Synthesis of 2,3-Dihydrothieno[2,3-b]-1,4-dithiine, 2,3-Dihydrothieno-[3,2-b]-1,4-oxathiine, 2,3-Dihydrothieno[2,3-b]-1,4-oxathiine and Their Transformation into Corresponding End-Capped Oligomers

Article abstract of DOI:10.1055/s-2003-41058

Three new heterocyclic parent compounds, 2,3-dihydrothieno[2,3-b][1,4]dithiine (TDT), 2,3-dihydrothieno[3,2-b][1,4]oxathiine (TOT), and 2,3-dihydrothieno[2,3-b][1,4]oxathiine, have been synthesized by acid-catalyzed transformations starting from 3-methoxy

Full text of DOI:10.1055/s-2003-41058

PAPER
2199
Synthesis of 2,3-Dihydrothieno[2,3-b]-1,4-dithiine, 2,3-Dihydrothieno-
[3,2-b]-1,4-oxathiine, 2,3-Dihydrothieno[2,3-b]-1,4-oxathiine and Their
Transformation into Corresponding End-Capped Oligomers1
N
ew End-
o
Capped Thio
n
phenes
a
nd O
a
ligothioph
s
enes Hellberg,* Tommi Remonen,2 Fredrik Allared, Johnny Slätt,3 Mats Svensson4
Organic Chemistry, Dept of Chemistry, Royal Institute of Technology, 100 44 Stockholm, Sweden
Fax +46(8)7912333; E-mail: jhel@kth.se
Received 19 May 2003; revised 14 July 2003
Abstract: Three new heterocyclic parent compounds, 2,3-dihy-
drothieno[2,3-b][1,4]dithiine
(TDT),
2,3-dihydrothieno[3,2-
S
S
S
b][1,4]oxathiine (TOT), and 2,3-dihydrothieno[2,3-b][1,4]oxathi-
ine, have been synthesized by acid-catalyzed transformations start-
ing from 3-methoxythiophene. Two of the new compounds have
been transformed to the corresponding end-capped dimeric, trimer-
ic and tetrameric oligothiophenes. These oligomers show very sta-
ble cationic and dicationic states as judged by cyclic voltammetry,
and their UV-Vis spectra are considerably red-shifted compared to
previously synthesized end-capped oligomers.
S
S
S
n-2
n-2
1
2
S
S
S
S
S
S
S
S
S
S
S
S
O
S
S
S
3
4
Key words: ring-closure, tandem reactions, oligothiophenes, Suzu-
ki coupling
S
S
O
S
O
S
S
S
S
Introduction
5
6
7
Figure 1 The structures of compounds 1–7
During the last decade, organic compounds have contrib-
uted significantly to the progress towards new light-
weight, high-performing materials in several electronic a more stable oligomer, resistant to further polymeriza-
applications, such as light-emitting diodes, superconduc- tion.7
tors and thin-film transistors.5 Polymeric materials have
The blocking terminal substituents can also play a more
been in focus of much research towards technological ap-
active role in building up the electroactive structure. Most
plications, while molecular materials have dominated the
organic superconductors are based on the -donor BEDT-
search for high conductivity and superconductivity. Poly-
mers hold several advantages in processing and synthesis,
but oligomers have become increasingly popular, giving
TTF [bis(ethylenedithio)tetrathiafulvalene, 3], and the
sulfur atoms in the fused dihydrodithiine ring in 3 are re-
sponsible for much of the interstack connections leading
well-defined materials with many of the classical poly-
to a two-dimensional material.8 We therefore decided that
mers’ advantages. In the sense of being model com-
the dihydrodithiino-fused thiophene 5 (TDT), would be a
pounds, oligomers offer the possibility to study the ‘true’
valuable building block for both condensed -donors and
behaviour of longer polymers.6 The oligomers can often
for ‘end-capped’ oligothiophenes. We were also interest-
be strictly purified, to give well-defined compounds,
ed in applying our experience of thioxane-condensed do-
whereas the dispersity and defect distribution for the com-
nor molecules like 4,9 to investigate compounds like 2,3-
monly used conjugated polymers varies significantly
from batch to batch. Optical and electronic properties vary
dihydrothieno[2,3-b][1,4]oxathiine (6) and 2,3-dihy-
drothieno[3,2-b][1,4]oxathiine (7, TOT)10 (Figure 1). Us-
only slightly with chain length above a certain critical
ing these parent compounds of surprisingly rare
number of repeating units. In the field of oligomer re-
heterocyclic systems,11 we should be able to synthesize
search, oligothiophenes 1 (Figure 1) have attracted a lot of
interesting new electroactive systems. Furthermore, elec-
interest, since it has been possible to synthesize and char-
troactive terminal groups should also pave the way for al-
acterize a large number of substituted and unsubstituted
ternative conduction pathways, which in turn could make
oligomers. Substitution may be chosen to enhance solubil-
these oligomers more attractive for high-mobility applica-
ity or affect electronic properties. If substituents are
tions such as field-effect transistors.
placed at the terminal positions (as in, e.g. 2), the result is
SYNTHESIS 2003, No. 14, pp 2199–2205
0
2.
1
0
.
