Convenient Electrocatalytic Synthesis of Azobenzenes from Nitroaromatic Derivatives Using SmI2

Article abstract of DOI:10.1021/acscatal.7b02940

The synthesis of azobenzenes has been a long-standing challenge. Their current preparation at a preparative or industrial scale requires stoichiometric amounts of environmentally unfriendly reactants. Herein, we demonstrate that the catalytic use of electrogenerated samarium diiodide (SmI₂) could promote, in one-step synthesis, the reduction of nitrobenzenes into azobenzenes in high yields under mild reaction conditions. This catalytic procedure contains many elements satisfying a sustainable chemical process for the preparation of one of the most widely wanted family of chemical compounds. The easy synthetic procedure, and the absence of precious metals, bases, and nonhazardous substances, already makes our catalytic procedure a serious alternative to currently available methods. This is a promising method for the efficient synthesis of both symmetrical and asymmetrical azo compounds with a high functional group tolerance.

Full text of DOI:10.1021/acscatal.7b02940

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Article
Convenient Electrocatalytic Synthesis of Azobenzenes
from Nitroaromatic Derivatives Using SmI2
Yu-Feng ZHANG, and Mohamed Mellah
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02940 • Publication Date (Web): 01 Nov 2017
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ACS Catalysis
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Convenient Electrocatalytic Synthesis of Azobenzenes from Nitroar-
omatic Derivatives Using SmI2
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AUTHOR NAMES. Yu-Feng Zhang1, Mohamed Mellah *1
AUTHOR ADDRESS. 1 ICMMO, Université Paris-Sud, CNRS UMR 8182, Université Paris-Saclay, F-91405 Orsay,
France.
ABSTRACT: The synthesis of azobenzenes has been a long-standing challenge. Their current preparation at a preparative
or industrial scale requires stoichiometric amounts of environmentally-unfriendly reactants. Herein, we demonstrate that
the catalytic use of samarium diiodide (SmI2) electrogenerated could promote in one-step synthesis the reduction of ni-
trobenzenes into azobenzenes in high yields under mild reaction conditions. This catalytic procedure contains many
elements satisfying a sustainable chemical process for the preparation of one of the most widely wanted family of chemi-
cal compounds. The easy synthetic procedure, the absence of precious metals, of bases, and non-hazardous substances
already puts our catalytic procedure as a serious alternative to current available methods. This is a promising method for
the efficient synthesis of both symmetrical and asymmetrical azo compounds with a high functional group tolerance.
KEYWORDS: Divalent samarium, electrosynthesis, catalysis, selective reduction, nitrobenzene, azobenzene
terms of sustainability and environmental considerations.
Although asymmetrical and symmetrical azobenzenes can
be readily prepared through azo coupling, Mills reaction,
INTRODUCTION
Since their discovery in the early 19th century, azoben-
zenes are considered as one of the largest and most versa-
tile class of organic dyes, accounting for approximatively
70% of worldwide production of industrial dyes.
