Oxidative transformation to naphthodithiophene and thia[7]helicenes by intramolecular scholl reaction of substituted 1,2-bis(2-thienyl)benzene precursors

Article abstract of DOI:10.1021/jo401807x

We present here a strategy to synthesize a variety of substituted naphthodithiophene building blocks through DDQ/acid-mediated oxidative cyclizations. The versatility of the Scholl reaction using the DDQ/acid system was demonstrated by the preparation of a novel substituted tetrathia[7]helicene where three new C-C bonds were formed in a one-pot procedure. The new DDQ/acid method was compared to the known strategies such as FeCl₃ oxidation and oxidative photocyclization. By protecting the 1,2-bis(2-thienyl)benzene precursors, it is possible to direct the intermediates to controlled cyclization and effectively suppressing the polymerization. The highly reactive α-position of the terminal thiophenes can allow for further functionalization. The efficient preparation of a variety of naphthodithiophene building blocks, the extension to a nonphotochemical synthesis of [n]helicenes, and the ease of isolation of the products are arguments for the use of DDQ/acid system for this Scholl reaction.

Full text of DOI:10.1021/jo401807x

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pubs.acs.org/joc
Oxidative Transformation to Naphthodithiophene and
Thia[7]helicenes by Intramolecular Scholl Reaction of Substituted
1,2-Bis(2-thienyl)benzene Precursors
Deepali Waghray, Christiaan de Vet, Konstantina Karypidou, and Wim Dehaen*
Molecular Design and Synthesis, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven,
Belgium
S
* Supporting Information
ABSTRACT: We present here a strategy to synthesize a
variety of substituted naphthodithiophene building blocks
through DDQ/acid-mediated oxidative cyclizations. The
versatility of the Scholl reaction using the DDQ/acid system
was demonstrated by the preparation of a novel substituted
tetrathia[7]helicene where three new C−C bonds were formed
in a one-pot procedure. The new DDQ/acid method was
compared to the known strategies such as FeCl3 oxidation and
oxidative photocyclization. By protecting the 1,2-bis(2-
thienyl)benzene precursors, it is possible to direct the intermediates to controlled cyclization and effectively suppressing the
polymerization. The highly reactive α-position of the terminal thiophenes can allow for further functionalization. The efficient
preparation of a variety of naphthodithiophene building blocks, the extension to a nonphotochemical synthesis of [n]helicenes,
and the ease of isolation of the products are arguments for the use of DDQ/acid system for this Scholl reaction.
thienyl oxidative cyclization using FeCl3.2a The highly reactive
α-position of thiophene was selectively substituted, thus
protecting the thiophene from polymerization. Later in 2006,
INTRODUCTION
■
During the past several decades, thiophene-based materials
have attracted significant interest due to their applications in
organic electronics.1 Incorporation of thiophene rings onto a
Pei and co-workers built on that knowledge and synthesized
helical polycyclic thiophene derivatives.10
polycyclic aromatic framework provides aromatic stability while
preserving the desirable physical properties such as high
In recent years, the Scholl reaction11 has gained significant
conductivity.2 Thia[n]helicenes,3 a subclass of heterahelicenes
interest due to its potential in the synthesis of several π-
conjugated materials. This reaction, first reported in 1910, is an
with alternating benzene and thiophene rings, are particularly
important. Heterahelicenes combine the electronic properties
afforded by their extensive π-conjugated system with the
intramolecular oxidative C−C bond formation between two
benzenoid rings to produce a biaryl linkage. The Scholl reaction
(chiro) optical properties associated to their helical structure.4
had been extensively utilized for the synthesis of planar
polycyclic aromatic compounds. This oxidative cyclodehydro-
Thiahelicenes are an extremely interesting class of conjugated
molecules being investigated for optoelectronic application.5
genation reaction can be accomplished by using a variety of
oxidants such as FeCl3 in DCM, CuCl2 or Cu(OTf)2 and AlCl3,
The potential of helicenes lies in developing strategies that
MoCl5 in DCM, Pd(OAc)4/BF3·Et2O MeCN, and quinones in
provide efficient access to a variety of helical frameworks,
the presence of strong acid.12 Rathore et al.13 have shown that
focusing on efficient routes that are amenable to scale-up and
also provide flexibility for functionalization.
dichlorodicyano-p-benzoquinone (DDQ, Ered = +0.60 V vs
SCE) in the presence of acid can readily oxidize a variety of
Arene cyclizations induced by chemical oxidants have
aromatic donors with an oxidation potential as high as ∼1.7 V
successfully led to the construction of aromatized ring
structures,6 but the study of oxidative carbon−carbon bond
vs SCE to the corresponding cation radicals and can be
employed for Scholl reactions.
Herein we report a detailed insight on the oxidative
cyclization of 1,2-bis(2-thienyl)benzene precursors. The α-
formation as a means of constructing discrete thiophene-based
materials has been relatively underexplored. In 1996, Larsen et
al. synthesized heterahelicenes by oxidative coupling of
stilbene-type precursors using FeCl3.7 Prior to the work of
positions of the terminal thiophenes were substituted with a
Swager et al.,8 the highly reactive nature of oxidized thiophene
variety of substituents ranging from electron-donating to
electron withdrawing groups, thus providing an opportunity
moieties toward polymerization has limited the development of
to investigate the influence of protecting groups on the
thiophene-centered oxidative cyclizations. Photochemical Mal-
lory-type cyclization9 using iodine as an oxidant was an
important variant of this strategy but has a variety of drawbacks
associated with it. Swager et al. described in detail the thienyl−
Received: August 16, 2013
Published: October 22, 2013
© 2013 American Chemical Society
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Table 1. Synthesis of α-Substituted Dithienylbenzenes 2a−d
entry
substituent (X)
reagents and conditions
yield (%)
1
2
3
4
Br
1, NBS, CHCl3/AcOH (8:2), 0 °C, 2 h
1, n-BuLi, MeI, THF −78 °C, 3 h
1, n-BuLi, TMSCl, THF −78 °C, 3 h
1, n-BuLi, DMF, THF −78 °C, 3 h
2a, 93
2b, 84
2c, 93
2d, 96
Me
TMS
CHO
oxidative cyclization. The most suitable reaction conditions
were employed to further examine its applicability in the
synthesis of thia[n]helicenes.
