A Convoluted Polyvinylpyridine-Palladium Catalyst for Suzuki-Miyaura Coupling and C?H Arylation

Article abstract of DOI:10.1002/adsc.202000742

The development of highly active and reusable supported catalysts for Suzuki-Miyaura coupling and catalytic C?H arylation is important for fundamental and applied chemistry, with these reactions being used to produce medical compounds and functional materials. Herein, we found that a mesoporous composite made of a linear poly(4-vinylpyridine) and tetrachloropalladate acted as a dual-mode catalyst for a variety of cross-coupling reactions, with both Pd nanoparticles and a Pd complex catalyst being observed under different conditions. The polyvinylpyridine-palladium composite 1 was readily prepared via the molecular convolution of poly(4-vinylpyridine) and sodium tetrachloropalladate to provide a hardly soluble polymer-metal composite. The Suzuki-Miyaura coupling and the C?H arylation of aryl chlorides and bromides with arylboronic acids, thiophenes, furans, benzene, and anisole proceeded in the presence of 0.004 mol% (40 mol ppm) to 1 mol% Pd of 1 to afford the corresponding coupling products in high yields. Furthermore, the catalyst was reused without an appreciable loss of activity. Pharmaceutical compounds and functional materials were synthesized via the coupling reactions. N₂ gas adsorption/desorption analysis indicated that the catalyst had a mesoporous nature, which played a crucial role in the catalysis. In the Suzuki-Miyaura couplings, in situ generated palladium nanoparticles in the polymer matrix were catalytically active, while a polymeric Pd(II) complex was crucial in the C?H arylations. These catalytic species were investigated via XAFS, XPS, far-infrared absorption, and Raman spectroscopies, as well as DFT calculations. (Figure presented.).

Full text of DOI:10.1002/adsc.202000742

Title: A Convoluted Polyvinylpyridine-Palladium Catalyst for Suzuki–
Miyaura Coupling and C-H Arylation
Authors: Aya Ohno, Takuma Sato, Toshiaki Mase, Yasuhiro Uozumi,
and Yoichi M. A. Yamada
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.202000742
Link to VoR: https://doi.org/10.1002/adsc.202000742

10.1002/adsc.202000742
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
A Convoluted Polyvinylpyridine-Palladium Catalyst for
Suzuki–Miyaura Coupling and C-H Arylation
Aya Ohno,a Takuma Sato,a Toshiaki Mase,b Yasuhiro Uozumi,*b and Yoichi M. A.
Yamada*a
a
RIKEN Center for Sustainable Resource Science, Hirosawa, Wako, Saitama 351-0198, Japan.
[phone: +81-(0)48-467-9404, fax: +81-(0)48-467-1423, e-mail: ymayamada@riken.jp]
Institute for Molecular Science (IMS), Myodaiji, Okazaki, Aichi 444-8787, Japan.
b
[phone: +81-(0)564-59-5571, fax: +81-(0)564-59-5574, e-mail: uo@ims.ac.jp]
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract. The development of highly active and reusable Furthermore, the catalyst was reused without an appreciable
supported catalysts for Suzuki-Miyaura coupling and catalytic loss of activity. Pharmaceutical compounds and functional
C-H arylation is important for fundamental and applied materials were synthesized via the coupling reactions. N2 gas
chemistry, with these reactions being used to produce medical adsorption/desorption analysis indicated that the catalyst had
compounds and functional materials. Herein, we found that a a mesoporous nature, which played a crucial role in the
mesoporous composite made of a linear poly(4-vinylpyridine) catalysis. In the Suzuki-Miyaura couplings, in situ generated
and tetrachloropalladate acted as a dual-mode catalyst for a palladium nanoparticles in the polymer matrix were
variety of cross-coupling reactions, with both Pd catalytically active, while a polymeric Pd(II) complex was
nanoparticles and a Pd complex catalyst being observed under crucial in the C-H arylations. These catalytic species were
different conditions. The polyvinylpyridine-palladium investigated via XAFS, XPS, far-infrared absorption, and
composite 1 was readily prepared via the molecular Raman spectroscopies, as well as DFT calculations.
convolution of poly(4-vinylpyridine) and sodium
tetrachloropalladate to provide a hardly soluble polymer-
metal composite. The Suzuki-Miyaura coupling and the C-H
arylation of aryl chlorides and bromides with arylboronic
acids, thiophenes, furans, benzene, and anisole proceeded in
the presence of 0.004 mol% (40 mol ppm) to 1 mol% Pd of 1
to afford the corresponding coupling products in high yields.
Keywords: Molecular Convolution; Polymeric Palladium;
poly(4-vinylpyridine); Suzuki-Miyaura coupling; C-H
Arylation
and MacGlacken[23] have both covered the topic of
direct Csp2-H arylation using heterogeneous palladium
catalysts in their useful reviews. Catalysts used for this
purpose include Pd/C,[24,25] Pd(OH)2/C,[26] Pd on metal
oxides,[27] Pd nanoparticles on supports,[28-32] and
polymer-supported palladium catalysts;[33,34] however,
the reusability of these catalysts has not been
investigated in depth. Although we acknowledge these
pioneering works, we decided to develop highly active
and reusable polymeric palladium catalysts for the
direct C-H arylation of various substrates, including
heteroaromatic systems. Additionally, development of
highly active and reusable heterogeneous catalysts for
Suzuki–Miyaura coupling of aryl chlorides are still in
progress. We believe that cutting-edge concepts are
important for the development of highly active and
reusable polymeric palladium.
