Wacker Oxidation of Terminal Alkenes Over ZrO2-Supported Pd Nanoparticles Under Acid- and Cocatalyst-Free Conditions

Article abstract of DOI:10.1002/cssc.201701016

Highly efficient Wacker oxidation of aromatic or aliphatic terminal alkenes into methyl ketones and benzofurans is developed by using reusable Pd⁰ nanoparticles (NPs) supported on ZrO₂ under acid- and cocatalyst-free conditions. Molecular oxygen or air can be utilized as the terminal oxidant, which results in the formation of H₂O as the only theoretical byproduct. The activation of the Pd NPs by O₂ plays an important role in promoting this reaction. Interestingly, PdO supported on ZrO₂ showed no activity. Additionally, the Pd particle size significantly affects the catalytic activity, with an apparent optimal diameter of 4–12 nm. In addition to the heterogeneous catalyst forms, the Pd NPs can be generated from a Pd⁰ complex during the reaction, and these particles are even recyclable.

Full text of DOI:10.1002/cssc.201701016

DOI: 10.1002/cssc.201701016
Full Papers
Wacker Oxidation of Terminal Alkenes Over
ZrO -Supported Pd Nanoparticles Under Acid- and
2
Cocatalyst-Free Conditions
[a]
[a]
[a]
[a]
Zhenzhong Zhang, Yuhya Kumamoto, Taishin Hashiguchi, Tetsuya Mamba,
[
a]
[a]
[b]
[c]
Haruno Murayama, Eiji Yamamoto, Tamao Ishida,* Tetsuo Honma, and
Makoto Tokunaga*
[a]
Highly efficient Wacker oxidation of aromatic or aliphatic termi-
nal alkenes into methyl ketones and benzofurans is developed
ing this reaction. Interestingly, PdO supported on ZrO showed
2
no activity. Additionally, the Pd particle size significantly affects
the catalytic activity, with an apparent optimal diameter of 4–
12 nm. In addition to the heterogeneous catalyst forms, the Pd
0
by using reusable Pd nanoparticles (NPs) supported on ZrO2
under acid- and cocatalyst-free conditions. Molecular oxygen
or air can be utilized as the terminal oxidant, which results in
0
NPs can be generated from a Pd complex during the reaction,
the formation of H O as the only theoretical byproduct. The ac-
and these particles are even recyclable.
2
tivation of the Pd NPs by O plays an important role in promot-
2
Introduction
Wacker oxidation, which was developed by the researchers at
Wacker Chemie in 1959, is a practical synthetic method to
access methyl ketones from terminal alkenes at both laborato-
issues; the removal of associated ligands is often difficult, and
the complexity of these catalytic systems may preclude their
application on a large scale.
[
1]
ry and industrial scales. Conventionally, this reaction occurs in
an aqueous, acidic solution containing a catalytic amount of
Heterogeneous catalysts possess many advantages in organ-
ic syntheses in terms of easy separation and recycling. There-
fore, immobilizing Pd particles on supports such as metal
[
2]
PdCl , with Cu salts serving as a redox cocatalyst. Despite the
2
[
8]
[9]
[10]
widespread applications of this reaction, several drawbacks
remain. For example, using Cu or other metal cocatalysts is not
highly compatible with the goals of green sustainable chemis-
oxides, montmorillonite, heteropolyanions, and other ma-
[
11]
terials is recognized as a positive and valuable approach for
advancing traditional reactions. For instance, cationic Pd NPs
[
1]
try. Moreover, the general requirement of an acidic environ-
ment or excess chloride ions not only causes reactor corrosion
but also lowers product selectivity by generating chlorinated
deposited on TiO can act as an active and recyclable catalyst
2
[
8d]
for Wacker oxidation. The preparation of Pd/Fe O can allow
3
4
[8f,h]
the recycling of Pd catalysts by magnetic separation.
Re-
[
1,3]
byproducts.
To overcome these limitations, many efforts
cently, Donck et al. presented carbon-nanotube-supported
multicomponent Pd metal and PdO (approximate ratio of 1:1)
that exhibited good activity for Wacker oxidation of styrene
[
4]
[5]
have focused on refining the oxidants, solvents, and addi-
[
6]
[7]
tives and replacing metal cocatalysts with organic ligands.
However, most of these techniques simultaneously create new
[11c]
and its derivatives.
