Effective heterogeneous catalyst for suzuki-miyaura cross-coupling in aqueous media: Melamine cyanurate complex containing Pd species

Article abstract of DOI:10.1246/bcsj.20170296

A melamine cyanurate complex catalyst containing Pd(II) ions (denoted Pd/M-CA) was effective for Suzuki-Miyaura cross-coupling reactions in environmentally benign aqueous media at room temperature. The reaction conditions, such as the type of Pd species in Pd/M-CA, loading amounts of Pd, solvent, and substrate concentration, were investigated for optimization. In the presence of Pd/M-CA under the optimal conditions, cross-coupling reactions of a wide range of structurally diverse aryl halides and arylboronic acids containing functional groups proceeded smoothly to provide the corresponding products in high yields. In addition, the Pd/M-CA could be reused at least 5 times while maintaining high yields. The reduction of Pd(II) ions in Pd/M-CA to Pd(0) by NaBH4 enhanced the catalytic activity to provide a high turnover number (TON) of 17600 and turnover frequency (TOF) of 880 h-1.

Full text of DOI:10.1246/bcsj.20170296

Advance Publication Cover Page
Effective Heterogeneous Catalyst for Suzuki-Miyaura Cross-Coupling in Aqueous
Media: Melamine Cyanurate Complex Containing Pd Species
Daisuke Nagai,* and Hiroki Goto
Advance Publication on the web November 1, 2017
doi:10.1246/bcsj.20170296
©
2017 The Chemical Society of Japan

Effective Heterogeneous Catalyst for Suzuki-Miyaura Cross-Coupling in Aqueous
Media: Melamine Cyanurate Complex Containing Pd Species
1
1
Daisuke Nagai,* and Hiroki Goto
1Division of Molecular Science, Faculty of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-
8
515, Japan
E-mail: < daisukenagai@gunma-u.ac.jp >
Daisuke Nagai received Ph.D. Degree from Tokyo Institute of Technology in 2002 under Professor Takeshi Endo. He
worked as a postdoctoral fellow in Yamagata University under Professor Takeshi Endo from 2002 to 2005. He was
appointed to Assistant Professor in Gunma University under Professor Takeshi Yamanobe in 2005. His current research
interests are polymer chemistry, metal recovery materials, and organic-inorganic hybrid materials.
Abstract
complex results in precipitation of a metal/M-CA complex,
A melamine cyanurate complex catalyst containing Pd(II)
which can be separated by filtration. The system operates
efficiently, providing the third largest Pd(II) recovery amount
(0.596 gPd/gM-CA) among reported adsorbents.
ions (denoted Pd/M-CA) was effective for Suzuki-Miyaura
cross-coupling reactions in environmentally benign aqueous
media at room temperature. The reaction conditions, such as the
type of Pd species in Pd/M-CA, loading amounts of Pd, solvent,
and substrate concentration, were investigated for optimization.
In the presence of Pd/M-CA under the optimal conditions, cross-
coupling reactions of a wide range of structurally diverse aryl
halides and arylboronic acids containing functional groups
proceeded smoothly to provide the corresponding products in
high yields. In addition, the Pd/M-CA could be reused at least 5
times while maintaining high yields. The reduction of Pd(II) ions
1
8,19
To isolate the metals from metal-recovered materials, the
organic components generally need to be burned. However, this
requires energy and can produce environmental pollutants, and
it also produces low-purity metals due to carbonization.
Therefore, the development of effective utilization of metal-
recovered materials is important from the viewpoints of
environment and material sciences. The M-CA complex was
selected as a candidate for an effective heterogeneous catalyst
because the M-CA system coordinates to the metal ions
homogeneously, which means that the metal ions should be
uniformly dispersed in the M-CA complex. In addition, the M-
CA complex contains multiple hydrogen bonds, in contrast to
heterogeneous catalysts formed by covalent bonds. The
interatomic distance between M and CA is approximately 2.85-
2.96 Å, which is 2 times greater than the general carbon-carbon
in Pd/M-CA to Pd(0) by NaBH
4
enhanced the catalytic activity
to provide a high turnover number (TON) of 17600 and turnover
-
1
frequency (TOF) of 880 h .
