A novel approach to a one-pot synthesis of unsubstituted oligo(α-thiophenes)

Article abstract of DOI:10.1021/jo025838e

Oligo(α-thiophenes) α-4T and α-8T were prepared by the following one-pot sequential conversions: thiophene (T) → α-2T → α-4T → α-8T. PdCl₂-induced coupling of a mono-α-mercuration derivative of each of the n-mers T, α-2T, and α-4T was applied in these conversions.

Full text of DOI:10.1021/jo025838e

etc. These, however, are statistical oligomerization sys-
tems which lack the critically required control over
oligoselectivity and therefore yield low amounts of the
particular oligomers.
We recently reported a conceptually novel approach to
an efficient one-pot stepwise synthesis of monodispersed
oligomeric PPV-analogues, having extended π-conjuga-
tion.10 Oligomers of the general structure
A Novel Ap p r oa ch to a On e-P ot Syn th esis
of Un su bstitu ted Oligo(r-th iop h en es)
Ludmila Buzhansky and Ben-Ami Feit*
Raymond and Beverly Sackler Faculty of Exact Science,
School of Chemistry, Tel-Aviv University,
Tel-Aviv 69978, Israel
feit@post.tau.ac.il
Received April 18, 2002
were prepared from the corresponding conjugated di-
functional symmetrical monomer (n ) 1). This approach
was based on repeatedly performing a sequence of two
reactions: a selective monofunctionalization of the con-
jugated dienic n-mer (starting with n ) 1), followed by
coupling of the product of this reaction to yield the
corresponding 2n-mer (eq 1). Reactions similar to those
Abstr a ct: Oligo(R-thiophenes) R-4T and R-8T were pre-
pared by the following one-pot sequential conversions:
thiophene (T) f R-2T f R-4T f R-8T. PdCl2-induced
coupling of a mono-R-mercuration derivative of each of the
n-mers T, R-2T, and R-4T was applied in these conversions.
Oligo(R-thiophenes) exhibit many of the desirable
electrooptical characteristics of the derived polymers.1
Their extensive R-electron conjugation led to a large
number of optoelectronic applications such as active
layers in field effect transistors,2 light emitting diodes,3
photovoltaic cells,4 etc. Despite the resulting wide interest
in unsubstituted oligo(R-thiophenes), the reported syn-
thetic routes to these oligomers (R-4T, R-5T, R-6T, R-8T)
are limited. A two-step-synthesis approach was used in
all the reported syntheses of these oligomers. First, an
R-monofunctionalized thiophene n-mer (n ) 1-4) was
prepared and then coupled to yield the target oligomers.
Coupling of R-lithiated thiophene n-mers in the presence
involved in the one-pot synthesis of the
oligomers10 were applied in a two-step dimerization of
thiophene to yield R,R′-bithienyl (Scheme 1):
It was, therefore, the purpose of the present research
to adjust and apply the approach used for preparing the
above-mentioned oligomeric PPV analogues10 for an
analogous novel one-pot synthesis of oligo(R-thiophenes).
A specific objective in this regard was the one-pot
stepwise conversion of thiophene to R-8T by the following
series of consecutive dimerization reactions: T f R-2T
f R-4T f R-8T.
We succeeded (in the first phase of this research) in
converting the above two-step synthesis of R,R′-bithenyl
(Scheme 1) into an efficient general one-pot dimerization,
applicable for converting oligo(R-thiophene) n-mers to the
corresponding 2n-mers. The dimerization of each thio-
phene R-2T and R-4T was performed by consecutively
carrying out, in one pot, the following two reactions: (a)
reaction of the substrate (R-nT, n ) 1, 2, 4) with Hg(TFA)2
to yield the corresponding mono-R-HgTFA derivative, and
(b) PdCl2-induced coupling of this mercuration product
(Scheme 1). The reactions were carried out in benzene
at low temperatures (0-5 °C) and in relatively short
reaction times.
