Synthesis of an azobenzene-linker-conjugate with tetrahedrical shape

Article abstract of DOI:10.1016/j.tetlet.2008.04.086

The synthesis of a tripodal linker system with an adamantane core unit and an azobenzene headgroup is reported for the preparation of photochromic SAMs on gold surfaces. For the final Sonogashira-coupling of the ethynylene-linker precursor 4 with the p-io

Full text of DOI:10.1016/j.tetlet.2008.04.086

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 4020–4025
Synthesis of an azobenzene-linker-conjugate with tetrahedrical shape
Sebastian Zarwell, Karola Ruck-Braun *
¨
Institut fu¨r Organische Chemie, Technische Universita¨t Berlin, Straße des 17. Juni 135, Sekr. C3, D-10623 Berlin, Germany
Received 19 March 2008; revised 10 April 2008; accepted 14 April 2008
Available online 18 April 2008
Abstract
The synthesis of a tripodal linker system with an adamantane core unit and an azobenzene headgroup is reported for the preparation
of photochromic SAMs on gold surfaces. For the final Sonogashira-coupling of the ethynylene-linker precursor 4 with the p-iodo substi-
tuted azobenzene 6 model studies were required to optimize the coupling step in the presence of a diazene functionality. The photochro-
mic properties of the photoswitch-linker-conjugate 1 were investigated in solution, and compared to the behavior of the precursor 6.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Azobenzene; Photoswitch; Tripod; Linker-conjugate; cis/trans-Isomerisation
Photochromic compounds have been widely investi-
gated as promising candidates for optical data storage,
microelectronics, non-linear optics and on/off-switching
of functional molecules in biology1–5 Especially azobenzene
derivatives have gained enormous interest in these research
fields. This class of photoactive compounds isomerizes
from the trans- to the cis-state by activation with light in
the UV/vis region or by electron transfer. The reverse reac-
tion can be achieved by illumination with UV/vis light of a
different wavelength or appears by thermal relaxation.1
The shift in structure causes a significant change in physical
properties, for example, absorption spectra or dipole
moment. During the past years there has been a tremen-
dous and consistent research effort for the development
of switchable azobenzene-modified surfaces by using Lang-
muir–Blodget-films or self-assembly.1,6 The investigation
and characterization of chromophore containing self
assembled monolayers (SAMs), which are formed by cova-
lent binding of sulfides or disulfides to a solid surface, for
example, gold or silver, are seen as an auspicious approach
toward the development of materials with novel optical
and electronical properties.6 Generally, one-dimensional
linkers like alkanethiols are used for the preparation of
SAMs on gold.
In most cases the alkyl chains are tilted in order to max-
imize the interchain van der Waals interactions. However,
the final orientation of a chromophore headgroup is not
always easy to predict.6–8
Moreover, p-aggregation of the attached chromophores
is a known problem that changes or even suppresses the
chromophore properties.6,9 Recently, Galoppini,9,12
Tour,10,11 and others.13–17 have developed three-dimen-
sional linkers with a tetrahedrical core unit for sufficient
control of the perpendicular shape and the headgroup to
surface distance. Additionally, it has been shown that tri-
pods are feasible linkers that prevent p-stacking or dimer-
ization of azobenzenes when adsorbed on gold or applied
as AFM-tip.18,19
Herein, we report the synthesis of a tripodal linkersys-
tem with an adamantane core and an azobenzene head-
group containing an additional nitrile functionality in p-
position. The nitrile acts as a reporter group for spectros-
copy on surfaces, for example, SFG (sum frequency gener-
ation) or HREELS (high resolution electron energy loss
spectroscopy). For immobilization on metal surfaces the
system provides three protected thiol groups (Fig. 1). The
classical thiol protection groups are thioethers or thioes-
ters. Due to the fact that the cleavage of thioethers requires
*
Corresponding author. Tel.: +49 30 314 26319; fax: +49 30 314 79651.
E-mail address: krueck@chem.tu-berlin.de (K. Ruck-Braun).
¨
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.04.086

