A new, improved and convenient synthesis of 4H-cyclopenta[2,1-b:3,4-b′]-dithiophen-4-one

Article abstract of DOI:10.1055/s-2002-31958

A new and efficient three-step synthesis of 4H-cyclopenta[2,1-b:3,4-b′]dithiophen-4-one (CDT) (1) is described. This was achieved by a one-pot, regiospecific synthesis of bis(2-iodo-3-thienyl)methanol (13), its subsequent oxidation to the bls(2-iodo-3-thienyl) ketone (14) which after Ullmann coupling yielded the title compound 1.

Full text of DOI:10.1055/s-2002-31958

PAPER
1053
A New, Improved and Convenient Synthesis of 4H-Cyclopenta[2,1-b:3,4-b ]-
dithiophen-4-one
Synthesis
a
of
4
H
-Cycl
c
openta[2,1-
e
b
:3,4-
b
’
]
dit
k
hiophen-4-one Z. Brzezi ski,*a John R. Reynoldsb
a
Department of Organic Chemistry, Institute of Chemistry, University of ód , Narutowicza 68, 90-136 ód , Poland
Fax +48(42)6786583; E-mail: jzb@krysia.uni.lodz.pl
b
Center for Macromolecular Science and Engineering, Department of Chemistry, University of Florida, Gainesville, FL 32611-7200, USA
E-mail: reynolds@chem.ufl.edu
Received 13 March 2002
The method that has been employed previously in prepa-
ration of compound 1 is a five-step procedure reported by
Gronowitz8 and Wynberg9 with some further
modifications2g,4b as depicted in path A of Scheme 1.
Abstract: A new and efficient three-step synthesis of 4H-cyclopen-
ta[2,1-b:3,4-b ]dithiophen-4-one (CDT) (1) is described. This was
achieved by a one-pot, regiospecific synthesis of bis(2-iodo-3-thie-
nyl)methanol (13), its subsequent oxidation to the bis(2-iodo-3-
thienyl) ketone (14) which after Ullmann coupling yielded the title Thus, di(3-thienyl)methanol (6) was prepared from 3-
compound 1.
thienyllithium [obtained via metal halogen exchange be-
tween 3-bromothiophene (5) and n-butyllithium] and
thiophene-3-carbaldehyde (3-ThCHO)8a or methyl
formate4b. The alcohol 6 was oxidized with chromium(VI)
oxide8a or pyridinium chlorochromate (PCC)2g,4b yielding
di(3-thienyl) ketone (7) which was subsequently protect-
ed with ethylene glycol furnishing 1,3-dioxolane 8.8b,9 In
the next step, compound 8 was reacted with n-butyllithi-
um affording the 2,2 -dilithioderivative which, upon reac-
tion with iodine, gave the corresponding 2,2 -
diiododioxolane 9.9 The diiododerivative 9, after subjec-
tion to Ullmann reaction followed by acidic deprotection,
afforded the ketone 1.9 However, the overall yield of the
process was low, in the range of 20 25%.
Recently, Breau10 has proposed a four-step approach to
the ketone 1 what is depicted in path B of Scheme 1. In
general, it is similar to Gronowitz and Wynberg method8,9
and its main advantage is omission of the synthesis of
dithienylmethanol 6 by adopting Hörnfeldt procedure11
for preparation of ketone 7 in the reaction of 3-thienyllith-
ium with N,N-dimethylcarbamoyl chloride. The other im-
provement was achieved by protection of the carbonyl
group using Noyori method12 for acetalization under apro-
tic conditions, as opposed to the classic acetalization un-
der protic conditions which was employed before.8b,9 The
acetal 8 obtained was then transformed in two steps into
the cyclopentadithiophenone 1 applying Wynberg meth-
od.9 These modifications resulted in some increase in ef-
ficiency, but the overall yield of synthesis of the ketone 1
was still below 40%.
