Recyclable gold nanoparticle catalyst for the aerobic alcohol oxidation and C-C bond forming reaction between primary alcohols and ketones under ambient conditions

Article abstract of DOI:10.1016/j.tet.2008.12.005

A recyclable gold catalyst is synthesized from readily available reagents by immobilizing gold nanoparticles in aluminum oxyhydroxide support through a simple sol-gel method. The catalyst showed the high activity even at room temperature in the aerobic oxidation of various alcohols and in the coupling reaction between primary alcohols and ketones.

Full text of DOI:10.1016/j.tet.2008.12.005

Tetrahedron 65 (2009) 1461–1466
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Recyclable gold nanoparticle catalyst for the aerobic alcohol oxidation and C–C
bond forming reaction between primary alcohols and ketones under ambient
conditions
a
b
b
a,
*
Sungjin Kim , Sang Won Bae , Jae Sung Lee , Jaiwook Park
a
Department of Chemistry, Pohang University of Science and Technology (POSTECH), San 31 Hyojadong, Pohang, Kyungbuk 790-784, Republic of Korea
Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyojadong, Pohang, Kyungbuk 790-784, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 October 2008
Received in revised form 3 December 2008
Accepted 3 December 2008
Available online 9 December 2008
A recyclable gold catalyst is synthesized from readily available reagents by immobilizing gold nano-
particles in aluminum oxyhydroxide support through a simple sol–gel method. The catalyst showed the
high activity even at room temperature in the aerobic oxidation of various alcohols and in the coupling
reaction between primary alcohols and ketones.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Heterogeneous catalyst
Gold nanoparticle
Aerobic alcohol oxidation
Room temperature
C–C bond formation
1. Introduction
temperature.8–11 Recently, Kobayashi et al. have developed a poly-
12
mer incarcerated gold catalyst (PI Au), which shows very high
activity and selectivity in the aerobic oxidation of alcohols even at
room temperature. However, multistep procedure and strong re-
ductant are still required for preparing the gold catalyst. We herein
report a recyclable gold catalyst that can be made from readily
available reagents by a one-pot procedure without pre- and post-
treatments and strong reductant. The catalyst is highly active for
the aerobic oxidation of a wide range of alcohols at room temper-
ature. Furthermore, the catalyst can be used for C–C bond forming
reaction between primary alcohols and ketones under ambient
conditions.
The selective oxidation of alcohols is one of the most important
1
reactions in the organic synthesis. Generally, several metal-based
2
oxidants and halooxoacids have been used for alcohol oxidation.
Although these reagents are quite general in scope, they cannot be
appropriate in green chemistry point of view, because the oxidation
requires them in stoichiometric amount and produces problematic
byproducts. Hence, the development of efficient and environment-
friendly catalysts for alcohol oxidation, particularly that of aerobic
3
oxidation catalysts, is ongoing research interest. For aerobic oxi-
dation of alcohols, there are noticeable heterogeneous catalysts
4
,5
such as the hydroxyapatite (HAP) bound RuHAP and PdHAP, as
6
well as Ru/Al
2
O
3
. However, they often require high reaction tem-
2. Results and discussion
peratures, which are not applicable to thermally unstable com-
pounds. Since Haruta et al. reported CO oxidation at low
2.1. Synthesis of gold nanoparticle catalyst
7
temperature, several heterogeneous Au-based catalysts such as
8
9
10
Au/CeO
2
,
Au-Pd/TiO
2
,
and Au/Ga
3
Al
3
O
9
have been developed.
The catalyst was made from a solution of hydrogen tetra-
chloroaurate hydrate, aluminum tri-sec-butoxide, Pluronic P123 in
ethanol through a procedure similar to those reported previously
However, they also needed high reaction temperatures for alcohol
oxidation. Furthermore, in general, synthetic procedures for these
gold catalysts are not easy, which require pH adjustment, concen-
tration control, strong reductant, and/or calcination at high
13,14
by us for other metal nanoparticle catalysts (Scheme 1).