However, the majority of these catalysts
still require stoichiometric amounts of Cu salts or other cocata-
lysts. Thus, a sustainable catalytic system that does not need
the addition of an acid or a cocatalyst is quite desirable. One
successful example without a cocatalyst was accomplished
[
a] Dr. Z. Zhang, Y. Kumamoto, T. Hashiguchi, T. Mamba, Dr. H. Murayama,
Dr. E. Yamamoto, Prof. Dr. M. Tokunaga
Department of Chemistry, Graduate School of Sciences
Kyushu University
[8e]
7
44 Motooka, Nishi-ku, Fukuoka 819-0395 (Japan)
over supported Au–Pd NPs, but it was constrained by the re-
E-mail: mtok@chem.kyushu-univ.jp
quirement of high-pressure supercritical CO and a narrow sub-
2
[
b] Dr. T. Ishida
strate scope. Additionally, although heterogeneously catalyzed
Wacker oxidation has been well studied with several reusable
catalysts, the size effect of the Pd NPs and deactivation of the
catalyst after recycling, which are essential factors to translate
a laboratory method to industrial production, have rarely been
Research Center for Gold Chemistry
Graduate School of Urban Environmental Sciences
Tokyo Metropolitan University
1
-1 Minami-Osawa Hachioji, Tokyo 192-0397 (Japan)
E-mail: tamao@tmu.ac.jp
[8a,f]
[
c] Dr. T. Honma
Japan Synchrotron Radiation Research Institute (JASRI)
SPring-8, Sayo, Hyougo 679-8198 (Japan)
described in detail.
Very recently, our group reported a method to decarbony-
late furfural to furan catalyzed by a ZrO -supported Pd catalyst
2
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
cssc.201701016.
[12]
without additives. In that work, the Pd/ZrO catalytic activity
2
was significantly affected by the Pd particle size. We have also
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0
reported that supported Pd catalysts promote the CÀH acetox-
the previous report in which the BE values of Pd were 335.2
[
13]
[14]
ylation of terminal alkenes. Herein, we disclose an efficient
method to transform aromatic or aliphatic terminal alkenes
and 340.5 eV, respectively, the peaks of the ZrO -supported
2
Pd NPs shifted to a higher BE by approximately 0.4 eV, and
electron-deficient sites might be partially formed on the cata-
lyst because of the interaction between the Pd NPs and ZrO2.
into methyl ketones and benzofurans over reusable ZrO -sup-
2
0
ported Pd NPs (Scheme 1). The reactions are performed with-
out acid or a cocatalyst, and O serves as the sole oxidant, re-
Pd/ZrO was initially employed as the catalyst for the Wacker
2
2
sulting in H O as the only theoretical byproduct. The size
oxidation of styrene (1a) to acetophenone (2a), and several
solvents were screened (Table 1, entries 1–4). Although (N,N-di-
2
effect of the Pd NPs and the possible causes of catalyst deacti-
vation are discussed in this work.
[
a]
Table 1. Wacker oxidation of styrene over the supported Pd catalyst.
Entry Catalyst
Solvent
Gas
[atm]
Conv. Yield of 2a
[
b]
[b]
(
Pd 5 mol%) (2 mL)
[%]
[%]
1
2
3
4
5
6
7
8
9
10
11
12
1
1
15
Pd/ZrO
Pd/ZrO
Pd/ZrO
2
2
2
DMA
DMF
THF
MeOH
O
O
O
2
2
2
(2)
(2)
(2)
0
11
32
64
99
99
100
95
97
45
48
12
7
0
0
15
19
76
81
Scheme 1. The research purpose of this work.
Pd/ZrO2
2
O (2)
Pd/ZrO
Pd/ZrO
Pd/ZrO
2
2
2
MeOH/DMSO (1:1)
MeOH/DMSO (9:1)
MeOH/DMSO (9:1)
MeOH/DMSO (9:1) O (2)
MeOH/DMSO (9:1)
MeOH/DMSO (9:1) O (2)
MeOH/DMSO (9:1)
MeOH/DMSO (9:1) O (2)
MeOH/DMSO (9:1)
O
O
O
2
2
2
(2)
(2)
(2)
[
[
[
c]
c]
c]
[d]
98 (99)
Results and Discussion
Pd/Al O
38
79
16
17
3
0
89
0
2
3
2
Pd/CeO
Pd/TiO2
Pd/C
2
2
O (2)
[
c]
The catalysts were prepared by an impregnation method using
aqueous Pd(NO3)2 as a precursor, and several metal oxides
2
[c]
2
O (2)
[
c]
[13]
2
Pd(OH) /C
2
were examined as the support (see Experimental Section).