Key words: regiospecific lithiation, iodination, oxidation, Ullmann
coupling, conducting polymers
In recent years there has been a growing number of reports
dealing with polymers obtained from 4H-cyclopenta-
[2,1-b:3,4-b ]dithiophene and its derivatives.1 Among
them, polymers of the title 4H-cyclopenta-[2,1-b:3,4-b ]-
dithiophen-4-one (CDT) (1) as well as compounds 2, 3, 4
(Figure 1) and copolymers utilizing any of these precur-
sors as comonomers have attracted special interest2–7 for
their electroactivity, n-type dopability and electro-
chromism. These polymers and copolymers belong to a
very promising group of conducting polymers with the
lowest known band gaps in the range from 0.16 to 1.5
eV.2a–h,3,5,6,7b
R
R
S
S
Y
Z
O
X
X
S
S
S
S
S
S
S
S
1
2
3
4
Y, Z = electron-withdrawing groups
R = H, Me, benzo, CO2Me, S(CH2)nMe
X = O, S
Figure 1
We now wish to report a new method for the synthesis of
the title ketone 1 using a three-step protocol starting from
3-bromothiophene (5), as depicted in Scheme 2. The first,
key step was a one-pot process and involved formation of
3-thienyllithium [prepared via metal halogen exchange
between 3-bromothiophene (5) and n-butyllithium]
which, when treated with thiophene-3-carbaldehyde (3-
ThCHO), furnished the intermediate, lithium dithienyl-
methoxide 10. Methoxide 10 was then reacted with two
equivalents of n-butyllithium to generate the trilithiated
species 11, which, upon reaction with three equivalents of
The importance of the ketone 1 as a monomer itself, and the
fact that it is the key compound in preparation of the deriv-
atives 2, 3 and 4, causes its synthesis to be a crucial factor
deciding the availability of a great number of functional-
ized poly(4H-cyclopenta[2,1-b:3,4-b ]dithiophenes).
Synthesis 2002, No. 8, 04 06 2002. Article Identifier:
1437-210X,E;2002,0,08,1053,1056,ftx,en;P01102SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

1054
J. Z. Brzezi ski, J. R. Reynolds
Br
Br
Li
Path A
Path B
a
(1 equiv)
BuLi
BuBr
+
+
OH
a, b, c
S
S
S
5
5
b
j, k, l
O
S
S
d
Li
O
6
O
Li
7
S
S
+
S
S
S
e
m
15
10
c
O
O
O
Li
Li
O
8
S
S
+
f, g
O
S
S
Li
Li Li
S
16
11
O
d
9
OI
IO
S
S
I
I
+
h, i
S
S
I
I
I
S
O
17
12
e
OH
1
HO
S
S
+
Scheme 1 Reagents and conditions: a) BuLi (1 equiv), Et2O, –70
ºC8a or BuLi (1 equiv), THF–hexanes, –40 ºC r.t.,4b b) 3-ThCHO (1
equiv), Et2O, –70 ºC –10 ºC8a or HCOOMe (0.5 equiv), THF, r.t.,4b
c) H2O, –10 ºC r.t.8a or aq NH4Cl, AcOH–H2O, r.t.,4b d) CrO3,
AcOH–H2O, r.t.8a or PCC, CH2Cl2, 0 ºC r.t.,2g,4b e) HOCH2CH2OH,
TsOH (cat.), C6H6, reflux (–H2O),8b,9 f) BuLi (2 equiv), Et2O, –35
ºC r.t.,9 g) I2 (2 equiv), Et2O, r.t.,9 h) Cu (powder), DMF, reflux,9 i)
aq HCl, r.t.,9 j) BuLi (1 equiv), Et2O–hexanes, –78 ºC –40 ºC,10 k)
ClCONMe2 (0.5 equiv), –50 ºC –30 ºC,10 l) aq HCl, 0 ºC r.t.,10 m)
TMSOCH2CH2OTMS, TMSOTf (cat.), CH2Cl2, 0 ºC r.t.10
S
S
I
I
I
S
18
13
f
O
O
g
S
S
S
S
I
I
1
14
Scheme 2 Reagents and conditions: a) Et2O–hexanes, –78 ºC, 3 h,
b) 3-ThCHO (1 equiv), Et2O, –78 ºC, 0.5 h r.t., 0.5 h, c) BuLi (2
equiv), hexanes, –23 ºC, 2 h r.t., 1 h, d) I2 (3 equiv), Et2O, –23
ºC r.t., e) aq HI, r.t., f) PCC, CH2Cl2, r.t., 12 h, g) Cu (powder),
DMF, reflux, 15 h
iodine followed by hydrolysis, afforded the 2,2 -diiodoal-
cohol 13 in 78% yield. The lithiation reaction was re-
1
giospecific as evidenced by H and 13C NMR spectra of
the crude material. No signals for other regioisomers of
compound 13 were detected.
