Revisiting the Polymerization of Diphenylacetylenes with Tungsten(VI) Chloride and Tetraphenyltin: An Alternative Mechanism by a Metathesis Catalytic System

Article abstract of DOI:10.1002/anie.202005964

An alternative reaction mechanism of the polymerization of diphenylacetylelnes using a catalytic system composed of tungsten(VI) chloride and tetraphenyltin has been proposed through the optimization of reaction conditions and investigation of the effect of the electronic nature of diphenylacetylene monomers on the polymerizability. The detailed structures of the polymers have been suggested by mass spectrometric analysis of the obtained polymers and oligomers, which suggested that a phenyl group of tetraphenyltin has been introduced to an initiating end of the polymer chain. Mass spectrometric analysis also provided information about the termination processes of the polymerization. The experimental results strongly suggested that the polymerization of diphenylacetylenes using tungsten(VI) chloride and tetraphenyltin proceeds through a migratory insertion mechanism rather than the long-accepted metathesis mechanism.

Full text of DOI:10.1002/anie.202005964

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Accepted Article
Title: Revisiting the Polymerization of Diphenylacetylenes with
Tungsten(VI) Chloride and Tetraphenyltin: An Alternative
Mechanism by a Metathesis Catalytic System
Authors: Mami Miyairi, Tsuyoshi Taniguchi, Tatsuya Nishimura, and
Katsuhiro Maeda
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
Revisiting the Polymerization of Diphenylacetylenes with
Tungsten(VI) Chloride and Tetraphenyltin: An Alternative
Mechanism by a Metathesis Catalytic System
[
a]
[a]
[a]
[a],[b]
Mami Miyairi, Tsuyoshi Taniguchi,* Tatsuya Nishimura, and Katsuhiro Maeda*
[
a]
M. Miyairi, Dr. T. Taniguchi, Prof. Dr. Tatsuya Nishimura, Prof. Dr. Katsuhiro Maeda
Graduate School of Natural Science and Technology
Kanazawa University
Kakuma-machi, Kanazawa 920-1192 (Japan)
E-mail: tsuyoshi@p.kanazawa-u.ac.jp
E-mail: maeda@se.kanazawa-u.ac.jp
[
b]
Prof. Dr. Katsuhiro Maeda
Nano Life Science Institute (WPI-NanoLSI)
Kanazawa University
Kakuma-machi, Kanazawa 920-1192 (Japan)
Supporting information for this article is given via a link at the end of the document.
Abstract: An alternative reaction mechanism of the polymerization of
diphenylacetylelnes using catalytic system composed of
photoluminescent properties even in a solid state, high gas
permeability, and liquid Moreover,
crystallinity.[4]
a
tungsten(VI) chloride and tetraphenyltin has been proposed through
the optimization of reaction conditions and investigation of the effect
of the electronic nature of diphenylacetylene monomers on the
polymerizability. The detailed structures of the polymers have been
suggested by mass spectrometric analysis of the obtained polymers
and oligomers, which suggested that a phenyl group of tetraphenyltin
has been introduced to an initiating end of the polymer chain. Mass
spectrometric analysis also provided information about the
termination processes of the polymerization. The experimental
poly(diphenylacetylene)s carrying optically active pendants have
been reported to form a preferred-handed helical conformation
and show a good circularly polarized luminescence (CPL)
property and chiral recognition ability as a chiral stationary phase
for HPLC or an enantioselective menbrane owing to the helical
chirality.[5]
A catalytic system composed of WCl
the polymerization of diphenylacetylene.
6
-Ph
4
Sn is often used for
As an analog, the
[
3]
n
TaCl
5
- Bu
4
Sn system works as a catalyst to induce polymerization
[
3]
5 4
However, the TaCl Sn system is
-nBu
results
strongly
suggested
that
the
polymerization
of
of diphenylacetylenes.
diphenylacetylenes using tungsten(VI) chloride and tetraphenyltin
proceeds through a migratory insertion mechanism rather than the
long-accepted metathesis mechanism.
often inefficient in polymerization of diphenylacetylenes having
electron-withdrawing polar functional groups such as
esters.[
5d,g,6,7]
6 4
Even in such a case, the WCl -Ph Sn system
induces the polymerization but often suffers from low molecular
weights and low yields of obtained polymers.
