Nanosized Mn(acac)3 anchored on amino functionalized silica for the selective oxidative synthesis of 2-arylbenzimidazoles, 2-arylbenzothiazoles and aerobic oxidation of benzoins in water

Article abstract of DOI:10.1007/s10562-010-0540-8

Silica functionalized Mn(acac)₃ was prepared and employed for the one-pot oxidative synthesis of 2-arylbenzimidazoles, 2-arylbenzothiazoles; and oxidation of benzoins to benzils under air atmosphere using water as the reaction medium. Environmentally friendly procedure, chemoselectivity and excellent yields are main advantages of this procedure. In all the cases, the catalyst was found to be highly active and selective; passes hot filtration test successfully; and could be recycled several times with a slight loss of activity. Graphical Abstract: SiO₂-Mn(acac)₃ catalyzed synthesis of 2-arylbenzimidazoles, 2-arylbenzothiazoles and oxidation of benzoins in air [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-010-0540-8

Catal Lett (2011) 141:608–615
DOI 10.1007/s10562-010-0540-8
Nanosized Mn(acac)3 Anchored on Amino Functionalized Silica
for the Selective Oxidative Synthesis of 2-arylbenzimidazoles,
2-arylbenzothiazoles and Aerobic Oxidation of Benzoins in Water
•
Ravinderpal Kour Sodhi Satya Paul
Received: 5 October 2010 / Accepted: 8 December 2010 / Published online: 5 January 2011
Ó Springer Science+Business Media, LLC 2010
Abstract Silica functionalized Mn(acac)3 was prepared
and employed for the one-pot oxidative synthesis of
2-arylbenzimidazoles, 2-arylbenzothiazoles; and oxidation
of benzoins to benzils under air atmosphere using water as
the reaction medium. Environmentally friendly procedure,
chemoselectivity and excellent yields are main advantages
of this procedure. In all the cases, the catalyst was found to
be highly active and selective; passes hot filtration test
successfully; and could be recycled several times with a
slight loss of activity.
heterogenised through covalent or non-covalent attachment
to either inorganic or organic materials, particularly silica
have been successfully employed in aqueous media [15].
Benzimidazole and benzothiazole functionality leads to
highly active pharmaceuticals [16–20]. High profile bio-
logical activities of compounds containing benzimidazole
and benzothiazole structures has attracted considerable
attention for their synthesis. Commonly employed
synthesis of benzimidazoles involve the reaction between
1,2-phenylenediamines and carboxylic acids or their
derivatives (nitriles, amidates, orthoesters) in the presence
of strong acids such as polyphosphoric acid [21] or mineral
acids [22] and the thermal or acid promoted cyclization of
N-(N-arylbenzimidoyl)-1,4-benzoquinoneimines [23]. The
other popular strategies for their synthesis utilize o-nitro-
anilines as intermediates or resort to direct N-alkylation of
an unsubstituted benzimidazole [24–27]. Recently, strate-
gies have directed toward the synthesis of benzimidazoles
involving cyclocondensation of 1,2-phenylenediamines
with aldehydes under oxidative conditions [28–31]. In
addition to various methodologies reported for the syn-
thesis of benzothiazoles [32–39], the most common and
convenient involve the cyclocondensation of o-aminothio-
phenols with aldehydes under oxidative conditions [28–31,
40, 41]. In many of the methods for the synthesis of
benzimidazoles or benzothiazoles, stoichiometric amounts
of oxidizing agents such as amino-acid based prolinium
nitrate ionic liquid [42], K3Fe(CN)6 at 90 °C under basic
conditions [43]; excess of Mn(OAc)3 in AcOH at 110 °C
for 4 h [44]; CAN–H2O2 [28]; Cu3/2PMo12O40/SiO2 [29];
Fe(NO3)3–H2O2 [30]; ZrOCl2ꢀnH2O/montmorillonite K10
[31]; nano CuO in DMSO [45] etc. have been employed.
Recently, Chakraborti et al. [46] have reported environ-
mentally benign method for the synthesis of 2-arylbenzo-
thiazoles in water at 110 °C.
Keywords Silica functionalized Mn(acac)3 ꢀ
2-arylbenzimidazoles ꢀ 2-arylbenzothiazoles ꢀ Benzils ꢀ
Recyclable heterogeneous catalyst ꢀ Green procedure ꢀ
Aqueous medium ꢀ Air atmosphere
1 Introduction
In recent times, the development of heterogeneous catalysts
[1–6] for organic synthesis has taken lead over their
homogeneous counterparts because of simplified recovery,
recyclability and ease of handling, potential for incorpo-
ration in continuous reactors and microreactors; and more
environmentally safe disposal. Among various solid sup-
ports commonly used for the preparation of heterogeneous
catalysts, silica’s have received more attention [1, 7–14].
