Pd-Catalyzed Consecutive C?H-Arylation-Triggered Cyclotrimerization: Synthesis of Star-Shaped Benzotristhiazoles and Benzotrisoxazoles

Article abstract of DOI:10.1002/chem.201801849

Star-shaped π-extended molecules comprising discotic aromatic cores and peripheral π-conjugated arms have attracted significant attention as diverse optoelectronic materials in terms of their large π-surface, tunable self-assembly, enhanced charge transport and fluorescence, and liquid crystallinity. Although many efforts have been made in the construction of various aromatic discotic cores, a new class of C₃-symmetric star-shaped discotic π-molecules consisting of electron-deficient benzotristhiazole and benzotrisoxazole cores, which are expected to exhibit distinct optoelectronic properties, remains unexplored owing to the unachievable synthetic approaches involved in their synthesis. Herein, we report a novel and highly efficient Pd-catalyzed cyclotrimerization of the functionalized thiazoles or oxazoles for the construction of a new class of discotic molecules of benzotristhiazole and benzotrisoxazole central cores with star-shaped π-conjugated arms. The combination of [Pd₂(dba)₃]/XPhos (dba=dibenzylideneacetone) catalyst systems with the 4-bromo-substituted thiazole enables the formation of a sufficiently stable thiazole–Pd species that participates in the subsequent C?H arylations consecutively to form the corresponding cyclic trimer products. This new class of star-shaped discotic π-extended products showed tunable energy levels and high fluorescence quantum yields that make them promising candidates for optoelectronic applications.

Full text of DOI:10.1002/chem.201801849

A Journal of
Accepted Article
Title: Pd-Catalyzed Consecutive C-H Arylation Triggered
Cyclotrimerization: Synthesis of Star-shaped Benzotristhiazoles
and Benzotrisoxazoles
Authors: Zhanqiang Xu, Kazuaki Oniwa, Hiromasa Kikuchi, Ming Bao,
Yoshinori Yamamoto, Tienan Jin, and Masahiro Terada
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201801849
Link to VoR: http://dx.doi.org/10.1002/chem.201801849
Supported by

10.1002/chem.201801849
Chemistry - A European Journal
FULL PAPER
Pd-Catalyzed Consecutive C-H Arylation Triggered
Cyclotrimerization: Synthesis of Star-shaped Benzotristhiazoles
and Benzotrisoxazoles
Zhanqiang Xu,[a] Kazuaki Oniwa,[a] Hiromasa Kikuchi,[a] Ming Bao,[b] Yoshinori Yamamoto,[a,b] Tienan
Jin,*[a,b,c] and Masahiro Terada[a]
Abstract: Star-shaped π-extended molecules comprising discotic
aromatic cores and peripheral π-conjugated arms have attracted
significant attention as diverse optoelectronic materials in terms of
their large π-surface, tunable self-assembly, enhanced charge
transport and fluorescence, and liquid crystallinity. Although many
efforts have been made in construction of various aromatic discotic
cores, a new class of C3-symmetric star-shaped discotic π-molecules
consisting
of
electron-deficient
benzotristhiazole
and
benzotrisoxazole cores remains unexplored owing to the
unachievable synthetic approaches, which are expected to exhibit
distinct optoelectronic properties. Herein, we report a novel and highly
efficient Pd-catalyzed cyclotrimerization of the functionalized
thiazoles or oxazoles for the construction of a new class of discotic
molecules of benzotristhiazole and benzotrisoxazole central cores
with star-shaped π-conjugated arms. The combination of
Pd2(dba)3/XPhos catalyst systems with the 4-bromo-substituted
thiazole enables the formation of a sufficiently stable thiazole-Pd
species that participates in the subsequent C-H arylations
consecutively to form the corresponding cyclic trimer products. This
new class of star-shaped discotic π-extended products showed
tunable energy levels and high fluorescence quantum yields that
make them promising candidates in optoelectronic application.
Introduction
Star-shaped π-extended molecules with discotic polyaromatic or
polyheteroaromatic units as central cores, such as triphenylene,
truxene, and triazatruxene offer a large π-surface to enhance the
Scheme 1. Design of new catalytic reaction for synthesis of novel π-extended
discotic molecules of benzotristhiazole and benzotrisoxazole.