S. Zarwell, K. Ru¨ck-Braun / Tetrahedron Letters 49 (2008) 4020–4025
4021
phenyl)adamantane in 39% yield over two steps.20,21 The
following Sonogashira-coupling with trimethylsilylacety-
lene and the carboxylation with t-BuLi and CO2 gave pre-
cursor 2a as previously reported.12 By following the
literature protocol, compound 2a is obtained as a mixture
of the tri- and dicarboxylic acid (2a,b, see Scheme 1) which
was used in the next step to avoid the difficult separation by
column chromatography.12 Alcohol 3a was synthesized by
the reduction of acid 2a with LAH. 1,4-Dioxane as a more
polar solvent was used to avoid problems observed when
dissolving the lipophobic acid mixture 2a,b in Et2O. Under
these conditions the reduction of the acid functionalities is
accompanied by a cleavage of the silyl protection group
furnishing the free acetylene. Similar Si–C bond cleavage
reactions have been previously reported for alkylsilanes
using LAH in THF.22
CN
N
N
N
NC
N
405 nm
Δ, 460 nm
SAc
AcS
AcS
SAc
SAc
SAc
1
cis-
1
trans-
The final coupling of an azobenzene precursor to the lin-
ker-conjugate was planned via a palladium-mediated Sono-
gashira-coupling. Because of the strong affinity of sulfur
containing groups, especially free thiols, for transition met-
als like palladium, the protection of the sulfur atoms is
mandatory. Again thioesters are superior to thioethers,
since cleavage of the latter can also be initiated by palla-
dium catalysts. Also, the reduction of the azobenzene moi-
ety to the hydrazine with simultaneous formation of
disulfide dimers or polymers could be a problem without
protection.23 In the past, the coupling of alkynes with pal-
ladium in the presence of thioacetate functionalities was
successfully achieved by several groups.10,14,15
Fig. 1. Photoisomerization and thermal relaxation of the azobenzene-
linker-conjugate 1.
harsh conditions we decided to use the latter protection
approach. The three anchoring groups of the linker gener-
˚
ate a triangle footprint with a side length about 12–15 A.
However, it has been shown that the anchoring of all the
three thiols in systems with tetrahedral shape is elusive.11,14
Hence, for a better flexibility we inserted a methylene unit
between the anchoring moiety and the phenyl ring of the
adamantane core.
The synthesis of the linker building block 4 was accom-
plished in six steps, starting with the literature known
conversion of 1-bromoadamantane to 1,3,5,7-(4-tetraiodo-
Hence the introduction of the thioacetate groups was
carried out starting from 3a. An elegant method is the
H
SiMe3
LAH
+
+
2b
3b
dioxane, 0ºC -> rt, 4 h
HO2C
CO2H
HO
OH
CO2H
OH
3a
2a
H
R1
DIAD, PPh3, AcSH
3a
THF, 0ºC -> rt, 4 h
R2
R1
81%
R2
AcS
SAc
2b
: R1 = SiMe3, R2 = CO2H
3b: R1 = H, R2 = CH2OH
SAc
4
Scheme 1. Synthesis of the ethynylene-linker containing thioacetate anchoring groups.

