Catalytic oxidation of primary aromatic amines with sodium periodate catalyzed by Mn(III)salophen complex supported on polystyrene-bound imidazole

Article abstract of DOI:10.1007/BF03246052

The catalytic activity of Mn(III)salophen complex supported on polystyrene-bound imidazole, [Mn(salophen)Cl-PSI], was studied in the oxidation of primary aromatic amines in acetonitrile/water, using sodium periodate as an oxygen source. Amines were oxidized efficiently to their corresponding azo derivatives in the presence of this catalyst. The heterogeneous catalyst showed high stability and reusability in the oxidation reactions and could be reused several times without loss of its activity. The effect of different solvents was studied in the oxidation of p-toluidine and CH₃CN/H₂O was chosen as the solvent.

Full text of DOI:10.1007/BF03246052

J. Iran. Chem. Soc., Vol. 7, No. 3, September 2010, pp. 641-645.
JOUIRrNaAnLiaOnF THE
Chemical Society
Catalytic Oxidation of Primary Aromatic Amines with Sodium Periodate Catalyzed by
Mn(III)salophen Complex Supported on Polystyrene-Bound Imidazole
V. Mirkhani*, M. Moghadam, S. Tangestaninejad and S. Hajibagheri
Chemistry Department, Catalysis Division, University of Isfahan, Isfahan, 81746-73441, Iran
(Received 6 June 2008, Accepted 11 September 2009)
The catalytic activity of Mn(III)salophen complex supported on polystyrene-bound imidazole, [Mn(salophen)Cl-PSI], was
studied in the oxidation of primary aromatic amines in acetonitrile/water, using sodium periodate as an oxygen source. Amines
were oxidized efficiently to their corresponding azo derivatives in the presence of this catalyst. The heterogeneous catalyst
showed high stability and reusability in the oxidation reactions and could be reused several times without loss of its activity. The
effect of different solvents was studied in the oxidation of p-toluidine and CH3CN/H2O was chosen as the solvent.
Keywords: Amines, Oxidation, Manganese salophen, Polystyrene, Periodate
INTRODUCTION
decarboxylation of carboxylic acids, amines oxidation and
medicinal studies as models for mimicking the superoxide
dismutase [15-17]. The electronic and steric nature of the
metal complex can be tuned by introducing electron-
withdrawing and electron-releasing substituents and bulky
groups in the salen ligand. The catalytic activity of these
biomimetic catalysts decreases in a homogeneous medium,
because of the formation of inactive dimeric μ-oxo
manganese(IV) species [13,14]. The heterogenization of these
complexes on several supports is the subject of intense
research in order to facilitate catalyst separation from the
reaction mixture, simplify procedures for catalyst recycling
and maximizing the site isolation of catalytic centers to
minimize the possibility of inactive oxo-bridged dimer
formation [18-21].
Recently, we reported the use of supported Manganese(III)
salophen/periodate systems in the epoxidation of alkenes and
hydroxylation of alkanes [22-24]. This paper describes a
catalytic system based on a chloromethylated polystyrene
support modified with imidazole (PSI), which could act as an
axial ligand for Mn(salophen) by covalent attachment. The
catalytic activity and reusability of this catalyst in the
Transition metal complexes with Schiff bases and
porphyrin ligands have been successfully used as models for
the heme containing Cytochrom P-450 [1]. P-450 enzymes are
known to oxidize a very extensive range of endogenous and
exogenous organic compounds. These reactions are important
in biological systems because they are crucial steps in
biosynthesis,
cellular
biochemistry,
metabolism,
pharmacology and medicine [2,3]. Various single oxygen
atom donors such as NaClO, PhIO, KHSO5, H2O2 and NaIO4
have been used for oxidation reactions [4-10]. These
complexes catalyze the transfer of oxygen atoms to organic
substrates and the nature of products depends on several
factors such as substrate, oxidant, counter ion, solvent,
structure of salen ligands and the kind of axial ligand [11,12].
Metal complexes of salen and salophen ligands have been
used as reagents and catalysts in many reactions including
olefin epoxidation, nucleic acid modification, electrochemical
reduction, alkane hydroxylation, Diels-Alder transformation,
*Corresponding author. E-mail: mirkhani@sci.ui.ac.ir

