Electrosynthesis of Azobenzenes Directly from Nitrobenzenes

Article abstract of DOI:10.1002/cjoc.202100470

The electrochemical reduction strategy of nitrobenzenes is developed. The chemistry occurs under ambient conditions. The protocol uses inert electrodes and the solvent, DMSO, plays a dual role as a reducing agent. Its synthetic value has been demonstrated by the highly efficient synthesis of symmetric, unsymmetric and cyclic azo compounds.

Full text of DOI:10.1002/cjoc.202100470

Cite this paper: Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
Electrosynthesis of azobenzenes directly from nitrobenzenes
Yanfeng Ma,a Shanghui Wu,a Shuxin Jiang,a Fuhong Xiao,*,a Guo-Jun Deng*,a,b
a
Key Laboratory for Green Organic Synthesis and Application of Hunan Province, Key Laboratory of Environmentally Friendly Chemistry
and Application of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, China.
b
Guangdong Provincial Key Laboratory of Luminescence from Molecular Aggregates (South China University of Technology), Guangzhou
510640, China.
Keywords
Azo compounds | Nitro reduction | Electrochemistry | Nitrobenzenes
Main observation and conclusion
The electrochemical reduction strategy of nitrobenzenes is developed. The chemistry occurs under ambient conditions. The protocol
uses inert electrodes and the solvent, DMSO, plays a dual role as a reducing agent. Its synthetic value has been demonstrated by the
highly efficient synthesis of symmetric, unsymmetric and cyclic azo compounds.
Comprehensive Graphic Content
C (+)/Pt (-), 20 mA
Ph
N
Ph
O
NO2
NO
undivided cell
NO2
+
+
N
N
N
Ph
Ph
NH
4Cl, DMSO, rt.
R1
1a
2a
3a
4a
+
NO2
R2
C
Pt
R2
N
N
R1
metal free
undivided cell
mild conditions
DMSO as reductant
.
*E-mail: fhxiao@xtu.edu.cn; gjdeng@xtu.edu.cn
Chin. J. Chem. 2021, 39, XXX－XXX
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
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through the copyediting, typesetting, pagination and proofreading process which may lead to
differences between this version and the Version of Record. Please cite this article as doi:
10.1002/cjoc.202100470
This article is protected by copyright. All rights reserved.