Introduction
The use of both C-H arylations and Suzuki–Miyaura
couplings to prepare (hetero)aromatic biaryl
compounds is important not only for fundamental
catalytic chemistry but also for the production of
functional materials and pharmaceutical compounds.[1-
3]
In recent years, many homogeneous catalysts for
these reactions have been reported that utilize precious
transition metals such as palladium,[4-7] rhodium,[8]
ruthenium,[9] copper,[10] and iridium.[11] Converting
such systems from homogeneous to heterogeneous
catalysis is desirable because it would allow these
precious transition metal species to be readily
recovered and reused. While heterogeneous catalysts
including crosslinked polymeric resin-supported
palladium catalysts have been reported for Suzuki-
Miyaura coupling,[12-21] heterogeneous catalysts for
couplings using C-H bonds are still limited. Felpin[22]
We have developed a convolution methodology to
overcome the issues with the preparation of more
highly active and reusable immobilized metal catalysts
1
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10.1002/adsc.202000742
Advanced Synthesis & Catalysis
than polymeric resin (cross-linked polymer)-supported
ones (Figure 1a).[16,17,35-37] For example, we have
already reported a poly(N-isopropylacrylamide-co-N-
vinylimidazole) palladium dichloride catalyst (aka
MPPI-Pd) for the Suzuki–Miyaura coupling. MPPI-Pd
was a highly active catalyst for the coupling of some
aryl chlorides.[17] However, when using less reactive
electron-rich aryl chlorides such as chloroanisole, 0.1
mol% Pd of MPPI-Pd was required. Additionally,
MPPI-Pd was unsuitable for C-H arylations. Since
highly active and reusable palladium catalysts for the
Suzuki–Miyaura coupling of aryl chlorides and C-H
arylations are desired, we have developed polymeric
palladium catalysts for these reactions.
It was envisioned that some polymeric ligands should
activate palladium species and accelerate the targeted
coupling reactions. After screening a variety of
polymeric ligands, we found a simple, linear, and
commercially available linear polymer, poly(4-
vinylpyridine), showed the greatest increase in
palladium species activity. Herein, we report the
development of the polyvinylpyridine-palladium
catalyst 1, and its application to the C-H arylation of
thiophenes, furans, and benzenes with aryl chlorides
and bromides, as well as the Suzuki–Miyaura coupling
of aryl chlorides. The use of heterogeneous catalyst 1
loaded with 40 mol ppm Pd (0.004 mol% Pd) mediated
the Suzuki–Miyaura coupling to give the
corresponding biaryl products, and 1 loaded with 100
mol ppm Pd (0.01 mol% Pd) promoted the C-H
arylations to give their corresponding products. The
catalyst was reused without significant loss of catalytic
activity. Moreover, we found that the catalytically
active species in these couplings were different from
each other, with polymer–stabilized palladium
nanoparticles being present in the Suzuki-Miyaura
coupling, while a polymeric Pd(II) complex was active
in the C-H arylations. This shows that 1 acted as a
dual-mode catalyst for these cross-coupling reactions
(Figure 1b).
Figure 1. Suzuki-Miyaura coupling and C-H arylation by heterogeneous (supported) polymeric Pd dual-mode catalysts. [a]
Molecular convolution for the preparation of polymeric metal catalysts. [b] Dual mode catalyses of 1.
to a solution of poly(4-vinylpyridine) (2) (3 mol equiv
based on a monomer) in 2-propanol, after which
Results and Discussion
composite 1 quantitatively precipitated as a yellow
solid (Figure 2b) that was hardly soluble in water, 2-
propanol, ethyl acetate, dichloromethane, and hexane
(checked by treatment of 1 in these solvents at 90 °C
for 5 d). Polymeric Pd catalysts 1’ and 1’’ were also
prepared from 3 (1 mol equiv Pd) and 2 (2 and 5 mol
equiv of N, respectively). Polarized light imaging of 1
Preparation of polymeric pyridine palladium
catalyst 1
The poly(4-vinylpyridine)-palladium catalyst 1 was
prepared from commercially available materials in one
step via our molecular convolution methodology (Fig.
2a). An aqueous solution of Na2PdCl4 (3) was added
2
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10.1002/adsc.202000742
Advanced Synthesis & Catalysis
suggested that the material was homogeneously
dispersible and non-crystalline in nature (Figure 2c).
The structure of 1 was investigated using several
spectroscopic methods, including X-ray absorption
fine structure (XAFS). The oxidation state of
palladium in 1 was +2, as shown in the X-ray
absorption near edge structure (XANES) of the Pd K-
edge (Figure S8). Elemental analysis indicated that the
molar ratio of Pd:Cl:N was 1:2:3, which is very
consistent with the quantitative formation of 1 under
the preparation conditions used (Pd:N ratio = 1:3).
