Studies on the generation of metalating species equivalent to the Knochel-Hauser base in the dehydrobrominative polymerization of thiophene derivatives

Article abstract of DOI:10.1021/om201209z

Dehydrobrominative polycondensation of 2-bromo-3-hexylthiophene proceeded with TMPMgBr?LiBr and (TMP)₂Mg?2LiBr, formed by LiTMP and MgBr₂, which was found to serve as a surrogate of the Knochel-Hauser base TMPMgCl?LiCl, and head-to-tail-type regioregular poly(3- hexylthiophene) was obtained with high efficiency. The regioregular poly(3-hexylthiophene) was also found to be obtained by one-shot addition of 2-bromo-3-hexylthiophene as a monomer, magnesium amide, and a nickel catalyst, suggesting that complete formation of metalated thiophene species is not an essential requisite for the successful polymerization with a nickel catalyst. This method was employed for the preparation of tolyl-terminated polythiophene by an aryl group with narrow molecular weight distribution using (o-tolyl)NiCldppp as a catalyst.

Full text of DOI:10.1021/om201209z

Article
pubs.acs.org/Organometallics
Studies on the Generation of Metalating Species Equivalent to the
Knochel−Hauser Base in the Dehydrobrominative Polymerization
of Thiophene Derivatives
Shunsuke Tamba, Shinobu Mitsuda, Fujimaru Tanaka, Atsushi Sugie, and Atsunori Mori*
Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
S
* Supporting Information
ABSTRACT: Dehydrobrominative polycondensation of 2-bromo-3-hexylthiophene proceeded with TMPMgBr·LiBr and
(TMP)2Mg·2LiBr, formed by LiTMP and MgBr2, which was found to serve as a surrogate of the Knochel−Hauser base
TMPMgCl·LiCl, and head-to-tail-type regioregular poly(3-hexylthiophene) was obtained with high efficiency. The regioregular
poly(3-hexylthiophene) was also found to be obtained by one-shot addition of 2-bromo-3-hexylthiophene as a monomer,
magnesium amide, and a nickel catalyst, suggesting that complete formation of metalated thiophene species is not an essential
requisite for the successful polymerization with a nickel catalyst. This method was employed for the preparation of tolyl-
terminated polythiophene by an aryl group with narrow molecular weight distribution using (o-tolyl)NiCldppp as a catalyst.
3 and related compounds under mild conditions using the
Knochel−Hauser base, chloromagnesium 2,2,6,6-tetramethylpi-
peridide lithium salt (TMPMgCl·LiCl) to generate the
metalated species A. as shown in Scheme 1. and polymerization
INTRODUCTION
■
Polythiophenes have attracted much attention in materials
science. In particular, polythiophenes that are controlled as
head-to-tail-type (HT) regioregularity form a highly planar
structure with extended π conjugation and show remarkable
physical properties as materials such as organic TFT,1 thin-film
organic solar cells,2 and conductive polymer materials.3 Head-
to-tail-type poly(3-hexylthiophene) (1; HT-P3HT) has been
generally synthesized by dehalogenative (−X2) polycondensa-
tion from 2,5-dibromo-3-hexylthiophene (2), which was
developed by Rieke,4 McCullough,5 and Yokozawa,6 with a
Grignard reagent in the presence of a nickel or palladium
catalyst.7 On the other hand, dehydrobrominative (−HBr)
polycondensation, which employs 2-bromo-3-hexylthiophene
(3) as a monomer precursor, is also a powerful method and
shows advantages in terms of atom efficiency. McCullough
reported generation of metalated thiophene monomer using
deprotonation with lithium diisopropylamide (LDA) following
metal exchange with ZnCl2, which was the first report to
synthesize HT-P3HT (1) with 2-bromo-3-hexylthiophene with
a nickel catalyst.8 Recently, Takita, Ozawa, and their co-workers
developed the palladium-catalyzed polymerization of 3 leading
to HT-P3HT by utilizing a palladium-catalyzed C−H arylation
reaction.9 However, these polymerization reactions required
low temperature for lithiation and the following metal exchange
to zinc or extremely high temperature was necessary for the
polymerization by palladium catalysis. We have also reported
that C−H functionalization polycondensation takes place with
Scheme 1
took place in the presence of a nickel(II) catalyst to afford highly
