Synthesis and characterization of cyclobutenedione-bithiophene π-conjugated polymers: Acetal-protecting strategy for Kumada-Tamao-Corriu coupling polymerization between aryl bromide and Grignard reagents

Article abstract of DOI:10.1039/c9ra08275a

Cyclobutenedione is an aromatic ring that exhibits strong electron-withdrawing properties but is susceptible to undesired reactions with nucleophiles. Herein, Kumada-Tamao-Corriu coupling polymerization of a cyclobutenedione monomer whose carbonyl groups

Full text of DOI:10.1039/c9ra08275a

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Synthesis and characterization of
cyclobutenedione–bithiophene p-conjugated
polymers: acetal-protecting strategy for Kumada–
Tamao–Corriu coupling polymerization between
aryl bromide and Grignard reagents†
Cite this: RSC Adv., 2019, 9, 40863
*
*
Tomoyuki Ohishi,
Takuma Sone, Kohei Oda and Akihiro Yokoyama
Cyclobutenedione is an aromatic ring that exhibits strong electron-withdrawing properties but is
susceptible to undesired reactions with nucleophiles. Herein, Kumada–Tamao–Corriu coupling
polymerization of a cyclobutenedione monomer whose carbonyl groups are protected as acetals was
achieved. Hydrolysis of the acetals afforded donor–acceptor type p-conjugated polymers consisting of
cyclobutenedione as an acceptor unit and bithiophene as a donor unit. The acetal-protected monomer
was also subjected to Suzuki–Miyaura coupling polymerization. The absorption and emission spectra of
the deprotected polymers shifted to the longer wavelength compared with the acetal-protected polymers.
Received 11th October 2019
Accepted 2nd December 2019
DOI: 10.1039/c9ra08275a
rsc.li/rsc-advances
blue photoluminescence in solution. Huang et al. reported the
synthesis of p-conjugated cyclobutenedione polymers via
Suzuki–Miyaura and Stille coupling polymerization.10 The
prepared polymers exhibited broad and strong absorption
bands in UV-vis region and high electron affinity.
Introduction
Cyclobutenedione is a four-membered aromatic ring with two
carbonyl groups that render this unit strongly electron-
withdrawing. Among the various cyclobutenedione derivatives,
the most well-known is strongly acidic 3,4-dihydroxy-3-
cyclobutene-1,2-dione (squaric acid).1 Recently, squaric acid
bisamides (squaramides) have attracted signicant attention
and have been used as chemosensors2 and organocatalysts.3
The cyclobutenedione unit has been used for pyrolytic and
photolytic syntheses of functionalized benzoquinone and
phenol4 as well as cyclo[n]carbon,5 and can serve as a xed cis-
vinylene linker in photochromic derivatives6 and anticancer
agents.7
In the eld of polymer chemistry, the synthesis of poly-
squaramide is commonly achieved by polycondensation of
squaric acids or squaric acid diesters and diamines.8 The
reactions between squaric acid diesters and amines proceed
under mild conditions.1,2d Furthermore, the synthesis of p-
conjugated cyclobutenedione polymers has been reported by
Suh's group, who synthesized cyclobutenedione-containing
poly(phenylenevinylene)s by dehalogenation polycondensation
and Heck coupling polymerization.9 These polymers showed
Inspired by these studies, we expected that the cyclo-
butenedione unit could be used as an electron acceptor in the
same manner as maleimide11 and donor–acceptor-type p-
conjugated cyclobutenedione polymers would exhibit inter-
esting properties. However, only a few reports regarding the
synthesis of p-conjugated polymers with cyclobutenedione in
the main chain have been published to date.9,10 This is likely
due to the unstable and reactive nature of cyclobutenedione,
which can be transformed into highly reactive bisketenes under
heating and light irradiation.12 These compounds can subse-
quently be subjected to Diels–Alder cycloaddition, dimeriza-
tion, and coupling with alcohols.13 The cyclobutenedione units
in polymer main chains can also be converted into bisketenes.14
In addition, nucleophiles including organolithium and
Grignard reagents react with the carbonyl groups of cyclo-
butenedione.15 If a polymerization method applicable to the
synthesis of p-conjugated cyclobutenedione polymers can be
developed, it can provide a facile method with which to achieve
various donor–acceptor cyclobutenedione polymers.
Herein, we describe two approaches to donor–acceptor-type
p-conjugated polymers consisting of
a cyclobutenedione
Department of Materials and Life Science, Faculty of Science and Technology, Seikei
University, 3-3-1 Kichijoji-Kitamachi, Musashino, Tokyo 180-8633, Japan. E-mail:
t-ohishi@st.seikei.ac.jp; ayokoyama@st.seikei.ac.jp
acceptor and bithiophene donor. The rst approach involved
direct coupling of the cyclobutenedione unit and aromatic
rings. Because the reactivity of the cyclobutenedione carbon
atoms at the 3- and 4-positions is similar to carbonyl carbons,
the desired cyclobutenedione polymer could likely be
† Electronic supplementary information (ESI) available: Synthesis of the
monomer, the model reaction, 1H NMR spectra, 13C NMR spectra, GPC traces,
UV-vis spectra, and uorescence spectra not depicted in the manuscript. See
DOI: 10.1039/c9ra08275a
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 40863–40872 | 40863

View Article Online
RSC Advances
Paper
synthesized using similar methods to the synthesis of aryl received without purication. Squaric acid dichloride (1a)16 and
ketones. Therefore, Suzuki–Miyaura coupling of 3,4-dichloro-3- 3,4-bis[(4-methoxyphenyl)thio]-3-cyclobutene-1,2-dione (1b)17 were
cyclobutene-1,2-dione (squaric acid dichloride) (1a) or Lie- synthesized according to procedures described in the literature.
beskind–Srogl coupling of 3,4-bis[(4-methoxyphenyl)thio]-3-
cyclobutene-1,2-dione (squaric acid thioester) (1b) under Typical procedure of Suzuki–Miyaura coupling between
ketone synthesis conditions were studied as model reactions. squaric acid dichloride (1a) and phenylboronic acid (2a)
The second method used acetal-protected cyclobutenedione
monomer, which was subsequently subjected to Kumada–
Tamao–Corriu and Suzuki–Miyaura coupling polymerizations.