Azo(hetero)arenes are currently widely used in textile,
food, cosmetic and pharmaceutical industries.1 At a more
fundamental level, they have recently received increasing
attention due to their photochromic properties that stem
from the light-induced trans-cis isomerization of the N=N
double bond.2 The E/Z photoisomerization of the azo
functionality induces a remarkable change in their physi-
cal properties (e.g. electronic, geometry and polarity),3
from which chemists have taken advantage to develop
sophisticated protein probes,4 organic dyes,5 chemosen-
sors,6 smart surface materials1 and molecular machines.7
In these various research fields, the photophysical proper-
ties of the azobenzenes derivatives rely on their substitu-
tion pattern through electronic and steric effects. The
fine-tuning of these properties may help to design photo-
chromic molecules responding to the desired optimal
criteria, with an emphasis on their photoswitchable spec-
tral window. Such a control is critical for in vivo applica-
tions that require working in the non-hazardous red or
near infrared region.8
oxidative or reductive dimerization, these methods are
often fraught with major drawbacks such as the use of an
excess of reducing or oxidizing agents and/or of hazard-
ous reactants (e.g. aromatic amines). Furthermore, their
efficiency lowers with increasing steric congestion, so that
their versatility is determined by the capacity of both
substrates and products to endure harsh reaction condi-
tions. In order to tackle these limitations, several catalytic
strategies have been developed, relying either on a cata-
lytic oxidation of anilines or reduction of nitroarenes. To
date, only a few examples have been reported to produce
efficiently various symmetrical and asymmetrical azoben-
zenes using catalytic aerobic oxidation of anilines. They
are based on the use of Ag/C nanoparticles,10 Au/TiO2,11
Cu/base,12 MnO2,13 or meso-Mn2O3.14 These methods are
generally efficient and selective, providing azo com-
pounds with a high structural diversity. To cite but a few
disadvantages, they require i) the use of anilines known
for their toxic and environmental harmful, ii) the prepara-
tion of a heterogeneous catalyst complex and iii) adding
stoichiometric amounts of additives (e.g. strong bases).
Regarding the reduction strategy, less hazardous reac-
tants such as nitro aromatic compounds can yield selec-
tively azo compound in presence of gold-based catalyst
such as Au/TiO2,11 Au/CeO2,11 or (Au/ZnO2).15 Other het-
erogeneous transition-metals-based catalysts (Pd,16 Ru,17
Pt,18 Au19 and In20) can also promote such transformation.
Under those reaction conditions, various symmetrical and
Despite the significant efforts devoted to the prepara-
tion of diverse substituted azo-photoswitches,9 accessing
sterically constrained azobenzenes using common syn-
thetic procedures still remains a challenge, not only in
terms of efficiency, robustness and versatility, but also in
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mated to allow a complete conversion of the substrate into
asymmetrical azobenzenes were generated, but the yields
are moderate and the selectivity is in general low. In this
context, devising novel catalytic methods to achieve this
type of reaction are still of great importance with the aim
of being effective, selective, tolerant to manifold func-
tional groups and easy to implement.
1
2
3
4
azobenzene. To produce SmI2 from a samarium anode, a
charge of 2F is necessary and, 4 equivalents of SmII are theo-
retically needed to ensure the formation of the desired azo-
benzene. Therefore, under imposed current intensity of
50.10-3 A, the transformation of 1 mmol of nitrobenzene
would take around 4.5 hours of electrolysis.
5
6
7
8
9
To the best of our knowledge, the use of electrochemis-
try to synthesize azobenzenes was only demonstrated by
Kim et al. who employed magnesium electrodes for direct
reduction of nitrobenzenes. 21 We recently discovered that
electrogenerating catalytically active SmI2 as a reducing
agent could represent an elegant and cost-effective syn-
thetic method for C-C and C-heteroatom bond forming
transformations compared to all previous reported one.22
The guiding principle of this electrochemical process is
based on the facile regeneration of the active low oxida-
tion state catalytic species using a bare samarium elec-
trode as a cathode.23 We also found that the electrolytic
media plays a crucial role in the catalysis. Thus, perform-
ing the reaction in anhydrous THF in the presence of
ammonium additives (nBu4NI and nBu4NPF6) significant-
ly enhanced the reactivity of the SmII species.24 These
results have urged us to evaluate the potential of our
catalytic process to reduce efficiently and selectively other
functional groups. Herein, we disclose a simple and cost-
effective synthetic strategy for the reductive coupling of
nitroarenes to furnish a broad array of symmetrical and
asymmetrical azobenzenes in high yields (scheme 1) via
the catalytic electrogeneration of SmI2. This procedure
can be implemented at a preparative scale under mild and
safe conditions to provide a more than competitive alter-
native to the current existing methods.
Table 1. Optimization of the reductive coupling of nitro-
benzene by electrogenerated SmI2.