2
0
0
3
Advanced online publication: 24.09.2003
DOI: 10.1055/s-2003-41058; Art ID: T04503SS.pdf
© Georg Thieme Verlag Stuttgart · New York

2200
J. Hellberg et al.
PAPER
Results and Discussion
HS
S
NaHSO4
SO2Cl2
58%
SH
10
5
+
Our original approach to synthesize TDT (5) was to react
3-bromothiophene (8) with tert-butyllitihum and then sul-
fur, and treat the resulting sulfide anion with 1-bromo-2-
chloroethane to give the chloroethylthio-substituted
thiophene 9. Reaction with potassium thioacetate fol-
lowed by hydrolysis was anticipated to give the thiol,
which then could be cyclized to 5 after umpolung with
sulfuryl chloride. However, when 9 was treated with thio-
acetate, a mixture was formed, containing at least four dif-
ferent inseparable thiophene products (Scheme 1).
toluene, ∆
67%
S
SH
15
16
Scheme 3
dihydrothieno[2,3-b][1,4]dithiine (5) in up to 58% yield
after distillation.
The reaction mechanism for the transetherification proba-
bly involves rapid and reversible protonation at C-2,15 nu-
cleophilic attack at C-3, followed by elimination of
methanol.
Br
1) t-BuLi; S8
S
KSCOCH3
Complex
mixtures
Cl
We envisaged that an analogous ring-closure of com-
pound 18 would provide an efficient route to TOT (7). In
order to prepare 18, we treated a solution of 3-methox-
ythiophene (10) and mercaptoethanol (11) in dichlo-
romethane with sulfuryl chloride. To our surprise, the
product mixture contained not only the expected product
18 and the chlorinated by-product 17, but also the target
molecule 7 (Scheme 4).
2) BrCH2CH2Cl
S
S
8
9
Scheme 1
An important method for the synthesis of 3-alkoxy and 3-
alkylthio thiophenes is the acid-catalyzed trans-
etherification12 of 3-methoxythiophene (10).13 This
should allow for a very simple synthesis of 5. Transether-
ification of 3-methoxythiophene (10) with mercaptoetha-
nol (11) has been reported to give 12,12b that would
probably provide a fast route to the interesting and un-
known parent compound 7. In practice, however, only the
thioether 13 and small amounts of 14 were isolated14 (cf.
Scheme 2). Moreover, the mass balance indicated only
23% conversion after 3 hours. When the reaction was per-
formed without solvent at 90 °C, with excess mercapto-
ethanol and a catalytic amount of p-toluenesulfonic acid,
13 was formed selectively and rapidly. Higher reaction
temperatures resulted in extensive decomposition of start-
ing materials.
OMe
Cl
OMe
S
O
SO2Cl2
+
+
OH
10 + 11
S
S
S
S
17
18
7
Scheme 4
By adjusting the conditions, we were able to suppress
chlorination and achieve a good yield (61%) of the ring-
closed product TOT (7) by this tandem reaction. Presum-
ably, 7 can be formed directly via ring-closure of the ini-
tially formed sigma-complex from the reaction of 10 with
2-hydroxyethylsulfenyl chloride. However, equilibration
between different tautomers of protonated 18 is likely to
be faster than the cyclization.
SH
OMe
KHSO4
+
toluene, ∆
In order to support further the structure proposed for TOT
(7), we decided to prepare the regioisomer 6. Compound
13 was regioselectively brominated using NBS, and ring-
closure was done by Buchwald’s alkoxylation procedure16
(Scheme 5).
S
OH
11
10
S
O
O
S
SH
OH
S
S
S
S
S
S
NBS, CH2Cl2
94 %
14, 0.4%
12, 0%
13, 15%
OH
OH
Br
O
S
S
Scheme 2
13
19
In order to prepare 5, we treated 10 with an excess of
ethanedithiol (15), and we could easily isolate 16 in 67%
yield after short-path distillation (see Scheme 3). Addi-
tion of sulfuryl chloride to a dilute solution of 16 in dieth-
yl ether effected ring-closure to the target molecule 2,3-
S
CuI, phenanthroline, Cs2CO3
toluene, 100 °C, 29 %
S
6
Scheme 5
Synthesis 2003, No. 14, 2199–2205 © Thieme Stuttgart · New York

PAPER
New End-Capped Thiophenes and Oligothiophenes
2201
Inspired by the results of Cava et al.,19 we decided to apply
the Suzuki methodology for the higher oligomers. The
original Suzuki–Gronowitz procedure20 with sodium car-
bonate in dimethoxyethane–water gave inseparable mix-
tures of different oligomers. Change of solvent and base/
activator proved to be essential.21 Thus, the bis(pinacolbo-
ronic ester)-substituted thiophene 2519 (Scheme 8) was re-
acted with 20 in refluxing dioxane with cesium fluoride,
to give the bis(ethylenedithio) end-capped trimer 28 in an
useful yield (Scheme 9). This procedure was also success-
ful when 25 reacted with 21 to yield the dihydrothioxino
end-capped trimer 29 in good yield.
Oligomer Synthesis
Having the building blocks TDT (5) and TOT (7) in hand,
we started to transmogrify these to the corresponding oli-
gomeric electroactive systems. Monobromination of TDT
(5) occurred smoothly with NBS in a dichloromethane–
acetic acid mixture, giving 20 in excellent yield. Applying
the same procedure on TOT (7) produced the brominated
dihydrothioxino analogue 21 (Scheme 6). Both 20 and 21
showed limited stability in air and light. It is therefore best
to prepare these immediately prior to their further conver-
sions.
