Polyoxometalate compound: a highly efficient heterogeneous catalyst for aerobic alcohol oxidation

Article abstract of DOI:10.1016/j.tetlet.2006.07.149

Oxidation of alcohols to aldehydes and ketones has been studied using atmospheric oxygen and a catalytic amount of Na_6.3Fe_0.9[AlMo₁₁O₃₉]· 2H₂O in toluene under heating (80 or 110 °C) in high yields. Secondary alcohols can be chemoselectively converted into ketones in the presence of primary hydroxyl groups.

Full text of DOI:10.1016/j.tetlet.2006.07.149

Tetrahedron Letters 47 (2006) 7171–7174
Polyoxometalate compound: a highly efficient
heterogeneous catalyst for aerobic alcohol oxidation
a,b
a
a
a,
*
Jianmin Wang, Liang Yan, Guang Qian and Xiaolai Wang
a
State key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, China
b
Graduate School of the Chinese Academy of Sciences, 100000, China
Received 2 July 2006; revised 28 July 2006; accepted 31 July 2006
Available online 22 August 2006
Abstract—Oxidation of alcohols to aldehydes and ketones has been studied using atmospheric oxygen and a catalytic amount of
39]Æ2H
Na6.3Fe0.9[AlMo11
converted into ketones in the presence of primary hydroxyl groups.
O
2
O in toluene under heating (80 or 110 °C) in high yields. Secondary alcohols can be chemoselectively
Ó 2006 Elsevier Ltd. All rights reserved.
The oxidation of alcohols into the corresponding alde-
hydes or ketones is one of the most important functional
group transformations in organic synthesis. Recently,
the use of molecular oxygen as terminal oxidant has re-
ceived great attention for both economic and environ-
mental benefits, and many highly efficient systems have
been developed for catalytic aerobic alcohol oxidation
ever, a major challenge in liquid-phase oxidation with
these systems is to prevent leaching of the active species.
The search for suitable green aerobic catalysts is still a
significant challenge.
1
In this letter, we report a highly efficient catalytic system
made up of a novel polyoxometalate catalyst 1 (Na6.3
-
2
3,14,20
4
5
6
7
8
9
10
using Pd, Ru,
Cu, Co, V, Ph, Os, Ni, Zr,
Fe0.9[AlMo O ]Æ2H O): A solid catalyst is indiscerpti-
11
39
2
1
1
12
M(II)-radical catalysts, M-TEMPO, polyoxometa-
ble in both water and organic solvents which has high
selectivity and are active for the aerobic oxidation of a
variety of alcohols (Scheme 1). Catalytic results indicate
that the polyoxometalate catalyst is a green, selective,
and efficient aerobic catalyst for alcohol oxidation. It
can selectively oxidize alcohols to their corresponding
carbonyls under mild aerobic conditions without using
1
3
14
lates,
Ru-biomimetic-coupled systems,
bimetallic
1
5
16
17
Mo–Cu, and Os–Cu systems, perruthenate, Pt,
1
8
and Pt/Bi catalysts,
manganese oxide octahedral
1
9
20
molecular sieves, Ru-hydroxyapatite, lignin peroxi-
dase,2 Noble-metal, polymer encapsulation of metallo-
1
22
2
3
24
phthalocyanines, and others.
sacrificial reducing agents and pure O . This reaction
2
Although these procedures for the oxidation with air/
system displays an important merit, that is, more facile
catalyst/product separation and catalyst reuse.
molecular O are efficient, most of them are homoge-
2
neous systems and share common drawbacks because
of which catalyst/product separation and catalyst reuse
are difficult. The immobilization (heterogenization) of
catalytically active species onto solid supports (insoluble
bulk materials, inert solid supports such as metal
hydroxides and polymers) display advantages, for exam-
ple, no co-catalyst requirement and convenient separa-
tions, but their applications are limited, either to
activated alcohols or by the need for pure oxygen. How-
Preparation of catalyst precursor (Na3.5H5.5AlMo O Æ
11
39
9H O): Sodium molybdate (13.3 g, 55 mmol) is
2
dissolved in 30 ml of water in a separate 100 ml beaker
containing a magnetic stirring bar. To this solution,
catalyst 1
RCH OH
RCHO
2
o
air, toluene, 80 or 110 C
Keywords: Indiscerptible polyoxometalate; Oxidation of alcohol;
Atmospheric oxygen; Catalyst separation.
R = alkyls, aryls
*
Corresponding author. E-mail: wangjianmin060102@163.com
Scheme 1.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.149

