Visible-light-promoted oxidative dehydrogenation of hydrazobenzenes and transfer hydrogenation of azobenzenes

Article abstract of DOI:10.1039/c9gc01618j

Azo compounds are widely used in the pharmaceutical and chemical industries. Here, we report the use of a non-metal photo-redox catalyst, Eosin Y, to synthesize azo compounds from hydrazine derivatives. The use of visible-light with air as the oxidant makes this process sustainable and practical. Moreover, the visible-light-driven, photo-redox-catalyzed transfer hydrogenation of azobenzenes is compatible with a series of hydrogen donors such as phenyl hydrazine and cyclic amines. Compared with traditional (thermal/transition-metal) methods, our process avoids the issue of over-reduction to aniline, which extends the applicability of photo-redox catalysis and confirms it as a useful tool for synthetic organic chemistry.

Full text of DOI:10.1039/c9gc01618j

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DOI: 10.1039/C9GC01618J
Visible-light-promoted oxidative dehydrogenation of hydrazobenzenes and transfer
hydrogenation of azobenzenes are realized using Eosin Y as photo-catalysis

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DOI: 10.1039/C9GC01618J
ARTICLE
Visible-Light-Promoted Oxidative Dehydrogenation of
Hydrazobenzenes and Transfer Hydrogenation of Azobenzenes
Xianya Wang,a,b,# Xianjin Wang,a,# Chungu Xia,*,a Lipeng Wu*,a
Received 00th January 20xx,
Accepted 00th January 20xx
Azo compounds are widely used in the pharmaceutical and chemical industries. Here, we report the use of a non-metal
photo-redox catalyst, Eosin Y, to synthesize azo compounds from hydrazine derivatives. The use of visible-light with air as
oxidant makes this process sustainable and practical. Moreover, the visible-light-driven, photo-redox-catalyzed transfer
hydrogenation of azobenzene is compatible with a series of hydrogen donors such as phenyl hydrazine and cyclic amines.
Compared with traditional (thermal/transition-metal) methods, our process avoids the issue of over-reduction to aniline,
which extends the applicability of photo-redox catalysis and confirms it as a useful tool for synthetic organic chemistry.
DOI: 10.1039/x0xx00000x
Introduction
R2
H
R2
Azo compounds are widely used in the chemical industry as
dyes and pigments,1 photo-responsive switches and polymers,2-
[Co, Rh, Pd, Cu]
N
N
(a)
N
H
N
R1
R1
K3Fe(CN)6, KMnO4,
KClO3/H2SO4, O2, H2O2
4
food additives, and therapeutic agents.5 In addition, azo
compounds have shown potential application in drug delivery6
and electronics.7 There exist numerous methods for the
preparation of azo compounds, such as oxidative coupling of
anilines,8, 9 reduction of nitroaromatics,10, 11 coupling of diazo
salts with aromatic compounds,12 and the Mills reaction.13
precious metals; strong oxidants; tedious workup
R2
R2
H
N
(b)
tBuOK
N
N
N
R1
H
R1
NH3 (I), air
strong base; unfriendly medium
Alternatively,
the
oxidative
dehydrogenation
of
a
H
R2
R2
hydrazobenzene (hydrazine) derivatives provides
N
Na (Hg)
(c)
N
N
R1
N
H
straightforward route to azobenzenes. In this context, the use
of stoichiometric amounts of strong oxidants, such as K3Fe(CN)6,
KMnO4, and KClO3/H2SO4, has been reported. Other methods
have used O2 or H2O2 as the oxidant, which is believed to be
more sustainable; however, transition metals such as Co,14 Rh,15
Cu,16 and Pd17 were indispensable in these syntheses (Scheme
1a). Recently, Hashimoto and coworkers have reported the
R1
stoichiometric metal reagents; large amounts of inorganic wastes
H
R2
R2
N
[Pd, Au]
H2
(d)
N
N
N
R1
H
R1
over reduction to aniline, poor functional group tolerance
this work: Eosin Y catalyzed mutual transformations!
oxidative dehydrogenation of hydrazobenzenes using
a
Eosin Y, Air
catalytic amount of tBuOK (Scheme 1b). However, use of liquid
NH3 as solvent necessitated an operating temperature of -75°C,
which limited its practical application. Thus, the discovery of a
novel, environmentally friendly, and practical method for the
oxidative dehydrogenation of hydrazobenzenes to azobenzene
is keenly pursued.
