Solid-phase synthesis of unsymmetrical trans-stilbenes

Article abstract of DOI:10.1021/cc900099g

A small library of fluorescent and unsymmetrical trans-stilbene compounds was efficiently produced by a solid-phase synthetic approach using the sulfonate-based traceless linker system. Nickel(0)-catalyzed cleavage/cross-coupling of polymer-bound stilbenesulfonates with aryl Grignard reagents generated the desired trans-stilbenes at high yield by the multifunctional release facilitating the additional diversification of the library. The luminescent properties of trans-stilbenes are assessed, and the influence of alkyl substituents on their photophysical properties is discussed.

Full text of DOI:10.1021/cc900099g

J. Comb. Chem. 2010, 12, 45–50
45
Solid-Phase Synthesis of Unsymmetrical trans-Stilbenes
Chul-Hee Cho, Chul-Bae Kim, and Kwangyong Park*
School of Chemical Engineering and Materials Science, Chung-Ang UniVersity, 221 Heukseok-Dong,
Dongjak-Gu, Seoul 156-756, Korea
ReceiVed July 10, 2009
A small library of fluorescent and unsymmetrical trans-stilbene compounds was efficiently produced by a
solid-phase synthetic approach using the sulfonate-based traceless linker system. Nickel(0)-catalyzed cleavage/
cross-coupling of polymer-bound stilbenesulfonates with aryl Grignard reagents generated the desired trans-
stilbenes at high yield by the multifunctional release facilitating the additional diversification of the library.
The luminescent properties of trans-stilbenes are assessed, and the influence of alkyl substituents on their
photophysical properties is discussed.
Introduction
However, the systematic relationship between the lumines-
cent properties of materials and their chemical structure
remains unclear. Therefore, it is important to prepare and
characterize a variety of structurally similar luminophores
to elucidate the influence of chemical modifications on the
photophysical properties. During the course of our studies
on the development of organic fluorescent materials emitting
blue light for OLEDs, we became interested in developing
an efficient synthetic methodology for preparing a stilbene
library. A combinatorial strategy was necessary to produce
a large array of different stilbene derivatives quickly.
Organic fluorescent materials have attracted attention
because of their various potential applications such as
fluorescent dyes for labeling biomolecules,1 fluorescent
tracers,2 fluorescent sensors and switches,3 fluorochromes in
dental restorations,4 and optical brighteners.5 Recently, they
have gained particular interest for application in organic light-
emitting diodes (OLEDs), in which the emissive layer is
composed of organic fluorescent compounds.6
Stilbene derivatives, which are famous because of their
wide range of biological activities,7 are attracting much
attention for application in OLEDs. Stilbene derivatives have
been widely investigated as short subunits of poly(p-
phenylenevinylene), which was the first light-emitting poly-
mer to be discovered,8 to gain an understanding of the
emission mechanism and the relationship between the
substitution on the conjugated skeleton and the emitted light.9
Moreover, stilbene and aminostilbene derivatives exhibit blue
photoluminescence10 and hole transport11 properties. Stilbene
dendrimers have recently attracted considerable attention as
efficient luminescent materials.12
Dimerization reaction has been the most popular method
for the preparation of symmetrical stilbenes. The oxidative
dimerization of methylarenes,13 eliminative dimerization of
benzyl halides,14 and reductive dimerization of benzalde-
hydes15 have been reported for preparing symmetrical
stilbenes. On the other hand, unsymmetrical stilbenes have
often been prepared by the Wittig16 or Horner-Wadsworth-
Emmons (HWE) reactions17 of substituted benzaldehydes
with benzyl phosphonium ylides or phosphonates. The Heck
reaction has also been applied for the synthesis of stilbene
derivatives.18 We have previously reported our efforts in
applying the nickel-catalyzed coupling reaction of neopen-
tyloxysulfonylarenes with arylmagnesium halides to the
synthesis of unsymmetrical stilbene derivatives.19
Combinatorial synthesis, coupled with high-throughput
screening and integrated data management systems, is a
powerful tool to produce large and diverse libraries of
structurally related compounds. The combinatorial approach,
which has played an important role in the search for lead
molecules in the pharmaceutical industry,20 is now widely
utilized to accelerate the discovery of new materials.21
Inorganic luminescent22 and magnetic23 materials, catalysts,24
organic/inorganic sensor materials,25 and organometallic26
and polymeric luminescent materials27 have been investigated
through the use of combinatorial approaches. The parallel/
combinatorial preparation of organic fluorescent materials
has also been reported.28
Polymer-supported parallel/combinatorial synthesis is the
most popular synthetic strategy for the combinatorial prepa-
ration of large libraries of organic substrates. A library of
hydroxystilbene analogues has also been prepared by this
method and characterized to investigate their biological
activities.29 During the polymer-supported reaction process,
substrates are usually linked to the polymeric support through
an ether, ester, or amide bond.30 Accordingly, most organic
compounds prepared by this approach are destined to possess
a functional moiety, typically hydroxyl, amino, or carboxyl
group, that is the trace of the former linkage with the support.
However, a majority of blue emitting organic materials are
nonfunctionalized hydrocarbons.31 Therefore, the polymer-
supported preparation of a fluorescent stilbene library for
OLEDs requires the efficient traceless linker technology32
Organic fluorescent materials with high quantum efficiency
and longtime stability are required for OLED fabrication.
* To whom correspondence should be addressed. E-mail: kypark@
cau.ac.kr.
10.1021/cc900099g CCC: $40.75  2010 American Chemical Society
Published on Web 12/04/2009

