Aerobic oxidation of alcohols under mild conditions catalyzed by novel polymer-incarcerated, carbon-stabilized gold nanoclusters

Article abstract of DOI:10.1002/adsc.200800319

Polymer-incarcerated, carbon-stabilized gold nanoclusters were found to be highly active in heterogeneous catalysis for the oxidation of secondary alcohols using molecular oxygen in aqueous medium. After optimization of catalyst preparation methods, highly loaded and highly effective catalysts were obtained, and a broad range of secondary alcohols could be oxidized by using these catalysts under mild conditions. The catalysts could be recovered and reused several times without significant loss of activity. Moreover, kinetics of the oxidation reaction with (±)-1-phenylethanol was investigated.

Full text of DOI:10.1002/adsc.200800319

COMMUNICATIONS
DOI: 10.1002/adsc.200800319
Aerobic Oxidation of Alcohols under Mild Conditions
Catalyzed by Novel Polymer-Incarcerated, Carbon-Stabilized
Gold Nanoclusters
CØline Lucchesi,a Takeshi Inasaki,a Hiroyuki Miyamura,a Ryosuke Matsubara,a
and Shu¯ Kobayashia,*
a
Department of Chemistry, School of Science and Graduate School of Pharmaceutical Sciences, The HFRE Division,
ERATO Japan Science Technology Agency (JST), Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
Fax : (+81)-3-5684-0634; e-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
Received: May 22, 2008; Published online: August 13, 2008
Dedicated to Professor Ryoji Noyori on the occasion of his 70th birthday.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800319.
Abstract: Polymer-incarcerated, carbon-stabilized
gold nanoclusters were found to be highly active in
heterogeneous catalysis for the oxidation of secon-
dary alcohols using molecular oxygen in aqueous
medium. After optimization of catalyst preparation
methods, highly loaded and highly effective cata-
lysts were obtained, and a broad range of secondary
alcohols could be oxidized by using these catalysts
under mild conditions. The catalysts could be recov-
ered and reused several times without significant
loss of activity. Moreover, kinetics of the oxidation
reaction with (Æ)-1-phenylethanol was investigated.
ty. However, an increase of gold loading amounts was
hampered by aggregation during the incarceration
step, which is well-known to decrease the catalytic ac-
tivity of gold-based catalysts significantly.[5] In order
to circumvent this drawback, we incorporated carbon
black (CB),[6] whose partially graphitic structure[7] was
expected to enhance the stability of gold nanoclusters
during the preparation step. In addition, its high spe-
cific surface area might allow a high dispersion of the
gold nanoclusters on the support. We describe here
preparation and characterization of new gold nano-
cluster catalysts stabilized by both a polymer and
carbon black (PI/CB-Au). The oxidation of various al-
cohols by molecular oxygen catalyzed by PI/CB-Au is
also reported.
Keywords: gold; nanoparticles; oxidation; water
Two synthetic procedures for the preparation of PI/
CB-Au were evaluated. The first procedure leading to
PI/CB-Au 1a (Scheme 1) involves preliminary loading
The selective oxidation of alcohols is an important of gold onto carbon black (CB), isolation, and finally
transformation in synthetic organic chemistry.[1] While combination with polymer 2. The second procedure is
conventional oxidation methods often require stoi- a one-pot synthesis of PI/CB-Au 1b (Scheme 2). It in-
chiometric amounts of metallic oxidants which induce volves in situ reduction of the gold precursor, Au-
a large quantity of by-products, a current challenge is
A
the use of molecular oxygen with a heterogeneous re- 2 and CB. In both cases, the final stepis the cross-
usable catalyst related to green sustainable chemistry. linking of the polymer upon heating without solvent.
Recently, we reported the effective oxidation of a For comparison, carbon black-supported gold nano-
wide range of alcohols using polymer-incarcerated cluster (CB-Au) was also synthesized (Scheme 3).
gold(0) catalysts (PI-Au).[2,3] In these catalysts, gold
The catalytic activities of the prepared catalysts in
nanoclusters are stabilized by weak interactions with the aerobic oxidation of alcohols were assessed using
a polystyrene-based matrix, and cross-linking of the (Æ)-1-phenylethanol as a model substrate (Scheme 4).
polymer matrix prevents the metal from leaching to The yields and the characteristics of each catalyst are
the reaction medium (incarceration).[4] In addition, summarized in Table 1. Despite the relatively small
this structure allows simple and efficient recovery and mean diameters of gold nanoclusters, CB-Au was in-
reuse of the catalyst without significant loss of activi- active (Table 1, entry 1). This is presumably due to
1996
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1996 – 2000

Aerobic Oxidation of Alcohols under Mild Conditions
COMMUNICATIONS
Scheme 1. Synthesis of PI/CB-Au 1a.
Scheme 2. Synthesis of PI/CB-Au 1b.
Scheme 3. Synthesis of CB-Au.
and Table 1, entry 2). On the other hand, the one-pot
procedure leading to PI/CB-Au 1b proved to be suc-
cessful (Scheme 2 and Table 2, entry 3). The diameter
of the gold nanoclusters was small and remained con-
stant even after heating. The final loading amount
was 0.25 mmolgÀ1, which implied that the entire gold
precursor was reduced and loaded onto the final ma-
Scheme 4. Standard procedure for assessment of the catalyt-
ic activity of CB-Au and PI/CB-Au.
the highly hydrophobic nature of carbon black (CB), terial without any leaching. Moreover, PI/CB-Au 1b
which may not be suitable for the oxidation. The pro- was shown to be highly efficient in the oxidation of
cedure involving the isolation leading to PI/CB-Au 1a (Æ)-1-phenylethanol, affording acetophenone quanti-
resulted in aggregation of gold nanoclusters upon tatively.[8,9] Using the standard procedure illustrated in
heating, which led to low catalytic activity (Scheme 1 Scheme 4, an increase of the gold loading amount
Table 1. Characterization of catalysts CB-Au and PI/CB-Au 1a and 1b.
Entry
Catalyst
MC/CB-Au
Diameter [nm]
PI/CB-Au
Loading [mmolg]À1
Oxidation
Yield [%]
Diameter [nm]
[b]
1[a]
2[c]
3[c]
CB-Au
PI/CB-Au 1a
PI/CB-Au 1b
1–2
3.8
2.7
-
0.43
0.17
0.25
0
9
>99
10.9
2.9
[a]
Targeted Au loading amount is 0.51 mmolgÀ1.
CB-Au was not heated at 1508C.
[b]
[c]
Targeted Au loading amount is 0.25 mmolgÀ1.
Adv. Synth. Catal. 2008, 350, 1996 – 2000
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1997

CØline Lucchesi et al.
Oxidation
COMMUNICATIONS
Table 2. Increase of the loading amount for PI/CB-Au.[a]
Entry
Catalyst
Targeted loading [mmolg]À1
MC/CB-Au
Diameter [nm]
PI/CB-Au[a]
Diameter [nm]
Loading [mmolg]À1
Yield [%]
1
2
3
4
1b
1c
1d
1e
0.25
0.50
0.75
1.00
2.7
2.7
2.2
1.8
2.9
5.1
3.8
3.8
0.25
0.40
0.52
0.60
>99
90
91
86
from 0.25 mmolgÀ1 (5 wt%) to 1.00 mmolgÀ1 (20 proved to be less reactive, thus needing longer reac-
wt%) was investigated (Table 2). Although the incor- tion times (entries 8–12). Finally, as for secondary al-
poration ratio decreased when the loading amounts cohols bearing heteroatoms, 2-thiophenylethanol re-
were
increased,
the
loading
amount
of acted smoothly to afford the corresponding ketone
0.60 mmolgÀ1could be reached and successfully em- (entry 13), although 2-pyridylethanol proved to be
ployed in the aerobic oxidation of (Æ)-1-phenyletha- less reactive (entry 14). It should be noted that no
nol with a slight loss of catalytic activity.
leaching of the metal was observed in all entries.
The PI/CB-Au catalyst could be recovered by
Several secondary alcohols were then oxidized in
the presence of 1 mol% of 1b (Table 3). Aromatic simple filtration and reused without significant loss of
and allylic alcohols were oxidized smoothly to afford catalytic activity (Table 4). No gold leaching was de-
the corresponding ketones in quantitative yields tected during the investigation.
(Table 4, entries 1–7). Secondary aliphatic alcohols
Table 3. [a] General synthetic procedure is depicted in Sche-
Table 3. (Continued)
me 2.Table 3Oxidation of various secondary alcohols cata-
Entry
Substrate
Time [h]
24
Yield[a] [%]
lyzed by PI/CB-Au 1b.
7
>99
Entry
1
Substrate
Time [h]
3
Yield[a] [%]
8
48
48
48
99
86
82
>99
9
2
3
3
6
>99
>99
10
11
12
48
48
57
62
4
5
24
48
>99
>99
13
48
48
99
58
14
6
48
95
[a]
Determined by GC analysis.
1998
asc.wiley-vch.de
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1996 – 2000

Aerobic Oxidation of Alcohols under Mild Conditions
COMMUNICATIONS
Table 4. Recovery and reuse of PI/CB-Au 1b.
mechanism of this reaction and the exact role of CB
are in progress.
Experimental Section
Run 1
Run 2
Run 3
Run 4
Run 5
Preparation of PI/CB-Au 1b
Yield [%]
>99
>99
>99
>99
>99
To a solution of copolymer 2 (2.1 g) in diglyme (32 mL) at
08C were successively added carbon black (CB, 499.6 mg),
NaBH4 (43.1 mg in 8 mL diglyme, 1.14 mmol) and AuPPh3Cl
(139 mg in 20 mL diglyme, 0.28 mmol). After stirring for 3 h
from 08C to room temperature, diethyl ether (120 mL) was
added dropwise while stirring. The resulting MC/CB-Au was
filtered, washed several times with diethyl ether and dried
under vacuum for 24 h. The cross-linking stepwas epr-
formed at 1508C for 3 h without solvent to afford PI/CB-Au
1b.
Au leaching
no
no
no
no
no
For obtaining a mechanistic insight, the kinetic
study of the oxidation of (Æ)-1-phenylethanol was
performed using PI/CB-Au 1b (0.25 mmolgÀ1). The
observed straight curve (Figure 1) is characteristic of
a zero-order kinetics, which suggests either that the
uptake of O2 into the reaction medium is a rate-limit-
ing step, or that the reaction mainly occurs in a spa-
tially limited hydrophobic environment, where the
concentration of the substrate is almost constant
throughout the reaction.
In summary, we have developed the aerobic oxida-
tion of alcohols catalyzed by novel polymer-incarcer-
ated, carbon-stabilized gold nanoclusters. The inclu-
sion of carbon black (CB) to the composition of the
catalyst, together with an amphiphilic copolymer, en-
hances the stability of gold nanoclusters probably via
synergistic p-p interactions between the three compo-
nents, which enables us to increase the metal loading
amount upto 0.60 mmolg À1. Secondary alcohols could
be oxidized smoothly to afford analytically pure prod-
ucts in excellent yields after simple phase separation
without further purification. The catalyst could be re-
covered and reused several times without significant
loss of activity or metal leaching in the reaction
medium. Further investigations to clarify the precise
Typical Procedure for the Oxidation of Alcohols
(Æ)-1-Phenylethanol
(30.5 mg,
0.25 mmol),
K2CO3
(103.7 mg, 0.75 mmol), PI/CB-Au 1b (10 mg, 0.0025 mmol),
water (1.5 mL) and benzotrifluoride (1.5 mL) were com-
bined in a round-bottomed flask. The mixture was stirred
for 3 h under an O2 atmosphere at 308C. After the reaction,
the catalyst was filtered and washed with diethyl ether and
water. The aqueous layer was extracted with diethyl ether.
The yield was determined by GC analysis using anisole as
an internal standard. The leaching of gold in both aqueous
and organic layers was checked by ICP analysis.
Acknowledgements
This work was partially supported by a Grant-in-Aid for Sci-
entific Research fromthe Japan Society of the Promotion of
Sciences (JSPS).
References
[1] a) C. L. Hill, Advances in Oxygenated Processes, Vol. 1,
JAI Press, London, 1988; b) M. Hudlicky, Oxidations in
Organic Chemistry, American Chemical Society: Wash-
ington DC, 1990.
[2] H. Miyamura, R. Matsubara, Y. Miyazaki, S. Kobayashi,
Angew. Chem. 2007, 119, 4229; Angew. Chem. Int. Ed.
2007, 46, 4151.
[3] For recent reviews on heterogeneous gold catalysis:
a) A. S. K. Hashmi, Angew. Chem. 2005, 117, 7150;
Angew. Chem. Int. Ed. 2005, 44, 6990; b) D. Astruc, F.
Lu, J. Ruiz Aranzaes, Angew. Chem. 2005, 117, 8062;
Angew. Chem. Int. Ed. 2005, 44, 7852; c) A. S. K.
Hashmi, G. J. Hutchings, Angew. Chem. 2006, 118, 8064;
Angew. Chem. Int. Ed. 2006, 45, 7896; d) A. S. K.
Hashmi, Chem. Rev. 2007, 107, 3180; e) T. Ishida, M.
Haruta, Angew. Chem. 2007, 119, 7288; Angew. Chem.
Int. Ed. 2007, 46, 7154.
Figure 1. Kinetics of the oxidation reaction of (Æ)-1-phenyl-
ethanol with PI/CB-Au 1b.
[4] For related immobilized catalysts: a) R. Nishio, S.
Wessely, M. Sugiura, S. Kobayashi, J. Comb. Chem.
Adv. Synth. Catal. 2008, 350, 1996 – 2000
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1999