Therefore, the local structure of 1 was proposed as
trans-PdCl2L2 (L = pyridine unit of polymer), and the
(b)
(c)
Figure 2. (a) Preparation of polyvinylpyridine-PdCl2
catalysts 1, 1’, and 1’’. (b) Visual appearance of 1 (yellow
solid). (c) Polarized microscopy image of 1.
optimized
geometry
of
trans-PdCl2(4-
methylpyridine)2 was obtained by DFT using the
PBE0 functional with the polarized triple-zeta basis set
(Figures 3a and 3b). A fitting analysis of the extended
X-ray absorption fine structure (EXAFS) of 1 with the
DFT-calculated model indicated the validity of this
local Pd structure. Thus, the best fit was obtained and
the Pd-N and Pd-Cl bond lengths were estimated to be
2.026 and 2.302 Å, respectively (Figure 3c). The
trans-configuration of the palladium center was
confirmed from the absorption pattern of the Cl-Pd-Cl
stretching vibrations in the far-infrared (FIR) region
and the corresponding Raman spectrum (Figure S16).
Curve fitting analysis of the X-ray photoelectron
spectroscopy (XPS) spectrum indicated the existence
of free pyridine units as a minor peak component as
well as coordinating pyridines as the main peak with a
positive binding energy shift (Figures S18 and S19).
(a)
(a)
(b)
(c)
Figure 3. (a) Plausible polymeric structure of 1 (Pd:N =1:3).
(b)
DFT-optimized
structure
of
trans-PdCl2(4-
3
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10.1002/adsc.202000742
Advanced Synthesis & Catalysis
methylpyridine)2 as a model for EXAFS analysis. (c)
Fourier-transform of k3-weighted Pd K-edge EXAFS of 1
and the best fit. R factor: 0.014, k-range: 3-15 Å-1, R-range:
1.2-2.2 Å, R(Pd-N) = 2.026(16) Å, R(Pd-Cl) = 2.302(10) Å,
2(Pd-N) = 0.00304(121) Å2, 2(Pd-Cl) = 0.00347(55) Å2.
PdCl2(pyridine)2, and a mixture of (NH4)2PdCl4 and
pyridine (Pd:pyridine = 1:3), were also much lower,
affording 6a in 18%, <1%, and 28% yields,
respectively (Table 1, entries 7–9). The synergistic
combination of an appropriate polymeric ligand and a
palladium salt can therefore be seen to provide a much
higher catalytic activity when coupling this less
reactive substrate.
Polyvinylpyridine-palladium composite-catalyzed
Suzuki-Miyaura coupling of aryl chlorides
Table 1. Suzuki–Miyaura coupling of chloroanisole (4a)
with phenylboronic acid (5a) catalyzed by a variety of
palladium catalyst candidates a)
Suzuki-Miyaura couplings were investigated using a
variety of catalysts, including our polymeric palladium
composites (poly(4-vinylpyridine)-palladium 1, 1’,
and 1’’) as heterogeneous catalysts and several Pd (II)
complexes as homogeneous catalysts (Table 1). We
chose an electron-rich and unreactive aryl chloride, 4-
chloroanisole (4a), as a widely used benchmark
substrate. When the reaction of 4a with phenylboronic
acid (5a; 1.2 mol equiv) was performed using 1 (0.004
mol% = 40 mol ppm, Pd:N = 1:3), K3PO4 (3.0 mol
equiv), and tetrabutylammonium bromide (TBAB; 1.0
mol equiv) in H2O at 95 °C for 20 h, the desired
coupling product, 4-methoxybiphenyl (6a) was
obtained in 93% yield (Table 1, entry 1). The turnover
number and the turnover frequency in this catalytic
reaction reached 20,000, and 1162 h-1, respectively.
Note that the molar ratio of Pd to N (polymeric ligand)
in the polyvinylpyridine-palladium catalyst was
critical in terms of the yield of the product. Thus, using
a similar polyvinylpyridine-palladium composite 1’
(Figure 2a, Pd:N = 1:2) promoted the coupling to give
6a in 74% yield (Table 1, entry 2), while the use of
catalyst 1’’ (Figure 2a, Pd:N = 1:5) gave less than 1%
of the product (Table 1, entry 3).
Interestingly, gas adsorption/desorption isotherms
revealed that the 1:2 and 1:3 composites had
mesoporous features (type IV isotherms with H2
hysteresis), whereas the 1:5 composite and the starting
polymer (i.e., poly(4-vinylpyridine) powder) were
non-porous (Figures S1–S7). The Barrett-Joyner-
Halenda (BJH) plot of 1 (Pd:N = 1:3) shows that its
pore size distribution peaked at around 19 nm (Figure
4). The specific surface areas of 1 (Pd:N = 1:3), 1’
(Pd:N =1:2), 1’’ (Pd:N = 1:5), and poly(4-
vinylpyridine) 2 were 366 (N2), 350 (N2), 4.7 (Kr), and
2.6 (Kr) m2g-1, respectively, based on their Brunauer-
Emmett-Teller (BET) plots. Therefore, the
morphology and catalytic activity of the
polyvinylpyridine-palladium composites strongly
depended on the Pd:N ratio, even though the local
palladium structures were intrinsically identical as
indicated by XAFS (trans-PdCl2L2, Figure S9).
