Synthesis and photochemical properties of oligo-ortho-azobenzenes

Article abstract of DOI:10.1021/jo201996w

Azobenzenes have attracted great interest in recent years because of their ability to change conformation upon irradiation. This property has been featured in several applications not only in organic chemistry but also in biology. Even though monoazobenzenes have been extensively studied and documented in the literature, only a few methods are available for the synthesis of oligo-ortho-azobenzenes. Also, their photochemical properties have not been reported so far. This study shows an efficient strategy for the preparation of oligo-ortho-azobenzenes and the investigation of their photochemical properties. It is demonstrated that the absorption spectra are highly influenced by the substituents. Interestingly, none of the ortho-bis-, tris-, or tetra-azobenzenes showed any E → Z isomerization. Only the ortho-nitrogen-substituted monoazobenzenes' photochromic behavior upon UV irradiation was observed.

Full text of DOI:10.1021/jo201996w

Article
pubs.acs.org/joc
Synthesis and Photochemical Properties of Oligo-ortho-azobenzenes
Silvia Bellotto,†,‡ Raphael Reuter,† Christian Heinis,‡ and Hermann A. Wegner*,†
†Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland
‡Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fed
erale de Lausanne (EPFL), CH-1015 Lausanne,
́ ́
Switzerland
S
* Supporting Information
ABSTRACT: Azobenzenes have attracted great interest in recent years because of
their ability to change conformation upon irradiation. This property has been
featured in several applications not only in organic chemistry but also in biology.
Even though monoazobenzenes have been extensively studied and documented in
the literature, only a few methods are available for the synthesis of oligo-ortho-
azobenzenes. Also, their photochemical properties have not been reported so far.
This study shows an efficient strategy for the preparation of oligo-ortho-
azobenzenes and the investigation of their photochemical properties. It is
demonstrated that the absorption spectra are highly influenced by the substituents. Interestingly, none of the ortho-bis-, tris-,
or tetra-azobenzenes showed any E → Z isomerization. Only the ortho-nitrogen-substituted monoazobenzenes' photochromic
behavior upon UV irradiation was observed.
́
azobenzenes, however, has only been scarcely addressed. Bleger
INTRODUCTION
■
et al. investigated the photochromic and thermochromic
behavior of four different para-bis(azo) connected by
biphenyls.17 Furthermore, the photochemical properties of a
meta- and a para-bis(azo) derivative have been studied and
compared. It has been shown that while the (E,E)-m-
bisazobenzene has a similar behavior to the (E)-azobenzene,
the (E,E)-p-azobenzene presents a red shift in the absorption
spectrum and a lower photoreactivity upon UV irradiation.18
The corresponding ortho-bis-azobenzenes, though, have not
been investigated so far. One explanation might be the
difficulties in the preparation of these structural themes with
the known methods, making their synthesis quite challenging.
Only a few examples have been reported in the literature. At the
end of the 19th century, Mendola published a diazotization by
sodium nitrite.19 The synthesis of ortho-bis-azobenzenes,
performed by Mills reaction in acetic acid and sodium acetate,
was reported by Ruggli et al.20−22 More recently, in addition
to the preparations previously published by our group,5 the
treatment of azoxybenzenes with sodium sulphide23 or a
diazotization with sodium nitrite24 have also been described for
their synthesis. Herein, we present an efficient strategy for the
preparation of oligo-ortho-azobenzenes and show the influence
of conjugated azobenzene units and substituents in the ortho
position on the photochemical properties.
Since their discovery in the 19th century, azocompounds have
been extensively used as dyes and pigments.1 Therefore, several
synthetic strategies have been developed to prepare azobenzene
derivatives. Among all, the most relevant and common methods
found in literature are the oxidation of anilines, the reduction
of nitro-substituted aromatic compounds, the Mills reaction2
(condensation of an aniline with a nitroso compound), and the
diazo-coupling via diazonium salts.3 Recently, a new synthetic
strategy was proposed, in which a Pd-catalyzed coupling
reaction was employed.4 In addition to their use as colorants, a
well-known property of azobenzenes is the photoisomerization
from the E to the Z isomer and back.5 The two isomers can be
switched with ultraviolet light. The more intense absorption at
320 nm corresponds to the π−π* (S2 state) transition, and
irradiation at this wavelength leads to E → Z isomerization,
whereas the Z isomer absorbs at longer wavelengths, around
430 nm, which is due to the n−π* (S1 state) transition inducing
Z → E isomerization. Since the E isomer is more stable by
approximately 50 kJ/mol, the Z isomer can also easily relax
back via Z → E isomerization upon heating.6 Upon UV
irradiation, the distance between the para carbon atoms
decreases from about 9.0 Å in the E isomer to 5.5 Å in the Z
form.7 Thanks to this property, azo compounds have been
employed for different uses and applications, such as in light
controlled polymers,8 liquid crystals,9 surfaces,10 and catalysts,11
and were more recently found to be promising in controlling
the structural and functional changes in biomolecules.
Azobenzene derivatives, indeed, have been used for the
photoswitching of proteins, like ion channels and receptors,
and for the photocontrol of peptides,12 nucleic acids, lipids, and
carbohydrates.13
RESULTS AND DISCUSSION
■
Even though several procedures are known for the synthesis of
azobenzenes, none of them proved to be suitable for the pre-
paration of ortho-nitrogen-substitued azobenzenes. The oxidative
Compounds containing isolated azobenzenes have been
Received: September 27, 2011
Published: October 17, 2011
already thoroughly investigated;14−16 the interplay of multiple
© 2011 American Chemical Society
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coupling with manganese dioxide, potassium superoxide, the
reductive coupling with zinc and sodium hydroxide, a Pd-
catalyzed strategy, and the Mills reaction were evaluated; none
of them gave the expected product. For this reason, a method
previously developed in our group was used,5 in which the Mills
reaction was modified using toluene as solvent and AcOH in
stoichiometric amounts. Several ortho-nitrogen-substituted
azobenzenes have been synthesized using these optimized
conditions (Table 1).
Table 2. Synthesis of 2,2″-Dinitrogen-Substituted Bisazo-
benzenes 4
product
(yield %)
a
Table 1. Synthesis of 2,2′-Dinitrogen-Substituted Azo-
entry
R1
R2
T/°C
time
condition
benzenes 3
1
2
3
4
5
6
7
NH2
NH2
NH2
H
NHAc
H
60
60
rt
2.5 d
21 h
15 min
21 h
21 h
21 h
21 h
B
B
A
A
A
A
A
C
4a (16)
4b (16)
4c (81)
4d (17)
4e (44)
4f (66)
4g (2)
NO2
NO2
NHAc
NO2
H
60
60
60
60
NHAc
NHAc
NHAc
4g (44)
R1
R2
T/°C
product (yield %)
a
entry
(A) AcOH; (B) toluene, AcOH (4 equiv); (C) 2 (4 equiv), toluene,
1
2
NH2
NH2
NH2
H
NHAc
H
60
60
rt
3a (70)
3b (33)
3c (4)
AcOH (4 equiv).
3
NO2
NO2
H
The method was shown to be efficient for the preparation of
the nonsymmetric 2,2″-disubstitued bis-ortho-azobenzenes. Indeed,
compounds 4f and 4g were obtained in good to moderate
yields starting from o-phenylenediamine (1a) (Scheme 1).
4
60
60
60
60
60
60
60
3d (24)
3e (n.r.)
3f (n.r.)