1
. Introduction
The Suzuki-Miyaura cross-coupling reaction is an
important carbon-carbon bond-forming reaction in organic
1
7
covalent bond length (1.54 Å). Seiche et al. reported that a
rhodium complex with a bidentate ligand, prepared by self-
assembly through hydrogen bonding of the two monodentate
ligands, catalyzes hydroformylation of functional terminal
alkenes with greater catalytic activity and regioselectivity than
the rhodium complex with bidentate ligands containing covalent
1
-7
8
synthesis. Its broad use for synthesis of natural products,
9
10
11
pharmaceuticls, agrochemicals, and polymers demonstrates
its broad impact on the chemical sciences. A transition metal
such as Pd often is used as a catalyst because of its efficiency.
However, homogeneous catalytic systems usually require
cumbersome and costly workup procedures to obtain pure
products, and the recovery of precious and toxic metals is time-
consuming. Therefore, development of metal-immobilized
heterogeneous catalysts is desirable because they can be
separated easily by filtration and reused in subsequent
2
0
bonds.
They demonstrated through molecular orbital
calculations that the flexible hydrogen bonds of the ligands
2
1
promoted the catalytic activity and regioselectivity. In addition,
M-CA is easily prepared through simple mixing of M and CA
and metal ions, eliminating costly and troublesome procedures.
These results prompted the use of Pd(II)-recovered M-CA
(Pd/M-CA) as a heterogeneous catalyst for effective utilization
of metal-recovered material. The present article describes the
Pd/M-CA-catalyzed Suzuki-Miyaura coupling of various
substrates in environmentally benign aqueous media at room
temperature.
1
2-14
reactions.
These advantages make heterogeneous catalysts
suitable for continuous flow systems in large-scale industrial
syntheses. However, heterogeneous catalysts must be prepared
by heterogeneous coordination of the supports to the metal ions,
causing aggregation of metal ions, which reduces catalytic
activity.
Investigation into recovery of precious metals (e.g., Pd, Au,
Pt) using organic materials led to the development of highly
efficient and selective processes for precious metal recovery
based on the complexation of melamine (M) with cyanuric acid
2. Experimental
Materials. Melamine (M, Tokyo Kasei Kogyo, >98.0%)
and cyanuric acid (CA, Tokyo Kasei Kogyo, >98.0%) were
commercially available and used as received. Sodium
1
5
(CA). Melamine is a water-soluble ligand that associates with
CA through hydrogen bonding, affording a 1:1 supramolecular
tetrachloropalladate (II) (Na
>98.0%), palladium acetate (II) [Pd(OAc)
2
PdCl
4
, Tokyo Kasei Kogyo,
, Wako Pure
1
6,17
complex (melamine cyanurate, M-CA).
The addition of M to
2
an aqueous solution of metal ions results in rapid and highly
efficient homogenous coordination between M and metal ions.
