Nickel(II)-catalyzed cross-coupling polycondensation of thiophenes via C-S bond cleavage

Article abstract of DOI:10.1021/om4010737

Cross-coupling polycondensation of thiophene derivatives occurs via C-S bond cleavage in the presence of a nickel catalyst. Head to tail type (HT) regioregular poly(3-hexylthiophene) is obtained by a nickel(II)-catalyzed deprotonative C-H functionalization polycondensation of 2-(phenylsulfonyl)-3- hexylthiophene with stoichiometric TMPMgCl·LiCl or with the catalytic secondary amine/RMgX. Debrominative Grignard metathesis (GRIM) polymerization with 5-bromo-2-(phenylsulfonyl)-3-hexylthiophene also proceeds by the catalysis of the nickel(II) complex to afford the corresponding polythiophene.

Full text of DOI:10.1021/om4010737

Communication
pubs.acs.org/Organometallics
Nickel(II)-Catalyzed Cross-Coupling Polycondensation of Thiophenes
via C−S Bond Cleavage
Shunsuke Tamba, Kanta Fuji, Karin Nakamura, and Atsunori Mori*
Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
S
* Supporting Information
ABSTRACT: Cross-coupling polycondensation of thiophene
derivatives occurs via C−S bond cleavage in the presence of a
nickel catalyst. Head to tail type (HT) regioregular poly(3-
hexylthiophene) is obtained by a nickel(II)-catalyzed depro-
tonative C−H functionalization polycondensation of 2-
(phenylsulfonyl)-3-hexylthiophene with stoichiometric
TMPMgCl·LiCl or with the catalytic secondary amine/
RMgX. Debrominative Grignard metathesis (GRIM) polymer-
ization with 5-bromo-2-(phenylsulfonyl)-3-hexylthiophene
also proceeds by the catalysis of the nickel(II) complex to
afford the corresponding polythiophene.
ransition-metal-catalyzed cross coupling has attracted
considerable attention in organic synthesis, and a wide
cleavage with a transition-metal catalyst is shown to take place
in the polymerization of thiophene derivatives.
T
range of organometallic reagents and organic electrophiles have
been employed.1 Among those, the coupling reaction with
organic halides as an electrophile has been studied so far,
whereas there has been fewer remarks on the use of an organic
compound bearing a C−S bond as an electrophile for cross-
coupling.2 Recent advances in cross-coupling chemistry enabled
the reaction of organic sulfinates through the oxidative cleavage
of the C−S bond (eq 1)3 and the thus formed organometallic
As a monomer precursor we have first chosen 2-(phenyl-
sulfonyl)-3-hexylthiophene (1), and 1 was subjected to
deprotonation with Knochel−Hauser base8 at room temper-
ature for 30 min. Addition of 1 mol % of NiCl2(dppe) as a
catalyst to the reaction mixture and further stirring at 50 °C for
24 h induced polymerization to afford polythiophene 2 with Mn
= 9300 (Mw/Mn = 1.80), although the reaction was slower than
that of the related halothiophene that proceeded to completion
within a few hours at room temperature.6a,10 The head-to-tail
(HT) regioregularity of the obtained polymer was confirmed by
1H NMR analysis, showing 99% of the HT selectivity. Worthy
of note is that carbon−carbon bond formation by transition-metal
catalysis occurred via C−S bond cleavage, which is a new class of
cross-coupling polycondensation. On the other hand, polymer-
ization with 3-hexylthiophen-2-yl phenyl sulfide (3) as a
monomer was examined under similar conditions to afford
P3HT (2) with much lower yield and molecular weight (Mn =
1610). A sulfoxide, 2-phenylsulfinyl-3-hexylthiophene (4), also
reacted in a similar manner to give the corresponding polymer
with Mn = 3840 (Mw/Mn = 1.56) in 74% yield. The results
show that phenyl sulfone serves as the most effective leaving
group in the polymerization of thiophene (Scheme 1)
intermediate allowed the C−C bond formation. In contrast,
there have been fewer studies on the direct oxidative cleavage of
C−S bond of sulfoxides and sulfones by a transition-metal
catalyst (eq 2)4 due to the difficulties in the oxidative addition
of low-valent metallic species to the C−S bond.
