Mono-substituted Keggin-polyoxometalate complexes as effective and recyclable catalyst for the oxidation of alcohols with hydrogen peroxide in biphasic system

Article abstract of DOI:10.1016/j.tetlet.2005.08.040

Mono-substituted Keggin-polyoxymetalate complex Na₆ [SiW ₁₁ZnH₂O₄₀]?12H₂O was demonstrated to be an effective catalyst for the selective oxidation of alcohols in the presence of hydrogen peroxide as oxidant. The reaction was carried out in an aqueous/oil biphasic system, which allowed easy recovery of catalyst, under relative mild conditions. The catalyst could be reused five times without appreciable loss of activity.

Full text of DOI:10.1016/j.tetlet.2005.08.040

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 7023–7027
Mono-substituted Keggin-polyoxometalate complexes as
effective and recyclable catalyst for the oxidation of alcohols
with hydrogen peroxide in biphasic system
a,c
a
a
a,c,
*
Jianmin Wang, Liang Yan, Guixian Li, Xiaolai Wang,
a
a,b
Yong Ding and Jishuan Suo
a
State key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics,
Chinese Academy of Sciences, Lanzhou 730000, China
b
Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, China
c
Graduate School of the Chinese Academy of Sciences, China
Received 4 July 2005; revised 9 August 2005; accepted 11 August 2005
Available online 25August 2005
Abstract—Mono-substituted Keggin-polyoxymetalate complex Na [SiW ZnH O ]Æ12H O was demonstrated to be an effective
6
11
2
40
2
catalyst for the selective oxidation of alcohols in the presence of hydrogen peroxide as oxidant. The reaction was carried out in
an aqueous/oil biphasic system, which allowed easy recovery of catalyst, under relative mild conditions. The catalyst could be reused
five times without appreciable loss of activity.
Ó 2005 Elsevier Ltd. All rights reserved.
The selective oxidation of alcohols to carbonyl com-
pounds, which are important precursors of various fine
chemicals including drugs, vitamins, and fragrances,
tion. For the construction of such biphasic system, the
use of water-soluble catalyst was often involved. Ini-
tially, a water-soluble palladium–phenanthroline com-
plex has been used to catalyze the title reaction using
1
–3
is a key transformation in fine chemical industry.
Because of the advantageous properties of hydrogen
6
the biphasic system. However, the system still suffers
4
peroxide, a number of useful procedures have been
from the use of both an expensive and a noble metal.
Moreover, the preparation of water-soluble organic
ligand was also a hard task.
developed for H O -mediated oxidation of alcohols cata-
2
2
lyzed by tungsten systems, such as tungstic acid, quater-
nary ammonium tetrakis(diperoxotungsto)phosphates,
sodium tungstate + [(n-C H ) N]Cl, and sodium tung-
Transition metal polyoxometalates have proved to be
effective catalysts for the oxidation of hydrocarbons with
hydrogen peroxide or molecular oxygen over the past
4
9 4
5
state + quaternaryammoniumhydrogen sulfate. These
types of reactions with the catalysts dissolved in the
organic phase inherently require separation of the cata-
lyst from the product, invariably a chromatographic pro-
cedure, for recovery of the catalyst.
7
decade. In particular, transition metal mono-substi-
(
nÀm)À
tuted Keggin-type heteropolyacids, [XW MO ]
1
1
39
(X = P, Si, B), in which a transition metal cation,
m+
M
, is coordinated to the binding sites of lacunary het-
nÀ
In order to overcome this drawback, an alternative
approach has been developed using a biphasic system,
with two immiscible liquid phases, such that the homo-
geneous catalyst is soluble in one phase (e.g., water) and
the products in the other phase (organic). The oxidation
product can then be obtained by a simple phase separa-
eropolyanions [XW O ] , have generated substantial
1
1
39
interest as oxidative catalysts. This is because their ther-
mal and chemical stability and redoxic and acidic prop-
erties could be controlled at the atomic/molecular
levels by changing the constituent elements (other transi-
tion or main group metals) without affecting the Keggin-
8
type primary structure. Previously, Ronny Neumannꢀs
group demonstrated feasibility for the application of
Keywords: Oxidation of alcohol; Mono-substituted Keggin-polyoxo-
metalate; Hydrogen peroxide; Biphasic catalysis.
water-soluble polyoxometalates, Na12
À [WZnZn (H O) -
2 2 2
*
Corresponding author. E-mail: wangjianmin060102@yahoo.com.cn
(ZnW O ) ], in the biphasic oxidation of alcohols and
9
34 2
0040-4039/$ - see front matter Ó 2005Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.08.040

7024
J. Wang et al. / Tetrahedron Letters 46 (2005) 7023–7027
9
contributed more than 10 papers in this area. To expand
the applications of water-soluble polyoxometalates
in the biphasic oxidation of alcohols, now, we reported
the oxidation of alcohols with aqueous hydrogen perox-
ide using a mono-substituted Keggin-polyoxometalate,
Na [SiW ZnH O ]Æ12H O, as catalyst in biphasic sys-
Having the best catalyst in hand, various alcohols,
including primary and secondary alcohols, were submit-
ted to the condition of Table 1 and the results are listed
in Table 2. Secondary alcohols, such as 2-pentanol, 2-
hexanol, and cyclooctanol, were converted to the corre-
sponding ketones with H O using SiW Zn as catalyst
6
11
2
40
2
2
2
11
tem without the use of organic solvent. Compared with
conventional catalytic systems, this methodology offers
significant improvements with regard to catalytic effi-
ciency, simplicity in operation, cost efficiency, and green
in a short period with excellent conversions and good
selectivity. Indeed, secondary allylic alcohols, such as
2-clycohexen-1-ol, 1-octen-3-ol could also be oxidized
effectively to the corresponding a,b-unsaturated
ketones. Primary alcohols, for instance, ethanol, 1-pent-
anol, benzyl alcohol, are all worked well under our con-
ditions, but the formation of the corresponding
carboxylic acids was proved to be the predominant reac-
tion, and only slight amount of corresponding aldehydes
was detected after the reaction. It is worth noting that
1,3-diol, 2-ethylhexane-1,3-diol underwent the same
oxidation reaction under our conditions to give high
selectivity of the partial oxidation product, 2-ethyl-1-
hydroxy-3-hexanone, which implies that primary alco-
hols were more difficult to oxidize than secondary
1
0
aspects avoiding toxic catalysts and solvents.
The catalyst was prepared by the following procedure:
8 g (0.009 mol) H SiW O Æ7H O and 100 ml water
2
4
12 40
2
were mixed and heated to 95 °C; 7.2 g NaHCO
3
(
0.085mol) was then added to the aqueous solution,
adjusting the pH value to 6; subsequently, 10 ml hot
ZnNO Æ6H O (3.1 g, 0.013 mol ZnNO Æ6H O) solution
3
2
3
2
was added carefully. After 10 min of stirring at 95 °C,
the solution was quickly filtered. The filtrate was dried
at 95 °C for 2 h, and a white powder was obtained.
The structure of the powder was confirmed as Na6
alcohols over SiW Zn catalyst. It is well known that
11
[
SiW ZnH O ]Æ12H O (abbreviated as SiW Zn) from
benzyl alcohol has higher reactivity as compared with
that of linear aliphatic primary and secondary alcohols.
On the one hand, benzyl alcohol could be completely
converted to benzoic acid even at room temperature
within 2 h. On the other hand, our experimental results
also show that SiW Zn catalytic system does not induce
1
1
2
40
2
11
IR, UV spectroscopic data, and elemental analysis.
Other catalysts were synthesized according to the stan-
dard method.
1
1
Initially, oxidation of cyclohexanol was used as the
model reaction to test the activities of various catalysts
[
V) and the results are listed in Table 1. It was observed
that SiW Zn was the best catalyst for the reaction, and
cyclohexanol was completely consumed after 7 h reac-
tion with exclusive selectivity to cyclohexanone. Other
M-containing catalysts are also capable of catalyzing
the model reaction, but their efficiencies seem to be
slightly inferior as compared with that of SiW Zn. It
should be pointed out here, besides the unreacted cyclo-
hexanol, the only product was cyclohexanone by the
analysis of GC–MS, indicating the specific character of
our catalysts for the oxidation of cyclohexanol. When
SiW , the transition metal was omitted, was used as a
1
1
the unproductive decomposition of H O to any great
2
2
(
nÀm)À
SiW MO ]
(M = Zn, Cu, Ni, Co, Fe, Mn, Cr,
extent and enables the economic use of the oxidant,
namely, molar ratio of H O to cyclohexanol is 1.5:1.
1
1
39
1
2
2
2
With a higher molar ratio, the product selectivity is
decreased to some extent; for the alcohols with relatively
poor reactivity, for example, 2-octanol, a 5-fold molar
excess of H O was required.
1
1
2
2
One of the main aims of using mono-substituted Keg-
gin-polyoxometalate complexes as catalysts was to study
the possibility of its recycle and reuse. We found that the
catalyst, SiW Zn, could be easily recycled by a simple
1
1
1
1
procedure. For example, in the oxidation of cyclohexa-
nol, the product, cyclohexanone, could be decanted
from the biphasic system. To prevent the accumulation
of water in the reused experiments, the recovered aque-
ous solution could be heated to 90 °C for 0.5h under
vacuum (12 mmHg) before each run. The remaining
SiW Zn aqueous residue was therefore recharged with
1
2
catalyst, only unreacted cyclohexanol was detected.
These results indicated that the transition metal in cata-
lysts might play a pivotal role in the present oxidation
reaction.
1
1
cyclohexanol and hydrogen peroxide as above. Reaction
workup was carried out again. We found that the reac-
tion was carried out five times in consecutive run with
only a slight decrease in activity (Table 2). Table 2 also
lists ToN of catalyst for different alcohol oxidation reac-
tions. Compared with Neumannꢀs catalyst, the SiW Zn
Table 1. Oxidation of cyclohexanol with hydrogen peroxide over
various polyoxometalate catalysts at 363 K
a
Catalyst
Conversion of
Selectivity of
cyclohexanol (%)
cyclohexanone (%)
1
1
SiW12
(
0
70
69
70.4
52.9
43.8
52.8
38.6
100
0
100
94.3
97.7
ꢀ100
100
100
100
100
IV)
is more effective for various alcohol oxidation reactions.
SiW11
V
(VI)
SiW11Cr
SiW11Mn
SiW11Fe
SiW11Co
SiW11Ni
SiW11Cu
SiW11Zn
(IV)
Figure 1 shows IR of the fresh, recycled and SiW Zn
11
(III)
treated with 30% H O The IR spectrum of fresh
2
2.
(II)
SiW Zn shows six peaks at 1016, 975, 921, 884, 793,
11
(II)
À1
and 532 cm , indicating that the SiW Zn is a Keg-
11
gin-type structure polyoxometalates.
(II)
8
b,13
There is not
(II)
an obvious difference between the IR spectra of the recy-
cled and fresh samples. After addition of hydrogen
a
Reaction condition: cyclohexanol/30% H
2
O
2
/catalysts = 670:1005:1,
À1
temperature: 363 K, reaction time: 7 h, 10 ml water.
peroxide, a new peak at 623 cm was observed. It is

J. Wang et al. / Tetrahedron Letters 46 (2005) 7023–7027
7025
a
Table 2. Selective oxidation of alcohol with aqueous hydrogen peroxide by SiW11Zn
Substrates
Eff. conv. of
(%)
Alcohols
conv. (%)
Product selectivity (%)
TON
2 2
H O
b
i
Benzyl alcohol
99
99
99
99
91
89
99
99
99
99
86
99
100
100
100
100
77
58
96
98
Benzoic acid (100)
4-Bromo-benzoic acid (100)
4-Nitrobenzoic acid (100)
544(250 )
366
417
b
4
4
1
-bromo-benzyl alcohol
b
-Nitrobenzyl alcohol
c
i
-Phenyethanol
Acetophenone (100)
Acetaldehyde (7)
Pentanal (8)
471(250 )
Ethanol
-Pentanol
Propan-2-ol
Acetic acid (93)
Pentanaonic acid (92)
942
i
1
340(165 )
i
Acetone (95)
830(233 )
2
2
2
2
2
-Butanol
-Pentanol
-Hexanol
-Octanol
2-Butanone (100)
2-Pentanone (100)
2-Hexanone (100)
2-Octanone (100)
2-Ethyl-1-hydroxy-
i
100
100
91
577(235 )
697
i
356(228 )
i
-Ethyl-1,3-hexanediol
100
Butanoic acid (7)
398(250 )
3
-hexanone (93)
2,2,4-Trimethyl-1-hydroxy
-pentanone (97)
2
,2,4-Trimethyl-1,3-pentanediol
99
100
385
3
i
Cyclohexanol
Cyclooctanol
99
100
98
Cyclohexanone (100)
600(250 )
d
9
6
e
99
100
Cyclooctanone (100)
479
(238 )
f
i
7
8
9
6
578
3
95
g
195
233
638
488
401
406
h
93
2
1
-Cyclohexen-1-ol
-Octen-3-ol
99
99
99
99
100
100
99
2-Cyclohexen-1-one (100)
1-Octen-3-one (100)
Menthone (100)
Menthol
Borneol
100
Camphor (100)
a
b
c
d
e
f
2 2
Reaction condition: 2 ml substrates; 0.1 g of catalyst; 4 ml H O (30%); temperature 363 K; reaction time: 9 h; 10 ml water.
2
2
ml substrates; 0.1 g of catalyst; 2 ml H
ml substrates; 0.1 g of catalyst; 3 ml H
2
O
2
(30%); reaction time: 2 h.
2
O (30%); reaction time: 7 h.
2
The fifth run.
SiW11Zn as catalyst.
Na12
À
[WZnZn
PW12 40 as catalyst (Q = cetylpyridinium).
[PO (W(O)(O ] as catalyst. The remaining H
Calculated from Ref. 9d.
2 2 2 9 34 2
(H O) (ZnW O ) ] as catalyst.
g
h
i
Q
3
O
Q
3
4
2
)
2
)
4
2 2
O after reaction was estimated by titration with KI solution.
stage, the exact structure of the peroxometal species
and the activating site are not clear. However, several
possible structures for the active peroxometal species
could be deduced as shown in Figure 2 according to
1
5
the previous analogy research.
As to the reaction mechanism, based on the hypothesis
about the structures of peroxometal species mentioned
above, it could yet be deduced as shown in Figure 3:
at the beginning of the reaction, SiW Zn catalyst
1
1
reacted with 30% H O to produce species, which may
2
2
be the active intermediate for the oxidation of alcohol,
then peroxometal species reacted with alcohol to afford
the corresponding product.
In summary, we have described an efficient catalytic oxi-
dation procedure of alcohols with hydrogen peroxide
using mono-substituted Keggin-polyoxometalate com-
plexes as catalysts. Many primary and secondary alco-
hols could be converted to the corresponding ketones
and carboxylic acids with good to excellent conversion
and selectivity. This biphasic system based on SiW Zn
permits extensive recycling of the catalyst without signif-
icant loss in activity. IR investigation revealed that per-
oxometal might be the active species for the oxidation of
alcohol.
Figure 1. IR spectra of SiW11Zn before during and after reaction: (a)
fresh SiW11Zn; (b) used SiW11Zn; (c) SiW11Zn in 30% H
O .
2 2
most likely attributed to the formation of a peroxometal
1
4
species. After decomposition of the excess hydrogen
peroxide with aqueous KI, the IR spectrum is almost
identical to that of fresh SiW Zn. The results of IR
1
1
1
1
characterization indicated that the SiW Zn is a stable
1
1
8
catalyst in the reaction system. But at this research