46 Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 1
Scheme 1. Library Design for Stilbene Compounds 6
Cho et al.
that enables the release of unfunctionalized stilbenes from
the support. In addition, a multifunctional cleavage strategy33
is highly desirable to facilitate the additional diversification
of the library with the release of the target compound.
However, difficulties have been experienced in trying to
isolate the known synthetic strategy appropriate for our
purpose.
Herein, we present the novel parallel syntheses and
photophysical studies of fluorescent unfunctionalized stilbene
derivatives. The sulfonate-based traceless and multifunctional
linker system34 was applied for the solid-phase synthesis of
unsymmetrical trans-stilbene derivatives 6 via the nickel(0)-
catalyzed cleavage/cross-coupling reaction of polymer-bound
trans-stilbenesulfonates 4 with aryl Grignard reagents 5
(Scheme 1). Polymer-supported synthesis of unfunctionalized
stilbenes has not previously been reported to the best of our
knowledge. The results of this study are presented and
discussed below.
simple filtration and underwent the next step without any
further purification.
The progress of all of the solid-phase reactions employed
in these sequences was monitored by using FT-IR spectros-
copy (Figure 1). Sulfonylation and the subsequent methy-
lation of the hydroxyethylmethyl resin were identified by
the disappearance of the IR bands corresponding to the
hydroxy group at 3583 and 3455 cm-1, and by the appear-
ance of new bands corresponding to the sulfate group of 1
at 1362 and 1182 cm-1. The presence of a strong new band
at 1705 cm-1 and the absence of the absorption band at 1576
cm-1 corresponding to the C-Br bond clearly demonstrated
the efficient substitution of the Br group by formyl group in
2. Solid-phase HWE reaction of 2 with diethyl benzylphos-
phonate was confirmed by the complete disappearance of
the formyl band on the IR spectrum of 4{1}.
Stilbene derivatives 6 were generated by the traceless
multifunctional cleavage/cross-coupling reactions of stilbe-
nesulfonates 4 with aryl Grignard reagents 5 using a Ni(0)
catalyst. Treatment of a mixture of 4 and [1,1′-bis(diphe-
Result and Discussion
Polymer-supported 4-bromobenzenesulfonate 1 was pre-
pared by treating hydroxyethylmethyl resin34 with 4-bro-
mobenzenesulfonyl chloride in the presence of Et3N for
48 h.35 The yield of 1 was calculated as 91% based on the
loading level of the resin, which was determined by elemental
analysis. The remaining hydroxyl group was converted into
a methoxy group by treatment with excess CH3I. Polymer-
bound 4-formylbiphenylsulfonate 2 was prepared by the
Suzuki-Miyaura coupling of 1 with 4-formylphenylboronic
acid in the presence of Pd(PPh3)4. The liberation of arene-
sulfonate via the catalytic cleavage of the C-O bond by the
Pd(0) catalyst was not observed by our standard analysis.
Formylbiphenylsulfonate 2 was allowed to react with ben-
zylphosphonates 3 to form polymer-bound stilbenesulfonates
4 by constructing the central vinyl moiety of the stilbene
skeletons. All intermediates 1, 2, and 4 could be isolated by
Figure 1. FT-IR analysis of the polymer-bound intermediates 1, 2,
and 4{1}.

