Pd(II)-hydrotalcite-catalyzed selective oxidation of alcohols using molecular oxygen

Article abstract of DOI:10.1246/bcsj.74.165

A novel heterogenized Pd catalyst, Pd supported by hydrotalcite [Pd(II)-hydrotalcite], was synthesized by a simple operation from commercially available hydrotalcite, Pd(OAc)₂ and pyridine. The catalyst was effective for the oxidation of a wide range of alcohols using molecular oxygen as a sole oxidant. With this catalytic system, various alcohols were readily converted to the corresponding aldehydes or ketones selectively in high to excellent yields. Sterically less hindered substrates were converted to the corresponding ketones much faster than the hindered ones. For example, cyclohexanol was converted to cyclohexanone in 79% yield for 15 hr, while larger-sized cyclic alcohols were oxidized to the corresponding ketones only in 32-77% yields in the same reaction time. The catalyst was also applicable to the oxidation of unsaturated alcohols such as geraniol and nerol without any isomerization of an alkenic part. Modified Pd(II)-hydrotalcite [Pd(II)-hydrotalcite(m)] could be reused several times while keeping its activity.

Full text of DOI:10.1246/bcsj.74.165

© 2001 The Chemical Society of Japan
165
Bull. Chem. Soc. Jpn., 74, 165–172 (2001)
Pd(II)-Hydrotalcite-Catalyzed Selective Oxidation of Alcohols Using
Molecular Oxygen
Nobuyuki Kakiuchi, Takahiro Nishimura, Masashi Inoue, and Sakae Uemura*
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University,
Sakyo-ku, Kyoto 606-8501
(Received August 22, 2000)
A novel heterogenized Pd catalyst, palladium supported by hydrotalcite [Pd(II)-hydrotalcite], has been synthesized.
The catalyst is found to be effective for the oxidation of a wide range of alcohols using molecular oxygen as a sole oxi-
dant. In this catalytic system, various alcohols are readily converted to the corresponding aldehydes or ketones selective-
ly in high to excellent yields. It is noteworthy that the catalyst is also applicable to the oxidation of unsaturated alcohols
such as geraniol and nerol without any isomerization of an alkenic part. Another characteristic property of this heteroge-
neous catalyst is that a shape selectivity depending on the structure of alcohols is observed in some cases. The catalyst
also has advantages such as ease of handling, easy preparation, and reusability for several times.
The increasing environmental and economical concerns
have led chemists to develop the clean and high-performance
catalytic reactions. Especially, much attention has been paid to
the heterogeneous or heterogenized catalysts1,2 because of their
unique properties, such as reusability and molecular recogni-
tion effect. For example, the application of these catalysts for
aerobic oxidation of alcohols is a target of interest to many
chemists. Immobilization of transition metal salts to various
kinds of supports such as activated carbon, alumina, silica,
clays and polymers is an important method to produce such ef-
fective heterogeneous catalysts. For recent examples, the aero-
bic oxidation of alcohols was successfully carried out using
heterogenized catalysts such as polymer- or mesoporous solid
MCM-41-supported TPAP [tetrapropylammonium tetraoxoru-
thenate(VII)]3,4 and TiO2-supported palladium cluster.5b,5c Ru-
thenium-doped hydrotalcites (layered basic clay minerals)
were also reported as efficient heterogeneous catalysts for the
oxidation of alcohols under O2.6 Quite recently, monomeric ru-
thenium supported by hydroxyapatite (RuHAP) was also syn-
thesized for the same purpose.7
ing, and drying under vacuum at room temperature. In this
process [Pd(OAc)2(py)2] complex11 was initially formed and
then it was adsorbed on the hydrotalcite support. The Pd con-
tent in the Pd(II)-hydrotalcite was 0.16 mmol g–1, as estimated
by inductively coupled plasma (ICP) atomic emission spec-
trometry analysis. Hydrotalcite has a layered structure consist-
ing of positively charged brucite-like layers and negatively
charged counter ions and water molecules located in the inter-
layers.12 Thus, we first investigated the spacing between layers
of newly prepared Pd(II)-hydrotalcite. Spacing (d003) of both
commercially available hydrotalcite and the Pd(II)-hydrotal-
cite was estimated to be almost the same by X-ray diffraction
(XRD) analysis. These results suggest that the palladium salt
does not exist between brucite layers by an anion exchange.
The possibility of the substitution of any cations in brucite lay-
ers by the palladium salt is also low because of a stable struc-
ture of the brucite. So, we postulated that the palladium salt is
located on the external surface of thin plate-like crystals of hy-
drotalcite (not between layers). For the purpose of confirming
our assumption and disclosing the dispersion state of the palla-
dium salt, the Pd(II)-hydrotalcite was analyzed by transmission
electron microscopy (TEM). At low magnification, a number
of small particles were observed on the plate-like crystals of
the hydrotalcite. However, because of severe dehydration of
hydrotalcite by irradiation of the electron beam, the high mag-
nification image of surface morphology of Pd(II)-hydrotalcite
could not be obtained.
