Linear polystyrene-stabilized palladium nanoparticles-catalyzed C-C coupling reaction in water

Article abstract of DOI:10.1021/jo200485q

Linear polystyrene-stabilized PdO nanoparticles (PS-PdONPs) were prepared in water by thermal decomposition of Pd(OAc)₂ in the presence of polystyrene. The immobilization degree of palladium was dependent on the molecular weight of polystyrene, while the size of the Pd nanoparticles was not. Linear polystyrene-stabilized Pd nanoparticles (PS-PdNPs) were also prepared using NaBH₄ and phenylboronic acid as reductants. The catalytic activity of PS-PdONPs was slightly higher than that of PS-PdNPs for Suzuki coupling reaction in water. PS-PdONPs exhibited high catalytic activity for Suzuki and copper-free Sonogashira coupling reactions in water and recycled without loss of activity.

Full text of DOI:10.1021/jo200485q

ARTICLE
pubs.acs.org/joc
Linear Polystyrene-Stabilized Palladium Nanoparticles-Catalyzed
CꢀC Coupling Reaction in Water
Atsushi Ohtaka,*,† Takuto Teratani,† Ryohei Fujii,† Kanako Ikeshita,† Takahiro Kawashima,†
Koichi Tatsumi,† Osamu Shimomura,† and Ry^oki Nomura*,†,‡
†Department of Applied Chemistry, Faculty of Engineering and ‡Nanomaterials and Microdevices Research Center,
Osaka Institute of Technology, 5-16-1 Ohmiya, Osaka, Osaka 535-8585, Japan
S Supporting Information
b
ABSTRACT: Linear polystyrene-stabilized PdO nanoparticles
(PS-PdONPs) wereprepared in water by thermaldecomposition
of Pd(OAc)2 in the presence of polystyrene. The immobilization
degree of palladium was dependent on the molecular weight of
polystyrene, while the size of the Pd nanoparticles was not. Linear
polystyrene-stabilized Pd nanoparticles (PS-PdNPs) were also
prepared using NaBH4 and phenylboronic acid as reductants.
The catalytic activity of PS-PdONPs was slightly higher than that
of PS-PdNPs for Suzuki coupling reaction in water. PS-PdONPs
exhibited high catalytic activity for Suzuki and copper-free
Sonogashira coupling reactions in water and recycled without loss of activity.
Pd nanoparticles on its surface.19 Kobayashi et al. described that the
1. INTRODUCTION
palladium catalyst would be immobilized by interaction between the
π electrons of benzene rings of the polystyrene used as a polymer
backbone and the vacant orbitals of the metal.20 Encouraged by these
results, we have investigated the immobilization of Pd nanoparticles
onto polystyrene in water to develop a recyclable PdNPs catalyst
system. Consequently, we found that PdO nanoparticles have only to
The efficiency of heterogeneous catalysis in organic synthesis1
can be improved by employing nanosized catalysts because of their
extremely small size and large surface-to-volume ratio. Recently, it
has been demonstrated that palladium nanoparticles (PdNPs) as
catalysts offer significant potential for a wide range of applications
in organic synthesis.2 PdNPs aretypically formed in the presence of
a protecting agent, because they tend to precipitate or aggregate
and lose their catalytic activity. Popular protecting agents include
tetraalkylammonium salts,3 functionalized polymers,4 dendrimers,5
and ionic liquids.6 The surface properties of these PdNPs and their
catalytic activity are crucially controlled by the nature of these
stabilizers.
21
be generated by thermal decomposition of Pd(OAc)2 in the
presence of polystyrene.22 Here we report a simple method for
preparation of linear polystyrene-supported palladium nanoparticles
in water. Additionally, the catalytic activity of the resultant Pd
nanoparticles is also discussed.