The
ꢀ
solution was heated at 120 C for 10 min to give greenish-brown
15
suspension containing gold nanoparticles. Here the ethanol
served as both solvent and reductant. Water was added into the
suspension to form a red gel of aluminum oxyhydroxide. The
*
Corresponding author. Tel.: þ82 54 279 2117; fax: þ82 54 279 3399.
E-mail address: pjw@postech.ac.kr (J. Park).
ꢀ
resulting gel was washed with acetone, filtered, and dried at 120 C
0
040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.12.005

1462
S. Kim et al. / Tetrahedron 65 (2009) 1461–1466
HAuCl4 hydrate
+
EtOH
+
Pluronic P123
Table 1
1
) H2O
Oxidation of 1-phenylethanol using gold catalysts
Entry Catalyst (mol %) Base
Solvent Oxidant T ( C) Time (h) Yielda (%)
Cs CO
Al(sec-OBu)3
2) filtration
Au/AlO(OH)
1
ꢀ
120 °C, 10 min 3) wash with acetone
4) dry
1
2
3
4
5
6
7
8
9
1 (0.30)
1 (0.30)
1 (0.30)
1 (0.30)
1 (0.30)
1 (0.30)
1 (0.30)
1 (0.30)
1 (0.30)
2
3
Toluene
Cs CO3 Toluene Air
O
2
25
25
25
25
25
25
25
25
25
50
25
25
3
6
3
3
3
2
2
2
2
>99
>99
32
2
3
84
70
60
40
2
K
K
3
2
PO
CO
4
Toluene
Toluene
Toluene
Toluene
O
O
O
O
O
2
2
2
2
2
Scheme 1. Preparation of Au/AlO(OH) (1).
3
Et
Cs
Cs
3
N
2
2
CO
CO
3
for 2 h to give reddish powder (Au/AlO(OH), (1)). The reddish
powder was characterized by transmission electron microscopy
TEM), X-ray diffraction (XRD), X-ray photo-electron spectroscopy
XPS), inductively coupled plasma (ICP), and nitrogen isotherm
experiment. Gold nanoparticles of 5–13 nm size and aluminum
oxyhydroxide fibers were observed in the TEM images (Fig. 1). By
the XRD and XPS data, we confirmed that the nanoparticles are
metallic gold.16 ICP results implicated that almost all of the gold
source is entrapped in the support.
3
CH
2
Cl
2
Cs CO3 Hexane O2
2
(
(
Cs
Na
CO
2
CO
3
EtOAc
O
O
O
2
2
2
b
c
1
0
Au/CeO
PI Au (1.0)
Au/Ga Al
2
(0.66)
2
CO
3
H
2
O
5
5
40
51
>99
>99
1
1
1
K
d
2
3
TFT/H
2
O
2d
3
3
O
9
(3.0)
Toluene Air
a
b
c
Determined by GC.
Ref. 8.
Ref. 12.
d
Ref. 10.
2
.2. Aerobic oxidation of alcohols at room temperature
ꢀ
8
only in 51% even at 50 C after 5 h by using 0.66 mol % of Au. PI Au
reported by Kobayashi et al. can oxidize alcohols at room temper-
ature, but the oxidation of 1-phenylethanol takes 5 h even with
1 mol % of Au under oxygen atmosphere in a mixed solvent of tri-
We tested the catalytic activity of 1 in the aerobic oxidation of 1-
phenylethanol. Acetophenone was obtained in quantitative yield in
h at room temperature under O balloon by using only 0.30 mol %
3
2
12
of Au in the presence of 3 equiv of cesium carbonate (Table 1). Even
in the air, the oxidation was completed in 6 h. Cesium carbonate
was clearly more efficient than potassium phosphate, potassium
carbonate, or triethylamine (entries 1–5). The oxidation in tolu-
ene was faster than that in dichloromethane, hexane, or ethyl ac-
etate (entries 6–9). Notably, our catalyst 1 shows an activity higher
than those of reported heterogeneous Au catalysts in the aerobic
oxidation, possibly due to its highly porous structure of the support
fluorotoluene and water. Although Au/Ga
3 3 9
Al O is a noticeable
catalyst for the aerobic oxidation of alcohols at room temperature
without base, the activity is far lower than that of our catalyst
system under the ambient conditions.