Recently, we reported the oxidation of alcohols using a ho-
mogeneous palladium catalyst under atmospheric pressure of
oxygen,8 and also succeeded in the immobilization of the ho-
mogeneous palladium catalyst on hydrotalcite (Mg6Al2-
(OH)16CO3•4H2O) and its application for aerobic oxidation of
alcohols.9 In this paper we describe the full scope of this aero-
bic oxidation using Pd(II)-hydrotalcite as catalyst.10
Next, we attempted to investigate the presence of pyridine in
Pd(II)-hydrotalcite. Elemental analysis showed that 0.35 wt%
of N atom was contained in Pd(II)-hydrotalcite, which was
quite close to a theoretical percentage of nitrogen (0.45 wt%
calculated from the result of ICP atomic emission analysis, as-
suming that Pd atoms exist in the [Pd(OAc)2(py)2] form in
Results and Discussion
Preparation and Characterization of Pd(II)-Hydrotal-
cite. Pd(II)-hydrotalcite was prepared by mixing Pd(OAc)2
(1.67 mmol), pyridine (4.18 mmol) and hydrotalcite (10.0 g) in
toluene (100 mL) at 80 °C for 1 h, followed by filtration, wash-

166 Bull. Chem. Soc. Jpn., 74, No. 1 (2001)
Aerobic Oxidation of Alcohols
Pd(II)-hydrotalcite). Further evidence of the existence of pyri-
dine in Pd(II)-hydrotalcite was obtained by thermogravimetry/
mass spectrometry (TG/MS) analysis. The MS peak of pyri-
dine (m/z = 79) desorbed from Pd(II)-hydrotalcite was ob-
served at around 160 °C.13 We also carried out control experi-
ments to rule out the possibility of adsorption of pyridine on
hydrotalcite; that is, another sample was prepared by treating
hydrotalcite only with pyridine in toluene and by washing the
resultant solid in the same way as in the preparation of Pd(II)-
hydrotalcite. In this sample, pyridine was not detected by ele-
mental analysis and TG/MS. These results suggest that pyri-
dine exists as a ligand on palladium.
Finally, we tried to identify the palladium-pyridine complex
itself which might be supported on Pd(II)-hydrotalcite. For
this purpose, several physical analyses such as cross polariza-
tion-magic angle spinning (CP-MAS) 13C NMR, absorption
and diffuse reflectance of IR, UV, and thermogravimetry-dif-
ferential thermal analysis (TG-DTA) were employed, but un-
fortunately, no exact and clear information was obtained. In
order to know whether the palladium complex in Pd(II)-hydro-
talcite contains acetate or not, Pd(II)-hydrotalcite was washed
with an excess amount of pyridine repeatedly to compel the
palladium complex to leach from Pd(II)-hydrotalcite; then the
resulting filtrate was concentrated. As a result, the yellowish
solids, which mainly consisted of [Pd(OAc)2(py)2] complex,14
were obtained.
Chart 1.
Table 1. Pd(II)-Hydrotalcite-Catalyzed Oxidation of
Benzylic Alcohols Using Molecular Oxygena)
Entry Substrate Product
Time
h
Conv. Isolated
%
yield/%
1
1a
1a
1a
1b
1c
1d
1e
1f
2a
2a
2a
2b
2c
2d
2e
2f
2
12
12
2
2
5
8
2
2
2
2
3
3
6
6
11
11
100
98
94
quant.b)
91b)
87b)
94
2c)
3d)
4
98
5
6
7
8
9
10
11
12e)
13e)
14
15
16e)
17e)
98
92
100
96
97
95
Thus, we concluded that the palladium-pyridine complex
was immobilized on the external surface of brucite layers of
Pd(II)-hydrotalcite.15
97
97
87
84
100
98
90
1g
1h
1i
2g
2h
2i
93b)
84b)
69
Pd(II)-Hydrotalcite-Catalyzed Oxidation of Alcohols.
The oxidation of alcohols was performed as follows: Pd(II)-hy-
drotalcite (300 mg, Pd: 0.05 mmol), alcohol (1.0 mmol) and
pyridine (0.2 mmol) were mixed in toluene and the mixture
was stirred at 80 °C under atmospheric pressure of oxygen.16
The results of the oxidation of primary and secondary alcohols
are shown in Chart 1 and Table 1. In the oxidation of primary
benzylic alcohols (1a–h), substrates were smoothly consumed
to afford the corresponding aldehydes (2a–h) in high yields
(Table 1, Entries 1, 4–10), irrespective of the presence of either
electron-donating substituents (1b–e) or electron-withdrawing
substituents (1f–h). However, 3-nitrobenzyl alcohol (1i) yield-
ed the corresponding aldehyde (2i) in a slightly lower yield
(69%) in spite of the high conversion of 1i. Secondary benzyl-
ic alcohols (1j and k) were also converted to the corresponding
ketones (2j and k) in excellent yields for 3 h (Entries 12 and
13), although some excess pyridine (1 mmol) was needed for
sufficient reaction without loss of a catalytic activity.
Non-activated primary and secondary aliphatic alcohols
(3a–d) were also transformed into the corresponding carbonyl
compounds (4a–d) using Pd(II)-hydrotalcite, although reac-
tions were relatively slow (Entries 14–17). The reaction rate of
secondary alcohols was lower than that of primary ones (En-
tries 16 and 17).
