Synthesis of a rigid tetrahedral linker with thioether end groups

Article abstract of DOI:10.1055/s-0033-1340494

The synthesis of a tetrahedral thio end-functionalized rigid oligo(p-phenylene ethynylene) (OPE) based on a convergent approach is reported.

Full text of DOI:10.1055/s-0033-1340494

PAPER
▌475
paper
Synthesis of a Rigid Tetrahedral Linker with Thioether End Groups
Synthesis of a Rigid Tetrahedral Thioether Linker
Florian Kretschmer,a,b Andreas Wild,a,b Martin D. Hager,a,b Ulrich S. Schubert*a,b
a
Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstr. 10, 07743 Jena, Germany
b
Jena Center for Soft Matter (JCSM), Friedrich Schiller University Jena, Philosophenweg 7, 07743 Jena, Germany
Fax +49(3641)948202; E-mail: ulrich.schubert@uni-jena.de
Received: 28.10.2013; Accepted after revision: 05.12.2013
now, uncertainty remained over the exact binding mode of
such linkers. Yan and co-workers reported the synthesis
of rigid quadratic and trigonal planar linkers but, instead
of binding three or four gold nanoparticles, the chelating
effect allowed only two particles to be bridged.12 In con-
trast, Novak et al. reported the successful formation of di-
mers and trimers with similar linkers.13
Abstract: The synthesis of a tetrahedral thio end-functionalized
rigid oligo(p-phenylene ethynylene) (OPE) based on a convergent
approach is reported.
Key words: alkynes, cross-coupling, nanostructures
There has been increasing interest in the synthesis and
self-assembly of noble metal nanoparticles in recent
years. In particular, gold and silver particles have gathered
attention because they feature a localized surface plasmon
resonance in the visible region and because of their high
environmental stability.1 Their tunable size and structure
allows devices with tailor-made optical properties to be
fabricated that could be used in applications for energy
conversion,2 medicinal therapy,3 sensors,4 and optical de-
vices.5 Moreover, their optical response is not only gov-
erned by their dimension or shape but, in fact, also by the
spatial arrangement of the particles to each other. Hence,
it is crucial to implement methods that enable the precise
formation of ordered plasmonic nanoparticle assemblies.
Top-down approaches involve the pattering of large pre-
formed metal structures, whereas the top-up strategy uti-
lizes patterned surfaces that are subsequently
functionalized with nanoparticles. The bottom-up ap-
proach relies on the assembly of nanoparticles, for exam-
ple through chemical reactions or electrostatic
interactions. Click,6 Diels–Alder,7 and oligonucleotide8
reactions have successfully been used to create clusters
with a precise number of particles. Moreover, mediated by
polymers, clusters with dozens of particles can be synthe-
sized.9 Thiols, and to a lesser extent amines, show strong
affinity towards gold surfaces. Molecules with a defined
geometry, for example thio end-functionalized linear link-
ers therefore allow ordered nanoparticle structures to be
created. The electromagnetic interactions between the
particles and, hence, the optical response is strongly de-
pendent on the interparticle distance. As a result, a pivotal
point is to utilize molecules with a defined distance be-
tween the functional moieties. Whereas α,ω-alkane thiols
have been employed to assemble gold nanoparticles, the
flexibility of the alkyl chain prevents a stable gap between
the particles.10 To avoid this problem, rigid linker mole-
cules based on arylene vinylenes and arylene ethynylenes
were introduced as a design principle.11 However, until
Here we report the synthesis of a tetrahedral molecule. By
attaching 8 to a large gold nanoparticle surface, the most
probable structure is that three arms bind to the surface of
the particle. The fourth arm stands out perpendicular to
the surface and is able to bind another particle. If instead,
compared to the linker size, small nanoparticles are em-
ployed, each arm would be able to bind one particle, re-
sulting in a tetrahedral cluster.
Different strategies can be utilized to create these kinds of
linkers.14 In the divergent approach, starting from a tetra-
phenylmethane core, in consecutive steps more phenyl
ethynylene units could be coupled onto the core. Howev-
er, side reactions or incomplete functionalization of all
arms results in structures with similar properties (e.g., Rf
values) and, as a result, separation of these impurities
from the desired product becomes an increasingly tedious
task the longer the arm becomes. The second strategy in-
volves the convergent approach, whereby the arm is syn-
thesized first, followed by final attachment to the core.
