Synthesis of 2,9-dialkylated phenanthro[1,2-b:8,7-b′]dithiophenes via cross-coupling reactions and sequential Lewis acid-catalyzed regioselective cycloaromatization of epoxide

Article abstract of DOI:10.1016/j.tetlet.2014.05.035

Phenanthro[1,2-b:8,7-b′]dithiophene (PDT) was prepared via Suzuki-Miyaura or Negishi cross-coupling of a 2-thienylboron or -zinc compound with 1,4-dibromobenzene, followed by Lewis acid-catalyzed regioselective cycloaromatization of the epoxide. A series of 2,9-dialkylated phenanthro[1,2-b:8,7-b′]dithiophene (PDT) derivatives could also be synthesized in good yields by Suzuki-Miyaura coupling of the brominated PDT with alkylboranes by introducing linear alkyl substituents.

Full text of DOI:10.1016/j.tetlet.2014.05.035

Tetrahedron Letters xxx (2014) xxx–xxx
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of 2,9-dialkylated phenanthro[1,2-b:8,7-b0]dithiophenes
via cross-coupling reactions and sequential Lewis acid-catalyzed
regioselective cycloaromatization of epoxide
Keita Hyodo a, Hikaru Nonobe a, Shuhei Nishinaga a, Yasushi Nishihara a,b,
⇑
a Division of Earth, Life, and Molecular Sciences, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka, Kita-ku, Okayama 700-8530, Japan
b Japan Science and Technology Agency, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
a r t i c l e i n f o
a b s t r a c t
Phenanthro[1,2-b:8,7-b0]dithiophene (PDT) was prepared via Suzuki–Miyaura or Negishi cross-coupling
of a 2-thienylboron or -zinc compound with 1,4-dibromobenzene, followed by Lewis acid-catalyzed reg-
ioselective cycloaromatization of the epoxide. A series of 2,9-dialkylated phenanthro[1,2-b:8,7-b0]dithi-
ophene (PDT) derivatives could also be synthesized in good yields by Suzuki–Miyaura coupling of the
brominated PDT with alkylboranes by introducing linear alkyl substituents.
Article history:
Received 26 March 2014
Revised 26 April 2014
Accepted 9 May 2014
Available online xxxx
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Phenacenes
Suzuki–Miyaura coupling
Negishi coupling
Boron
Zinc
Organic field-effect transistors (OFETs) have attracted consider-
able interest as key components in future ubiquitous electronics
due to advantages such as flexibility, light weight, and ease of
design.1 One particular acene-type molecule, pentacene,2 has
served as the active semiconducting layer in OFETs owing to the
of (Z)-alkenylboronates with polyhalobenzene and sequential
intramolecular double cyclization via CAH activation. This protocol
is also applicable to the synthesis of phenanthro[1,2-b:8,7-b0]dithi-
ophene (PDT) by replacing the terminal phenyl rings in picene with
thiophene rings, aiming at increased intermolecular
p–p interac-
high field-effect mobility (
l
) of 5.5 cm2 Vꢀ1 sꢀ1 that it has shown
tions due to the large atomic radius of sulfur, which may enhance
its performance in OFETs.15 Results showed that OFET devices
fabricated with thin films of PDT formed by thermal deposition
exhibited carrier mobility as large as 1.1 ꢁ 10–1 cm2 Vꢀ1 sꢀ1, and
suggested that fabrication using a solution process might be possi-
ble owing to PDT’s high solubility in common organic solvents.
However, we found that this synthetic strategy is not suitable
for the large-scale synthesis of PDT because in some cases a mix-
ture of stereoisomers of the coupled products is formed through
(E)/(Z) isomerization upon Suzuki–Miyaura coupling, leading to a
lower yield of the desired products. To produce derivatives of
PDT for use as organic semiconductors, a more efficient synthetic
method is highly desirable. Here we report a new synthetic route
to the PDT core structure using cross-coupling reactions. Further-
more, the solubility of PDT might be improved by introducing long
alkyl chains, as this may induce a self-assembly process by the ‘fas-
tener effect’, leading to high crystallinity in thin films. Some exam-
ples of this have already been reported in alkyl-substituted
picene16 and alkylated thienoacene.17,18
in a thin-film transistor,3 and its state-of-the-art value of 40 cm2 -
Vꢀ1 sꢀ1 in single crystals.4 However, pentacene is unstable under
atmospheric conditions, and readily photodegrades owing to its
relatively high HOMO energy (ꢀ5.0 eV), which arises from its
extended
p
-conjugation.5 Recently, a phenacene-type molecule,
picene, incorporating the same number of benzene rings as penta-
cene, has become the focus of considerable interest because it
becomes superconductive6 with alkali-metal doping, and also
shows high field-effect mobility in a transistor (1.1 cm2 Vꢀ1 sꢀ1 in
a thin-film OFET).7 Moreover, a picene-based FET is stable in air
because picene has a larger energy bandgap (Eg = 3.3 eV) and a
lower HOMO energy (ꢀ5.5 eV) than pentacene.8 The potential util-
ity of phenacene-type molecules makes the development of more
efficient synthetic methods9 and further improvement in their
OFET properties10–12 matters of some importance.