Entry Catalyst
Yield of 6a (%)
1
2
3
4
5
6
7
8
1 (Pd:N= 1:3)
93
74
<1
38
47
10
18
<1
1’ (Pd:N = 1:2)
1’ ’ (Pd:N = 1:5)
poly(vinylimidazole)2PdCl2
MPPI-Pd
PA-TAP-Pd
(NH4)2PdCl4
trans-PdCl2(pyridine)2
9
10
(NH4)2PdCl4 + pyridine (1:3) 28
Pd(PPh3)4 29
a) Conditions: 4a (1.5 mmol), 5a (1.8 mmol), Pd catalyst (60
nmol), K3PO4 (4.5 mmol), TBAB (1.5 mmol), H2O (0.6 mL),
95 °C, 20 h, mixing at 800-900 rpm.
For comparison, poly(vinylimidazole)2PdCl2 and our
polymeric Pd catalysts, MPPI-Pd17 (composite of
poly(N-isopropylacrylamide-co-N-vinylimidazole)
and PdCl2, Pd:imidazole = 1:2) and PA-TAP-Pd16
(composite of poly(N-isopropylacrylamide-co-N-
diphenylstyrylphosphine) and PdCl2, Pd:P = 1:2) gave
6a in 38%, 47%, and 10% yields, respectively (Table1,
entries 4–6). The catalytic activities of their
homogeneous counterparts, (NH4)2PdCl4, trans-
Figure 4. Barrett-Joyner-Halenda (BJH) pore size
distribution plot of 1 (Pd:N = 1:3) derived from the N2
desorption branch of the isotherm.
4
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10.1002/adsc.202000742
Advanced Synthesis & Catalysis
Table 2. Suzuki–Miyaura couplings of a variety of aryl
Chlorides (4) with arylboronic acids (5) catalyzed by 40 mol
ppm of 1 a)
recovered and reused without significant loss of
catalytic activity, and no detectable amount of Pd was
found in the reaction mixture as confirmed by
inductively coupled plasma mass spectrometry (ICP-
MS) analysis (see Supporting Information for details).
Moreover, fenbufen (6p), a nonsteroidal anti-
inflammatory drug (NSAID), was directly synthesized
without any functional group protection in 95% yield.
For large-scale synthesis, a 0.5-mol-scale reaction was
investigated (Scheme 2): The reaction of 4b (63 g)
with 5a (73 g) was performed with 1 (10 mg) in water
to produce 79 g of the biaryl compound 6b (94% yield).
Entry 4 R1, X
5 R2
5a
Yield of 6 (%)
6b 99
1
4b 4-Me, Cl
2
3
4
4c 2-Me, Cl
4d 3,5-(MeO)2, Cl
4e 4-Ac, Cl
4f 4-CF3, Cl
4g 4-NO2, Cl
4b
6c 92
6d 98
6e 92
6f 99
5a
5a
5a
5a
5
6b)
7b)
6g 81
5a
5b 4- 6h 89
MeO
8
9
6i 99
5c 4- 6j 98
4e
4b
5b
Ac
5c
10b)
11c)
6k 90
4e
4e
5d 4- 6l 75
Ph
5a
5a
5a
12d)
13d)
14d)
4a-Br 4-MeO, Br
4b-Br 4-Me, Br
4b-I 4-Me, I
6a 94
6b 99
6b 99
a)conditions: 4 (1.5 mmol), 5 (1.8 mmol), 1 (60 nmol),
K3PO4 (4.5 mmol), TBAB (1.5 mmol), H2O (0.6 mL), 95 °C,
20 h, mixing at 800-900 rpm; b) 40 h; c) in H2O–4-methyl-2-
pentanone (0.6 mL each); d) without TBAB in 5 h
Since 1 exhibited high catalytic activity for the
coupling of 4a to 5a, the Suzuki-Miyaura coupling of
a variety of aryl chlorides with different arylboronic
acids was performed with 0.004 mol% (40 mol ppm)
Pd of 1 (Table 2). Both electron-poor and rich aryl
chlorides and arylboronic acids were suitable
substrates and reactants in the polyvinylpyridine-
palladium 1-catalyzed cross-coupling, affording the
corresponding biaryl compounds in yields of up to
99%. The reaction of arylbromides, and iodotoluene
was also carried out with 1 (0.004 mol% Pd) to give
the corresponding products 6a-b in 94-99% yields
(Entries 12-14). Evangelisti and co-workers reported
palladium nanoparticles supported on poly(4-
vinylpyridine) cross-linked with divinylbenzene (also
known as cross-linked polyvinylpyridine Pd
nanoparticles) for the Suzuki-Miyaura coupling.[15] In
their report, the reaction of arylbromides and -iodides
was performed with 0.15 mol% Pd in DMA/H2O. The
reaction of 4-nitrochlorobenzene, an activated aryl
chloride, was shown as the reaction of aryl chlorides
to give 4-nitrobiphenyl in 16% yield.
Scheme 1. Suzuki-Miyaura couplings of 3-pyridyl chloride
(4h), 1,4-dichlorobenzene (4i), and 1,3,5-trichlorobenzene
(4j), and reusability of 1.
A heteroaromatic chloride as well as di- and tri-
chlorobenzenes were also suitable substrates for this
reaction (Scheme 1). Various functional materials
were readily produced under similar conditions in 98–
99% yields. Polyvinylpyridine-palladium 1 was
Scheme 2. The 0.5-mol-scale Suzuki-Miyaura coupling.