intermolecular charge transfer, facile self-assemble into columnar
superstructure, strong fluorescence, and liquid crystallinity with
flexible peripheral alkyl chains.[1] For example, the triazatruxene-
based materials having electron-rich fused carbazole units have
emerged in recent years as promising light-emitting and hole
transporting materials employed in optoelectronics such as high
performance organic light-emitting diodes and Perovskite solar
cells (Scheme 1a).[1d,2] On the other hand, the introduction of
electron-accepting thiazole units into π-conjugated cooligomers
or polymers lowers the lowest unoccupied molecular orbital
(LUMO) and highest occupied molecular orbital (HOMO) energy
levels and enhances the intermolecular interaction, which has
become one of the promising strategies for designing high
performance n- or p-channel organic semiconductors.[3] In this
context, a new class of C3-symmetric star-shaped discotic π-
[a]
Dr. Z. Xu, Dr. K. Oniwa, H. Kikuchi, Prof. Y. Yamamoto, Prof. T. Jin,
Prof. M. Terada
Department of Chemistry, Graduate School of Science
Tohoku University
6-3 Azaaoba Aramaki Aoba-ku, Sendai, 980-8578, Japan
E-mail: tjin@m.tohoku.ac.jp
Prof. M. Bao, Prof. Y. Yamamoto, Prof. T. Jin
State Key Laboratory of Fine Chemicals and School of Chemistry
Dalian University of Technology
Dalian 116023, China
Prof. T. Jin
Research and Analytical Center for Giant Molecules, Graduate
School of Science
Tohoku University
[b]
[c]
6-3 Azaaoba Aramaki Aoba-ku, Sendai, 980-8578, Japan
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
This article is protected by copyright. All rights reserved.

10.1002/chem.201801849
Chemistry - A European Journal
FULL PAPER
molecules consisting of an electron-deficient BTT or BTO core
(Scheme 1a) are expected to exhibit remarkable optoelectronic
properties through the modulation of the HOMO and LUMO
energy levels and the intermolecular interaction. However, this
new class of star-shaped π-molecules has not been successfully
developed due to the unachievable synthetic approaches and still
remains a significant challenge. Although a sole synthetic method
of the BTT core was reported through the cyclotrimerization of 2-
(dialkylamino)thiazol-4(5H)-ones with phosphoryl trichloride
under refluxing conditions, this method is limited to the strong
electron-donating 2-dialkylamine-substituted substrates that
could not be utilized for synthesizing our target molecules
(Scheme 1b and vide infra).[4] Consequently, the development of
novel synthetic methods towards the star-shaped BTTs and BTOs
with a wide range of peripheral π-conjugated arms is highly
desirable.
In preliminary experiments, we examined the reported synthetic
method of BTT[4] and the Yamaguchi’s dimerization catalyst
systems (Scheme S1).[6] The cyclotrimerization of 2-(4-
propoxyphenyl)thiazol-4(5H)-one with phosphoryl trichloride,
which has been successfully applied in the synthesis of
dialkylamine-substituted BTTs,[4] was found to be unsuccessful to
produce the corresponding BTT 2a, affording 4-chloro-2-(4-
propoxyphenyl)thiazole as a sole product instead. In addition, the
Yamaguchi’s conditions (Pd(OAc)2/PPh3/PivOH/K2CO3) in the
cyclotrimerization of our designed substrate of 4-bromo-2-(4-
propoxyphenyl)thiazole (1a) was almost ineffective, producing the
expected BTT 2a in less than 2% yield. Consequently, we decided
to screen a variety of reaction conditions to implement the
designed cyclotrimerization of 1a.
Table 1. Optimization of reaction conditions for cyclotrimerization of 4-bromo-2-
arylthiazole 1a[a,b]
The transition-metal-catalyzed C-H arylation of thiazoles has
become an elegant strategy for the synthesis of arylthiazoles in
terms of step economic and environmental requirements.[5] Owing
to the inherent reactivity of the chemically nonequivalent three C-
H bonds on the parent thiazole, the direct C-H arylation turned out
to be particularly useful in the regioselective functionalization of
thiazoles by controlling the catalyst systems. On the other hand,
although the annulative C-H arylation of thiazoles has rarely been
reported, this methodology has been demonstrated to offer new
opportunities for constructing structurally intriguing novel π-