We have recently reported the synthesis of picenes13 and ful-
minene14 by the palladium-catalyzed Suzuki–Miyaura coupling
To explore a new synthetic route to PDT, we investigated the
palladium-catalyzed Suzuki–Miyaura coupling of commercially
⇑
Corresponding author. Tel./fax: +81 86 251 7855.
E-mail address: ynishiha@okayama-u.ac.jp (Y. Nishihara).
http://dx.doi.org/10.1016/j.tetlet.2014.05.035
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Hyodo, K.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.05.035

2
K. Hyodo et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Pd(dba)2 (5 mol %)
[HPtBu3]BF4 (10 mol %)
CHO
OHC
KF (4.6 equiv)
H2O (6 equiv)
CHO
B(OH)2
+
Br
Br
THF, reflux, 8 h
78%
S
S
S
2
3
1
(2.2 equiv)
Scheme 1.
O
O
available 3-formyl-2-thiopheneboronic acid (1) with 1,4-dibromo-
benzene (2) affording the corresponding coupled product 3 in 78%
yield (Scheme 1).19
We screened various reaction conditions to develop an efficient
synthetic route to 3. The results are summarized in Table 1. To our
surprise, we found that chemoselective CAH zincation across 3-
formylthiophene (4) occurred at the 2-position (adjacent to the
formyl group) of the thiophene ring, using TMPZnClꢂLiCl.20 Negishi
coupling of the in situ generated 2-thienylzinc reagent with 1,4-
Me3SI (2.4 equiv)
KOH (5.5 equiv)
CHO
OHC
MeCN, 60 ºC, 3 h
quant
S
S
S
S
5
3
Scheme 2.
dibromobenzene (2) using
a catalyst system of Pd(dba)2
Following this, sequential epoxidation of 3 gave the desired
product 5 quantitatively (Scheme 2).26,27 We next screened the
reaction conditions of acid-mediated28 and -catalyzed29 Friedel–
Crafts-type cycloaromatization of 5 and the results are summa-
rized in Table 2. Attempted reactions with MeSO3H and BF3ꢂOEt2
did not proceed, even with the addition of excess reagent. With a
stoichiometric amount of Sc(OTf)3, PDT (6) was obtained in 32%
yield (entry 1). However, the catalytic variant of Sc(OTf)3 was not
effective (entry 2). We then explored catalytic reactions with other
Lewis acids M(OTf)n, but yields of 6 were insufficient (entries 3–6).
To our delight, 10 mol % of InCl3 was found to give better results
and afforded 6 in 46% yield (entry 7). Increasing the amount of
InCl3 to 20 mol % improved the yield to 50% (entry 8). Varying
the concentrations, we found that lower concentrations gave
(dba = dibenzylideneacetone) with a phosphonium salt, [HPtBu3]-
BF4 used as a precursor of the phosphine ligand proceeded at reflux
to furnish 3 in 65% yield (entry 1).21 However, other palladium pre-
cursors and phosphine-based ligands including biarylphosphine
(Sphos)22 and PdCl2(dppf)ꢂC6H6 (dppf = 1,10-bis(diphenylphos-
23
phino) ferrocene) were found to be inferior (entries 2–4).
PEPPSI-IPr ((PEPPSI = pyridine-enhanced precatalyst preparation
stabilization and initiation, IPr = 1,3-diisopropylimidazol-2-
ylidene),24 recently introduced by Organ, displayed a modest
catalytic activity to afford 3 in 52% yield as determined by
NMR (entry 5). With palladacycle precatalysts25 utilized in the
sp2–sp2 Negishi couplings, compound 3 was obtained in lower
yields (entries 6–10).