S
S
NBS
a) n-BuLi, TMEDA
O
O
Br
Br
S
S
S
S
CH2Cl2/HOAc
98%
S
S
S
B
B
S
O
O
O
S
O
b)
5
20
B
Oi-Pr
25
O
O
24
NBS
S
CH2Cl2/HOAc
92%
O
O
a)
b)
7
21
B
B
S
S
O
S
S
O
Scheme 6
26
27
Scheme 8
The first obvious oligomer is of course the dimer, and
dimerization of a haloaromatic can be achieved by several
methods. Oxidative coupling of the lithiated aromatic is a
versatile method which has been used extensively.17 How-
ever, treatment of 20 with n-butyllithium and then cop-
per(II) chloride resulted only in reduction of 20, giving 5
as the sole product. This demonstrates the low stability of
metalated, chalcogen-rich aromatics, in accord with pre-
vious observations in our group. In the same way, direct
lithiation of 5 with LDA failed, probably due to the rela-
tive inertness of 5 under these conditions. We found in-
stead that the nickel-catalyzed method of Iyoda18 gave a
modest yield of dimer 22 (TDT2T) from 20 (Scheme 7).
Reduction to the thiophene 5 is the major side reaction.
Preparation of 23 turned out to be a more difficult task.
Analogously with the dithia analogue, lithiation of 2
failed both by direct or halogen–metal exchange methods.
A series of procedures for homocoupling were tested and
best yields of the dimer 23 (TOT2T) were obtained with a
bipyridine-ligated nickel catalyst (Scheme 7). This meth-
od gave only a few percent of reduced material.
Pd(PPh3)4,
CsF
S
S
20 + 25
dioxane,
reflux
Ar, 65%
S
S
S
S
S
28 TDT3T
Pd(PPh3)4,
CsF
O
O
21 + 25
20 + 27
dioxane,
reflux
Ar, 85%
S
S
S
S
S
29 TOT3T
Pd(PPh3)4,
CsF
S
S
dioxane,
reflux
S
S
S
S
S
S
S
S
S
S
Ar, 35%
30 TDT4T
Pd(PPh3)4,
CsF
O
O
21 + 27
dioxane,
reflux
Ar, 46%
S
S
S
S
S
31 TOT4T
(Ph3P)2NiBr2
Scheme 9
Br
S
S
S
S
S
S
S
S
S
Zn, TBAI, THF, 29%
22 TDT2T
20
To reach beyond trimers, we decided to synthesize the
bithiophene homologue of 25 by direct dilithiation of
bithiophene 26 with n-butyllithium/TMEDA and subse-
quent reaction with the mixed boronic ester 24. This pro-
cedure gave the bifunctional compound 2722 in good yield
(Scheme 8).
O
O
O
NiCl2, PPh3, bipy
Br
S
S
S
Zn, THF, DMF, 26%
23 TOT2T
21
Scheme 7
The tetramers were now readily obtained with the analo-
gous CsF/Pd(0) procedure, i.e. 20 and 27 gave the dihy-
Synthesis 2003, No. 14, 2199–2205 © Thieme Stuttgart · New York

2202
J. Hellberg et al.
PAPER
drodithiino end-capped tetramer 30. Reaction between 21
and 27 produced the dihydrothioxino end-capped tetramer
31 in modest, but still useful yield (Scheme 9). The tet-
ramers are highly insoluble materials, which simplifies
their isolation.
Table 2 Cyclic Voltammetric Data for Oligomersa
Compound
EC3Tb
E11/2 (V)
0.38
0.32
0.26
0.42
0.40
0.35
0.32
0.32
0.29
E21/2 (V)
0.79 (irr.)
0.66
E (V)
0.41
EC4Tb
0.34
0.29
0.37
0.20
0.16
0.42
0.24
0.14
EC5Tb
0.55
UV-Vis Spectroscopy
TDT2T (22)
TDT3T (28)
TDT4T (30)
TOT2T (23)
TOT3T (29)
TOT4T (31)
0.79
The UV-Vis spectrum was recorded for each oligomer in
chloroform (see Table 1). The spectra are more or less
identical for the ethylenedithio and thioxano oligomer se-
ries. This clearly illustrates that the two different end-cap-
ping moieties have the same influence on the electronic
spectra. The spectra show a considerable red-shift induced
by the heterofusion when compared to end-capped oligo-
mers, ECnT (2), synthesized by Bäuerle et al.7 Compared
to the ECnT series, the values are red-shifted for the trim-
er and tetramer by 36 nm and 17 nm respectively. It seems
that the heterobridge induces a red-shift in the UV-Vis
spectrum, which is equal to a one-unit extension of the oli-
gomer number.
0.60
0.51
0.74c
0.56
0.43
a Measured at 30 °C in CH2Cl2, 0.15 M Bu4NPF6 with 100 mV/s scan
rate. Values are given vs. the ferrocene-ferrocenium couple.
b Literature values for oligomers synthesized by Bäuerle et al.7
c The second half-potential for TOT2T was recorded at 7.0 V/s.
with only -methylsulfanyl group show an even lower
E, the trimer coming down to only 130 mV.23
Table 1 UV/Vis Data for Oligomers
Compound
EC3Ta
max (nm)
375
max (eV)
3.31
Conclusion
EC4Ta
411
432
380
411
428
374
412
431
3.02
2.87
3.27
3.02
2.90
3.32
3.02
2.88
We have synthesized three new parent heterocyclic com-
pounds 5, 6 and 7. The compounds 5 and 7 have been used
as building blocks for the synthesis of end-capped bi–, ter
and quaterthiophenes. The synthesis clearly shows the
versatility and usefulness of 3-methoxythiophene as a
starting material for electronic materials.