First author et al.
Report
synthesis using nitroarenes as the readily available coupling part-
ners, herein, we report an environment-friendly and efficient
electrochemical strategy for the reductive coupling of nitroarenes
into azobenzenes under mild reaction conditions. Notably, this
method uses inert electrodes while the solvent, DMSO, plays a
dual role as a reducing agent, therefore, providing a green syn-
thetic approach for azobenzene derivatives.
Background and Originality Content
Azobenzene derivatives are an important class of molecules,
which are widely used in dyestuffs, colourants, photoresponsive
materials, and therapeutic drugs.[1] Recently, due to the strong
coordination ability of nitrogen atom, many chemists utilized the
azo group as a directing group for ortho C-H functionalization.[2]
Accordingly, all sorts of preparation methods for the synthesis of
these compounds have been developed. The traditional synthetic
method is primarily based on coupling of aryl diazonium salts with
nucleophiles.[3] Yet this method requires preparation of reactive
diazonium salts by oxidation of the corresponding aromatic
amines with nitrous acid. Due to the electrophilic substitution
mechanism, this kind of reaction is limited to the combination of
electron rich and electron deficient aromatic substrates.
Significant progress has been achieved by employing direct
dehydrogenation of aromatic anilines or diarylhydrazines (Scheme
1, a).[4] Yet these methods suffer from major issues such as using
excessive additives and oxidants or hard preparation of starting
materials. The direct dehydrogenation of diaryl hydrazines with
the liberation of molecular hydrogen would be an alternative
method for preparing aromatic azo compounds. [5]
Nitroarenes are easily accessible and stable compounds, and
can be converted into functionalized amines by reduction reac-
tion.[6] In this regard, our group demonstrated a series of proto-
cols by direct amination of nitroarenes to form corresponding
amines or N-containing heterocycles.[7] Likewise, preparation of
azobenzenes through a reduction of nitrobenzenes with reducing
reagents is highly attractive (Scheme 1, b).[8] But this approach is
limited to operations that require stoichiometric amounts of met-
al reductant or high-pressure hydrogen as a reducing reagent. For
example, Guo and coworkers reported a Cu/graphene photocata-
lytic reduction coupling reactions of aromatic nitro compounds to
corresponding azo compounds. [9]
Results and Discussion
Table 1 Optimization of the reaction conditiona.
NO2
N
NH
4Cl (3 equiv)
N
DMSO, rt, 5 h
C(+)/Pt(-), I = 20 mA
1a
2a
undivided cell
Entry
1
Variation from the standard reaction conditions
none
Yieldb (%)
76
2
NH4Br instead of NH4Cl
NH4OAc instead of NH4Cl
TBAB instead of NH4Cl
nBu4NBF4 instead of NH4Cl
DMF instead of DMSO
EtOH instead of DMSO
Pt as anode
20
3
25
4
n.r.
5
40
6
trace
trace
38
7
8
9
10 mA instead of 20 mA, 10 h
25 mA instead of 20 mA, 4 h
no NH4Cl
trace
51
10
11
12
n.r.
no electric current DMSO
n.r.
aReaction conditions: anode: graphite rod (6 mm) and cathode: Pt foil (1.0
x 1.0 cm2), constant current = 20 mA, 1a (0.5 mmol), NH4Cl (1.5 mmol),
DMSO (8 mL) in an undivided cell, rt, air, 5 h. b Isolated yield based on 1a
Scheme 1. Previous reports on catalytic approaches to the synthesis
of azobenzene derivatives.
NH2
(a) Oxidative
coupling
Mg or Sm
Previous works
R
R
We primary chose nitrobenzene 1a as the substrate for reduc-
tive homodimerization with carbon/platinum electrodes and
NH4Cl electrolyte to determine the optimized conditions. Nitro-
benzene 1a reacted efficiently in an undivided cell with 20 mA for
5 h, azobenzene 2a was synthesized in 76% isolated yield (Table 1,
entry 1). Then we investigated the influence of electrolytes such
as NH4Br, NH4OAc, TBAB (n-Bu4NBr) or n-Bu4NBF4, it was found to
be ineffective for this electrochemical reduction reaction (Table 1,
entries 2-5). The employment of other media such as DMF and
EtOH also failed to give acceptable yield of azobenzene 2a, which
indicates DMSO may play roles more than solvent (Table 1, entries
6-7). During the optimization process, (methylsulfonyl) methane
was detected by GC-MS when utilizing DMSO. When a reticulated
Pt was used as the anode, the yield of azo product 2a was dra-
matically decreased (Table 1, entry 8). Either lower (10 mA) or
higher (25 mA) constant current (compared to 20 mA) leads to
lower yields (Table 1, entries 9-10). We did not observe azo prod-
uct 2a when the reaction was carried out in the absence of cur-
rent or electrolyte (Table 1, entries 11-12).
Under the optimized conditions, we investigated various aro-
matic nitro compounds for this reductive homocoupling reaction
(Scheme 2). A 76% isolated yield of azo product 2a was achieved
using nitrobenzene (1a) as model substrate. Electron-rich aro-
matic nitro compounds, such as methyl, ethyl and methoxy groups
in the para position, reacted smoothly to furnish the correspond-
ing azo products 2b-2d in excellent yields. Halo-substituted nitro-
benzenes (1e-1h) could also be used. It should be noted that the
use of 4-iodo-nitrobenzene 1h gave access to product 2h in 73%
excess oxidant
stoichiometric reagent
NO2
Mg or Sm as reductant
N
Ar
Ar
N
R
NO2
(b) Reductive
coupling
C
Pt
metal free
undivided cell
mild conditions
This work
H2/metal reductant
cat.: Au, Pt, Pd...
DMSO as reductant
NO2
X
n
C
Pt
X
n
N
N
This work
N
O2
n = 0, 2
X = C, O, S
Electrochemical synthetic methods have been considered as a
green and viable synthetic tool since it employs electrons as rea-
gents to avoid waste production.[10] The nitroaryl electroreduction
has attracted considerable attention over the past decade,
especially for the synthesis of anilines.[11] To date, very few exam-
ples have been reported on the use of electrochemistry to synthe-
size azobenzenes from nitroarenes. In 2006, Kim and co-workers
employed magnesium electrodes for the direct reduction of ni-
trobenzenes to synthesize azobenzenes.[12] Noteworthy, this reac-
tion suffers from the use of a sacrificial Mg electrode and narrow
substrate scope. In 2017, Mellah carried out the same reaction,
using stoichiometric Sm as a reagent that was electrogenerated
from Sm anode.[13] As part of our continuing efforts in molecular
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© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
Chin. J. Chem. 2021, 39, XXX－XXX

