Composite tetraheteroarylenes and related higher cyclic oligomers of heteroarenes produced by palladium-catalyzed direct coupling

Article abstract of DOI:10.1246/bcsj.20190255

Substantial research interests have been focused on cyclic π-conjugated molecules owing to their unique chemical and physical properties. By constructing hybrid aromatic arrays within these cyclic systems, new series of composite macrocycles would be prov

Full text of DOI:10.1246/bcsj.20190255

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Composite Tetraheteroarylenes and Related Higher Cyclic Oligomers of
Heteroarenes Produced by Palladium-Catalyzed Direct Coupling
Keita Fukuzumi, Yuji Nishii,* and Masahiro Miura*
Advance Publication on the web October 9, 2019
doi:10.1246/bcsj.20190255
© 2019 The Chemical Society of Japan
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Composite Tetraheteroarylenes and Related Higher Cyclic Oligomers of
Heteroarenes Produced by Palladium-Catalyzed Direct Coupling
Keita Fukuzumi,1 Yuji Nishii,*2 and Masahiro Miura*1
1 Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871
2 Frontier Research Base for Global Young Researchers, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871
y_nishii@chem.eng.osaka-u.ac.jp, miura@chem.eng.osaka-u.ac.jp
Yuji Nishii
Yuji Nishii is an assistant professor of the Graduate School of Engineering, Osaka University. He received his Ph.D. degree
from Osaka University in 2015 under the guidance of Prof. K. Mashima. He then assumed the current position. He is a
member of Otsu Academy Award Fellow. His current research interests are focusing on the organic synthetic chemistry
utilizing organo- and transition metal-catalyses, involving direct functionalization of aromatic compounds. He worked as
a visiting researcher with Prof. P. Knochel at Ludwig-Maximilians-Universität München in 2011, and with Prof. T. Agapie
at California Institute of Technology in 2014.
Masahiro Miura
Masahiro Miura is a professor of the Graduate School of Engineering, Osaka University. He received his Ph.D. degree
from Osaka University in 1983 under the guidance of Prof. S. Kusabayashi and Prof. M. Nojima. He started his academic
career as assistant professor at Osaka University in 1984. He was promoted to associate professor in 1994 and to full
professor in 2005. He also worked as a Humboldt fellow with Prof. K. Griesbaum at Karlsruhe University of Germany in
1990–1991. His current research interests include transition-metal catalysis and the synthesis of functional molecules,
especially new π-conjugated aromatic substances.
Abstract
Substantial research interests have been focused on cyclic
thiophene-containing cyclo[9]pyrrole as a 34-electron aromatic
expanded porphyrin.7 Tanaka and Osuka also described a
thiophene-pyrrol hybrid macrocycle as a precursor for an
aza[8]circulene.8 The synthesis of these “composite” molecules,
however, has been a significant challenge, and thus, their low
availability has obstructed the systematic study on their
properties. Our group recently reported a Pd-catalyzed synthesis
of heteroarene-fused COTs through the dehydrogenative
cyclodimerization of biheteroaryls (Scheme 1).9,10,11
Additionally, the first synthesis and characterization of
composite tetraheteroarylenes from unsymmetrical biheteroaryls
were achieved. It was worth noting that a thiophene-thiazole
hybrid tetraarylene provided chiral crystals via the spontaneous
resolution, which was evidenced by a Flack parameter12 of
0.001(7) in the X-ray crystallographic analysis; however, this
compound was only obtained as an inseparable mixture of two
regioisomers under the C–H/C–H coupling protocol.
-conjugated molecules owing to their unique chemical and
physical properties. By constructing hybrid aromatic arrays
within these cyclic systems, new series of composite
macrocycles would be provided. Our group has been studying
the construction of such hybrids by adopting the palladium-
catalyzed direct coupling of heteroaromatics. We herein report
the synthesis and characterization of thiophene-thiazole
composite macrocycles. X-ray diffraction analyses elucidated
that some of these coupling products exhibit characteristic
helical assemblies in the solid state. Additionally, we synthesized
a
heteroarene-fused cyclooctatetraene composed of four
different heteroaryl fragments.
Keywords: Palladium, Macrocycles, Cross-coupling
1. Introduction
The construction of cyclic -conjugated oligomers of
arenes has been an intriguing research topic because of their
unusual optical, electronic, magnetic, and molecular inclusion
properties as contrasted to the parent linear aromatics.1 Among
these macrocycles, tetraarylenes are the smallest class of
compounds in which four aromatic fragments are ortho-linked
each other to establish an arene-fused cyclooctatetraene (COT)
core at the center of the molecules. Considerable attention has
been paid to these compounds over the decades owing to their
unique characteristics involving chirality, conformational
flexibility, and redox-active nature of the central ring.2,3 Notably,
a series of arene-fused COTs have been of key motifs for
investigating the antiaromaticity4,5 and the Baird aromaticity.6
Meanwhile, construction of hybrid aromatic arrays within
such macrocyclic scaffolds would provide a wide variety of
novel -conjugated systems. For example, Sessler, Kim, and
Bucher reported an electrochemical synthesis of a novel
Scheme 1. Tetraarylene synthesis through the Pd-catalyzed
dehydrogenative cyclodimerization of biaryls.