A sensible metathesis mechanism has been proposed in the
polymerization of diphenylacetylenes using the WCl -Ph Sn
6 4
Introduction
system (Figure 1b) because this system and its related systems
are pioneering catalysts of olefin metathesis reactions as
aforementioned.[1] However, it is unclear how metal carbenoids
6
A catalytic system composed of tungsten(VI) hexachloride (WCl )
and alkylating reagent, such as alkyl aluminum, alkyl lithium, alkyl
magnesium or alkyl tin reagent, is one of the pioneering catalytic
systems for olefin metathesis reactions.[1] Tungsten carbenoid
complexes have been identified as active species inducing
metathesis reactions, and they can be formed by α-hydrogen
atom elimination of alkylated tungsten(VI) species generated from
the catalytic system.[1d] In current organic synthesis, such an “ill-
defined” catalytic system has been replaced by more useful “well-
defined” metal carbenoid complexes such as Schrock and Grubbs
catalysts as metathesis catalysts.[2] However, an ill-defined
catalytic system with tungsten(VI) chloride is still used for the
that induce the polymerization of diphenylacetylenes are formed
Sn system (Figure 1a). If nBu
from the WCl
6
-Ph
Sn as
4
4
Sn is used
instead of Ph
4
a
co-catalyst, tungsten carbenoid
nPrCH=WCl
n
4
might be formed by α-proton elimination of BuWCl
5
n
formed by transmetallation between WCl
6
and Bu
4
Sn as
aforementioned.[1d,2a] However, this is not the case in the WCl
Ph Sn system because there is no α-proton in PhWCl formed by
transmetallation between WCl and Ph Sn. Tungsten vinylidene
6
-
4
5
6
4
complexes might be formed by 1,2-migration of a carbon
substituent of a tungsten–alkyne complex formed in the presence
of an acetylene monomer. The formation of vinylidene complexes
is likely in the tungsten-catalyzed polymerization of terminal
alkynes such as phenylacetylene because 1,2-hydrogen atom
migration of tungsten–alkyne complexes could easily occur.[8,9]
On the other hand, there are only limited examples of metal
vinylidene complexes from diphenylacetylenes.[10] Otherwise,
4
Ph Sn might work as a reducing reagent to generate an active
polymerization
of
disubstituted
acetylenes
such
as
diphenylacetylenes because well-defined carbenoid complexes
are often inefficient catalysts in such a polymerization due to a
steric factor.[3]
Poly(diphenylacetylene)s
are
potential
π-conjugated
polymers as functional materials in practical use because they
have excellent thermal stability, mechanical properties and
photochemical
resistance
and
they
exhibit
good
1
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
metathesis catalyst which might be a tungsten(IV) species formed
improvement
of
tungsten-catalyzed
polymerization
of
by double transmetallation between WCl
6
and Ph
4
Sn followed by
diphenylacetylenes and structural elucidation of the obtained
poly(diphenylacetylene)s, and we propose an alternative to the
metathesis polymerization mechanism based on experimental
results.
reductive elimination.[
1c,f,h,8b]
In any case, there is very poor
evidence of the formation of tungsten carbenoid species or any
species that induce a metathesis reaction in the polymerization of
6 4
diphenylacetylenes with WCl -Ph Sn.
Results and Discussion
Investigation of Reaction Conditions and Monomers in the
Polymerization
First, we reinvestigated the polymerization of diphenylacetylene
1
6 4
a using 5 mol% of WCl -Ph Sn (1:1). The polymerization was
conducted by heating a mixture of monomer 1a and catalysts in
toluene at 110 °C. The corresponding polymer poly-1a with a low
molecular weight was obtained in a low yield (entry 1, Table 1),
whereas we previously obtained poly-1a with a higher molecular
4
weight (M
conditions.[
n
= 1.3–1.5 × 10 ) in a good yield (~80%) under similar
5g,11]
These results indicate that the reproducibility of
the polymerization of diphenylacetylenes with WCl
6
-Ph
4
Sn is low
or moderate.[
13]
Replacement of the solvent with chlorobenzene
did not affect the results (entry 2). Polar solvents such as 1,4-
dioxane could not be used for the polymerization due to
deactivation of the catalyst. However, we found that the
replacement of WCl
6 4
with tungsten(VI) oxychloride (WOCl )
significantly improved the results in terms of yield and molecular
weight of poly-1a (entries 3 and 4) and the reproducibility of the
4 6
instead of WCl
was very good.[13]
polymerization using WOCl
The replacement of Ph Sn with more electron-rich 4-methylphenyl
and 4-methoxyphenyl derivatives gave good results (entries 5 and
), whereas the polymerization hardly proceeded when an
electron-deficient 4-trifluoromethylphenyl derivative was used
instead of Ph Sn (entry 7). Clearly, these results suggest that the
4
6
Figure 1. Unclear formation of an active metathesis catalyst from WCl
6 4
-Ph Sn
4
(
a), proposed metathesis mechanism of the polymerization of
a
electronic nature of the co-catalyst affects the polymerization.