3g (4)
5
NO2
NO2
NHAc
NHAc
NHAc
H
6
NO2
NHAc
NO2
H
7
8
3h (48)
3i (11)
3i (72)
Scheme 1. One-Pot Preparation of Nonsymmetric 2,2″-
Disubstitued Bis-ortho-azobenzenes
9
10
NHAc
The efficiency of the reactions was shown to be dependent
on the substituents. The reaction of entry 3 (Table 1) pro-
ceeded at rt because of the electron-withdrawing effect of the
nitro group making the nitroso coupling partner 2 more
reactive. However, 2-nitroaniline did not react either with
nitrosobenzene (2b) (Table 1, entry 5) or 2-nitronitrosoben-
zene (2c) (Table 1, entry 6). Because of the presence of the
nitro group in ortho position, the nucleophilicity of the amino
group is greatly reduced toward an attack at the nitroso
compound.
In order to obtain ortho-bis-azobenzenes, 2-amino-substituted
azobenzenes 3a, 3b, and 3l were subjected to the optimized
Mills condition (AcOH, toluene). Interestingly, the classical
condition (AcOH as solvent) gave, in most cases, superior
results.
The reactivity of the nitroso compounds 2a, 2b, and 2c was
similar to the one observed for the 2-substitued-anilines
influenced by the electron-withdrawing or -donating effect of
the substituents. Indeed, thanks to the presence of the nitro
group, the reaction of the diamine with 2-nitronitrosobenzene
(2c) (Table 2, entry 3) was completed after a few minutes at rt
under classical Mills conditions, whereas the other products
were only obtained after longer reaction times at 60 °C. For
example, when 2-nitrosoacetanilide (2a) was used, the reaction
was completed after 2.5 days and the product was obtained in
low yield (Table 2, entry 1). The reaction of entry 7 (Table 2)
gave 2-phenylbenzotriazole as the main product. Only when 4
equiv of nitrosobenzene (2b) was used, the desired compound was
obtained as the major product (Table 2, entry 7, condition C).
The different reactivity of o-phenylenediamine (1a) and 2-
amino-substituted azobenzenes opens the possibility to obtain
2,2″-dinitrogen-substituted bisazobenzenes in a one-pot procedure.
The same procedure was followed in the preparation of the
symmetric 2,2″-disubstitued bis-ortho-azobenzenes (Scheme 2).
When nitrosobenzene (2b) and o-phenylenediamine (1a)
were subjected to the modified Mills conditions, the product
4h was isolated after 3 days in 33% yield. However, the reac-
tion with the 2-nitronitrosobenzene did not give the desired
product.
Unfortunately, the same protocol was not applicable for the
synthesis of symmetric 2,2″′-disubstitued tris-ortho-azobenzenes
5 (Scheme 3).
Therefore, a stepwise approach was chosen to access 2,2″′-
disubstitued tris-ortho-azobenzenes with different substituents
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reactions gave first compound 4a in 59% and the final product
5f in 58% yield, respectively.
Scheme 2. One-Pot Preparation of the Symmetric 2,2″-
Disubstitued Bis-ortho-azobenzene 4h
With all the different oligo-ortho-azobenzenes in hand, the
photochemical properties were studied. The absorption spectra
were acquired, and the photoisomerization behavior was
assessed. In particular, the influence of the conjugated systems
and of the substituents was investigated. First, the absorption
spectra of the ortho-nitrogen-substituted monoazobenzenes
were measured in chloroform. All of them showed the typical
π−π* transition at around 320 nm. However, depending on the
substituents, specific characteristics can be distinguished. In all
the amino substituted azobenzenes, where a free NH2 group is
present, the spectra present a broad peak at around 450 nm.
The presence of the nitro group causes a slight bathochromic
shift of the spectrum. This shift is also observed when the
second substituent is an acetamido group or a free amino
group. In the case where the nitro group is the only substituent,
the spectrum is highly similar to the one of the azobenzene
(Figure 1a). If all the acetamido substituted azobenzenes are
compared, the presence of shoulders at around 400 nm is
observed (see the Supporting Information).
Scheme 3. Attempted One-Pot Preparation of Symmetric
2,2″-Disubstitued Tris-ortho-azobenzenes 5
All the ortho-nitrogen substituted monoazobenzenes were
irradiated at 365 nm, and the switching ability was analyzed by
UV-spectroscopy25 (Figure 1b−d; see the Supporting In-
formation for details). Compound 3i followed the classical well-
known behavior of azobenzene (Figure 1d). Indeed, the
absorbance increased at 450 nm and decreased at 320 nm
until it reached the photostationary state after a few seconds
of irradiation. It completely relaxed back after 2 h at rt.
Compounds 3a, 3b (Figure 1b), 3c, 3d (Figure 1c), 3g, 3h, and
3l, after exposure to UV light, showed a decrease in absorbance
for all transitions (see the Supporting Information for details).
The photoisomerization was reversible even after several cycles
of irradiation for all the monoazobenzenes. Furthermore, all
compounds except 3d (Figure 1c) showed a higher absorbance
than the initial state after incubation at rt, especially if kept
in the dark (3c, 3g, 3h, 3l, see the Supporting Information),
and could be completely converted back after exposure to
visible light indicating a mixture of E and Z isomers in the
photostationary state at these conditions. It was also shown that
the presence of electron-withdrawing groups caused only small
changes in the absorbance (compound 3d, Figure 1c, for com-
pounds 3c and 3h, see the Supporting Information). Bandara
et al. have also demonstrated the influence of substituents on
the photoswitching behavior of azobenzene compounds. They
describe that in their system, based on azo-(aminomethyl)-
pyridine compounds, the presence of electron-donating groups
reduces the photoisomerization properties. Similar to what we
observed, they reported a decreased absorbance for both
transitions upon UV irradiation.26 This behavior is unusual,
since the Z isomer of azobenzene normally shows an increased
intensity for the n−π* transition. Consequently a photo-
chemical E → Z isomerization is observed after irradiation
with visible light, whereas for most azobenzenes a Z → E
isomerization takes place if irradiated at 450 nm.
(Scheme 4). The modified Mills reaction was found to be the best
procedure. It was observed that the higher the number of the
azobonds, the longer the reaction time. Indeed, the reaction
required 8 days to provide the product in acceptable yield. The
synthesis of the 2,2⁗-disubstitued tetra-ortho-azobenzenes was
performed only for the dinitro-ortho-substituted 6. As expected
from the reactivity previously observed, the reaction was finished
after only 4 days. The product was obtained in a moderate yield
(25%) compared to the tris-ortho-azobenzenes 5a and 5b.
Other nonsymmetric 2,2″-disubstitued tris-ortho-azobenzenes
were synthesized according to the strategy shown in Scheme 5
relying on the one-pot procedure to prepare the key inter-
mediate 3a. The deprotection of compound 4g gave the amino-
substituted bisazobenzene 4b, followed by Mills reaction. The
coupling with 2-nitrosoacetanilide (2a) and nitrosobenzene
(2b) did not provide any of the desired product. When 2-
nitronitrosobenzene (2c) was used, the reaction occurred and
the corresponding tris-ortho-azobenzene 5c was obtained after
6 days in 21% yield. Again, this reactivity can be rationalized by
the reduced electron density of the nitroso functionality due to
the electron-withdrawing effect of the nitro group.
The absorption spectra of the bis-ortho-azobenzenes showed
a similar behavior as for the monoazobenzenes (Figure 2).