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N
N
Sm anode, e-
nBu4NI (2 equiv.)
NH2
NO2
N
N
O
+
+
nBu4NPF6 (0.04 M)
THF, 20 °C
1
2
2a
2b
Entry
MeOH
t(h) Conv [%]
Selectivity
(equiv.)
2
2a
35
75
0
2b
16
0
1
10
5
100
100
100
35
42
23
95
37
2
3
4
100
0
5
5
0
0[a]
60
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Reaction conditions: R-ArNO2 (1 mmol, 1 equiv.), nBu4NI
(2.0 mmol, 2 equiv.) and nBu4NPF6 (0.04 M) in THF (50
mL) at 20 °C for 5 h. [a] 6 equivalent of SmI2 prepared by
Kagan’s method. Yields given are isolated yields.
Scheme 1. Catalytic synthesis of azobenzenes from
nitroarenes using electrogenerated SmI2.
previous works:
Based on the initial Kagan’s report,25 we studied the ef-
fect of proton donors as additives on the selectivity (Table
1). In the presence of 10 equivalents of MeOH, complete
conversion was achieved within 5 hours to give the de-
sired 1,2-diphenyldiazene 2 as a major product (42 %).
Aniline 2a and azoxybenzene 2b were also obtained in 35
% and 16 % yields, respectively. On the other hand, with
100 equivalents of MeOH, the selectivity of the reaction
decreased dramatically providing azobenzene (2) in 23%
yield along with aniline (2a) (75%). In contrast, when the
electrolysis was performed without proton donor, azo-
benzene (2) was obtained as the sole product in 95 %
yield. The same reaction was attempted using SmI2 (8
equivalents) prepared according to Kagan’s method in
anhydrous THF. However, only 35% conversion was ob-
served after 60 hours with a poor selectivity towards ani-
line (2a) (54%) (Entry 4, table 1). This result is in agree-
ment with the results reported by Kagan et al.25 This dif-
ference of behavior, when compared to our method, can
be attributed to the specific reactivity of the electrogener-
ated SmI2, which is freshly produced in the electrolytic
medium. As noted in our previous report on the homo-
coupling of aliphatic ketone and the reduction of 1-
chlorododecane,23, 24 it proved to be also more reactive.
Control experiments were then performed. First of all, the
H2 and/or strong base
cat. : Au,11,15,19 Pt,18 Pd16 or Ru17
T = 40-120°C
(R') R
R
NO2
NO2
(R') R
N
+
N
this work
R
SmI2 cat. e-
THF, 20 °C
38 examples (up to 95%)
In a seminal study from Kagan’s group, it was reported
that SmI2 was capable of promoting the reductive cou-
pling of nitrobenzene into the corresponding azoben-
zene.24 However, the reaction required the use of 8 equiv-
alents of SmI2 and 2 equivalents of MeOH, while provid-
ing only a mixture of aniline and azobenzene with a low
conversion (50%). Based on these results, we decided to
explore the reduction of nitrobenzene (1) by electrogen-
erated SmI2 for a selective access to azobenzenes.
First, the electrochemical process was conducted using a
samarium anode to generate continuously a stoichiometric
amount of SmI2 (Table 1). The electrolysis was conducted at a
constant current density of 2.5 A. cm-2 during the time esti-
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ACS Catalysis
SmX2 (10 mol%)
Sm cathode
TMSCl (2 equiv.)
samarium anode was replaced by an inert carbon anode
and electrolysis was strictly performed in the established
1
2
3
4
5
6
7
8
NO2
NO
N
N
N
N
+
+
nBu4NPF6 (0.04 M) THF, 20 °C
conditions. After the electrolysis time estimated (4.5 h),
nitrobenzene (1) was fully recovered indicating that the
direct reduction at the cathode did not occur. We also
checked that no chemical transformation took place
without application of any current intensity. Both control
experiments confirm that the coupling is mediated by
electrogenerated SmI2.