5
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10.1002/adsc.202000742
Advanced Synthesis & Catalysis
2
0.05
0.02
0.01
0.003
0.1
0.1
0.1
0.1
0.1
0.05
0.02
0.1
0.1
0.2
0.1
0.1
0.1
9a 91
9a 88
9a 83
9a 47
9b 83
9c 79
9d 97
9e 82
9f 89
9f 81
9f 73
9g 88
9h 78
9i 74
9j 70
9k 58
9l 87
7a
7a
7a
7a
7a
7a
7a
7a
8a
8a
8a
8a
8b 2-Me
8c 3-Me
8d 4-Me
8e 4-CF3
8a
8a
8a
8b
8d
8f 4-OMe
8g 4-Cl
8a
Polyvinylpyridine-palladium
composite
1-
3b),c)
4d),e)
5d),e)
6
catalyzed C-H arylation of thiophenes, furans,
benzene, and anisole
Since 1 exhibited high catalytic activity and reusability
when used for the Suzuki-Miyaura coupling, C-H
arylation of various thiophene and furan derivatives,
benzene, and anisole using polyvinylpyridine-
palladium 1 was investigated. While there are many
reports on homogeneous catalysis of this reaction,[5-
7,38] the development of heterogeneous alternatives is
still in progress.[22,23] Especially, the C-H arylation
with aryl chlorides,[7,39,40] less reactive reactants as
compared with aryl bromides and iodides, with
heterogeneous catalysts is still challenging although
the reaction is important for preparation of organic
functional materials and pharmaceutical compounds.
Since 1 exhibited high catalytic activity for the Suzuki-
Miyaura coupling with reusability, we challenged to
develop C-H arylation of thiophenes, furans, benzene,
and anisole by using 1. The reaction of 2-
hexylthiophene (7a) and bromobenzene (8a) was
carried out with K2CO3, pivalic acid (PivOH), and 0.1
mol% Pd of 1 in dimethylacetamide (DMA) at 120 °C
for 20 h to afford the corresponding phenylated
thiophene 9a in 93% yield (Table 3, entry 1). The
catalytic loading of 1 was decreased to 500, 200, and
100 mol ppm (0.05, 0.02, and 0.01 mol%), affording
9a in yields of 91–83% (table 3, entries 2–4), and then
to 30 mol ppm (0.003 mol%), where turnover number
and frequency of 15000 and 780 h-1 were achieved
(Table3, entry 5). The arylation with o-, m-, and p-
7d)
8d)
9d)
10
7b Me
7b
11c)
12c),d) 7b
13
7b
7b
14d)
15c),d) 7b
16b),c) 7b
17
18
7c Cl
7d
8f
t-Bu-
NH-SO2
7a
19f)
20g)
0.05
0.05
9a 83
9a 77
8a
8a
7a
a) conditions: 7 (1.0 mmol), 8 (2.0 mmol), 1 (as described),
K2CO3 (1.5 mmol), PivOH (0.3 mmol), DMA (3 mL), Ar,
b)
c)
120 °C, 20 h, mixing at 1000 rpm; 130 °C; 40 h;
d)140 °C; e) 65 h; f) 1‘ was used; g) 1‘‘ was used.
substituted
bromotoluenes
8b–d
gave
the
corresponding products 9b–d in up to 97% yield
(Table3, entries 6–8). The reaction of a variety of
thiophenes and aryl bromides proceeded smoothly to
give the corresponding coupling products 9e–9l in up
to 89% yield, with chloro- and t-Bu-NHSO2 moieties
remaining intact (Table 3, entries 9–18). Of particular
note is product 9l, which is an intermediate for the
synthesis of AR-C123196, a candidate compound for
the treatment of inflammatory and allergic conditions
such as asthma and rhinitis.[41,42] As control
experiments, the reactions of 7a and 8a were
performed with 1’ and 1’’ (0.05 mol% Pd) to give 9a
in 83% and 77% yield, respectively (Entries 19 and 20).
The catalyst 1 is also the best in the C-H arylation (vs.
Entry 2).
Table 3. C-H arylations of a variety of thiophenes (7) with
arylbromides (8) catalyzed by 0.003–0.1 mol% Pd of 1 a)
Scheme 3. Application of C-H arylation to the synthesis of
functional materials.
Several functional materials were prepared using 1, as
shown
in
Scheme
3.
2,5-Bis(4-
chlorophenyl)thiophene (9m), hexyl 4T (9n),[43] and 2-
phenylbenzo[b]thiophenes (9o and 9p) were readily
synthesized in one step with yields of 73–85%.