In the lithiation of compound 10, we utilized the ortho-di-
recting effect of the methoxide group.13 Surprisingly, al-
though ortho metalation reactions of thiophenes
substituted at the 3-position are very well known and
widely employed in synthesis,13c,14 there are only a few
examples of such conversions for both, 3-thienylmethanol
itself and substituted 3-thienylmethanols. The only two
cases we are aware of were reported by Snieckus15a who
metalated lithium 3-thienylmethoxides having an aryl or
heteroaryl substituted methylene groups with sec-butyl-
lithium, and recently by May15b who lithiated unsubstitut-
ed 3-thienylmethanol with n-butyllithium. The authors
claimed only the formation of products arising from ortho
lithiation in position 2 of thiophene ring.
It cannot be excluded that iodination of the trilithioderiv-
ative 11 proceeded via the hypoiodite 12 since, when only
two equivalents of iodine were used, the yield of the di-
iodoalcohol 13 dropped to 51% and also the monoiodoal-
cohol 1916 (Figure 2) was formed in 16% yield. In the
literature there are a few examples for the formation of hy-
poiodites in the reaction of alkoxides with iodine that sup-
ports our hypothesis.17
After the synthesis, in addition to the 2,2 -diiodoalcohol
13 we also isolated, as a side product, 1-(2-iodo-3-thie-
nyl)pentan-1-ol (18) in 14% yield, irrespectively whether
two or three equivalents of iodine were used. As depicted
in Scheme 2, the iodoalcohol 18 was formed in the reac-
tion of n-butyllithium with thiophene-3-carbaldehyde
Synthesis 2002, No. 8, 1053–1056 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Synthesis of 4H-Cyclopenta[2,1-b:3,4-b ]dithiophen-4-one
1055
Bis(2-iodo-3-thienyl)methanol (13) and 1-(2-Iodo-3-thie-
nyl)pentan-1-ol (18)
OH
4'
4
3
3'
All reactions were carried out under argon in a flask equipped with
a mechanical stirrer, a thermometer and a dropping funnel.
5'
5
2
2'
S
S
1'
1
I
BuLi soln in hexanes (20.0 mmol) was added via a syringe to a
stirred soln of 3-bromothiophene (5) (3.26 g, 20.0 mmol) in Et2O
(25 mL) at 78 ºC. Stirring was continued for 3 h and subsequently
a soln of thiophene-3-carbaldehyde (2.24 g, 20.0 mmol) in Et2O (25
mL) was added via syringe. The reaction mixture was stirred at –78
ºC for 0.5 h, allowed to reach r.t., and stirred for a further 0.5 h. Af-
ter cooling to –23 ºC, BuLi soln in hexanes (40.0 mmol) was added
via syringe. Stirring at –23 ºC was continued for 2 h, then the result-
ing mixture was allowed to reach r.t., and stirred for another 1 h. Af-
ter cooling to –23 ºC, a soln of I2 (15.99 g, 63.0 mmol) in Et2O (100
mL) was added dropwise. The reaction mixture was allowed to
warm-up to r.t., a 10% w/w aq soln of Na2SO3 (50 mL) was added
with vigorous stirring and the H2O layer was acidified with a 10%
w/w aq soln of hydriodic acid to pH ~ 5. The layers were separated,
the etheral layer was washed with H2O (20 mL portions) until neu-
tral, and dried over anhyd MgSO4. The solvent was distilled off un-
der reduced pressure, the solid residue obtained was triturated with
a mixture of benzene–hexane, 1:2 (10 mL). The solid was filtered
off and washed on a filtration funnel with benzene–hexane, 1:2
(3 5 mL) yielding 6.02 g of the product 13. From the filtrate, sol-
vents were distilled off under reduced pressure, the residue was sub-
jected to column chromatography on silica gel using CH2Cl2 as
eluent and two fractions were collected.
