Oxidative Coupling of Aromatic Amines and Nitrosoarenes: Iodine-Mediated Formation of Unsymmetrical Aromatic Azoxy Compounds

Article abstract of DOI:10.1002/adsc.201800495

I₂/DABCO (iodine/1,4-diazabicyclo[2.2.2]octane)-mediated oxidative coupling of nitrosobenzenes with aromatic amines was revealed to lead to the production of unsymmetrical aromatic azoxy compounds, instead of azo compounds reported previously in Mills reaction. Our study illustrates that various aromatic amines can be efficiently coupled with nitrosobenzenes to produce unsymmetrical azoxy product, in which more than thirty unsymmetrical azoxybenzenes have been successfully prepared. The applicability to a broad range of substrates, scalability to large scales and mild reaction conditions make this new synthetic protocol very practical, providing a convenient and direct access to unsymmetrical azoxybenzenes. (Figure presented.).

Full text of DOI:10.1002/adsc.201800495

Title: Oxidative Coupling of Aromatic Amines and Nitrosoarenes:
Iodine-Mediated Formation of Unsymmetrical Aromatic Azoxy
Compounds
Authors: Xiaochun Yu, Weijie Ding, Panyu Ge, Shun Wang, and
Jichang Wang
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800495
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10.1002/adsc.201800495
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Oxidative Coupling of Aromatic Amines and Nitrosoarenes:
Iodine-Mediated Formation of Unsymmetrical Aromatic Azoxy
Compounds
Xiaochun Yu,a,* Weijie Ding,a Panyu Ge,a Shun Wang,a,* and Jichang Wangb,*
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.
Department of Chemistry and Biochemistry, University of Windsor, Windsor, ON, N9B 3P4, Canada. E-mail:
b
jwang@uwindsor.ca.
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))
the oxidation of aromatic amines and reduction
Abstract.
I2/DABCO
(iodine/1,4-
diazabicyclo[2.2.2]octane)-mediated oxidative coupling of process of aromatic nitro make it very challenging to
nitrosobenzenes with aromatic amines was revealed to lead
to the production of unsymmetrical aromatic azoxy
compounds, instead of azo compounds reported previously
in Mills reaction. Our study illustrates that various aromatic
amines can be efficiently coupled with nitrosobenzenes to
produce unsymmetrical azoxy product, in which more than
achieve high yields and selectivity.
So far, several methods have been developed for
the synthesis of unsymmetrical azoxybenzenes. The
reaction of Gringard reagent with N-substituted
tosyloxydiimide N-oxide needs inert atmosphere
protection and anhydrous condition.[14] Moreover, N-
hydroxy-N-phenylnitrous amide, as the starting
material for the synthesis of N-substituted–
tosyloxydiimide N-oxide, its preparation process is
tedious. Other methods such as oxidation of
unsymmetrical azoarenes,[15] selective substitution of
azoxybenzes,[16] condensation of nitrosoarens with N-
thirty
unsymmetrical
azoxybenzenes
have
been
successfully prepared. The applicability to a broad range of
substrates, scalability to large scales and mild reaction
conditions make this new synthetic protocol very practical,
providing a convenient and direct access to unsymmetrical
azoxybenzenes.
arylhydroxylamines,[17]
and
condensation
of
Keywords: Unsymmetrical azoxybenzenes; Nitrosoarenes;
Aromatic amines; Iodine; Oxidative coupling
nitroarenes with aryliminodimagnesium[18] suffer
from the use of expensive metal,[16] poor
selectivities,[15,17] availability of starting material,[17]
and harsh reaction conditons.[18] All of these issues
greatly limit the utilities of the methods and make
them not so practical. Therefore, a convenient access
to unsymmetrical azoxybenzenes is highly desirable.
