Palladium-catalyzed direct C-H arylation of thieno[3,4-b]pyrazines: Synthesis of advanced oligomeric and polymeric materials

Article abstract of DOI:10.1002/ejoc.201200769

The first examples are reported of an efficient regioselective direct C-H arylation of thieno[3,4-b]pyrazine (TP) and its 2,3-dimethyl derivative with bromoalkylthiophenes (BATs), under Heck experimental conditions using Pd(OAc)₂/Bu₄NBr as the catalytic system, giving rise to a variety of valuable aryl-substituted thienopyrazines. The obtained results suggested that the 2-position of the TP moiety is less reactive towards C-H arylation than the 5-and 7-positions. Moreover, the 3-position of the TP moiety showed almost no significant reactivity when all other positions were arylated. The C-H arylation of 2,3-dimethyl-TP with an excess amount of BATs proceeded smoothly, affording the corresponding diarylated thienopyrazine derivatives in excellent yields, without any additional products. Compared to usual cross-coupling reactions, the present synthetic methodology has been used to prepare interesting donor-acceptor π-conjugated polymeric materials in a facial manner in a simple way. Microwave-assisted polymerization proved to be efficient for obtaining reasonable molecular weight copolymers ranging from 18.8 to 24.3 kg mol^-1. Incorporating the thienopyrazine unit into polyhexylthiophene chains affected the photophysical and electrochemical properties. The optical band gaps were estimated to be in the range of 1.63-1.06 eV. All copolymers exhibited a diffraction peak at around 2θ = 5.72° corresponding to a d spacing of 15.43 A, which was assigned to an interchain spacing between polymer main chains similar to that found in P3HT. Moreover, a peak around 2θ = 23.09 (3.84 A) was also observed and is believed to be related to π-π stacking of the polymer backbones.

Full text of DOI:10.1002/ejoc.201200769

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FULL PAPER
DOI: 10.1002/ejoc.201200769
Palladium-Catalyzed Direct C–H Arylation of Thieno[3,4-b]pyrazines:
Synthesis of Advanced Oligomeric and Polymeric Materials
Nabiha I. Abdo,[a,b] Ashraf A. El-Shehawy,*[a,c] Ahmed A. El-Barbary,[b] and Jae-Suk Lee*[a]
Keywords: Synthetic methods / Cross-coupling / Heck reaction / Arylation / Microwave chemistry / Polymers /
Heterocycles / Donor-acceptor systems
The first examples are reported of an efficient direct regio-
selective direct C–H arylation of thieno[3,4-b]pyrazine (TP)
and its 2,3-dimethyl derivative with bromoalkylthiophenes
(BATs), under Heck experimental conditions using Pd(OAc)2/
Bu4NBr as the catalytic system, giving rise to a variety of
used to prepare interesting donor–acceptor π-conjugated
polymeric materials in a facial manner in a simple way. Mi-
crowave-assisted polymerization proved to be efficient for
obtaining reasonable molecular weight copolymers ranging
from 18.8 to 24.3 kgmol–1. Incorporating the thienopyrazine
valuable aryl-substituted thienopyrazines. The obtained re- unit into polyhexylthiophene chains affected the photophysi-
sults suggested that the 2-position of the TP moiety is less
reactive towards C–H arylation than the 5- and 7-positions.
Moreover, the 3-position of the TP moiety showed almost no
significant reactivity when all other positions were arylated.
The C–H arylation of 2,3-dimethyl-TP with an excess amount
of BATs proceeded smoothly, affording the corresponding di-
arylated thienopyrazine derivatives in excellent yields, with-
out any additional products. Compared to usual cross-cou-
pling reactions, the present synthetic methodology has been
cal and electrochemical properties. The optical band gaps
were estimated to be in the range of 1.63–1.06 eV. All copoly-
mers exhibited a diffraction peak at around 2θ = 5.72° corre-
sponding to a d spacing of 15.43 Å, which was assigned to
an interchain spacing between polymer main chains similar
to that found in P3HT. Moreover, a peak around 2θ = 23.09
(3.84 Å) was also observed and is believed to be related to π-
π stacking of the polymer backbones.
modification of such organic materials is an attractive issue
in organic synthesis.
Introduction
Conventionally, many π-conjugated polymers are synthe-
sized by using a variety of transition-metal-catalyzed cross-
coupling reactions (e.g., Kumada, Suzuki, Negishi, and
Stille) that allow the formation of carbon–carbon bonds.[5,6]
However, these synthetic methodologies require the prior
preparation of bifunctional organometallic reagents as
monomers, and the carbon–carbon bond-forming reaction
often produces a stoichiometric amount of toxic biproduct
such as organostannyl compounds. Moreover, the reactions
mostly proceed in two steps via an organometallic interme-
diate, and their chemocompatibility is often limited. In par-
ticular, palladium-catalyzed aryl–aryl coupling has become
an attractive alternative to traditional C–C cross-coupling
reactions due to the minimization of stoichiometric metallic
waste and the costs associated with the preparation of start-
ing materials.[6–8] Direct regioselective arylation of several
heterocyclic compounds such as 3,4-ethylenedioxy-
thiophene,[8c] alkoxythiophenes,[8a] pyrazines,[9] thiazoles,[10]
uracil,[11] and others[7,8] with aryl/heteroaryl halides has
been reported.
In recent years, π-conjugated polymers have received a
considerable level of interest due to their promising optical
and electronic properties.[1–3] Such materials have found a
variety of advanced technological applications in the fields
of photovoltaic light-emitting diodes, field-effect transis-
tors, and electrochromic devices[1–4] owing to their low cost,
compatibility, and tunable intrinsic properties (electronic,
optical, conductivity, and stability). Thus, the development
of synthetic methodologies for the facile synthesis and/or
[a] Department of Nanobio Materials and Electronics, School of
Materials Science and Engineering, and Research Institute for
Solar & Sustainable Energies (RISE), Gwangju Institute of
Science and Technology (GIST),
1 Oryong-dong, Buk-gu, Gwangju 500-712, Republic of South
Korea
Fax: +82-62-9702304
E-mail: jslee@gist.ac.kr
Homepage: https://mse.gist.ac.kr/~fps/
[b] Department of Chemistry, Faculty of Science, Tanta University,
Tanta 31527, Egypt
[c] Department of Chemistry, Faculty of Science, Kafr El-Sheikh
University,
During the past decade, BHJ solar cells based on blends
of poly(3-hexylthiophene) (P3HT) and PCBM have been
widely investigated and promising power conversion effi-
ciencies (PCEs; up to 6%) have been reported.[1,12] How-
Kafr El-Sheikh 33516, Egypt
Fax: +20-47-3215175
E-mail: elshehawy2@yahoo.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201200769.
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FULL PAPER
ever, a relatively large band gap of 1.9 eV and a highest
occupied molecular orbital (HOMO) energy level of 5.1 eV
prevent P3HT/PCBM-based BHJ solar cells from reaching
higher PCE values.[13] The use of intramolecular charge
transfer (ICT) from an electron-rich unit (donor) to an elec-
tron-deficient unit (acceptor) has become a promising ap-
proach to obtain low band gap π-conjugated copoly-
mers.[1,4] A variety of thienopyrazine (TP) derivatives have
been shown to be excellent precursors for the production of
promising low band gap π-conjugated polymers, especially
for photovoltaic solar cell applications.[14] However, for
these compounds to be fully utilized in optoelectronic appli-
cations, general and direct synthetic routes must be devel-
oped that allow access to a large number of different func-
tionalities. C–H arylation methodologies seem to be an ef-
Scheme 1. Synthesis of thieno[3,4-b]pyrazine derivatives 4a,b and
5a,b; Reagents and conditions: (i) conc. H2SO4/fuming H2SO4/fum-
ing HNO3, ice bath to room temp., 3 h; (ii) Sn/HCl, overnight 0 °C;
ficient synthetic approach that could be used to provide (iii) for 4a: glyoxal, aq. K2CO3, room temp. 3 h; for 4b: dimeth-
ylglyoxal, CH2Cl2/EtOH/Et3N/reflux, 50 °C; (iv) for 5a: NBS,
highly purified organic materials, which is an essential issue
AcOH/CHCl3, room temp., overnight; for 5b: NBS, DMF, 0 °C to
for optoelectronic applications.
room temp., 7 h.