O
1
2
2b
2c
nitrobenzene 1
azobenzene 2
azoxybenzene 2b
nitrosobenzene 2c
100
80
60
40
20
0
9
After demonstrating that electrogenerated SmI2 can be
a powerful reductant for the conversion of nitrobenzene
into 1,2-diphenyldiazene (2) in THF, we turned our atten-
tion to the development of a catalytic version. For its
implementation, the catalytic procedure was accom-
plished in two steps as previously reported for various C-
C bond forming reactions.23 The first step consists in the
in situ electrogeneration of SmI2 from the samarium elec-
trode used as an anode in the presence of nBu4NI. This
pre-electrolysis was achieved in an undivided cell charged
with anhydrous THF containing nBu4NPF6 under argon at
a constant current intensity of 50×10−3 A, which prevents
any manipulation of the sensitive catalyst out of the elec-
trochemical cell. In order to perform the second electroly-
sis step, nitrobenzene (1) and 2 equivalents of trimethylsi-
lyl chloride (TMSCl) were then introduced. In a precedent
report, we demonstrated that TMSCl was able to promote
the cleavage the SmIII-O bonds by metal exchange,
providing the corresponding silyl ether and SmIIIX3.23 The
use of TMSCl will release chloride ions during the reduc-
tion, and it is well known that chloride has a significantly
higher affinity for SmII even in the presence of excess
iodide.26 In view of this, the active catalytic species will be
rather of the SmIIX2 form in this case according the ligand
exchange.
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0
50
100
150
200
250
300
350
Time (min)
Figure 1. Time-conversion plot for the catalytic
reduction of nitrobenzene into azobenzene in anhydrous
THF (0.02 M) by electrogenerated SmI2 (10 mol%).
Scheme 2. Probing mechanism intermediates for
the catalytic reduction of nitrobenzene.
Path a
3 equiv. e-
SmX2
O
NO2
N
2 equiv. e-
SmX2
O
N
2 equiv. e-
SmX2
N
N
N
2c
1
2b
2
(80%)
1 equiv. e-
SmX2
Path b
Path a: experiment using nitrosobenzene as starting ma-
terial under electrocatalytic homocoupling conditions
mediated by SmIIX2. Path b: experiment using azoxyben-
zene as starting material under electrocatalytic homocou-
pling conditions mediated by SmIIX2.
A subsequent electron transfer from the samarium
cathode allows ultimately the regeneration of the SmIIX2
active species from SmIIIX3 free in solution. The polarity of
the samarium electrode is then simply reversed to act as a
cathode during the whole electrolysis process. Electroca-
talysis is therefore carried out by applying a constant
current intensity of −50 mA. Gratifyingly, nitrobenzene
led selectively to the desired azobenzene (2) in 88 %
yield.
The concentration of nitrobenzene (1) was found to de-
crease continuously during the electrolysis process, while
the concentration of azobenzene (2) increased over the
same period, but after a delayed time of 60 minutes.
Meanwhile, the formation of azoxybenzene (2b) reached
a threshold of 40% conversion after 140 minutes, before
its concentration decreases in the second half of the elec-
trocatalysis. The evolution of the concentration of
azoxybenzene suggests a reaction pathway, in which the
nitroaromatic compound is initially reduced to the azoxy
compound and then to the corresponding azobenzene
(Fig. 1). Besides, when azoxybenzene (2b) or nitrosoben-
zene (2c) were subjected independently to the same reac-
tion conditions (Scheme 2, paths a and b), they both led
to azobenzene (2) as a sole product within one and four
hours (1 and 3 equivalents of electrons), respectively. We
observed that azoxybenzene (2b) was converted into
azobenzene (2) at a rate four times faster than nitro-
sobenzene (2c), which is in agreement with their respec-
We conducted also additional experiments to gain
some insight into the mechanism of this catalytic process
by following the transformation from nitrobenzene (1)
into azobenzene (2) through gas chromatography (GC)
analysis (Fig. 1).