Scheme 2. Overview of Oxidative Cyclization toward
Naphthodithiophene Building Block
RESULTS AND DISCUSSION
■
Our approach makes use of 1,2-bis(octyloxy)-4,5-bis(2-
thienyl)benzene (1)14 as a versatile building block for further
substitution, study of substituent effect on the oxidative
cyclization, and finally, synthesis of thia[7]helicene. The choice
of the substituents was made, from electron-donating to
electron-withdrawing groups (OMe, Me, TMS, Br, CHO),
which could provide a possibility of postcyclization function-
alization. Substitution of building block 1 with bromine was
done using NBS and a CHCl3/AcOH mixture as the solvent at
0 °C to furnish the dibrominated compound 2a in 93% yield.
Further substitution with the methyl, trimethylsilyl, or formyl
group was accomplished by lithiation of compound 1 with n-
BuLi and THF as solvent at −78 °C and subsequently treating
the lithiated thiophenes with methyl iodide, trimethylsilyl
chloride, and dimethylformamide to furnish the disubstituted
compounds 2b, 2c, and 2d in 84%, 93%, and 96% yields,
respectively (Table 1).
Though the reaction was efficient, scale-up, was limited. To
overcome the drawbacks associated with the photochemical
cyclization we opted for oxidative cyclization. As a starting
point we used the unsubstituted compound 1 to further study
the effect of different oxidants. As described by Swager et al.,
the use of FeCl3 as an oxidant on the unsubstituted thiophene
moieties always led to polymerization. Building on that
knowledge, we wanted to compare the use of the DDQ/acid
system on the unsubstituted benzodithiophene building block
1. Complete polymerization was observed in the reaction using
DDQ/MeSO3H at 0 °C for 1 h. Changing the acid to BF3·EtO2
did not have a significant effect on the reaction outcome (Table
2).
Substitution with the methoxy group on the building 1 was
not straightforward; hence, compound 2e was prepared by
Stille coupling of 1,2-dibromo-4,5-bis(octyloxy)benzene with
the in situ prepared stannyl derivative of 2-methoxythiophene,
using 10 mol % of Pd(PPh3)4 and toluene as the solvent
(Scheme 1). Compound 2e was obtained in 95% yield.
Table 2. Oxidative Cyclization on Unsubstituted
Benzodithiophene Building Block
Scheme 1. Synthesis of Dimethoxy-Substituted
Dithienylbenzene 2e
entry
reagents and conditions
1, hν, I2, toluene, 15 h
yield of 3 (%)
a
1
82
b
c
2
3
4
1, FeCl3 , CH2Cl2/CH3NO2 , 0 °C, 2 h
polymerization
polymerization
d
e
1, DDQ /MeSO3H , CH2Cl2, 0 °C, 1 h
d
f
1, DDQ /BF3·Et2O , CH2Cl2, 0 °C, 1 h
polymerization
a
b
c
d
Previous work. 6 equiv FeCl3. CH2Cl2/CH3NO2 10:1. 1.0 equiv of
DDQ. CH2Cl2/acid 9:1. 10 equiv of Lewis acid.
e
f
A bromine-substituted benzodithiophene derivative was
employed next in the Scholl reaction. Oxidative cyclization of
compound 2a was performed in accordance with the procedure
of Pei et al. using FeCl3 (6 equiv per C−C bond) and CH2Cl2
as the solvent at room temperature. Complete decomposition
was observed owing to the large excess of FeCl3 used. After a
series of experiments, oxidative cyclization of compound 2a
with 2.2 equiv of FeCl3 and CH2Cl2/CH3NO2 (10:1) as solvent
at 0 °C for 3 h gave the cyclized compound 3a in 64% yield and
a small amount (8%) of debrominated byproduct (charac-
The dithienylbenzene derivatives 1 and 2a−e were subjected
to oxidative cyclization to investigate the influence of the α-
substituents. The reagents used in the oxidative cyclization
reactions are FeCl3 and DDQ/acid as described by Rathore et
al.13 and the chloranil/acid system (Scheme 2). Oxidative
photocyclization was also carried out for comparison.
In our previous work,14 we synthesized the unsubstituted
naphthodithiophene building block via oxidative photochemical
cyclization of compound 1 to give compound 3 in 82% yield.
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terized by NMR, see the Supporting Information for 3a′). A
reaction performed in the absence of CH3NO2 also furnished
compound 3a in 50% yield. The use of Fe(acac)3 as an
alternative source of iron(III) did not lead to any product
formation. We further investigated the quinone/acid systems.
Oxidative cyclization of compound 2a using DDQ/MeSO3H in
CH2Cl2 (CH2Cl2/acid 9:1) at 0 °C furnished compound 3a in
65% yield, which was very close to the yield obtained from the
FeCl3 reaction. However, at gram scale using a slight excess of
DDQ (1.5 equiv)/MeSO3H compound 3a was obtained in 85%
yield. This drastic increase in yield can be attributed to the
explanation given by Rathore et al.13 that if the oxidation
potential of the final product to its radical cation is lower in
value than the actual oxidation potential of the starting material
further dehydrogenation can be arrested. They proposed to
solve this by adding 0.5 equiv of DDQ more. The use of a
Lewis acid like BF3·OEt2 proved to be more efficient compared
to MeSO3H. The yield of compound 3a increased to 93%. The
other system employed in the study was chloranil/acid. Using
similar conditions like DDQ, the chloranil/MeSO3H combina-
tion gave the desired product in 50% yield. The chloranil/BF3·
OEt2 combination furnished the compound 3a in good yields
(74%) although the reaction time was substantially increased
from 45 min to 20 h (Table 3).
photochemical cyclization proved to be very efficient, and
compound 3b was obtained in 80% (Table 4).