Recently, water has been found to be a promising medium
for heterogenised homogeneous catalysis. Catalysts
R. K. Sodhi ꢀ S. Paul (&)
Department of Chemistry, University of Jammu, Jammu
180 006, India
e-mail: paul7@rediffmail.com
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Nanosized Mn(acac)3 Anchored on Amino Functionalized Silica
609
The oxidation of benzoins to benzils is an important
synthetic transformation in organic synthesis because of
their applications as photosensitive and synthetic agents in
organic and pharmaceutical chemistry [47–52]. The most
straight forward method for the synthesis of benzils is the
oxidation of benzoins. Numerous oxidizing reagents have
been reported for the oxidation of benzoins to benzils in
homogeneous and heterogeneous media [53, 54]. To
overcome the problems associated with homogeneous
catalysis, we have recently reported a series of silica
functionalized metal catalysts for the selective oxidation of
benzoins to benzils in toluene at 100 °C [54]. Thus, there is
still a need to develop new method which utilizes water as
solvent and hence contribute toward green chemistry.
In recent times, transition-metal catalyzed oxidations
using oxygen as green oxidant are highly desirable
[55–58]. Among various transition metals, manganese is
probably the most versatile in promoting or catalyzing
oxidation reactions [59–65]. Various supported manganese
complexes have been prepared and used for various oxi-
dation reactions [66–71]. Recently, silica functionalized
Mn(acac)2 has been reported for the oxidation of limonene
using t-BHP as an oxidant [72]. Most desirable oxidations
are those which could be carried out under air atmosphere
without using additional oxidant.
micrographs (TEM) were recorded in a H7500 Hitachi. The
amount of the manganese supported on silica was deter-
mined by stirring the sample in dil. HNO3 and subjecting to
AAS analysis on GBC Avanta-M atomic absorption spec-
trometer manufactured by GBC Scientific agencies.
2.2 Preparation of Silica Functionalized Mn(acac)3
[SiO2-Mn(acac)3]
Silica (K100, 0.063–0.200 mm) was activated by refluxing
in a mixture of conc. HCl and distilled water (1:1) for 12 h
and then washed thoroughly with distilled water and dried
at 110 °C for 12 h. Activated silica (10 g) was added to the
solution of 3-aminopropyl(trimethoxy) silane (1.79 g,
10 mmol) in dry toluene (150 mL) and refluxed for 24 h.
The 3-aminopropylsilica (AMPS) was filtered off, washed
with hot toluene (100 mL) and dried at 110 °C for 5 h to
give the surface bound amino groups (AMPS). For the
preparation of SiO2-Mn(acac)3, a mixture of organically
modified silica (AMPS, 5 g) and Mn(acac)3 (0.85 g,
2.5 mmol) in dry toluene (100 mL) was stirred at 120 °C
for 16 h. The catalyst was filtered off, washed with toluene
(250 mL) till the washings were colourless and dried in a
hot air oven at 110 °C for 5 h. In order to remove any
physisorbed Mn(acac)3, the catalyst was conditioned by
refluxing for 12 h each in xylene at 130 °C (2 9 2 h),
ethanol at 78 °C (2 9 2 h) and acetonitrile at 80 °C
(2 9 2 h). Finally, the catalyst was dried in hot air oven at
110 °C for 5 h. The complete preparation procedure of
SiO2-Mn(acac)3 is represented in Scheme 1.
In this paper, we report the simple preparation of
silica functionalized Mn(acac)3 [SiO2-Mn(acac)3] and its
application for the selective oxidative synthesis of 2-aryl-
benzimidazoles or 2-arylbenzothiazoles by the one-pot
condensation of aromatic aldehydes and o-phenylenedi-
amine or o-aminothiophenols, respectively; and selective
oxidation of benzoins, under air atmosphere using water as
the reaction medium.
MeO
OH
MeO Si
MeO
NH
2
O
OH
OH
O
O
Si
NH
2
o
Dry toluene, 120
reflux, 24 h
C
2 Experimental
AMPS
2.1 General Remarks
Mn(acac)
3
Dry toluene,
o
The chemicals used were either prepared in our laborato-
ries or purchased from Aldrich Chemical Company and
Merck. The products were characterized by comparison of
their physical data with those of known samples or by their
spectral data. The 1H NMR data were recorded in CDCl3 or
DMSO-d6 on Bruker DPX 200 (200 MHz) spectrometer
using TMS as an internal standard. The IR spectra were
recorded on Perkin-Elmer FTIR spectrophotometer using
KBr windows and mass spectra were recorded using
Bruker Esquires 3000 (ESI). XRD spectra were recorded
on Bruker AXSD8 X-ray diffraction spectrometer and
scanning electron microscope (SEM) using Jeol make
T-300 scanning electron microscope. Transmission electron
120 C, 16 h
O
O
O
Si
N
O
Mn
O
O
O
O
Scheme 1 Synthesis of silica functionalized Mn(acac)3 [SiO2-
Mn(acac)3]
123

610
R. K. Sodhi, S. Paul
2.3 SiO2-Mn(acac)3 Catalyzed Oxidative Synthesis
of 2-arylbenzimidazoles and 2-arylbenzothiazoles
Table 2 SiO2-Mn(acac)3 catalyzed selective oxidation of benzoins to
benzils at 100 °C in water under air atmosphere
Entry Ar
Time Yield m.p./Lit. m.p. ( °C)
(min) (%)a
A mixture of 1,2-phenylenediamine (0.8 mmol) [for entries
3, 6, 9, 10, 1,2-phenylenediamine (1 mmol) was used] or
o-aminothiophenol (1 mmol), aromatic aldehyde (1 mmol)
and SiO2-Mn(acac)3 (0.2 g, 6.8 mol% Mn) in water (7 mL)
was stirred at 70 °C under air atmosphere for an appro-
priate time (Table 1). On completion (monitored by TLC),
the reaction mixture was cooled to room temperature and
filtered. The residue was washed with hot ethyl acetate
(40 mL), followed by distilled water (200 mL). The
1
2
3
4
5
C6H5
45
45
60
50
99
97
97
96
94
96–97/95 [89]
4-MeC6H4
4-MeOC6H4
4-ClC6H4
103–104/104–105 [89]
134–135/135 [89]
197–199/199 [89]
103–104/104–104.5 [90]
4-t-C4H9C6H4 50
Reaction conditions: benzoin (1 mmol), K2CO3 (1.5 mmol), SiO2-
Mn(acac)3 (0.15 g, 5.1 mol% Mn), water (5 mL) and 100 °C as the
reaction temperature
a
Isolated yields
Table 1 SiO2-Mn(acac)3-catalyzed selective oxidative synthesis of
2-arylbenzimidazolesa and 2-arylbenzothiazolesb in water at 70 °C
under air atmosphere
organic layer was washed with water and dried over
anhydrous Na2SO4. Finally, the product was purified by
passing through column of silica gel and elution with
EtOAc: pet ether.