In a continuation of our interest in the synthesis of oligo-
and polythiophenes,[15] we wish to report herein a facial ap-
proach to the synthesis of several π-conjugated oligomers
and copolymers based on hexylthiophene and thienopyraz-
ine units through the direct C–H arylation of thienopyraz- product 7a could also be prepared in 89% yield through
ine derivatives under “Heck-type” experimental conditions Suzuki cross-coupling reaction of 5,7-dibromothieno[3,4-b]-
(Jeffery conditions).[16] However, the direct C–H arylation pyrazine (5a) with 2-(3-hexylthiophen-2-yl)-4,4,5,5-tetra-
of thienopyrazine moieties has not yet been shown. The de- methyl-1,3,2-dioxaborolane (9). The spectroscopic data
sign of the synthesized copolymers reported herein was were found to be in good agreement with the proposed
based on P3HT to capitalize on the attractive properties of structure.
this well-known conjugated polymer. The major weakness
Under microwave irradiation, C–H arylation of 4a with
of P3HT Ϫ the relatively narrow spectral absorption 6a (molar ratio = 1:3) afforded the same arylated products
breadth from 350 to 650 nm Ϫ is solved in our approach by 7a and 8a within only five minutes with almost the same
copolymerization with thienopyrazine acceptor monomers. ratio of the reaction products when compared with the
same reaction under thermal reaction conditions (Table 1,
entries 2 and 6). Longer reaction times (up to 30 min) had
no significant effect on the reaction progress (Table 1, en-
Results and Discussion
try 7). Interestingly, C–H arylation of 4a with an excess
Thieno[3,4-b]pyrazines 4a and 4b and their correspond- amount of 2-bromo-3-methylthiophene (6b) (molar ratio of
ing 5,7-dibromo derivatives 5a and 5b, respectively, were se- 4a/6b = 1:5) afforded a mixture of the diarylated product
lected for our study. They were synthesized in good yields 7b (77% yield) and triarylated product 8b (18%) (Scheme 2
by using literature procedures[17] with some modifications, and Table 1, entry 8). Although the introduction of a
as shown in Scheme 1. All compounds were purified by methyl group instead of a hexyl chain in the thiophene ring
flash silica gel column chromatography (see the Supporting showed some improvement in the yield of the diarylated
Information for spectra).
product over that of the triarylated product, we could not
To obtain insight into the mode of Pd-catalyzed C–H detect any additional aryl coupling products either by TLC
arylation of the thienopyrazine moiety, thermal C–H aryl- or by GC analysis. The aforementioned results suggested
ation of thieno[3,4-b]pyrazine (4a) with 2-bromo-3-hexyl- that the 2-position of the thienopyrazine moiety was less
thiophene (6a), using Pd(OAc)2/Bu4NBr as the catalytic reactive towards C–H arylation than positions 5 and 7.
system, was first investigated (Scheme 2). A mixture of the Moreover, under the employed reaction conditions, the 3-
di- and tri-arylated products 7a and 8a was obtained in 50 position of the TP moiety 4a showed almost no significant
and 22% yield, respectively, along with the starting material reactivity after all other positions were arylated. This is
4a, when two molar equivalents of 6a was employed. The probably due to the high steric hindrance imposed during
same reaction with a molar ratio of 4a/6a of 1:3 furnished the fourth substitution. C–H arylation of the diarylated
the diarylated product 7a in 66% yield and triarylated prod- products 7 with 6 afforded the corresponding triarylated
uct 8a in 25% yield with almost no starting material re- products 8 along with significant amounts of the starting
maining. Increasing the reaction time, temperature, and/or diarylated compounds (Table 1, entries 9 and 10), which
the molar ratio of 4a/6a (up to 1:5 molar ratio) resulted in were recovered by column chromatography. We were not
no significant change in the ratio of the resulting products able to identify any significant other aryl coupling products
(Table 1, entries 3–5). It is worth noting that the diarylated either by TLC or by GC analysis.
2
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Palladium-Catalyzed Direct C–H Arylation
Scheme 2. C–H arylation of thieno[3,4-b]pyrazine derivatives with 2-bromo-3/4-alkylthiophenes; Reagents and conditions: C–H arylation
[DMF, KOAc, Bu4NBr, Pd(OAc)2, thermal or microwave]; Suzuki cross-coupling [Pd(PPh3)4, toluene, 24 h, 90 °C].
Table 1. Reaction conditions and yields for the C–H arylation reactions of thienopyrazine derivatives with bromoalkylthiophenes.
Entry
Reactants
Molar ratio Time
Temp. [°C]
Heating
Product (yield [%])[a]
1
2
3
4
5
6
7
8
4a
4a
4a
4a
4a
4a
4a
4a
7a
7b
4a
11
4b
4b
4b
16
6a
6a
6a
6a
6a
6a
6a
6b
6a
6b
10
10
6a
10
10
10
1:2
1:3
1:3
1:3
1:5
1:3
1:3
1:5
1:1.5
1:1.5
1:3
1:1.5
1:3
1:3
1:2
12 h
12 h
24 h
12 h
80
80
80
100
80
80
80
80
80
80
80
80
80
80
80
80
thermal
thermal
thermal
thermal
7a (50)
7a (66)
7a (65)
7a (64)
7a (68)
7a (69)
7a (68)
7b (77)
8a (35)
8b (52)
11 (60)
12 (29)
14 (93)
15 (95)
15 (78)
15 (93)
8a (22)
8a (25)
8a (24)
8a (25)
8a (23)
8a (26)
8a (28)
8b (18)
7a (54)
7b (42)
12 (33)
11 (61)
–
12 h
thermal
5 min
30 min
5 min
5 min
5 min
5 min
5 min
5 min
5 min
5 min
5 min
microwave
microwave
microwave
microwave
microwave
microwave
microwave
microwave
microwave
microwave
microwave
9
10
11
12
13
14
15
16
–
16 (15)
–
1:1.5
[a] Yield after flash silica gel column chromatography.
As expected, under microwave irradiation, C–H arylation try 12). The diarylated product 11 could also be prepared
of 4a with 2-bromo-4-hexylthiophene (10) (molar ratio of in 85% yield through Suzuki cross-coupling reaction of 5a
4a/10 = 1:3) proceeded in a manner similar to that of 4a with 2-(4-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-di-
with 6a, affording a mixture of the corresponding di- and oxaborolane (13).
tri-arylated products 11 and 12, respectively (Scheme 2 and
Under the same Heck experimental conditions with mi-
Table 1, entry 11). C–H arylation of 11 with 10 afforded the crowave irradiation, C–H arylation of 2,3-dimethyl-TP 4b
corresponding triarylated product 12 along with a signifi- with 6a or 10 (molar ratio of 4b/6a or 10 = 1:3) proceeded
cant amount of unreacted starting material 11 (Table 1, en- smoothly, affording the corresponding 5,7-diarylated thieno-
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Scheme 3. C–H arylation of 2,3-dimethylthieno[3,4-b]pyrazine derivatives with 2-bromo-3/4-hexylthiphenes; Reagents and conditions: C–
H arylation [DMF, KOAc, Bu4NBr, Pd(OAc)2, microwave]; Suzuki cross-coupling [Pd(PPh3)4, toluene, 24 h, 90 °C].
pyrazine derivatives 14 and 15, respectively, in excellent same chemical shift (but not with the same relative inten-
yields (Scheme 3; Table 1, entries 13 and 14, respectively). sities) at ca. 8.54 and ca. 8.50 ppm assigned to the α-meth-
However, the same reaction of 4b with 10 (1:2 molar ratio) ine proton (N=CH–C=N; integration value: 1 H) were ob-
afforded a mixture of the mono- and di-arylated products served for the triarylated products 8 and 12, respectively.
16 and 15 in a 15 and 78% yield, respectively (Table 1, en-
Although there are several possible mechanistic pathways
try 15). Thus, an excess amount of the bromoalkylthio- for the reaction, including via Pd0/PdII, the selectivity ob-
phene is essential for completion of the reaction. The C–H served in the C–H arylation reaction of TP by the catalysis
arylation of monoarylated product 16 with 10 (1:1.5 molar of a palladium complex can be conveniently explained by a
ratio) afforded exclusively the diarylated product 15 in high reaction mechanism based on a similar reasoning reported
yield (Table 1, entry 16). The diarylated products 14 and 15 for the mechanisms of PdII/PdIV catalyzed cross-coupling
could also be prepared in good yields through the Suzuki reactions.[18] A plausible PdII/PdIV mechanism was pro-
cross-coupling reaction of 5b with 9 or 13, respectively. The posed for the selectivity observed in the C–H arylation of
aforementioned results suggested that the success of the C– TP as shown in Scheme 4. C–H activation and subsequent
H arylation of TP derivatives can allow the synthesis of C–C bond formation would provide the desired C–C cou-
polyaryl-TP derivatives by a combination of Suzuki cross- pled products.
coupling and palladium-catalyzed C–H arylation reactions.
We took advantage of the easy direct C–H arylation of
1
As anticipated, the H NMR spectra of diarylated prod- TP moieties to introduce a simple and efficient synthetic
ucts 7 and 11 displayed single peaks at ca. 8.55 and ca. methodology to synthesize some donor–acceptor type π-
8.40 ppm, respectively, originating from the α-methine pro- conjugated copolymers (Scheme 5). Based on previous re-
tons adjacent to the nitrogen atoms (N=CH–CH=N; inte- ports,[15b,18e,19] microwave-assisted polymerization proved
gration value: 2 H). Moreover, single peaks with almost the to be particularly efficient for obtaining copolymers with
Scheme 4. Mechanistic pathways for the C–H arylation of TP moiety.
4
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Palladium-Catalyzed Direct C–H Arylation
higher Mn and lower PDI values than those obtained under analysis of the soluble parts of P1Ј and P3Ј showed maxi-
thermal reaction conditions. However, the desired copoly- mum average molecular weights (Mn) of 5.86 and 5.47 kg/
mers were obtained mostly in good yields with high purity. mol with PDI (polydispersity index) values of 3.91 and 4.26,
Therefore, all polymerizations were performed using micro- respectively, as determined by GPC analysis using polysty-
wave irradiation. After trying several reaction conditions, rene standards.