Oxidative Lactonization of Diols. In the oxidation of α,
ω-primary diols, oxidative lactonization occurred to give the
corresponding lactones (Chart 2, Table 2, Entries 1–4).17 Un-
1j
2j
quant.b)
1k
3a
3b
3c
3d
2k
4a
4b
4c
4d
95
97
86
97
85
100
100
93
92
a) Reaction conditions; Pd(II)-hydrotalcite (300 mg, 0.05 mmol
Pd), alcohol (1.0 mmol), pyridine (0.2 mmol), toluene (10 mL), 80
◦C, O2. b) GLC yield. c) 1 mol% Pd(II)-hydrotalcite, for 12 h. d)
10 fold scale reaction using 1 mol% Pd(II)-hydrotalcite for 12 h.
e) Pyridine (1.0 mmol) was used.
fortunately, little chemoselectivity was observed in the reaction
of an unsymmetrical diol in our system (Table 2, Entry 4).18
Oxidation of Unsaturated Alcohols. The oxidation of
alkenic alcohols using either Pd(II)-hydrotalcite system or
Pd(OAc)2/pyridine/MS3A system (homogeneous catalytic sys-
tem; abbreviated as MS-system)8b was performed for compari-
son; the results are summarized in Table 3. In the oxidation of
alkenic alcohols, a quite excess of pyridine (25 times as much
as a standard condition) was necessary to complete the reaction
(footnotes of Table 3). Such an excess of pyridine may prevent
the complexation of Pd(II)-intermediates with an alkenic moi-
ety which might accelerate the reduction of Pd(II).19 When the
oxidation of cinnamyl alcohol using both catalytic systems was
compared, Pd(II)-hydrotalcite system showed a slightly higher

N. Kakiuchi et al.
Bull. Chem. Soc. Jpn., 74, No. 1 (2001) 167
good method for the selective aerobic oxidation of allylic alco-
hols. In order to explain this superiority of heterogeneous cata-
lyst compared with homogeneous one in the oxidation of allyl-
ic alcohols, we tried several experiments. Geranial was kept
under the reaction conditions of either Pd(II)-hydrotalcite sys-
tem or MS-system (Eq. 1, Table 4). In both systems, the E/Z
ratio of geranial did not change much (Entries 2 and 3). The re-
sult indicates that the geometrical isomerization might be due
to the interaction between an intermediate Pd(II) species such
as Pd(II)H, which was generated during the reaction of sub-
strates, and allylic moieties which exist in both substrates and
products in the reaction media. In the MS-system, such isomer-
ization could not be prevented even by using excess pyridine
(Table 3, Entries 7 and 9). Although the reason for preserva-
tion of the E/Z geometry in products in the case of Pd(II)-hy-
drotalcite system is not yet clear, the complexation of the palla-
dium species with an alkenic part may be inhibited because of
the steric bulkiness of hydrotalcite surface.
Chart 2.
Table 2. Pd(II)-Hydrotalcite-Catalyzed Oxidation of Di-
ols Using Molecular Oxygena)
The oxidation of alkynic alcohols (Chart 3) was next at-
tempted. An alkynic alcohol 7a which has an internal car-
bon−carbon triple bond was converted to the corresponding al-
dehyde 8a in low yield (23%, after 24 h), while alcohols which
have a terminal alkynic moiety such as 7b and 7c did not give
the corresponding ketones because of non-controlled polymer-
ization.
Entry Substrate Product Time/h
Isolated yield/%b)
1
2
3
4
5a
5b
5c
5d
6a
6b
6c
2.5
2.5
2
56
88
24c)
6d+6e
5
58 (6d/6e = 44/56)d)
a) Reaction conditions; Pd(II)-hydrotalcite (300 mg, 0.05 mmol
Pd), alcohol (1.0 mmol), pyridine (0.2 mmol), toluene (10 mL), 80
◦C , O2. b) Substrates were completely consumed. c) GLC yield.
d) Determined by 1H NMR.
Recycle of the Catalyst. Pd(II)-hydrotalcite can be easily
separated from the reaction mixture by simple filtration after
the reaction. The recovered Pd(II)-hydrotalcite could be re-
used for the next oxidation (Table 5), although a slight decrease
of the catalytic activity was observed in the third use of Pd(II)-
hydrotalcite for the oxidation of benzyl alcohol (Table 5, Entry
1; first: 98%, second: 89%, third: 77%). In order to prevent this
deactivation of catalyst, another modified Pd(II)-hydrotalcite
[abbreviated as Pd(II)-hydrotalcite(m), Pd: 0.092 mmol g–1
(estimated by ICP atomic emission analysis): ca. a half amount
of Pd content compared with the so far described Pd(II)-hydro-
talcite] was prepared by a similar method. It was eventually
disclosed that Pd(II)-hydrotalcite(m) could be reused without
an appreciable loss of the catalytic activity in the same reaction
(Entry 2; first: 96%, second: 97%, third: 90%). With Pd(II)-hy-
drotalcite(m) the oxidation of 4-methoxybenzyl alcohol giving
4-methoxybenzaldehyde proceeded well even when used for
the fourth time (Entry 3; first: 84%, second: 88%, third: 84%,
fourth: 85%).
Possible reasons of the deactivation of the catalysts (espe-
cially in the case Pd(II)-hydrotalcite) might be 1) leaching of
Pd(II) salt and 2) reduction of an active Pd(II) species to Pd(0).
In order to clarify whether Pd(II) salt leaches or not under the
reaction conditions, we carried out the following experiment.