19
Figure 2
(3-ThCHO) and subsequent generation of dilithioderiva-
tive 16 by regiospecific ortho lithiation of the alkoxide 15
followed by iodination and hydrolysis. Presence of n-bu-
tyllithium in the reaction mixture resulted from existing
equilibrium in metal halogen exchange between 3-bro-
mothiophene (5) and n-butyllithium what is very well
known fact.18 It can be theorized that the iodination reac-
tion also proceeded via the hypoiodite 17, similarly to the
formation of alcohol 13.
In the second step, alcohol 13 was oxidized with pyridin-
ium chlorochromate (PCC) giving the 2,2 -diiodoketone
14 in 94% yield. Finally, ketone 14 was subjected to an
Ullmann coupling reaction and afforded the cyclopenta-
dithiophenone 1 in 90% yield. Both of these steps pro-
ceeded in almost quantitative yields as evidenced by the
1H and 13C NMR spectra of the crude materials after reac-
tions.
After the solvent was distilled off under reduced pressure from the
first fraction eluted, 0.95 g of the product 13 was obtained which
was combined with the main crop obtained after trituration; total
yield 6.97 g (78%); mp 103–110 °C. The analytical sample was ob-
tained as a tan solid after recrystallization from a mixture benzene–
hexane, 1:1; mp 116–118°C.
1H NMR (200 MHz, CDCl3): = 2.37 (d, 1 H, J = 3.2, OH), 5.75 (d,
1 H, J = 2.9, HOCH), 6.92 (d, 2 H, J = 5.6, 4- and 4 -H), 7.43 (d, 2
H, J = 5.6, 5- and 5 -H).
In conclusion, we have developed a new method for the
preparation of 4H-cyclopenta[2,1-b:3,4-b ]dithiophen-4-
one (CDT) (1), the precursor of a new class of conducting
polymers. The reported procedure is particularly useful
because of its convenience (only three-step synthesis) and
efficiency (66% overall yield).
13C NMR (50 MHz, CDCl3): = 71.6 (HOCH), 75.2 (2- and 2 -C),
126.8 (4- and 4 -C or 5- and 5 -C), 131.3 (4- and 4 -C or 5- and 5 -
C), 146.5 (3- and 3 -C).
Melting points were determined using a Boetius hot stage apparatus
and are uncorrected. The yields are given on pure products and were
not optimized. The 1H NMR spectra were recorded at 200 MHz and
the 13C NMR spectra were recorded at 50 MHz on a Varian Gemini
200 BB spectrometer. All spectra were obtained using CDCl3 as the
solvent and Me4Si ( = 0 ppm) as the internal standard. High reso-
lution mass spectra (HRMS) were recorded by using chemical ion-
ization on a Finnigan MAT 95 spectrometer. Analytical thin layer
chromatography tests (TLC) were carried out on Whatman silica gel
plates [PE SIL G/UV (fluorescence UV254), layer thickness 250 m]
and visualized by UV lamp at 254 nm in an iodine chamber. Col-
umn chromatography separations and purifications were performed
on silica gel 60 (0.063 0.100 mm) from Merck using 30 g of silica
gel per 1 g of the separated or purified material. n-Butyllithium (1.6
M soln in hexanes) was obtained from Aldrich and titrated before
use.19 Pyridinium chlorochromate (PCC) (98%) and dimethylfor-
mamide (DMF) (99.8%, anhyd) were obtained from Aldrich and
used as received. Thiophene-3-carbaldehyde (3-ThCHO) (97%)
and 3-bromothiophene (5) (97%) were obtained from Lancaster and
distilled under reduced pressure prior to use. Iodine (pure for anal-
ysis) was obtained from POCh and used as received. Copper pow-
der (copper bronze) (99%) from Aldrich was activated before use.20
Et2O (pure) was obtained from POCh and freshly distilled over so-
dium benzophenone ketyl. CH2Cl2 (pure) was obtained from POCh
and dried over 4A molecular sieves activated for 6 h at 210 °C.
Anal. Calcd for C9H6I2OS2: C, 24.12; H, 1.35; S, 14.31. Found: C,
24.20; H, 1.36; S, 14.41.
