1090
N. S. Kus
singlet between d = 4.5 and 6.5 ppm owing to the NH2
protons in the 1H NMR spectra confirmed the products.
Use of molecular oxygen is a selective and convenient
method for transformation of primary aromatic amines to
corresponding diazenes. This environmentally friendly
method avoids use of materials such as chromic acid,
Schiff base complexes, and permanganate, which should
allow application in organic synthesis. This cheap method
uses only water as a solvent and the procedure is simple
and mild, with environmentally friendly reaction condi-
tions and good yields.
O2
NH2
N
N
Subcritical water
R
R
R
Scheme 1
Table 1 Oxidation of primary aromatic amines to azo derivatives
with molecular oxygen (110 °C, 2.1 kPa O2 pressure)
Entry
R
Time (h) M.p. (Lit. M.p.) (°C) Ref. Yield (%)
70–71 (68–71) [12] 80
203–204 (203–205) [12] 65
190 (189) [12] 59
1
H
3
4
4
5
3
4
6
4
4
3
3
6
4
2
4-Br
3
4-Cl
In conclusion, I developed a practical procedure for
oxidation of primary aromatic amines to their corre-
sponding diazene compounds, in subcritical water with
molecular oxygen. This simple, economical, and environ-
mentally friendly method is useful for transformation of
primary aromatic amines to corresponding diazenes.
4
4-NO2
4-CH3
4-OCH3
4-OC2H5
4-CN
222–224 (222–223) [12] 55
143–145 (144–145) [12] 95
5
6
164 (165)
[12] 85
[12] 85
[18] 45
[19] 65
[12] 95
[12] 85
[4] 53
[20] 85
7
128 (130–131)
235 (230–231)
137 (136)
8
9
2-Cl
10
11
12
13
2-CH3
2-OCH3
2-CN
56 (54)
152 (143–145)
59 (57–58)
130 (130–132)
Experimental
All chemical reagents were commercially purchased from
standard chemical suppliers. The starting materials were
purified (distilled or crystallized) before use. 1H NMR
spectra were recorded on a Bruker 400 MHz spectrometer,
and infrared spectra were obtained as film on NaCl plates
for liquids and KBr pellets for solids on a Win FirstÒ
Satellite Model infrared recording spectrophotometer.
All column chromatography was performed on silica gel
(60 mesh, Merck).
2-OC2H5
Isolated yields based on starting aromatic amine. Products identified
by comparison of physical and spectroscopic properties [melting
points, mass spectra, infrared spectra, and nuclear magnetic resonance
(NMR) spectra] with literature values
as uncharacterized tar. Oxidation of arylamines with elec-
tron-withdrawing substituents such as –Br and –Cl resulted
in formation of corresponding oxide products in 50–60%
yield because of the unreacted starting materials. As the
reaction went on, an increase in the time led to decomposi-
tion of the starting materials.
General procedure for the oxidation reaction
The amount of oxygen dissolved in water at atmospheric
pressure was determined according to Henry’s Law [17]
(1.296 9 10-3 mol O2/kg of H2O at 25 °C). The amount
of oxygen was regulated by the oxygen pressure [17]. All
oxidations were performed by adding 50 cm3 water, one
molar equivalent of substrate, and two molar equivalents of
oxygen. Moreover, increase of oxygen pressure beginning
from 0.53 until 2.1 kPa increased the yields, but increase
over 2.1 kPa resulted in decomposition of the starting
materials.
Oxidation reactions were carried out at 110 °C in a 200-
cm3 stainless-steel reactor. A glass vessel was inserted into
the reactor, and the oxidation occurred without contact
with the stainless-steel reactor surface to avoid the catalytic
effect of steel and corrosion. The stainless-steel reactor,
containing 50 cm3 water and the substrate (5 mmol), was
heated to the desired temperature (110 °C) from the bottom
by means of a hotplate and surrounded by heat-resistant
material to prevent loss of heat. All the valves of the
reactor were tightly closed during preheating. Then, the
desired oxygen pressure (2.1 kPa) was applied to the vessel
through a stainless-steel tube directly into the liquid phase.
After the reaction was complete, the mixture was cooled to
room temperature and filtered, and the residue was washed
with CHCl3 (3 9 40 cm3). After drying over MgSO4, the
solvents were evaporated in vacuum and the products were
purified by column chromatography on silica gel (60 mesh,
Merck, with 50 cm glass column, eluent: CCl4–Et2O). The
All products were characterized by comparison of their
thin-layer chromatography (TLC), melting points, and
1
infrared (IR) and H nuclear magnetic resonance (NMR)
spectra with those of authentic samples. The IR spectra of all
the products showed bands between 1,630 and 1,575 cm-1
,
ascribed to the azo compound, and no absorption bands were
observed for the substrate NH2 group (between 3,500 and
3,300 cm-1). This result clearly indicates complete con-
version of amino derivatives to their corresponding azo
compounds. Furthermore, the disappearance of a broad
1
products were characterized by H NMR, 13C NMR, and
IR spectroscopy.
123
Kus, Nermin Simsek