extended polycycles. For example, Yamaguchi et al. reported a
Pd-catalyzed annulative C-H arylation of bithiazoles having an
active C-H bond at the 5-position and a C-Br bond at the 4'-
position of the bithiazoles toward synthesis of cyclic thiazole
tetramers (Scheme 1c).[6] In light of the precedent C-H arylation
of thiazoles[5,6] and our recent efforts to develop new C-H bond
transformations for the synthesis of π-extended polycycles,[7] we
sought to exploit a new Pd-catalyzed annulative C-H arylation of
thiazoles and oxazoles toward construction of a new class of star-
Entry
Pd-Cat
Ligand
Solvent
2a
[%]
3a
[%]
1a
[%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18[c]
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
[(η3-C3H5)PdCl]2
Pd(OPiv)2
Pd(OAc)2
Pd(acac)2
PdCl2
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
Pd2(dba)3
XPhos
XPhos
XPhos
XPhos
XPhos
XPhos
XPhos
XPhos
XPhos
XPhos
BrettPhos
SPhos
RuPhos
PCy3
CH3CN
nPrCN
iPrCN
54
63
71
67
(84)
77
48
43
14
trace
13
23
9
0
0
0
0
0
0
0
0
0
0
0
0
0
23
73
14
8
24
68
68
0
nBuCN
tBuCN
tBuCN
tBuCN
tBuCN
tBuCN
tBuCN
tBuCN
tBuCN
tBuCN
tBuCN
tBuCN
tBuCN
tBuCN
tBuCN
0
0
12
27
15
34
0
35
11
5
shaped
π-molecules,
benzotristhiazoles
(BTTs)
and
benzotrisoxazoles (BTOs) with peripheral π-conjugated systems
(Scheme 1d). We reasoned that the key to success this
transformation is the formation of a sufficiently stable thiazole-Pd
species A from the readily available 4-bromo-2-arylthiazole
having an active C-H bond at the 5-position,[5e] which is expected
to participate in the formation of both thiazole dimer and trimer
through a consecutive intermolecular C-H arylation, thereby
suppressing the formation of the undesired cyclic tetramer from
the dimer and leading to the desired BTT by an intramolecular C-
H arylation of the trimer. We herein report a novel Pd-catalyzed
cyclotrimerization of 4-bromo-substituted 2-arylthiazoles and 2-
aryloxazoles via a consecutive C-H arylation process, which
offers a unique approach to construct the star-shaped discotic
molecules of BTTs and BTOs with various peripheral π-
conjugated arms that cannot be realized by the precedent
methods.
0
5
63
4
0
PPh3
TFP
Xantphos
XPhos
0
59
0
18
99
[a] Reaction conditions: 1a (0.3 mmol), Pd-catalyst (10 mol%), ligand (20 mol%),
CsOAc (3 equiv), tBuCN (0.15 M), 80 oC for 12 h under an argon atmosphere.
5 mol% of Pd2(dba)3 and (η3-C3H5)2Pd2Cl2 were used . 10 mol% of Xantphos
was used. [b] 1H NMR yields determined using CH2Br2 as an internal standard.
Isolated yield is shown in parenthesis. [c] 5-Bromo-2-(4-propoxyphenyl)thiazole
(1a') was used as a substrate instead of 1a.
The initial optimization of various solvents by using catalytic
amounts of Pd2(dba)3 (dba, dibenzylideneacetone, 5 mol%) and
XPhos (20 mol%) in the presence of CsOAc at 80 oC, showed that
the polar solvents of acetonitrile (54%) and DMF
(dimethylformamide, 41%) gave higher yields of the
Results and Discussion
This article is protected by copyright. All rights reserved.

10.1002/chem.201801849
Chemistry - A European Journal
FULL PAPER
corresponding product 2a compared to the less polar solvents of
toluene, THF (tetrahydrofuran), and 1,4-dioxane (<30%, Table 1,
entry 1 and Table S1). The prominent effect of acetonitrile
prompted us to further examine other nitrile solvents. Gratifyingly,
the yields of 2a were improved considerably with increasing the
steric bulk of the nitrile solvents and a significantly improved yield
of 2a (88%) was obtained in t-BuCN (Table 1, entries 2-5). It is
noted that the conditions with lower yields of 2a are mainly due to
the decomposition of 1a. Subsequently, other palladium catalysts
were screened in place of Pd2(dba)3. It was found that (η3-
C3H5)2Pd2Cl2 also exhibited a comparable catalytic activity, while
the yields of 2a were drastically diminished with Pd(OPiv)2,
Pd(OAc)2, and Pd(acac)2 catalysts due to the serious
decomposition of 1a (Table 1, entries 6-9). The typical Pd(II)
catalyst of PdCl2 was examined to be almost inactive (Table 1,
entry 10), indicating that the current reaction should be catalyzed
by a Pd(0) species. Taking into consideration the beneficial effect
of the XPhos ligand, we further studied its structural analogues.
