2
12
Chemistry Letters Vol.36, No.2 (2007)
Synthetic Application of PVP-stabilized Au Nanocluster Catalyst to Aerobic Oxidation
of Alcohols in Aqueous Solution under Ambient Conditions
1
1;2
ꢀ1
Hironori Tsunoyama, Tatsuya Tsukuda, and Hidehiro Sakurai
Research Center for Molecular-Scale Nanoscience, Institute for Molecular Science, Myodaiji, Okazaki 444-8787
1
2
CREST, Japan Science and Technology Agency, Kawaguchi 332-0012
(Received October 24, 2006; CL-061254; E-mail: hsakurai@ims.ac.jp)
Gold nanoclusters (ꢀ ¼ 1:3 nm) stabilized by poly(N-vinyl-
-pyrrolidone) (Au:PVP) were found to show a high catalytic
activity toward the aerobic oxidation of alcohols. Various
kinds of primary and secondary alcohols were converted to the
corresponding carboxylic acids and ketones, respectively, in
basic aqueous media at 300–360 K under air.
Table 1. Aerobic oxidation of primary alcohols
2
2
atom % Au:PVP
R CH OH
RCO H + (RCHO)
2
2
3
00 mol % K CO3
1
2
3
2
H O, under air
2
Temp Time
2
3
Recovery
Entry
R
/K
/h /% /%
of 1/%
1a,b
2c
3b
4b
5b
6c
7c
8c
9
Ph (1a) 300
Ph (1a) 300
6
6
0
0
85
98
0
0
0
Gold nanoclusters (NCs) supported on metal oxides have
paid much attention to develop aerobic oxidation catalysts since
1
–3
the landmark report by Haruta and co-workers. On the other
0
o-HOC6H4 (1b) 300
m-HOC6H4 (1c) 300
p-HOC6H4 (1d) 300
p-CH3C6H4 (1e) 300
p-MeOC6H4 (1f) 300
p-ClC6H4 (1g) 300
p-NO2C6H4 (1h) 300
2-Py (1i) 300
24
8
54
34
91
0
34
3
hand, catalytic activities of homogeneous or colloidal Au
4
52
0
NCs have been studied only recently. We have recently report-
8
3
ed that gold nanoclusters stabilized by the water-soluble polymer
poly(N-vinyl-2-pyrrolidone) (Au:PVP) perform as excellent
catalysts in the aerobic oxidation of benzylic alcohols in water
4
91
93
78
99
72
76
>99
94
95
0
3
0
0
6
0
0
5
under ambient conditions. Notable features of alcohol oxidation
24
3
0
0
1
1
1
0
1
2
0
8
catalyzed by Au:PVP clusters are listed as follows: 1) Au:PVP
is easily prepared from commercially available precursors.
Au:PVP is nonhazardous and can be stored for months either
3-Py (1j) 320
24
24
24
0
8
n-C5H11 (1k) 340
0
0
6
a
13
CH =CH(CH ) (1l) 320
2 2 3
0
0
as a solid powder or an aqueous dispersion. 2) Au:PVP behaves
as a quasi-homogeneous catalyst in aqueous solution and can be
1
4
Ph(CH2)3 (1m) 320
24 trace
0
5
–7
a
b
c
recovered by centrifugal ultrafiltration. 3) Au:PVP catalyzes
oxidation of primary benzylic alcohols to the corresponding
aldehydes/carboxylic acids in water under ambient conditions.
No additional co-oxidant such as peroxides is needed, which
Ester was formed in 10% yield. Ref. 5a. KOH was used as a base.
Au:PVP catalyst is a good reagent for this purpose. As shown
in Entries 1–9, most of the primary benzylic alcohols were read-
ily oxidized at 300 K to give the acids in excellent yields except
for o- and p-hydroxybenzyl alcohol (1b, 1d: Entries 3 and 5),
which were selectively converted to the corresponding alde-
5
enables safer operation. 4) The catalytic activity per unit cluster
surface area increases rapidly with decreasing size from ca. 10
to 1.3 nm. The smallest Au:PVP clusters (ꢀ ¼ 1:3 ꢁ 0:3 nm)
exhibit the highest activity probably due to their non-metallic
5
hydes as reported previously. By-products often observed
in the K CO conditions were the ester derivatives (Entry 1),
5
b
electronic structures. 5) At ambient temperature (300 K),
Au:PVP clusters can oxidize benzylic alcohols at a rate much
higher than that obtained for Pd:PVP clusters of comparable
size.5 In addition, gold atoms are not leached out from the
clusters whereas atomic leaching is often observed in Pd clus-
ters. These advantages of Au:PVP prompted us to investigate
the applicable scope of Au:PVP as an oxidation catalyst in
organic synthesis. We report reactions with various kinds of
alcohols and the possible reusability of the Au:PVP catalyst.
Au:PVP with an average cluster size of ꢀ ¼ 1:3 ꢁ 0:3 nm
was chosen as the catalyst since it has already been demonstrated
2
3
whose formation was suppressed by changing the base to
KOH due to fast promotion of hydrolysis (Entry 2). The pyridine
ring did not interfere with the reaction as shown in Entries 10 and
11. Contrary to the benzylic alcohols, the primary aliphatic alco-
hols required slightly elevated temperatures, such as 320 or
340 K, to undergo the oxidation (Entries 12–14). It is noteworthy
that the carbon–carbon double bond was tolerant to oxidation
under the reaction conditions (Entry 13). The initial rate con-
stants for oxidation of a series of para-substituted benzyl alco-
,8
1
2
hols (Entries 1 and 6–9) were determined. The rate constants
þ
5
–7
to be the most active in the size range of 1.3–10 nm. All the
reactions were performed in basic aqueous solution (K2CO3)
under aerobic conditions unless otherwise noted. Although
there are several catalytic reactions for direct oxidation from pri-
mary alcohols to the corresponding carboxylic acids, such as
showed a linear relationship against the ꢁp constants and the
ꢂ value was determined to be ꢂ0:66 by the least-squares meth-
od. This ꢂ value is comparable to those obtained in hydride
13
transfer or radical processes (usually between ꢂ0:3 and ꢂ1:5).
Next, oxidation of secondary alcohols was examined
(Table 2). In every case, the corresponding ketone derivative 2
was obtained as a sole product with recovery of 1, and no other
products such as esters (Baeyer–Villiger product) were detected.
As for the acyclic alcohols, slightly elevated temperatures were
9
a
9b
9c
TEMPO/NaClO, TEMPO/NaClO2, TEMPO/ArI(OAc)2,
RuCl3/NaIO4, RuCl3/TCCA, Na2WO4/H2O2,9f or PCC/
H5IO6, examples under aerobic (or O2) conditions are still
9d
9e
9g
rare.1
0,11
The results in Table 1 clearly demonstrate that the
Copyright Ó 2007 The Chemical Society of Japan