Palladium(0)-catalyzed synthesis of cross-conjugated polymers: Transformation into linear-conjugated polymers through the diels-alder reaction

Article abstract of DOI:10.1002/anie.201200303

Making the shift: The title reaction of a propargylic bis(carbonate) with diboron compounds leads to new cross-conjugated polymers 1 having 2,3-butadienylene moieties. 1 is then converted into the linear-conjugated polymers 2 through a Diels-Alder reactio

Full text of DOI:10.1002/anie.201200303

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DOI: 10.1002/anie.201200303
Polymerization
Palladium(0)-Catalyzed Synthesis of Cross-Conjugated Polymers:
Transformation into Linear-Conjugated Polymers through the
Diels–Alder Reaction
Noriyuki Nishioka, Shotaro Hayashi, and Toshio Koizumi*
Palladium(0)-catalyzed reactions of propargylic compounds
with nucleophiles have been shown to be effective for the
synthesis of heterocyclic compounds,[1] allylic compounds,[2]
and allenes.[3] As an application of these reactions to polymer
synthesis, we have reported that the palladium(0)-catalyzed
polycondensation of propargylic carbonates with bifunctional
nucleophiles gives exomethylene-containing polymers.[4]
Recently, a palladium(0)-catalyzed coupling reaction of
propargylic bis(carbonate)s with organoboron compounds
has been reported by Bçhmer and Grigg, and the reaction
produced diarylated dienes in moderate to good yields.[5] The
results prompted us to undertake a synthetic investigation of
cross-conjugated polymers by palladium(0)-catalyzed cross-
coupling polymerization between a propargylic bis(carbon-
ate) and diboron compounds. Conjugated polymers have
received much attention because of their various applications
such as light-emitting diodes,[6] chemical sensors,[7] and
electronic devices.[8] The most studied frameworks for con-
jugated polymers are linearly conjugated p systems. In con-
trast to the conjugated polymers, cross-conjugated polymers
have been less studied. However, cross-conjugated com-
pounds have recently attracted increasing attention because
of their potential for various applications.[9] For example,
poly(arylene-1,1-vinylene)s are synthesized by Weber and co-
workers[10] and Itami et al.[11] Cross-conjugated polymers
would present new opportunities for material science.
Poly(arylene-2,3-butadienylene)s, which have sequential
1,1-vinylene groups in the monomer unit, have not been
reported so far. We anticipated that cross-conjugated poly-
mers having 2,3-butadienylene moieties in the main chain
would be quite interesting because the conversion of them
into conjugated polymers would easily be accomplished
through reactions such as the Diels–Alder reaction. This
approach would be a new protocol for the synthesis of
conjugated polymers. We report herein the synthesis of new
cross-conjugated polymers by palladium(0)-catalyzed cross-
coupling polymerization of a propargylic bis(carbonate) with
diboron compounds, and their transformation into conjugated
polymers.
We first examined the palladium(0)-catalyzed reaction of
the propargylic bis(carbonate) 1 with 3,5-dimethylphenyl
boronic acid (2) as a model reaction (Table 1). The reaction of
Table 1: Palladium(0)-catalyzed cross-coupling reaction of propargylic
bis(carbonate) 1 with 3,5-dimethylphenyl boronic acid 2.[a]
Entry
Cat. Pd
Ligand
Pd [mol%]
Yield [%][b]
1
2
3
4
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
[Pd2(dba)3]·CHCl3
[Pd2(dba)3]·CHCl3
[Pd2(dba)3]·CHCl3
PPh3
10
10
10
10
10
5
18
27
72
0
92
84
62
P(o-Tolyl)3
S-Phos
X-Phos
S-Phos
S-Phos
S-Phos
5[c]
6[c]
7[c]
2
[a] Reaction conditions: 1 (0.5 mmol), boronic acid 3 (1.0 mmol), Pd/P
(1:2), 2mK2CO3 aq. (1.0 mL), toluene (2 mL), reflux, 24 h. [b] Yield of
isolated product. [c] Pd/P (1:1). dba=dibenzylidene acetone.
1 with 2 was conducted using the conditions reported by
Bçhmer and Grigg, that is, Pd(OAc)2 and PPh3 in toluene at
658C.[5] The expected 2,3-diarylated-1,3-butadiene 3 was
obtained as a cross-conjugated product. However, the yield
was low (21%) and the conversion of 1 was 55%. Carbonate
1 was completely consumed when the reaction was carried out
at reflux, but the yield did not increase (entry 1). Various
phosphine ligands were examined for the reaction.
P(o-Tolyl)3 was also not effective (entry 2), however, the
yield of 3 significantly increased with use of 2-dicyclohexyl-
phosphino-2,6-dimethoxybiphenyl (S-Phos; entry 3). Using 2-
dicyclohexylphosphino-2,4,6-triisopropylbiphenyl (X-Phos)
did not afford 3 at all (entry 4). [Pd2(dba)3]·CHCl3 was more
effective than Pd(OAc)2 as a palladium source, with the yield
of 3 increasing up to 92% (entry 5). A lower loading of the
palladium (5 mol% of Pd) did not cause a significant decrease
in the yield (entry 6). The palladium(0)-catalyzed cross-
coupling reaction of 1 with 9,9-dioctylfluorene-2-boronic
acid (4) gave the corresponding diarylated diene 5 in good
yield (Scheme 1).
