TEMPO catalyzed oxidative dehydrogenation of hydrazobenzenes to azobenzenes

Article abstract of DOI:10.1039/d0ob00103a

A metal-free direct oxidative dehydrogenation approach for the synthesis of azobenzenes from hydrazobenzenes has been developed by using TEMPO as an organocatalyst for the first time. The reaction proceeded in open air under mild reaction conditions. A wide range of hydrazobenzenes readily undergo dehydrogenation to give the corresponding azobenzenes in excellent yields.

Full text of DOI:10.1039/d0ob00103a

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DOI: 10.1039/D0OB00103A
COMMUNICATION
TEMPO Catalyzed Oxidative Dehydrogenation of
Hydrazobenzenes to Azobenzenes
Haiping Lv,b,c,§ Ronibala Devi Laishram,c,§ Yong Yang,*a Jiayan Li,c Dandan Xu,c Yong Zhan,a Yang
Received 00th January 20xx,
Accepted 00th January 20xx
Luo,a Zhimin Su,a Sagar More,c and Baomin Fan*b,c
DOI: 10.1039/x0xx00000x
reaction provided efficient methodology, the use of liquid
ammonia as a medium limited the reaction applicability. Huang
and co-workers recently reported the electrochemical strategy
for the dehydrogenation of hydrazobenzenes to generate
azobenzenes.9 Balaraman and co-workers reported the
dehydrogenation of hydrazobenzenes utilizing [Ru(bpy)3]Cl2 as
photocatalyst and Co(dmgH)2(py)Cl as the proton-reduction
catalyst.10a Recently, our group also reported the
dehydrogenation of hydrazobenzenes harnessing the visible
light using the organic photocatalyst Acr+-Me at ambient
temperature.10b Shortly after we reported, Wu and co-worker
A metal-free direct oxidative dehydrogenation approach for the
synthesis of azobenzenes from hydrazobenzenes has been
developed by using TEMPO as organocatalyst for the first time. The
reaction proceeded in open air under a mild reaction condition. A
wide range of hydrazobenzenes readily undergo the
dehydrogenation to give the corresponding azobenzenes in
excellent yields.
Compounds having N=N double bond are of great interest in
various fields. For instance, azobenzenes have profound
applications in agrochemical, chemical and pharmaceutical
industries as dyes and pigments, indicators, food additives,
radical reaction initiators and therapeutic agents.1 Also,
azobenzenes have potential applications in areas of nonlinear
optics, optical storage media, chemosensors, liquid crystals,
molecular shuttles, nanotubes, and in the manufacture of
protective eye glasses and filters.2 Numerous methods have
been reported for the preparation of azobenzenes such as
oxidative coupling of anilines,3 reductive coupling of aromatic
nitro compounds,4 azo coupling reaction,5 Mills reaction,6 the
Wallach reaction.7 Direct dehydrogenation of hydrazobenzenes
is also one of the indispensable methods for the preparation of
azobenzenes. These dehydrogenations can be achieved
homogenously by using transition metal salts8a-d and
heterogeneously by using rGO-1, RhNPS, Meso-Mn2O3.8e-g
Wang and co-workers reported the dehydrogenation of
hydrazobenzenes using potassium tert-butoxide in liquid
ammonia under air at room temperature (rt).8h Although the
also
developed
visible-light-promoted
oxidative
dehydrogenation of hydrazobenzenes utilizing Eosin Y as a
photocatalyst.10c
Previous works
[Fe], [Ti], [Pd], [Rh], [Mn], [r-GO-1]
Ref 8a-8f
tBuOK, NH3(l), air, rt
Ref 8g
H
Ar1
Ar1
N
N
N
N
Ar2
Ar2
Azobenzenes
H
Pt(+)-Pt(-), 10 mA, NaOAc, Air
Ref 9
Hydrazobenzenes
[Ru(bpy)3]Cl2/ [Co], Acr+-Me/ O2, EY/ O2
Ref 10
This work
Metal-free
Additives free
Mild condition
Environmentally benign
High yields
Ar1
H
Ar1
TEMPO (10 mol%)
EtOH, 60 oC, air
N
N
N
N
Ar2
Ar2
H
15 examples
a. Chongqing Academy of Chinese Materia Medica, Chongqing, 400065, China.
b. School of Chemistry and Environment, Yunnan Minzu University, Kunming,
650500, China.
c. Key Laboratory of Chemistry in Ethnic Medicinal Resources, Yunnan Minzu
University, Kunming, 650500, China.
Scheme 1 Oxidative dehydrogenation of hydrazobenzenes
E-mail addresses: neimen@163.com (Yong Yang), FanBM@ynni.edu.cn (Baomin
Fan).
