Pd(II)-hydrotalcite-catalyzed oxidation of alcohols to aldehydes and ketones using atmospheric pressure of air

Article abstract of DOI:10.1021/jo010338r

A heterogenized Pd catalyst, Pd(II)-hydrotalcite (palladium(II) acetate-pyridine supported by hydrotalcite) catalyzes the aerobic oxidation in toluene of a variety of primary and secondary alcohols into the corresponding aldehydes and ketones in high yields using atmospheric pressure of air as a sole oxidant under mild conditions. This catalyst is also effective for the oxidation of allylic alcohols, especially such as geraniol and nerol, without any isomerization of an alkenic part. The catalyst can be easily prepared from all commercially available reagents and reused several times.

Full text of DOI:10.1021/jo010338r

6620
J . Org. Chem. 2001, 66, 6620-6625
P d (II)-Hyd r ota lcite-Ca ta lyzed Oxid a tion of Alcoh ols to Ald eh yd es
a n d Keton es Usin g Atm osp h er ic P r essu r e of Air
Nobuyuki Kakiuchi, Yasunari Maeda, Takahiro Nishimura, and Sakae Uemura*
Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University,
Sakyo-ku, Kyoto 606-8501, J apan
uemura@scl.kyoto-u.ac.jp
Received March 30, 2001
A heterogenized Pd catalyst, Pd(II)-hydrotalcite (palladium(II) acetate-pyridine complex supported
by hydrotalcite) catalyzes the aerobic oxidation in toluene of a variety of primary and secondary
alcohols into the corresponding aldehydes and ketones in high yields using atmospheric pressure
of air as a sole oxidant under mild conditions. This catalyst is also effective for the oxidation of
allylic alcohols, especially such as geraniol and nerol, without any isomerization of an alkenic part.
The catalyst can be easily prepared from all commercially available reagents and reused several
times.
In tr od u ction
atmospheric pressure of air a relatively narrow range of
substrates such as benzylic alcohols could be oxidized.16,17
For example, Sheldon et al. reported a quite sophisticated
catalytic system for oxidation of alcohols in water using
palladium salt as a catalyst and air as a sole oxidant,
where the oxidation was performed at 100 °C under 30
bar pressure of air.7j Recently, we have reported that the
combination of Pd(OAc)2/pyridine/MS3A could catalyze
the aerobic oxidation of alcohols using pure oxygen.18 This
novel homogeneous oxidation system was also expanded
Oxidation of alcohols to the corresponding aldehydes
or ketones is one of the most fundamental reactions in
organic chemistry.1 Many effective reaction systems have
been developed using a stoichiometric amount of chro-
mium salts2 or other oxidants such as oxalyl chloride3
and hypervalent iodines.4 Although these traditional
procedures are quite useful in laboratory-scale reactions,
the waste of heavy metals or stoichiometric reagents and
the generation of undesirable coproducts damage the
environment seriously in bulk-scale reaction. From the
standpoint of the so-called green and sustainable chem-
istry, another approach to construct the cleaner catalytic
system for this reaction has been demanded. Recently,
metal-catalyzed oxidation of alcohols using clean and
cheap oxidants such as H2O2,5 O2, and/or air has been
investigated. Especially, air is the most suitable oxidant
because it is safe, clean, and available free. Metal-
catalyzed aerobic oxidation of alcohols is regarded as the
most attractive and ideal technology considering its atom-
efficiency, and many procedures using metal catalysts
such as Ru,6 Pd,7 Co,8 Cu,9 Pt,10 Rh,11 V,12 Os,13 Ce,14 and
Ni15 have so far been reported. However, the utilities of
atmospheric pressure of air are still limited in most of
these aerobic oxidation systems, where pure oxygen or a
high pressure of air under severe reaction conditions was
generally required in order to maintain the activity of
the metal catalysts, and even in the rare example using
to
palladium-reusable
system
using
a
clay
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10.1021/jo010338r CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/30/2001

Pd(II)-Hydrotalcite-Catalyzed Oxidation of Alcohols
J . Org. Chem., Vol. 66, No. 20, 2001 6621
as a support.19 Thus, palladium(II) acetate-pyridine
complex was found to be easily supported by hydrotalcite
(Mg6Al2(OH)16CO3‚4H2O, basic clay mineral)20 to give a
heterogeneous palladium catalyst (abbreviated as Pd(II)-
hydrotalcite).19a,b For the purpose of constructing “greener”
oxidation system, we investigated the Pd(II)-hydrotal-
cite-catalyzed oxidation of alcohols using atmospheric
pressure of air instead of pure molecular oxygen. The
successful result of this catalytic reaction is described in
this paper.