However, the use of BrettPhos, SPhos, RuPhos, and PCy3 as
monodentate ligands instead of XPhos resulted in very low yields
of 2a with poor mass balances despite of their slight structural
changes (Table 1, entries 11-14). Although the common
monodentate ligand of triphenylphosphine (PPh3) exhibited a very
poor activity, tri(2-furyl)phosphine (TFP) was an effective ligand
for achieving a good yield of 2a (Table 1, entries 15 and 16). The
bidentate phosphine ligand of Xantphos was examined to be
inactive for forming 2a but favorable for producing the dimer 3a
(Table 1, entry 17). Among the bases tested, the stronger bases
of CsOAc and RbOAc afforded higher yields of 2a than NaOAc
and KOAc (Table S1). The effect of the reaction temperature
revealed that in comparison with the high yield of 2a at 80 oC (88%,
o
Table 1, entry 5), the reaction at 110 C lowered the yield (58%)
due to the partial decomposition of 1a, while the reaction became
much slow at 60 oC, resulting in low conversion of 1a (55%, Table
S1). In sharp contrast, the use of 5-bromo-2-(4-
propoxyphenyl)thiazole (1a') having a less active C-H bond at the
4-position resulted in no reaction under optimal conditions,
demonstrating the importance of the C-H bond activity for the
current transformation. (Table 1, entry 18). The blank experiments
in the absence of Pd-catalyst or ligand or base resulted in no
reaction. It is concluded that the combination of Pd2(dba)3 and
XPhos is a superior catalyst system in the presence of CsOAc in
t-BuCN at 80 °C for rendering the current cyclotrimerization
Table 2. Synthesis of various benzotristhiazoles[a,b]
[a] Reaction conditions: 1 (0.4 mmol), Pd2(dba)3 (5 mol%), XPhos (20 mol%), CsOAc (3 equiv), t-BuCN (0.15 M), 80 oC for 12 h under an argon atmosphere. [b]
Isolated yields are shown.
This article is protected by copyright. All rights reserved.

10.1002/chem.201801849
Chemistry - A European Journal
FULL PAPER
effectively (Table 1, entry 5). The structure of 2a was determined
from the 2-arylthiazoles with electron-rich aryl groups, the
reaction with 2-arylthiazole having an electron-withdrawing ester
group on the phenyl ring was sluggish, giving a mixture of the
corresponding BTT and cyclic tetramer in low yields with
decomposition of the remaining starting substrate (Scheme S2).
Several control experiments were performed to gain insights
into the reaction mechanism. When the reaction of 1a under the
standard conditions was stopped after 1 hour, we obtained a
mixture of the recovered 1a (57%), the thiazole dimer 3a (23%),
the thiazole trimer 4a, and the corresponding product 2a (16%),
1
unambiguously by the high resolution mass (HRMS), H and 13
C
1
NMR analysis (see ESI). Particularly, the H NMR showed only
two aromatic C-H peaks of the phenyl ring and the 13C NMR
showed seven aromatic carbon peaks of the phenyl and thiazole
moieties, demonstrating the C3-symmetric structure of 2a.
The substrate scope of various 4-bromothiazoles was studied
under the optimized conditions as shown in Table 2. The sterically
bulky groups of t-butyl and trimethylsilyl at the para-position of the
phenyl ring have little steric effect on the reaction outcome,
affording the corresponding BTTs 2b and 2c in 73% and 52%
yields, respectively. However, the arylthiazole having a sterically
bulky mesityl group at the 2-position of the thiazole ring lowered
the yield of 2d. Importantly, the 2-arylthiazoles having a strong
electron-donating N,N-dimethyl- or N,N-diarylaniline group
afforded novel donor-acceptor type molecules 2e and 2f in good
to high yields, which are expected to be potential candidates as
new carrier transporting and light emitting functional materials in
optoelectronics. Regardless of the steric hindrance, the 9-
anthracene-substituted arylthiazole was able to undergo the
present cyclotrimerization, giving the desired π-extended BTT 2g
in moderate yield. Regarding to the prospective application of the
π-cooligomers as new organic semiconductors, the present
method was successfully applied in the synthesis of 2-bipheny-,
4-biphenyl-, and 2,2'-bithienyl-substituted BTTs 2h-j, in which the
pendant long alkyl chains make them soluble in various organic
solvents. Remarkably, a novel styryl-substituted π-extended BTT
2k could be synthesized in a high yield of 92% upon the
cyclotrimerization of (E)-4-bromo-2-(4-(hexyloxy)styryl)thiazole
under the standard conditions. One of the limitations is that in
comparison with the perfect selectivity for the formation of BTTs
1
which were confirmed by the crude H NMR spectra combining
with the HRMS analysis (Scheme 2a). The crude mixture was
completely converted to the desired product 2a after 11 hours
upon being conducted with the same conditions. Notably, the
formation of a thiazole-Pd-XPhos species A was supported by the
HRMS analysis from the crude mixture. In addition, the thiazole
trimer 4a prepared by an alternative method readily underwent a
Pd-catalyzed intramolecular C-H arylation to afford 2a in 98%
yield under the standard conditions in diluted t-BuCN because of
the low solubility of 4a (Scheme 2b). These experimental
observations suggest that the current cyclotrimerization takes
place through the sequential formation of 3a to 4a to 2a, and the
stable Pd-complex A which forms from the reaction of Pd(0) with
1a involves in the reaction. Hence the complex A should be a key
species for successful implementation of the current
transformation. Moreover, we tested the reactions of the thiazole
dimer 3a by using two different conditions (Scheme 2c). The
reaction of 3a under our standard conditions produced the cyclic
thiazole tetramer 7a in less than 10% yield with the predominant
recovery of 3a (78%), while the dimerization reaction of 3a
Scheme 2. Control experiments for reaction mechanism.
