Oligoazobenzenes - Switching in a new dimension

Article abstract of DOI:10.2533/chimia.2010.180

A new synthesis of cyclotrisazobenzene with different substituents was developed with yields of up to 30%. Solid-state structures of cyclotrisazobenzene as well as the tert-butyl derivative were obtained. Also, the photochromic properties and the complexa

Full text of DOI:10.2533/chimia.2010.180

180ꢀ CHIMIAꢀ2010,ꢀ64,ꢀNo.ꢀ3ꢀ
doi:10.2533/chimia.2010.180ꢀꢀ
Laureates: awards and Honors, sCs FaLL Meeting 2009
Chimiaꢀ64ꢀ(2010)ꢀ180–183ꢀ ©ꢀSchweizerischeꢀChemischeꢀGesellschaft
Oligoazobenzenes – Switching in a New
Dimension
RaphaelꢀReuter§,ꢀNikꢀHostettler,ꢀMarkusꢀNeuburger,ꢀandꢀHermannꢀA.ꢀWegner*
§SCS–DSMꢀNutritionalꢀProductsꢀAwardꢀWinnerꢀ(PosterꢀPresentation)
Abstract:ꢀAꢀnewꢀsynthesisꢀofꢀcyclotrisazobenzeneꢀwithꢀdifferentꢀsubstituentsꢀwasꢀdevelopedꢀwithꢀyieldsꢀofꢀupꢀtoꢀ
30%.ꢀSolid-stateꢀstructuresꢀofꢀcyclotrisazobenzeneꢀasꢀwellꢀasꢀtheꢀtert-butylꢀderivativeꢀwereꢀobtained.ꢀAlso,ꢀtheꢀ
photochromicꢀpropertiesꢀandꢀtheꢀcomplexationꢀbehaviourꢀwithꢀalkaliꢀmetalꢀionsꢀofꢀthisꢀclassꢀofꢀmoleculesꢀwereꢀ
investigated.
Keywords:ꢀAzobenzenesꢀ·ꢀMacrocyclesꢀ·ꢀMillsꢀreactionꢀ·ꢀMolecularꢀswitchesꢀ·ꢀPhotochromicꢀcompounds
1. Introduction
makes cyclic azobenzenes, or azobenze- we initiated a research program to investi-
nophanes a particularly interesting class gate the efficient preparation and the prop-
Since their discovery in the mid 19th cen- of compounds.[3] In a few cases, where a erties of such macrocycles.[11] Since the
tury, azo-compounds have always been distorted geometry is present, the usual be- overall yield of the reported synthesis of
important products as dyes and pigments haviour can be reversed, and the Z-isomer cyclotrisazobenzene was low, a new and
for the chemical industry. In recent years, becomes the thermally more stable form.[4] efficient synthetic strategy was developed.
the scope of azo-compounds has widened
The cavity of macrocyclic azobenzenes This approach also allows the introduction
beyond their traditional use.[1] Due to its also has the potential to act as a host for of functional groups.
EZ photoisomerization the azo moi- cations,[5] similar to crown ethers[6] or ca-
ety has been applied in many examples lixarenes.[7]
to bring about structural changes in mol-
A special class of azobenzenophanes 2. Synthesis of
ecules and hence, to alter their physical are cyclotrisazobenzenes 1 (Fig. 1), in Cyclotrisazobenzene
and chemical properties reversibly by UV- which three aryl units are bridged in ortho-
irradiation.[2] However, in most cases only positions by azobenzene units to create a
One of the oldest and probably still the
a photostationary state with mixtures of the fully conjugated π-system. Dreiding and most important protocol for preparing azo-
E- and Z-isomer is obtained. Additionally, coworkers synthesized cyclotrisazoben- compounds is the Mills reaction.[12] An im-
the thermodynamically unfavourable Z- zene in an overall yield of 2.6% from N-(l- portant advantage of this method is that a
isomer often has a short lifetime.
pyridinio)-2-nitroanilide.[8] The synthesis large variety of substituted non-symmetric
It has been shown that strain, which is of the precursor could be further improved azobenzenes can be prepared under mild
induced when azo-bonds are incorporated by the work of Skrabal et al.[9] Because of conditions by condensation of an aniline
into macrocyclic structures, can extend its prospective photochromic behaviour with a nitrosoarene. The major limitation
the lifetime of the unstable Z-isomer. This and use as a molecular gripper (Fig. 2),[10] consists in the accessibility and stability of
the nitrosoarenes, which tend to dimerize
or decompose. Recently, a two-step pro-
cedure based on Buchwald-Hartwig cou-
pling of Boc-protected hydrazoaryls and
halogenated arenes was reported, which
is particularly efficient in the synthesis of
azobenzenophanes.[13] For the synthesis of
symmetrical azobenzenes, a larger variety
of procedures is known, either by oxida-
tion of anilines or reduction of nitroben-
zenes.[14]
In our retrosynthetic analysis we envi-
sioned that the last azo-bond to be formed
had to be generated either by reductive
N
N
N
N
N
N
1
Fig.ꢀ1.ꢀStructureꢀofꢀcyclotrisazobenzene.
Fig.ꢀ2.ꢀPossibleꢀ
switchingꢀofꢀ
cyclotrisazobenzene.
*Correspondence:ꢀDr.ꢀH.ꢀA.ꢀWegner
UniversityꢀofꢀBasel
DepartmentꢀofꢀChemistry
St.ꢀJohanns-Ringꢀ19
hv
CH-4056ꢀBasel
Tel.:ꢀ+41ꢀ61ꢀ267ꢀ1147
Fax:ꢀ+41ꢀ61ꢀ267ꢀ0976
E-mail:ꢀhermann.wegner@unibas.ch

