Benzodithiophene and benzotrithiophene as π cores for two-and three-blade propeller-shaped ferrocenyl-based conjugated systems

Article abstract of DOI:10.1002/ejoc.201701045

The syntheses of linear and star-shaped bis- and tris(ferrocenyl) derivatives of benzo[1,2-b:4,5-b′]dithiophene and benzo[1,2-b:3,4-b′:5,6-b′′]trithiophene are achieved through one-pot CuI/TMEDA-catalyzed (TMEDA = tetramethylethylenediamine) multiple annulations of bromoethynylbenzenes with sodium sulfide. In addition, the preparation of the parent benzotrithiophene in a good yield with a short reaction time is achieved through the threefold annulation of 1,3,5-trifluoro-2,4,6-tris(trimethylsilyl)ethynylbenzene. The computed structural and electronic features of these ferrocenyl derivatives as well as their UV/Vis spectra and electrochemistry are discussed, and the results provide insights into the effect of the presence of three rather than two ferrocenyl units. To the best of our knowledge, 2,5,8-tris(ferrocenyl)benzo[1,2-b;3,4-b′;5,6-b′′]trithiophene is the first organometallic complex containing benzotrithiophene.

Full text of DOI:10.1002/ejoc.201701045

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A Journal of
Accepted Article
Title: Benzodithiophene and Benzotrithiophene as π-Core for Two- and
Three-Blade Propeller-Shaped Ferrocenyl-Based Conjugated
Systems
Authors: Saverio Santi, Serena Rossi, Annalisa Bisello, Roberta
Cardena, and Laura Orian
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201701045
Link to VoR: http://dx.doi.org/10.1002/ejoc.201701045
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Benzodithiophene and Benzotrithiophene as π-Core for Two- and
Three-Blade Propeller-Shaped Ferrocenyl-Based Conjugated
Systems
Serena Rossi,[a] Annalisa Bisello,[a] Roberta Cardena,[a] Laura Orian and Saverio Santi*[a]
Abstract: The syntheses of linear and star-shaped bis and
tris(ferrocenyl) derivatives of benzo[1,2-b:4,5-b’]dithiophene and
benzo[1,2-b:3,4-b’:5,6-b’’]trithiophene are described via one-pot
Recently, many efforts have been focused on benzo[1,2-b:4,5-
b’]dithiophene (BDT) and benzo[1,2-b:3,4-b’:5,6-b’’]trithiophene
(BTT) (Scheme 1) as potential π-core for a new class of organic
semiconductors since they contain two or three identical
thiophene moieties with C2h or C3h symmetries that enable two-
dimensional and three-dimensional molecular extensions.
BDT is a fused dithiophene which has been widely used as
semiconductors9 and building block in organic solar cells.10 BTT
is a fused trithiophene with a sulfur-rich, planar and extended π-
system.11 Among the possible isomeric members of the
benzotrithiophene family, BTT has been studied thoroughly and
used as a core for the construction of star-shaped oligomers and
dendrimers.12
In addition, it was reported that the peculiar aromatic structure
containing three thiophene units fused to a central benzene ring
confers a completely planar and highly electron-rich system,
thus making it a promising candidate for organic photovoltaic
(OPV) devices.13 Despite its particular molecular structure and
potential use as a building block for organic electronic devices,
the application of BTT remains limited. BDT and BTT derivatives
have become major components or key precursors for the
design and synthesis of novel material. Some patents14 describe
the methods to provide these cores at industrially applicable
level and innovative materials having liquid crystalline and
semiconductor properties.
CuI/TMEDA
catalyzed
manifold
annulation
of
bromoethynylbenzenes with sodium sulfide. In addition, an approach
to prepare in good yield and short reaction time the parent
benzotrithiophene is achieved via threefold annulation of 1,3,5-
trifluoro-2,4,6-tris(trimethylsilyl)ethynylbenzene.
The
computed
structural and electronic features of these ferrocenyl derivatives and
their UV-vis spectra and the electrochemistry are discussed,
providing insight on the presence of three rather than two ferrocenyl
units. At the best of our knowledge 2,5,8-tris(ferrocenyl)benzo[1,2-
b;3,4-b’;5,6-b’’]trithiophene is the first organometallic complex
containing benzotrithiophene.