A round-bottomed ask equipped with a three-way stopcock
was heated under reduced pressure and subsequently cooled to
room temperature under an argon atmosphere. Then, 2a
We choose acetal as a protecting group, because it is stable
(146 mg, 1.20 mmol), PdCl2(PPh3)2 (7 mg, 0.01 mmol), K3PO4-
under basic conditions but easily hydrolyzed under aqueous
$nH2O (718 mg, 3.00 mmol), and 1a (76 mg, 0.50 mmol) were
added to the ask, and the atmosphere in the ask was replaced
with argon. Aer addition of dry toluene (2.5 mL) to the ask
acidic conditions. Finally, the optical properties of the obtained
cyclobutenedione polymers were investigated.
using a syringe, the ask was evacuated and lled with argon
ꢁ
three times, and the reaction mixture was stirred at 110 C for
Experimental
3 h. Subsequently, ethyl acetate was added at room temperature
and the mixture was washed successively with saturated
aqueous NaHCO3, water, and brine, and dried over anhydrous
Measurements
The 1H and 13C NMR spectra were obtained using a JEOL ECA-
500 instrument. The internal standards used for the 1H and 13
C
MgSO4. The mixture was ltered using Celite and the solvent
was distilled off under reduced pressure. The crude product
NMR spectra in CDCl3 were tetramethylsilane (0.00 ppm) and
the midpoint of CDCl3 (77.0 ppm), respectively. The Mn and Mw/
Mn values of the polymers were measured using a TOSOH HLC-
8220 gel permeation chromatography (GPC) unit (eluent, THF;
calibration, polystyrene standards) with two TSK-gel columns
(Multipore HXL-M) and a TOSOH HLC-8320 GPC unit (eluent,
CHCl3; calibration, polystyrene standards) containing two TSK-
gel columns (2 ꢀ SuperMultiporeHZ-M). IR spectra were
recorded using a JASCO FT/IR-470 plus and UV-vis spectra were
recorded using a JASCO V-650. The uorescence spectra were
recorded using a JASCO FP-6500 instrument. Electrospray
ionization (ESI) mass spectra were recorded on a Thermo Fisher
1
(brown solid, 70 mg) was analyzed by H NMR.
Typical procedure of coupling reaction between 1a and 2a in
the presence of copper(I)-2-thiophenecarboxylate (CuTC)
A round-bottomed ask equipped with a three-way stopcock
was heated under reduced pressure and subsequently cooled to
room temperature under an argon atmosphere. Then, 2a
(244 mg, 2.00 mmol), PPh3 (14 mg, 0.053 mmol), 1a (75 mg, 0.50
mmol), and CuTC (95 mg, 0.50 mmol) were added to the ask
and the atmosphere was replaced with argon. Pd(dba)2 (14 mg,
0.024 mmol) was added to the ask and the atmosphere in the
ask was replaced with argon. Aer addition of dry diethyl ether
(15 mL) to the ask using a syringe, the ask was evacuated and
lled with argon three times, and the mixture was subsequently
stirred at room temperature for 49 h and ltered using Celite at
room temperature. The solvent was distilled off under reduced
pressure and the crude product (brown solid, 393 mg) was
Scientic
Q Exactive Hybrid Quadrupole-Orbitrap Mass
Spectrometer.
Materials
3,4-Dihydroxy-3-cyclobutene-1,2-dione (TCI), 4,4,5,5-tetramethyl-
1,3,2-dioxaborolane (TCI), tri(2-furyl)phosphine (TFP; TCI), p-
toluenesulfonic acid monohydrate (TsOH$H2O; TCI), cesium
carbonate (Cs2CO3; TCI), 9,9-dioctyl-9H-uorene-2,7-diboronic
acid bis(pinacol) ester (TCI), phenylboronic acid (Kanto), 2-thio-
pheneboronic acid (Kanto), triphenylphosphine (PPh3; Kanto), 2-
dicyclohexylphosphino-20,60-dimethoxybiphenyl (SPhos; Kanto),
1
analyzed by H NMR.
Typical procedure of Liebeskind–Srogl coupling between 3,4-
bis[(4-methoxyphenyl)thio]-3-cyclobutene-1,2-dione (1b) and
2-thiopheneboronic acid (2b)
tri-potassium phosphate n-hydrate (K3PO4$nH2O; Kanto), A round-bottomed ask equipped with a three-way stopcock
lithium chloride (LiCl; Kanto), 4-methoxythiophenol (Aldrich), was heated under reduced pressure and subsequently cooled to
2.0 M solution of isopropylmagnesium chloride (iPrMgCl) in THF room temperature under an argon atmosphere. Then, 1b
(Aldrich), 1,4-benzenediboronic acid bis(pinacol) ester (Wako), (70 mg, 0.20 mmol) and dry THF (10 mL) were added to the ask
sodium carbonate (Na2CO3; Wako), triuoroacetic acid (TFA; and the solution was deoxygenated by bubbling argon for 5 min.
Wako), dehydrated DMF (Wako), dehydrated benzene (Kanto), CuTC (95 mg, 0.50 mmol) and TFP (4 mg, 0.02 mmol) were
dehydrated toluene (Wako), dehydrated THF (Wako), dehydrated added to the ask, which was then evacuated and lled with
diethyl ether (Wako), copper(I)-2-thiophenecarboxylate (CuTC; TCI), argon three times. Pd2(dba)3 (7 mg, 0.007 mmol) and 2-thio-
[1,3-bis(diphenylphosphino)propane]nickel(II) dichloride (Ni(dppp) pheneboronic acid (2b) (51 mg, 0.40 mmol) were added to the
Cl2; TCI), tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4; ask, which was then evacuated and lled with argon three
Kanto), bis(triphenylphosphine)palladium(II) dichloride (PdCl2(- times. The mixture was stirred at 55 ꢁC for 19 h, cooled to room
PPh3)2; Aldrich), bis(dibenzylideneacetone)palladium(0) (Pd(dba)2; temperature, and ltered using Celite and SiO2. The ltrate
Aldrich), tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3; Ald solvent was distilled off under reduced pressure. The crude
rich), and palladium(II) acetate (Pd(OAc)2; Wako) were used as- material was puried via column chromatography on a silica gel
40864 | RSC Adv., 2019, 9, 40863–40872
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
(ethyl acetate/hexane ¼ 1/5) and recrystallization from ethyl 4H), 1.70–1.50 (m, 4H), 1.39–1.16 (m, 12H), 0.90–0.76 (m, 6H);
acetate to afford 3b (45 mg, 56%) as an ocher solid: mp 202.0– IR (KBr) 2927, 2853, 2360, 1459, 1265, 1220, 1094, 1038, 985,
202.7 ꢁC. 1H NMR (500 MHz, CDCl3) d 8.43 (dd, J ¼ 4.0 and 949, 803 cmꢂ1
.
