Facile, product-selective reduction of azoxyarenes into azoarenes or hydrazoarenes by aluminium/hydrazine hydrate

Article abstract of DOI:10.1080/00397910500182275

Azoxyarenes, on treatment with hydrazine hydrate in presence of aluminium powder in methanol, undergo reduction. The reactions have been carried under microwave irradiation as well at reflux to get the corresponding hydrazoarenes and azoarenes as reduced products. The reaction is very fast, which gives excellent yield of the product. Substituents such as OCH₃, OC ₂H₅, and Cl are unaffected. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1080/00397910500182275

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Facile, Product‐Selective
Reduction of Azoxyarenes into
Azoarenes or Hydrazoarenes by
Aluminium/Hydrazine Hydrate
H. M. Nanjundaswamy a b & M. A. Pasha a
a Department of Chemistry, Central College
Campus , Bangalore University , Bangalore, India
b Chemical Examination Section , Public Health
Institute , Bangalore, India
Published online: 18 Aug 2006.
To cite this article: H. M. Nanjundaswamy & M. A. Pasha (2005) Facile,
Product‐Selective Reduction of Azoxyarenes into Azoarenes or Hydrazoarenes by
Aluminium/Hydrazine Hydrate, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 35:16, 2163-2168
To link to this article: http://dx.doi.org/10.1080/00397910500182275
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Synthetic Communicationsw, 35: 2163–2168, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500182275
Facile, Product-Selective Reduction
of Azoxyarenes into Azoarenes
or Hydrazoarenes by Aluminium/
Hydrazine Hydrate
H. M. Nanjundaswamy
Department of Chemistry, Central College Campus,
Bangalore University, Bangalore, India and Chemical Examination
Section, Public Health Institute, Bangalore, India
M. A. Pasha
Department of Chemistry, Central College Campus,
Bangalore University, Bangalore, India
Abstract: Azoxyarenes, on treatment with hydrazine hydrate in presence of
aluminium powder in methanol, undergo reduction. The reactions have been carried
under microwave irradiation as well at reflux to get the corresponding hydrazo-
arenes and azoarenes as reduced products. The reaction is very fast, which gives
excellent yield of the product. Substituents such as OCH3, OC2H5, and Cl are
unaffected.
Keywords: Aluminium powder, azoarenes, azoxyarenes, hydrazine hydrate,
hydrazoarenes
The reduction of azo and azoxyarenes has received a good deal of attention in
recent years because reduction of –N–N– multiple bonds is an important
reaction in connection with structural determination of azodyes. Reports are
available in the literature for the reduction of azoxy and azoarenes into
Received in India April 19, 2005
Address correspondence to M. A. Pasha, Department of Chemistry, Central College
Campus, Bangalore University, Bangalore 560 001, India. E-mail: m_af_pasha@
yahoo.co.in
2163

2164
H. M. Nanjundaswamy and M. A. Pasha
hydrazoarenes by using Raney nickel/NH2NH2, Pd/NH2NH2, Mg/NH4Cl,[1–5]
Al/KOH in methanol,[6] and a complex of the coenzyme dihydrolipoamide
and iron(II).[7]
In continuation of our work on hydrazine hydrate for functional group
transformations,[8–10] we report the reduction of azoxyarenes into hydrazo-
arenes or azoarenes in presence of aluminium metal and hydrazine hydrate.
It has been found that 99% hydrazine hydrate in methanol under microwave
irradiation is an exceedingly convenient system to achieve the transformation
of substituted azoxyarenes to the corresponding hydrazoarenes (Scheme 1),
which are summarized in Table 1.
The reaction proceeds smoothly and is completed within 1.5–2 min
without affecting substituents such as OH, OCH3, OC2H5, and Cl. Hydrazo-
arenes that are obtained are of high purity. Further, to compare the reaction
at different conditions with microwave irradiation, the reactions were
performed by stirring at room temperature and at reflux with same substrates,
which afforded hydrazoarenes after 24 h at 258C. At reflux, azoarenes are
recovered within 10 min but hydrazoarenes are not. Table 2 lists the results
of the conversion of various azoxyarenes are not to the corresponding
azoarenes at reflux.
In conclusion, we have demonstrated the synthesis of hydrazoarenes and
azoarenes from azoxyarenes with hydrazine hydrate in the presence of
aluminium powder as catalyst.
EXPERIMENTAL
All azoxyarenes are either commercial grade or prepared according to
standard procedures[11] and purified before use. Hydrazine hydrate and
other solvents were purchased from Merck and BDH, purified, and dried
prior to use. Yields refer to the isolated yields of the products after purification
by column chromatography using 60–120 mesh silica gel with a suitable
Scheme 1.

