A facile synthesis of dithieno[3,2-b:6,7-b]fluorenes via a tandem annulation-reduction

Article abstract of DOI:10.1039/c3ra41927d

A concise and facile synthesis of 2,8-disubstituted dithieno[3,2-b:6,7-b] fluorenes starting from 2,7-dihydroxyfluorenone is reported. The key step involved is the tandem annulation-reduction of a 3,6-dichloro-2,7-di(alkynyl) fluorenone using Na₂S·9H₂O as a sulfur surrogate and reductant. The final dithiophene-fused fluorenes are obtained in reasonable yields (23-43%). The Royal Society of Chemistry.

Full text of DOI:10.1039/c3ra41927d

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A facile synthesis of dithieno[3,2-b:6,7-b]fluorenes via
a tandem annulation–reduction3
Cite this: RSC Advances, 2013, 3, 17707
Xiaoli Xiong, Qiancai Liu,* Jun Zhang, Min Zhu, Yanmei Wang and Shiming Deng
Received 19th April 2013,
Accepted 20th July 2013
DOI: 10.1039/c3ra41927d
www.rsc.org/advances
A concise and facile synthesis of 2,8-disubstituted dithieno[3,2-
b:6,7-b]fluorenes starting from 2,7-dihydroxyfluorenone is
reported. The key step involved is the tandem annulation–
thiophene-substituted acenes have been widely utilized as
important p-cores for the development of semiconductors, from
both small organic molecules and polymers, in the fields of
organic field-effect transistors (OFETs) and organic photovoltaics
(OPVs). Recently, Takimiya et al. successfully demonstrated the
selective and efficient synthesis of several isomeric naphtho-
dithiophenes (NDTs, Fig. 1), and fully investigated their
properties. NDTs are commonly indicated as useful cores for
optoelectronic devices, but have rarely been studied due to
difficulties in their preparation and the limited availability of
reduction of
Na S?9H as
a
a
3,6-dichloro-2,7-di(alkynyl)fluorenone using
sulfur surrogate and reductant. The final
2
2
O
dithiophene-fused fluorenes are obtained in reasonable yields
23–43%).
(
p-Conjugated polymers can be utilized as organic semiconductors,
which are applicable as active materials in organic electronic
devices, such as organic light emitting diodes (OLEDs), organic
field-effect transistors (OFETs), and organic photovoltaics (OPVs),
and energy storage materials. In recent years, much attention has
been devoted to preparing conjugated organic molecules and
polymers containing fused heterocycles, especially those with
thiophene and benzo[b]thiophene nuclei, which have found a
huge number of applications in materials science. Thiophene-
fused polycyclic (hetero)aromatics have aroused great interest due
to their excellent charge transport properties. For example,
thiophene- or benzothiophene-based derivatives have been
2d,e,3a–c
suitable precursors.
In contrast to dithiophene-fused
carbocyclic acenes, the synthesis of dithiophene-fused hetero-
acenes has not been extensively studied, despite the fact that
heteroatoms might have a strong and unique influence on the
properties of small organic molecules or polymers. A few
examples have been reported concerning dithiophene-fused
carbazoles suitable for materials chemistry, for example, Geng
et al. reported the synthesis of thiophene-fused congeners with
dithieno-carbazoles as building blocks for the construction of
semiconducting polymers for application as OFETs (Fig. 1,
1,2,5a,6
3g,h
applied in the fields of OLEDs and OFETs.
In particular,
CDTs).
acene-dithiophene systems with two thiophene rings fused at the
Therefore, any new strategy for the synthesis of benzothio-
phene cores represents an important development and particu-
larly appealing are those syntheses that allow the easy and facially
regioselective construction of five-membered rings fused to
3i,j
ends of a carbocyclic acene, such as benzodithiophenes (BDTs),
3
k,l
naphthodithiophenes (NDTs),
and anthradithiophenes
3m–o
(ADTs),
offer numerous advantages over other types of acenes,
owing to the ease of functionalization at the a-position of
thiophene, for typical transition metal catalyzed cross-coupling
polymerizations such as the Suzuki, Stille and Yamamoto
couplings or the direct (hetero)arylation polycondensation (DArP
3p,q
or DHArP) recently developed by Lerclerc and others.