regioregular poly-3-substituted thiophenes with high efficiency.10
During the course of our studies on the development of efficient
Received: December 5, 2011
Published: March 12, 2012
© 2012 American Chemical Society
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Article
polymerization processes, our interest has been focused on the
development of a new class of metalating species that is
equivalent to Knochel−Hauser base11 (magnesium amide) at the
C−H bond of thiophene. We herein describe that several
metalating reagents and the metalating protocols are found to be
equally effective for polythiophene synthesis.12
performance of Mg(TMP)2,14 which was generated by the
reaction of 2 equiv of LiTMP and MgBr2·Et2O. The obtained
species also underwent polymerization to afford the polymer
1 in excellent yield and reasonable molecular weight with
relatively narrow molecular weight distribution (entry 2). It was
also found that 1 was obtained in 93% yield when 2 equiv of
monomer 3 was employed toward Mg(TMP)2, suggesting that
both amides on the magnesium atom were utilized for
the metalation of 3 (entry 3). The use of 0.5 equiv of the
magnesium ate complex [(TMP)3Mg]Li,15 which was
generated by the reaction of 3LiTMP and MgBr2·Et2O, toward
3 was found to give the polymer 1 in 72% yield (Mn = 13 500,
Mw/Mn = 1.72) (entry 4). On the other hand, polymerization
with equimolar amounts of [(TMP)3Mg]Li and 3 resulted in
no polymerization under similar conditions (entry 5). The
result can be explained by the fact that the organometallic
species thought to be formed as [Thienyl−Mg(TMP)2]Li,
which was confirmed by the reaction with iodine leading to the
corresponding iodothiophene in 94% yield, involves difficulties
in further transmetalation to nickel due to the steric effect of
two hindered TMP groups. Polymerization of 3 with 0.33 equiv
of [(TMP)3Mg]Li through [(Thienyl)3Mg]Li with NiCl2dppe
as a catalyst afforded P3HT (1) in only 19% yield; in contrast,
the palladium-catalyzed cross-coupling of [(Thienyl)3Mg]Li
with an aryl halide was reported to take place (entry 6).15c
The reaction with the metalating system of zinc in place of
magnesium using 1:1 LiTMP and ZnEt2, by which
[TMPZnEt2]Li would be formed, also resulted in no reaction
(entry 7). Although polymerization with an ate complex of zinc
has been unsuccessful,16 metalation of 2-bromo-3-hexylthio-
phene (3) was also found to take place. Treatment of the
metalated species [TMPZnEt2]Li with iodine at room temper-
ature for 30 min in THF afforded the corresponding iodide
2-bromo-3-hexyl-5-iodothiophene in 98% yield.
RESULTS AND DISCUSSION
■
We first examined the use of lithium 2,2,6,6-tetramethylpiper-
idide (LiTMP) and magnesium bromide, which would form
TMPMgBr·LiBr, as shown in Scheme 2.13 Table 1 summarizes
Scheme 2
Table 1. Polymerization of 2-Bromo-3-hexylthiophene
a
(3) with Various Metalating Reagents
Concerning the metalated species of 3-substituted thiophenes,
it is also intriguing to know if a one-shot procedure, in which the
addition of necessary substrate, reagents, and the catalyst is
carried out simultaneously, can be achieved. However, it is well-
known that complete consumption of the alkyl Grignard reagent
RMgX leading to the metalated thiophene is necessary prior to
polymerization with a nickel catalyst in the debrominative poly-
condensation of 2,5-dibromo-3-substituted thiophene 2. Other-
wise, the remaining RMgX serves as a terminating agent
of the propagating Ni−Br polymer end to afford the alkylated
polymer end via formation of the Ni−alkyl bond and thus
induces immediate reductive elimination5e,6b (Scheme 3).
b
metalating
3/M
yield
(%)
c
c
entry
system (M)
(mol/mol)
Mn
Mw/Mn
1
2
3
4
5
6
7
TMPLi + MgBr2
2TMPLi + MgBr2
2TMPLi + MgBr2
3TMPLi + MgBr2
3TMPLi + MgBr2
3TMPLi + MgBr2
TMPLi + ZnEt2
1.0
1.0
2.0
2.0
1.0
3.0
1.0
52
>99
93
72
0
14 000
19 000
12 100
13 500
1.49
1.69
1.64
1.72
19
0
4 600
1.34
a
The reaction was carried out with 3 and metalating reagent in THF
for the metalation, and 1.0 mol % of nickel catalyst was employed for
the polycondensation. Isolated yield. Mn and Mw/Mn values were
b
c
Scheme 3
estimated by SEC analysis using CHCl3 as an eluent.