Ta ble 1. P d -Ca ta lyzed Oxid a tion of Ben zyl Alcoh ol
u n d er 1 Atm Air a
Resu lts a n d Discu ssion
a
Op tim iza tion of Rea ction Con d ition s. First, the
oxidation of benzyl alcohol using previously reported
homogeneous and heterogeneous Pd(II)-catalytic sys-
tems18,19a,b was performed under atmospheric pressure
of air in balloon (Table 1). In Pd(OAc)2/pyridine or Pd-
(OAc)2/pyridine/MS3A system (homogeneous system),
benzaldehyde was obtained in good yield within 3 h
(Table 1, entries 1 and 2), and Pd(II)-hydrotalcite system
gave the best result (98% yield, entry 3). Furthermore,
the recovered catalyst could be used for further reaction
(90% yield).
Reaction conditions: Pd-catalyst (0.05 mmol), benzyl alcohol
(1.0 mmol), pyridine (0.2 mmol), toluene (10 mL), air (balloon, 1
atm) at 65 °C for 3 h. Conversion of benzyl alcohol. c MS3A (500
b
mg) was used.
Ta ble 2. Effect of Rea ction Tem p er a tu r e for
P d (II)-Hyd r ota lcite-Ca ta lyzed Oxid a tion of Alcoh ols
u n d er 1 Atm Air a
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(16) Aerobic oxidation using atmospheric pressure of air, for example
see: refs 6i, 6u, 7i, 7k, 13a, 13c, and 14.
(17) Aerobic oxidation using atmospheric pressure of oxygen or air,
for example see: refs 6g, 6k, 9f, 9h, 9i, 9k, 9l, 9m.
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1 2000, 4301.
a
Reaction conditions: Pd(II)-hydrotalcite (0.05 mmol), alcohol
(1.0 mmol), pyridine (0.2 mmol), toluene (10 mL), air (balloon, 1
atm). Conversion of alcohol. c The color of Pd(II)-hydrotalcite
b
d
turned black. Isolated yield.
Table 2 shows the result of aerobic oxidation of benzyl
alcohol and dodecan-1-ol using Pd(II)-hydrotalcite as a
catalyst. At 80 °C or 70 °C, benzyl alcohol was converted
into benzaldehyde in quantitative yield. In these reac-
tions, however, the color of the catalyst turned from
white-yellow to black, showing the formation of an
inactive Pd-black by the reduction of Pd(II) species. On
the other hand, benzaldehyde was obtained in 98% yield
without any formation of such Pd-black at 65 °C. In the
oxidation of dodecan-1-ol at 80 °C, Pd-black was formed
during the reaction resulting in the slow reaction (70%
yield of dodecanal, after 13 h), while dodecanal was
obtained in 91% yield at 65 °C for 10 h.
Next, the effect of the amount of pyridine on the
oxidation of benzyl alcohol was investigated. As shown
in Figure 1, when pyridine was not added, oxidation did
not proceed efficiently; at least 4 mol equiv of pyridine
to Pd(II) was needed to maintain the catalytic activity.
