Mesoporous Titania-Iron(III) Oxide with Nanoscale Porosity and High Catalytic Activity for the Synthesis of β-Amino Alcohols and Benzimidazole Derivatives

Article abstract of DOI:10.1002/cctc.201500563

A mesoporous TiO₂-Fe₂O₃ mixed oxide material (MTF-1E) with nanoscale porosity and a high BET surface area was synthesized using sodium dodecyl sulfate (SDS) as a structure-directing agent. The material was characterized by

Full text of DOI:10.1002/cctc.201500563

DOI: 10.1002/cctc.201500563
Full Papers
Mesoporous Titania-Iron(III) Oxide with Nanoscale Porosity
and High Catalytic Activity for the Synthesis of b-Amino
Alcohols and Benzimidazole Derivatives
[a]
[b]
[a]
[b]
[a]
Susmita Roy, Biplab Banerjee, Noor Salam, Asim Bhaumik,* and Sk. Manirul Islam*
A mesoporous TiO -Fe O mixed oxide material (MTF-1E) with
under solvent-free conditions at room temperature for the syn-
thesis of a series of b-amino alcohols. It also showed a very
high catalytic efficiency for the synthesis of benzimidazole de-
rivatives in water. The catalyst can be recovered easily from
the reaction medium and reused six times without a significant
decrease in its catalytic activity and selectivity.
2
2
3
nanoscale porosity and a high BET surface area was synthe-
sized using sodium dodecyl sulfate (SDS) as a structure-direct-
ing agent. The material was characterized by powder XRD,
high-resolution TEM, N2 sorption, and NH3 temperature-pro-
grammed desorption studies. The catalyst shows an excellent
regioselectivity for the ring-opening of epoxides with amines
Introduction
Porous nanomaterials with high specific surface areas have at-
tracted wide interest because of their potential as green ad-
sorbents and catalysts, which can provide a sustainable solu-
sary. Epoxide ring-opening (ERO) by the reaction of amines
with epoxides in particular is an elegant strategy to produce
[
19]
biologically and synthetically important b-amino alcohols.
[
1–3]
tion to environmental concerns and waste management.
Therefore, a large number of strategies for the synthesis of b-
amino alcohols have been developed in this context. ERO with
amines has been well developed as an efficient tool, in which
reactions are optimized by changing the physical reaction pa-
Mesoporous materials are particularly demanding in this con-
text because they have uniform pores of nanoscale dimensions
[4–6]
and plenty of well-distributed active sites,
which could help
[
20]
the diffusion of large reactant molecules through the pore
rameters, such as conventional heating,
microwave heat-
[
7–8]
[21]
[22]
channels.
These materials have been explored in gas ad-
ing, and ultrasound heating, and the use of polar reaction
[
9]
[10]
[11]
[23]
sorption, sensing,
catalysis,
as energy storage materi-
[13]
media/ionic liquids,
solvents such as dichloromethane
[12]
[24]
[25]
als, and in biological applications. A wide range of organic
transformations that involve bulky substrate molecules, such
(DCM),
fluoroalcohols,
or water at different pH condi-
[
26]
tions.
This method is not suitable for poorly nucleophilic
[14]
[15]
as the epoxide ring-opening reaction, acid- or base-cata-
amines as their regioselectivities are low. A large number of
[
16]
[17]
[27]
[28]
lyzed reactions, photocatalysis, and size- and shape-selec-
promoters, acid catalysts,
transition metal halides,
and
[
18]
[29]
tive isomerization are performed very efficiently over these
materials with nanoscale porosity.
Lewis acids are employed to achieve the desired yield of the
products. However, these catalysts mostly require long reaction
times, elevated temperatures, high pressures, and stoichiomet-
ric amounts of catalyst. As Fe is abundant in nature, nontoxic,
inexpensive, and environmentally benign, it is worthwhile to
explore Fe-based porous catalysts in the ERO reaction with
As a result of strict environmental regulations, environmen-
tally benign, efficient, economical, and green chemical routes
for the synthesis of organic fine chemicals have become neces-
[
30]
+
amines.
In this context, the benzimidazole ring is an important phar-
[
a] S. Roy, N. Salam, Dr. S. M. Islam
Department of Chemistry
[
31]
University of Kalyani
Nadia, 741235, West Bengal (India)
Fax: (+91)33-2582-8282
macophore in modern drug design and discovery. In addi-
tion, benzimidazoles are very important intermediates in or-
[
32]
ganic reactions.