Table 4. Oxidative Cyclization Reaction of
Dimethyldithienylbenzene
entry
reagent and conditions
yield of 3b (%)
1
2
3
4
5
6
2b, FeCl3, CH2Cl2/CH3NO2, 0 °C, 3 h
2b, DDQ/BF3·OEt2, CH2Cl2, 0 °C, 20 min
2b, DDQ/MeSO3H, CH2Cl2, 0 °C, 3 h
2b, DDQ/CH3COOH, CH2Cl2, 0 °C, 1 h
2b, DDQ, CH2Cl2, 0 °C, 2 h
46
16
polymerization
polymerization
No conversion
80%
2b, hυ, I2, toluene, 24 h
Trimethylsilyl-protected benzodithiophene derivative 2c was
subjected to oxidative cyclization under the optimized
conditions. The trimethylsilyl group was cleaved under the
given reaction conditions furnishing the deprotected compound
1, but no trace of product was observed. When an additional
2.2 equiv of FeCl3 or 1.5 equiv of DDQ was added, it led to
complete polymerization of the starting material. Photo-
chemical cyclization of 2c led to the formation of the desired
product 3c in 80% yield with no trace of deprotection (Table
5).
Table 5. Oxidative Cyclization of
Bis(trimethylsilyl)dithienylbenzene
Table 3. Oxidative Cyclization Reactions of Dibromo
Substituted Benzodithiophene 2a
entry
1
reagents and conditions
yield of 3c (%)
entry
reagents and conditions
yield of 3a (%)
b
2c, FeCl3, CH2Cl2/CH3NO2,
0 °C, 30 min
2c, DDQ/BF3·OEt2, CH2Cl2,
0 °C, 20 min
deprotection and polymerization
1
2
2a, FeCl3 , CH2Cl2/CH3NO2, rt, 1 h
dec
c
d
2a, FeCl3 , CH2Cl2/CH3NO2 , 0 °C, 3 h
64
a
c
d
2
3
deprotection and polymerization
80
3
2a, FeCl3 , CH2Cl2/CH3NO2 , 0 °C, 3 h
52
c
4
5
6
2a, FeCl3 , CH2Cl2, 0 °C, 3 h
50
2c, hν, I2, toluene, 24 h
2a, Fe(acac)3, CH2Cl2/CH3NO2, 40 °C, 24 h
no conversion
g
2a, DDQ/MeSO3H , CH2Cl2, 0 °C, 20 min
65
85
93
50
a
e
g
7
2a, DDQ /MeSO3H , CH2Cl2, 0 °C, 20 min
Methoxy-protected benzodithiophene derivative 2e was
subjected to oxidative cyclization using FeCl3 and CH2Cl2/
CH3NO2 combination at 0 °C for 5 min. Product formation
was observed (by TLC), but this product immediately
polymerized under the given reaction conditions. The DDQ/
acid system also did not lead to product formation. The less
reactive chloranil/BF3·OEt2 system was employed and gave
compound 3e in 8% yield and 16% of dimer (confirmed by
NMR; see the Supporting Information for 3e′) was formed.
Chloranil with a catalytic amount of Lewis acid proved to be
more efficient and gave compound 3e in 55% yield and only
traces of dimer. In the presence of DDQ or chloranil alone, no
conversion of starting material 2e was observed. In comparison,
photochemical cyclization gave compound 3e in 26% yield.
This leads us to conclude that although the oxidative cyclization
is fast with methoxy-substituted dithienylbenzenes, the product
formed is very susceptible to oxidation, which may cause
further polymerization in the given reaction conditions (Table
6).
When the methodology for electron-withdrawing dialdehyde
derivative was expanded, compound 2d was subjected to
oxidative cyclization using an FeCl3 and DDQ/acid system.
The presence of the aldehyde had a significant effect, as there
was no conversion observed. This could be attributed to the
fact that the oxidation potential of the compound 2d exceeds
1.7 V vs SCE as described by Rathore et al.13 The
photochemical cyclization also did not lead to any product
formation (Table 7).
e
f
8
2a, DDQ /BF3·OEt2 , CH2Cl2, 0 °C, 20 min
e
g
9
2a, chloranil /MeSO3H , CH2Cl2, 0 °C, 45 min
e
f
10
2a, chloranil / BF3·OEt2 , CH2Cl2, 0 °C, 20 h
74
a
b
c
Reaction on gram scale. 6 equiv of FeCl3 per C−C bond. 2.2 equiv
d
e
of FeCl3 per C−C bond. CH2Cl2/CH3NO2 10:1. 1.5 equiv of DDQ
or chloranil. 10 equiv of Lewis acid. CH2Cl2/acid 9:1 in entries 6, 7,
and 9.
f
g
To conclude, on the basis of the series of experiments (Table
2), FeCl3, CH2Cl2/CH3NO2, and DDQ/BF3·OEt2 combination
proved to be very efficient for the oxidative cyclization of the
compound 2a. These two methods were extended to other
substituted benzodithiophene derivatives.
Thus, compound 2b was subjected to oxidative cyclization
using FeCl3 and CH2Cl2/CH3NO2 at 0 °C for 3 h. Compound
3b was obtained in a relatively poor 46% yield which was rather
unexpected, as the electron-donating methyl group in principle
should have a stabilizing effect on the intermediates. The
DDQ/acid system gave unexpected results as well. Only 16% of
compound 3b was obtained when DDQ/BF3·OEt2 was used.
Variation of the acid from MeSO3H to acetic acid did not have
a significant effect on the reaction, and complete polymer-
ization was observed. The byproducts isolated were unidenti-
fied mixtures, and this led us to conclude that highly reactive
species, possibly radical in nature, were formed and the methyl
group could not be used as an efficient protecting group on
thiophenes in oxidative cyclization process. Nevertheless,
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Table 6. Oxidative Cyclization of Methoxy-Substituted Dithienylbenzene
entry
reagent and conditions
yield of 3e (%)
1
2
3
4
5
6
2e, FeCl3, CH2Cl2/CH3NO2, 0 °C, 5 min
2e, DDQ/BF3·OEt2, CH2Cl2, 0 °C, 5 min
2e, DDQ/MeSO3H, CH2Cl2, −10 °C, 5 min
2e, chloranil/BF3·OEt2, CH2Cl2, 0 °C, 5 min
2e, chloranil/BF3·OEt2 (cat), CH2Cl2, −10 °C, 15 min
2e, hν, I2, toluene, 15 h
polymerization
no product trace
no product trace
8 and 16 dimer
55
26
by the preparation of a novel hexaalkoxy-substituted
tetrathia[7]helicene where three new C−C bonds were formed
in a one-pot procedure. Palladium-mediated coupling between
1,2-dibromo-4,5-bis(octyloxy)benzene and in situ prepared
monostannyl derivative of compound 1 gave the highly
conjugated helicene precursor 4 in one step. Compound 1
was monolithiated using n-BuLi and then treated with
tributylstannyl chloride to obtain the monostannyl derivative
in quantitative yield. Since protodestannylation was observed
during chromatographic purification, the stannyl derivative was
used without further purification. Stille coupling of the in situ
prepared monostannyl derivative of 1 and 1,2-dibromo-4,5-
bis(octyloxy)benzene using 10 mol % of Pd(PPh3)4 and
toluene as the solvent gave the compound 4 in 63% yield.