Scheme 2. Palladium-catalyzed direct C–H coupling for the synthesis of a composite tetraheteroarylene and related higher
heteroarylenes.
To address this issue, we have undertaken the
construction of hybrid cyclic skeletons through the Pd-
catalyzed C–H/C–Br direct coupling method.13 In this report,
we describe the synthesis and characterization of a series of
thiophene-thiazole hybrid macrocyclic compounds. Single
crystal X-ray analyses revealed that the created coupling
products form characteristic helical assemblies, and among
them, the new COT specifically affords chiral crystals. In
addition, we have achieved the first synthesis of a
tetraarylene which is composed of four different heteroaryl
fragments.
responsible for the thiophen-thiophene coupling.
The compound 3 has a tab-shape structure with a bent
angle 5a (Chart 1) of 31.2–33.4 degrees in the COT ring.
Fascinatingly, the crystal was classified into a space group
P21 (monoclinic), and its absolute configuration was
estimated by a Flack parameter of 0.04(2). The chirality was
derived from the helical assembly along the a-axis
underpinned by two characteristic C–H···N hydrogen
bonding interactions (ca. 2.6 Å) between the thiophene and
thiazole fragments (Figure 1c). This result contrasts with the
fact that each of other tetraarylenes crystallizes into a
“Pringles-like” columnar (or slipped columnar) packing
structure.3i,3j This is probably because the hydrogen bonding
interaction is much stronger than the – stacking interaction
between electronically non-biased aromatic systems.
2. Results and Discussion
Our study was initiated with the preparation of a thiophene-
thiazole composite tetraarylene 3 through the palladium-
catalyzed direct coupling of a bithiophene 1 with a dibromo-
bithiazole 2. After screening various reaction parameters, the
target compound 3 was found to form up to 6% yield in the
presence of Pd(OAc)2, PPh3, PivOH, and K2CO3 in DMF.
However, further improvement of the productivity of this
compound was difficult because the competing formation of
higher cyclic/acyclic oligomers accounted for the majority of the
reaction mixture. Incidentally, the higher cyclic oligomers
seemed to be invaluable compounds which should be
characterized. Thus, we carried out the reaction at a preparative
scale and isolated a series of cyclized products using size-
exclusion chromatography (Scheme 2). The tetraarylene 3 was
given in 3.5% yield. As the 12-membered ring compounds, a 1:2
coupling product 4a and a 2:1 coupling product 4b were
obtained in 7.6% yield and 2.9% yield, respectively. In addition,
16-membered ring compounds 5a (1:3 coupling product) and 5b
(2:2 coupling product) could be isolated. The connectivity of 5b,
in which bithiophene and bithiazole fragments are alternately
coupled, was confirmed by the highly symmetric signals
observed in its NMR measurements, showing only one 1H signal
for the thiophene C–H. The structures of 3, 4a, and 5a were
unambiguously determined by X-ray crystallographic analyses
(Figure 1 and 2). The formation of these cyclic compounds
implies that not only thiophene-thiazole (C-H/C-Br) coupling,
but also thiophene-thiophene (C-H/C-H) oxidative coupling and
thiazole-thiazole (C-Br/C-Br) reductive coupling occur under
the palladium-catalyzed conditions. The thiazole-thiazole
linkage would be formed via disproportionation or
protodebromination in the intermediary heteroarylpalladium(II)
species with liberating a palldium(II) salt which would be
Figure 1. (a,b) Molecular structure of 3 drawn with 40% thermal
probability ellipsoids. Hydrogen atoms and solvent molecules
are omitted for clarity. (c) Helical pattern in the crystal structure.

compounds also exhibited fluorescence in visible region with
emission maxima at 460–560 nm, albeit the internal quantum
efficiencies were low ( = 0.02–0.03). It was obvious that
the spectral shapes were primarily influenced by the numbers
of arylene units within the central rings, and the
thiophene/thiazole compositional ratio had a minimal impact
on their optical properties. Substantial bathochromic shifts
both in the absorption and fluorescence bands were found for
the 12-membered ring compounds 4 as compared to the other
cyclo-oligomers. This indicates the relatively high
conformational flexibility of 4 which ensures a longer
effective conjugation length. In accordance with these
observations, 1H NMR spectra of 4a and 4b showed two and
three nonequivalent thiophene C–H peaks, respectively,
probably due to the conformational isomerism. The ratio of
these signals were reversibly shifted in valuable-temperature
(VT-NMR) measurements (see the Supporting Information).
In contrast, quasi-bimodal emission bands were found around
500 nm in the fluorescence spectra of 5 even though these
compounds have the largest ring size among the obtained
coupling products, suggesting the higher conformational
rigidity of these macrocycles.
Chart 1. Bent angle for the COT skeleton.