4022
S. Zarwell, K. Ru¨ck-Braun / Tetrahedron Letters 49 (2008) 4020–4025
Table 1
CN
Oxone®
NO
Model Sonogashira-reactions with azobenzene 6 and TMSA
DCM, H2O,
rt, 1.5 h
95%
H2N
NC
SiMe3
5
I
Pd-cat., TMSA
N
N
N
N
I
I
NH2
NC
NC
N
N
8
6
acetic acid, rt, 72 h
NC
86%
Entry Cat./ligand
Conditions
3% Pd(PhCN)2Cl2, 6% 2% CuI, DIPA, dioxane, 8 h at
[(t-Bu)3PH]BF4 40 °C, 12 h at rt
3% Pd(PPh3)2Cl2, 10% 5% CuI, NEt3, THF, 18 h at
(%)a
6
1
2
3
4
5
6
7
60
24
42
Scheme 2. Synthesis of azobenzene 6.
PPh3
3% Pd(PPh3)4
45 °C
5% CuI, NEt3, THF, 18 h at
45 °C
activation of the alcohol functionality with triphenylphos-
phine and azodicarboxylate followed by a nucleophilic sub-
stitution with acids furnishing the corresponding esters.24
Alcohol 3a was treated with diisopropylazodicarboxylate,
acetyl mercaptan and an excess of triphenylphosphine,10
to give acetate 4 in 81% yield.
The azobenzene coupling partner 6 was prepared by a
two-step synthesis (Scheme 2). The iodo-substituted com-
pound was chosen due to the better reactivity towards
the oxidative addition by the active palladium catalyst
thereby compensating the higher costs. Firstly, p-amino-
benzonitrile was oxidized with OxoneÒ in a two-phase
reaction with water/DCM to give 4-nitrosobenzonitrile 5
in 95% yield (Scheme 2) as previously described by us.25
Subsequent condensation of the latter with p-iodoaniline
in acetic acid afforded azobenzene 6 as an orange solid in
86% isolated yield.
5% Pd(PPh3)2Cl2, 10% 10% CuI, NEt3, toluene, DMF, 76
PPh3
5% Pd(OAc)2, 10%
PPh3
10% Pd(PPh3)4
18 h at 50 °C
10% CuI, NEt3, toluene, DMF, 68b
18 h at 50 °C
10% CuI, NEt3, toluene, DMF, 87
18 h at 50 °C
5% CuI, DIPA, toluene, DMF,
2 h at 40 °C, 48 h at rt
5% Pd(OAc)2, 10%
PPh3
80c
Conversion to azobenzene 8 determined by 1H NMR.
1:1 mixture of azobenzene 8 and hydrazine 7.
Isolated yield of hydrazine 7.
a
b
c
Sonogashira conditions employed, the reducing reagent is
not clearly identified yet and thus more detailed examina-
tions are necessary. However, the hydrazine was easy trans-
ferred to the azobenzene by oxidization with manganese
dioxide in 83% yield (Scheme 3). Fortunately, azobenzene
8 was obtained with 10 mol % Pd(PPh3)4 in a toluene/
DMF mixture at 50 °C (Table 1, entry 6) with 87% conver-
sion by avoiding the use of Pd(OAc)2/PPh3. Analogously,
the tripodal linker 4 was reacted with azobenzene 6 in
22 h to yield the final photoswitch-linker-conjugate 1 in
61% after work-up and purification by flash chromatogra-
phy (Scheme 4).33
Photoisomerization of azobenzenes 6 and 1 can be
achieved by illumination with light and was investigated
by UV/vis spectroscopy.34
The UV/vis spectra of the azobenzene-linker-conjugate
1 in the trans-state and of the cis-/trans-mixture in
the photo-stationary state (pss) were compared to the
For the Sonogashira-coupling reaction the optimal con-
ditions were screened by a model reaction using azobenz-
ene 6 and trimethylsilylacetylene.26
By using standard Sonogashira-conditions in THF only
a low conversion of azobenzene 6 could be monitored
(Table 1, entries 1 and 2).26 Even with bulky electron-rich
phosphine ligands, successfully applied by Fu27 for the cou-
pling of arylhalides, only a moderate conversion of the
starting material was detected. Interestingly, with
Pd(OAc)2 and triphenylphosphine as ligand coupling and
the reduction of the azo-functionality gave hydrazine 7 in
80% isolated yield (Table 1, entry 7). While the reduction
of stabilized azo-compounds with electron-withdrawing
groups (DEAD, DIAD) is easily induced by triphenylphos-
phine, there are only few and less detailed publications
about the reduction of azoarenes under Sonogashira-cou-
pling conditions.28 However, for entry 7 (Table 1) triphen-
ylphosphine was used in catalytic amounts only. Thus, a
catalytic activation of the azo-functionality by the
SiMe3
5% Pd(OAc)2,
10% PPh3, CuI,
DIPA, TMSA
I
H
N
N
N
toluene, DMF,
40 ºC, 2 h,
rt, 48 h
N
H
NC
NC
6
palladium center is another reasonable suggestion.29
A
7
80%
comparable transformation of azoarenes to hydrazines
published by Muniz occurs with a (bathocuproine)palla-
dium complex and acetic acid in DCM. Apparently, in
the coupling and reduction of 6 furnishing 7, trimethylsilyl-
acetylene can act as a proton donor. It must be noted that
the reduction of the N@N double bond also easily occurs
in the presence of free thiols and moisture,23 or in the pres-
ence of a variety of transition metals.30 Anyhow, under the
SiMe3
MnO2
N
toluene, rt, 1 h
N
83 %
NC
8
Scheme 3. Formation of hydrazine 7 and subsequent oxidation to
azobenzene 8.