Concentration of the substrate was also an important
factor. When the oxidation of benzyl alcohol was per-
formed using toluene (0.2 M), the color of Pd(II)-
hydrotalcite turned black and the yield of benzaldehyde
decreased (77%). Therefore, we decided the optimum
reaction condition to be the use of alcohol, Pd(II)-
hydrotalcite (5 mol %), pyridine (20 mol %) in toluene
(0.1 M) under 1 atm air (in balloon) at 65 °C.
Oxid a tion of Ben zylic a n d Alip h a tic Alcoh ols. We
performed the oxidation of various benzylic and aliphatic
alcohols under the above-described optimum condition,
the results of which are listed in Table 3. Primary
benzylic alcohols were readily oxidized to the correspond-
(20) Cavani, F.; Trifiro`, F.; Vaccari, A. Catal. Today 1991, 11, 173.

6622 J . Org. Chem., Vol. 66, No. 20, 2001
Kakiuchi et al.
The excess pyridine may prevent from the formation of
Pd(II)-alkene complex which might accelerate the reduc-
tion of Pd(II). Citronellol and 10-undecen-1-ol were
converted to the corresponding aldehydes in good yields
(Table 4, entries 1 and 2). A variety of allylic alcohols
were also efficiently oxidized by Pd(II)-hydrotalcite
(Table 4, entries 3, 5-8, 10, 11). The selective oxidation
of geraniol [(E)-isomer] and nerol [(Z)-isomer] proceeded
smoothly to give the corresponding aldehydes in 90% and
85% yields without any change of E/Z ratios (Table 4,
entries 6-8). Similarly, trans-4,8-dimethyl-3,7-nonadien-
2-ol and farnesol were oxidized to the corresponding
ketone and aldehyde, keeping their E/Z ratios unchanged
(Table 4, entries 10 and 11). We have already reported
that the geometrical isomerization occurred to change the
E/Z ratios of products in the oxidation of these substrates
using Pd(OAc)2/pyridine/MS3A system.19b We suppose
that the interaction of Pd(II)-intermediate species such
as Pd(II)-H with allylic moieties in both substrates and
products causes such isomerization in Pd(OAc)2/pyridine/
MS3A system. Although details are not yet known, such
interaction may be prevented by the steric bulkiness of
hydrotalcite surface to result in no isomerization.19b For
comparison, the representative data using oxygen as an
oxidant are also shown in Table 4 (entries 4, 9, and 12).
Oxid a tion of Diols. We carried out the oxidation of
decane-1,10-diol under the above optimum condition, but
the reaction could not proceed completely (Table 5, entry
1). Even the addition of an excess pyridine was found to
be ineffective (entry 2). However, its oxidation proceeded
smoothly under 3 atm of air at 80 °C to afford decanedial
2 as the major product with a small amount of hydroxy
aldehyde 1 (entry 3). In the case of cyclohexane-1,4-diol,
the reaction could not proceed completely and a ca. 4:1
mixture of mono- and diketones was obtained even under
3 atm of air (entry 4), while vic-diol such as 1,2-
diphenylethane-1,2-diol was converted to the correspond-
ing diketone in 74% yield (entry 5). Oxidative lactoniza-
tion took place to give the corresponding lactone in the
oxidation of R,ω-primary diol in 76% isolated yield under
3 atm air (entry 6).21 In the case of this substrate, the
oxidation under 1 atm oxygen gave a slightly better result
than that of under 3 atm air (entry 7).
Oxid a tion of Ben zyl Alcoh ol u n d er 3 a tm of Air .
From the viewpoint of economical and environmental
concerns, we attempted to reduce the amount of solvent.
As described above, higher concentration of alcohol
decreased the reaction rate when the oxidation was
performed under 1 atm of air. However, it was disclosed
that the oxidation under 3 atm of air smoothly proceeded
even at higher concentration (Table 6, entries 1 and 2).
Furthermore, even in the case of reactions using 1 mol
% catalyst, benzaldehyde was obtained in high yield
within 5 h (entries 3 and 4).