Figure 1 depicts the evolution of the various species
during the course of the electrocatalytic reduction of
nitrobenzene (1). Based on the retention times of pure
samples, three compounds were clearly identified, namely
nitrobenzene (1), azoxybenzene (2b), and azobenzene (2).
On the other hand, no other potential intermediates,
such as nitrosobenzene (2c) or aniline, were detected by
this method.
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tive reactivities. These experiments confirm that nitro- merical azobenzenes, which is much more challenging.
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8
sobenzene (2c) might be the first intermediate in the
catalytic process, but would react too quickly to be de-
tected by GC analysis. This is in total agreement with the
mechanism of nitrobenzene reduction by SmII intermedi-
ates, which was previously examined in detail by Brady et
al.27 The use of the well-defined {Sm[N(SiMe3)2]2(thf)}2
complex as a reductant lead to identify nitrosobenzene
then azobenzene as intermediates. In their case, azoben-
zene was further reduced to aniline in presence of water.
The method of choice involves usually the treatment of
diazonium salts with electron-rich aromatic compounds
in a stoichiometric manner.9, 28
Since we were able to isolate p-methoxynitrosobenzene
(8a)
via
a
stoichiometric
reduction
of
p-
methoxynitrobenzene, this reaction was also attempted in
the presence of aniline to get the asymmetrical azoben-
zene by condensation (see scheme 3). How ever the cross-
coupling product was never detected and only symmet-
rical 1,2-bis(4-methoxyphenyl)diazene was isolated after
full conversion. This result supports the hypothesis that
the dimerization is favored and implies the existence of a
radical intermediate (8), which results from the nitro-
sobenzene monoelectronic reduction. It then gives rise to
a fast radical process instead of a condensation between
azoxybenzene and aniline (Mills reaction) 9.
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When the reaction was carried out with the more elec-
tron rich p-methoxynitrobenzene under stoichiometric
electrosynthetic conditions in the presence of 10 equiva-
lents
of
MeOH,
the
corresponding
p-
methoxynitrosobenzene was isolated in 65% yield. This
result supports the hypothesis of the initial formation of
the nitroso compound.
We then turned our attention on assessing the scope
and the limitations of this catalytic procedure. Using our
optimized electrolysis conditions, diverse nitrobenzene
derivatives were converted into the corresponding azo-
benzenes in high yields (up to 95%) with an excellent
selectivity (> 99%) (Table 2). After purification, no trace
of samarium was detected all physical and spectral data
are in total accordance with those given in the literature
(see SI). Nitrobenzene (1), nitrotoluenes and dimethylni-
trobenzenes were examined and gave the corresponding
azobenzenes (Table 2, 3-7) in excellent yields (80-94%).
The reaction was compatible with both electron-rich (8,
9, 11-13) and electron-poor (14, 15, 19-23) arenes, providing
the desired azobenzenes in yields ranging from 53% to
95%. On the other hand, in the case of 2-
methoxynitrobenzene, azobenzene 9 was obtained with a
low conversion and poor selectivity (Table 2). These re-
sults might be explained by the sequestering of the metal
center by the oxygen and nitrogen functionalities know-
ing the high oxophilicity of samarium (D0(Sm-O) = 138
kcal/mol). Interestingly, the good yield (80%) obtained
for azobenzene 7 derived from 2,6-dimethylnitrobenzene
indicates that reaction is relatively unaffected by steric
hindrance. Nitroaromatics bearing halogen substituents
were also converted into the corresponding azobenzenes
without observing any dehalogenation process (Table 2,
16-20). Of note, our procedure can be performed on a
large-scale without a drop of yield. Thus, 10 mmol of p-
methoxynitrobenzene gave azobenzene (8) in 85% yield
(1.02 g) after 50 hours of electrolysis. The established
catalytic procedure proved to be versatile and efficient for
the synthesis of various functionalized symmetrical azo-
benzenes. Moreover, in these catalytic conditions the
samarium electrode is poorly consumed and can be re-
used more than one hundred times. These results encour-
aged us to apply this procedure to the synthesis of asym-
Once formed, nitrosobenzene should be even more eas-
ily reduced than the parent nitrobenzene to be rapidly
converted into the corresponding azoxybenzene, follow-
ing a pathway that involves successive transfer of elec-
trons (scheme 3). To ascertain this radical pathway, we
carried out the catalytic reduction of nitrobenzene in the
presence of 2,6-di-tert-butyl-4-methylphenol and TEMPO
as radical inhibitors. In both cases, no traces of azoben-
zene were observed. Furthermore, the reaction did not
give any product resulting from the trapping of radical
intermediates by 2,6-di-tert-butyl-4-methylphenol or by
TEMPO. We assumed that the radical species (8b) formed
from nitrosobenzene (8a) quickly dimerize to form
azoxybenzene 8c (intermediate clearly detected by GC
analysis) through successive electron transfers, delivering
finally azobenzene (8).