Pd-catalyzed C–H arylation of unsubstituted TP 4a with
Under the same Heck experimental conditions, the Pd-
2,5-dibromo-3-hexylthiophene (17a) or 5,5Ј-dibromo-3,3Ј- catalyzed C–H arylation of 2,3-dimethylthieno[3,4-b]pyr-
dihexyl-2,2Ј-bithiophene (18a) (1:1 molar ratio) with micro- azine (4b) with 17a or 18a (1:1 molar ratio) afforded copoly-
wave irradiation afforded partially soluble copolymers (P1Ј mers exclusively in high yields (92 and 94%, respectively)
1
and P3Ј, respectively, Scheme 5) after only 15 min. How- with identical H NMR spectroscopic data to those of co-
ever, longer reaction times and/or higher temperatures af- polymers P2 and P4 prepared through the Stille cross-cou-
forded copolymers that were very sparingly soluble in most pling of equimolar ratios of 5b with 17b or 18b, respectively.
1
common organic solvents. H NMR spectra of the soluble All copolymers were purified by successive Soxhlet extrac-
parts of the copolymers P1Ј and P3Ј were completely dif- tions with methanol and acetone to remove catalyst and
ferent to those of the copolymers P1 and P3 obtained oligomers. The synthesized copolymers (except P1Ј and
through the Stille cross-coupling of 5a with (3-hexylthio- P3Ј) are readily soluble in most common organic solvents
phene-2,5-diyl)bis(tributylstannane) (17b) or 3,3Ј-dihexyl- (Ͼ5 mgmL–1). They showed Mn values ranging from 18.8
5,5Ј-bis(tributylstannyl)-2,2Ј-bithiophene (18b) (1:1 molar to 24.3 kg/mol with PDI values of around 1.71–1.54 (see
ratio), respectively (Scheme 5). Based on the postulated Table 2).
mechanism of C–H arylation of TP (see Scheme 4), it is
The copolymers P1–P4 showed high thermal stability,
believed that cross-linking occurred through multiple C–H with decomposition temperature (Td, 95 wt.-% residues;
arylation at positions 2, 5, and 7 of the TP moiety (see the Figure 1) in the range of 285–394 °C as determined by ther-
Supporting Information for their spectroscopic data). GPC mal gravimetric analysis (TGA) (Table 2). The glass transi-
Scheme 5. Synthesis of π-conjugated copolymers P1–P4; Polymerization conditions: Stille cross-coupling: DMF, [Pd(PPh3)4], microwave;
C–H arylation, DMF, KOAc, Bu4NBr, Pd(OAc)2, microwave.
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Table 2. Polymerization results and thermal properties of copoly- thiophene alternating polymer (P1 and P2 type) has an in-
mers P1–P4.[a]
trinsically smaller band gap than TP-bithiophene alternat-
Polymer Mn [kg/mol][b] PDI (Mw/Mn)[b] Yield [%]
Td [°C][c]
ing polymers (P3 and P4 type), which is also supported by
theoretical calculations.[14a]
P1
P2
P3
P4
18.8
1.57
51.0
285.2
19.9 (17.7)[d]
23.6
1.71 (1.65)[d]
1.54
47.0 (81.0)[d] 283.7
85.0
393.8
24.3 (22.8)[d]
1.66 (1.61)[d]
81.0 (84.0)[d] 392.3
[a] All polymerizations were carried out using the Stille cross-cou-
pling method, expect where otherwise stated. [b] Calculated from
GPC (eluent CHCl3, 30 °C, polystyrene standards). [c] Determined
by TGA analysis under a nitrogen atmosphere at a heating rate of
10 °C/min. [d] Values in parentheses are for those polymers ob-
tained from polymerization using the C–H arylation method.
tion and melting temperatures were not observed, as re-
vealed by differential scanning calorimetry (DSC). The high
thermal stability of these polymers is adequate for applica-
tion in optoelectronic devices.
Figure 2. UV/Vis absorption spectra of copolymers P1–P4 in
chlorobenzene solutions and annealed spin-coated thin films.
Table 3. Optical and electrochemical properties of copolymers P1–
P4.
Figure 1. TGA thermograms of copolymers P1–P4.
UV/Vis absorption
Solution
Cyclic voltammetry[b,c]
The copolymers P1–P4 exhibited broad absorption ex-
tending into the near-infrared region with absorption max-
ima near 609–738 nm (see parts a and b of Figure 2 and
Table 3). Introduction of TP moieties into the P3HT back-
bone is observed to significantly decrease the optical band
gap and lead to the formation of a dual band absorption in
solutions and thin films, in the range of 300–1116 nm,
which most likely arises from the π-π* and ICT transitions,
respectively. There is a redshift in the absorption spectra for
copolymers in thin films relative to those in chlorobenzene
solutions; especially, P1 and P2 are low band gap polymers
with broader absorption bands than P3 and P4. A compari-
son of the optical absorption properties suggests that λmax
decreases as the number of thiophene moieties increases,
and indicates that the geometry of the studied polymers
ec
Film
λmax λonset Eg
HOMO LUMO Eg
op
op
λmax λonset Eg
[eV]
[eV]
[eV]
[nm] [nm] [eV][a] [nm] [nm] [eV][a]
P1 717.8 1049.9 1.18
P2 705.8 1092.1 1.13
P3 624.3 744.5 1.66
P4 578.7 690.8 1.79
[a] Optical band gap (Egop) was calculated from the intersection of
the tangent on the low energetic edge of the absorption spectrum
with the baseline. [b] The onset potentials were obtained from the
intersection of the two tangents down at the rising current and the
baseline changing current of the CV curves. [c] The polymer thin
films were prepared from chlorobenzene solutions of the polymers
spin-coated on ITO glass substrates.
737.9 1166.2 1.06 –4.85 –3.56 1.29
713.7 1146.2 1.08 –4.70 –3.51 1.19
635.5 813.5 1.52 –5.11 –3.42 1.69
609.2 759.7 1.63 –5.27 –3.43 1.84
Cyclic voltammetry (CV) was used to determine the
probably varied as the thiophene content increased. As the HOMO and LUMO energy levels (EHOMO and ELUMO
,
number of thiophene moieties in the polymer structure in- respectively) of the synthesized copolymers P1–P4. The cy-
creases the acceptor content is reduced, resulting in decreas- clic voltammograms are shown in Figure 3 and the derived
ing ICT.[20] The lower band gaps for P1 and P2 than those EHOMO, ELUMO, and electrochemical band gaps (Egec) are
observed for P3 and P4 might be explained by a twisted summarized in Table 3. The estimated Egec values are some-
op
conformation of the head-to-head bi(hexylthiophene) moi- what higher than the Eg estimated in thin films, most
ety. Although this effect might make only a minor contri- likely owing to an interface barrier present between the
bution to the band gaps, it has been reported that a TP- polymer film and the electrode surface.[21] The EHOMO level
6
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Palladium-Catalyzed Direct C–H Arylation
is higher for the supposedly more planar polymers incorpo- cate that these copolymers are able to retain the face-on
rated with one HT unit [P1 (–4.85 eV), and P2 (–4.70 eV)] orientation when blended with the electron acceptor
than for the more twisted polymers incorporated with two (PC60BM).
HT units [P3 (–5.11 eV), and P4 (–5.27 eV)]. Thus, the for-
mer class of copolymers with the extended π-conjugation
show lower Egec values (1.29 and 1.19 eV, respectively) than
the latter (1.69 and 1.84 eV). The estimated LUMO levels
Conclusions
Efficient direct regioselective C–H arylation of thieno-
[3,4-b]pyrazine derivatives with BATs under “Heck-type”
experimental conditions give rise to a variety of valuable
of P1–P4 (–3.42 to –3.56 eV) are higher than that of PCBM
(–4.3 eV),[22] allowing a favorable photoinduced electron
transfer from the LUMO of the donor to the LUMO of the
aryl-substituted TPs. The success of the C–H arylation of
acceptor.
TP derivatives can allow the synthesis of polyaryl-TP deriv-
atives by a combination of Suzuki cross-coupling and palla-
dium-catalyzed C–H arylation reactions. Thus, the present
method would be suitable for the combinatorial synthesis
of a variety of aryl-substituted thienopyrazines, which
would be potentially effective for the construction of thien-
opyrazine libraries. Compared to usual cross-coupling reac-
tions, a more facile method for the synthesis of some HT-
TP-based copolymers through direct C–H arylation of TP
derivatives, using the commercially available catalyst
Pd(OAc)2, have been developed. Incorporation of the TP
moiety into P3HT chains leads to a significant reduction in
the optical and electrochemical band gaps as compared to
P3HT. The high thermal stability, broad absorption spectra,
and appropriate HOMO/LUMO positions make P1–P4
promising candidates as donor materials in BHJ solar cells,
which is the subject of our on-going studies.
Figure 3. Cyclic voltammograms of copolymers P1–P4.
