Synthesis of Oligo(thienylene-vinylene) by Regiocontrolled Deprotonative Cross-Coupling

Article abstract of DOI:10.1021/acs.orglett.5b03567

Concise synthesis of oligo(thienylene-vinylene) with a head-to-tail type structure is achieved by regioselective deprotonative coupling of 3-hexylthiophene. The palladium catalyzed reaction of 3-hexylthiophene with (E)-2-(2-bromoethenyl)-3-hexylthiophene takes place to afford head-to-tail type trans-1,2-dithienylethene. Further extension of a vinylthiophene unit is similarly performed in an iterative manner. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.orglett.5b03567

Letter
pubs.acs.org/OrgLett
Synthesis of Oligo(thienylene-vinylene) by Regiocontrolled
Deprotonative Cross-Coupling
Shota Tanaka,† Yuta Fukui,† Naoki Nakagawa,† Kohei Murakami,† Takurou N. Murakami,*,‡
Nagatoshi Koumura,‡ and Atsunori Mori*,†
†Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
‡Research Center for Photovoltaics (RCPV), National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1
Higashi, Tsukuba, Ibaraki 305-8565, Japan
S
* Supporting Information
ABSTRACT: Concise synthesis of oligo(thienylene-vinylene)
with a head-to-tail type structure is achieved by regioselective
deprotonative coupling of 3-hexylthiophene. The palladium
catalyzed reaction of 3-hexylthiophene with (E)-2-(2-bromoe-
thenyl)-3-hexylthiophene takes place to afford head-to-tail type
trans-1,2-dithienylethene. Further extension of a vinylthio-
phene unit is similarly performed in an iterative manner.
ligothiophene and polythiophene derivatives have
attracted much attention as π-conjugated material for
of aldehyde with phosphate, McMurry coupling6b of thiophene
aldehydes, and cross-coupling reactions by transition metal
catalysis.7 Although these reactions are effective tools to
introduce carbon−carbon double bond adjacent to the
thiophene ring, a subsequent transformation is necessary
prior to an additional extension of the unit structure. Thus,
multistep reactions are required to achieve preparation of a
certain number of oligomers. We have been successful in the
stepwise synthesis of head-to-tail-type oligothiophenes by a
nickel-catalyzed coupling reaction to form a thiophene−
thiophene bond via regioselective deprotonation at the C−H
bond of thiophene, with which each thiophene unit can be
extended in a single manipulation.4a,b Thus, the well-defined
monodisperse oligomer was shown to be obtained in a highly
concise manner. We envisaged that a similar approach with
regioselective deprotonation of 3-alkylthiophene 1 to react with
(E)-2-(2-bromoethenyl)-3-alkylthiophene 2 gives (E)-1,2-di-
thienylethene in a regioregular manner. In addition, repeating
the regioselective deprotonation of the obtained oligomer and
the following coupling brings about a practical stepwise
synthesis of oligo(thienylene-vinylene)s, which would also
involve only a single step in each extension of the thienylene-
vinylene unit (Scheme 1). Herein, we describe the synthesis of
head-to-tail-type regioregular oligo(thienylene-vinylene)s, cata-
lyzed by a palladium complex.