Entry
R
X
Time Yield m.p./Lit. m.p. ( °C)
(h)
(%)c
1
C6H5
NH
3
90
95
93
92
90
88
88
90
87
89
93
93
93
92
93
92
92
92
91
94
290–292/292 [82]
2
3d
4-OMeC6H4
4-MeC6H4
4-ClC6H4
4-ClC6H4
4-NO2C6H4
3-NO2C6H4
2-NO2C6H4
NH 3.5
225–226/226 [82]
2.4 SiO2-Mn(acac)3 Catalyzed Oxidation of Benzoins
to Benzils
NH
NH
NH
NH
NH
NH
4
5
8
4
4
4
271–272/270 [82]
4
288–290/288–291 [82]
288–290/288–291 [82]
313–315/316 [82]
5e
6d
7
A mixture of benzoin (1 mmol), K2CO3 (1.5 mmol) and
SiO2-Mn(acac)3 (0.15 g, 5.1 mol% Mn) in water (5 mL) was
stirred at 100 °C for an appropriate time (Table 2). On
completion (monitored by TLC), the reaction mixture was
cooled to room temperature and filtered. The residue was
washed with hot ethyl acetate (30 mL), followed by distilled
water (150 mL). The organic layer was washed with water
and dried over anhydrous Na2SO4. Finally, the product was
obtained after crystallization with EtOAc: pet ether.
204–205/204–206 [82]
278–279/277–279 [83]
202-205/203-205 [82]
285–287/288 [82]
8
9d
10d
11
12
13
14
15f
16
17
18
19
20
a
C6H5CH = CH NH 4..5
2-Furyl
NH
S
5
C6H5
5
113–114/115–116 [41]
117–119/119–121 [41]
84–85/86 [84]
4-OMeC6H4
4-MeC6H4
4-OHC6H4
4-ClC6H4
4-ClC6H4
4-BrC6H4
4-NO2C6H4
3-NO2C6H4
2-Furyl
S
3
S
5
S
5
229–230/231–232 [85]
114–115/116.5 [86]
114–115/116.5 [86]
131/130–133 [87]
The structures of the products synthesised were con-
1
firmed by FTIR, H NMR, mass spectral data and com-
S
3.5
8
S
parison with authentic samples prepared according to
S
4
literature procedures.
S
3.5
3.5
3.5
224–225/224–226 [41]
184–186/181–183 [41]
104–105/106–107 [88]
S
S
3 Results and Discussion
Reaction conditions: aldehyde (1 mmol), 1,2-phenylenediamine
(0.8 mmol), SiO2-Mn(acac)3 (0.2 g, 6.8 mol% Mn), water (7 mL)
and 70 °C as the reaction temperature
Reaction conditions: aldehyde (1 mmol), o-aminothiophenol
(1 mmol), SiO2-Mn(acac)3 (0.2 g, 6.8 mol% Mn), water (7 mL) and
3.1 Catalyst Preparation and Characterization
b
SiO2-Mn(acac)3 was prepared by refluxing Mn(acac)3 with
(3-aminopropyl)silica in dry toluene (Scheme 1) following
the procedure reported for the synthesis of MCM-41-
Mn(acac)2 with slight modification [72]. A similar meth-
odology has also been reported for the preparation of
MCM-41-L/Mn(acac)2 [73] using EtOH and HCl in addi-
tion to toluene which we have used in our present proce-
dure. The characterization of SiO2-Mn(acac)3 was done by
FTIR, TGA, XRD, SEM, TEM and AAS analysis.