Solid-Phase Synthesis of Unsymmetrical trans-Stilbenes
Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 1 47
Table 1. Solid-Phase Preparation of Unsymmetrical trans-Stilbenes 6a
a Reactions between 4 (0.282 mmol) and 5 (1.97 + 1.41 mmol) in THF (6 mL) were carried out at 66 °C (6 mL) in the presence of dppfNiCl2 (0.113
mmol). b Reactions between 1 (10.4 mmol) and boronic acid (34.2 mmol) in DME (230 mL) were carried out at 85 °C in the presence of Pd(PPh3)4
(0.933 mmol) and aqueous Na2CO3 (0.0900 mmol). c Reactions between 2 (1.86 mmol) and 3 (7.44 mmol) DME (40 mL) were carried out at 85 °C in
the presence of NaH (7.07 mmol). d The calculated overall yields of 6 from 1 are based on the loading level of 1.
nylphosphino)ferrocene]dichloronickel (dppfNiCl2) with 15
equiv of 5 produced the desired unsymmetrical stilbenes 6
via the nucleophilic aromatic substitution of the alkyloxy-
sulfonyl group by aryl nucleophiles. The reactions were
performed in tetrahydrofuran (THF) at 66 °C for 60 h.
Stilbenes 6 were isolated as reasonably pure white solids by
simple precipitation using methanol after the standard
workup. Even though a large amount of biphenyls derived
by the dimerization of 5 was produced in these reactions,
these biphenyls were easily removed by washing with
methanol during the precipitation process. The catalytic
substitution of arenesulfonates by nucleophiles36 was not
observed by gas chromatography (GC) and nuclear magnetic
resonance (NMR) analyses. Crude products were purified
further by column chromatography for more precise calcula-
tion of isolated yields and characterization of structures.
All reactions resulted in good overall yields (Table 1),
which were calculated on the basis of the initial loading of
1. Isolated yields of 37-50% for three steps are compatible
or better than the theoretical combined yields of the corre-
sponding solution-phase syntheses.19 Overall yields were
dependent more on the type of phosphonates than on the
type of Grignard reagents. The use of different kinds of
phosphonate 3 induced a 7-10% variation in the overall
yields (entries 1, 4, and 7 and entries 3, 6, and 9), while a
change of Grignard reagents 5 generated only a 4-6%
variation (entries 1-3 and entries 4-6). Only trans-stilbenes
were obtained within the limits of the NMR analysis, as