The use of water instead of organic solvents, on the other hand, is
of significant interest with respect to environmentally friendly organic
synthesis.7 Several research groups have developed PdNPs catalyzed
reactions in water, including Suzuki coupling reaction,8 Stille coupling
reaction,9 Heck reaction,8aꢀd,10 Sonogashira coupling reaction,8a,b,11
and π-allylic substitution.12 However, recovery and reuse of the
catalysts was difficult to achieve in these cases due to a significant loss
of PdNPs during workup and/or morphology change (Ostwald
ripening).8k,13 In contrast, PdNPs stabilized by a hydrogel of poly
(N-isopropylacrylamide-co-poly[2-methacrylic acid 3-(bis-carboxy-
methylamino)-2-hydroxypropyl ester] (PNIPAM-co-PMACHE),14
eꢀshell poly(styrene-co-4-vinylpyridine) microspheres,15 polyanili-
ne,16 poly(amidoamine)-based dendrimer captured in microporous
polymer,17 and fluorous ligands18 have been demonstrated as easily
reusable for Suzuki, Heck, and Sonogashira coupling reactions in
water. On the other hand, Mayer et al. have reported that higher
hydrophobicity of the latex type results in improved accumulation of
2. RESULTS AND DISCUSSION
2.1. Preparation and Characterization of PS-PdONPs and
PS-PdNPs. Linear polystyrene-stabilized PdO nanoparticles (PS-
PdONPs, 1a) were prepared by thermal decomposition of Pd-
(OAc)2 in the presence of polystyrene (Mn = 6.0 ꢁ 103).23
The mixture of Pd(OAc)2 and linear polystyrene was added to
1.5 mol Lꢀ1 aqueous K2CO3 solution (Scheme 1).24 After the
3
mixture was stirred at 90 °C for 1 h, the color turned black. An
X-ray diffraction (XRD) pattern of 1a is presented in Figure 1a.
Besides the broad diffraction with 2θ ranging from 12° to 28°
ascribed to the polystyrene, five diffraction peaks assigned to
PdO (JCPDS no. 41-1107, Figure 1b) are observed clearly.
Figure 1c shows a transmission electron microscopy (TEM)
Received: March 8, 2011
Published: April 11, 2011
r
2011 American Chemical Society
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image of 1a, which has a fairly uniform particle size of 2.3 ( 0.3 nm.
ascribed to polystyrene, five other diffraction peaks were assigned to
the Pd nanoparticles (JCPDS no. 46-1043, Figure 3b). PS-PdNPs
(1a0 and 1a00) were examined by TEM, showing nanoparticles of
about 4.1 ( 0.7 nm (Figure 4a) and 1.7 ( 0.2 nm (Figure 4b),
respectively.
Inductively coupled plasma-atomic emission spectroscopy (ICP-
AES) revealed that 1a contained an average of 2.5 mmol gꢀ1 of Pd.
3
Next, the effect of polystyrene on immobilization was investigated
(Table 1). The loading of palladium decreased with an increase in
the molecular weight of polystyrene (entries 1ꢀ5). In addition, the
lowest palladium loading level was found for cross-linked polystyr-
ene (entry 6). However, the precise reason is as yet unclear, and in
fact the size of palladium nanoparticles was independent of the
molecular weight and cross-linking (Figure 2).
2.2. Suzuki Coupling Reaction in Water. To test the potency
of 1a as a catalyst, the Suzuki coupling reaction of bromobenzene
(2a) with 4-methylphenylboronic acid (3a) in 1.5 M KOH
aqueous solution was examined as a test reaction (Table 2).
The reaction proceeded efficiently at 80 °C for 1 h to give
4-methylbiphenyl (4a) in 99% yield (entry 1). More importantly,
the workup was performed under organic solvent-free conditions
where water was the only solvent used. When the reaction was
completed, the reaction mixture was filtered with hot water. The
catalyst was recovered on a filter, and the filtrate was cooled to
room temperature to give 4a quantitatively as colorless crystals in
high purity. The reusability of the catalyst was also examined. The
catalyst was recycled at least 10 times without any loss of activity.
The average yield of 4a in consecutive reactions promoted by the
first through tenth recovered catalysts was 99%. No leaching of
palladium into the reaction solutions occurred during the reac-
tion, as confirmed by ICP-AES. ICP-AES analysis also revealed
that quantitative recovery of palladium was achieved. Similar
sizes of palladium nanoparticles were observed by TEM after the
recycling experiments. However, the distribution was changed
slightly, probable due to Ostwald ripening (Figure 5).
Linear polystyrene-stabilized Pd nanoparticles (PS-PdNPs, 1a0
and 1a00) were also prepared using NaBH4 and phenylboronic acid as
reductants. An XRD pattern of PS-PdNPs is presented in Figure 3. In
addition to the broad diffraction with 2θ ranging from 12° to 30°
Scheme 1. Preparation of Polystyrene-Stabilized PdO
Nanoparticles
In contrast, the PS-PdNPs (1a0 and 1a00) exhibited catalytic
activities slightly lower than those of PS-PdONPs (entries 4
and 5), probably due to the presence of oxygen25 and size
effect.13b Neither Pd/C nor Pd/Al2O3 were effective catalysts
under these reaction conditions (entries 6 and 7). Although a
similar result was obtained with 1f, the catalytic activity
decreased in the third consecutive run as previously reported
(entries 8 and 9).8h Given these results, the PS-PdONPs were
determined to be the best catalyst for Suzuki coupling reaction
in water.
