Application of lead and ammonium formate as a new system for the synthesis of azo compounds

Article abstract of DOI:10.1055/s-2002-20950

Aromatic nitro compounds containing additional reducible substituents such as acid, phenol, halogen, ester functions are reduced to the corresponding symmetrically substituted azo compounds by employing lead and ammonium formate in methanol or tetrahydrofuran or dioxane medium at reflux temperature. The conversion occurs without hydrogenolysis or hydrogenation of -Cl, -OCH₃, -OC₂H₅, -CO₂H, moieties, which are prone to undergo reduction. The conversion is reasonably fast, clean and high yielding.

Full text of DOI:10.1055/s-2002-20950

4
60
SHORT PAPER
Application of Lead and Ammonium Formate as a New System for the
Synthesis of Azo Compounds
Synthesis of
A
h
zo Compoun
a
ds nkare Gowda, D. Channe Gowda*
Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore, Karnataka, 570 006 India
Fax +91(82)1421263 ; E-mail: dcgowda@yahoo.com
Received 15 October 2001; revised 29 January 2002
By using this method, we can synthesize a number of
symmetrically substituted azo compounds 2 directly from
nitro compounds 1 in a single step. Synthesis of unsym-
Abstract: Aromatic nitro compounds containing additional reduc-
ible substituents such as acid, phenol, halogen, ester functions are
reduced to the corresponding symmetrically substituted azo com-
pounds by employing lead and ammonium formate in methanol or metrically substituted azo compounds leads to the forma-
tetrahydrofuran or dioxane medium at reflux temperature. The con- tion of a mixture, which needs extensive purification and
version occurs without hydrogenolysis or hydrogenation of Cl,
yields are low (less than 30%). Moreover, ammonium for-
OCH , OC H , CO H, moieties, which are prone to undergo re-
27
28
3
2
5
2
mate in the presence of Raney Ni, 5% Pt/C or 10% Pd/
C
duction. The conversion is reasonably fast, clean and high yielding.
29
directly converts nitro compounds to amines. This
Key words: catalytic transfer hydrogenation, nitro compounds, azo
compounds, lead ammonium formate, reduction
new system reduced with ease a wide variety of nitro com-
pounds to corresponding azo compounds and many func-
tional groups are tolerated. The reduction of nitro
compounds to azo compounds was completed within one
Reduction is an important tool in the hands of organic to three hours. The course of reaction was monitored by
chemists. There are many methods available for the re- thin layer chromatography and IR spectra. The disappear-
1
-5
duction of organic compounds. However, these meth- ance of asymmetric and symmetric stretching bands near
520 cm 1 and 1345 cm 1 due to N O of NO and ap-
ods have one or more limitations. Heterogeneous catalytic
1
2
5
6-8
1
transfer hydrogenation and metal mediated reactions
are gaining more potential due to their simple work up and
pearance of strong band between 1630 1575 cm due to
N=N stretching in the IR spectrum clearly indicates the
conversion. There is no absorption between 3500 3300
3
,5,9-17
selectivity. Lead and its compounds
are widely used
cm 1 indicating the absence of NH group. The work-up
in organic synthesis, but lead powder has not been used in
the field of heterogeneous catalytic transfer hydrogena-
2
and isolation of the products were easy. Thus, all the com-
tion. There are few methods reported in the literature for pounds reduced to azo compounds (few examples are list-
1
8
the synthesis of intermediates like aryl hydroxyl amines,
ed in Table) and were characterized by comparison with
hydrazo compounds and azo the thin layer chromatography, IR spectra and melting
Most of the methods documented in the points of the authentic samples. A control experiment us-
1
6,19
20
azoxy compounds,
2
1-26
compounds.
literature are associated with cyclization, rearrangement ing nitro compounds with ammonium formate but without
and isomerization in the presence of strong acid or alka- lead powder, does not yield the desired product. Another
line medium.
control experiment using nitro compound with lead pow-
der in the absence of ammonium formate also did not
yield the desired product. In order to test the selectivity,
reduction was attempted with p-dichloro benzene, p-
chloro-m-cresol, -naphthol, cinnamic acid, acetanilide,
benzoic acid, anisole, phenyl acetate, under similar condi-
tions. However, the reaction failed to give any reduced
product. The yields are virtually quantitative and analyti-
cally pure.
Here, we wish to report the synthesis of azo compounds 2
by catalytic transfer hydrogenation of nitro arenes 1 by us-
ing lead powder with ammonium formate in methanol or
tetrahydrofuran or in dioxane medium at reflux tempera-
ture. Among the solvents used, methanol is the best one
(
Scheme 1).
The scope of this new general procedure is shown in Ta-
ble. In most cases, the reactions were completed within
1
3 hours. Lead powder can be reused after thorough
washing. These results demonstrate a rapid versatile and
selective reducing system for wide variety of nitro com-
pounds in the presence of other functional groups for e. g.
Scheme 1
–
CO H, Cl, OC H , CH . The reduction was also car-
2 2 5 3
ried out with the nitro compounds bearing bromomethyl
sulphonic acid, oximino, amino and dialkyl amino groups.
In these cases, the bromomethyl, dialkyl amino and ox-
imino groups are compatible under the experimental con-
ditions. However, in the case of amino substituted nitro
Synthesis 2002, No. 4, 15 03 2002. Article Identifier:
1
437-210X,E;2002,0,04,0460,0462,ftx,en;E07001SS.pdf.
Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881
©