Several methodologies have been devel-
E-mail: manir65@rediffmail.com
+
oped to synthesize benzimidazole derivatives. The classical
method for the synthesis of 2-substituted benzimidazoles in-
volves the treatment of 1,2-phenylenediamines with carboxylic
[
b] B. Banerjee, Prof. A. Bhaumik
Department of Materials Science
Indian Association for the Cultivation of Science
A&B, Raja S.C. Mullick Road, Jadavpur, Kolkata 700032 (India)
Fax: (+91)33-2473-2805
2
[33]
[34]
acids or their derivatives (nitriles, imidates, or orthoesters)
under strongly acidic conditions and sometimes combined
with very high temperatures (i.e., polyphosphoric acid, 453 K)
E-mail: msab@iacs.res.in
+
[
] These authors contributed equally to this work.
[
35]
or the use of microwave irradiation. Often these derivatives
are generated by the condensation of phenylenediamines with
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500563.
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[
36]
aldehydes under oxidative conditions using various oxidizing
agents and catalysts. Although the reaction can be promoted
efficiently under these conditions, the catalysts employed are
often homogeneous in nature, and some of these methods
suffer from one or more disadvantages, such as the high cost
of the catalysts, the use of hazardous organic solvents, pro-
longed reaction times, the occurrence of several side reactions,
difficulty in the separation of the products from the reaction
mixture, and the strong oxidizing nature of the reagents.
Therefore, the discovery of mild, stable, cheap, recyclable, and
eco-friendly heterogeneous catalysts for the synthesis of 2-sub-
stituted benzimidazoles continues to attract the attention of
researchers. Furthermore, a heterogeneous catalyst would be
more environmentally acceptable if the reaction can be per-
formed in water.
Figure 2. Wide-angle XRD pattern of MTF-1E.
In this context, we have designed a mesoporous TiO -Fe O
3
2
2
mixed oxide through an anionic-surfactant-assisted soft-tem-
[
37]
plating pathway. We characterized the material and used it
as a catalyst for the synthesis of b-amino alcohols by the ERO
reaction with amines under solvent-free reaction conditions at
room temperature and for the synthesis of benzimidazole de-
rivatives in water.
Results and Discussion
Small-angle powder X-ray diffraction (PXRD) patterns of the as-
synthesized and template-extracted mesoporous TiO -Fe O
3
2
2
mixed oxide samples are shown in Figure 1. A well-ordered,
Figure 3. Dark-field STEM image of a) MTF-1E and its elemental mapping for
b) Ti, c) Fe, and d) O.
To understand the composition and distribution of individual
components of the mixed oxide MTF-1E, we recorded a dark-
field scanning transmission electron microscopy (STEM) image
Figure 1. Small-angle PXRD pattern of a) as-synthesized MTF-1 and b) MTF-
1E.
(
Figure 3) and performed elemental mapping at different parts
of the specimen. Elemental maps of Ti, Fe, and O are shown in
Figure 3b, c, and d, respectively. These maps suggest the pres-
2
D hexagonal mesophase is observed in the small-angle PXRD
pattern of the as-synthesized mixed oxide material. Peaks are
indexed and correspond to the (100), (110), and (200) planes
of the 2D hexagonal mesophase. For the extracted sample,
there is a shift in the peak positions towards higher 2q values,
which indicates a decrease in the d spacing of the mesophase
during the removal of the template. The wide-angle PXRD pat-
tern of MTF-1E is shown in Figure 2. The pattern shows that
the solid material is a mixture of anatase TiO and a-Fe O
3
ence of more TiO2 particles than Fe O3 at the mesoporous
2
MTF-1E surface. To check the surface acidity of MTF-1E, we
measured the temperature-programmed desorption of NH3
(NH -TPD). The NH -TPD profile of MTF-1E is shown in Figure 4.
3
3
The profile shows broad desorption peaks in the range of 325–
650 K. Furthermore, the desorption profile was deconvoluted
to give two well-separated peaks centered at 375 and 500 K.
These two peaks could be attributed to the weak and moder-
ately strong acidic sites present in MTF-1E. The total surface
2
2
phases (JCPDS File Card No. 21-1272 and JCPDS File Card No.