1.1 Hz, 2H, thiophene-H), 7.97 (dd, J ¼ 4.9 and 1.0 Hz, 2H,
P3 (116 mg, 64%), Mn ¼ 10 100, Mw/Mn ¼ 1.83; 1H NMR (500
thiophene-H), 7.39 (dd, J ¼ 4.9 and 3.7 Hz, 2H, thiophene-H); MHz, CDCl3) d 7.76–7.68 (m, 2H), 7.51 (br s, 2H), 7.48–7.39 (m,
13C NMR (126 MHz, CDCl3) d 193.0, 173.1, 135.2, 134.0, 129.3; 4H), 4.31 (m, 4H), 4.09 (m, 4H), 2.74–2.65 (m, 4H), 2.07–1.95 (m,
IR (KBr) 1765, 1754, 1568, 1513, 1418, 1405, 1387, 1372, 1154, 4H), 1.70–1.59 (m, 4H), 1.37–1.22 (m, 12H), 1.20–1.01 (m, 24H),
1116, 1062, 868, 726 cmꢂ1; ESI-MS calcd for C12H6NaO2S2+ m/z 0.87–0.82 (m, 6H), 0.81–0.76 (m, 6H); IR (KBr) 3429, 2935, 2858,
268.9701 (M + Na)+, found m/z 268.9691.
1608, 1547, 1458, 1269, 1038, 984, 949, 818, 721, 698, 579 cmꢂ1
.
Polymerization of monomer 4 by Kumada–Tamao–Corriu
coupling
Synthesis of 3,4-bis(3,30-dihexyl-2,20-bithiophen-5-yl)-3-
cyclobutene-1,2-dione (7)
A round-bottomed ask equipped with a three-way stopcock
containing LiCl (33 mg, 0.78 mmol) was heated under reduced
pressure and subsequently cooled to room temperature under
a nitrogen atmosphere. A solution of 4 (396 mg, 0.600 mmol) in
dry THF (3.3 mL) was added to the ask under a nitrogen
To a solution of 6 (84 mg) in CHCl3/1,4-dioxane (4/1 (v/v), 1.4
mL), H2O (0.11 mL) and TFA (0.22 mL) were added. Aer stirring
at 40 C for 3 h, the solvent was removed under reduced pres-
ꢁ
sure. The crude material was puried via column chromatog-
raphy on a silica gel (ethyl acetate/hexane ¼ 1/30) to afford 7
(56 mg, 75%) as a red solid: mp 76.3–77.6 ꢁC. 1H NMR (500
MHz, CDCl3) d 8.30 (s, 2H, thiophene-H), 7.40 (d, J ¼ 5.2 Hz, 2H,
thiophene-H), 7.01 (d, J ¼ 5.2 Hz, 2H, thiophene-H), 2.61 (t, J ¼
7.9 Hz, 4H, thiophene-CH2CH2C3H6CH3), 2.56 (t, J ¼ 7.7 Hz, 4H,
thiophene-CH2CH2C3H6CH3), 1.64–1.52 (m, 8H, thiophene-
CH2CH2C3H6CH3), 1.34–1.17 (m, 24H, thiophene-CH2CH2C3-
H6CH3), 0.85 (t, J ¼ 6.9 Hz, 6H, thiophene-CH2CH2C3H6CH3),
0.80 (t, J ¼ 7.0 Hz, 6H, thiophene-CH2CH2C3H6CH3); 13C NMR
(126 MHz, CDCl3) d 193.2, 171.9, 144.8, 143.4, 140.7, 135.6,
129.1, 128.6, 126.8, 126.7, 31.55, 31.51, 30.7, 30.6, 29.05, 29.03,
29.01, 28.7, 22.53, 22.52, 14.0; IR (KBr) 3082, 2924, 2854, 1766,
1581, 1458, 1408, 1319, 1200, 1130, 1053, 887, 725 cmꢂ1; ESI-MS
ꢁ
atmosphere and the mixture was cooled to ꢂ20 C and stirred
i
for 20 min. A 2.0 M solution of PrMgCl in THF (0.30 mL, 0.60
mmol) was added and the mixture was stirred at ꢂ20 ꢁC for 1 h.
Aer a suspension of Ni(dppp)Cl2 (13 mg, 0.024 mmol) in THF
(1.2 mL) was added to the ask using a syringe, the mixture was
stirred at 0 ꢁC for 2 h and 40 ꢁC for 48 h aerwards. The reaction
was quenched by adding methanol and the solvent was subse-
quently removed under vacuum. CH2Cl2 was added to the
residue and the insoluble material was removed by suction
ltration and thoroughly washed with CH2Cl2. Aer the removal
of the solvent in vacuo from the ltrate, the residue was again
dissolved in CH2Cl2 and poured into methanol with vigorous
stirring. The precipitated polymer was collected and dried in
vacuo to afford P1 (229 mg, 76%). Mn ¼ 7400, Mw/Mn ¼ 10.8. 1H
NMR (500 MHz, CDCl3) d 7.42 (br s, 2H), 4.26 (m, 4H), 4.06 (m,
4H), 2.53 (t, J ¼ 7.2 Hz, 4H), 1.57–1.51 (m, 4H), 1.27–1.21 (m,
12H), 0.86–0.79 (m, 6H); 13C NMR (126 MHz, CDCl3) d 143.2,
133.0, 131.8, 130.8, 130.4, 114.1, 65.9, 31.6, 30.7, 29.1, 28.8, 22.6,
14.0.
+
calcd for C44H59O2S4 m/z 747.3392 (M + H)+, found m/z
747.3386.