diphenylacetylenes (b) and the polymerization of 1a with WCl
possible cis-transoid structure of poly-1a (c).
6 4
-Ph Sn and the
n
4
Bu Sn did not work as a good co-catalyst (entry 8), and this result
is similar to that of previous reports for the polymerization of
diphenylacetylenes.[ The use of twice the amount of Ph
6]
4
Sn
Very recently, we have reported the synthesis of one-handed
helical poly(diphenylacetylene)s bearing carboxy groups based
on macromolecular helicity induction through noncovalent
interaction with chiral amines and its subsequent memory of the
helicity, as well as their detailed helical structures including the
main-chain configuration (cis or trans) and conformation (cisoid or
transoid) and also the absolute helical handedness (right- or left-
handed helix) and its helix-sense excess.[11] In that study, a
symmetrically substituted poly(diphenylacetylene) bearing
carboxy groups was synthesized by polymerization of the
diphenylacetylene monomer 1a having ester groups at the para-
4 4
[WOCl -Ph Sn (1:2)] significantly inhibited the polymerization,
indicating that the present polymerization is sensitive to the
amount of a co-catalyst (entry 9). In previous works, a mixture of
WCl
polymerization,
polymerization of 1a using WCl
6 4
and Ph Sn was often aged in a solvent prior to the
[5,6,8b]
but this “aging” procedure did not work in the
and WOCl (entries 10 and 11).
6
4
Polymerization at a lower temperature (80 °C) resulted in lower
yield and lower molecular weight of the polymer (entry 12).
4 4
Next, we applied the WOCl -Ph Sn system to other
representative diphenylacetylene monomers (Table 2). First, we
tested the polymerization of asymmetrically substituted
diphenylacetylenes analogues 1b and 1c having a bromo atom
and a trifluoromethyl group instead of an ester group, respectively.
Both polymerizations were readily completed under the standard
conditions (the same conditions as those shown in entry 3 in
Table 1) and gave the corresponding polymers poly-1b and poly-
1c in good yields (entries 2 and 3). On the other hand, the
polymerization of asymmetrically substituted diphenylacetylene
analogue 1d omitting an ester group from 1a was not completed
even after 24 h under the standard conditions, and poly-1d
having a low molecular weight was obtained in a low yield (entry
positions using WCl
6
-Ph
4
Sn followed by hydrolysis of the ester
groups in the obtained poly-1a (Figure 1c).[
5g,11]
Various
spectroscopic analyses of the obtained polymers combined with
theoretical calculations suggested that the obtained
poly(diphenylacetylene)s have a highly cis-stereoregular (cis-
transoidal) structure. However, it is difficult to clearly rationalize
such a high cis-stereoregularity from the viewpoint of the
polymerizatin reaction of diphenylacetylenes through
methathesis mechanism.
a
Therefore, we decided to reinvestigate the polymerization of
diphenylacetylenes using tungsten(VI) chloride and Ph Sn to
4
4
4). However, we found that the use of twice the amount of Ph Sn
elucidate the polymerization mechanism through the optimization
of polymerization conditions. In this paper, we describe the
(10 mol%) improved the result unlike the polymerization of 1a
(entry 5). The polymerization of diphenylacetylene 1e having a p-
2
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RESEARCH ARTICLE
alkoxy group was sluggish under the conditions using 5 and 10
Ph
4
Sn, and poly-1f was obtained in a good yield (entry 9). These
mol% of Ph
amount of Ph
4
Sn (entries 6 and 7), but the use of an increased
Sn (20 mol%) afforded the corresponding polymer
results suggest that the polymerization is greatly affected by the
electronic nature of diphenylacetylene monomers and the amount
4
poly-1e in a good yield (entry 8). Similarly, 1f having a p-alkoxy
4
of Ph Sn used.