This article is protected by copyright. All rights reserved.

10.1002/chem.201801849
Chemistry - A European Journal
FULL PAPER
proceeded sufficiently under Yamaguchi’s conditions,6 affording
7a in 84% yield; conversely, the Yamaguchi’s conditions was
almost inactive for the reaction of the thiazole monomer 1a. These
results indicate that our Pd(0)/XPhos catalyst system is much less
favorable than the Yamaguchi’s Pd-catalyst system for the
oxidative addition of Pd(0) to the C-Br bond of 3a, but favorable
for the oxidative addition to C-Br bond of the monomer 1a. In other
words, the low activity of our Pd-catalyst for the C-Br bond of 3a
implies that the thiazole-Pd species A might prefer to undergo a
C-H arylation with the C-H bond of 3a to form 4a, thus hampering
the dimerization of 3a to 7a.
On the basis of the experimental results, we proposed the
reaction mechanism as outlined in Scheme 3. The initial oxidative
addition of the Pd(0)-XPhos complex to the C-Br bond of
arylthiazole 1a generates the thiazole-Pd-XPhos species A,
which could be stabilized by the XPhos ligand and t-BuCN. Next,
the intermolecular C-H arylation of the Pd-thiazole A with 1a in
the presence of CsOAc, followed by the reductive elimination of
the bithiazole-Pd intermediate B would afford the thiazole dimer
3a and the Pd(0) species (cycle A). Once the dimer 3a forms, it
may smoothly undergo the second intermolecular C-H arylation
with the stable Pd-thiazole species A assisted by CsOAc to give
the corresponding thiazole trimer 4a through the reductive
elimination of the intermediate C (cycle B). While the detailed C-
H arylation mechanism remains to be studied, the D/H exchanges
were observed in the reaction of the deuterated arythiazole 1a-d
with a C-D bond at the 5-position of thiazole under the standard
conditions after 2 hours, giving rise to the formation of the
nondeuterated substrates 1a and dimer 3a along with the
corresponding deuterated substrates 1a-d and dimer 3a-d, and
the desired product 2a (Scheme 2d). This result suggests that the
intermolecular C-H arylation pathways in cycles A and B should
be reversible. In addition, the reaction of 1a with the same Pd-
catalyst system in the absence of CsOAc or with CsOAc in the
absence of the Pd-catalyst resulted in no reaction and no D/H
exchange was detected (Scheme S4), suggesting the
indispensable role of bases in the C-H arylation step and the
deprotonation pathway of 1a with CsOAc could be excluded.
Finally, the intramolecular C-H arylation of the trimer 4a with the
Pd(0) species followed by reductive elimination affords the
corresponding product 2a.
Taking into consideration the important role of the oxazole unit
in π-molecules for intense fluorescence property and electronic
interactions between π-systems,[8] we next turned our attention to
the construction of a new framework of BTO by extension of the
current cyclotrimerization reaction. It was found that under the
standard conditions, 4-bromo-2-(4-propoxyphenyl)oxazole (5a)
was able to undergo the Pd-catalyzed cyclotrimerization, giving
rise to the formation of the corresponding BTO 6a albeit in a low
yield of 33%. A brief screening of the reaction conditions revealed
that the change of other phosphine ligands or bases did not
improve the yield of 6a (Table S2). However, when the solvent of
t-BuCN was replaced with other polar solvents such as CH3CN
(52%), DMF (71%, Table 3), and DMAc (66%), the yields of 6a
increased obviously. The optimal conditions was further
employed for the synthesis of π-extended BTOs (Table 3). The 4-
bromooxazole
having
an
electron-donating
N,N-bis(4-
(hexyloxy)phenyl)aniline group at the 2-position of oxazole
afforded the corresponding BTO 6b in 46% yield. Similarly, the π-
extended star-shaped BTOs 6c and 6d having p-biphenyl and
styryl groups could be synthesized from the corresponding 4-
bromooxazoles in 52% and 42% yields, respectively.
Scheme 3. Proposed Reaction Mechanism.
This article is protected by copyright. All rights reserved.

