648 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 648±649$
Efficient Catalytic Oxidation of Primary Aromatic
Amines to Azo Derivatives by Manganese(III)
Tetraphenylporphyrin$
Mohammad Hossein Habibi,* Shahram Tangestaninejad and
Valiollah Mirkhani
Department of Chemistry, Esfahan University, Esfahan, 81744, Iran
The oxidation of primary aromatic amines to the corresponding azo derivatives has been observed in catalytic systems
containing manganese(III) tetraphenylporphyrin and sodium periodate in the presence of heterocyclic nitrogen bases
acting as axial ligands.
Table 1 Effect of various axial ligands on oxidation of para-
toluidine to azo product in 90 mina
The catalytic role of metallo-porphyrins for hydroxylation
of alkanes,1,2 epoxidation of alkenes,3,4 demethylation of
N-methylbenzylamine,5 oxidation of nitroso6 and primary
aromatic amines7±10 to nitro derivatives has been observed.
In this report, we describe a periodate-metalloporphyrin
system for oxidation of primary aromatic amines to azo
derivatives.
Axial ligand
Azo yield (%)b
Turnover per h
Imidazole
100
25
19
12
55.55
13.88
10.55
6.67
4-Methylpyridine
2-Methylpyridine
Pyridine
Without axial ligand
10
5.55
One important aspect of this catalytic system is the
modi®cation of the oxidation rate by addition of a small
amount of imidazole to the mixture. The corresponding
results of the eect of various axial ligands on oxidation
of para-toluidine are presented in Table 1. The formation
of azo product in the absence of axial ligand is very slow
and the yields are always below 10% within 90 min, whereas
100% GLC yield of azo product is obtained during the
same period in the catalyzed reaction with imidazole as the
axial ligand. The yield of azo product in the oxidation
of para-toluidine decreased in the following order using
dierent axial ligands: imidazole ꢀ 4-methylpyridine>
2-methylpyridine >pyridine.
Reactions were performed at room temperature in air
in CH2Cl2±H2O containing the primary aromatic amines,
periodate, axial ligand and tetraphenylporphyrinato-
manganese(III) chloride (MnTPPCl) in 83:166:17:1 ratios,
respectively. This catalytic system led to oxidation of
primary aromatic amines RC6H4NH2 1±13 to azo deriva-
tives (1a±13a) (Scheme 1) in good isolated yields (38±85%)
(Table 2).
aReaction conditions: para-toluidine (1 mmol), MnTPPCl
(0.012 mmol), axial ligand (0.2 mmol), NaIO4 (2 mmol),
tetrabutylammonium bromide (0.05 mmol), CH2Cl2/H2O
(10 ml/10 ml). bGLC yields based on starting para-toluidine.
essential for the above oxidation of primary aromatic
amines with MnTPPCl as catalyst. The highest coordinative
capability of imidazole compared to other axial ligands and
amines is shown by the pronounced spectral changes of
the MnTPPCl Soret band at 477.5 nm in the presence of
imidazole.
The oxidation product of benzylamine under the above
conditions was benzaldehyde with 90% isolated yield and
100% selectivity.
Experimental
MnTPPCl was prepared according to the literature pro-
a typical reaction, a 50 ml ¯ask was charged
cedures.11,12 In
with primary aromatic amine (1 mmol), MnTPPCl (0.012 mmol),
imidazole (0.2 mmol), CH2Cl2 (10 ml), NaIO4 (2 mmol) in H2O
(10 ml) and 0.05 mmol of tetrabutylammonium bromide as a phase
transfer catalyst. The reaction was magnetically stirred at room
temperature for 1±5 h. The progress of the reaction was monitored
by gas chromatography and the products were separated by column
chromatography with silica gel. All the oxidation products were
clearly identi®ed by IR, 1H NMR and UV±VIS spectral data.
MnTPPCl; NaIO4; room temp:
.
RC6H4NH2
4 RC6H4N NC6H4R
CH2Cl2 ±H2O; Imidazole; Bu4NBr
1±13
1a±13a
1
2
3
4
5
6
7
R p-Et
8
9
R o-CN
R o-Cl
R p-Me
R o-Me
R p-OMe
R o-OMe
R m-OMe
R H
Table 2 Oxidation of primary aromatic amines (1±13) to azo
derivatives (1a±13a) with NaIO4 catalyzed by MnTPPCl in the
presence of imidazolea
10 R p-NO2
11 R m-NO2
12 R o-NO2
13 R p-Br
Amine
Azo yield (%)b
Reaction time/h
1
2
3
4
5
6
7
8
9
10
11
12
13
85
80
80
80
80
80
70
60
50
48
63
40
38
2
1
2
1.5
2
Scheme 1
3
2
3
4
5
5
Control experiments carried out on the amines showed
that, in the absence of catalyst, no oxidation to azo com-
pounds occurs.
The eect of axial ligands clearly indicate that an electron
donating axial substituent coordinated to the metal is
5
1.5
aReaction conditions: aromatic amine (1 mmol), MnTPPCl
(0.012 mmol), axial ligand (0.2 mmol), NaIO4 (2 mmol),
tetrabutylammonium bromide (0.05 mmol), CH2Cl2/H2O
(10 ml/10 ml). bIsolated yields based on starting aromatic amine.
*To receive any correspondence.
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).
Habibi, Mohammad Hossein