group readily polymerized under the condition using 20 mol% of
Table 1. Polymerization Results of Diphenylacetylene 1a.[a]
Entry
[W]
R in R
4
Sn
solvent
poly-1a
Yield [%][b]
M
n
[c]
[c]
w n
M /M
1
2
3
4
5
6
7
8
9
1
1
1
WCl
WCl
6
6
Ph
Ph
Ph
Ph
4-Me-Ph
4-MeO-Ph
4-CF
nBu
Ph
Ph
Ph
toluene
PhCl
toluene
PhCl
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
11
17
91
89
86
89
<5
<5
16
18
<5
48
8,000
7,700
1.35
1.37
1.71
1.60
1.67
1.48
1.14
1.36
1.33
1.40
1.17
1.95
WOCl
WOCl
WOCl
WOCl
WOCl
WOCl
WOCl
WCl
WOCl
WOCl
4
4
4
4
4
4
4
21,900
19,500
21,200
27,600
6,100
9,400
7,400
7,100
6,300
3
-Ph
[
d]
[
e]
e]
f]
0
1
2
6
[
[
4
4
Ph
15,800
[
a] Conditions: 1a (2 mmol), [W] (0.1 mmol), R
insoluble fraction. [c] Determined by SEC based on polystyrene standards (THF, 40 °C). [d] 10 mol% of Ph
Ph Sn in toluene was stirred for 10 min at room temperature before addition of monomer 1a. [f] At 80 °C.
4
Sn (0.1 mmol) in solvent (2 mL) for 5‒24 h at 110 °C in a sealed tube. [b] Yield obtained from MeOH–THF (3:1)-
4
Sn was used. [e] A mixture of a tungsten catalyst and
4
Table 2. Polymerization Results of Various Diphenylacetylenes 1a‒f.[a]
Entry
R1
R2
1
poly-1a‒f
Yield [%][b]
M
n
[c]
[c]
w n
M /M
1[d]
2
3
CO
CO
CO
CO
CO
n
n
n
C
C
C
H
H
H
H
H
CO
Br
nC
H
2 7 15
1a
1b
1c
1d
1d
1e
1e
1e
1f
91
70
74
26
56
13
38
63
72
21,900
29,300
26,800
4,700
14,000
7,900
8,500
10,800
12,000
1.71
4.26
1.78
1.33
1.59
1.68
2.55
2.20
2.53
2
2
2
2
2
7
7
7
7
7
15
15
15
15
15
CF
H
3
4
nC
nC
OMe
OMe
[
e]
5
6
7
8
9
H
n
C
C
C
5
H
H
H
11
11
11
[
[
[
e]
n
n
5
5
f]
OMe
f]
n
O C
8
H
17
H
[
a] 1a‒f (2 mmol), WOCl
fraction. [c] Determined by SEC based on polystyrene standards (THF, 40 °C). [d] The same data shown in Entry 3 in Table 1. [e] 10 mol% of Ph
0 mol% of Ph Sn was used.
4
(0.1 mmol), Ph
4
Sn (0.1 mmol) in toluene (2 mL) for 6‒24 h at 110 °C in a sealed tube. [b] Yield obtained from MeOH–THF (3:1)-insoluble
4
Sn was used. [f]
2
4
Characterization of Poly(diphenylacetylene)s
To get information on the structures of the obtained
poly(diphenylacetylene)s, we performed MALDI-TOF-MS
the molecular ion at m/z 3334.9544 corresponds to
+
+
C
216
H
270
O
28Na [M + Na] (calcd, 3334.9596) and would be
composed of 7-mer of 1a (C210 28) and an additional C
unit. This suggests that a terminal structure of the polymer has a
phenyl group (C ), but a hydrogen atom (H) is somehow lost
H
266
O
6 4
H
analysis (DIT+NaCl) of poly-1 having a low molecular weight (M
2,880, M /M = 1.14) obtained by polymerization with the
WOCl -Ph Sn system. A series of peaks having a 462 u
n
=
w
n
6 5
H
4
4
from the polymer structure. The loss of a hydrogen atom
suggests an oxidation process is somewhere involved in the
reaction, but two hydrogen atoms are lost in a typical oxidation
difference corresponding to a monomer unit was observed up to
m/z 4,000 in the HRMS spectrum (Figure 2a). Representatively,
3
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RESEARCH ARTICLE
Figure 2. MALDI-TOF-MS spectra of poly-1a (a, b, d) and poly-1g (c) and their proposed structures 2‒4 (e). (a) poly-1a (WOCl
4 4
-Ph Sn) corresponding to 2a, 2a
‒
2H and 2a ‒ 4H (b) poly-1a (WCl
-Ph Sn) corresponding to 2a ‒ 2H and 2a ‒ 4H, (c) poly-1g (WOCl -Ph Sn) corresponding to 2b ‒ 4H, (d) poly-1a (WOCl -(4-
6 4 4 4 4
MeO-Ph) Sn) corresponding to 2c ‒ 2H (△) and 4c (◯).