Removal of acetal
To a solution of P1 (50 mg, Mn ¼ 7400, Mw/Mn ¼ 10.8) in CHCl3/
1,4-dioxane (4/1 (v/v), 1.4 mL), TsOH$H2O (380 mg, 2.0 mmol)
ꢁ
was added and the mixture was stirred at 40 C for 18 h. Aer
Polymerization of monomer 4 and 5 by Suzuki–Miyaura
coupling
CHCl3 addition, the solution was washed with H2O and dried
over anhydrous MgSO4. Aer solvent removal in vacuo, the
A round-bottomed ask equipped with a three-way stopcock residue was again dissolved in CHCl3 and poured into methanol
was heated under reduced pressure and subsequently cooled to under vigorous stirring. The precipitated polymer was collected
room temperature under an argon atmosphere. Then, 4 and dried in vacuo to afford P4 (40 mg, 97%, Mn ¼ 7200, Mw/Mn
1
(132 mg, 0.200 mmol), 1,4-benzenediboronic acid bis(pinacol) ¼ 11.1). H NMR (500 MHz, CDCl3) d 8.31 (br s, 2H), 2.68–2.18
ester (5a) (66 mg, 0.20 mmol), and Cs2CO3 (286 mg, 0.880 mmol) (m, 4H), 1.62 (m, 4H), 1.25–1.18 (m, 12H), 0.88–0.75 (m, 6H); 13
C
were added to the ask, which was evacuated and lled with NMR (126 MHz, CDCl3) d 192.6, 171.8, 145.8, 137.6, 129.7, 31.4,
argon three times. Pd(PPh3)4 (12 mg, 0.010 mmol) was added to 30.6, 29.9, 29.0, 22.5, 14.0; IR (KBr) 2931, 2854, 1759, 1573, 1411,
the ask and the atmosphere was replaced with argon. Aer 1157, 1038, 852 cmꢂ1
.
1
addition of dry toluene (2.0 mL) to the ask using a syringe, the
P5 (3.4 mg, 11%, Mn ¼ 4580, Mw/Mn ¼ 2.74); H NMR (500
ask was evacuated and lled with argon three times. The MHz, CDCl3) d 8.33 (br s, 2H), 7.68–7.43 (m, 4H), 2.88–2.46 (m,
mixture was stirred at 120 ꢁC for 24 h and ltered using Celite. 4H), 1.78–1.44 (m, 4H), 1.41–1.07 (m, 12H), 0.93–0.68 (m, 6H);
The ltrate solvent was distilled off under reduced pressure. IR (KBr) 3435, 2925, 2856, 1764, 1575, 1431, 1385, 1260, 1203,
The residue was again dissolved in CHCl3 and the solution was 1098, 1021, 805 cmꢂ1
poured into methanol under vigorous stirring. The precipitated
.
P6 (27 mg, 33%, Mn ¼ 11 500, Mw/Mn ¼ 1.76); 1H NMR (500
polymer was collected and dried in vacuo to afford P2 (40 mg, MHz, CDCl3) d 8.33 (br s, 2H), 7.86–7.74 (m, 2H), 7.58–7.44 (m,
1
34%). Mn ¼ 4070, Mw/Mn ¼ 1.80. H NMR (500 MHz, CDCl3) 4H), 2.87–2.73 (m, 4H), 2.12–1.94 (m, 4H), 1.77–1.54 (m, 4H),
d 7.55–7.38 (m, 6H), 4.28 (m, 4H), 4.08 (m, 4H), 2.71–2.61 (m, 1.41–1.22 (m, 12H), 1.20–0.97 (m, 24H), 0.91–0.82 (m, 6H), 0.81–
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 40863–40872 | 40865

View Article Online
RSC Advances
Paper
0.57 (m, 6H); IR (KBr) 3440, 2925, 2853, 1762, 1571, 1532, 1428, carboxylate (CuTC) promoted the palladium-catalyzed
1204, 1099, 821 cmꢂ1
.
coupling reactions of acid chlorides and arylboronic acids,19
the reaction of 1a and 2a was performed under similar condi-
tions, but 1a was decomposed by a side reaction (entry 3).
Next, we focused on the Liebeskind–Srogl coupling reac-
tion20 using bisarylthiocyclobutenedione 1b and 2-thio-
pheneboronic acid (2b) in the presence of CuTC and
a palladium catalyst as a model reaction of the polymerization
of 1b and 2,5-thiophenediboronic acid (Scheme 2). Because the
Results and discussion
Model reaction for direct Suzuki–Miyaura and Liebeskind–
Srogl coupling polymerization of benzene-1,4-diboronic acid
and squaric acid dichloride
To investigate whether donor–acceptor p-conjugated polymers
can be obtained by direct coupling of squaric acid dichloride
(1a) and benzene-1,4-diboronic acid, a model reaction was
investigated using 1a and phenylboronic acid (2a) (Scheme 1).
The previously reported conditions of Suzuki–Miyaura coupling
reaction between acyl chloride and arylboronic acid were used.18
First, reaction of 1a and 2a was performed in the presence of
1.0 mol% of Pd(OAc)2 and 3.4 equivalents of Na2CO3 in H2O/
PEG-2000 at 60 ꢁC.18a The 1H NMR spectrum of the crude
product revealed that 1a was hydrolyzed (Table 1, entry 1).
When the coupling reaction was performed in non-aqueous
solvent using 2.0 mol% of PdCl2(PPh3)2 and 6.0 equivalents of
tri-potassium n-hydrate (K3PO4$nH2O) in toluene,18b the 1H
NMR spectrum of the crude product indicated that 1a was
decomposed by a side reaction other than hydrolysis (entry 2).
As Nishihara et al. reported that copper(I) thiophene-2-
˜
Pena-Cabrera group reported that the Liebeskind–Srogl
coupling of 1b and 3-thiopheneboronic acid (the regioisomer of
2b) affords the desired product in 94% yield,17 the reaction of 1b
and 2b was performed under similar conditions using 2.5
equivalents of CuTC, 9 mol% of tri(2-furyl)phosphine (TFP), and
ꢁ
3.5 mol% of Pd2(dba)3 in THF at 55 C. The reaction for 19 h
afforded 3b in 56% yield (entry 4). When Pd(PPh3)4 was used
instead of Pd2(dba)3, the yields of 3b were not improved (entry
5). In addition, using Pd(OAc)2/SPhos resulted in no reaction
(entry 6). Based on the conditions used for entry 4, increasing
CuTC did not increase the yields (entry 7). When an increased
amount of Pd2(dba)3 (7.0 mol%) was used, the yield of 3b
improved to 63% (entry 8), but because the model reaction did
Scheme 1 Model reaction for the direct Suzuki–Miyaura coupling Scheme 2 Model reaction for the direct Liebeskind–Srogl coupling
polymerization of squaric acid dichloride (1a) and benzene-1,4- polymerization of 3,4-bis[(4-methoxyphenyl)thio]-3-cyclobutene-
diboronic acid.
1,2-dione (1b) and 2,5-thiophenediboronic acid.
Table 1 Suzuki–Miyaura and Liebeskind–Srogl coupling reactions of 1 and 2a
Entry 1 (mmol) 2 (equiv.) Pd cat. (mol%) Ligand (mol%) Base (equiv.)