3-0800, respectively).
À1
0
acidity calculated for this material is 4.3 mmolg , which is
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Scheme 1. Regioselective ring-opening of epoxides with amines.
Table 1. Optimization of reaction conditions for the ERO reaction over
[a]
2 2 3
mesoporous TiO -Fe O mixed oxide.
Figure 4. NH
3
-TPD profile of MTF-1E.
[b]
Entry
Catalyst amount [mg]
Solvent
t [h]
Yield [%]
1
2
3
4
5
6
7
8
9
–
30
30
30
30
20
20
20
10
20
–
24
5
5
5
5
5
4
3
3
2
trace
87
71
91
96
96
96
96
82
83
DCM
toluene
H
–
–
–
–
–
–
2
O
10
[
(
a] Reaction conditions: 1,2-epoxy-3-phenoxy propane (1 mmol), aniline
1 mmol), MTF-1E (20 mg), without solvent, RT; [b] Yields refer to isolated
pure product.
The ERO of 1,2-epoxy-3-phenoxy propane with aniline as
a nucleophile was used as a model reaction to test the efficacy
of the mesoporous MTF-1E material, and the results are given
in Table 1. In the absence of catalyst, only traces of b-amino al-
cohol product are obtained in 24 h under solvent-free condi-
tions at room temperature (Table 1, entry 1), and inferior re-
Figure 5. N
2
adsorption (*) and desorption (*) isotherms of MTF-1E at 77 K.
The pore size distribution using the Barrett–Joyner–Halenda (BJH) method is
shown in the inset.
sults were obtained with DCM, toluene, and H O (Table 1, en-
2
a reasonably high surface acidity for a mesoporous mixed
oxide material. The high surface acidity of MTF-1E motivated
us to explore its catalytic potential in the regioselective ring-
opening reaction of epoxides and benzimidazole synthesis.
tries 2–4). We optimized the quantity of catalyst at room tem-
perature under solvent-free conditions for this reaction
(Table 1, entries 5–9). It was observed that the use of just
20 mg of MTF-1E (0.04 mmol of Fe) is sufficient for the comple-
tion of the reaction in 3 h with 98% yield of the corresponding
product at room temperature (Table S1, entry 8). Notably,
larger amounts of catalyst did not improve the results (Table 1,
entries 2–5). However, with less than 20 mg of the catalyst, the
reaction was incomplete and resulted in a low yield of the
product (Table 1, entry 9). Under the above optimized reaction
conditions, the catalyst was employed in the ERO of different
epoxides, namely, 1,2-epoxy-3-phenoxy propane, styrene
oxide, cyclopentene oxide, cyclohexene oxide, cis epoxides,
and various terminal epoxides with anilines, substituted ani-
lines, and aliphatic amines, and the results are summarized in
Table 2. Significantly, in all these cases only a single regioiso-
mer is obtained in excellent yield with no traces of the bis
adduct. In most cases, the MTF-1E catalyst is highly efficient,
and the reaction proceeded smoothly to afford the desired
product b-amino alcohol in a high isolated yield (89–98%)
within a short reaction time (2–4 h). Styrene oxide underwent
The N adsorption–desorption isotherm of MTF-1E is shown
2
in Figure 5. This isotherm can be classified as type IV, character-
istic of mesoporous materials with a long and very broad capil-
lary condensation step. The increase in the N uptake at P/P =
2
0
0
.10–0.90 indicates the presence of mesopores in the sample.
The BET surface area of the calcined mixed oxide material is
2
À1
3
67 m g . The pore size distribution of the sample estimated
by employing the Barrett–Joyner–Halenda (BJH) method is
shown in the inset of Figure 5. The observed peak pore width
(
w) for this sample was 1.69 nm.
ERO reaction
The catalytic activity of the mesoporous TiO -Fe O mixed
2
2
3
oxide was investigated in the ERO reaction for a series of epox-
ides with aromatic amines (Scheme 1).
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results in terms of product yields
Table 2. ERO reaction of various epoxides with different anilines using MTF-1E as catalyst.