4
4
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Angewandte Chemie International Edition
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RESEARCH ARTICLE
process. This suggests that the primary structure of the polymer
is 2a having a hydrogen atom at another terminal end, but an
oxidative coupling between two aromatic rings of the polymer
would compete to give the structure including a phenanthrene ring
structure or a structure capped by a phenyl group. As another
supportive fact, small signals tentatively assigned as methyl and
methoxy groups were observed in H NMR spectra of poly-1a
1
synthesized with (4-Me-Ph)
Ph) Sn (Entry 6 in Table 1), respectively (see the Supporting
Information).
To confirm that a phenyl group is introduced to the polymer
initiating end from Ph Sn, we tested the reaction of
hydrogen atoms were lost from the structure 2a. Representatively, diphenylacetylene (1g) with stoichiometric amounts of WOCl and
the molecular ion at m/z 3332.9373 corresponds to (4-Me-Ph) Sn (Scheme 1). The starting diphenylacetylene was
4
Sn (Entry 5 in Table 1) and (4-MeO-
3
a. We also performed MALDI-TOF-MS analysis (DCTB + NaCl)
of poly-1a (M = 2,500, M /M = 1.19) synthesized with the WCl
Ph Sn system and observed a similar series of peaks based on
the polymer. However, each molecular ion indicated that four
4
n
w
n
6
-
4
4
4
4
+
+
C
216
H
268
O
28Na [M + Na] (calcd, 3332.9439) and would be
fully consumed by heating the mixture in toluene at 100 °C for 30
min, and 1,2-diphenyl-1,2-di-p-tolylethene (5) as a mixture of
geometrical isomers was isolated along with mixtures of dimers 6,
though the reaction was somewhat complex due to production of
oligomers. Both isomers of compound 5 are known compounds,
and the spectral data accorded with those of a previous report.[
Production of 5 clearly indicates that one of the phenyl groups in
composed of 7-mer of 1a (C210 28), a phenyl group (C H )
H
266
O
6 5
and a terminal hydrogen atom (H), but there are four hydrogen
atoms missing (4H) (Figure 2b). This clearly indicates that further
oxidative coupling occurred at another site when the WCl
system was used probably because WCl is a stronger oxidant
than WOCl With this information in hand, we scrutinized the
HRMS spectrum of the case of the WOCl -Ph Sn system again,
6 4
-Ph Sn
15]
6
4
.
4
4
Ph
poly(diphenylacetylene)s synthesized with tungsten(VI) chloride
and Ph Sn. Dimers 6 could not be isolated as pure forms, but
4
Sn is introduced to an initiating end of the
and the isotope distribution of each molecular ion was slightly
different from that of the calculation as shown in Figure 2a,
indicating a mixture of 2a and their oxidized structures (2a ‒ 2H
4
mass spectrometric analysis of the mixture of dimers 6 indicated
the production of structures similar to 2b, 3b and 4b (see Figures
S3 and S4). These products suggest the involvment of a
migratory insertion in the reaction.
(3a) and 2a ‒ 4H).
For comparison, simple poly(diphenylacetylene) (poly-1g)
(
(
M
n
= 1,900, M
w
/M
n
= 1.16) was prepared from diphenylacetylene
4 4
PhC≡CPh, 1g) with the WOCl -Ph
Sn system,[ and MALDI-
14]
TOF-MS analysis (DIT) was performed. A series of peaks having
a 178 u difference corresponding to a monomer unit was clearly
observed up to m/z 3,500 in the HRMS spectrum, and the
representative molecular ion at m/z 2210.9540 corresponds to
+
122 [M]+ (calcd, 2210.9541) (Figure 2c). This would be
C
174
H
6 5
composed of 12-mer of 1g (C204H120), a phenyl group (C H ) and
a terminal hydrogen atom (H), and there are four hydrogen atoms
missing (4H), indicating the over-oxidized structure 2b ‒ 4H
Scheme 1. The stoichiometric reaction of diphenylacetylene (1g) and WOCl
(4-Me-Ph) Sn.
