Selective aerobic oxidation of allyl phenyl ether to methyl ketone by palladium–polyoxometalate hybrid catalysts

Article abstract of DOI:10.1016/j.mcat.2020.111178

In this study, we report that selective aerobic oxidation of allyl phenyl ethers is attained by a Pd catalyst/polyoxometalate hybrid system to yield corresponding methyl ketones in water-enriched acetonitrile. The Pd(OAc)₂/H₅PV₂Mo₁₀O₄₀ system exhibits higher conversions and yields of corresponding methyl ketone by Wacker-type oxidation of allyl phenyl ether as compared with the conventional PdCl₂/CuCl₂ system. The higher yields are attributed to the efficient re-oxidation of Pd⁰ to Pd²⁺ by H₅PV₂Mo₁₀O₄₀ using O₂ as an oxidant as evidenced by electrochemical measurements. A reduced species of H₅PV₂Mo₁₀O₄₀ by Pd⁰ during the catalytic oxidation is revealed by UV–vis spectral measurements. The use of PdCl₂ in place of Pd(OAc)₂ in combination with [PV₂Mo₁₀O₄₀]^5? bearing tetraalkylammonium counter cations has also exhibited comparable conversions and product yields in the Wacker-type oxidation of allyl phenyl ethers. Para-substituted allyl phenyl ether derivatives are successfully oxidized in the Pd catalyst/polyoxometalate system to yield corresponding methyl ketones. The initial rate of products of para-substituted methyl ketones depended on the electronic effect of the substituents in which allyl phenyl ethers with electron-donating groups have accelerated the initial rate in the Pd catalyst/polyoxometalate system.

Full text of DOI:10.1016/j.mcat.2020.111178

Molecular Catalysis 496 (2020) 111178
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Selective aerobic oxidation of allyl phenyl ether to methyl ketone by
palladium–polyoxometalate hybrid catalysts
,
,c
Satoru Tamuraa, Yoshihiro Shimoyamab, Dachao Hongb c,*, Yoshihiro Konb
a Institute for Energy and Material/Food Resources, Technology Innovation Division, Panasonic Corporation, 1006 Kadoma, Kadoma City, Osaka, 571-8508, Japan
b Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki,
305-8565, Japan
c Global Zero Emission Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this study, we report that selective aerobic oxidation of allyl phenyl ethers is attained by a Pd catalyst/pol-
yoxometalate hybrid system to yield corresponding methyl ketones in water-enriched acetonitrile. The Pd
(OAc)2/H5PV2Mo10O40 system exhibits higher conversions and yields of corresponding methyl ketone by
Wacker-type oxidation of allyl phenyl ether as compared with the conventional PdCl2/CuCl2 system. The higher
yields are attributed to the efficient re-oxidation of Pd0 to Pd2+ by H5PV2Mo10O40 using O2 as an oxidant as
evidenced by electrochemical measurements. A reduced species of H5PV2Mo10O40 by Pd0 during the catalytic
oxidation is revealed by UV–vis spectral measurements. The use of PdCl2 in place of Pd(OAc)2 in combination
Allyl phenyl ether
Polyoxometalate catalyst
Pd catalyst
Hybrid catalysis
Wacker-type oxidation
with [PV2Mo10O40
]
5ꢀ bearing tetraalkylammonium counter cations has also exhibited comparable conversions
and product yields in the Wacker-type oxidation of allyl phenyl ethers. Para-substituted allyl phenyl ether de-
rivatives are successfully oxidized in the Pd catalyst/polyoxometalate system to yield corresponding methyl
ketones. The initial rate of products of para-substituted methyl ketones depended on the electronic effect of the
substituents in which allyl phenyl ethers with electron-donating groups have accelerated the initial rate in the Pd
catalyst/polyoxometalate system.
