Synthesis of Unsymmetrical Aromatic Azoxy Compounds by Silver-Mediated Oxidative Coupling of Aromatic Amines with Nitrosoarenes

Article abstract of DOI:10.1002/adsc.201801175

A silver(I) oxide-mediated synthesis of unsymmetrical aromatic azoxy compounds has been successfully achieved, wherein oxidative coupling reactions between aromatic amines and nitrosoarenes take place in ethanol under air. This reaction has very high economic value because silver(I) oxide is the only oxidant required and no other additive is needed. The resulted silver particles can be easily recovered, while the only other byproduct is water. This new procedure is compatible with various functional groups and proceeds under mild reaction conditions.

Full text of DOI:10.1002/adsc.201801175

Title: Synthesis of Unsymmetrical Aromatic Azoxy Compounds by
Silver-Mediated Oxidative Coupling of Aromatic Amines with
Nitrosoarenes
Authors: Weijie Ding, Shengshi Xu, Xiaochun Yu, and Shun Wang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
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using the Digital Object Identifier (DOI) given below. The VoR will be
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801175
Link to VoR: http://dx.doi.org/10.1002/adsc.201801175

10.1002/adsc.201801175
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Synthesis of Unsymmetrical Aromatic Azoxy Compounds by
Silver-Mediated Oxidative Coupling of Aromatic Amines with
Nitrosoarenes
Weijie Ding,a Shengshi Xu,a Xiaochun Yu,a,* and Shun Wanga,*
a College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325035, P. R. China.
Fax: 86-577-86689300, E-mail: xiaochunyu@wzu.edu.cn, shunwang@wzu.edu.cn.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
and high atom economy procedure for the synthesis
unsymmetrical aromatic azoxy compounds has been of unsymmetrical azoxybenzenes from simple
Abstract.
A
silver(I) oxide-mediated synthesis of
successfully achieved, wherein oxidative coupling reactions
between aromatic amines and nitrosoarenes take place in
ethanol under air. This reaction has very high economic
value because silver(I) oxide is the only oxidant required
and no other additive is needed. The resulted silver
particles can be easily recovered, while the only other
byproduct is water. This new procedure is compatible with
various functional groups and proceeds under mild reaction
conditions.
starting materials remains highly desirable in modern
organic synthesis.
We have recently developed a practical and
convenient method for the synthesis of
unsymmetrical azoxybenzenes through the I2-
mediated oxidative coupling reaction of aromatic
amines with nitrosoarenes.[12] However, some
shortcomings still need to be addressed, such as the
use of large amount of oxidant used, which leads to
much waste and post-processing. Meanwhile, the
reaction of aromatic amines with strongly electron-
withdrawing groups (–NO2, –CF3, –CO2Me) is
significantly inhibited by the strong iodination effect.
To overcome these problems, appropriate oxidants
should be selected. Fortunately, in our studies, we
found that silver could promote oxidative coupling
reactions between aromatic amines and nitrosoarenes,
which would solve the problems encountered
previously.[12] Ag(I) oxidants have been found to be
generally superior to other common oxidants in
various oxidative coupling reactions.[13] Herein, we
applied silver(I) as an oxidant for the synthesis of
unsymmetrical azoxybenzenes as a green chemistry
approach with high atom economy.
We started our investigation by examining the
reaction of 4-chloroaniline (1a) and nitrosobenzene
(2a) to obtain optimized reaction conditions (Table 1).
An initial attempt, using 0.15 equiv. of silver salt to
promote the reaction of 1a and 2a, was not efficient.