After the solvent was distilled off under reduced pressure from the
second fraction eluted, the product 18 was obtained as a light brown
liquid; yield 0.82 g (14%). The analytical sample was obtained after
repeated column chromatography on silica gel using a mixture ben-
zene–hexane, 1:1 as eluent.
1H NMR (200 MHz, CDCl3): = 0.90 (t, 3 H, J = 6.9, CH3), 1.07–
1.52 (m, 4 H, CH2), 1.52–1.94 (m, 2 H, HOCHCH2), 2.10 (br s, 1 H,
OH), 4.68 (t, 1 H, J = 6.8, HOCH), 6.93 (d, 1 H, J = 5.5, 4-H), 7.44
(d, 1 H, J = 5.3, 5-H).
13C NMR (50 MHz, CDCl3): = 14.0 (CH3), 22.5 (CH2), 27.7
(CH2), 37.3 (HOCHCH2), 71.7 (HOCH), 73.7 (2-C), 125.7 (4- or 5-
C), 131.2 (4- or 5-C), 148.7 (3-C).
HRMS: m/z calcd for C9H13IOS, 295.973183; found: 295.973500.
Bis(2-iodo-3-thienyl) Ketone (14)
To a stirred soln of alcohol 13 (4.48 g, 10.0 mmol) in CH2Cl2 (100
mL) solid PCC (3.23 g, 15.0 mmol) was added in one portion at r.t.
Stirring at r.t. was continued for 12 h and from the reaction mixture
about half volume of the solvent was distilled off under reduced
pressure. The resulting slurry was purified by column chromatogra-
phy on 75 g of silica gel using CH2Cl2 as eluent. From the eluate,
solvent was distilled off under reduced pressure and the solid ob-
Synthesis 2002, No. 8, 1053–1056 ISSN 0039-7881 © Thieme Stuttgart · New York

1056
J. Z. Brzezi ski, J. R. Reynolds
tained was recrystallized from MeOH giving the product 14 as a
yellow solid. Yield 4.20 g (94%), mp 97–98.5 °C.
(4) (a) Gunatunga, S. R.; Jones, G. W.; Kalaji, M.; Murphy, P.
J.; Taylor, D. M.; Williams, G. O. Synth. Met. 1997, 84, 973.
(b) Beyer, R.; Kalaji, M.; Kingscote-Burton, G.; Murphy, P.
J.; Pereira, V. M. S. C.; Taylor, D. M.; Williams, G. O.
Synth. Met. 1998, 92, 25. (c) Kalaji, M.; Murphy, P. J.;
Williams, G. O. Synth. Met. 1999, 101, 123. (d) Mills, C.
A.; Taylor, D. M.; Murphy, P. J.; Dalton, C.; Jones, G. W.;
Hall, L. M.; Hughes, A. V. Synth. Met. 1999, 102, 1000.
(e) Kalaji, M.; Murphy, P. J.; Williams, G. O. Synth. Met.
1999, 102, 1360.
1H NMR (200 MHz, CDCl3): = 7.05 (d, 2 H, J = 5.6, 4- and 4 -H),
7.47 (d, 2 H, J = 5.6, 5- and 5 -H).
13C NMR (50 MHz, CDCl3): = 81.3 (2- and 2 -C), 129.8 (4- and
4 -C or 5- and 5 -C), 131.6 (4- and 4 -C or 5- and 5 -C), 143.2 (3-
and 3 -C), 185.5 (C=O).
Anal. Calcd for C9H4I2OS2: C, 24.23; H, 0.90; S, 14.38. Found: C,
24.16; H, 0.85; S, 14.69.
(5) (a) Huang, H.; Pickup, P. G. Acta Polym. 1997, 48, 455.
(b) Huang, H.; Pickup, P. G. Chem. Mater. 1998, 10, 2212.
(c) Huang, H.; Pickup, P. G. Chem. Mater. 1999, 11, 1541.
(6) Fusalba, F.; El Mehdi, N.; Breau, L.; Belanger, D. Chem.
Mater. 1999, 11, 2743.