The scope of the reaction was studied using various aryl
halides and arylboronic acids (Table 3). Both the electron-rich
and electron-deficient aryl bromides were reactive, affording the
desired coupling products in high yields (entries 1ꢀ5). 2-Bro-
motoluene also underwent the Suzuki coupling reaction to give
the corresponding product in 92% yield (entry 6). It is note-
worthy that the reaction proceeded well and the catalyst could
be recycled even with an aryl chloride (entry 9). However,
the reaction of chlorobenzene (2h) with 3a gave a low yield
(entry 10).
2.3. Copper-Free Sonogashira Coupling Reaction with 1a
in Water. The palladium-catalyzed reaction of aryl halides with
terminal alkynes, known as Sonogashira coupling, is recognized
as a powerful and reliable synthetic method for the formation of
Figure 1. (a) XRD patterns of 1a; (b) JCPDS data (no. 41-1107) for PdO;
(c) TEM micrograph of 1a (scale bar = 20 nm); (d) size distribution of 1a.
Table 1. Preparation of PS-PdONPs Using Several Polystyrene
entry
polystyrene
PS-stabilized PdO nanoparticles
loading,a mmol gꢀ1
size of Pd,b nm
3
1
2
3
4
5
6
linear (Mn = 6.0 ꢁ 103)
linear (Mn = 1.8 ꢁ 104)
linear (Mn = 3.8 ꢁ 104)
linear (Mn = 9.6 ꢁ 104)
linear (Mn = 4.3 ꢁ 105)
cross-linked (2% DVB)
1a
1b
1c
1d
1e
1f
2.5
1.8
1.5
1.0
0.4
0.3
2.3 ( 0.3
2.2 ( 0.3
2.2 ( 0.3
2.1 ( 0.2
2.2 ( 0.2
2.4 ( 0.2
a Determined by ICP-AES. b Determined by TEM.
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Figure 3. (a) XRD patterns of 1a0. (b) JCPDS data (no. 46-1043) for Pd.
Figure 4. TEM micrographs and size distributions of 1a0 and 1a00 (scale
bar = 20 nm).
conditions. Representative results are summarized in Table 4.
The coupling of iodobenzene (2i) with phenylacetylene (5a)
took place smoothly in water at 80 °C in the presence of 3 equiv
of Et3N and 1.5 mol % palladium of 1a to give 6a in 99% yield
(entry 1). After the reaction, the catalyst 1a was recovered and
successively subjected to second through fifth runs of the
coupling reaction under the same conditions to afford 6a in
99%, 99%, 97%, and 98% yields, respectively (entries 2ꢀ5). ICP-
AES analysis of the aqueous phases revealed barely detectable
levels of palladium residue, although the size distribution of
nanoparticles was changed slightly (Figure 6). Both electron-rich
and electron-deficient aryl iodides were applicable, affording the
desired coupling products in excellent yields (entries 6ꢀ9).
Sterically hindered aryl iodides such as 2-iodotoluene (2n) and
1-iodonaphthalene (2o) also underwent the Sonogashira cou-
pling with 5a to give the corresponding products in 74% and 84%
yields, respectively (entries 10 and 11). The coupling of various
terminal alkynes bearing electron-withdrawing groups and elec-
tron-donating groups took place with 2i to give the correspond-
ing product in 99%, 91%, and 97% yields, respectively (entries
Figure 2. TEM micrographs and size distributions of 1bꢀ1f (scale bar =
20 nm).
substituted acetylenes.26 Poly(ethylene glycol) (PEG)-stabilized
PdNPs and in situ generated PdNPs have high catalyitc activity
for the Sonogashira coupling reaction in water under copper-free
conditions.8b,11b However, it is difficult to reuse the catalyst
because the PdNPs are dispersed in water. Recently, Cacchi et al.
have reported that perfluoro-tagged PdNPs immobilized on silica
gel was reused many times for the Sonogashira coupling reaction
in water under copper-free conditions.18b Our continuing inter-
est in the catalytic utility of PS-PdONPs led us to examine
the Sonogashira coupling in water with 1a under copper-free
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Table 2. Suzuki Coupling Reaction in Water
entry
catalyst
1a
run
yield (%)
1
2
3
4
5
6
7
S
9
first run
99a (73)b
99
tenth run
Figure 5. TEM micrographs and size distributions of 1a after recycling
experiments (scale bar = 20 nm).
first to tenth consecutive
av 99
99 (57)b
99 (49)b
61
1a0
1a00
3. CONCLUSION
Pd/C
Pd/AI2O3
1f
In summary, a highly efficient heterogeneous palladium
catalyst for Suzuki and copper-free Sonogashira coupling reac-
tions in water was prepared with a simple procedure. The
immobilization degree of palladium was dependent on the
molecular weight of polystyrene, while the size of the nanopar-
ticles was not. The polystyrene-supported PdO nanoparticles
were easily recycled with no loss of activity. ICP-AES analysis
confirmed that no palladium was leached into either the aqueous
or organic solutions and that the recovery of palladium was
achieved quantitatively. However, the size distribution of nano-
particles was changed slightly, which suggests that Ostwald
ripening occurred during the reaction.