5
of CuCl2 , FeCl3,6 and ferric acetylacetonate6 has often
been used. Oligo(R-thiophenes) were also prepared by
reacting the R-magnesium halide derivatives with R-bro-
mothiophene in the presence of NiCl2(dppp).7 All existing
approaches involve isolation and purification of each
n-mer prior to its conversion to the corresponding (n +
1)-mer or 2n-mer. The recently reported solid-phase
synthesis of oligo(R-T)8 represents a significant improve-
ment in this regard.
A general, relatively efficient one-pot approach to long-
chain conjugated oligomers was recently introduced in
an excellent review on linear monodispersed conjugated
oligomers.9 It consists of homo- or hetero-coupling reac-
tions of symmetrical conjugated oligomers, heterocycles,
(1) (a) Fichou D., Ed. Handbook of Oligo- and Polythiophenes; Wiley-
VCH: Weinheim, Germany, 1999. (b) Casado, J .; Hernandez, V.; Hotta,
S.; Navarrete, L. J . Adv. Mater. 1998, 10, 11458. (c) Leeuw, D. M.
Synth. Met. 1993, 55-57, 3597-3602.
(2) (a) Gorowitz, G. Adv. Mater. 1998, 10, 375. (b) Garnier, F. Chem.
Phys. 1998, 227, 253. (c) Dodabalapur, A.; Torsi, L.; Katz, H. E. Science
1995, 268, 270.
The experimental conditions involved and the results
obtained are summarized in Table 1. The corresponding
dimerization products, R-2T from T (entries 1, 2), R-4T
from R-2T (entry 5), and R-8T from R-4T (entry 7),
respectively, were obtained in high yields.
(3) (a) Marks, F. N.; Biscarini, F.; Zamboni, R.; Taliani, C. Europhys.
Lett. 1995, 54, 523. (b) Neureiter, H.; Gebauer, W.; Vaterlein, C.;
Sokolowski, M.; Bauerle, P.; Umbach, E. Synth. Met. 1994, 67, 173.
(4) Videlot, C.; El Kassmi, A.; Fichou, D. Sol. Energy Mater. Sol.
Cells 2000, 63, 69.
(5) Kagan, J .; Arora, S. Heterocycles 1983, 20, 1937.
(6) (a) Kagan, J .; Arora, S. Tetrahedron Lett. 1983, 24, 4043. (b)
Segura, J .; Martin, N. J . Mater. Chem. 2000, 10, 2403.
(7) Cunningham, D.; Laguren-Davidson, L.; Mark, H.; Pham, C.;
Zimmer, H. J . Chem. Soc., Chem. Commun. 1987, 1021.
(8) Malenfant, P. R. L.; Frechet, J . M. J . Chem. Commun. 1998,
2657.
(9) Martin, R.; Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350
and relevant references therein.
(10) Feit, B.-A.; Buzhansky, L. J . Chem. Soc., Perkin Trans. 1 2000,
1777.
10.1021/jo025838e CCC: $22.00 © 2002 American Chemical Society
Published on Web 09/19/2002
J . Org. Chem. 2002, 67, 7523-7525
7523

SCHEME 1
TABLE 1. On e-P ot Step w ise P r ep a r a tion of Oligo(r-th iop h en es) Exp er im en ta l Con d ition sa a n d Resu lts
reaction time, min
entry
starting thiophene
t1
t2
product(s)
yieldb (%)
1
2
3
T
T
T
15
5
15
10
15
15
15
15
15
15
15
15
15
R-2T
R-2T
89
92
76 + (5)
15
15
15
15
15
15
15
15
15
R-4T + (R-2T)
4
T
R-8T + (R-4T) + (R-6T)
70 + (18) + (2-3)
5
6
R-2T
R-2T
R-4T
88
R-8T + (R-4T) + (R-6T)
81 + (7) + (2-3)
7
R-4T
R-8T
80
a
b
The temperature of the reaction mixture throughout the reaction was 0-5 °C. Isolated yields of the target n-mer.