(
89–95%) for these epoxides
[a]
[b]
Entry
1
Epoxide
Amine
Product
t [h]
Yield [%]
91
(Table 2, entries 3–6). Further-
more, to understand the effect
of nucleophilicity in the ERO re-
action, various substituted ani-
lines, namely, 2-methoxy-, 4-me-
thoxy-, 4-methyl-, and 2-cholora-
niline were also used as a nucleo-
phile with meso-stilbene oxide
as model substrates for the cata-
lyst MTF-1E. In the presence of
both electron-donating and elec-
tron-withdrawing groups in sub-
stituted anilines, the ERO reac-
tion proceeds smoothly (Table 2,
entries 7–10). The use of hetero-
cyclic secondary amines, such as
morpholine and pyrrolidine, for
the ERO reaction of cyclohexene
oxide also gave excellent prod-
uct yields (Table 2, entries 11 and
C
C
6
6
H
5
5
NH
NH
2
2
3
2
3
H
3
3
96
92
C
6
H
5
NH
2
4
5
6
7
C
C
C
6
6
6
H
H
H
5
5
5
NH
NH
NH
2
2
2
4
4
3
3
91
95
92
91
2-OMeC
4-OMeC
6
H
H
4
NH
NH
2
2
1
2). The advantage of this proce-
dure over other reported meth-
ods is the formation of the b-
amino alcohols as the only prod-
uct through attack at the termi-
nal carbon atom of the epoxides.
Furthermore, the work-up is very
simple, and the yields are gener-
ally high. To the best of our
knowledge, these results repre-
sent the highest regioselectivity
reported to date for the nucleo-
philic ring-opening of epoxides
with amines.
8
9
6
4
3
4
3
91
89
90
6 4 2
4-MeC H NH
10
6 4 2
2-ClC H NH
1
1
morpholine
pyrrolidine
4
3
92
93
12
1
1
1
3
4
5
C
C
C
6
6
6
H
H
H
5
5
5
NH
NH
NH
2
2
2
2
90
98
89
We also performed the ERO of
1
,2-epoxy-3-phenoxy
propane
2.5
3
with aniline in the presence of
TiO and Fe O as catalysts, and
[c]
2
2
3
the results are summarized in
Table 3. If we used nonporous
[
a] Reaction conditions: epoxide (1 mmol), amine (1 mmol), MTF-1E (20 mg, 0.04 mmol of Fe), without solvent,
RT; [b] Yields refer to isolated pure products. [c] 4% more substituted regioisomer 2-(phenylamino)propan-1-ol
was also obtained.
(
TiO2 and Fe O ) and mesopo-
2 3
rous (TiO2 and Fe O ) catalysts
2
3
for the synthesis of b-amino al-
cohols, the conversions and the
ring-opening if it was reacted with aniline to yield a regioselec-
tive product through the preferential attack at the benzylic
selectivity of the desired products were quite low compared to
those of MTF-1E. We also performed the reaction with a physi-
cal mixture of mesoporous TiO and Fe O as a catalyst and
[
30f]
and more-hindered terminal carbon atom (Table 2, entry 1).
2
2
3
However, the reaction of aryloxy epoxides and other aliphatic
terminal epoxides (epichlorohydrin, allyl glycidyl ether, propyl-
ene oxide) with aniline afforded the major secondary alcohol
regioisomer regioselectively (Table 2, entries 2–3 and 13–15)
through nucleophilic attack at a less sterically hindered carbon
atom. Meso epoxides, such as cyclopentene oxide, cyclohexene
oxide, cycloheptene oxide, and cis-stilbene oxide also partici-
pated in the ERO reaction with aniline. We obtained excellent
found no particular improvement in the product yield com-
pared to that of MTF-1 (Table 3, entry 5).
The yield and product selectivity were verified by NMR spec-
troscopy. The results given in Table 3 indicate clearly that the
mesoporous MTF-1E material with a high BET surface area, po-
rosity, and self-assembled nanostructure is responsible for the
high yields and exclusive regioselectivity for the desired b-
amino alcohol as the product.
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Table 3. Synthesis of b-amino alcohol (1-phenoxy-3-(phenylamino)pro-
[
a]
pan-2-ol) in the presence of different catalysts.
[b]
Entry
Catalyst
TiO
Yield [%]
1
2
3
4
5
6
7
2
21
54
32
60
63
96
8
Fe
Mesoporous TiO
Mesoporous Fe O
3
2 3
O
2
2
Mixture of mesoporous TiO
MTF-1E
Without catalyst
2 2 3
and mesoporous Fe O
[
(
a] Reaction conditions: 1,2-epoxy-3-phenoxy propane (1 mmol), aniline
1 mmol), catalyst (20 mg), without solvent, RT, 3 h; [b] Yields refer to iso-
lated pure product.