EC5Ta
TDT2T (22)
TDT3T (28)
TDT4T (30)
TOT2T (23)
TOT3T (29)
TOT4T (31)
NMR spectra were recorded on a Bruker AM 400 (400 MHz) or
Bruker AMX 500 (500 MHz) spectrometer. Mass spectra were re-
corded on a Finnigan SSQ 7000 spectrometer. Elemental analyses
were performed by Analytische Laboratorien GmbH, Lindlar, Ger-
many. High-Resolution MS was performed at the Department of
Organic Chemistry, Stockholm University and at the Department of
Bioorganic chemistry, Lund University. THF was distilled from so-
dium/benzophenone, all other solvents were distilled or HPLC
grade. MgSO4 was used as drying agent during work-up in all reac-
a Literature values for oligomers synthesized by Bäuerle et al.7
Cyclic Voltammetry
The cyclic voltammetric behaviors of the new oligomers tions. Starting materials were commercially available and used as
received. Melting points were measured on a Bibby Sterilin Stuart
were recorded in dichloromethane (Table 2). The differ-
SMP3 melting point apparatus, or by DSC on a Mettler-Toledo
ent influences of the two fused six-membered heterocy-
DSC820, 1st heating, 10 °C/min, N2 (80 mL/min), 150–350 °C. Cy-
clic voltammetry was performed at 25 °C in CH2Cl2 at 100 mV/s
with Bu4NPF6 (0.15 M) as electrolyte and measured vs. a SCE ref-
cles are clear. Thioxano condensation lowers the first
oxidation potential by 60–100 mV compared to the dihy-
drodithiino series. This clearly shows the superior -do-
nating ability of oxygen. The values for Bäuerle’s
oligomers fall in between these values. All oligomers syn-
thesized in this work nicely show reversible first and sec-
ond oxidation half-waves, except TOT2T (23), for which
we were unable to record a reversible redox pair at normal
sweep rates. In the second oxidation, our two series show
their ability to host charge and reduce the on-site coulom-
bic repulsion; the difference between the two oxidation
values ( E) is considerably lower (150–200 mV) than for
the comparison series. However, oligomers end-capped
erence electrode. The halfwave potential for the ferrocene/ferroce-
nium couple was 0.42 V in our system.
2-(3-Thienylsulfanyl)ethanol (13) and 3-[3-Thienoxyethylsulfa-
nyl)thiophene (14)
(Unoptimized Procedure, cf. Scheme 2)
3-Methoxythiophene (10; 1.14 g, 9.99 mmol), mercaptoethanol (11;
964 mg, 12.3 mmol) and oven-dried KHSO4 (182 mg, 1.34 mmol)
were heated in refluxing toluene (25 mL) for 3 h. After cooling to
r.t., the mixture was filtered through glass wool and concentrated to
a yellow oil (362 mg, 23% of theory). Chromatography on silica
Synthesis 2003, No. 14, 2199–2205 © Thieme Stuttgart · New York

PAPER
New End-Capped Thiophenes and Oligothiophenes
2203
gel, eluting with toluene, CH2Cl2 and then 30% MeOH in CH2Cl2
2-(3-Thienylsulfanyl)ethanethiol (16)
gave 13 (242 mg, 15%) and 14 (10 mg) as yellow oils.
3-Methoxythiophene (10; 15.00 g, 0.131 mol) and ethanedithiol
(15; 32 mL, 0.38 mol) in toluene (300 mL) was heated to 110 °C.
NaHSO4 (2 g) was added in 4 portions during 2 h. The MeOH–tol-
uene azeotrope was continuously collected and when no more azeo-
trope could be observed, the temperature was raised and the toluene
was distilled off. Distillation under reduced pressure (12 mm Hg)
removed the ethanedithiol (31–35 °C) and 3-methoxythiophene
(38–40 °C). Further distillation under reduced pressure at 90–
110 °C/10–2 mbar to a trap cooled by dry ice gave 15.55 g of pure
16 (67%).
13
1H NMR (CDCl3): = 7.34 (dd, J1 = 3.0 Hz, J2 = 5.0 Hz, 1 H), 7.26
(dd, J1 = 1.3 Hz, J2 = 3.0 Hz, 1 H), 7.06 (dd, J1 = 1.3 Hz, J2 = 5.0
Hz, 1 H), 3.72 (dt, J1 = J2 = 6.0 Hz, 2 H), 3.02 (t, J = 6.0 Hz, 2 H),
2.03 (t, J = 6.0 Hz, 1 H).
13C NMR (CDCl3): = 130.63, 130.59, 127.0, 125.6, 60.8, 39.0.
MS (EI): m/z (%) = 160 (48, M+), 116 (100).
1H NMR (CDCl3): = 7.32 (dd, J1 = 4.9 Hz, J2 = 3.1 Hz, 1 H), 7.23
(dd, J1 = 3.1, J2 = 1.2 Hz, 1 H), 7.04 (dd, J1 = 4.9 Hz, J2 = 1.2 Hz, 1
H), 3.01 (m, 2 H), 2.69 (m, 2 H), 1.70 (t, 1 H).
13C NMR (CDCl3): = 130.4, 130.3, 126.6, 125.6, 39.5, 24.5.