Running title
Chin. J. Chem.
NO2
NO2
yield and C–I bond is suitable for this reaction system. An isolated
yield was reduced to 51% using 4-nitrobenzonitrile (1i) as the re-
actant. Interestingly, steric factors do not seem to play a role,
whether the substituents were at ortho or meta position. For ex-
ample, p-methylnitrobenzene (1b), m-methylnitrobenzene (1k),
o-methylnitrobenzene (1r) proceeded smoothly to give the corre-
sponding azo products in 73-87% yields. Nitrobenzenes with two
functional groups also reacted well giving the aromatic azo prod-
ucts in good yields (2v and 2w). It is noteworthy that the ethynyl
group is well tolerated under this reaction condition, and the
ethynyl-substituted aromatic azo compounds 2x could be used for
further Sonogashira coupling or addition reactions. In this reac-
tion, sensitive and active hydroxy group substituent on benzene
ring was well tolerated, yielding azobenzene products (2y and 2z)
in good yields.
C (+)/Pt (-), 20 mA
undivided cell
R2
N
+
N
R2
R1
R1
NH4Cl, DMSO, rt, 5 - 6 h,
1
1
2
N
N
Br
H3CO
N
N
H3C
Br
, 57%
, 51%
2bg
2dg
N
N
CN
N
N
OMe
H3C
F
, 53%
, 43%
2bi
2ed
N
N
OMe
N
N
OMe
Scheme 2 Symmetric aromatic azo compounds under standard
H3C
conditionsa.
, 47%
, 51%
2ad
2bd
C (+)/Pt (-), 20 mA
undivided cell
NO2
R
N
Reaction conditions: anode: graphite rod (6 mm) and cathode: Pt foil (1.0 x
1.0 cm2), constant current = 20 mA, 1 (0.25 mmol), NH4Cl (1.5 mmol),
DMSO (8 mL) in an undivided cell, rt, air, 5-6 h. Isolated yields.
R
N
R
NH4Cl, DMSO, rt, 5 - 6 h,
1
2
, R = H, 76%
2a
Scheme 4. Control experiments under various conditions.
, R = CH , 87%
R
N
2b
, R = CH3
, 85%
N
R
R
2c
2d
2e
2CH
, R = OMe, 835%
, R = F, 62%
NO2
C (+)/Pt (-), 20 mA
undivided cell
X
N
, R = CH , 73%
2k
2l
N
n
3
, R = Cl, 84%
, R = Br, 78%
X
, R = Cl, 86%
, R = Br, 55%
2m
2f
n
2g
2h
NH
4Cl, DMSO, rt, 5-6 h,
R
, R = I, 73%
N
N
, R = I, 60%
2n
2o
2p
2q
N
O2
, R = CN, 51%
, R = CH
, R = CF , 57%
2i
2j
, R = CN3, 63%
5
6
n = 0, 2
2CN, 73%
, R = NO , 71%
X = C, O, S
2
R
, R = CH , 83%
CH3
N
N
2r
3
N
N
, R = F, 63%
, R = Cl, 73%
2s
CH3
N
N
N
N
N
N
N
R
N
2t
H3C
, R = Br, 67%
2u
H3C
O
, 73%
Br
Br
2v
, 75%
, 64%
, 51%
Br
, 60%
6d
Cl
6a
6b
6c
N
N
N
N
OCH3
Cl
H3CO
N
N
N
N
N
N
, 65%
, 71%
2w
2x
O
O
S
, 67%
6e
, 58%
, 64%
6g
6f
N
N
CH2OH
N
N
CH2CH2OH
HOH2C
Reaction conditions: anode: graphite rod (6 mm) and cathode: Pt foil (1.0 x
1.0 cm2), constant current = 20 mA, 5 (0.5 mmol), NH4Cl (1.5 mmol), DMSO
(8 mL) in an undivided cell, rt, air, 5-6 h. Isolated yields.
HOH2CH2C
, 73%
2y
, 83%
2z
Furthermore, the treatment of 2,2'-dinitro-1,1'-biphenyl (5a)
gave intramolecular cyclization product (6a) in 75% yield. The
target product (6b) can be obtained when using
4,4'-dibromo-2,2'-dinitro-1,1'-biphenyl. It is noteworthy that eight
membered ring product 6c was obtained when using
1,2-bis(2-nitrophenyl)ethane (5c). Other eight membered ring
products 6d-6g can also be obtained in moderate yields (Scheme
4). We also conducted cyclic voltammetry (CV) experiments (see
Supporting Information, Fig S3). The reductive peak of 1b was
recorded at -1.143 V vs SCE and 5a at -1.280 V vs SCE in ethanol.
aReaction conditions: anode: graphite rod (6 mm) and cathode: Pt foil (1.0
x 1.0 cm2), constant current = 20 mA, 1a (0.5 mmol), NH4Cl (1.5 mmol),
DMSO (8 mL) in an undivided cell, rt, air, 5 h. b Isolated yield based on 1a.
Next, the formation of unsymmetric aromatic azo compounds
were also explored (Scheme 3). Unsymmetric azo product 2bg as a
major product was selectively achieved in 57% yield when
p-methylnitrobenzene (1b) was reacted with equimolar amount of