That is, the oxidation of benzyl alcohol using Pd(II)-hydrotal-
cite was allowed to proceed for 30 min, and the catalyst was fil-
tered at 80 °C. Then, the filtrate containing the product benzal-
dehyde and the unreacted benzyl alcohol was stirred under O2
at 80 °C. As a result, it was revealed that the leaching of a
small amount of Pd(II) salt from Pd(II)-hydrotalcite occurred
[a further conversion of benzyl alcohol to benzaldehyde (ca.
activity than MS-system to give cinnamaldehyde in 95% isolat-
ed yield (Entry 1). In the oxidation of other allylic alcohols, a
similar trend was observed (Entries 2–4). Alkenic alcohols
such as 10-undecen-1-ol and citronerol could also be smoothly
oxidized to give the corresponding aldehydes in good yields
without affecting the alkenic moieties (Entries 5 and 6). Next,
the oxidation of nerol [(Z)-isomer] and geraniol [(E)-isomer]
was carried out. Using MS-system, the yields of aldehydes
were low (39 and 56%, respectively) and E/Z ratios of the prod-
ucts were seriously changed (E/Z ratios were 31/69 and 63/37,
respectively).20 On the other hand, Pd(II)-hydrotalcite-cata-
lyzed reaction smoothly proceeded to give the corresponding
aldehydes in 89% and 91% yields without any geometrical
isomerization (E/Z ratios were 6/94 and 96/4) (Entries 7 and 9).
Further, in these oxidations reaction rates dramatically in-
creased using Pd(II)-hydrotalcite. In the cases of trans-4,8-
dimethyl-3,7-nonadien-2-ol and farnesol (Entries 10 and 11),
the difference between Pd(II)-hydrotalcite and MS-system is
smaller than that in the former two substrates, but E/Z ratios
were also much changed by using the MS-system (8/2). It is
well known that Pd(II) species are generally not such effective
catalysts for oxidation of allylic alcohols because of their
strong complexation with unsaturated carbon−carbon bonds.10a
Recently, such drawbacks in palladium chemistry have been
overcome and some good examples have been reported on pal-
ladium-catalyzed oxidation of allylic alcohols into the corre-
sponding aldehydes and ketones. Our results also provide a

168 Bull. Chem. Soc. Jpn., 74, No. 1 (2001)
Aerobic Oxidation of Alcohols
Table 3. Pd(II)-Hydrotalcite or Pd(OAc)2/Pyridine/MS3A System-Catalyzed Oxidation of Alkenic Alcohols Using Molecular
Oxygena)
Pd(II)-hydrotalcite systemb)
Pd(OAc)2/pyridine/MS3A systemc)
Entry
Substrate
Product
Time
h
Isolated yield
Time
h
Isolated yield
d)
d)
%
%
1
2
3
5
95
97
(100)
(100)
4
91
93
(96)
12
(100)
3
3
97
(100)
4
83
(90)
4e)
3
96
93
94
(100)
(100)
(98)
4
17
6
87
91
81
(100)
(95)
5
8
6
4.5
(86)
7f)
4.5
89
(100)
15
39
(71)
E/Z = 6/94
E/Z = 31/69
8f),g)
9f)
12
45
(58)
E/Z = 95/5
91
E/Z = 96/4
4.5
(98)
15
15
15
56
(76)
(91)
(80)
E/Z = 63/37
10f)
11f)
4.5
4.5
90
(98)
72
E/Z = 9/1
E/Z = 8/2
91
(100)
61
E/Z = 9/1
E/Z = 8/2
a) Reaction conditions; alcohol (1.0 mmol), pyridine (5.0 mmol), toluene (10 mL), O2, 80 ◦C. b) Pd(II)-hydrotalcite (300 mg, 0.05
mmol Pd). c) Pd(OAc)2 (0.05 mmol), MS3A (500 mg). d) The value in parentheses is the conversion of the alcohol (%). e) GLC yield.
f) E/Z ratio was determined by 1H NMR. g) Pyridine (0.2 mmol) was used.
9%) was observed after 21 h]. In contrast, using Pd(II)-hydro-
talcite(m) in the same experiment, such further oxidation did
not proceed even after 24 h. To determine the exact amount of
palladium salt in the filtrate which leached from Pd(II)-hydro-
talcite and Pd(II)-hydrotalcite(m), we performed ICP atomic
emission analysis for the filtrate after the reaction. As a result,
14% of Pd leached from Pd(II)-hydrotalcite was detected,
while in the case of Pd(II)-hydrotalcite(m) only 0.8% leaching
was observed. These results showed that the leaching of Pd
can be much diminished by decreasing the amount of Pd com-
plex supported on hydrotalcite.
carried out the oxidation of a variety of cyclic alcohols having
the different sizes and substituents using Pd(II)-hydrotal-
cite(m).21 Sterically less hindered substrates were converted to
the corresponding ketones much faster than the hindered ones.
For example, cyclohexanol was converted to cyclohexanone in
79% yield for 15 h (Table 6, Entry 1), while larger-sized cyclic
alcohols were oxidized to the corresponding ketones only in
32–77% yields in the same reaction time (Entries 2–7). On the
other hand, all alcohols shown in Table 6 were converted into
the corresponding ketones completely within 2 h in MS-sys-
tem. The reason of the differences of the reaction rate accord-
ing to the shape of alcohols using Pd(II)-hydrotalcite(m) may
Shape Selective Oxidation of Cyclic Alcohols. Next, we

N. Kakiuchi et al.
Bull. Chem. Soc. Jpn., 74, No. 1 (2001) 169
Table 6. Pd(II)-Hydrotalcite(m)-Catalyzed Oxidation of
Cyclic Alcohols Using Molecular Oxygena)
(1)
Isolated yield
Entry
Substrate
Product
b)
%
1
2
79c)
43
(80)
Table 4. Attempts for Isomerization of E/Z Ratio of Geranial
Entry
Pd cat.