52
first run
99
3rd run
53
a Isolated by crystallization under organic solvent-free conditions. b Yield
in parentheses refers to an experiment at 60 °C.
12ꢀ14). 1-Octyne (5e) and trimethylsilylacetylene (5f) reacted
with 2i to generate the corresponding products in good yields
(entries 15 and 16). However, the reaction of 5a with bromo-
benzene (2a) gave a low yield (entry 17).
2.4. Synthesis of Benzo[b]furan with 1a in Water. The
Sonogashira coupling reaction of o-halophenols with terminal
alkynes is an efficient procedure for the synthesis of benzo-
[b]furan derivatives. Pal et al. found that Pd/C was active for the
synthesis of benzo[b]furan derivatives in water, although this
method required a cocatalyst and phosphine ligand.27 Uozumi
et al. reported the preparation of benzo[b]furan derivatives using
amphiphilic polystyrene-PEG resin-supported Pd catalyst under
copper-free conditions although no recycling experiments was
reported.28 Ranu et al. developed a very simple method using
in situ generated PdNPs.11a However, loss of activity was observed in
recycling experiment due to agglomerization of PdNPs after
each cycle.
4. EXPERIMENTAL SECTION
4.1. Preparation of PS-PdONPs (1a). To a screw-capped vial
with a stirring bar were added 9.0 mg of polystyrene (85 μmol of styrene
unit), Pd(OAc)2 5.5 mg (25 μmol), and 1.5 M aqueous K2CO3 solution
(3 mL). After stirring at 90 °C for 1 h, the reaction mixture was filtered
with hot water. Subsequently, the polystyrene-stabilized Pd nanoparticles
were washed with hot water (5 ꢁ 1.0 mL) and acetone (5 ꢁ 1.0 mL).
4.2. Preparation of PS-PdNPs by Reduction with NaBH4
(1a0). To a screw-capped vial with a stirring bar were added 1.8 mg of
polystyrene (17 μmol of styrene unit), Pd(OAc)2 1.0 mg (5 μmol), and
water (1 mL). After adding anethanolsolution of NaBH4 (0.01 mol Lꢀ1
)
PS-PdONPs proved to function efficiently for the Sonogashira
coupling in water under copper-free conditions; therefore, our
attention was turned to the synthesis of benzo[b]furan deriva-
tives. The reaction of o-halophenol with 1.1 equiv of terminal alkyne
was performed in the presence of 1a (1.5 mol % of Pd) and 3 equiv
of Et3N in water (Table 5). The coupling of 2-iodophenol (2p)
with phenylacetylene (5a) was accomplished in 20 h to afford
2-phenylbenzofuran (7a) in 97% yield (entry 1). In addition, the
catalytic deactivation of reused 1a was not observed at all (entries
2ꢀ5). ICP-AES analysis confirmed that no leaching of Pd into
either the aqueous or organic solutions occurred. A TEM image
of the recovered catalyst revealed a fairly uniform particle size
(Figure 7). To establish the scope of the reaction, the reaction
of various o-halophenols with terminal alkynes was next exam-
ined. The reaction of 4-ethynyltoluene (5c) and 4-ethynylani-
sole (5d) bearing electron-donating groups at their para
positions gave the corresponding coupling products in 93%
and 94% yields, respectively (entries 6 and 7). 1-Octyne (5e)
and 1-ethynyl-4-fluorobenzene (5g) also underwent the reac-
tion with 2p under similar conditions to afford the correspond-
ing benzofuran in 71% and 95% yields, respectively (entries 8
and 9). 4-Hydroxy-3-iodobiphenyl (2q) reacted with 5a to
generate 2,5-diphenylbenzofuran (7f) in 90% yield (entry 10).
3
dropwise, the mixture was stirring at 25 °C for 1 h, and then the aqueous
solution was decanted. Subsequently, the polystyrene-stabilized Pd nano-
particles were washedwith water (5 ꢁ 1.0mL) and acetone (5 ꢁ 1.0 mL).
4.3. Preparation of PS-PdNPs by Reduction with Phenyl-
boronic Acid (1a00). To a screw-capped vial with a stirring bar were
added 1.8 mg of polystyrene (17 μmol of styrene unit), Pd(OAc)2 1.0 mg
(5 μmol), phenylboronic acid 9.1 mg (0.75 mmol), and water (1 mL).