Laureates: awards and Honors, sCs FaLL Meeting 2009ꢀ
CHIMIAꢀ2010,ꢀ64,ꢀNo.ꢀ3ꢀ 181
Schemeꢀ1.ꢀ
Retrosyntheticꢀanalysis.
cyclotrisazobenzene was obtained. Ring
closure was achieved by treating the di-
amine 8 with Pb(OAc)4 as a key step of the
synthesis. A problem of this protocol was
its low selectivity. When the method was
employed according to Dreiding’s original
procedure, the corresponding bisbenzotri-
azol was obtained as the major product. It
was found that the addition of NEt3 pre-
vented this unwanted side reaction, leading
to an improvement of the yield from 26%
to 51%. Two more derivatives, either with
a bromide or a tert-butyl substituent were
prepared, according to the same synthetic
pathway (Scheme 2).
NR2
Oxidative or
N
N
Reductive
N
N
Azocoupling
N
N
N
N
N
N
R = H, O
NR2
2
1
Mills Reaction
R2N
NH2
NH2
Mills Reaction
H2N
N
We also established a second procedure
in which the first two azo-bonds can be
created in a one-pot two-step strategy via
Mills reaction, starting with ortho-phenyl-
enediamine. The first coupling could only
be achieved in toluene under the same con-
ditions, as used in the first strategy. The
second coupling, though, took place when
the polarity of the solvent was enhanced
by increasing the amount of acetic acid.
After deprotection of the obtained bis-azo-
compound 10 and oxidative coupling with
Pb(OAc) the unsubstituted macrocycle
1a could4be obtained in an overall yield of
30% (Scheme 3).
N
3
4
or oxidative azo-coupling. To favour the generating the last azo-bond under oxida-
formation of a macrocycle, rather than an tive conditions. Our first strategy starts
intermolecular reaction, the metal tem- with the preparation of 2,2’-diaminoazo-
plate effect could be of use. The bis-azo benzene (5) from ortho-phenylenediamine
precursor could be prepared either by two (4) by oxidative coupling. The use of KO2
sequential Mills coupling reactions, or in a instead of MnO2 as an oxidant significantly
one-pot reaction in which both azo-bonds increased the yield of the reaction.[15] The
would be formed in a single step. Cheap next azo-bond was installed by the Mills
ortho-phenylenediamine (4) would be a reaction with 2-nitrosoacetanilide (6). This
suitable starting material (Scheme 1).
reaction proceeds only in diluted condi-
In our initial synthetic attempts, a tions.A solvent screening showed that non-
route was established where the cycliza- polar solvents lead to an increased yield.
tion would be done reductively. An effi- Consequently, chloroform initially used as
cient route to dinitrobis-azo-compound 9 solvent was changed to toluene using only
was prepared. However, no protocol could four equivalents of acetic acid. After de-
be found which was suitable for the last protection of the acetyl protecting groups
cyclization step. Therefore, we focused on under basic conditions, the precursor 8 of
3. Solid-State Structures
By slow evaporation of a solution of
the tert-butyl substituted macrocycle (1b)
in tert-butylmethylether, crystals could be
obtained which were suitable for X-ray
structural analysis. The structure of the
unsubstituted derivative (1a) has already
beendiscussedbyDreidingandco-workers
earlier.[16] In both cases, for the tert-butyl
substituted 1b as well as the unsubstituted
cyclotrisazobenzene 1a the structure can
only be described in pairs of molecules.
For 1a, two molecules are arranged in a
face-to-face arrangement, caused by π–π-
stacking interactions. These interactions
are suppressed in tert-butylcyclotrisazo-
benzene (1b). The high steric demand of
the tert-butyl group forces the two mol-
ecules to align in a different manner next
to each other (Fig. 3).
NO
NH2
NHAc
N
N
NH2
NH2
R
KO2
NH2
6 a-c
N
N
N
N
AcOH, toluene, 60 °C, 3 d
toluene, r.t., 3 d
65%
R
H2N
5
6a: 64%
6b: 71%
6c: 39%
4
NHAc
7 a-c
4 steps
39 %
7a: 95%
KOH
7b: quant.
7c: 95%
H2O, EtOH, 100 °C, 2 h
NO2
NH2
N
N
N
N
N
N
4. Binding Studies
Pb(OAc)4, NEt3
N
N
N
N
N
N
DCM, r.t., 30 min
N
There are different heavy metal com-
plexes of the Schiff’s base macrocycles
reported.[17] ESI-MS can be utilized to
measure binding constants of alkali metal
ion complexes of crown ethers.[18] Unfor-
tunately, all attempts to detect first row
transition metal complexes of 1a failed.
To investigate the gas phase binding prop-
erties of macrocycle 1a with different al-
kali metal ions, an equimolar solution of
R
N
8a: 51%
8b: 49%
8c: 42%
NO2
R
NH2
8 a-c
1 a-c
9
R = H
t-Bu
Br
a
b
c
ꢀSchemeꢀ2.ꢀSynthesisꢀofꢀcyclotrisazobenzeneꢀandꢀderivatives.