On the other hand, no significant C–H···N interaction
was found in the structure of 4a (Figure 2a and 2b). The
central 12-membered ring was twisted and folded as an
infinity symbol, which is structurally relevant to a previously
reported hexa[2,3-thienylene] macrocycle.14 The compound
5a exhibited an intramolecular stacking association in
between two opposite bithiazole units, and its distance was
approximately 3.14–3.15 Å (Figure 2c and 2d). Although the
proper location of the bithiophene unit in the structure of 5a
was not feasibly determined by the crystallographic analysis,
it should be placed at the edge of the molecule since the
crystals of 5a displayed one-dimensional helical assembly as
similar to that found in the structure of 3. However, 5a
afforded achiral crystals which contain both the right-handed
and left-handed helical patterns with a space group Pba2
(orthorhombic).
Finally, we challenged ourselves to synthesize a
tetraarylene composed of four different arylene fragments
(Scheme 3). Two unsymmetrical biaryls 6 and 7 were
prepared as coupling partners, and these were subjected to the
standard reaction conditions. To our delight, the desired
compound 8 was obtained as a mixture of regioisomers
(8a:8b = 63:37) in 29% combined yield. Replacement of the
ligand by 1,2-bis(diphenylphosphino)ethane (dppe)
improved both the yield and the selectivity (39%, 8a:8b =
67:33). Interestingly, the competing formation of higher
oligomers were fairly retarded in these reactions while the
clear explanation for this has not been in hand.15 After
numerous attempts, the major isomer 8a was successfully
isolated by recrystallization, and its structure was confirmed
by single-crystal X-ray diffraction analysis. This represents
the first synthesis and characterization of tetraarylene with all
different aryl subunits.
Figure 2. Molecular structure of 4a (a,b) and 5a (c,d) drawn with
20% thermal probability ellipsoids. Hydrogen atoms and solvent
molecules are omitted for clarity.
Scheme 3. Tetraarylene synthesis through the palladium-
catalyzed direct C–H coupling.
The positions of two sulfur atoms in 8a can be explicitly
assigned as illustrated in Figure 4, whose structure and unit
cell parameters are similar to those of 3. Bent angles of the
COT moiety ranges within 27.3–31.7 degrees, and the largest
angle was given to the thiophen unit. This can be attributed
to the C–H/C–H as well as S/S repulsive interactions against
the adjacent furan and thiazole fragments. The crystal
exhibited helical packing pattern along the a-axis as
expected; however, the structure was refined as a two-
component inversion twin with a Flack parameter of 0.44(7).
Figure 3. UV-vis absorption and emission spectra of 3 (red), 4a
(blue), 4b (light blue), 5a (green), and 5b (light green) in CHCl3
(5.0×10-6 mol/L).
The UV-vis spectra of the cyclic coupling products 3–5
were measured for CHCl3 solutions (Figure 3). All of these

times. The combined organic layer was dried over Na2SO4 and
concentrated in vacuo. The crude material was subjected to silica
gel chromatography (eluent: hexane) to give 2 in 73% yield (698
mg, 1.46 mmol). NMR data were identical with those reported
in the literature.16 1H NMR (400 MHz, CDCl3) δ 7.44-7.47 (m,
6H), 7.93-7.97 (m, 4H); 13C NMR (100 MHz, CDCl3) δ 109.0,
126.6, 129.2, 130.8, 133.1, 147.5, 167.9.
2-Phenyl-4-(5-phenylthiophen-3-yl)furan (6)
Figure 4. Molecular structure of 8a drawn with 15% thermal
probability ellipsoids. Hydrogen atoms and solvent molecules
are omitted for clarity.
3. Conclusion
In summary, as demonstrated in this study, the palladium-
catalyzed direct coupling reaction can be a convenient method
for the generation of composite heteroarene-fused COTs and
higher macrocyclic homologs. Although we should admit that
the productivity and generality of the present protocol need to be
improved, it has unveiled unique characteristics of these
compounds forming helical assemblies and chiral crystals.
Further studies on the chiroptical properties of the composite
heteroarylenes is in underway.
[Step 1] A two-neck round-bottom flask equipped with an
N2 balloon and
a rubber cap was charged with 2,4-
4. Experimental
dibromothiophene (2.42 g, 10 mmol), phenylboronic acid (1.22
g, 10 mmol), Pd(PPh3)4 (231 mg, 2.0 mol%), and Na2CO3 (1.06
g, 10 mmol). Toluene (20 mL) and ethanol (4 mL) were added
via syringe, and the mixture was refluxed overnight. The
resulting suspension was diluted with H2O and was extracted
with EtOAc three times. The combined organic layer was dried
over Na2SO4 and concentrated in vacuo. The crude material was
subjected to silica gel chromatography (eluent: hexane) to give
4-bromo-2-phenylthiophene as white solid in 84% yield (2.00 g,
8.4 mmol). NMR data were identical with those reported in the
literature.10 1H NMR (400 MHz, CDCl3) δ 7.18 (d, J = 1.4 Hz,
1H), 7.21 (d, J = 1.4 Hz, 1H), 7.30-7.35 (m, 1H), 7.37-7.42 (m,
2H), 7.54-7.58 (m, 2H).
General information
All manipulations were performed under N2 using standard
Schlenk techniques unless otherwise noted. Toluene, DMF, and
Et2O were dried and deoxygenated by a Glass Counter Solvent
Dispending System (Nikko Hansen & Co., Ltd.). DCM was
degassed with N2 bubbling and dried over molecular sieves 4A.