Table 1
Optimization of Negishi cross-coupling of 2 with 4
Pd cat. (5 mol %)
Ligand (5 mol %)
Br
Br
CHO
CHO
OHC
TMPZnCl·LiCl
(2.2 equiv)
CHO
ZnCl·LiCl
2
reflux, 3 h
S
THF, rt, 1 h
S
S
S
4
3
(2.2 equiv)
P1: L = Xphos
P2: L = Sphos
P3: L = PPh3
PCy2
OMe
PCy2
iPr
NH2
Pd OMs
MeO
iPr
L
P4
: L = P(o-tol)3
iPr
XPhos
P5: L = dppf
SPhos
P1-P5
Entry
Pd cat.
Ligand
Yielda (%)
1b
2b
3b
4
5
6
7
8
9
10
Pd(dba)2
Pd(OAc)2
Pd(dba)2
[HPtBu3]BF4
SPhos
P(o-tol)3
—
—
—
—
—
—
—
65
46
9
55
52
41
36
15
3
PdCl2(dppf)ꢂC6H6
PEPPSIꢂIPr
P1
P2
P3
P4
P5
4
a
NMR yields based on 2.
10 mol % of phosphine ligand was used.
b
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K. Hyodo et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Table 2
Since the 2-position of the thiophene ring is reported to be
readily functionalized, borylation,33 bromination,34 and stannyla-
tion,35 as well as various linear alkyl groups, were introduced onto
the PDT core (Scheme 4). First, bromination of PDT 6 with a slight
excess of butyllithium followed by addition of bromine afforded 8
in 89% yield. Starting from the synthesized 8, 2,9-dialkylated PDTs
9a–9f were obtained in 69–80% yields by Suzuki–Miyaura coupling
reactions36 with an excess of alkylboranes, derived from the
hydroborylation of various terminal alkenes with 9-BBN dimer.37,38
In summary, we have developed a more efficient and readily
scalable synthetic method for the preparation of the parent PDT
molecule by sequential Suzuki–Miyaura or Negishi coupling, epox-
idation, and InCl3-catalyzed cyclization. We have successfully used
this route to prepare a new class of 2,9-dialkylated PDT derivatives.
These diverse synthetic strategies to introduce various alkyl groups
onto the common core at the final stage should be applicable to the
production of other functionalized PDT molecules. Further study to
elucidate the effects of the introduced alkyl chains on the FET char-
acteristics of 9c–9f is currently underway in our laboratories.
Screening Lewis acids for electrophilic double cycloaromatization of 5a
O
O
Lewis acid (X mol %)
DCE (Y M), reflux, 24 h
S
S
S
S
5
6 (
)
PDT
Entry
Lewis acid
(X mol %)
(Y M)
Yieldb (%)
1
2
3
4
5
6
7
8
Sc(OTf)3
Sc(OTf)3
Bi(OTf)3
Hf(OTf)4
Cu(OTf)2
In(OTf)3
InCl3
InCl3
InCl3
InCl3
240
10
10
10
10
10
10
20
20
20
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.025
0.005
32
<1
22
0
<1
26
46
50
9c
10c
58 (58)
64
a
b
c
DCE = 1,2-dichloroethane.
NMR yields. An isolated yield is shown in parenthesis.
Reaction time was 12 h.
Acknowledgments
higher yields of up to 64% (entries 9 vs 10), presumably due to the
suppression of an undesired intermolecular side reaction. Consid-
ering the longer reaction time and the greater solvent consump-
tion, we adopted the reaction conditions in entry 9 as optimum.
With conditions optimized, we could achieve the gram-scale syn-
thesis of PDT (6).30
In regard to the regioselectivity of cyclization reaction, we have
only observed the formation of traces of the corresponding
anthra[1,2-b:5,6-b0]dithiophene (ADT) (60),31 which is a structural
isomer of 6. We speculate that the selective formation of 6 can
be attributed to an electronic effect. As shown in Scheme 3, a prim-
itive DFT calculation disclosed a higher electron density localized
on the indicated (red dot) carbon atom on the second side of inter-
mediate 7 to undergo electrophilic cyclization, which favors one of
the two possible Friedel–Crafts-type cycloaromatizations on that
side.32 Detailed calculations to gain deeper insight into this
regioselectivity are in progress.
This study was partly supported by the Program for Promoting
the Enhancement of Research Universities from MEXT, Japan and a
Special Project of Okayama University. We gratefully thank Dr.
Masayuki Iwasaki (Okayama University) for HRMS measurements
and Ms. Megumi Kosaka and Mr. Motonari Kobayashi at the
Department of Instrumental Analysis, Advanced Science Research
Center, Okayama University, for the measurements of elemental
analyses and the SC-NMR Laboratory of Okayama University for
the NMR spectral measurements.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.05.
