Synthesis of the anti and syn isomers of thieno[f,f′]bis[1] benzothiophene. Comparison of the optical and electrochemical properties of the anti and syn isomers

Article abstract of DOI:10.1021/jo048010w

We report isomer-pure synthesis of thieno[2,3-f:5,4-f′]bis-[1] benzothiophene and thieno[3,2-f:4,5-f′]bis[1]benzothiophene, the anti and syn isomers of a pentacyclic compound consisting of alternating thiophene and benzene rings. The optical and electrochemical properties of both are reported. In the anti isomer, the ribbonlike embedding of three thiophene units leads to a near-planar molecule with favorable π-π stacking behavior in the solid state as shown by X-ray crystal structure analysis.

Full text of DOI:10.1021/jo048010w

Synthesis of the anti and syn Isomers of
Thieno[f,f ′]bis[1]benzothiophene.
Comparison of the Optical and
Electrochemical Properties of the anti and
syn Isomers1
SCHEME 1. Ladder-Type Aromatics
†
‡
Brigitte Wex, Bilal R. Kaafarani,
§
,†
Kristin Kirschbaum, and Douglas C. Neckers*
Center for Photochemical Sciences, Bowling Green State
University, Bowling Green, Ohio 43403, Department of
Chemistry, American University of Beirut, Beirut, Lebanon,
and Department of Chemistry, University of Toledo,
Toledo, Ohio 43606
is unreactive with dienophiles even under drastic condi-
tions. Similar stability trends were shown recently in
more extended fused compounds. Anthra[b:b′]dithiophenes
have improved stability toward oxidation and solubility
neckers@photo.bgsu.edu
Received November 9, 2004
7
compared to pentacene. This overall decrease in reactiv-
ity vanishes as the acene unit in the center of the
molecule is elongated. Thiaheterohelicenes are also com-
posed of alternating thiophene and benzene rings fused
on the [a,c]-bond of the benzene. These compounds can
easily be prepared via a photochemical “stilbene-phenan-
8
threne” cyclization. Starting with the pentacyclic rep-
resentative thieno[3,2-e:4,5-e′]bis[1]benzothiophene (3),
the compounds lose planarity as they twist out of plane
to accommodate the crowdedness. This twist results in
We report isomer-pure synthesis of thieno[2,3-f:5,4-f ′]bis-
9
[
1]benzothiophene and thieno[3,2-f:4,5-f ′]bis[1]benzothio-
the formation of enantiomeric helical structures. Thieno-
phene, the anti and syn isomers of a pentacyclic compound
consisting of alternating thiophene and benzene rings. The
optical and electrochemical properties of both are reported.
In the anti isomer, the ribbonlike embedding of three
thiophene units leads to a near-planar molecule with favor-
able π-π stacking behavior in the solid state as shown by
X-ray crystal structure analysis.
acenes are composed only of fused thiophenes. Therein,
the pentacyclic representative, bis(thieno[2′,3′:4,5]thieno-
[
3,2-b:2′,3′-d])thiophene (4), exhibits less stability com-
10
pared to pentacene, indicating the importance of the
benzene units for stability.
Synthetic strategies for heteronuclear, benzo-fused,
ladder-type materials include cyclization, annulation to
the heterocyclic ring, vacuum pyrolysis, and ring closure
at the heteroatom.11 Viable approaches for large-scale
Pentacene (1, Scheme 1), a member of the acene series
of linear polycyclic aromatic hydrocarbons, is a funda-
12
synthesis for such compounds are scarce. Recent syn-
theses include double condensation of thiophenedicar-
boxaldehyde with 1,4-cyclohexanedione under basic con-
ditions, followed by the reduction of the generated dione.
An inseparable mixture of anthradithiophene isomers
mental component of organic field-effect transistors
2
(
OFET). Attempts to rectify its shortcomings such as
3
poor solubility, limited stability in solution, and unfavor-
able stacking in the solid state are ongoing.4
7
When fused into ladder-type aromatic compounds, a
was obtained. Dibenzo[b,b′]thieno[2,3-f:5,4-f ′]bis[1]benzo-
5
sulfur atom affords unique properties. Carruthers dem-
thiophene was synthesized via the intramolecular acid-
onstrated that, unlike anthracene, naphtho[2,3-b]thio-
phene undergoes no cycloaddition with benzyne. Wynberg
and co-workers showed that benzo[1,2-b:5,4-b′]dithiophene
(6) Wynberg, H.; de Wit, J.; Sinnige, H. J. M. J. Org. Chem. 1970,
35, 711.