Entry 7 R1
8 R2
1 (mol% Yield of
Pd)
9 (%)
1
7a C6H13 8a H
0.1
9a 93
6
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10.1002/adsc.202000742
Advanced Synthesis & Catalysis
Table 4. C-H arylations of various thiophenes (7) with aryl
chlorides (4) catalyzed by 1 a)
Entry
10 R1 4 R2
1
Yield
of 11
(%)
(mol%
Pd)
Entry
1b)
7 R1
4 R2
1(mol
Yield
of
% Pd)
9
1 (1st use)b)
4l H
1.0
11a 67
10a
C4H9
(%)
4l Hc)
1.0
0.1
9f 75
7b
Me
7b
7b
7b
7a
C6H13
7a
7a
7a
2 (2nd use) b)
1.0
1.0
0.4
0.2
0.1
1.0
0.4
1.0
11a 68
11a 66
11a 72
11a 68
11a 62
11b 70
11c 78
11d 69
11e 82
11f 75
11g
10a
10a
10a
10a
10a
10a
10a
10a
10a
10a
10b
H
4l
4l
4l
4l
3 (3rd use) b)
2b),d)
4lc)
9f 67
9g 82
9g 81
9a 60
4
3 (1st use) e)
4 (2nd use) e)
5b)
4m 2-CN 1.0
5c)
6c)
7d)
8
1.0
1.0
4m
4l
4lc)
4b 4-Me
4g 4-NO2
4m 2-CN
4o 4-CHO 1.0
4p 4-COPh 1.0
6
7
8
1.0
0.1
0.05
0.02
1.0
9r 86
9r 90
9r 76
9r 61
9e 84
9s 72
9t 80
9u 74
4m
4m
4m
4m
9
10
11
12e)
9d),f)
10
11
12
13
7a
7a
7a
7a
1.7
4l
4f 4-CF3
4g 4-NO2 1.0
4n 4-CN 1.0
4o 4-
CHO
4p 4-
COPh
53f)
a) conditions: 10 (1.0 mmol), 4 (2.0 mmol), 1 (as described),
K2CO3 (1.5 mmol), PivOH (0.3 mmol), TBAB (1.0 mmol),
1.0
7a
b)
DMA (3 mL), Ar, 100 °C, 20 h, mixing at 1000 rpm;
110 °C; c) 40 h;d) 120 °C;e) conditions: 10 (0.6 mmol), 4 (2.4
mmol), 1 (1.7 mol%), K2CO3 (1.8 mmol), PivOH (0.36
mmol), TBAB (0.6 mmol), DMA (3 mL), Ar, 110 °C, 20 h,
mixing at 1000 rpm; f) yield of 2,5-diphenylfuran.
14
1.0
9v 98
7a
a) conditions: 7 (1.0 mmol), 8 (2.0 mmol), 1 (as described),
KOAc (2.0 mmol), TBAB (1.0 mmol), DMA (3 mL), Ar,
130 °C, 20 h, mixing at 1000 rpm; b) without DMA; c) 4 (5.0
mmol); d) 65 h; e) 140 °C; f) 120 °C.
The C-H arylation of thiophenes with aryl chlorides by
heterogeneous catalysis was also attempted (Table 4).
The reaction of 2-methylthiophene (7b) and
chlorobenzene (4l), a non-reactive aryl chloride, was
carried out using 1. We were pleased to find that the
reaction proceeded with 1.0 mol% and even 0.1 mol%
Pd of 1 to afford 2-methyl-5-phenylthiophene (9f) in
75% and 67% yield, respectively (Table 4, entries 1
and 2). The reaction of 7b with electron-deficient 2-
chlorobenzonitrile (4b) gave the product 9g in 82%
yield, and the catalyst was reused to afford 9g in 81%
yield (Table 4, entry 3 and 4). The reaction between 7a
and 4m gave 9r in a 86% yield (Table 4, entry 6), and
when the catalyst loading was reduced to 0.1, 0.05, and
0.02 mol% Pd, the achieved yields were 90%, 76%,
and 61%, respectively, with a turnover number of
3,000 (Table 4, entries 7–9). Substituted
chlorobenzenes 4f, 4g, 4n, 4o, and 4p were also
suitable reactants in the conversion, affording their
corresponding products in 72%–98% yield (Table 4,
entries 10–14).
Scheme 4. C-H arylations of benzene and anisole.
The C-H arylation of furans is rather difficult owing to
their instability. However, 2-butylfuran (10a) reacted
with chlorobenzene (4l) to give 2-butyl-5-phenylfuran
(11a) in 67% yield when using 1 as a catalyst (Table 5,
entry 1). The catalyst was reused without loss of
catalytic activity (Table 5, entries 2 and 3). Various
furans and aryl chlorides were coupled to give the
corresponding arylated furans 11b–g in 53–82% yield
(Table 5, entries 4–12). The C-H arylations of benzene
and anisole are among the most difficult C-H bond
functionalizations.[11,44] The reaction of 4-tolylbromide
(8d) with benzene (12a) was carried out with 1 to give
4-methylbiphenyl (6b) in 58% yield (Scheme 4). The
Table 5. C-H arylations of a variety of furans (10) with aryl
chlorides (4) catalyzed by 1 a)
7
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10.1002/adsc.202000742
Advanced Synthesis & Catalysis
reaction of bromobenzene (8a) and anisole (12b) was
also performed with 1 to give a mixture of
methoxybiphenyls with a combined yield of 53%.
The black color of the recovered catalyst powder from
the Suzuki-Miyaura coupling suggested the in situ
reduction of Pd(II) complexes to Pd(0) nanoparticles.