The
integration of thiophene rings and carbocyclic acenes into a rigid
and coplanar entity could enhance p-electron delocalization, and
might also induce strong p–p stacking, useful for efficient charge
transport in materials chemistry. These dithiophene-fused or
Department of Chemistry, East China Normal University, 500 Dongchuan Rd,
Shanghai 200241, China. E-mail: qcliu@chem.ecnu.edu.cn; Tel: +86(21)54340097
3
9
Electronic supplementary information (ESI) available. CCDC reference number
19150. For ESI and crystallographic data in CIF or other electronic format see DOI:
0.1039/c3ra41927d
Fig. 1 Molecular structures of some representative dithiophene-fused carbocycles
1
and heterocycles.
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polycyclic aromatic hydrocarbons or heteroarenes. Currently there
are three major approaches towards the synthesis of the
benzothiophene unit: 1) the electrophilic cyclization reaction of
4
an o-alkynyl thioanisole; 2) the annulation of an o-haloalkynyl-
benzene with sodium sulfide at high temperature or a thiolation–
5
annulation reaction within a transition-metal catalysed system;
and 3) the coupling cyclization reaction of o-bromoalkynylben-
6
zenes with various thiol surrogates. In these methods, the
synthesis of key intermediates such as o-alkynyl thioanisole and
o-haloalkynylarenes is difficult due to the limited number of
suitable precursors. Other factors such as the solvent, temperature
and the catalyst system can also play key roles in the ring
formation of thiophene. For example, a high reaction temperature
3a
is an essential factor in Takimiya’s procedure, in which 1-halo-2-
arylethynyl)arenes can be used as precursors to efficiently
(
Scheme 1 The synthesis route to thieno-fluorenes.
construct benzothiophene derivatives, especially dithiophene- or
trithiophene-fused heteroarenes. Meanwhile, a transition metal
catalyst such as in the CuI–TMEDA system can efficiently promote
the synthesis of different benzothiophenes under mild condi-
of 2,7-dihydroxy-fluorenone with sulfuryl chloride (SO
glacial acetic acid in the temperature range from 42 to 45 uC
strictly below 45 uC) proceeded smoothly to afford 3,6-dichloro-
,7-dihydroxy-fluorenone (8) in high yield (90%) as previously
2 2
Cl ) in
5b
tions.
(
2
Due to the important roles of fluorene, thiophene and
thiophene-fused polyaromatic hydrocarbons (PAHs) or polymers
in materials science, the development of efficient and regioselec-
tive methods for the synthesis of hitherto unknown thiophene-
fused fluorenones and their derivatives is of considerable current
importance. These materials might offer important cores for
further evaluation of thiophene-based semiconductors in the
fields of solar cells and organic field transistors. As part of our
continued interest in the construction of different sulfur-contain-
ing heterocycles such as thiophene-fused fluorenones, fluorenes
8a,b
described. This was followed by a reaction with triflic anhydride
3.5 equiv.) in the presence of pyridine as a base in dichlor-
omethane, to form the bis(triflate) (9) in an isolated yield of
(
5a,6,8c
93%.
