Thienothiophenes. Part 2. Synthesis, metallation and bromine→lithium exchange reactions of thieno[3,2-b]thiophene and its polybromo derivatives

Article abstract of DOI:10.1039/a701877k

Methods for the large-scale synthesis of thieno[3,2-b]thiophene [including a catalytic vapour-phase reaction (at 550°C) between 2-(2-thienyl)ethanol and carbon disulfide], its 2-carboxylic acid and its 3,6-dibromo and 2,3,5,6-tetrabromo derivatives are reported. With 2 mol equiv. of butyllithium thieno[3,2-6]thiophene gives its 2,5-dilithiated derivative and its 3,6-dibromo derivative gives the 3,6-dilithiated compound. By quenching with suitable electrophilic reagents these dilithiated compounds have been converted into various 2,5- or 3,6-disubstituted thieno[3,2-6]thiophenes, respectively. Likewise 2,3,5,6-tetrabromothieno[3,2-b]thiophene has been converted into 2,5-disubstituted 3,6-dibromothieno[3,2-b]thiophenes.

Full text of DOI:10.1039/a701877k

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Thienothiophenes. Part 2.1 Synthesis, metallation and
bromine→lithium exchange reactions of thieno[3,2-b]thiophene and
its polybromo derivatives
Lance S. Fuller,a Brian Iddon*,b,c and Kevin A. Smithb
a Synthetic Chemicals Ltd., Four Ashes, Nr. Wolverhampton, UK WV10 7BP
b Division of Chemical Sciences, Science Research Institute, University of Salford, Salford,
UK M5 4WT
c Present address: House of Commons, Westminster, London, UK SW1A 0AA
Methods for the large-scale synthesis of thieno[3,2-b]thiophene [including a catalytic vapour-phase
reaction (at 550 ЊC) between 2-(2-thienyl)ethanol and carbon disulfide], its 2-carboxylic acid and its
3,6-dibromo and 2,3,5,6-tetrabromo derivatives are reported. With 2 mol equiv. of butyllithium
thieno[3,2-b]thiophene gives its 2,5-dilithiated derivative and its 3,6-dibromo derivative gives the
3,6-dilithiated compound. By quenching with suitable electrophilic reagents these dilithiated
compounds have been converted into various 2,5- or 3,6-disubstituted thieno[3,2-b]thiophenes,
respectively. Likewise 2,3,5,6-tetrabromothieno[3,2-b]thiophene has been converted into
2,5-disubstituted 3,6-dibromothieno[3,2-b]thiophenes.
Polythiophene, poly(alkylthiophenes) and other oligomers and
polymers containing thiophene rings are attracting consider-
able attention currently as molecules with potential electro-
optical properties for use as molecular devices.2–11 These poly-
mers are more stable in the environment and often possess
better electrical conducting properties than some others, e.g.
polyacetylene or polypyrrole. Indeed, the most stable molecular
switching devices discovered to date are thiophene deriv-
atives.12–15 This interest in thiophenes has been extended
recently to thieno[3,2-b]thiophenes.11,16–34 However, research in
this area has been impeded by the lack of good synthetic routes
to thieno[3,2-b]thiophene and its derivatives.24,25,27,29,35–38 In this
paper we report convenient reproducible methods for the large-
scale syntheses of thieno[3,2-b]thiophene, its 2-carboxylic acid
and its 3,6-dibromo and 2,3,5,6-tetrabromo derivatives and the
synthesis of 2,5- and 3,6-disubstituted derivatives through
dilithiation of the parent system or via Br→Li exchange reac-
tions involving 3,6-dibromo- or 2,3,5,6-tetrabromo-thieno-
[3,2-b]thiophene.
aqueous lithium hydroxide in THF24 gave the corresponding
acid 4 (90%).
Thermal decarboxylation of thiophene and thienothio-
phenecarboxylic acids is possible in quinoline in the presence of
copper.11,24,35,36,45,46 Thieno[3,2-b]thiophene-2-carboxylic acid 4
was decarboxylated in this way, to give the parent ring system
5 (88% yield).24,35 Use of a Woods-metal bath allowed us to
control the temperature at a constant 260 ЊC (cf. ref. 24;
decarboxylation at 160 ЊC) whilst, following removal of the
bulk of quinoline with hydrochloric acid, flash chromatography
of the crude product on silica24 allowed efficient removal of the
final amounts of quinoline and gave the thieno[3,2-b]thiophene
in higher yields and better quality than previous work-up
procedures allowed.35
Thiophene42 is prepared commercially by a vapour-phase
catalytic reaction between various C4 starting materials (e.g.
butanol) and carbon disulfide.47,48 Derivatives of thiophene (e.g.
3-methylthiophene from Me2CHCH2CH2OH)47,48 and benzo-
[b]thiophene (from C2 substituted benzene derivatives and H2S
᎐
The preferred route for construction of the thieno[3,2-b]-
thiophene ring system, which is based on some of our earlier
work,1,39,40 is summarised in Scheme 1. The starting material,
3-bromothiophene 1,41 is available commercially.42
in the order of reactivity: PhC᎐CH > PhCH᎐CH PhCH2-
᎐
᎐
2
CH2SH > PhCHMeSH > PhCOMe49)47–51 may be prepared
similarly. For thiophene synthesis an alkali-promoted chrome–
alumina catalyst is used.47,48 We now report that thieno[3,2-b]-
thiophene 5 can be prepared similarly from C2 substituted
thiophenes and carbon disulfide.