the results. The reaction of TMPH with n-butyllithium at 0 °C
in anhydrous THF and then addition of magnesium bromide,
which was formed in situ by the reaction of Mg and 1,2-
dibromoethane in diethyl ether, was performed and the re-
action mixture thus obtained was subjected to the polymer-
ization of 2-bromo-3-hexylthiophene (3) in the presence of
1.0 mol % of NiCl2dppe. The polymerization reaction took
place similarly to the case of that with Knochel−Hauser base to
afford the corresponding polymer 1 in 52% yield (Mn = 14 000,
Mw/Mn = 1.49) (entry 1). The result suggests that magnesium
amide generated by lithium amide and a magnesium salt also
forms metalating species and is equally as effective as the
Knochel−Hauser base. We next examined the polymerization
Accordingly, synthesis of HT-P3HT by the established GRIM
(Grignard metathesis) method with 2 has been generally carried
out by 2 and RMgX to give A of Scheme 1 and the following
addition of a nickel catalyst. It has also been necessary to form
the similar intermediate A in the deprotonation of 2-bromo-3-
hexylthiophene (3) with a lithium amide such as LDA prior to
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metal exchange with a zinc salt.17 On the other hand, complete
deprotonation with magnesium amide prior to nickel-catalyzed
polymerization may not be required, since the magnesium
amide R2NMgX(·LiCl) is potentially inert to thienyl−Ni−X
species, which is the propagating end of the polymer.
When the polymerization by GRIM was carried out by the
addition of 2,5-dibromo-3-hexylthiophene (2) to a mixture of
an equimolar amount of ethyl magnesium chloride and 2.0 mol %
of NiCl2dppe, only 36% of P3HT (1) was obtained with a
much lower Mn of 3600 (Mw/Mn = 2.29), suggesting termina-
tion by the remaining Grignard reagent.18 The result is in sharp
contrast to the one-shot polymerization with the nickel catalyst,
TMPMgCl·LiCl, and 2-bromo-3-hexylthiophene (3), which
brought about excellent conversion at room temperature within
3 h, affording the polymer 1 with Mn = 12 700 (Mw/Mn = 1.86)
(Scheme 4).
also found that the addition of the related base prepared with
LiTMP and MgBr2 to a mixture of 3 and NiCl(o-tolyl)dppp
similarly afforded HT-P3HT (entry 4). Figure 1 shows the
1H NMR spectrum of the tolyl-terminated polymer using
TMPMgCl·LiCl as a base, in which the methyl group derived
from the terminal tolyl group was clearly observed. The average
degree of polymerization was estimated to be ca. 59 (Mn =
9900) by the calculation of the integration ratio of the methyl
(tolyl) and the methylene (thiophene side chain) protons.
CONCLUSION
■
In conclusion, we have successfully generated a thienyl metallic
species, which is a precursor of the corresponding regioregular
polythiophene, by the reaction of LiTMP and MgBr2.
The presumably formed metalating agent TMPMgBr·LiBr
exhibited reactivity similar to that of the Knochel−Hauser base
(TMPMgCl·LiCl) for the proton abstraction at the C−H bond
of thiophene derivatives, and the thienyl metal species thus
obtained underwent polymerization. Another metalating species,
(TMP)2Mg, was also found to be available for the metalation
and further polymerization. These metalating agents can be
generated by the common chemicals BuLi, secondary amine,
and MgBr2 (Mg and (CH2Br)2), suggesting a facile preparation
method for regioregular polythiophene. It is also remarkable
that magnesium amide does not serve as a poisoning agent for
the dehydrobrominative polymerization of thiophene, which is
in marked contrast with the fact that the remaining alkyl
Grignard reagent induces undesired termination of GRIM
polymerization (debrominative polythiophene synthesis); thus,
complete conversion toward thienyl magnesium species is not
necessary, allowing the polymerization procedure in a one-shot
manner.
Scheme 4
We also examined the one-shot polymerization reaction of
the monomer 3, which was carried out by the addition of a
magnesium amide species to the mixture of 3 and a nickel
catalyst. As summarized in Table 2, the polymerization of
2-bromo-3-hexylthiophene (3) was carried out with an equimolar
amount of Knochel−Hauser base at 25 °C for 3 h in the
presence of 2.0 mol % of NiCl2dppp. The corresponding HT-
P3HT was obtained in 90% yield with Mn = 10 500 (Mw/Mn =
1.84) and an HT selectivity of 98% (entry 1). The use of
NiCl2dppe as a catalyst improved Mw/Mn to 1.54. However, the
molecular weight of the polymer was slightly lower (Mn = 5400,
entry 2). The reaction with NiCl(o-tolyl)dppp, which was
generated by NiCl(o-tolyl)(PPh3)2 and DPPP in situ, resulted
to afford the tolyl-terminated P3HT in 70% yield with Mn = 9500
and the remarkably narrow Mw/Mn = 1.1419 (entry 3). It was
EXPERIMENTAL SECTION
■
General Methods. All the reactions were carried out under a
nitrogen atmosphere. 1H NMR (300 MHz) and 13C NMR (75 MHz)
spectra were measured on a Varian Gemini 300 instrument as CDCl3
solutions. The chemical shifts were expressed in ppm with CHCl3
(7.26 ppm for 1H) or CDCl3 (77.0 ppm for 13C) as internal standards.