The scope of our catalyst system was investigated for the oxi-
dation of other benzylic alcohols and aliphatic alcohols under
oxygen balloon at room temperature (Table 2). Secondary benzylic
alcohols were oxidized with 0.30 mol % of Au successfully (entries
1–7). The electronic effect of the substituent of phenyl ring was not
significant (entries 1–3). Cyclopropyl group survived during
10
17
2
18
(
630 m /g) and good dispersion of the gold nanoparticles. In the
case of Au/CeO , acetophenone is obtained from 1-phenylethanol
2
Figure 1. TEM images of 1: (a) high resolution (2 nm bar scale), (b) low resolution (20 nm bar scale), (c) size distribution of Au nanoparticles.

S. Kim et al. / Tetrahedron 65 (2009) 1461–1466
1463
Table 2
a
Oxidation of various alcohols using 1 at room temperature
Entry
1
Substrate
Product
Time (h)
3
Yield (%)
>99b
b
OH
O
OH
O
>
99
2
4
c
9
9
Cl
Cl
OH
O
b
b
>
99
3
4
5
4
c
9
3
MeO
MeO
O
OH
>99
c
9
9
OH
O
O
O
b
b
>
99
5
6
7
c
9
6
OH
>99
25
c
9
2
OH
b
>99
7
8
6
c
9
6
OH
O
24
Traceg
CF3
OH
CF3
O
b,d
>
99
9
8
c,d
9
5
OH
O
1
0
19
7
>99b,d
11
OH
O
99b,d
12
72
83c,g
H
H
H
H
H
H
HO
O
OH
O
1
3
24
24
25b,g
50b,g
N
N
OH
O
14
O
O
OH
O
86b,e
77c,f
1
5
9
6
OH
O
16
O
OH
77c,g
72c,h
17
24
24
O
O
OH
18
Ph
O
Ph
a
ꢀ
The reaction was performed on 0.50 mmol of substrate dissolved in 2.0 mL of toluene with 0.30 mol % Au and Cs
Determined by GC.
Isolation yield.
2
CO
3
(1.5 mmol) at 25 C under O
2
balloon.
b
c
d
e
f
%
of Au (1.0 mol %) was used.
Benzyl benzoate was formed in 14%.
Cinnamyl cinnamate was formed in 23%.
Au (3 mol %) was used.
g
h
Au (5 mol %) was used.

1464
S. Kim et al. / Tetrahedron 65 (2009) 1461–1466
Table 3
(entry 16). Meanwhile, ester formation was the major reaction in
Recycling test for 1a
the oxidation of primary aliphatic alcohols (entries 17–18). The
corresponding esters were formed in 77% and 72% in the oxidation
of cyclohexylmethanol and in that of 3-phenylpropan-1-ol, re-
spectively. The ester formation from the oxidation of primary
alcohols by gold catalysts can be explained by the following
OH
O
catalyst 1 (0.30 mol %)
rt, toluene, O2 balloon
Cs2CO3 (3 equiv), 3 h
19
mechanism. Aldehyde formed initially is reacted with alcohol to
produce hemiacetal, which is further oxidized into ester.
Reuse
Yieldb (%)
First
>99
Second
Third
Fourth
Fifth
>99
>99
>99
>99
a
After washing the catalyst with toluene (3 mL) three times, 1.5 equiv of Cs
was added for each run.
2 3
CO
2
.3. Recycling test and scale-up experiments
b
Determined by GC.
We tested the recyclability of 1 in the oxidation 1-phenylethanol
under the conditions described in Table 2. After each run, the
resulting solution was separated by decantation, and 1.5 equiv of
2 3
Cs CO was added for next run. Even in the fifth use loss of the
Table 4
Scale-up experiments
a
catalytic activity was not observed (Table 3). Leached metal species
was not detected by inductively coupled plasma analysis for the
solution phase in the recycling test, whereas the TEM images of the
recovered catalyst showed that the average size of gold nano-
Entry
1
Substrate
Product
Time (h) Yield (%)
OH
O
16
>99b
particles slightly increases during the test. Scale-up experiments
were carried out for 1-phenylethanol, 2-octanol, and 3-phenyl-
propan-1-ol in 10 mmol scale (Table 4). The reaction efficiencies
were almost same as those observed in 0.5 mmol scale reactions.