4
-
similar to that of poly-1a synthesized with WCl
6
-Ph
4
Sn. Probably,
-Ph Sn
4
oxidative coupling was promoted even with the WOCl
4
4
system because phenyl groups of poly-1g are more electron-rich
than those of poly-1a having an ester group.
Discussion of the Polymerization Mechanism
We presumed that the origin of the phenyl group would be
The experimental results are inconsistent with the metathesis
polymerization mechanism shown in Figure 1b, and we therefore
propose a new polymerization mechanism involving a migratory
insertion reaction as shown in Figure 3. First, the starting
Ph
spectrometric analysis of poly-1a synthesized using (4-MeO-
Ph) Sn instead of Ph Sn to check if a 4-methoxyphenyl group was
similarly introduced to the polymer. In MALDI-TOF-MS analysis
DCTB) of poly-1a (M = 2,100, M /M = 1.11), a series of peaks
having a 462 u difference that was identified as structure 2c ‒ 2H
4
Sn used as a co-catalyst. Therefore, we focused on mass
4
4
6 4
tungsten(VI) chloride WCl or WOCl is converted to phenylated
(
n
w
n
tungsten(VI) species 7 or 8, respectively, by a transmetallation
1f,8b]
process with Ph
4
Sn (Figure 3a).[
The degree of phenylation
4
would depend on the amount of Ph Sn used.
(
3c) was detected (Figure 2d). Representatively, the molecular
+
21 [M]+ (calcd,
ion at m/z 2417.4227 corresponds to C157
H
196
O
Since the experimental results indicated that a phenyl group
was introduced to the initiating end of the polymer chain, the
polymerization of diphenylacetylenes would be initiated by a 1,2-
insertion reaction of phenylated tungsten(VI) species 7 or 8 to a
triple bond of a monomer to form alkenyltungsten(VI) species 9
(Figure 3b).[16] Subsequently, the chain would propagate from 9
by repetition of the 1,2-insertion reaction to the monomer.
Masuda and coworkers reported that the metathesis
2
417.4264) and would be composed of 5-mer of 1a (C150
and an additional C O unit corresponding a 4-methoxyphenyl
group (C O) minus a hydrogen atom (H) (in other words, C
H ‒ 2H). In this spectrum, another series of peaks was detected
190 20
H O )
7 6
H
7
H
7
7 7
H O
+
and a representative peak at m/z 2525.4801 corresponds to
+
C
164
H
204
O
22 [M]+ (calcd, 2525.4839) composed of 5-mer of 1a
(C
150
H
190
O
20) and an additional C14
additional unit corresponds to two 4-methoxyphenyl groups
2), suggesting that the structure 4c has 4-
14 2
H O unit (Figure 2d). This
polymerization of diphenylacetylenes using
a well-defined
(C
7
H
7
O
×
ruthenium carbene complex (Grubbs-Hoveyda catalyst) gave a
polymer having a backbone structure different from that of the
polymer synthesized using the WCl -Ph
6 4
Sn system.[17] This might
methoxyphenyl groups at both polymer ends. Interestingly, a
molecular ion indicating the loss of a hydrogen atom from 4c was
not detected. These results indicate that the polymerization of
be rationalized by a difference of the polymerization mechanism.
Since organometallic 1,2-insertion to multiple bonds typically
occurs in a syn-selective manner,[ the backbone of the resultant
polymer would have a cis-rich structure from a mechanistic
diphenylacetylenes using the WOCl
the addition of a phenyl group to a triple bond from Ph
at least two possible termination processes to form a cyclic
4
-Ph
4
Sn system is initiated by
18]
4
Sn and has
5
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RESEARCH ARTICLE
viewpoint, and this is consistent with our recent report.[11] The
moderate E-selectivity in production of compound 5 from 1g might
undoubtedly, the termination process by the intramolecular
oxidative aromatic coupling reaction would become a serious
problem because electron-rich aromatic rings have a lower
oxidation potential. Indeed, promotion of oxidative aromatic
coupling was suggested in the mass spectrometric analysis of
simple poly(diphenylacetylene) (Figure 2c). Therefore, multi-
phenylated tungsten(VI) species, which could be formed in the
4
be the result of isomerization of the primary product by WOCl as
a Lewis acid or HCl formed in situ under the heating conditions.