S. Zarwell, K. Ru¨ck-Braun / Tetrahedron Letters 49 (2008) 4020–4025
4023
CN
N
N
CN
Pd(PPh3)4, CuI, NEt3
N
+
N
toluene, DMF, 50 ºC, 22 h
I
61%
AcS
SAc
6
SAc
4
AcS
SAc
SAc
1
Scheme 4. Synthesis of the azobenzene-linker-conjugate 1 by Sonogashira-coupling.
corresponding spectra of azobenzene 6 (Fig. 2). The spectra
of trans-azobenzene 1 in DCM (c = 2.63 Â 10À5 mol/L)
shows an intense absorption maximum at kmax = 376 nm
Table 2
UV/vis data of the azobenzenes and rates of thermal back reaction in
DCM at 25 °C
*
kmax,trans
kmax,pss
kiso
(nm)
krev
(minÀ1
t1/2
(h)
that corresponds to the p–p transition band, which is
(nm) (e)a
(nm) (e)a
)
red shifted by 31 nm with respect to the iodo-substituted
compound 6, due to the more extended p conjugation
(see Table 2 for the data of both the compounds). The n–
6
1
345 (29219) 302 (7407), 440 (197)
288, 400 6.01 Â 10À4 19.2
376 (37167) 288 (21670), 364 (19726) 320, 444 2.70 Â 10À3 4.28
[dm3molÀ1cmÀ1].
a
*
*
p
transition band of 1 is overlapped by the p–p band
in contrast to the two separated bands observed for com-
pound 6. The considerable increase of absorption below
k = 270 nm and the shoulder at about k = 288 nm can be
assigned to the ethynylene–phenylene backbone.
band and a simultaneous increase of the shoulder at
around k = 288 nm. The corresponding pss was reached
within minutes. Isomerisation of trans-6 to cis-6 occurs
via the irradiation of 365 nm light on a similar timescale
with a blue-shift of the transition band from k = 345 to
The quotient-functions of azobenzene 6 and azobenzene
1 attain their maxima at k = 364 nm and k = 395 nm,
respectively. Irradiation of azobenzene 1 with vis light
(405 nm used instead of 395 nm) causes an appreciable
*
k = 302 nm. Due to the clear separation of the n–p and
*
*
p–p bands also an increasing absorption at k = 440 nm
decrease and hypsochromic shift of the p–p transition
was observed.
Without light cis-isomers of azobenzenes slowly revert
back to the thermodynamic more favorable trans-isomers
(shown in Fig. 3). Both spectra show two isosbestic points
at k = 288 nm and k = 400 nm for azobenzene 6 and at
k = 320 nm and k = 444 nm for azobenzene 1, indicating
a clear conversion of the isomers into each other without
the formation of side products. By monitoring the change
of absorption at the optimal irradiation wavelength during
the relaxation, the kinetic of the process was determined.
Thereby first-order characteristics were found for the
thermal backreactions. The obtained time constants are
shown in Table 2 for each azo-compound, indicating that
the half-life time of the conjugated azobenzene 1 is by a
factor of 5 smaller than the half-life time of azobenzene
6. The results are in agreement with the data published
for systems with analogous p-conjugation.31
1.0
trans
pss 1
trans
pss 6
1
6
0.5
0.0
300
350
400
450
500
550
600
λ [nm]
In conclusion, an azobenzene-linker-conjugate with
acetate protected thiol anchoring groups to address gold
surfaces has been synthesized from the corresponding
iodo-substituted azobenzene 6 and the ethynylene-linker
Fig. 2. UV/vis absorption spectra of azobenzenes 6 (trans = dotted line,
pss = dashed-dotted line, c = 3.88 Â 10À5 mol/L) and 1 (trans = solid line,
pss = dashed line, c = 2.63 Â 10À5 mol/L) in DCM.

