Multi-wall carbon nanotube supported manganese(III) porphyrin: An efficient and reusable catalyst for oxidation of 2-imidazolines with sodium periodate

Article abstract of DOI:10.1007/s11243-012-9651-y

The oxidation of 2-substituted imidazolines with sodium periodate catalyzed by tetrakis(p-aminophenyl)-porphyrinatomanganese(III) chloride, [Mn(TNH ₂PP)Cl], supported on functionalized multi-wall carbon nanotubes is reported. A wide variety of 2-imidazolines were efficiently converted to their corresponding imidazoles by this catalytic system. When the same reaction was subjected to ultrasonic irradiation, the reaction times were reduced significantly and the product yields were increased. This catalyst could be reused several times without significant loss of activity. The effects of reaction parameters such catalyst amount, choice of solvent, and the effects of ultrasonic irradiation on the catalytic activity have been investigated.

Full text of DOI:10.1007/s11243-012-9651-y

Transition Met Chem (2013) 38:1–5
DOI 10.1007/s11243-012-9651-y
Multi-wall carbon nanotube supported manganese(III) porphyrin:
an efficient and reusable catalyst for oxidation of 2-imidazolines
with sodium periodate
•
Hadi Kargar Majid Moghadam Valiollah Mirkhani
•
•
•
•
Shahram Tangestaninejad Iraj Mohammadpoor-Baltork
Simindokht Rezaei
Received: 4 August 2012 / Accepted: 21 September 2012 / Published online: 21 October 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract The oxidation of 2-substituted imidazolines with
sodium periodate catalyzed by tetrakis(p-aminophenyl)-
porphyrinatomanganese(III) chloride, [Mn(TNH2PP)Cl],
supported on functionalized multi-wall carbon nanotubes is
reported. A wide variety of 2-imidazolines were efficiently
converted to their corresponding imidazoles by this cata-
lytic system. When the same reaction was subjected to
ultrasonic irradiation, the reaction times were reduced
significantly and the product yields were increased. This
catalyst could be reused several times without significant
loss of activity. The effects of reaction parameters such cat-
alyst amount, choice of solvent, and the effects of ultrasonic
irradiation on the catalytic activity have been investigated.
metabolism, pharmacology, and medicine [1]. Several
efforts have been made to mimic the function of these
enzymes [2–4]. Oxo metalloporphyrins, in which the metal
possesses high oxidation number, have been used as syn-
thetic models for cytochrome P-450. Various single oxygen
donors such as iodosylbenzene [5], hypochlorites [6],
amine N-oxides [7], alkyl hydroperoxides [8], hydrogen
peroxide [9], and sodium periodate [10] in the presence of
both natural cytochrome P-450 and artificial metallopor-
phyrin complexes, are able to oxidize organic compounds.
However, the major problem in the application of
homogenous metalloporphyrins is their degradation in the
reaction media, and formation of l-oxo bridged catalyti-
cally inactive dimers in solution. Immobilization onto solid
organic or inorganic supports can solve these problems
[11], as well as making the catalyst reusable and avoiding
contamination of the reaction media.
Introduction
Iron porphyrins are known to provide the active site of the
P-450 monooxygenase enzymes, which are responsible for
a vast number of oxidations in nature including very dif-
ficult substrates ranging from medium chain alkanes such
as n-heptane and n-octane, to steroidal compounds and
polycyclic aromatic hydrocarbons as well as very large
molecules such as triterpenes and cyclosporine. These
reactions are important in biological systems because they
are crucial steps in biosynthesis, cellular biochemistry,
Among the metalloporphyrins, supported manganese por-
phyrins have been widely used as catalysts for the oxidation of
organic compounds [12–19]. Previously, our research group
has reported the immobilization of manganese porphyrins on
silica, polystyrene, and multi-wall carbon nanotubes via
covalent attachment, and used these materials for the catalytic
oxidation of organic compounds with NaIO4 [10–17].
Many imidazole derivatives possess biological activities
[20]. The most common route for the preparation of imi-
dazoles is by oxidation of 2-imidazolines. Therefore, oxi-
dation of imidazolines to their corresponding imidazoles is
biologically and pharmaceutically very important. 2-Imi-
dazolines are easily prepared from nitriles and ethylene-
diamine [21] and can be oxidized by DMSO [22],
KMnO4@SiO2[23], KMnO4@K10 [24], KMnO4@Al2O3
[25], trichloroisocyanuric acid [26], KMnO4 in refluxing
dioxane [27], activated carbon–O2 [28], microwave-assis-
ted MnO2 [29], manganese(III) porphyrins [30, 31], and
H. Kargar (&) Á S. Rezaei
Department of Chemistry, Payame Noor University,
P.O. Box 19395-3697, Tehran, Islamic Republic of Iran
e-mail: hadi_kargar@yahoo.com
M. Moghadam Á V. Mirkhani Á S. Tangestaninejad Á
I. Mohammadpoor-Baltork
Department of Chemistry, Catalysis Division, University of
Isfahan, Isfahan 81746-73441, Islamic Republic of Iran
123