However, this result of geometrical selectivity in compound 5
could not be simply applied to the structure of the polymer
because the rigid internal polyene backbone of the polymer might
have a higher registance to geometrical isomerization by acids.
Structures 2a‒c and 3a‒c suggested by mass spectrometric
analysis of the polymer indicate two possible termination
processes in the polymerization (Figure 3c and 3d). The
phenanthrene structure 3a‒c could be formed by intramolecular
oxidative coupling between two aromatic rings by the tungsten(VI)
species 12 at the propagating chain end (R’ = polymer) and/or
external ones (R’ = Cl or Ph) because tungsten(VI) chloride
derivatives could oxidize aromatic compounds via single electron
transfer due to a high oxidation potential.[19,20] In addition,
hydrogen chloride (HCl) generated by this redox process would
protonate an active tungsten(VI) species 10 to give polymer 2
having a hydrogen atom at the terminal end. The second
termination process would be caused by reductive elimination of
the tungsten(VI) species 13 having a phenyl group(s) at the
propagating chain end to form the end-capped polymer 4 and
reduced tungsten(IV) species (Figure 3d). The tungsten(VI)
species 13 could be formed when the polymerization was induced
from tungsten(VI) species having multiple phenyl groups or when
a chloride on tungsten(VI) species 12 at the propagating chain
end is replaced with a phenyl group by transmetallation with
4
presence of an increased amount of Ph Sn, could suppress the
termination process by oxidative aromatic coupling reactions
because such a chloride-less species would be a milder oxidant
than mono-phenylated tungsten(VI) species. In this case,
however, it should be noted that another termination process by
an end-capping reaction might be a problem due to the use of
4
excess amounts of Ph Sn (Figure 3d).
4
unreacted Ph Sn. Compound 5 in Scheme 1 would be produced
from 9 by this mechanism.
Since a molecule oxidized from the end-capped polymer 4
was not detected in the mass spectrometric analysis, the primary
redox reaction shown in Figure 3c might occur via intramolecular
electron transfer processes of 10 at the propagating chain end,
and oxidative aromatic coupling and protonation would follow. On
the other hand, further oxidation would be an intermolecular
6 4
process when it occurs (e.g., WCl -Ph Sn system or
polymerization of PhC≡CPh).
Given the proposed mechanism, we suggest the reason why
the use of WOCl improved the results of polymerization of
4
diphenylacetylene and the reproducibility. Probably, termination
by the redox process becomes a serious problem due to the high
oxidation potential of WCl
6 4
(Figure 3c), and the use of WOCl
could suppress the termination because the oxidation potential of
[
21]
WOCl
results of mass spectrometric analysis of poly-1a synthesized
with WOCl -Ph Sn and WCl -Ph Sn because over-oxidation of
polymer side chains was suggested in the latter catalytic system
4
is lower than that of WCl
6
.
This is consistent with the
4
4
6
4
(Figure 2a and 2c).
The termination process by the redox reaction could also
explain why the polymerization is sensitive to the amount of
Ph Sn used as the co-catalyst depending on monomers (Tables
and 2). Strongly electron-deficient monomers such as 1a–c
were readily polymerized under the conditions using the smallest
amount of Ph Sn (5 mol%) (entries 1‒3 in Table 2), and the use
of a larger amount of Ph Sn resulted in low monomer conversion
entries 3 and 9 in Table 1). On the other hand, this tendency
reverses in the polymerization of electron-richer monomers such
as 1d–f (entries 4-9 in Table 2). The rate of the redox reaction
would depend on the electron density of aromatic rings of 11 and
tungsten(VI) species 12. In the case of electron-rich monomers,
4
1
Figure 3. The proposed mechanism of the polymerization of diphenylacetylenes
with tungsten(VI) chloride (WCl or WOCl ) and tetraphenyltin (Ph Sn).
