Selective oxidation of alcohols to aldehydes by hydrogen peroxide using hexamolybdochromate(III) as catalyst

Article abstract of DOI:10.1007/BF03246030

Anderson type hexamolybdochromate(III) was utilized as a catalyst for facile conversion of various aliphatic, benzylic and heterocyclic alcohols to corresponding aldehydes and ketones in good yields. The reaction was carried out in 50% aq. acetonitrile us

Full text of DOI:10.1007/BF03246030

J. Iran. Chem. Soc., Vol. 7, No. 2, June 2010, pp. 441-446.
JOUIRrNaAnLiaOnF THE
Chemical Society
Selective Oxidation of Alcohols to Aldehydes by Hydrogen Peroxide Using
Hexamolybdochromate(III) as Catalyst
S.P. Mardur and G.S. Gokavi*
Kinetics and Catalysis Laboratory, Department of Chemistry, Shivaji University
Kolhapur-416004, Maharashtra State, India
(Received 24 April 2009, Accepted 26 July 2009)
Anderson type hexamolybdochromate(III) was utilized as a catalyst for facile conversion of various aliphatic, benzylic and
heterocyclic alcohols to corresponding aldehydes and ketones in good yields. The reaction was carried out in 50% aq. acetonitrile
using hydrogen peroxide as the oxidant at 50 °C. The reaction was found to involve oxidation of the catalyst to its active Cr(V)
intermediate by hydrogen peroxide.
Keywords: Anderson type, Alcohols, Oxidation, Homogeneous catalysis, Hydrogen peroxide
INTRODUCTION
from oximes and phenylhydrazones respectively, have also
been reported.
Oxidation is a core technology wherein the general
methods are unacceptable for practical synthesis. The heavy
metal oxidants form toxic wastes, and known organic
stoichiometric oxidants are usually very expensive [1-2]. Thus,
there is need for clean, safe oxidation procedures. Molecular
oxygen is obviously an ideal oxidant [3], but aerobic oxidation
is often difficult to control. Hydrogen peroxide, H2O2, is a
very attractive oxidant for liquid-phase reactions [4]. It can
oxidize organic compounds with an atom efficiency of 47%
and with the generation of water as the only theoretical co-
product. It is also waste-avoiding oxidant only when it is used
in a controlled manner without organic solvents and other
toxic compounds. These oxidations, however, require an
efficient catalyst. Other methods for the preparation of
carbonyl compounds utilizing sodium nitrate in the presence
of an acidic ionic liquid [5] and methyl triphenylphosphonium
peroxydisulfate [6], either from the corresponding alcohols or
Polyoxomolybdates have been attracting considerable
interest over recent years as oxidation catalysts for both gas
and liquid phase oxidation reactions [7-11]. They were utilized
as co-catalysts in various oxidations of terminal alkenes [12],
followed later on by various catalytic oxidations of alcohols,
ketones [13] and amines [14-15], phenols [16], alkyl aromatics
[17], dienes [18], alkanes [19] and sulfur containing
compounds [20]. Although Keggin and Dawson are known to
be active catalysts in several [8] industrial processes, the
application of Anderson type heteropolyoxomolybdates is only
starting. The Anderson structure may be described as an
isopolyoxometalate containing a crown of six octahedrons
sharing edges. The center may be occupied or not. In the
chromium-containing compound structure, there are six
hydrogen atoms, the positions of which are considered to be
the six OH groups bridging the central atom to the
molybdenum-containing crown octahedrons. The structure has
been confirmed by Perloff [21] who found that it resembles
that of [TeMo6O24]6-. Most of the polyoxometalate catalysis
*Corresponding author. E-mail: gsgokavi@hotmail.com