THF and MeOH were purchased as dehydrated solvents and
used as received. Silica gel column chromatography was
performed using Wakosil® C-200. Nuclear magnetic resonance
spectra were measured at 400 MHz (1H NMR) and at 100 MHz
(13C NMR) in 5 mm NMR tubes. 1H NMR chemical shifts were
reported in ppm relative to the resonance of TMS ( 0.00) or the
residual solvent signals at 7.26 for CDCl3 and at 5.32 for
CD2Cl2. 13C NMR chemical shifts were reported in ppm relative
to the residual solvent signals at 77.2 for CDCl3. Melting
points were measured with Mettler Toledo MP90. High
resolution mass spectra (HRMS) were recorded by FAB or
APCI-TOF. HPLC analyses were carried out with JASCO
EXTREMA (PU-4180/MD4015/CO4065) equipped with an
YMC CHIRAL ART Cellulose-SB column at 350 nm detection.
GC-MS spectra were recorded on Shimadzu GCMS-QP2010 SE
with a CBP-1 column (0.5 mm × 25 m). Preparative gel
permeation chromatography (GPC) was conducted with Showa
Denko H-2001, H-2002 column (eluent: CHCl3) or YMC T2000
(eluent: EtOAc) column. Absorption spectra were recorded on
JASCO V-750 spectrometer. Fluorescence and excitation
spectra were recorded on JASCO FP8500 spectrometer.
Quantum efficiency was determined using JASCO FP8500
spectrometer equipped with an integration sphere system
JASCO ILF-835.
[Step 2] Synthesized according to the literature
procedure.18 A two-neck round-bottom flask equipped with an
N2 balloon and a rubber cap was charged with 4-bromo-2-
phenylthiophene (1.28 g, 5.38 mmol) and Et2O (10 mL). To a
solution was added n-BuLi (1.6 mol/L in hexane, 5.91 mmol)
dropwise at -78 °C. The mixture was stirred for 30 min at this
temperature, followed by the slow addition of tributyltin chloride
(3.8 mL, 5.91 mmol). The resulting solution was allowed to
warm to room temperature, and stirred for an additional 2 h. The
resulting suspension was diluted with H2O and was extracted
with EtOAc three times. The combined organic layer was dried
over Na2SO4 and concentrated in vacuo. The crude material
was subjected to silica gel (containing ca. 10wt% K2CO3)
chromatography (eluent: hexane) to give 2-phenyl-4-
(tributylstannyl)thiophene in 62% yield (1.50 g, 3.34 mmol).
1
Colorless oil; H NMR (400 MHz, CD2Cl2) δ 0.88-0.93 (t, J =
7.3 Hz, 9H), 1.07-1.12 (m, 6H), 1.30-1.40 (m, 6H), 1.53-1.61 (m,
6H), 7.24-7.30 (m, 2H), 7.33-7.40 (m, 3H), 7.60-7.65 (m, 2H);
13C NMR (100 MHz, CDCl3) δ 10.3, 13.8, 27.4, 29.2, 126.4 (2C),
127.3, 129.0 (2C), 129.5, 131.4, 134.8, 140.1, 144.7; HRMS
(FAB) m/z calcd for C22H35SSn [M+H]+ 451.1478, found
451.1478.
[Step 3] Synthesized according to the literature
procedure.19 A two-neck round-bottom flask equipped with an
N2 balloon and a rubber cap was charged with 2-furylboronic
5,5'-Dibromo-2,2'-diphenyl-4,4'-bithiazole (2)
A two-neck round-bottom flask equipped with an N2
balloon and a rubber cap was charged with 2,2'-diphenyl-4,4'-
bithiazole9 (641 mg, 2.0 mmol), NBS (730 mg, 4.1 mmol). DCM
(10 mL) was added via syringe, and the mixture was stirred at
room temperature overnight. The resulting suspension was
diluted with Na2CO3aq and was extracted with EtOAc three

acid (1.00 g, 8.93 mmol), iodobenzene (696 mg, 4.47 mmol),
Pd(PPh3)4 (514 mg, 10 mol%), and Cs2CO3 (1551 mg, 4.76
mmol). Toluene (10 mL) and methanol (2.5 mL) were added via
syringe, and the mixture was refluxed overnight. The resulting
suspension was diluted with H2O and was extracted with EtOAc
three times. The combined organic layer was dried over Na2SO4
and concentrated in vacuo. The crude material was subjected to
silica gel chromatography (eluent: hexane) to give 2-phenylfuran
as pale yellow oil in 80% yield (514 mg, 3.57 mmol). NMR data
were identical with those reported in the literature.20 1H NMR
(400 MHz, CDCl3) δ 6.48 (dd, J = 1.8, 3.3 Hz, 1H), 6.64-6.67
(m, 1H), 7.23-7.29 (m, 1H, overlapped with CDCl3), 7.35-7.42
(m, 2H), 7.45-7.50 (m, 1H), 7.65-7.71 (m, 2H).