Further bromination of 4 with NBS using CHCl3/AcOH as
Table 7. Oxidative Cyclization of Aldehyde-Protected
Benzodithiophene
entry
reagents and conditions
yield of 3d (%)
1
2
3
2d, FeCl3, CH2Cl2/CH3NO2, 0 °C to rt, 30 h
2d, DDQ/BF3·OEt2, CH2Cl2, 0 °C to rt, 30 h
2d, hν, I2, toluene, 72 h
no conversion
no conversion
no conversion
From these sets of experiments we could conclude that
oxidative conditions may replace the photochemical reaction on
a large scale, although the effectiveness of the reagents used in
the Scholl reaction is dependent on the substituents present on
the α-position of thiophenes.
Expanding on this knowledge, the versatility of the Scholl
reaction using the DDQ/acid system was further demonstrated
Scheme 3. Stille Coupling and Oxidative Cyclization toward the Synthesis of Benzo-Fused Tetrathia[7]helicenes
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saturated NaHCO3 solution, H2O, and brine, dried over MgSO4, and
concentrated. Purification by column chromatography using CH2Cl2/
petroleum ether (20:80) gave compound 2a (2.45 g, 93%) as a yellow
solid: mp 51−53 °C; MS (EI) m/z 656.4, [MH]+; HRMS (EI) calcd
solvent at 0 °C gave the dibrominated helicene precursor 5 in
80% yield. Compound 5 was subjected to oxidative cyclization
under the optimized conditions used for brominated
benzodithiophene 2a using DDQ/BF3·OEt2 (1.5 equiv per
C−C bond, 10 equiv of Lewis acid), CH2Cl2 as the solvent at 0
°C for 1.5 h. The desired tetrathia[7]helicene was obtained in
70% yield after purification. The reaction was straightforward
and confirms the increasing reactivity after each subsequent
cyclization. This is in agreement with the studies of Rempala et
al.15 that report that the energy barrier becomes lower with
every subsequent intramolecular cyclization; i.e., each cycliza-
tion is faster than the preceding one. The driving force given for
this phenomenon was the increasing electron delocalization
(Scheme 3).
For comparison, oxidative cyclization using an FeCl3/
CH2Cl2/CH3NO2 system (2.2 equiv of FeCl3 per C−C
bond) furnished the helicene in 64% yield. Photochemical
cyclization on compound 4 using UV light and iodine as the
oxidant furnished the helicene (6a) in 60% yield. Though the
yield was comparable, the reaction time was 72 h, and high
dilution conditions were required. The DDQ/acid system
proved to be superior in terms of reaction time and purity.
Problems such as chlorination can be overcome by using the
DDQ/acid system.
1
for C30H40Br2O2S2 656.0816, found m/z = 657.0894 [M + H]+; H
NMR (300 MHz, CDCl3) δ 6.91 (d, J = 3.8 Hz, 2H), 6.89 (s, 2H),
6.62 (d, J = 3.8 Hz, 2H), 4.02 (t, J = 6.6 Hz, 4H), 1.85−1.78 (m, 4H),
1.47 (m, 4H), 1.29 (m, 16H), 0.88 (s, 6H); 13C NMR (75 MHz,
CDCl3) δ 149.1, 144.2, 129.9, 127.2, 125.5, 115.9, 112.1, 69.5, 31.9,
29.5, 29.4, 29.3, 26.1, 22.8, 14.3.
Synthesis of 5,5′-(4,5-Bis(octyloxy)-1,2-phenylene)bis(2-methyl-
thiophene) (2b). To a solution of 1,2-bis(octyloxy)-4,5-bis(2-thienyl)-
benzene (1) (1.0 g, 2.0 mmol) in dry THF (40 mL), n-BuLi (2.4 mL,
2.5 M, 6 mmol) was added dropwise at −78 °C under argon, and the
reaction mixture was allowed to warm to room temperature. After 1.5
h, the reaction was cooled to −78 °C, and MeI (1.25 mL, 20.0 mmol)
was added. The solution was quenched with a saturated NH4Cl
solution at 0 °C, extracted with EtOAc, dried over MgSO4, and
concentrated. Purification by column chromatography using CH2Cl2/
petroleum ether (30:70) gave compound 2b (0.887 g, 84%) as an off-
white solid: mp 26−28 °C; MS (EI) m/z 526, [MH]+; HRMS (EI)
calcd for C32H46O2S2 526.2939, found m/z = 527.3011 [M + H]+; 1H
NMR (300 MHz, CDCl3) δ 6.94 (s, 2H), 6.64 (d, J = 3.4 Hz, 2H),
6.59 (dd, J1 = 3.6 Hz, J2 = 3.4 Hz, 2H), 4.01 (t, J = 6.6 Hz, 4H), 2.45
(s, 6H), 1.87−1.77 (m, 4H), 1.47−1.5 (m, 4H), 1.28−1.44 (m 16H),
0.89 (s, 6H); 13C NMR (75 MHz, CDCl3) δ 148.4, 140.8, 139.9,
126.5, 125.1, 116.2, 69.4, 31.9, 29.5, 29.4, 29.3, 26.1, 22.8, 15.4, 14.2.
Synthesis of (5,5′-(4,5-Bis(octyloxy)-1,2-phenylene)bis(thiophene-
5,2 diyl))bis(trimethylsilane) (2c). To a solution of 1,2-bis(octyloxy)-
4,5-bis(2-thienyl)benzene (1) (0.500 g, 1.0 mmol) in THF (20 mL)
was added n-BuLi (1.2 mL, 2.5 M, 3.00 mmol) dropwise at −78 °C
under argon, and the reaction mixture was allowed to warm to room
temperature. After 1 h, trimethylsilyl chloride was added at −78 °C.