After coordination, subsequent addition of CA to the metal/M
Chemical, 46.5–49.5%], tris(dibenzylideneacetone)dipalladium
(0) (ALDRICH, 97.0%) were purchased commercially and used
as received. Iodobenzene (Tokyo Kasei Kogyo, >98.0%), 1-

iodo-4-nitrobenzene (Tokyo Kasei Kogyo, >99.0%), 4-
iodoanisole (Tokyo Kasei Kogyo, >99.0%), bromobenzene
aqueous cyanuric acid (CA, 20 mL) to the mixture resulted in
instant precipitation. The precipitate was separated by
filtration, and the concentration of Pd(II) in the filtrate was
measured by atomic absorption spectrometry (AAS),
resulting in a load amount of 80 mgPd/gM-CA. Elemental
analysis of the complex showed that the ratio of M to CA was
1:1. Based on these results and the amount of Pd(II) loaded,
as determined by AAS, the ratio of Pd/M/CA was 1/5/5. To
examine the coordination site, IR spectra of Pd/M-CA with
(Tokyo Kasei Kogyo, >99.0%), 4-bromonitrobenzene (Wako
Pure Chemical, 98.0%), and 4-bromotoluene (Wako Pure
Chemical, 98.0%) were purchased commercially and used as
received. Phenylboronic acid (Tokyo Kasei Kogyo, >97.0%), 4-
tolylboronic acid (Aldrich, 97.0%), 2-tolylboronic acid (Aldrich,
9
7.0%), and 4-(trifluoromethyl)phenylboronic acid (Tokyo
Kasei Kogyo, >97.0%) were purchased commercially and used
as received. Sodium acetate (Wako Pure Chemical, >98.5%),
potassium acetate (Wako Pure Chemical, >97.0%), sodium
hydrogen carbonate (Wako Pure Chemical, >98.5%), potassium
hydrogen carbonate (Wako Pure Chemical, >99.5%), sodium
carbonate (Kanto Chemical, >99.8%), potassium carbonate
different loading amounts (48 mgPd/gM-CA; 81 mgPd/M-CA; 122
mgPd/gM-CA) were obtained (see Supporting Information,
Table S1). The absorption peak corresponding to C=N (1537
-
1
cm ) shifted to
a higher wavenumber, while those
-1
-1
corresponding to C=O (1782 cm ) and NH
2
(1741 cm ) did
(
>
Kanto Chemical, >99.5%), cesium carbonate (Kanto Chemical,
98.0%), sodium hydroxide (Wako Pure Chemical, >97.0%),
not change position significantly. These results indicate that
the imine group in M coordinated predominantly to Pd(II).
This tendency is consistent with the nucleophilicity of the
imine group in M, which is greater than those of the primary
amine group in M and the carbonyl group in CA. The XPS
analysis of the Pd/M-CA complex showed Pd3d5/2 and
and potassium hydroxide (Wako Pure Chemical, >85.0%) were
purchased commercially available and used as received.
Undecane (Tokyo Kasei Kogyo, >99.5%) was purchased
commercially and used as received. Ethanol (Wako Pure
Chemical, 99.5%) was distilled before use.
Pd3d3/2 peaks at 337.7 and 343.0 eV, respectively, which
were assigned to Pd(II) species (Figure 1a).2
2,23
The
Instruments.
Gas chromatographic analysis was
conducted using a Shimadzu GC-2014ATF/SPL instrument
equipped with an ULBON-HR-1 capillary column and flame
ionization detector (FID), with helium as the carrier gas. Flame
atomic absorption spectrometry was conducted using a Hitachi
polarized Zeeman atomic absorption spectrometer (AAS) Z-
EDX/SEM analysis showed the presence of Pd and Cl species
(Figure 1b). The Cl species originated from Na PdCl
therefore, Pd(II)Cl is contained in the complex. The SEM
2
4
;
2
images showed a needle-type structure, suggesting a high
surface area (Fig. 1c). In addition, EDX/SEM mapping
images showed that the Pd species (red dots) were uniformly
dispersed on the M-CA, indicating the potential high catalytic
activity of Pd/M-CA (Fig. 1d). Therefore, we examined the
utility of Pd/M-CA as a heterogeneous catalyst for Suzuki-
Miyaura cross-coupling, which is a favorable and versatile
method for carbon-carbon bond formation.
2
310. X-ray photoelectron spectroscopy (XPS) spectra were
obtained using a Kratos AXIS-NOVA instrument. Scanning
electron microscopy (SEM) was performed using a Hitachi
S3000N instrument at an acceleration voltage of 1.5 kV. Energy
dispersive X-ray analysis (EDX/SEM) measurements were
obtained with a Horiba EX-200K/Hitachi S3000N instrument.