[Step 4] A two-neck round-bottom flask equipped with an
N2 balloon and a rubber cap was charged with diisopropylamine
(0.26 ml, 1.8 mmol), t-BuOK (202 mg, 1.8 mmol), and THF (5
mL). To a solution was added n-BuLi (1.6 mol/L in hexane, 1.71
mmol) dropwise at -78 °C, and the mixture was stirred for 30
min. A solution of 5-bromo2-phenylfuran (254 mg, 1.14 mmol)
in THF (10 mL) was added dropwise, and the temperature was
slowly brought up to -20 °C. After stirring for 2 h at this
temperature, the resulting mixture was diluted with H2O (10 mL)
and was extracted with EtOAc three times. The combined
organic layer was dried over Na2SO4 and concentrated in vacuo.
The crude material was subjected to silica gel chromatography
(eluent: hexane) to give 4-bromo-2-phenylfuran as white solid in
70% yield (176 mg, 0.80 mmol). NMR data were identical with
those reported in the literature.21 1H NMR (400 MHz, CDCl3) δ
6.68 (m, 1H), 7.27-7.33 (m, 1H, overlapped with CDCl3), 7.37-
7.42 (m, 2H), 7.46 (m, 1H), 7.60-7.65 (m, 2H).
[Step 5] A two-neck round-bottom flask equipped with an
N2 balloon and a rubber cap was charged with 2-phenyl-4-
(tributylstannyl)thiophene (356 mg, 0.80 mmol), 4-bromo-2-
phenylfuran (176 mg, 0.80 mmol), and Pd(PPh3)4 (45.6 mg, 5
mol%). Toluene (5 mL) was added via syringe, and the mixture
was heated at 100 °C for 16 h. The resulting suspension was
diluted with H2O and was extracted with EtOAc three times. The
combined organic layer was dried over Na2SO4 and concentrated
in vacuo. The crude material was subjected to silica gel
chromatography (eluent: hexane) and further purification was
carried out with GPC (EtOAc) to give 6 in 52% yield (125 mg,
0.42 mmol). NMR data were identical with those reported in the
literature.9 1H NMR (400 MHz, CDCl3) δ 6.90 (s, 1H), 7.27 (d,
J = 1.3 Hz, 1H), 7.28-7.34 (m, 2H), 7.38-7.44 (m, 4H), 7.45 (d,
J = 1.3 Hz, 1H), 7.62-7.68 (m, 2H), 7.68-7.74 (m, 3H).
[Step 1] Synthesized according to the literature
procedure.22 A two-neck round-bottom flask equipped with an
N2 balloon and a rubber cap was charged with 1,3-thiazolidine-
2,4-dione (834 mg, 7.12 mmol), P2O5 (4.78 g, 33.7 mmol), and
tetrabutylammonium bromide (5.32 g, 16.5 mmol). Toluene (16
mL) was added via syringe, and the mixture was refluxed
overnight. The resulting suspension was diluted with Na2CO3aq
and was extracted with Et2O three times. The combined organic
layer was dried over Na2SO4 and concentrated in vacuo. The
crude material was subjected to silica gel chromatography
(eluent: hexane/EtOAc = 3/1) to give 2,4-dibromothiazole as
white solid in 84% yield (1.45 g, 5.97 mmol). NMR data was
identical with that reported in the literature.22 1H NMR (400
MHz, CDCl3) δ 7.21 (s, 1H).
[Step 2] Synthesized according to the literature
procedure.23 Pd(OAc)2 (49.8 mg, 2.5 mol%) and Xantphos (129
mg, 2.5 mol%) were dissolved in THF (10 mL) under an N2
atmosphere. After stirring for 5 min at room temperature, this
solution was transferred to a two-neck round-bottom flask
equipped with an N2 balloon and a rubber cap containing 2,4-
dibromothiazole (2.16 g, 8.88 mmol), phenylboronic acid (1.16
g, 9.5 mmol), K3PO4 (5.66 g, 26.6 mmol), and THF (20 mL).
The mixture was heated at 60 °C overnight. The resulting
suspension was diluted with H2O and was extracted with EtOAc
three times. The combined organic layer was dried over Na2SO4
and concentrated in vacuo. The crude material was subjected to
silica gel chromatography (eluent: hexane/EtOAc = 10/1) to give
4-bromo-2-phenylthiazole as white solid in 83% yield (1.76 g,
7.33 mmol). NMR data were identical with those reported in the
literature.23 1H NMR (400 MHz, CDCl3) δ 7.22 (s, 1H), 7.42-
7.47 (m, 3H), 7.91-9.97 (m, 2H).
5-Bromo-4-(5-bromo-2-phenylthiazol-4-yl)-2-phenyloxazole
(7)
[Step 3] Synthesized according to the literature
procedure.24 A two-neck round-bottom flask equipped with an
N2 balloon and a rubber cap was charged with 4-bromo-2-
phenylthiazole (717 mg, 3.0 mmol) and Et2O (20 mL). To a
solution was added n-BuLi (1.6 mol/L in hexane, 3.6 mmol)
dropwise at -78 °C, and the mixture was stirred at -50 °C for 30
min. Then the reaction mixture was cooled again to -78 °C,
followed by the slow addition of tributyltin chloride (1.1 mL, 4.1
mmol). After stirring for 10 min, the resulting solution was
allowed to warm to room temperature and stirred for an
additional 1 h. The resulting suspension was diluted with H2O
and was extracted with EtOAc three times. The combined
organic layer was dried over Na2SO4 and concentrated in vacuo.