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
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COMMUNICATION
Journal Name
TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl)oxyl) is stable, temperature was lowered to 20 °C (entry 12). Gratifyingly, the
low weight radical that shows valuable redox behaviour.11 It has highest reaction yield (99%) was obtainDeOdI:w10h.1e0n3V9ti/ehDwe0AOrrtiBecl0ae0cO1tn0iloi3nnAe
been extensively used as a catalyst in the area of polymer temperature was increased to 60 °C (entry 13). Lowering the
science, trapping radical processes, oxidation of alcohols to catalytic loading lowered the yield of reaction as well as slower
carbonyl compounds, dehydrogenation of N-heterocycles, the reaction while increasing the catalytic loading to 20% has no
cyclization reaction.12,13 However, to the best of our knowledge, appreciable effect on the yield of the reaction (entries 14 and
there has been no reports on the application of TEMPO as 15). Thus, we optimized the reaction at 10%. The reaction
catalyst for the direct oxidative dehydrogenation of NH-NH. proceeded smoothly under oxygen atmosphere in 4 h and
These facts about TEMPO inspired us to explore new methods obtained the product in 92% yield (entry 16). This study
for the preparation of azobenzenes through oxidative indicates that the most optimal condition for dehydrogenation
dehydrogenation of hydrazobenzenes using TEMPO as catalyst of hydrazobenzene is by using TEMPO as organocatalyst in EtOH
in open air. Herein, we describe a mild and efficient direct under aerobic condition.
oxidative dehydrogenation of hydrazobenzenes for the first
time utilizing the commercially available, and easy to handle evaluate the scope and the generality of the oxidative
TEMPO as the catalyst. dehydrogenation by subjecting a series of hydrazobenzenes
With the established optimal conditions, we sought to
A preliminary investigative study was carried out using (Table 2). Various functional groups on the aromatic rings
hydrazobenzene 1a as the model substrate under aerobic exerted little influence and were well-tolerated in this TEMPO
condition (Table 1). Pleasingly, our plan to dehydrogenate promoted direct oxidative dehydrogenation. The reaction
hydrazobenzene without metal was achieved by using 10 mol% proceeded in excellent yields with both electron activating and
of TEMPO in acetonitrile (MeCN) at 40 °C affording the desired electron deactivating substituted substrates. Position of the
product 2a in 95% yield (entry 1). In order to maximize the yield substituents also has not much effect on the reactivity of the
of the reaction, we screened different other organocatalysts reaction. Unsymmetrical hydrazobenzenes with electron-
such as 4-Hydroxy-TEMPO, 4-Amino- TEMPO, 4-Oxo-TEMPO, donating substituents such as 4-CH3O, 4-NH2, 4-CH3, 4-tBu, 2-
Pyridine-N-oxide but there was no improvement in the yield of CH3, and 3-CH3 were well-tolerated giving the desired products
the desired product (entries 2-5). Next, we investigated the in excellent yields (2b-2g). Electron-withdrawing halogen
effect of different polar and non-polar solvents on the reactivity substituents such as Cl, Br and I either at 2, 3 or 4-positions as
of the reaction (entries 6-11). Among them ethanol was found well as 4-CF3 or 4-CO2Me group on the phenyl ring were
as the best solvent giving the desired product in 97% yield in 30 smoothly converted to the desired products in excellent yields
h (entry 10). No improvement in the yield was observed when (2g-2n). The symmetrical hydrazobenzenes with different
the reaction
substituents such as CH3, CF3 in the 4-position of the phenyl
rings were well tolerated giving the corresponding products in
excellent yields
Table 1. Optimization of the reaction conditions for the oxidative
dehydrogenation of hydrazobenzenea)
Table 2. Scope of the TEMPO catalyzed oxidative dehydrogenation
of hydrazobenzenesa)
NH
HN
N
Organocatalyst (10 mol%)
Solvent, Temp., air
N
2a
1a
R
NH
HN
R
Entry
Organocatalyst Solvent
Temp
(°C)
40
Time
(h)
30
Yield
(%)b)
95
N
TEMPO (10 mol%)
EtOH, 60 oC, air
R'
N
R'
1a-q
2a-q
1
2
TEMPO
MeCN
MeCN
4-Hydroxy-
TEMPO
40
48
94
3
4-Amino-
TEMPO
MeCN
40
30
88
4
5
4-Oxo-TEMPO
Pyridine-N-
oxide
MeCN
MeCN
40
40
60
72
89
42
6
7
8
9
TEMPO
TEMPO
TEMPO
TEMPO
DCE
40
40
40
40
24
24
24
48
92
91
96
94
Toluene
EtOAc
1,4-
dioxane
EtOH
THF
EtOH
EtOH
EtOH
EtOH
EtOH
10
11
12
13
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
TEMPO
40
40
20
60
60
60
60
30
36
64
12
36
8
97
92
97
99
92
98
92
14c)
15d)
16e)
4
a) Reaction conditions: 1a (0.3 mmol), organocatalyst (10 mol%), solvent (2
mL), Air. b) Isolated yield. c) Organocatalyst loading 5%. d) Organocatalyst
loading 20% . e) Under O2.