The high efficiency of these dimerization reactions of
R-nT (n ) 1, 2, 4) clearly points to an exclusive mercu-
ration (in the presence of 1 equiv of Hg(TFA)2) at one of
the two R-positions of the oligo(R-thiophene). This sug-
gests that the presence of the HgTFA substituent ef-
fectively suppresses the mercuration at the other unsub-
stituted R-position.
The relatively high efficiency observed in each of the
three one-pot dimerization reactions (to yield R-2T, R-4T,
and R-8T) led to the application of this experiment
protocol for conducting the following one-pot conversions,
without isolating any of the intermediate products: (a)
thiophene to R-4T, (b) thiophene to R-8T, and (c) R-2T to
R-8T. The results are summarized in Table 1. This novel
approach to oligo(R-thiophenes) and the derived meth-
odology may be presented as follows (eq 2):
the previous cycle. The one-pot 8T to R-8T conversion
(entry 4) was performed in an impressively short reaction
time of 90 min. Similarly, relatively high yields of R-4T
and R-8T were obtained in the one-pot 4T f R-4T (entry
3) and R-2T f R-8T (entry 6) conversions, respectively.
Trace amounts of R-6T (2-3%) were isolated from the
reaction mixtures of each of the 8T f R-8T (entry 4) and
the 4 R-2T f R-8T (entry 6) conversions. No R-6T was
detected in that of the 4T f R-4T (entry 3) conversion.
The trimer R-3T was not detected in any of the reactions
outlined in Table 1. R-6T could have been formed in the
2 R-3T f R-6T and in the R-2T + R-4T f R-6T coupling
reactions.
Two coupling reactions (eqs 3 and 4) were carried out
(under competition conditions) to verify at which phase
of the one-pot stepwise 8T f R-8T and 4 R-2T f R-8T
conversions the R-6T was formed:
T (1 equiv.) + R-2T (1 equiv) a, b8
R-2T(main product) + T (3)
R-2T (1 equiv) + R-4T (1 equiv) a, b8
R-4T(main product) + R-2T + R-6T (6%) (4)
Thus, for example, the one-pot 8T f R-8T conversion
(entry 4) involved steps 1 and 2 (eq 2) to yield (in situ)
R-2T, the reaction time being t1 and t2, respectively. This
dimer was twice more subjected (at the end of t2) to the
same two steps, to yield the corresponding dimerization
products R-4T and R-8T consecutively. Each cycle of the
two steps used half an equivalent of each reactant from
(a) Hg(TFA)2, (1 equiv); (b) PdCl2 (1 equiv), Et3N (3 equiv)
None of R-3T and R-4T were detected in reaction 3. The
lack of formation of R-3T excludes the formation of R-6T
in a 2 R-3T f R-6T coupling pathway in the 8T f R-8T
synthesis.
7524 J . Org. Chem., Vol. 67, No. 21, 2002

On the basis of the results obtained it might be
reasonably concluded that formation of R-6T in the 8T
f R-8T conversion occurred only in the 2 R-4T f R-8T
coupling phase, and only in the presence of small
amounts of R-2T.
The results of the present research reflect an essential
conceptual element with regard to the feasibility of the
present novel one-pot stepwise synthesis of oligo-R-
thiophenes, namely, the reactivity of R-nT decreases
(significantly) with increase of n. The above outlined
formation of R-6T (and not of R-3T) suggests that the
difference in reactivity between R-nT and R-(n + 1)T or
R-(n + 2)T decreases with increase of n.
°C. One equivalent of PdCl2 and 3 equiv of triethylamine were
then introduced to the reaction mixture, which was stirred for
a further 15 min (t2) at 0 °C. Methylene chloride, carbon black,
and water were added to the reaction mixture and the mixture
was filtered. The aqueous layer was separated and extracted
with methylene chloride, the organic layers were combined, dried
over anhydrous MgSO4, and filtered, and the solvent was
removed from the filtrate. The residue was subjected to column
chromatography on silica gel to yield the corresponding pure
dimerization product. The highly insoluble R-8T was purified
as described below and not by column chromatography.