The crude material was subjected to silica gel (containing ca.
10wt% K2CO3) chromatography (eluent: hexane/EtOAc = 9/1)

to give 2-phenyl-4-(tributylstannyl)thiazole as pale yellow oil in
74% yield (1.00 g, 2.22 mmol). NMR data were identical with
those reported in the literature.25 1H NMR (400 MHz, CDCl3) δ
0.72-1.73 (m, 27H), 7.32 (s, 1H), 7.38-7.46 (m, 3H), 7.98-8.03
(m, 2H).
134.2, 144.5, 162.5, 168.3; HRMS (APCI) m/z calcd for
C18H11N2OSBr2 [M+H]+ 462.8933, found 462.8929.
Pd-catalyzed direct coupling of 1 with 2 (Scheme 2)
A two-neck round-bottom flask equipped with an N2
balloon and a rubber cap was charged with 19 (318 mg, 1.0
mmol), 2 (476 mg, 1.0 mmol), Pd(OAc)2 (46 mg, 20 mol%),
PPh3 (104 mg, 40 mol%), PivOH (112 mg, 1.2 mmol), and
K2CO3 (830 mg, 6.0 mmol). DMF (20 mL) was added via
syringe, and the mixture was heated at 100 °C for 16 h. The
resulting suspension was diluted with H2O and was extracted
with CHCl3 three times. The combined organic layer was dried
over Na2SO4 and concentrated in vacuo. The crude material was
subjected to short-pad silica gel chromatography (eluent:
hexane/EtOAc = 3/1) and GPC (CHCl3) to roughly separate the
products by the molecular sizes. Further purification was carried
out as described below.
[Step 4] Synthesized according to the literature
procedure.26 A two-neck round-bottom flask equipped with an
N2 balloon and a rubber cap was charged with 2-phenyl-4,5-
dihydrooxazole (1.47 g, 10 mmol), NBS (5.35 g, 30 mmol), and
AIBN (82 mg, 5.0 mol%). CCl4 (20 mL) was added via syringe,
and the mixture was refluxed overnight. The resulting mixture
was filtered through a pad of Celite eluting with EtOAc, and the
filtrate was washed with saturated Na2S2O3aq. The organic layer
was dried over Na2SO4 and concentrated in vacuo. The crude
material was subjected to silica gel chromatography (eluent:
hexane/EtOAc = 5/1) to give 5-bromo-2-phenyloxazole as
brown solid in 24% yield (540 mg, 2.4 mmol). NMR data were
identical with those reported in the literature.26 1H NMR (400
MHz, CDCl3) δ 7.10 (s, 1H), 7.44-7.50 (m, 3H), 7.97-8.03 (m,
2H).
[Step 5] Synthesized according to the literature
procedure.25 A two-neck round-bottom flask equipped with an
N2 balloon and a rubber cap was charged with 5-bromo-2-
phenyloxazole (540 mg, 2.4 mmol) and THF (10 mL). To a
solution was added freshly-prepared LDA (3.63 mmol in 5 mL
THF) dropwise at -78 °C, and the mixture was stirred for 2 h at
this temperature. The reaction was quenched with H2O (10 mL),
and the resulting suspension was extracted with EtOAc three
times. The combined organic layer was dried over Na2SO4 and
concentrated in vacuo. The crude material was subjected to silica
gel chromatography (eluent: hexane/EtOAc = 20/1) to give 4-
bromo-2-phenyloxazole as white solid in 98% yield (530 mg,
2.37 mmol). NMR data were identical with those reported in the
literature.25 1H NMR (400 MHz, CDCl3) δ 7.43-7.51 (m, 3H),
7.69 (s, 1H), 7.99-8.06 (m, 2H).
Isolation of 3
Analytically pure compound 3 was obtained by washing
the crude material with EtOAc (22.0 mg, 3.5% yield). Single
crystals suitable for X-ray analysis were obtained from CHCl3
1
solution layered with hexane. Orange solid, m.p. >300 °C, H
NMR (400 MHz, CDCl3) δ 7.28 (s, 2H), 7.31-7.36 (m, 2H), 7.38-
7.46 (m, 10H), 7.57-7.63 (m, 4H), 7.97-8.05 (m, 4H); 13C NMR
(100 MHz, CDCl3) δ 125.6, 125.9, 126.9, 128.2, 128.5, 129.0,
129.2, 129.7, 130.6, 133.3, 133.5, 138.3, 147.2, 148.3, 169.4;
HRMS (APCI) m/z calcd for C38H23N2S4 [M+H]+ 635.0739,
found 635.0744.
Isolation of 4a and 4b
The crude material was subjected to silica gel
chromatography (eluent: hexane/EtOAc = 3/1) to give 4a (36.2
mg, 7.6% yield) and 4b (13.8 mg, 2.9% yield). Single crystals of
4a suitable for X-ray analysis were obtained from DCM solution
layered with hexane.