70 °C as the reaction temperature
c
Isolated yield by column chromatography
d
Reaction conditions: in case of entries 3, 6, 9, 10, aldehyde
(1 mmol), 1,2-phenylenediamine (1 mmol), SiO2-Mn(acac)3 (0.2 g,
6.8 mol% Mn), water (7 mL) and 70 °C as the reaction temperature
e
The reaction was carried out using 4-chlorobenzaldehyde (20 mmol),
1,2-phenylenedimine (16.6 mmol), SiO2-Mn(acac)3 (2 g, 67.9 mol%
Mn), water (30 mL) and 70 °C as the reaction temperature
f
The reaction was carried out using 4-chlorobenzaldehyde (20 mmol),
o-aminothiophenol (20 mmol), SiO2-Mn(acac)3 (2 g, 67.9 mol% Mn),
water (25 mL) and 70 °C as the reaction temperature
The FTIR of 3-aminopropylsilica (AMPS) displays
characteristic –CH2 stretching bands at 2939 and
123

Nanosized Mn(acac)3 Anchored on Amino Functionalized Silica
611
400
100
53
57 54
100
0
10
20
30
40
50
60
70
Position [°2Theta] (Copper (Cu))
Fig. 1 XRD diffraction patterns of SiO2-Mn(acac)3
Fig. 2 SEM image of SiO2-Mn(acac)3
2970 cm-1. After schiff condensation, the –CH2 stretching
band shifted to 2928 cm-1 and the characteristic band due
to C = N bond appears at 1637 cm-1
.
The stability of SiO2-Mn(acac)3 was determined by
thermal analysis. The TGA curve showed an initial weight
loss up to 110 °C which may be attributed to loss of the
residual solvent and water trapped onto the surface of sil-
ica. Further, weight loss occurred from 195 to 550 °C,
which was due to the loss of organic functionality. Thus,
the catalyst is stable up to 195 °C and it is safe to carry out
the reaction at 70 or 100 °C under heterogeneous
conditions.
The powdered XRD patterns for SiO2-Mn(acac)3
showed the peaks in the pattern which were indexed on the
basis of crystallographic data for the known structure of
silica (Fig. 1). Four major reflection patterns were found in
the XRD pattern corresponding to 2h = 36°, 44°, 59° and
64°. The microstructure and morphology was studied using
the SEM. The surface of SiO2-Mn(acac)3 was found to be
fine homogeneous powder with porous structure (Fig. 2).
The TEM images provide a direct observation of the
morphology and distribution of manganese onto SiO2-
Mn(acac)3. The regular arrangement of the pores can be
clearly observed. The Mn is uniformly distributed with
average diameter of 7.5 nm (Fig. 3). Further, no bulk
aggregation of the metal occurred indicating that the Mn
was finely dispersed onto the surface of silica.
Fig. 3 TEM image of SiO2-Mn(acac)3
2-arylbenzimidazoles (Scheme 2), 4-methoxybenzaldehyde
and 1,2-phenylenediamine were selected as the test sub-
strates. After carrying out series of reactions under different
set of conditions, the optimized conditions selected are:
aldehyde (1 mmol), 1,2-phenylenediamine (0.8 mmol),
SiO2-Mn(acac)3 (0.2 g, 6.8 mol% Mn), water (7 mL) and
70 °C as the reaction temperature. In the cyclocondensation
of aldehydes with 1,2-phenylenediamine, there are possi-
bilities of two products, [43, 74, 75], 2-arylbenzimidazoles
and 1,2-disubstituted benzimidazoles. The present proce-
dure gives 2-arylbenzimidazoles selectively and no 1,2-
disubstituted benzimidazoles have been detected.
The amount of the manganese supported onto silica was
determined by AAS analysis. It was found that 0.0173 g of
Mn per g of SiO2-Mn(acac)3 was present.
3.2 Catalyst Testing for the Synthesis of 2-
arylbenzimidazoles and 2-arylbenzothiazoles
In case of 2-arylbenzothiazoles, the reaction was carried
out using 4-methoxybenzaldehyde (1 mmol), o-amino-
thiophenol (1 mmol), SiO2-Mn(acac)3 (0.2 g, 6.8 mol% of
Mn), deionized water (7 mL) and atmospheric air as the
To test the activity of SiO2-Mn(acac)3 as completely
heterogeneous catalyst for the one-pot synthesis of
123

612
R. K. Sodhi, S. Paul
1/2 O2
Fig. 4 Proposed mechanism for
the SiO2-Mn(acac)3 catalyzed
oxidative synthesis of 2-
arylbenzimidazoles and
III
R
X
N
O
Mn
XH
H
R
2-arylbenzothiazoles
+
B
+
NH
2
H2O
O
°
H2O, 70
C
V
A
Mn
O2 (air)
X
R
X
N
H
H
N
R
Mn
O H
X = NH, S
source of oxygen under stirring at 70 °C for 3 h
(Scheme 2). It was found that 2-(4-methoxyphenyl)ben-
zothiazole was obtained in 93% isolated yield. Since thiols
are good nucleophiles and SET agents, but no substitution
of the halogen atom (4-chloro and 4-bromobenzaldehyde)
or the nitro group (4-nitrobenzaldehyde), dealkylation
(4-methoxybenzaldehyde) or reduction of nitro group took
place, which demonstrated high chemo-selectivity of the
reactions. No competitive dithioacetal formation was
observed under the present conditions, though dithioacetal
formation is the common reaction of aldehydes with thiols.