Mirkhani et al.
N
N
N
O
N
O
Mn
Cl
NH2
N N
NaIO4/CH3CN/H2O, RT
R
R
R
Scheme 1
oxidation of primary aromatic amines with NaIO4 were
investigated (Scheme 1).
phase systems), dichloromethane, chloroform and carbontetra-
chloride (two phase systems with Bu4NBr as phase transfer
catalyst) with water, the 1:1 acetonitrile/water mixture was
chosen as the reaction medium whose higher catalytic activity
was observed. The results are shown in Table 1.
EXPERIMENTAL
Instruments and Reagents
All materials were commercial reagent grade.
Chloromethylated polystyrene (4-5% Cl content, 2% cross-
linked with divinylbenzene) was purchased from Merck.
Amines were obtained from Merck or Fluka and were passed
through a column containing active alumina to remove
oxidation impurities. [Mn(salophen)Cl-PSI] was synthesized
according to our previously reported method [22].
Catalytic Oxidation of Primary Aromatic Amines
with NaIO4 Catalyzed by Mn(salophen)-PSI
First, to find the optimized conditions, the oxidation of p-
toluidine with NaIO4 in the presence of catalytic amounts of
Mn(salophen)-PSI was investigated. The optimum conditions
which were obtained for the oxidation of p-toluidine by this
catalytic system, were the catalyst, oxidant, and substrate in a
molar ratio of 1:2:10, respectively.
Catalytic Experiments
Oxidation of various primary aromatic amines was
performed under the same reaction conditions which were
All reactions took place at room temperature under air in a
25 ml flask equipped with a magnetic stirrer bar. In a typical
experiment, a solution of NaIO4 (2 mmol) in H2O (3 ml) was
added to a mixture of amine (1 mmol), Mn(salophen)Cl-PSI
(0.096 mmol) in CH3CN (3 ml). The reaction mixture was
stirred at room temperature. The progress of reaction was
monitored by TLC. After the reaction was completed, the
reaction products were extracted with CH2Cl2 (2 × 10 ml) and
purified on a silica-gel plate or a silica-gel column (eluent:
Table 1. Effect of Solvent on the Oxidation of p-Toluidine
Catalyzed by Mn(salophen)Cl-PSI at Room
Temperaturea
Row
Solvent
Conversion (%)b after
30 min
87
51
18
20
1
CCl4-Et2O). IR and H NMR spectral data confirmed the
1
2
3
4
5
CH3CN/H2O
CH3COCH3/H2O
CHCl3/H2O
CH2Cl2/H2O
CCl4/H2O
identities of the products.
RESULTS AND DISCUSSION
15
aReaction conditions: amine (1 mmol), NaIO4 (2 mmol),
catalyst (0.096 mmol), CH3CN (3 ml), H2O (3 ml).
bIsolated yield.
Effect of Solvent on the Oxidation of p-Toluidine with
NaIO4 Catalyzed by Mn(salophen)-PSI
From among the mixtures of acetonitrile, acetone (single
642

Catalytic Oxidation of Primary Aromatic Amines
obtained for p-toluidine. The obtained results showed that the
oxidation reactions of p-toluidine with NaIO4 under agitation
with magnetic stirring. At the end of each reaction, the catalyst
was separated from the reaction mixture by filtration, washed
with water and acetonitrile and dried carefully before using it
in the subsequent runs. After the use of the catalyst for four
consecutive times, the azo yield was 87% (Table 3). The
filtrates were used for the determination of manganese
leaching. The results showed that only small amounts of Mn
was detected in the filtrates after the two first runs by atomic
absorption spectroscopy.
catalyst was an efficient one for oxidation of amines under
agitation with magnetic stirring. As shown in Table 2, the azo
derivatives were obtained in 60-87% isolated yields. It is clear
that the electron rich amines are more reactive than the
electron poor ones.
A blank experiment in the presence of the oxidant using
the same experimental conditions as in the absence of the
catalyst was also investigated in the oxidation of p-toluidine.
The results showed that NaIO4 had poor ability to oxidize the
p-toluidine at room temperature.
Such a finding confirmed the strong covalent bonding of
the polymer and the metallosalophen, in which the
Mn(salophen) catalyst was not leached from the polymer
during the oxidation reaction.
Catalyst Reuse and Stability
The stability of Mn(salophen)-PSI was studied in repeated
Table 2. Oxidation of Primary Aromatic Amines to Azo Derivatives with NaIO4 Catalyzed
by Mn(salophen)Cl-PSIa
Row
1
Amine
Conversion (%)b
87
Time (h)
0.5
TOF
H3C
NH2
18.13
NH2
CH3
2
85
0.5
17.71
NH2
OMe
3
4
86
77
1
1
17.92
16.04
MeO
NH2
NH2
5
6
74
67
1
1
15.42
13.96
MeO
Br
NH2
NH2
7
60
3
12.50
Cl
aReaction conditions: amine (1 mmol), NaIO4 (2 mmol), catalyst (0.096 mmol), CH3CN (3
ml), H2O (3 ml). bBased on aromatic amine.
643

Mirkhani et al.
Table 3. The Results of [Mn(salophen)Cl-PSI] Catalyst Recovery and the Manganese
Leached in the Oxidation of p-Toluidine with NaIO4
Run
Conversion (%)a
Time (h)
Mn leached (%)b
1
2
3
4
87
82
80
80
0.5
0.5
0.5
0.5
0.5
0.2
0
0
aIsolated yield. bDetermined by atomic absorption spectroscopy.
Table 4. Oxidation of Primary Aromatic Amines to azo Derivatives with NaIO4
Catalyzed by Mn(III)salophen and Mn(III)salophen-PSI
Row
Amine
Conversion (%)/time (h)
Heterogeneous
Homogeneous
88/0.2
H3C
NH2
1
2
87/0.5
85/0.5
NH2
CH3
NH2
OMe
83/0.2
82/0.2
3
86/1
MeO
NH2
4
5
77/1
74/1
85/0.2
83/0.2
NH2
MeO
Br
NH2
6
7
67/1
60/3
61/0.5
65/0.5
NH2
Cl
644

Catalytic Oxidation of Primary Aromatic Amines
Comparison of Homogeneous Mn(salophen)Cl and
Heterogeneous Mn(salophen)Cl-PSI in Amine
Oxidation with Sodium Periodate
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In order to show the effect of supporting on the catalytic
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the homogeneous catalyst. On the other hand, the
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without loss of its activity.
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CONCLUSIONS
In this paper, we demonstrated the use of
[Mn(salophen)Cl-PSI] catalyst in the oxidation of primary
aromatic amines to azo derivatives with NaIO4. This catalytic
system showed high activity in the oxidation of various
primary aromatic amines under mild reaction conditions. Easy
preparation and handling of the catalyst, commercial
availability of support, mild reaction conditions (room
temperature), no need for axial ligand in the catalyst system
and facial and effective catalyst recovery and recycling make
this catalytic system a useful method for oxidizing of amines.
ACKNOWLEDGMENTS
The financial support of this work by the Research Council
of the University of Isfahan is gratefully acknowledged
(Project No. 860308).
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