Reduced graphene oxide as a recyclable catalyst for dehydrogenation of hydrazo compounds

Article abstract of DOI:10.1016/j.tetlet.2014.06.097

Reduced graphene oxide served as a reusable and efficient carbocatalyst for aerobic oxidative dehydrogenation of hydrazo compounds. Azo compounds were obtained in high yields under mild reaction conditions.

Full text of DOI:10.1016/j.tetlet.2014.06.097

Tetrahedron Letters xxx (2014) xxx–xxx
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Tetrahedron Letters
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Reduced graphene oxide as a recyclable catalyst for dehydrogenation
of hydrazo compounds
Li-Sha Bai a, Xiao-Min Gao a, Xuan Zhang a, Fei-Fei Sun a,b, Ning Ma a,b,
⇑
a Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China
b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Reduced graphene oxide served as
a
reusable and efficient carbocatalyst for aerobic oxidative
Received 25 March 2014
Revised 15 June 2014
Accepted 23 June 2014
Available online xxxx
dehydrogenation of hydrazo compounds. Azo compounds were obtained in high yields under mild
reaction conditions.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Reduced graphene oxide
Carbocatalyst
Hydrazo compounds
Azo compounds
Aerobic oxidation
Azo compounds have found important applications in industry
and laboratory as organic dyes, pigments, indicators, food addi-
tives, radical initiators, and therapeutic agents.1 The well known
synthetic routes toward these compounds include azo coupling,
Wallach rearrangement of azoxy compounds, coupling of primary
arylamines with aromatic nitroso and nitro compounds, and reduc-
tive coupling of nitro aromatics.2 Besides, azo derivatives have
been obtained by oxidative dehydrogenation of N,N0-disubstituted
hydrazines. The general methods to achieve this transformation
involve the use of stoichiometric or excess oxidants, such as
Pb(CH3CO2)4, HgO, FeCl3, N-bromosuccinimide in pyridine, and
NaNO2 in acetic anhydride.3 However, these oxidative systems
often generated wastes which were hard to recycle.
Catalytic oxidative dehydrogenations of arylhydrazines to azo
compounds have been reported. Utilizing oxygen or H2O2 as clean
oxidants, NH4VO3, CuCl2, Co(II) complexes, and TiCl3/HBr system
were able to catalyze this transformation.4a–c Also, FeSO4 acted as
a catalyst using a mixture of KClO3 and H2SO4 as oxidative system.4d
Recently, Gozin et al. reported that TiCl3/HBr system was a more
efficient catalytic system than NH4VO3 with H2O2 as oxidant.5
However, some drawbacks, such as toxicity and non-reusability of
the catalyst as well as potential benzidine rearrangement which
probably takes place under acid catalysis and leads to undesired
suspect carcinogenic products, may exist in the above reactions.
Graphene (GP) and related carbon materials have attracted
recurring attention in the past decade.6 As a type of chemically pre-
pared graphene, reduced graphene oxide (rGO) is obtained by
reducing exfoliated graphite oxide (GO), which is the strongly oxi-
dized product of graphite. The reducing agents involved in rGO
preparation include hydrazine, H2, NaBH4, NaHSO3, HI, etc. Among
them, hydrazine is most commonly used.7 Because of easy prepa-
ration, GO- and rGO-based carbon materials have been intensively
investigated as robust and reusable catalysts.8 GO showed catalytic
activities to activate oxygen in dehydrogenation of alcohols to car-
bonyl compounds,9a oxidative coupling of amines to imines,9b,c and
dehydrogenative aromatization of saturated nitrogen-containing
heterocycles.9d Also, it has been reported as an acid catalyst in a
variety of organic transformations by others and us.10 In compari-
son to GO, rGO catalysis in organic conversions has been seldom
investigated. Ma and co-workers disclosed the catalytic activity
of rGO in reduction of nitrobenzene.11a Oxidative degradation of
phenols and methylene blue in water by peroxymonosulfate has
been described.11b More recently, we demonstrated that rGO could
efficiently catalyze the synthesis of bis(aminothiocarbonyl)disul-
fides from secondary amines and CS2 through aerobic oxidation.11c
Herein, we propose a new rGO-catalyzed oxidative dehydrogena-
tion of hydrazo compounds to their corresponding azo compounds.
In order to optimize the reaction conditions, we chose the
oxidative dehydrogenation of hydrazobenzene (1a) as the model
reaction. The employed rGO (rGO-1) was prepared by treating
GO with hydrazine hydrate, which is the most common method
⇑
Corresponding author. Tel.: +86 22 27403475; fax: +86 22 27403475.
E-mail address: mntju@tju.edu.cn (N. Ma).
http://dx.doi.org/10.1016/j.tetlet.2014.06.097
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Bai, L.-S.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.06.097