Gronowitz et al.43 have synthesized 3-bromothiophene-2-
carbaldehyde 2 previously in 70% yield from 2,3-dibromo-
thiophene through its successive treatment with butyllithium
(see also ref. 44) and N,N-dimethylformamide (DMF). We
found that better yields (80% after distillation; 95% crude,
which was pure enough to carry forward) of the aldehyde 2
could be obtained more conveniently and on a larger scale by
deprotonation of 3-bromothiophene 1 with lithium diiso-
propylamide (LDA–THF, 0 ЊC) followed by a N-formyl-
piperidine quench. With DMF as the quenching reagent the
yield fell to 60% whilst use of methyl formate gave only the
bithienylmethanol derivative 9 (84%). At Ϫ78 ЊC we found that
deprotonation of 3-bromothiophene occurred in both α-
positions of the thiophene ring but, as the temperature
increased to 0 ЊC, the reaction showed greater selectivity for
deprotonation at position-2. When o-bromoaldehyde 2 was
treated with ethyl 2-sulfanylacetate (DMF–K2CO3, ambient
temp.)1,24,40 it gave ethyl thieno[3,2-b]thiophene-2-carboxylate 3
(81%; 95% for smaller scale reactions). Its hydrolysis with
2-(2-Thienyl)ethanol was prepared in 68% yield from 2-
thienyllithium and ethylene oxide (oxirane) and treated with
carbon disulfide (ratio 1:2). We varied the catalyst contact time
and reaction temperature (see Experimental section for details)
and analyzed the liquid condensate by gas liquid chrom-
atography (GLC). With a contact time of 7.5 s the yield of
thieno[3,2-b]thiophene 5 obtained increased from 5% at 480 ЊC
to 7% at 500 ЊC to 12% at 525 ЊC to 16% at 550 ЊC. With an
increased contact time of 10 s three runs were completed at 500,
525 and 550 ЊC; the yields of product were 8, 10 and 6%,
respectively. Using 2-(2-thienyl)ethanol as the feedstock it was
possible to prepare multigram quantities of thieno[3,2-b]-
thiophene 5 at 550 ЊC with a 7.5 s contact time. We have made
no attempts to optimize conditions further, e.g. by varying the
feedstock: CS2 ratio.
We found it more convenient to prepare 1-(2-thienyl)ethanol
(82%) from 2-thienyllithium and acetaldehyde. However, with a
J. Chem. Soc., Perkin Trans. 1, 1997
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contact time of 7.5 s and by varying the reactor temperature
from 425 to 550 ЊC at 25 ЊC intervals the best yield of
thieno[3,2-b]thiophene 5 produced from this feedstock was
only 5% (at 475–500 ЊC). Commercially available 2-acetyl-
thiophene was converted also into thieno[3,2-b]thiophene 5
(best yield 9% with a contact time of 7.5 s at 550 ЊC) but con-
siderable carbonisation of the catalyst occurred with this feed-
stock and continuous flow of the reactor was not possible for
more than 3 h before regeneration of the catalyst became neces-
sary. Increased contact times exacerbated the carbonisation
problem and resulted in lower yields of product.
With all these reactions varying amounts of α-terthienyl
(identified using GLC analysis by comparison with an authen-
tic sample) were formed also. In the synthesis of benzo[b]thio-
phene from various C2 substituted benzene derivatives Venuto
et al.49 detected varying yields of 2,5-diphenylthiophene in their
products. This became the major product at 400 ЊC. It is pos-
sible, therefore, that α-terthienyl can be produced in greater
quantities by our vapour-phase process but we have not investi-
gated this possibility. The best yield (4%) obtained by us was
using 2-(2-thienyl)ethanol as the feedstock with a contact time
of 7.5 s at 480 ЊC. At 550 ЊC (our preferred operating temper-
ature for thieno[3,2-b]thiophene 5 production) α-terthienyl was
present in the product in only trace (<5%) amounts and was
removed by flash chromatography of the crude product on
silica.
Br
Br
S
i
iii
CO2Et
ii
CHO
S
1
S
2
S
3
iv
S
v
S
S
5
CO2H
S
Br
vi
4
S
Br
Br
S
6
vii
Br
Br
S
S
viii
Br
Br
S
S
Br
Br
7
8
Scheme 1 Reagents and conditions: i, LDA, THF, 0 ЊC; ii, N-formyl-
piperidine; iii, HSCH2CO2Et, K2CO3, DMF; iv, LiOH,THF; v, Cu,
quinoline, 260 ЊC; vi, Br2, aq. AcOH; vii, LDA, THF, Ϫ80 ЊC, then Br2;
viii, Zn, AcOH.
By contrast with thiophenecarboxylic acids,52–54 bromo-
decarboxylation of thieno[3,2-b]thiophenecarboxylic acids has
not been investigated to date. We have investigated bromo-
decarboxylation of the acid 4 under a range of reaction condi-
tions; both bromine–acetic acid52 and bromine–aqueous
sodium hydroxide53 have been used. In glacial acetic acid only
bromination of the ring system was observed with no
accompanying bromodecarboxylation. Addition of water to
the acetic acid caused decarboxylation; as more water was
added more di- and tri-bromo derivatives were obtained until
or CO2Me33]. The only dilithiations carried out to date are
dilithiation of the parent system 5 followed by carbonation
(CO2) or formylation (DMF) of the resulting dilithium deriv-
ative 10, which gave the dicarboxylic acid 1135 or dialdehyde
12,60 respectively. We now report similar syntheses of the 2,5-
disubstituted thieno[3,2-b]thiophenes 13–16 (73, 62, 66 and
41% yields, respectively). We also prepared the dialdehyde 12
[86% crude yield, mp 269–270 ЊC (lit.,60 mp of crude product,
272–275 ЊC); this compound decarbonylates on attempted
recrystallisation, e.g. from acetic acid] but using N-formyl-
piperidine instead of DMF as the quenching reagent. Bugge35
used butyllithium in refluxing diethyl ether to synthesize the
dilithium compound 10 but we used butyllithium in ether at
much lower temperatures (cf. ref. 60).