Note: All 13C NMR data were reported as appeared in the
spectra. 1H NMR data of 4a and 4b were reported as appeared in
the spectra because they showed complicated signals due to the
[Step 6] A two-neck round-bottom flask equipped with an
N2 balloon and a rubber cap was charged with 2-phenyl-4-
(tributylstannyl)thiazole (1.00 g, 2.22 mmol), 4-bromo-2-
phenyloxazole (530 mg, 2.37 mmol), and Pd(PPh3)4 (128 mg,
5.0 mol%). Toluene (10 mL) was added via syringe, and the
mixture was heated at 100 °C for 16 h. The resulting suspension
was diluted with H2O and was extracted with EtOAc three times.
The combined organic layer was dried over Na2SO4 and
concentrated in vacuo. The crude material was subjected to silica
gel chromatography (eluent: hexane/EtOAc = 5/1) to give the
target compound as white solid in 60% yield (400 mg, 1.32
conformational isomerism. VT-NMR (VT
=
variable
temperature) measurements of 4 displayed a reversible spectral
change (see the Supporting Information).
4a: Yellow solid, M.p. >300 °C; 1H NMR (400 MHz,
CD2Cl2, -60 °C) δ 6.81-6.93 (m, 4H), 7.12-7.16 (m, 3H), 7.20-
7.52 (m, 19H), 7.62-7.67 (m, 3H), 7.98-8.00 (m, 3H); 13C NMR
(100 MHz, CDCl3) δ 123.8, 124.6, 125.5, 125.5, 125.7, 126.0,
126.3, 126.4, 126.7, 127.1, 127.2, 127.2, 127.4, 127.7, 127.8,
128.1, 128.2, 128.3, 128.6, 128.7, 128.7, 128.8, 129.0, 129.2.
130.1, 130.2, 130.3, 130.4, 130.6, 130.7, 131.2, 133.0, 133.1,
133.2, 133.2, 133.3, 133.4, 133.4, 133.5, 133.5, 133.6, 135.2,
138.3, 144.0, 144.5, 144.6, 145.0, 146.0, 146.4, 147.2, 148.7,
149.6, 166.7, 167.0, 167.2, 167.8, 168.4, 168.9; HRMS (APCI)
m/z calcd for C56H33N4S6 [M+H]+ 953.1024, found 953.1029.
4b: Yellow solid, m.p. >300 °C, 1H NMR (400 MHz,
CD2Cl2, -60 °C) δ 6.81 (s, 1H), 6.81-7.00 (m, 4H), 7.12-7.50 (m,
24H), 7.64-7.70 (m, 4H), 8.00-8.02 (m, 2H); 13C NMR (100
MHz, CDCl3) δ 124.4, 124.8, 125.6, 125.7, 125.8, 125.9, 126.0,
126.2, 126.2, 126.3, 126.4, 126.4, 127.1, 127.4, 127.5, 127.7,
127.8, 127.9, 128.0, 128.2, 128.2, 128.6, 128.7, 128.8, 128.9,
128.9, 129.0, 129.0, 129.2, 129.2, 130.0, 130.1, 130.5, 130.9,
131.1, 132.4, 132.7, 133.3, 133.5, 133.6, 133.7, 133.7, 133.7,
133.8, 133.9, 134.1, 134.3, 135.7, 137.0, 138.1, 143.6, 143.8,
144.4, 144.7, 146.0, 146.7, 148.6, 166.4, 167.1, 167.9; HRMS
(APCI) m/z calcd for C58H35N2S6 [M+H]+ 951.1119, found
951.1123.
1
mmol). M.p. 175-176 °C; H NMR (400 MHz, CDCl3) δ 7.44-
7.52 (m, 6H), 7.76 (s, 1H), 8.00-8.05 (m, 2H), 8.12-8.16 (m, 2H),
8.22 (s, 1H); 13C NMR (100 MHz, CDCl3) δ 114.7, 126.8 (2C),
126.8 (2C), 127.5, 128.9 (2C), 129.1 (2C), 130.4, 130.7, 133.6,
135.6, 138.1, 148.7, 162.1, 168.8; HRMS (APCI) m/z calcd for
C18H13N2OS [M+H]+ 305.0743, found 305.0752.
[Step 7] A two-neck round-bottom flask equipped with an
N2 balloon and a rubber cap was charged with the thiazole-
oxazole biaryl (304 mg, 1.0 mmol) and NBS (712 mg, 4.0 mmol).
CHCl3 (10 mL) was added via syringe, and the mixture was
heated at 60 °C overnight. The resulting suspension was diluted
with H2O and was extracted with CHCl3 three times. The
combined organic layer was dried over Na2SO4 and concentrated
in vacuo. The crude material was subjected to silica gel
chromatography (eluent: hexane/EtOAc = 5/1) and further
purification was carried out with GPC (CHCl3) to give 7 as white
1
solid in 32% yield (150 mg, 0.32 mmol). M.p. 188-189 °C; H
NMR (400 MHz, CDCl3) δ 7.44-7.52 (m, 6H), 7.92-7.98 (m, 2H),
8.08-8.14 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 107.7, 121.2,
126.6 (4C), 126.6, 129.0 (2C), 129.2 (2C), 130.8, 131.1, 133.0,
Isolation of 5a and 5b
The crude material was subjected to silica gel
chromatography (eluent: hexane/EtOAc = 3/1) to give 5a (4.1

mg, 1.0% yield) and 5b (34.4 mg, 5.4% yield). Single crystals of
5a suitable for X-ray analysis were obtained from DCM solution
layered with hexane.