No disulfide formation was observed, which clearly indi-
cates the high selectivity of the procedure.
70 °C for 8 h. After the usual work-up, 2-(4-chloro-
phenyl)benzimidazole was obtained in 90% isolated yield
(Entry 5, Table 1) and 2-(4-chlorophenyl)benzothiazole
was obtained in 92% isolated yield (Table 1, entry 16).
3.3 Catalyst Testing for the Oxidation of Benzoins
to Benzils
The most common method for the synthesis of benzils
involves the oxidation of benzoins. However, the important
aspects for this oxidation include selectivity, toxicity of
oxidants and environmentally friendly procedure. Recently,
we have reported [54] a selective method for the oxidation
of benzoins in air atmosphere using covalently anchored
Pd, Co and Ni complexes onto silica using toluene as
solvent. To make the procedure still more green, the
activity of SiO2-Mn(acac)3 was also tested for the oxida-
tion of benzoins in air using deionized water as solvent
(Scheme 3). Benzoin was selected as the test substrate and
reaction was carried out under different set of conditions
with respect to different solvents, temperatures and
amounts of the catalyst. After carrying out series of reac-
tions, the optimum conditions selected are: benzoin
(1 mmol), SiO2-Mn(acac)3 (0.15 g, 5.1 mol% Mn), K2CO3
(1.5 mmol), water (5 mL) and 100 °C as the reaction
temperature.
To demonstrate the versatility of the catalytic system for
the synthesis of 2-substituted benzimidazoles and benzo-
thiazoles, aldehydes substituted with various electron-
withdrawing and electron-donating groups were chosen
and excellent results were obtained (Table 1). Heterocyclic
aldehydes also worked well and corresponding products
were obtained in excellent yields (Table 1, entries 10, 20).
To scale-up the reaction conditions, the reactions were
carried out by stirring a mixture of 1,2-phenylenediamine
(1.79 g, 16.6 mmol in case of benzimidazoles) and
o-aminothiophenol (2.5 g, 20 mmol in case of benzo-
thiazoles), 4-chlorobenzaldehyde (2.72 g, 20 mmol), SiO2-
Mn(acac)3 (2 g, 67.9 mol% Mn) and water (30 mL) at
NH2
XH
N
SiO2-Mn(acac)3
H2O, Air, 70°C
OH
O
R-CHO
R
SiO2-Mn(acac)3, H2O
K2CO3, 100 °C, Air
Ar
Ar
X
Ar
Ar
X = NH, S
O
O
Scheme 2 SiO2-Mn(acac)3 catalyzed selective oxidative synthesis of
2-arylbenzimidazoles and 2-arylbenzothiazoles under air atmosphere
in water
Scheme 3 SiO2-Mn(acac)3 catalyzed selective oxidation of benzoins
to benzils under air atmosphere in water
123

Nanosized Mn(acac)3 Anchored on Amino Functionalized Silica
613
Table 3 Effect of the catalyst on the SiO2-Mn(acac)3 catalyzed synthesis of 2-arylbenzimidazoles, 2-arylbenzothiazoles and benzils under air
atmosphere using water as a solvent
Entry
Catalyst
2-Arylbenzimidazolesa
2-Arylbenzothiazolesb
Benzilsc
Time (h)
Time (h)
Yield (%)
Time (h)
Yield (%)
Yield (%)
1
2
3
4
a
No catalyst
Activated SiO2
AMPS
10
10
10
3
No reaction
Traces
Traces
95
10
10
10
3
20
25
25
93
8
96
97
98
99
8
7.5
0.75
SiO2-Mn(acac)3
Reaction conditions: 4-methoxybenzaldehyde (1 mmol), 1,2-phenylenediamine (0.8 mmol), activated silica or AMPS or SiO2-Mn(acac)3
(0.2 g), water (7 mL) and 70 °C as the reaction temperature
b
Reaction conditions: 4-methoxybenzaldehyde (1 mmol), o-aminothiophenol (1 mmol), activated silica or AMPS or SiO2-Mn(acac)3 (0.2 g),
water (7 mL) and 70 °C as the reaction temperature
c
Reaction conditions: benzoin (1 mmol), K2CO3 (1.5 mmol), activated silica or AMPS or SiO2-Mn(acac)3 (0.15 g), water (5 mL) and 100 °C as
the reaction temperature
To demonstrate the generality and versatility of the
procedure, benzoins substituted with different groups at
C-4/C-40 possessing both electron-withdrawing and elec-
tron-donating groups were subjected to oxidation under the
selected conditions and excellent results were obtained
(Table 2).
further significant conversion was observed. This indicates
that no active species was present in the supernatant
(determined by AAS analysis).
In order to find out the role of SiO2-Mn(acac)3 as the
heterogeneous catalyst, the reaction with test substrates
(entry 2, Table 1 for 2-arylbenzimidazoles; entry 12,
Table 1 for benzothiazoles; and entry 1, Table 2 for ben-
zoins) was also carried out in the presence of activated
silica, 3-aminopropyl silica (AMPS) and without using any
catalyst. The results are summarised in Table 3. It is clear
that SiO2-Mn(acac)3 catalyzes the reactions.