1. Introduction
such as hydrolysis that yield alcohols [15–17]. To avoid the hydrolysis of
ether groups, some modifications have been developed for the Pd
Aerobic oxidation reactions play an important role in organic
transformations. Selective aerobic oxidation of ethylene into acetalde-
hyde is known as the Wacker oxidation, in which PdCl2 and CuCl2 are
used as a catalyst and a re-oxidation mediator of Pd0, respectively, in an
HCl aqueous solution [1–7]. The PdCl2/CuCl2 system has been applied
for selective oxidation of terminal alkenes with long-chain alkyl, [8–10]
aromatic [11], or cyclic [12] groups into corresponding ketones. How-
ever, the use of CuCl2 yields undesired by-products of chlorides in
long-chain olefins [13,14]. The corrosive condition of the PdCl2/CuCl2
system can be a bottleneck for broad applications to other alkenes with
functional groups.
catalyst systems [18–20]. Alternative oxidants, solvents, and organic
media were explored for the selective oxidation of allyl ethers [19,
21–24]. It should be a meaningful model to use allyl phenyl ethers, that
are utilized as a raw material to gain versatile useful chemicals in
electronic devices, as substrates for the Wacker-type oxidation [25]. For
example, Fe2(SO4)3 (1.5 equiv) as an alternative oxidant was reported to
oxidize allyl phenyl ether into corresponding ketone in good yield [23].
Similarly, Dess–Martin periodinane as an oxidant was also reported to
oxidize allyl phenyl ether into corresponding methyl ketone [24]. In
these reactions, oxidants generate large amounts of waste that are
difficult to handle and remove from a product. As a consequence, it is
required to develop environmentally benign catalytic systems for the
Wacker-type oxidation of allyl ethers using molecular oxygen as an ideal
oxidant.
In the Wacker-type oxidation reactions, allyl ethers bearing two
reactive sites (allyl and ether groups) are regarded as difficult substrates
because the oxygen atom in the vicinity of an allyl group can potentially
coordinate to a Pd center, which can result in multiple side reactions
Polyoxometalates (POMs) are promising candidates for catalytic
* Corresponding author at: Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), 1-
1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan.
E-mail address: hong-d@aist.go.jp (D. Hong).
https://doi.org/10.1016/j.mcat.2020.111178
Received 30 June 2020; Received in revised form 10 August 2020; Accepted 11 August 2020
Available online 1 September 2020
2468-8231/© 2020 Elsevier B.V. All rights reserved.

S. Tamura et al.
Molecular Catalysis 496 (2020) 111178
oxidation reactions using molecular oxygen as an oxidant [26–31].
POMs are composed of molecular metal oxide anions that exhibit
attractive features of redox properties [31]. In addition, the selection of
counter cations of POMs, such as metal or organic cations, enables the
control of their solubility to be used in a wide range of solvents [32].
Notably, the phosphovanadomolybdates, H3+xPVxMo12ꢀ xO40 (x = 0–2),
of the Keggin structure, which contains molybdenum and vanadium as
addenda atoms, are reported to be efficient catalysts for aerobic oxida-
tion and found to oxidize Pd0 to a Pd2+ species with molecular oxygen
[28,33–35]. For example, PdSO4/H3+xPVxMo12ꢀ xO40 systems are used
for the oxidation of but-1-ene to butan-2-one in the presence of O2 [34].
Furthermore, the Pd(OAc)2/H3PMo12O40/tetrabutylammonium acetate
system was reported to selectively oxidize alcohols to aldehydes using
O2 as an oxidant [35]. In this context, the combination of Pd catalysts
and POMs as re-oxidation mediators is expected to be a potential system
for the Wacker-type oxidation of allyl ethers into corresponding ketones.
However, Wacker-type oxidation of allyl phenyl ethers has yet to be
clarified in a Pd catalyst/POM system.
allyl phenyl ether (0.10 M) at room temperature. The mixture was
heated to target temperature and stirred for 16 h under ambient air.
After the reaction, the mixture with biphenyl as an internal standard was
quantified by GC-FID (Shimadzu, GC-2014). The temperature of the GC-
FID program was kept 120 ℃ for 2 min, raised to 280 ℃ with a ramp rate
of 10 ℃/min and then kept for 2 min. The same procedure was used for
1H NMR spectroscopic analysis but with the use of CD3CN/D2O (7:1, v/
v) instead of CH3CN/H2O (7:1, v/v) as a reaction solution. Acetophe-
none was used as an internal standard for the quantification of products
in the 1H NMR spectroscopic analysis.
2.4. UV–vis spectroscopic analysis of reaction mixture
Pd(OAc)2 (10 mM), H5PV2Mo10O40 (3.0 mM) were mixed in 4.0 mL
of MeCN/H2O (7:1, v/v) mixed solution containing allyl phenyl ether
(0.10 M) at RT under air. The reaction solution (100 μL) was pretreated
with a syringe filter (pore size: 0.20 μm) to remove undissolved solid and
then diluting in 2.9 mL of MeCN for UV–vis spectral measurements.