After stirring at 90 °C for 1 h, the aqueous solution was decanted.
Subsequently, the polystyrene-stabilized Pd nanoparticles were washed
with water (5 ꢁ 1.0 mL) and acetone (5 ꢁ 1.0 mL).
4.4. Determination of Loading of the Palladium. 1a (2.9 mg)
was placed in a screw-capped vial, and then was added 13 M nitric acid
(5 mL). The mixture was heated at 80 °C to dissolve completely. After
cooling to room temperature, the solution was adjusted to 50 mL by
water and then the amount of Pd metal was measured by ICP-AES
analysis (15.3 ppm). The amount of Pd in 1bꢀ1f, 1a0, and 1a00 was as
follows: 1b, 11.1 ppm; 1c, 9.38 ppm; 1d, 6.12 ppm; 1e, 2.53 ppm; 1f,
1.56 ppm; 1a0, 15.5 ppm; 1a00, 15.5 ppm.
4.5. Typical Procedures for Suzuki Coupling Reaction. To a
screw-capped vial with a stirring bar were added 0.5 mmol of bromoben-
zene, 0.75 mmol of p-methylphenylboronic acid, 1a (2.9 mg, 1.5 mol % of
Pd), and 1.5 M aqueous KOH solution (1 mL). After stirring at 80 °C for
1 h, the reaction mixture was filtered with hot water. Subsequently, the
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Table 3. Suzuki Coupling Reaction of Various Aryl Halides with Arylboronic Acids in Water
a Reaction time = 12 h. b Yield in parentheses refers to a second set of experiment. c Reaction time = 20 h.
1
catalyst was washed with hot water (5 ꢁ 1.0 mL). The filtrate was cooled
to room temperature to afford crystals of 4-methylbiphenyl. The recov-
ered 1a was dried in vacuo and reused. Furthermore, the amount of Pd
metal in the recovered 1a determined by ICP-AES analysis was 15.3 ppm.
4-Methylbiphenyl (4a)8f. 1H NMR (CDCl3) δ 7.60ꢀ7.55 (m, 2 H),
7.50ꢀ7.47 (m, 2 H), 7.43ꢀ7.39 (m, 2 H), 7.33ꢀ7.29 (m, 1 H),
7.25ꢀ7.20 (m, 2 H), 2.38 (s, 3 H); 13C NMR (CDCl3) δ 141.1,
138.3, 136.9, 129.4, 128.7, 128.7, 126.9, 21.1. CAS registry number: 644-
08-6.
2,40-Dimethylbiphenyl (4f)31. H NMR (CDCl3) δ 7.25ꢀ7.20 (m,
8 H), 2.40 (s, 3 H), 2.28 (s, 3 H); 13C NMR (CDCl3) δ 141.8, 140.0,
136.4, 135.4, 130.2, 129.8, 129.0, 128.7, 127.0, 125.7, 21.27, 20.5. CAS
registry number: 611-61-0.
4-Methoxybiphenyl (4g)8f. 1H NMR (CDCl3) δ 7.58ꢀ7.51 (m, 4 H),
7.42 (t, J = 7.6 Hz, 2 H), 7.31 (t, J = 7.6 Hz, 1 H), 6.98 (d, J = 8.7 Hz, 2 H),
3.85 (s, 3 H); 13C NMR (CDCl3) δ 159.1, 140.7, 133.7, 128.6, 128.0,
126.6, 126.6, 114.2, 55.3. CAS registry number: 613-37-6.
4-Trifluoromethylbiphenyl (4h)8f. H NMR (CDCl3) δ 7.76ꢀ7.68
1
4-Acetyl-40-methylbiphenyl (4b)29. 1H NMR (CDCl3) δ 8.02 (d, J =
8.7 Hz, 2 H), 7.67 (d, J = 8.7 Hz, 2 H), 7.53 (d, J = 8.4 Hz, 2 H), 7.27
(d, J = 8.4 Hz, 2 H), 2.63 (s, 3 H), 2.41 (s, 3 H); 13C NMR (CDCl3)
δ 197.8, 145.7, 138.2, 136.9, 135.5, 129.7, 128.9, 127.1, 126.9, 26.6, 21.2.
CAS registry number: 5748-38-9.
(m, 4 H), 7.68ꢀ7.58 (m, 2 H), 7.51ꢀ7.38 (m, 3 H); 13C NMR (CDCl3)
δ 144.7, 139.7, 129.3 (q, J = 32.3 Hz), 129.0, 128.2, 127.6, 127.4, 127.3,
125.6 (q, J = 3.2 Hz), 124.3 (q, J = 272.1 Hz). CAS registry number: 398-
36-7.
