Electrochemical dehydrogenation of hydrazines to azo compounds

Article abstract of DOI:10.1039/c9gc00515c

A strategy for the electrochemical dehydrogenation of hydrazine compounds is disclosed under ambient conditions. This protocol proceeded smoothly in ethanol by employing electrons as clean oxidants. Its synthetic value is well demonstrated by the highly efficient synthesis of symmetric and unsymmetric azo compounds. It is an environmentally friendly transformation and the present protocol was effective on a large scale.

Full text of DOI:10.1039/c9gc00515c

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Electrochemical Dehydrogenation of Hydrazines to Azo
Compounds
Ke-Si Dua and Jing-Mei Huanga,*
Received 00th January 20xx,
Accepted 00th January 20xx
A strategy for the electrochemical dehydrogenation of hydrazine compounds is disclosed under ambient conditions. This
protocol proceeded smoothly in ethanol by employing electrons as clean oxidants. Its synthetic value is well demonstrated
by the highly efficient synthesis of symmetric and unsymmetric azo compounds. It is an environment-friendly transforma-
tion and the present protocol was effective on a larger scale.
DOI: 10.1039/x0xx00000x
www.rsc.org/
electrochemical strategy for the dehydrogenation of a NH-NH bond
to construct a N=N double bond.
Introduction
Azo compounds have attracted wide attention due to their
extensive applications as food additives, therapeutic agents, organic
dyes, radical reaction initiators, indicators and photochemical
switches.1 As a consequence, various methods for the preparation
of azo compounds have been developed over the past few decades,
including the oxidative coupling of anilines,2 Mills reaction,3
electrophilic reactions of diazonium salts4 and reduction of nitro
aromatics.5 In addition, the direct dehydrogenation of hydrazo
derivatives has demonstrated to be an indispensable method for
the preparation of azo compounds, which largely depends on
stoichiometric reagents, for examples, K3Fe(CN)6, CAN, n-Bu4NIO4,
MnO2, NaNO2/NaHSO4, KClO3/H2SO4, HgO and KMnO4.6 Some other
reported protocols, which employed molecular oxygen or H2O2 as
oxidants are appealing in terms of green and sustainable chemistry.
However, catalysts or additives, such as CuCl,7 cumene,8 Fe(Pc),9 t-
BuOK/NH3(l),10 TiCl3/HBr,11 a carbon nanotube-rhodium system,12
Results and discussion
We initially chose hydroazobenzene 1a as the model substrate to
find out the optimized conditions. To our delight, 1a reacted
efficiently to afford the desired product 2a in 97% yield at 10 mA in
Table 1 Optimization of reaction conditionsa
H
Pt (+) - Pt (-): I = 10 mA
3.5 h, rt, undivided cell
N
N
N
N
H
0.1 M NaOAc, EtOH
1a
2a
Yield
(%)b
Entry
1
Variation from the standard conditions
none
97
15
reduced graphene oxide,13 Pd(OAc)2,14 NH4VO3 and a Co(II)
2
5 mA instead of 10 mA, 7 h.
15 mA instead of 10 mA, 2 h 20 min.
LiClO4 instead of NaOAc
n-Bu4NBF4 instead of NaOAc
0.05 M NaOAc instead of 0.1 M NaOAc
RVC instead of Pt as anode
DMSO instead of EtOH
59
88
83
69
58
83
51
73
81
n.d.
complex16 were also included. Very recently, a photochemical
dehydrogenation of hydrazines was reported by Balaraman’s group.