Introduction
Benzo[b]thiophene (BT, Scheme 1) and its related derivatives
represent an important class of fused-thiophene compounds due
to their wide range of biological properties1,2 and also various
applications in material science.3
S
The preparation of multicomponent molecules with specific
redox, optoelectronic and conductive properties is currently
S
S
S
S
fundamental
for
modern
technology.15
In
particular
S
(multi)ferrocenyl compounds with conjugated spacer groups and
displaying multielectron redox chemistry are of particular interest
owing to their unpaired electron density migration properties.16,17
Ferrocene is one of the most employed organometallic
components. Its stability, redox properties, specific electron
donor character and well-developed functionalization chemistry
make it a primary candidate for testing the communication
properties in conjugated systems. The literature reports that
thiophene spacers exhibit better communication compared to
phenyl spacers in donor-acceptor systems, while ferrocene is a
widely studied strong donor.16a,18 Therefore, in this work we have
combined the ferrocenyl moiety with both BTT and BDT.
Examples of bimetallic BDT derivatives are rather rare19 and at
the best of our knowledge no BTT trimetallic complexes have
been reported so far.
BT
BTT
BDT
Scheme 1. Fused-thiophene compounds: benzo[b]thiophene (BT), benzo[1,2-
b:4,5-b’]dithiophene (BDT) and benzo[1,2-b:3,4-b’:5,6-b’’]trithiophene (BTT).
In particular, (multi)thiophene fused-aromatic compounds are
attracting current interest as promising electronic materials for
organic conductors,4 organic light-emitting diode5 photovoltaic
cells6 and field-effect transistors.7 For this reason thiophene-
based-π-conjugated oligomers have been widely investigated as
organic semiconductors.8
[a]
Department of Chemical Sciences
University of Padova
Via Marzolo 1, 35131 Padova, Italy
E-mail:saverio.santi@unipd.it
On the basis of these premises, our aim was to synthetize
end-capped ferrocenyl BTT and BDT conjugated systems,
2,5,8-tris(ferrocenyl)benzo[1,2-b:3,4-b’:5,6-b’’]trithiophene
and 2,6-bis(ferrocenyl)benzo[1,2-b:4,5-b’’]dithiophene
(1)
(2)
Supporting information for this article is given via a link at the end of
the document
(Scheme 2). In fact, for the further development of new materials
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10.1002/ejoc.201701045
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based on BTT and BDT, it is of primary importance to plan
effective synthetic methods.
Recently, transition-metal-catalyzed carbon-sulfur bond
forming reactions have been developed.24 In this context, Zhang
worked out a copper-catalyzed thiolation-annulation reaction of
bromoalkynylbenzenes
giving
differently
substituted
Fc
S
benzo[b]thiophenes with sodium sulfide in the presence of CuI
TMEDA in DMF at 80 °C for 24 h (Scheme 3b).21
S
S
Fc
Fc
Accordingly,
we
prepared
1,3,5-tribromo-2,4,6-
Fc
tris(trimethylsilyl)ethynylbenzene25 (3b) starting from 1,3,5-
tribromo-2,4,6-triiodo-benzene26 (Scheme 4). Howsoever, this
step proceeded in low yield (30%). Besides, the thioannulation
of 3b under Zhang’s conditions proceeded in low yield (Table 1,
entry c). Reaction of 3b under Takimiya’s conditions once more
gave only traces of BTT (Table 1, entry d).
S
S
Fc
2
1
Scheme 2. 2,5,8-tris(ferrocenyl)benzo[1,2-b:3,4-b’:5,6-b’’]trithiophene (1) and
2,6-bis(ferrocenyl)benzo[1,2-b:4,5-b’]dithiophene (2) complexes.
(Me)3Si
Cl
Si(Me)3
Cl
I
6 mol% CuI
1 mol% Pd Cl2(PPh3)2
I
I
Results and Discussion
Cl
Cl
ethynyltrimethylsilane
6 mol% mol PPh3
iPr2NH, THF
Cl
Cl
Takimiya and co-workers20 proposed a new route to introduce
sulfur functional groups to aromatic rings with Na2S⋅9H2O in N-
methyl-2-pyrrolidone (NMP) at 180 °C for 12 h (Scheme 3a).
