Controllable synthesis of azoxybenzenes and anilines with alcohol as the reducing agent promoted by KOH

Article abstract of DOI:10.1080/00397911.2019.1566472

Nitrobenzene and its derivatives can be selectively reduced to the corresponding azoxybenzene and aniline compounds with alcohols as the hydrogen source and KOH as the promoter only by simple changes of reaction conditions.

Full text of DOI:10.1080/00397911.2019.1566472

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Controllable synthesis of azoxybenzenes and
anilines with alcohol as the reducing agent
promoted by KOH
Rui Ping Wei & Feng Shi
To cite this article: Rui Ping Wei & Feng Shi (2019): Controllable synthesis of azoxybenzenes and
anilines with alcohol as the reducing agent promoted by KOH, Synthetic Communications
To link to this article: https://doi.org/10.1080/00397911.2019.1566472
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2019.1566472
Controllable synthesis of azoxybenzenes and anilines with
alcohol as the reducing agent promoted by KOH
Rui Ping Weia,b and Feng Shia
aState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
b
Chinese Academy of Sciences, Lanzhou, China; University of Chinese Academy of Sciences,
Beijing, China
ABSTRACT
ARTICLE HISTORY
Received 25 May 2018
Nitrobenzene and its derivatives can be selectively reduced to the
corresponding azoxybenzene and aniline compounds with alcohols
as the hydrogen source and KOH as the promoter only by simple
changes of reaction conditions.
KEYWORDS
Alcohols; aniline;
azoxybenzene; nitroarenes;
selective reduction
GRAPHICAL ABSTRACT
Introduction
The reductive transformation of the functional groups such as NO2 is one of the most
important transformations in synthetic organic chemistry.[1] A lot of high-value chemi-
cals, such as azoxybenzenes, azobenzenes, diphenylhydrazines, anilines, and indoles,[2–5]
which are widely used in industry as dyes, pigments, food additives, and drugs, can be
obtained via the selective reduction of nitrobenzenes with appropriate reducing agents.
Over the past few decades, various transition metals, such as Pd,[6,7] In,[8] Pt,[9] Au,[10]
Cu,[11] Mo[12] and reducing agents such as zerovalent metals,[13–19] H2,[6,9,20] CO/
H2O,[10] HCOOH,[12] NH2NH2,[21,22] NaBH4,[23–25] hydrosilane,[8] formates,[11] and ole-
fins,[7] have been employed in the reduction of nitrobenzene sequentially and have
CONTACT Feng Shi
fshi@licp.cas.cn
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, No. 18, Lanzhou, China.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