[
c]
3
4
PdO/ZrO
Pd/ZrO
Pd/ZrO2
2
O
2
(2)
According to high-angle annular dark-field scanning transmis-
sion electron microscopy (HAADF-STEM) images, the mean di-
ameters of the Pd particles in Pd/Al O , Pd/TiO , and Pd/ZrO
2
[c]
2
MeOH/DMSO (9:1) Air (10)
MeOH/DMSO (9:1) N (1)
90
2
2
2
3
2
[
(
a] Reaction conditions: 1a (1 mmol), catalyst (Pd 5 mol%), solvent
2 mL), H O (12 equiv.), gas, 808C, 20 h. [b] GC analysis with tridecane as
were 4.4, 4.8, and 3.9 nm, respectively (Figure S1). Next, Pd/
2
ZrO was characterized by X-ray photoelectron spectroscopy
an internal standard. [c] The catalyst and solvent were treated in an auto-
clave under O (10 atm) at 808C for 1 h prior to the reaction. [d] The
value in parentheses is the isolated yield.
THF=tetrahydrofuran, DMF=N,N-dimethylformamide, DMA=N,N-dime-
thylacetamide, DMSO=dimethyl sulfoxide.
2
(
XPS) analysis (Figure 1). No nitrogen was observed in the
2
À
spectra, revealing that the NO3 group was completely re-
moved during the calcination in air. Furthermore, the binding
energy (BE) values of Pd supported on ZrO were confirmed to
2
be 335.6 eV for Pd3d5/2 and 340.9 eV for Pd3d3/2. In contrast to
[
5a–d]
[2b]
methylacetamide) DMA,
N,N-dimethylformamide (DMF),
[11c]
and tetrahydrofuran (THF)
efficient solvents to promote Wacker oxidation, methanol
MeOH) gave better results than those solvents (Table 1,
have been previously reported as
(
entry 4). The conversion increased remarkably when dimethyl
sulfoxide (DMSO) was added to MeOH (Table 1, entry 5). The
optimal MeOH/DMSO ratio was 9:1 (v/v), by which 2a was ob-
tained in 81% yield with 7% methyl 2-phenylacetate as a by-
product (Table 1, entry 6; also see Table S1, entries 1–3). This
byproduct might be formed by anti-Markovnikov selective
Wacker oxidation of 1a followed by oxidative esterification
with MeOH. Furthermore, the treatment of Pd/ZrO2 under
high-pressure O in MeOH/DMSO prior to the reaction provid-
2
ed excellent results, affording 2a in 98% yield (Table 1,
entry 7). Subsequently, the role of the solvent was investigated.
In contrast with a report that MeOH can participate in the
Wacker-type reaction to form a methyl vinyl ether intermedi-
[5d]
ate, neither the intermediate nor the target product was de-
Figure 1. XPS spectra of 10 wt% Pd/ZrO
2
. The curves correspond to Pd (red),
Zr (blue), raw data (open diamond), and the fitting line (solid line).
tected under our reaction conditions in the absence of H O.
2
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These results demonstrated that H O acted as the nucleophile
2
[a]
Table 2. Scope of Pd/ZrO
2
-catalyzed Wacker oxidation.
in this reaction and that an appropriate ratio of MeOH/DMSO
promoted the activation of Pd NPs by O . Next, several metal-
Entry Substrates
Products
Yield
2
[
b]
[%]
oxide-supported Pd catalysts were investigated, but their activ-
ities were lower than that of Pd/ZrO (Table 1, entries 8–10).
2
Although Al O -supported Pd displayed high catalytic activity,
2
3
1
a poor material balance was caused by the generation of sev-
eral dimer byproducts and other polymerized compounds. The
1
b
R
1
=H, R
2
=Me, R
3
=H
2b
2c
2d
98
87
48 (81)
use of commercially available Pd/C and Pd(OH) /C resulted in
1c R =H, R =tBu, R =H
1d
2
1
1
3
1
1
2
3
[
c]
considerably worse yields (Table 1, entries 11–12). Therefore,
the size of the Pd NPs that were deposited on the metal oxide
and the nature of the support could greatly influence the cata-
lytic performance. Additionally, no product was observed when
R
R
R
R
=Me, R
=Me
=H, R
=MeO, R
2
=Me,
1
1
e
f
2
=MeO, R
=H, R
3
3
=H 2e
=H 2 f
2g
99
98
91
71
76
2
1g R =H, R =F, R =H
1h
1
1
1
1
2
2
2
3
PdO/ZrO was used as the catalyst, suggesting that crystalline
R
R
=H, R
=H, R
=Cl, R
3
=H
2h
2
i
=OAc, R
3
=H 2i
PdO was inactive in this reaction (Table 1, entry 13; also see
Table S1, entry 4). Because air is widely regarded as a safer oxi-
2
3
1j
2j
82
83
dant than pure O in industrial production, we confirmed that
2
the reaction could proceed well with a good material balance
under air pressure of 10 atm (Table 1, entry 14). No product
was detected if the reaction was performed in the absence of
O , which highlighted that O was essential to this reaction
1k
2k
2
2
4
5
6
1l
2l
68
86
55
(
Table 1, entry 15).