UV–vis spectra of the solution were performed before reaction and after
2 h reaction at 60 ℃. The control experiments for UV–vis spectroscopic
analysis were conducted without the addition of Pd(OAc)2 or allyl
phenyl ether in the reaction solution under the same procedure.
Herein, we report the Wacker-type oxidation of allyl phenyl ether
derivatives by
a catalytic system that uses Pd catalysts and
5ꢀ
[PV2Mo10O40
]
with various cations as re-oxidation mediators. The
yields of the corresponding methyl ketones were achieved up to 74% in
the Pd(OAc)/H5PV2Mo10O40 system for the Wacker-type oxidation of
allyl ethers using O2 as an oxidant. The yields were significantly
improved compared with the conventional PdCl2/CuCl2 system. In this
study, we have demonstrated that Pd catalyst/H5PV2Mo10O40 can act as
an efficient catalytic system for the Wacker-type oxidation of allyl
phenyl ethers under mild conditions.
2.5. Electrochemical measurements
Cyclic voltammograms of POMs (6.0–10 mM) and CuCl2 (6.0 mM)
were measured in MeCN under Ar atmosphere. All cyclic voltammo-
grams were measured in three cycles. Electrolyte: 0.10 M TBAPF6, W.E.:
Glassy carbon, C.E.: Pt wire, R.E.: Ag/AgNO3. Scan rate: 100 or 50 mV/s.
Temperature: RT.
2. Experimental
2.1. General
3. Results and discussion
Allyl phenyl ether, tetraethylammonium bromide, tetrabutylammo-
nium bromide, tetrahexylammonium bromide, and tetradecylammo-
nium bromide were purchased from Tokyo Chemical Industry Co., Ltd.
Allyl 4-methoxy–, 4-fluoro–, 4-chloro–, and 4-bromophenyl ethers were
synthesized according to literature procedure [36]. H3+xPVxMo12ꢀ xO40
(x = 0–2) were obtained commercially from Nippon Inorganic Colour &
Chemical Co., Ltd. Tetraoctylammonium bromide and all palladium(II)
reagents were purchased from Fujifilm Wako Pure Chemical Corp. All
solvents for catalytic reactions were used without further purification.
Gas chromatography was performed using Shimadzu GC-2014 equipped
with an FID equipped with a TC-1 column (GL Science, 0.25 mm ×30
m).1H NMR spectral measurements were performed on a JEOL JNM-ECX
400 spectrometer using acetophenone as an internal standard. UV–vis
spectral measurements at RT were performed using a SCINCO UV–vis
spectrophotometer (S-3100). Electrochemical measurements were
recorded on a potentiostat/galvanostat instrument (HZ-7000, HOKUTO
DENKO). Purified water (18.2 MΩ cm) was obtained from a Milli-Q
system (Direct-Q3 UV, Millipore).
We used various Pd salts as Pd catalysts and POMs as re-oxidation
mediators for the Wacker-type oxidation of allyl phenyl ether in a
CH3CN/H2O (7:1, v/v) mixed solution at 60 ℃ under air atmosphere for
16 h. Products were quantified by gas chromatography with flame
1
ionization detector (GC-FID) and also confirmed by H NMR spectros-
copy.
Three
products,
phenoxyacetone
(methyl
ketone),
phenoxypropan-1-al (aldehyde), and phenol were obtained from all the
reactions (Table 1). Firstly, we have examined the effect of Pd catalysts
in the Wacker-type oxidation reactions using H5PV2Mo10O40 as a re-
oxidation mediator (Table1, entries 1–5). The use of PdCl2 in the
Wacker-type oxidation reaction showed good conversion (87%) and
afforded methyl ketone and aldehyde in yield of 51% and 13%,
respectively (Table 1, entry 1). This result is consistent with previous
studies that methyl ketones formed by Wacker-type oxidation reactions
are typically favored in accordance with Markovnikov’s rule [5,37–40].