Indeed, as previously described, the nitro group caused a small
shift in the spectra, whereas the presence of the amino group
was easily detectable because of the broad and high absorbance
at around 450 nm. Once again, the acetamido-substituted
compounds showed the typical pattern with three maxima
at 340, 400, and 470 nm. Furthermore, close to the π−π*
Accordingly, a similar strategy gave 2-nitro-2‴-acetamido-
tris-ortho-azobenzene (5f) as presented in Scheme 6. The
ortho-substituted diamine 3l was obtained by deprotection of
2-acetamido-2′-aminoazobenzene (3a). The following two Mills
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Scheme 4. Synthesis of Nonsymmetric 2,2‴-Disubstitued Tris-ortho-azobenzenes
transition at around 350 nm, an additional shoulder can be
noticed, which is present in all bis-ortho-azobenzenes. In the
case of the diamino-substituted derivative 4i and 2-nitro-
2‴acetamido-bisazobenzene (4f) (see the Supporting Informa-
tion), only a broadening of the absorption can be seen.
In contrast to the switching behavior observed for mono-
azobenzenes, none of bis-ortho-azobenzenes switched upon UV
irradiation. Cisnetti and co-workers have demonstrated that
(E,E)-m-bisazobenzene behaves similarly to azobenzene and
that (E,E)-p-bisazobenzene also isomerizes although a
decreased photoreactivity, has been observed.18 The authors
explain this fact by the larger electronic coupling in the
p-bisazobenzene compared to the m-bisazobenzene, which they
confirmed by quantum chemical calculations. If this increased
electronic communication is the reason for the reduced
isomerization, o-bisazobenzenes should exhibit an even lower
isomerization tendency. Indeed, in our case, all the tested
compounds showed complete inertness toward photoisomeri-
zation. One possible alternative explanation would be that the
back-isomerization is faster than the minimum time required
for the measurement (5 s). Flash photolysis studies have not
been done so far to rule out this option.
additional absorption caused by the acetamido group is less
evident. The spectrum of tetra-azobenzene 6 is comparable to
the one of 4d and 5c because of the presence of the nitro
groups. None of these compounds show isomerization upon
UV irradiation as it was previously observed for the bis-ortho-
azobenzenes.
CONCLUSIONS
■
A stepwise convenient strategy for the preparation of oligo-
ortho-azobenzenes and a one-pot procedure for the synthesis of
bis-ortho-azobenzenes have been presented. The photochemical
properties of these compounds have been thoroughly
investigated. It has been shown that the absorption spectra
exhibit common features depending on the substituent.
Additionally, only the ortho-nitrogen-substituted monoazoben-
zenes isomerized upon UV irradiation. Furthermore, the
presence of electron-withdrawing groups induced a decreased
photochromism. Current efforts in the group address the
reason for the inhibition of the photochromism by substitution
with another azobenzene moiety in the ortho position.
EXPERIMENTAL SECTION
■
2-Nitrosoacetanilide (2a). A solution of o-phenylenediamine
(1a) (10.0 g, 92.5 mmol, 1.00 equiv) in 150 mL of EtOAc was cooled
down below 5 °C. Then, an ice-cold solution of acetic anhydride (9.12 mL,
97.1 mmol, 1.05 equiv) in 80 mL of EtOAc was added, and the
An analogous behavior was also observed for the tri- and
tetra-azobenzenes (Figure 3). The amino group causes also in
these cases an increased absorbance at around 450 nm. The
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Scheme 5. Synthesis of Nonsymmetric 2,2‴-Disubstitued
Tris-ortho-azobenzenes
Scheme 6. Synthesis of 2-Nitro-2‴-acetamido-tris-ortho-
azobenzenes (5f)
(d, J = 8.0 Hz, 1H). The compound 2c is in equilibrium with its dimer,
and the analytical data correspond to the literature.30
General Procedure for the Mills Reaction Using AcOH
(Condition A). A solution of amine (1.00 equiv) in acetic acid
(7 mL/mmol) was degassed with a nitrogen stream for 15 min. Then,
the nitroso compound 2 (1.00 equiv) was added. The mixture was
stirred at 60 °C for 21 h, and the solvent was removed under reduced
pressure. The residue was purified by flash column chromatography.
General Procedure for the Mills Reaction Using AcOH/
Toluene (Condition B). A solution of amine (1.00 equiv) in toluene
(7 mL/mmol) was degassed with a nitrogen stream for 15 min. Then,
the nitroso compound (1.00 equiv) and acetic acid (4.00 equiv) were
added. The mixture was stirred at 60 °C for 21 h, and the solvent was
removed under reduced pressure. The residue was purified by flash
column chromatography.
General Procedure for the Mills Reaction Using AcOH/
Toluene (Condition C). A solution of amine (1.00 equiv) in toluene
(7 mL/mmol) was degassed with a nitrogen stream for 15 min. Then,
the nitroso compound (4.00 equiv) and acetic acid (4.00 equiv) were
added. The mixture was stirred at 60 °C for 21 h, and the solvent was
removed under reduced pressure. The residue was purified by flash
column chromatography.
2-Acetamido-2′-amino-ortho-azobenzene (3a). The title
compound 3a was prepared according to general procedures
(condition B) from o-phenylenediamine (1a) (0.821 g, 7.58 mmol,
1.00 equiv) and 2-nitrosoacetanilide (2a) (1.25 g, 7.71 mmol, 1.00
equiv). The residue was purified by flash column chromatography
(SiO2, hexane/EtOAc, 1:1), and the title compound was obtained as
red solid in 70% yield (1.35 g): mp 148−150 °C (lit. 143 °C);31 1H
NMR (400 MHz, CDCl3) δ 9.95 (s, 1H), 8.63 (d, J = 8.3 Hz, 1H),
7.70 (dd, J = 8.1, 1.6 Hz, 1H), 7.62 (dd, J = 8.1, 1.3 Hz, 1H), 7.45−
7.37 (m, 1H), 7.29−7.22 (m, 1H), 7.18−7.10 (m, 1H), 6.87−6.71 (m,
2H), 5.40 (s, 2H), 2.27 (s, 3H); 13C NMR (101 MHz, CDCl3)
δ 168.6, 145.0, 137.5, 135.5, 133.2, 131.8, 124.4, 123.5, 122.2, 120.5,
120.2, 118.0, 117.4, 25.5.
mixture was stirred for 20 min. The precipitate that was obtained was
filtered with a glass filter under vacuum to give 5.13 g (37%) of 2-
aminoacetanilide: H NMR (400 MHz, CDCl3) δ 9.10 (s, 1H), 7.15
1
(dd, J = 7.8, 1.2 Hz, 1H), 6.92−6.85 (m, 1H), 6.70 (dd, J = 8.0, 1.3 Hz,
1H), 6.53 (td, J = 7.6, 1.4 Hz, 1H), 4.84 (s, 2H), 2.03 (s, 3H). The
analytical data correspond to the literature.27 The acetylated diamine
(5.00 g, 33.3 mmol, 1.00 equiv) was suspended in 50 mL of CH2Cl2.