Next, the substrate scope of this reaction was investigated.
With the optimized reaction conditions, various styrene deriva-
tives containing electron-donating or electron-withdrawing
groups on the phenyl rings efficiently underwent the oxidative
transformation, affording the corresponding methyl ketones in
moderate to excellent yields (Table 2, entries 1–5). In particular,
the reactions with substrates 1l and 1m, which were protect-
ed by acid-sensitive groups, proceeded smoothly to give the
desired products (Table 2, entries 4–5). Hydrolysis of the sub-
strates was not observed during these processes, indicating
that acid-free conditions were achieved. We also confirmed
that terminal aliphatic alkenes could be converted into aliphat-
1m
1n
2m
2n
7
8
9
1o
1p
1q
2o
2p
2q
90
68
77
ic ketones over Pd/ZrO using DMSO as the sole solvent; the
2
product yields were strongly affected by the substrate struc-
tures (Table 2, entries 6–8). In the case of 1o and 1p, their
chain length affected the reactivity. Moreover, this heterogene-
ous catalyst could also be used to synthesize benzofuran deriv-
atives from 2-allylphenols through a Wacker-type cyclization
1
0
1r
2r
66
[
a] Reaction conditions: substrate (1 mmol), 10 wt% Pd/ZrO
2
(Pd 5 mol%),
procedure (Table 2, entries 9–10), in which PdCl and sodium
2
MeOH/DMSO=9:1 (v/v, 2 mL), H O (12 equiv.), O (2 atm), 808C, 20 h. The
catalyst was treated in the presence of MeOH/DMSO in an autoclave
2
2
[5a]
acetate were required in the homogeneous catalytic system.
The recyclability of the Pd/ZrO was tested by synthesizing
under O (10 atm) at 808C for 1 h prior to the reaction. [b] Isolated yields.
2
2
2
a. The catalyst was filtered from the reaction mixture, washed
[c] The value in parenthesis was the yield of the reaction performed at
1008C for 36 h. [d] DMSO (2 mL) was used as the solvent.
with MeOH and CH Cl , dried under air at 708C overnight, and
2
2
MOM=methoxymethyl, Boc=tert-butyloxycarbonyl.
then used for the next run. As demonstrated in Figure 2, the
catalyst could be used at least six times without any notable
loss of catalytic activity. The yield of 2a decreased in the
7
th run, but 67% yield was still obtained in the 8th run. As dis-
served in Cat.-1st. We also checked the concentration of resid-
ual Pd in the solution after filtration. The result showed that
0.02 ppm Pd was detected in the solution (diluted 25 times),
indicating that 0.01 mol% Pd species was leached from the
catalyst into the solvent during the reaction. The Pd loading
was maintained at 9.0 wt% after the 8th run (Table S2). To fur-
ther understand the leaching phenomenon, the Wacker oxida-
cussed in our previous work, the leaching of Pd species and
the aggregation of Pd particles were considered to cause the
[
15]
deactivation of the catalyst. To estimate the leaching of Pd,
the amount of Pd in the catalyst after the 1st and 8th runs (ab-
breviated as Cat.-1st and Cat.-8th, respectively) was deter-
mined by microwave plasma atomic emission spectroscopy
(MP-AES). The results demonstrated that 9.4 wt% Pd was de-
tion kinetics over Pd/ZrO and a filtration test were studied
2
posited on the fresh catalyst and the same Pd loading was ob-
(Figure 3). The reaction was almost halted after the catalyst
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Figure 2. Recyclability of 10 wt% Pd/ZrO
Reaction conditions: 1a (1 mmol), 10 wt% Pd/ZrO
DMSO=9:1 (v/v, 2 mL), H O (12 equiv.), O (2 atm), 808C, 20 h. The catalyst
was treated in the presence of MeOH/DMSO in an autoclave under O
10 atm) at 808C for 1 h prior to the reaction.
2
catalyst for Wacker oxidation.
2
(Pd 5 mol%), MeOH/
Figure 4. XRD patterns of (a) ZrO
2
, (b) fresh catalyst, (c) the catalyst after the
2
2
1st run (Cat.-1st), and (d) the catalyst after the 8th run (Cat.-8th).
2
(
at 45.98, assigned to the (220) crystalline plane of PdO (PDF:
1-075-6703), was observed for Cat.-8th. Therefore, Pd NP ag-
0
gregation was considered to be one of the causes of catalyst
deactivation. Furthermore, the appearance of PdO on the cata-
lyst could not be neglected.