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R. P.WEI AND F. SHI
TABLE 1. Optimization of reaction conditions for azoxybenzene synthesisa.
O
N
NO2
NH2
N
N
O
N
N
+
+
+
a
b
c
d
Yields(%)b
Entry
Solvent
Base
KOH
KOH
KOH
KOH
Hydrogen source
Conversion (%)b
a
b
c
d
1c
Acetonitrile
1,4-dioxane
Toluene
Cyclohexane
Octane
Octane
Octane
Octane
Octane
Octane
Octane
Octane
–
Octane
Octane
Octane
Octane
Octane
Octane
1-propanol
1-propanol
1-propanol
1-propanol
1-propanol
1-propanol
1-propanol
1-propanol
1-propanol
1-propanol
1-propanol
1-propanol
1-propanol
Methanol
42
56
65
88
91
100
65
0
5
0
0
0
0
0
7
5
0
0
2
6
4
6
5
12
0
0
0
7
0
0
15
0
3
14
26
19
0
0
0
1
0
1
1
1
0
0
2
1
2
1
0
0
1
0
0
0
2c
28
28
33
42
17
22
0
3c
4c
5c
KOH
6
7
8
9
t-BuOK
NaOH
Na2CO3
K2CO3
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
0
0
10
11d
12e
13f
14
15
16
17
18
19
100
96
99
74
56
99
100
90
98
22
74
77
77
19
21
70
63
29
51
0
0
10
11
18
0
0
0
0
0
0
Ethanol
1-butanol
2-propanol
2-butanol
Tert-butanol
aReaction conditions: Nitrobenzene (1 mmol), hydrogen source (1 mmol), base (2 mmol), solvent (2 mL), 50 ꢀC, 24 h, Ar.
bConversion and yields were determined by GC-FID with biphenyl as an external standard.
c10 h.
d2 mmol 1-propanol.
e3 mmol 1-propanol.
f2 mL 1-propanol as the solvent.
The bold values is to emphasize that this reaction condition is the optimal condition for the synthesis of azoxybenzene.
yielded good results. Besides, recently, some photo- and electro-catalysis systems for the
reduction of nitrobenzene have been reported.[26–31] When compared with these trad-
itional hydrogenation reactions, catalytic transfer hydrogenation using cheap and easily
available alcohols as the hydrogen source under moderate conditions has obvious
advantages with respect to selectivity, economy, safety, and pro-environment, accord-
ingly it has been adopted as a clean process for the reduction of nitro functional group.
Various different alcohols, such as secondary[32,33] and primary alcohols[34,35] and bio-
mass-based polyols,[36,37] have been used as good reducing agents in the reduction of
nitrobenzenes. Despite the rapid progress in catalytic transfer hydrogenation, only one
reductive product is selectively obtained in most cases. The selective transformation of
nitrobenzene into more than one reductive product using a single reducing system has
been still a challenge. Encouragingly, an example of the selective transformation of
nitrobenzene into more than one reductive product has been recently reported.[34] In
the presence of Ru/C and equivalent base additives (KOH or NaOH), nitrobenzenes
were selectively reduced into azoxybenzenes, azobenzenes, and anilines with ethanol as
a hydrogen source. However, the use of noble metal and excess of base additives are
undesirable from a view of industrial application. Herein, we proved that nitrobenzenes
can be selectively reduced to azoxybenzene and anilines compounds with alcohols as
the hydrogen source and KOH as the promoter.

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3
Results and discussion
In order to explore the optimum reaction conditions, the reduction of nitrobenzene to
azoxybenzene was used as a model reaction, and the effects of solvents, bases, and
hydrogen sources were investigated under argon atmosphere. As shown in Table 1, in
the case of 1-propanol as the hydrogen source and KOH as the promoter, the conver-
sion of nitrobenzene gradually increased along with the enhancement of the solvent
polarity (Entries 1–5). Among different solvents tested, octane proved to be the suitable
reaction medium for the synthesis of azoxybenzene and 42% yield was obtained (Entry
5). Lower yields were provided in the case of other solvents (Entries 1–4). In this cata-
lyst-free system, the importance of the base as a promoter is undoubted. Therefore, a
series of common base additives such as KOH, NaOH, t-BuOK, Na2CO3, and K2CO3
were tested and compared (Entries 6–10). Clearly, KOH as the promoter exhibited the
best catalytic performance in the synthesis of azoxybenzene and 74% yield was gained
at 50 ꢀC after 24 h (Entry 10). Much lower yields were obtained by changing the base to
t-BuOK and NaOH (Entries 6–7). It is noteworthy that in the case of Na2CO3 and
K2CO3, no target product was observed (Entries 8–9), which could be attributed to their
weak alkalinity. All these results implied that the choice of proper base is important in
this reaction. Further, the amount of 1-propanol as the hydrogen source was also opti-
mized. A slight increase in the yield of azoxybenzene could be observed if 2 equiv., 1-
propanol was applied (Entry 11). However, no further increase was gained in the case
of 3 equiv. of 1-propanol (Entry 12). A sharp decrease in the yield of azoxybenzene was
also observed when directly employing 1-propanol as the reaction medium (Entry 13).
Besides, the effects of alcohols as the hydrogen source were also explored. In the pres-
ence of KOH as the promoter and octane as a solvent, various alcohols including pri-
mary alcohols (methanol, ethanol, 1-propanol, and 1-butanol), secondary alcohols (2-
propanol and 2-butanol), and tertiary alcohols (tert-butanol) were applied in the select-
ive reduction of nitrobenzene to azoxybenzene. The results showed that 1-propanol
gave the highest yield of azoxybenzene (Entry 10), followed by ethanol, 1-butanol, 2-
butanol, 2-propanol, and methanol (Entries 14–18). As expected, no target product was
generated in the presence of tert-butanol (Entry 19). In all the above catalytic trans-
formation, nitrosobenzene as a by-product was generated in a small quantity. By com-
paring the conversion and yields of all the observed products, it is obvious that the
poor balance was obtained in some cases, which implied the formation of some
unknown by-products that could not be detected by GC-MS and GC-FID. From the
above, the optimal reaction condition for the synthesis of azoxybenzene is as follows:
nitrobenzene (1 mmol), 1-propanol (1 mmol), KOH (2 mmol), octane (2 mL), 50 ꢀC,
24 h, Ar. It should be highlighted that only equimolar alcohol as the hydrogen source is
required in this selective reduction of nitrobenzene into azoxybenzenes, which means
propionaldehyde should not be the final oxidation product because more than 1 mmol
H2 was obtained from 1-propanol as the hydrogen source. A reasonable speculation is
that propionaldehyde can further provide hydrogen in the reaction system. In order to
verify this speculation, the control experiment in which propionaldehyde is used as a
hydrogen source to reduce nitrobenzene was performed. As a result, the reduction of
products including azoxybenzene and aniline are observed, and the yields were respect-
ively 26% and 6% when 1 mmol propionaldehyde was used as the hydrogen source.