Lim ita tion s. Pd(II)-hydrotalcite could act as an ef-
fective catalyst for the oxidation of a wide variety of
alcohols, but some limitations in substrates exist (Figure
2). The oxidation of 5 and 6 could not proceed. The
oxidation of 7 and 8 was very slow, and some unidentified
products were produced. In the cases of 9 and 10, which
had an alkynic part, undesirable polymerization occurred
to give a complex mixture. When the oxidation of alco-
F igu r e 1. Time profile of the aerobic oxidation of benzyl
alcohol: effect of the amount of pyridine to product yield.
ing aldehydes in 76-98% yields (Table 3, entries 1, 2,
4-9). It should be noted that this catalytic system showed
the compatibility with either electron-donating or electron-
withdrawing substituents in the benzene ring although
the oxidation of the compound having a substituent at
ortho-position was slightly slow. The amount of Pd(II)-
hydrotalcite could be reduced to 1 mol % to the substrate
(30 h, entry 5). A slightly larger-scale oxidation of
p-methoxybenzyl alcohol (1.385 g, 10.0 mmol) was found
to give p-methoxybenzaldehyde in high isolated yield
(1.244 g, 9.1 mmol, entry 6). When the oxidation of
2-phenylethanol was attempted for 5 h under the above
optimum condition, acetophenone was obtained in 75%
yield (entry 10), but its yield became quantitative using
more pyridine (entry 11). Other secondary benzylic
alcohols were also easily converted to the corresponding
ketones in excellent yields. The oxidation of R-ketol such
as benzoin proceeded to give the corresponding diketone
in 98% yield, although longer reaction time (32 h) and
more pyridine were required (entry 14). The saturated
aliphatic primary and secondary alcohols could be oxi-
dized to the corresponding aldehydes and ketones selec-
tively in high yields (entries 15, 16, 18, 19, and 21). The
oxidation of aliphatic alcohols was slower than that of
benzylic ones. The undesired byproducts such as the
corresponding carboxylic acids, and their esters were
hardly produced in the oxidation of these alcohols. For
comparison, the representative data using oxygen as an
oxidant are also shown in Table 3 (entries 3, 12, 17, and
20). The results of entries 2 and 16 showed that the
oxidation in an open flask equipped with CaCl2 drying
tube (in place of air balloon) also proceeded smoothly to
give the corresponding products in similar yields.
Oxid a tion of Alk en ic Alcoh ols. The oxidation of
alkenic alcohols using Pd(II)-hydrotalcite under air was
carried out, the typical results of which are shown in
Table 4. There are still few reports about effective
oxidation of alkenic alcohols by palladium catalysts
because of their strong complexation with unsaturated
carbon-carbon double bonds. It has been revealed that
Pd(II)-hydrotalcite was an effective catalyst for the
aerobic oxidation of variety of alkenic alcohols using
molecular oxygen in our previous reports.19a,b Reactions
also proceeded smoothly under atmospheric air, and
several alkenic alcohols were converted into the corre-
sponding aldehydes and ketones in high to excellent
yields. The addition of a quite excess of pyridine was
essential for the sufficient conversion of alkenic alcohols.
(21) Procter, G. In Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Ley, S. V., Eds.; Pergamon: Oxford, U. K., 1991; Vol. 7;
pp 312-318.

Pd(II)-Hydrotalcite-Catalyzed Oxidation of Alcohols
J . Org. Chem., Vol. 66, No. 20, 2001 6623
Ta ble 3. P d (II)-Hyd r ota lcite-Ca ta lyzed Oxid a tion of Alcoh ols u n d er 1 Atm Air a
a
Reaction conditions: Pd(II)-hydrotalcite (0.05 mmol), alcohol (1.0 mmol), pyridine, toluene (10 mL), air (balloon, 1 atm) at 65 °C.