To study the dependence of the catalytic activity on the size
of the NPs, different size Pd NP catalysts were prepared by
varying the Pd loading from 1 to 30 wt%. The mean diameters
and metal dispersions of these catalysts are summarized in
[17]
Table 3. Interestingly, the Pd NPs with an approximate diam-
eter range of 4–12 nm exhibited excellent activity, whereas
smaller or larger Pd NPs showed relatively poor Wacker oxida-
tion activity. This phenomenon differs from that in a previous
report in which the smaller and better dispersed Pd NPs gener-
[14]
ally demonstrated better catalytic performance.
According
to the dynamics study on the oxidation of Pd/ZrO reported by
2
Bell and co-workers, O can be dissociatively adsorbed on the
2
surface of the Pd NPs to form a monolayer of adsorbed O; the
amorphous oxide layer tends to transform into PdO under
Figure 3. Yield change versus reaction time for the synthesis of 2a from 1a
over Pd/ZrO ).
(~), and after removal of the catalyst after 5 h by filtration (
&
2
[18]
high temperature. Therefore, we presumed that the forma-
tion of PdO, which has been confirmed as an inactive species
was removed by filtration after 5 h, suggesting that the main
cause of catalyst deactivation was not Pd leaching.
To evaluate the aggregation of Pd NPs on the catalyst, CO
pulse chemisorption was used to identify the particle sizes and
[
a]
Table 3. Effect of Pd NP size on the Wacker oxidation of styrene.
[
16]
metal dispersion. The Pd particle size increased with consec-
utive runs, growing from an initial 4.2 nm to 25.1 nm after the
Pd loading
wt%]
Mean diameter
[nm]
Pd dispersion
[%]
Conv.
[%]
Yield of 2a
[%]
[
b]
[b]
[c]
[c]
[
8
th run (Table S2 and Figure S2). This result was in accordance
1
2
5
1
2
0.8
1.9
2.9
4.2
11.8
27.4
100
58.3
37.7
27.0
11.8
4.1
47
100
100
100
99
26
47
59
98
97
60
with the powder X-ray diffraction (XRD) study of the catalysts.
As demonstrated in Figure 4, the diffraction peak at 39.08,
which was assigned to (111) crystalline planes of metallic Pd
0
0
(PDF: 00-087-0637), had different intensities for each sample.
30
78
Specifically, the peak of the fresh and Cat.-1st samples were
not clearly observed owing to the small Pd particles and over-
[
a] Reaction conditions: 1a (1 mmol), Pd/ZrO
2
(Pd 5 mol%), MeOH/
DMSO=9:1 (v/v, 2 mL), H O (12 equiv.), O (2 atm), 808C, 20 h. The cata-
2
2
lap with the ZrO peak. In contrast, the peak of Cat.-8th was
2
lyst and solvent were treated in an autoclave under O (10 atm) at 808C
for 1 h prior to the reaction. [b] Determined by CO pulse chemisorption
analysis. [c] GC analysis with tridecane as an internal standard.
2
clear, suggesting that Pd particle aggregation had occurred. In
addition, compared with the fresh catalyst, the diffraction peak
&
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for Wacker oxidation, was faster on the small Pd NPs than on
the large ones. However, the excessively large Pd NPs resulted
in poor catalytic activity, which might be caused by a lack of
sufficient surface Pd atoms on the catalyst.
trometry (EDS) of Pd (Figure 6). Additionally, these particles
could be recovered by simple centrifugation and reused to
afford the product in a moderate yield (Table 4, entry 3). When
compared with the recyclability of Pd/ZrO , the decreased
2
To confirm the chemical state of Pd, the fresh catalyst and
Cat.-1st were characterized by X-ray absorption fine structure
product yield in the second run might arise from the lack of a
support to disperse the Pd NPs. Therefore, we suggested that
the reactions were promoted by the appropriately sized Pd
particles generated in both heterogeneous and homogeneous
catalytic systems, although the possibility of other reaction
pathways, such as direct promotion by a single-atom Pd spe-
cies, could not be excluded.
(
(
XAFS). The Pd K-edge X-ray absorption near edge structure
XANES) spectra are shown in Figure 5. The spectra were com-
pared with those of standard Pd foil and PdO. The absorption
edges of both the fresh catalyst and Cat.-1st appeared in the
same region as those for the Pd foil, indicating that the cata-
0
lysts were present as Pd .
3 4
Figure 6. (a) HAADF-STEM image of Pd particles generated from [Pd(PPh ) ]
and (b) STEM-EDS elemental mapping of Pd.