Scheme 3. Proposed mechanism based on a radical
process and successive electron transfers mediated
by SmIIX2.
NH2
NO
NO2
SmX2
SmX2
N
no product
e-
O
e-
FAST
SLOW
MeO
OMe
OMe
8a
8b
dimerization
FAST
SmX2
MeO
N
N
O
MeO
N
e-
OMe
N
OMe
8
8c
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1
2
Table 2. SmIIX2-catalyzed reduction of nitroarenes into symmetrical azobenzenes.
3
4
5
6
1) Sm anode, nBu4NI (20 mol%)
NO2
R
2) Sm cathode, TMSCl (2 equiv.)
THF, 20 °C, 5 h
N
R
N
R
7
8
9
Me
Me
Me
Me
Me
Me
Me
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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35
Me
N
N
N
N
N
N
N
N
N
N
N
N
Me
Me
Me
Me
7 (80%)
Me
Me
Me
Me
4
5
(88%)
2 (88%)
3
(88%)
(94%)
6 (82%)
OMe
OMe
H2N
MeO
BnO
N
N
N
N
N
N
N
N
N
N
MeO
NH2
OMe
OBn
8
9
10
11
12
(93%)
(85%)
(12%)
(95%)
(91%)
OMe
MeO2C
NC
Cl
N
Br
N
N
N
N
N
N
N
N
N
N
CO2Me
CN
Cl
N
Br
13
14
15
16
17
(80%)
(85%)
(61%)
(80 %)
(90%)
Cl
Cl
Cl
N
F
F3C
F
N
N
F
N
N
Cl
N
N
N
N
N
Cl
N
N
Cl
18
CF3
F
Cl
Cl
19
20
21
22
23
(81)%
(53%)
(91%)
(85%)
(81%)
(74%)
Reaction conditions: R-ArNO2 (1 mmol, 1 equiv.), TMSCl (2 mmol, 2 equiv.) and nBu4NPF6 0.04 M in THF (50 mL) in the
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presence of 10 mol% of SmI2 (determined by coulometry) at 20 °C for 5 h. Yields given are isolated yields
.
Based on these results, we focused on the cross cou-
pling reactions of p-methoxynitrobenzene and nitroben-
zene using different molar ratios.10, 12a, 13, 14 A 1:3 molar ratio
of p-methoxynitrobenzene to nitrobenzene proved to be
optimal for the synthesis of the desired asymmetrical
azobenzene (24), which was obtained in 83% yield with a
high selectivity (> 99%) (Table 3).
withdrawing groups underwent homocoupling more
slowly than those bearing electron-donating groups, so
that an excess of the less electron-rich nitrobenzene had
to be used to increase the yield of the desired asymmet-
rical product. Therefore, a 1:3 molar ratio was employed
according to the nature of electronic substituents to fur-
nish the corresponding products in high yields (77-90%)
(Table 3, 34-39).To illustrate this method, methyl yellow
(39 in Table 3), which can be used as a pH indicator, was
prepared in 85% yield from a direct cross-coupling be-
tween nitrobenzene and N,N-dimethyl-4-nitroaniline.