Conversion of alcohol. c GLC yield. Open air system using CaCl2 drying tube. e Oxygen (balloon, 1 atm) at 80 °C. f Pd(II)-hydrotalcite
(0.01 mmol; 1 mol % Pd) was used. 10 Fold scale reaction.
b
d
g
hols, which had a nitrogen atom such as 11 and 12, was
performed, no reaction was observed even using excess
pyridine probably due to the complexation of palladium
with the nitrogen atom. 3-Thiophenemethanol 13 was
converted to the corresponding aldehyde only in low yield
even using excess pyridine (13%, after 14 h).
to the corresponding aldehydes and ketones, respectively,
using atmospheric pressure of air as a sole oxidant in
toluene at 65 °C. The results are intrinsically quite
similar to those obtained using molecular oxygen, but the
air oxidation proceeds at lower temperature. This catalyst
can be easily recovered and reused for several times.
These features match to today’s demands for construction
of clean and environmentally friendly catalytic reactions.
Recyclin g of th e Ca ta lyst. Pd(II)-hydrotalcite can
be recovered easily from the reaction media by simple
filtration after the reaction and could be reused for the
next run (Table 7). In the air oxidation of benzyl alcohol
using Pd(II)-hydrotalcite, benzaldehyde was obtained in
the following yield; first: 98%, second: 90%, third: 72%
(entry 1). In the third run, the yield of benzaldehyde
slightly decreased, mainly because of leaching of Pd(II)
species.22 It has been revealed that leaching of palladium
could be avoided by using a slightly modified catalyst,
abbreviated as Pd(II)-hydrotalcite(m) in which Pd con-
tent was ca. a half amount compared with the standard
Pd(II)-hydrotalcite (see Experimental Section). As ex-
pected, the loss of catalytic activity was prevented in
some extent (entry 2; first: 98%, second: 94%, third:
82%).
Exp er im en ta l Section
Gen er a l Meth od s. 1H NMR spectra were obtained in
CDCl3 at 300 or 400 MHz with Me4Si as an internal standard.
13C NMR spectra were obtained at 75.5 or 100 MHz. GLC
analyses were carried out with a Shimadzu GC-14A instru-
ment (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 cyclodode-
cane as an internal standard. Analytical thin layer chromatog-
raphies (TLC) were performed with Merck silica gel 60 F-254
plates. Column chromatographies were performed with Merck
silica gel 60.
Ma t er ia ls. Pd(OAc)2 was purchased from Wako Pure
Chemical Ind., Ltd., and used without further purification.
Hydrotalcite (Mg6Al2(OH)16CO3‚4H2O, brand name KYOWAAD
Con clu sion s
Pd(II)-hydrotalcite which can be prepared by simple
operation from commercially available hydrotalcite, Pd-
(OAc)2, and pyridine, efficiently catalyzes the aerobic
oxidation of a variety of primary and secondary alcohols
(22) By ICP atomic emission analysis, the amount of leached
palladium in oxidation of benzyl alcohol under O2 was estimated; in
the case of Pd(II)-hydrotalcite, av. 14%; Pd(II)-hydrotalcite(m), av.
0.8%. See ref 19b.

6624 J . Org. Chem., Vol. 66, No. 20, 2001
Ta ble 4. P d (II)-Hyd r ota lcite-Ca ta lyzed Oxid a tion of Alk en ic Alcoh ols u n d er 1 Atm Air a
Kakiuchi et al.
a
Reaction conditions: Pd(II)-hydrotalcite (0.05 mmol), alcohol (1.0 mmol), pyridine (5.0 mmol), toluene (10 mL), air (balloon, 1 atm)
at 65 °C. Conversion of alcohol. c Oxygen (balloon, 1 atm) at 80 °C. E/Z ratio was determined by 1H NMR. e Open air system using
b
d
CaCl2 drying tube. f E/Z ratio of neral was 4/96.