6
(
7) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J. J. Am. Chem.
Soc. 1998, 120, 664.
†
Bowling Green State University.
(8) (a) Groen, M. B.; Schadenberg, H.; Wynberg, H. J. Org. Chem.
1971, 36, 2797. (b) Wynberg, H. Acc. Chem. Res. 1971, 4, 65. (c) Larsen,
J.; Bechgaard, K. Acta Chem. Scand. 1996, 50, 71.
‡
American University of Beirut.
§
University of Toledo.
(
1) Contribution no. 551 from the Center for Photochemical Sciences.
(9) (a) Konno, M.; Saito, Y.; Yamada, K.; Kawazura, H. Acta
Crystallogr. 1980, B36, 1680. (b) Yamada, K.; Nakagawa, H.; Kawa-
zura, H. Bull. Chem. Soc. Jpn. 1986, 59, 2429. (c) Nakagawa, H.;
Yamada, K.; Kawazura, H. J. Chem. Soc., Chem. Commun. 1989, 1378.
(10) (a) Kobayashi, K. Phosphorus, Sulfur Silicon Relat. Elem. 1989,
43, 187. (b) Mazaki, Y.; Kobayashi, K. Tetrahedron Lett. 1989, 30, 3315.
(11) (a) Pandya, L. J.; Pillai, G. N.; Tilak, B. D. J. Sci. Ind. Res.
1959, 18B, 198. (b) Andrews, M. D. Product Class 6: Diben-
zothiophenes. In Science of Synthesis: Houben-Weyl Methods of
Molecular Transformations; Thomas, E. J., Ed.; Thieme: Stuttgart,
2001; p 211.
(
2) (a) Nelson, S. F.; Lin, Y.-Y.; Gundlach, D. J.; Jackson, T. N. Appl.
Phys. Lett. 1998, 72, 1854. (b) Sakamoto, Y.; Suzuki, T.; Kobayashi,
M.; Gao, Y.; Fukai, Y.; Inoue, Y.; Sato, F.; Tokito, S. J. Am. Chem.
Soc. 2004, 126, 8138. (c) Klauk, H.; Halik, M.; Zschieschang, U.;
Schmid, G.; Radlik, W.; Weber, W. J. Appl. Phys. 2002, 92, 5259. (d)
V o¨ lkel, A. R.; Street, R. A.; Knipp, D. Phys. Rev. 2002, B66, 195336/1.
(
e) Kwon, O.; Coropceanu, V.; Gruhn, N. E.; Durivage, J. C.; Laquin-
danum, J. G.; Katz, H. E.; Cornil, J.; Br e´ das, J. L. J. Chem. Phys. 2004,
20, 8186.
3) Weidkamp, K. P.; Afzali, A.; Tromp, R. M.; Hamers, R. J. J. Am.
Chem. Soc. 2004, 126, 12740.
4) (a) Anthony, J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4,
5. (b) Sheraw, C. D.; Jackson, T. N.; Eaton, D. L.; Anthony, J. E. Adv.
Mater. 2003, 15, 2009.
5) Carruthers, W. J. Chem. Soc. 1963, 4477.
1
(
(12) (a) Yamamoto, T.; Ogawa, S.; Sato, R. Tetrahedron Lett. 2004,
45, 7943. (b) Hartough, H. D.; Meisel, S. L. Compounds with Condensed
Thiophene Rings; Interscience Publishers: New York, 1954. (c) Ted-
jamulia, M. L.; Tominaga, Y.; Castle, R. N.; Lee, M. L. J. Heterocycl.
Chem. 1983, 20, 1143.
(
1
(
10.1021/jo048010w CCC: $30.25 © 2005 American Chemical Society
4502
J. Org. Chem. 2005, 70, 4502-4505
Published on Web 04/27/2005

SCHEME 2. Synthesis of Dialdehydes 6 and 8
added a solution of thiophene-2,5-dicarboxaldehyde (6).