The Catalyst was reused without loss of its catalytic
activity (Scheme 1; An SEM image of the recovered
catalyst is shown in Figure S23.). Transmission
electron microscopy (TEM) observations of the
recovered 1 showed the formation of nanoparticles
with mean diameters of 3.7±1.0 nm (Figures S21 and
S22). The oxidation state of Pd in the recovered
catalyst from the Suzuki-Miyaura coupling was zero
(i.e., metallic), as shown by Pd K-edge XANES
(Figure S8). According to EXAFS, the coordination
number of Pd decreased from 12 in bulk Pd to 7.8 after
use, which supports the formation of metallic
nanoclusters and nanoparticles whose Pd-Pd bond
lengths are estimated to be 2.794 Å (Figure 6). Curve
fitting analysis of the N 1s XPS spectrum revealed
three peak components: (a) free pyridine, (b) +1.6 eV,
and (c) +3.1 eV (Figure 7). We assigned the main peak
(b) and the minor peak component (c) to face-to-face
(6-C,N) and edge-to-face (1-N) interactions of
pyridine with the surface of the metallic palladium
nanoparticles, respectively, as supported by DFT
calculations on a model Pd20 cluster and 4-
methylpyridine system (Figure 8, see Supporting
Information Figures S8-S16 for details).[45] In other
words, Pd nanoparticles prefer to interact with poly(4-
vinylpyridine) in a face-to-face manner.
Reusability and structure of the recovered catalyst
from the Suzuki-Miyaura coupling
[a]
[b]
Figure 5. Time course experiments of (a) the Suzuki-
Miyaura coupling of 4b and 5a (Table 1. Entry 1) and (b)
the C-H arylation of 7a and 8a (Table 3, Entry 1).
Figure 6. Fourier-transform of k3-weighted Pd K-edge
EXAFS of the recovered catalyst from the Suzuki-Miyura
coupling and the best fit. R factor: 0.003, k-range: 3–19 Å-1,
R-range: 2.0–2.9 Å, mean coordination number = 7.8(5), R
= 2.794(2) Å, 2 = 0.00482(21) Å2. The spectrum was
measured in transmission mode at 10 K.
During time course experiments of the Suzuki-
Miyaura coupling of 4b and 5a (Table 1. Entry 1) and
the C-H arylation of 7a and 8a (Table 3, Entry 1),
induction period was obserbed in the Suzuki-Miyaura
coupling (Figure 5). We also found that the colors of 1
when used in the Suzuki-Miyaura coupling and the C-
H arylation were different. While the as-prepared 1
was yellow, the catalysts recovered from Suzuki-
Miyaura couplings and C-H arylations were black and
yellowish brown, respectively. We built a hypothesis
that 1 worked as a polymeric Pd nanoparticle catalyst
in the Suzuki-Miyaura coupling and as a polymeric Pd
complex catalyst in the C-H arylation, i.e., as a dual-
mode catalyst. This hypothesis was tested as outlined
below.
8
This article is protected by copyright. All rights reserved.

10.1002/adsc.202000742
Advanced Synthesis & Catalysis
Scheme 5. Reusability of 1 in C-H arylations.
Figure 7. N 1s XPS spectrum of the recovered catalyst from
the Suzuki-Miyaura coupling. Dashed lines indicate the
fitting curves. Binding energy shift values in eV relative to
free N of pyridine.
These results suggest that the coordination-like
interaction between pyridine and Pd(0) nanoparticles
maintains the insoluble polymer matrix of the catalyst
even after the completion of the reaction. Since the
catalyst is highly active and reusable (Scheme 1), in-
situ generated palladium nanoparticles work as the
heterogeneous catalyst for the Suzuki-Miyaura
coupling. Pd nanoparticles were dispersed in the
polymer matrix, and no detectable leaching of Pd was
observed in the solution phase of the reaction by ICP-
MS analysis (less than the limit of the detection of
ICP-MS, <0.06 ppb).
Reusability and structure of the recovered catalyst
from the C-H arylation
The molar ratio of the (hetero)arenes and aryl halides
used as starting materials for the C-H arylation
drastically affected the reusability of the catalyst. For
example, the reaction of 2-methylthiophene (7b; 1 mol
equiv) with bromobenzene (8a; 2 mol equiv)
proceeded in the presence of polyvinylpyridine-
palladium 1 (1 mol% Pd) to give the corresponding
product 9f in excellent yield with good reusability: 1st
use, 89%; 2nd use, 93%; 3rd use, 95%; 4th use, 93%
yield (Scheme 5a; An SEM image of the recovered
catalyst is shown in Figure S24.). Note that the
recovered catalyst under these conditions was
yellowish brown, and so it (SEM images of the
recovered catalyst are shown in Figure S24.) was
supposed that it acted as a polymeric Pd complex in
this reaction. However, reactions under similar
conditions but with the molar ratio of the starting
materials swapped gave the product in 85% (1st use)
and 5% yields (2nd use) (Scheme 5b). In this case, the
catalyst powder turned black after the first use, which
suggests that metallic nanoparticles formed during the
1st use and were no longer very active for the catalysis
of the C-H arylation. For this reason, we focused on
the analysis of the catalyst recovered from reactions
using excess aryl halides, i.e., where the catalyst was
reusable (Scheme 5a). In fact, the Pd K-edge XANES
Figure 8. Face-to-face (left) and edge-to-face (right)
interactions based on a model system using a Pd20
cluster and 4-methylpyridine at the level of
M06/LANL2DZ (for Pd) + 6-31G(d,p). Palladium
atoms are partially omitted for clarity.
9
This article is protected by copyright. All rights reserved.

10.1002/adsc.202000742
Advanced Synthesis & Catalysis
spectrum of the recovered catalyst that was
catalytically active for the C-H arylation indicated that
the oxidation state of palladium was +2. Elemental
analysis suggested that the Pd:Br ratio was nearly 1:1.