An acetylene-functionalized core and an iodo-functional-
ized arm could be coupled through a Sonogashira reac-
tion. Star-star coupling of the core, however, leads to a
complex mixture of byproducts. Hence, a more versatile
approach is the utilization of an iodo-substituted core and
the respective arm. The creation of large rigid OPE struc-
tures can be accomplished in most cases by utilizing the
bifunctional compound C, which can be coupled onto an
acetylene moiety through a Sonogashira reaction (Scheme
1).15 Subsequent cleavage of the SiMe3 protecting group
followed by reaction with C allows the number of repeat-
ing units to be increased in consecutive steps. Another
strategy involves the orthogonal deprotection of com-
pound B, which can be converted either into compound C
or into compound D.16 The latter can be coupled with the
corresponding iodo compound followed by conversion of
the triazene moiety with, for example iodomethane to give
the elongated iodo compound.
SYNTHESIS 2014, 46, 0475–0478
Advanced online publication: 03.01.2014
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DOI: 10.1055/s-0033-1340494; Art ID: SS-2013-T0708-OP
© Georg Thieme Verlag Stuttgart · New York

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F. Kretschmer et al.
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showed only poor solubility, which hampers purification
as well as potential applications.
I
NH2
A
Hydroquinone was found to be a suitable starting material
for linkers with increased solubility because it allows the
introduction of long alkyl side chains through etherifica-
tion of the hydroxyl groups. Moreover, halogenation with
N-bromosuccinimide (NBS) and iodine followed by a
Sonogashira reaction gave compound 1, which is similar
in its structure to C. The strength of the gold–sulfur inter-
action is, to a large extent, dependent on the nature of the
thio moiety.17
1. HCl (aq), NaNO2, pyrrolidine
2. TMSA, Et3N, Pd(PPh3)4, CuI
Me3Si
N N N
B
MeI
KF
Thiol groups and disulfides bind most strongly; however,
they interfere with the palladium-catalyzed cross-cou-
pling reaction. Whereas thioacetates can be utilized as
protecting groups for thiols, we found that they are also
cleaved off to a significant extent during the reaction, thus
hampering multiple consecutive synthetic steps with this
moiety. Hence, although offering a less strong gold–sulfur
interaction, thioethers were chosen due to their good
chemical stability. Therefore, 1 was coupled with p-eth-
ynylthioanisole 2 in a Sonogashira reaction to give 3 as a
yellow solid (Scheme 2). It should be noted that 1 can con-
tain substantial amounts of the disubstituted trimethylsi-
lylacetylene (TMSA) compound where the bromine is
additionally substituted by TMSA. Both are difficult to
separate by column chromatography, however, upon sub-
sequent coupling with 2 only the bromo-substituted com-
pound reacts. Compound 3 can then easily be separated
from the disubstituted compound by column chromatog-
Me3Si
I
N N N
C
D
Sonogashira reaction
Me3Si
N N N
E
Scheme 1 Schematic representation of the synthesis of OPEs14
p-Iodoaniline (A) is a common starting material for both
approaches; however, incorporation of side chains that
improve the solubility is difficult to achieve. For an in-
creasing number of repeating units, the solubility of the
OPEs decreases due to extensive π-stacking. Experiments
conducted with linear linkers devoid of side-chains
OC6H13
OC6H13
Et3N, CuI
Pd(PPh3)2Cl2
Me3Si
Br
SMe
+
Me3Si
SMe
80 °C, 20 h, 63%
2
C6H13
O
C6H13
O
1
3
OC6H13
OC6H13
OC6H13
1, Et3N, CuI
THF–MeOH, KF
r.t., 24 h, 93%
Pd(PPh3)2Cl2
Me3Si
SMe
SMe
SMe
80 °C, 20 h, 73%
C6H13
O
C6H13O
C6H13
O
4
5
OC6H13
OC6H13
OC6H13
OC6H13
THF–MeOH, KF
r.t., 20 h, 92%
Me3Si
SMe
C6H13
O
C6H13O
C6H13
O
C6H13O
5
6
I
OC6H13
OC6H13
THF, Et3N, CuI
Pd(PPh3)4
6
+
SMe
I
I
C
r.t., 24 h, 60%
4
C6H13
O
C6H13O
8
I
7
Scheme 2 Schematic representation of the synthesis of the tetrahedral linker
Synthesis 2014, 46, 475–478
© Georg Thieme Verlag Stuttgart · New York

PAPER
Synthesis of a Rigid Tetrahedral Thioether Linker
477
Anal. Calcd for C32H44O2SSi: C, 73.79; H, 8.51; S, 6.16. Found: C,
74.00; H, 8.46; S, 6.16.