O
organic advanced materials. In particular, these compounds are
expected to be applied for organic electronic materials such as
organic transistors,1 organic solar cells,2 and liquid crystals.3 It
is therefore important to develop concise and practical
synthesis of oligo- and polythiophenes in materials science as
well as synthetic organic chemistry. We have been engaged in
practical synthesis of oligothiophenes and polythiophenes with
a head-to-tail (HT) type regioregular structure, which generally
exhibits higher performance as a material because of its
minimum steric repulsion leading to planar π-conjugated
conformation, using a transition-metal-catalyzed cross-coupling
reaction at the C−H bond forming a thiophene−thiophene
bond.4
The related π-conjugated oligomers and polymers of
thiophenes bearing a vinylene spacer between two thiophene
rings have also been of much interest as advanced materials by
further extended π-conujugation and cancellation of steric
hindrance from the direct connection of thiophene−thio-
phene.5 Various (thienylene-vinylene)s have been prepared to
date and revealed to show a lower band gap than corresponding
oligothiophenes or polythiophenes.6 Indeed, development of a
practical synthesis of a thienylene-vinylene polymer with a
head-to-tail-type structure has been a major concern and several
syntheses employing transition metal catalysis produced
thienylene-vinylene polymers.6g−i On the other hand, it is
also intriguing to afford thienylene-vinylenes with a well-
defined molecular size and structure via stepwise construction
by repeating unit reactions affording the corresponding
oligomers with monodispersity.7 Several synthetic pathways
to well-defined thienylene-vinylene have been shown so far,
namely, by a Horner−Wadsworth−Emmons (HWE) reaction6a
We first focused our concern on the development of the
efficient synthesis of thiophene-substituted vinyl bromide 2a
with a readily available thiophene derivative 2-formyl-3-
hexylthiophene 1a-CHO. As shown in Scheme 2, the reaction
Received: December 16, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03567
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Organic Letters
Letter
dithienylethene 3aa was obtained in a moderate yield (71%)
after stirring at room temperature for 24 h. The reaction with
NiCl2(PPh3)IPr at an elevated temperature (60 °C) slightly
increased the yield to 89%. Other nickel catalysts with bidentate
diphosphine as a ligand, NiCl2(dppp) or NiCl2(dppf), afforded
3aa in only 44% and 36% yields, respectively, accompanied by
unreacted 2a, which was confirmed by TLC analysis. A
palladium catalyst was also shown to undergo the reaction.
When ubiquitous palladium complex PdCl2(PPh3)2 (2 mol %)
was employed as a catalyst, 3aa was obtained in 62% yield. The
reaction of the palladium catalyst with bulky alkylphosphine
ligand Pd(PtBu3)2 also took place to afford 3aa in 42% yield.
Higher catalyst performance was observed in the reaction with
palladium bearing NHC ligand Pd-PEPPSI-IPr11 (77% yield),
and the reaction at 120 °C in toluene with Pd-PEPPSI-IPr
resulted in giving a quantitative yield whereas the reaction in
hexane was shown to be less effective (at 60 °C, 59% yield).
We next studied the reaction with a variety of thiophene
derivatives with several trans-vinyl bromides. The deprotonative
metalation of thiophenes at the 5-position proceeded in a
regioselective manner. In addition to 3-hexylthiophene (1a),
unsubstituted thiophene, 3-methylthiophene, branched terthio-
phene,4b and 3-arylthiophene bearing a 4-methoxyphenyl group
underwent the coupling reaction with 2a and the corresponding
heteroarene-vinylene-heteroarene 3 in 70−86% yields. trans-
Vinyl bromide bearing unsubstituted thiophene and furan
reacted with 2a to afford 3ab and 3af in 45% and 37% yields,
respectively. Vinyl bromide bearing an aryl substituent also
reacted with 1a to provide 3ag in 71% yield. These results are
summarized in Table 2.
Scheme 1. Synthetic Strategy of Thienylene-Vinylene
Oligomers
Scheme 2. Synthesis of E-Alkenyl Bromide 2a
of 1a-Br with a Grignard reagent effected halogen−metal
exchange. Following treatment with N,N-dimethylformamide
(DMF), the formyl group was introduced at the 2-position to
afford 1a-CHO in 90% yield. The Corey−Fuchs reaction of 1a-
CHO with PPh3 and carbon tetrabromide (CBr4) afforded
vinylidene bromide 1a-CCBr2 in 90% yield.8 Selective
debromination leading to trans-vinyl bromide 2a was achieved
by the reaction of dimethylphosphite in 61% yield.9
The reaction of the thus obtained vinyl bromide 2a with 3-
hexylthiophene (1a) was examined with several transition-metal
catalysts. Treatment of 1a with EtMgCl in the presence of 10
mol % 2,2,6,6-tetramethylpiperidine (TMPH) induced depro-
tonative metalation regioselectively after stirring for 24 h under
reflux in THF.4 The metalated thiophene was then subjected to
the coupling reaction with 2a in the presence of a transition
metal catalyst. Results are summarized in Table 1. When 2a and
a nickel(II) catalyst bearing N-heterocyclic carbene (NHC)
NiCl2(PPh3)IPr,10 which showed excellent catalyst perform-
ance in thiophene-thiophene coupling,4 was added to a THF
solution of metalated thiophene, head-to-tail-type trans-1,2-
Table 2. Regioselective Cross-Coupling of Various 3-
a
Substituted Thiophenes
a
Table 1. Coupling of 1a with Vinyl Bromide 2a
b
entry
catalyst
solvent
yield/%
c
1
NiCl2(PPh3)IPr
NiCl2(PPh3)IPr
NiCl2(dppp)
NiCl2(dppf)
THF
THF
THF
THF
THF
THF
THF
toluene
hexane
71
89
44
36
62
42
77
98
59
2
3
4
5
6
7
PdCl2(PPh3)2
Pd(PtBu3)2
Pd-PEPPSI-IPr
Pd-PEPPSI-IPr
Pd-PEPPSI-IPr
d
8
9
a
Unless noted, the reaction was carried out with 1a (0.5 mmol),
EtMgCl (0.6 mmol), 2a (0.6 mmol), and catalyst (0.01 mmol) in THF
a
The reaction was carried out with 3-substituted thiophene (0.5
b
c
at 60 °C for 24 h. Isolated yield. The reaction was carried out at
mmol), EtMgCl (0.6 mmol), vinyl bromide (0.6 mmol), and Pd-
PEPPSI-IPr (0.01 mmol) in toluene at 120 °C for 24 h. Isolated yield.