80° C, 24 h
Si(Me)3
3a (45%)
They
reported
that
1,3,5-trichloro-2,4,6-triiodobenzene,
X
(Me)3Si
Si(Me)3
accessible by iodination of 1,3,5-trichlorobenzene with periodic
acid, can be utilized as starting compound material for the
preparation of 1,3,5-trichloro-2,4,6-tris[(trimethylsilyl)ethynyl]-
benzene (3a) with ethynyltrimethylsilane in Sonagashira-
coupling conditions (Scheme 4). Compound 3a was the
precursor of the cyclization reactions affording the fused-
thiophene BTT. However, as recently reported by Zhang21 and
Sanz,22 the utility and applicability of the above reactions suffer
from the harsh reaction conditions and low yield.
X
3 mol% CuI
3 mol% Pd Cl2(PPh3)2
I
I
X
X
ethynyltrimethylsilane
3% PPh3
X
X
I
iPr2NH, THF
80° C, 16 h
Si(Me)3
3b (X = Br, 31%)
3c (X = F, 98%)
Fc
X
Fc
X
I
6 mol% CuI
I
I
1 mol% Pd Cl2(PPh3)2
ethynylferrocene
6% mol PPh3
iPr2NH, reflux
24 h
X
X
X
Na2S 9H2O
X
X
(a)
Takimiya conditions
NMP, 180°C
12 h
Fc
R
S
4a (X = Br, 65%)
4b (X = Cl, 55%)
X = Cl, Br
Br
R
Na2S 9H2O
Zhang conditions
(b)
CuI, TMEDA
DMF, 80 °C
Scheme 4. Synthesis of substituted tris(ethynylbenzenes).
R
24 h
R = H, alkyl, aryl
Interestingly, it was reported the annulation reaction of
trifluorotrialkynylbenzenes with CsOH⋅H2O or RNH2 obtaining
Scheme 3. Syntheses of benzo[b]thiophenes.
benzotrifuranes
and
benzotripyrrol27
and
of
fluoro-
phenylacetylene derivatives with KOH or Na2S giving
benzofuranes and benzothiophenes.28 In particular, transition-
metal-free thiolation annulation of 2-fluorophenylacethylene with
Actually, we tested the thioannulation reaction of 3a under
Takimiya’s conditions but we obtained only traces of BTT (Table
1, entry a). The same reaction in the presence of CuI and
TMEDA in NMP at 120 °C for 24 h was unsuccessful (Table 1,
entry b).Therefore, we changed strategy towards a more
appropriate route for the preparation of BTT.
Na2S⋅9H2O in DMSO at 90 °C for 10
h
gave 2-
phenylbenzo[b]thyophene in outstanding yield (91%).
We
tested
this
method
on
1,3,5-trifluoro-2,4,6-
tris[(trimethylsilyl)ethynyl]benzene (3c),29 synthesized starting
from 1,3,5-trifluoro-2,4,6-triiodobenzene30, obtaining BTT in
discrete yield (Table 1, entry e).
Alternative routes for the synthesis of BTT from 1,3,5-
trichlorobenzene,12d
hydroxyarenes23
and
2,3-
dibromothiophene12e were reported, but they required several
steps and very long reaction time.
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Fc
Table 1. Optimization of the Thioannulation Reaction Conditions
Fc
Fc
Br
T
time
(h)
yield
(%)[f
Product
substrate
catalyst
ligand
solvent
S
Na2S 9H2O
S
(°C)
Fc
BTT[a]
BTT[b]
BTT[c]
BTT[d]
BTT[e]
BTT[f]
1[f]
1[g]
1[h]
1[i]
2[f]
3a
3a
3b
3b
3c
3c
4a
4b
4b
4b
5
-
-
NMP
NMP
DMF
NMP
DMSO
DMF
DMF
DMF
NMP
DMSO
DMF
180
120
80
12
24
24
12
10
2
<5
-
30 mol%CuI,
60 mol%TMEDA
DMF, 110 °C
2 h
Br
Br
CuI
CuI
-
TMEDA
S
Fc
TMEDA
10
<5
45
78
98
<5
7
Fc
-
180
90
-
-
4a
1 (98%)
CuI
CuI
CuI
-
TMEDA
TMEDA
TMEDA
-
110
110
110
180
60
2
Scheme 6. Synthesis of 1.