1
9
A mild and effective reducing agent NaCNBH
3 2
/ZnI
yielded pure 12 in 80%. Because of the instability of 11,
20
4 3 4
traditional reducing agents such as LiAlH /AlCl , NaBH /
TFA,21 and even TMSCl/NaI did not produce the desired
product 12 in satisfactory yields. Careful monitoring of
the lithiation conditions (time and temperature) allowed
near quantitative formation of dialdehyde 13. Effective
cyclization of dialdehyde 13 occurred when refluxed in
benzene in the presence of Amberlyst-15 using a Dean-
Stark trap. The product 2-syn was isolated in 46% by
sublimation and recrystallization from ethanol.
22
The synthesis of 2-anti was accomplished in a similar
induced coupling reaction of aromatic methyl sulfoxides.13
Similarly, this approach yielded a mixture of three
isomers.
four-step procedure starting from 2,3-dibromothiophene
(
9, Scheme 3). Reaction of 10 with dialdehyde 8 is
followed by the reduction using NaCNBH /ZnI to yield
3
2
We have now developed a new approach for the
synthesis of pentacyclic ladder-type aromatic compounds
of alternating thiophene and benzene units in which the
thiophenes are fused at the [a,d]-bonds of the benzene
units. In contrast to their carbon analogues, thiophene-
terminated heteroacenes are expected to be more stable
in air and more soluble, pack well in the solid state, and
be easily modified at the terminal thiophene units. Our
strategy in preparing pure 2 employed a tandem in-
tramolecular cyclodehydration (i.e., hetero-Bradsher re-
1
4 in 43%. Double bromine-lithium exchange at the
â-positions of the terminal thiophene units of 14 is
followed by formylation using DMF to yield dialdehyde
1
5 in 54% yield. The acid-catalyzed cyclodehydration
furnished 2-anti as a pale yellow, crystalline solid in 24%
yield. The overall yield of 2-anti was 6%, and the overall
yield of 2-syn was 34%. As expected, both compounds
exhibit reasonable solubility in dichloromethane of 1-2
mg/mL (room temperature).
The UV-vis absorption spectra of 2-anti and 2-syn
Figure S15) showed that the longest wavelength absorp-
tion band of the anti isomer (375 nm) is 12 nm red-shifted
relative to the maximum absorption of the syn isomer
1
4
action ) as the key step. This approach essentially
eliminates the possibility of isomer formation for either
compound.
(
Following isomer-pure synthesis, we report a first
comparison of the optical and electronic properties of
thieno[3,2-f:4,5-f ′]bis[1]benzothiophene (2-syn)15 and
thieno[2,3-f:5,4-f ′]bis[1]benzothiophene (2-anti). The pack-
ing behavior of 2-anti is reported as well.
(
363 nm). The longest wavelength absorption band of the
2
3
parent dibenzo[b,d]thiophene (DBT) is at 327 nm. The
fluorescence emission of 2-syn exhibits a 12 nm blue-shift
from that of 2-anti (Table 1). Both isomers show an
equally small Stokes shift, indicating that they are of
equal rigidity. Emission quantum yields vary signifi-
cantly by isomer and are listed in Table 1.24 The emission
quantum yields of both isomers of 2 are compared to DBT
Compound 2-syn was synthesized from 2,5-thiophene-
dicarboxaldehyde (6), which was prepared from 2,5-
16
dibromothiophene (5, Scheme 2). The key starting
material for the synthesis of 2-anti was 3,4-thiophene-
dicarboxaldehyde17 (8). Previous synthetic reports of 8
were unsatisfactory in that they either yielded a complex
(
Φ
F
) 0.025, Φ
) 0.47).25 The fluorescence quantum
P
yield of 2-anti is 0.14, a 6-fold increase relative to that
of the parent. In comparison, the fluorescence quantum
yield of 2-syn (0.01) is comparable to that of the parent
compound. The phosphorescence emission quantum yield
of 2-syn is 0.56 in a frozen matrix as expected as a result
of the intramolecular heavy atom effect. In contrast, the
phosphorescence is effectively shut off in the anti isomer
where the quantum yield is only 0.04. This observation
suggests that the electronic excited state structures and
nonradiative decay channels vary considerably with
constitution of the isomers of 2.
