Facile synthesis of azo compounds from aromatic nitro compounds using magnesium and triethylammonium formate

Article abstract of DOI:10.1071/CH03143

Magnesium/triethylammonium formate is a convenient reagent for the reduction of aromatic nitro compounds to corresponding symmetrically substituted azo compounds. Various azo compounds containing additional reducible substituents, including halogen, nitrile, acid, phenol, ester, and methoxy functions, have been synthesized in a single step by the use of this reagent. The conversion is reasonably fast, clean, high yielding, and occurs at room temperature in methanol.

Full text of DOI:10.1071/CH03143

Short Communication
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2004, 57, 609–610
Facile Synthesis of Azo Compounds from Aromatic Nitro Compounds
using Magnesium and Triethylammonium Formate
G. R. Srinivasa,A K. Abiraj,A and D. Channe GowdaA,B
A Department of Studies in Chemistry, University of Mysore, Mysore, Karnataka 570006, India.
B Author to whom correspondence should be addressed (e-mail: dcgowda@yahoo.com).
Magnesium/triethylammonium formate is a convenient reagent for the reduction of aromatic nitro compounds to
corresponding symmetrically substituted azo compounds. Various azo compounds containing additional reducible
substituents, including halogen, nitrile, acid, phenol, ester, and methoxy functions, have been synthesized in a
single step by the use of this reagent. The conversion is reasonably fast, clean, high yielding, and occurs at room
temperature in methanol.
Manuscript received: 4 June 2003.
Final version: 16 September 2003.
HCO2HNEt3/Mg
Azo compounds have been widely utilized as dyes and
2 X–Ar–NO2
X–Ar–NϭN–Ar–X
MeOH, r.t.
analytical reagents. They can also be used as indicators in
chemical laboratories and as stains in the biological field.
There are many methods available for the synthesis of azo
compounds.[1–5] Most of the methods documented in the lit-
erature are associated with cyclization, rearrangement, and
isomerization in strongly acid and alkaline media. Nowadays,
heterogeneous catalytic transfer hydrogenation has proved to
be a potent choice for reduction of organic compounds.[6–11]
In comparison with catalytic hydrogenation or other reduc-
tion methods, catalytic transfer hydrogenation has real and
potential advantages such as low cost, rapidity, mild con-
ditions that usually avoid strong acids or bases, selectivity,
simple operation and work-up, and broad applicability.
The application of triethylammonium formate as a hydro-
gen donor in the catalytic transfer hydrogenation of a variety
of organic compounds has been reported.[12–14] In our previ-
ous study, we have successfully employed magnesium for
the transfer hydrogenolysis of some commonly used pro-
tecting groups in peptide synthesis.[15] Here we report the
synthesis of symmetrical functionalized azo compounds by
the reductive coupling of nitro arenes using magnesium and
triethylammonium formate in methanol at room temperature
(Scheme 1).
Inspection of the data in Table 1 clearly shows that the
method can be conveniently applied for the synthesis of
several structurally different, symmetrically substituted azo
compounds. Synthesis of unsymmetrically substituted azo
compounds leads to the formation of a mixture, which needs
extensive purification and yields are low (less than 30%).
This new system reduced with ease a wide variety of nitro
compounds to the corresponding azo compounds, and many
other reducible functional groups are tolerated. The reduc-
tion of nitro compounds to azo compounds was completed
Scheme 1. Reductive coupling of nitro arenes. X = Cl, Br, CN, CH3,
OCH3, COOH, COCH3, OH.
within two to three hours at room temperature. The course
of the reaction was monitored by thin layer chromatogra-
phy and IR spectroscopy. The disappearance of asymmetric
and symmetric stretching bands near 1520 and 1345 cm–1
—
due to the N O bond of NO and appearance of strong
---
2
band between 1630 and 1575 cm–1 due to N N stretching
clearly indicates the conversion. The work-up and isola-
tion of the products were simple. Thus all the compounds
reduced to azo compounds were characterized by compar-
1
ison of their TLC, IR and H NMR spectra, and melting
points with authentic samples. Two control experiments, car-
riedoutusingnitrocompoundswitheithertriethylammonium
formate or magnesium powder, did not yield the desired
products.
The reduction was also carried out with the nitro com-
pounds bearing bromomethyl, sulphonic acid, oximino,
amino, and dialkyl amino groups. In these cases, the bromo
methyl, dialkyl amino, and oximino groups are compatible
under the experimental conditions. But, in the case of amino-
substituted nitro compounds, a mixture of products is yielded,
probably due to the coupling of reduction intermediates with
the free amino group. Nitro sulphonic acids gave a precipitate
insoluble in the solvents employed and, thus, this system is
not helpful in obtaining azo compounds. This procedure will
therefore be of general use for the preparation of functional-
izedazocompounds, specificallyincaseswheremildreaction
conditions are required, and it is less expensive compared to
existing methods.
© CSIRO 2004
10.1071/CH03143
0004-9425/04/060609