4.6. Typical Procedures for Sonogashira Coupling Reac-
tion. To a screw-capped vial with a stirring bar were added iodobenzene
(52.0 mg, 0.5 mmol), phenylacetylene (56.2 mg, 0.55 mmol), 1a (2.9
mg, 1.5 mol % of Pd), triethylamine (152 mg, 1.5 mmol), and H2O
(1.0 mL). After stirring at 80 °C for 6 h, the reaction mixture was cooled
to room temperature by immediately immersing the vial in water
(∼20 °C) for about 10 min. Subsequently, the aqueous phases were
removed, and recovered catalyst was washed with H2O (5 ꢁ 3.0 mL)
and diethyl ether (5 ꢁ 3.0 mL), which were then added to the aqueous
phase. The aqueous phase was extracted eight times with diethyl ether.
The combined organic extracts were dried over MgSO4 and concentrated
under reduced pressure. The recovered 1a was dried in vacuo and reused.
The resulting product was purified by PTLC (hexane/CH2Cl2) and
analyzed by 1H NMR.
4-Methyl-40-(trifluoromethyl)biphenyl (4c)8f. 1H NMR (CDCl3)
δ 7.67 (m, 4 H), 7.50 (d, J = 8.4 Hz, 2 H), 7.28 (d, J = 8.4 Hz, 2 H),
2.41 (s, 3 H); 13C NMR (CDCl3) δ 144.6, 138.1, 136.8, 129.7, 129.2
(q, J = 32.3 Hz), 127.2, 127.1, 125.6 (q, J= 4.2 Hz), 124.3(q, J =271.7 Hz),
21.1. CAS registry number: 97067-18-0.
1
4,40-Dimethylbiphenyl (4d)30. H NMR (CDCl3) δ 7.47 (d, J =
8.1 Hz, 4 H), 7.23 (d, J = 8.1 Hz, 4 H), 2.38 (s, 6 H); 13C NMR (CDCl3)
δ 138.2, 136.7, 129.4, 126.8, 21.1. CAS registry number: 613-33-2.
4-Methoxy-40-methylbiphenyl (4e)8f. 1H NMR (CDCl3) δ 7.51
(d, J = 9.0 Hz, 2 H), 7.45 (d, J = 8.1 Hz, 2 H), 7.22 (d, J = 8.1 Hz,
2 H), 6.96 (d, J = 9.0 Hz, 2 H), 3.84 (s, 3 H), 2.38 (s, 3 H); 13C NMR
(CDCl3) δ 158.9, 137.9, 136.3, 133.7, 129.4, 127.9, 126.6, 114.2, 55.3,
21.0. CAS registry number: 53040-92-9.
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Table 4. Sonogashira Coupling Reaction of Various Aryl Halides with Alkynes in Water
a Reaction was carried out at 90 °C for 24 h.
Diphenylacetylene (6a)26c
.
1H NMR (CDCl3) δ 7.56ꢀ7.52 (m,
127.9, 123.6, 115.4, 114.0, 89.3, 88.0, 55.3. CAS registry number:
7380-78-1.
4 H), 7.36ꢀ7.33 (m, 6 H); 13C NMR (CDCl3) δ 131.2, 128.3, 128.2,
123.42, 89.3. CAS registry number: 501-65-5.
1-Phenyl-2-(o-tolyl)acetylene (6f)26c. 1H NMR (CDCl3) δ 7.56ꢀ7.48
(m, 3 H), 7.38ꢀ7.31 (m, 3 H), 7.24ꢀ7.14 (m, 3 H), 2.52 (s, 3 H); 13C
NMR (CDCl3) δ 140.2, 131.8, 131.5, 129.4, 128.3, 128.1, 125.6, 123.5,
123.0, 93.3, 88.3, 20.7. CAS registry number: 14635-91-7.
40-(Phenylethynyl)acetophenone (6b)26c. 1H NMR (CDCl3) δ 7.95
(d, J = 8.1 Hz, 2 H), 7.62 (d, J = 8.1 Hz, 2 H), 7.61ꢀ7.55 (m, 2 H),
7.38ꢀ7.35 (m, 3 H), 2.62 (s, 3 H); 13C NMR (CDCl3) δ 197.3, 136.1,
131.7, 131.7, 128.8, 128.4, 128.3, 128.2, 122.6, 92.7, 88.6, 26.6. CAS
registry number: 1942-31-0.
1-Phenyl-2-(p-trifluoromethylphenyl)acetylene (6c)26c 1H NMR
.
