Facile synthesis of polyaromatic bisarylethynes using a diborylethyne synthon

Full text of DOI:10.1002/bkcs.10695

Note
DOI: 10.1002/bkcs.10695
BULLETIN OF THE
D. Jung and Y. K. Kang
KOREAN CHEMICAL SOCIETY
Facile Synthesis of Polyaromatic Bisarylethynes Using a Diborylethyne
Synthon
*
Daero Jung and Youn Kyung Kang
Department of Chemistry, Sangmyung University, Seoul 03016, Korea. *E-mail: younkang@smu.ac.kr
Received November 4, 2015, Accepted December 3, 2015, Published online March 21, 2016
Keywords: Bisarylethyne, Bisarylacetylene, Diborylethyne, Suzuki–Miyaura, 1,2-Bis(40,40,50,50-tetra-
methyl[10,30,20]dioxaborolan-20-yl)-ethyne, One-pot reaction
Ethyne-bridged arylene compounds are important classes of
chemical structures that have been studied in broad areas of
fundamental research as well as applications.1–4 The ethyne
bridge has a characteristic structural feature that enforces
attached aromatic units in linear and coplanar geometry,
which plays an important role in efficient intramolecular elec-
tronic interaction.5,6 Many conjugated oligomers,7–9
polymers,10,11 and dendrimers12 that employ the ethyne group
as a major connection unit between the aromatic building
blockshave beenreported. Theethyne bridgeexhibits notonly
such linear rigidity but also structural flexibility that gives rise
to the formation of three dimensional helical structures in fol-
damers13,14 or single-walled carbon nanotube–poly(p-ary-
lene)ethynylene adduct.15,16
The simplest form of ethyne-bridged arylene compound is
bisarylethyne.17–23 The energy gap between highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital ((LUMO) of the individual aryl unit can be reduced sig-
nificantlybyefficientorbitalinteractionthrough twospcarbon
atoms that segregate each aryl unit with only ~4 Å distance,
and thus gives rise to bathochromic shifts of both absorption
and emission spectra. For example, an anthracene that absorbs
light below 400 nm wavelength region becomes a visible light
chromophore when two of them are connected by the ethyne
group.24,25 More than 5000 cm−1 of bathochromic shift can be
achieved by such simple change of chemical structure. And a
Q-band absorption of meso-substituted metal porphyrins in
the central visible region shifts to the near IR region when
two porphyryl units are coupled through an ethyne group.1
These types of molecules have interesting features of
polymorphism,25–27 structure-dependent fluorescence,25,28
unusually high fluorescence quantum yield,29 intramolecular
charge transfer dynamics in the excited state,24 and nonlinear
optical property.30 Similar modification with ethene bridge,
however, which is often employed in the architecture of con-
jugated polymer, cannot afford the coplanar geometry of two
anthracene units due to the steric hindrance between the
anthryl proton in positions 1 and 9 and the ethenyl protons.31
Coplanar conjugated geometries possible for the polypheny-
lenevinylenes or bisarylethenes become cascade structures
in the case of bisanthrylethene. Facile synthesis of ethyne-
bridged polyaromatic dimer is thus important for the study
of solar energy conversion,32 sensors,33 imaging,34 and
molecular electronic devices.35,36
A series of bisarylethyne compounds have already been
synthesized by different approaches. Nakagawa et al. have
synthesized polyaromatic bisarylethynes by means of pyrolytic
decomposition of β-ketoalkylidene-triphenylphosphorane.19
Pd-catalyzedSonogashiracross-couplingreactionbetweenaryl
halide and aryl ethyne is the most widely used method after-
wards. Besidesitsiterative silane protection/deprotectionofter-
minal ethynylenes,22 the production of butadiyne side product
is the major obstacle of this method. Alkyne metathesis of aryl
propyne was the alternative approach that prevents butadiyne
side products; but this method also requires pre-propynylation
ofaryl halide.37 Moreover, the use of protic polar solvents isnot
allowed for this method. Recently, a couple of facile one-pot
synthetic methods to generate bisarylethynes have been
reported. One is the utilization of trimethylsilylacetylene in
the presence of PdCl2(PPh3)2/CuI catalyst system with 1,8-dia-
zabicyclo[5.4.0]undec-7-ene (DBU) and H2O as a base and an
additive,respectively.22 Anotheroneistheutilizationofpropio-
lic acid or 2-butynedioic acid in the presence of PdCl2(PPh3)2/
dppb catalyst system with DBU as a base.23 These methods are
similar to the Sonogashira reaction but a variety of bisary-
lethyne compounds can be prepared in single step. However,
authors of both works were only interested in making bisary-
lethynes from simple phenyl-based aromatics.
Recently, the synthesis of poly(p-aylrene ethynylene)
(PPES) using a new ethyne synthon, 1,2-bis(40,40,50,50-tetra-
methyl[10,30,20]dioxaborolan-20-yl)-ethyne (B2C2), has been
reported.38 Pd-catalyzed Suzuki–Miyaura polycondensation
reaction between B2C2 and diiodoarene was performed even
in the aqueous phase without an inert atmosphere. However,
the utilization of B2C2 for the synthesis of bisarylethynes
has not been reported yet. Herein, we report the synthesis of
series polyaromatic bisarylethyne molecules from the reaction
between bromoarenes and B2C2 via Pd-catalyzed Suzuki–
Miyaura cross-coupling reaction (Scheme 1).