Moreover, the catalytic role of the mesoporous TiO -Fe O
3
2
2
over mesoporous TiO or mesoporous Fe O is very interesting.
2
2
3
The enhanced visible-light photocatalytic activity for the TiO2-
Fe O mixed oxide system is attributed to the increase in the
2
3
specific surface area, the separation of photoinduced electron–
[38]
Figure 6. Plausible reaction pathway for the ring-opening of epoxides over
MTF-1E.
hole pairs, and the hollow micro architecture.
Murugesan
et al. reported that the incorporation of Fe O into TiO leads
2
3
2
to a facile electron transfer between the valence band (VB)
and the conduction band (CB) of the composite TiO -Fe O
3
2
2
compared to pure that of TiO because of the decrease of the
Table 4. Comparison of the catalytic activity of the present catalyst with
2
that of other related reported systems.
band gap between the VB and CB of pure TiO . As a result,
2
[
38c]
a hole is generated in the VB of the TiO -Fe O composite.
2
2
3
Catalyst
Fe(ClO
Reaction conditions
Yield [%] Ref.
The presence of the hole in the VB increases the Lewis acidic
characteristics of the complex, which initiate electron transfer
from oxygen atoms to the metal complex and results in the in-
creased efficiency of the catalytic activity and reaction rate.
Neria et al. reported that the doping of Fe O on the TiO2
4
)
2
10 mol% catalyst, DCM, 208C, 90
[30c]
[30d]
[30e]
16 h.
Mesoporous alumino- 12 mol% catalyst, DCM, RT, 6 h 70
silicate
Mesoporous-Zr-beta
catalyst
308 K, 0.5 h
80.7
2
3
matrix increases the Lewis acid strength of TiO -Fe O because
2
2
3
TiO
2
-ZrO
2
H
2
O, RT, 1.2 h
86
92
[30f]
this
[
39]
of the formation of new acid on the surface of TiO -Fe O .
MTF-1E
4 mol% catalyst, without sol-
vent, RT, 3 h
2
2
3
Epoxides can act as Lewis bases through direct conjugation
with Lewis acids by their nonbonded electron pairs, which en-
hances the rate of the ERO. Hence, a plausible reaction mecha-
nism that takes into account the role of the Lewis acidic as-
pects of mesoporous TiO -Fe O as a catalyst for ERO with
work
2
2
3
amine is shown in Figure 6.
We have compared the activity of our present mesoporous
TiO -Fe O mixed oxide catalyst with existing state-of-the-art
2
2
3
Scheme 2. Synthesis of benzimidazoles catalyzed by MTF-1E.
heterogeneous catalysts used for regioselective ERO (Table 4).
(
Table 5, entries 3 and 5–13). Next we examined the effect of
Synthesis of benzimidazole derivatives
the temperature and time by varying these parameters in the
representative reaction. The best result in terms of yield and
Encouraged by the satisfactory results with the ERO reaction,
we extended the scope of this catalyst successfully to the syn-
thesis of benzimidazoles (Scheme 2).
time is obtained if the reaction is performed in H O at 408C for
2
3 h (Table 5, entry 12). A decrease of the amount of catalyst
(10 mg) led to a decrease of the product yield (Table 5,
entry 14).
Initially, to optimize the conditions for the synthesis of ben-
zimidazoles, a series of experiments was performed with the
variation of solvent, temperature, and time for a representative
reaction between benzaldehyde (1 mmol) and 1,2-phenylene-
diamine (1 mmol) in the presence of MTF-1E (20 mg,
The optimized reaction conditions were then applied for the
synthesis of a series of benzimidazoles to explore the scope
and generality of the reaction. Aromatic aldehydes that contain
electron-donating and electron-withdrawing groups reacted ef-
ficiently with 1,2-phenylenediamines in the presence of a cata-
lytic amount of MTF-1 at 408C to afford the corresponding
0
.04 mmol of Fe), and the results are summarized in Table 5.