E/Z ratioa)
(45)
(49)
(43)
(64)
1
2
3
none
Pd(OAc)2/MS3A
Pd(II)-hydrotalcite
90/10
92/8
94/6
3
4
5
39
32
58
a) E/Z ratio was determined by 1H NMR.
6
7
77
35
(79)
(36)
Chart 3.
Table 5. Recycling of Pd(II)-Hydrotalcite and Pd(II)-
Hydrotalcite(m) in the Oxidation of Benzylic Alcohols
Using Molecular Oxygena)
Entry
Catalyst
Substrate
Time Number GLC
a) Reaction conditions; Pd(II)-hydrotalcite(m) (600 mg, 0.05
mmol Pd), alcohol (1.0 mmol), pyridine (1.0 mmol), toluene (10
mL), 80 ◦C, O2. b) Based on the starting substrate. The value in
parentheses is the conversion of alcohol (%). c) GLC yield.
h
2
of use yield/%
1a) Pd(II)-hydrotalcite
benzyl alcohol
first
98
second
third
89
77
2a) Pd(II)-hydrotalcite(m) benzyl alcohol
5
5
first
second
third
first
second
third
96
97
90
84c)
88c)
84c)
85c)
3b) Pd(II)-hydrotalcite(m) 4-methoxy-
benzyl alcohol
fourth
a) Reaction conditions;
Pd(II)-hydrotalcite or Pd(II)-
hydrotalcite(m) (900 mg or 1800 mg, 0.15 mmol Pd), benzyl
alcohol (3.0 mmol), pyridine (1.5 mmol), toluene (30 mL), 80 ◦C,
O2. b) Reaction conditions; Pd(II)-hydrotalcite(m) (600 mg, 0.05
mmol Pd), 4-methoxybenzyl alcohol (1.0 mmol), pyridine (0.5
mmol), toluene (10 mL), 80 ◦C, O2. c) Isolated yield (%).
be the steric bulkiness of Pd(II)-species immobilized on the ex-
ternal surface of hydrotalcite.
Reaction Scheme. We propose a plausible reaction path-
way as illustrated in Scheme 1 for the oxidation of alcohols in
this system, the pathway being intrinsically the same as that
proposed in the homogeneous system, in which the formation
of H2O2 was detected by KI-starch test.8b In the case of Pd(II)-
hydrotalcite system, the result of KI-starch test was negative,
suggesting that the decomposition of H2O2 may occur. Fur-
Scheme 1. Plausible reaction pathway.
thermore, the study of an oxygen uptake in the oxidation of
benzyl alcohol supported this reaction pathway. In the oxida-
tion of benzyl alcohol using catalytic amounts of Pd(OAc)2 and
pyridine, ca. 1:1 ratio of the amount of oxygen uptake to the

170 Bull. Chem. Soc. Jpn., 74, No. 1 (2001)
Aerobic Oxidation of Alcohols
Fig. 1. Measurement of O2 uptake using gas burette.
leaching of Pd was performed with a Shimadzu ICPS-1000 se-
quential plasma spectrometer.
yield of benzaldehyde was observed, whereas ca. 1:2 ratio was
observed when the reaction was carried out in the presence of
Pd(II)-hydrotalcite (Fig. 1). In MS-system, a 2:3 ratio was ob-
served.8b These results indicated that a generated H2O2 could
be decomposed by MS3A as well as hydrotalcite and that hy-
drotalcite works more effectively than MS3A for the decompo-
sition.
Conclusion. Pd(II)-hydrotalcite was newly prepared by a
simple operation from commercially available hydrotalcite,
Pd(OAc)2 and pyridine. This novel clay compound efficiently
catalyzed the aerobic oxidation of various kinds of alcohols, in-
cluding unsaturated ones. Modified Pd(II)-hydrotalcite [Pd(II)-
hydrotalcite(m)] could be reused several times while keeping
its catalytic activity.
Materials. Pd(OAc)2 was purchased from Wako Pure Chemi-
cal Ind., Ltd., and used without further purification. Hydrotalcites
(Mg6Al2(OH)16CO3•4H2O, brand name KYOWAAD® 500 and
ALCAMAC<L>®) were kindly supplied by Kyowa Chemical Ind.,
Ltd. Pyridine was purchased and used without further purifica-
tion. Toluene was distilled before use. MS3A powder was com-
mercially available from Nacalai Tesque Chemical Co., Inc.,
which was activated by calcination (by a gas burner) just before
use. All of alcohols except for three that are described below were
commercially available and were purified by known methods just
before use. Geranial was obtained by Swern oxidation of geraniol.
[Pd(OAc)2(py)2] complex was prepared by the reported method.11
2,2-Dimethyl-1,5-pentanediol (Table 2, Entry 4, 5d).