raphy. The conversion of the reaction and the attachment
of the phenylene-ethynylene units can be easily followed
by thin-layer chromatography. Both the Rf value and the
fluorescence of the product change significantly as the
length of the conjugated π system increases. In the follow-
ing step, cleavage of the trimethylsilyl group was
achieved through conversion with potassium fluoride un-
der an inert atmosphere to yield acetylene 4. Subsequent-
ly, a second Sonogashira reaction was applied to react 4
with compound 1 to give 5 in 73% yield. Cleavage of the
trimethylsilyl groups was accomplished in the same way
as described before to yield the linker arm 6. Through a
Sonogashira reaction of the arm with tetrakis(4-iodo-
phenylmethane) 7, finally the tetrahedral linker 8 could be
synthesized. The compound shows good solubility in
common organic solvents such as chloroform, tetrahydro-
furan and N,N-dimethylformamide.
(4-{[4-Ethynyl-2,5-bis(hexyloxy)phenyl]ethynyl}phenyl)(meth-
yl)sulfane (4)
Compound 3 (970 mg, 1.86 mmol) and KF (540 mg, 9.29 mmol)
were dissolved in a 1:1 mixture of MeOH and THF (40 mL). After
stirring at r.t. under an argon atmosphere for 24 h, toluene (100 mL)
was added and the mixture was washed three times with H2O and
then dried with Na2SO4. The solvent was removed under reduced
pressure and the crude product was purified by column chromatog-
raphy (SiO2; CHCl3–hexane, 1:1) to obtain 4.
Yield: 780 mg (93%); yellow solid; mp 81 °C.
IR (FTIR): 3287, 2940, 2851, 2357, 1504, 1389, 1273, 1219, 1030,
864, 818, 733, 637 cm–1.
1H NMR (300 MHz, CD2Cl2): δ = 7.44 (d, J = 8.2 Hz, 2 H), 7.22 (d,
J = 8.2 Hz, 2 H), 6.99 (s, 1 H), 6.98 (s, 1 H), 3.99 (m, 4 H,
2 × OCH2), 3.38 (s, 1 H, CH), 2.50 (s, 3 H, SCH3), 1.80 (m, 4 H,
2 × CH2), 1.54 (m, 4 H, 2 × CH2), 1.36 (m, 8 H, 4 × CH2), 0.91 (m,
6 H, 2 × CH3).
13C NMR (75 MHz, CD2Cl2): δ = 154.39, 153.59, 140.08, 131.94,
125.93, 119.62, 117.95, 116.88, 114.81, 112.59, 94.79, 86.02,
82.35, 80.19, 69.87, 69.81, 31.81, 31.76, 29.51, 29.38, 25.94, 25.81,
22.87, 22.82, 15.29, 14.03, 14.02.
In conclusion, a novel, rigid tetrahedral linker for the po-
tential self-assembly of gold nanoparticles has been syn-
thesized. The formation of plasmonic superstructures with
this compound is under investigation.
HRMS (ESI): m/z [M + H] calcd for [C29H36O2S + H]+: 449.2509;
found: 449.2488.
Reagents and solvents were purchased from Carl Roth, Sigma–
Aldrich, Fluka, Merck and VWR. All chemicals were used as re-
ceived. THF was distilled over sodium/benzophenone under an ar-
gon atmosphere. Triethylamine was distilled over KOH under an
Anal. Calcd for C29H36O2S: C, 77.63; H, 8.09; S, 7.15. Found: C,
77.66; H, 8.30; S, 6.95.
1
argon atmosphere. H and 13C NMR spectra were recorded with a
({4-[(2,5-Bis(hexyloxy)-4-{[4-(methylthio)phenyl]ethynyl}phe-
nyl)ethynyl]-2,5-bis(hexyloxy)phenyl}ethynyl)trimethylsilane
(5)
Bruker AC 300 at 298 K. Chemical shifts are reported in parts per
million (ppm, δ scale) and were calibrated against residual protio-
solvent. ESI-Q-TOF-MS measurements were performed with a mi-
crOTOF (Bruker Daltonics) mass spectrometer. Elemental analyses
were carried out with a Vario ELIII – Elementar Euro and an EA
from HekaTech. Melting points were determined with a Stuart
SMP3. IR spectra were measured with a Shimadzu IRAffinity-1
FTIR-spectrophotometer. Compounds 1,18 2,19 and 720 were pre-
pared as reported.