MS (EI): m/z (%) = 176 (44, M+), 116 (100).
HRMS: m/z calcd for C6H8OS2: 160.0017; found: 160.0024.
14
1H NMR (CDCl3): = 7.34 (dd, J1 = 4.9 Hz, J2 = 3.1 Hz, 1 H), 7.26
(dd, J1 = 3.1 Hz, J2 = 1.2 Hz, 1 H), 7.17 (dd, J1 = 5.3 Hz, J2 = 3.1
Hz, 1 H), 7.08 (dd, J1 = 4.90 Hz, J2 = 1.2 Hz, 1 H), 6.73 (dd, J1 = 5.3
Hz, J2 = 1.5 Hz, 1 H), 6.19 (dd, J1 = 3.1 Hz, J2 = 1.5 Hz, 1 H).
Anal. Calcd for C6H8S3: C, 40.87; H, 4.57; S, 54.55. Found: C,
41.01; H, 4.55.
13C NMR (CDCl3): = 157.2, 130.7, 130.3, 126.5, 125.3, 124.9,
119.5, 97.8, 68.8, 34.3.
2,3-Dihydrothieno[2,3-b][1,4]dithiine (TDT, 5)
MS (EI): m/z (%) = 242 (8, M+), 143 (100).
To a solution of 2-(3-thienylsulfanyl)ethanethiol (16; 18.14 g, 0.103
mol) in Et2O (1.5 L) under argon, was added SO2Cl2 (14.02 g, 0.104
mol) in one portion. After 3 h at r.t., the mixture was washed several
times with H2O and aq 2 M NaOH, dried and concentrated to yield
17.64 g of crude product. The product was distilled at 85–90 °C un-
der reduced pressure (5·10–3 mbar) to a flask cooled with dry ice;
yield: 10.39 g (58%).
2-(3-Thienylsulfanyl)ethanol (13)
3-Methoxythiophene (10; 10.00 g, 87.59 mmol), mercaptoethanol
(11; 12.38 g, 158.5 mmol, 1.81 equiv), and p-TsOH·H2O (53 mg,
0.28 mmol) were heated at 90 °C under a stream of N2. After 3 h,
NMR indicated 90% conversion. The product was diluted with tol-
uene, and distilled from K2CO3 at 15 mmHg, collecting the fraction
between 100–165 °C. The product was obtained as a yellowish oil
(7.451 g, 53%); analyses as above.
1H NMR (CDCl3): = 7.09 (d, J = 5.2 Hz, 1 H), 6.73 (d, J = 5.2 Hz,
1 H), 3.35–3.32 (m, 2 H), 3.30–3.27 (m, 2 H).
13C NMR (CDCl3): = 127.1, 123.2, 121.8, 121.6, 28.8, 28.0.
MS (EI): m/z (%) = 174 (100, M+), 70 (32), 146 (59).
2-[3-(2-Bromothienyl)sulfanyl]ethanol (19)
2-(3-Thienylsulfanyl)ethanol (13; 3.185 g, 19.87 mmol) was dis-
solved in CH2Cl2 (120 mL) and cooled in an ice–water bath for 30
min. NBS (3.926 g, 22.16 mmol, 1.12 equiv) was added in portions
over 5 min. The mixture was left with stirring for 90 min, and the
organic layer was washed with 10% aq Na2S2O3, aq 2 M NaOH and
H2O, dried (MgSO4), and concentrated to give 19 as a yellowish oil
(4.471 g, 94% of theory), sufficiently pure for further transforma-
tions.
1H NMR (CDCl3): = 7.27 (d, J = 5.7 Hz, 1 H), 6.96 (d, J = 5.7 Hz,
1 H), 3.65 (t, J = 5.9 Hz, 2 H), 2.99 (t, J = 5.9 Hz, 2 H), 2.32 (s, 1 H).
13C NMR (CDCl3): = 131.6, 131.3, 126.8, 116.7, 60.8, 39.0.
Anal. Calcd for C6H6S3: C, 41.35; H, 3.47; S, 55.18. Found: C,
41.50; H, 3.49.
6-Bromo-2,3-dihydrothieno[2,3-b][1,4]dithiine (20); Typical
Procedure
2,3-Dihydrothieno[2,3-b][1,4]dithiine (5; 10.39 g, 59.6 mmol) was
dissolved in a mixture of AcOH (100 mL) and CH2Cl2 (50 mL). The
reaction mixture was cooled to 0 °C, and NBS (10.61 g, 59.6 mmol)
was added in portions during 30 min. After 3 h at r.t., the solution
was diluted with CH2Cl2 (200 mL) and washed with 10% aq
Na2S2O3 and H2O. The aqueous phases were back-extracted with
CH2Cl2, and the combined organic fractions were washed with 2 M
NaOH and H2O, dried and concentrated to yield 14.76 g (98%) of
crude 20. The purity of the product was sufficient for further trans-
formations.
MS (EI): m/z (%) = 237.9 (44, M+), 114.9 (46), 69.0 (52), 45.0
(100).
HRMS: m/z calcd for C6H7BrOS2: 237.9122; found: 237.9129.
2,3-Dihydrothieno[2,3-b][1,4]oxathiine (6)
1H NMR (CDCl3): = 6.68 (s, 1 H), 3.33–3.30 (m, 2 H), 3.27–3.24
(m, 2 H).
13C NMR (CDCl3): = 129.6, 124.4, 123.1, 107.9, 28.9, 28.4.