10.1002/chem.201801849
Chemistry - A European Journal
FULL PAPER
attributed to the nature of the pendant N,N-diarylamine
chromophore. The optical energy gaps (ΔEopt) estimated from the
intersection of the UV/Vis absorption and fluorescence spectra
show that the π-extended compounds 2f-k and 6b-c lead to
smaller ΔEopts compared to that of 2a and 6a. The HOMO energy
level was calculated from the oxidation potentials by cyclic
voltammetry and the LUMO energy level was estimated from the
HOMO and ΔEopt (Tables 4 and S3, and Figure S2). BTTs and
BTOs have relatively low-lying HOMOs around -5.56 eV to -5.97
eV and LUMOs around -2.40 to -2.93 eV compared to the
triazatruxene-based compounds,[2a] owing to the electron-
deficient nature of the thiazole and oxazole moieties.
Exceptionally, the BTT 2f and the BTO 6b bearing strong
electron-donating N,N-diarylamine groups possess much high-
lying HOMO energy levels of -5.26 eV and -5.12 eV, which are
comparable with that of the hole transporting materials of spiro-
OMeTAD (-5.22 eV)[9] and triazatruxene KR131 (-5.30 eV)[2a] in
high performance perovskite solar cells, suggesting the suitable
potentials of our new star-shaped π-systems for the efficient
charge-carrier transport.
Table 3. Synthesis of benzotrisoxazoles[a,b]
[a] Reaction conditions: 5 (0.4 mmol), Pd2(dba)3 (5 mol%), XPhos (20 mol%),
CsOAc (3 equiv), DMF (0.5 M), 80 oC under an argon atmosphere. [b] Isolated
yields are shown.
The normalized ultraviolet/visible (UV/Vis) absorption and
fluorescence spectra of the π-extended BTTs and BTOs in diluted
chloroform are shown in Figure 1 and the values are summarized
in Table 4 (also Figure S1 and Table S3). The UV/Vis absorption
maxima (λmax) and absorption onsets (λonset) of BTTs are red-
shifted as compared to the corresponding BTOs. The strong λmax
between 300 and 420 nm are assigned to the π-π* transitions of
the conjugated systems, which shift bathochromically with
increasing the π-conjugation length (2a and 6a vs 2f-k and 6b-d).
The N,N-diarylamine-substituted BTT (2f) and bithienyl-
substituted BTT (2j) show the most extended λmax at 408 nm and
415 nm, respectively. The fluorescence spectra show the similar
trend of emission maxima with the UV/Vis absorption at the region
of 385−493 nm and the relatively large Stock shifts around
70−131 nm imply the weak structural rigidity of BTTs and BTOs.
In general, the 2-aryl-substituted BTOs 6a-c in diluted chloroform
exhibit higher fluorescence quantum yields of 0.77 to 0.68 as
compared to the corresponding BTTs 2a-j presumably due to the
reduced heavy atom effect from sulfur to oxygen atom, while the
2-styryl-substituted BTO 6d is measured to be weakly fluorescent
(0.07). The high quantum yield (0.74) of the BTT 2f should be
Figure 1. UV/Vis absorption and fluorescence spectra of selected
benzotristhiazoles and benzotrisoxazoles. The UV/Vis absorption (solid line)
and fluorescence spectra (dash line) of 2f-k (a), and 6b-c (b) in diluted
chloroform.
This article is protected by copyright. All rights reserved.

10.1002/chem.201801849
Chemistry - A European Journal
FULL PAPER
Table 4 Optical and electrochemical properties of benzotristhiazoles and benzotrisoxazoles
Compound
λabsmax (nm)[a]
λonset (nm)[a]
λemmax (λexc) (nm)[b]
Фf[b]
ΔEopt (eV)[c]
HOMO (eV)[d]
LUMO (eV)[e]
2a
2f
2i
281, 340
295, 408
280, 352
415
278, 382
309
283, 383
338
292, 360
380
451
393
455
424
347
442
372
406
410 (400)
490 (400)
422 (410)
493 (410)
462 (430)
385 (370)
514 (360)
404 (340)
475 (420)
0.1
3.26
2.75
3.15
2.72
2.92
3.57
2.80
3.33
3.05
-5.78
-5.26
-5.81
-5.65
-5.56
-5.97
-5.12
-5.75
-5.74
-2.52
-2.51
-2.66
-2.93
-2.64
-2.40
-2.32
-2.42
-2.69
0.74
0.16
0.42
0.08
0.70
0.68
0.77
0.07
2j
2k
6a
6b
6c
6d
[a] UV/Vis absorption and fluorescence spectra were measured in chloroform. [b] Absolute fluorescence quantum yield (Фf) was measured by a photon-counting
method using an integration sphere. [c] Optical energy gap (ΔEopt) was estimated from the intersection of normalized UV/Vis absorbance and fluorescence spectra
according to the equation, ΔEopt (eV)=1240/λonset. [d] HOMO was calculated from the oxidation potential. First oxidation potentials measured by cyclic voltammetry
(CV) using Ag/AgCl as a reference electrode, Pt wire as a counter electrode, glassy carbon as a working electrode, and Bu4NPF6 (0.1 M) as a supporting electrolyte
in dichloromethane. The scan rate is 50 mV s-1 and the Fc/Fc+ (-4.80 eV) was used as external standard. [e] The LUMO energy was calculated from the HOMO
energy and the optical energy gap.