The reaction mixture was quenched with a saturated solution of
NH4Cl after 30 min and then allowed to come to room temperature.
The mixture was extracted with EtOAc, dried over MgSO4, and
concentrated. The crude product was purified by column chromatog-
raphy using CH2Cl2/petroleum ether (20:80) to give compound 2c
(0.596 g, 93%) as a yellow liquid: MS (EI) m/z 643 [MH]+; HRMS
(EI) calcd for C36H58O2S2Si2 642.3417, found m/z = 643.3487 [M +
H]; 1H NMR (300 MHz, CDCl3) δ 7.05 (d, J = 3.0 Hz, 2H), 6.99 (s,
2H), 6.84 (d, J = 3.0 Hz, 2H), 4.03 (t, J = 6.4 Hz, 4H), 1.85−1.81 (m,
4H), 1.41−1.55 (m, 4H), 1.18−1.40 (m, 16H), 0.89 (s, 6H), 0.28 (s,
18H); 13C NMR (75 MHz, CDCl3) δ 148.6, 140.4, 133.9, 128.0,
126.4, 116.0, 69.4, 31.9, 29.5, 29.4, 29.4, 26.1, 22.8, 14.2, 0.1.
CONCLUSION
■
The optimization of the cyclization reaction leading to the
(substituted) naphthodithiophene building block has proved
that the Scholl reaction using DDQ/acid can be a good
candidate to replace the photochemical cyclization on larger
scale or as an alternative to the known FeCl3/CH3NO2
method. Furthermore, the substituent on the α-position
seems to have a significant influence on the outcome of the
oxidative cyclization. The versatility of the DDQ/acid system
was shown in the synthesis of tetrathia[7]helicenes.
EXPERIMENTAL SECTION
■
General Experimental Methods. NMR spectra were acquired on
commercial instruments (300 and 400 MHz), and chemical shifts (δ)
are reported in parts per million (ppm) referenced to tetramethylsilane
(1H) or the internal (NMR) solvent signal (13C). Mass spectra were
acquired (EI, 70 eV ionization energy). Exact mass measurements
were performed in the EI mode at a resolution of 10000 and also on a
quadrupole orthogonal acceleration time-of-flight mass spectrometer.
Samples were infused at 3 μL/min, and spectra were obtained in
positive (or negative) ionization mode with a resolution of 15000
(fwhm) using leucine enkephalin to lock the mass. Melting points (not
corrected) were determined using a Reichert Thermovar apparatus.
For column chromatography, 70−230 mesh silica 60 was used as the
stationary phase. Chemicals received from commercial sources were
used without further purification. K2CO3 (anhydrous, granulated) was
finely ground (with mortar and pestle) prior to use. All solvents were
used as received from commercial sources and not explicitly dried
prior to use (H2O ≤ 0.1%). All photochemical reactions were
performed in a photochemical reactor equipped with interchangeable
light sources (250, 300, 350 nm lamps).
Synthesis of 5,5′-(4,5-Bis(octyloxy)-1,2-phenylene)bis(2-carbalde-
hydethiophene) (2d). To a solution of 1,2-bis(octyloxy)-4,5-bis(2-
thienyl)benzene (1) (1.0 g, 2.0 mmol) in dry THF (40 mL) was added
n-BuLi (2.4 mL, 2.5 M, 6 mmol) dropwise at −78 °C under argon, and
the reaction mixture was allowed to warm to room temperature. After
1.5 h, the reaction was cooled to −78 °C and DMF was added (1.54
mL, 20.0 mmol). The reaction was quenched with saturated solution
of NH4Cl, extracted with EtOAc, dried over MgSO4, and concentrated.
Column chromatography using EtOAc/petroleum ether (20:80) gave
compound 2d (1.063 g, 96%), as a yellow low melting solid: mp 36−
39 °C; MS (EI) m/z 555 [MH]+; HRMS (EI) calcd for C32H42O4S2
1
554.2525, found m/z = 555.2599 [M + H]; H NMR (300 MHz,
CDCl3) δ 9.84 (s, 2H), 7.59 (d, J = 3.8 Hz, 2H), 6.98 (s, 2H), 6.87 (d,
J = 4.0 Hz, 2H), 4.06 (t, J = 6.4 Hz, 4H), 1.87−1.83 (m, 4H), 1.51−
1.47 (m, 4H), 1.46−1.31 (m, 16H), 0.89 (s, 6H). 13C NMR (75 MHz,
CDCl3) δ 182.9, 152.6, 149.9, 136.6, 128.4, 125.1, 115.8, 69.6, 31.9,
29.4, 29.4, 29.2, 26.1, 22.8, 14.2.
Synthesis of 5,5′-(4,5-bis(octyloxy)-1,2-phenylene)bis(2-methox-
ythiophene) (2e). To a solution of 2-methoxythiophene (0.600 g, 5.20
mmol) in dry THF (30 mL) was added n-BuLi (2.52 mL, 6.31 mmol,
2.5 M in hexanes) at 0 °C under argon, and the mixture was stirred for
1 h. To this was added tributyltin(IV) chloride (2.05 mL, 6.31 mmol)
at 0 °C. The reaction mixture was stirred at this temperature for 1 h.
The mixture was filtered through a plug of silica gel to remove
inorganic salts. The filtrate was concentrated to give the monostannyl
Experimental and Characterization Data. Synthesis of 1,2-
Bis(octyloxy)-4,5-bis(2-thienyl)benzene (1). This compound has been
prepared according previously reported literature procedure.14 Ma-
1
terial identity was confirmed by mp, MS, and H and 13C NMR.