The 2,5-dilithiated compound 10 has been prepared also
from 2,5-dibromothieno[3,2-b]thiophene via a Br→Li exchange
reaction and reacted with 2 mol equiv. of fluorenone.61 2-
Bromothieno[3,2-b]thiophene and its 3-, 5- and 6-methyl-,
3-phenyl- and 3,5- and 3,6-dimethyl derivatives undergo
Br→Li exchange and the resulting lithiated derivatives have
been treated with butyl borate, to give the corresponding
boronic acids [TTB(OH)2].62 Their oxidation with hydrogen
peroxide leads to the formation of tautomeric hydroxy-
thieno[3,2-b]thiophenes. Thieno[3,2-b]thiophene-3-carbalde-
hyde has been prepared (57% yield) via a Br→Li exchange reac-
tion of 3-bromothieno[3,2-b]thiophene followed by a DMF
quench45 of the resulting thieno[3,2-b]thien-3-yllithium com-
pound; this lithium derivative has also been treated with N,N-
dimethylacetamide to give 3-acetylthieno[3,2-b]thiophene,62
and with bis(3-thienyl) disulfide and bis(phenylsulfonyl) sulfide
to give sulfides (82 and 23% yields, respectively),11,29,30 whilst
2,3,5-tribromothieno[3,2-b]thiophene 6 has been converted into
the 3,5-dibromo compound via Br→Li exchange followed
by an aqueous quench.36 Successive treatment of 2,3,5-tri-
even 2,3,5,6-tetrabromothieno[3,2-b]thiophene
7 began to
form. For the synthesis of the tetrabromo compound 731,55 the
preferred procedure involved initial monobromination of the
carboxylic acid 4 in acetic acid. After addition of the first mol
equiv. of bromine at ambient temperature water was added
until all the monobrominated thieno[3,2-b]thiophene-2-
carboxylic acid was precipitated. Then the mixture was heated
under reflux when a further 1.5 mol equiv. of bromine was
added to it. The initial precipitate redissolved and the mixture
was heated further until thin-layer chromatographic (TLC)
analysis indicated absence of the intermediate monobromin-
ated acid (~ 3 h, depending on scale) when more bromine (4 mol
equiv.) was added to the refluxing mixture. After a further 12 h
the sole product was the tetrabromo compound 7 (70%).
Bromodecarboxylation of thieno[3,2-b]thiophene-2-carboxylic
acid 4 in 10% aqueous sodium hydroxide gave the 2,3,5,6-
tetrabromo derivative 7 but in only 18% yield; again, insolubil-
ity of initial products was a problem but the known36 instability
of 2-bromothieno[3,2-b]thiophene, even at ambient temper-
ature, probably contributed greatly to the low yield of this reac-
tion. 2,3,5,6-Tetrabromothieno[3,2-b]thiophene 7 can be pre-
pared also by elemental bromination of the parent system 5
[e.g. in carbon disulfide (87%)31].
With zinc and acetic acid36,56,57 we converted 2,3,5,6-tetra-
bromothieno[3,2-b]thiophene 7 (Scheme 1) into 3,6-dibromo-
thieno[3,2-b]thiophene 8 (76%).
Thieno[3,2-b]thiophene 526–28,33,35,36,45,58 and its 2-ethyl59 and
3,6-dimethyl18,28,33 derivatives have been monometallated with
butyllithium in an α-position and the resulting lithiated deriv-
ative used to introduce various α-substituents [Br,36 I,26 MgBr,18
ZnCl,27 SnBu3,27 B(OH)2,27 SeLi→SMe,28 TeLi→TeMe (both
with MeI),28 SLi→SR (with MeBr or C18H37Br),33 SMe,58
SLi→SEt (with EtI),59 SO2Me,33 SO2Li→SO2NH2 (with N-
chlorosuccinimide followed by ammonia),33 CHO,45 CO2H,35
bromo-6-methylthieno[3,2-b]thiophene
with
butyllithium
(Et2O, Ϫ70 ЊC), magnesium bromide and tert-butyl perbenzoate
gave a mixture of 3,5-dibromo-6-methylthieno[3,2-b]thiophene
and 3,5-dibromo-2-tert-butoxy-6-methylthieno[3,2-b]thiophene
(major product).62 The latter product was converted into its 3,5-
dilithiated derivative by treatment with butyllithium and, fol-
lowing hydrolysis with water, this gave 2-tert-butoxy-6-methyl-
thieno[3,2-b]thiophene.62
3466
J. Chem. Soc., Perkin Trans. 1, 1997

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abundance ratios were as expected for the compounds contain-
Br
ing bromine.
CHOH
Catalytic vapour-phase conversions were carried out by con-
tinuous pumping of a 2:1 (by volume) mixture of carbon
disulfide and the thiophene feedstock through a stainless-steel
tube (ca. 20 cm long and 1 cm i.d.) packed for 5 cm with car-
borundum through which a thermocouple passed (the pre-
heater) followed by 5 cm3 of the alkali-promoted chrome–
alumina catalyst.47,48 Product condensates were collected below
the reactor in a glass separating funnel and analyzed both by
TLC and GLC. Both the pre-heater and reactor zones were
heated by an insulated salt bath. Regeneration of the catalyst
was achieved by heating it in situ with an air flow at 500 ЊC.
Camlab Polygram silica G/UV254 plates were used for TLC
and flash chromatography was performed using gradient
elution techniques on Merck Kieselgel type 60H silica.