4
R. P.WEI AND F. SHI
Table 2. Selective reduction of nitrobenzenes to the corresponding azoxybenzenea.
Entry Substrates
Products
Conversion(%)b Yields (%)c
NO2
O
N
1
100
100
100
72
75
75
N
O
N
NO2
2
3
N
N
O
N
NO2
Cl
NO2
O
N
4
100
100
100
56
70
55
N
N
N
Cl
Cl
Cl
Cl
O
N
NO2
NO2
5
Cl
Br
O
N
6d
Br
Br
aReaction conditions: Nitrobenzene (1 mmol), 1-propanol (1 mmol), KOH (2 mmol), octane (2 mL), 50 ꢀC, 24 h, Ar.
bConversion was determined by GC-FID with biphenyl as an external standard.
cIsolated yield.
dThe mixture of 1 mL dichloromethane and 1 mL octane as the reaction solvent.
It means that propionaldehyde can be used as the hydrogen source to further provide
hydrogen in the reaction system. In order to explore the final oxidation product from
1
1-propanol, the reaction mixture after the reaction was determined by H NMR and
obvious signal peaks at 9.7 and 12.0 ppm could be observed, which implied the forma-
tion of propionaldehyde and propanoic acid. Therefore, the final oxidation products
from 1-propanol consist of propionaldehyde and propanoic acid.
Under the optimum conditions, a series of substituted nitrobenzenes are converted to
the corresponding azoxybenzenes, and the results are shown in Table 2. Clearly, nitro-
benzene and its derivatives with a methyl group at different positions are well-tolerated
and good yields (72–75%) of the desired azoxybenzenes are obtained (Entries 1–3),
which implies that the position of electron-donating groups has little effect on
the reactivity. However, a lower yield was obtained when nitrobenzene was substituted
with Cl at the 4 position (Entry 4), which may be attributed to the weak
electron-withdrawing property. Interestingly, the adverse effect could be alleviated if the
substitution of Cl group is at the 3 position of nitrobenzene (Entry 5). Besides, the
dehalogenation phenomenon was observed when nitrobenzene with Br group at the 4
position was used as the substrate, which led to a lower yield of the target product
(Entry 6).