Therefore, it is proposed that the structure of the
recovered catalyst was PdBrPhL2 (L = pyridine unit of
polymer), which is the final product of the oxidative
addition of bromobenzene to a presumed Pd(0)
intermediate. The fitting result of Pd K-edge EXAFS
based on the DFT-optimized (PBE0 functional with
polarized triple-zeta basis set) geometry of trans-
PdBrPh(4-methylpyridine)2 showed good agreement
with the experimental data (Figures 9a and 9b). Pd-N,
Pd-C, and Pd-Br bond lengths were estimated to be
1.976, 2.024, and 2.438 Å, respectively. Because it is
hard to strictly distinguish between N and C using
EXAFS, we also performed IR absorption
measurements after a deuterium labeling experiment
using bromobenzene-d5 as a reagent in order to
confirm the incorporation of the phenyl group into the
complex (Scheme S2 and Figure S17, see Supporting
Information for experimental details). Moreover, Br
K-edge EXAFS also suggested the existence of a Pd-
Br bond with an interatomic distance of 2.438 Å
(Figure S14). Notably, this structure corresponds to a
precursor state for the concerted metalation-
deprotonation mechanism in Pd-catalyzed C-H
arylations under homogeneous conditions, as proposed
in pioneering works by Fagnou et al.46,47 Considering
the above, the in situ generated PdBrPhL2 structure
should be an active species in C-H arylations using 1.
Figure 10 summarizes the structural changes that the
catalyst undergoes in the studied reactions. The
Suzuki-Miyaura coupling and the C-H arylation
reactions were catalyzed by polymer-stabilized
palladium nanoparticles and organopalladium(II)
species, respectively. The in situ generated active
catalysts were recoverable and reusable without
significant loss of activity. From the above, it is
concluded that 1 acted as a dual-mode catalyst for the
Suzuki-Miyaura coupling and C-H arylation reactions.
[b]
Figure 9. (a) DFT-optimized structure of trans-PdBrPh(4-
methylpyridine)2 as a model for EXAFS analysis. (b)
Fourier-transform of k3-weighted Pd K-edge EXAFS of the
recovered catalyst from the C-H arylation and the best fit. R
factor: 0.013, k-range: 3–15 Å-1, R-range: 1.2–2.4 Å, R(Pd-
C) = 1.976(26) Å, R(Pd-N) = 2.024(26) Å, R(Pd-Br) =
2.438(15) Å, 2 (Pd-C) = 0.00384(410) Å2, 2 (Pd-N) =
0.00369(124) Å2, 2 (Pd-Br) = 0.00806(71) Å2.
[a]
10
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10.1002/adsc.202000742
Advanced Synthesis & Catalysis
Figure 10. Suzuki-Miyaura coupling and C-H arylation using 1.
times. The combined organic layer was washed with water
and brine, dried over MgSO4, and concentrated under
reduced pressure. The residue was purified by column
chromatography over silica gel (eluent: mixture of hexane
and EtOAc) to give product 9.
Conclusion
We developed a polyvinylpyridine-palladium catalyst
1 for the Suzuki-Miyaura coupling of aryl chlorides
and the C-H arylation of (hetero)arenes with aryl
halides. The reactions proceeded in the presence of
0.004 mol% (40 mol ppm) to 1 mol% of 1 to afford the
corresponding products in high yields, and the catalyst
showed good reusability. Using X-ray absorption fine
structure (XAFS) analysis, we found that the
composite 1 was a dual-mode catalyst that allowed the
in situ generation of Pd(0) nanoparticles dispersed in a
polymer matrix that were catalytically active in the
Suzuki-Miyaura coupling, while a polymeric Pd(II)
complex was an active species in the C-H arylation
(Figure 9). Moreover, we also found that the final state
of the catalyst and its reusability depended on the
initial molar ratio of the starting materials in case of C-
H arylations. This means that the reusability of
heterogeneous catalysts is potentially affected by the
consumption of one of the starting materials from the
solution phase after the completion of the reaction.
Acknowledgements
We thank Dr. Tetsuo Honma (JASRI, SPring-8) for assistance with
the XAFS studies (SPring-8, BL14B2). We would like to thank
RIKEN for assistance with the XPS (Dr. Aiko Nakao, RIKEN),
elemental analysis (materials characterization support team,
CEMS,
RIKEN), and ESI-MS
(molecular
structure
characterization unit, CSRS, RIKEN) measurements.
We gratefully acknowledge financial support from the JST ACT-C
(#JPMJCR12ZC), the JST ACCEL (#JPMJAC1401), the JSPS
(#24550126,
#20655035,
and
#15K05510),
AMED
(#19ak0101115h), the Takeda Science Foundation, the Naito
Foundation, and RIKEN.
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12
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10.1002/adsc.202000742
Advanced Synthesis & Catalysis
FULL PAPER
A Convoluted Polyvinylpyridine-Palladium
Catalyst for Suzuki–Miyaura Coupling and C-H
Arylation
Adv. Synth. Catal. Year, Volume, Page – Page
Aya Ohno, Takuma Sato, Toshiaki Mase, Yasuhiro
Uozumi,* and Yoichi M. A. Yamada*
13
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