Although polar solvents such as DMF and DMSO showed a fair
performance, H O was the best solvent for this reaction
2
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of intermediate B under ambient atmosphere in the presence
of the catalyst afforded the desired product C with the release
of the catalyst for the next catalytic cycle. Porous iron phos-
phonate with a high surface acidity behaves similarly in the
synthesis of benzimidazoles in the reaction between the aro-
Table 5. Optimization of reaction conditions for the synthesis of 2-phe-
nylbenzimidazole.
[a]
[
40a]
matic aldehydes and 1,2-phenylenediamine.
[b]
Entry
T [8C]
Solvent
t [h]
Yield [%]
Conclusions
1
2
3
4
5
6
7
8
9
70
70
70
70
70
60
50
40
30
RT
40
40
40
40
DMSO
DMF
5
5
5
5
5
5
5
5
5
5
4
3
2
3
88
87
97
–
We have designed the synthesis of mesoporous TiO -Fe O
3
H
–
2
O
2
2
mixed oxide and explored its application in the synthesis of b-
amino alcohol and benzimidazole derivatives. The mesoporous
mixed oxide material showed a good BET surface area, high
chemical stability, and excellent catalytic activity. The notable
advantages offered by this protocol are the use of a green sol-
vent such as water, simplicity in operation, general applicabili-
ty, mild reaction conditions, high regioselectivity, compatibility
of a wide range of functionalities, which includes the thio-
phene and pyridine moieties, recyclability of the catalyst, and
the high yield of products. Thus, the green and cost-effective
catalytic pathways over mesoporous TiO -Fe O reported
H
H
H
H
H
H
H
2
O
2
O
2
O
2
O
2
O
2
O
2
O
97
97
97
97
85
75
97
97
82
83
1
0
1
1
1
1
1
2
H
2
O
O
O
3
4
H
H
2
[
c]
2
[
(
a] Reaction conditions: benzaldehyde (1 mmol), o-phenylenediamine
1.2 mmol), MTF-1E (20 mg), H O (4 mL). [b] Yields refer to isolated pure
2
2
2
3
product. [c] 10 mg of catalyst used.
herein may find large-scale application in the synthesis of
a wide range of value-added fine chemicals.
benzimidazoles in high yields (89–97%; Table 6, entries 1–7).
Under the same reaction conditions, polycyclic aldehydes such
as 1-naphthaldehyde gave the corresponding benzimidazoles
in 89% yield (Table 6, entry 10). Notably, heterocyclic aldehydes
such as pyridine-3-carbaldehyde and thiophene-2-carbalde-
hyde took part smoothly in the reaction to give the desired
products in 91–93% yield (Table 6, entries 11–12). Many effi-
cient heterogeneous catalysts have been reported for the syn-
thesis of b-amino alcohols (through ERO) and benzimidazole
derivatives. As in the previous catalytic reactions, we per-
formed the benzimidazole synthesis using benzaldehyde
Experimental Section
Mesoporous TiO -Fe O mixed oxide was synthesized by the follow-
2
2
3
ing experimental procedure. First, sodium dodecyl sulfate (SDS;
1.44 g) was dissolved in distilled water (5 mL). It was then stirred
for 30 min to obtain a clear micellar solution. After that, anhydrous
FeCl (0.82 g) dissolved in distilled water (5 mL) was added slowly
3
to the micellar solution. The resulting mixture was stirred for
3
0 min. Then, titanium isopropoxide (Ti(OiPr) ; 2.84 g) dissolved in
4
2
-propanol (5 mL) was added slowly to the premixed solution. The
pH of the solution was then maintained at ꢀ1.0 by the addition of
a 2n HCl solution. The resulting solution was stirred for 3 h, and
the mixture was kept under freezing conditions (277 K) for 36 h.
After that, the resultant solid was collected by filtration. The solid
was dried under vacuum and further dried in an oven at 348 K.
The as-synthesized solid was extracted in dilute HCl-containing
ethanol medium and designated as MTF-1E. This template-extract-
ed mesoporous iron-titanium mixed oxide material was employed
as the catalyst for the synthesis of b-amino alcohols and benzimi-
(
1 mmol), o-phenylenediamine (1.2 mmol), and TiO or Fe O as
2 2 3
the catalyst, and the results are summarized in Table 7. If non-
porous reference catalysts (TiO and Fe O ) were used for the
2
2
3
synthesis of benzimidazole, the conversions were very low
compared to that with our mesoporous TiO -Fe O mixed
2
2
3
oxide (Table 7).