CuTC (equiv.) Temp. Solvent
Yield of 3b (%)
1
2
3
4
5
6
7
8
1a (0.50) 2a (2.5) Pd(OAc)2 (1.0)
1a (0.50) 2a (2.4) PdCl2(PPh3)2 (2.0) —
—
Na2CO3 (3.4)
K3PO4$nH2O (6.0) —
—
60 ꢁC H2O/PEG-2000 (1.5 g/1.5 g) 0
110 ꢁC Toluene (2.5 mL)
rt Diethyl ether (15 mL)
0
1a (0.50) 2a (4.0) Pd(dba)2 (5.0)
1b (0.20) 2b (2.0) Pd2(dba)3 (3.5)
1b (0.20) 2b (2.0) Pd(PPh3)4 (3.5)
1b (0.20) 2b (2.0) Pd(OAc)2 (3.5)
1b (0.20) 2b (2.0) Pd2(dba)3 (3.5)
1b (0.20) 2b (2.0) Pd2(dba)3 (7.0)
PPh3 (10)
TFP (9)
—
SPhos (10)
TFP (9)
TFP (9)
—
—
—
—
—
—
1.0
0
2.5
2.5
2.5
5.0
2.5
55 ꢁC THF (10 mL)
55 ꢁC THF (10 mL)
55 ꢁC THF (10 mL)
55 ꢁC THF (10 mL)
55 ꢁC THF (10 mL)
56
44
0
45
63
a
b
The reaction of 1 and 2 was performed as indicated. Yield was calculated from the 1H NMR spectra of the products aer column
chromatography.
40866 | RSC Adv., 2019, 9, 40863–40872
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
not proceed quantitatively, the Liebeskind–Srogl coupling to 8 h using 4 equivalents of the Ni catalyst, P1 with the highest
reaction was difficult to apply to polymerization.
Mn and Mw values was obtained in a good yield (entry 5). The
obtained polymer showed signals derived from the acetal group
at 4.6–4.2 ppm in the 1H NMR spectrum (Fig. 1a) and from the
cyclobutene ring in the 13C NMR spectrum (Fig. S1†). These
results demonstrated that conjugated polymers containing
a cyclobutene ring can be synthesized under Kumada–Tamao–
Corriu coupling conditions using an acetal-protecting strategy.
Synthesis and Kumada–Tamao–Corriu coupling
polymerization of the acetal monomer
Introducing various polymerizable functional groups at the 2-
and 5-positions of the 3-alkylthiophene is relatively facile and
the synthesis of p-conjugated polymers composed of alkylth-
iophene has been achieved by various coupling reactions.21 For
the p-conjugated polymers containing alkylthiophene and
cyclobutenedione units, Huang et al. reported Suzuki–
Miyaura10a and Stille coupling polymerizations10b using 3,4-
bis(5-bromo-4-hexylthiophen-2-yl)-3-cyclobutene-1,2-dione as
a monomer. However, only the two articles have reported the
synthesis of p-conjugated cyclobutenedione polymers by
coupling reactions likely because of the high susceptibility of
the cyclobutenedione unit to nucleophiles and bases, limiting
the available polymerization methods. We thought that the
yields of the model reactions described in the previous section
did not improved for the same reason. Therefore, to suppress
side reactions at the cyclobutenedione units induced by bases
and nucleophiles, an acetal-protected cyclobutenedione
monomer was used. For the synthesis of p-conjugated polymers
with carbonyl groups by coupling reaction, the Jenekhe and Jia
groups reported the synthesis of poly(3-alkanoylthiophene) via
Kumada–Tamao–Corriu coupling polymerization of a thio-
phene monomer whose acyl group was protected as an acetal.22
Thus, the acetal groups are compatible with Grignard reagents.
In addition, acetal groups can be easily hydrolyzed under
aqueous acidic conditions. Therefore, we tried to synthesize the
polymer P1 by Kumada–Tamao–Corriu coupling polymerization
of the acetal monomer 4 (Scheme 3). The synthesis of monomer
4 was described in the ESI (Scheme S1†).
Suzuki–Miyaura coupling of the acetal monomer
To expand the scope of the acetal monomer polymerization, the
model reaction of Suzuki–Miyaura coupling polymerization was
performed using 1 equivalent of 4 and 2.2 equivalents of 3-
hexylthiophene-2-boronic acid pinacol ester in the presence of
5 mol% of Pd(PPh3)4 and 4.4 equivalents of Cs2CO3 in toluene at
120 ꢁC (Scheme S2†).23 This reaction afforded the target model
compound in good yield (83%).
Next, to conduct Suzuki–Miyaura coupling polymerization,
the introduction of boronic acid pinacol ester (Bpin) to 4 was
examined. 4 was reacted with n-BuLi in THF at ꢂ78 ꢁC, then
4,4,5,5-tetramethyl-1,3,2-dioxaborolane was added and the
reaction was carried out at room temperature for 22 h.24 The 1H
NMR spectrum of the crude product showed that Bpin was
introduced to 4 but the product could not be separated from the
by-products via silica gel column chromatography. Similarly,
aer the reaction of n-BuLi with 4 in THF at ꢂ78 ꢁC, 2-
isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane or triiso-
propyl borate was added.25,26 However, the 1H NMR spectrum of
the crude products showed small signals of 4 and those corre-
sponding to many by-products. Further, 4 (1.0 equivalent) and
bis(pinacolato)diboron (4.0 equivalents) were reacted in the
presence of PdCl2(dppf) (0.20 equivalents) and potassium
acetate (6.0 equivalents) in 1,4-dioxane at 80 ꢁC for 22 h, but the
desired product was not obtained.
Therefore, to synthesize a polymer by Suzuki–Miyaura
coupling, 4 was polymerized with commercially available bis(-
boronic acid pinacol ester) monomers. 1,4-Phenylenediboronic
acid pinacol ester (5a) and 9,9-dioctyluorene-2,7-diboronic
acid pinacol ester (5b) were used as monomers because their
polymerization with 4 should afford polymers with different
conjugation lengths. The monomer 4 was reacted with an equal
amounts of 5a or 5b in the presence of 5 mol% of Pd(PPh3)4 and
4.4 equivalents of Cs2CO3 in toluene at 120 ꢁC for 24 h (Scheme
4). GPC elution curves of the crude products showed peaks
corresponding to P2 (Mn ¼ 4070, Mw/Mn ¼ 1.80) and P3 (Mn ¼
10 100, Mw/Mn ¼ 1.83) and the 1H NMR spectra also agreed with
the corresponding polymers (Fig. S3b and S4b†). Therefore, the
target polymers (P2 and P3) were obtained under the same
conditions as those used for the model reaction.