The
compound was prepared by the hydroboration-oxidation of 2,2-
dimethyl-4-pentenal. Colorless oil; 1H NMR (270 MHz) δ 0.89 (s,
6H), 1.25–1.35 (m, 2H), 1.48–1.60 (m, 2H), 1.87 (br s, 2H), 3.33
(s, 2H), 3.64 (t, J = 6.3 Hz, 2H); 13C NMR (100 MHz) δ 24,0, 27.0,
34.3, 34.8, 63.6, 71.3.
trans-4,8-Dimethyl-3,7-nonadien-2-ol (Table 3, Entry 10).
The compound was prepared by the reaction of trans-3,7-dimeth-
yl-2,6-octadien-1-one (geranial) with MeLi.22 Colorless oil;
1H NMR (400 MHz) δ 1.23 (d, J = 6.3 Hz, 3H), 1.36 (br s, 1H),
1.60 (s, 3H), 1.68–1.70 (m, 6H), 1.97–2.12 (m, 4H), 4.58 (dq, J =
8.6, 6.3 Hz, 1H), 5.07–5.11 (m, 1H), 5.20–5.23 (m, 1H); 13C NMR
(75.5 MHz) δ 16.4, 17.7, 23.6, 25.7, 26.4, 39.4, 64.8, 123.9, 129.1,
131.7, 137.6.
4-Tridecyn-1-ol (Chart 3, 7a). The compound was prepared
by the reported procedure.23 Colorless oil; 1H NMR (400 MHz) δ
0.88 (t, J = 6.8 Hz, 3H), 1.27–1.39 (m, 10H), 1.44–1.51 (m, 2H),
1.74 (tt, J = 6.4, 6.4 Hz, 2H), 1.86 (br s, 1H), 2.13 (dt, J = 6.8, 2.4
Hz, 1H), 2.14 (dt, J = 6.8, 2.4 Hz, 1H), 2.28 (dt, J = 6.8, 2.4 Hz,
1H), 2.29 (dt, J = 6.8, 2.4 Hz, 1H), 3.76 (t, J = 6.2 Hz, 2H);
13C NMR (75.5 MHz) δ 14.1, 15.5, 18.7, 22.7, 28.9, 29.1, 29.2,
29.2, 31.7, 31.9, 62.0, 79.3, 81.1.
General Procedure for Preparation and Characterization of
Pd(II)-Hydrotalcite and Pd(II)-Hydrotalcite(m). To a mixture
of Pd(OAc)2 (375 mg, 1.67 mmol) and toluene (100 mL) in a 200
mL two necked flask was added pyridine (331 mg, 4.18 mmol) at
Experimental
General. 1H and 13C NMR spectra were measured on JEOL
EX-400, JNM-AL300, and JEOL GSX270 spectrometers for solu-
tions in CDCl3 with Me4Si as an internal standard. GLC analyses
were carried out with a Shimadzu GC-14A instrument (2 m × 3
mm glass column packed with 5% OV®-17 on Chromosorb® W,
5.0 µm film thickness) and Shimadzu fused silica capillary column
(HiCap CBP10-S25-050) using helium as a carrier gas. GLC
yields were determined using bibenzyl, cyclododecane or pentam-
ethylbenzene as an internal standard. Analytical thin layer chro-
matographies (TLC) were performed with Merck silica gel 60 F-
254 plates. Column chromatographies were performed with Mer-
ck silica gel 60. XRD data were obtained on a Shimadzu XD-D1
diffractometer using Cu Kα radiation and a carbon monochrome-
ter. IR spectra were recorded with a Nicolet Impact 400 FT-IR
spectrometer. UV spectra were analyzed with a Shimadzu MSP
200 spectrophotometer equipped with a diffuse-reflectance unit.
CP-MAS 13C NMR spectra were measured on a JEOL GSX270
spectrometer with pentamethylbenzene as an external standard.
TEM analysis was performed by a Hitachi H 800 transmission
electron microscope. TG/MS analysis was done with a Shimadzu
QP 1000 mass spectrometer equipped with a Shimadzu TG-50
thermogravimeter. TG-DTA data were obtained with a Shimadzu
TG-50A thermal analyzer. ICP atomic emission analysis for

N. Kakiuchi et al.
Bull. Chem. Soc. Jpn., 74, No. 1 (2001) 171
80 °C. The brown suspension turned to a yellow-white suspension
when pyridine was added. Hydrotalcite (KYOWAAD® 500, 10.0
g)24 was added and the mixture was stirred vigorously for 1 h at 80
°C. Then, the resulting slurry was cooled to 0 °C, followed by fil-
tration and washing with diethyl ether (20 mL × 2). The resulting
solid was dried under vacuum at room temperature to give ca. 10 g
of a light-yellow powder of Pd(II)-hydrotalcite. The Pd content in
the Pd(II)-hydrotalcite was 0.16 mmol g–1 as estimated by ICP
atomic emission analysis. Elemental analysis, Pd(II)-hydrotalcite:
N, 0.35% [Calcd N, 0.45 wt% calculated from the result of ICP
atomic emission analysis, assuming that Pd atoms exist in the
[Pd(OAc)2(py)2] form in Pd(II)-hydrotalcite]. The values of spac-
ing d003 were estimated by XRD analysis as follows: commercially
available hydrotalcite (KYOWAAD® 500), 7.81 Å, and Pd(II)-hy-
drotalcite, 7.75 Å. Pd(II)-hydrotalcite(m) was prepared by a simi-
lar method to that described above using a half scale of Pd(OAc)2
(188 mg), pyridine (165 mg), and toluene (50 mL) with 10 g of hy-
drotalcite. The Pd content in the Pd(II)-hydrotalcite(m) was 0.092
mmol g–1 estimated by ICP atomic emission analysis.