To study the reaction pathway, we tested the Wacker oxida-
1
8
tion of 1-octene (1o) over Pd/ZrO by using water- O (97 at%
2
18
O) as a reagent instead of H O. The product contained
2
2
Figure 5. Pd K-edge XANES spectra of Pd foil, PdO, fresh Pd/ZrO , and the
catalyst after the 1st run (Cat.-1st).
18
18
6
5 at% O (detected by GCMS), suggesting that water- O
served as a nucleophile to react with the substrate, and the
oxygen atom exchange occurred between water and O . As
2
II
Generally, soft Lewis acidic Pd catalysts such as PdCl and
discussed, O was an indispensable component in this reaction.
2
2
Pd(OAc) are required in homogeneous catalytic systems. In
In earlier studies of aerobic oxidative processes, several forms
2
contrast, we found that Wacker oxidation can be promoted by
of activation of Pd NPs by O were proposed (Scheme 2). For
2
0
0
the very weak acidic Pd species. To test the activity of the ho-
instance, Pd species can be promoted by DMSO to produce a
0
mogeneous Pd catalyst, [Pd(PPh ) ] or [Pd (dba) ·CHCl ] (dba=
peroxypalladacycle intermediate in a homogeneous catalytic
3
4
2
3
3
[
19a]
dibenzylideneacetone) were employed in place of Pd/ZrO . In-
system.
Thus, the generation of intermediate B1 was sug-
2
0
terestingly, the reactions proceeded under the acid- and chlo-
ride-free conditions to give 2a in excellent yield (Table 4, en-
tries 1 and 2). To the best of our knowledge, there is no report
of Wacker oxidation with such a good performance using
gested in our Pd /DMSO/MeOH/O2 system. Alternatively,
weakly adsorbed O species also contribute to the formation
2
cationic Pd species, as illustrated in B2. Dissociated O atoms
on the Pd NPs should also be considered, such as the partial
I
[
Pd(PPh ) ] or [Pd (dba) ·CHCl ]. Additionally, the Pd particle
formation of Pd species on the catalyst surface (intermedia-
3
4
2
3
3
[19b]
formation was observed in the homogeneous reaction mixture
by HAADF-STEM and verified by energy dispersive X-ray spec-
te B3).
Among these intermediates, B1 was depicted as a
typical example to propose the reaction pathway. After forma-
tion of this intermediate, the terminal alkenes can coordinate
with the activated Pd catalyst to give intermediate C, which
can be attacked by H O to afford intermediate D according to
Markovnikov’s rule. Then, the product is generated and the
catalyst returns to its initial state after the elimination steps
[
a]
2
Table 4. Homogeneously catalyzed Wacker oxidation of styrene.
[
b]
[b]
Entry
Catalyst
[Pd(PPh
Conv. [%]
Yield of 2a [%]
(
Scheme 2).
1
2
3
)
4
3
] (Pd 5 mol%)
·CHCl ] (Pd 5 mol%)
98
100
84
96
95
63
[Pd (dba)
2
3
[c]
3
Recovered solid from entry 1
Conclusions
[a] Reaction conditions: 1a (1 mmol), catalyst, MeOH/DMSO=9:1 (v/v,
2
mL), H O (12 equiv.), O (2 atm), 808C, 12 h. [b] GC analysis with tride-
2
2
Wacker oxidation promoted by Pd/ZrO combined with O or
2
2
cane as an internal standard. [c] The catalyst used in this entry was the
solid recovered from entry 1 by filtration.
air under acid- and cocatalyst-free conditions was developed.
0
The optimal particle size and the discovery of a Pd complex
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2
Scheme 2. A possible reaction pathway promoted by O -activated Pd NPs.
that can efficiently catalyze the Wacker oxidation are reported
for the first time in this work. As a result of the wide substrate
scope and good recyclability, this catalyst is also expected to
have significant potential in industrial applications. As revealed
by characterization of the catalyst and optimization of the re-
action conditions, the interaction between the Pd NPs and
particle size 40–100 mm). High-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) observations
were performed using a JEOL JEM-ARM200F at Kyushu University.
The Pd content in the catalysts and the Pd leaching into the reac-
tion solutions were analyzed by microwave plasma atomic emis-
sion spectrometry (MP-AES) with an Agilent 4100 MP-AES. The re-
action solution was filtered to remove solid catalysts and the fil-
trate was analyzed by MP-AES. X-ray absorption fine structure
(XAFS) measurements were performed at the BL14B2 beamline of
ZrO and the activation of the Pd NPs by O may have signifi-
2
2
cant roles in initiating the reaction. Further studies on the reac-
tion mechanism are in progress.
[20]
SPring-8 (Hyogo, Japan). The XAFS samples were ground with
boron nitride in an agate mortar and were compacted into pellets.