Oxone (30.7 g, 49.9 mmol, 1.50 equiv), dissolved in 200 mL of water,
was added and stirred fiercely for 1 h at rt. The organic phase was
separated, and the aqueous phase was extracted with 100 mL of
CH2Cl2. The combined organic phases were washed with 1 m HCl
(100 mL), saturated aq NaHCO3 (100 mL), and water (100 mL). It
was then dried over MgSO4, concentrated, and purified by flash
column chromatography (SiO2, hexane/EtOAc, 2:1), and the product
was obtained as green solid (2.41 g, 44% yield): mp 107−108 °C (lit.
mp 106−107 °C);28 1H NMR (400 MHz, CDCl3) δ 10.79 (s, 1H),
8.85 (dd, J = 8.6, 1.0 Hz, 1H), 7.71 (ddd, J = 8.5, 7.2, 1.4 Hz, 1H), 7.32
(s, 1H), 7.22−7.08 (m, 1H), 2.35 (s, 3H). The analytical data
correspond to the literature.5
2-Nitronitrosobenzene (2c). Oxone (20.0 g, 32.6 mmol, 1.50
equiv) was treated with 24 mL of concentrated H2SO4. The
suspension was then poured onto 130 g of crushed ice, and the
mixture was stirred at rt until all of the ice had melted. Then,
2-nitroaniline (3.00 g, 21.7 mmol, 1.00 equiv) was added and stirred at
rt for 20 h. The mixture was extracted twice with 150 mL of CH2Cl2
and dried over MgSO4, the solvent was removed, and the title
compound was obtained as a brown solid (3.12 g, 94%): mp 119−122
°C (lit. 135−137 °C);29 1H NMR (400 MHz CDCl3) δ 8.17 (d, J =
8.0 Hz, 1H), 7.96 (t, J = 8.0 Hz, 1H), 7.77 (t, J = 8.0 Hz, 1H), 6.63
2-Amino-ortho-azobenzene (3b). The title compound 3b was
prepared according to general procedures (condition B) from o-phenyl-
enediamine (1a) (0.820 g, 7.58 mmol, 1.00 equiv) and nitrosobenzene
(2b) (812 mg, 7.58 mmol, 1.00 equiv). The residue was purified by
flash column chromatography (SiO2, hexane/EtOAc, 7:1), and the
title compound was obtained as brown solid in 33% yield (498 mg):
1
mp 58−62 °C; H NMR (400 MHz, CDCl3) δ 7.87−7.82 (m, 3H),
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Figure 1. Absorption spectra (a) and photoisomerization of representative ortho-amino-, nitro-, and acetamido-substituted monoazobenzenes (3b, 3d, and
3i, respectively; b−d) in chloroform.
7.53−7.47 (m, 2H), 7.45−7.39 (m, 1H), 7.22 (ddd, J = 8.5, 7.1, 1.6
Hz, 1H), 6.83 (ddd, J = 8.2, 7.1, 1.3 Hz, 1H), 6.77 (dd, J = 8.2, 1.1 Hz,
1H), 5.90 (s, 2H). The analytical data correspond to the literature.32
2-Amino-2′-nitro-ortho-azobenzene (3c). The title compound
3c was prepared according to general procedures (condition B) from
o-phenylenediamine (1a) (0.820 g, 7.58 mmol, 1.00 equiv) and 2-
nitronitrosobenzene (2c) (1.15 g, 7.58 mmol, 1.00 equiv). The residue
was purified by flash column chromatography (SiO2, hexane/EtOAc,
2:1) followed by preparative TLC (SiO2, hexane/CHCl3, 1:1), and the
title compound was obtained as brown solid in 4% yield (73.2 mg):
δ 7.98−7.89 (m, 3H), 7.72−7.65 (m, 2H), 7.62−7.49 (m, 4H); 13C
NMR (101 MHz, CDCl3) δ 152.6, 147.6, 145.6, 133.2, 132.4, 130.6,
129.4 (2C), 124.2, 123.8 (2C), 118.6.
2,2′-Diacetamido-ortho-azobenzene (3g). The title com-
pound 3g was prepared according to general procedures (condition
B) from N-(2-aminophenyl)acetamide (495 mg, 3.30 mmol, 1.00
equiv) and 2-nitrosoacetanilide (2a) (541 mg, 3.30 mmol, 1.00 equiv).
The residue was purified by flash column chromatography (SiO2,
DCM/EtOAc, 1:1), and 41.3 mg of orange product were obtained (4%
yield): mp 273−275 °C; 1H NMR (400 MHz, CDCl3) δ 9.38 (s, 2H),
8.68 (d, J = 8.4 Hz, 2H), 7.64 (d, J = 8.1 Hz, 2H), 7.52 (t, J = 7.8 Hz,
2H), 7.17 (t, J = 7.7 Hz, 2H), 2.30 (s, 6H). The analytical data
correspond to the literature.34
1
mp 95−97 °C; H NMR (400 MHz, DMSO) δ 8.05 (dd, J = 8.1, 1.2
Hz, 1H), 7.92 (dd, J = 8.1, 1.2 Hz, 1H), 7.84−7.76 (m, 1H), 7.68−
7.60 (m, 2H), 7.36 (s, 2H), 7.26 (ddd, J = 8.4, 6.9, 1.6 Hz, 1H), 6.88
(dd, J = 8.4, 1.0 Hz, 1H), 6.69 (ddd, J = 8.2, 6.9, 1.2 Hz, 1H); 13C
NMR (101 MHz, DMSO) δ 146.7, 145.0, 144.2, 135.9, 134.1, 133.4,
130.0, 127.7, 124.1, 118.0, 117.4, 115.9; MS (EI, 70 eV) m/z (%) =
242 (57%) [M+], 92 (100%); C12H10N4O2 (242.24) calcd C 59.50, H
4.16, N 23.13; found C 59.35, H 4.14, N 22.88.
2-Acetamido-2′nitro-ortho-azobenzene (3h). The title com-
pound 3h was prepared according to general procedures (condition B)
from N-(2-aminophenyl)acetamide (500 mg, 3.31 mmol, 1.00 equiv)
and 2-nitronitrosobenzene (2c) (506 mg, 3.33 mmol, 1.00 equiv). The
residue was purified by flash column chromatography (SiO2, hexane/
EtOAc, 1:1). The solvent was evaporated and the orange product was
2-Nitro-ortho-azobenzene (3d). The title compound 3d was
prepared according to general procedures (condition B) from aniline
(0.827 mL, 9.07 mmol, 1.00 equiv) and 2-nitronitrosobenzene (2c)
(1.38 g, 9.07 mmol, 1.00 equiv). The residue was purified by flash
column chromatography (SiO2, hexane/EtOAc, 3:1), and the title
compound was obtained as an orange-brown solid in 24% yield (502
mg): mp 66−70 °C (lit. 67−68 °C);33 1H NMR (400 MHz, CDCl3)
1
obtained in 48% yield (449 mg): mp 143−147 °C; H NMR (400
MHz, CDCl3) δ 10.27 (s, 1H), 8.74 (d, J = 8.5 Hz, 1H), 7.93 (dd, J =
8.0, 1.4 Hz, 2H), 7.81 (dd, J = 8.0, 1.5 Hz, 1H), 7.74 (td, J = 7.7, 1.4
Hz, 1H), 7.61 (ddd, J = 8.0, 7.4, 1.5 Hz, 1H), 7.57−7.47 (m, 1H), 7.22
(ddd, J = 8.6, 7.3, 1.3 Hz, 1H), 2.30 (s, 3H); 13C NMR (101 MHz,
CDCl3) δ 167.0, 146.2, 144.8, 138.9, 135.0, 134.6, 133.4, 130.9, 126.7,
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2.28 (s, 3H); 13C NMR (101 MHz, CDCl3) δ 168.7, 152.5, 138.9,
136.1, 133.1, 132.0, 129.5 (2C), 123.5, 122.8 (2C), 121.3, 120.4, 25.5.