Preparation of Pd/M-CA. Melamine (23.8 mg, 0.189
mmol) was added to a 0.200 mM aqueous solution of sodium
(a)
(b)
2 4
tetrachloropalladate(II) (Na PdCl , 400 mL), and the mixture
Pd3d5/2
Pd3d3/2
was stirred at room temperature for 1 h. Subsequent addition
of a 10.4 mM aqueous solution of cyanuric acid (20.0 mL) to
the mixture resulted in instant precipitation. The precipitate
was separated by filtration, and the concentration of Pd(II) in
the filtrate was measured by atomic absorption spectrometry
N
O
Cl
ClPd
Pd
Cl
PdII
PdII
Pd
(
AAS). The loading amount was calculated based on the
following equation:
Loading amount (gPd/gM-CA) = atomic weight (g/mol) of
3
50 348 346 344 342 340 338 336 334 332 330
0
1
2
3
4
5
6
binding energy (eV)
keV
(c)
(d)
metal × loading amount (mol) / weight of melamine and
cyanuric acid used (g)
Anal. Found for Pd/M-CA: C, 27.99; H, 3.73; N, 48.43
Suzuki-Miyaura cross-coupling reaction catalyzed
by Pd/M-CA complex (typical procedure). Pd/M-CA
(
(
H
7.10 mg, 0.500 mol% to iodobenzene), phenylboronic acid
146 mg, 1.20 mmol), K CO (415 mg, 3.00 mmol), 1/1 (v/v)
O/ethanol solvent (3.00 mL), iodobenzene (0.110 mL, 1.00
10m
10m
2
3
2
mmol), and undecane (0.300 mL) were added to a test tube
under nitrogen atmosphere. After stirring for 20 h at room
temperature, the Pd/M-CA was separated by filtration, and
diethyl ether (30 mL) was added to the filtrate. The filtrate
was washed with water (10 mL) three times. The organic
phase was analyzed by GC (internal standard; undecane),
which indicated an 80% yield of the product.
Figure 1. (a) XPS spectrum, (b) EDX spectrum, (c) SEM
image, and (d) EDX/SEM mapping of Pd (red dos) of Pd/M-
CA complex (loading amount = 81 mgPd/gM-CA).
Suzuki-Miyaura cross-coupling reaction catalyzed
with Pd/M-CA complex catalyst. To examine the efficacy
of the Pd/M-CA catalysts, the Suzuki-Miyaura cross-
coupling reaction of iodobenzene and phenylboronic acid
catalyzed with Pd/M-CA (0.5 mol% of Pd to iodobenzene)
3
. Results and Discussion
Preparation of Pd/M-CA complex. The Pd/M-CA
catalyst was initially prepared by adding melamine (M, 0.189
was carried out in the presence of K
reaction, the catalyst was separated by filtration and the yield
was monitored by GC because the loss of workup is overlooked
which gave biphenyl in 43% yield (GC yield) (Scheme 1,
2 3 2
CO in H O/EtOH. After
mmol)
tetrachloropalladate(II) (Na
stirred at room temperature. Subsequent addition of 10.4 mM
to
an
aqueous
solution
of
sodium
,
2
PdCl
4
, 400 mL), and the mixture

Table 1, entry 1). Investigation of other reaction conditions,
such as the type of palladium species in the M-CA complex,
loading amounts of Pd, the base selected, and solvent used,
was carried out to determine the optimal conditions.
strong base such as NaOH and KOH, nucleophilic addition
of hydroxide ion to phenylboronic acid also occurred,
resulting in the formation of the phenylborate that was
unreactive in transmetalation25 (reaction mechanism for
Suzuki-Miyaura coupling is shown in Scheme S1). The
At first, the catalytic activity of Pd/M-CA was compared
with that of M-CA containing Pd(OAc)
2
, which is widely
2 3
base K CO was chosen for the present system (entry 6).