The establishment of appropriate reaction conditions often
requires an extensive screening procedure. Targeting for the
polyaromatic bisarylethynes, we chose 9-bromoanthracene
as a starting material for the optimization of reaction condi-
tions. Although it is typical to start a screening procedure
Bull. Korean Chem. Soc. 2016, Vol. 37, 576–579
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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KOREAN CHEMICAL SOCIETY
Table 1. Pd-catalyzed Suzuki–Miyaura cross-coupling reactions of
Pd, Base
Solvent
O
O
O
O
arylbromide with B2C2.a
Ar
Ar
Ar-X
+
B
B
Ar–Br
Ar-≡-Ar
Yieldb
69
Scheme 1. Synthesis of bisarylethyne from B2C2 ethyne synthon.
for the selection of a suitable catalyst system, we initially per-
formed test reactions to choose a proper solvent to adjust sol-
vent polarity. With the representative known reaction
conditions that utilize PdCl2(PPh3)2 and aqueous K2CO3 as
a catalyst and a base, respectively, we tested the reactions with
seven different solvent systems that have broad windows of
solvent polarity (Table S1, entries 1–7, Supporting Informa-
73
81
ꢀ
tion). The reactions were carried out at 80 C until no
73c
more progress of the reaction was monitored by thin-layer
chromatography. Two days of reaction time was necessary
to complete the reaction. Results indicated that the
reaction gave higher yields as the polarity of the solvent
increased. Nonpolar toluene did not produce the desired
1,2-bis(9-anthryl)ethyne at all. Mildly polar DME and
DME/EtOH mixture gave only16and 5%yields, respectively.
Moderately polar n-BuOH also did not give the product at all.
When DMF or DMF/EtOH mixture was used, the product
yields increased significantly (48 and 40%, respectively).
Addition of EtOH did not improve reaction yield. Interest-
ingly, CH3CN was not effective for the reaction (0%) in spite
of its substantial polarity.
a
Reaction conditions: Aryl bromide = 100 mg (3 equiv to B2C2),
B2C2 = 36 mg, Pd(PPh3)4 = 10 mol% to B2C2, base = 1 M K2CO3
(6 equiv), solvent = DMF, temperature = 80 ꢀC, reaction
time = 2 days.
b
c
Isolated yield.
Reaction time = 1 day.
Pd(PPh3)4, 6 equiv of 1 M K2CO3, DMF of 30 times of the
volume of the aqueous base at 80 C for 2 days.
Thenext parameter that wetried tooptimizewasthecatalyst
system (Table S1, entries 7–12). Among six different catalyst
species, the best result was obtained when Pd(PPh3)4 was used
(53%, Table S1, entry 11). The performances of PdCl2(PPh3)2
and Pd(PtBu3)2 follow next that gave 48 and 42% yields,
respectively. Thus, we tested the reactions with these catalyst
systems with another base of aqueous K3PO4. The yields were
reduced significantly by the change of the base when
PdCl2(PPh3)2 and Pd(PtBu3)2 catalysts were used (24 and
13%, respectively, Table S1, entries 13, 15). However, the
reduction of the yield was only minor for Pd(PPh3)4 (45%,
Table S1, entry 14), indicating that Pd(PPh3)4 is a more versa-
tile catalyst.
The work byMio et al. suggested that the amount of water is
crucial for the optimization of the reaction conditions in the
synthesis of bisarylethynes.22 Inspired by their work, we
tested the effect of solvent/water ratio (Table S1, entries
11, 16–18). Even in the absence of water, the desired product
was formed in 51% yield, which is similar to that obtained by
the preliminary reaction conditions with a DMF/water ratio of
20. The reaction yield was further increased to 69% when the
ratio was increased to 30 (Table 1, entry 17). However, when
the ratio was reduced to 10, the yield was only 43%.
ꢀ
With the optimized reaction conditions at hand, we synthe-
sized three representative polyaromatic bisarylethynes, 1,2-
bis(9-anthryl)ethyne (1), 1,2-bis(9-phenanthrenyl)ethyne
(2), 1,2-bis(1-pyrenyl)ethyne (3) as well as 1,2-bis[(10,20-
bis[20,60-bis(300,300-dimethyl-100-butyloxy)phenyl]porphina-
to)zinc(II)-5-yl]ethyne. The results are summarized in Table 1.