18
mixture of isomers or were based on starting materials
that were not readily available.17 Successful preparation
of 8 from 7 was achieved in a one-pot procedure by
stepwise replacement of the halogens (Scheme 2). The
first bromine-lithium exchange was carried out using
n
BuLi. This salt was quenched with a stoichiometric
amount of DMF, and without further hydrolysis, the
second bromine-lithium exchange was carried out using
t
the stronger base BuLi. The kinetically formed organo-
lithium salt was quenched with DMF quickly to avoid
rearrangements. Sublimation and recrystallization re-
sulted in 8 in 34% yield.
Crystal packing is of pivotal concern for efficient
2
6
charge-transport in devices such as OFETs. Single-
crystal X-ray analysis revealed that 2-anti27 crystallized
Synthesis of 2-syn was carried out according to Scheme
. To a solution of 3-bromo-2-lithiothiophene (10) was
3
(
19) Lau, C. K.; Dufresne, C.; Belanger, P. C.; Pietre, S.; Scheigetz,
J. J. Org. Chem. 1986, 51, 3038.
(
13) Sirringhaus, H.; Friend, R. H.; Wang, C.; Leuninger, J.; M u¨ llen,
K. J. Mater. Chem. 1999, 9, 2095.
14) Ahmed, M.; Ashby, J.; Ayad, M.; Meth-Cohn, O. J. Chem. Soc.,
Perkin Trans. 1 1973, 1099.
15) Wex, B.; Kaafarani, B. R.; Neckers, D. C. J. Org. Chem. 2004,
(20) (a) Yamada, M.; Ikemote, I.; Kuroda, H. Bull. Chem. Soc. Jpn.
1988, 61, 1057. (b) Aubry, J.-M.; Pierlot, C.; Rigaudy, J.; Schmidt, R.
Acc. Chem. Res. 2003, 36, 668.
(
(21) Nutaitis, C. F.; Patragnoni, R.; Goodkin, G.; Neighbour, B.;
Obaza-Nutaitis, J. Org. Prep. Proced. Int. 1991, 23, 403.
(22) Nenajdenko, V. G.; Baraznenok, I. L.; Balenkova, E. S. J. Org.
Chem. 1998, 63, 6132.
(
6
1
1
9, 2197.
(
16) Feringa, B. L.; Hulst, R.; Rikers, R.; Brandsma, L. Synthesis
988, 316.
(23) Perkampus, H.-H. UV-Atlas of Organic Compounds; VCH:
Weinheim, 1992; p H15/1.
(17) Robba, M.; Roques, B.; Bonhomme, M. Bull. Soc. Chim. Fr.
967, 2495.
18) (a) Trofimenko, S. J. Org. Chem. 1964, 29, 3046. (b) Dubus, P.;
Decroix, B.; Morel, J.; Pastour, P. Bull. Soc. Chim. Fr. 1976, 628.
(24) A complete photophysical characterization of these new materi-
als is currently under way.
(
(25) Zander, M.; Kirsch, G. Z. Naturforsch. 1989, 44, 205.
J. Org. Chem, Vol. 70, No. 11, 2005 4503

SCHEME 3. Preparation of 2-syn and 2-antia
a
n
n
Regents and conditions: (a) BuLi, THF/Et2O (1:5), -78 °C; (b) NaCNBH3/ZnI2, DCE; (c) BuLi, Et2O, -78 °C; (d) DMF; (e) ∆, Amberlyst-
5, benzene.