Table 1. Reduction of azoxyarenes into hydrazoarenes by Al/hydrazine hydrate in methanol under microwave irradiation
Substrate
Producta
Mp (8C)
Time
(min)
Yieldb
(%)
Entry
R
R0
R
R0
Found
126
Lit.[11]
126
1
2
3
4
5
6
7
H
H
H
H
1.75
1.75
1.75
1.50
1.50
1.50
2.00
91
90
90
90
90
85
90
2-CH3
3-CH3
4-CH3
2-OCH3
2-OC2H5
4-Cl
20-CH3
30-CH3
40-CH3
20-OCH3
20OC2H5
40-Cl
2-CH3
3-CH3
4-CH3
2-OCH3
2-OC2H5
4-Cl
20-CH3
30-CH3
40-CH3
20-OCH3
20OC2H5
40-Cl
164–166
37
165
38
133–134
102–103
88
134
102
89
123–124
124
Caution: Hydrazine hydrate is toxic and corrosive; handle carefully.
aProducts are characterized by comparison of TLC, melting point, and IR spectra of authentic samples.
bIsolated yields.

Table 2. Reduction of azoxyarenes into azoarenes by Al/hydrazine hydrate in methanol at reflux
Substrate
Producta
Mp (8C)
Time
(h)
Entry
R
R0
R
R0
Yieldb
Found
Lit.[12]
1
2
H
H
H
H
0.167
0.167
0.167
0.167
0.167
0.167
0.167
0.167
0.167
0.167
92
91
92
92
91
90
90
90
90
90
67–68
55
54–55
68
55
2-CH3
3-CH3
4-CH3
2-OCH3
4-OCH3
4-OC2H5
2-Cl
20-CH3
30-CH3
40-CH3
20-OCH3
40-OCH3
4-OC2H5
20-Cl
2-CH3
3-CH3
4-CH3
2-OCH3
4-OCH3
4-OC2H5
2-Cl
2-CH3
3-CH3
4-CH3
2-OCH3
4-OCH3
4-OC2H5
2-Cl
3
54–55
144–145
143
4
145–146
142–144
159–161
156–158
137–138
100–101
185–187
5
6
160
7
157–159
137
101
8
9
10
3-Cl
4-Cl
30-Cl
40-Cl
3-Cl
4-Cl
3-Cl
4-Cl
185
Caution: Hydrazine hydrate is toxic and corrosive, handle carefully.
aCharacterized by IR spectral analysis and on GC with authentic samples.
bIsolated yields.

Reduction of Azoxyarenes
2167
eluent. Melting points were obtained by the capillary method and are uncor-
rected. Analytical TLC was performed on precoated aluminum plates with
Merck silica gel 60 F-254 as the adsorbent. The developed plates were air
dried and irradiated with UV light. GC analysis was performed on a
Shimadzu GC-MS QP 5050A instrument. IR spectra were recorded on
Nicolet 400D FT-IR spectrometer.
General Procedure 1
A mixture of azoxybenzene (0.2 g, 1 mmol), hydrazine hydrate (99–100%,
0.5 mL), aluminium powder (0.27 g, 10 mg atom), and methanol (5 mL) in a
Pyrex cylindrical tube were irradiated in a commercial microwave oven
(unmodified, LG, Little Chef, MS-194W, 230 V ꢀ50 Hz working at 160 W)
for 1.5–2 min and poured into ice. The solid was filtered, dried, and recrystal-
lized from 95% ethanol to get hydrazobenzene (0.175 g, 95%).
General Procedure 2
These substrates and quantities were refluxed for 10min, filtered on a bed of
celite, and washed with diethyl ether. The filtrate was washed successively
with saturated brine solution and water, and dried over anhydrous sodium
sulfate. Solvent removal and silica-gel column chromatography gave azoarene.
ACKNOWLEDGMENTS
H. M. Nanjundaswamy thanks the Joint Director of Public Health Institute and
the Commissioner of H & F W Services, Bangalore, India, for encouragement.
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