2,2′-Diamino-ortho-azobenzene (3l). A solution of o-phenyl-
enediamine (1a) (0.764 g, 7.07 mmol, 1.00 equiv) in toluene (50 mL)
was degassed with a nitrogen stream for 15 min. Then, 2-
nitrosoacetamide (1.16 g, 7.07 mmol, 1.00 equiv) and acetic acid
(1.62 mL) were added. The mixture was stirred at 60 °C for 21 h, and
the solvent was removed under reduced pressure. The residue was
purified by flash column chromatography (SiO2, hexane/EtOAc, 2:1),
and the compound 3a was obtained as red solid (0.792 g, 44% yield).
Then, a solution of 2-acetamido-2′-amino-ortho-azobenzene (3a) (740
mg, 2.91 mmol, 1.00 equiv) in ethanol (138 mL) was treated with a
solution of KOH (12.1 g, 215 mmol, 74.0 equiv) in ethanol (80 mL)
and water (32 mL). The mixture was heated to 90 °C. After 1 h, the
mixture was poured onto ice (500 g) and extracted with CH2Cl2 (3 ×
100 mL). It was dried over Na2SO4 and concentrated to yield 0.583 g
(94%): mp 136−138 °C; 1H NMR (400 MHz, CDCl3) δ 7.68 (dd, J =
8.0, 1.4 Hz, 2H), 7.21−7.14 (m, 2H), 6.83−6.74 (m, 4H), 5.49 (s, 4H).
The analytical data correspond to the literature.5
2-Acetamido-2″-amino-ortho-bisazobenzene (4a). The title
compound 4a was prepared according to general procedures
(condition B) from 2,2′-diaminodiazobenzene (3l) (300 mg, 1.41
mmol, 1.00 equiv) and 2-nitrosoacetanilide (2a) (230 mg, 1.41 mmol,
1.00 equiv). After 2.5 days, the solvent was removed under reduced
pressure, and the residue was purified by flash column chromatography
(SiO2, hexane/EtOAc, 2:1). The product was obtained as a red solid
Figure 2. Absorption spectra of a representative ortho-amino-, nitro-,
acetamido-, and hydrogen-substituted bis-azobenzenes (4b, 4d, 4g,
and 4h, respectively) in chloroform.
1
(80.3 mg, 16%): mp 158−162 °C; H NMR (400 MHz, CDCl3) δ
10.12 (s, 1H), 8.69 (d, J = 8.3 Hz, 1H), 7.88 (dd, J = 8.1, 1.5 Hz, 1H),
7.85 (dd, J = 8.1, 1.5 Hz, 1H), 7.81 (dd, J = 7.9, 1.5 Hz, 1H), 7.72 (dd,
J = 7.7, 1.6 Hz, 1H), 7.62−7.44 (m, 3H), 7.20 (ddd, J = 15.3, 8.4, 1.4
Hz, 2H), 6.87−6.81 (m, 1H), 6.74 (dd, J = 8.2, 1.0 Hz, 1H), 6.31 (s,
2H), 2.00 (s, 3H). The analytical data correspond to the literature.5
2-Amino-ortho-bisazobenzene (4b). The title compound 4b
was prepared according to general procedures (condition B) from 2,2′-
diaminodiazobenzene (3l) (0.250 g, 1.19 mmol, 1.00 equiv) and
nitrosobenzene (2b) (130 mg, 1.19 mmol, 1.00 equiv). The residue
was purified by flash column chromatography (SiO2, hexane/EtOAc,
7:1). The product was obtained as a brown solid (57.1 mg, 16%): mp
97−100 °C; 1H NMR (400 MHz, CDCl3) δ 7.97 (dd, J = 8.0, 1.6 Hz,
1H), 7.95−7.88 (m, 3H), 7.76 (dd, J = 8.0, 1.4 Hz, 1H), 7.62−7.45
(m, 5H), 7.21 (ddd, J = 8.5, 7.1, 1.6 Hz, 1H), 6.86 (ddd, J = 8.2, 7.1,
1.3 Hz, 1H), 6.74 (dd, J = 8.3, 1.1 Hz, 1H), 6.69 (s, 2H); 13C NMR
(101 MHz, CDCl3) δ 153.3, 148.1, 147.9, 141.5, 138.0, 133.2, 132.3,
131.7, 131.4, 130.4, 129.3 (2C), 123.3 (2C), 117.3, 117.2, 117.1,
116.4; HRMS (ESI) m/z calcd for C18H15N5 [M + H+] 302.1406,
+
found 302.1400.
2-Amino-2″-nitro-ortho-bisazobenzene (4c). To a round-
bottom flask was added the 2,2′-diaminoazobenzene (3l) (80.0 mg,
0.377 mmol, 1.00 equiv) and 2-nitronitrosobenzene (2c) (57.3 mg,
0.377 mmol, 1.00 equiv) followed by AcOH (11 mL). The mixture
was stirred for 15 min. Then, water was added (50 mL/mmol), and
the liquid was carefully turned basic with sodium carbonate (2 M
Na2CO3 in H2O, 5 mL/1 mL AcOH). The mixture was then extracted
with CH2Cl2 (3 × 20 mL/mmol) and dried over MgSO4, and the
solvent was removed. The residual black solid was resuspended in
CH2Cl2 and purified by flash column chromatography (SiO2, DCM/
hexane 1:1). The product was obtained as a brown solid (106 mg,
Figure 3. Absorption spectra of the disubstitued tris- and tetra-ortho-
azobenzenes in chloroform.
124.5, 123.6, 120.9, 120.3, 25.4; MS (EI, 70 eV) m/z (%) = 284 (48%)
[M+], 134 (100%); C14H12N4O3 (284.27) calcd C 59.15, H 4.25,
N 19.71; found C 59.09, H 4.25, N 19.85.
2-Acetamido-ortho-azobenzene (3i). The title compound 3i
was prepared according to general procedures (condition B) from
aniline (0.294 mL, 3.22 mmol, 1.00 equiv) and 2-nitrosoacetanilide
(2a) (529 mg, 3.22 mmol, 1.00 equiv). The residue was purified by
flash column chromatography (SiO2, hexane/EtOAc, 1:1), and 555 mg
of yellow product were obtained (72% yield). The same product was
also prepared according to general procedures (condition B) from
2-acetamido-2′-amino-ortho-azobenzene (3a) (470 mg, 3.13 mmol,
1.00 equiv) and nitrosobenzene (2b) (357 mg, 3.33 mmol, 1.07
equiv). The residue was purified by flash column chromatography
(SiO2, hexane/EtOAc, 2:1), and 80.1 mg of yellow product was
obtained (11%): mp 124−127 °C (lit. 127−129 °C);35 1H NMR (400
MHz, CDCl3) δ 10.12 (s, 1H), 8.68 (d, J = 8.4 Hz, 1H), 7.91−7.81
(m, 3H), 7.58−7.50 (m, 3H), 7.50−7.45 (m, 1H), 7.21−7.14 (m, 1H),
1
81%): mp 111−113 °C; H NMR (400 MHz, CDCl3) δ 7.97 (d, J =
7.0 Hz, 2H), 7.91 (d, J = 8.1 Hz, 1H), 7.77 (d, J = 8.1 Hz, 1H), 7.72−
7.67 (m, 1H), 7.64 (t, J = 7.6 Hz, 1H), 7.62−7.56 (m, 2H), 7.51 (t, J =
7.6 Hz, 1H), 7.22 (d, J = 6.9 Hz, 1H), 6.87 (dd, J = 8.1, 7.0 Hz, 1H),
6.75 (d, J = 8.3 Hz, 1H), 6.62 (s, 2H); 13C NMR (101 MHz, CDCl3)
δ 149.0, 147.7, 147.4, 146.2, 141.7, 138.0, 133.3, 133.1, 133.0, 132.6,
130.6, 130.4, 124.3, 119.0, 117.31, 117.30, 117.26, 117.0; HRMS (ESI)
m/z calcd for C18H15N6O2 [M + H+] 347.1251, found 347.1260.