Apart from serving as precursors for the synthesis of
azobenzene, hydrazobenzenes themselves also have plenty of
applications: for example, substituted diphenyl hydrazines have
R2
H
R2
N
N
N
visible light
(e)
R1
N
R1
H
HD = hydrogen donor
Eosin Y, HD
sustainable, mild, facile, practical, excellent functional group tolerance
Scheme 1. Known processes for mutual transformation of
azobenzene and hydrazobenzene, and the process reported herein.
potential value for evaluating Alzheimer’s disease.18
Traditionally, hydrazobenzenes were prepared from
azobenzene using stoichiometric amounts of metallic reducing
reagents such as sodium amalgam,19 zinc in alcoholic
ammonia,20 and stannous chloride21 in ethanolic sodium
a. State Key Laboratory for Oxo Synthesis and Selective Oxidation, Suzhou Research
Institute of LICP, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of
Sciences, Lanzhou, 730000 (P. R. China)
b. University of Chinese Academy of Sciences
# These authors contributed equally
Email: cgxia@licp.cas.cn; lipengwu@licp.cas.cn
†Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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hydroxide, along with concomitant formation of large amounts Table 1. Condition optimization for the oxidative
View Article Online
of inorganic waste (Scheme 1c). Although catalytic dehydrogenation from 1a to 2a.a
DOI: 10.1039/C9GC01618J
H
hydrogenation of azobenzene by transition metals has also
been proposed,22 it often suffers from over-reduction of
azobenzene to anilines (Scheme 1d). Moreover, functional
groups such as nitro, iodo, and ester cannot survive the
reduction conditions. Thus, the development of a new, mild,
and facile method for selective azobenzene-to-hydrazobenzene
reduction is highly desirable.
EosinY (2 mol%)
Ph
N
Ph
N
N
H
Ph
N
Ph
CFL/air, 20 h, solvent
1a
2a
Entries
Catalysts
Solvent
Yields[%]b
86
1
[Ru(bpy)3]Cl2·6H2O
Ir(bpy)3
Rose Bengal
DCA
MeCN
MeCN
MeCN
MeCN
MeCN
THF
2
62
3
54
Over the past decade, visible-light photo-redox catalysis has
emerged as an enabling platform for the development of novel
organic reactions with high synthetic efficiency and functional-
group tolerance.23-29 In this context, photo-redox catalysis has
been used to perform oxidation and dehydrogenation
reactions. For example, the activation of O2 for transformation
of amines and alkenes using photo-redox catalysis has been
comprehensively developed.30-33 Recently, a range of dual-
catalysis systems combining a photo-redox catalyst and a
cobalt-based proton-reduction catalyst have been established
for dehydrogenation reactions.34-37 Interestingly, there is only
one report of using a Ru–Co dual-metal system for azobenzene
synthesis from hydrazobenzene.37 In the area of azobenzene
reduction, Pt/TiO2 and Au/CeO2 systems using alcohol as a
sacrificial electron-and-proton donor are known.38, 39 However,
the former system needs UV light for catalyst photoexcitation,
and the latter needs a strong base and generates aniline as a
side product. Herein, we wish to report a facile and practical
mutual transformation of azobenzene and hydrazobenzene
using Eosin Y as a photo-redox catalyst (Scheme 1e). Our work
represents a very rare example of using photo-redox catalysis
as the sole catalyst to realize the mutual transformation of two
kinds of compound.
