Aqueous manganese-mediated reductive coupling of nitroarenes to azoxybenzenes

Article abstract of DOI:10.1080/00397910600602669

Azoxy compounds have been prepared in good yields by reductive coupling of aromatic nitro compounds with manganese and a catalytic amount of acetic acid in aqueous conditions. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910600602669

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Aqueous Manganese‐Mediated Reductive
Coupling of Nitroarenes to Azoxybenzenes
Dilip K. Dutta a
a Natural Products Chemistry Division , Regional Research Laboratory ,
Jorhat, Assam, India
Published online: 18 Aug 2006.
To cite this article: Dilip K. Dutta (2006) Aqueous Manganese‐Mediated Reductive Coupling of Nitroarenes to
Azoxybenzenes, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 36:13, 1903-1906, DOI: 10.1080/00397910600602669
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Synthetic Communicationsw, 36: 1903–1906, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600602669
Aqueous Manganese-Mediated Reductive
Coupling of Nitroarenes to Azoxybenzenes
Dilip K. Dutta
Natural Products Chemistry Division, Regional Research Laboratory,
Jorhat, Assam, India
Abstract: Azoxy compounds have been prepared in good yields by reductive coupling
of aromatic nitro compounds with manganese and a catalytic amount of acetic acid in
aqueous conditions.
Keywords: Azoxy compound, nitroarene, manganese, aqueous
Azoxy compounds have received much attention because of their physiologi-
cal activities and utilization in liquid crystal systems.[1] The most common
approach for the synthesis of azoxybenzene is the reduction of aromatic
nitro compounds by a variety of homogeneous and heterogeneous catalysts.
Classical methods are reduction with glucose/NaOH,[2] sodium arsenate,[3]
Mg reagent,[4] KBH4,[5] PH3,[6] and yellow phosphorus.[7] Thallium
mediated reduction[8] of nitrobenzene is another well-documented method
for the preparation of azoxybenzene. Zinc in presence of acetic acid and
acetic anhydride[9] as well as Se[10] and Te[11] reagents were also used as
efficient reagents for this conversion. The use of lanthanide reagents such as
Sm[12] also has been reported. More recently, metallic Bi[13a] have been
found to produce azoxy compounds in good yields.
Aqueous manganese-mediated reactions have not been reported for this
transformation to the best of our knowledge. Besides, manganese-mediated
Received in India December 19, 2005
Address correspondence to Dilip K. Dutta, Natural Products Chemistry Division,
Regional Research Laboratory, Jorhat 785 006, Assam, India. Fax: þ91-376-2370-
011; E-mail: dkdutta@yahoo.com
1903

1904
D. K. Dutta
reductions are scarse in organic syntheses although the metal has a stronger
reduction potential than
zinc and is abundant in nature.[14] As a sequel to our works with metal,[15] we
decided to explore the possibility of employing this metal in a water-mediated
reaction in combination with a catalytic amount of acetic acid to carry out the
desired transformation.
We found that 2.5 equivalents of manganese powder in the presence of a
catalytic amount of acetic acid in water and THF (1:1) can trigger the
reductive coupling of aromatic nitro compounds to form azoxy compounds
in good yields. The reaction conditions are summarized in Table 1 without
optimization of all the parameters. We found that catalytic amount of acetic
acid was sufficient to initiate the reaction. It was also observed that the
yield of the reaction becomes low in the absence of an inert atmosphere,
which may be due to the aerial oxidation of manganese metal. The reaction
conditions were compatible with functional groups such as Cl, CN, COCH3,
CH55CH, CO2R, and OMe, present in the substrates. In case of p-nitroaceto-
phenone, it produces an azoxy compound in good yield rather than pinacol
when reacted with SmI3.[17] The yield of the substrates bearing CN, Cl,
CH3, and CH55CH were excellent as compared to some other methods.
Table 1. Reductive coupling of nitroarenes to azoxybenzenes
Mp (8C)
Entry
R
Time (h) Yield (%)a
Found
Lit.
1
2
H
12
15
14
16
12
12
14
12
15
12
85
83
80
75
90
85
78
92
70
90
35
66–67
39
36[16a]
4-CH3
3-CH3
2-CH3
4-Cl
70[16d]
39[16d]
3
4
57–58
154–55
96–97
55–56
220–21
204
59–60[16d]
155–56[16b]
97[16b]
5
6
3-Cl
2-Cl
7
56[16b]
8
4-CN
4-CO2Me
MeO2CCH55CH
221[16c]
9
10
207[16c]
218–219
272–74
117–118
190–191
221–223[12]
276–78
11
12
4-MeO
COCH3
15
15
21
71
118–119[16e]
191–193[12]
aYield conforms to a reaction condition where 2.5 equivalents of Mn powder and
a catalytic amount of acetic acid per equivalent of substrate are used. The spectral
data were in accord with those of authentic samples.

Coupling of Nitroarenes to Azoxybenzenes
1905
However, p-nitroanisole recorded the lowest yield of the azoxy compounds,
with a large bulk of the starting material remaining unreacted. The yield of
the product was significantly decreased in the case of the electron-donating
group. The addition of water as a cosolvent improves the yield of the
products. The present procedure also bars overreduction, as is the case in
some other ones.
Although the exact mechanism of the reaction cannot be proposed at this
stage, we believe that reduction of the nitroarene can produce both the nitroso
and corresponding hydroxylamine by a SET process from metallic Mn to nitro
compounds, which combine together to form azoxy compounds.[18]
GENERAL PROCEDURE
To a stirred solution (10 ml, 1:1 H2O:THF) of the nitro compound (entry 8,
2 mmol, 0.296 g) under nitrogen atmosphere was added acetic acid
(0.02 mmol, 0.012 g), followed by manganese powder (5 mmol, 0.275 g).
Stirring continued at room temperature for 12 h, after which reaction
mixture was poured into a saturated solution of NaHCO3 (20 ml). The
aqueous solution was extracted with CHCl3 (2 ꢀ 25 ml). The combined
organic extract was dried over anhydrous Na2SO4 and evaporated to
dryness to yield the crude azoxy compound. Recrystallization with
methanol yielded the pure compound (0.415 g, 92%).
In conclusion, the present method deserves a better place because of its
eco friendly nature. However the actual beauty of the reaction lies in the
fact that it is the simplest of all the methods reported so far, besides being
extremely cost effective and chemoselective. Newer ventures with this
reagent are under way.
ACKNOWLEDGMENT
The author sincerely expresses his gratitude to the late Parijat Sarmah for his
help in some of the experiments. The author thanks P. G. Rao, director, for
providing necessary facilities for this work.
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