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5
Table 3. Optimization of reaction conditions for aniline synthesisa.
Yields(%)b
Conversion
Entry
KOH
T (ꢀC)
(%)b
a
b
c
d
1
2
3
4
5
6
3 mmol
3 mmol
3 mmol
2 mmol
4 mmol
5 mmol
80
100
120
100
100
100
100
21
0
0
4
0
5
9
0
13
5
4
41
67
67
60
67
66
1
0
0
1
0
0
100
100
100
100
100
0
aReaction conditions: Nitroarenes (1 mmol), 2-propanol (2 mL), Ar, 24 h.
bConversion and yields were determined by GC-FID with biphenyl as an external standard material.
During the investigation on the influence of different alcohols as hydrogen sources in
the synthesis of azoxybenzene, it is found that the considerable aniline product was gen-
erated when 2-propanol was used as the hydrogen source, which means that it will be
possible to obtain the desired aniline product with 2-propanol as the hydrogen source
by further optimizing the reaction conditions. Considering the aniline as the finally
reductive product of nitrobenzene, more hydrogen is undoubtedly necessary for the
reductive transformation of nitrobenzene to aniline. Therefore, 2-propanol (2 mL) was
directly applied as the reaction medium as well as a hydrogen source. Accordingly, the
reaction temperature and the amount of KOH were further optimized. As shown in
Table 3, 41% yield of aniline was obtained when the reduction reaction was performed
at 80 ꢀC in the presence of 3 mmol KOH (Entry 1). Gratifyingly, a better result was
obtained by increasing the reaction temperature to 100 ꢀC, and nitrobenzene was select-
ively converted into aniline in 67% yield (Entry 2). However, further increase of the
reaction temperature did not lead to a higher yield of aniline (Entry 3). A slight drop in
the yield of aniline could be observed if the amount of KOH was reduced to 2 mmol
(Entry 4). The yield of aniline could not be increased even if more KOH was introduced
(Entries 5 and 6). So, the optimal reaction condition for the synthesis of aniline is as
follows: 1 mmol nitrobenzene, 3 mmol KOH, 2 mL 2-propanol, 100 ꢀC, 24 h, Ar.
With the optimum conditions in hand, the scope and generality of this catalyst-free
system were explored subsequently. As shown in Table 4, nitrobenzene could be select-
ively reduced to aniline in moderate yield (67%) under the optimized reaction condi-
tions (Entry 1). Almost similar result was obtained when 1-methyl-3-nitrobenzene was
used as the starting material (Entry 3). Interestingly, up to 92% yield of the target prod-
uct was obtained if the methyl group was at the vicinal position of the nitro group
(Entry 2), which may be ascribed to the better electron-donating property. However, an
opposite phenomenon was observed when the methyl group was changed into an elec-
tron-withdrawing Cl group. Only 41% yield of 2-chloroaniline was obtained (Entry 4),
which is much lower than 81% yield of 3-chloroaniline (Entry 5). This can be explained
by the stronger electron-withdrawing property and steric effect of the vicinal Cl sub-
stituent. The dehalogenation phenomenon of the halogenated nitrobenzene, especially
Br substituted one, often occurs in the reductive transformation of nitrobenzenes, which
leads to a poor yield of the desired product. Gratifyingly, the dehalogenation problem
does not seem to occur in our system and good yield (85%) of 4-bromoaniline
was obtained (Entry 6). Besides, the more sterically hindered substrate such as

6
R. P.WEI AND F. SHI
Table 4. Selective reduction of nitroarenes to the corresponding aromatic aminesa.
Entry Substrates
Products
Conversion(%)b Yields (%)b
NO2
NH2
1
100
100
67
92
NH2
NO2
2
NH2
NO2
3
4
5
100
100
100
68
41
81
NH2
Cl
NO2
Cl
Cl
NO2
Cl
NH2
aReaction conditions: Nitroarenes (1 mmol), 2-propanol (2 mL), KOH (3 mmol), 100 ꢀC, 24 h, Ar.
bConversion and yields were determined by GC-FID with biphenyl as an external standard material.
1,3-dimethyl-2-nitrobenzene was also explored and 56% yield of 2,6-dimethylaniline was
provided (Entry 7).
Conclusion
In summary, a catalyst-free system for the reductive transformation of nitrobenzenes to
more than one product was developed for the first time. A series of nitrobenzene and
its derivatives can be selectively converted into the corresponding azoxybenzene and
aniline products only by changing alcohols and reaction temperature. This work pro-
vides a practical and simple method for the controllable synthesis of valuable azoxyben-
zene and aniline.
Experiment section
General
All commercially available compounds and solvents were purchased and used as
received, unless otherwise noted. NMR spectra were measured using a Varian NMR sys-
tem at 400 MHz (1H) and 101 MHz (13C). All spectra were recorded in CDCl3 and
chemical shifts (d) were reported in p.p.m. relative to tetramethylsilane referenced to
R
V
the residual solvent peaks. Melting points were determined on a SGW X-4 optical
micro melting point instrument.