On the other hand, remarkable progress has recently been
shown in cross-coupling polymerization of thiophene deriva-
tives leading to conjugated polythiophenes, in which highly
reactive nickel catalysts play a key role in successful
polymerization.5−7 Accordingly, we envisaged that such a
coupling reaction may achieve C−S bond cleavage, as shown in
eq 3. Herein, we describe that cross-coupling via C−S bond
Polymerization with sulfonylthiophene 1 was examined
under several conditions, as shown in Table 1. The reaction
in the presence of 3 mol % of NiCl2(dppe) proceeded in THF,
1,4-dioxane, toluene, and cyclopentyl methyl ether (CPME) to
afford the corresponding polymers in reasonable yields.
Polythiophene 2 of a higher molecular weight was obtained
by decreasing the catalyst loading of the nickel complex;
Received: November 5, 2013
Published: December 30, 2013
© 2013 American Chemical Society
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Communication
Scheme 1. Deprotonative Polymerization via C−S Bond
Cleavage in the Presence of Ni Catalyst
Table 2. Generation of Thienyl Magnesium Species with a
Grignard Reagent and a Catalytic Amount of Amine
a
b
amine
time
(h)
yield
(%)
X
(amt (mol %))
RMgCl
EtMgCl
temp (°C)
−SO2Ph
Et2NH (10)
Et2NH (10)
Et2NH (10)
Et2NH (10)
Et2NH (1)
room
temp
1
1
1
1
1
1
1
65
−H
EtMgCl
EtMgCl
room
temp
1
Table 1. Nickel-Catalyzed Polymerization of
Sulfonylthiophene 1 with Knochel−Hauser Base
a
−Cl
room
temp
9
NiCl2(dppe) amt
(mol %)
temp (°C),
yield
(%)
M /
wb
−SO2Ph
−SO2Ph
−SO2Ph
−SO2Ph
iPrMgCl·
room
temp
86
50
95
13
b
solvent
time (h)
Mn
Mn
LiCl
iPrMgCl·
60
60
60
1
THF
50, 24
50, 24
60, 24
60, 24
60, 24
50, 24
50, 8
43
74
53
86
50
79
9300
3200
6800
6300
6500
7400
15900
6900
2530
1.80
1.30
1.94
2.26
1.81
1.71
1.68
1.47
1.33
LiCl
3
THF
c
DMP (1)
iPrMgCl·
3
dioxane
toluene
CPME
THF
LiCl
3
none
iPrMgCl·
3
LiCl
2
a
The reaction was carried out with 2-substituted 3-hexylthiophene
c
1.5
1.5
1.5
THF
(79)
(0.2 mmol), Grignard reagent (0.2 mmol), and amine (0.02 or 0.002
mmol) in 0.4 mL of THF at room temperature. The conversion was
estimated by H NMR analysis after quenching the reaction mixture
with iodine. DMP = cis-2,6-dimethylpiperidine.
c
THF
50, 4
(22)
b
c
THF
50, 2
(8)
1
a
c
The reaction was carried out with 1 and 1.0 equiv of TMPMgCl·LiCl
in THF for the metalation, and 1.0−3.0 mol % of nickel catalyst was
employed for the polycondensation. Mn and Mw/Mn values were
estimated by SEC analysis using CHCl3 as an eluent. Conversion of 1
was estimated by H NMR analysis.
b
iodinated thiophene in 95% yield, whereas the reaction in the
absence of amine under similar conditions resulted in poor
deprotonation (13% yield).
c
1
With the method for catalytic generation of metallic species
i
with 10 mol % of Et2NH and PrMgCl·LiCl at room
however, a nonlinear increase of the average molecular weight
was observed over time (see Figure S2), indicating that the
curve was slightly out of the theoretical line and suggesting that
polymerization took place partially in a step-growth manner.
Although the observed enhancement in the reactivity in cross-
coupling via C−S bond cleavage is explained by the
intramolecular catalyst transfer of nickel species,5 a higher
reaction temperature may have resulted in the partial step-
growth polymerization.
temperature for 1 h, polymerization in the presence of 1.5
mol % of NiCl2(dppe) was indeed carried out at 50 °C for 24 h.
P3HT (2) was obtained in 50% isolated yield with Mn = 11900
(Mw/Mn = 1.27), as shown in Scheme 2.