A comparison of the results obtained for our present catalyt-
ic system with those reported in the literature is summarized
dazole derivatives. Individual TiO2 and Fe O3 materials were also
prepared without using SDS in the synthesis for the comparison of
catalytic activity.
2
[40]
in Table 8. Compared with these data, it is clear that our
mesoporous TiO -Fe O mixed oxide material demonstrates
2
2
3
a better catalytic efficiency for the synthesis of benzimidazole
derivatives.
Catalysis
A possible reaction mechanism for the synthesis of benzimi-
dazoles over the mesoporous TiO -Fe O mixed oxide catalyst
In a typical catalytic reaction for ERO, a mixture of MTF-1E (20 mg),
an epoxide, (1 mmol), and an amine (1 mmol) were put in a 5 mL
round-bottomed flask and stirred vigorously for a given time
2
2
3
is illustrated in Figure 7. Here the aromatic aldehyde is first ac-
tivated by the Lewis acid sites of the MTF-1E catalyst by coor-
dination with oxygen atoms. This facilitates the attack of the
amine on the carbonyl carbon atom of benzaldehyde followed
by condensation with the amino group of 1,2-phenylenedia-
mine to result in imine A, which affords intermediate B upon
(
monitored by TLC) under solvent-free conditions. After the reac-
tion the contents were dissolved in chloroform (15 mL) and filtered
to recover the catalyst. The catalyst after washing with chloroform
three times (10 mL each) was reactivated in an air oven at 608C
and reused six times. The chloroform layer was transferred to a sep-
arating funnel and washed twice with water. This was followed by
a wash with a saturated solution of sodium chloride. The organic
layer was then dried with anhydrous sodium sulfate. The crude
intramolecular nucleophilic attack of the remaining NH group
2
in the presence of catalyst. Finally, the oxidative aromatization
ChemCatChem 2015, 7, 2689 – 2697
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hyde (1 mmol, 106 mg) was stirred
Table 6. Synthesis of 2-benzimidazoles from aldehydes and diamines.
in H O (4 mL) in the presence of
2
[a]
1
2
[b]
the MTF-1E catalyst (20 mg) in air
at 408C for 3 h (monitored by
TLC). After the reaction was com-
plete, the catalyst was filtered,
washed with water and then ace-
tone and dried in an oven. The fil-
trate was then extracted with ethyl
acetate (4”10 mL) and washed
with water. The organic layers
were dried over anhydrous Na SO
Entry
1
R
R
Product
t [h]
Yield [%]
97
H
H
C
6
H
5
3
2
3
4-ClC
2-ClC
6
H
H
4
4
4
4
94
91
H
6
2
4
and evaporated to obtain the
crude product benzimidazole,
which was then recrystallized from
EtOH to afford pure 2-phenylbenzi-
4
5
6
7
H
H
H
H
4-NO
2
C
6
H
4
3
4
3
4
96
89
91
90
1
13
4-OCH
3
C
6
H
4
midazole. The
H and C NMR
spectra of this product were in
good agreement with those of an
authentic sample.
3 6 4
4-CH C H
6 4
4-OHC H
Recyclability of the catalyst
For a heterogeneous catalyst, it is
important to examine its ease of
separation and reusability. The re-
usability of the MTF-1E catalyst
was investigated in synthesis of 2-
phenylbenzimidazole and a repre-
sentative ERO reaction between
8
9
2-Cl
C
C
6
6
H
H
5
5
3
4
91
90
2-CH
3
3
1
0
4-CH
4
89
1
,2-epoxy-3-phenoxy propane and
aniline. After the completion of the
reaction, the contents were centri-
fuged to separate the solid catalyst
from the reaction mixture. The cat-
alyst was then washed thoroughly
with distilled water then acetone
and dried in an oven at 908C for
6 h before reuse. The catalyst can
be recycled efficiently and reused
for six times without an apprecia-
ble decrease in the product yield
1
1
H
H
5
4
93
91
12
[
a] Reaction conditions: aldehyde (1 mmol), diamine (1.2 mmol), MTF-1E (20 mg, equivalent to 0.04 mmol of
Fe), H O (4 mL), 408C. [b] Yields refer to isolated pure products.