1
TABLE 1. Spectroscopic Properties and Thermal
The stability of 2-anti and 2-syn was tested under air
and nitrogen atmospheres by thermogravimetric analysis
(TGA). Under a nitrogen atmosphere, the onset temper-
ature for decomposition is the highest for 2-syn, reaching
Stability of 2-anti and 2-syn
a
fl
max
b
c
ph
max
d
e
on (air)f
)f
λ
max
λ
φ
fl
λ
φ
ph
T
Ton (N
2
2
-anti 375 nm 384 nm 0.14 523 nm 0.04 344.23
01 nm
-syn 363 nm 372 nm 0.01 471 nm 0.56 341.57
90 nm
375.04
400.91
34
4
00 °C, about 70 °C higher compared to pentacene. Both
4
2
isomers of 2 exhibit comparable stability under air and
start to decompose around 340 °C (Table 1). This consti-
tutes an increase of 20 °C compared to the reported
3
a
Longest wavelength absorption band in dichloromethane.
b
c
34
Wavelength of maximum of fluorescence emission. Fluorescence
decomposition temperature of 319 °C of pentacene.
quantum yield. Standard: anthracene (EtOH) φfl ) 0.27; 9-bro-
Compound 2-anti exhibits a quasi-reversible oxidation
at 1.15 V, whereas 2-syn exhibits a reversible oxidation
at a half wave potential of 1.12 V vs Ag/AgNO , respec-
3
2
8 d
moanthracene (EtOH) φfl ) 0.02.
Wavelength of maximum of
e
phosphorescence emission. Phosphorescence quantum yield. Stan-
dard: biphenyl (EPA, 77 K) φph ) 0.24.29 f Onset decomposition
temperature as observed by TGA.
tively. Formation of an intensely colored film from 2-anti
on the electrode surface and the gradual appearance of
lower potential waves upon repeated cyclization indicates
electropolymerization of the generated radical cations. A
reversible redox behavior is desired for OFET materials
to avoid trapping of charge carriers at the injection site.
Our future work therefore includes a suitable substitu-
tion and end-capping of the terminal thiophene groups
to prevent oxidative coupling of the generated radical
cations.35
We have developed a viable synthetic procedure for the
synthesis of 3,4-thiophenedicarboxaldehyde from the
readily available 3,4-dibromothiophene. We have suc-
cessfully synthesized two new representatives of penta-
cyclic, [a,d]-fused ladder-type, sulfur-containing materi-
als. Our synthetic approach yields isomer-pure 2-anti
and 2-syn. The absorption and emission properties were
compared to DBT. The phosphorescence of 2-anti is
substantially decreased compared to the isomer and the
parent compound. Isomer 2-anti packs in a herringbone-
fashion with good molecular overlap. The redox behavior
indicates that substitution of the terminal thiophenes will
be favorable to obtain reversibility and to block electro-
polymerization.
1
in the orthorhombic space group Pmn2 with two mol-
ecules in the unit cell. The central atom S2 resides on
the crystallographic mirror plane that relates the two
halves of the molecule. Molecules of 2-anti are nearly
planar with the highest deviation of atoms from the best
plane through the entire molecule of (0.174(2) Å.
The crystals of 2-anti exhibit a herringbone-packing
pattern with favorable molecular overlap along the c-axis
of the unit cell. The alternating π-stacked columns are
tilted at an angle of 50.24° (viewed down a-axis, Figure
1
). In comparison, this tilt-angle is 51.9° in pentacene.30
Tight packing in the solid state increases the material’s
stability toward oxygen in that it decreases the amount
of oxygen diffusing into the bulk material.31 The effective
volume occupation is expressed as the Kitaigorodskii
packing index (KPI).32 Compound 2-anti has a packing
coefficient of 0.75 compared to 333 with 0.72, whereas
pentacene shows with a KPI of 0.76, the highest percent
of filled space.
(
26) Mas-Torrent, M.; Hadley, P.; Bromley, S. T.; Ribas, X.; Tarr e´ s,
J.; Mas, M.; Molins, E.; Veciana, J.; Rovira, C. J. Am. Chem. Soc. 2004,
26, 8546.
27) Crystallographic data are deposited at the Cambridge Crystal-
1
(
lographic Data Center (CCDC no. 254385).
Experimental Section
(
(
28) Melhuish, W. H. J. Phys. Chem. 1965, 65, 229.
29) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
General experimental details can be found in Supporting
Information.
chemistry, 2nd ed.; Marcel Dekker Inc.: New York, 1993.
(
30) Mattheus, C. C.; Dros, A. B.; Baas, J.; Meetsma, A.; de Boer, J.
L.; Palstra, T. T. M. Acta Crystallogr. 2001, C57, 939.