2-[3-(2-Bromothienyl)sulfanyl]ethanol (19; 1.405 g, 5.875 mmol),
Cs2CO3 (3.860 g, 11.85 mmol, 2.02 equiv), CuI (122 mg, 0.641
mmol, 11%), 1,10-phenanthroline monohydrate (238 mg, 1.20
mmol, 20%) and toluene (15 mL) were charged in a vial, which was
sealed and heated at 100 °C for 68 h, allowed to cool to r.t. and left
at r.t. for 24 h. The mixture was diluted with Et2O and filtered
through a short plug of silica gel to yield crude 6 as a yellow oil (854
mg). Chromatography on silica gel, with gradient elution going
from hexane to 20% EtOAc in hexane gave 6 as a yellow oil (263
mg, 29%).
HRMS: m/z calcd for C6H5BrS3: 251.8737; found: 251.8733.
2,3-Dihydrothieno[3,2-b][1,4]oxathiine (TOT, 7)
Mercaptoethanol (11; 6.85 g, 88 mmol) was dissolved in CH2Cl2
(500 mL) under argon, and cooled to 0 °C. The argon source was re-
moved, the system closed and SO2Cl2 (11.82 g, 88 mmol) was add-
ed. After 15 min, 3-methoxythiophene (10; 10.00 g, 88 mmol) was
added. The cooling bath was removed and the reaction mixture was
left with stirring overnight. After cautiously opening the system, aq
2 M NaOH was added to the reaction mixture. The organic layer
was washed several times with aq 2 M NaOH and H2O. After drying
and filtration through neutral Al2O3, the solution was concentrated
to yield an oil. After a quick distillation from Na2CO3 (8·10–3 mbar,
1H NMR (CDCl3): = 6.53 (d, J = 6.0 Hz, 1 H), 6.50 (d, J = 6.0 Hz,
1 H), 4.48 (m, 2 H), 3.12 (m, 2 H).
13C NMR (CDCl3): = 151.3, 124.5, 110.8, 104.2, 68.6, 25.3.
MS (EI): m/z (%) = 158 (79), 130 (21), 102 (100).
HRMS: m/z calcd for C6H6OS2: 157.9860; found: 157.9860.
Synthesis 2003, No. 14, 2199–2205 © Thieme Stuttgart · New York

2204
J. Hellberg et al.
PAPER
75–80 °C), the oil was purified by chromatography on silica gel us-
ing 10% toluene in hexanes as eluent; yield: 8.48 g (61%).
1H NMR (CDCl3): = 6.94 (d, J = 5.5 Hz, 1 H), 6.63 (d, J = 5.5 Hz,
Isopropoxy-4,4,5,5-tetramethyl[1,3,2]dioxaborolane (24; 8.38 g,
45.0 mmol) was added and the mixture left to attain r.t. overnight
(16 h). After dilution with CH2Cl2 and washing with aq 10%
NaHCO3 and brine, the solution was dried (MgSO4) and concentrat-
ed to give a brownish-white solid (6.80 g, 93% of theory) which was
recrystallized from MTBE to give pure 27 (4.15 g, 57%).
1 H), 4.45 (m, 2 H), 3.10 (m, 2 H).
13C NMR (CDCl3): = 148.3, 119.81, 119.78, 103.6, 66.4, 25.8.
13C NMR (DMSO-d6): = 148.0, 120.4, 119.8, 102.7, 66.0, 25.0.
MS (EI): m/z (%) = 158 (100, M+), 130 (42), 102 (49).
1H NMR (CDCl3): = 7.52 (d, J = 3.7 Hz, 2 H), 7.29 (d, J = 3.7 Hz,
2 H), 1.35 (s, 24 H).
13C NMR (CDCl3): = 144.3, 138.4, 129.1 (br s), 126.0, 84.6, 25.2.
MS (EI): m/z (%) = 418 (100, M+), 218 (17).
Anal. Calcd for C6H6OS2: C, 45.54; H, 3.82. Found: C, 45.44; H,
3.76.
Anal. Calcd for C20H28S2BO4: C, 57.44; H, 6.75. Found: C, 57.42;
H, 6.65.
6-Bromo-2,3-dihydrothieno[3,2-b][1,4]oxathiine (21)
The same procedure as for 20 was used. Compound 7 (2.99 g, 18.9
mmol) gave 4.14 g (92%) of 21.
1H NMR (CDCl3): = 6.60 (s, 1 H), 4.43 (m, 2 H), 3.09 (m, 2 H).
13C NMR (CDCl3): = 147.7, 122.8, 106.6, 104.7, 66.6, 26.0.
HRMS: m/z calcd for C6H5BrOS2: 235.8965; found: 235.8965.
TDT3T (28); Typical Procedure
6-Bromo-2,3-dihydrothieno[2,3-b][1,4]dithiine (20; 2.68 g, 10.60
mmol) was dissolved in dioxane (120 mL) under argon. Pd(PPh3)4
(0.62 g, 0.54 mmol) was added, and after 5 min 25 (1.78 g, 5.30
mmol), and CsF (9.66 g, 63.6 mmol) were added. The mixture was
refluxed for 48 h. The reaction mixture was concentrated and parti-
tioned between CH2Cl2 and H2O. The organic layer was dried and
concentrated to give a dark brownish powder as crude product. The
product was treated with absolute EtOH overnight and filtered to
yield a light-brown powder (1.40 g, 62%). This powder contained
the trimer and small (<5%) amounts of the dimer. Pure fractions
could be obtained either by treatment overnight with CHCl3 and fil-
tering off the pure trimer, or by chromatography.