035. These data include MOL files and InChiKeys of the most
important compounds described in this article.
electron-rich
O
S
S
S
S
S
S
electron-poor
6'
7
6
(PDT)
up to 64%
(ADT)
trace
Scheme 3.
nBuLi (2.2 equiv)
Br2 (2.4 equiv)
-78 °C to rt, 6 h
THF
-78 °C to rt, 1 h
S
S
Br
S
S
Br
6
8
89%
8
Pd(dba)2 (10 mol %)
[HPtBu3]BF4 (20 mol %)
KOH (6 equiv)
9-BBN dimer
(1.5 equiv)
R
THF, 60 °C, 1 h
Alkyl
S
S
Alkyl
reflux, 6 h
(3 equiv)
9a
: 80% (Alkyl = C7H15
9d
)
)
: 75% (Alkyl = C12H25
9e: 73% (Alkyl = C13H27
9f
: 69% (Alkyl = C14H29)
)
)
9b: 77% (Alkyl = C8H17
9c
: 78% (Alkyl = C10H21
)
Scheme 4.
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4
K. Hyodo et al. / Tetrahedron Letters xxx (2014) xxx–xxx
TMPZnClꢂLiCl (0.5 M in THF, 37.4 mL, 18.7 mmol, 2.2 equiv) at 25 °C and the
References and notes
reaction mixture was then stirred at this temperature for 1 h. 1,4-
Dibromobenzene (2) (2 g, 8.5 mmol, 1 equiv), Pd(dba)2 (244 mg, 0.425 mmol,
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temperature. The resulting reaction mixture was stirred at reflux for 3 h,
quenched with water (50 mL), and extracted with chloroform (100 mL ꢁ 3).
The combined organic layers were washed with brine and dried over MgSO4.
Filtration and evaporation afforded a pale yellow solid. Recrystallization gave
product 3 (1.65 g, 5.5 mmol, 65%) as a yellow solid.
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Miwa, K.; Sato, H.; Yamano, A.; Matsushita, T.; Uemura, T.; Takeya, J. Adv.
Mater. 2013, 25, 6392; (e) Kang, I.; Park, S. M.; Lee, D. H.; Han, S.-H.; Chung, D.
S.; Kim, Y.-H.; Kwon, S.-K. Sci. Adv. Mater. 2013, 5, 199; (f) Ota, S.; Minami, S.;
Hirano, K.; Satoh, T.; Le, Y.; Seki, S.; Aso, Y.; Miura, M. RSC Adv. 2013, 3, 12356;
(g) Kurihara, N.; Yao, A.; Sunagawa, M.; Ikeda, Y.; Terai, K.; Kondo, H.; Saito, M.;
Ikeda, H.; Nakamura, H. Jpn. J. Appl. Phys. 2013, 52. 05DC11/1.
18. During the preparation of this manuscript, the synthesis of 9f (n = 14) by a
different protocol has been disclosed. Dötz, F.; Weitz, T.; Jiao, C.; Noguchi, H.;
Sweemeng, A.; Zhou, M.; Iori, D.; Mishra, A. K. PCT Int. Appl. 2013, WO 2013-
168048.
33. Ishiyama, T.; Nobuta, Y.; Hartwig, J. F.; Miyaura, N. Chem. Commun. 2003, 2924.
34. Arsenyan, P.; Paegle, E.; Belyakov, S. Tetrahedron Lett. 2010, 51, 205.
35. Cheng, S.-W.; Chiou, D.-Y.; Lai, Y.-Y.; Yu, R.-H.; Lee, C.-H.; Cheng, Y.-J. Org. Lett.
2013, 15, 5338.
19. Synthesis of
3 by Suzuki–Miyaura coupling. To a solution of 3-formyl-2-
thiopheneboronic acid (1) (2.6 g, 16.5 mmol, 2.2 equiv) in THF (150 mL) in a
300 mL 2-neck round-bottom flask under an argon atmosphere were added
Pd(dba)2 (216 mg, 0.4 mmol, 5 mol %), [HPtBu3]BF4 (218 mg, 0.8 mmol,
10 mol %), KF (2 g, 34.5 mmol, 4.6 equiv), H2O (0.81 mL, 51 mmol, 6 equiv)
and 1,4-dibromobenzene (2) (2.6 g, 8.5 mmol, 1 equiv) were added at room
temperature. The resulting reaction mixture was stirred at 80 °C for 8 h,
quenched with water (50 mL), and extracted with chloroform (100 mL ꢁ 3).
The combined organic layers were washed with brine and dried over MgSO4.