A. Schreivogel, M. Tosoni, Angew. Chem. Int. Ed. 2007, 46, 4832-4887;
Angew. Chem. 2007, 119, 4916-4973; d) X.-C. Li, C.-Y. Wang, W.-Y. Lai,
W. Huang, J. Mater. Chem. C 2016, 4, 10574-10587; (e) A. Rapakousiou,
Conclusions
R. Sakamoto, R. Shiotsuki, R. Matsuoka, U. Nakajima, T. Pal, R.
Shimada, A. Hossain, H. Masunaga, S. Horike, Y. Kitagawa, S. Sasaki,
In conclusion, we have designed and developed a novel
K. Kato, T. Ozawa, D. Astruc, H. Nishihara, Chem. Eur. J. 2017, 23,
consecutive C-H arylation for cyclotrimerization of 4-bromo-
substituted 2-arylthiazoles and 2-aryloxazoles by establishing a
8443-8449.
[2]
a) K. Rakstys, A. Abate, M. I. Dar, P. Gao, V. Jankauskas, G. Jacopin,
E. Kamarauskas, S. Kazim, S. Ahmad, M. Grätzel, M. K. Nazeeruddin, J.
Am. Chem. Soc. 2015, 137, 16172-16178; b) Y.-F. Xie, S.-Y. Ding, J.-M.
Liu, W. Wang, Q-Y. Zheng, J. Mater. Chem. C 2015, 3, 10066-10069; c)
Z. Lu, C. Li, T. Fang, G. Li, Z. Bo, J. Mater. Chem. A 2013, 1, 7657-7665;
d) A. Benito-Hernández, U. K. Pandey, E. Cavero, R. Termine, E. M.
García-Frutos, J. L. Serrano, A. Golemme, B. Gómez-Lor, Chem. Mater.
2013, 25, 117-121; e) S. W. Shelton, T. L. Chen, D. E. Barclay, B. Ma,
ACS Appl. Mater. Interfaces 2012, 4, 2534-2540; f) E. M. García-Frutos,
U. K. Pandey, R. Termine, A. Omenat, J. Barberä, J. L. Serrano, A.
Golemme, B. Gómez-Lor, Angew. Chem. Int. Ed. 2011, 50, 7399-7402;
Angew. Chem. 2011, 123, 7537-7540; g) M. Talarico, R. Termine, E. M.
García-Frutos, A. Omenat, J. L. Serrano, B. Gómez-Lor, A. Golemme,
Chem. Mater. 2008, 20, 6589-6591; h) W.-Y. Lai, Q.-Y. He, R. Zhu, Q.-
Q. Chen, W. Huang, Adv. Funct. Mater. 2008, 18, 265-276; i) W.-Y. Lai,
R. Zhu, Q.-L. Fan, L.-T. Hou, Y. Cao, W. Huang, Macromolecules 2006,
39, 3707-3709; j) B. Gómez-Lor, B. Alonso, A. Omenat, J. Serrano,
Chem. Commun. 2006, 5012-5014.
new palladium catalyst system and molecular engineering. The
combination of Pd2(dba)3/XPhos catalysts with CsOAc in t-BuCN
solvent provides a unique synthetic approach to the selective
construction of novel star-shaped π-extended BTTs and BTOs
that cannot be attainable by the precedent synthetic approaches
and catalyst systems. The formation of a stable thiazole-Pd-
XPhos species is crucial for promoting the present consecutive C-
H arylation sufficiently. The highly tunable HOMO and LUMO
energy levels triggered by the electron-deficient nature of the
thiazole and oxazole moieties, high fluorescence quantum yields,
and unique star-shaped discotic structures make them promising
candidates for application as optoelectronic materials. Further
application of the present π-materials in organic electronics and
extension of the present synthetic strategy to a new class of star-
shaped π-extended molecules with various discotic
heteropolyaromatic cores are in progress.
[3]
a) Y. Lin, H. Fan, Y. Li, X. Zhan, Adv. Mater. 2012, 24, 3087-3106; b) W.
Li, H. E. Katz, A. J. Lovinger, J. G. Laquindanum, Chem. Mater. 1999,
11, 458-465; c) S. Ando, R. Murakami, J.-i. Nishida, H. Tada, Y. Inoue,
Tokito, Y. Yamashita, J. Am. Chem. Soc. 2005, 127, 14996-14997; d) K.