Quantitative GLC was carried out on a Polar BPX 70
column (25 cm long; 0.22 mm i.d.) using a Pye Unicam GCD
machine fitted with a flame ionisation detector (H2 pressure 11
psi; air pressure 13 psi). Helium was the carrier gas (inlet
pressure 12 psi; linear velocity 32.2 cm sϪ1). The injector and
detector temperatures were maintained at 200 and 300 ЊC,
respectively, whilst the oven temperature was raised for each run
from 50 to 250 ЊC at a rate of 8 ЊC minϪ1, after which the tem-
perature was held at 250 ЊC for 5 min. Commercially available
5-chlorobenzo[b]thiophene was used as an internal standard
and products were identified by comparing their retention times
with those of authentic samples.
S
2
9
R
S
S
R
R
S
S
R
8 R = Br
10 R = Li
17 R = Li
11 R = CO2H
12 R = CHO
13 R = SMe
14 R = SPh
18 R = COMe
19 R = SMe
20 R = SPh
21 R = SiMe3
15 R = SiMe3
16 R = SiMe2But
Br
S
R
R
S
Br
22 R = Li
23 R = Me
24 R = SMe
All reactions involving organolithium reagents were carried
out under dry, oxygen-free nitrogen or argon and reagents and
solvents were dried by standard procedures. Reagents were
transferred with syringes through a rubber septum cap fitted to
the reaction flask.
25 R =
Me
26 R = SiMe3
Light petroleum refers to the fraction of bp range 40–60 ЊC
unless stated otherwise. Ether refers to diethyl ether. In all cases
organic extracts were combined, dried (MgSO4), filtered and
evaporated under reduced pressure on a rotary evaporator.
Small-scale distillations were carried out with a Kugelrohr
bulb-to-bulb distillation apparatus and the ‘bp’ temperatures
recorded are those of the oven at the time of distillation. Mps
were recorded with a Gallenkamp mp apparatus and are
uncorrected.
We treated 3,6-dibromothieno[3,2-b]thiophene 8 with 2 mol
equiv. of butyllithium (THF, Ϫ100 ЊC) and quenched the result-
ing lithium compound 17 with dimethyl disulfide, diphenyl
disulfide and trimethylsilyl chloride, which gave compounds 19
(71% yield in ether), 20 (64% in hexane–THF), and 21 (42% in
ether; 62% in hexane–THF), respectively. It is important that
the temperature is kept low during these reactions; we shall
report on the stability of 3,6-dilithiated thieno[3,2-b]thiophenes
elsewhere. Attempts to synthesize the 3,6-diacetyl compound 18
through quenching the dilithiated derivative 17 with acetonitrile
or N,N-dimethylacetamide were not very successful. Only the
latter reagent allowed compound 18 to be isolated, albeit in
very low (16%) yield [the major product was 3-acetylthieno-
[3,2-b]thiophene (28% yield)].
We treated the tetrabromo compound 7 with 2 mol equiv. of
butyllithium (THF, ambient temp.) and quenched the resulting
dilithiated derivative 22 with dimethyl sulfate, dimethyl
disulfide, 2-methylcyclohexanone and trimethylsilyl chloride,
which gave compounds 23 (55%), 24 (54%), 25 (55%; after
dehydration of the initially produced carbinol) and 26 (75%),
respectively.
α-Terthienyl was supplied by Aldrich and thiophene,42 2-
acetylthiophene, 3-bromothiophene,42 and 5-chlorobenzo[b]-
thiophene by Synthetic Chemicals Ltd.
2-(2-Thienyl)ethanol
Ethylene oxide was bubbled through a stirred solution of 2-
thienyllithium in tetrahydrofuran [prepared by addition of
butyllithium (1.5 mol dmϪ3 in hexane; 172 cm3, 260.0 mmol)
to thiophene (21.7 g, 260.0 mmol) in tetrahydrofuran (500 cm3)]
at 0 ЊC. When TLC analysis of the reaction mixture indicated
absence of thiophene the reaction was quenched by addition of
an excess of hydrochloric acid (1.0 mol dmϪ3) and then
extracted with ether. The combined organic extracts were dried
(MgSO4) and distillation of the solvents gave the product (22.5
g, 68%), bp 65 ЊC at 0.1 mmHg (lit.,63 44% yield and bp 122–
126 ЊC at 17 mmHg; prepared by reaction of 2-thienyl-
magnesium bromide with ethylene oxide); δH 3.05 (2 H, t, J 6.3,
CH2), 3.81 (2 H, t, J 6.3, CH2OH), 6.85 (1 H, d, J3,4 3.5, H-3),
6.93 (1 H, dd, J4,3 3.5, J4,5 5.0, H-4) and 7.14 (1 H, d, J5,4 5.0,
H-5).
Experimental
IR spectra were recorded with a Perkin–Elmer 1710 FT IR
spectrometer (liquids as films and solids as Nujol mulls between
sodium chloride plates); 1H NMR spectra were recorded in
deuteriochloroform (CDCl3) with a Bruker AC300 FT (300.13
MHz) instrument with tetramethylsilane as internal standard
unless stated otherwise; J values in Hz. Low-resolution mass
spectra were recorded using a Finnegan 4500 machine and
high-resolution mass spectra were obtained with a Kratos
Concept 1S mass spectrometer, all operating at 70 eV (CI
unless stated otherwise). Reported molecular weights are given
for the 79Br isotope unless stated otherwise. The isotopic
1-(2-Thienyl)ethanol
Butyllithium (1.4 mol dmϪ3 in hexane; 59.5 cm3, 83.0 mmol)
was added to a stirred solution of thiophene (7.0 g, 83.0 mmol)
in tetrahydrofuran (250 cm3) at 0 ЊC and the resulting mixture
was stirred at 0 ЊC for a further 1 h prior to addition of
acetaldehyde (4.7 cm3, 3.70 g, 84.0 mmol). After a further 1 h
the reaction mixture was quenched by addition of an excess of
J. Chem. Soc., Perkin Trans. 1, 1997
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hydrochloric acid (1.0 mol dmϪ3) and extracted with ethyl
acetate to give 1-(2-thienyl)ethanol (8.73 g, 82%), bp (Kugel-
rohr distillation) 65 ЊC at 0.2 mmHg (lit.,64 86.5% yield and bp
90–92 ЊC at 6.0 mmHg; prepared by reaction of 2-
thienylmagnesium bromide with acetaldehyde); δH 1.52 (3 H, d,
J 6.5, Me), 3.30 (1 H, br s, OH), 5.02 (1 H, q, J 6.5, CH), 6.91 (2
H, m, H-3/4) and 7.18 (1 H, m, H-5).cf. 65
chromatographed on silica. Light petroleum eluted compound
5 (6.70 g, 88%), mp 55–56 ЊC (lit.,24,35,66 mp 55–57, 55–56 and
56–56.5 ЊC); δH 7.25 (2 H, d, J 5.0, H-3/6) and 7.37 (2 H, d, J
5.0, H-2/5); m/z 140 (Mϩ, 100%).