Synthesis of 5,5′-(4,5-Bis(octyloxy)-1,2-phenylene)bis(2-bromo-
thiophene) (2a). To a solution of 1,2-bis(octyloxy)-4,5-bis(2-thienyl)-
benzene (1) (2.0 g, 4.0 mmol) in CHCl3/AcOH (135 mL, 8:2) was
added NBS (1.57 g, 8.83 mmol) at 0 °C. The reaction mixture was
allowed to stir for an additional 2 h at room temperature. The reaction
was quenched with NaHCO3 and subsequently washed with a
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derivative (02.12 g, quantitative) as a brown oil. This compound was
used immediately in the following reaction without further
purification. To a solution of monostannyl compound (2.12 g, 5.07
mmol) and 1,2-dibromo-4,5-bis(octyloxy) benzene (1.00 g, 2.03
mmol) in toluene (35 mL) was added Pd(PPh3)4 (0.242 g, 10 mol %)
under argon, and the mixture was refluxed for 12 h. Toluene was
removed under reduced pressure, and the crude product was purified
by column chromatography using CH2Cl2/petroleum ether (30:70) as
eluent to give compound 2e (1.08 g, 95%) as a off-white solid: mp
30−32 °C; MS (EI) m/z 558 [MH]+; HRMS (EI) calcd for
(0.518 g, 52%) was obtained as pale yellow solid. Material identity was
1
confirmed by H NMR and 13C NMR.
Synthesis of 2,5-Dimethyl-8,9-bis(octyloxy)naphtho[1,2-b:4,3-b′]-
dithiophene (3b). Synthesis according to general procedure of
oxidative cyclization. FeCl3: Compound 2b (0.155 g, 0.29 mmol),
FeCl3 (105 mg, 0.67 mmol), MeNO2 (3 mL), CH2Cl2 (30 mL).
Compound 3b (0.072 g, 46%) was obtained as a pale white solid: mp
114−118 °C; MS (EI) m/z 524 [MH]+; HRMS (EI) calcd for
1
C32H44O2S2 524.2783, found m/z = 525.2853 [M + H]; H NMR
(300 MHz, CDCl3) δ 7.30 (s, 2H), 7.28 (s, 2H), 4.15 (t, J = 6.6 Hz,
4H), 2.68 (s, 6H), 1.94−1.89 (m, 4H), 1.47−1.56 (m, 4H), 1.21−1.45
(m, 16H), 0.89 (s, 6H); 13C NMR (75 MHz, CDCl3) δ 149.1, 138.7,
133.4, 132.2, 121.4, 121.1, 106.3, 69.4, 31.9, 29.5, 29.4, 29.3, 26.2, 22.8,
16.2, 14.2.
1
C32H45O4S2 558.2838, found m/z = 558.2834; H NMR (300 MHz,
CDCl3) δ 6.90 (s, 2H), 6.51 (d, J = 3.9 Hz, 2H), 6.06 (d, J = 3.9 Hz,
2H), 4.01 (t, J = 6.4 Hz, 4H), 3.85 (s, 6H), 1.84−1.77 (m, 4H), 1.48−
1.43 (m, 4H), 1.41−1.28 (m, 16H), 0.94−0.88 (m, 6H); 13C NMR
(75 MHz, CDCl3) δ 166.7, 148.4, 129.0, 126.5, 124.1, 116.1, 103.6,
69.5, 60.2, 31.9, 29.5, 29.4, 29.3, 26.1, 22.8, 14.2.
General Experimental Procedures for Thienyl−Thienyl
Oxidative Cyclization. 1. Oxidative Cyclization Using FeCl3. 2.2
equiv of anhydrous FeCl3 was dissolved in CH3NO2 and added
dropwise to a solution of the substituted 1,2-bis(2-thienyl)benzene
building blocks in dry CH2Cl2 at 0 °C under argon for 3 or 4 h. After
completion, methanol was added, and the reaction mixture was
concentrated and purified by column chromatography.
2. Oxidative Cyclization Using DDQ/acid. 1.5 equiv of DDQ was
added to a 0.01 M solution of the substituted 1,2-bis(2-thienyl)-
benzene building blocks in dry CH2Cl2/acid (9:1) (or 10 equiv of
Lewis acid) under an argon atmosphere at 0 °C. After completion, the
reaction was quenched with NaHCO3. The organic layer was
separated, washed with additional NaHCO3 and brine, dried over
MgSO4, and concentrated. The crude product was purified by column
chromatography.
3. Photochemical Cyclization. To an argon-purged solution of 1,2-
bis(2-thienyl)benzene building block in toluene (3 mM), I2 was added
followed by a large excess of 2-methyloxirane. The reaction mixture
was irradiated using a photochemical reactor (350 nm). After
completion, the reaction mixture was washed with a saturated solution
of Na2S2O3. The organic layer was separated, washed with brine, dried
over MgSO4, and concentrated. Purification was done using column
chromatography.
Photochemical cyclization: Compound 2b (0.20 g, 0.38 mmol), I2
(0.097 g, 0.38 mmol), propylene oxide (2.2 mL, 38.8 mmol), toluene
(125 mL). Column chromatography using EtOAc/petroleum ether
(5:95) gave compound 3b (0.162 g, 81%) as a pale white solid.
1
Material identity was confirmed by H NMR and 13C NMR.
DDQ/acid system: Compound 2b (0.150 g, 0.285 mmol), DDQ
(0.097 g, 0.427 mmol), BF3·OEt2 (0.75 mL, 2.85 mmol, 48%), CH2Cl2
(30 mL). Column chromatography using EtOAc/petroleum ether
(5:95) gave compound 3b (0.026 g, 16%) as a pale white solid.
1
Material identity was confirmed by HNMR and 13CNMR
Synthesis of (8,9-Bis(octyloxy)naphtho[1,2-b:4,3-b′]dithiophene-
2,5-diyl)bis(trimethylsilane) (3c). Synthesis according to the general
procedure for oxidative cyclization did not give any product 3c.
Photochemical cyclization: Compound 2c (0.100 g, 0.155 mmol), I2
(0.040 g, 0.155 mmol), propylene oxide (1.0 mL, 15.5 mmol), toluene
(52 mL). Column chromatography using CH2Cl2/petroleum ether
(20:80) gave compound 3c (0. 080 g, 80%) as white solid: mp 53−75
°C; MS (EI) m/z 641 [MH]+; HRMS (EI) calcd for C36H56O2S2Si2
1
640.3260, found m/z = 641.3362 [M + H]; H NMR (300 MHz,
CDCl3) δ 7.85 (s, 2H), 7.47 (s, 2H), 4.18 (t, J = 6.6 Hz, 4H), 1.97−
1.89 (m, 4H), 1.35−1.26 (m, 20 H), 0.89−088 (m, 6H); 13C NMR
(75 MHz, CDCl3) δ 149.4, 139.3, 139.3, 133.5, 130.01, 106.5, 69.2,
31.9, 29.5, 29.4, 29.3, 26.2, 22.8, 14.2, 0.1.