Mills reaction products (4a) were formed with
specific selectivity in the presence of silver nitrate or
silver triflate (Table 1, entries 2 and 6). Oxidative
coupling products 3a were obtained with 13%
conversion and 99% selectivity, accompanied by 1%
of the oxidative dimerization product (5a) of 1a,
when 0.15 equiv. of commercially available silver
oxide was added to the reaction mixture (Table 1,
entry 4). To achieve a higher yield, an increasing
amounts of silver(I) oxide was used (Table 1, entries
7–10). In the presence of 1.0 equiv. of Ag2O, 1a and
2a reacted to give an improved conversion of 91%
Keywords: Aromatic amines; Nitrosoarenes; Oxidative
coupling; Silver oxide; Unsymmetrical azoxybenzenes
Azoxyarenes have been widely applied in dyes,
reducing agents, polymerization inhibitors, and
chemical stabilizers due to their conjugated system
and polar functionality.[1,2] In the light of their
importance, the development of azoxyarene
derivatives with varied functional groups, such as
unsymmetrical azoxyarenes, has become a hot
research topic. The direct oxidation of aromatic
amines[3] and reduction of nitroarenes[4] are well
established methods for the preparation of azoxy
compounds. However, only symmetrical azoxyarenes
can be obtained.
To introduce different functional groups into azoxy
compounds, much effort has been directed toward the
C–H functionalization of symmetrical azoxyarenes,
with
ortho-acylation,[5]
halogenation,[6]
acetoxylation,[7] alkoxylation,[7,8] sulfonamidation,[9]
acylamidation,[10] and alkenylation[11] achieved to
form unsymmetrical azoxyarenes. Unfortunately,
these reactions suffer from the severe drawback that
functional groups can only be introduced in the
ortho-position of the synthesized azoxybenzenes,
which limits the scope of functionality. Furthermore,
environmentally unfriendly oxidants and relatively
low yields and selectivity have limited their real
applications. Therefore, developing a low-cost, green,
1
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10.1002/adsc.201801175
Advanced Synthesis & Catalysis
with 95% selectivity. The conversion was further
increased to 97% by prolonging the reaction time to
48 h (Table 1, entry 9). When the amount of Ag2O
was further increased to 1.1 equiv., the reaction went
to completion (Table 1, entry 10). Other metallic
oxides were also explored, but a lower efficiency was
obtained (see Table S1 in the supporting information
(SI)).
oxidative coupling reaction to form azoxyarenes with
moderate to excellent yields. We did not observe any
steric effect on this reaction, with sterically hindered
amines, such as 2-chloro (3c), 2-fluoro (3g), 2-phenyl
(3m), 2-methoxy (3p), and even 2-tert-butyl aniline
(3k), showing high reactivity and producing the
corresponding azoxyarenes in high yields. However,
aromatic amines were observed to have an electronic
effect on this transformation. For example, 2-
(phenylethynyl)aniline reacted with 2a at 65 C to
give 2-phenylethynyl product 3ab in 52% isolated
yield with 20% unidentified products. Furthermore,
aromatic amines bearing strongly electron-
withdrawing substituents, such as –COPh, –CN, –
CO2Me, –NO2, and –CF3, exhibited slightly lower
reactivity, affording corresponding products 3q–3y in
67%–82% yields using an elevated temperature (80
C) and longer reaction time (48 h). Heteroaromatic
amines, such as pyridin-3-amine and quinolin-3-
amine, were also tolerated in the oxidative coupling
reaction, affording corresponding unsymmetrical
products 3z and 3aa, respectively, in moderate yields.
Next, we tested nitrosoarenes bearing an electron-
Table 1. Optimization of reaction conditions[a].
Sel. (%)b
Conv.
Entry
[M]
Equiv.
Solvent
(%)b
3a
4a
5a
1
2
3
AgOAc
AgNO3
Ag2CO3
0.15
0.15
0.15
Toluene
Toluene
Toluene
13
35
-
30
-
-
70
100
-
-
-
-
4
5
6
7
Ag2O
AgBr
AgOTf
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
0.15
0.15
0.15
0.5
1.0
1.0
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
DMSO
DMF
CH3CN
THF
Dioxane
DCE
13
-
99
-
-
-
-
1
-
-
17
43
91
97
100
99
92
94
92
71
92
99
90
100
-
-
donating
or
electron-withdrawing
group.