Aromatic
azoxy
compounds
are
versatile
intermediates and precursors in the preparation of
dyes, pigments and pharmaceuticals, and have also
been used as reducing agents, chemical stabilizers,
and food additives.[1] Selective oxidation of aromatic
amines and reduction of nitroarenes are the most
common methods for the preparation of azoxy
compounds. Attempts have been made for oxidative
coupling of aromatic amines using different oxidants
such as H2O2,[2] O2,[3] peracetic acid,[4] lead salts,[5]
mangances salts,[6] ferrates,[7] mercury salts,[8] etc. A
variety of reduction processes have also been
developed to obtain the desired products, in which
Scheme 1. Different products from the reaction of
aromatic amines with nitrosoarenes.
alcohol,[9] H2,[9a,10] NH2NH2·H2O[11] or other reductant
such as zinc
the reducing reagents.
and bimuthium[12b,13] were utilized as
[12]
Despite the tremendous amounts of efforts towards
the synthesis of azoxy derivatives, azoxy compounds
provided by these two routes are symmetric, and
unsymmetrical aromatic azoxy compounds could not
be prepared via them. Meanwhile, the complexity of
In Mills reaction aromatic nitroso derivatives react
with aromatic amines and give the corresponding
unsymmetrical azobenzenes in good yield, which
involves the attack of aromatic amines on the nitroso
derivatives that leads to azobenzenes after
1
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10.1002/adsc.201800495
Advanced Synthesis & Catalysis
dehydration of the intermediate [Scheme 1a].[1d] In
this study, we found unsymmetrical azoxy
compounds were formed [Scheme 1b] through the
reactions of aromatic nitroso derivatives with
aromatic amines using I2/DABCO as the promoters.
To the best of our knowledge, it is the first
convenient and practical process for unsymmetrical
azoxy compounds from aromatic amines with
nitrosobenzenes under mild conditions.
The condensation of 4-chloroaniline 1a and
nitrosobenzene 2a was chosen as a model reaction to
optimize reaction parameters (Table 1). In the initial
attempt, 1a was treated with 2a in the presence of I2
(1.5 equiv.) in toluene at 65 °C under air for 24 hr,
and 4-Chloro-2-iodoaniline 6a was formed as a major
product in 75% GC yields (Table 1, entry 1). Various
only moderate yields with an elevated reaction
temperature and longer reaction time (80oC and 48hr).
And some unreacted amines were detected by GC,
possibly because strongly electron-withdrawing
substituted aromatic amines are less basic and
nucleophilic, resulting in less efficient condensation
reactions. Naphthalen-1-amine was not a suitable
substrate, its reaction led to the product in 57% GC
yields (Table 2, 3n), accompanied with a Mills
product (5%), unidentified product (20%), and
unreacted material (18%). Similar to electron-
withdrawing substituted phenylamines, the reaction
between heteroaromatic amines with nitrosobenzene
is not efficient and only forms the corresponding
products with a moderate yield at a high temperature
and long reaction time (80oC and 48hr) (Table 2, 3o,
bases such as CH3COONa, NaOH, pyridine, CH3ONa, 3q-3t). Pyridin-4-amine failed to react with
DABCO, TEA (triethylamine), piperidine, EDA
(ethylenediamine), and HMTA (hexamethylene
tetramine) were screened. It was found that the
reaction selectivity was very different with respect to
the added base. Specifically, When CH3COONa was
added, the formation of 6a was decreased greatly,
companied with the substantial increase of Mills
product 4a (Table 1, entry 2). When NaOH was
employed, there was no iodinated product, while 4a
and 5a (oxidative dehydrogenative coupling product
of 1a) formed (Table 1, entry 3). Interestingly, when
DABCO was employed as base, dehydrogenative
coupling compound 3a, unsymmetrical azoxybenzene
was formed as a major product (96% selectivity with
94% conversion) (Table 1, entry 6). The molecular
structure of product 3g[19] was confirmed by single
crystal X-ray diffraction analysis (SI, Figure S1) and
NMR spectra. In the next step, the loadings of I2 and
DABCO were optimized (SI, Table S1 and S2). To
obtain satisfactory conversion, 1.5 equivalent of I2
and 3.0 equivalent of DABCO were needed for this
transformation. Increasing reaction temperature led to
lower selectivity. Good conversion with a high
selectivity can be obtained with a low reaction
temperature and long reaction time (SI, Table S3).