4
46
5
Eosin Y
95
6
Eosin Y
67
7
Eosin Y
DMSO
Dioxane
Tol
66
8
Eosin Y
73
9
Eosin Y
79
10
11
12
13
14c
Eosin Y
DMF
59
Eosin Y
EtOH
DCM
58
Eosin Y
54
Eosin Y
H2O
75
Eosin Y
MeCN
96
athe reaction were performed in a 20 mL vial with 1a (0.2
mmol), photo-redox catalysts (2 mol%), solvents 1.0 mL and the
vial were open to air under irradiation using CFL as the light
b
1
source; yields were determined by H NMR using CH2Br2 as
internal standard; cwith green LED light.
With the optimal reaction conditions in hand (0.2 mmol 1, 2
mol% Eosin Y, CFL, 1 mL MeCN under ambient conditions) we
evaluated the oxidative dehydrogenation strategy to synthesize
different azobenzene derivatives (Figure
1 and 2). For
symmetrical hydrazobenzene derivatives (Figure 1), substrates
with both electron-rich and electron-deficient substituents
underwent efficient dehydrogenation to afford the
corresponding desired products in excellent yields (72-99%, 2a-
2o). We found that sterically hindered substrates such as 2b (2-
Me) and 2e (3-iPr) provided slightly poorer yields (85% and
81%). Substrates with electron-withdrawing groups like -F, -Cl, -
Br, and -CF3 reacted well and gave the corresponding
azobenzenes in very good yields (up to 93%). The slightly lower
yields with electron-withdrawing groups indicated that
electronic effects also played a role during the reaction.
Furthermore, disubstituted hydrazobenzenes were also
suitable substrates for the reaction and afforded the desired
products in high yields (84-90%, 2m-2o). The excellent
functional-group compatibility and mild conditions favor the
implementation of this reaction in late-stage functionalization
of complex structural motifs.
Results and Discussion
We focused our initial studies on the oxidative dehydrogenation
of diphenyl hydrazine (1a, 0.2 mmol) under ambient conditions
with a 22 W white compact fluorescent lamp (CFL). We first
evaluated different photo-redox catalysts in various solvents
(Table 1). With transition metal-based photo-redox catalysts,
azobenzene was obtained in yields of 86% with
[Ru(bpy)3]Cl2·6H2O and 62% with Ir(bpy)3 in MeCN (Table 1,
entries 1 and 2). Then, different organic photo-redox catalysts
were tested. We found that Eosin Y gave the best yield (95%),
even better than the transition metal-based photo-redox
catalysts (Table 1, entry 5). With Eosin Y as photo-redox catalyst,
various solvents were tested. To our delight, all solvents
produced azobenzene (2a) in moderate to very good yields (54-
95%) (Table 1, entries 5-13). However, MeCN was still the best
solvent. Notably, even with H2O as the solvent, a 75% yield of
2a was still observed (Table 1, entry 13). When the color of the
excitation light was changed to green, the amount of 2a
produced was almost unchanged (96%, Table 1, entry 14).
Similar to symmetrical hydrazobenzenes, unsymmetrically
substituted hydrazobenzenes showed very good reactivity
(Figure 2). Substrates with both electron-rich and electron-
deficient substituents undertook facile dehydrogenation to
afford the corresponding products in excellent yields (up to
2 | J. Name., 2012, 00, 1-3
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90%). Various substituents such as -Me, -MeO, -F, -Cl, and
DOI: 10.1039/C9GC01618J
disubstituted hydrazobenzenes were well tolerated (products
R1
R1
View Article Online
H
N
N
N
EosinY (2 mol%)
N
R1
R1
H
CFL/air, 20h, MeCN
4a–4i), which showed the synthetic value of our process.
1
2
Me
Me
Follow-up chemistry was investigated to demonstrate the more
general applicability of our strategy. Interestingly, when phenyl
hydrazine 3j was examined, m-cresol 4j was detected as the main
product with 86% yield (Scheme 2a). This might because of the
further denitrogenative formation of aryl radical quenched by H2O.
Changing one phenyl group of diphenyl hydrazine to a stronger
electron-withdrawing ester group (3k), we obtained only 21% of the
dehydrogenation product 4k (Scheme 2b), consistent with the lower
reactivity of strong electron-withdrawing groups. Thus,
unsurprisingly, the use of diethyl hydrazine-1,2-dicarboxylate 3l
resulted in no reaction at all (Scheme 2c). However, by adding 1
equivalent of CaCl2 as an additive, the yield of 4k was increased to
60%.