95
95
95
94
98
98
98
97
97
96
98
86
5
5
5
5
1
1
1
2
2
3
1
13
Nitrosoarenes with a strongly electron-donating
group exhibited comparatively lower reactivity, while
the reaction of N,N-dimethyl-4-nitrosoaniline with
aromatic amines gave a lower yield than the reactions
of chloro, bromo, ester, and methyl-substituted
nitrosoarenes. Substrates used in this study were then
extended to sterically hindered 1-chloro-2-
8
9c
10
11
12
13
14
15
16
17
18
-
1
1
1
1
1
1
1
1
1
nitrosobenzene
and
1-methyl-2-nitrosobenzene,
which produced the corresponding azoxyarenes in
good yields. Notably, ortho-halogenation products
3au and 3av, which are difficult to prepare through
the C–H functionalization of azoxyarenes owing to
their unavailability, were conveniently synthesized
using our method.[6] The limitation of this method
was that no reaction occurred between strongly
electron-rich nitrosobenzenes and strongly electron-
poor aromatic amines. For example, N,N-dimethyl-4-
nitrosoaniline did not react with 4-nitroaniline.
EtOH
n-hexane
[a] All experiments were conducted using p-ClC6H4NH2
(0.20 mmol), C6H5NO (0.22 mmol), [M], and solvent (1.0
[b]
mL) at 65 C under air for 24 h, unless otherwise noted.
Conversion and selectivity were measured by monitoring
p-ClC6H4NH2 using GC–MS. [c] Reaction time extended to
48 h.
The solvent effect was then evaluated (Table 1,
entries 11–18). Solvents including DMSO, DMF,
CH3CN, THF, DCE (ClCH2CH2Cl), and EtOH were
found to be highly effective media for this
transformation. In contrast, 1,4-dioxane and n-hexane
showed lower reactivity. EtOH was selected as the
reaction solvent for subsequent experiments because
it was the most green solvent among those tested. The
influence of temperature was investigated (Table S2,
SI), with complete conversion achieved at room
temperature with a prolonged reaction time (120 h).
Higher temperatures had nearly no effect on reaction
selectivity.
With optimal reaction conditions in hand, we
explored the scope and limitations of these oxidative
coupling reactions (Table 2). Various aromatic
amines bearing different functional groups, including
ester, ketone, nitrile, nitro, alkynyl, fluoro, chloro,
bromo, and iodo substituents, underwent the
Scheme 1. Gram-scale synthesis.
To demonstrate the scalability and practicality of the
above silver-mediated oxidative coupling method, we
performed a gram-scale reaction using 1a and
nitrosobenzene as the substrates under the optimized
reaction
conditions,
with
unsymmetrical
azoxybenzene 3a obtained in 85% isolated yield after
reacting for 24 h in ethanol (Scheme 1). Furthermore,
a series of repeat reactions showed that the silver
recovery was more than 99% (see SI for details).
2
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10.1002/adsc.201801175
Advanced Synthesis & Catalysis
Table 2. Substrate scope.
a All experiments were conducted with Ar1NH2 (0.20 mmol), Ar2NO (0.22 mmol), Ag2O (0.22 mmol), and EtOH (1.0 mL)
at 65 C under air for 24 h, unless otherwise noted. b Reacted at 80 C for 48 h.
3
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10.1002/adsc.201801175
Advanced Synthesis & Catalysis
after 24 h. This result indicated that azobenzene was
not an intermediate in this transformation. A high GC
yield of 3a was obtained when the reaction was
performed under N2 protection, confirming that
oxygen in air was not involved in the reaction, with
silver oxide acting as the oxidant (Scheme 2C). To
identify whether the oxygen atom in azoxybenzene
came from nitrosobenzene or silver oxide, we reacted
1a and 1b in the presence of different amounts of
silver acetate (Table S3 in SI). During the
optimization process, we found that silver acetate
promoted the oxidative coupling reaction (Table 1,
entry 1). Furthermore, we also found that the yield of
3a increased with an increasing amount of silver
acetate, which clearly demonstrated that silver acetate
might also act as the oxidant to produce 3a. The
above study led us to propose that the oxygen atom of
3a was not derived from silver oxide.