Compound 4 (1.2 g, 2.67 mmol) and 1 (1.21 g, 2.67 mmol) were dis-
solved in Et3N (50 mL). The solution was purged with argon for 45
min, then Pd(PPh3)2Cl2 (18.9 mg 2.67 µmol) and CuI (5 mg, 2.67
µmol) were added and the solution was heated to 80 °C and stirred
at this temperature overnight. Et2O (100 mL) was added and the
mixture was washed with H2O (3 × 100 mL). The organic phase
was then dried over Na2SO4 and the solvent was removed under re-
duced pressure. The crude product was purified by column chroma-
tography (SiO2; CHCl3–hexane, 1:1) and then recrystallized from
EtOH to obtain 5.
[(2,5-Bis(hexyloxy)-4-{[4-(methylthio)phenyl]ethynyl}phe-
nyl)ethynyl]trimethylsilane (3)
Compound 1 (3.06 g, 6.75 mmol), 2 (1 g, 6.75 mmol), and CuI (13
mg, 6.8 µmol) were dissolved in Et3N (50 mL). The solution was
purged with argon for 1 h and subsequently Pd(PPh3)2Cl2 (48 mg,
6.8 μmol) was added. The solution was heated to 80 °C and stirred
at this temperature for 20 h. The solvent was then removed under re-
duced pressure and the residue was dissolved in CHCl3 (100 mL).
The organic phase was washed three times with H2O, dried over
Na2SO4 and finally the solvent was removed under reduced pres-
sure. The crude product was purified by column chromatography
(SiO2; CH2Cl2–hexane, 1:1) to obtain 3.
Yield: 1.6 g (73%); yellow solid; mp 70 °C.
IR (FTIR): 2947, 2851, 2361, 2149, 1497, 1389, 1273, 1215, 1034,
841 cm–1.
1H NMR (300 MHz, CD2Cl2): δ = 7.48 (d, J = 8.0 Hz, 2 H), 7.26 (d,
J = 8.2 Hz, 2 H), 7.05 (s, 1 H), 7.04 (s, 1 H), 7.00 (s, 1 H), 6.98 (s,
1 H), 4.04 (m, 8 H, 4 × OCH2), 2.54 (s, 3 H, SCH3), 1.87 (m, 8 H,
4 CH2), 1.56 (m, 8 H, 2 CH2), 1.39 (m, 16 H, 8 × CH2), 0.94 (m,
12 H, 4 × CH3), 0.29 (s, 9 H, 3 × SiCH3).
13C NMR (75 MHz, CD2Cl2): δ = 154.68, 154.14, 154.07, 153.91,
140.37, 132.29, 126.31, 120.08, 117.81, 117.59, 117.54, 117.41,
114.91, 114.63, 114.48, 114.34, 101.76, 100.62, 95.2, 92.05, 91.87,
86.67, 70.26, 70.24, 70.17, 70.1, 32.2, 29.92, 29.86, 26.34, 26.29,
26.26, 26.24, 23.24, 15.66, 14.40, 0.19.
Yield: 2.2 g (63%); yellow solid; mp 67 °C.
IR (FTIR): 2940, 2859, 2156, 1504, 1389, 1215, 1030, 891, 837,
810, 760, 637 cm–1.
1H NMR (300 MHz, CD2Cl2): δ = 7.46 (d, J = 8.2 Hz, 2 H), 7.25 (d,
J = 8.2 Hz, 2 H), 7.00 (s, 1 H), 6.96 (s, 1 H), 4.02 (m, 4 H,
2 × OCH2), 2.54 (s, 3 H, SCH3), 1.84 (m, 4 H, 2 × CH2), 1.58 (m,
4 H, 2 × CH2), 1.39 (m, 8 H, 4 × CH2), 0.94 (m, 6 H, 2 × CH3), 0.29
(s, 9 H, 3 × SiCH3).
13C NMR (75 MHz, CD2Cl2): δ = 155.4, 154.7, 141.09, 133.01,
127.01, 120.76, 118.43, 118.09, 115.49, 114.88, 102.47, 101.26,
95.82, 87.23, 70.88, 70.84, 32.91, 32.89, 30.62, 30.6, 27.02, 27.0,
23.95, 16.38, 15.14, 15.12, 0.91.
HRMS (ESI): m/z [M + H] calcd for [C52H72O4SSi + H]+: 821.4993;
found: 821.4938.
Anal. Calcd for C52H72O4SSi: C, 76.05; H, 8.84; S, 3.9. Found: C,
76.35; H, 8.86; S, 4.29.