5
OH
Ph
O
2d
24
97b
2.4. Coupling of acetophenone with benzyl alcohol under
O
O
3e
30
75c
ambient conditions
OH
Ph
Ph
a
The reaction was performed on 10 mmol of substrate dissolved in 30 mL of
We tested the potency of our catalyst system for the coupling of
ketones with primaryalcohols (Table 5). Indeed, chalconewasproduced
as the major product in 54% yield from the reaction of acetophenone
with benzyl alcohol under the conditions for the oxidation of benzyl
alcohol inTable 2 (entry 1). Chalcone was obtained in 94% when 1 mol %
of Au was used (entry 2). Cesium carbonate was a more effective base
than potassium phosphate or potassium carbonate (entries 3 and 4).
Our palladium catalyst, which is composed of palladium nanoparticles
embedded in aluminum oxyhydroxide, also has been used in the cou-
ꢀ
toluene with 0.30 mol % Au and Cs
2
CO
3
(3 equiv) at 25 C under O
2
balloon.
b
Determined by GC.
Isolation yield.
Au (1.0 mol %) was used.
Au (5.0 mol %) was used.
c
d
e
oxidation (entry 5). tert-Butyl group slowed down the oxidation
rate significantly (entry 6). Trifluoromethyl group inhibited the
oxidation (entry 8). The oxidation of aliphatic alcohols was much
slower than that of benzylic alcohols (entries 9–12). However, the
oxidation can be accelerated by increasing the amount of 1. Cho-
lestanone was obtained in 83% yield from cholestanol in 72 h using
pling of ketones with primary alcohols to produce
a,b-unsaturated
ꢀ
ketones, but the reaction temperature should be higher than 80 C to
20
complete the reactionwithin 20 h (entry 5). An homogeneous iridium
complex, [IrCl(cod)] , can be used for the catalytic -alkylation of ke-
2
a
ꢀ
3
mol % of Au (entry 12). Alcohols containing heteroaromatic rings
tones in the presence of phosphine ligands at 100 C. However, the
major product is ketone formed by the hydrogenation of the in-
were not successful substrates of our catalyst system (entries 13
and 14). The oxidation of primary alcohols did not stop at the stage
of aldehyde. Benzaldehyde was produced in 86% yield with forming
benzyl benzoate in 14% yield (entry 15). The ester formation was
also the side reaction in the oxidation of cinnamyl alcohol
termediate
a,b-unsaturated ketone under the conditions using excess
21a
primary alcohol (entry 6). As heterogeneous catalysts for the
a-al-
kylation of ketones, a polymeric palladium catalyst and a ruthenium
catalyst have been reported. The palladium catalyst is active under
Table 5
Coupling with acetophenone and benzyl alcohol using 1
O
O
O
OH
cat.
+
+
+
Ph
OH
Ph
Ph
Ph
Ph
Ph
Ph
Ph
2
3
4
ꢀ
Conv.a (%)
Product yieldb (%)
Entry
Catalyst (mol %)
Base (equiv)
T ( C)
Time (h)
2
3
4
1
2
3
4
5
6
7
8
1 (0.3)
1 (1.0)
1 (1.0)
1 (1.0)
Cs
Cs
2
CO
CO
3
(3)
(3)
(3)
(3)
(3)
25
25
25
25
80
100
100
180
24
24
24
24
20
4
54
94
88
20
95
d
54
94
88
20
95
d
d
d
d
d
d
d
d
86
91
85
d
d
d
d
d
10
d
d
2
3
K
3
K
2
K
3
PO
CO
PO
4
3
Pd/AlO(OH) (0.20)
[IrCl(cod)]
Pd/viologen polymer (5.0)
Ru/HT (0.75)
4
2
(1.0)
KOH (0.1)
Ba(OH)
None
2
(1)
24
20
d
85
a
Conversion of acetophenone.