Figure 1. Visual appearance.
(a) Reaction mixture at room temperature before heating. (b1,b2)
Reaction mixture turned brown after the reaction, where silver
particles were formed. (c1,c2) Silvery or black solids were
collected from the reaction mixture by filtration and washed with
ultrasonication.
Scheme 2. Control experiments.
Figure 2. XRD diffraction patterns of the
collected silver particles.
We noticed that the black silver oxide had turned
silvery when the reaction was finished (Figure 1). The
XRD pattern of a silver particle sample collected after
reaction completion is shown in Figure 2. The XRD
patterns exhibited diffraction peaks at 2θ = 37.9,
43.9, 64.3, and 77.3, representing (111), (200),
(220), and (311) planes of Ag particles, which
supported that silver oxide had been reduced to silver
particles during the reaction and, therefore, acted as
an oxidant in this transformation.
Scheme 3. Proposed reaction mechanisms.
Based on the above experimental evidence and
existing mechanisms for the oxidative dimerization of
aniline,[14] we have proposed a reaction mechanism
for this transformation, as shown in Scheme 3.
Initially, 1a reacts with silver(I) oxide to generate
radical cation I. Subsequent deprotonation affords
active radical II, which can be trapped by BHT to
give 6. Alternatively, the addition of radical II to
nitrosoarene 2a produces radical intermediate III.
Next, aided by silver(I) oxide, radical intermediate III
generates species IV. Product 3a is then formed by
the deprotonation of intermediate IV, accompanied by
the formation of Ag(0).
To probe the reaction mechanism, several control
experiments were conducted. As shown in Scheme
2A, using butylhydroxytoluene (BHT) as a radical
scavenger in the model reaction completely inhibited
the formation of 3a. Instead, corresponding trapping
product 6 was detected as a byproduct by GC–MS
(Figure S1), suggesting that a radical intermediate
was involved in this silver-mediated synthesis of
unsymmetrical
aromatic
azoxy
compounds.
Furthermore, the oxidation reaction of azobenzene
was investigated in the presence of 1.1 equiv. of
Ag2O (Scheme 2B), with no azoxybenzene detected
4
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10.1002/adsc.201801175
Advanced Synthesis & Catalysis
QP2010 Plus spectrometer (EI). 1H NMR spectra were
recorded on Bruker AVANCE III-500 spectrometers.
Chemical shifts (in ppm) were referenced against TMS in
CDCl3 (δ 0 ppm). 13C NMR spectra were obtained using
the same NMR spectrometers and calibrated with CDCl3 (δ
77.00 ppm). High-resolution mass spectra were recorded
on an ESI-Q-TOF mass spectrometer.
Scheme 4. Synthetic applications.
General procedure for the synthesis of
unsymmetrical aromatic azoxy compounds
Finally, using 3a as a representative sample, we
investigated the synthetic potential of this
transformation. As shown in Scheme 4, based on a
literature procedure,[15] 3a underwent regioselective
C–H activation/cyclization with an alkyne to
construct 2H-indazole 7a in 70% isolated yield, which
is recognized as an effective pharmacophore.[16]
A solution of aromatic amines (0.20 mmol, 1.0 equiv.),
nitrosoarenes (0.22 mmol, 1.1 equiv.), and Ag2O (0.22
mmol, 1.1 equiv.) in EtOH (1.0 mL) in a sealed (or open to
air) 10-mL Schlenk tube was stirred at 65 C for 24 h.
Reaction progress was monitored by TLC and/or GC–MS.