Oniwa, H. Kikuchi, T. Kanagasekaran, H. Shimotani, S. Ikeda, N. Asao,
Y. Yamamoto, K. Tanigaki, T. Jin, Chem. Commun. 2016, 52, 4926-4929.
A. Rössler, P. Boldt, Synthesis 1998, 7, 980-982.
Acknowledgements
This work was supported by Japan Society for Promotion of
Science (JSPS) KAKENHI Grant Number 15KK0180 on Fostering
Joint International Research.
[4]
[5]
a) Y. Segawa, T. Maekawa, K. Itami, Angew. Chem. Int. Ed. 2014, 54,
66-81; Angew. Chem. 2015, 127, 68-83; b) I. V. Seregin, V. Gevorgyan,
Chem. Soc. Rev. 2007, 36, 1173-1193; c) B. Liégault, D. Lapointe, L.
Caron, A. Vlassova, K. Fagnou, J. Org. Chem. 2009, 74, 1826-1834; d)
S. Kirchberg, S. Tani, K. Ueda, J. Yamaguchi, A. Studer, K. Itami, Angew.
Chem. Int. Ed. 2011, 50, 2387-2391; Angew. Chem. 2011, 123, 2435-
2439; e) S. Tani, T. N. Uehara, J. Yamaguchi, K. Itami, Chem. Sci. 2014,
5, 123-135.
Keywords: consecutive C-H arylation • benzotristhiazole •
benzotrisoxazole • cyclotrimerization • star-shaped π-molecule
[1]
a) T. Wöhrle, I. Wurzbach, J. Kirres, A. Kostidou, N. Kapernaum, J.
Litterscheidt, J. C. Haenle, P. Staffeld, A. Baro, F. Giesselmann, S.
Laschat, Chem. Rev. 2016, 116, 1139-1241; b) F. Goubard, F. Dumur,
RSC Adv. 2015, 5, 3521-3551; c) S. Laschat, A. Baro, N. Steinke, F.
Giesselmann, C. Hägele, G. Scalia, R. Judele, E. Kapatsina, S. Sauer,
[6]
[7]
K. Mouri, S. Saito, S. Yamaguchi, Angew. Chem. Int. Ed. 2012, 51, 5971-
5975; Angew. Chem. 2012, 124, 6073-6077.
a) T. Jin, J. Zhao, N. Asao, Y. Yamamoto, Chem. Eur. J. 2014, 20, 3554-
3576; b) J. Zhao, K. Oniwa, N. Asao, Y. Yamamoto, T. Jin, J. Am. Chem.
This article is protected by copyright. All rights reserved.

10.1002/chem.201801849
Chemistry - A European Journal
FULL PAPER
Soc. 2013, 35, 10222-10225; c) J. Zhao, N. Asao, Y. Yamamoto, T. Jin,
J. Am. Chem. Soc. 2014, 136, 9540-9543; d) X. Zhang, W. Si, M. Bao,
N. Asao, Y. Yamamoto, T. Jin, Org. Lett. 2014, 16, 4830-4833; e) J. Zhao,
Z. Xu, K. Oniwa, N. Asao, Y. Yamamoto, T. Jin, Angew. Chem. Int. Ed.
2016, 55, 259-263; Angew. Chem. 2016, 128, 267-271; f) X. Zhang, Z.
Xu, W. Si, K. Oniwa, M. Bao, Y. Yamamoto, T. Jin, Nat. Commun. 2017,
8, 15073.
[8]
[9]
a) T. G. Pavlopoulos, P. R. Hammond, J. Am. Chem. Soc. 1974, 96,
6568-6579; b) S. A. Jenekhe, J. A. Osaheni, Science 1994, 265, 765-
768.
S. Kazim, M. K. Nazeeruddin, M. Grätzel, S. Ahmad, S. Angew. Chem.
Int. Ed. 2014, 53, 2812-2824; Angew. Chem. 2014, 126, 2854-2867.
This article is protected by copyright. All rights reserved.

10.1002/chem.201801849
Chemistry - A European Journal
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
A novel and highly efficient Pd-
catalyzed cyclotrimerization of the
functionalized thiazoles or oxazoles
has been developed for the
Zhanqiang Xu, Kazuaki Oniwa,
Hiromasa Kikuchi, Ming Bao, Yoshinori
Yamamoto, Tienan Jin,* and Masahiro
Terada
construction of a new class of
symmetric benzotristhiazole and
benzotrisoxazole central cores with
star-shaped π-conjugated arms. The
resulting π-extended
Page No. – Page No.
Pd-Catalyzed Consecutive C-H
Arylation Triggered
Cyclotrimerization: Synthesis of Star-
shaped Benzotristhiazoles and
Benzotrisoxazoles
benzotristhiazoles and
benzotrisoxazoles showed tunable
molecular orbital energies and high
fluorescence quantum yields
This article is protected by copyright. All rights reserved.