For polymerization, 4 was reacted with an equimolar amount
i
of PrMgCl in the presence of 1.3 equivalents of LiCl in THF at
0
ꢁC. Subsequently, 4 mol% Ni(dppp)Cl2 was added and the
reaction was performed at 40 ꢁC for 24 h. Aer purication, the
polymer P1 with Mn and Mw values of 4500 and 11 800,
respectively, was obtained in 44% yield (Table 2, entry 1). To
perform the bromine–magnesium exchange reaction under
i
ꢁ
milder conditions, 4 was reacted with PrMgCl at ꢂ20 C. Aer
Ni catalyst addition and polymerization at 40 ꢁC, P1 with
slightly increased Mn and Mw values was obtained (entry 2).
Increasing or decreasing the Ni catalyst loading to 8 mol% and
2 mol%, respectively, decreased the molecular weights of P1
(entries 3 and 4). When the polymerization time was extended
Hydrolysis of the acetal groups
Initially, as a model reaction, the hydrolysis of the acetal groups
of 6 was investigated under aqueous acidic conditions (Fig. 2).22
When 6 was treated with triuoroacetic acid (TFA; 29 equiva-
lents) and distilled water (61 equivalents) in a mixed solvent of
Scheme 3 Polymerization of 4 via Kumada–Tamao–Corriu coupling. CHCl3/1,4-dioxane (4/1, v/v) at 40 ꢁC for 3 h, the 1H NMR
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 40863–40872 | 40867

View Article Online
RSC Advances
Paper
Fig. 1 1H NMR spectra (500 MHz, CDCl3) of (a) P1 and (b) P4, and (c) GPC profiles (eluent: CHCl3) of P1 and P4.
spectrum of the product corresponded to target compound 7 shied slightly toward lower molecular weight region (Mn
¼
and acetal signals were not detected (Fig. 2). Therefore, the 7200, Mw/Mn ¼ 11.1) compared to P1 (Mn ¼ 7400, Mw/Mn ¼ 10.8;
acetal was easily hydrolyzed under acidic conditions without Fig. 1c). Furthermore, the 1H NMR spectrum of the products P4
side reactions, affording 7 in good yield (75%). However, acetal did not show acetal-methylene protons at approximately 4.6–
hydrolysis of P1 under the same conditions resulted in precip- 4.2 ppm (Fig. 1b) and the 13C NMR spectrum contained signals
itate formation as the reaction progressed and the 1H NMR derived from the cyclobutenedione ring (Fig. S2†). Therefore,
spectrum of the crude product showed residual acetal signals. the acetal groups were removed without polymer decomposi-
Next, hydrolysis using p-toluenesulfonic acid monohydrate tion and P4 containing cyclobutenedione rings in the main
(TsOH$H2O; 20 equivalents to the repeating unit) was per- chain was obtained in high yield (97%). This demonstrates that
formed in a mixed solvent of CHCl3/1,4-dioxane (4/1, v/v) at the use of an acid hydrate in non-aqueous solvents enabled
ꢁ
40 C for 18 h. Deprotection of P1 proceeded homogeneously hydrolysis of the acetal group without polymer precipitation.
and the product was puried by precipitation in a large excess of Hydrolysis of the acetal groups in P2 and P3 under the same
methanol. The GPC elution curve of the deprotected polymer P4 conditions afforded P5 (11%) and P6 (33%), respectively
Table 2 Polymerization of monomer 4a
Tempb (ꢁC)
Ni(dppp)Cl2 (mol%)
Timec (h)
Mn
Mw
Yield (%)
d
d
Entry
1
2
3
4
5
0
4
4
8
2
4
24
24
24
24
48
4500
4700
1100
4100
7400
11 800
15 700
5700
10 000
80 100
44
22e
49
44
76
ꢂ20
ꢂ20
ꢂ20
ꢂ20
a
i
Monomer 4 was initially reacted with PrMgCl (1.0 equiv.) in the presence of LiCl (1.3 equiv.) in THF for 1 h. Aer Ni(dppp)Cl2 addition, the
b
i
c
reaction was performed at 40 ꢁC. The temperature for the reaction of 4 with PrMgCl in the presence of LiCl in THF for 1 h. Reaction time
aer Ni(dppp)Cl2 addition. d Determined by GPC based on PSt standards (eluent: CHCl3). The purication procedure was performed twice.
e
40868 | RSC Adv., 2019, 9, 40863–40872
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Scheme 4 Polymerization of 4 and 5 under Suzuki–Miyaura coupling conditions.
(Scheme 5, Fig. S3c and S4c†). The low yields of P5 and P6 are P1 in CHCl3 showed unimodal peaks at maximum absorption
not due to side reactions and can instead be attributed to a large wavelengths (labs) of 374 and 393 nm, respectively, whereas 7
loss of the products during precipitation purication.
(435 and 353 nm) and P4 (433 and 340 nm) exhibited bimodal
absorption peaks (Fig. 3a). The labs of P1 shied to 19 nm
longer than 6, whereas that of P4 was shorter than 7. The
bimodal UV-vis absorption curves of 7 and P4 are consistent
with previously reported results showing donor–acceptor-type
p-conjugated polymers containing cyclobutenedione units
exhibited two absorption bands.10a The absorption peak at the
shorter wavelength side seems to arise from the p–p* transition
Optical properties of the p-conjugated cyclobutenedione
polymers
The optical properties of P1 and P4, which have shorter conju-
gation lengths than P2, P3, P5, and P6, were compared with
model compounds 6 and 7 (Fig. 3). The UV-vis spectra of 6 and
Fig. 2 1H NMR spectra (500 MHz, CDCl3) of (a) 6 and (b) 7.
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 40863–40872 | 40869

View Article Online
RSC Advances
Paper
Scheme 5 Hydrolysis of acetal groups in P2 and P3.
the spectrum of P4, a broad absorption band was observed at
a longer wavelength (approximately 480 nm), which was not
observed for 7. This indicates that the conjugation length of P4
is longer than that of 7.
In contrast, the labs of the acetal-protected polymers, P2 and
P3, and deprotected polymers P5 and P6 in CHCl3 were 394, 407,
443 and 473 nm, respectively (Fig. S5a and S5b†). The labs of P2
and P5 were observed at similar wavelengths compared to P1
and P4, respectively, suggesting that conjugation length did not
increase when a benzene ring was introduced into the mono-
mer unit of the polymer main chain. In contrast, the longer labs
of P3 and P6 compared to P1 and P4, respectively, indicates that
the polymer conjugation length increased upon introduction of
a uorene ring to the monomer unit.