Various nitrobenzene derivatives were subjected to the-
se
optimized
reaction
conditions
with
p-
methoxynitrobenzene (Table 3, 24-33). In each case,
asymmetrical azobenzenes were obtained with high con-
version rates (> 99%) and a complete control of the selec-
tivity. We noticed that nitrobenzenes bearing electron-
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1
2
Table 3. SmIIX2-catalyzed reduction of nitroarenes into asymmetrical azobenzenes.
3
4
5
6
7
8
9
1) Sm anode, nBu4NI (20 mol%)
NO2
NO2
R
2) Sm cathode, TMSCl (2 equiv.)
THF, 20 °C, 5 h
N
R
+
R'
N
R'
3 equiv.
MeO
MeO
MeO
MeO
Me
MeO
Me
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
N
N
N
N
N
N
N
N
N
N
Me
CF3
28
(93%)
Me
27 (81%)
24
(83%)
25 (80%)
26 (80%)
MeO
F
MeO
MeO
Cl
MeO
MeO
CN
N
N
N
N
N
Cl
N
Cl
N
N
N
N
Cl
29
30
31
32
33
(90%)
(92%)
(95%)
(95%)
(86%)
Cl
Cl
Me
N
Cl
N
N
N
N
N
N
N
N
N
N
N
N
CF3
CF3
39 (85%)
34 (88%)
35 (77%)
36 (77%)
37 (90%)
38 (80%)
Reaction conditions: R-ArNO2 (0.25 mmol, 1 equiv.), R’ArNO2 (0.75 mmol, 3 equiv.), TMSCl (2.0 mmol, 2 equiv.) and
nBu4NPF6 (0.04 M) in THF (50 mL) in the presence of 10 mol% of SmI2 (determined by coulometry) at 20 °C for 5 h. Yields
given are isolated yields.
CONCLUSION
ASSOCIATED CONTENT
Supporting Information. Complete experimental methods
and characterisations.This material is available free of charge
via the Internet at http://pubs.acs.org
In conclusion, we have developed a facile, efficient and
safe alternative for synthesizing a wide variety of azoben-
zene derivatives, including symmetrical and asymmetrical
ones, is possible. The efficiency and the wide functional
tolerance of this procedure provide a new tool in our
arsenal to access azobenzenes. The use of electrochemis-
try to perform catalytic reactions in mild conditions em-
ploying SmI2 as a reductant, the absence of precious met-
als, bases, and non-hazardous substances make our pro-
tocol a great alternative to current methods. During the
screening of the substrate scope (Table 2), we found that
electron-donating substituents in ortho position were not
reactive in dimerization step. We looked closely to the
reaction mechanism, which allowed us to identify clearly
various intermediates in the catalytic process. Results
from preliminary mechanistic experiments suggest that
the formation of azobenzenes is accomplished by succes-
sive monoelectronic reductions. The key step is the re-
duction of nitrosobenzene into a radical anionic interme-
diate. The fast dimerization of this intermediate to gener-
ate an azoxybenzene followed by a subsequent monoelec-
tronic transfert mediated by SmIIX2 catalyst seems to be
the reaction pathway in this catalytic process.
AUTHOR INFORMATION
Corresponding Author
* Mohamed.mellah@u-psud.fr
Author Contributions
The manuscript was written through contributions of all
authors. / All authors have given approval to the final version
of the manuscript.
ACKNOWLEDGMENT
We thank Université Paris-Sud, CNRS, and the China Schol-
arship Council (CSC) for financial support.
ABBREVIATIONS
TMSCl, trimethylsilyl chloride; GC, gas chromatography;
TEMPO, 2,2,6,6-Tetramethylpiperidinyloxy.
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