb)
/%
Amines
Product
H3C
N
NH2
12
10
177.2
98.0
H3C
2¹
a) Conditions: amine 1 mmol, diketone 1.2 mmol, TiO2-SO4 100 mg, simple physical grinding, solvent free. b) Yield with
respect to amines. c) Hydrazone.
2¹
Table 3. Comparison of Efficiency of TiO2-SO4 over Other Acid Catalyst Used in the Synthesis of 1-Benzyl-2,5-dimethylpyrrole
from Benzylamine and 1,5-Diketone
Entry Catalyst
Conditions
Time/min
Yielda)/%
Reference
1
2
3
4
5
6
Bi(NO3)3¢5H2O
Sc(OTf)3
¡-Zr(KPO4)2
RTc)/dichloromethane
RT/solvent free
RT/solvent free
600
25
120
6
3
3
96.0
93.0
81.0
86.0
13
14
16
20
17
UO2(NO3)2¢6H2O RT/methanol/Ultrasonic irradiation
SSAb)
TiO2-SO4
RT/solvent free/physical grinding
RT/solvent free/physical grinding
98.0 (98.0, 96.0, 95.0, 95.0, 90.0)d)
98.0 (98.0, 98.0, 98.0, 96.0, 94.0)e) Present work
2¹
a) Yield with respect to benzylamine. b) SSA: silica sulfuric acid. c) RT: room temperature. d) Reusability of SAA. e) Reusability
2¹
of TiO2-SO4
.
H
H
2
Acidic sites of
TiO2-SO4
O
O
+
+
2−
O O
H
H
H
2
OH
O
O
O
+
+
N
R
H
H
H
N
R
H
HO
OH2
OH
O
N
N
R
H
R
H
H
Acidic sites of
TiO2-SO4
OH2
HO
+
+
2−
-2H2O
N
N
R
R
2¹
Scheme 2. Proposed mechanism for the condensation reaction of amines with £-diketone catalyzed by TiO2-SO4
.
plete, ethanol was added to the reaction mixture and the
insoluble catalyst was separated by filtration. The separated
Conclusion
catalyst after removal of solvent was dried at 100 °C for 4 h
in an oven and used five times without any treatment. The
efficiency of the catalyst was 94% at the 5th run. The catalyst
was stable and no appreciable loss in its catalytic activity was
observed even up to fifth run as shown in Figure 3.
In conclusion, sulfated titania is introduced as an excellent
solid acid catalytic system for the synthesis of pyrrole
derivatives by simple grinding at room temperature. An acid-
catalyzed mechanism is proposed. Its higher activity over
TiO2 is due to many fold increase in Brønsted acidity. This

K. Ravi et al.
Bull. Chem. Soc. Jpn. Vol. 86, No. 3 (2013)
375
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1 mmol, £-diketone: 1.2 mmol, solvent: solvent free, cata-
lyst: 0.1 g of sulfated titania, time: 3 min (grinding at room
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Authors are thankful to CSIR, New Delhi, India for financial
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Supporting Information
Mass spectrum of compounds Figures S1-S9. This material
is available free of charge on the web at http://www.csj.jp/
journals/bcsj/.
26 B. Krishnakumar, R. Velmurugan, M. Swaminathan, Catal.
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