Experimental Section
The crystallinity of the spin-coated blend films of copoly-
mers (P1–P4)/PC60BM prepared from CB solutions and an-
nealed at 120 °C was evaluated by X-ray diffraction (XRD)
Materials: Unless otherwise stated, all manipulations and reactions
involving air-sensitive reagents were performed under a dry, oxy-
gen-free nitrogen atmosphere. All reagents and solvents were ob-
measurements. All copolymers exhibited a diffraction peak
at around 2θ = 5.72°, which corresponds to a d spacing of
15.43 Å (Figure 4). We assign this peak to the interchain
spacing between polymer main chains, similar to those
found in P3HT.[23] The observed increase in the intensity of
diffraction peaks of P1 and P2 suggests that an increase of
the HT crystallinity led to a better order in the blend, which
would led to higher Jsc in BHJ solar cells. Moreover, a peak
around 2θ = 23.09 (3.84 Å) is also observed and is believed
to be related to π-π stacking of the polymer backbones. The
π-stacking peak observed for the entire polymer series indi-
tained from commercial sources and dried by using standard pro-
cedures before use. 2-Bromo-3-hexylthiophene (6a), 2-bromo-3-
methylthiophene (6b), and 2-(3-hexylthiophen-2-yl)-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane (9) were purchased from Sigma–Ald-
rich and used without further purification. 2,5-Dibromo-3-hexyl-
thiophene (17a)[6a] and 3,3Ј-dihexyl-5,5Ј-bis(tributylstannyl)-2,2Ј-
bithiophene (18b)[17] were prepared according to described pro-
cedures. All reactions were monitored by TLC until completion.
Instrumentation and Methods: 1H and 13C NMR spectra were mea-
1
sured with a Varian spectrometer (400 MHz for H and 100 MHz
for 13C) in CDCl3 at 25 °C with TMS as the internal standard.
Chemical shifts are reported in ppm units; coupling constants (J)
are given in Hz. Flash column chromatography was performed with
Merck silica gel 60 (particle size 230–400 mesh ASTM). Neutral-
ized silica gel was prepared by adding triethylamine to the silica
gel in the same eluent used for column chromatography. Analytical
thin-layer chromatography (TLC) was conducted with Merck
0.25 mm 60F silica gel precoated aluminum plates and UV-254 flu-
orescent indicators. The UV/Vis/NIR absorption spectra were ob-
tained with a Varian Cary UV/Vis/NIR-5000 spectrophotometer
on the pure polymer samples. Cyclic voltammetry (CV) measure-
ments were performed with a Potentiostate/Galvanostate (SP-150
OMA Company) instrument. The supporting electrolyte was tetra-
butylammonium hexafluorophosphate (TBAPF6) in acetonitrile
(0.1 m) at a scan rate of 50 mV s–1. A three-electrode cell was used,
and a Pt wire and silver/silver chloride [Ag in 0.1 m KCl] were used
Figure 4. XRD profiles of copolymers P1–P4.
Eur. J. Org. Chem. 0000, 0–0
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N. I. Abdo, A. A. El-Shehawy, A. A. El-Barbary, J.-S. Lee
FULL PAPER
as the counter and reference electrodes, respectively. The HOMO
with ether and/or ethyl acetate. The combined organic fractions
were washed with water, dried with anhydrous Na2SO4, and con-
centrated by rotary evaporation without heating to give a light-
and LUMO energy levels (EHOMO and ELUMO) of the copolymers
were determined from the following relationships:[24] EHOMO
=
–Eox – 4.72 and ELUMO = –Ered – 4.72 (in eV), where Eox and Ered brown oil. Analytical samples were prepared by dissolving the oil
are the onset oxidation and reduction potentials, respectively, of
the polymers vs. the Ag/AgCl reference electrode. The polymer thin
films for electrochemical measurements were spin-coated from a
polymer/chlorobenzene solution on ITO glass slides, ca. 10 mg/mL.
XRD experiments were performed with a Bruker D8 advanced
model diffractometer with Cu-Kα radiation (λ = 1.542 Å) at a gen-
erator voltage of 40 kV and a current of 40 mA. Gel permeation
chromatographic (GPC) analysis was conducted with a Shimadzu
(LC-20A Prominence Series) instrument. Chloroform was used as
a carrier solvent (flow rate: 1 mL/min, at 30 °C), and calibration
curves were made with standard polystyrene samples. Microwave-
assisted polymerizations were performed with a focused microwave
synthesis system CEM (Discover S-Class System). Low tempera-
ture reactions were performed in a low temperature bath (PSL1810-
SHANGHA EYELA). A KD Scientific syringe pump (KDS-100)
was used to deliver accurate and precise amounts of reagents dur-
ing the dropwise addition processes.
in a minimal amount of CH2Cl2 and purified by column
chromatography using hexane as the eluting solvent to give thi-
1
eno[3,4-b]pyrazine (4a; 0.57 g, 76%) as a light-tan solid. H NMR
(400 MHz, CDCl3):[17a,17b] δ = 8.51 (s, 2 H, N=CH-CH=N), 8.03
(s, 2 H, 2ϫ CHS) ppm. 13C NMR (100 MHz, CDCl3): δ = 144.41,
142.80, 118.00 ppm. C6H4N2S (136.17): calcd. C 52.92, H 2.96, N
20.57, S 23.55; found C 53.07, H 3.11, N 20.50, S 23.63.
2,3-Dimethylthieno[3,4-b]pyrazine (4b): A mixture of dimethylgly-
oxal (1.63 g, 19 mmol), 3,4-diaminothiophene dihydrochloride (3;
3.5 g, 18.70 mmol), and triethylamine (95 mL) in dichloromethane
(170 mL) and ethanol (170 mL) was stirred at 50 °C for 8 h. After
cooling, the reaction mixture was extracted with dichloromethane/
water. The organic layer was collected and dried with sodium sul-
fate. The organic solvent was evaporated and the crude product
thus obtained was further purified using silica gel flash column
chromatography (chloroform/ethyl acetate, 4:1) to generate 4b
(2.45 g, 80%) as a light-tan solid. 1H NMR (400 MHz, CDCl3):[17c]
δ = 2.55 (s, 6 H, 2ϫ N=C-CH3), 7.71 (s, 2 H, 2ϫ CHS) ppm. 13C
NMR (100 MHz, CDCl3): δ = 153.14, 141.69, 115.92, 23.62 ppm.
C8H8N2S (164.23): calcd. C 58.51, H 4.91, N 17.06, S 19.52; found:
C 58.59, H 5.06, N 17.01, S 19.60.
2,5-Dibromo-3,4-dinitrothiophene (2): Into a high-vacuum-dried
three-necked flask containing a mixture of concentrated sulfuric
acid (12.4 mL), fuming sulfuric acid (19.1 mL), and fuming nitric
acid (10.5 mL), cooled with an ice bath, 2,5-dibromothiophene (1;
3.35 mL, 7.2 g, 29.7 mmol) was added slowly in a dropwise fashion
to maintain a temperature of 15 °C. After complete addition, the
reaction mixture was stirred for an additional 3 h at a temperature
of 25–30 °C and then poured into 100 g of ice. Upon melting of
the ice, the yellow solid residue was collected by vacuum filtration
and recrystallized from methanol (8.0 g, 81%). 13C NMR
(100 MHz, DMSO):[7a] δ = 139.62, 116.66 ppm. C4Br2N2O4S
(331.93): calcd. C 14.47, Br 48.15, N 8.44, S 9.66; found: C 14.43,
Br 48.31, N 8.37, S 9.72.
5,7-Dibromothieno[3,4-b]pyrazine (5a): Toa solution of thieno[3,4-b]-
pyrazine (4a; 2.0 g, 14.7 mmol) in chloroform/acetic acid (1:1;
60 mL) cooled to 0 °C, NBS (5.75 g, 32.3 mmol) was added in three
portions. The reaction mixture was stirred overnight at room tem-
perature, followed by the addition of an equal amount of water.
The chloroform layer was separated and washed thoroughly with
water and then dried with anhydrous Na2SO4. The organic layer
was evaporated under reduced pressure without heating to give a
greenish yellow solid. The resulting crude solid was washed several
times with diethyl ether (by decantation method, until the ether
became colorless). Further purification was performed by silica gel
flash column chromatography (hexane/CH2Cl2; 1:1) to afford the
pure product 5a (1.62 g, 75%) as a yellow solid. 1H NMR
(400 MHz, CDCl3):[17b] δ = 8.53 (s, 2 H, N=CH-CH=N) ppm. 13C
3,4-Diaminothiophene Dihydrochloride (3): Concentrated HCl
(230 mL, 37%) and 2,5-dibromo-3,4-dinitrothiophene (2; 12.8 g,
38.0 mmol) were combined in a thoroughly dried flask and cooled
in an ice bath. Tin metal (31.9 g, 269 mmol) was slowly added
within 1 h to maintain a temperature of 25–30 °C. After complete
addition, the reaction was stirred until all the tin was consumed,
and the flask was then kept in a refrigerator overnight. The solid
precipitate was recovered by vacuum filtration and washed with
diethyl ether and acetonitrile. The product·2H+ salt 3 is extremely
stable, whereas the free amino product is very susceptible to oxi-
dation. For this reason, this precursor was commonly stored as
the product·2H+ salt. When desired, for conversion into the free
diaminothiophene, the product·2H+ salt was dissolved in water
(600 mL) and cooled with an ice bath. The solution was made basic
with 4 n Na2CO3, followed by extraction with diethyl ether. The
organic layer was then dried with anhydrous Na2SO4 and concen-
trated by rotary evaporation without heating to give a white crys-
talline product of 3,4-diaminothiophene (2.6 g, 61%). 1H NMR
(400 MHz, CDCl3):[17a] δ = 6.16 (s, 2 H, 2ϫ CHS), 3.36 (br. s, 4
H, 2ϫ NH2) ppm. 13C NMR (100 MHz, CDCl3): δ = 137.2,
101.7 ppm. C4H6N2S (114.17): calcd. C 42.08, H 5.30, N 24.54, S
28.09; found C 42.18, H 5.50, N 24.66, S 27.92.