In the uorescence spectra, the emission maximum wave-
lengths of 6, 7, P1, and P4 in CHCl3 were observed at 473, 557,
543, and 604 nm, respectively (Fig. 3b). Compared to the model
compound and polymer with the same cyclobutenedione unit (6
vs. P1; 7 vs. P4), the emission maximum wavelengths of the
prepared polymers were longer than those of the model
compounds. In both cases of the model compounds (6 vs. 7) and
the polymers (P1 vs. P4), removal of acetal groups red-shied
the emission maximum wavelengths. This was also observed
for other polymers, and the emission maxima of the depro-
tected polymers P5 (lem ¼ 602 nm) and P6 (lem ¼ 559 nm) were
longer than those of the corresponding acetal polymers P2 (lem
¼ 546 nm) and P3 (lem ¼ 491 nm; Fig. S5c and S5d†). When the
chloroform solutions of the model compounds and polymers
were irradiated using a UV lamp (365 nm, 4 W), 6, P1, and P4
emitted blue, yellowish, and red light, respectively, but the
solution of 7 was non-emissive (Fig. 4b). In addition, the solu-
tions of P2 and P3 emitted yellowish-green light and those of P5
and P6 emitted red light (Fig. 4d). It is unclear the reason why
the solution of 7 did not emit, but these results demonstrated
that the luminescence character depends on the electronic
states (acetal-protected vs. deprotected) of the cyclobutene unit
and on the molecular weight (small molecule vs. polymer).
Furthermore, absorption and uorescence peaks of polymers
P4–P6 were longer than those of previously reported
Fig. 3 (a) UV-vis absorption and (b) photoluminescence spectra of 6
(red), 7 (blue), P1 (green), and P4 (orange) in CHCl3 (1.0 ꢀ 10ꢂ5 M); PL
excitation at the longest absorption maximum wavelength.
of the polymer repeating units, and the absorption peak on the
longer wavelength side will be due to an intramolecular charge
transfer transition between the donor and the acceptor units. In
40870 | RSC Adv., 2019, 9, 40863–40872
This journal is © The Royal Society of Chemistry 2019

View Article Online
Paper
RSC Advances
Notes and references
1 F. R. Wurm and H.-A. Klok, Chem. Soc. Rev., 2013, 42, 8220–
8236.
2 (a) R. Prohens, G. Martorell, P. Ballester and A. Costa, Chem.
Commun., 2001, 1456–1457; (b) V. Ramalingam,
M. E. Domaradzki, S. Jang and R. S. Muthyala, Org. Lett.,
´
2008, 10, 3315–3318; (c) E. Sanna, L. Martınez, C. Rotger,
´
´
˜
S. Blasco, J. Gonzalez, E. Garcıa-Espana and A. Costa, Org.
Lett., 2010, 12, 3840–3843; (d) A. Rostami, A. Colin, X. Y. Li,
M. G. Chudzinski, A. J. Lough and M. S. Taylor, J. Org.
Chem., 2010, 75, 3983–3992; (e) M. Porel, V. Ramalingam,
M. E. Domaradzki, V. G. Young Jr, V. Ramamurthy and
R. S. Muthyala, Chem. Commun., 2013, 49, 1633–1635; (f)
R. B. P. Elmes, P. Turner and K. A. Jolliffe, Org. Lett., 2013,
15, 5638–5641.
´
3 For review, see: J. Aleman, A. Parra, H. Jiang and
Fig. 4 Pictures of 6, 7, and P1–P6 (a and c) under fluorescent lamp
irradiation and (b and d) under UV lamp irradiation (365 nm).
K. A. Jørgensen, Chem.–Eur. J., 2011, 17, 6890–6899.
4 (a) L. S. Liebeskind, S. Iyer and C. F. Jewell Jr, J. Org. Chem.,
1986, 51, 3065–3067; (b) L. S. Liebeskind, Tetrahedron, 1989,
45, 3053–3060; (c) L. S. Liebeskind, K. L. Granberg and
J. Zhang, J. Org. Chem., 1992, 57, 4345–4352; (d)
C. S. Tomooka, H. Liu and H. W. Moore, J. Org. Chem.,
1996, 61, 6009–6012.
5 (a) Y. Rubin, C. B. Knobler and F. Diederich, J. Am. Chem.
Soc., 1990, 112, 1607–1617; (b) K. Kaiser, L. M. Scriven,
F. Schulz, P. Gawel, L. Gross and H. L. Anderson, Science,
2019, 365, 1299–1301.
cyclobutenedione conjugated polymers.9,10 These results indi-
cate that acetal-protecting strategy is a useful method for
synthesizing conjugated polymers with a high degree of poly-
merization and signicant conjugation length.
Conclusions
In summary, the synthesis of donor–acceptor-type p-conjugated
polymers consisting of cyclobutenedione as an acceptor and
bithiophene as a donor was developed. When the carbonyl
groups of cyclobutenedione were protected as acetals, the
reaction proceeded under Kumada–Tamao–Corriu coupling
conditions using Grignard reagents. Furthermore, the p-
conjugated cyclobutenedione polymers whose repeating units
exhibited different conjugation lengths were synthesized by
Suzuki–Miyaura coupling reaction. The acetal groups were
smoothly removed under acidic conditions. The absorption and
uorescence spectra of the prepared polymers demonstrated
that acetal removal red-shied the absorption and emission
maximum wavelengths and generated bimodal peaks likely due
to intramolecular donor–acceptor interaction. A synthetic study
of the cyclobutenedione polymers with other p-conjugated
lengths is currently underway.
6 M. M. Krayushkin, V. Z. Shirinian, L. I. Belen'kii,
A. Y. Shadronov, A. Y. Martynkin and B. M. Uzhinov,
Mendeleev Commun., 2002, 12, 141–143.
7 Z.-Y. Liu, Y.-M. Wang, Y.-X. Han, L. Liu, J. Jin, H. Yi, Z.-R. Li,
J.-D. Jiang and D. W. Boykin, Eur. J. Med. Chem., 2013, 65,
187–194.
8 (a) Y. Imai, M. Shiratori, T. Inoue and M. Kakimoto, J. Polym.
Sci., Part A: Polym. Chem., 2002, 40, 2648–2655; (b)
´
A. Rostami, C. J. Wei, G. Guerin and M. S. Taylor, Angew.
Chem., Int. Ed., 2011, 50, 2059–2062; (c) A. Rostami,
´
G. Guerin and M. S. Taylor, Macromolecules, 2013, 46,
6439–6450.