Next, reactions between bis(triflate) (9) and various ethynyl
derivatives were carried out using the palladium-catalyzed
i
Sonogashira coupling conditions, Pd(PPh ) Cl /CuI/ Pr NH in
3
2
2
2
DMF, affording 3,6-dichloro-2,7-dialkynyl (or aryl-alkynyl) fluor-
enones 10a–10k. Considering the solubility and functional group
diversity, even alkyl or aryl acetylenes could be employed to
perform this coupling reaction, all of the desired diethynyl
derivatives could be obtained at room temperature. After the
usual work up and purification by column chromatography,
7
and spirobifluorenes, herein we report, to the best of our
knowledge, the first synthesis of dithiophene-fused fluorenes
using a well-designed synthetic route. In the context of our studies,
we developed a novel and general protocol for the synthesis of 2,8-
disubstituted dithieno[3,2-b:6,7-b]fluorenes, which provides novel
and unprecedented access to target molecules that are difficult to
prepare using other synthetic strategies, in terms of the number of
synthetic steps and availability of key precursors.
compounds 10a–10k could be efficiently isolated in yields of 72–
5a,6
88%.
With these intermediates in hand, our investigation
began with the attempted annulation of substrate 10a using
Na S?9H O in refluxing N-methylpyrrolindone (NMP) as described
2
2
The above mentioned methods for the construction of
benzothiophene, generally require an o-haloalkynylbenzene, which
can be prepared by the reaction of a 2-halo-1-iodobenzene or
in Takimiya’s procedure. A white solid product was obtained in an
isolated yield of 25%. To our surprise, the methylene signal
appeared as a singlet in the proton NMR spectrum (4.11 ppm,
2-halophenol triflate derivative with suitable acetylenes, under
3
fluorene is at 3.92 ppm, see the ESI for details). A carbonyl signal
13
typical transition metal-catalyzed Sonogashira cross-coupling
conditions. Due to the limited availability of 2,3,6,7-tetrahalo-
fluorenones (e.g., 2,7-dibromo-3,6-diiodofluorenone or 3,6-
dibromo-2,7-diiodofluorenone, as well as their chloro congeners),
we sought a different synthetic strategy that could be applied to
dithieno-fluorenones. Inspired by Takimiya’s procedures to obtain
NDTs, we chose 2,7-dihydroxy-fluorenone as a starting material,
which contains a central fluorenone component, with the further
consideration that triflates derived from the hydroxy groups could
act as excellent leaving groups in the palladium-catalyzed
was also not found in the C NMR spectrum, a signal for the
methylene of fluorene was observed instead (35.82 ppm).
Delighted by this unexpected result, we subsequently converted
the other 3,6-dichloro-2,7-di(ethynyl)-fluorenones (10b–10k) into
dithieno-fluorenes in the same manner. In all cases, the reactions
proceeded smoothly to afford fluorenes 11a–11k in isolated yields
of 23–43%. (Scheme 1 and Table 1.)
Typically, one can approach the synthesis of thiophene-fused
aromatic ketones or quinones such as anthrathiophenediones
using the reaction between an o-halogenated arylacetylene and
5a,6
9
Sonogashira coupling.
Na S?9H O in a suitable solvent with or without a catalyst.
2
2
In our case, in order to obtain suitable precursors like those
reported by Takimiya, we decided to introduce two chlorides to 3
and 6 positions of the 2,7-dihydroxyfluorenone. The chlorination
However, no example of a tandem cyclization–reduction with
Na S?9H O as the sole reagent, involving the direct reduction of
2
2
the carbonyl group of the aromatic ketone or quinone during the
1
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Table 1 The different dithieno[3,2-b:6,7-b]fluorenes synthesised
Entry
1
Precursor 10
Product 11 (yield)
2
3
4
5
6
7
8
9
1
1
0
1
reaction process, has ever been reported. In order to prove the role
of Na S?9H O as a reductant in this reaction, we first used
fluorenone as a model, and found that the conversion of
fluorenone could be completed with Na S?9H O in refluxing
as a separable by-product (isolated 22%). However, only part of
fluorenone was consumed if the temperature was decreased to 110
uC. Further repetitions indicated that fluorenone could be fully
recovered, if the same reaction was carried out at room
temperature. For further validation of this unprecedented reduc-
2
2
2
2
NMP, affording fluorene (46% isolated yield), with 9,99-bifluorene
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