The highest conversion (100%) and methyl ketone yield (70%) were
achieved by Pd(OAc)2 (Table 1, entry 2). The use of Pd(NO3)2 and Pd
(OCOCF3)2 has also exhibited high conversions as well as Pd(OAc)2 in
the Wacker-type oxidation reaction; however, the amount of phenol as
the by-product was larger than that of Pd(OAc)2 (Table 1, entries 3 and
4). No products were obtained by PdCl2(PPh3)2 in the Wacker-type
oxidation (Table 1, entry 5). This can be ascribed to the strong coordi-
nation of the PPh3 ligand to the Pd center preventing the coordination of
the allyl moiety to Pd2+. Therefore, we demonstrate that a halogen-free
catalytic system composed of Pd(OAc)2 and H5PV2Mo10O40 has exhibi-
ted selective oxidation of allyl phenyl ether into corresponding methyl
ketone using O2 as an oxidant. Additionally, when the re-oxidation
mediator of H5PV2Mo10O40 was replaced to H4PVMo11O40 and
H3PMo12O40 (Table 1, entries 6 and 7), the conversions and methyl
ketone yields gradually decreased in the Wacker-type oxidation re-
actions. The decrease of the conversions and methyl ketone yields may
be reflected in the reduction potentials of the POMs that H3PMo12O40
has a lowest oxidation power (E1/2 = +0.20 V vs Ag/AgNO3) among
2.2. Synthesis of TA5PV2Mo10O40 (n = 2–10)
Protons of H5PV2Mo10O40 were exchanged to tetraalkylammonium
(TA) ions (n = 2–10) by a conventional method: Tetraalkylammonium
bromide in methanol was slowly added to a methanol solution con-
taining H5PV2Mo10O40 and stirred at RT. The formed precipitate was
filtered and recrystallized in acetonitrile/ethanol solution to give
TA5PV2Mo10O40. The obtained samples were characterized by FT-IR to
confirm the structure and presence of TA ions.
2.3. General procedure for Wacker-type oxidation of allyl phenyl ethers
A Pd catalyst (10 mol%) and a POM (3.0 mol%) re-oxidation medi-
ator were added to the CH3CN/H2O (7:1, v/v) mixed solution containing
2

S. Tamura et al.
Molecular Catalysis 496 (2020) 111178
Table 1
Conversions and products yield obtained in Wacker-type oxidation of allyl phenyl ether.
Entry
Pd catalyst
Re-oxidation mediator
Conv. (%)
Methyl Ketone Yield (%)
Aldehyde Yield (%)
Phenol Yield (%)
1
PdCl2
H5PV2Mo10O40
H5PV2Mo10O40
87
100
99
98
2
51
70
58
52
0
13
14
9
13
9
2
Pd(OAc)2
Pd(NO3)2
Pd(OCOCF3)2
PdCl2(PPh3)2
Pd(OAc)2
Pd(OAc)2
PdCl2
3
H5PV2Mo10 40
O
24
27
0
4
H5PV2Mo10 40
O
11
0
5
H5PV2Mo10O40
6
H4PVMo11O40
H3PMo12O40
CuCl2
100
41
35
28
34
65
23
19
16
18
12
6
14
6
7
8
7
5
9a
10b
PdCl2
CuCl2
7
4
PdCl2
H5PV2Mo10O40
6
7
Solvent: CH3CN/H2O (7:1, v/v), reaction time: 16 h, reaction temperature: 60 ℃, [Allyl phenyl ether]: 100 mM, [Pd catalyst]: 10 mM, [Re-oxidation mediator]: 3.0
mM. Products were quantified by GC-FID.
a
[CuCl2]: 10 mM.
b
Under Ar.
those POMs (Fig. S1 in the Supplementary materials).
In comparison with the higher conversion and methyl ketone yield
by the combination of PdCl2 and H5PV2Mo10O40, the use of PdCl2 and
CuCl2, the conventional Wacker catalysts, results in a low conversion
(34%) with 19% of methyl ketone yield in the Wacker-type oxidation
reactions (Table 1, entry 8). The result demonstrates that the conversion
and product yields are significantly improved by the use of
H5PV2Mo10O40 in the Wacker-type oxidation reactions. Additionally,
the increase of CuCl2 concentrations has suppressed the conversion to
28% (Table 1, entry 9), suggesting that CuCl2 may disturb the coordi-
nation of the allyl moiety to Pd2+. This also indicates the advantage of
H5PV2Mo10O40 as a re-oxidation mediator for the oxidation of Pd0 to
Pd2+. The enhancement of the re-oxidation process can be explained by
the difference in reduction potentials between H5PV2Mo10O40 and
CuCl2. The reduction potential of H5PV2Mo10O40 (E1/2 = +0.43 V vs Ag/
AgNO3) is higher than that of CuCl2 (E1/2 = +0.18 V vs Ag/AgNO3),
which is determined by electrochemical measurements in MeCN
(Fig. S2). A blank test was also examined by the combination of PdCl2
and H5PV2Mo10O40 in the absence of O2. The conversion was dramati-
cally suppressed to 34% under argon atmosphere (Table 1, entry 10),
indicating that O2 was indispensable for the Wacker-type oxidation
reactions.