+
2-Nitro-ortho-bisazobenzene (4d). The title compound 4d
was prepared according to general procedures (condition A) from
2-amino-ortho-azobenzene (3b) (200 mg, 1.01 mmol, 1.00 equiv) and
2-nitronitrosobenzene (2c) (154 mg, 1.01 mmol, 1.00 equiv). The
mixture was stirred for 21 h at 60 °C. The solvent was removed under
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reduced pressure, and HCl (1 M aq solvent; 25 mL) was added. The
mixture was then extracted with CH2Cl2 (2 × 30 mL) and purified by
flash column chromatography (SiO2, hexane/EtOAc, 7:1). An oily
orange solution was obtained (57.2 mg) in 17% yield: mp 61−65 °C;
1H NMR (400 MHz, CDCl3) δ 8.01−7.97 (m, 2H), 7.97−7.93 (m,
1H), 7.83−7.78 (m, 1H), 7.76 (dd, J = 7.9, 1.4 Hz, 1H), 7.69 (dd, J =
5.2, 2.0 Hz, 2H), 7.67−7.61 (m, 1H), 7.61−7.50 (m, 5H); 13C NMR
(101 MHz, CDCl3) δ 153.1, 149.2, 147.6 (2C), 146.0, 133.3, 132.5,
131.7, 131.2, 130.7, 129.3 (2C), 124.3, 123.5 (2C), 119.11, 119.09,
117.9; HRMS (ESI) m/z calcd for C18H14N5O2+ [M + H+] 332.1142,
found 332.1148.
2,2″-Diacetamido-ortho-bisazobenzene (4e). The title com-
pound 4e was prepared according to general procedures (condition A)
from 2-acetamido-2′-amino-ortho-azobenzene (3a) (187 mg, 0.735
mmol, 1.00 equiv) and 2-nitrosoacetanilide (2a) (123 mg, 0.749
mmol, 1.02 equiv). The residue was purified by flash column
chromatography (SiO2, hexane/EtOAc, 1:2), and 130 mg of product
were obtained (44%): mp 182−184 °C; 1H NMR (400 MHz, CDCl3)
δ 10.04 (s, 2H), 8.69 (d, J = 8.3 Hz, 2H), 7.86 (dd, J = 8.1, 1.4 Hz,
2H), 7.76 (dd, J = 6.0, 3.4 Hz, 2H), 7.65 (dd, J = 6.0, 3.4 Hz, 2H), 7.51
(t, J = 7.8 Hz, 2H), 7.18 (t, J = 7.7 Hz, 2H), 1.91 (s, 6H). The
analytical data correspond to the literature.5
with a nitrogen stream for 15 min. Then, nitrosobenzene (2b) (594 mg,
5.55 mmol, 3.00 equiv) and AcOH (215 μL, 3.75 mmol, 2.00 equiv)
were added. The mixture was stirred at 60 °C for 18 h, and more
AcOH (108 mL) was added. After 2 days the solvent was removed
under reduced pressure. The residue was purified by flash column
chromatography (SiO2, hexane/EtOAc, 7:1), and the solid that was
obtained was recrystallized from EtOH to obtain 177 mg of a brown
solid (33%): mp 106−109 °C; 1H NMR (400 MHz, CDCl3) δ 8.01−
7.94 (m, 4H), 7.79−7.73 (m, 2H), 7.61−7.46 (m, 8H). The analytical
data correspond to the literature.23
General Procedure for One-Pot Preparation of Nonsym-
metric 2,2″-Disubstitued Bis-ortho-azobenzenes (Condition D). A
solution of o-phenylendiamine (1a) (1.00 equiv) in toluene (7 mL/mmol)
was degassed under a nitrogen stream for 15 min. Then, the first
nitroso compound (1.10 equiv) and AcOH (4.00 equiv) were
added. The mixture was stirred at 60 °C for 18 h, and the second
nitroso compound (1.00 equiv) and more acetic acid (toluene/
acetic acid, 1:3) were added. After 1 day the solvent was removed
under reduced pressure. The residue was purified by flash column
chromatography.
One-Pot Preparation of 2,2″-Disubstitued Bis-ortho-azoben-
zenes (4f). The title compound 4f was prepared according to general
procedures (condition D) from o-phenylenediamine (1a) (250 mg,
2.31 mmol, 1.00 equiv), 2-nitrosoacetanilide (2a) (417 mg, 2.54
mmol, 1.10 equiv), and 2-nitronitrosobenzene (2c) (352 mg, 2.31,
1.00 equiv). The residue was purified by flash column chromatography
(SiO2, hexane/EtOAc, 2:1), and the product 4f was obtained as an
orange solid (450 mg, 50%).
2-Acetamido-2″-nitro-ortho-bisazobenzene (4f). The title
compound 4f was prepared according to general procedures
(condition A) from 2-acetamido-2′-amino-ortho-azobenzene (3a)
(0.101 g, 0.393 mmol, 1.00 equiv) and 2-nitronitrosobenzene (2c)
(59.8 mg, 0.393 mmol, 1.00 equiv). The residue was purified by flash
column chromatography (SiO2, hexane/EtOAc, 2:1), and 103 mg of
1
One-Pot Preparation of 2,2″-Disubstitued Bis-ortho-azoben-
zenes (4g). The title compound 4g was prepared according to
general procedures (condition D) from o-phenylendiamine (1a) (1.80 g,
16.6 mmol, 1.00 equiv), 2-nitrosoacetanilide (2a) (2.73 g, 16.6 mmol,
1.00 equiv), and nitrosobenzene (2b) (7.13 g, 66.6, 4.00 equiv). The
residue was purified by flash column chromatography (SiO2, hexane/
EtOAc, 2:1), and the product was obtained in 23% yield (1.32 g).