(b) A 2:1 mixture (v/v) of carbon disulfide and 2-(2-thienyl)-
ethanol was pumped at a rate of 2.76 cm3 hϪ1 into the reactor
maintained at 550 ЊC. The condensate was washed with 20%
aqueous sodium hydroxide and then flash chromatographed on
silica. Light petroleum (bp 60–80 ЊC) eluted compound 5
(16%), mp 55–56 ЊC, identical in all other respects with the
sample prepared as described in (a).
3-Bromothiophene-2-carbaldehyde 2
3-Bromothiophene 1 (44.7 g, 274.0 mmol) was added dropwise
to a stirred solution of lithium diisopropylamide (LDA)
[prepared by addition of butyllithium (1.37 mol dmϪ3 in
hexane; 200 cm3, 274.0 mmol) to diisopropylamine (38.40 cm3,
27.73 g, 274.0 mmol)] in tetrahydrofuran (500 cm3) at 0 ЊC and
the resulting mixture was stirred for a further 30 min at this
temperature prior to addition of N-formylpiperidine (30.38
cm3, 30.96 g, 274.0 mmol). The mixture was stirred further until
TLC analysis indicated that all the starting material had been
consumed (~ 3 h) when an excess of 20% aqueous ammonium
chloride was added to it. Extraction of the mixture with ether
gave 3-bromothiophene-2-carbaldehyde 2 (41.9 g, 80%), bp
(Kugelrohr distillation) 75 ЊC at 0.2 mmHg (lit.,43 70% crude
yield and bp 113–115 ЊC at 10.0 mmHg; prepared from 2,3-
dibromothiophene); νmax/cmϪ1 1734 (CO); δH 7.11 (1 H, d, J4,5
5.0, H-4), 7.69 (1 H, dd, J 1.4, J5,4 5.0, H-5) and 9.92 (1 H, d,
J 1.4, CHO); m/z (CI) 227 (Mϩ ϩ 35, 100%), 225 (Mϩ ϩ 35,
99%), 210 (Mϩ ϩ 18, 52%), 208 (Mϩ ϩ 18, 50%), 192 (Mϩ, 6%)
and 190 (Mϩ, 4%). The crude product (49.6 g, 95%) from an
identical experiment was found to be pure enough to use in the
following reaction.
Bis(3-bromo-2-thienyl)methanol 9
3-Bromothiophene 1 (20.0 g, 123.0 mmol) was added dropwise
to a solution of lithium diisopropylamide (LDA)[prepared by
addition of butyllithium (1.5 mol dmϪ3 in hexane; 82.0 cm3,
123.0 mmol) to diisopropylamine (17.3 cm3, 12.49 g, 123.0
mmol)] in tetrahydrofuran (200 cm3) at 0 ЊC and the resulting
mixture was stirred at this temperature for a further 30 min.
Methyl formate (8.0 cm3, 7.79 g, 130.0 mmol) was then added to
the mixture after which it was stirred until no further starting
material could be detected by TLC analysis (3 h). An excess of
20% aqueous ammonium chloride was added to the mixture
which was then extracted with ether to give, on work-up, the
crude product which was chromatographed on silica. Light
petroleum–ethyl acetate (ratio 6:1) eluted compound 9 (18.5 g,
84%) as an oil; νmax/cmϪ1 3349 (OH); δH 3.02 (1 H, d, J 3.0, OH),
6.40 (1 H, d J 3.0, ArCH), 6.93 (2 H, d, J4,5 5.0, H-4) and 7.26 (2
H, d, J5,4 5.0, H-5); m/z 354 (Mϩ, 2%), 339 (Mϩ Ϫ 17, 39%), 337
(Mϩ Ϫ 17, 70%) and 335 (Mϩ Ϫ 17, 35%) (Found: Mϩ ϩ 1,
352.8289. C9H6Br2OS2 requires M ϩ 1, 352.8306).
Ethyl thieno[3,2-b]thiophene-2-carboxylate 3
3-Bromothiophene-2-carbaldehyde 2 (25.71 g, 134.0 mmol) was
added to a stirred mixture of ethyl 2-sulfanylacetate (14.8 cm3,
16.22 g, 135.0 mmol), potassium carbonate (25.0 g) and N,N-
dimethylformamide (250 cm3) at ambient temperature and the
resulting mixture was stirred for a further 72 h. Then it was
poured into water (500 cm3) and extracted with dichloro-
methane. The combined extracts were dried (MgSO4), filtered
and distillation of the solvents under reduced pressure gave the
ester 3 (23.0 g, 81%), bp (Kugelrohr distillation) 120–125 ЊC at
0.1 mmHg; νmax/cmϪ1 1707 (CO); δH 1.37 (3 H, t, J 7.0, Me),
4.34 (2 H, q, J 7.0, CH2), 7.24 (1 H, d, J6,5 5.0, H-6), 7.55 (1 H,
d, J5,6 5.0, H-5) and 7.97 (1 H, s, H-3); m/z 247 (Mϩ ϩ 35, 18%),
230 (Mϩ ϩ 18, 100%) and 213 (Mϩ ϩ 1, 35%) (Found: C, 51.1;
H, 3.9; S, 30.6. C9H8S2O2 requires C, 50.9; H, 3.8; S, 30.2%).