Ta ble 5. P d (II)-Hyd r ota lcite-Ca ta lyzed Oxid a tion of Diols u n d er 3 Atm Air a
a
b
Reaction conditions: Pd(II)-hydrotalcite (0.05 mmol), alcohol (1.0 mmol), pyridine, toluene (10 mL), air (3 atm) at 80 °C. Conversion
of alcohol. c Air (balloon, 1atm) at 65 °C. Oxygen (balloon, 1 atm) at 80 °C.
d
Ta ble 6. P d (II)-Hyd r ota lcite-Ca ta lyzed Oxid a tion of
fore use. Air was not dried. All of the alcohols except for
one described below were commercial products and purified
by known methods just before use. Pd(II)-hydrotalcite and
Pd(II)-hydrotalcite(m) were prepared by using the reported
procedure.19b The Pd content in the Pd(II)-hydrotalcite and
Ben zyl Alcoh ol u n d er 3 Atm Air a
catalyst
(mmol)
toluene
(mL)
time
(h)
conv
(%)b
GLC yield
(%)
entry
Pd(II)-hydrotalcite(m) was 0.16 mmol g-1 and 0.092 mmol g-1
,
1
2
3
4
0.05
0.05
0.01
0.01
10
5
3
1
1
5
5
97
100
93
95
96
91
95
respectively, as estimated by ICP atomic emission analysis.
tr a n s-4,8-Dim eth yl-3,7-n on a d ien -2-ol (Ta ble 4, en tr y
10). The compound was prepared by the reaction of trans-3,7-
dimethyl-2,6-octadien-1-one (geranial) with MeLi.23 A colorless
oil; 1H NMR (400 MHz, CDCl3) 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 Hz, 6.3 Hz, 1H), 5.07-5.11 (m, 1H),
5.20-5.23 (m, 1H); 13C NMR (75.5 MHz, CDCl3) δ 16.4, 17.7,
23.6, 25.7, 26.4, 39.4, 64.8, 123.9, 129.1, 131.7, 137.6.
2
97
a
Reaction conditions: Pd(II)-hydrotalcite, benzyl alcohol (1.0
mmol), pyridine (0.20 mmol), toluene, air (3 atm) at 80 °C.
bConversion of benzyl alcohol.
500) was kindly supplied by Kyowa Chemical Ind., Ltd.
Pyridine was purchased and used without further purification.
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 be-
(23) Adam, W.; Mitchell, C. M.; Paredes, R.; Smerz, A. K.; Veloza,
L. A. Liebigs Ann./ Recueil 1997, 1365.

Pd(II)-Hydrotalcite-Catalyzed Oxidation of Alcohols
J . Org. Chem., Vol. 66, No. 20, 2001 6625
determined by 1H and 13C NMR and GC/MS. GLC yields were
determined using cyclododecane as an internal standard.
tr a n s-4,8-Dim eth yl-3,7-n on a d ien -2-on e (Ta ble 4, en tr y
10). A colorless oil; 1H NMR (400 MHz, CDCl3) 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, CDCl3) δ 17.7, 19.3, 25.7, 26.2, 31.7,
41.2, 123.1, 123.6, 132.5, 158.2, 198.7.
10-Hyd r oxyd eca n a l (1). A white solid; 1H NMR (300 MHz,
CDCl3) δ 1.26-1.43 (m, 11H), 1.52-1.67 (m, 4H), 2.42 (td, J
) 7.3 Hz, 1.6 Hz, 2H), 3.62-3.66 (m, 2H), 9.76 (t, J ) 1.6 Hz,
1H); 13C NMR (75.5 MHz, CDCl3) δ 22.1, 25.7, 29.2, 29.3, 29.3,
29.4, 32.8, 43.9, 63.1, 203.0.
1
1,10-Deca n ed ia l (2). A colorless oil; H NMR (300 MHz,
CDCl3) δ 1.26-1.43 (m, 8H), 1.60-1.65 (m, 4H), 2.43 (td, J )
7.3 Hz, 1.7 Hz, 4H), 9.76 (t, J ) 1.7 Hz, 2H); 13C NMR (75.5
MHz, CDCl3) δ 22.0, 29.1, 29.1, 43.9, 202.9.
Gen er a l P r oced u r e for P d (OAc)2/P yr id in e/MS3A Sys-
tem -Ca ta lyzed Oxid a tion of Alcoh ols Usin g Air (1 a tm ).