(CDCl3) δ 7.65ꢀ7.54 (m, 6 H), 7.39ꢀ7.35 (m, 3 H); 13C NMR
(CDCl3) δ 131.8, 131.7, 129.9 (q, J = 34 Hz), 128.8, 128.4, 127.1,
125.7, 125.3 (q, J = 3.2 Hz), 122.5, 91.7, 87.9. CAS registry number:
370-99-0.
1-Phenyl-2-(p-tolyl)acetylene (6d)26c. 1H NMR (CDCl3) δ 7.54ꢀ7.49
(m, 2 H), 7.43 (d, J= 8.1 Hz, 2 H), 7.37ꢀ7.30 (m, 3 H), 7.14 (d, J =8.1 Hz,
2 H), 2.36 (s, 3 H); 13C NMR (CDCl3) δ 138.4, 131.5, 131.4, 129.1,
128.3, 128.0, 123.4, 120.1, 89.5, 88.7, 21.5. CAS registry number:
185817-85-0.
1-(p-Methoxyphenyl)-2-phenyl acetylene (6e)26c. 1H NMR (CDCl3)
δ 7.53ꢀ7.45 (m, 4 H), 7.37ꢀ7.26 (m, 3 H), 6.88 (d, J = 9.0 Hz, 2 H),
3.82 (s, 3 H); 13C NMR (CDCl3) δ 159.6, 133.0, 131.4, 128.3,
Figure 6. TEM micrographs and size distributions of 1a after Sonoga-
shira coupling reaction (scale bar = 20 nm).
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Table 5. Synthesis of Benzo[b]furans in Water
1-(1-Naphthyl)-2-phenyl acetylene (6g)26c. 1H NMR (CDCl3) δ 8.45
(d, J = 7.5 Hz, 1 H), 7.85 (t, J = 8.1 Hz, 2 H), 7.77 (dd, J = 7.2 Hz, 1.2 Hz,
1 H), 7.67ꢀ7.63 (m, 2 H), 7.60 (dt, J = 6.9 Hz, 1.5 Hz, 1 H), 7.53 (dt, J =
6.9 Hz, 1.5 Hz, 1 H), 7.45 (dd, J = 8.4 Hz, 7.2 Hz, 1 H), 7.43ꢀ7.35
(m, 3 H); 13C NMR (CDCl3) δ 133.3, 133.2, 131.7, 130.4, 128.8, 128.4,
128.3, 128.2, 126.8, 126.4, 126.2, 125.3, 123.4, 120.9, 94.3, 87.5. CAS
registry number: 4044-57-9.
1-Phenyl-1-octyne (6h)26d. 1H NMR (CDCl3) δ 7.41ꢀ7.37 (m, 2 H),
7.29ꢀ7.25 (m, 3 H), 2.40 (t, J = 7.2 Hz, 2 H), 1.75ꢀ1.29 (m, 8 H), 0.91
(t, J = 7.2 Hz, 3 H); 13C NMR (CDCl3) δ 131.5, 128.1, 127.4, 124.1,
90.5, 80.5, 31.4, 28.7, 28.6, 22.6, 19.4, 14.1. CAS registry number:
16967-02-5.
Figure 7. TEM micrographs and size distributions of 1a after the
synthesis of 2-phenylbenzofuran (scale bar = 20 nm).
1-Phenyl-2-(trimethylsilyl)acetylene (6i)32. 1H NMR (CDCl3) δ
7.51ꢀ7.45 (m, 2 H), 7.33ꢀ7.28 (m, 3 H), 0.25 (s, 9 H); 13C NMR
(CDCl3) δ 131.9, 128.5, 128.2, 123.1, 105.1, 94.1, ꢀ0.03. CAS registry
number: 13735-81-4.
1 H), 7.03 (s, 1 H); 13C NMR (CDCl3) δ 155.9, 154.8, 132.5, 130.4,
129.2, 128.8, 128.5, 128.4, 124.9, 124.2, 122.9, 120.9, 111.2, 101.3. CAS
registry number: 1839-72-1.
4.7. Typical Procedures for Synthesis of Benzo[b]furan. To
a screw-capped vial with a stirring bar were added 2-iodophenol (110 mg,
0.5 mmol), phenylacetylene (56.2 mg, 0.55 mmol), 1a (2.9 mg, 1.5 mol %
of Pd), triehtylamine (152 mg, 1.5 mmol), and H2O (1.0 mL). After
stirring at 80 °C for 20 h, the reaction mixture was cooled to room
temperature by immediately immersing the vial in water (∼20 °C) for
about 10 min. Subsequently, the aqueous phases were removed, and
recovered catalyst was washed with H2O (5 ꢁ 3.0 mL) and diethyl ether
(5 ꢁ 3.0 mL), which were then added to the aqueous phase. The aqueous
phase was extracted eight times with diethyl ether. The combined organic
extracts were dried over MgSO4 and concentrated under reduced
pressure. The recovered 1a was dried in vacuo and reused. The resulting
product was purified by PTLC (hexane/CH2Cl2) and analyzed by
1H NMR.