2,3,2 ,3 -Tetrahydro-6,6 -bi(thieno[2,3-b][1,4]dithiinyl)
(TDT2T, 22)
6-Bromo-2,3-dihydrothieno[2,3-b][1,4]dithiine (20; 5.00 g, 19.75
mmol) was dissolved in THF (40 ml) under N2. To the solution was
added Bu4NI (7.30 g, 19.75 mmol), Zn (2.00 g, 29.6 mmol) and
NiBr2(PPh3)2 (1.41 g, 1.89 mmol). The mixture was heated to 50 °C
for 2 h and left at r.t. overnight. Most of the solvent was removed
under reduced pressure, and the mixture was diluted with Et2O. Fil-
tration through a pad of neutral alumina and concentration gave
2.14 g of crude product. An additional 1.53 g of crude product was
extracted from the alumina with CH2Cl2. Recrystallization from
EtOH or MeOH gave in total 1.00 g (29%) of the dimer.
1H NMR (CDCl3): = 6.93 (s, 2 H), 6.77 (s, 2 H), 3.36–3.28 (m, 8
H).
13C NMR (CDCl3): = 135.2, 132.8, 124.3, 124.1, 123.3, 121.3,
28.8, 28.0.
1H NMR (CDCl3): = 6.68 (s, 2 H), 3.37–3.28 (m, 8 H).
13C NMR (CDCl3): = 132.2, 124.0, 123.2, 121.2, 28.8, 27.9.
MS (EI): m/z (%) = 346 (100, M+), 318 (40), 242 (21).
MS (EI): m/z (%) = 428 (46, M+), 400 (18), 280 (100).
Anal. Calcd for C16H12S7: C, 44.83; H, 2.82. Found: C, 44.57; H,
2.74.
Anal. Calcd for C12H10S6: C, 41.59; H, 2.91. Found: C, 41.55; H,
3.04.
TOT3T (29)
In a procedure analogous to the synthesis of 28, 21 (2.11 g, 8.9
mmol) was reacted with 25 (1.57 g, 4.67 mmol) in the presence of
CsF (4.07 g, 26.8 mmol) and Pd(PPh3)4 (0.26 g, 0.24 mmol), to give
1.14 g (65%) of pure 29; mp (DSC) 246 °C.
2,3,2 ,3 -Tetrahydro-6,6 -bi(thieno[3,2-b][1,4]oxathiinyl)
(TOT2T, 23)
Anhyd NiCl2 (531 mg, 4.18 mmol), Ph3P (5.24 g 20.0 mmol) and
2,2 -bipyridine (648 mg, 4.15 mmol) were mixed with anhyd DMF
(20 mL) in an oven-dried flask and heated at 70 °C under a stream
of N2 for 70 min. 6-Bromo-2,3-dihydrothieno[3,2-b][1,4]oxathiine
(21; 4.71 g, 19.9 mmol) was added as a solution in anhyd THF (20
mL), followed, after another 10 min, by Zn (3.31 g, 50.6 mmol). Af-
ter 10 min, the outlet for N2 was removed and the mixture was
stirred at 80 °C for 22 h. After cooling to r.t., the mixture was dilut-
ed with CH2Cl2, washed with 2 M HCl, aq 10% NaHCO3, dried and
evaporated to afford 7.36 g of a dark brown oil. The oil was treated
with Et2O, and the insoluble material was purified by chromatogra-
phy on silica gel with gradient elution using mixtures of heptane and
EtOAc. Analytically pure 23 was obtained as yellow crystals; yield:
827 mg (26%); mp 181–183 °C.
1H NMR (CDCl3): = 6.92 (s, 2 H), 6.68 (s, 2 H), 4.60 (m, 4 H),
3.13 (m, 4 H).
13C NMR (CS2–CDCl3): = 148.3, 135.9, 131.0, 123.7, 116.2,
104.0, 66.3, 26.7.
MS (EI): m/z (%) = 396 (100, M+), 264 (74).
Anal. Calcd for C16H12S5O2: C, 48.46; H, 3.05. Found: C, 48.40; H,
2.97.
TDT4T (30); Typical Procedure
Compound 20 (2.14 g, 8.46 mmol) was dissolved in dioxane (120
mL) under argon. To this solution was added Pd(PPh3)4 (0.49 g,
0.42 mmol). After 5 min, 27 (1.77 g, 4.23 mmol) and CsF (7.71 g,
50.8 mmol) were added, and the mixture was heated at reflux for 48
h. The mixture was concentrated, and partitioned between CH2Cl2
and H2O. The organic layer was dried, filtered through Celite and
concentrated to give 3.30 g of a dark oil as crude product. The prod-
uct was treated with EtOH, yielding a crystalline residue, which was
treated with CHCl3 to give 200 mg of NMR pure tetramer. The
Celite was transferred to a Soxhlet apparatus and extracted with
EtOH, followed by extraction with CHCl3 for 2 d. The CHCl3 ex-
tract was concentrated to afford 556 mg of product. The total yield
of tetramer was 756 mg (35%). Analytically pure fractions could be
obtained by chromatography through Al2O3 with toluene as eluent;
mp (DSC) 276 °C.
1H NMR (CDCl3): = 6.60 (s, 2 H), 2.45 (m, 4 H), 3.10 (m, 4 H).