Pd K-edge (24.3 keV) XAFS spectra were measured using a Si (311)
double crystal monochromator in transmission mode. The spectral
analysis was performed using the XAFS analysis software
Experimental Section
[21]
Materials
Athena. The Pd NP diameters and the NP dispersion on the cata-
lyst were estimated by CO pulse chemisorption using a BELCAT Mi-
Palladium nitrate aqueous solution (Pd(NO ) ·H O) and palladium
3
2
2
crotracBEL. The catalyst (50 mg) was pretreated in He
chloride (PdCl ) were purchased from Tanaka Kikinzoku KK and
2
À1
(
(
50 mLmin ) from RT to 4008C for 35 min and 5% O in He
2
used as received. Al O , TiO (P-25), ZrO (RC-100), and CeO were
2
3
2
2
2
À1
50 mLmin ) at 4008C for 15 min, and then, the sample was treat-
supplied by Mizusawa Chemicals, Nippon Aerosil Co., Ltd., Daiichi
Kigenso Kagaku Kogyo, and Shinetsu Chemical Co., Ltd., respec-
tively. Palladium on activated carbon (5 wt% Pd/C) and palladium
À1
ed in 5% H in Ar (50 mLmin ) at 2008C for 15 min. After pretreat-
2
ment, a CO pulse was introduced into the cell at 508C for 15 min,
and the amount of CO adsorbed was measured. The Pd/CO ratio
was set as 1 for the calculation of Pd dispersion (D). The Pd parti-
cle diameters (d, nm) were estimated by the following equation:
hydroxide on activated carbon (20 wt% Pd(OH) /C) were supplied
2
by Wako Pure Chemical Industries and Sigma–Aldrich, respectively.
All starting materials and reagents were commercial grade.
[22]
d=1.1/D. The XRD patterns were obtained on a Rigaku Mini-
Flex600 with
a
high-intensity CuKa radiation source (l=
À1
0
.154178 nm) at a scanning rate of 208min .
Instruments
Conversions and product yields were analyzed by gas chromatog-
raphy (GC) using an Agilent GC 6850 Series II equipped with a
flame-ionization detector (FID) and a J&W HP-1 column (0.25 mm
thickness, 0.25 mm I.D., 30 m) using tridecane as an internal stan-
Catalyst preparation
1
13
dard. H and C NMR spectroscopy spectra were recorded on a
JEOL JNM-ECS400 spectrometer at 400 and 100 MHz, respectively.
Pd/metal oxide catalysts were prepared by an impregnation
method. The palladium loading was adjusted to 10 wt%. Palladium
nitrate aqueous solution (Pd(NO ) ·H O) (1140 mL) was dissolved in
1
H assignment abbreviations are the following: singlet (s), doublet
3
2
2
(
d), triplet (t), double of doublet (dd), and multiplet (m). Analytical
a small volume of water. A support (1 g), such as ZrO , was added
2
thin-layer chromatography (TLC) was performed with Merck TLC
silica gel 60 F254 plates. Column chromatography was performed
on silica gel (Kanto Chemicals, Silica gel 60n, spherical, neutral,
to the aqueous solution and stirred at room temperature for
30 min. After impregnation, H O was removed by vacuum freeze
2
drying or evaporation. The obtained catalyst was calcined at 5508C
&
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for 4 h to obtain PdO/ZrO . Then, PdO/ZrO was reduced under a
funnel. After the addition, the reaction mixture was warmed to
room temperature and stirred for 4 h. The residue was extracted
with CH Cl (10 mLꢁ3). The organic layers were combined, washed
2
2
À1
flow of pure H (20 mLmin ) at 2008C for 2 h to obtain Pd/ZrO .
2
2
2
2
with brine (10 mL), dried over Na SO , and concentrated. The crude
2
4
General experimental procedure for catalysis reactions
product was purified by column chromatography hexane/ether
30:1, v/v) to give a pale-yellow oil 2-methoxy-4-(methoxyme-
thoxy)-1-vinylbenzene (1l) in 74% yield.
(
An autoclave was charged with catalyst (Pd 5 mol%), solvent (total
amount of MeOH and DMSO was 2 mL), and a magnetic stirring
bar. The autoclave was purged and filled with O until the pressure
2
reached 10 atm and then stirred at 808C for 1 h. After the pretreat-
ment process, the substrate (1 mmol) and H O (12 equiv.) were
2
added into the autoclave. The autoclave was purged and filled
with O until the pressure reached 2 atm. The reaction was stirred
2
at 808C for 20 h. After the reaction, the mixture was filtered, and
the filtrate was analyzed by GC using tridecane as an internal stan-
dard.