2927, 2855, 2724, 2353, 2232, 1731, 1464, 1455, 1435, 1411,
1386, 1358, 1332, 1116, 1058, 895, 848, 722 cm–1. 1H NMR (300
MHz) δ 0.88 (t, J = 6.8 Hz, 3H), 1.27–1.36 (m, 10H), 1.41–1.51
(m, 2H), 2.11 (dt, J = 7.1, 2.3 Hz, 1H), 2.13 (dt, J = 6.9, 2.4 Hz,
1H), 2.46–2.51 (m, 2H), 2.59–2.65 (m, 2H), 9.79 (t, J = 1.4 Hz,
1H); 13C NMR (75.5 MHz) δ 12.2, 14.1, 18.7, 22.7, 28.9, 28.9,
29.1, 29.2, 31.9, 43.0, 77.7, 81.7, 201.1. Found: C, 80.47; H,
11.37%. Calcd for C13H22O: C, 80.35; H, 11.41%.
General Procedure for Recycling of the Catalyst. First run
of the oxidation of benzylic alcohols catalyzed by Pd(II)-hydrotal-
cite or Pd(II)-hydrotalcite(m) was performed using the procedure
described above. Recovered Pd(II)-hydrotalcite or Pd(II)-hydro-
talcite(m) was washed with diethyl ether (20 mL × 2) and dried un-
der vacuum at room temperature before use for the next run. The
method of the test for Pd-leaching was as follows: the usual oxida-
tion of benzyl alcohol was allowed to proceed for 30 min, and the
catalyst was removed by filtration at 80 °C. Then the filtrate con-
taining the product benzaldehyde and unreacted benzyl alcohol
was stirred under O2 at 80 °C. The reaction was monitored with
GLC using cyclododecane as an internal standard. By ICP atomic
emission analysis, the amount of leached palladium in oxidation of
benzyl alcohol under the conditions shown in Table 5 was estimat-
ed; in the case of Pd(II)-hydrotalcite, av 14%; Pd(II)-hydrotal-
cite(m), av 0.8%.
The residue which was obtained by washing Pd(II)-hydrotalcite
with pyridine was analyzed by 1H and 13C NMR. Major peaks of
1H and 13C NMR spectra were the same as those of the
[Pd(OAc)2(py)2] complex.
General Procedure for Pd(II)-Hydrotalcite or Pd(II)-Hy-
drotalcite(m)-Catalyzed Oxidation of Alcohols Using Molecu-
lar Oxygen.9 A typical experimental procedure is as follows: to
a suspension of the Pd(II)-hydrotalcite (300 mg, 0.05 mmol as Pd)
or Pd(II)-hydrotalcite(m) (600 mg, 0.05 mmol as Pd) in toluene (6
mL) in a 20 mL two-necked flask was added pyridine (0.2–5
mmol) and the mixture was stirred. Then, oxygen gas was intro-
duced into the flask from an O2-balloon under atmospheric pres-
sure, and the mixture was heated to 80 °C for ca. 10 min. Next, an
alcohol (1 mmol) in toluene (4 mL) was added and the mixture
was stirred vigorously for 2 h (or some appropriate time) at 80 °C
under oxygen. After the reaction, the catalyst was separated by fil-
tration through a glass filter. Removal of the solvent from the fil-
trate under the reduced pressure left an oily residue which was
subjected to column chromatography (Merck silica gel 60; elu-
ents, hexane−diethyl ether) to give a product. Products obtained
were determined by 1H and 13C NMR and GC/MS.
General Procedure for Pd(OAc)2/Pyridine/MS3A System-
Catalyzed Oxidation of Unsaturated Alcohols Using Molecu-
lar Oxygen.8b An improved procedure for the oxidation of un-
saturated alcohols using homogeneous catalyst is as follows: to a
suspension of the Pd(OAc)2 (11.2 mg, 0.05 mmol) in toluene (4
mL) in a 20 mL two-necked flask were added pyridine (5 mmol)
and MS3A (500 mg), and the mixture was stirred at room tempera-
ture. Then, oxygen gas was introduced into the flask from an O2-
balloon under atmospheric pressure. Next, an alcohol (1 mmol) in
toluene (6 mL) was added at room temperature and the mixture
was heated to 80 °C and stirred vigorously for an appropriate time
under oxygen. After the reaction, the mixture was filtered through
a pad of Florisil. Removal of the solvent from the filtrate under the
reduced pressure left an oily residue which was subjected to col-
umn chromatography (Merck Silica gel 60; eluents, hexane−dieth-
yl ether) to give a product.
The authors are grateful to Kyowa Chemical Ind., Ltd. for
the gift of hydrotalcites (Mg6Al2(OH)16CO3•4H2O, brand name
KYOWAAD® 500 and ALCAMAC<L>®) for this study. We
also thank Dr. Tatsuya Takeguchi (Kyoto Univ.) for his help in
CP-MAS 13C-NMR analysis and Dr. Shinji Iwamoto (Kyoto
Univ.) for his help in ICP atomic emission analysis. T. N.
gratefully acknowledges a Fellowship of the Japan Society for
the Promotion of Science forYoung Scientists.