Screening the solvent indicates that the selectivity of
the reaction is independent of the solvent polarity,
while the conversion rate is sensitive to it in this
transformation (SI, Table S4).
At the above optimized reaction conditions, the
I2/DABCO system were then applied to various
aromatic amines (Table 2). The results show that the
method tolerates various functional groups. Halo,
methoxy, and alkyl substituted aromatic amines,
including the sterically more hindered ortho-
substituted ones, reacted efficiently to give the target
unsymmetrical aromatic azoxy compounds with a
good yield (3a-3d, 3h-3m). It is noteworthy that
halogens survived well, leading to the production of
halo-substituted unsymmetrical azoxybenzene. These
substance are potentially useful in other synthesis, as
they bear chloro, bromo and iodo groups. The
reactions of electron-deficient aromatic amines, such
as CH3OCO-, NO2-, and CF3-, were not efficient
(Table 2, 3e-3g), in which their target products have
nitrosobenzene, and was recovered intact. This is
most likely due to a combination of electronic effect
and solubility, as pyridin-4-amine is not soluble in
toluene. Quinolin-3-amine was not a suitable
substrate neither, and only 27% GC yield of product
was detected, accompanied with Mills product (9%),
iodinated byproduct (41%), and unreacted amine
(23%)(Table 2, 3u).
Table 1. Bases screening for the synthesis of
unsymmetrical azoxybenzene.
Sel.(%)b)
Entrya)
Base
Conv.(%)b)
3a 4a 5a 6a
1
2
3
4
5
6
7
8
9
-
92
82
80
90
52
94
96
8
-
18
-
-
82
41
-
48
100
2
CH3COONa
NaOH
Pyridine
TEA
DABCO
CH3ONa
Piperidine
Piperazine
EDA
10 49
-
-
-
96
-
-
25
28 12
-
40 60
52
-
-
-
-
2
39 12 49
-
7
-
-
-
-
100
68
60
15
5
44
10
11
HMTA
-
100
a)
All the experiments were carried out with p-ClC6H4NH2
(0.2 mmol), C6H5NO (0.22 mmol), I2 (0.3 mmol), Base
(0.6 mmol), Toluene (1 mL), 65oC, air, 24hr, unless
otherwise noted. b) Conversion and selectivity based on p-
ClC6H4NH2 and measured by GC-MS.
2
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10.1002/adsc.201800495
Advanced Synthesis & Catalysis
Table 2. The oxidative coupling reactions of aromatic amines and nitrosobenznes for synthesis of
unsymmetrical azoxybenzenes in the presence of I2/DABCO.
a) All the experiments were carried out with Ar1NH2 (0.2 mmol), Ar2NO (0.22 mmol), I2 (0.3 mmol), Base (0.6 mmol),
Toluene (1 mL), 65oC, air, 24hr, unless otherwise noted. b) 80oC, 48hr.
Several nitrosoarenes were also employed to react
with substituted aromatic amines. The electronic
effect of substitutent on nitrosobenzenes was
observed again. Electron-donating substituted
3
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10.1002/adsc.201800495
Advanced Synthesis & Catalysis
nitrosobenzenes showed a negative response to the
oxidative coupling reaction. For example, when N,N-
dimethylamino was attached to the nitrosobenzene,
its reaction was not so efficient and only afforded the
products with a moderate yield with 80oC and 48hr
period (Table 2, 3v-3y). In contrast, electron-
withdrawing group substituted ones showed a
positive effect on this transformation. For example,
the reaction of ethyl 4-nitrosobenzoate produces
azoxybenzenes with a good yield (Table 2, 3ad-3ah).
were obtained in both of cases (Scheme 3-C, D). All
these results implicate that the oxygen atom of
product comes from the oxygen of nitrosobenzene.