In order to expand the reaction protocol for general synthe-
sis of bisarylethynes, we applied the established reaction con-
ditions to a series of aryl halides. However, the product yields
of bisarylethynes with simple phenylene-based aryl halides
were not as satisfactory as we expected. Therefore we
attempted to devise other reaction conditions. As the synthesis
of PPES with B2C2 was successful via microwave-assisted
Suzuki–Miyaura polycondensation,38 we adopted its reaction
conditions with slight modifications. Elaborate screening pro-
cedures similar to those performed for the optimization
of 1 gave reaction conditions that produced series bisary-
lethynes in moderate to high yields (Table 2). Aryl iodides that
include iodobenzene, 1-iodo-4-methoxybenzene, and 2-
iodothiophene gave desired bisarylethynes in 90, 86, and
99% yields, respectively. When aryl bromides were used,
the product yields were reduced; bromobenzene, 1-bromo-
The final parameter for the optimization of the reaction con-
ditions was the reaction temperature (Table S1, entries
4-methoxybenzene,
1-bromo-4-trifluoromethyl-benzene,
ꢀ
and 1-bromonaphthalene gave corresponding bisarylethynes
in 80, 26, 53, and 57% yields, respectively.
11, 19–22). The reaction yields maximized at 80 C; further
ꢀ
increase to 90 C resulted in a slight reduction of product yield
Elongation of conjugation through ethyne bridge usually
gives rise to a significant bathochromic shift of both
(61%). The final reaction conditions are as follows: to the
given amount of B2C2, 3 equiv of arylbromide, 10 mol% of
Bull. Korean Chem. Soc. 2016, Vol. 37, 576–579
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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KOREAN CHEMICAL SOCIETY
Table 2. Pd-catalyzed Suzuki–Miyaura cross-coupling reactions of
Table 3. Fluorescence excitation and emission data of
compounds 1–3.a
arylhalide with B2C2.a
Ar–X
Ar-≡-Ar
Yieldb
90
λmax
λmax
Stoke
Compound
(ex), nm
(em), nm
Φf
shift, cm−1
1b
2c
3d
a
462
368
433
494
445
442
0.60
1.00
0.81
1402
4702
470
86
99
Experimental conditions: solvent = CH2Cl2, T = 23 ꢁ 1 ꢀC.
b
c
d
λex = 440 nm, λem = 510 nm.
λex = 360 nm, λem = 470 nm.
λex = 400 nm, λem = 460 nm.
80
26
53
57
a
Reaction conditions: aryl halide = 67.6 μmol (4 equiv to B2C2),
B2C2 = 4.8 mg, Pd(OAc)2 = 6 mol% to B2C2, P(tBu)3 = 3.5 mg,
i
(P(tBu)3/Pd(OAc)2 = 4), base = Pr2NH (6 equiv), solvent = DMSO,
additive = dibutylhydroxytoluene (BHT, 1 mg),
ꢀ
temperature = 200 C microwave, reaction time = 5 min.
Yield based on 1H-NMR.
b
Figure 2. Frontier molecular orbital (10 highestoccupied and10 low-
est unoccupied) energy diagram of three diarylethynylene com-
pounds calculated at the B3LYP/6-31g(d) level. Gradual
destabilization of HOMO energy levels and concomittant stabiliza-
tion of LUMO energy levels are guided by the dotted lines.
reported,17,19,20 we measured fluorescence excitation spectra
to closely examine the origin of their fluorescence signatures.
Figure 1 displays spectral features of three compounds and
spectroscopic data are summarized in Table 3.
It is worth noting that both compounds 1 and 2 have signif-
icant Stoke shifts of 1402 and 4702 cm−1, respectively, with
broadened spectral features for emission spectra indicating
that large degrees of structural relaxations are pertinent in
the excited state. Compound 3, however, exhibited only
470 cm−1 of Stoke shift with characteristic mirror image of
excitation and emission spectra. E0,0 values estimated by the
intersection of excitation and emission spectra are 3.01,
2.84, and 2.61 eV for compounds 2, 3, and 1, respectively,
exhibiting gradual decreasing of S1 state energies. Fluores-
cence quantum yields also decreased in the same order;
1.00, 0.81, and 0.61 for compounds 2, 3, and 1, respectively.
Perhaps the energy gap law is the major background for the
sharply decreasing fluorescence quantum yield upon decreas-
ing E0,0 values. Time-dependent density functional theory cal-
culations performed in the gas phase indicate that the lowest
energy absorptions of all three compounds are solely
HOMO-to-LUMO transitions. The frontier orbital energy
Figure 1. Excitation and emission spectra of (a) 1,2-bis(9-phenan-
threnyl)ethyne (2), (b) 1,2-bis(1-pyrenyl)ethyne (3), and (c) 1,2-bis
(9-anthryl)ethyne (1).
absorption and emission spectral envelopes. We measured
fluorescence emission spectra of compounds 1–3. As the
absorption spectra of these compounds have already been
Bull. Korean Chem. Soc. 2016, Vol. 37, 576–579
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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bilization of LUMO levels for compounds 2, 3, and 1, which
are in accordance with experimentally determined E0,0 values.
This work clearly demonstrates an efficient electronic interac-
tion between two polyaromatic units through ethyne bridge.
In conclusion, we have developed a new synthetic protocol
in which Pd-catalyzed Suzuki–Miyaura cross-coupling reac-
tions between aryl halide and B2C2 have been adapted for
the synthesis of polyaromatic bisarylethynes in high yield.
The three prepared compounds, 1–3, show significant batho-
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Acknowledgments. This work was supported by the 2012
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Bull. Korean Chem. Soc. 2016, Vol. 37, 576–579
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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