After reaction completion, the crude reaction mixture was
cooled to room temperature, filtered to remove silver
In summary, we have developed a simple and particles, and the solvent was removed under reduced
pressure. The resulting residue was purified by column
chromatography on silica gel with EtOAc and petroleum
ether as eluent to give unsymmetrical aromatic azoxy
compounds. Characterization data of the products are
provided in the Supporting Information.
efficient approach to the construction of azoxyarenes
using a commercially available silver-mediated
oxidative coupling reaction of aromatic amines with
nitrosoarenes. This reaction takes place in ethanol
solution under air without other additives. This new
protocol enables a variety of functionalized amines
and nitrosoarenes to be transformed into the
corresponding products under very mild reaction
conditions. This transformation has several
advantages, including a high atom economy, simple
recovery of silver particles, an environmentally
friendly solvent, no use of moisture-sensitive starting
materials, and mild reaction conditions with a wide
substrate scope. Furthermore, the synthetic utility of
this method was demonstrated by a gram-scale
reaction and the synthesis of 2H-indazole 7a.
Investigations into the detailed reaction mechanism
and potential medicinal activities are underway in our
laboratory.
Typical procedure for the synthesis of 2-(4-
chlorophenyl)-3-phenyl-2H-indazole
To a dried 10-mL round-bottom flask equipped with a
magnetic stir bar were added [Cp*RhCl2]2 (7.7 mg, 0.0125
mmol), AgSbF6 (17.3 mg, 0.05 mmol), Cu(OAc)2 (90.8 mg,
0.5 mmol), Zn(OTf)2 (36.3 mg, 0.1 mmol), 1,2-
diphenylethyne (106.9 mg, 0.6 mmol), and 2-(4-
chlorophenyl)-1-phenyldiazene oxide (116.0 mg, 0.5
mmol). The vessel was evacuated and backfilled with N2
before adding 1,1,1,3,3,3-hexafluoro-2-propanol (1.0 mL).
The mixture was heated to 80 C and stirred for 24 h. After
reaction completion, the reaction mixture was cooled to
room temperature and the solvent was removed under
reduced pressure. The resulting residue was purified by
column chromatography on silica gel (petroleum
ether/EtOAc = 30:1, v/v) to afford 2-(4-chlorophenyl)-3-
phenyl-2H-indazole as a light yellow solid (106.0 mg, 70%
yield).
Experimental Section
General remarks
Acknowledgements
4-Nitrosobenzoate, 1-chloro-4-nitrosobenzene, 1-bromo-4-
nitrosobenzene, 1-chloro-2-nitrosobenzene, and 1-methyl-
2-nitrosobenzene were synthesized according to the
literature.[17] Silver(I) oxide (99%), 2-fluoroaniline (99%),
2-aminodiphenyl (97%), 4-methoxy-phenylamine (99%),
Financial support from the National Natural Science Foundation
of China (Nos. 21302143 and 51572198), Natural Science
Foundation of Zhejiang Province (Nos. LY13B020006 and
LZ17E020002), the Graduate Scientific Research Foundation of
Wenzhou University, and the Xinmiao Talent Planning
Foundation of Zhejiang Province (No.2018R429055) is greatly
appreciated. We are very grateful to Jichang Wang, from
University of Windsor, and Simon Partridge, PhD, from Liwen
Bianji, for their assistance during the revision of the manuscript.
(4-aminophenylhenyl)(phenyl)methanone
aminobenzonitrile (98%), 2-aminobenzonitrile (98%), 2-
aminobenzoic acid methyl ester (98%), 4-
(98%),
4-
aminobenzotrifluoride (98%), 3-aminoquinoline (97%),
nitrosobenzene (97%), and N,N-dimethyl-4-nitrosoanline
(98%) were purchased from Energy Chemical. All other
compounds were purchased from Aladdin. All reagents
were used without further purification. Melting points were
recorded on Digital Melting Point Apparatus WRS-1B and
are uncorrected. Crude reaction mixtures were analyzed
using an Agilent 7890 GC System with Shimadzu GCMS-
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10.1002/adsc.201801175
Advanced Synthesis & Catalysis
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Advanced Synthesis & Catalysis
UPDATE
Synthesis of Unsymmetrical Aromatic Azoxy
Compounds by Silver-Mediated Oxidative
Coupling of Aromatic Amines with Nitrosoarenes
Adv. Synth. Catal. Year, Volume, Page – Page
Weijie Ding, Shengshi Xu, Xiaochun Yu* and
Shun Wang*
7
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