Angewandte
Chemie
The Stille cross-coupling reaction has allowed significant
[1]
advances in the synthesis of new organic molecules. This
reaction also had a huge impact in macromolecular chemistry,
[
1,2]
especially for the synthesis of conjugated polymers.
Despite its great versatility, the Stille reaction involves
drawbacks such as the formation of a stoichiometric amount
of toxic by-products and in some cases, some instability of the
organometallic reagents.
Recently, the development of reactions called “direct
[
3]
arylations” has received much attention. These reactions
allow the formation of carbon–carbon bonds between aro-
matic units having activated hydrogen atoms without the use
of organometallic intermediates. These reactions are mostly
[
4]
developed for the synthesis of small molecules. Indeed, up
until now, only a few publications have reported the use of
Scheme 1. Comparison of the Stille (P1*) versus direct heteroarylation
(P1) polymerization approach.
[5]
direct arylation in polymerization reactions. Moreover,
there are very few examples showing the coupling between
thiophenes or thiophene derivatives by direct heteroarylation
despite the fact that these moieties are particularly important
monomers for plastic electronics.
[
6c]
previous studies (yield of 71% for the soluble fraction in
chloroform). In agreement with polystyrene-calibrated size-
exclusion chromatography (SEC) data reported for similar
[
6c,7,9]
copolymers,
Stille-polymerized P1* shows an M of only
n
In parallel, the thieno[3,4-c]pyrrole-4,6-dione (TPD) unit
has become a building block of choice for the development of
new conjugated polymers for organic solar cells (power
conversion efficiencies up to 7.3%) and more recently, for
organic field-effect transistors (hole mobilities up to
9 kDa. This relatively low molecular weight might be related
to the loss of some functional groups during the Stille
polymerization. In parallel, for the preparation of P1 using
direct heteroarylation polycondensation reactions, several
reaction conditions were tested (Table 1). The optimal
reaction conditions were obtained with ligand L1 and catalyst
1. For the synthesis of P1, palladium acetate (2) is not as
efficient as catalyst 1 and this is probably due to the higher
[
6]
2
À1 À1 [7]
0
.6 cm V s ). Interestingly, the imide group may act as
[
8]
an orienting and activating group for the hydrogen atoms at
the 2- and 2’-positions (Scheme 1), and on this basis, this
monomer could be a good candidate for direct heteroaryla-
tion polycondensation reactions.
[
5a]
thermal stability of the Herrmann catalyst. The temper-
ature, time, solvent, and concentration were kept constant for
this work and were based on general procedures reported in
Ultimately, by merging the advantages of plastic solar
cells with new ecofriendly synthetic procedures, new oppor-
tunities for the production of green energy from green
materials may emerge. The use of more environmentally
friendly materials, which produce no tin by-products, could
also show improved performance and stability. In this regard,
our present work was devoted to the development of
a catalytic system for the synthesis of TPD-based polymers
using direct heteroarylation polycondensation reactions
instead of the standard Stille cross-coupling reactions.
As shown in Scheme 1, P1 was synthesized by both Stille
and direct heteroarylation polymerization reactions. This
copolymer is an analogue of other TPD-based copolymers
recently reported as being used in highly efficient plastic solar
[3a–h,5a–c]
the literature.
A high M of 56 kDa (PDI of 2.6) with
n
a yield of 96% (soluble fraction in chloroform) was obtained
when using catalyst 1 and ligand L1 (Table 1, entry 1). Other
examples of polymerizations of TPD-based monomers using
the same catalytic system are shown in the Supporting
Table 1: Reaction conditions for the polymerization of P1 by direct
heteroarylation.
Entry
Cat (mol%)
L (mol%)
Mn
kDa]
PDI
DP
80
[
[
[
[
[
a]
a]
a]
a]
1
2
3
4
5
6
7
(P1)
(P1)
(P1)
(P1)
(P1)
(P1)
1 (4)
1 (4)
1 (4)
2 (4)
2 (5)
2 (5)
L1 (8)
L2 (8)
L3 (8)
L1 (8)
L1 (15)
56
2.6
[b]
[b]
[b]
–
–
–
–
[c]
[c]
[c]
–
–
21
2.5
33
[6c,9]
[7]
cells
and field-effect transistors. Stille polymerization
[a]
[a]
[c]
[c]
[c]
–
–
–
was carried out essentially following methods from those
[d]
L1 (15)
9
9
1.8
1.5
14
14
[
e]
[e]
(P1*)
[
*] P. Berrouard, Dr. A. Najari, Dr. A. Pron, D. Gendron, P.-O. Morin,
J.-R. Pouliot, J. Veilleux, Prof. Dr. M. Leclerc
Department of Chemistry, Universitꢀ Laval
Quebec City, QC, G1V 0A6 (Canada)
E-mail: mario.leclerc@chm.ulaval.ca
Homepage: http://www.chm.ulaval.ca/poly_conducteurs/fr/mari-
oleclerc.html
[a] P1 was synthesized by direct heteroarylation following the procedure
described in the experimental section. [b] No polymerization reaction
occurred. [c] All the reaction product was recovered with acetone Soxhlet
extraction and no further characterization was made on these materials.
[d] Reaction time was 44 h instead of 22 h. [e] P1* was synthesized by
Stille cross-coupling following the procedure described in the exper-
imental section. M =number-average molecular weight, PDI=polydis-
n
[
**] This work was supported by grants from the NSERC. The authors
thank Professors F.-G. Fontaine and F. Ozawa for useful discus-
sions and J. Delisle-Labrecque and W.-O. Caron for their technical
assistance.
persity index, DP=degree of polymerization.
Angew. Chem. Int. Ed. 2012, 51, 2068 –2071
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim