A nanoplatinum catalyst for aerobic oxidation of alcohols in water

Article abstract of DOI:10.1002/anie.200603900

(Chemical Equation Presented) Aerobic exercise: A dispersion of nanoparticles of platinum in an amphiphilic polystyrene-polyethylene glycol resin (ARP-Pt) contains particles with a mean diameter of 5.9 nm and a narrow size distribution throughout the resin. ARP-Pt is a readily recyclable catalyst for the aerobic oxidation of a wide variety of alcohols in water with an oxygen or air atmosphere under heterogeneous conditions (see picture).

Full text of DOI:10.1002/anie.200603900

Communications
DOI: 10.1002/anie.200603900
Oxidation Catalysts
A Nanoplatinum Catalyst for Aerobic Oxidation of Alcohols in
Water**
Yoichi M. A. Yamada, Takayasu Arakawa, Heiko Hocke, and Yasuhiro Uozumi*
The development of highly active immobilized catalysts for
the aerobic oxidation of a variety of alcohols in aqueous
media is a tremendously important topic in chemistry today.[1]
Yet aerobic oxidation of nonactivated alcohols, such as
aliphatic and alicyclic alcohols, using immobilized catalysts
generally proceeds poorly.[2] There have been reports of
heterogeneous metal catalysts being applied to the aerobic
oxidation of various alcohols, but the catalysts were regularly
ARP-Pt (4) was prepared by complexation of commer-
cially available PS-PEG-NH2 (1; TentaGel S NH2) with
Zeiseꢀs salt (2·H2O) in water followed by reduction
(Scheme 1). High-resolution transmission electron micro-
used in harmful organic solvents[3] and/or under vigorous
conditions.[4] Over the last fewyears, our group,
Kaneda
[5]
et al.,[6] Sheldon et al.,[7] and Corma et al.[8] reported the
aerobic oxidation of some aliphatic and alicyclic alcohols in
water with reusable catalysts.[9] While acknowledging the
pioneering work in this area, we believe the development of a
novel catalyst system that exhibits a wide range of substrate
tolerance under mild and aqueous conditions still remains a
major challenge. Consequently, we devised a dispersion of
nanoparticles of palladium in an amphiphilic polystyrene–
polyethylene glycol (PS-PEG) resin (ARP-Pd; ARP: amphi-
philic resin particles) that catalyzes the aerobic oxidation of
alcohols in water at 1008C.[10]
Scheme 1. Preparation of ARP-Pt (4).
scopy (TEM) analysis of 4 revealed that the Pt nanoparticles
in it had a mean diameter of 5.9 nm with a very narrow size
distribution throughout the resin, and that the density of the
Pt particles was uniform in the resin (Figure 1). Energy-
dispersive spectroscopy/scanning electron microscopy (EDS/
SEM) analysis also revealed a uniform dispersion of Pt
throughout the resin.[15] To the best of our knowledge, a
uniform dispersion of nanoparticles into the entire region of
an insoluble resin with uniform density distribution was
realized for the first time.[16]
However, despite such progress, the reactivity and versa-
tility of ARP-Pd for the oxidation of alicyclic and aliphatic
alcohols was inadequate (see below). We therefore consid-
ered platinum nanoparticles,[11,12] since platinum has a stron-
ger oxidizing ability than palladium (Pt/Pt2+
versus Pd/Pd2+
E
ox = + 1.12 V
E
ox = + 0.95 V). However, deactivation of
platinum by oxygen is problematic for its efficient reuse,[13]
and Pt particles possess a latent tendency to explode under
oxidative conditions in organic solvents.[14] Our concept for
the preparation of ARPs, where metal particles would be
stabilized by amphiphilic resins in aqueous media, should
overcome these drawbacks. Herein, we report the develop-
ment of an ARP-Pt and its application to the aerobic
oxidation in water of a wide variety of alcohols, including
not only benzylic and allylic but also alicyclic and aliphatic
alcohols. Notably, ARP-Pt promoted the reaction efficiently
at 608C with high recyclability.
Figure 1. a) High-resolution TEM image of 4. b) Histogram of the size
distribution of the Pt particles (n: number of Pt particles per mm2;
d: diameter of the particles).
[*] Dr. Y. M. A. Yamada, T. Arakawa, Dr. H. Hocke, Prof. Dr. Y. Uozumi
Institute for Molecular Science (IMS)
Myodaiji, Okazaki, Aichi 444-8787 (Japan)
Fax: (+81)564-59-5574
With an amphiphilic resin catalyst containing Pt nano-
particles of uniform size and density distribution in hand, we
conducted the oxidation of a variety of primary and secondary
alcohols to the corresponding carbonyl compounds. Repre-
sentative data are summarized in Table 1, where several
results from Pd catalysis are included for comparison (Pt
versus Pd). The oxidation of 5a (20 mmol, 2.2 g) with
1.0 mol% ARP-Pt[17] was performed at 608C for 24 h to
E-mail: uo@ims.ac.jp
[**] This work was supported by the CREST program, sponsored by the
JST. We also thank the JSPS, MEXT, and the Uehara Memorial
Foundation for partialfinancialsupport of this work.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
704
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 704 –706

Angewandte
Chemie
Table 1: The aerobic oxidation of alcohols 5 with ARP-Pt (4) as catalyst.[a]
catalyst exhibited lower activity even at a higher reaction
temperature (Table 1, runs 16, 18, and 25). Notably, the
aerobic oxidation of 6a with ARP-Pt 4110, which was
prepared from 10-mm-diameter PS-PEG resin beads, gave a
similar result to that obtained with 4 (1 = 90 mm; Table 1,
runs 3 and 9). These observations indicate that the oxidation
took place not only on the surface of the polymer beads but
also throughout the polymer matrix, where the Pt nano-
particles were dispersed with uniform size and density
distribution.
The workup of the reactions was performed under
organic-solvent-free conditions by extraction with supercrit-
ical carbon dioxide, and recovered 4 was readily reused
without further purification and/or reactivation.[19,20] In gen-
eral, the catalytic activity of Pt nanoparticle catalysts seriously
declines, while catalytic deactivation of reused 4 was not
observed at all (Table 1, run 3–7). In addition, no change of
the particle structure was observed by TEM. Notably, ARP-
Pt (4) was stable, reusable, and not flammable under aerobic
and aqueous conditions, and was stored in an open vessel for
at least one year.
Run Substrate/ R1
Product
R2
4
t [h] Yield
[mol%]
[%]
1[b] 5a/7a
Ph
Ph
1.0
5.0
5.0
24
8
24
99
97
82
81
84
92
90
79
86
80
82
93
87
72
80
<2
81
<2
93
87
93
95
87
82
29
84
81
2[b]
3
4
6a/8a
CH3
(2nd use)
(3rd use)
(4th use)
(5th use)
10
5
6
7
8
(under air)
36
24
36
36
18
15
36
12
20
12
20
12
12
60
36
15
60
20
30
36
9
5.0[c]
10
10
10
5.0
10
11
6b/8b
6c/8c
Ph
Ph
C7H15
(CH2)3OBn
12[b] 5b/7b
PhCH CH2
=
=
13
14
15
16
17
18
19
20
21
6d/8d
6e/8e
6 f/8 f
PhCH CH2 CH3
À
=
À
CH CH(CH2)3
5.0
À
(CH2)4
À
5.0
5.0[d]
5.0
À
(CH2)5
À
À
6g/8g
In conclusion, we have developed a highly active and
reusable catalytic system for the aerobic oxidation of a wide
variety of alcohols, including benzylic, allylic, alicyclic, and
aliphatic alcohols, in aqueous media with high recyclability
and stability.
5.0[d]
5.0
À
À
6h/8h
6i/8i
(CH2)6
(CH2)7
À
5.0
20
10
5.0
(under air)
22[b] 5c/7c
n-C7H15
n-C6H13
23
24
25
26
27
6j/8j
(under air)
CH3
20
5.0[d]
10
6k/8k
6l/8l
n-C5H11
n-C4H9
C2H5
n-C3H7
Experimental Section
10
PS-PEG resin-supported dichloro(ethylene)platinum complex (3): A
mixture of PS-PEG amino resin 1 (3.2 g; loading value of amino
residue: 0.31 mmolgÀ1) and Zeiseꢀs salt 2·H2O (372 mg, 1.01 mmol) in
water (20 mL) was shaken at room temperature for 1 h. The mixture
was filtered, and the resulting resin beads were rinsed three times with
water and dried in vacuo to give 3 (3.5 g; loading value of platinum
residue: 0.28 mmolgÀ1). IR: n˜ = 1602, 3595, 3728 cmÀ1; swollen-resin
magic-angle spinning (SR-MAS) 13C NMR (100 MHz, CDCl3, 258C):
d = 39.9, 44.4, 70.1, 74.0, 104.2, 125.2, 127.5, 144.8 ppm.
[a] All the reactions were carried out with 4 at 608C in water under
atmospheric oxygen unless otherwise noted (Bn=benzyl). [b] 1 mol
equivalent of K2CO3 was added. [c] ARP-Pt made of 10-mm-diameter resin
was used (4110). [d] With ARP-Pd under O2 at 1008C.
give 7a in 99% yield of isolated product (2.4 g; Table 1,
run 1). Activated alcohols, such as benzylic alcohols 6a–c and
allylic alcohols 5b, 6d, and 6e, were transformed to the
corresponding carbonyl compounds 7b and 8a–e in 72–93%
yield (Table 1, runs 3 and 10–14).[18]
ARP-Pt (4): A mixture of 3 (3.5 g; loading value of platinum
residue: 0.28 mmolgÀ1) and benzyl alcohol (5 mL) in water (24 mL)
was shaken at 808C for 12 h. The mixture was filtered, and the
resulting resin beads were rinsed three times with water and three
times with acetone, then dried in vacuo to give 4 (3.4 g; loading value
of platinum residue: 0.29 mmolgÀ1). IR: n˜ = 3557, 3661 cmÀ1; SR-
MAS 13C NMR (100 MHz, CDCl3, 258C): d = 40.1, 61.3, 66.6, 70.2,
104.0, 125.4, 127.8, 145.1 ppm.
General procedure for the catalytic aerobic oxidation of primary
alcohols in water: A mixture of 4 (0.002 mmol of effective platinum),
primary alcohol (0.2 mmol), and potassium carbonate (0.2 mmol) in
water (2 mL) was stirred at 608C under oxygen gas at atmospheric
pressure. After cooling, the mixture was washed with tert-butyl
methyl ether and acidified with 5% hydrochloric acid. The mixture
was extracted with ethyl acetate (5 ” 1 mL). The extract was dried
over magnesium sulfate and concentrated in vacuo to give the
corresponding carboxylic acid.
We were pleased to find that the aerobic oxidation of a
variety of nonactivated alicyclic alcohols, cyclopentanol (6 f),
cyclohexanol (6g), cycloheptanol (6h), and cyclooctanol (6i),
also took place smoothly to provide the corresponding
alicyclic ketones in 80–93% yield (Table 1, runs 15, 17, 19,
and 20). Moreover, it was found that the nonactivated
aliphatic primary and secondary alcohols, 1-, 2-, 3-, and 4-
octanol (5c, 6j–l), were efficiently converted into the
corresponding aliphatic carbonyl compounds in 81–95%
yield (Table 1, runs 22, 23, 26, and 27) under similar con-
ditions. Chemoselective oxidation was also accomplished, in
that alkenes and a benzyl ether were tolerated (Table 1,
runs 11–14).
General procedure for the catalytic aerobic oxidation of secon-
dary alcohols in water: A mixture of 4 (0.002 mmol of effective
platinum) and the secondary alcohol (0.2 mmol) in water (2 mL) was
stirred at 608C under oxygen gas at atmospheric pressure. After
cooling, the mixture was filtered to give the resin beads 4 that held the
resulting ketone.
Atmospheric air was likewise applied as an oxidant in this
catalytic system. Thus, the efficient conversion of benzylic,
alicyclic, and aliphatic secondary alcohols 6a, 6i, and 6j was
achieved to afford 8a, 8i, and 8j in 79, 93, and 82% yield,
respectively (Table 1, runs 8, 21, and 24). The ARP-Pd
Post-treatment procedure A: extract with EtOAc: The resin
beads were rinsed three times with ethyl acetate (3 ” 1 mL) and the
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Communications
aqueous layer was extracted with ethyl acetate (3 ” 1 mL). The
combined extracts were dried over magnesium sulfate and concen-
trated in vacuo to give the corresponding ketones 8.
Post-treatment procedure B: extract with scCO2: The resin beads
were extracted with supercritical (sc) carbon dioxide (296 atm, 408C,
24 min) to afford the corresponding ketones 8. The recovered catalyst
beads were reused without any further purification or activation.
[10] For recent examples of transition-metal complexes supported on
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Received: September 22, 2006
Published online: December 13, 2006
Keywords: alcohols · heterogeneous catalysis · oxidation ·
.
platinum · polymers
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[15] Additional high-resolution TEM and EDS/SEM data are
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[17] Total loading of Pt species (0.02 mmol) was 1212 Pt atoms; the
number of surface atoms was estimated from the average
diameter of the particles.
[18] Control experiments were carried out at 60 and 1608C without
Pt species: the aerobic oxidation of 6a did not proceed at 608C at
all, whereas at 1608C under neat conditions 8a was obtained in
64% yield (24 h; not optimized); Y. Uozumi, Y. M. A. Yamada,
unpublished results.
[19] The obtained 2-octanone (Table 1, entry 23) was not contami-
nated with platinum residue (checked by inductively coupled
plasma (ICP) analysis). Over 99.9% of Pt was retained in the
resin: ICP analysis showed that leaching of Pt to the aqueous
solution was less than 1 ppm, and the aqueous filtrate did not
showany catalytic activity.
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[20] A referee has argued that the oxidation with leached homoge-
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706
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