SEC (size exclusion chromatography) analyses were performed with a
standard HPLC system equipped with a UV detector using CHCl3 as
eluent with a Tosoh TSKgel GMHHR-M. Molecular weights and
molecular weight distributions were estimated on the basis of the
calibration curve obtained by six polystyrene standards (Mn = 2630−
355 000). TMPMgCl·LiCl was prepared by following the literature
procedure20 and stored in the freezer as a 1.0 M THF solution.
a
Table 2. One-Shot Polymerization with Knochel−Hauser Base
b
c
c
d
entry
Ni cat.
base
yield (%)
Mn, Mw/Mn
HT (%)
1
2
3
4
NiCl2dppp
NiCl2dppe
TMPMgCl·LiCl
TMPMgCl·LiCl
TMPMgCl·LiCl
2LiTMP+ MgBr2
90
87
70
81
10 500, 1.84
5 400, 1.58
9 500, 1.14
15 000, 1.33
98
96
99
99
NiCl(o-tolyl)(PPh3)2 + DPPP
NiCl(o-tolyl)(PPh3)2 + DPPP
a
The reaction was carried out with 1.0 mmol of 3, 1.0 equiv of metalating reagent, and 2.0 mol % of nickel catalyst in 10 mL of THF at room
b
c
d
temperature for 3 h. Isolated yield. Mn and Mw/Mn values were estimated by SEC analysis using CHCl3 as an eluent. HT selectivity was estimated
by H NMR analysis.
1
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1
Figure 1. H NMR spectrum of tolyl-terminated P3HT using TMPMgCl·LiCl as a base (Table 2, entry 3), with protons of Thienyl−CH2−C5H11
(left) and CH3−C6H4− (right) being shown.
19a
NiCl2dppe21 and NiCl(o-tolyl)(PPh3)2
were prepared according
ASSOCIATED CONTENT
■
to the literature procedures. MgBr2·Et2O22 and [ZnEt2(TMP)]Li16b
were prepared according to the literature procedures. Other materials
were purchased and used without further purification.
Polymerization of 2-Bromo-3-hexylthiophene with LiTMP/
MgBr2 in the Presence of NiCl2dppe. BuLi in hexane (1.65 M,
S
* Supporting Information
Figures giving NMR spectra of head-to-tail-type poly
(3-hexylthiophene) and that of end-functionalized P3HT with
a tolyl group. This material is available free of charge via the
Internet at http://pubs.acs.org.
n
0.6 mL, 1.0 mmol) was added dropwise to a solution of TMPH
(200 μL, 1.2 mmol) in THF (1 mL) at −78 °C, and the reaction
mixture was stirred at 0 °C for 0.5 h. MgBr2·Et2O (0.5 mmol) was
added in several portions to the solution of the formed LiTMP at 0 °C
over 10 min. Then the reaction mixture was stirred at 0 °C for 2 h to
produce a solution of (TMP)2Mg in THF. Subsequently, 2-bromo-3-
hexylthiophene (3; 200 μL, 1.0 mmol) was added dropwise to
the solution of the formed (TMP)2Mg at room temperature. After the
mixture was stirred at 25 °C for 0.5 h, THF (8 mL) and NiCl2dppe
(5.2 mg, 0.01 mmol) were successively added. The resulting mixture
was stirred at 25 °C for 3 h. Hydrochloric acid (1.0 M, 20 mL) and
methanol (50 mL) were added to form a precipitate. The mixture was
filtered, and the residue was washed with methanol repeatedly to leave
a dark purple solid, which was dried under reduced pressure to afford
156 mg of poly(3-hexylthiophene) (1; 93%, Mn = 12 100, Mw/Mn = 1.64).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: amori@kobe-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by KAKENHI (B) by the
Japan Society for the Promotion of Science (JSPS). We thank
Shuji Okamoto and Hikaru Meguro of Soken Chemical &
Engineering Co., Ltd. for their kind donation of thiophene
monomers.
1
The regioregularity was estimated by H NMR analysis (thienyl-CH2
signals) at the δ 2.80 (H-T) and δ 2.60 (H−H) signals (H-T
regioregularity 98%). 1H NMR (300 MHz): δ 0.92 (t, J = 6.6 Hz, 3H),
1.20−1.52 (m, 6H), 1.60−1.80 (m, 2H), 2.81 (t, J = 7.6 Hz, 2H), 6.98
(s, 1H). 13C NMR (75 MHz): δ 14.1, 22.7, 29.3, 29.5, 30.5, 31.7,
128.6, 130.5, 133.7, 139.9.
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