To gain insight into the catalytic cycle of the Wacker-type oxidation
of allyl phenyl ether, the effects of reaction temperature on the con-
Fig. 1. Temperature dependence of conversions and product yields in the
Wacker-type oxidation of allyl phenyl ether. Solvent: CH3CN/H2O (7:1, v/v),
reaction time: 16 h, [Allyl phenyl ether]: 100 mM, [Pd(OAc)2]: 10 mM,
[H5PV2Mo10O40]: 3.0 mM.
versions and product yields were investigated using the Pd(OAc)2/
H5PV2Mo10O40 system as shown in Fig. 1. The results revealed that high
conversions of allyl phenyl ether were attained within the range of
30–70 ℃. The highest yield for methyl ketone was obtained to be 74%
when the reaction was performed at 30 ℃. The yields of methyl ketone
and aldehyde decreased with increase in temperature, whereas the
phenol yield increased. This result agrees with the fact that hydrolysis of
ether moieties is generally facilitated with execution at high tempera-
ture [41]. When the reaction was performed at 80 ℃, black precipitates
derived from Pd0 nanoparticles were observed in the solutions, resulting
in a low conversion (41%). Thus, the results indicate that the
reaction time and after 2 h at 60 ℃ (Fig. 2a), which was not observed on
the reaction solution in the absence of the Pd catalyst or allyl phenyl
ether. This suggests that the characteristic absorption band around 720
nm is derived from the reduced species of H5PV2Mo10O40. The charac-
teristic absorption band was also observed in a separate experiment in
which hydroquinone, a reductant, was added into H5PV2Mo10O40 in
6ꢀ
MeCN (Fig. 2b), indicating the formation of [PVVVIVMo10O40
]
Wacker-type oxidation of allyl phenyl ether by the Pd(OAc)2/H5PV2
-
(POMred). Thus, these results clearly indicate that the formed species in
the reaction solution is attributed to POMred. As POMred is observed in
the catalytic reactions, the rate-determining step could be involved in
the oxidation of POMred by molecular oxygen in the Pd(OAc)2/
H5PV2Mo10O40 system for the Wacker-type oxidation reactions.
According to previously reported literature, [6,42] the catalytic cycle
in the Wacker-type oxidation of allyl phenyl ether using the Pd/POMs
system is displayed in Scheme 1. The Wacker-type oxidation of allyl
Mo10O40 system is accelerated at low temperatures and competed with
phenol production derived from the hydrolysis of ether moieties at high
temperatures.
We have also measured the UV–vis spectral changes to monitor in-
termediates formed during the Wacker-type oxidation reactions in the
catalytic system of Pd(OAc)2/H5PV2Mo10O40 (Fig. 2). The reaction so-
lution containing Pd(OAc)2, H5PV2Mo10O40, and allyl phenyl ether
exhibited a characteristic absorption band around 720 nm at initial
3

S. Tamura et al.
Molecular Catalysis 496 (2020) 111178
Fig. 2. (a) UV–vis spectra of reaction solution containing H5PV2Mo10O40 (3.0 mM), Pd(OAc)2 (10 mM), and allyl phenyl ether (0.10 M) for reaction time of 0 and 2 h
at 60 ℃, and the solutions in the absence of Pd(OAc)2 or allyl phenyl ether. (b) UV–vis spectral changes in POMox (0.50 mM) with addition of 1.5 mM of hydro-
quinone as reductant to form POMred in MeCN. Inset: The absorption changes monitored at 765 nm against the hydroquinone concentrations.
phenyl ether by Pd2+ results in the formation of Pd0 that is then oxidized
Pd2+. Furthermore, the temperature dependence of yield and conversion
in the use of PdCl2/TA5PV2Mo10O40 (n = 6) was also investigated during
the Wacker-type oxidation of allyl phenyl ether, and the highest con-
versions and yields were obtained at 60 ℃ (Fig. S4). Meanwhile, it was
found that the alkyl chain lengths (n) of TA cations has less affected the
conversions and product yields in the PdCl2/TA5PV2Mo10O40 (n = 2–10)
(Table 2, entries 3–7). Thus, these results demonstrate that the combi-
nation of Pd sources with POMs bearing alternative counter cations is
critical for the Wacker-type oxidation of allyl phenyl ether.