[*] N. Nishioka, Dr. S. Hayashi, Prof. T. Koizumi
Department of Applied Chemistry, National Defense Academy
1-10-20 Hashirimizu, Yokosuka, Kanagawa 239-8686 (Japan)
E-mail: tkoizumi@nda.ac.jp
Homepage: http://www.mod.go.jp/nda/
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201200303.
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erate Mn value was produced even when 0.4 equivalents of 6
were used (entry 3). N-Octylcarbazole-3,6-bis(ethyleneboro-
nate) (7) and 2,5-dioctyloxyphenylene-1,4-diboronic acid (8)
also gave the corresponding cross-conjugated polymers P2
and P3 (entries 4 and 5). The structures of the obtained
polymers P1–P3 were confirmed by FT/IR analysis and NMR
spectroscopy. All the polymers were soluble in common
organic solvents such as THF, toluene, and CHCl3. The
thermal stability of these polymers was investigated by
thermal gravimetric analysis (TGA) under a helium atmos-
phere. All the polymers showed good thermal stability with
5% weight loss at temperatures (Td) above 3008C (see
Figure S1 in the Supporting Information).
Scheme 1. Palldium(0)-catalyzed cross-coupling reaction of 1 with 9,9-
dioctylfluorene-2-boronic acid (4).
On the basis of the results of the model reaction, we next
investigated the palladium(0)-catalyzed cross-coupling poly-
merization of 1 with 9,9-dioctylfluorene-2,7-diboronic acid
bis(1,3-propanediol ester) (6; Table 2). The polymerization of
1 with 6 (1 equiv) proceeded to give the corresponding cross-
conjugated polymer P1 in 54% yield, but the Mn value was
rather low (entry 1). We reported that the palladium(0)-
catalyzed polycondensation of bis(phenol)s with an excess of
propargylic carbonates proceeded successfully to afford
higher molecular weight polyethers.[4] This success resulted
from because a small amount of propargyl carbonate was
decomposed by the palladium catalyst used. Therefore, we
tried the polymerization with 6 by using excess 1. When
0.8 equivalents of 6 was used, the corresponding polymer P1
was obtained quantitatively, and the Mn value increased to
7500 (entry 2). The corresponding polymer P1 having a mod-
To convert the cross-conjugated polymers P1–P3 into
conjugated polymers, the Diels–Alder reaction of P1–P3 with
dienophiles was explored. Among the dienophiles tested, N-
phenyl-1,3,4-triazoline-2,5-dione (PTAD) was most reactive.
The Diels–Alder reactions of each of the polymers P1–P3
with PTAD (1 equiv) were carried out in THF, and were
complete within a few minutes, successfully delivering the
expected linear-conjugated polymers DP1 (Mn = 9800, Mw/
Mn = 5.57), DP2 (Mn = 3300, Mw/Mn = 2.39), and DP3 (Mn =
6400, Mw/Mn = 2.56). In contrast, the Diels–Alder reaction of
the cross-conjugated product 3 with PTAD required a longer
reaction time (1 h) and excess PTAD (3 equiv) to complete its
transformation into the corresponding conjugated product.
The representative 1H NMR spectra of P1 and DP1 are
shown in Figure 1. Complete consumption of P1 was con-
firmed by the disappearance of the signals for the diene
groups, and the appearance of signals corresponding to the
methylene groups of DP1 were observed. The FT/IR
spectrum of DP1 showed a strong absorption band at n =
1779 cmÀ1 and is attributed to the carbonyl group. In the
13C NMR spectrum of DP1, the carbonyl carbon atom was
observed at d = 152.57 ppm. These data support the structure
Table 2: Palladium(0)-catalyzed cross-coupling polymerization of prop-
argylic bis(carbonate) 1 with diboron compounds 6–8.[a]
[c]
[c]
Entry Diboron
compounds
Equiv. Polymer Yield
[%][b]
Mn
Mw/Mn
1
2
3
4
5
6
1.0
0.8
0.4
0.8
0.8
P1
P1
P1
P2
P3
54
2700 1.68
6
6
7
8
quant. 7500 4.22
94
81
82
5400 3.49
2700 2.71
6400 2.81
Figure 1. 1H NMR spectra of P1 and DP1 in CDCl3 (300 MHz). The
disappearance of the signal corresponding to the diene protons
(a: d=5.34 and 5.55 ppm) of P1 and appearance of those for the
methylene protons (a’: d=4.65 ppm) of DP1 show the complete
transformation of P1 into DP1 through the Diels–Alder reaction of P1
with PTAD.