7172
J. Wang et al. / Tetrahedron Letters 47 (2006) 7171–7174
a
2
Table 1. Selective oxidation of alcohol with air by Na6.3Fe0.9[AlMo11O39]Æ2H O
Entry
Substrate
Method
Time
Product
Yield (%)
69
Selectivity (%)
99
TON
138
b
OH
OH
1
A
1
O
O
2
3
4
5
6
7
A
A
A
A
A
A
1
1
1
4
1
3
96
96
93
87
98
98
99
99
99
99
99
99
192
192
186
43.5
196
65.3
OH
O
OH
O
Cl
Cl
O N
2
2
O N
OH
O
OH
O
CH O
CH O
3
3
OH
O
OH
OH
O
8
9
A
9
93
99
21.7
OH
B
B
A
A
3
4
1
4
94
90
97
95
99
99
99
99
65
45
OH
O
1
1
1
0
1
2
N
O
N
O
194
190
OH
OH
O
O
S
S
1
1
1
3
4
5
OH
B
B
B
6
5
5
88
92
89
99
99
99
29
20
36
O
OH
O
OH
O
O
OH
1
1
6
7
B
B
5
4
88
92
99
99
35
46
OH
O
a
Reaction conditions: alcohol (2 mmol), catalyst (0.01 mmol), toluene (6 ml), atmospheric pressure; method A: 80 °C (oil bath temperature); method
B: 110 °C (oil bath temperature). Selectivities are based on the GC with area normalization. All yields are for pure, isolated products.
Na3.5H5.5AlMo11O39Æ9H O as catalyst.
2
b

J. Wang et al. / Tetrahedron Letters 47 (2006) 7171–7174
7173
Table 2. Recycling of the catalytic system for the oxidation of benzyl
alcohol to aldehyde
and 2-pyridinemethanol, were oxidized to the corre-
sponding aldehydes in high yields. No oxidation was ob-
served with S and N heteroatoms. In addition, a mixture
of primary and secondary alcohols was next subjected to
oxidation. When cyclohexanol and hexyl alcohol were
allowed to react, the former oxidized to cyclohexanone
in 90% yield and the latter gave hexanal in <10% yield.
This clearly reveals that this method can be applied for
the chemoselective oxidation of secondary alcohols in
the presence of primary hydroxy groups.
a
catalyst
OH
O
o
air, toluene, 80 or 110 C
Run
Time
Conversion (%)
Yield (%)
1
2
3
1
1
1
100
100
100
96
95
96
a
Reaction conditions: alcohol (2 mmol), catalyst (0.01 mmol), toluene
6 ml), atmospheric pressure, 80 °C (oil bath temperature). Selectiv-
(
Furthermore, a catalyst recycle experiment (2 mmol of
benzyl alcohol, 6 ml of toluene, 80 °C, 1 h) carried out
simply by filtration of catalyst and re-addition of benzyl
alcohol (2 mmol) showed 96%, 95%, and 96% yields in
three reaction cycles with benzaldehyde as the only
product. There was no discernible loss in activity or
selectivity. To confirm whether the oxidation proceeded
heterogeneously, we have checked the progress of the
oxidation of the filtrate under the similar reaction condi-
tions; Al, Mo, and Fe ions in the solution could not be
detected by ICP analysis after the alcohol oxidation
reaction.
ities are based on the GC with area normalization. All yields are for
pure, isolated products.
1
6.5 ml of 4 M HCl is added in 1 ml portions over 1 min,
with vigorous stirring. Then, a solution containing Al-
(
NO ) Æ9H O (1.89 g, 5 mmol) is poured into the molyb-
3
3
2
date solution, and pH is adjusted to between 2 and 3 by
addition of the 4 M HCl. The solution is kept at 90 °C
for 1 h. The yellow crystal is collected at room tempera-
À1
ture. IR (KBr, m/cm ) 1053.71, 956.27, 880.51, 784.13,
7
01.08, 627.12, 577.28.
Preparation of catalyst (Na6.3Fe0.9[AlMo O ]Æ2H O):
In conclusion, the oxidation of alcohols to aldehydes
and ketones has been studied with molecular oxygen
in the presence of recyclable catalyst 1 in high yields.
This reaction provides a new environmentally friendly
route to the conversion of alcoholic functions to car-
bonyl groups. Aldehydes do not undergo further oxida-
tion to carboxylic acids.
11
39
2
3
g of yellow crystal (Na3.5H5.5AlMo O Æ9H O) is dis-
11 39 2
solved in 20 ml water in a separate 100 ml beaker con-
taining a magnetic stirring bar. To this solution, a
solution of 0.55 g Fe(NO ) Æ6H O is added. The solution
3
3
2
is kept at room temperature for 1 h. The yellow solid
product is collected on a sintered glass funnel, washed
À1
with water and acetone, dried in air. IR (KBr, m/cm
036.10, 970.77, 912.04, 708.93, 661.44, 506.55.
)
1
Typical procedure for the oxidation: Alcohol (2 mmol)
and catalyst 1 (0.01 mmol) were stirred at 80 °C in tolu-
ene (6 ml) under air pressure for the appropriate time
(Tables 1 and 2). After filtering the catalyst, the filtrate
was analyzed by GC and passed through a short pad
of silica gel using ethyl acetate and hexane (1:19) as elu-
ent or distilled to provide the analytically pure aldehydes
and ketones.
To begin with, the oxidation of benzyl alcohol was
examined as a standard substrate in the presence of a
catalytic amount of 1 in toluene under air pressure. As
expected, the oxidation took place affording benzalde-
hyde in 99% yield when the reaction mixture was
allowed to stir at 80 °C for 1.0 h. In contrast, the
corresponding homogeneous process using AlMo O
39
1
1
was less effective and provided benzaldehyde only in
9%. This suggested that Mo ions in the polyoxometa-
6
References and notes
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