Determined by GC.
b

S. Kim et al. / Tetrahedron 65 (2009) 1461–1466
1465
Table 6
a
Coupling with various ketones and alcohols using of 1
Entry
1
Ketone
Alcohol
Time (h)
24
Product
Yieldb (%)
90
O
O
OH
OH
O
O
O
2c
3d
4
24
24
24
85
82
86
O
O
OH
OH
OH
O
O
O
Cl
Cl
5
24
70
MeO
MeO
O
O
6e
OH
OH
30
30
30
79
O
O
7e
78
n-C H
5
11
n-C5H11
O
O
8e–g
OH
Trace
n-C H
n-C H
Ph
5
11
5
11
O
O
9e–g
OH
30
Trace
Ph
Ph
a
ꢀ
The reaction was performed on 1.0 mmol of ketone and 3.0 mmol of alcohol dissolved in 3.0 mL of toluene with 1.0 mol % Au and Cs
CO
2 3
(3 equiv) at 25 C under O balloon.
2
b
c
d
e
f
Isolation yield.
Second reuse.
Third reuse.
Alcohol (4.0 mmol) was used.
Au (3.0 mol %) was used.
In GC the ratio of 3-phenylpropanol and 3-phenylpropyl 3-phenylpropanoate was about 3:1.
g
atmospheric conditions without organic solvent although the activity is
The catalyst is highly active for the aerobic oxidation of a wide
range of alcohols at room temperature in the presence of base. In
addition, the catalyst system is effective for the coupling of primary
21b
lower than the iridium catalyst (entry 7). The heterogeneous ruthe-
nium catalyst dose not require base, but the reaction temperature
ꢀ
21c
should be high (180 C) (entry 8).
alcohol with ketone to produce
under ambient conditions.
a,b-unsaturated ketone selectively
2
.5. Coupling with various ketones and alcohols under
ambient conditions
4
4
. Experimental section
.1. Synthesis of Au/AlO(OH) (1)
On the basis of the results obtained from the coupling of aceto-
phenone and benzyl alcohol, we tested the reaction with various
ketones and alcohols (Table 6). Under the conditions similar to the
entry 2 of Table 5, a,b-unsaturated ketones were obtained as the
major products in 70–90% yields. Our gold catalyst was reusable
although additional cesium carbonate was required in reuse (en-
Hydrogen tetrachloroaurate hydrate (49% Au) (120 mg,
.23 mmol), Pluronic P123 (1.5 g) (EO20PO80EO20 (EO¼ethylene
0
oxide, PO¼propylene oxide)), and absolute ethanol (7.0 g) were
placed in a 100 mL flask equipped with condenser. The reaction
0
tries 1–3). Acetophenone derivatives such as 4 -chloroaceto-
ꢀ
mixture was heated at 120 C for 10 min. The yellow solution
0
phenone and 4 -methoxyacetophenone were also coupled with
turned into greenish-brown suspension. To the resulting suspen-
sion, water (8 mL) was added slowly to form red gel. After cooling
to room temperature, the red gel was filtered, washed with acetone,
and dried at 120 C for 2 h to give 1 as red powder (2.0 g; 2.8 wt % of
Au). The gold content was estimated by ICP analysis.
benzyl alcohol successfully (entries 4 and 5). The coupling reaction
of aliphatic ketone with benzyl alcohol was slower than those of
aromatic ketones (entries 6 and 7). Increasing the amount of benzyl
ꢀ
alcohol to 4 equiv produced the corresponding
a,b-unsaturated
ketones in about 80% after 30 h. Meanwhile, the reaction of ali-
phatic ketone (or aromatic ketone) with aliphatic primary alcohol
did not afford the corresponding coupling product in significant
yield (entries 8 and 9).