Synthesis of 2,5-Dimethoxy-8,9-bis(octyloxy)naphtho[1,2-b:4,3-
b′]dithiophene (3e). Photochemical cyclization: Compound 2e (0.100
g, 0.179 mmol), I2 (0.045 g, 0.179 mmol), propylene oxide (1.0 mL,
17.9 mmol), toluene (60 mL). Column chromatography using
CH2Cl2/petroleum ether (20:80) gave compound 3e (0. 026 g,
26%) as a white solid: mp 125−127 °C; MS (EI) m/z 556 [MH]+;
HRMS (EI) calcd for C32H44O4S2 556.2681, found m/z = 557.2748
[M + H]+; 1H NMR (300 MHz, CDCl3) δ 7.13 (s, 2H), 6.59 (s, 2H),
4.12 (t, J = 6.6 Hz, 4H), 4.04 (s, 6H), 1.93−1.88 (m, 4H), 1.54−1.51
(m, 4H), 1.38−0.91 (m, 16H), 0.89−0.87 (m, 6H); 13C NMR (75
MHz, CDCl3) δ 164.8, 148.8, 129.6, 123.9, 120.9, 105.5, 98.6, 69.3,
60.2, 31.9, 29.5, 29.4, 29.3, 26.2, 22.8, 14.2.
Synthesis of 8,9-Bis(octyloxy)naphtho[1,2-b:4,3-b′]dithiophene
(3). This compound has been prepared according to the previously
reported literature procedure14 starting from 1,2-bis(octyloxy)-4,5-
bis(2-thienyl)benzene. Material identity was confirmed by mp, HRMS,
1
and H and 13C NMR.
Synthesis of 2,5-Dibromo-8,9-bis(octyloxy)naphtho[1,2-b:4,3-b′]-
dithiophene (3a). Synthesis according to general procedure of
oxidative cyclization using the quinone/acid system: Compound 2a
(1.00 g, 1.52 mmol), DDQ (0.517 g, 2.28 mmol), MeSO3H (14 mL),
CH2Cl2 (130 mL). Column chromatography using CH2Cl2/petroleum
ether (20:80) gave compound 3a (0.847 g, 85%) as pale yellow solid:
mp 122−124 °C; MS (EI) m/z 654 [MH]+; HRMS (EI) calcd for
C30H38Br2S2O2 652.0680, found m/z = 652.0772; 1H NMR (300
MHz, CDCl3) δ 7.54 (s, 2H, 3-Th), 7.16 (s, 2H), 4.16 (t, J = 6.6 Hz,
4H), 1.95−1.90 (m, 4H), 1.25 (m, 20H), 0.89 (s, 6H); 13C NMR (75
MHz, CDCl3) δ 149.9, 135.9, 130.7, 125.8, 120.9, 112.7, 105.6, 69.4,
31.9, 29.6, 29.5, 29.2, 26.2, 22.8, 14.3.
Chloranil/Lewis acid system: Compound 2e (0.100 g, 0.179 mmol),
DDQ (0.044 g, 0.179 mmol), BF3·OEt2 (cat), CH2Cl2 (30 mL).
Column chromatography using EtOAc/petroleum ether (5:95) gave
compound 3e (0.055 g, 55%) as a pale white solid. Material identity
1
was confirmed by H NMR and 13C NMR
Synthesis of 5,5′-(4,5-Bis(octyloxy)-1,2-phenylene)bis(2-(4,5-bis-
(octyloxy)-2-(thiophene-2-yl)phenyl)thiophene) (4). To a solution of
1,2-bis(octyloxy)-4,5-bis(2-thienyl)benzene (1) (1.50 g, 3.00 mmol) in
dry THF (80 mL) was added n-BuLi (1.44 mL, 2.5 M, 3.6 mmol)
dropwise at −78 °C under argon and the mixture allowed to stir for 1
h. Tributyltin chloride (1.17 mL, 3.60 mmol) was added at −78 °C,
and the reaction mixture was filtered over a silica plug, The filtrate was
concentrated to give the monostannyl derivative (2.30 g, quantitative)
as a brown oil. This compound was used immediately in the following
reaction without further purification. To a solution of monostannyl
compound (2.30 g, 2.92 mmol) and 1,2-dibromo-4,5-bis(octyloxy)-
benzene (0.575 g, 1.16 mmol) in toluene (50 mL) was added
Pd(PPh3)4 (0.134 g, 10 mol %) under argon, and the mixture was
refluxed for 12 h. Toluene was removed under reduced pressure, and
the crude product was purified by column chromatography using
CH2Cl2/petroleum ether (30:70) as eluent to give compound 4 (1.10
Compound 2a (0.200 g, 0.304 mmol), DDQ (0.103 g, 0.46 mmol),
BF3·OEt2 (0.82 mL, 3.04 mmol, 48%), and CH2Cl2 (30 mL).
Compound 3a (0.185 g, 93%) was obtained as pale yellow solid.
1
Material identity was confirmed by H NMR and 13C NMR
Chloranil/acid: Compound 2a (0.200 g, 0.304 mmol), chloranil
(0.075 g, 0.347 mmol), MeSO3H (3 mL), CH2Cl2 (27 mL).
Compound 3a (0.101 g, 50%) was obtained as pale yellow solid.
1
Material identity confirmed by H NMR and 13C NMR
Chloranil/acid: Compound 2a (0.15 g, 0.228 mmol), chloranil
(0.084 g, 0.347 mmol), BF3·OEt2 (0.60 mL, 2.28 mmol), CH2Cl2 (18
mL). Compound 3a (0.110 g, 74%) was obtained as pale yellow solid.
1
Material identity was confirmed by H NMR and 13C NMR
FeCl3: Compound 2a (1.0 g, 1.52 mmol), FeCl3 (0.544 g, 3.35
mmol), CH3NO2 (17.3 mL), CH2Cl2 (173.3 mL). Compound 3a
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g, 63%) as a yellow solid: mp 51−53 °C; MS (EI) m/z 1326 [MH]+;
HRMS (EI) calcd for C82H118O6S4 1326.7811, found m/z =
AUTHOR INFORMATION
Corresponding Author
*E-mail: wim.dehaen@chem.kuleuven.be.