2-Amino-2‴-acetamido-tris-ortho-azobenzene (5a). The title
compound 5a was prepared according to general procedures
(condition B) from 2,2″-diamino-bis-ortho-azobenzene (4i) (250 mg,
0.790 mmol, 1.00 equiv) and 2-nitrosoacetanilide (2a) (131 mg, 0.869
mmol, 1.10 equiv). After 8 d the solvent was evaporated, the reaction
mixture was purified by flash column chromatography (SiO2, hexane/
EtOAc, 2:1 to 1:3), and the product was obtained in 17% yield (73.1
mg): mp 177−180 °C; 1H NMR (400 MHz, CDCl3) δ 10.81 (s, 1H),
8.68 (d, J = 8.4 Hz, 1H), 7.99 (dd, J = 8.0, 1.6 Hz, 1H), 7.91 (dd, J =
3.5, 1.4 Hz, 1H), 7.89 (dd, J = 3.5, 1.4 Hz, 1H), 7.88−7.85 (m, 1H),
7.72−7.57 (m, 5H), 7.51−7.42 (m, 2H), 7.24−7.19 (m, 1H), 7.19−
7.13 (m, 1H), 6.84−6.78 (m, 1H), 6.67 (dd, J = 8.2, 1.0 Hz, 1H), 6.53
(s, 2H), 1.74 (s, 3H); 13C NMR (101 MHz, CDCl3) δ 169.7, 148.3,
148.1 (2C), 147.7, 141.71, 139.3, 137.9, 134.5, 133.3, 132.54, 132.50,
132.2, 131.6, 131.3, 130.2, 127.0, 123.4, 120.6, 119.5, 118.0, 117.8,
117.24, 117.22, 116.9, 25.2; HRMS (ESI) m/z calcd for C26H22N8O+
[M + H+] 463.1989, found 463.1997.
orange product were obtained (66%): mp 136−138 °C; H NMR
(400 MHz, DMSO) δ 10.19 (s, 1H), 8.32−8.27 (m, 1H), 8.15 (dd, J =
7.8, 1.5 Hz, 1H), 8.01 (dd, J = 8.0, 1.3 Hz, 1H), 7.88−7.71 (m, 5H),
7.66 (dd, J = 7.8, 1.5 Hz, 1H), 7.62 (dd, J = 8.0, 1.3 Hz, 1H), 7.59−
7.53 (m, 1H), 7.27−7.20 (m, 1H), 1.97 (s, 3H); 13C NMR (101 MHz,
CDCl3) δ 169.4, 148.8, 147.3, 147.1, 146.1, 139.4, 134.8, 133.6, 133.4,
132.8, 131.5, 130.1, 126.2, 124.4, 123.5, 120.6, 119.1, 118.8, 117.8,
25.2; MS (EI, 70 eV) m/z (%) = 388 (51%) [M+], 106 (100%);
C20H16N6O3 (388.39) calcd C 61.85, H 4.15, N 21.64; found C 61.46,
H 4.17, N 21.50.
2-Acetamido-ortho-bisazobenzene (4g). The title compound
4g was prepared according to general procedures (condition C) from
2-acetamido-2′-amino-ortho-azobenzene (3a) (195 mg, 0.767 mmol,
1.00 equiv) and nitrosobenzene (2b) (337 mg, 3.15 mmol, 4.00
equiv). After 3 h, the reaction was finished, and the solvent was
removed under reduced pressure. The residue was purified by flash
column chromatography (SiO2, hexane/EtOAc, 1:1), and 116 mg
1
(44%) of orange product were obtained: mp 112−113 °C; H NMR
(400 MHz, CDCl3) δ 10.49 (s, 1H), 8.70 (dd, J = 8.4, 1.1 Hz, 1H),
7.99 (dd, J = 8.1, 1.6 Hz, 1H), 7.92−7.87 (m, 2H), 7.85 (dd, J = 7.8,
1.6 Hz, 1H), 7.71 (dd, J = 7.7, 1.7 Hz, 1H), 7.61 (dqd, J = 14.8, 7.3,
1.6 Hz, 2H), 7.55−7.45 (m, 4H), 7.25−7.19 (m, 1H), 1.72 (s, 3H);
13C NMR (101 MHz, CDCl3) δ 169.4, 153.0, 149.7, 148.1, 147.1,
139.4, 135.2, 133.3, 131.8, 131.2, 129.4 (2C), 125.1, 123.4, 123.3 (2C),
120.6, 120.2, 118.0, 25.0; MS (EI, 70 eV) m/z (%) = 343 (83%) [M+],
106 (100%); C20H17N5O (343.39) calcd C 69.96, H 4.99, N 20.40;
found C 69.84, H 4.96, N 20.42.
2-Amino-tris-ortho-azobenzene 5b. The title compound 5b
was prepared according to general procedures (condition B) from 2,2″-
diamine-bis-ortho-azobenzene (4i) (250 mg, 0.790 mmol, 1.00 equiv)
and nitrosobenzene (2b) (84.6 mg, 0.790 mmol, 1.00 equiv). After 6
days the solvent was removed, the reaction mixture was purified by
flash column chromatography (SiO2, hexane/EtOAc, 7:1), and the
2,2″-Diamino-bis-ortho-azobenzene (4i). A solution of 2,2″-
diacetamido-ortho-bisazobenzene (4e) (3.76 g, 9.39 mmol, 1.00 equiv)
in ethanol (341 mL) was treated with a solution of KOH (30.4 g, 542
mmol, 57.7 equiv) in ethanol (200 mL) and water (77 mL). The
mixture was heated to 90 °C. After 1.5 h, the mixture was poured onto
ice (1.20 kg), extracted with CH2Cl2 (3 × 200 mL), dried over
Na2SO4, and concentrated to yield a red oil (2.96 g), which crystallized
1
product was obtained in 50% yield (128 mg): mp 156−159 °C; H
NMR (400 MHz, CDCl3) δ 7.95 (dd, J = 8.0, 1.6 Hz, 3H), 7.92−7.88
(m, 1H), 7.82 (dd, J = 8.0, 1.3 Hz, 1H), 7.79−7.75 (m, 1H), 7.70 (dd,
J = 7.3, 2.1 Hz, 1H), 7.64−7.54 (m, 3H), 7.54−7.43 (m, 4H), 7.22−
7.15 (m, 1H), 6.84 (t, J = 7.0 Hz, 1H), 6.69 (d, J = 8.2 Hz, 1H), 6.64
(s, 2H); 13C NMR (101 MHz, CDCl3) δ 153.1, 148.7, 148.5 (2C),
148.0, 141.5, 137.9, 133.3, 132.3, 132.1, 131.5, 131.2, 131.0, 130.3,
129.3 (2C), 123.4 (2C), 118.61, 118.57, 117.4, 117.3, 117.0, 116.5;
1
in the refrigerator overnight (99%): mp 95−98 °C; H NMR (400
MHz, CDCl3) δ 7.88 (dd, J = 8.1, 1.5 Hz, 2H), 7.81−7.75 (m, 2H),
7.53−7.46 (m, 2H), 7.21 (ddd, J = 8.4, 7.1, 1.6 Hz, 2H), 6.87−6.81
(m, 2H), 6.75 (dt, J = 8.2, 3.9 Hz, 2H), 6.27 (s, 4H). The analytical
data correspond to the literature.5
HRMS (ESI) m/z calcd for C24H19N7 [M + H+] 406,1775, found
+
406.1779.
One-Pot Preparation of Symmetric 2,2″-Disubstitued Bis-
ortho-azobenzenes (4h). A solution of ortho-phenylendiamine (1a)
(200 mg, 1.85 mmol, 1.00 equiv) in toluene (36 mL) was degassed
2-Nitro-tris-ortho-azobenzene 5c. The title compound 5c
was prepared according to general procedures (condition B) from
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2-amino-ortho-bisazobenzene (4b) (300 mg, 0.996 mmol, 1.00 equiv)
in toluene (10.0 mL) and 2-nitronitrosobenzene (2c) (167 mg, 1.10
mmol, 1.10 equiv) After 6 days the solvent was removed under
reduced pressure, and the product was purified by flash column
chromatography (SiO2, hexane/EtOAc, 7:1 to 1:3) to give the product
in 21% yield (91.1 mg): mp 124−126 °C; 1H NMR (400 MHz,
CDCl3) δ 7.95−7.81 (m, 5H), 7.80−7.70 (m, 2H), 7.69−7.55 (m,
5H), 7.54−7.38 (m, 5H); 13C NMR (101 MHz, CDCl3) δ 152.9,
149.1, 148.5, 148.4, 147.6, 145.7, 135.0, 133.2, 132.3, 131.7, 131.6,
131.5, 131.0, 130.6, 129.2 (2C), 124.0, 123.3 (2C), 120.0, 118.9, 118.7,
(3) Hamon, F.; Djedaini-Pilard, F.; Barbot, F.; Len, C. Tetrahedron
2009, 65, 10105−10123.