This reaction has been achieved under dual catalytic conditions with
[Ru(bpy)3]Cl2 as the photoredox catalyst and Co(dmgH)2(py)Cl as the
proton-reduction catalyst.17
3
4
5
6
Electroorganic synthesis has been recognized as an environ-
mentally friendly methodology since redox reagents can be
replaced by an electrical current or generated in situ in the course
of electrolysis.18 Electrochemical anodic oxidation along with the
cathodic proton reduction has been envisioned as a promising
method, these reactions usually generates H2 as the only
theoretical byproduct.19 With these critical features in mind and our
interest in the application of electrochemical methods to organic
7
8
9
MeOH instead of EtOH
10
11
under N2
no electric current
aUnless otherwise noted, the mixture of 1a (0.3 mmol),
NaOAc (0.5 mmol) in EtOH (5 mL) was electrolyzed at
constant current (10 mA) in an undivided cell at room
temperature for 3.5 hours, anode and cathode: Pt foil (1.0
× 1.5 cm2).
synthesis,20 we sought to develop
a green and efficient
a. Key Laboratory of Functional Molecular Engineering of Guangdong Province,
School of Chemistry and Chemical Engineering, South China University of
Technology, Guangzhou, Guangdong 510640, China
bYield determined by GC analysis with n-dodecane as the
internal standard; n.d. = not detected.
Fax: (+86)-20-8711-0622; phone: (+86)-208711-0695; e-mail: chehjm@scut.edu.cn
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an undivided cell for 3.5 h (Table 1, entry 1). The electrolysis at Table 3 Scope of unsymmetric hydrazo compoundsa,b
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DOI: 10.1039/C9GC00515C
higher (15 mA) or lower (5 mA) current blunted the product
formation (Table 1, entries 2-3). When other electrolytes, such as
LiClO4 or n-Bu4NBF4 was used instead of NaOAc, the yield of the
desired product was reduced (Table 1, entries 4-5). Through an
investigation of the amount of electrolyte, we found that the yield
of the target product reduced when it decreased to 0.05 M (Table 1,
entry 6). When a reticulated vitreous carbon (RVC) was used as the
anode, the yield decreased (83%, Table 1, entry 7). The influence of
the solvent was also studied. When other solvents, for instance,
DMSO or MeOH was used instead of EtOH, the yield of the desired
product dropped (Table 1, entries 8-9). Hence, ethyl alcohol, one of
the least expensive chemicals, was the best solvent. Reaction
carried out under nitrogen produced a lower yield (81%, Table 1,
entry 10). No desired product could be obtained without an electric
current in this reaction (Table 1, entry 11).
To demonstrate the utility of this method, we then examined the
scope of the hydrazo compounds. First, the reactivity of symmetric
aromatic hydrazo compounds with substituents on the benzene ring
was studied (Table 2). In general, both electron-withdrawing and
electron-donating groups were tolerated. Substrates with methyl,
ethyl and methoxyl groups provided the desired products in good
yields (2b, 91%; 2c, 92% and 2d, 86%). The substrate bearing a
H
Pt (+) - Pt (-), 10 mA
N
R1
N
R1
rt, 1.5 - 4 h, undivided cell
N
N
R
R
H
0.1 M NaOAc, EtOH 5 mL
1
2
F
O
N
2k
N
N
N
N
N
N
c
2l
, 88%
, 96%
N
2m
, 97%
O
Cl
Br
N
N
N
N
2n
2o
, 97%
N
, 96%
2p
, 90%
CF3
O
O
O
O
N
N
N
N
N
d
d
2s
, 71%
2q
, 93%
O
2r
, 86%
N
N
2t
, 89%
Table 2 Scope of symmetric hydrazo compoundsa
aReaction conditions: anode and cathode: Pt foil (1.0 × 1.5 cm2),
constant current = 10 mA, 1 (0.3 mmol), 0.1 M NaOAc, EtOH (5
mL) in an undivided cell at room temperature under air for 3 - 4
h.
Pt (+) - Pt (-), 10 mA
rt, 3 - 4 h, undivided cell
H
R
R
N
N
N
N
R
0.1 M NaOAc, EtOH 5 mL
H
R
bIsolated yields.
1
2
cconstant current = 5 mA.
dn-Bu4NBF4 instead of NaOAc, DMF instead of EtOH.