13C NMR (CDCl3): = 148.5, 131.2, 115.8, 103.6, 66.7, 26.3.
MS (EI): m/z (%) = 314 (100, M+), 287 (21), 182 (33), 154 (32).
5,5 -Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,2 -
bithiophene (27)
Bithiophene (26; 2.91 g, 17.5 mmol) with TMEDA (4.69 g, 40.4
mmol) in hexanes (dried over CaH2) under N2 was cooled to –78 °C,
and n-BuLi in hexanes (1.5 M, 25 ml, 38 mmol) was added over 5
min. After 10 min, the cooling bath was removed, and 45 min later,
the mixture was refluxed for 30 min, and again cooled to –78 °C. 2-
Synthesis 2003, No. 14, 2199–2205 © Thieme Stuttgart · New York

PAPER
New End-Capped Thiophenes and Oligothiophenes
2205
1H NMR (CDCl3): = 7.02 (d, J = 3.7 Hz, 2 H), 6.96 (d, J = 3.9 Hz,
(6) Müllen, K.; Wegner, G. Adv. Mater. 1998, 10, 433.
2 H), 6.80 (s, 2 H), 3.33 (m, 8 H).
13C NMR (CDCl3): = 136.4, 135.9, 133.4, 124.9, 124.8, 124.7,
123.8, 121.9, 29.7, 28.8.
(7) Bäuerle, P.; Segelbacher, U.; Maier, A.; Mehring, M. J. Am.
Chem. Soc. 1993, 115, 10217.
(8) (a) Schultz, A. J.; Wang, H. W.; Williams, J. M. J. Am.
Chem. Soc. 1986, 108, 7853. (b) Whangbo, M.-H.; William,
J. M.; Schultz, A. J.; Emge, T. J.; Beno, M. A. J. Am. Chem.
Soc. 1987, 109, 90.
MS (EI): m/z (%) = 510 (100, M+), 482 (26), 362 (37).
Anal. Calcd for C20H14S8: C, 47.03; H, 2.76. Found: C, 46.81; H,
2.94.
(9) Hellberg, J.; Moge, M.; von Schütz, J.-U.; Törnroos, K.-W.
Synth. Met. 1997, 86, 1877.
(10) In order to have a useful shorthand nomenclature for the
oligothiophenes, we named TDT for thienodithiine 5 and
TOT for thienooxathiine 7. The oligothiophenes were then
called TDTnT for the dithiino end-capped series, and
TOTnT for the oxathiino end-capped series, where n is the
number of thiophene units, as for the ECnT oligomers7. For
instance, TOT3T 29 is a terthiophene, which is end-capped
with thioxano rings.
(11) A literature search with Beilstein Commander and Scifinder
Scholar in May 2003 found 20 substances containing 5 as a
substructure, other than those reported by us in preliminary
communications. Only 3 of these contain a peripheral
ethylenedithio bridge. No compound with the substructure 6
was found, and only 3 compounds based on 7.
TOT4T (31)
In a procedure analogous to the synthesis of 30, 21 (1.98 g, 8.4
mmol) was reacted with 27 (1.75 g, 4.18 mmol) in the presence of
CsF (3.76 g, 24.8 mmol) and Pd(PPh3)4 (0.26 g, 0.24 mmol), to give
0.934 g (47%) of pure 31; mp (DSC) 302 °C.
1H NMR (CDCl3): = 7.27 (d, J = 3.7 Hz, 2 H), 7.20 (d, J = 3.7 Hz,
2 H), 6.99 (s, 2 H), 4.60 (m, 4 H), 3.15 (m, 4 H).
13C NMR (DMSO-d6, 95 °C): = 149.5, 136.5,135.8, 130.6, 126.2,
125.4, 117.3, 105.3, 67.3, 26.5.
MS (EI): m/z (%) = 478 (97, M+), 450 (24), 346 (100), 214 (56).
Anal. Calcd for C20H14S6O2: C, 50.12; H, 2.95. Found: C, 49.93; H,
2.90.
(12) (a) Zotti, G.; Gallazzi, M. C.; Zerbi, G.; Meille, S. V. Synth.
Met. 1995, 73, 217. (b) Faid, K.; Leclerc, M. Chem.
Commun. 1996, 2761. (c) Goldoni, F.; Langeveld-Voss, B.
M. W.; Meijer, E. W. Synth. Commun. 1998, 28, 2237.
(13) 3-Methoxythiophene (10) is commercially available, but it
can also be readily synthesized from 3-bromothiophene:
Keegstra, M. A.; Peters, T. H. A.; Brandsma, L. Synth.
Commun. 1990, 20, 213.
Acknowledgment
Financial support from the Royal Institute of Technology, Swedish
Natural Science Research Council, and the Ernst Johnson foundati-
on is gratefully acknowledged.
References
(14) The NMR shifts of the thiophene protons of the main
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(1) Parts of this paper have been presented at ICSM
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101, 103.
(2) Present address: Acreo AB, Bredgatan 34, SE-602 21
Norrköping, Sweden.
(3) Present address: Unit for Organic Chemistry, Department of
Biosciences, Karolinska Institute and Södertörn University
College, Novum Research Park, SE-141 57 Huddinge,
Sweden.
(4) Present address: AstraZeneca R&D, SE-151 85 Södertälje,
Sweden..
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Synthesis 2003, No. 14, 2199–2205 © Thieme Stuttgart · New York