(4) Lim, Y.-K.; Lee, K.-S.; Cho, C.-G. Org. Lett. 2003, 5, 979−982.
(5) Reuter, R.; Hostettler, N.; Neuburger, M.; Wegner, H. A. Eur. J.
Org. Chem. 2009, 32, 5647−5652.
(6) Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89, 1915−1925.
(7) Yim, K. S.; Fuller, G. G. Phys. Rev. 2003, 67, 041601.
(8) Petrova, T. S.; Mancheva, I.; Nacheva, E.; Tomova, N.;
Dragostinova, V.; Todorov, T.; Nikolova, L. J. Mater. Sci.: Mater.
Electron. 2003, 14, 823−824.
118.3, 117.9; HRMS (ESI) m/z calcd for C24H17N7O2 [M + H+]
+
(9) Pan, X.; Wang, C.; Xu, H.; Wang, C.; Zhang, X. Appl. Phys. B:
Lasers Opt. 2007, 86, 693−697.
436.1516, found 436.1522.
2-Acetamido-2‴-nitro-tris-ortho-azobenzene 5f. The title
compound 5f was prepared according to general procedures
(condition B) from 2-acetamido-2″-amino-ortho-bisazobenzene (4a)
(368 mg, 1.03 mmol, 1.00 equiv) in toluene (17 mL) and 2-nitro-
nitrosobenzene (2c) (172 mg, 1.13 mmol, 1.10 equiv). After 6 d the
solvent was removed under reduced pressure, and the product was
purified by flash column chromatography (SiO2, hexane/EtOAc, 2:1)
to give the product in 58% yield (293 mg): mp 162−165 °C; 1H NMR
(400 MHz, CDCl3) δ 10.83 (s, 1H), 8.62 (dd, J = 8.4, 1.1 Hz, 1H),
7.92 (dd, J = 8.0, 1.6 Hz, 1H), 7.87−7.80 (m, 3H), 7.76 (ddd, J = 13.4,
7.9, 1.8 Hz, 2H), 7.70−7.57 (m, 4H), 7.55−7.44 (m, 3H), 7.44−7.38
(m, 1H), 7.20−7.13 (m, 1H), 1.70 (s, 3H); 13C NMR (101 MHz,
CDCl3) δ 169.6, 149.2, 148.2, 147.7, 147.6, 147.0, 145.5, 139.2, 134.2,
133.3, 133.0, 132.3, 132.1, 131.6, 131.4, 130.8, 127.5, 124.1, 123.3,
120.6, 120.5, 118.7, 118.5, 118.4, 118.1, 25.0; MS (FAB) m/z (%) =
493 (81%) [M+], 345 (100%); C26H20N8O3 (492.49) calcd C 63.41,
H 4.09, N 22.75; found C 63.42, H 4.14, N 22.60.
(10) El Halabieh, R. H.; Mermut, O.; Barrett, C. J. Pure Appl. Chem.
2004, 76, 1445−1465.
(11) Stoll, R. S.; Hecht, S. Angew. Chem., Int. Ed. 2010, 49, 5054−
5075.
(12) For a recent example of peptides controlled by two isolated
azobenzene units see: Samanta, S.; Woolley, G. A. ChemBioChem
2011, 12, 1712−1723.
(13) Beharry, A. A.; Woolley, G. A. Chem. Soc. Rev. 2011, 40, 4422−
4437.
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Soc. 1953, 49, 735−744.
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8203.
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1983, 1021−1024.
́ ́
(17) Bleger, D.; Dokic, J.; Peters, M. V.; Grubert, L.; Saalfrank, P;
Hecht, S. J. Phys. Chem. B 2011, 115, 9930−9940.
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S.; Negri, F.; Pieraccini, S.; Spada, G. P. Chem.Eur. J. 2004, 10,
2011−2021.
2,2⁗-Dinitro-tetra-ortho-azobenzene 6. The title compound 6
was prepared according to general procedures (condition B) from
diamine-bis-ortho-azobenzene (4i) (250 mg, 0.790 mmol, 1.00 equiv)
and 2-nitronitrosobenzene (2c) (361 mg, 2.37 mmol, 3.00 equiv).
After 4 days the solvent was removed, the reaction mixture was
purified by flash column chromatography (SiO2, hexane/EtOAc, 2:1),
and the product was obtained in 25% yield (114 mg): mp 149−151
(19) Meldola, R. J. Chem. Soc. 1883, 43, 425−442.
(20) Ruggli, P.; Iselin, E. Helv. Chim. Acta 1947, 30, 733−738.
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̈
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1
°C; H NMR (400 MHz, CDCl3) δ 7.90−7.83 (m, 4H), 7.80−7.74
(m, 2H), 7.64 (tt, J = 5.0, 2.5 Hz, 4H), 7.59−7.46 (m, 10H); 13C
NMR (101 MHz, CDCl3) δ 148.7, 148.4, 147.7 (2C), 145.6, 133.2,
132.3, 131.7, 131.6, 130.9, 124.0, 119.6, 119.0, 118.8, 117.8; HRMS
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(25) Hand-held UV lamp, 8 W, 365 nm, rt, in chloroform.
(ESI) m/z calcd for C30H20N10O4 [M + H+] 585.1742, found
+
(26) Bandara, H. M. D.; Cawley, S.; Gascon
́
, J. A.; Burdette, S. C.
585.1752.
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1714−1720.
ASSOCIATED CONTENT
■
S
* Supporting Information
(28) Tie, C.; Gallucci, J. C.; Parquette, J. R. J. Am. Chem. Soc. 2006,
128, 1162−1171.
1H NMR and 13C NMR spectra of compounds 3c, 3h, 4b, 4c,
4d, 4f, 4g, 5a, 5b, 5c, 5f, 6; absorption spectra of all ortho-
amino, acetamido and nitro substituted monoazobenzenes and
ortho-amino, acetamido, nitro and hydrogen substituted bis-
azobenzenes; photoisomerization of compounds 3a, 3c, 3g, 3h,
3l. This material is available free of charge via the Internet at
http://pubs.acs.org.
(29) Tibiletti, F.; Simonetti, M.; Palmisano, G.; Parravicini, M.;
Imbesi, F.; Tollari, S.; Penoni, A.; Nicholas, K. M. Tetrahedron 2010,
66, 1280−1288.
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1981, 403−407.
AUTHOR INFORMATION
■
(32) Kaiya, T.; Fujiwara, T.; Kohda, K. Chem. Res. Toxicol. 2000, 13,
993−1001.
Corresponding Author
*E-mail: hermann.wegner@unibas.ch.
(33) Roling, P. V.; Kirt, D. D.; Dill, J. L.; Hall, S.; Hollstrom, C.
J. Organomet. Chem. 1976, 116, 39−53.
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1971, 19, 277−286.
ACKNOWLEDGMENTS
■
The authors thank SystemsX.ch for financial support. H.A.W. is
indebted to the Fonds der Chemische Industrie for a Liebig
fellowship. Heiko Gsellinger, University of Basel, is acknowl-
edged for help in the NMR analysis.
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