1
2-(4-Methylphenyl)benzofuran (7b)33. H NMR (CDCl3) δ 7.76
(d, J = 8.4 Hz, 2 H), 7.55 (d, J = 8.1 Hz, 1 H), 7.49 (d, J = 8.1 Hz, 1 H),
7.28ꢀ7.17 (m, 4 H), 6.97 (s, 1H), 2.40 (s, 3 H); 13C NMR (CDCl3)
δ 156.1, 154.7, 138.6, 129.5, 129.3, 127.7, 124.8, 124.0, 122.8, 120.7,
111.1, 100.5, 21.4. CAS registry number: 25664-48-6.
2-(4-Methoxyphenyl)benzofuran (7c)33. 1H NMR (CDCl3) δ 7.80
(d, J = 9.0 Hz, 2 H), 7.57ꢀ7.48 (m, 2 H), 7.28ꢀ7.18 (m, 2 H), 6.98
(d, J = 9.0 Hz, 2 H), 6.89 (s, 1 H), 3.86 (s, 3 H); 13C NMR (CDCl3)
δ 159.9, 156.0, 154.7, 129.5, 126.4, 123.7, 123.3, 122.8, 120.5, 114.2,
111.0, 99.6, 55.3. CAS registry number: 19234-04-9.
2-Hexylbenzofuran (7d)34. 1H NMR (CDCl3) δ 7.51ꢀ7.39 (m, 2 H),
7.23ꢀ7.14 (m, 2 H), 6.37ꢀ6.36 (m, 1 H), 2.76 (t, J = 7.5 Hz, 2 H),
1.74 (tt, J = 7.5 Hz, 7.5 Hz, 2 H), 1.45ꢀ1.22 (m, 6 H), 0.92ꢀ0.86
(m, 3 H); 13C NMR (CDCl3) δ 159.8, 154.6, 129.0, 123.0, 122.3, 120.1,
110.7, 101.7, 31.6, 28.9, 28.4, 27.6, 22.6, 14.1. CAS registry number:
39195-67-0.
2-Phenylbenzofuran (7a)33. 1H NMR (CDCl3) δ 7.87 (d, J = 7.2 Hz,
2 H), 7.58 (d, J = 7.6 Hz, 1 H), 7.52 (d, J = 7.6 Hz, 1 H), 7.45 (t, J = 7.2 Hz,
2 H), 7.33 (t, J = 7.2 Hz, 1 H), 7.28 (t, J = 7.2 Hz, 1 H), 7.23 (t, J = 7.2 Hz,
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2-(4-Fluorophenyl)benzofuran (7e)33. 1H NMR (CDCl3) δ 7.87ꢀ7.81
(m, 2 H), 7.60ꢀ7.56 (m, 1 H), 7.53ꢀ7.50 (m, 1 H), 7.32ꢀ7.21 (m, 2 H),
7.17ꢀ7.11 (m, 2 H), 6.98 (s, 1 H); 13C NMR (CDCl3) δ 164.5, 161.2,
154.8, 126.8, 126.7, 124.3, 123.0, 120.9, 116.0, 115.7, 111.1, 101.0. CAS
registry number: 69976-38-1.
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2,5-Diphenylbenzofuran (7f)35. 1H NMR (CDCl3) δ 7.88 (d, J = 7.5 Hz,
2 H), 7.78ꢀ7.75 (m, 1 H), 7.65ꢀ7.33 (m, 10 H), 7.06 (s, 1 H); 13C NMR
(CDCl3) δ 154.5, 141.6, 136.6, 130.4, 129.7, 128.8, 128.7, 128.6, 128.5,
127.4, 126.9, 124.9, 124.0, 119.4, 111.3, 101.5. CAS registry number: 121045-
39-4.
’ ASSOCIATED CONTENT
1
Supporting Information. Copies of H NMR and 13C
S
b
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Tel: þ81-6-6954-4729. Fax: þ81-6-6957-2135. E-mail: otaka@
chem.oit.ac.jp; nomura@chem.oit.ac.jp.
’ ACKNOWLEDGMENT
We are grateful to the Nanomaterials and Microdevices
Research Center (NMRC) of OIT for financial and instrumental
support. This work was supported by the Joint Studies Program
(2010) of the Institute for Molecular Science. We wish to thank
Prof. Uozumi for general discussions. We also thank Dr. G.
Hamasaka, Dr. T. Shinagawa, and Dr. Y. Kashiwagi for assistance
with TEM, ICP-AES, and XRD measurements.
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