N
N
N
N
N
N
Me
Me
2a, 95%
2b, 85%
2c, 83%
iPr
OMe
Et
N
N
N
N
N
N
MeO
Et
iPr
2d, 99%
2e, 81%
2f, 96%
Cl
F
F
N
N
N
N
N
N
Cl
F
F
H
2g, 83%
2h, 93%
2i, 77%
OH
N
NH2
EosinY (2 mol%)
(a)
CF3
CFL/air, 20h, MeCN
Br
N
Me
F3
C
N
N
N
Me
N
Br
N
CF3
3j
4j
, 86% yield
F3
C
2j, 70%
2k, 86%
O
O
2l, 92%
H
N
EosinY (2 mol%)
iPr
N
N
iPr
(b)
(c)
N
H
O
O
Cl
CFL/air, 20h, MeCN
Me
F
Me
H
3k
4k
, no additve: 21%
with CaCl2: 60%
N
Me
N
F
N
N
N
Me
N
F
H
Me
O
O
H
H
N
Cl
EosinY (2 mol%)
Me
F
O
Et
O
N
Et
Et
N
H
O
Et
N
O
CFL/air, 20h, MeCN
2m, 90%
2n, 90%
2o, 84%
H
O
O
3l
4l
: 0%
Figure 1. Visible light mediated synthesis of symmetrical azobenzene:
the reactions were performed in 20 mL vial with 1 (0.2 mmol), Eosin Y
(2 mol%), MeCN 1.0 mL and the vial were placed under ambient
conditions with irradiation for 20 h, yields of isolated products are
reported.
Scheme 2. Further extension of our strategy.
To better understand the reaction mechanism, control
experiments were performed (Scheme 3). First, standard reaction
was performed without Eosin Y, we found that there was only 10%
of 2a produced (Scheme S1). When the reaction with
hydrazobenzene was performed under N2 atmosphere, only an 4%
yield of azobenzene was produced, which indicated the key role of
air (O2) for this transformation (Scheme 3a). When the reaction was
performed without light, only a 2% yield of azobenzene was detected
(Scheme 3b). Those experiments proved the key role of Eosin Y, light
and O2 in the reaction process. Then, in a fluorescence quenching
study, we found that hydrazobenzene could efficiently quenched the
excited state of Eosin Y (Supporting information, Figure S1a), while
the quenching ability of saturated O2 was barely discernible
(Supporting information, Figure S1b).
R2
H
R2
N
N
EosinY (2 mol%)
N
N
H
R1
R1
CFL/air, 20h, MeCN
3
4
Me
Me
OMe
N
N
Ph
N
N
Ph
N
Ph
N
4a
4b
4c
, 90%
, 90%
, 77%
F
F
Cl
N
N
N
Ph
N
Ph
N
Ph
N
4d
4e
4f
, 80%
, 87%
, 95%
H
N
N
EosinY (2 mol%)
N
N
Me
Cl
Cl
Cl
(a)
H
CFL/N2, 20 h, MeCN
N
N
N
Ph
N
Me
Ph
N
Me
Ph
N
1a
2a
, yield 4%
4g
4h
4i
, 81%
, 89%
, 80%
H
N
N
N
EosinY (2 mol%)
N
(b)
Figure 2. Visible light promoted synthesis of unsymmetrical azobenzene: the
reactions were performed in 20 mL vial with 3 (0.2 mmol), Eosin Y (2 mol%),
MeCN 1.0 mL and the vial were placed under ambient conditions with
irradiation for 20 h, yields of isolated products are reported.
H
dark/air, 20 h, MeCN
1a
2a
, yield 2%
Scheme 3. Control experiments for mechanism studies.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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H
N View Article Online
HD
N
N
DOI: 10.103H9/C9GC01618J
EosinY (2 mol%)
CFL/N2, 20 h, MeCN
N
H
N
2a
1a
H
SET
HD
= hydrogen donor
Ph
N
Ph
Ph
N
Ph
N
H
.