Throughout our exploration, it was noticed that both
iodine and DABCO are essential for the reaction. No
reaction occurred in the absence of iodine. It was also
observed that the selectivity of the reaction was very
different with or without DABCO. To understand
how DABCO and I2 worked in the reaction, we
1
deployed H NMR techniques to monitor the above
reaction as shown in Figure 1. It was seen that upon
the addition of 0.5 equivalents of I2 into a
[D6]DMSO solution of DABCO (1.0 equiv.), the
singlet signal (2.6 ppm) of DABCO disappeared,
accompanied with the appearance of a broad
multiplet (2.77-2.95 ppm) and a triplet (3.14 ppm)
signal, which resulted from the interaction between I2
with DABCO. When 1.5 equivalents of I2 was added
to a [D6]DMSO solution of 1a (1.0 equiv.), the two
doublet signals (6.53 and 7.00 ppm) of aromatic
hydrogen atoms of 1a disappeared, and 6a was
formed in 65% (1H NMR yield). Another unidentified
compound was produced, which also has two doublet
signals (7.32 and 7.54 ppm). It was speculated to be
the N-iodinated product such p-ClC6H4NHI, an
unstable intermediate, and could also be converted
into 1a through a hydrogen-iodine exchange, since
we detected only 6a (85%) and 1a (15%) after 24 hr
through GC-MS.[20] However, once DABCO was
added into mixture [D6]DMSO solution of 1a and I2,
not only did the iodinated product 6a decrease greatly,
but N-iodinated product of 1a could not be detected,
while the interaction signals of I2 with DABCO still
unchanged. We suggest that the above result was due
to nucleophilicity difference of nitrogen in DABCO
and 1a. DABCO has stronger nucleophilicity than 1a,
and the bond between iodine and nitrogen of DABCO
is formed more easily than a bond between iodine and
nitrogen of 1a, moreover, this formed bond is so
strong that electrophilic aromatic iodination product
6a cannot occur efficiently and its yield decreases
greatly. We did another experiment in which we
added N-iodosuccinimide (NIS) to the reaction of 1a
with 2a instead of I2. It was found that similar results
were obtained that the GC yields of iodinated
products (6a and diiodoaniline) were 71% without
addition of DABCO, while their yields decreased
greatly with the addition of DABCO (SI, Table S5).
When nitrosobenzene was added to the mixture
solution of iodine, DABCO and 1a, the oxidative
coupling reaction occured immediately.
Scheme 2. Large scale reaction for the synthesis of
unsymmetrical azoxybenzene.
The reaction was then carried out on a much larger
scale to examine the scalability and practicality of the
above oxidative coupling of aromatic amines with
nitrosobenzene. As shown in Scheme 2, 80-mmol
scale reaction of 1a and 2a successfully provides 81%
isolated yield of 3a under the optimized reaction
conditions. All the above experimental results clearly
revealed the generality, practicality and synthetic
potential of the present method.
To decipher the reaction mechanism, control
experiments were carried out. The addition of
TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) to
the standard reaction, which is a representative
radical inhibitor, did not affect the product yield
(Scheme 3-A). It was also observed that conducting
the reaction in the dark or under daylight (Scheme 3-
B) had no impacts on the reaction yield and
selectivity. These results suggest that the process
involves the formation of ionic rather than radical
intermediates.
Scheme 3. Control experiments for exploration
reaction mechanism.
Based on those control experiments, a plausible
mechanism for the oxidative dehydrogenative is
proposed in Scheme 4. First, nucleophilic attack on
nitrosobenzene by the lone pair electrons of an
aromatic amines occurs, affording a nucleophilic
addition intermediate. Subsequently, DABCO
The reactions of 1a with 2a under optimized
conditions were also investigated under N2 or O2
atmosphere, respectively. Similar GC yields of 3a
4
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10.1002/adsc.201800495
Advanced Synthesis & Catalysis
abstracted a proton from the resulted ammonium ion,
yielding the intermediate A. The nitrogen lone pair
electron of A attacked iodine atom of DABCO-I2 ion
pair, followed by deprotonation of the initially
formed ammonium ion of intermediate B which
undergoes elimination of iodide anion (I-) and gives a
unsymmetrical azoxy compound.