CØline Lucchesi et al.
COMMUNICATIONS
2006, 8, 459; b) Y. Miyazaki, H. Hagio, S. Kobayashi,
Org. Biomol. Chem. 2006, 4, 2529; c) Y. Miyazaki, H.
Hagio, S. Kobayashi, Org. Biomol. Chem. 2006, 4, 3537;
d) H. Oyamada, R. Akiyama, H. Hagio, T. Naito, S. Ko-
bayashi, Chem. Commun. 2006, 1, 4297; e) T. Matsumo-
to, M. Ueno, J. Kobayashi, H. Miyamura, Y. Mori, S. Ko-
bayashi, Adv. Synth. Catal. 2007, 349, 531; f) R. Nishio,
M. Sugiura, S. Kobayashi, Chem. Asian J. 2007, 2, 983;
g) T. Inasaki, M. Ueno, S. Miyamoto, S. Kobayashi, Syn-
lett 2007, 20, 3209; h) H. Oyamada, T. Naito, S. Miyamo-
to, R. Akiyama, H. Hagio, S. Kobayashi, Org. Biomol.
Chem. 2008, 6, 61; i) H. Miyamura, M. Shiramizu, R.
Matsubara, S. Kobayashi, Chem. Lett. 2008, 37, 360; j) T.
Matsumoto, M. Ueno, N. Wang, S. Kobayashi, Chem.
Asian J. 2008, 3, 196.
C. Della Pina, R. Matarrese, M. Rossi, A. Siani, Appl.
Catal. A Gen. 2005, 291, 204; c) G. J. Hutchings, S. Car-
rettin, P. Landon, J. K. Edwards, D. Enache, D. W.
Knight, Y.-J. Xu, A. F. Carley, Top. Catal. 2006, 38, 223;
d) S. Demirel, K. Lehnert, M. Lucas, P. Claus, Appl.
Catal. B Environ. 2007, 70, 637.
[7] G. Leu, Y. Liu, D. D. Werstler, D. G. Cory, Macromole-
cules 2004, 37, 6883.
[8] Extensive optimization of cross-linking conditions re-
vealed that 1508C and 3 h are the best conditions for the
cross-linking step. See Supporting Information for de-
tails.
[9] A preliminary kinetic study has shown that the initial re-
action rate using PI/CB Au 1b was 1.38 times faster than
that using PI Au[2] in the oxidation of (Æ)-1-phenyletha-
nol.
[5] M. Haruta, Catal. Today 1997, 36, 153.
[6] For selected examples of Au/C catalysts: a) L. Prati, F.
Porta, Appl. Catal. A Gen.2005, 291, 199; b) M. Comotti,
2000
asc.wiley-vch.de
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1996 – 2000