Filtration and evaporation afforded a pale yellow solid. Recrystallization gave
product 3 (1.74 g, 6.6 mmol, 78%) as a yellow solid. Mp = 175 °C. FT-IR (KBr,
cmꢀ1): 3329 (s), 3124 (w), 3096 (m), 3047 (s), 3024 (s), 2837 (m), 2816 (w),
2735 (w), 1672 (w), 1409 (w), 1375 (w), 1217 (w), 840 (m), 823 (m), 748 (m).
1H NMR (300 MHz, CDCl3, rt): d 7.35 (d, J = 5.4 Hz, 2H), 7.61 (d, J = 5.4 Hz, 2H),
7.63 (s, 4H), 9.95 (s, 2H); 13C{1H} NMR (150 MHz, CDCl3, rt): d 125.8, 127.1,
130.5, 132.6, 137.5, 154.2, 185.2. Anal. Calcd for C16H10O2S2: C, 64.40; H, 3.38.
Found: C, 64.47; H, 3.20.
36. Nagao, K.; Ohmiya, H.; Sawamura, M. Synthesis 2012, 44, 1535.
37. Miyaura, N.; Ishiyama, T.; Ishikawa, M.; Suzuki, A. Tetrahedron Lett. 1986, 27,
6369.
38. Typical procedure for alkylation of 8: To a solution of terminal alkene (0.6 mmol,
3 equiv) in anhydrous THF (4 mL) in a 20 mL Schlenk under argon was added 9-
BBN dimer (75 mg, 0.3 mmol, 1.5 equiv) at room temperature. The reaction
mixture was stirred at 60 °C for 1 h, then cooled to room temperature before
adding Pd(dba)2 (11.5 mg, 0.02 mmol, 10 mol %), [HPtBu3]BF4 (11.6 mg,
0.04 mmol, 20 mol %), powdered KOH (67.3 mg, 1.2 mmol, 6 equiv), and
compound 8 (89.6 mg, 0.2 mmol, 1 equiv). The reaction mixture was stirred
at 85 °C for 6 h, quenched with water (10 mL), and extracted with chloroform
(30 mL ꢁ 2). The combined organic layers were washed with brine and dried
over MgSO4. Filtration and evaporation afforded a pale yellow solid. Column
chromatography on silica gel (chloroform/hexane = 2:3) gave 9a in 80% yield,
which was further purified by recrystallization from hexane to give analytically
pure samples as a white solid. Mp = 175–176 °C. FT-IR (KBr, cmꢀ1): 2953 (m),
2926 (w), 2883 (m), 2850 (m), 1463 (m), 1433 (m), 1361 (s), 1292 (m), 817 (w),
723 (w). 1H NMR (600 MHz, CDCl3, rt): d 0.90 (t, J = 7.2 Hz, 6H), 1.26–1.40 (m,
12H), 1.42–1.47 (m, 4H), 1.83 (quintet, J = 7.2 Hz, 4H), 3.00 (t, J = 7.2 Hz, 4H),
7.17 (s, 2H), 7.89 (d, J = 9 Hz, 2H), 8.09 (s, 2H), 8.60 (d, J = 9 Hz, 2H); 13C{1H}
NMR (150 MHz, CDCl3, rt): d 14.1, 22.7, 29.1, 29.1, 30.9, 31.4, 31.8, 119.9, 121.5,
121.9, 123.3, 126.6, 127.1, 137.7, 138.0, 146.4. Anal. Calcd for C32H38S2: C,
78.96; H, 7.87. Found: C, 78.58; H, 7.78.
20. (a) Mosrin, M.; Knochel, P. Org. Lett. 2009, 11, 1837; (b) Mosrin, M.; Monzon, G.;
Bresser, T.; Knochel, P. Chem. Commun. 2009, 5615; (c) Crestey, F.; Knochel, P.
Synthesis 2010, 1097; (d) Bresser, T.; Mosrin, M.; Monzon, G.; Knochel, P. J. Org.
Chem. 2010, 75, 4686; (e) Haag, B.; Mosrin, M.; Ila, H.; Malakhov, V.; Knochel, P.
Angew. Chem., Int. Ed. 2011, 50, 9794.
21. Synthesis of 3 by Negishi coupling. To a solution of 3-thiophenecarboxaldehyde
(4) (2.10 g, 18.7 mmol, 2.2 equiv) in anhydrous THF (130 mL) in a 300 mL of
2-neck round-bottom flask under an argon atmosphere was added a solution of
Please cite this article in press as: Hyodo, K.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.05.035