NMR (100 MHz, CDCl3):
δ = 145.61, 140.60, 105.78 ppm.
C6H2Br2N2S (293.97): calcd. C 24.51, H 0.69, Br 54.36, N 9.53, S
10.91; found: C 24.44, H 0.67, Br 54.49, N 9.56, S 11.05.
5,7-Dibromo-2,3-dimethylthieno[3,4-b]pyrazine (5b): A solution of
NBS (1.14 g, 6.42 mmol) in DMF (30 mL) was added dropwise to
a
solution of 2,3-dimethylthieno[3,4-b]pyrazine (4b; 0.48 g,
2.92 mmol) in DMF (100 mL), which was cooled in an ice bath.
After complete addition, the reaction mixture was warmed to room
temperature and stirred for 6 h. The reaction mixture was poured
into water and then extracted with chloroform. The organic layers
were combined, washed with sodium chloride solution and dried
with sodium sulfate. Further purification was performed using
flash silica gel chromatography (chloroform) to generate 5b (0.79 g,
85%) as a tan solid. 1H NMR (400 MHz, CDCl3):[17c] δ = 2.68 (s, 6
H, 2ϫ N=C-CH3) ppm. 13C NMR (100 MHz, CDCl3): δ = 155.18,
139.27, 103.20, 23.42 ppm. C8H6Br2N2S (322.02): calcd. C 29.84,
H 1.88, Br 49.63, N 8.70, S 9.96; found: C 30.05, H 1.92, Br 49.42,
N 8.77, S 9.88.
Thieno[3,4-b]pyrazine (4a): 3,4-Diaminothiophene dihydrochloride
(3; 1.03 g, 5.52 mmol) was added to 5% aqueous Na2CO3 (60 mL).
Glyoxal (0.36 g, 6.1 mmol) was then added dropwise within 5 min
as an aqueous solution, which was prepared by diluting 0.885 g of
a 40% glyoxal solution in water (15 mL). The reaction mixture was
stirred at room temperature for at least 3 h and extracted repeatedly
(3-Hexylthiophene-2,5-diyl)bis(tributylstannane) (17b): nBuLi (2.5 m
in hexane, 14.85 mmol, 5.94 mL) was added dropwise within a
period of 60 min to a solution containing of 3-hexylthiophene (1 g,
5.94 mmol) in anhydrous THF (100 mL) and cooled to –80 °C. The
reaction mixture was stirred for 3 h at –80 °C. Tributyltin chloride
8
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Palladium-Catalyzed Direct C–H Arylation
(4.56 mL, 14.85 mmol) was added dropwise within 30 min to the
reaction mixture. After being stirred at –80 °C for 30 min, the reac-
tion mixture was then allowed to return to room temperature and
stirred further for 2 h, followed by quenching with aqueous NH4Cl.
The reaction mixture was extracted with ethyl acetate followed by
washing of the organic layer with water before drying over anhy-
drous Na2SO4. The solvent was evaporated and the crude mixture
of products was purified by flash column chromatography on silica
gel pretreated with triethylamine to afford the desired product 17b
Thermal C–H Arylation of 4a with 2-Bromo-3-hexylthiophene (6a).
Typical Procedure: Potassium acetate (0.59 g, 6.0 mmol), tetrabut-
ylammonium bromide (TBAB; 0.64 g, 2.0 mmol), 2-bromo-3-hex-
ylthiophene (6a; 0.49 g, 2.0 mmol), and palladium acetate (0.04 g,
0.2 mmol) were added to a stirred solution of 4a (0.14 g, 1.0 mmol)
in anhydrous DMF (25 mL). The resulting suspension was heated
under reflux for 12 h at a temperature of 80 °C. The reaction mix-
ture was cooled to room temperature and a mixture of CH2Cl2
(50 mL) and water (25 mL) was added. The organic layer was sepa-
rated and concentrated under reduced pressure. The resulting dark
oil was purified by flash chromatography on silica gel (n-hexane/
EtOAc, 10:2) to generate the product 7a (0.23 g, 50% yield) as a
1
(4.21 g, 95%). H NMR (400 MHz, CDCl3): δ = 7.06 (s, 1 H, CH-
C-hexyl), 2.76–2.73 (t, J = 12 Hz, 2 H, ArCH2, hexyl), 1.62 [m, 14
H, ArCH2CH2, hexyl, 6ϫ (CH2)2CH2, butyl], 1.44–1.40 [m, 24 H,
6ϫ (CH2)2CH2, butyl], 1.25–1.15 [m, 6 H, CH3(CH2)3, hexyl], violet solid, and 2,5,7-tris(3-hexylthiophen-2-yl)thieno[3,4-b]pyr-
0.99–0.91 (m, 21 H, CH3, hexyl, 6ϫ CH3, butyl) ppm. 13C NMR
(100 MHz, CDCl3): δ = 144.30, 136.80, 136.05, 125.58, 31.64,
30.91, 29.25, 29.09, 27.54, 27.37, 22.77, 14.12, 13.74, 10.78 ppm.
C34H68SSn2 (746.39): calcd. C 54.71, H 9.18, S 4.30, Sn 31.81;
found: C 54.59, H 9.23, S 4.40.
azine (8a; 0.14 g, 22% yield) as a sticky violet oil (see Table 1, en-
try 1). The same procedure was applied with changing molar ratios
of 4a/6a; reaction time and reaction temperature (see Table 1, en-
tries 2–5).
Compound 8a: 1H NMR (400 MHz, CDCl3): δ = 8.54 (s, 1 H,
N=CH-C=N), 7.17–7.16 (d, J = 4 Hz, 3 H, 3ϫ CHS), 7.05–7.04
(d, J = 4 Hz, 3 H, 3ϫ CH-C-hexyl), 2.86–2.84 (m, 6 H, 3ϫ
ArCH2CH2), 1.71–1.67 (m, 6 H, 3ϫ ArCH2CH2), 1.30–1.27 [m, 18
5,5Ј-Dibromo-3,3Ј-dihexyl-2,2Ј-bithiophene (18a): NBS (2.26 g,
12.70 mmol) was added, in one portion, to an ice-cold solution of
3,3Ј-dihexy1-2,2Ј-bithiophene[15b] (1.7 g, 5.08 mmol) in anhydrous
THF (65 mL). After stirring for 30 min at 0 °C, the reaction mix-
ture was stirred overnight at room temperature, followed by
quenching with a saturated aqueous solution of NH4Cl. The reac-
tion mixture was extracted with ethyl acetate followed by washing
of the separated organic layers with water. The collected organic
layers were dried with anhydrous Na2SO4, and the organic solvent
was removed under vacuum. The crude product thus obtained was
purified using silica gel flash column chromatography (n-hexane/
EtOAc, 10:2) to generate the desired product 18a (2.35 g, 94%
H, 3ϫ CH3(CH2)3], 0.89–0.86 (t,
J = 12 Hz, 9 H, 3ϫ
CH2CH3) ppm. 13C NMR (100 MHz, CDCl3): δ = 151.46, 144.06,
141.96, 139.63, 137.68, 125.99, 125.48, 123.81, 31.65, 30.26, 29.67,
29.28, 22.59, 14.05 ppm. C36H46N2S4 (635.02): calcd. C 68.09, H
7.30, N 4.41, S 20.20; found: C 68.15, H 7.44, N 4.36, S 20.11.
Microwave-Assisted C–H Arylation of 4a with 2-Bromo-3-hexylthio-
phene (6a). Typical Procedure: Potassium acetate (1.18 g,
12.0 mmol), TBAB (1.28 g, 4.0 mmol), 2-bromo-3-hexylthiophene
(6a; 1.48 g, 6.0 mmol), and palladium acetate (0.09 g, 0.40 mmol)
were added to a stirred solution of 4a (0.27 g, 2.0 mmol) in anhy-
drous DMF (25 mL). The resulting suspension was irradiated with
microwave for 5 min at a temperature of 80 °C. The reaction mix-
ture was then cooled to room temperature followed by the addition
of CH2Cl2 (25 mL) and water (25 mL). The organic layer was sepa-
rated and concentrated and the resulting dark oil was purified by
flash chromatography on silica gel (n-hexane/EtOAc, 10:2) to gen-
erate the product 7a (0.64 g, 69%) as a violet solid, and 8a (0.33 g,
26% yield) as a sticky violet oil (see Table 1, entry 6). The same
procedure was applied for the reaction of 4a with 6a for 30 min
under microwave irradiation (see Table 1, entry 7).
1
yield) as a yellow oil. H NMR (400 MHz, CDCl3): δ = 6.90 (s, 2
H, 2ϫ CH-C-Br), 2.44–2.40 (t, J = 16 Hz, 4 H, 2ϫ ArCH2), 1.66
(m, 4 H, 2ϫ ArCH2CH2), 1.25–1.22 [m, 12 H, 2ϫ CH3(CH2)3],
0.87–0.84 (t, J = 12 Hz, 6 H, 2ϫ CH2CH3) ppm. 13C NMR
(100 MHz, CDCl3): δ = 143.81, 131.37, 128.83, 112.35, 31.56,
30.49, 28.96, 28.71, 22.53, 14.05 ppm. C20H28Br2S2 (492.37): calcd.