Our catalytic system in water is advantageous to some of
the earlier reported methods. For example, Yang et al. [76]
have reported the synthesis of 2-arylbenzothiazoles at
100 °C using CTAB; Azizi et al. [77] used PTSA at
100 °C; Chakraborti et al. [46] carried out the reaction
without using any catalyst at 110 °C. Very few catalytic
systems have been reported for the synthesis of 2-aryl-
benzimidazoles in water. For example, Bahrami et al. [78]
used H2O2/HCl in water at 80 °C and more recently, using
SDS micelles at 25 °C [79]; Mukhopadhyay et al. [80]
have used boric acid and glycerol at 80 °C. However, our
catalytic system is successful at 70 °C with similar or
higher yields and moreover, recyclable under the reaction
conditions.
In case of oxidation catalysis, the most important point
is the deactivation and recyclability of the catalyst. To
test this, a series of five consecutive runs were carried
out with the same catalyst by the cyclocondensation
of 1,2-phenylenediamine and 4-methoxybenzaldehyde [in
case of 2-(4-methoxyphenyl)benzimidazole]; o-amino-
thiophenol and 4-methoxybenzaldehyde [in case of 2-(4-
methoxyphenyl)benzothiazole]; and oxidation of benzoin
to benzil. The results are shown in Fig. 4. Thus, there is
no change in the activity of the catalyst up to fifth use.
Further, there is no change in the amount of Mn in
SiO2-Mn(acac)3 after fifth use (as determined by AAS
analysis).
3.4 Heterogeneous Catalysis and Recyclability
When using a supported metal catalyst, two important
issues need to be addressed to qualify this as purely het-
erogeneous catalyst. One is the possibility that some active
metal migrates from the solid to the liquid phase and that
this leached manganese would become responsible for a
significant part of the catalytic activity. To rule out the
contribution of homogeneous catalysis in the results shown
in Table 1, one reaction in case of entry 2 was carried out
in the presence of SiO2-Mn(acac)3 until the conversion was
37% and at that point, SiO2-Mn(acac)3 was filtered off at
the reaction temperature after 1 h. The liquid phase was
then filtered to another flask and allowed to react, but no
A cyclic mechanism has been proposed for the oxidative
synthesis of 2-arylbenzimidazoles and 2-arylbenzothiaz-
oles using SiO2-Mn(acac)3 in the presence of atmospheric
air (Fig. 5). An oxygen molecule from air reacts with
manganese to form oxo complex ‘A’ which deprotonate
2,3-dihydrobenzimidazole or 2,3-dihydrobenzothiazole
yielding the product ‘B’. MnIII is regenerated back with the
elimination of water molecule. The formation of oxo
complex ‘A’ from MnIII and O2 has already been reported
elsewhere [81]. A similar mechanism as reported earlier
[54] has been proposed for the oxidation of benzoins to
benzils using SiO2-Mn(acac)3.
123

614
R. K. Sodhi, S. Paul
100
95
90
85
80
9. Webb JD, MacQuarrie S, McEleney K, Crudden CM (2007) J.
Catal. 252:97
10. Budroni G, Corma A, Garcia H, Primo A (2007) J. Catal. 251:345
11. Qiu H, Sarkar SM, Lee D-H, Jin M-J (2008) Green Chem 10:37
12. Margelefsky EL, Zeidan RK, Davis ME (2008) Chem. Soc. Rev.
37:1118
13. Wang L, Reis A, Seifert A, Philippi T, Ernst S, Jia M, Thiel WR
(2009) Dalton Trans 17:3315
14. Sarkar K, Nandi M, Islam M, Mubarak M, Bhaumik A (2009)
Appl Catal A Gen 352:81–86
15. Minakata S, Komatsu M (2009) Chem. Rev. 109: 711 and ref-
erences cited therein
16. Gravatt GL, Baguley BC, Wilson WR, Denny WA (1994) J Med
Chem 37:4338
17. Roth T, Morningstar ML, Boyer PL, Hughes SH, Buckheit RW
Jr, Michejda CJ (1997) J Med Chem 40:4199
18. Bradshaw TD, Wrigley S, Shi DF, Shulz RJ, Paull KD, Stevens
MFG (1998) Br. J Cancer 77:745
19. Hutchinson I, Chua MS, Browne HL, Trapani V, Bradshaw TD,
Westwelland AD, Stevens MFG (2001) J Med Chem 44:1446