N
N
N
N
N
N
2b
2c
, 92%
, 91%
2a
, 97%
strong electron-withdrawing group CF3, could be transformed to
the corresponding product in an excellent yield (2h, 91%). Notably,
halide substituents remained intact in the reaction (2e, 92%; 2f,
95%; 2g, 91%), which provide the opportunity for further
derivatization. Heteroaromatic hydrazo compounds were also
investigated and good yields were obtained for 1,2-di(pyridin-4-
yl)diazene (2i, 86%) and 1,2-bis(5-methylisoxazol-3-yl)diazene (2j,
83%).
O
Cl
F
N
N
N
N
N
N
O
Cl
F
c
2f
, 95%
2d
, 86%
2e
, 92%
CF3
Br
N
N
N
N
N
N
N
N
F3C
Br
2g
, 91%
2h
, 91%
2i
, 86%
Next, unsymmetric aromatic hydrazo compounds were also
explored (Table 3). Unsymmetric hydrazo compounds with different
substituents on the benzene ring were unaffected and the
transformations of these substrates proceeded smoothly in good
yields (2k-2q). Alkyl 2-phenylazocarboxylates have recently received
attention from synthetic chemists, as these azo compounds showed
distinct reactivity in many transformations.9,21 The dehydroge-
nation of alkyl 2-phenylhydrazinecarboxylates were then studied. It
was found that DMF as a solvent and n-Bu4NBF4 as an electrolyte
were optimal for the synthesis of tert-butyl 2-phenylazocarboxylate
(2r) and benzyl 2-phenylazocarboxylate (2s) in the yields of 86% and
71%, respectively. Benzoyl-2-phenylhydrazine could also react
O
N
N
N
N
O
2j
, 83%
aReaction conditions: anode and cathode: Pt foil (1.0 × 1.5 cm2),
constant current = 10 mA, 1 (0.3 mmol), 0.1 M NaOAc, EtOH (5
mL) in an undivided cell at room temperature under air for 3 -
4 h.
bIsolated yields.
cconstant current = 5 mA.
2 | J. Name., 2012, 00, 1-3
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cathodic reduction of the proton-hydrogen led to the formation of
Scheme 1 Scale-up experiment.
View Article Online
DOI: 10.1039/C9GC00515C
hydrogen gas. As shown in Table 1, when the reaction was carried
a)
out under nitrogen, the productivity of 2a dropped to a 81% yield
(entry 10). Therefore, a minor pathway might be involved. O2•- was
generated from the cathodic reduction of O2 followed by the
oxidation of 1a with the O2•- to afford the dehydrozination
product.13,25
H
Pt (+) - Pt (-), 40 mA
undivided cell, rt, 22 h
N
N
N
N
H
0.1 M NaOAc, EtOH 200 mL
1a,
12 mmol (2.21 g)
2a,
97%
N
b)
O
O
H
Pt (+) - Pt (-), 40 mA
undivided cell, rt, 10 h
N
N
O
N
O
H
0.1 M n-Bu4NBF4, DMF 100 mL
1r,
6 mmol (1.25 g)
2r
, 79%
smoothly to provide benzoylazobenzene (2t, 89%). Thus, the
reaction was shown to be a reliable method for the synthesis of 2-
arylazocarboxylates and their derivatives.
Fig. 1 Cyclic voltammogram of 0.1 M NaOAc solution in EtOH at
room temperature: a) none, b) 1a (0.01 M), c) 1j (0.01 M). Cyclic
voltammogram of 0.1 M n-Bu4NBF4 solution in DMF at room
temperature: d) none, e) 1r (0.01 M). The voltammogram was
obtained with Pt wire as auxiliary electrode and a saturate calomel
electrode (SCE) as a reference electrode. The scan rate was 0.1 V/s
on a platinum disk electrode (d = 2 mm).