{4-[(4-{[4-Ethynyl-2,5-bis(hexyloxy)phenyl]ethynyl}-2,5-
bis(hexyloxy)phenyl)ethynyl]phenyl}(methyl)sulfane (6)
Compound 5 (830 mg, 1.01 mmol) and KF (460 mg, 7.91 mmol)
were dissolved in a 1:1 mixture of MeOH and THF (40 mL). After
stirring at r.t. under an argon atmosphere overnight, toluene (100
mL) was added to the solution. The mixture was washed three times
HRMS (ESI): m/z [M + Na] calcd for C32H44O2SSiNa+: 543.2723;
found: 543.2712.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 475–478

478
F. Kretschmer et al.
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with H2O, then dried with Na2SO4. The solvent was removed under
reduced pressure and the crude product was purified by column
chromatography (SiO2; CH2Cl2–hexane, 1:1) to obtain 6.
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Yield: 700 mg (92%); yellow solid; mp 82 °C.
IR (FTIR): 3291, 2920, 2851, 2361, 1732, 1505, 1420, 1389, 1204,
1030, 860, 818, 617 cm–1.
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1H NMR (300 MHz, CD2Cl2): δ = 7.45 (d, J = 8.0 Hz, 2 H), 7.23 (d,
J = 8.2 Hz, 2 H), 7.05–6.95 (m, 4 H), 4.01 (m, 8 H, 4 × OCH2), 3.39
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8 H, 4 × CH2), 1.36 (m, 16 H, 8 × CH2), 0.90 (m, 12 H, 4 × CH3).
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13C NMR (75 MHz, CD2Cl2): δ = 155.11, 154.51, 154.46, 154.23,
140.74, 132.66, 126.68, 120.44, 118.81, 117.92, 117.84, 117.79,
115.68, 115.07, 114.78, 113.49, 95.58, 92.43, 92.06, 87.01, 83.15,
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HRMS (ESI): m/z [M + H] calcd for [C49H64O4S + H]+: 749.4598;
found: 749.4546.
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78.79; H, 8.84; S, 4.05.
Tetrakis[4-({4-[(2,5-bis(hexyloxy)-4-{[4-(methylthio)phe-
nyl]ethynyl}phenyl)ethynyl]-2,5-bis-(hexyloxy)phenyl}eth-
ynyl)phenyl]methane (8)
Compound 6 (280 mg, 0.37 mmol), 7 (70 mg, 85.5 μmol),
Pd(PPh3)4 (23 mg, 20 μmol) and CuI (3.8 mg, 20 μmol) were dis-
solved in THF (5 mL). The solution was purged 20 min with nitro-
gen and subsequently Et3N (5 mL) was added. The solution was
stirred at r.t. for 24 h, then heated to 80 °C for 3 h. Subsequently, the
solvent was removed under reduced pressure and the crude product
was purified by column chromatography (SiO2; CH2Cl2–hexane,
3:2) to obtain 8.
Yield: 170 mg (60%); dark-yellow solid; mp 90 °C.
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IR (FTIR): 2929, 2859, 2361, 1732, 1508, 1420, 1377, 1277, 1215,
1018, 860, 818 cm–1.
1H NMR (300 MHz, CD2Cl2): δ = 7.46 (m, 16 H), 7.22 (m, 16 H),
7.10–7.03 (m, 16 H), 4.05 (m, 32 H, 16 × OCH2), 2.51 (s, 12 H,
4 × SCH3), 1.86 (m, 32 H, 16 × CH2), 1.53 (m, 32 H, 16 × CH2),
1.36 (m, 64 H, 32 × CH2), 0.90 (m, 48 H, 16 × CH3).
13C NMR (75 MHz, CD2Cl2): δ = 153.72, 153.57, 153.50, 146.06,
139.79, 131.72, 130.97, 125.73, 121.50, 119.51, 116.99, 116.95,
116.84, 114.16, 114.04, 113.94, 113.88, 94.62, 94.41, 91.52, 92.42,
86.43, 86.10, 69.70, 69.63, 69.60, 65.00, 31.63, 30.59, 29.34, 29.29,
25.75, 25.68, 22.66, 15.09, 13.83.
HRMS (ESI): m/z [M + H] calcd for [C221H268O16S4 + H]+:
3308.9174; found: 3309.2124.
Anal. Calcd for C221H268O16S4: C, 80.22; H, 8.16; S, 3.88. Found: C,
80.10; H, 8.36; S: 3.96.
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Acknowledgment
Financial support from the research initiative PhoNa (Photonic na-
nomaterials), which is supported by the German Federal Ministry of
Education and Research in the program ‘Spitzenforschung und
Innovation in den Neuen Ländern’ (support code 03IS2101A), is
acknowledged.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.SnuIomprifgtooatrinSugpIoi
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