2
L.-S. Bai et al. / Tetrahedron Letters xxx (2014) xxx–xxx
to prepare rGO. It was characterized by FT-IR, XRD, XPS, TG, and
TEM. FT-IR spectra of GO and rGO-1 are shown in Figure 1. For
GO, the following characteristic peaks were observed: 3347 cmꢀ1
for hydroxyl OAH, 1726 cmꢀ1 for carbonyl C@O, 1619 cmꢀ1 for
aromatic C@C, 1411 cmꢀ1 for carboxyl CAO, 1223 cmꢀ1 for epoxy
CAO, and 1053 cmꢀ1 for CAO. This indicates that GO contains
abundant oxygen-containing functional groups. After reduction
by hydrazine hydrate, the peaks associated to oxygen-functional
groups decreased significantly, and the peak at 1726 cmꢀ1 for
carbonyl C@O totally disappeared. The peak at 1619 cmꢀ1 moved
to 1555 cmꢀ1, indicating formation of aromatic C = C groups.
Figure 2 shows the X-ray diffraction (XRD) patterns of GO and
rGO-1. For GO, a sharp diffraction peak displays at 11.8°, corre-
sponding to an interlayer d-spacing of 0.749 nm as the oxygen-
functional groups located on the graphene sheet. After being
reduced by hydrazine hydrate, no sharp peaks were observed, indi-
cating that most of the oxygen-containing groups were removed
during the reduction process. XPS analysis showed that rGO-1
had a C/O ratio of 7.0, which was much greater than that of GO
(1.2). This also supported the removal of most of the oxygen-con-
taining groups.
Figure 2. XRD patterns of GO, (A) pristine rGO-1, (B) rGO-1 after being used one
time, (C) and rGO-1 after being used five times (D).
TG analysis of rGO-1 is shown in Supplementary data. There
was no apparent mass loss at around 200 °C, indicating that little
amount of oxygen-functional groups remained after reduction of
GO. The morphology of rGO-1 was observed by TEM (see Supple-
mentary data). TEM image showed large aggregated or spread
graphene sheets with numerous wrinkles.
transformation (entry 18, Table 1). The reaction was sluggish in
the absence of rGO-1 (entry 19, Table 1). Thus, the optimum reac-
tion conditions were found as 10 wt% of rGO-1, 30 °C, EtOH as sol-
vent, and under open-air.
The dehydrogenations of various hydrazo compounds (1b–k)
were examined and the results are summarized in Table 2. For
the substrates having electron-donating groups, the reactions took
place readily to give the corresponding azo products in good to
excellent yields (entries 1–2, 4–8). Changing rGO loading to
5 wt % and 2 wt % led to extension of the reaction time and
decrease of the yields (see Supplementary data, Table 1S). Sym-
metric substrates in the reaction were prepared by the reductive
coupling of the corresponding nitro compounds. Among them,
3,30-dimethyl hydrazobenzene was difficult to isolate, so its etha-
nol solution was directly used without purification and the yield
of 2c was based on m-nitrotoluene. In the cases of hydrazo com-
pounds having an electron-withdrawing group, the oxidative
dehydrogenation proceeded more slowly (entries 9 and 10). Exten-
sion of reaction time did not result in the increase of yields. Chang-
ing rGO loading to 5 wt% resulted in a poor yield of 1i even for a
long reaction time under reflux (see Supplementary data, Table 1S).
After dehydrogenation reaction of hydrazobenzene (1a), rGO-1
was recovered by filtering the solid out, washing thoroughly with
ethanol, and drying. Then it was reused in the reaction. At sixth
run, no obvious loss of catalytic activity was observed (Table 3).
There was no significant change in IR spectra (Figure 1) and TG
curves (see Supplementary data) of rGOs before first run, after first
run, and after fifth run.
As reaction temperature was raised from 10 °C to 30 °C, the
reaction time was reduced and the yield of azobenzene (2a)
increased (entries 1–3, Table 1). However, when the temperature
changed from 30 °C to 40 °C, the yield of 2a decreased (entry 4,
Table 1). Furthermore, the amount of catalyst was changed. The
yield of 2a reached to 97% satisfactorily when the loading of
rGO-1 was 10 wt % (entries 5–8, Table 1). Subsequently, different
solvents were screened (entries 9–13, Table 1). The results demon-
strated that ethanol, acetone, and acetonitrile could serve as the
solvent but ethanol was the best choice. Meanwhile, the reaction
was carried out under nitrogen atmosphere. The yield of 2a was
fairly poor, which indicated that oxygen in air could be the final
oxidant (entry 14, Table 1). In addition, other rGOs were prepared
in the presence of NaBH4, NaHSO3, and hot water, which were
marked as rGO-2, rGO-3, and rGO-4, respectively. Their catalytic
activities were much lower than that of rGO-1 (entries 15–17,
Table 1). While graphite power was not able to catalyze this
At this stage it is hard to propose a detailed mechanism of this
reaction. Recently, Loh et al. reported that unpaired electrons in the
holes and at the edges of ba-GO sheets could activate O2 to form
O2Åꢀ and led to aerobic oxidative coupling of benzylamines.9c The
ba-GO in their research was prepared by treatment of GO with base
and acid sequentially. Without regard to the carboxyls at the edges,
the structure of basal plane of ba-GO was very similar with that of
rGO.12 Enlightened by their mechanism study, we envisage that
rGO might activate O2 to form O2Åꢀ similarly, then hydrazo
compound 1 was oxidized by O2Åꢀ to produce azo compound 2
and H2O2 (Scheme 1). Meanwhile, an electron was released and
transferred back to rGO, then rGO structure with unpaired
electrons was recovered. However, pristine GO has less unpaired
electrons at the edges of its sheets,9c so it showed lower catalytic
activity than rGO (entries 3 vs 18, Table 1).
Figure 1. FT-IR of pristine rGO-1, (A) rGO-1 after being used one time, (B) rGO-1
after being used five times, (C) and GO (D).
Please cite this article in press as: Bai, L.-S.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.06.097