5ꢀ
by [PVV2Mo10O40
]
(POMox) to regenerate Pd2+. The Reduced POM,
6ꢀ
which is assigned to [PVVVIVMo10O40
]
(POMred) by the cyclic vol-
tammograms in Fig. S1, reacts with O2 to produce POMox. Based on the
results in Table 1 and UV–vis spectra in Fig. 2, the use of H5PV2Mo10O40
of can prevent from the formation of Pd nanoparticles, resulting in the
high conversions and methyl ketone yields in the Wacker-type oxidation
of allyl phenyl ether.
We have also investigated the influence of the counter cations of
POM on the Wacker-type oxidation of allyl phenyl ether because the
cation–POM interaction is known to be crucial to control their properties
[19]. We have employed tetraalkylammonium (TA) cations with
Substituent effects of allyl phenyl ether were investigated for the
Wacker-type oxidation in the PdCl2/TA5PV2Mo10O40 (n = 6) system
(Fig. 3a). The corresponding methyl ketones quantified by1H NMR
yielded in the reactions using para-substituted allyl phenyl ether de-
rivatives (X = H, OMe, F and Cl) as substrates (Fig. S5). Fig. 3b shows
the time courses of the corresponding methyl ketones obtained in the
reactions. The initial rates for the production of methyl ketones were
calculated from the slopes and determined to be 3.74 × 10–5 M min–1 for
H, 4.53 × 10–5 M min–1 for OMe, 2.54 × 10–5 M min–1 for F and 2.46 ×
10–5 M min–1 for Cl. An allyl phenyl ether derivative having an electron-
donating group (OMe) exhibited the highest initial rates as compared
with those having electron-withdrawing groups (F and Cl). The initial
rates of the Wacker-type oxidations were dependent on the Hammett
parameters [43] of the substituents of the allyl phenyl ethers (Fig. 3c).
The Hammett plots obtained for the para-substituted allyl phenyl ether
different alkyl chain lengths (n) as counter cations of [PV2Mo10O40
]
5ꢀ to
examine the effect of the counter cations. TA5PV2Mo10O40 (n = 2–10)
were prepared by a typical method of cation exchange and characterized
by Fourier transform infrared (FT-IR) spectra (Fig. S3). Unexpectedly,
Pd(OAc)2/TA5PV2Mo10O40 (n = 6) exhibited low conversions in the
Wacker-type oxidation of allyl phenyl ether (Table 2, entry 1). After the
reaction, a black precipitate was observed in the solution, indicating
insufficient re-oxidation of Pd0 to Pd2+ in the combination of Pd(OAc)2
and TA5PV2Mo10O40 (n = 6). On the other hand, the PdCl2/TA5PV2
-
Mo10O40 (n = 6) system showed a relatively high conversion (93%) and
afforded methyl ketone (56%) and aldehyde (15%) with a higher yield
(Table 2, entry 2) compared with that of obtained from the
PdCl2/H5PV2Mo10O40 system (Table 1, entry 1). In the case of
PdCl2/TA5PV2Mo10O40 (n = 6) system, no black precipitate was
observed after the reaction, suggesting efficient re-oxidation of Pd0 to
derivatives showed a negative slope (ρ = –1.18) as shown in Fig. 3c. The
results demonstrate that the initial rates lineally depend on the σp value
of the substituents. Thus, the use of allyl phenyl ether derivatives having
Scheme 1. Catalytic cycle in Wacker-type oxidation of allyl phenyl ether derivatives by Pd catalyst/POM hybrid system in this study.
4

S. Tamura et al.
Molecular Catalysis 496 (2020) 111178
Table 2
Dependence of alkyl chain length of counter cations to POM in the Wacker-type oxidation of allyl phenyl ether.