4.2. Aerobic oxidation of 1-phenylethanol at room
temperature
3
. Conclusion
Cs
Au) were added to
0.50 mmol) in toluene (2 mL) and the reaction mixture was stirred
at room temperature for 3 h under O balloon.
2
CO
3
(490 mg, 1.5 mmol, 3 equiv) and 1 (12 mg, 0.30 mol % of
a
solution of 1-phenylethanol (61 mg,
We have developed a new recyclable gold catalyst that can be
made from readily available reagents through simple procedures.
2

1466
S. Kim et al. / Tetrahedron 65 (2009) 1461–1466
4
.3. Recycling test of 1
4. Yamaguchi, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc.
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. Mori, K.; Hara, T.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 2004,
26, 10657–10666.
6. Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2002, 41, 4538–4542.
2
5
After the oxidation reaction for 1-phenylethanol (61 mg,
.50 mmol) with 1 (12 mg, 0.30 mol % of Au) and Cs CO (490 mg,
2 3
1
0
7
8
9
. (a) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301–309;
1
.5 mmol, 3 equiv) in toluene (2 mL), solid containing the catalyst
was recovered by decanting the solution. For next run 1.5 equiv of
Cs CO was added.
(b) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252–255.
. Abad, A.; Concepcion, P.; Corma, A.; Garcia, H. Angew. Chem., Int. Ed. 2005, 44,
4066–4069.
. Enache,D. I.;Edwards, J. K.; Landon,P.;Solsona-Espriu, B.; Carley, A.F.;Herzing, A. A.;
Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362–365.
2
3
4
.4. Reactions of benzyl alcohol with ketones
1
0. Su, F.-Z.; Liu, Y.-M.; Wang, L.-C.; Cao, Y.; He, H.-Y.; Fan, K.-N. Angew. Chem., Int.
Ed. 2008, 47, 334–337.
1. (a) Hu, J.; Chen, L.; Zhu, K.; Suchopar, A.; Richards, R. Catal. Today 2007, 122,
77–283; (b) Haider, P.; Baiker, A. J. Catal. 2007, 248, 175–187; (c) Deng, J.-P.;
1
The reaction of benzyl alcohol with acetophenone is typical.
2
2 3
Cs CO (980 mg, 3.00 mmol) and 1 (69 mg, 1.0 mol % of Au) were
Shih, W. C.; Mou, C.-Y. ChemPhysChem 2005, 6, 2021–2025; (d) Porta, F.; Prati, L.;
Rossi, M.; Coluccia, S.; Martra, G. Catal. Today 2000, 61, 165–172; (e) Biella, S.;
Prati, L.; Rossi, M. J. Catal. 2002, 206, 242–247; (f) Porta, F.; Prati, L. J. Catal. 2004,
added to a solution of acetophenone (120 mg, 1.00 mmol) and
benzyl alcohol (324 mg, 3.00 mmol) in toluene (3 mL) in a 10 mL
tube. The tube with a rubber stopper was connected with an oxy-
gen balloon through a needle, and the mixture was stirred at room
temperature for 24 h.
224, 397–403; (g) Choudhary, V. R.; Dhar, A.; Jana, P.; Jha, R.; Uphade, B. S. Green
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13. Pluronic P123 was essential for the preparation of 1, but more than 95% of the
employed amount was recovered from the filtration step.
14. (a) Kwon, M.-S.; Kim, N.; Park, C.-M.; Lee, J.-S.; Kang, K.-Y.; Park, J. Org. Lett.
Acknowledgements
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We are grateful for the financial supports from Korea Research
Foundation (KRF-2008-314-C00203) and the Korean Ministry of
Education through the BK21 project for our graduate program.
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1
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16. See, Supplementary data.
7. Without base, the oxidation was too slow to observe the production of aceto-
phenone under the standard conditions in Table 1.
1
Supplementary data
1
Characterization data for 1, and GC data and NMR spectra for the
products. Supplementary data associated with this article can be
found in the online version, at doi:10.1016/j.tet.2008.12.005.
1
8. There are many factors for the aerobic catalytic oxidation of alcohols with
heterogeneous gold nanoparticles. See the recent paper for the catalyst para-
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1
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