C 48.79, H 5.73, Br 32.46, S 13.02; found: C 48.67, H 5.81, Br
32.29, S 13.15.
Suzuki Cross-Coupling of 5,7-Dibromothieno[3,4-b]pyrazine (5a)
with 2-(3-Hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborol-
ane (9): A mixture of 5,7-dibromothieno[3,4-b]pyrazine (5a; 0.5 g,
1.70 mmol), 2-(3-hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-di-
oxaborolane (9; 1.25 g, 4.25 mmol), and [Pd(PPh3)4] (0.5 g,
43.5 mol-%) was dissolved in toluene (35 mL) and degassed with
nitrogen for 20 min, followed by the addition of an aqueous solu-
tion of K2CO3 (2 m, 30 mL). The reaction mixture was stirred vig-
orously and heated under reflux at 90 °C for 12 h. The reaction
mixture was allowed to return to room temperature followed by
extraction with CH2Cl2. The organic layers were collected, washed
with water, and dried with anhydrous Na2SO4. The organic solvent
was evaporated under reduced pressure and the crude product thus
obtained was purified by flash silica gel column chromatography
(n-hexane/EtOAc, 10:2) to generate the desired product 5,7-bis(3-
hexylthiophen-2-yl)thieno[3,4-b]pyrazine (7a; 0.70 g, 89%) as a vi-
olet solid. 1H NMR (400 MHz, CDCl3):[8a] δ = 8.55 (s, 2 H,
N=CH-CH=N), 7.43–7.41 (d, J = 8 Hz, 2 H, 2ϫ CHS), 7.06–7.04
(d, J = 8 Hz, 2 H, 2ϫ CH-C-hexyl), 2.87–2.83 (t, J = 16 Hz, 4 H,
2ϫ ArCH2CH2), 1.70–1.67 (m, 4 H, 2ϫ ArCH2CH2), 1.28–1.25
[m, 12 H, 2ϫ CH3(CH2)3], 0.86–0.83 (t, J = 12 Hz, 6 H, 2ϫ
Microwave-Assisted C–H Arylation of 4a with 2-Bromo-3-methyl-
thiophene (6b): Potassium acetate (0.59 g, 6.0 mmol), TBAB (0.64 g,
2.0 mmol), 2-bromo-3-methylthiophene (6b; 0.89 g, 5.0 mmol), and
palladium acetate (0.04 g, 0.2 mmol) were added to a stirred solu-
tion of 4a (0.14 g, 1.0 mmol) in anhydrous DMF (25 mL). The re-
sulting suspension was heated overnight at a temperature of 80 °C.
The reaction mixture was cooled to room temperature followed by
addition of CH2Cl2 (25 mL) and water (25 mL). The organic layer
was separated and concentrated and the resulting dark oil was puri-
fied by flash chromatography on silica gel (n-hexane/EtOAc, 10:2)
to generate the 5,7-bis(3-methylthiophen-2-yl)thieno[3,4-b]pyrazine
(7b) as a deep violet solid (0.25 g, 77%) and 2,5,7-tris(3-methylthio-
phen-2-yl)thieno[3,4-b]pyrazine (8b) as a violet solid (0.08 g, 18%)
(see Table 1, entry 8). The same procedure was also applied for the
reaction of 7 with 6 (molar ratios = 1:1.5), which afforded the prod-
uct 8 along with starting material 7 (recovered by silica gel flash
column chromatography) (see Table 1, entries 9 and 10).
CH2CH3) ppm. 13C NMR (100 MHz, CDCl3): δ = 144.03, 141.94, Compound 7b: 1H NMR (400 MHz, CDCl3): δ = 8.54 (s, 2 H,
139.68, 129.52, 127.01, 126.71, 126.08, 31.64, 30.47, 29.88, 29.27, N=CH-CH=N), 7.38–7.37 (d, J = 4 Hz, 2 H, 2ϫ CHS), 6.98–6.97
22.58, 14.05 ppm. C26H32N2S3 (468.74): calcd. C 66.62, H 6.88, N (d, J = 4 Hz, 2 H, 2ϫ CH-C-hexyl), 2.50 (s, 6 H, 2ϫ CH3) ppm.
5.98, S 20.52; found: C 66.71, H 6.81, N 5.67, S 20.44.
13C NMR (100 MHz, CDCl3): δ = 143.82, 139.30, 136.18, 130.87,
Eur. J. Org. Chem. 0000, 0–0
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N. I. Abdo, A. A. El-Shehawy, A. A. El-Barbary, J.-S. Lee
FULL PAPER
126.67, 125.47, 123.81, 16.29 ppm. C16H12N2S3 (328.47): calcd. C
58.50, H 3.68, N 8.53, S 29.29; found: C 58.66, H 3.60, N 8.42, S
29.30.
Suzuki Cross-Coupling Reaction of 5b with 9: A mixture of com-
pound 5b (0.5 g, 1.55 mmol), 9 (1.14 g, 3.88 mmol), and [Pd-
(PPh3)4] (0.5 g, 43.5 mol-%) was dissolved in toluene (35 mL) and
degassed with nitrogen for 20 min, followed by the addition of an
aqueous solution of K2CO3 (2 m, 30 mL). The reaction mixture was
stirred vigorously and heated under reflux at 90 °C for 12 h. After
cooling, the reaction mixture was extracted with CH2Cl2 and the
organic layers were collected, washed with water, and finally dried
with anhydrous Na2SO4. The solvent was evaporated under re-
duced pressure, and the crude product thus obtained was purified
by silica gel flash column chromatography (n-hexane/EtOAc, 10:2)
Compound 8b: 1H NMR (400 MHz, CDCl3): δ = 8.44 (s, 1 H,
N=CH-C=N), 7.17–7.16 (d, J = 4 Hz, 3 H, 3ϫ CHS), 6.90–6.89
(d, J = 4 Hz, 3 H, 3ϫ CH-C-hexyl), 2.47 (s, 9 H, 3ϫ CH3) ppm.
13C NMR (100 MHz, CDCl3): δ = 151.47, 143.99, 141.96, 138.99,
133.51, 126.31, 125.47, 123.73, 15.65 ppm. C21H16N2S4 (424.63):
calcd. C 59.40, H 3.80, N 6.60, S 30.21; found: C 59.30, H 3.91, N
6.53, S 30.16.
Suzuki Cross-Coupling of 5,7-Dibromothieno[3,4-b]pyrazine (5a)
with 2-(4-Hexylthiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborol-
ane (13): A mixture of compound 5a (0.5 g, 1.70 mmol), 2-(4-hexyl-
to
generate
5,7-bis(3-hexylthiophen-2-yl)-2,3-dimethylthieno-
[3,4-b]pyrazine (14; 0.71 g, 92%) as a dark-red solid.
Microwave-Assisted C–H Arylation of 4b with 6a: Potassium acetate
(0.59 g, 6.0 mmol), TBAB (0.64 g, 2.0 mmol), 6a (0.74 g,
3.0 mmol), and palladium acetate (0.04 g, 0.2 mmol) were added to
a stirred solution of 4b (0.16 g, 1.0 mmol) in anhydrous DMF
(25 mL). The resulting suspension was irradiated with microwave
for 5 min at a temperature of 80 °C. The reaction mixture was then
cooled to room temperature followed by the addition of a mixture
of CH2Cl2 (25 mL) and water (25 mL). The organic layer was sepa-
rated and concentrated under reduced pressure. The resulting dark
oil was purified by flash chromatography on silica gel (n-hexane/
EtOAc, 10:2) to afford 5,7-bis(3-hexylthiophen-2-yl)-2,3-dimeth-
ylthieno[3,4-b]pyrazine (14; 0.46 g, 93% yield) as a dark-red solid.
1H NMR (400 MHz, CDCl3): δ = 7.39–7.38 (d, J = 4 Hz, 2 H, 2ϫ
CHS), 7.05–7.04 (d, J = 4 Hz, 2 H, CH-C-hexyl), 2.91–2.88 (t, J =
12 Hz, 4 H, 2ϫ ArCH2CH2), 2.66 (s, 6 H, 2ϫ N=C-CH3), 1.76–
1.73 (m, 4 H, 2ϫ ArCH2CH2), 1.47–1.31 [m, 12 H, 2ϫ CH3-
(CH2)3], 0.91–89 (t, J = 8 Hz, 6 H, 2ϫ CH2CH3) ppm. 13C NMR
(100 MHz, CDCl3): δ = 152.82, 141.13, 138.37, 135.67, 126.34,
125.46, 124.17, 31.68, 30.45, 29.99, 29.36, 23.53, 22.59, 14.05 ppm.