Then, the scalability of this electrocatalytic transformation was
evaluated. Excellent yield (97%) could be obtained on a 12 mmol
scale of 1a (Scheme 1, a). Similarly, a gram-scale reaction on
unsymmetric hydrazo compound 1r afforded the desired product in
79% yield (Scheme 1, b). These results showed the great potential
of this electrochemical reaction in practical synthesis.
The synthetic utility was further explored by the dehydrazination
reaction of hydrazines. When the phenylhydrazine 3a was
examined under the standard conditions, the dehydrazinated
product 4a was isolated in 65% (solvent: EtOH) and 71% (solvent:
DMF) yields, respectively (Scheme 2, a). Heteroaromatic hydrazine
3b was also successfully dehydrazinated in a moderate yield (4b,
53%), thus indicating its potential application for other
heteroaromatic hydrazines (Scheme 2, b). It was suggested that the
dehydrogenated intermediate, an azo compound should be
generated during the reaction.22 To the best of our knowledge, it is
a simple and mild method for the dehydrazination of hydrazines.23
Cyclic voltammetry (CV) experiments were also implemented (Fig
1). The oxidative peak of 1a was recorded at 0.60 V vs SCE and 1j at
1.19 V vs SCE in ethanol. The oxidative peak of 1r was observed at
0.86 V vs SCE in DMF. Based on the above results and the literature
reports,19,24 it was proposed that the product was generated by the
anode oxidation through two-electron transfer. Meanwhile,
Experimental
General information
Unless otherwise noted, all the chemicals were purchased
commercially, and used without further purification. Proton nuclear
magnetic resonance (1H NMR) and carbon nuclear magnetic
resonance (13C NMR) spectroscopy were performed on Bruker
1
Advance III-400 spectrometers (400 MHz for H NMR, 100 MHz for
13C NMR). Electrolysis reactions were conducted using a Model
QJ3003T (32 V) power supply purchased from Qiujing Instruments
(Shenzhen) Co., Ltd. Cyclic voltammetry (CV) analysis was
performed on Ingsens 1030 electrochemical workstation (Ingsens
Instruments (Guangzhou) Co., Ltd., China), using
a platinum
electrode (d = 2 mm) as working electrode, a Pt wire as counter
electrode and saturated calomel electrode (SCE) as a reference
electrode. Cyclic voltammograms were recorded at 0.1 V/s scan
rate.
Scheme 2 Dehydrazination of hydrazines
General procedure for the synthesis of azo compounds
a )
To a round-bottomed flask (5 mL), 1 (0.3 mmol), NaOAc (0.5 mmol),
EtOH (5 mL) were added. The reaction flask was equipped with Pt
foils as anode (1.0 × 1.5 cm2) and cathode (1.0 ×× 1.5 cm2). The
solution was electrolyzed under a constant current (10 mA) for 3 - 4
h at ambient temperature. After electrolysis, the mixture was
poured into water and extracted with ethyl acetate (3 × 15 mL). The
combined organic layer was washed with brine (10 mL) and dried
over MgSO4, filtered and concentrated. The resulting mixture was
purified by silica gel column chromatography to afford 2.
H
N
Pt (+) - Pt (-), 10 mA
2.5 h, rt, undivided cell
NH2
electrolyte
O2N
NO2
O2N
NO2
4a
3a
EtOH (0.1M NaOAc), yield: 65%
DMF (0.1M n-Bu4NBF4), yield: 71%
b )
Pt (+) - Pt (-), 10 mA
2.5 h, rt, undivided cell
NH2
N
N
0.1 M NaOAc, EtOH
N
H
3b
4b
, 53%
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3
1
Spectroscopic data of products
32.7 Hz,), 126.4 (q, JCF = 3.7 Hz), 123.8(q, JCF = 271.7 Hz), 123.3
ppm.