[a] Reaction conditions: 1 (0.25 mmol), diboron compounds 6–8
(0.1–0.25 mmol), [Pd2(dba)3]·CHCl3 (2.5 mol%), S-Phos (5.0 mol%),
2mK2CO3 aq. (1.0 mL), toluene (2 mL), reflux, 24 h. [b] Insoluble in
MeOH. Yields are based on the diboron compound. [c] Estimated by
GPC (PSt).
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of DP1, and the structures of DP2 and DP3 were similarly
confirmed. These polymers are composed of an all-cis
conformation. Similar all-cis conjugated polymers were also
synthesized by Suzuki–Miyaura cross-coupling polymeri-
zation of 2,3-dibromomaleimide with diboronic acids,[12]
however, this procedure gave low-molecular-weight poly-
mers, probably as a result of debromination of 2,3-dibromo-
maleimide.
The optical properties of the cross-conjugated polymers
P1–P3 and linear-conjugated polymers DP1–DP3 were
measured in a THF solution. The representative UV/Vis
absorption and fluorescence spectra of P1 and DP1 are shown
in Figure 2. The absorption maximum of P1 was observed at
a Diels–Alder reaction of P1 with maleic anhydride in toluene
at reflux for 24 hours. We anticipated that the DP4 would
show a different fluorescence spectrum if intramolecular
charge transfer occurs between the fluorene and the dien-
ophile moieties. The observed fluorescence spectrum of DP4
was similar to that of DP1 (see Figure S4 in the Supporting
Information), thus suggesting that the fluorescence spectra of
the conjugated polymers DP1–DP4 were hardly affected by
the dienophile moiety.
A large Stokes shift (131 nm) was observed for a P1 film
prepared by spin coating onto a glass plate (Figure 2, dashed
line). P2 and P3 films also showed large Stokes shifts (see
Figures S2 and S3 in the Supporting Information). These
phenomena were similar to those of cis-linked conjugated
polymers DP1–DP4. X-ray diffraction, as reported by van
Walree et al., of 2,3-diphenyl-1,3-diene shows s-gauche to be
the preferred geometry with a face-to-face arrangement of
the phenyl groups in the solid state.[13] The films of P1–P3 do
consist mainly of the units in an s-gauche conformation. It has
also been reported that multilayered chromophoric groups
show large Stokes shift.[14] Therefore, the observed large
Stokes shifts for DP1–DP4 would be based on the energy
transfer between the arylene moieties.
In conclusion, a series of new cross-conjugated polymers
(P1–P3) were synthesized by palladium(0)-catalyzed cross-
coupling polymerization between propargylic bis(carbonate)
1 and diboron compounds. The cross-coupling polymerization
proceeded efficiently when using the [Pd2(dba)3]·CHCl3/
S-Phos catalyst system. Various diboron compounds could
be used in this polymerization, and the obtained cross-
conjugated polymers P1–P3 were easily be converted into the
linear-conjugated polymers DP1–DP4 through a Diels–Alder
reaction. We succeeded in changing the optical properties of
P1–P3, such that, the maximum emission peaks in the
fluorescence spectra of the linear-conjugated polymers
DP1–DP4 showed large redshifts compared to those of the
cross-conjugated polymers P1–P3. Cross-conjugated poly-
mers obtained by our method have the potential to be
transformed into a variety of functionalized polymers includ-
ing conjugated polymers. Additional investigations, including
the synthesis of conjugated polymers from other dienophiles,
are now in progress.
Figure 2. Normalized absorption and fluorescence spectra of P1
(solid), DP1 (dotted) in THF, and for the P1 film (dashed).
lmax = 325 nm. The conjugated polymer DP1 exhibited
a broad shoulder around l = 350 nm along with lmax
=
320 nm. Polymers P2, DP2, P3, and DP3 also showed similar
behavior (see Figures S2 and S3 in the Supporting Informa-
tion). These results indicate that the conjugation path of
DP1–DP3 was not extended along the polymer backbone.
This phenomenon is probably due to interruption of the p-
electron delocalization caused by the twist of the polymer
backbone having an all-cis configuration.
The fluorescence spectra of the polymers were recorded
with the excitation wavelength corresponding to the max-
imum absorption wavelength. The cross-conjugated polymer
P1 showed two fluorescent emission peaks at l = 361 and
386 nm along with a shoulder around l = 440 nm. Interest-
ingly, a large redshift was observed in the fluorescent
spectrum of the conjugated polymer DP1 (lem = 473 nm),
and the fluorescent emission changed from blue to green. The
fluorescence spectra of polymer DP2 (l = 458 nm) and DP3
(l = 470 nm) also showed redshifts compared to those of the
original polymer P2 (l = 398 nm) and P3 (l = 413 nm),
respectively. These linear-conjugated polymers DP1–DP3
showed large Stokes shifts (ca. 150 nm). There are two
possible reasons for these large redshifts: 1) intramolecular
charge transfer between the fluorene and the dienophile
moieties or 2) energy transfer between the arylene moieties.
To clarify the reason for the large Stokes shift, we next
synthesized another linear-conjugated polymer, DP4, through
Received: January 12, 2012
Published online: February 28, 2012
À
Keywords: C C coupling · conjugation · Diels–Alder reaction ·
palladium · polymers
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