Notes
■
1
1
2
1326.7794; H NMR (300 MHz, CDCl3) δ 7.08 (dd, J = 3.3 Hz, J
= 4.7 Hz, 2H), 6.97 (s, 2H), 6.96 (s, 2H), 6.93 (s, 2H), 6.64 (d, J = 3.5
Hz, 2H), 6.60 (d, J = 3.5 Hz, 2H), 4.04−3.98 (m, 12H), 1.84−1.79
(m, 12 H), 1.46−1.44 (m, 12), 1.31−1.28 (m, 48H), 0.90−0.85 (m,
18H); 13C NMR (75 MHz, CDCl3) δ 148.7, 148.6, 142.9, 142.8,
126.9, 126.7, 126.5, 126.4, 126.3, 125.3, 116.3, 116.1, 69.5, 31.9, 29.5,
29.4, 29.3, 26.1, 22.8, 14.2.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the FWO (Fund for Scientific Research - Flanders),
the K.U. Leuven, and the Ministerie voor Wetenschapsbeleid
(IAP-IV-27) for continuing financial support. We also thank the
Hercules Foundation of the Flemish Government (Grant No.
20100225-7) for mass spectrometry.
Synthesis of 5,5′-(4,5-Bis(octyloxy)-1,2-phenylene)bis(2-(2-(5-bro-
mothiophene-2-yl)-4,5-bis(octyloxy)phenyl)thiophenes 5. To a
solution of compound 4 (200 mg, 0.15 mmol) in CH2Cl2/AcOH
(1:1, 20 mL), NBS (0.059 g, 0.33 mmol) was added under argon at 0
°C. After 1 h, the reaction was quenched with NaHCO3 and
subsequently washed with a saturated NaHCO3 solution, H2O and
brine, dried over MgSO4, and concentrated. Purification by column
chromatography using CH2Cl2/petroleum ether (35:65) gave
compound 5 (0.180 g, 80%) as a yellow oil: MS (EI) m/z 1485
[MH]+; HRMS (EI) m/z [M + Na]+ calcd for C82H116Br2O6S4Na
1505.5919, found m/z = 1505.5892 [M + Na]+; 1H NMR (600 MHz,
CDCl3) δ 6.95 (s, 2H), s (6.94, 2H), 6.70−6.72 (t, J = 4.1 and 4.0 Hz,
4H), 6.69 (d, J = 3.2 Hz, 2H), 6.60 (d, J = 3.2 Hz, 2H), 4.01 (t, J = 6.4
Hz, 12 H), 1.83−1.81 (m, 12H), 1.41−1.49 (m, 12H), 1.21−1.40 (m,
48H), 0.88 (s, 18H); 13C NMR (75 MHz, CDCl3) δ 148.8, 148.7,
144.7, 143.2, 142.2, 129.6, 127.3, 127.0, 126.9, 126.4, 126.2, 125.6,
116.1, 115.8, 111.8, 69.5, 69.5, 31.9, 29.5, 29.4, 29.3, 26.1, 22.8, 14.2.
Synthesis of 5,6,10,11,15,16-Hexakis(octyloxy)tribenzotetra-
thia[7]helicene (6a). To an argon-purged solution of compound 4
(0.100 g, 0.07 mmol) in toluene (25 mL) was added I2 (0.059 g, 0.23
mmol) followed by a large excess of 2-methyloxirane. The reaction
mixture was irradiated using a photochemical reactor (350 nm). After
completion, the reaction mixture was washed with a saturated solution
of Na2S2O3. The organic layer was separated, washed with brine, dried
over MgSO4, and concentrated. Purification was done using column
chromatography using CH2Cl2/petroleum ether (30:70) as eluent to
give compound 6a (0.060 g, 60%) as a yellow solid: mp 159−161 °C;
MS (EI) m/z 1322 [MH]+; HRMS (EI) calcd for C82H112O6S4
1343.7239 [M + Na]+, found m/z = 1343.7179 [M + Na]+; 1H
NMR (300 MHz, CDCl3) δ 7.63 (s, 2H), 7.58 (s, 2H), 7.51 (s, 2H),
7.00 (d, J = 5.4 Hz, 2H), 6.79 (d, J = 5.4 Hz, 2H), 4.31−4.22 (m,
12H), 2.00−1.96 (m, 12H), 1.59−1.55 (m, 12H), 1.39−1.33 (m,
48H), 0.93−0.89 (m, 18H); 13C NMR (75 MHz, CDCl3) δ 150.1,
149.7, 135.4, 134.7, 133.3, 129.9, 128.2, 126.6, 122.4, 121.6, 121.3,
106.5, 106.1, 106.0, 69.4, 32.0, 29.5, 29.4, 29.3, 26.3, 22.8, 14.2.
Synthesis of 2,19-Dibromo-5,6,10,11,15,16-hexakis(octyloxy)-
tribenzotetrathia[7]helicene (6b). To a solution of compound 5
(0.150 g, 0.101 mmol) in dry CH2Cl2 (20 mL) was added BF3.OEt
(0.27 mL, 48% solution in Et2O, 1.01 mmol dropwise under argon at 0
°C. Subsequently, DDQ (107.7 mg, 0.474 mmol) was added, and the
reaction was allowed to stir for 1.5 h. The reaction was quenched with
a saturated solution of NaHCO3 and subsequently washed with
NaHCO3 and brine. The crude product was dried over MgSO4,
concentrated, and purified by column chromatography using CH2Cl2/
petroleum ether (25:75) to give compound 6b (0.105 g, 70%) as a
yellow solid: mp 232−240 °C; MS (EI) m/z 1480 [MH]+; HRMS
(EI) calcd for C82H110Br2O6S4 1476.5552, found m/z = 1499.5450 [M
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NMR (75 MHz, CDCl3) δ 150.3, 150.2, 149.9, 135.9, 135.3, 133.6,
132.6, 129.9, 128.2, 127.3, 122.3, 121.9, 121.5, 110.1, 106.2, 105.8,
105.6, 69.4, 32.0, 29.6, 29.5, 29.3, 26.3, 26.2, 22.8, 14.2.
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