(E)-1,2-diphenyldiazene (2a).10 The crude product was purified by
column chromatography on silica gel (petroleum ether/ethyl 1,2-bis(5-methylisoxazol-3-yl)hydrazine (2i).10 The crude product
1
acetate = 50:1). Orange solid (52.9 mg, 93%), H NMR (400 MHz, was purified by column chromatography on silica gel (petroleum
CDCl3): δ 7.92 (d, J = 7.4 Hz, 4H), 7.53 - 7.46 (m, 6H) ppm; 13C NMR ether/ethyl acetate = 10:1). Yellow solid (47.8 mg, 83%), H NMR
1
(100 MHz, CDCl3): δ 152.7, 130.99, 129.1, 122.9 ppm.
(400 MHz, CDCl3): δ 6.41 (s, 2H), 2.52 (s, 6H) ppm; 13C NMR (100
MHz, CDCl3): δ 173.9, 172.0, 92.0, 12.9 ppm.
(E)-1,2-di-p-tolyldiazene (2b).10 The crude product was purified by
column chromatography on silica gel (petroleum ether/ethyl (E)-1,2-di(pyridin-4-yl)diazene (2j).10 The crude product was
acetate = 50:1). Yellow solid (57.2 mg, 91%), 1H NMR (400 MHz, purified by column chromatography on silica gel (petroleum
1
CDCl3): δ 7.85 (d, J = 8.0 Hz, 4H), 7.34 (d, J = 8.0 Hz, 4H), 2.47 (s, 6H) ether/ethyl acetate = 10:1). Red solid (47.5 mg, 86%) H NMR (400
ppm; 13C NMR (100 MHz, CDCl3): δ 150.9, 141.2, 129.7, 122.7, 21.5 MHz, CDCl3): δ 8.86 (d, J = 4.4 Hz, 4H), 7.74 (d, J = 4.1 Hz, 4H) ppm;
ppm.
13C NMR (100 MHz, CDCl3): δ 156.6, 151.7, 116.3 ppm.
4,4’-Diethylazodibenzene (2c).26 The crude product was purified by (E)-1-phenyl-2-(p-tolyl)diazene (2k).10 The crude product was
column chromatography on silica gel (petroleum ether/ethyl purified by column chromatography on silica gel (petroleum
1
acetate = 50:1). Yellow solid (65.7 mg, 92%), 1H NMR (400 MHz, ether/ethyl acetate = 50:1). Yellow solid (56.5 mg, 96%), H NMR
CDCl3): δ 7.83 (d, J = 7.7 Hz, 4H), 7.31 (d, J = 7.8 Hz, 4H), 2.71 (d, J = (400 MHz, CDCl3): δ 7.90 (d, J = 7.9 Hz, 2H), 7.83 (d, J = 7.8 Hz, 2H),
7.5 Hz, 4H), 1.27 (t, J = 7.6 Hz, 6H) ppm; 13C NMR (100 MHz, CDCl3): 7.52 - 7.40 (m, 3H), 7.29 (d, J = 7.9 Hz, 2H), 2.41 (s, 3H) ppm; 13C
δ 151.1, 147.5, 128.5, 122.9, 28.9, 15.5 ppm.
NMR (100 MHz, CDCl3): δ 152.8, 150.9, 141.6, 130.7, 129.8, 129.1,
122.9, 122.8, 21.6 ppm.
4,4’-Dimethoxyazodibenzene (2d).5e The crude product was purify-
ed by column chromatography on silica gel (petroleum ether/ethyl (E)-1-(4-methoxyphenyl)-2-phenyldiazene (2l).10 The crude product
acetate = 30:1). Brown solid (62.5 mg, 86%),1H NMR (400 MHz, was purified by column chromatography on silica gel (petroleum
CDCl3): δ 7.89 (d, J = 8.2 Hz, 4H), 7.01 (d, J = 8.5 Hz, 4H), 3.88 (s, 6H) ether/ethyl acetate = 30:1). Brown solid (56.0 mg, 88%), 1H NMR
ppm; 13C NMR (100 MHz, CDCl3): δ 161.6, 147.1, 124.4, 114.2, 55.6 (400 MHz, CDCl3): δ 7.92 - 7.86 (m, 4H), 7.49 - 7.40 (m, 3H), 6.98 (d,
ppm.