2,3,5-Tribromothieno[3,2-b]thiophene 6
Bromine (0.62 cm3, 1.93 g, 12.0 mmol) was added dropwise to a
stirred solution of the acid 4 (2.0 g, 11.0 mmol) in acetic acid
(500 cm3) at ambient temperature and the resulting mixture was
stirred for a further 1 h. After this water (100 cm3) was added to
the mixture which was then heated under reflux. More bromine
(2.47 cm3, 7.68 g, 48.0 mmol) was added to this hot, stirred
solution which was then heated under reflux until no starting
material could be detected by TLC analysis. The white precipi-
tate of compound 6 (3.84 g, 93%) was filtered off, washed with
10% aqueous sodium thiosulfate and recrystallised from
trichloromethane, mp 140–142 ЊC (lit.,36 mp 140–143.5 ЊC); δH
7.21 (1 H, s, H-6).
2,3,5,6-Tetrabromothieno[3,2-b]thiophene 7
Thieno[3,2-b]thiophene-2-carboxylic acid 4
(a) Bromine (2.06 cm3, 6.41 g, 40.1 mmol) was added dropwise
to a stirred solution of the acid 4 (7.41 g, 40.1 mmol) in acetic
acid (1 l) at ambient temperature and the resulting mixture was
stirred for a further 1 h before it was diluted with water (100
cm3) to give a precipitate. The mixture was then heated under
reflux when more bromine (2.06 cm3, 6.41 g, 40.1 mmol) was
added to it. When TLC analysis showed that the intermediate
monobrominated acid had all reacted (~ 3 h) more bromine
(8.20 cm3, 160.0 mmol) was added dropwise to the mixture dur-
ing 15 min. The resulting solution was then heated at 60–70 ЊC
and stirred for a further 12 h. After this the white precipitate
was filtered off, washed with water and recrystallised from
tetrahydrofuran, to give compound 7 (12.65 g, 70%), mp 229–
230 ЊC [lit.,31,55 mp 237–238.5 (sealed tube) and 229–230 ЊC];
m/z 460 (Mϩ, 13%), 458 (Mϩ, 68%), 456 (Mϩ, 100%), 454 (Mϩ,
68%) and 452 (Mϩ, 12%) (Found: Mϩ, 452.6254. C6Br4S2
requires M, 452.6255).
A stirred mixture of the ester 3 (15.0 g, 71.0 mmol), aqueous
lithium hydroxide (1.0 mol dmϪ3; 150 cm3) and tetrahydrofuran
(150 cm3) was heated under reflux for 3 h (until TLC analysis
indicated absence of starting material) when the solvent was
distilled off under reduced presssure and conc. hydrochloric
acid (300 cm3) was added to the residue. The precipitate
was filtered off, washed with water and dried in a vacuum
desiccator, to give the acid 4 (11.76 g, 90%), mp 221–222 ЊC
(lit.,24,35,66 mp 218–220 and 219–220 ЊC); νmax/cmϪ1 1672
(CO); δH([2H6]-DMSO; 300 MHz) 7.50 (1 H, d, J6,5 5.0, H-6),
7.91 (1 H, d, J5,6 5.0, H-5) and 8.09 (1 H, s, H-3); m/z 202
(Mϩ ϩ 18, 100%), 184 (Mϩ, 75%) and 140 (Mϩ Ϫ 44, 10%).
Thieno[3,2-b]thiophene 5
(a) A stirred solution of the acid 4 (10.0 g, 54.3 mmol), copper
powder (2.0 g) and quinoline (80 cm3) was heated at 260 ЊC in a
Woods-metal bath. When no further bubbles of carbon dioxide
gas could be seen escaping from the mixture (~ 30 min) the
reaction mixture was cooled to ambient temperature. TLC
analysis confirmed the absence of starting material. Ether (100
cm3) was added to the mixture and most of the quinoline was
removed by repeated washing of the resulting solution with
hydrochloric acid (1.0 mol dmϪ3), after which the residue was
(b) 2,3,5-Tribromothieno[3,2-b]thiophene 6 (0.3 g, 0.79
mmol) in tetrahydrofuran (5.0 cm3) was added to a stirred solu-
tion of LDA [prepared by addition of butyllithium (1.5 mol
dmϪ3 in hexane; 0.53 cm3, 80.0 mmol) to diisopropylamine
(0.12 cm3, 0.86 g, 0.85 mmol)] in tetrahydrofuran (50 cm3) at
Ϫ80 ЊC and the resulting mixture was stirred for a further 5 min
3468
J. Chem. Soc., Perkin Trans. 1, 1997

View Article Online
prior to addition of bromine (0.5 cm3, 1.56 g, 9.7 mmol). The
resulting mixture was allowed to warm slowly to ambient tem-
perature, after which it was poured into water (50 cm3). The
resulting precipitate was filtered off, washed with 10% aqueous
sodium thiosulfate (10 cm3) and dried in an oven, to give
compound 7 (0.283 g, 78%), mp 229–230 ЊC, identical in all
respects with the sample prepared as described in (a).