C28H36N2S3 (496.79): calcd. C 67.69, H 7.30, N 5.64, S 19.36;
found: C 67.77, H 7.39, N 5.55, S 19.30.
thiophen-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane[15a]
(13;
1.25 g, 4.25 mmol), and [Pd(PPh3)4] (0.5 g, 43.5 mol-%) was dis-
solved in toluene (35 mL) and degassed with nitrogen for 20 min,
followed by the addition of an aqueous solution of K2CO3 (2 m,
30 mL). The reaction mixture was vigorously stirred and heated
under reflux at 90 °C for 12 h. After cooling to room temperature,
the reaction mixture was extracted with CH2Cl2 and the organic
layers were separated, washed thoroughly with water, and finally
dried with anhydrous Na2SO4. The solvent was evaporated under
vacuum and the resulting crude product was purified using silica
gel flash column chromatography (n-hexane/EtOAc, 10:2) to gener-
ate 5,7-bis(4-hexylthiophen-2-yl)thieno[3,4-b]pyrazine (11; 0.67 g,
85% yield) as a deep-violet solid. 1H NMR (400 MHz): δ = 8.40
(s, 2 H, N=CH-CH=N), 7.39 (s, 2 H, 2ϫ CHS), 6.90 (s, 2 H, 2ϫ
CH-C-hexyl), 2.57–2.54 (t, J = 12 Hz, 4 H, 2ϫ ArCH2CH2), 1.63–
1.56 (m, 4 H, 2ϫ ArCH2CH2), 1.22–1.19 [m, 12 H, 2ϫ CH3-
(CH2)3], 0.80–0.78 (t, J = 8 Hz, 6 H, 2ϫ CH2CH3) ppm. 13C NMR
(100 MHz, CDCl3): δ = 144.14, 143.76, 138.99, 133.71, 126.40,
121.61, 31.68, 31.58, 30.40, 29.01, 22.64, 14.10 ppm. C26H32N2S3
(468.74): calcd. C 66.62, H 6.88, N 5.98, S 20.52; found: C 66.44,
H 6.75, N 6.09, S 20.43.
Suzuki Cross-Coupling Reaction of 5b with 13: A mixture of 5b
(0.5 g, 1.55 mmol), 13[15a] (1.14 g, 3.88 mmol), and [Pd(PPh3)4]
(0.5 g, 43.5 mol-%) was dissolved in toluene (35 mL) and degassed
with nitrogen for 20 min, followed by the addition of an aqueous
solution of K2CO3 (2 m, 30 mL). The reaction mixture was vigor-
ously stirred and heated under reflux at 90 °C for 12 h. After cool-
ing to room temperature, the reaction mixture was extracted with
CH2Cl2, and the organic layers were separated, washed thoroughly
with water, and finally dried with anhydrous Na2SO4. The solvent
was evaporated under vacuum, and the resulting crude product was
purified using silica gel flash column chromatography (n-hexane/
EtOAc, 10:2) to generate 5,7-bis(4-hexylthiophen-2-yl)-2,3-dimeth-
Microwave-Assisted C–H Arylation of 4a with 2-Bromo-4-hexylthio-
phene (10). Typical Procedure: Potassium acetate (0.59 g,
6.0 mmol), TBAB (0.64 g, 2.0 mmol), 2-bromo-4-hexylthio-
phene[15a] (10; 0.74 gm, 3.0 mmol) and palladium acetate (0.04 g,
0.2 mmol) were added to a stirred solution of 4a (0.14 g, 1.0 mmol)
in anhydrous DMF (25 mL). The resulting suspension was irradi-
ated with microwave for 5 min at a temperature of 80 °C. The reac-
tion mixture was then cooled to room temperature followed by add-
ing CH2Cl2 (25 mL) and water (25 mL). The organic layer was sep-
arated and concentrated under reduced pressure. The resulting dark
oil was purified by flash chromatography on silica gel (n-hexane/
EtOAc, 10:2) to generate 5,7-bis(4-hexylthiophen-2-yl)thieno[3,4-b]-
pyrazine (11; 0.28 g, 60%) as a deep-violet solid and 2,5,7-tris(4-
hexylthiophen-2-yl)thieno[3,4-b]pyrazine (12; 0.21 g, 33%) as a
sticky violet oil. The same procedure was also applied for the reac-
tion of 11 with 10 (molar ratios = 1:1.5), which afforded the prod-
uct 12 (0.18 g, 29%) along with starting material 11 (0.29, 61%;
recovered by silica gel flash column chromatography) (see Table 1,
entry 13).
1
ylthieno[3,4-b]pyrazine (15; 0.68 g, 88%) as a deep-violet solid. H
NMR (400 MHz, CDCl3): δ = 7.37 (s, 2 H, 2ϫ CHS), 6.86 (s, 2
H, 2ϫ CH-C-hexyl), 2.71–2.68 (t,
J = 12 Hz, 4 H, 2ϫ
ArCH2CH2), 2.57 (s, 6 H, 2ϫ N=C-CH3), 1.67–1.58 (m, 4 H, 2ϫ
ArCH2CH2), 1.30–1.19 [m, 12 H, 2ϫ CH3(CH2)3], 0.88–0.80 (t, J
= 8 Hz, 6 H, 2ϫ CH2CH3) ppm. 13C NMR (100 MHz, CDCl3): δ =
153.24, 143.52, 137.86, 134.33, 125.77, 123.82, 120.94, 31.69, 31.57,
30.40, 29.00, 23.72, 22.62, 14.10 ppm. C28H36N2S3 (496.79): calcd.
C 67.69, H 7.30, N 5.64, S 19.36; found: C 67.55, H 7.19, N 5.60,
S 19.44.
Compound 12: 1H NMR (400 MHz, CDCl3): δ = 8.50 (s, 1 H,
N=CH-C=N), 7.36 (s, 3 H, 3ϫ CHS), 7.26 (s, 3 H, 3ϫ CH-C-
hexyl), 2.64–2.59 (m, 6 H, 3ϫ ArCH2CH2), 1.70–1.63 (m, 6 H, 3ϫ
ArCH2CH2), 1.33–1.25 [m, 18 H, 3ϫ CH3(CH2)3], 0.91–0.88 (t, J Microwave-Assisted C–H Arylation of 4b with 10. Typical Pro-
= 12 Hz, 9 H, 3ϫ CH2CH3) ppm. 13C NMR (100 MHz, CDCl3): cedure: Potassium acetate (0.59 g, 6.0 mmol), TBAB (0.64 g,
δ = 151.46, 144.06, 143.74, 139.84, 139.56, 137.67, 125.48, 123.81, 2.0 mmol), 10[15a] (0.49 g, 2.0 mmol), and palladium acetate (0.04 g,
31.65, 31.55, 30.66, 29.84, 22.59, 14.05 ppm. C36H46N2S4 (635.02): 0.2 mmol) were added to a stirred solution of 4b (0.16 g, 1.0 mmol)
calcd. C 68.09, H 7.30, N 4.41, S 20.20; found: C 68.00, H 7.42, N
4.35, S 20.24.
in anhydrous DMF (25 mL). The resulting suspension was micro-
wave irradiated for 5 min at a temperature of 80 °C. The reaction
10
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Job/Unit: O20769
/KAP1
Date: 21-08-12 17:15:18
Pages: 13
Palladium-Catalyzed Direct C–H Arylation
mixture was cooled to room temperature followed by the addition
of a mixture of CH2Cl2 (25 mL) and water (25 mL). The organic
layer was separated, concentrated, and the resulting dark oil was
purified by flash chromatography on silica gel (n-hexane/EtOAc,
10:0.2) to generate 5,7-bis(4-hexylthiophen-2-yl)-2,3-dimethyl-
thieno[3,4-b]pyrazine (15; 0.39 g, 78% yield) as a deep-violet solid
(48 h), and dried under vacuum prior to GPC and 1H NMR analy-
ses.
Poly[(thieno[3,4-b]pyrazine-5,7-diyl)-alt-(3-hexylthiophene-2,5-diyl)]
(P1): 1H NMR (400 MHz, CDCl3): δ = 8.53 (br. s, 2 H, N=CH-
CH=N), 7.00 (br. s, 1 H, CHC-hexyl), 2.64 (br. s, 2 H, ArCH2CH2),
1.86–1.69 (br. s, 2 H, ArCH2CH2), 1.32–1.25 [m, 6 H, CH3(CH2)
3], 0.89 (br. s, 3 H, CH2CH3) ppm. (C16H16N2S2)n (300.44)n: calcd.
C 63.96, H 5.37, N 9.32, S 21.35; found: C 63.87, H 5.44, N 9.25,
S 21.20.
and
5-(4-hexylthiophen-2-yl)-2,3-dimethylthieno[3,4-b]pyrazine
(16; 0.05 g, 15% yield) as a sticky violet oil.
The same procedure was applied for the reaction of 4b with 10 with
a molar ratio of 1:3. The product 15 was obtained as the sole prod-
uct in 95% yield, after silica gel flash column chromatography (see
Table 1, entry 14). Moreover, the same procedure was also applied
for the reaction of 16 with 10 (molar ratio = 1:1.5), which afforded
the product 15 in 93% yield, after silica gel flash column
chromatography (see Table 1, entry 16).
Poly[(thieno[3,4-b]pyrazine-2,5,7-triyl)-2,5-bis(3-hexylthiophene-2,5-
diyl)] (P1Ј): GPC analysis of the soluble parts of P1Ј showed a
maximum Mn value of 5855 with a PDI value of 3.91. 1H NMR
(400 MHz, CDCl3): δ = 8.56 (br. s, 1 H, N=CH-C=N), 7.00 (br. s,
2 H, 2ϫ CHC-hexyl), 2.68 (br. s, 4 H, 2ϫ ArCH2CH2), 1.86 (br.
s, 4 H, 2ϫ ArCH2CH2), 1.32–1.25 [m, 12 H, 2ϫ CH3(CH2)3], 0.88
(br. s, 6 H, 2ϫ CH2CH3) ppm. (C26H29N2S3)n (465.72)n: calcd. C
67.05, H 6.28, N 6.02, S 20.66; found: C 67.00, H 6.21, N 5.89, S
20.53.