J = 8.7 Hz, 2H), 3.83 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3): δ 162.1,
152.9, 147.1, 130.4, 129.1, 124.8, 122.6, 114.3, 55.6 ppm.
(E)-1,2-bis(4-fluorophenyl)diazene (2e).10 The crude product was
purified by column chromatography on silica gel (petroleum (E)-1-(4-fluorophenyl)-2-phenyldiazene (2m).10 The crude product
ether/ethyl acetate = 50:1). Dark brown solid (60.2 mg, 92%), 1H was purified by column chromatography on silica gel (petroleum
1
NMR (400 MHz, CDCl3): δ 7.92 - 7.88 (m, 4H), 7.19 - 7.15 (m, 4H) ether/ethyl acetate = 50:1). Yellow solid (58.2 mg, 97%), H NMR
ppm; 13C NMR (100 MHz, CDCl3): δ 164.4 (d, JCF = 252.1 Hz), 149.0 (400 MHz, CDCl3): δ 7.95 -7.89 (m, 4H), 7.53 - 7.46 (m, 3H), 7.21 -
1
(d, 4JCF = 2.3 Hz), 124.8 (d, 3JCF = 9.1 Hz), 116.1 (d, 2JCF = 23.0 Hz) ppm. 7.17 (m, 2H) ppm; 13C NMR (100 MHz, CDCl3): δ 164.4 (d, JCF
=
1
3
250.4Hz), 152.5, 149.2, 131.0, 129.1, 124.9 (d, JCF = 8.9Hz), 122.8,
116.0 (d, 2JCF = 22.7Hz) ppm.
(E)-1,2-bis(4-chlorophenyl)diazene (2f).10 The crude product was
purified by column chromatography on silica gel (petroleum
ether/ethyl acetate = 50:1). Yellow solid (71.4 mg, 95%): H NMR (E)-1-1(4-chloropheny)2-phenyldiazene (2n).10 The crude product
1
(400 MHz, CDCl3): δ 7.86 (d, J = 8.2 Hz, 4H), 7.49 (d, J = 8.3 Hz, 4H) was purified by column chromatography on silica gel (petroleum
1
ppm; 13C NMR (100 MHz, CDCl3): δ 150.8, 137.3, 129.4, 124.2 ppm.
ether/ethyl acetate = 50:1). Yellow solid (63.1 mg, 97%), H NMR
(400 MHz, CDCl3): δ 7.93 - 7.87 (m, 4H), 7.53 - 7.49 (m, 5H) ppm. 13
NMR (100 MHz, CDCl3): δ 152.7, 151.0, 137.0, 131.3, 129.4, 129.2,
C
(E)-1,2-bis(4-bromophenyl)diazene (2g).10 The crude product was
purified by column chromatography on silica gel (petroleum
124.3, 123.1 ppm.
1
ether/ethyl acetate = 50:1). Yellow solid (92.8 mg, 91%), H NMR
(400 MHz, CDCl3): δ 7.82 (d, J = 8.1 Hz, 4H), 7.67 (d, J = 8.2 Hz, 4H) (E)-1-(4-bromophenyl)-2-phenyldiazene (2o).10 The crude product
ppm; 13C NMR (100 MHz, CDCl3): δ 151.2, 132.4, 125.8, 124.4 ppm.