4-bromonitrobenzene (1g). Similar yield was obtained when
4-methoxyaniline (1d) was treated with 4-bromoaniline (1g). This
heterocoupling approach is also applicable for the synthesis of
push-pull-type azobenzenes 2bi and 2ed in moderate yields. The
cross-dimerization reactions of an electron-rich p-methoxy nitro-
benzene (1d) with p-methylnitrobenzene (1b) and nitrobenzene
(1a) also lead to give unsymmetric azobenzenes 2bd and 2ad in
47% and 51% yield, respectively.
Scheme 5. Time-conversion plot for the catalytic reduction of ni-
trobenzene into azobenzene.
Scheme 3. Unsymmetrically substituted Azo Compound preparation
under Standard conditions.
Chin. J. Chem. 2021, 39, XXX－XXX
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First author et al.
Report
C (+)/Pt (-), 20 mA
undivided cell
Ph
N
Ph
O
NO
In summary, we demonstrated an efficient and low ener-
gy-consuming electrochemical reduction procedure for nitro to
construct a N=N double bond under mild reaction conditions. This
protocol was carried out under ambient conditions in DMSO and
provided a straightforward route for the synthesis of symmetric,
unsymmetric and cyclic azo compounds.
NO2
+
+
N
N
N
Ph
Ph
NH
4Cl, DMSO, rt.
1a
2a
3a
4a
Experimental
General procedure for the electrochemical reaction
In
an
undivided
three-necked
flask
(25
mL),
4-methylnitrobenzene (0.5 mmol, 69 mg), NH4Cl (1.5 mmol, 81 mg)
and DMSO (8 mL) were continuously added. The flask was
equipped with platinum plate electrode (1.0×1.0 cm2) as the
cathode and graphite rod (Φ = 6 mm) as the anode. The reaction
mixture was stirred and electrolyzed at a constant current of 20
mA under air at room temperature for 5 h. After the reaction was
complete, the mixture was diluted with water (30 mL) and then
extracted by EtOAc (10 mL×3). The combined organic phases were
dried over anhydrous Na2SO4, filtered, concentrated in vacuo and
the crude product was obtained. The pure product was obtained
by silica gel chromatography using petroleum ether/ ethyl acetate
(100:1) as eluent.
In order to better understand this potential reaction pathway,
we then carried out reaction time tracking experiment of nitro-
benzene (1a) conversion to azobenzene (2a) through gas chroma-
tography (GC) analysis (Scheme 5). In the early stage of electro-
chemical reduction coupling reaction, the concentration of raw
material nitrobenzene (1a) decreased continuously, while the
concentration of azo product azobenzene (2a), intermediate
product azoxybenzene (3a) and nitrosobenzene (4a) increased
slowly. Meanwhile, the highest conversion of nitrobenzene (4a)
was 8% in 150 min, and then it was completely converted after
210 min. At the same time, the intermediate product azoxyben-
zene (3a) reached a threshold of 42% conversion, and then the
concentration decreased to 18%. The evolution of the concentra-
tion of azoxybenzene (3a) and nitrosobenzene (4a) suggests a
reaction pathway, in which the nitroaromatic compound is initially
reduced to nitrosobenzene (4a), which quickly dimerize to form
azoxybenzene (3a), and finally to the corresponding azobenzene
(2a).
Supporting Information
The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2021xxxxx.
Acknowledgement
This work was supported by the National Natural Science
Foundation of China (21871226, 21502160, 21572194), Scientific
Research Fund of Hunan Provincial Education Department
(19B564).
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At the cathode
NO2
NO
O
N
+ e-
+ 2 e-
+ 2 e-
Ph
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dimerization
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Chin. J. Chem. 2021, 39, XXX－XXX