Scheme 2. Polymerization of 1 with a Nickel Catalyst after
Deprotonation with Catalytic Amine and Grignard Reagent
It was also found that deprotonation of 1 was achieved with
the combination of a Grignard reagent and a catalytic amount
of Et2NH, which resulted in catalytic in situ generation of
magnesium amide. Such deprotonation has been achieved in
unsubstituted 3-hexylthiophene and 2-chloro-3-hexylthiophene,
whereas that of bromothiophene has been unsuccessful, due to
undesired bromine−magnesium exchange with a Grignard
reagent.6b,9 Deprotonation reactions of 1 and several related
derivatives were studied as summarized in Table 2. The result
was confirmed by quenching the generated anion with iodine,
leading to 5-iodo-3-hexyl-2-(phenysulfonyl)thiophene. The use
of EtMgCl and 10 mol % of Et2NH was found to result in a
reasonable metalation efficiency to afford the iodide in 65%
yield. In contrast, deprotonation of 3-hexylthiophene or 2-
chloro-3-hexylthiophene under similar conditions hardly took
place, although these reactions have been achieved under more
harsh conditions.10 Better deprotonation efficiency was
Concerning the polymerization mechanism of halothio-
phenes, it has been considered that the initiation reaction in
the polymerization is reductive tail to tail homocoupling of
metalated sulfonylathiophene and oxidative addition of Ni(0)
species into the C(thiophene)−S bond.11 As shown in Scheme
3, a propagation reaction thus occurs, after initial homocoupling
with the metalated monomer A to give B, by the incorporation
of monomer B at the terminal C−S bond. Accordingly, an end
group of C is the terminal thiophene bearing an SO2Ph group.
ESI-MS analysis of the reaction mixture revealed the formation
i
−
achieved with PrMgCl·LiCl and 10 mol % Et2NH to afford
of Ph-SO2 ([Mfound] = 141.0012), suggesting that the
the iodide in 86% yield. It should be pointed out that
deprotonation with a decreased amount of Et2NH to only 1
mol % also resulted in giving metalated thiophene in 50% yield
in 1 h. Switching the secondary amine to cis-2,6-dimethylpiper-
idine (DMP) improved the yield dramatically. Metalation in the
presence of 1 mol % of DMP smoothly proceeded to give
phenysulfonyl group served as a leaving group, forming Ph-
SO2MgCl(LiCl) species.
An end group of the polymer 2 was also found by
measurement of the 1H NMR spectrum to be SO2Ph, as
shown in Figure 1a.12 A proton signal corresponding to the
terminal thiophene ring Hb was observed at 6.95 ppm by the
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Organometallics
Communication
with 1.5 mol % of NiCl2(dppe) afforded the corresponding
polymer 2 in 54% isolated yield15 (Mn = 4600; Mw/Mn = 1.42).
In conclusion, we have shown nickel-catalyzed polymer-
ization with sulfonylthiophene, which occurs via unprecedented
C−S bond cleavage, when NiCl2(dppe) was employed as a
catalyst. Generation of the polymerizable organometallic
species was performed in a deprotonative manner with 1 by
the use of stoichiometrically or catalytically generated
magnesium amide. The deprotonation reaction of thiophene
was revealed to occur under conditions milder than those for
the related halothiophenes due to its improved acidity by the
effect of an electron-withdrawing sulfonyl group. The GRIM-
type halogen−metal exchange was also found to furnish a
similar metalated monomer species, and the following nickel-
(II)-catalyzed polymerization led to the polymer 2.
Scheme 3. Proposed Mechanism of the Cross-Coupling
Polymerization via C−S Bond Cleavage
ASSOCIATED CONTENT
* Supporting Information
■
S
Text and figures giving experimental details and spectroscopic
characterization data of compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for A.M.: amori@kobe-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by Kakenhi B (No.
22350042 and 25288049) from the Japan Society for the
Promotion of Science (JSPS) and Special Coordination Funds
for Promoting Science and Technology, Creation of Innovation
Centers for Advanced Interdisciplinary Research Areas
(Innovative Bioproduction Kobe), from MEXT, Japan. S.T.
thanks the JSPS for a Research Fellowship for Young Scientists.
Figure 1. 1H NMR spectra of 2 (a) and desulfonylated polymer 5 (b).
electron-withdrawing effect of the PhSO2 group. The phenyl-
sulfonyl group was efficiently removed by nickel-catalyzed
t
reductive desulfonylation with BuMgCl to give polythiophene
5. Desulfonylation with 2 was carried out by excess amounts of
tBuMgCl and NiCl2(dppe) as a catalyst at 60 °C for 16 h to
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