2
Transition Met Chem (2013) 38:1–5
[Mn(TNH2PP)Cl@MWCNT] (0.012 mmol) in CH3CN
(6 mL), and the reaction mixture was stirred at room
temperature or exposed to ultrasonic irradiation. The pro-
gress of the reaction was monitored by TLC (eluent:
EtOAc/MeOH: 4/1). After the reaction was completed, the
catalyst was filtered off. The solvent was evaporated by
rotary evaporator, and the resulting crude material was
purified by column chromatography on alumina to afford
N
N
Catalyst, NaIO4
CH3CN/H2O
MS or US
R
R
N
H
N
H
1a-h
2a-h
Scheme 1 Oxidation of imidazolines with NaIO4 catalyzed by
[Mn(TNH2PP)Cl@MWCNT]
1
manganese(III) salophen [32] catalytic systems. Never-
theless, some of these reagents suffer from limitations such
as low yields of the products, very long reaction times,
toxicity, the use of large excess of the reagents, and harsh
reaction conditions. Therefore, the development of mild
and efficient methods for the oxidation of 2-imidazolines to
their corresponding imidazoles is of great interest.
the pure imidazole. IR and H NMR spectral data con-
firmed the identity of all the products.
Results and discussion
Catalytic oxidations
Here, oxidation of 2-substituted imidazolines to 2-imi-
dazoles with NaIO4 in the presence of tetrakis(p-amino-
phenyl)porphyrinatomanganese(III) chloride supported on
functionalized multi-wall carbon nanotubes, [Mn(TNH2PP)
Cl@MWCNT], under mechanical stirring (MS) or ultra-
sonic irradiation (US), is reported (Scheme 1).
Scheme 2 shows the preparation route for the catalyst.
The high catalytic activity of [Mn(TNH2PP)Cl@
MWCNT] in the epoxidation of alkenes [10] prompted us
to explore its catalytic activity in the oxidation of 2-imi-
dazoline with sodium periodate under agitation with mag-
netic stirring or ultrasonic irradiation. First, the amount of
catalyst was optimized in the oxidation of 2-phenyl imi-
dazoline with sodium periodate under MS. As shown in
Table 1, the highest yield was obtained in the presence of
100 mg (12 lmol) of [Mn(TNH2PP)Cl@MWCNT] for
1 mmol of the imidazoline. Next, the effect of different
solvents was investigated. A 2:1 mixture of H2O/CH3CN
proved to be the best solvent for the oxidation of 2-imi-
dazolines (Table 2). The amount of oxidant was also
optimized (Table 3). Optimum results were obtained with
2 mmol of NaIO4 for 1 mmol of the imidazoline; increas-
ing the amount of oxidant to 2.5 mmol did not affect the
yield, but decreasing it to lower amounts reduced the yield.
Under these optimized conditions, a wide range of 2-aryl-
2-imidazolines were oxidized to their corresponding
imidazoles in good to excellent yields under MS (Table 4).
In the absence of catalyst, only 6–10 % yields of imida-
zoles were obtained. The high catalytic activity of this
catalytic system may be related to the isolation of the
catalytic sites.
Experimental
All materials were of commercial reagent grade and were
purchased from Merck or Fluka. The 2-substituted imi-
dazolines were synthesized by the reported procedures [21].
Tetra(4-aminophenyl)porphyrin, (TNH2PP), was prepared
according to the literature [33]. The [Mn(TNH2PP)Cl]
catalyst was prepared according to the procedure reported
for metallation of porphyrins [34]. The [Mn(TNH2PP)Cl@
MWCNT] catalyst was prepared and characterized
according to our reported procedure [17]. Specifications of
the MWCNT-COOH used in this study were as following:
outside diameter (20–30 nm), inside diameter (5–10 nm),
length (30 lm), –COOH content (1.5 %), and specific
1
surface area ([110 m2/g). H NMR spectra were recorded
on a Bruker–Avance AQS 400 MHz. All yields refer to
isolated products. A UP 400S ultrasonic processor equipped
with a 3 mm wide and 140 mm long probe, which was
immersed directly into the reaction mixture, was used for
sonication. The operating frequency was 24 kHz, and the
output power was set at 0–400 W as required. The total
volume of the solution was 15 mL. The temperature
reached to 40 °C during sonication. MWCNTs (multi-wall
carbon nanotubes containing -COOH groups, purity 96 %)
were purchased from Shenzen NTP Factory (China).
The exact mechanism of the reaction is not clear at
present. However, a plausible explanation is that the cata-
lyst is first transformed to the corresponding oxo compound
1 (Scheme 3), which then reacts with imidazoline via
intermediates 2 and 3. Elimination of water from inter-
mediate 3 then leads to the product.
When the same reactions were carried out under US
irradiation, the reaction times were reduced to 1 h
(Table 4). This enhancement may be due to the breakup of
agglomerates during the sonication process thorough mix-
ing of the reactants, and/or production of hot spots upon
sonication [35].
General catalytic procedure
A solution of NaIO4 (2 mmol) in H2O (12 mL) was added
to a mixture of the appropriate 2-imidazoline (1 mmol) and
123