20. Horton DA, Bourne GT, Smythe ML (2003) Chem Rev 103:893
21. Preston PN (1981) Benzimidazoles and congeneric tricyclic
compounds. In: Weissberger A, Taylor EC (eds) In the chemistry
of heterocyclic compounds, vol 40. Wiley, New York, Part 1,
pp 6–60
22. Grimmett MR (1984) Imidazoles and their benzo derivatives. In:
Katritzky AR, Rees CW (eds) In Comprehensive heterocyclic
chemistry, vol 5. Pergamon, Oxford, pp 457–487
Benzimidazoles
Benzothiazoles
Benzils
1
2
3
4
5
Number of runs
Fig. 5 Recyclability of SiO2-Mn(acac)3. Reaction conditions: Benz-
imidazoles-1,2-phenylenediamine (0.8 mmol), 4-methoxybenzalde-
hyde (1 mmol), SiO2-Mn(acac)3 (0.2 g, 6.8 mol% Mn), water
(7 mL) at 70 °C for 3.5 h; benzothiazoles-o-aminothiophenol
(1 mmol), 4-methoxybenzaldehyde (1 mmol), SiO2-Mn(acac)3
(0.2 g, 6.8 mol% Mn), water (7 mL) at 70 °C for 3 h; benzils–
benzoin (1 mmol), SiO2-Mn(acac)3 (0.15 g, 5.1 mol% Mn), K2CO3
(1.5 mmol), water (5 mL) at 100 °C for 45 min
`
23. Benincori T, Sannicolo F (1988) J. Heterocycl. Chem. 25:1029
24. Kim BH, Han R, Kim JS, Jun YM, Waik W, Lee BM (2004)
Heterocycles 63:41
4 Conclusion
25. Itoh T, Nagata K, Ishikawa H, Obsawa A (2004) Heterocycles
63:2769
26. Tumelty D, Cao K, Holmes CP (2001) Org. Lett. 3:83
27. Kilburn JP, Lau J, Jones RCF (2000) Tetrahedron Lett. 41: 5419
and references cited therein
28. Bahrami K, Khodaei MM Naali F (2008) J. Org. Chem. 73: 6835
and references cited therein
29. Fazaeli R, Aliyan H (2009) Appl. Catal. A: Gen. 353:74
30. Bahrami K, Khodaei MM, Naali F (2009) Synlett. 569 and ref-
erences cited therein
31. Rostamizadeh S, Amani AM, Aryan R, Ghaieni HR, Norouzi L
(2008) Monatsh. Chem. 140:547
In conclusion, we have reported a novel heterogeneous
SiO2-Mn(acac)3 for the selective oxidative synthesis of 2-
arylbenzimidazoles and 2-arylbenzothiazoles; and selective
oxidation of benzoins to benzils under air atmosphere in
water. The salient features of our catalytic system includes
high selectivity, mild reaction conditions, molecular oxy-
gen from air as oxidant, recyclability, high yield of prod-
ucts and water as the reaction medium.
32. Couture A, Grandclaudon P (1984) Heterocycles 22:1383
33. Tale RH (2002) Org. Lett. 4:1641
34. Alagille D, Baldwin RM, Tamagnan GD (2005) Tetrahedron Lett
46:1349
35. Itoh T, Mase T (2007) Org. Lett. 9:3687
36. Paul S, Gupta M, Gupta R (2002) Synth Commun 32:3541
37. Heuser S, Keenam M, Weichert AG (2005) Tetrahedron Lett
46:9001
Acknowledgments We thank the Head, Regional Sophisticated
Instrumentation Centre, Nagpur University, Nagpur for AAS analysis;
and Head, SAIF, Punjab University Chandigarh for TEM, SEM and
XRD.
References
38. Terashima M, Ishii M (1982) Synthesis 6:484
39. Chakraborti AK, Rudrawar S, Kaur G, Sharma L (2004) Synlett
1533
40. Hari A, Karan C, Rodrigues WC, Miller BL (2001) J Org Chem
66:991
41. Al-Qalaf F, Mekheimer RA, Sadek KU (2008) Molecules
13:2908
42. Rostamizadeh S, Aryan R, Ghaieni HR, Amani AM (2009) J.
Heterocycl. Chem. 46:74
1. Yin L, Liebscher J (2007) Chem. Rev. 107: 133 and references
cited therein
2. Li H, Wang L, Li P (2007) Synthesis 1635
3. Budarin V, Clark JH, Hardy JJE, Luque R, Milkowski K, Tavener
SJ, Wilson AJ (2006) Angew Chem Int Ed Engl 45:3782
4. Choudhary D, Paul S, Gupta R, Clark JH (2006) Green Chem. 8:
479 and references cited therein
43. Hutchinson I, Stevens MFG, Westwel AD (2000) Tetrahedron
Lett 41:425
44. Mu XJ, Zou JP, Zeng RS, Wu JC (2005) Tetrahedron Lett
46:4345
45. Saha P, Ramana T, Purkait N, Ali MA, Paul R, Punniyamurthy T
(2009) J Org Chem 74:8719
5. Jamwal N, Gupta M, Paul S (2008) Green Chem 10:999
6. Shamim T, Gupta M, Paul S (2009) J. Mol. Catal. A: Chem.
10:1931
7. Lu Z-L, Lindner E, Mayer HA (2002) Chem Rev 102:3543
8. Clark JH, Macquarrie DJ, Tavener SJ (2006) Dalton Trans
36:4297
123