used as a catalyst for the Suzuki-Miyaura reaction.1 The
-7
reaction of iodobenzene and phenylboronic acid catalyzed by
Table 2. Effect of base on Suzuki-Miyaura reaction.a)
Pd(OAc)
but gave a lower yield than that catalyzed by Pd/M-CA (entry
). Examination of the Pd(OAc) /M-CA complex using SEM
2
/M-CA was conducted under the above conditions,
b)
base yield (%)b)
2
2
entry
base
yield (%)
entry
revealed that the complex was spherical (Fig. S1), whereas
Pd/M-CA was needle-shaped. These results indicate that the
1
2
3
4
5
NaOAc
KOAc
23
21
46
50
31
6
7
8
9
K
2
CO
3
66
36
20
16
surface area of Pd(OAc)
2
/M-CA is smaller than that of Pd/M-
CA, resulting in the lower yield.24 Next, the effect of loading
Cs
2
CO
3
amount of Pd in Na PdCl /M-CA was examined. The yield of
2
4
NaHCO
KHCO
Na CO3
3
NaOH
KOH
the product increased with loading amount (entries 3-6). A
decrease in loading amount increased the amounts of M and
CA in the Pd/M-CA complex, which disrupted progress of the
reaction. From this table, a loading amount of 81 mgPd/gM-CA
was selected for the subsequent experiments.
3
2
a)Conditions: [iodobenzene]
Pd/M-CA = 0.5 mol% of Pd to iodobenzene; [K
= 0.33 M; [phenylboronic acid]
= 0.36 M;
3 0
] = 1.0 M;
0
0
2
CO
H
2
O/EtOH (v/v = 3/2); room temperature; reaction time = 20 h. b)
Scheme 1
Determined by GC analysis (internal standard: undecane).
Pd/M-CA
Finally, the effects of the H
substrate concentration were examined (Table 3). When the
reaction was conducted at various H O/EtOH mixing ratios
entries 1-7), a ratio of 1/1 (v/v) H O/EtOH gave the best
results (entry 4) because H O is a good solvent for K CO and
EtOH promoted solubility of the substrates and the fast
2
O/EtOH ratio and reaction
I
(0.5 mol% of Pd)
+
(
HO) B
base, H2O/EtOH
rt, 20 h
2
2
(
2
2
2
3
Table 1. Effect of Pd species type and Pd loading
amount on Suzuki-Miyaura reaction.
reduction of Pd(II) to Pd(0).2
6-28
Reaction with 1/1 (v/v)
O/EtOH at various concentrations of the substrate (entries
and 8-10), revealed that the best results were obtained when
a)
H
4
2
loading amount
yield (%)b)
the substrate concentration was 0.33 M (entry 4). Therefore,
the optimal conditions were: loading amount on Pd/M-CA:
entry
Pd species
(mgPd/gM-CA)
8
1 mgPd/gM-CA; base: K
2 3 2
CO ; H O/EtOH (v/v) = 1/1;
1
2
3
4
5
6
Na
2
PdCl
4
46
35
43
28
37
43
66
66
concentration of substrate: 0.33 M.
Pd(OAc)
2
Table 3. Effect of H
2
Oa/)EtOH (v/v) mixing ratio and
Na
Na
Na
Na
2
2
2
2
PdCl
PdCl
PdCl
PdCl
4
4
4
4
20
substrate concentration.
46
yield (%)b)
entry
2 0
H O/EtOH (v/v) [iodobenzene]
81
1
2
3
4
5
6
7
8
9
H
2
O
0.33
0.33
0.33
0.33
0.33
0.33
0.33
1.0
44
66
66
80
74
77
47
49
74
49
121
4/1
3/2
a)Conditions: [iodobenzene]
Pd/M-CA = 0.5 mol% of Pd to iodobenzene; [K
H
2
O/EtOH (v/v = 3/2); room temperature; reaction time = 20 h. b)
= 0.33 M; [phenylboronic acid]
= 0.36 M;
3 0
] = 1.0 M;
0
0
2
CO
1/1
Determined by GC analysis (internal standard: undecane).