References
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Our previous reports about organic reactions using hetero-
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1
Colorless oil; H NMR (400 MHz) δ 1.61 (s, 3H), 1.69 (s, 3H),
2.13–2.18 (m, 10H), 5.06–5.08 (m, 1H), 6.07 (s, 1H); 13C NMR
(75.5 MHz) δ 17.7, 19.3, 25.7, 26.2, 31.7, 41.2, 123.1, 123.6,
132.5, 158.2, 198.7.
6
K. Kaneda, T. Yamashita, T. Matsushita, and K. Ebitani, J.
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4-Tridecyn-1-al (Chart 1, 8a). Colorless oil; IR (neat) 2955,

172 Bull. Chem. Soc. Jpn., 74, No. 1 (2001)
Aerobic Oxidation of Alcohols
7
K. Yamaguchi, K. Mori, T. Mizugaki, K. Ebitani, and K.
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a) T. Nishimura, T. Onoue, K. Ohe, and S. Uemura, Tetrahe-
for maintaining the oxidation state of Pd(II). The absence of addi-
tional pyridine might cause the formation of Pd(0) species, result-
ing in a low catalytic activity. Actually, the color of the powder of
the recovered catalyst changed from yellow-white to gray after the
reaction without a further addition of pyridine, while such color
change was not observed in the reaction with additional pyridine.
17 G. Procter, in “Comprehensive Organic Synthesis,” ed. by
B. M. Trost and I. Fleming, Pergamon, Oxford (1991), Vol. 7, pp.
312–318.
8
dron Lett., 39, 6011 (1998). b) T. Nishimura, T. Onoue, K. Ohe,
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similar catalytic system to the oxidation of tert-cyclobutanols: c) T.
Nishimura, K. Ohe, and S. Uemura, J. Am. Chem. Soc., 121, 2645
(1999).
9
T. Nishimura, N. Kakiuchi, M. Inoue, and S. Uemura,
Chem. Commun., 2000, 1245.
18 In our previous report, we have also shown that a chemo-
selectivity between the primary and secondary hydroxyl group was
hardly observed in the oxidation of diol 5d using Pd(OAc)2/pyri-
dine/MS3A system (Ref. 8b). However, see for selective oxidation
of 5d; Y. Tamaru, Y. Yamada, K. Inoue, Y. Yamamoto, and Z.
Yoshida, J. Org. Chem., 48, 1286 (1983).
19 We suppose that the present reaction proceeds in a similar
pathway to the homogeneous catalytic system as previously de-
scribed (See, Ref. 8b). Thus, pyridines coordinate to Pd(II) as
ligands and stabilize the Pd(II)-hydride species to avoid the reduc-
tive elimination of HX from HPdX species to give Pd(0).
20 In our previous report, we failed the oxidation of geraniol in
MS-system. However, this limitation could be overcome by slight-
ly improving the procedure. See experimental section.
21 The difference of the reaction rates by the size of alcohols
was little when the oxidation of these cyclic alcohols was carried
out using Pd(II)-hydrotalcite as a catalyst.
10 As examples of palladium-catalyzed aerobic oxidation, see
for example: a) T. F. Blackburn and J. Schwartz, J. Chem. Soc.,
Chem. Commun., 1977, 157. b) M. Hronec, Z. Cvengrosová, and J.
Kizlink, J. Mol. Catal., 83, 75 (1993). c) E. Gómez-Bengoa, P.
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287, 1636 (2000).
11 [Pd(OAc)2(py)2] complex was prepared by treatment of
Pd(OAc)2 with pyridine, see: a) T. A. Stephenson, S. M.
Morehouse, A. R. Powell, J. P. Heffer, and G. Wilkinson, J. Chem.
Soc., 1965, 3632. b) S. V. Kravtsova, I. P. Romm, A. I. Stash, and
V. K. Belsky, Acta Crystallogr., C52, 2201, (1996).
12 F. Cavani, F. Trifirò, and A. Vaccari, Catal. Today, 11, 173
(1991).
13 [Pd(OAc)2(py)2] complex decomposed at 185 °C; see Ref.
11a.
22 W. Adam, C. M. Mitchell, R. Paredes, A. K. Smerz, and L.
A. Veloza, Liebigs Ann./Recueil, 1997, 1365.
23 Y. Torisawa, A. Hashimoto, M. Nakagawa, H. Seki, R.
Hara, and T. Hino, Tetrahedron, 47, 8067 (1991).
14 [Pd(OAc)2(py)2] complex was detected by 1H and 13C NMR
of the crude mixture.
15 Although it is not clear what kinds of interactions let a pal-
ladium–pyridine complex be immobilized on hydrotalcite, there is
a possibility of ionic bonding between an acetoxy carbonyl moiety
of the palladium−pyridine complex and hydroxyl groups on the
surface of the hydrotalcite. Short comment about this kind of inter-
action was stated (See Ref. 5c).
24 KYOWAAD® 500 has the same structure of hydrotalcite as
ALCAMAC<L>®. Although both hydrotalcites can be used as a
support, the former showed more reproducible results than the lat-
ter one. In our previous communication (Ref. 9), we described the
brand name of hydrotalcite as ALCAMAC<L>® incorrectly. KY-
OWAAD® 500 is correct.
16 The presence of excess pyridine (to palladium) is essential