A typical experimental procedure for the oxidation of unsatur-
ated alcohols using the title system is as follows: to a
suspension of 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
temperature. Then, unpurified air was introduced into the
flask from a balloon under atmospheric pressure. Next, an
alcohol (1 mmol) in toluene (6 mL) was added at room
temperature, and the mixture was heated to 65 °C and stirred
vigorously for 3 h (or some appropriate time) under air in
balloon. After the reaction, the mixture was treated as
described above to obtain a product.
Gen er a l P r oced u r e for P d (II)-Hyd r ota lcite-Ca ta lyzed
Oxid a tion of Alcoh ols Usin g Air (3 a tm ). A typical experi-
mental procedure is as follows: a mixture of Pd(II)-hydro-
talcite (60-300 mg, 0.01-0.05 mmol as Pd), pyridine (0.04-
0.20 mmol), alcohol (1 mmol) and toluene (2-10 mL) in a high-
polymer autoclave with a glass container was stirred vigorously
with a magnetic stirrer under unpurified air pressure (initial
pressure of 3 atm) at 80 °C for 3 h (or some appropriate time).
After the reaction, the mixture was treated as described above
to obtain a product.
F igu r e 2. Limitations.
Ta ble 7. Recyclin g of P d (II)-Hyd r ota lcitea
GLC
pyridine time time of conv yield
(mmol) (h)
use (%)b (%)
entry
1
catalyst
Pd(II)-hydrotalcite
0.2
3
first
98
98
90
72
98
94
82
second 90
third
11 first
73
99
2
Pd(II)-hydrotalcite(m)
0.5
second 98
third 85
a
Reaction conditions: Pd(II)-hydrotalcite (0.05 mmol), benzyl
alcohol (1.0 mmol), pyridine, toluene (10 mL), air (balloon, 1 atm)
at 65 °C. Conversion of benzyl alcohol.
b
Gen er a l P r oced u r e for P d (II)-Hyd r ota lcite-Ca ta lyzed
Oxid a tion of Alcoh ols Usin g Air (1 a tm ). A typical experi-
mental procedure is as follows: to a suspension of Pd(II)-
hydrotalcite (300 mg, 0.05 mmol as Pd) and toluene (6 mL) in
a 30 mL two-necked flask was added pyridine (0.2-5.0 mmol),
and the mixture was stirred at room temperature. Then,
unpurified air was introduced into the flask from a balloon
under atmospheric pressure and the mixture was heated to
65 °C for ca. 10 min (caution: it is possible that one is working
within explosion limit of toluene/ oxygen mixtures in the gas
phase at temperature lower than ca. 40 °C). Next, an alcohol
(1.0 mmol) in toluene (4 mL) was added, and the mixture was
stirred vigorously with a magnetic stirrer for 3 h (or some
appropriate time) at 65 °C under air in balloon. After the
reaction, the catalyst was separated by filtration through a
glass filter. Removal of the solvent from the filtrate under the
reduced pressure left an oily residue, which was subjected to
column chromatography (Merck silica gel 60; eluents, hexane-
diethyl ether) to give a product. Products obtained were
Gen er a l P r oced u r e for Reu se of th e Ca ta lyst. First run
of the oxidation of benzylic alcohols catalyzed by Pd(II)-
hydrotalcite or Pd(II)-hydrotalcite(m) was performed by the
above-described procedure. Recovered Pd(II)-hydrotalcite or
Pd(II)-hydrotalcite(m) was washed with hexane (20 mL × 2)
and dried under vacuum at room temperature before use for
the next run.
Ack n ow led gm en t. We thank Kyowa Chemical Ind.,
Ltd. for the gift of hydrotalcite (Mg6Al2(OH)16CO3‚4H2O,
brand name KYOWAAD 500) for this study. T.N.
gratefully acknowledges a Fellowship of the J apan
Society for the Promotion of Science for Young Scientists.
J O010338R