2/3
Next, an appropriate base was identified for the Pd/M-
CA system for the Suzuki-Miyaura cross-coupling reaction
1/4
(Table 2). Nine widely available bases that have been used
in Suzuki-Miyaura cross-coupling were considered.
Reactions of iodobenzene and phenylboronic acid in the
presence of weak bases, such as NaOAc and KOAc, and in
the presence of strong bases, such as NaOH and KOH, gave
low product yields (entries 1, 2 and 8, 9, respectively). In
the Suzuki-Miyaura reaction, the base acts as an accelerator
of transmetalation with phenylboronic acid by nucleophilic
addition of the base to the Pd(II)-aryl halide complex. Since
NaOAc and KOAc are weak bases, nucleophilic addition of
acetate ion did not proceed effectively. In the presence of a
EtOH
1/1
1/1
0.50
0.25
1
0
1/1
Conditions: [phenylboronic acid]
0
= 1.1 equivalent to [iodobenzene]
0
;
Pd/M-CA = 0.5 mol% of Pd to iodobenzene; [K
2
CO = 1.0 M; room
3 0
]

temperature; reaction time = 20 h. b) Determined by GC analysis (internal
standard: undecane).
The results also demonstrated that the Pd/M-CA
catalytic system could tolerate arylboronic acids with various
substituents (Scheme 3, Table 5). Reaction of iodobenzene
and 4-tolylboronic acid under similar conditions gave the
product in quantitative yield (entry 1). In contrast,
arylboronic acid containing an electron-withdrawing group (-
To expand the scope of the reaction substrate, various
aryl halides and arylboronic acids were used for Suzuki-
Miyaura coupling. The results are summarized in Tables 4
and 5. The effect of the substitution group for aryl iodide was
summarized in Table 4. Reactions of aryl iodides containing
electron-withdrawing and -donating groups, such as -OMe
3
CF ) exhibited lower reactivity (entry 2) because the electron
density of the carbon bonded with boron decreased, hindering
progress of nucleophilic addition of the carbon to Pd(II)
during the transmetalation. Increasing the reaction
temperature also was effective (entry 3). Substitution of
methyl groups at the ortho position of arylboronic acid did
not disrupt the reaction progress (entry 4).
2
and -NO , with phenylboronic acid yielded the corresponding
products in high yields (entries 2 and 3). When aryl bromides
were used instead of aryl iodides, product yield decreased
because the halogen-carbon bond energy of bromobenzene
was stronger than that of iodobenzene, hindering progress of
oxidative addition of Pd(0) to bromobenzene (entries 4-6).
Contrary to expectation, the yield of the reaction of aryl
bromide containing electron-donating group (-OMe) is higher
than that of aryl bromide having electron-withdrawing group
Scheme 3
Pd/M-CA
X
X
I
(1.2 mol% of Pd)
+
(
2
-NO ) (entries 5 and 6). This is due to that 4-
(
HO)2B
K CO 20 h
2
3,
bromonitrobenzene is insoluble in EtOH, resulting in the
heterogeneous progress of reaction. Although the reaction
H2O/EtOH v/v = 1/1)
2
was conducted in 1/1 (v/v) H O/THF homogeneous system,
the product was not obtained, suggesting the hindering of
reduction of Pd(II) to Pd(0) without EtOH. Increasing the
reaction temperature to 80 °C produced the greatest reactivity
of the all reagents and the coupling products were obtained in
high yields (entries 8-10).
Table 5. Suzuki-Miyaura reaction of iodobenzene and
a)
various boronic acids catalyzed with Pd/M-CA.
entry
X
T (°C)
yield (%)b)
quant.
15
1
2
3
4-Me
rt
rt
Scheme 2
4-CF
4-CF
3
Pd/M-CA
X
+
K2CO3
(
HO)2B
3
80
quant.
R
H2O/EtOH (v/v = 1/1)
R
4
2-Me
rt
quant.