48 Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 1
Cho et al.
Table 2. Absorption and Emission Specifications of Stilbene Derivatives 6
solutiona
solid
c
d
entry
1
product
λabsb(nm)
λem (nm)
stokes shift (nm)
72
ΦF
λem (nm)
chromaticity
mp (°C)
6{1}
327
399
0.31
0.49
0.75
0.38
0.39
0.57
0.48
0.49
0.54
441
x: 0.1470
y: 0.1437
x: 0.1487
y: 0.1297
x: 0.1492
y: 0.1205
x: 0.1451
y: 0.1616
x: 0.1496
y: 0.1684
x: 0.1484
y: 0.1709
x: 0.1426
y: 0.1186
x: 0.1574
y: 0.0784
x: 0.1484
y: 0.1344
315-317
2
3
4
5
6
7
8
9
6{2}
6{3}
6{4}
6{5}
6{6}
6{7}
6{8}
6{9}
333
333
336
335
336
335
335
339
400
400
400
402
402
401
404
402
67
67
64
67
66
66
69
63
442
447
460
466
477
465
459
450
c
326-328
281-283
314-316
329-330
299-301
293-294
313-315
307-309
a Solution of 6 in chloroform solution. b λabs represents the maximum absorption wavelength of 6. λem represents the maximum fluorescence
wavelength of 6. d The quantum yields were determined using 4-phenyl-trans-stilbene in cyclohexane as the reference standard (ΦF ) 0.60).
influence on the red shift of fluorescence wavelength (entries
4-6). The methyl group also considerably increased the y
value of chromaticity of the fluorophore.31
Conclusions
A small library of fluorescent trans-stilbene compounds
was produced and characterized by employing a solid-phase
parallel synthetic approach. The unsymmetrical and unfunc-
tionalized trans-stilbene derivatives 6 were generated in good
Figure 2. Quantum yields of 6 in chloroform.
yield by the nickel(0)-catalyzed cleavage/cross-coupling of
polymer-bound trans-stilbenesulfonate 4 with aryl nucleo-
phile 5. The luminescent properties of 6 were assessed, and
the influence of the alkyl substituents on their photophysical
expected. The coupling constants of a pair of doublets
generated by two vinylic protons of 6{4-9} were 16-17
Hz, which is the typical coupling constant between two trans
properties was discussed. The efficiency of the sulfonate-
vicinal vinylic protons, in the 1H NMR spectra. The vinylic
based traceless and multifunctional linker system was
protons of 6{1-3} formed a singlet in the 1H NMR spectra.
The absorption and emission properties of stilbenes 6 were
characterized and are summarized in Table 2. All of 6
demonstrated in the preparation of various unfunctionalized
hydrocarbons by facilitating the additional diversification of
the library with the traceless release of the final products.
dissolved in CHCl3 displayed similar spectra with the
We expect this approach to be useful for the rapid parallel
absorption λmax between 333 to 339 nm and the emission
or combinatorial synthesis of conjugated hydrocarbon librar-
ies for various purposes.
λmax between 399 and 404 nm. The absorption and emission
λmax spectra of the solution slightly increased with the
addition of the alkyl substituents, within a small margin.31,37
However, there was no consistency in Stokes shifts of the
solutions with substituents. The fluorescence quantum yields
for 6 in CHCl3 were calculated from the steady-state
spectroscopic measurements using the techniques described
by Demas and Crosby.38 The values are given in Table 2
and graphically illustrated in Figure 2. The quantum ef-
ficiency was considerably enhanced with the addition of an
alkyl group on the terphenyl moiety, while the substitution
on the phenyl moiety had no discernible effect. Especially,
the presence of a tert-butyl group on the terphenyl side
remarkably increased the quantum yield (entries 3, 6 and
9). Dramatic red shifts, which are generally attributed to π-π
stacking, of about 47-75 nm were observed in the emission
maximum of the solid-state stilbenes relative to their solu-
tions.39 While the emission λmax spectra of the solids also
increased in the presence of alkyl substituents as the solution,
the methyl group on the phenyl moieties had a greater
Experimental Section
Preparation of Polymer-Bound Formylbiphenylsul-
fonate 2. Polymer-bound bromobenzenesulfonate 1 (0.691
mmol/g, 15 g) and Pd(PPh3)4 (0.933 mmol, 1.08 g) were
swollen in DME (230 mL) under Ar atmosphere for 30 min
at room temperature. 4-Formylphenylboronic acid (34.20
mmol, 5.13 g) dissolved in a minimum amount of EtOH/
DME (1:1) and 2 M aqueous Na2CO3 (30 mL) were added
to the reactor, and the reaction mixture was stirred at 85 °C
for 24 h. The resin was filtered, washed with DME, 1%
aqueous HCl, water, and MeOH, and dried in vacuo. The
terminal functional group was determined by FT-IR analysis.
FT-IR (KBr) 2912, 1705, 1596, 1491, 1448, 1362, 1176, 949,
814, 754, 696.
General Procedure for the Preparation of Polymer-
Bound Stilbenesulfonates 4. A mixture of polymer-bound
formylbiphenylsulfonate 2 (2.5 g, 1.0 equiv), benzyl phos-

Solid-Phase Synthesis of Unsymmetrical trans-Stilbenes
Journal of Combinatorial Chemistry, 2010 Vol. 12, No. 1 49
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dispersion in oil, 3.10 mmol) in DME (40 mL) was swollen
under Ar atmosphere. The brown mixture was heated at 85
°C for 48 h. The resin was filtered, washed with DME, 1%
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General Procedure for Cross-Coupling Reaction of 4
with 5. To a stirred solution of 4 (0.282 mmol) and
dppfNiCl2 (0.113 mmol, 77.4 mg) in THF (6 mL), was
slowly added Grignard reagent 5 (2.830 mmol) at room
temperature under Ar atmosphere. The reaction mixture was
stirred at 66 °C of THF for 36 h and cooled to room
temperature. An additional 5.0 equiv of 4 (1.415 mmol) was
added to the solution, which was heated for 24 h. The resin
was filtered and washed with THF, 1% aqueous HCl, water,
and excess CH2Cl2. The drained solution mixture was poured
into a separatory funnel containing CH2Cl2 and 1% aqueous
HCl. The combined organic layer was washed with water,
dried over MgSO4, and concentrated in vacuo. To precipitate
the product 6, MeOH was added to the concentrated mixture.
The suspension was filtered through a small pad of silica
gel on a sintered glass filter. The solid remaining on the silica
gel was washed with MeOH, collected by elution with
CH2Cl2, and concentrated under reduced pressure. The
resulting crude product was purified by column chromatog-
raphy on silica gel using n-hexane/CHCl3 as the eluent
system to afford the corresponding products 6.
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1
Supporting Information Available. H NMR for com-
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