d
b
room temperature. The reaction was performed at 120 °C.
B
DOI: 10.1021/acs.orglett.5b03567
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Organic Letters
Letter
With the optimized coupling conditions, further extension of
the vinylthiophene unit was performed as shown in Scheme 3.
(thienylene-vinhylene)s was performed by a regioselective
cross-coupling reaction with 3-bromo-9-ethyl-carbazole (6).
Treatment of 3aa with a Knochel−Hauser base (TMPMgCl·
LiCl) induced deprotonation regioselectively, and palladium-
catalyzed coupling with 6 followed to afford the corresponding
coupling product 3aa-Cz in 70% yield. The reaction of 4
bearing three thiophenes and two vinylenes also proceeded in a
similar manner to afford 4-Cz in 43% yield. The obtained 3aa-
Cz and 4-Cz were subjected to the treatment of POCl3/DMF
leading to the formylated products (98% and 63%,
respectively), which were transformed to the corresponding
cyanoacrylates TMFN-1 and TMFN-2 both in quantitative
yields2b (Scheme 4).
Scheme 3. Stepwise Extension of Vinylthiophene Unit
Scheme 4. Synthesis of MK-Dye Analogs of Thienylene-
Vinylene Oligomers
Selective metalation of 3aa was carried out in a similar manner
with EtMgCl and 10 mol % of TMP-H under reflux for 24 h
resulting in deprotonation at the less-hindered position. The
following reaction with vinyl bromide 2a in the presence of the
Pd-PEPPSI-IPr catalyst gave the thienylene-vinylene oligomer
bearing three thiophene units and two C−C double bond 4 in
78% yield. The regioselective cross-coupling of 4 with 2a also
proceeded to afford oligomer 5 with four thiophenes and three
vinylenes in 58% yield.
Measurements of UV−vis absorption and photoluminescent
spectra of oligo(thienylene-vinylene)s 3aa, 4, and 5 revealed an
increase in molar absorbance coefficient ε as the extension of
the repeating thienylene-vinylene unit (Table 3). Remarkable
Table 3. UV−vis Absorption and Photoluminescent Spectra
of Thienylene-Vinylene Oligomers with Regioregular Head-
to-Tail Structure
The obtained products TMFN-1 and TMFN-2 were
employed as an organic dye for DSSCs. Table 4 shows the
a
b
c
compd
log ε
λabs/nm
λem/nm
Φ × 103
Table 4. DSSC Performance Parameters of TMFN-1 and
d
3aa: TVT
4: (TV)2T
5: (TV)3T
4.42
4.66
4.88
350
417
512
526
5
a
TMFN-2 Dyes
d
424
3
compd
Jsc/mA cm−2
Voc/V
FF
η%
472
0.2
a
Absorption spectrum measured as a chloroform solution (1.0 × 10−5
TMFN-1
TMFN-2
8.69
6.47
0.710
0.671
0.707
0.695
4.37
3.02
b
M). Photoluminescent spectrum as a 1.0 × 10−6 M chloroform
solution. The quantum yield Φ was estimated based on 7-
diethylamino-4-methylcoumarin as 0.01 mM solution of ethyl acetate
(Φ = 0.99). λabs of head-to-tail-type bithiophene 2T: 300 nm; 3T:
341 nm; 4T: 367 nm.
c
a
For details, see Supporting Information.