Table 4. Suzuki-Miyaura reaction of aromatic halides
and phenylboronic acid catalyzed with Pd/M-CA.
Conditions: [iodobenzene]
0
= 0.33 M; [phenylboronic acid] = 0.36 M;
0
a)
Pd/M-CA = 1.2 mol% of Pd to iodobenzene; [K
H
2
O/EtOH (v/v = 1/1); reaction time = 20 h. b) Determined by GC analysis
2
CO ] = 1.0 M;
3 0
b)
(internal standard: undecane).
entry
X
I
R
H
Pd (mol%) T (°C) time (h) yield (%)
In the reaction of iodobenzene and phenylboronic acid,
a control experiment was conducted to confirm that the
insoluble catalyst promotes the reaction under heterogeneous
conditions. The reaction was performed under identical
2
1
2
3
4
5
6
7
8
9
1.2
1.2
1.2
2.5
2.5
2.5
2.5
2.5
2.5
2.5
rt
rt
rt
rt
rt
rt
rt
20
20
20
72
72
72
72
80
80
80
98
92
I
OMe
I
NO
2
quant.
76
conditions as that in Table 3, entry 4 [1/1 (v/v) H O/EtOH,
0
.33 M iodobenzene, 0.36 M phenylboronic acid, 0.5 mol%
Br
Br
Br
Br
Br
Br
Br
H
of Pd to iodobenzene in Pd/M-CA, 1.0 M K CO , rt, 20 h
reaction time]. After 1 h reaction (63% yield of product), the
catalyst was subjected to membrane filtration (pore size =
2
3
Me
50
0
.45 m), and the filtrate stirred for additional 20 h. Gas
NO
NO
H
2
34
chromatography showed that reaction did not proceed further,
indicating that the reaction proceeded under heterogeneous
conditions. In addition, AAS showed that only 0.05% of the
original catalyst loading leached. To investigate the economy
of the reaction, the reusability of Pd/M-CA was examined for
Suzuki-Miyaura coupling of iodobenzene and phenylboronic
c)
2
0
d)
80
80
80
quant.
quant.
d)
d)
Me
NO
acid in 3/2 (v/v) H
2
O/EtOH at room temperature for 20 h
1
0
2
86
(Figure 2). After the first reaction, which gave 97% yield of
Conditions: [iodobenzene]
0
= 0.33 M; [phenylboronic acid]
0
= 0.36 M;
the corresponding product, Pd/M-CA was recovered by
filtration. The recovered Pd/M-CA was washed successively
with tetrahydrofuran, water, and methanol, and dried in vacuo.
The catalyst was subjected to 5 more reaction cycles under
similar conditions, which gave the product in high yields,
O/EtOH (v/v = 1/1). b) Determined by GC analysis
[
K
2
CO
3 0 2
] = 1.0 M; H
(
internal standard: undecane). C)
H
2
O/THF (v/v = 1/1) was used as a
solvent. d)80 °C is the temperature of oil bath.

indicating the reusability of Pd/M-CA. The slight decrease in
the yield was due to loss of catalyst during workup.
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7
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.
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0
0
1
2
3
4
5
6
1
2
3
4
5
6
cycle number
2
017, 58, 2236.
cycle
P. Pomanaranski, S. Samanta, P. Roszkowski, J. K. Maurin,
Figure 2. Reusability of Pd/M-CA in Suzuki-Miyaura
Z. Czarnocki, Tetrahedron Lett. 2016, 57, 4713.
reaction of iodobenzene and phenyboronic acid. Conditions:
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[
iodobenzene]
Pd/M-CA = 0.5 mol% of Pd to iodobenzene; [K
M; H O/EtOH (v/v = 1/1); room temperature; reaction time =
0 h. Determined by GC analysis (internal standard:
undecane).