+
H
Me
1a
SH
NHNH2
A
N2H4.H2
O
N
Eosin Y*
Eosin Y-.
N
H
H
O2
HD1
HD2
HD3
HD4
HD5
56%
yields of 1a:
60%
82%
50%
50%
: E1/2ox = +0.58 V vs SCE
Eosin Y
-.
Eosin Y
1a
O2
Figure 4. Eosin Y catalyzed transfer hydrogenation of azobenzene using
a series of hydrogen donor: the reactions were performed in 20 mL vial
with 2a (0.2 mmol), Eosin Y (2 mol%), MeCN 1.0 mL and the
corresponding hydrogen donor (0.42 mmol), the vial were placed under
ambient conditions with irradiation for 20 h, yields were determined
using by 1H NMR using CH2Br2 as internal standard.
: E1/2red = +0.83 V vs SCE
.OOH
H2O2
N
Ph
N
Ph
Ph
N
Ph
N
H
.
2a
B
Figure 3. Plausible reaction mechanism for oxidative azobenzene
synthesis.
Conclusions
In conclusion, we have developed the first visible-light-driven,
Based on our mechanistic studies and recent literature
reports,37, 40, 41 we proposed a reaction mechanism as depicted
in Figure 3. Eosin Y is first excited under visible light to produce
its excited state species, Eosin Y*, which undergoes a single
electron transfer with hydrazobenzene to form the
corresponding radical anion of Eosin Y and radical cation A. The
Eosin Y radical anion then relay-transfers one electron to O2 to
form a superoxide anion, and Eosin Y returns to its ground state.
Then, the superoxide anion gains one proton from A to produce
B and a peroxide radical. Finally, the peroxide radical reacts with
B to produce azobenzene and hydrogen peroxide.
photo-redox-catalyzed
oxidative
dehydrogenation
of
hydrazobenzene to azobenzene using Eosin Y as the catalyst.
The use of visible light and air as oxidant make this process
sustainable and practical. Thus, a series of symmetrical and
unsymmetrical azobenzenes were successfully synthesized in
excellent yields (up to 99%) with broad functional-group
tolerance. Moreover, the interconversion from azobenzene to
hydrazine derivatives was also realized. Thus, the visible-light-
driven, photo-redox-catalyzed transfer hydrogenation of
azobenzene was developed and found to be compatible with
hydrogen donors such as phenyl hydrazine and cyclic amines.
Compared with traditional (thermal/transition-metal) catalytic
methods, our process avoids over-reduction to aniline, which
extends the applicability of photo-redox catalysis.
Recently, the photo-redox transfer hydrogenation of imines
(with polar
π bonds) was reported using thiols and
triethylamine as hydrogen source and one-electron donor.42-44
However, no analogous transfer hydrogenation of non-polar π
bond-containing azobenzenes (N=N) has been reported. Given
the virtual absence of previous examples, we investigated
whether Eosin Y was able to catalyze the transfer hydrogenation
of azobenzene to hydrazobenzene with the hope of performing
the reaction under mild conditions and thus suppressing the
over-reduction of hydrazobenzene to aniline. The reaction was
performed with 0.1 mmol of azobenzene 2a, 0.2 mmol of p-
thiocresol, and 2% of Eosin Y in 1 mL of MeCN under visible light
irradiation. We were delighted to find that a 91% yield of
hydrazobenzene 1a was produced (Figure 4). Inspired by this
result, a series of potential hydrogen donors were tested. We
found that apart from widely used thiols, phenyl hydrazine and
hydrazine were also good transfer hydrogenation reagents,
producing hydrazobenzene in 82% and 50% yields, respectively.
Moreover, cyclic amines were also able to hydrogenate
azobenzene to hydrazobenzene with 50% and 56% yields,
respectively (Figure 4).
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (91845108) and Natural Science Foundation of Jiangsu
Province (Y81266JZQ3) for generous financial support for our
programs. We also thank the National Program for Thousand
Young Talents of China for support to start the Lab and Prof.
Liang-Qiu Lu and Quanquan Zhou for helpful suggestions.
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