was purified by column chromatography on silica gel (petroleum
1
ether/ethyl acetate = 50:1). Yellow solid (75.2 mg, 96%), H NMR
(E)-1,2-bis(4-(trifluoromethyl)phenyl)diazene (2h).10 The crude
product was purified by column chromatography on silica gel
(petroleum ether/ethyl acetate = 30:1). Red solid (86.7 mg, 91%),
(400 MHz, CDCl3): δ 7.92 (d, J = 7.3 Hz, 2H), 7.80 (d, J = 7.9 Hz, 2H),
7.65 (d, J = 8.0 Hz, 2H), 7.53 - 7.51 (m, 3H) ppm; 13C NMR (100 MHz,
CDCl3): δ 152.5, 151.4, 132.3, 131.3, 129.2, 125.4, 124.4, 123.0 ppm.
1H NMR (400 MHz, CDCl3): δ 8.06 (d, J = 8.2 Hz, 4H), 7.83 (d, J = 8.2
Hz, 4H) ppm; 13C NMR (100 MHz, CDCl3): δ 154.1, 133.0 (q, JCF
=
4-acethylazobenzene (2p).5e The crude product was purified by
2
column chromatography on silica gel (petroleum ether/ethyl
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acetate = 15:1). Brown solid (60.6 mg, 90%), 1H NMR (400 MHz,
CDCl3): δ 8.12 - 8.10 (m, 2H), 7.98 - 7.94 (m, 4H), 7.54 - 7.52 (m, 3H),
2.66 (s, 3H) ppm; 13C NMR (100 MHz, CDCl3): δ 197.5, 155.1, 152.6,
138.4, 131.8, 129.4, 129.2, 123.2, 122.9, 26.9 ppm.
Acknowledgements
View Article Online
We are grateful to the National Natural DSOciIe: 1n0c.e10F39o/uCn9dGaCt0i0o5n15oCf
China (Grant Nos. 21672074 and 21372089) for financial
support.
(E)-1-phenyl-2-(4-(trifluoromethyl)phenyl)diazene (2q).10 The cru-
de product was purified by column chromatography on silica gel
(petroleum ether/ethyl acetate = 30:1). Yellow solid (69.8 mg, 93%),
1H NMR (400 MHz, CDCl3): δ 7.99 - 7.92 (m, 4H), 7.77 - 7.74 (m, 2H),
7.52 - 7.51 (m, 3H) ppm; 13C NMR (100 MHz, CDCl3): δ 154.5, 152.5,
132.2, (q, 2JCF = 32.4 Hz), 131.8, 129.2, 126.3 (q, 3JCF = 3.9 Hz), 124.0
(q, 1JCF = 271.7 Hz), 123.2, 123.0 ppm.
Notes and references
1
2
3
a) D. R. Waring, G. Hallas, The chemistry and application of
dyes, Plenum, New York, 1990. b) K. Hunger, Industrial Dyes:
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tert-Butyl 2-Phenylazocarboxylate(2r).7 The crude product was
purified by column chromatography on silica gel (petroleum
ether/ethyl acetate = 5:1). Orange solid (53 mg, 86%), 1H NMR (400
MHz, CDCl3): δ 7.90 (d, J = 8.1 Hz, 2H), 7.57 - 7.49 (m, 3H), 1.66 (s,
9H) ppm; 13C NMR (100 MHz, CDCl3): δ 161.3, 151.6, 133.3, 129.2,
123.5, 85.0, 27.9 ppm.
Benzyl 2-Phenylazocarboxylate(2s).7 The crude product was puri-
fied by column chromatography on silica gel (petroleum ether/ethyl
acetate = 5:1). Orange solid (51 mg, 71%), 1H NMR (400 MHz, CDCl3):
δ 7.92 (d, J = 7.5 Hz, 2H), 7.59 - 7.56 (m, 1H), 7.53 - 7.48 (m, 4H),
7.42 - 7.37 (m, 3H ), 5.47 (s, 2H) ppm; 13C NMR (100 MHz, CDCl3): δ
162.1, 151.7, 134.7, 133.9, 129.4, 128.9, 128.8, 123.8, 69.9 ppm.