Compound 16: 1H NMR (400 MHz, CDCl3): δ = 7.35 (s, 1 H,
CHS), 6.95 (s, 1 H, CH-C-hexyl), 6.84 (s, 1 H, hexyl-C-CH-S),
2.71–2.68 (t, J = 12 Hz, 2 H, ArCH2CH2), 2.54 (s, 6 H, 2ϫ N=C-
CH3), 1.67–1.57 (m, 2 H, ArCH2CH2), 1.33–1.18 [m, 6 H,
CH3(CH2)3], 0.82–0.79 (t, J = 12 Hz, 3 H, CH2CH3) ppm. 13C
NMR (100 MHz, CDCl3): δ = 153.20, 143.49, 137.88, 134.35,
127.07, 125.73, 123.73, 120.90, 115.05, 31.70, 31.58, 30.40, 29.02,
23.69, 22.64, 14.10 ppm. C18H22N2S2 (330.51): calcd. C 65.41, H
6.71, N 8.48, S 19.40; found: C 65.33, H 6.79, N 8.53, S 19.44.
Poly(2,3-dimethylthieno[3,4-b]pyrazine-5,7-diyl)-alt-(3-hexylthio-
phene-2,5-diyl) (P2): 1H NMR (400 MHz, CDCl3): δ = 6.98 (s, 1
H, CHC-hexyl), 2.66 (br. s, 8 H, ArCH2CH2, 2ϫ N=C-CH3), 1.73
(br. s, 2 H, ArCH2CH2), 1.30–1.25 [br. s, 6 H, CH3(CH2)3], 0.89–
0.88 (br. s, 3 H, CH3) ppm. (C18H22N2S2)n (330.51)n: calcd. C 65.41,
H 6.71, N 8.48, S 19.40; found: C 65.32, H 6.81, N 8.59, S 19.29.
General Procedure for the Microwave-Assisted Stille Cross-Coupling
Polymerization: Into a thoroughly dried screw-capped glass tube,
equimolar ratios of each of the desired dibromo and bis(tributyl-
stannyl) derivatives (0.5 mmol) were dissolved in anhydrous DMF
(10 mL), and the mixture was degassed with N2 for at least 30 min,
followed by the addition of [Pd(PPh)4] (5 mol-% relative to Br).
The capped glass tube was then placed into the microwave reactor
and irradiated under the following conditions: 5 min at 100 °C,
5 min at 120 °C, and 30 min at 150 °C. The end-capping process
was performed in two steps; end-capping with phenylboronic acid
pinacol ester followed by bromobenzene. The end-capping condi-
tions were as follow: 2 min at 100 °C, 2 min at 120 °C, and 5 min at
150 °C. After the final end-capping process, the microwave screw-
capped glass tube was then allowed to come to room temperature
and the reaction mixture was poured into methanol. The crude
polymer was collected by filtration and washed repeatedly with
methanol. The residual solid was loaded into an extraction thimble
and washed successively with methanol (24 h), followed by acetone
(24 h), and dried under vacuum prior to GPC and 1H NMR analy-
ses.
Poly[(thieno[3,4-b]pyrazine-5,7-diyl)-alt-(3,3Ј-dihexyl-2,2Ј-bithio-
phene-5,5Ј-diyl)] (P3): H NMR (400 MHz, CDCl3): δ = 8.50 (br.
1
s, 2 H, N=CH-CH=N), 7.58 (br. s, 2 H, 2ϫ CHC-hexyl), 2.64 (br.
s, 4 H, 2 ϫ ArCH2CH2), 1.67–1.66 (br. s, 4 H, 2ϫ ArCH2CH2),
1.29 [br. s, 12 H, 2 ϫ CH3(CH2)3], 0.88–0.87 (br. s, 6 H, 2 ϫ
CH2CH3) ppm. (C26H30N2S3)n (466.72)n: calcd. C 66.91, H 6.48, N
6.00, S 20.61; found: C 66.78, H 6.54, N 5.87, S 20.69.
Poly[(thieno[3,4-b]pyrazine-2,5,7-triyl)-2,5-bis(3,3Ј-dihexyl-2,2Ј-bi-
thiophene-5,5Ј-diyl)] (P3Ј): GPC analysis of the soluble parts of P2Ј
showed a maximum Mn value of 5470 with a PDI value of 4.26.
1H NMR (400 MHz, CDCl3): δ = 8.50 (br. s, 1 H, N=CH-C=N),
7.58 (br. s, 4 H, 4ϫ CHC-hexyl), 2.64–2.58 (br. s, 8 H, 4ϫ ArCH2),
1.65–1.60 (br. s, 8 H, 4ϫ ArCH2CH2), 1.29–1.26 [br. s, 24 H, 4ϫ
CH3 (CH2 )3 ], 0.87–0.84 (br. s, 12 H, 4 ϫ CH2 CH3) ppm.
(C46H57N2S5)n (798.28)n: calcd. C 69.21, H 7.20, N 3.51, S 20.08;
found: C 69.43, H 7.38, N 3.45, S 19.87.
Poly(2,3-dimethylthieno[3,4-b]pyrazine-5,7-diyl)-alt-(3,3Ј-dihexyl-
1
2,2Ј-bithiophene-5,5Ј-diyl) (P4): H NMR (400 MHz, CDCl3): δ =
7.52 (br. s, 2 H, 2ϫ CHC-hexyl), 2.65–2.63 (br. s, 10 H, 2ϫ
General Procedure for the Microwave-Assisted C–H Arylation Poly-
merization: Into a thoroughly dried screw-capped glass tube, equi-
molar ratios of each of the desired dibromo- and thieno[3,2-b]pyr-
azine derivatives (0.5 mmol from each of them) was dissolved in
anhydrous DMF (20 mL) and purged with nitrogen for 20 min. Po-
tassium acetate (0.29 g, 3.0 mmol), TBAB (0.32 g, 1.0 mmol), and
palladium acetate (0.02 g, 0.1 mmol) were then added under a
stream of nitrogen. The capped tube was then placed into the mi-
crowave reactor and irradiated under the following conditions:
5 min at 100 °C, 5 min at 120 °C, and 30 min at 150 °C. End-cap-
ping was performed in one step with phenylboronic acid pinacol
ester. The end-capping conditions were as follow: 2 min at 100 °C,
2 min at 120 °C, and 5 min at 150 °C. The microwave screw-capped
glass tube was then allowed to return to room temperature, and
the reaction mixture was poured into methanol. The crude polymer
was collected through filtration and washed successively with meth-
anol. The residual solid was loaded into an extraction thimble and
washed repeatedly with methanol (48 h), followed by acetone
ArCH2CH2, 2ϫ N=C-CH3), 1.67–1.59 (br. m,
4 H, 2ϫ
ArCH2CH2), 1.35–1.29 [br. m, 12 H, 2ϫ CH3(CH2)3], 0.91–0.85
(br. m, 6 H, 2ϫ CH3) ppm. (C28H34N2S3)n (494.78)n: calcd. C
67.97, H 6.93, N 5.66, S 19.44; found: C 68.13, H 6.85, N 5.77, S
19.36.
Supporting Information (see footnote on the first page of this arti-
cle): NMR spectra of compounds 4a,b, 5a,b, 7a,b, 8a,b, 11, 12, 14,
15, 16 and copolymers P1, P1Ј, P2, P3, P3Ј, and P4.
Acknowledgments
This work was supported by the Program for Integrated Molecular
Systems (PIMS) and the World Class University (WCU) program,
Korea (project number R31-20008-000-10026-0). A. A. E.-S. would
like to thank the Korean Brain Pool Program supported by the
Korean Federation of Science and Technology Societies (KOFST).
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
11

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Date: 21-08-12 17:15:18
Pages: 13
N. I. Abdo, A. A. El-Shehawy, A. A. El-Barbary, J.-S. Lee
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Received: June 8, 2012
Published Online: ᭿
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Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O20769
/KAP1
Date: 21-08-12 17:15:18
Pages: 13
Palladium-Catalyzed Direct C–H Arylation
C–H Arylation
N. I. Abdo, A. A. El-Shehawy,*
A. A. El-Barbary, J.-S. Lee* ........... 1–13
Palladium-Catalyzed Direct C–H Aryl-
ation of Thieno[3,4-b]pyrazines: Synthesis
of Advanced Oligomeric and Polymeric
Materials
Keywords: Synthetic methods / Cross-cou-
pling / Heck reaction / Arylation / Micro-
wave chemistry / Polymers /
Regioselective C–H arylation of thieno-
[3,4-b]pyrazine derivatives with heteroaryl
bromides under “Heck-type” conditions
gives donor–acceptor π-conjugated oligo-
meric and copolymeric materials in a facial
manner. The thienopyrazine unit altered
the photophysical and electrochemical
properties. Diffraction peaks indicate inter-
chain spacing similar to P3HT and suggest
π-π stacking of the backbones.
Heterocycles / Donor-acceptor systems
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
13