7026
J. Wang et al. / Tetrahedron Letters 46 (2005) 7023–7027
H
O
O
O
O
OH2
O
OH2
O
W
O
W
W
O
W
M
M
O
O
O
O
O
O
W
W
W
W
Si
Si
O
OH
OH2
W
O
W
M
O
O
M = W or Zn
O
O
W
W
Si
Figure 2. Possible structure of the peroxometal species.
H
H O
2
2
O
M
M
OH
OH2
OH2
R R CHOH
1
2
M
= SiW11ZnH O
2 40
H
or R CH OH
3
2
M = Zn or W
OH
O
OH
M
H
M
H
O
R R C=O
1
2
CR R (R )
1
2
3
or R CHO
3
H O
2
2
OOH
O
H
R3
R3
-
H O
OH
2
OH
Figure 3. Possible mechanism for oxidation of alcohols.
References and notes
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7
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(4 ml) aqueous hydrogen peroxide (30%) was added. The
biphasic mixture was stirred at 90 °C for 9 h. At the end of
the reaction, the reaction mixture was cooled to room
temperature, and 10 ml of di-n-butyl ether was added.
Then, the organic phase was separated and analyzed by
gas chromatograph (PE Autosystem XL), using chloro-
benzene as internal standard, with a flame ionization
detector and a 30 m SE-54 capillary column or GC–MS
(Agilent6890N/5973N).
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004, 346, 339.
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1
2
0 ml water, and 0.02 mol (2 ml) alcohol were added into a
5ml round-bottomed flask equipped with a magnetic
stirring bar and reflux condenser. The mixture was stirred
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