2
(
Figure 8).
Table 7. Synthesis of 2-phenylbenzimidazole in the presence of different
catalysts.
[a]
[b]
Entry
Catalyst
TiO
Fe O
2 3
MTF-1E
Yield [%]
Characterization
1
2
3
4
2
10
41
97
–
PXRD patterns of different samples were recorded by using
a Bruker D8 Advance X-ray diffractometer operated at a voltage of
4
0 kV and
a current of 40 mA using Ni-filtered CuKa (l=
Without catalyst
0
.15406 nm) radiation. UV/Vis diffuse reflectance spectra were re-
[
(
[
a] Reaction conditions: benzaldehyde (1 mmol), o-phenylenediamine
1.2 mmol), MTF-1E (20 mg), (4 mL), 408C, reaction time=3 h.
b] Yields refer to the isolated pure product.
corded by using a Shimadzu UV 2401PC coupled with an integrat-
2
H O
ing sphere attachment. BaSO was used as the background stan-
4
dard. Thermogravimetric analysis (TGA) was performed by using
a Mettler Toledo TGA/DTA 851e. TEM images of the mesoporous
mixed oxide were obtained by using a JEOL JEM 2010 transmission
product of b-amino alcohol was then purified by flash column
chromatography (ethyl acetate/petroleum ether) to obtain the cor-
responding pure product as a single isomer. All compounds were
characterized based on their spectroscopic data ( H and C NMR)
and by comparison with those reported in the literature.
electron microscope with an operating voltage of 200 kV. N sorp-
2
tion isotherms were obtained by using a Quantachrome Autosorb
1C surface area analyzer at 77 K. Before the measurement, the sam-
ples were degassed at 423 K for approximately 4 h under high
1
13
1
13
vacuum. H and C NMR spectra were recorded by using Bruker
DPX-200, DPX-300, DPX-400, and DPX-500 instruments. The FTIR
spectra of the samples were recorded by using a PerkinElmer FTIR
In a typical catalytic reaction for the synthesis of benzimidazole,
a mixture of o-phenylenediamine (1 mmol, 108 mg) and benzalde-
7
83 spectrophotometer. NH -TPD was performed by using a Micro-
3
ChemCatChem 2015, 7, 2689 – 2697
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
meritics Chemisorb 2720 equipped with a thermal conductivity de-
tector. In a typical experiment for the TPD analysis, a quantity of
MTF-1E material was placed in a quartz cell at 393 K under a contin-
Table 8. Comparison of the catalytic activity of MTF-1E with other related
reported systems.
À1
uous He flow at a flow rate of 30 mLSTP min for ꢀ5 h followed by
Catalyst
Reaction conditions
26 min, MeOH, RT
9 mol% catalyst, solvent 95
free, 298 K, 24 h
Yield [%] Ref.
cooling to ambient temperature. The catalyst was then saturated
Fe/MgO
94
[36f]
[36g]
À1
by NH gas at a flow rate of 30 mL min for ꢀ30 min. The physi-
3
STP
Cu3/2PMo12
O
40/SiO
2
cally adsorbed NH was removed by He gas flow. The TPD experi-
3
ment was then performed by increasing the temperature at
Iron phosphonate
14.3 mol% catalyst, EtOH, 90
RT, 3 h
[40a]
[40b]
À1
a ramp rate of 58Cmin . The results were calculated by compari-
II
Cu -containing nanosilica tri- 25 min, 323 K
azine dendrimer
95
son with a standard TPD calibration curve.
MTF-1E
4 mol% catalyst, H
08C, 3 h
2
O,
97
this
work
4
Acknowledgements
S.M.I. acknowledges the Council of Scientific and Industrial Re-
search (CSIR) and Department of Science and Technology (DST),
New Delhi, India for funding. We acknowledge DST for providing
support to the University of Kalyani under Purse and FIST pro-
gramme. B.B. acknowledges the Council of Scientific and Industri-
al Research (CSIR) for a SPM research fellowship. N.S. acknowl-
edges UGC, New Delhi for providing his senior research fellow-
ship. A.B. wishes to thank DST-SERB, New Delhi for a extramural
project grant and DST Unit on Nanoscience for providing instru-
mental facilities at IACS.
Keywords: amino alcohols · heterogeneous catalysis · iron ·
mesoporous materials · titanium
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