9 (a) J. K. Jeong, S. J. Choi, T. H. Rhee, N. C. Yang and
D. H. Suh, Polym. Bull., 1999, 42, 559–565; (b) N. C. Yang,
J. K. Jeong, S. J. Choi, T. H. Rhee and D. H. Suh, Macromol.
Rapid Commun., 1999, 20, 586–590.
10 (a) E. F. Huo, Y. Zou, H. Q. Sun, Y. Huang, Z. Y. Lu and
Q. Jiang, Chin. Chem. Lett., 2011, 22, 1326–1330; (b)
D. Yang, Z. Guan, L. Yang, Y. Huang, Q. Wei, Z. Lu and
J. Yu, Sol. Energy Mater. Sol. Cells, 2012, 105, 220–228.
11 (a) N. C. Yang and D. H. Suh, Macromol. Rapid Commun.,
2001, 22, 335–338; (b) N. C. Yang and D. H. Suh, Polymer,
2001, 42, 7987–7992; (c) L.-H. Chan, Y.-D. Lee and
C.-T. Chen, Macromolecules, 2006, 39, 3262–3269; (d)
M. Nakamura, K. Yamabuki, T. Oishi and K. Onimura, J.
Polym. Sci., Part A: Polym. Chem., 2013, 51, 4945–4956; (e)
K. Wei, G. Wen, Y. Zhao, Z. Lin, X. Mei, L. Huang and
Q. Ling, J. Mater. Chem. C, 2016, 4, 9804–9812.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by a JSPS KAKENHI Grant Number
JP26870195. Some of this work was also supported by the
Cooperative Research Program of ‘‘Network Joint Research
Center for Materials and Devices”. The authors thank Prof.
Tsutomu Yokozawa and Dr Yoshihiro Ohta, Kanagawa Univer-
sity, for the GPC measurement (eluent, CHCl3).
This journal is © The Royal Society of Chemistry 2019
RSC Adv., 2019, 9, 40863–40872 | 40871

View Article Online
RSC Advances
Paper
12 (a) G. Maier and C. Rohr, Liebigs Ann., 1996, 307–309; (b) 20 L. S. Liebeskind and J. Srogl, J. Am. Chem. Soc., 2000, 122,
N. Fu, A. D. Allen, S. Kobayashi, T. T. Tidwell, S. Vukovic, 11260–11261.
T. Matsuoka and M. Mishima, J. Org. Chem., 2008, 73, 21 (a) C.-A. Dai, W.-C. Yen, Y.-H. Lee, C.-C. Ho and W.-F. Su, J.
1768–1773.
Am. Chem. Soc., 2007, 129, 11036–11038; (b) M. Urien,
H. Erothu, E. Cloutet, R. C. Hiorns, L. Vignau and
H. Cramail, Macromolecules, 2008, 41, 7033–7040; (c)
B. W. Boudouris, F. Molins, D. A. Blank, C. D. Frisbie and
M. A. Hillmyer, Macromolecules, 2009, 42, 4118–4126; (d)
Q. Meng, X.-H. Sun, Z. Lu, P.-F. Xia, Z. Shi, D. Chen,
M. S. Wong, S. Wakim, J. Lu, J.-M. Baribeau and Y. Tao,
Chem.–Eur. J., 2009, 15, 3474–3487; (e) N. Delbosc,
J. D. Winter, S. Moins, A. Persoons, P. Dubois and
O. Coulembier, Macromolecules, 2017, 50, 1939–1949.
13 (a) A. D. Allen, J. Ma, M. A. McAllister, T. T. Tidwell and
D.-c. Zhao, Acc. Chem. Res., 1995, 28, 265–271; (b) I. Egle,
W.-Y. Lai, P. A. Moore, P. Renton, T. T. Tidwell and
D.-c. Zhao, J. Org. Chem., 1997, 62, 18–25; (c) M. M. Dejmek
and R. Selke, Angew. Chem., Int. Ed., 1998, 37, 1540–1542;
(d) K. Piech, T. Bally, A. D. Allen and T. T. Tidwell, J. Org.
Chem., 2013, 78, 2908–2913.
14 Z. Y. Wang and L. Suzzarini, Macromolecules, 1996, 29, 1073–
1075.
15 (a) J. L. Kraus, Tetrahedron Lett., 1985, 26, 1867–1870; (b) 22 C. Wang, F. S. Kim, G. Ren, Y. Xu, Y. Pang, S. A. Jenekhe and
M. W. Reed, D. J. Pollart, S. T. Perri, L. D. Foland and
H. W. Moore, J. Org. Chem., 1988, 53, 2477–2482; (c)
L. A. Paquette, Eur. J. Org. Chem., 1998, 1709–1728.
16 J. M. Malicka, A. Sandeep, F. Monti, E. Bandini, M. Gazzano,
C. Ranjith, V. K. Praveen, A. Ajayaghosh and N. Armaroli,
Chem.–Eur. J., 2013, 19, 12991–13001.
L. Jia, J. Polym. Sci., Part A: Polym. Chem., 2010, 48, 4681–
4690.
23 F. Hermerschmidt, A. S. Kalogirou, J. Min, G. A. Zissimou,
S. M. Tuladhar, T. Ameri, H. Faber, G. Itskos, S. A. Choulis,
T. D. Anthopoulos, D. D. C. Bradley, J. Nelson, C. J. Brabec
and P. A. Koutentis, J. Mater. Chem. C, 2015, 3, 2358–2365.
˜
17 A. Aguilar-Aguilar, L. S. Liebeskind and E. Pena-Cabrera, J. 24 J. Li, M. Yan, Y. Xie and Q. Qiao, Energy Environ. Sci., 2011, 4,
Org. Chem., 2007, 72, 8539–8542. 4276–4283.
18 (a) B. Xin, Y. Zhang and K. Cheng, J. Org. Chem., 2006, 71, 25 R. Fernando, Z. Mao and G. Sauve, Org. Electron., 2013, 14,
´
5725–5731; (b) Y. Urawa and K. Ogura, Tetrahedron Lett.,
2003, 44, 271–273.
19 Y. Nishihara, Y. Inoue, M. Fujisawa and K. Takagi, Synlett,
2005, 15, 2309–2312.
1683–1692.
26 K. Adachi, H. Hirao, K. Matsumoto and T. Kubo, Org. Lett.,
2014, 16, 5870–5873.
40872 | RSC Adv., 2019, 9, 40863–40872
This journal is © The Royal Society of Chemistry 2019