SHORT PAPER
Synthesis of Azo Compounds
461
Table Reduction of Nitro Compounds to Azo Compounds using
Ammonium Formate/Lead Powder
mined by using Thomas-Hoover melting point apparatus and are
uncorrected. IR spectra were recorded on SHIMADZU FTIR- 8300
spectrometer. For preparative tlc, the plates were prepared from
Kieselgel 60 GF254, Merck, Darmstadt and for column chromatog-
raphy 60 120 mesh silica gel was obtained from SISCO Research
Laboratories.
Nitro
Compound. 1
Reaction Product 2
Time (h)
Yield Melting Point
(%) (ºC)
Lit.30
Found
66 68
54 56
Nitrobenzene
1.0
azobenzene
92
68
55
General procedure
A suspension of an appropriate nitro compound (1 g) lead powder
(2 g, 325 mesh size, 99.5% pure, packed under Ar) in MeOH or in
any other suitable solvent (10 mL) was stirred and refluxed with
o-Nitrotoluene 1.75
m-Nitrotoluene 1.25
m-Nitroanisole 1.5
2,2 -dimethy- 92
lazobenzene
ammonium formate (3 g) under N atm. After completion of the re-
2
3,3 -dimethy- 94
lazobenzene
55 56
90 93
55
91
action (monitored by TLC), the reaction mixture was filtered
through celite pad, washed with solvent. The combined filtrate and
washings were evaporated under vacuum. The residue was taken in
CHCl or Et O (15 mL), washed with sat. brine soln (2 15 mL) and
3,3 -di-
93
ethoxyazoben-
zene
3
2
finally with H O. The organic layer was dried over anhyd MgSO
2
4
and the solvent was removed using rotary evaporator. For further
purification, the residue was purified either by preparative TLC or
by column chromatography.
m-Chloroni-
trobenzene
1.5
3,3 -dichloro- 92
azobenzene
101 102 101
130 133 131
o-Nitroanisole 2.0
2,2 -di-
90
ethoxyazoben-
zene
References
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azobenzene
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144 146 144
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compounds, a mixture of products are obtained, probably
due to the coupling of reduction intermediates with the
free amino group. Nitro sulphonic acids gave insoluble
precipitates and thus, this procedure is not helpful to those
type of compounds. Furthermore, this system does not re-
quire a pressure apparatus, strong acid or alkaline medi-
um. Ammonium formate also has the advantage of being
readily available, inexpensive, stable, and nontoxic. It
may be added to the reaction in a single portion and prod-
ucts can be easily separated from the reaction mixture.
This procedure will therefore be of general use for the
preparation of azo compounds, specifically in cases where
mild reduction is required and it is less expensive com-
pared to existing methods.
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S. Gowda, D. C. Gowda
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Synthesis 2002, No. 4, 460–462 ISSN 0039-7881 © Thieme Stuttgart · New York