Transition Met Chem (2013) 38:1–5
3
Scheme 2 Preparation of the
catalyst
SOCl2
COCl
COOH
Reflux, 1 h
Mn(TNH2PP)Cl
Toluene, 80 oC
Et3N, 48 h
NH2
N
N
N
N
O
C
NH
Mn
NH2
NH2
Table 1 Optimization of the catalyst amounts in the oxidation of
2-phenyl imidazoline with NaIO4
Table 3 The effect of oxidant amount on the oxidation of 2-phenyl
imidazoline catalyzed by [Mn(TNH2PP)Cl@MWCNT] at room
temperature
Entry
Catalyst amount (mg, mmol)
Yield (%)a
Time (h)
Row
Oxidant amount (mmol)
Time (h)
Yield (%)a
1
2
3
4
5
0
6
28
61
67
86
10
10
10
10
10
1
2
3
4
5
6
2.5
2
10
10
10
10
10
10
86
86
68
43
19
5
25 (0.003 mmol)
50 (0.006 mmol)
75 (0.009 mmol)
100 (0.012 mmol)
1.5
1
0.5
0
Reaction conditions: 2-phenyl imidazoline (1 mmol), catalyst, NaIO4
(2 mmol), CH3CN/H2O (6 ml/12 ml)
a
Isolated yield
Reaction conditions: 2-phenyl imidazoline (1 mmol), NaIO4, catalyst
(0.012 mmol), CH3CN (6 mL), H2O (12 mL)
a
Isolated yield
Table 2 The effect of solvent on the oxidation of 2-phenyl imidaz-
oline with NaIO4 catalyzed by [Mn(TNH2PP)Cl@MWCNT] at room
temperature
Catalyst reusability
Entry
Solvent
Time (h)
Yield (%)a
The stability of the supported catalyst was monitored using
multiple sequential oxidations of 2-phenyl imidazoline
with sodium periodate under magnetic stirring or ultrasonic
irradiation. For each of the repeated reactions, the catalyst
was recovered, washed exhaustively with water followed
by diethyl ether, and dried before being used with fresh
2-phenyl imidazoline and sodium periodate. In both cases,
the catalyst was consecutively reused five times without a
significant loss of activity (Table 5).
1
2
3
4
5
6
7
CH3CN:H2O (1:1)
CH3CN:H2O (1:2)
CH3CN:H2O (2:1)
CH3COCH3:H2O (1:2)
CH3OH:H2O (1:2)
CH3CH2OH:H2O (1:2)
CH2Cl2:H2O (1:2)
10
10
10
10
10
10
10
70
86
60
50
65
55
35
Reaction conditions: 2-phenyl imidazoline (1 mmol), NaIO4 (2 mmol),
catalyst (0.012 mmol)
The catalytic behavior of the separated liquid was tested
by addition of fresh 2-phenyl imidazoline and sodium
a
Isolated yield
123

4
Transition Met Chem (2013) 38:1–5
Scheme 3 Proposed reaction
mechanism
H2O
MWCNT
Mn
-
IO4
H
H
N
N
-
IO3
R
R
N
N
O
H
H
4
O
MWCNT
Mn
Mn
N
N
N
R
1
3
R
N
H
H
5
O
H
N
R
Mn
MWCNT
N
H
H
2
Table 4 Oxidation
of 2-imidazolines with
NaIO4 catalyzed by
[Mn(TNH2PP)Cl@MWCNT]
under various reaction
conditions
Row Imidazoline
Imidazole
Yield (%)b
MS (10 h)
US (1 h)
91
N
N
86
1
N
N
H
(1a)
(2a)
H
N
N
66
78
Me
Me
Cl
2
N
H
N
H
(2b)
(1b)
N
N
69
74
80
76
Cl
3
4
N
H
N
H
(1c)
(2c)
N
N
N
N
(1d)
(1e)
H
(2d)
H
Cl
N
Cl
N
N
N
88
93
5
N
H
N
(2e)
H
N
N
82
88
66
88
93
71
6
7
8
N
H
N
N
N
(1f)
(2f)
H
N
N
Reaction conditions:
2-imidazoline (1 mmol),
catalyst (0.012 mmol), NaIO4
(2 mmol), CH3CN/H2O
(6 ml/12 ml), T = 25 °C for
MS and 40 °C for US
N
N
N
H
N
(2g)
(1g)
H
N
N
MeO
MeO
N
H
N
H
(1h)
(2h)
a
Isolated yield
periodate to the filtrates from each run. Oxidation reactions
using these filtrates under the usual conditions gave the
same results as for blank experiments.
Conclusion
In conclusion, we have described an efficient and biomi-
metic method for oxidation of 2-imidazolines to their
corresponding 2-imidazoles with sodium periodate under
magnetic stirring and ultrasonic irradiation catalyzed by
manganese(III) porphyrin supported on MWCNTs. The
Furthermore, no difference was observed in the diffuse
reflectance UV–vis spectra of fresh and reused catalysts
(Fig. 1), which confirms the stability of the catalyst over
successive catalytic cycles.
123

Transition Met Chem (2013) 38:1–5
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