2 | J. Name., 2012, 00, 1-3
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Journal Name
COMMUNICATION
OCH3
NH2
View Article Online
DOI: 10.1039/D0OB00103A
N
N
N
N
N
N
N
N
Scheme 3. Control Experiments
2a, 12h, 99% yield
tBu
2b, 12h, 96% yield
2c, 12h, 95% yield
2d, 12h, 98% yield
Cl
Based on the literature14 and experimental results, we
proposed a mechanism for the oxidative dehydrogenation of
hydrazobenzenes (Scheme 4). The reaction was initiated by the
abstraction of a hydrogen atom from hydrazobenzene 1a by the
TEMPO with the generation of intermediate 1a’ and TEMPOH.
The generation of TEMPOH was confirmed by GCMS study (see
ESI). The TEMPOH is readily oxidized by air to regenerate
TEMPO which subsequently abstracts another hydrogen atom
from the intermediate 1a’ to give the final product 2a.
N
N
N
N
N
N
N
CH3
N
CH3
2f, 15h, 97% yield
2h, 15h, 99% yield
2e, 12h, 95% yield
2g, 12h, 99% yield
Br
I
N
N
N
N
N
N
N
N
Br
Br
2k, 15h, 96% yield
2l, 15h, 92% yield
2j, 15h, 93% yield
2i, 15h, 95% yield
O
O
CF3
N
N
N
N
N
N
H2O
O2
2o, 12h, 98% yield
2n, 15h, 93% yield
2m, 12h, 99% yield
TEMPOH
H
N
TEMPO
N
H
tBu
N
N
CF3
N
N
N
N
N
N
1a
2a
N
N
tBu
TEMPOH
H
1a'
TEMPO
[ref. 15]
F3C
2q, 15h, 95% yield
2r, 24h, NR
2p, 15h, 96% yield
a) Reaction conditions: 1 (0.3 mmol), TEMPO (10 mol%), Air, EtOH (2 mL),
O2
H2O
60 °C, Isolated yields.
Scheme 4. Plausible mechanism for the dehydrogenation of
hydrazobenzene.
t
(2o and 2p). Notably, the introduction of bulkier Bu-group at
the 4-position was equally efficient, giving the desired product
in excellent yield 96% (2q). However, the hydrazine derivatives
with aliphatic substituent may not applicable, as no reaction
took place by using 2r.
To further extend the utility of the methodology, we carried
out the scale-up reaction as shown in Scheme 2. This method
can be easily scaled up to 3.68 g of the substrate giving the
desired product in excellent yield.
In summary, we have developed an efficient, metal-free
direct oxidative dehydrogenation for the preparation of
azobenzenes from hydrazobenzenes using TEMPO as
organocatalyst in open air for the first time. This methodology
provides a mild, simple, and environmentally benign approach
for the preparation of various azobenzenes. Furthermore, the
utility of the reaction on a gram scale is also demonstrated.
H
N
TEMPO (10 mol%)
EtOH, 60 oC, air, 18h
N
N
H
N
2a
1a
Conflicts of interest
20 mmol
3.57 g, 98%
There are no conflicts to declare.
Scheme 2. Scalable synthesis of 2a.
As control experiments, we carried out the reaction in Argon
Author Contributions
atmosphere but unfortunately the yield of the desired product § These authors contributed equally.
was very low (10%) even after 72 h of reaction (Scheme 3a).
Next, the reaction was carried out in absence of TEMPO.
However, both of the reactions in the air or oxygen were
Acknowledgements
incomplete and the desired product was furnished only in trace
after 72 h (Scheme 3b). Thus, suggested that both the
components were necessary for the transformation.
We are grateful to the National Natural Science Foundation of
China (21572198), the Chongqing Science & Technology Bureau
(cstc2017zdcy-zdyfx0013), the Applied Basic Research Project
of Yunnan Province (2017FA004, 2018FB021) and Yunnan
Provincial Key Laboratory Construction Plan Funding of
Universities for their financial support.
a)
NH
HN
N
TEMPO (10 mol%)
N
EtOH, 60 oC, Ar, 72 h
2a, Yield = 10%
1a
b)
NH
HN
N
N
No organocatalyst
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EtOH, 60 oC, air or O2, 72 h
2a, Trace
1a
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Organic & Biomolecular Chemistry
View Article Online
DOI: 10.1039/D0OB00103A
TEMPO Catalyzed Oxidative Dehydrogenation of Hydrazobenzenes to
Azobenzenes
Haiping Lv,b,c,§ Ronibala Devi Laishram,c,§ Yong Yang,*a Jiayan Li,c Dandan Xu,c Yong Zhan,a Yang
Luo,a Zhimin Su,a Sagar More,c and Baomin Fan*b,c
Graphical Abstract:
TEMPO=
N
·
O
H
Ar1
cat. TEMPO
EtOH, Air
Ar1
N
* Metal-free
N
N
N
* Additives free
* Mild condition
* Environmently benign
* High yields
Ar2
H
Ar2
* 15 examples