In conclusion, the feasibility of a conceptually novel
approach for the one-pot synthesis of oligo(R-thiophenes)
from the monomers (thiophene and R-2T) has been
demonstrated. This approach consists of performing
sequentially the required number of cycles of two reac-
tions, namely, monofunctionalization of the R-nT (n ) 1,
2, 4) and coupling of the product of this reaction.
It is our opinion that the present one-pot synthesis of
oligo(R-T), as well as the previous one-pot synthesis of
oligo-PPV analogues (having extended π-conjugation in
the repeat unit),10 represents a general concept that may
be applicable to a one-pot synthesis of conjugated oligo-
mers from the corresponding symmetrical difunctional
conjugated monomers. Such oligomerizations may be
achievable provided that compatible monofunctionaliza-
tion and subsequent coupling reactions can be elucidated
for a specific monomer of this type.
Gen er a l P r oced u r e for th e On e-P ot syn th esis of Oligo-
(r-th iop h en es) fr om th e Cor r esp on d in g Mon om er s (Equ a -
tion 6). The general procedure below was applied for the
following pairs of n and m values: 4, 1; 8, 1; and 4, 2.
A solution of the monomer (0.5 mmol) was dissolved in
benzene (8 mL) and cooled to a temperature of 0 °C, which was
maintained throughout the whole synthesis. One equivalent of
mercuric trifluoroacetate was added to the solution and the
mixture was stirred for 15 min (t1). Palladium chloride (1 equiv)
and triethylamine (3 equiv) were then added and stirring was
continued for 15 min (t2). A second portion of mercuric trifluo-
roacetate (0.5 equiv) was added and the reaction mixture was
stirred for 15 min (t1). Palladium chloride (0.5 equiv) and
triethylamine (1.5 equiv) were then added and stirring was
continued for a further 15 min (t2). If needed, a third addition
of Hg(TFA)2 (0.25 equiv), PdCl2 (0.25 equiv), and Et3N (0.75
equiv) was performed by using the same experimental conditions
as described above. Workup of the reaction mixture was per-
formed, as described above in the general procedure for the one-
pot dimerization of oligo-RT. The octamer R-8T obtained (Table
1, entries 4 and 6) contained trace amounts of R-6T, which was
removed from the R-8T by applying a workup protocol reported
in the literature.13
Exp er im en ta l Section
Gen er a l. Thiophene (Aldrich, 99%) and 2,2′-bithiophene
(Aldrich, 97%) were used as purchased. Benzene (AR) was dried
over sodium wire and distilled from this mixture before use.
Triethylamine was kept over sodium hydroxide and distilled
from this mixture before use. PdCl2 (Merck, 99%) and mercuric
trifluoroacetate (Merck, 98%) were dried in a vacuum for 12 h
before use.
The reaction setup consisted of a 50-mL flask fitted with a
self-sealing rubber septum, equipped with magnetic stirring and
a dry nitrogen inlet. Liquid materials were introduced into the
The structures of the oligomers R-2T,6a R-4T,6a and R-8T,4
which were determined on the basis of their 1H NMR and mass
spectral data, were found to be identical with the corresponding
literature-reported data.
reaction flask with
a hypodermic syringe. All glass parts,
syringes, and needles were thoroughly dried at 130 °C and
assembled while warm. Reaction products were purified by flash
column chromatography with H-60 type silica (Merck).
J O025838E
Gen er a l P r oced u r e for t h e On e-P ot Dim er iza t ion of
Oligo(r-th iop h en es) (Equ a tion 5). A benzene solution (5-8
mL) of the oligo(R-thiophene) (1 mmol) was cooled to 0 °C. An
equivalent amount of mercuric trifluoroacetate was added to this
solution. The reaction mixture was stirred for 15 min (t1) at 0
(11) Sokolov, V.; Bashilov, V.; Reutov, O. J . Organomet. Chem. 1978,
162, 271.
(12) Scully, F.; Davis, R. J . Org. Chem. 1978, 43, 1468.
(13) Katz, H. E.; Torsi, L.; Dodabapur, A. Chem. Mater. 1995, 7,
2235-2237.
J . Org. Chem, Vol. 67, No. 21, 2002 7525