265 ЊC (from light petroleum–ethyl acetate); νmax/cmϪ1 1625
(CO); δH 2.60 (6 H, s, COMe) and 8.19 (2 H, s, H-2/5) (Found:
MHϩ, 225.0052. C10H8O2S2 requires MH, 225.0044) and 3-
acetylthieno[3,2-b]thiophene (28%), mp 80–81 ЊC (from light
petoleum) (lit.,62 mp 80–81 ЊC); νmax/cmϪ1 1661 (CO); δH 2.60
(3 H, s, COMe), 7.24 (1 H, d, J5,6 5.0, H-5), 7.50 (1 H, dd, J6,5
5.0, J6,2 1.5, H-6) and 8.10 (1 H, d, J2,6 1.5, H-2) (Found: MHϩ,
182.9944. C8H6OS2 requires MH, 182.9938) (ether as the
solvent); 3,6-bis(phenylsulfanyl)thieno[3,2-b]thiophene 20 (64%;
reaction carried out in hexane–tetrahydrofuran), mp 134–
136 ЊC (after chromatography on a silica column; light petrol-
eum as eluent); δH 7.29 (2 H, s, H-2/5) and 7.20–7.50 (10 H, m,
2 × SPh) (Found: C, 60.5; H, 3.3%; Mϩ ϩ 1, 356.9891. C18H12S4
requires C, 60.7; H, 3.4%; M ϩ 1, 356.9895); and 3,6-
bis(trimethylsilyl)thieno[3,2-b]thiophene 21 (42% in ether or
62% in hexane–tetrahydrofuran), mp 84–86 ЊC (after chroma-
tography on neutral alumina; light petroleum as eluent); δH 0.35
(18 H, s, SiMe3) and 7.44 (2 H, s, H-2/5) (Found: C, 50.7; H,
7.0%; Mϩ ϩ 1, 285.0627. C12H20Si2S2 requires C, 50.6; H, 7.1%;
M ϩ 1, 285.0623) were prepared similarly.
3,6-Dibromothieno[3,2-b]thiophene 8
Compound 7 (1.0 g, 2.20 mmol) was dissolved in acetic acid
(100 cm3) heated under reflux, after which zinc dust (0.15 g,
2.30 × 10Ϫ3 mol) was added to the solution; the resulting mix-
ture was heated under reflux for 30 min. A further quantity of
zinc dust (0.15 g, 2.30 × 10Ϫ3 mol) was added to the mixture
after cooling after which refluxing was continued for a further
30 min. The mixture was then cooled to ambient temperature
and diluted with water. The precipitate was filtered off, washed
with water, dried and chromatographed on silica. Light petrol-
eum eluted compound 8 (0.50 g, 76%), mp 120–122 ЊC (from
light petroleum); νmax/cmϪ1 727 (C-Br); δH 7.32 (2 H, s, H-2/5)
(Found: C, 24.6; H, 0.8%; Mϩ ϩ 1, 296.8050. C6H2Br2S2
requires C, 24.2; H, 0.7%; M ϩ 1, 296.8044). The product
decomposed slowly with release of bromine.
3,6-Dibromo-2,5-dimethylthieno[3,2-b]thiophene 23
Butyllithium (1.37 mol dmϪ3 in hexane; 2.10 cm3, 2.90 mmol)
was added to a stirred solution of compound 7 (0.64 g, 1.40
mmol) in tetrahydrofuran (50 cm3) at ambient temperature and
the resulting mixture was stirred for 30 min prior to addition
of freshly distilled dimethyl sulfate (0.3 cm3, 0.34 g, 2.70 mmol).
The reaction mixture was stirred overnight at ambient tempera-
ture, after which it was quenched with an excess of 20% aque-
ous ammonium chloride. Extraction with ether gave the prod-
uct which was flash chromatographed on silica. Light petrol-
eum eluted 3,6-dibromo-2,5-dimethylthieno[3,2-b]thiophene 23
(0.25 g, 55%), mp 171–173 ЊC (from ether); νmax/cmϪ1 1600
(C᎐C); δ 2.47 (6 H, s, Me) (Found: Mϩ, 323.8278. C8H6Br2S2
2,5-Disubstituted thieno[3,2-b]thiophenes 13–16
Butyllithium (1.5 mol dmϪ3 in hexane; 1.3 cm3, 1.95 mmol)
was added dropwise to a stirred solution of compound 5 (0.12
g, 0.86 mmol) in ether (50 cm3) at 0 ЊC and the resulting mixture
was stirred for a further 10 min; it was then quenched with
dimethyl disulfide (0.50 cm3, 0.52 g, 5.55 mmol). After a further
15 min the mixture was allowed to warm to ambient temper-
ature when it was diluted with water (50 cm3). Extraction of the
product with ether gave 2,5-bis(methylsulfanyl)thieno[3,2-b]-
thiophene 13 (0.14 g, 73%), mp 73–75 ЊC (from light petroleum–
᎐
H
ethyl acetate); νmax/cmϪ1 1632 (C᎐C); δH 2.50 (6 H, s, SMe) and
requires M, 323.8278).