To a 100 mL round bottom flask under N was added a solution of
2
2.5 g (11.3 mmol) di-tert-butyldicarbonate in 15 mL of dry THF, fol-
lowed by a solution of 1.12 g (9.4 mmol) 4-vinylaniline in 10 mL of
dry THF. The mixture was heated at 808C and then refluxed for
16 h. After cooling to room temperature, the reaction mixture was
partitioned between 25 mL of H O and 25 mL of CH Cl . The layers
Experimental procedure to remove the catalyst (Figure 3)
2
2
2
An autoclave was charged with Pd/ZrO2 (Pd 5 mol%), MeOH/
DMSO (9:1, v/v, total amount of MeOH and DMSO was 2 mL), and
a magnetic stirring bar. The autoclave was purged and filled with
were separated and the aqueous layer was washed twice with
5 mL of CH Cl . The organic layer was then washed with 25 mL of
2
2
2
H O, dried over Na SO , and concentrated to give a yellow residue,
2
2
4
O until the pressure reached 10 atm and then stirred at 808C for
2
which was further purified by column chromatography on silica by
eluting with hexane/EtOAc (20:1, v/v) to give the product 4-N-Boc-
styrene (1m) as a white solid in 73% yield.
1
h. After the pretreatment process, styrene (1 mmol) and H O
2
(
12 equiv.) were added to the autoclave. The autoclave was purged
and filled with O until the pressure reached 2 atm. The reaction
2
was stirred at 808C for 5 h. After cooling to room temperature, the
reaction mixture was filtered, and the filtrate was analyzed by GC
using tridecane as an internal standard. Then, the filtrate was re-
added to the autoclave and the autoclave was filled with 2 atm O2.
The reaction was stirred at 808C for 15 h. After the reaction, the re-
action mixture was analyzed by GC.
18
Labelling experiment with water- O
Experimental procedure for Pd-complex-catalyzed reactions
(
Table 4)
An autoclave was charged with 10 wt% Pd/ZrO (Pd 5 mol%),
DMSO (2 mL), 1-octene (1 mmol), water- O (12 equiv.), and a mag-
2
18
An autoclave was charged with [Pd(PPh ) ] or [Pd (dba) ·CHCl ] (Pd
5
3
4
2
3
3
netic stirring bar. The autoclave was purged and filled with O until
2
mol%), styrene (1 mmol), H O (12 equiv.), MeOH/DMSO (9:1, v/v,
2
the pressure reached 10 atm. The reaction was stirred at 908C for
total amount of MeOH and DMSO was 2 mL), and a magnetic stir-
ring bar. The autoclave was purged and filled with O2 until the
pressure reached 2 atm, and then stirred at 808C for 12 h. After
the reaction, the mixture was filtered and the filtrate was analyzed
by GC using tridecane as an internal standard.
2
4 h. After the reaction, the mixture was filtered, and the filtrate
was analyzed by GC using tridecane as an internal standard to give
the yield of 2-octanone as 45%, and GC-MS to calculate the per-
18
centage of C H O as 65 at%.
8
16
Acknowledgements
Synthesis of compounds 1l and 1m
This work was financially supported by the ALCA program of the
Japan Science and Technology Agency. The synchrotron radiation
experiments were performed at the BL14B2 of SPring-8 with the
approval of JASRI (2016A1521).
Keywords: alkenes · heterogeneous catalysis · oxidation ·
palladium · size effect
NaH (440 mg, 11.0 mmol, 1.5 equiv.) was mixed in dry THF (20 mL)
in a 100 mL round bottom flask at 08C under a N atmosphere.
2
[
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(
1.06 mL, 14 mmol, 2 equiv.) was slowly added from the dropping
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the same level from CO pulse chemisorption analysis.
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2
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Manuscript received: June 9, 2017
Revised manuscript received: July 4, 2017
Version of record online: && &&, 0000
&
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Terminal alkenes take a Wacker! ZrO -
supported Pd NPs with a diameter of
Z. Zhang, Y. Kumamoto, T. Hashiguchi,
T. Mamba, H. Murayama, E. Yamamoto,
T. Ishida,* T. Honma, M. Tokunaga*
2
0
4–12 nm exhibited high catalytic activity
for Wacker oxidation and Wacker-type
cyclization of terminal alkenes into
methyl ketones and benzofurans. The
reactions are performed under acid- and
&
& – &&
Wacker Oxidation of Terminal Alkenes
Over ZrO -Supported Pd Nanoparticles
2
cocatalyst-free conditions, and O is em-
Under Acid- and Cocatalyst-Free
Conditions
2
ployed as the sole oxidant.
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