(7) (a) Kozaki, M.; Tanaka, S.; Yamashita, Y. J. Chem. Soc.,
Chem. Commun. 1992, 1137. (b) Kozaki, M.; Tanaka, S.;
Yamashita, Y. J. Org. Chem. 1994, 59, 442.
(8) (a) Gronowitz, S.; Eriksson, B. Ark. Kemi 1963, 21, 335.
(b) Gronowitz, S.; Skramstad, J. E.; Eriksson, B. Ark. Kemi
1967, 28, 99.
(9) Jordens, P.; Rawson, G.; Wynberg, H. J. Chem. Soc. C 1970,
273.
(10) Lucas, P.; El Mehdi, N.; Ho, H. A.; Belanger, D.; Breau, L.
Synthesis 2000, 1253.
(11) Michael, U.; Hörnfeldt, A. B. Tetrahedron Lett. 1970, 5219.
(12) (a) Tsunoda, T.; Suzuki, M.; Noyori, R. Tetrahedron Lett.
1980, 21, 1357. (b) Noyori, R.; Murata, S.; Suzuki, M.
Tetrahedron 1981, 37, 3899.
(13) (a) Gschwend, H. W.; Rodriguez, H. R. Heteroatom-
Facilitated Lithiations. In Organic Reactions, Vol. 26;
Dauben, W. G., Ed.; John Wiley & Sons, Inc.: New York,
1979, 43; and references cited therein. (b) Gschwend, H.
W.; Rodriguez, H. R. Heteroatom-Facilitated Lithiations. In
Organic Reactions, Vol. 26; Dauben, W. G., Ed.; John
Wiley & Sons, Inc.: New York, 1979, 67; and references
cited therein. (c) Snieckus, V. Chem. Rev. 1990, 90, 879; and
references cited therein.
(14) Frejd, T. Organometallic Derivatives of Thiophenes. In
Thiophene and Its Derivatives, Part Five; Gronowitz, S.,
Ed.; John Wiley & Sons, Inc.: New York, 1992, 257–754;
and references cited therein.
4H-Cyclopenta-[2,1-b:3,4-b ]dithiophen-4-one (1)
To a stirred soln of ketone 14 (4.46 g, 10.0 mmol) in DMF (30 mL)
Cu powder (1.91 g, 30.0 mmol) was added in one portion at r.t. and
the reaction mixture was heated under reflux for 15 h in an appara-
tus protected against moisture with a tube filled with NaOH pellets.
After cooling to r.t., the solid was filtered off and washed with Et2O
(5 10 mL). The filtrate was transferred to a separatory funnel, H2O
(150 mL) was added, the layers were separated and the H2O layer
was extracted with Et2O (2 50 mL). The combined ether extracts
were washed with H2O (1 50 mL), and dried over anhyd MgSO4.
After the solvent was distilled off under reduced pressure, the re-
sulting solid was purified by column chromatography on silica gel
using a mixture CHCl3–benzene, 1:1 as eluent. From the eluate, the
solvents were distilled off under reduced pressure and the solid ob-
tained was recrystallized from hexane giving the product 1 as a pur-
ple solid. Yield 1.72 g (90%), mp 138.5–140.5 °C (Lit.9 mp 135–
136 °C).
1H NMR (200 MHz, CDCl3): = 6.97 (d, 2 H, J = 4.8, 3- and 5-H),
7.02 (d, 2 H, J = 4.8, 2- and 6-H).
13C NMR (50 MHz, CDCl3): = 121.7 (2- and 6-C or 3- and 5-C),
127.2 (2- and 6-C or 3- and 5-C), 142.4 (3a- and 4a-C or 7a- and 7b-
C), 149.2 (3a- and 4a-C or 7a- and 7b-C), 182.6 (C=O).
Acknowledgments
We gratefully acknowledge support from the AFOSR (F49620-00-
1-0047) and ONR (N00014-00-1-0164).
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C
NMR (50 MHz, CDCl3): = 70.6 (HOCH), 74.5 (2-C),
121.5, 126.0, 126.3, 126.6, 131.5, 143.7 (3- or 3 -C), 147.7
(3- or 3 -C). Anal. Calcd for C9H7IOS2: C, 33.55; H, 2.19; S,
19.91. Found: C, 33.64; H, 2.18; S, 19.70.
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