(4a), 1894802 (5a), and 1894918 (8a). Copies of the data can be
obtained free of charge via CCDC Website.
5a: Yellow solid, m.p. >300 °C; 1H NMR (400 MHz,
CDCl3) δ 6.64-6.74 (m, 8H), 6.94-7.00 (m, 4H), 7.00-7.07 (m,
8H), 7.34-7.38 (m, 2H), 7.41-7.47 (m, 4H), 7.48-7.54 (m, 8H),
7.68-7.74 (m, 4H), 8.08-8.15 (m, 4H); 13C NMR (100 MHz,
CDCl3) δ 115.3, 122.6, 125.0, 125.6, 125.7, 126.1, 127.1, 128.0,
128.2, 128.2, 129.1, 129.2, 130.4, 132.1, 132.7, 132.7, 133.9,
134.4, 145.3, 146.5, 147.2, 148.5, 166.0, 166.4, 169.3; HRMS
(APCI) m/z calcd for C74H43N6S8 [M+H]+ 1271.1309, found
1271.1299.
Acknowledgement
This work was supported by JSPS KAKENHI Grant No.
JP 19K15586 (Grant-in-Aid for Young Scientists) to Y.N. and JP
17H06092 (Grant-in-Aid for Specially Promoted Research) to
M.M.
Supporting Information
Additional experimental data, ORTEP drawings with
selected parameters, and copy of NMR spectra are provided.
5b: Yellow solid, m.p. >300 °C, 1H NMR (400 MHz,
CDCl3) δ 6.66-6.73 (m, 8H), 6.93-6.99 (m, 4H), 7.02-7.09 (m,
8H), 7.32-7.38 (m, 4H), 7.40-7.47 (m, 8H), 7.50 (s, 4H), 7.70-
7.75 (m, 8H); 13C NMR (100 MHz, CDCl3) δ 125.0, 125.7, 126.1,
126.5, 127.9, 128.2, 128.8, 129.2, 132.5, 133.0, 134.5, 137.3,
145.1, 146.7, 165.7; HRMS (APCI) m/z calcd for C76H45N6S8
[M+H]+ 1269.1404, found 1269.1378.
This
material
is
available
on
http://dx.doi.org/10.1246/bcsj.***.
References
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Table 1. Summary of optical properties of the coupling
products
Compd
3

abs [nm] mM-1cm-1
341, 297, 34360,
flu [nm] 
468
0.033
0.024
0.033
265
33220, 33860
2.
3.
4a
355, 308, 50760,
263
354, 267
355, 284
353, 284
555
39420, 54420
40880, 42840 550
46320, 87860 508, 483 0.030
44180, 79660 508, 489 0.031
4b
5a
5b
All measurements were carried out as CHCl3 solutions (5.0
× 10-6 M);  = molar extinction coefficient for each absorption
peak,  = internal quantum efficiency measured with an
integration sphere system (JASCO ILF-835).
Pd-catalyzed direct coupling of 6 with 7 (Scheme 3)
A vial equipped with a magnetic stir bar was charged with
6 (7.6 mg, 0.025 mmol), 7 (11.6 mg, 0.025 mmol), Pd(OAc)2
(1.1 mg, 20 mol%), dppe (2.0 mg, 20 mol%), PivOH (3.1 mg,
0.03 mmol), K2CO3 (20.8 mg, 0.15 mmol), and DMF (1 mL).
The vial was refilled with dry N2 and sealed. The mixture was
heated at 100 °C for 16 h. The resulting suspension was diluted
with H2O and was extracted with CHCl3 three times. The
combined organic layer was dried over Na2SO4 and concentrated
in vacuo. The crude material was subjected to silica gel
chromatography (eluent: hexane/EtOAc = 9/1) and GPC
(CHCl3) to give the product 8 as a mixture of two isomers (5.9
1
mg, 39% yield). The ratio of 8a and 8b was determined by H
NMR analysis. Analytically pure crystals of 8a were obtained by
recrystallization from CHCl3 solution layered with hexane.
8a: Orange solid, m.p. >300 °C, 1H NMR (400 MHz,
CDCl3) δ 6.73 (s, 1H), 7.22 (s, 1H), 7.31-7.39 (m, 2H), 7.41-7.50
(m, 10H), 7.61-7.66 (m, 2H), 7.69-7.74 (m, 2H), 8.00-8.08 (m,
2H), 8.13-8.21 (m, 2H); 13C NMR (100 MHz, CDCl3) δ 108.3,
124.2, 124.4, 125.1, 125.2, 125.9, 126.9, 126.9, 127.0, 128.5,
128.7, 128.9, 129.0, 129.0, 129.3, 129.8, 130.7, 131.0, 133.2,
133.4, 135.7, 140.6, 142.9, 146.1, 148.6, 156.8, 163.3, 170.6;
HRMS (APCI) m/z calcd for C38H23N2O2S2 [M+H] 603.1195,
found 603.1195. See the Supporting Information for the
absorption and emission spectra of 8a.
4.
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