182ꢀ CHIMIAꢀ2010,ꢀ64,ꢀNo.ꢀ3ꢀ
Laureates: awards and Honors, sCs FaLL Meeting 2009
Schemeꢀ3.ꢀSecondꢀ
generationꢀsynthesisꢀofꢀ
cyclotrisazobenzene.
that the host–guest interactions in solution
are very weak.
NHAc
NH2
N
N
6a (3 eq.), AcOH (4 eq.)
NH2
toluene, 60 °C
N
N
5. Isomerization Studies
then AcOH/toluene (3:1)
60 °C
4
The different azobenzenophanes were
subjected to UV irradiation to investigate
their photochromic properties. Cyclotri-
sazobenzene with one absorption band at
294 nm was irradiated at different wave-
length ranging from 280 nm to 350 nm.
Even after enduring irradiation, no change
in the absorption spectrum was observed.
A second experiment was done using la-
ser flash photolysis. Again, no switching
could be seen. We concluded that the strain
in this small azobenzenophane is too high
for the isomerization to occur, since the
structure would become too distorted (Fig.
4).
63%
NHAc
10
KOH, EtOH/H2O
90 °C, 2 h
92%
NH2
N
Pb(OAc)4, NEt3
DCM, r.t., 30 min
51%
N
N
N
N
N
N
N
N
N
NH2
1a
8a
Fig.ꢀ3.ꢀSolidꢀstateꢀ
structuresꢀofꢀtert-butyl-
cyclotrisazoꢀbenzene.
Fig.ꢀ4.ꢀAbsorptionꢀspectrumꢀofꢀ
cyclotrisazobenzeneꢀ(inꢀchloroform).
6. Conclusion
An efficient synthesis of cyclotrisazo-
benzene was developed. Solid-state struc-
tures of two members of the family were
obtained, which show a deviation of the
molecules from planarity. The photochro-
mic properties of this class of compounds
were investigated. However, their strained
structure prohibits isomerisation of the
azo-bonds. First results on triscycloazobi-
phenyls, which are larger members of this
family, indicate that these molecules do not
possess this limitation and show isomeri-
zation. Further studies towards such mac-
rocycles are ongoing.
Tableꢀ1.ꢀESI-MSꢀbindingꢀstudiesꢀofꢀ
cyclotrisazobenzeneꢀ1aꢀwithꢀdifferentꢀalkaliꢀ
cations.
1a, and different alkali metal chlorides
(LiCl, NaCl, KCl, RbCl) in methanol was
analyzed by ESI-MS (Table 1). The mass
spectrum of the solution showed all ex-
pected monomeric complexes [1a + M+] as
well as dimeric complexes [2 * 1a + M+].
The signals for all metal complexes show
intensities in the same order of magnitude.
However, the two highest intensities for the
1:1 complexes could be seen with sodium
and rubidium. Intensities of the signals,
representing the 2:1 complexes, decline
with decreasing cation size.
Apart from investigation of the com-
plexation behaviour of cyclotrisazoben-
zene 1a in the gas phase, 1H-NMR titration
experiments were performed to explore
complexation properties in solution. Mea-
surements of 1a in DMSO-d with increas-
ing amounts of NaI from 0.65 equiv. to 10
equiv. did not show any change in chemical
shifts of the aromatic protons, indicating
Entry
m/z
Complex
Relativꢀ
ESI-MS
intensity
1
2
3
4
5
6
7
8
9
313
319
335
351
397
631
647
663
708
[1aꢀ+ꢀH+]
1
[1aꢀ+ꢀLi+]
3.2
9.6
9.2
16.1
30.4
15.7
3.2
3.2
[1aꢀ+ꢀNa+]
[1aꢀ+ꢀK+]
Acknowledgements
[1aꢀ+ꢀRb+]
[2ꢀ*ꢀ1aꢀ+ꢀLi+]
[2ꢀ*ꢀ1aꢀ+ꢀNa+]
[2ꢀ*ꢀ1aꢀ+ꢀK+]
[2ꢀ*ꢀ1aꢀ+ꢀRb+]
We thank the Swiss National Science
Foundation and the Fonds der chemischen
Industrie for their financial support. The Swiss
Chemical Society and DSM are thanked for
awarding Raphael Reuter the SCS–DSM
Nutritional Products Award.
Received: January 13, 2010

Laureates: awards and Honors, sCs FaLL Meeting 2009ꢀ
CHIMIAꢀ2010,ꢀ64,ꢀNo.ꢀ3ꢀ 183
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