L.-S. Bai et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Table 1
Dehydrogenation of hydrazobenzenea
H
N
N
N
H
N
2a
1a
Entry
Catalyst
Catalyst loading (wt %)
T (°C)
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
rGO-1
rGO-1
rGO-1
rGO-1
rGO-1
rGO-1
rGO-1
rGO-1
rGO-1
rGO-1
rGO-1
rGO-1
rGO-1
rGO-1
rGO-2
rGO-3
rGO-4
GO
10
10
10
10
1
10
20
30
40
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
30
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
CH3COCH3
CH3CN
THF
MeOH
CH2Cl2
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
8
95
92
97
93
65
92
93
98
95
96
94
86
83
21c
75
78
77
51
13
13
2.5
2.5
2.5
5
10
5.5
3
2
5
10
10
10
10
10
10
10
10
10
10
10
10
4
4
10
10
10
5
10
10
10
10
10
10
Graphite
None
a
b
c
Reacted in an open vessel.
Isolated yield.
Under N2 atmosphere.
Table 2
Dehydrogenation of various hydrazo compoundsa
H
N
R
R
N
N
rGO-1 (10 wt%)
N
H
H2O2
°
30 C, EtOH
R'
R'
e−
1
2
Ar
N
N
Ar
Entry
Substrate (1)
Time (h)
Product (2)
Yieldb (%)
2
1
2
3
4
5
6
7
8
1a (R = R0 = H)
3
3
3
2
2.5
2
2.5
3.5
6
2a
2b
2c
2d
2e
2f
2g
2h
2i
98
97
69c
95
92
90
89
89
60d
81
98
1b (R = R0 = o-Me)
1c (R = R0 = m-Me)
1d (R = R0 = p-Me)
H
N
O2
O2
Ar
N
H
Ar
1
1e (R = p-OMe, R0 = H)
1f (R = p-OCH2Ph, R0 = H)
1g (R = p-OEt, R0 = p-Me)
1h (R = p-OCH2Ph, R0 = p-Me)
1i (R = R0 = o-Cl)
1j (R = R0 = p-Cl)
1k (R = p-OCH2Ph, R0 = p-Cl)
Scheme 1. Possible catalytic function of rGO.
9
10
11
In conclusion, we have found an efficient and facile method to
4.5
2.5
2j
2k
prepare aromatic azo compounds from their corresponding
hydrazo compounds under mild conditions. As an easily available
carbocatalyst involved in this oxidative dehydrogenation, rGO
can be easily recovered and reused at least five times without
any loss of catalytic activity. Another feature of this protocol is that
air acts as the oxidant. Further investigations on reaction
mechanism and other rGO-catalyzed dehydrogenative reactions
are ongoing in our laboratory.
a
b
c
Reacted in an open vessel.
Isolated yield.
Yield is based on m-nitrotoluene.
Reflux for 6 h.
d
Supplementary data
Table 3
Reuse of rGO
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.06.
097.
H
N
rGO-1 (10 wt%)
N
N
N
H
30 °C, EtOH, 5 h
open-air
2a
1a
References and notes
Run
1
2
3
4
5
6
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Isolated yield (%)
98
97
98
92
99
95
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Please cite this article in press as: Bai, L.-S.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.06.097