31) Yamada, M.; Ikemoto, I.; Kuroda, H. Bull. Soc. Chem. Jpn.
988, 61, 1057.
32) (a) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;
3
,4-Thiophenedicarboxaldehyde (8). 3,4-Dibromothiophene
(
7, 8.75 g, 4.0 mL, 36.17 mmol) was dissolved in 100 mL of
(
1
(
(34) Merlo, J. A.; Newman, C. R.; Gerlach, C. P.; Kelley, T. W.;
Utrecht University: Utrecht, The Netherlands, 2005. (b) Kitaigorod-
skii, A. I. Molecular Crystals and Molecules; Academic Press: New
York, 1973.
Muyres, D. V.; Fritz, S. E.; Toney, M. F.; Frisbie, C. D. J. Am. Chem.
Soc. 2005, 127, 3997.
(35) (a) Wakamiya, A.; Yamazaki, D.; Nishinaga, T.; Kitagawa, T.;
Komatsu, K. J. Org. Chem. 2003, 68, 8305. (b) Turbiez, M.; Fr e` re, P.;
Roncali, F. J. Org. Chem. 2003, 68, 5357.
(
33) Nakagawa, H.; Yamada, K.; Kawazura, H.; Miyamae, H. Acta
Crystallogr. 1984, C40, 1039.
4504 J. Org. Chem., Vol. 70, No. 11, 2005

FIGURE 1. Crystal packing of 2-anti as viewed along the a- (left) and b-axis (right). All hydrogens are omitted.
diethyl ether and the solution purged with Ar. After cooling to
the addition, DMF (1.53 mL, 1.45 g, 19.84 mmol) was added and
the reaction was stirred for 2 h. The mixture was poured into a
n
-
78 °C, a solution of BuLi (15.91 mL, 39.78 mmol) was added
dropwise over a period of 5 min. Then, DMF (3.07 mL, 2.91 g,
9.78 mmol) was added and the reaction was stirred for 1 h.
4
separatory funnel containing ice-cold saturated NH Cl. After
3
separation of the organic layer, the aqueous layer was extracted
t
BuLi (57.88 mL, 86.83 mmol) was chilled to - 78 °C and then
rapidly added. After 5 min, DMF (6.70 mL, 6.34 g, 86.83 mmol)
was added and the reaction was stirred overnight. The reaction
mixture was then poured onto 2 N HCl at - 20 °C and warmed
to room temperature. After separating the organic layer, the
aqueous layer was extracted with copious amounts of diethyl
ether. The combined organic layers were washed with brine and
with diethyl ether (2 × 100 mL) and the combined organic layers
4
(400 mL) washed with water and dried over MgSO . Recrystal-
lization of 15 from diethyl ether yielded a white solid, 1.62 g
(54%), mp 89-90 °C. An analytical sample was prepared using
column chromatography (4:1 hexanes/ethyl acetate as eluent),
1
mp 93-94 °C. H NMR (CDCl
3
, 300 MHz): δ 4.45 (4H, s), 7.04
(2H, s), 7.12 (2H, d, J ) 5.4 Hz), 7.39 (2H, d, J ) 5.4 Hz), 10.05
1
3
water and subsequently dried over MgSO
4
. After evaporation
3
(2H, s). C NMR (CDCl , 75 MHz): δ 27.8, 123.9, 124.4, 128.2,
of the solvent, the orange-brown solid was sublimed at 110 °C
under vacuum (ca. 28 in. Hg). The yellowish-white solid was
dissolved in hot cyclohexane and the supernatant was decanted.
This process was repeated until only a dark-brown residue
remained in the flask. The cyclohexane fractions were combined,
reheated, and left for crystallization in a freezer (-20 °C). This
process yielded 1.72 g (34%) of a yellowish-white solid, mp 76-
136.7, 138.0, 154.3, 184.7. Anal. Calcd for C16
12 2 3
H O S : C, 57.80;
H, 3.64. Found: C, 57.60; H, 3.79.
Thieno[2,3-f:5,4-f ′]bis[1]benzothiophene (2-anti). Com-
pound 15 (1.63 g, 4.90 mmol) was dissolved in dry benzene and
Amberlyst-15 (0.5 g) was added. The reaction was refluxed for
36 h while water was removed by means of a Dean-Stark trap.