6 4 4
4
4
(
Conclusion
We have revisited the polymerization of diphenylacetylene with
tungsten(VI) chloride and Ph Sn through optimization of the
conditions using diphenylacetylene monomer 1a having two ester
4
6
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RESEARCH ARTICLE
groups at the para-positions. As a result, we found that the
replacement of WCl with WOCl improved the results of the
6 4
polymerization of 1a, and it also worked in the polymerization of
other diphenylacetylene monomers. Mass spectrometric analysis
b) M. Teraguchi, J. Suzuki, T. Kaneko, T. Aoki, T. Masuda,
Macromolecules 2003, 36, 9694-9697; c) T. Fukushima, K. Tsuchihara,
Macromolecules 2009, 42, 5453-5460; d) C. K. M. Jim, J. W. Y. Lam, C.
W. T. Leung, A. Qin, F. Mahtab, B. Z. Tang, Macromolecules 2011, 44,
2427-2437; e) D. Lee, Y.-J. Jin, N. Suzuki, M. Fujiki, T. Sakaguchi, S. K.
of the polymer structure combined with the results of
a
Kim, W.-E. Lee, G. Kwak, Macromolecules 2012, 45, 5379-5386; f) B. A.
S. Jose, S. Matsushita, K. Akagi, J. Am. Chem. Soc. 2012, 134, 19795-
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Z. Tang, ACS Macro Lett. 2012, 1, 75-79; h) H. Kim, K.-U. Seo, Y.-J. Jin,
C.-M. Lee, M. Teraguchi, T. Kaneko, T. Aoki, G. Kwak, ACS Macro Lett.
4
stoichiometric reaction of diphenylacetylene (4) with WOCl -
Ph Sn suggested an initiating process by insertion of a phenyl
4
group from Ph
polymerization.
4
Sn and two termination processes in the
Interestingly, these processes indicated a
2016, 5, 622-625; i) K. Maeda, M. Maruta, K. Shimomura, T. Ikai, S.
migratory insertion polymerization mechanism rather than a
metathesis polymerization mechanism that has been believed for
a long time. It should be noted that the present study does not rule
Kanoh, Chem. Lett. 2016, 45, 1063-1065; j) K. Maeda, M. Maruta, Y.
Sakai, Y. Ikai, S. Kanoh, Molecules 2016, 21, 1487.
[
6]
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out the possibility of
a
metathesis mechanism in the
polymerization of other types of monomers, such as norbornenes
6 4
and mono-substituted acetylenes, using WCl -Ph
Sn.[
1,8]
Similarly, the mechanism of the polymerization of
diphenylacetylenes with the TaCl Sn system is still an
-nBu
interesting issue, though a previous report suggests a similarity
[7]
[8]
T. Sakaguchi, S. Azuma, T. Hashimoto, Synth. Met. 2016, 212, 174-179.
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5
4
n
[17]
6 4 5 4
between WCl -Ph Sn and TaCl - Bu Sn systems.
Our study serves as a starting point to develop substantially
improved catalytic systems for polymerization of various
diphenylacetylenes, which are essential for research and
development of novel functional materials based on
poly(diphenylacetylene)s.[22] More generally, we anticipate that
an discovery of the alternative pathway by “ill-defined” metathesis
catalysts will promote the development of novel “well-defined”
tungsten catalysts in organometallic and synthetic chemistry.
1
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This work was partially supported by JSPS KAKENHI (Grant-in-
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[12] A similar problem has been reported by another research group.5g
13] The obtained poly-1a showed almost the same H and 13C NMR spectra
Center Initiative (WPI), MEXT, Japan.
1
[
6 4
as those of the previously reported polymer obtained by WCl -Ph Sn,
Keywords: Diphenylacetylenes • Insertion • Polymerization •
Polymers • Tungsten
indicating both polymers have the same main chain structure (Figures
S1 and S2).
[
[
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7
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RESEARCH ARTICLE
X.-H. Xu, S.-M. Kang, N. Liu, Z.-Q. Wu, J. Am. Chem. Soc. 2018, 140,
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8
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Entry for the Table of Contents
A migratory insertion mechanism has been proposed for the polymerization of diphenylacetylenes by a classic metathesis catalytic
system composed of tungsten(VI) chloride (WCl or WOCl ) and Ph Sn. Experimental results suggest that the polymerization is
6
4
4
initiated by addition of a phenyl group coming from Ph
processes.
4
Sn to diphenylacetylene monomers and has two possible termination
9
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