Entry
Pd catalyst
Re-oxidation mediator
Conv. (%)
Methyl Ketone Yield (%)
Aldehyde Yield (%)
Phenol Yield (%)
1a
2a
3
Pd(OAc)2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
PdCl2
TA5PV2Mo10O40 n = 6
TA5PV2Mo10O40 n = 6
66
93
97
90
98
97
99
33
56
60
53
58
60
61
13
15
21
22
19
23
23
15
17
10
10
12
11
11
TA5PV2Mo10 40
O
n = 2
n = 4
n = 6
n = 8
n = 10
4
TA5PV2Mo10 40
TA5PV2Mo10 40
TA5PV2Mo10 40
TA5PV2Mo10 40
O
5
O
6
O
7
O
Solvent: CH3CN/H2O (7:1, v/v), reaction time: 16 h, reaction temperature: 60 ℃, [Allyl phenyl ether]: 100 mM, [Pd catalyst]: 10 mM, [Re-oxidation mediator]: 3.0
mM. Products were quantified by GC-FID.
a
At 70 ℃.
Fig. 3. (a) Schematic representation of the
Wacker-type oxidation of para-substituted allyl
phenyl ether derivatives (X = H, OMe, F and
Cl) in the PdCl2/TA5PV2Mo10O40 (n = 6) sys-
tem. (b) Time courses of the corresponding
methyl ketones quantified by 1H NMR in the
reaction solutions (CD3CN/D2O (7:1, v/v))
containing TA5PV2Mo10O40 (n = 6) (1.5 mM),
PdCl2 (5.0 mM), and para-substituted allyl
phenyl ether derivatives (0.10 M) at 25 ℃. (c)
Hammett plots for the Wacker-type oxidation of
para-substituted allyl phenyl ether derivatives.
electron-donating groups can accelerate the Wacker-type oxidation in
green oxidant in synthetic and catalytic chemistry.
the PdCl2/TA5PV2Mo10O40 (n = 6) system.
CRediT authorship contribution statement
4. Conclusions
Satoru Tamura: Data curation, Writing - original draft. Yoshihiro
Shimoyama: Data curation, Writing - review & editing, Funding
acquisition. Dachao Hong: Conceptualization, Writing - review &
editing, Funding acquisition, Supervision. Yoshihiro Kon: Conceptu-
alization, Writing - review & editing.
In summary, we have found that the Pd(OAc)2/H5PV2Mo10O40 sys-
tem successfully catalyzed the Wacker-type oxidation of allyl phenyl
ether, which is regarded as a difficult substrate that has two reactive
sites, into corresponding methyl ketone in good yield (74%) using mo-
lecular oxygen as an ideal oxidant at low temperature (30 ℃). The yield
of methyl ketone obtained in the Pd(OAc)2/H5PV2Mo10O40 system is
higher than that in the conventional PdCl2/CuCl2 system for the Wacker-
type oxidation of allyl phenyl ether. The electrochemical measurements
of H5PV2Mo10O40 and CuCl2 in MeCN revealed that efficient re-
oxidation of Pd0 to Pd2+ by H5PV2Mo10O40 resulted in a high conver-
sion and a yield for the Wacker-type oxidation. A reduced species of
H5PV2Mo10O40 observed during the catalytic reaction was confirmed by
UV–vis spectral measurements. Para-substituted allyl phenyl ether de-
rivatives are also oxidized to yield corresponding methyl ketones in the
PdCl2/TA5PV2Mo10O40 (n = 6) system. The Hammett plots indicate a
substituent effect that substrates with electron-donating groups have
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgements
D.H. gratefully acknowledges support from the Japan Society for the
Promotion of Science (JSPS) by the Leading Initiative for Excellent
Young Researchers (LEADER). Y.S. also gratefully acknowledges support
from Grants-in-Aid (20K15328) from JSPS. The authors acknowledge
Mr. Masakazu Takata for assisting the research.
enhanced the initial rates for Wacker-type oxidation of allyl phenyl ether
5ꢀ
derivatives. Finally, the use of [PV2Mo10O40
]
bearing appropriate
counter anions as re-oxidation mediators and Pd catalysts are key fea-
tures in the selective oxidation of allyl phenyl ethers under mild con-
ditions. Thus, we expect that the combination of Pd catalysts with POMs
can be applied to various selective oxidation reactions using O2 as a
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2020.111178.
5

S. Tamura et al.
Molecular Catalysis 496 (2020) 111178
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