After cooling, dichloromethane was added to dissolve the pre-
cipitate and the mixture was filtered through a cotton plug. The
1
7
1
7
7 °C (lit. 78-80 °C). H NMR (CDCl
3
, 300 MHz): δ 8.23 (2H,
13
s), 10.31 (2H, s). C NMR (CDCl
3
, 75 MHz): δ 137.6, 140.3, 185.8.
filtrate was washed with saturated NH
4
Cl (aqueous) and then
3
,4-Bis(3-bromo-2-thienylmethyl)thiophene (14). 2,3-Di-
dried over MgSO . Evaporation of the solvent furnished a brown
4
bromothiophene (9, 10.00 g, 41.33 mmol) was dissolved in diethyl
solid. Purification by column chromatography on silica gel using
hexanes as eluent yielded a pale white solid. Recrystallization
n
ether/THF (5:1, 200 mL). At -78 °C, BuLi (20.0 mL, 47.1 mmol)
was added dropwise and the solution was stirred for 5 min. A
chilled solution of 8 (3.0 g, 21.4 mmol) in THF (160 mL) was
subsequently added and the reaction was allowed to warm to
room temperature. After addition of water, the aqueous layer
was extracted with diethyl ether and the combined organic layers
from CHCl
3
yielded 0.35 g (24%) of pale-white crystals, mp 237-
238 °C. H NMR (CDCl , 300 MHz): δ 7.39 (2H, d, J ) 5.4 Hz),
7.54 (2H, d, J ) 5.4 Hz), 8.24 (2H, s), 8.68 (2H, s). C NMR
(CDCl , 75 MHz): δ 115.0, 117.0, 123.1, 127.8, 132.7, 136.9,
1
3
1
3
3
137.1, 139.8. Anal. Calcd for C16
C, 64.54; H, 2.95.
8 3
H S : C, 64.83; H, 2.72. Found:
4
washed with water and dried over MgSO . Evaporation of the
solvent yielded a yellow liquid, which was dissolved in 1,2-
dichloroethane. Zinc iodide (20.50 g, 64.21 mmol) and sodium
cyanoborohydride (20.18 g, 321.06 mmol) were added and the
reaction was stirred overnight. The reaction mixture was
subsequently filtered through Celite, washed with aqueous
Acknowledgment. This work was supported by the
National Science Foundation (DMR0091689). B.W. ac-
knowledges support from the McMaster Endowment
and a SPIE educational scholarship. We thank Mettler
Toledo (Bowen Greil and Mike Zemo) for generous help
with TGA.
saturated NH
4 4
Cl, dried over MgSO , and evaporated. Column
chromatography using hexanes yielded 3.99 g (43%) of a yellow
1
oil. H NMR (CDCl
3
, 300 MHz): δ 4.02 (4H, s), 6.93 (2H, d, J )
1
3
5
.1 Hz), 6.99 (2H, s), 7.14 (2H, d, J ) 5.1 Hz); C NMR (CDCl
5 MHz): δ 28.8, 109.3, 123.6, 124.1, 130.0, 137.6, 137.9. Anal.
: C, 38.72; H, 2.32. Found: C, 38.69; H,
3
,
Supporting Information Available: General experimen-
tal procedure; synthetic procedures for 6, 11-13, and 2-syn;
NMR spectra of compounds 6, 8, 11-15, 2-anti, 2-syn; cyclic
voltammogram of 2-syn and 2-anti; X-ray crystallographic
data and cif file of 2-anti. This material is available free of
charge via the Internet at http://pubs.acs.org.
7
Calcd for C14
.53.
,2′-[3,4-Thiophenediylbis(methylene)]bis[3-thiophen-
2 3
H10Br S
2
2
n
ecarboxaldehyde] (15). BuLi (8.27 mL, 19.86 mmol) was
chilled to -78 °C in diethyl ether. Compound 14 (3.92 g, 9.03
mmol) in diethyl ether was added dropwise. Ten minutes after
JO048010W
J. Org. Chem, Vol. 70, No. 11, 2005 4505