᎐
7.18 (2 H, s, H-3/6) (Found: C, 41.4; H, 3.45%; Mϩ ϩ 1,
232.9582. C8H8S4 requires C, 41.4; H, 3.45%; M ϩ 1, 232.9587).
The following compounds were prepared similarly: 2,5-bis-
(phenylsulfanyl)thieno[3,2-b]thiophene 14 (62%), mp 137–
139 ЊC (from light petroleum–ethyl acetate); νmax/cmϪ1 1671
3,6-Dibromo-2,5-bis(methylsulfanyl)thieno[3,2-b]thiophene 24
(54%), mp 126–128 ЊC (from light petroleum ); νmax/cmϪ1 1600
(C᎐C); δ 2.50 (6 H, s, SMe) (Found: C, 24.9; H, 1.3%; Mϩ ϩ 1,
᎐
H
388.7784. C8H6Br2S4 requires C, 24.6; H, 1.55%; M ϩ 1,
388.7798); and 3,6-dibromo-2,5-bis(trimethylsilyl)thieno[3,2-b]-
thiophene 26 (75%), mp 145–147 ЊC (from light petroleum);
νmax/cmϪ1 1644 (C᎐C); δ 0.34 (18 H, s, 2 × SiMe3); δC Ϫ0.96,
(C᎐C); δ 7.15–7.30 (10 H, m, 2 × SPh) and 7.40 (2 H, s, H-3/6)
᎐
H
(Found: C, 60.65; H, 3.3%; Mϩ ϩ 1, 356.9895. C18H12S4
requires C, 60.7; H, 3.4%; M ϩ 1, 356.9900): 2,5-bis(trimethyl-
silyl)thieno[3,2-b]thiophene 15 (66%), mp 108–109 ЊC (from
light petroleum–ethyl acetate); νmax/cmϪ1 1644 (C᎐C); δ 0.33
᎐
H
29.7, 108.7 and 138.9 (Found: MHϩ, 442.8800. C12H1879Br-
81BrSiS2 requires MH, 442.8814) were prepared similarly.
᎐
H
(18 H, s, SiMe3) and 7.31 (2 H, s, H-3/6) (Found: C, 50.5; H,
7.1%; Mϩ ϩ 1, 285.0618. C12H20Si2S2 requires C, 50.6; H, 7.1%;
M ϩ 1, 285.0623): and 2,5-bis(tert-butyldimethylsilyl)thieno-
[3,2-b]thiophene 16 (41%), mp 117–118 ЊC (from aqueous
ethanol); νmax/cmϪ1 1644 (C᎐C); δ 0.29 (12 H, s, SiMe2), 0.93
3,6-Dibromo-2,5-bis(6-methylcyclohex-1-enyl)thieno[3,2-b]-
thiophene 25
Butyllithium (1.37 mol dmϪ3 in hexane; 1.6 cm3, 2.20 mmol)
was added to a stirred solution of compound 7 (0.39 g, 0.86
mmol) in tetrahydrofuran (50 cm3) at ambient temperature
and the resulting mixture was stirred for a further 30 min. 2-
Methylcyclohexanone (0.25 g, 2.20 mmol) was added to the
mixture which was then stirred overnight. After this it was
washed with hydrochloric acid (2.0 mol dmϪ3; 50 cm3) and
extracted with ether to give the crude carbinol. This was dis-
solved in toluene (100 cm3) to which toluene-4-sulfonic acid (0.1
g) was added; the resulting mixture was heated under reflux for
8 h whilst water was azeotroped off in a Dean–Stark apparatus.
After this the toluene was distilled off and the residue was flash
chromatographed on silica. Light petroleum eluted the product
25 (0.25 g, 55%) as a colourless oil; νmax/cmϪ1 1577 (C᎐C); δ
᎐
H
(18 H, s, SiCMe3) and 7.31 (2 H, s, H-3/6) (Found: Mϩ ϩ 1,
369.1544. C18H32Si2S2 requires M ϩ 1, 369.1562).
3,6-Disubstituted thieno[3,2-b]thiophenes 19–21
Butyllithium (1.5 mol dmϪ3 in hexane; 0.5 cm3, 0.75 mmol)
was added dropwise to a stirred solution of compound 8 (0.095
g, 0.32 mmol) in ether (50 cm3) at Ϫ100 ЊC and the resulting
mixture was stirred for a further 5 min; it was then quenched at
Ϫ80 ЊC by addition of dimethyl disulfide (0.1 cm3, 0.10 g, 1.1
mmol). The reaction mixture was then allowed to warm up
slowly to ambient temperature when it was diluted with water
(50 cm3) and extracted with ether; work-up gave the crude
product which was chromatographed on silica to give with
light petroleum as eluent 3,6-bis(methylsulfanyl)thieno[3,2-b]-
thiophene 19 (0.03 g, 71%), mp 82–84 ЊC (from light petroleum);
δH 2.52 (6 H, s, SMe) and 7.14 (2 H, s, H-2/5) (Found: C, 41.3;
H, 3.4%; Mϩ ϩ 1, 232.9584. C8H8S4 requires C, 41.4; H, 3.45%;
M ϩ 1, 232.9587).
᎐
H
0.98 (6 H, d, J 7.0, 2 × Me), 1.24–2.31 (12 H, m, 6 × CH2),
2.77–2.79 (2 H, m, 2 × CHMe) and 6.06 (2 H, dd, J 3.0,
2 × ᎐CH) [Found: Mϩ (EI), 483.9532. C20H22Br2S2 requires M,
᎐
483.9531].
Acknowledgements
A mixture of 3,6-diacetylthieno[3,2-b]thiophene 18 (16% with
N,N-dimethylacetamide as the quenching reagent), mp 264–
We thank the EPSRC (CASE Award to K. A. S.) and Synthetic
Chemicals Ltd., Four Ashes, Nr. Wolverhampton for financial
J. Chem. Soc., Perkin Trans. 1, 1997
3469

View Article Online
33 P. Boldt, M. Blenkle, I. Cabrera, D. Lupo and W. Hickel, MCLCS &
support, Synthetic Chemicals Ltd. for supplies of thiophene,
2-acetylthiophene, 3-bromothiophene, and 5-chlorobenzo[b]-
thiophene, Mr R. A. Stewart for supervising the vapour-phase
catalytic conversions, Mrs R. Howard for recording low- and
high-resolution mass spectra, Dr M. A. Stuckey for recording
the 1H NMR spectra and Mr M. Hart (University of Manches-
ter) for supplying the microanalytical data (C, H and N
analyses).
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Paper 7/01877K
Received 18th March 1997
Accepted 6th August 1997
3470
J. Chem. Soc., Perkin Trans. 1, 1997