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4′4′-Bipyridyl-Catalyzed
Reduction
of
Nitroarenes
by
Chin. J. Chem. 2021, 39, XXX－XXX
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
www.cjc.wiley-vch.de

First author et al.
Report
Manuscript accepted: XXXX, 2021
Accepted manuscript online: XXXX, 2021
Version of record online: XXXX, 2021
(The following will be filled in by the editorial staff)
Manuscript received: XXXX, 2021
Manuscript revised: XXXX, 2021
www.cjc.wiley-vch.de
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
Chin. J. Chem. 2021, 39, XXX－XXX

Running title
Chin. J. Chem.
Entry for the Table of Contents
Electrosynthesis of azobenzenes directly from nitrobenzenes
Yan Ma,a Shanghui Wu,a Shuxin Jiang,a Fuhong Xiao,*,a Guo-Jun Deng*,a,b
Chin. J. Chem. 2021, 39, XXX—XXX. DOI: 10.1002/cjoc.202100XXX
NO2
NO2
R2
N
N
n
R1
+
R1
R2
Pt
C
NO2
This work
X
X
metal free
n
undivided cell
mild conditions
DMSO as reductant
N N
N
O2
n = 0, 2
X = C, O, S
Chin. J. Chem. 2021, 39, XXX－XXX
© 2021 SIOC, CAS, Shanghai, & WILEY-VCH GmbH
www.cjc.wiley-vch.de