24
12
8
The successful synthesis of 1 prompted us to apply it to for the
preparation of 2,6-bis(ferrocenyl)benzo[1,2-b:4,5-b’’]dithiophene
(2) (Scheme 7). The formation of 1,4-dibromo-2,5-
bis(ethynylferrocenyl)benzene (5) was obtained through
Sonagashira-coupling of 1,4-dibromo-2,5-diiodo-benzene32 with
ethynyl-ferrocene. Finally, the twofold thioannulation reaction of
5 with 30% mol of CuI, 40% mol of TMEDA and 6 eq. of
Na2S·9H2O produced the desired compound 2.
CuI
CuI
-
<5
55
TMEDA
110
2
Reaction conditions: All the reactions were performed under argon
atmosphere. a3a (0.5 mmol), Na2S⋅9H2O (3 mmol) in NMP (12 mL). b3a (0.15
mmol), Na2S⋅9H2O (1.35 mmol), CuI (30 mol%), ligand (60 mol%) in NMP (10
mL). c3b (0.2 mmol), Na2S⋅9H2O (1.8 mmol), CuI (30 mol%), ligand (60 mol%)
in DMF (15 mL). d3b (0.5 mmol), Na2S⋅9H2O (4.5 mmol) in NMP (12 mL). e3c
(0.25 mmol), Na2S⋅9H2O (2.25 mmol) in DMSO (4 mL). fSee experimental
Section. g4b (0.12 mmol), Na2S⋅9H2O (1.08 mmol), CuI (30 mol%), ligand (60
mol%) in DMF (9 mL). h4b (0.09 mmol), Na2S⋅9H2O (0.81 mmol) in NMP (6
mL). i4b (0.08 mmol), Na2S⋅9H2O (0.72 mmol), CuI (30 mol%) in DMSO (6 mL).
Br
Br
5
Br
Fc
6 mol% CuI
1 mol% PdCl2(PPh3)2
Finally, the threefold thioannulation reaction of the fluorinated
derivative 3c was improved in the presence of CuI and TMEDA
in DMF obtaining BTT in higher yield and shorter reaction time
(Scheme 5).
I
ethynylferrocene
6 mol% PPh3
iPr2NH, THF
I
Fc
Fc
Br
5 (81%)
Si(Me)3
(Me)3Si
F
S
Na2S 9H2O
S
Fc
S
Na2S 9H2O
30 mol%CuI,
40 mol% TMEDA
DMF, 110 °C
2 h
S
30 mol%CuI,
60 mol%TMEDA
DMF 110°C
2 h
F
F
S
2 (55%)
Si(Me)3
Scheme 7. Synthesis of 2.
BTT (78%)
3c
Scheme 5. Synthesis of BTT.
The UV−Vis electronic spectra of complexes 1 and 2 between
230 and 700 nm are reported in Figure 1. The spectra show
similarities, but the bands of 2 are much less intense. Thus, the
electronic structure and the spectral properties of both
compounds were theoretically investigated.
The preparation of the 2,5,8-tris(ferrocenyl)benzo[1,2-b;3,4-
b’;5,6-b’’]trithiophene (1) (Scheme 6) was achieved starting from
1,3,5-tribromo-2,4,6-tris(ethynylferrocene)benzene (4a) obtain-
ed through Sonagashira-coupling of 1,3,5-tribromo-2,4,6-triiodo-
benzene with ethynylferrocene (Scheme 4). The threefold
thioannulation of 4a proceeded in the presence of CuI and
TMEDA at 110 °C for 2 h. After appropriate work-up, compound
1 was isolated without further purifications in almost quantitative
yield as a red compound (Table 1).
6
5
4
3
2
Conversely,
the
reaction
of
1,3,5-trichloro-2,4,6-
ε
1
tris(ethynylferrocene)benzene31 (4b) in the same reaction
conditions, and also with DMSO/CuI or NMP, gave 1 in poor
yield (Table 1, entry g and h, respectively).
0
300 400 500 600 700
λ/ (nm)
Figure 1. UV-vis spectra of 0.4 mM 1 (red line) and 0.2 mM 2 (blue line) in
CH2Cl2. The absorbance was corrected for compound concentration and
optical path.
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