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7
Typical procedure for the synthesis of azoxybenzene: nitrobenzene (1 mmol), 1-pro-
panol (1 mmol), KOH (2 mmol), and octane (2 mL) were added to a pressure tube (38 mL)
equipped with a magnetic stirrer. Then, the pressure tube was exchanged with argon and
kept at 50 ꢀC for 24 h. After the reaction, the reaction mixture was cooled to RT, and then
biphenyl (30 mg) and 1,4-dioxane (7 mL) were added for quantitative analysis by GC-FID.
Typical procedure for the separation and purification of azoxybenzene: After the
reaction, petroleum ether (b.p. 30–60 ꢀC, 20 mL) was added to the reaction mixture,
then the reaction mixture was filtered using diatomite, and the filtrate was collected.
The filtrate was washed with dilute HCl (3 M) three times and dried with anhydrous
MgSO4. Finally, the pure product was obtained after removing the organic solvent by a
1
rotary evaporator and vacuum drying, and characterized by H and 13C NMR.
Typical procedure for the synthesis of aniline: nitrobenzene (1 mmol), KOH
(3 mmol), and 2-propanol (2 mL) were added to a pressure tube (38 mL) equipped with
a magnetic stirrer. Then, the pressure tube was exchanged with argon and kept at
100 ꢀC for 24 h. After the reaction, the reaction mixture was cooled to RT, and then
biphenyl (30 mg) and ethanol (7 mL) were added for quantitative analysis by GC-FID.
Azoxybenzene: m.p. 35–36 ꢀC (lit.[38] 35–36 ꢀC). 1H NMR (400 MHz, CDCl3) d
8.35–8.27 (m, 2H), 8.20–8.14 (m, 2H), 7.59–7.46 (m, 5H), 7.43–7.36 (m, 1H). 13C NMR
(101 MHz, CDCl3) d 148.32, 143.97, 131.57, 129.59, 128.73, 125.50, 122.32.
1
m,m0-Dimethylazoxybenzene: m.p. 33–34 ꢀC (lit.[38] 35–37 ꢀC). H NMR (400 MHz,
CDCl3) d 8.15–8.06 (m, 2H), 7.98 (t, 2H), 7.48–7.30 (m, 3H), 7.22 (dd, 1H), 2.45 (d,
6H). 13C NMR (101 MHz, CDCl3) d 148.40, 144.02, 138.96, 138.43, 132.26, 130.34,
128.52, 125.99, 122.76, 122.48), 119.49, 21.42.
p,p0-Dimethylazoxybenzene: m.p. 67–68 ꢀC (lit.[39] 66–68 ꢀC). 1H NMR (400 MHz,
CDCl3) d 8.18 (d, 2H), 8.12 (d, 2H), 7.28 (d, 4H), 2.43 (s, 3H), 2.41 (s, 3H). 13C NMR
(101 MHz, CDCl3) d 146.19, 141.86, 139.99, 129.27, 125.63, 123.76, 122.11, 21.53, 21.28.
1
m,m0-Dichloroazoxybenzene: m.p. 97–98 ꢀC (lit.[38] 97–98 ꢀC). H NMR (400 MHz,
CDCl3) d 8.31 (t, 1H), 8.26 (t, 1H), 8.20 (ddd, 1H), 8.00 (dt, 1H), 7.55 (ddd, 1H),
7.49–7.36 (m, 3H). 13C NMR (101 MHz, CDCl3) d 148.81, 144.51, 134.81, 134.39,
132.00, 130, 125.40, 124.05, 122.83, 120.60.
p,p0-Dichloroazoxybenzene: m.p. 155–156 ꢀC (lit.[38] 154–156 ꢀC). 1H NMR
(400 MHz, CDCl3) d 8.24 (d, 2H), 8.15 (d, 2H), 7.45 (dd, 4H). 13C NMR (101 MHz,
CDCl3) d 146.56, 142.22, 138.08, 135.25, 128.99, 127.05, 123.70.
p,p0-Dibromoazoxybenzene: m.p. 173–174 ꢀC (lit.[38] 171–173 ꢀC). 1H NMR
(400 MHz, CDCl3) d 8.18 (d, 1H), 8.16 (d, 1H), 8.08 (d, 1H), 8.07 (d, 1H), 7.64 (d, 1H),
7.62 (t, 1H), 7.61 (d, 1H), 7.59 (d, 1H). 13C NMR (101 MHz, CDCl3) d 147.01, 142.55,
131.98, 127.21, 126.45, 123.87, 123.59.
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