4024
S. Zarwell, K. Ru¨ck-Braun / Tetrahedron Letters 49 (2008) 4020–4025
References and notes
pss
1 h
2 h
1. Knoll; Sekkat Photoreactive Organic Thin Films; Elsevier Science:
USA, 2002.
3 h
4 h
2. Woolley, G. A. Acc. Chem. Res. 2005, 38, 486–493.
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1.0
0.5
0.0
6 h
8 h
10 h
14 h
18 h
22 h
26 h
30 h
34 h
38 h
44 h
trans
5. Lougheed, T.; Borisenko, V.; Hennig, T.; Ruck-Braun, K.; Woolley,
¨
G. A. Org. Biomol. Chem. 2004, 2, 2798–2801.
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λ [nm]
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1.0
0.5
0.0
2 h
3 h
4 h
6 h
8 h
10 h
14 h
18 h
22 h
trans
300
350
400
450
500
550
600
λ [nm]
Fig. 3. Thermal relaxation of azobenzenes 6 (top, c = 3.88 Â 10À5 mol/L)
and 1 (bottom, c = 2.63 Â 10À5 mol/L) measured in DCM at 25 °C.
24. Mitsunobu, O. Synthesis 1981, 1–28.
25. Priewisch, B.; Ruck-Braun, K. J. Org. Chem. 2005, 70, 2350–2352.
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precursor 4 by Sonogashira-cross-coupling. The UV/vis
data and the photochromic properties of the synthesized
azobenzene 1 in solution have been determined and
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M. A.; Tour, J. M. Tetrahedron 2003, 59, 8555–8570.
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33. 4-(((E)-4-(4-(3,5,7-Tris(4-(acetylthiomethyl)phenyl)adamantane-1-yl)-
phenylethynyl)phenyl)diazenyl)benzonitrile (1): In a flask, azobenzene
Acknowledgements
This work was supported by the Deutsche Forschungs-
gemeinschaft (SFB 658) and the Fonds der Chemischen
Industrie.
6
(99.6 mg, 299 lmol), tetrakistriphenylphosphine palladium(0)
(28.8 mg, 24.9 lmol) and copper(I) iodide (2.72 mg, 24.9 lmol) were
dissolved in a degassed mixture of toluene and DMF (2:1, 4 mL)
Supplementary data
under nitrogen atmosphere. Then tripod
4 (182 mg, 249 lmol)
dissolved in the same solvent mixture (4 mL), and triethylamine
(0.5 mL) was added via a syringe and the reaction mixture was stirred
at 50 °C for 12 h. Then the same amount of palladium-catalyst was
added, and stirring at 50 °C was continued until no further conversion
could be detected (TLC monitoring, 10 h). The mixture was poured
into water (20 mL) and the aqueous layer was separated and extracted
with ethyl acetate (30 mL). The combined organic layers were washed
with water (10 mL) and brine (10 mL), dried (MgSO4) and evaporated
to dryness. The crude product was purified by flash chromatography
The synthesis and complete characterisation (1H NMR,
1
13C NMR, mp, Rf, MS, HRMS, IR, copies of H, 13C);
UV/vis absorption spectra and graphical determination
of the half-life time of 1 and 6 are provided. Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2008.04.086.

S. Zarwell, K. Ru¨ck-Braun / Tetrahedron Letters 49 (2008) 4020–4025
4025
on silica gel (DCM/hexane 3:1) to give the trans-isomer of azobenzene
1 (142 mg, 152 lmol, 61%) as a red solid. Mp 144 °C; Rf: 0.57 (DCM/
hexane/acetonitrile 4:1:0.01); 1H NMR (400 MHz, CDCl3): d 7.99 (d,
J = 8.7 Hz, 2H), 7.94 (d, J = 8.6 Hz, 2H), 7.82 (d, J = 8.9 Hz, 2H),
7.68 (d, J = 8.7 Hz, 2H), 7.54 (d, J = 8.6 Hz, 2H), 7.45 (d, J = 8.8 Hz,
2H), 7.39 (d, J = 8.5 Hz, 6H), 7.28 (d, J = 8.3 Hz, 6H), 4.11 (s, 6H),
2.35 (s, 9H), 2.12 (s, 12H); 13C NMR (100.6 MHz, CDCl3): d 195.3,
154.6, 151.5, 150.3, 148.3, 135.6, 133.4, 132.6, 131.9, 129.0, 127.6,
125.4, 125.4, 125.3 123.6, 120.7, 118.6, 114.2, 93.0, 89.0, 47.2, 39.6,
39.2, 33.1, 30.5; IR (ATR): 3090 (w), 3052 (w), 3027 (w), 2998 (w),
2920 (m), 2897 (w), 2849 (w), 2226 (w), 2212 (w), 1912 (w), 1688 (vs),
1595 (m), 1511 (m), 1448 (w), 1410 (m), 1354 (s), 1132 (s), 1102 (m),
1018 (w), 957 (m), 852 (m), 838 (m); MS (FAB) m/z (%): 934 (<1,
M+), 391 (17), 149 (100). MS calcd for C58H51N3O3S3 934.2, found
934; Anal. Calcd for C58H51N3O3S3: C, 74.57; H, 5.50; N, 4.50.
Found: C, 74.09; H, 5.44; N, 4.59.
34. Due to the fact that both isomers show an absorption over the whole
spectral bandwidth, continuous illumination always leads to mixtures.
Hence, it is not possible to convert either isomer quantitatively into
the other by activation with light. However, the composition of the
isomers in the pss can be varied by changing the irradiation
wavelength.1 The highest ratio of either isomer in the corresponding
pss is obtained at a wavelength at which the quotient of the
corresponding extinction coefficients is maximized. The best wave-
length can be evaluated, for example, by dividing the UV/vis spectra
of the trans-isomer by the spectra of the cis-/trans-mixture in the pss.
The maximum of the resulting function indicates the optimal
irradiation wavelength for the trans- to cis-photoisomerisation.