Nanosized Mn(acac)3 Anchored on Amino Functionalized Silica
615
46. Chakraborti AK, Rudrawar S, Jadhav KB, Kaur G, Chankeshwara
SV (2007) Green Chem 9:1335
68. Sahoo S, Kumar P, Lefebvre F, Halligudi SB (2009) Appl. Catal.
A: Gen. 354:17
47. Morrison H, Danishefsky S, Yates P (1961) J Org Chem 26:2617
48. Vollhardt KPC, Schore NE (1994) Organic Chemistry, 2nd edn.
Freeman, New York, p 924
69. Ren W, Fu X (2009) J. Mol. Catal. A: Chem. 312:40
70. Castaman ST, Nakagaki S, Ribeiro RR, Ciuffi K-J, Drechsel SM
(2009) J. Mol. Catal. A: Chem. 300:89
49. Mitra AK, De A, Karchaudhuri N (1999) J Chem Res(S) 9:246
50. Vogel
71. Heravi MM, Oskooie HA, Malakooti R, Alimadadi B, Alinejad
H, Behbahani FK (2009) Catal Commun 10:819
72. Oliveira P, Machado A, Ramos AM, Fonseca IM, Braz Fernandes
FM, Botelho do Rego AM (2007) J Vital Catal Commun 8:1366
73. Fernandes TA, Nunes CD, Vaz PD, Calhorda MJ, Brandao P,
Rocha J, Goncalves IS, Valente AA, Ferreira LP, Godincho M,
Ferreira P (2008) Microporous Mesoporous Mater 112:14
74. Ravi V, Ramu E, Vijay K, Rao AS (2007) Chem Pharm Bull
55:1254
A (1978) Textbook of practical organic chemistry
including qualitative organic analysis, 4th edn. Longman, New
York, p 883
51. Joul JA, Mills K, Smith GF (1995) Heterocyclic chemistry, 3rd
edn. Chapman & Hall, London
52. Mousset C, Giraud A, Provot O, Hamze A, Bignon J, Liu J-M,
Thoret S, Dubois J, Brion J-D, Alami M (2008) Bioorg. Med.
Chem. Lett. 18:3266
53. Skobridis K, Theodorou V, Weber E (2006) ARKIVOC X:102
54. Shamim T, Chaudhary D, Mahajan S, Paul S Gupta R (2009)
Catal. Commun. 10: 1931 and references cited therein
55. Kamata K, Kasai J, Yamaguchi K, Mizuno N (2004) Org. Lett.
6:3577
56. Punniyamurthy T, Velusamy S, Iqbal J (2005) Chem Rev
105:2329
57. Matsumoto T, Ueno M, Wang N, Kobayashi S (2008) Chem.
Asian J. 3:196
58. Gligorich KM, Sigman MS (2009) Chem Commun 26:3854
59. Gilbert BC, Smith JRL, Newton MS, Oakes J, Prats RP (2003)
Org. Biomol. Chem. 1:1568
60. Kim SY, Park KH, Chung YK (2005) Chem Commun 10:1321
61. Mardani HR, Golchoubian H (2006) J. Mol. Catal. A: Chem.
259:197
62. Nasr-Esfahani M, Moghadam M, Tangestaninejad S, Mirkhani V,
Momeni AR (2006) Bioorg. Med. Chem. 14:2720
63. Sun Y, Tang N (2006) J. Mol. Catal. A: Chem. 255:171
64. Layfield RA (2008) Chem. Soc. Rev. 37:1098
65. Stamatis A, Doutsi P, Vartzouma C, Christoforidis KC,
Deligiannakis Y, Louloudi M (2009) J. Mol. Catal. A: Chem.
297:44
75. Jacob RG, Dutra LG, Radatz CS, Mendes SR, Perin G, Lenarda˜o
EJ (2009) Tetrahedron Lett 50:1495
76. Yang X-L, Xu C-M, Lin S-M, Chen J-X, Ding J-c, Wu H-Y, Su
W-K (2010) J. Braz. Chem. Soc. 21:37
77. Azizi N, Amiri AK, Baghi R, Bolourtchian M, Hashemi MM
(2009) Monatsh Chem 140:1471
78. Bahrami K, Khodaei MM, Kavianinia I (2006) J Chem Res (S)
12:783
79. Bahrami K, Khodaei MM, Nejati A (2010) Green Chem 12:1237
80. Mukhopadhyay C, Tapaswi PK, Butcher RJ (2009) Aust J Chem
62:140
81. Gilbert BC, Kamp NWJ, Smith JR, Oakes J (1998) J Chem Soc
Perkin Trans 2:1841
82. Xiangming H, Huiqiang M, Yulu W (2007) ARKIVOC Xiii:150
83. Heravi MM, Tajbakhsh M, Ahamdi AN, Mohajerani B (2006)
Monatsh. Chem. 137:175
84. Kiprianov AI, Ushenko IK, Gershun AL (1944) J Gen Chem
USSR (Engl Transl) 14:865
85. Bogert MT, Corbitt HB (1926) J Am Chem Soc 48:783
86. Cohen VI (1979) J Heterocycl Chem 16:13
87. Sharghi H, Asemani O (2009) Synth Commun 39:860
88. Melnikova EB, Elchaninov MM, Milov AA, Lukyanov BS
(2008) Chem Heterocycl Compd 44:1070
89. Pollack JRA, Stevens R (1965) Dictionary of organic compounds.
Eyre and Spottiswoode Ltd, Great Britain
90. Trisler JC, Frye JL (1965) J Org Chem 30:306
66. Guoyu Y, Ailing S, Wenfeng Z, Hailin Z, Denggao J (2007) Catal
Lett 118:275
67. Silv AR, Budarin V, Clark JH, Freire C, de Castro B (2007)
Carbon 45:1951
123