0
= 0.33 M; [phenylboronic acid]
0
= 0.36 M;
2
CO = 1.0
3
]
0
2
a)
2
1
4. Z. Dong, Z. Ye, Reusable, Adv. Synth. Catal. 2014, 356,
To further increase the catalytic activity of Pd/M-CA,
the Pd(II) in Pd/M-CA was reduced to Pd(0) by NaBH
because Pd(0) is the catalytically active species in the Suzuki-
Miyaura cross-coupling reaction. The Pd/M-CA complexes
3401.
4
,
15. D. Nagai, T. Kuribayashi, H. Tanaka, H. Morinaga, H.
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Cryst. Eng. Chem. 2013, 15, 5838.
4
were added to a dispersed aqueous solution of NaBH (4
equivalents to Pd) and stirred at room temperature. After
reduction, the reduced Pd/M-CA [Pd(0)/M-CA] was
separated by filtration and dried in vacuo. The XPS analysis
confirmed the reduction of Pd(II) to Pd(0) (Fig. S2). Reaction
of iodobenzene and phenylboronic acid catalyzed with
Pd(0)/M-CA (0.005 mol% to iodobenzene) was performed in
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2005, 347, 1488.
1
2
/1 (v/v) H O/EtOH at room temperature to afford the
product in 88% yield. The turnover number (TON) and
turnover frequency (TOF) of the catalyst was 17600 and 880
21. U. Gellrich, W. Seiche, M. Keller, B. Breit, Angew. Chem.
Int. Ed. 2012, 51, 11033.
-1
h , respectively, demonstrating that the catalytic activity was
improved by Pd(0)/M-CA over Pd(II)/M-CA. Examination of
the conditions for enhancing catalytic activity of Pd(0)/M-CA,
such as reduction and reaction conditions, is now under
progress.
22. A. Gneiwek, A. M. Ziokowski, J. J. Ziokowski, L. Kepinski,
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24. Based on the SEM images of Pd/M-CA complexes, the
surface areas were estimated by assuming Pd/M-CA and
Pd(OAc)
2
are columnar- and cube-shaped, respectively.
were
4
. Conclusions
The surface areas of Pd/M-CA and Pd(OAc)
estimated to be 7.56 and 1.83 m /m , respectively.
2
2
3
In conclusion, a Pd/M-CA complex catalyzed Suzuki-
Miyaura cross-coupling in an environmentally benign solvent at
ambient temperature with good activity. The generality of this
catalyst under these conditions was demonstrated for a wide
range of structurally diverse aryl halides and arylboronic acid
with many functional groups. The reduction of Pd(II) ions to
25. C. Amatore, D. G. Le, A. Jutand, Chem. Eur. J. 2013, 19,
10082.
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4
Pd(0) in Pd/M-CA by NaBH enhanced the catalytic activity to
-
1
afford a high TON of 17600 and TOF 880 h . This research
provides a novel approach to the effective utilization of metal-
recovered materials for heterogeneous organometallic catalysts.
Investigations to enhance the catalytic activity of Pd(0)/M-CA is
now under investigation.
28. T. Teranishi, M. Miyake, Chem. Mater. 1998, 10, 594.
Acknowledgements
The authors would like to thank Professor Takeshi
Yamanobe (Division of Molecular Science, Faculty of Science
and Technology, Gunma University) for helpful suggestion. This
work was financially supported by JSPS KAKENHI 17K059646.

Graphical Abstract
<Title>
Effective Heterogeneous Catalyst for Suzuki-Miyaura Cross-Coupling in Aqueous Media: Melamine Cyanurate Complex Containing
Pd Species
<Authors' names>
Daisuke Nagai and Hiroki Goto
<Summary>
Amelamine cyanurate complex catalyst containing Pd(II) ions (denoted Pd/M-CA) was effective for Suzuki-Miyaura cross-
coupling reactions in environmentally benign aqueous media at room temperature. The generality of this catalyst was demonstrated for
a wide range of structurally diverse aryl halides and arylboronic acids with functional groups. Further, the Pd/M-CA could be reused
at least 5 times while maintaining high yields.
<Diagram>