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
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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
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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.
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1999