4
5
For selected examples: a) K. Haghbeen, E. W. Tan, J. Org.
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1162.
Benzoylazobenzene(2t).7 The crude product was purified by column
chromatography on silica gel (petroleum ether/ethyl acetate = 10:1).
Orange solid (56 mg, 89%), 1H NMR (400 MHz, CDCl3): δ 8.06 (d, J =
7.5 Hz, 2H), 8.00 (d, J = 7.1 Hz, 2H), 7.67 - 7.65 (m, 1H), 7.59 - 7.50
(m, 5H ) ppm; 13C NMR (100 MHz, CDCl3): δ 182.1, 152.1, 134.5,
133.4, 130.9, 130.6, 129.4, 128.9, 123.6 ppm.
1,3-dinitrobenzene (4a).10 The crude product was purified by
column chromatography on silica gel (petroleum ether/ethyl
1
acetate = 5:1). Yellow solid, H NMR (400 MHz, d-DMSO): δ 8.84 (s,
1H), 8.66 (d, J = 8.2 Hz, 2H), 7.99 - 7.96 (m, 1H) ppm; 13C NMR (100
MHz, d-DMSO): δ 148.5, 132.1, 129.8, 119 ppm.
6
Pyridine(4b).27 The crude product was purified by column
chromatography on silica gel (petroleum ether/ethyl acetate = 20:1).
1
Colourless liquid, H NMR (400 MHz, CDCl3): δ 8.60 (s, 2H), 7.66 -
7.62 (m, 1H), 7.25 (s, 2H) ppm; 13C NMR (100 MHz, CDCl3): δ 149.8,
135.8, 123.6 ppm.
Conclusions
7
8
M. H. Kim, J. Kim, J. Org. Chem., 2018, 83, 1673.
A. Malekafzali, K. Malinovska, F. W. Patureau, New J. Chem.,
2017, 41, 6981.
T. Hashimoto, D. Hirose, T. Taniguchi, Adv. Synth. Catal.
2015, 357, 3346.
In conclusion, a new electrochemical dehydrogenation of
hydrazine compounds is disclosed. Carried out under ambient
conditions in ethanol, this protocol provided a straightforward
means for the synthesis of azo compounds with a broad
substrate scope. Importantly, the reaction can be conducted
on a gram scale. In view of its broad utility and high efficiency,
this transformation may have more applications in the future.
9
10 L. Wang, A. Ishida, Y. Hashidoko, M. Hashimoto, Angew.
Chem. Int. Ed., 2017, 56, 870.
11 E. Drug, M. Gozin, J. Am. Chem. Soc., 2007, 129, 13784.
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Lett. 2014, 55, 4545.
14 W. Gao, Z. He, Y. Qian, J. Zhao, Y. Huang, Chem. Sci., 2012, 3,
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15 F. Trischler, J. Therm. Anal., 1979, 16, 119.
Conflicts of interest
There are no conflicts to declare.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Green Chemistry
Page 6 of 7
ARTICLE
Journal Name
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6 | J. Name., 2012, 00, 1-3
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Page 7 of 7
Green Chemistry
View Article Online
DOI: 10.1039/C9GC00515C
Electrochemical Dehydrogenation of Hydrazines to Azo Compounds
Ke-si Du, Jing-Mei Huang*
-
+
Pt foil
Pt foil
H
N
R2
N
R2
R1
N
R1
N
H
electrolysis
1
2
R1 = Ar, Heteroaryl
R2 = Ar, Heteroaryl, COOt-Bu, COOBn, COC6H5
20 examples
up to 97% yield
A strategy for the electrochemical dehydrogenation of hydrazine compounds is disclosed under
ambient conditions. Its synthetic value is well demonstrated by the highly efficient synthesis of
symmetric and unsymmetric azo compounds. It is an environment-friendly transformation and the
present protocol was effective on a larger scale.