d
preliminary results related to photocurrent density−voltage
characteristics.12 The short-circuit photocurrent density (Jsc)
was found to be ca. 6.5−8.7 mA cm−2, and the open-circuit
photovoltage (Voc) was 0.7 V in TMFN-1 and TMFN-2. The
fill factor (FF) value was ca. 0.7, and the power conversion
efficiency (η%) was estimated to be 4.37 (TMFN-1) and 3.02
(TMFN-2), respectively, suggesting that a carbazole dye
bearing a thienylene vinylene moiety with a simple synthetic
pathway showed moderate performance for DSSCs, albeit
formation of the cell device has not yet been fully optimized to
such a specific structure.
shifts in the absorption and emission maxima were also
observed. From comparison of the absorption spectrum of 4a
bearing only two thiophene units (λmax = 424 nm) with that of
head-to-tail-type terthiophene 3T (λmax = 341 nm), a thiophene
oligomer bearing a vinylene spacer suggests more extended π-
conjugation.
Functionalization of the obtained thienylene-vinylene
oligomers 3aa, 4, and 5 was then carried out. Since a head-
to-tail-type oligothiophene bearing a carbazole moiety is well-
known as a MK-dye for organic dye-sensitized solar cells
(DSSCs) that exhibit high performance,2b we envisaged
synthesizing the related analog, which would show absorption
to longer wavelengths. Introduction of carbazole to oligo-
In conclusion, concise synthesis of oligo(thienylene-vinyl-
ene) with a head-to-tail-type structure was achieved by
regioselective deprotonative coupling of 3-hexylthiophene
(1a) with thiophene-substituted vinyl bromide 2a. By repeating
C
DOI: 10.1021/acs.orglett.5b03567
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Organic Letters
Letter
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thiophene and three vinylene units was successfully synthesized.
UV−vis absorption maxima of the obtained oligomers shifted
to a longer wavelength as the number of repeating units
extended. Functionalization of oligomers was achieved to
introduce carbazole and cyanoacrylate groups at both ends of
the oligomers, which are recognized as the analog of MK-dyes
employed for organic dyes for dye-sensitized solar cells
(DSSC), although certain optimization should be made in
the device fabrication to the specific conditions for thienylene
vinylenes to improve the performance.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03567.
(7) (a) Mullen, K.; Wegner, G. In Electronic Materials: The Oligomer
̈
Experimental section, spectroscopic data, and details on
characterization of TMFN-1 and TMFN-2 as DSSC
(PDF)
Approach; Wiley-VCH: Weinheim, 1998. (b) Martin, R. E.; Diederich,
F. Angew. Chem., Int. Ed. 1999, 38, 1350.
(8) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 13, 3769.
(9) (a) Hirao, T.; Masunaga, T.; Ohshiro, Y.; Agawa, T. J. Org. Chem.
1981, 46, 3745. (b) Abbas, S.; Hayes, C. J.; Worden, S. Tetrahedron
Lett. 2000, 41, 3215. (c) Kuang, C.; Senboku, H.; Tokuda, M.
Tetrahedron 2002, 58, 1491.
AUTHOR INFORMATION
Corresponding Authors
■
(10) Matsubara, K.; Ueno, K.; Shibata, Y. Organometallics 2006, 25,
3422.
*E-mail: amori@kobe-u.ac.jp.
*E-mail: takurou-murakami@aist.go.jp.
(11) PEPPSI: Pyridine-enhanced precatalyst preparation stabilization
and initiation. See: (a) O’Brien, C. J.; Kantchev, E. A. B.; Valente, C.;
Hadei, N.; Chass, G. A.; Lough, A.; Hopkinson, A. C.; Organ, M. G.
Chem. - Eur. J. 2006, 12, 4743. (b) Organ, M. G.; Calimsiz, S.; Sayah,
M.; Hoi, K. H.; Lough, A. J. Angew. Chem., Int. Ed. 2009, 48, 2383.
(12) For experimental details concerning DSSC cell formation: see
Supporting Information.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank KAKENHI B (No. 25288049) by JSPS and
ASTEP by JST for financial support. S.T. thanks JSPS for the
Research Fellowship for Young Scientists. This work was partly
supported by Special Coordination Funds for Promoting
Science and Technology, Creation of Innovation Centers for
Advanced Interdisciplinary Research Areas (Innovative Bio-
production Kobe), MEXT, Japan.
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