610
G. R. Srinivasa, K. Abiraj and D. C. Gowda
Table 1. Synthesis of Azo Compounds from Nitro Arenes using HCO2HNEt3/Mg
1H NMR spectra were obtained on an AMX-400 MHz spectrometer in CDCl3 as the solvent and TMS
as internal standard. All of the products are known and the isolated products gave IR spectra in agreement
with their structures
Nitro compound
Time [h]
Product
Yield of isolated
product [%]
Melting point [◦C]
Found
Lit.[16]
Nitrobenzene
p-nitrobiphenyl
p-Nitrophenol
o-Nitrotoluene
m-Nitrotoluene
m-Nitroanisole
m-Chloronitrobenzene
o-Nitroanisole
o-Chloronitrobenzene
p-Nitrotoluene
2.0
2.5
2.2
2.4
2.0
2.5
2.5
2.0
2.8
2.6
3.0
2.0
3.0
2.5
azobenzene
azobiphenyl
93
84
92
90
93
88
84
92
90
88
86
92
91
87
67–68
248–252
174–175
54–56
55–57
90–92
100–102
130–132
135–137
144–145
159–162
186–187
188–191
207–209
68
250
173–175
55
55
91
101
131
137
144
160
188
190
208
2,2ꢀ-dihydroxyazobenzene
2,2ꢀ-dimethylazobenzene
3,3ꢀ-dimethylazobenzene
3,3ꢀ-diethoxyazobenzene
3,3ꢀ-dichloroazobenzene
2,2ꢀ-diethoxyazobenzene
2,2ꢀ-dichloroazobenzene
4,4ꢀ-dimethylazobenzene
4,4ꢀ-diethoxyazobenzene
4,4ꢀ-dichloroazobenzene
1,1ꢀ-azonaphthalene
p-Ethoxynitrobenzene
p-Chloronitrobenzene
1-Nitronaphthalene
2-Nitronaphthalene
2,2ꢀ-azonaphthalene
Experimental
[4] R. O. Hutchins, D. W. Lamson, L. Rua, M. Cynthia, M. Bruce,
J. Org. Chem. 1971, 36, 803.
A suspension of an appropriate nitro compound (1 g) and magnesium
powder (2 g) in methanol (10 mL) was stirred with triethylammonium
formate (4 mL) under N2 at room temperature. After the completion
of the reaction (monitored by TLC), the reaction mixture was filtered
through celite and washed with solvent. The combined filtrate and
washings were evaporated under vacuum. The residue was taken into
15 mL chloroform or ether, washed twice with 15 mL saturated brine,
and finally with water. The organic layer was dried over anhydrous
magnesium sulfate, and evaporation of the organic layer followed by
purification either by preparative TLC or by column chromatography to
yield the desired product.
[5] G. W. Kabalka, R. S.Varma, in Comprehensive Organic Synthesis
(Eds B. Trost, I. Fleming) 1991, p. 363 (Pergamon: Oxford).
[6] O. Brieger, T. J. Nestrick, Chem. Rev. 1974, 74, 567.
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1985, 85, 129.
[8] S. Ram, R. E. Ehrenkaufer, Synthesis 1988, 91. doi:10.1055/S-
1988-27478
[9] G. K. Jnaneshwara, A. Sudalai, V. H. Deshpande, J. Chem.
Res. (S) 1998, 160. doi:10.1039/A705957D
[10] S. Gowda, K. Abiraj, D. C. Gowda, Tetrahedron Lett. 2002, 43,
1329. doi:10.1016/S0040-4039(01)02370-X
[11] S. Gowda, D. C. Gowda, Tetrahedron 2002, 58, 2211.
doi:10.1016/S0040-4020(02)00093-5
Acknowledgments
[12] S. Cacchi, P. G. Ciattini, E. Morera, G. Ortar, Tetrahedron Lett.
1986, 27, 5541. doi:10.1016/S0040-4039(00)85262-4
[13] N. A. Cortese, R. F. Heck, J. Org. Chem. 1977, 42, 3491.
[14] N. A. Cortese, R. F. Heck, J. Org. Chem. 1978, 43, 3985.
[15] D. C. Gowda, Tetrahedron Lett. 2002, 43, 311. doi:10.1016/
S0040-4039(01)02113-X
The authors wish to thank University Grants Commission,
New Delhi, for providing financial assistance.
References
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