Figure 1. Contrast 1H NMR spectrum with different reactants.
the synthesis of 3-acyl-2H-indazole 4a with 82%
isolated yield, which was a useful intermediate and
could be utilized to prepare 4-aminoquinoline
derivative.[21]
Scheme 5. Transformation of 3a into 3-acyl-2H-
indazoles 4a.
Scheme 4. A plausible mechanism for the oxidative
dehydrogenative coupling.
In summary, we discovered a novel approach to the
preparation of unsymmetrical aromatic azoxy
compounds. A series of unsymmetrical azoxy
compounds have been conveniently prepared through
the oxidative coupling reaction of nitrosobenzenes
and aromatic amines mediated by I2/DABCO. As far
as we know, it is the first example for the synthesis of
As shown in Scheme 5, we finally investigated the
synthetic potential of this reaction. Based on the
literature report,[14d] 3a could undergo a region- and
chemoselective [4+1] annulation with diazoester for
5
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10.1002/adsc.201800495
Advanced Synthesis & Catalysis
unsymmetrical azoxy compounds directly from
aromatic amines and nitrosobenzenes. The reaction is
operationally simple and is amenable to large scale,
greatly reducing the difficulty in synthesis of
unsymmetrical azoxy compounds. Further study of
this new route would open a new chapter of research
that brings more reactions to the synthesis and
application of unsymmetrical azoxy compounds.
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General Procedure for the Synthesis of 3
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A solution of 1 (0.2 mmol, 1.0 equiv.), 2 (0.22 mmol,
1.1 equiv.), I2 (0.3 mmol, 1.5 equiv.) and DABCO
(0.6 mmol, 3.0 equiv.) in toulune (1 mL) in a sealed
10 ml Schlenk tube was stirred at 65 oC. The progress
of reaction was monitored by TLC and/or GC-MS.
After completion of reaction, the crude reaction
mixture was cooled to room temperature, and the
solvent was then removed under reduced pressure.
Then, the residue was purified by column
chromatography on silica gel with EtOAc and
petroleum ether as eluent to give 3. The
characterization data of the products are given in the
Supporting Information.
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Typical Procedure for the Synthesis of 4a
Into a dried 100 ml round bottom flask equipped with
a magnetic stir bar, was added [Cp*RhCl2]2 (15.4 mg,
0.025 mmol), AgSbF6 (34.6 mg, 0.1 mmol), 3a (232.0
mg, 1.0 mmol). The vessel was evacuated and
backfilled with N2 before PivOH (114.9μL, 1.0
mmol), dimethyl 2-diazomalonate (237.0 mg, 1.5
mmol), and 1.0 ml of 1,2-dichloroethane were added.
The mixture was heated to 130 oC and stirred
overnight. After completion of reaction, the reaction
mixture was cooled to room temperature, and the
solvent was removed under reduced pressure. Then
the residue was purified by column chromatography
on silica gel (PE/EtOAc = 10/1, v/v) to afford 4a as a
light yellow solid (234.5 mg, 82% yield).
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Acknowledgements
Financial support from the National Natural Science Foundation
of China (Nos. 21302143, and 51572198), Natural Science
Foundation of Zhejiang Province (Nos. LY13B020006, and
LZ17E020002) are greatly appreciated.
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7
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10.1002/adsc.201800495
Advanced Synthesis & Catalysis
UPDATE
Oxidative Coupling of Aromatic Amines and
Nitrosoarenes: Iodine-Mediated Formation of
Unsymmetrical Aromatic Azoxy Compounds
Adv. Synth. Catal. Year, Volume, Page – Page
Xiaochun Yu,a,* Weijie Ding,a Panyu Ge,a Shun
Wang,a,* and Jichang Wangb,*
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