Mardur & Gokavi
Hexamolybdochromate(III)
30% H2O2
appear in 1 h. The solution was allowed to stand for 2 weeks
before the precipitate was filtered off and washed several
times with cold water. Reddish purple crystals were obtained.
50 0C,
R-CHO
R-OH
50% aq. acetonitrile
Quantitative Analysis of the Catalyst
Scheme 1
The complex Na3[CrMo6O24H6].8H2O was studied by AAS
analysis. 100 mg of recrystalised sample was dissolved in
doubly glass-distilled water. 5 ml of this stock solution was
diluted to 100 ml and used for AAS analysis of Cr and Mo
metals using Perkin-Elmer AAnalyst-300. The complex
Na3[CrMo6O24H6].8H2O shows (Theoretical): Na-5.6303%
(5.6052%), Cr-4.2227% (4.2242%) and Mo-46.217%
(46.2109%).
reported in the literature are based on the utilization of Keggin
or Dawson-type polyoxometalates. In continuation of our
work on organic oxidations catalysed by polyoxometalates
[22-23], we now present our studies on a novel system; the
oxidation of alcoholic substrates to aldehydes and ketones
using hydrogen peroxide catalysed by an Anderson-type
polyoxomolybdate, sodium hexamolybdochromate(III) as
shown in Scheme 1. The conversion of alcohols into
corresponding carbonyl compounds can also be effected by
using carcinogenic chromium reagents [24]. However, the
present method utilizes the chromium reagent in a catalytic
amount rather than stoichiometric amount.
General Procedure for Oxidation of Alcohols
To start with, the oxidation of 4-nitrobenzyl alcohol was
studied. In a typical experiment 4-nitrobenzyl alcohol (2.2
mmol) and H2O2 (4.5 mmol) were taken in 50:50% aqueous
acetonitrile mixture in a round-bottomed flask fitted with
reflux condenser. Catalyst Na3[CrMo9O24H6] (0.4 mmol) was
added. The reaction mixture was stirred at 50 °C for 2-3 h. The
progress of the reaction was monitored by TLC. After the
completion of the reaction, the resulting solution was extracted
with dichloromethane (20 ml ꢀ 2) and the organic phase was
concentrated to afford the crude 4-nitrobenzaldehyde, which
was further purified by column chromatography (petroleum
ether-ethyl acetate, 9:1). The oxidations of other alcohols were
then examined using the optimized reaction conditions. Since
the carbonyl compounds are all known compounds, they are
characterized by comparing the physical constants (M.P or B.
P.) of the product with their corresponding 2,4,DNP
derivatives.
EXPERIMENTAL
All the products are known compounds and were identified
by comparison of their physical and spectral data with those of
authentic samples. Melting points were determined in open
capillaries and are uncorrected. IR was recorded as neat films
or as KBr pellet on a Thermo Nicolet spectrometer. All
alcohols were commercial materials purchased from S. D. Fine
Chemicals (Mumbai India) and Lancaster. Acetonitrile and
dichloromethane were purchased from S. D. Fine Chemicals
(Mumbai India) and used without further purification. Yields
reported refer to the isolated products of the carbonyl
compounds.
RESULTS AND DISCUSSION
Catalyst Preparation
To study the catalytic oxidation of alcohols mediated by
Na3[CrMo6O24H6].8H2O, benzyl alcohol was chosen as a
model compound and the reaction conditions were optimized.
The effect of catalyst between 1.0 mmol to 4.0 mmol by
keeping the amount of hydrogen peroxide constant was
studied (Table 1) and the maximum yield of benzaldehyde was
obtained within three hrs when the concentration of thecatalyst
was 4.0 mmol. Therefore, keeping the catalyst concentration
The catalyst sodium hexamolybdochromate(III) was
prepared using the previously reported method [21]. The pH of
a solution containing 14.5 g of Na2MoO4.2H2O in 30 ml of
water was adjusted to 4.5 with concentrated HNO3. A second
solution was made by dissolving 4.0 g of Cr(NO3)3.9H2O in 5
ml of water. Both solutions were mixed together, and the
mixture was boiled for 1 min and filtered while hot. The
filtrate was set aside for crystallization and crystals started to
442

Selective Oxidation of Alcohols to Aldehydes by Hydrogen Peroxide
Table 1. Effect of Hydrogen Peroxide and Catalyst on the
(4.0 mmol) constant, other substrates were oxidized. The
reaction did not occur within 3 h, when only the catalyst or
only hydrogen peroxide was present (Table 1). To assess the
general scope of the catalytic oxidation, the saturated and
benzylic, primary and secondary alcohols were chosen as
substrates. The results of this study are shown in Table 2 for
various alcohols which show that the carbonyl compound is
the only product obtained in comparatively high yields. Not
surprisingly, the substituted benzylic substrates like 2-Nitro-,
4-Nitro-, 4-Chloro- and 4-Methoxybenzyl alcohols were more
reactive than the 4-Methylbenzyl alcohol. It is worth
mentioning that no oxidation was observed in the aromatic
ring of the benzylic substrates. Cinnamyl alcohol and furfuryl
alcohol were also oxidized neatly without forming any side
Yield of Benzaldehyde During Oxidation of Benzyl
Alcohol after 3 h
Catalyst (mmol)
H2O2 (mmol)
Yield (%)b
0.0
4.5
4.5
4.5
4.5
4.5
0.0
0
0.1
55
60
70
85
0
0.2
0.3
0.4
0.4
bIsolated yield.
Table 2. Sodium Hexamolybdochromate(III) Catalyzed Oxidation of Alcohols to Carbonyl Compounds by H2O2
Sr. No.
Reactant
Product
Reaction
time
Yield M.P./B.P. of
M.P. of 2,4-
DNP derivative
(oC)
(%)b
product (oC)
(h)
1
2-Propanol
Acetone
4
4
72
78
81
75
80
85
92
92
87
81
83
56
76
150
122
185
115
106
235
264
>300
263
231
254
2
n-Butanol
n-Butaraldehyde
Iso-butaraldehyde
Ethyl methyl ketone
n-Octaldehyde
3
2-Butanol
4
66
4
Sec-butanol
4
80
5
n-Octanol
4.5
3
170
179
45
6
Benzyl alcohol
2-Nitro benzyl alcohol
4-Nitro benzyl alcohol
4-Chloro benzyl alcohol
4-Methyl benzyl alcohol
Benzaldehyde
7
2-Nitobenzaldehyde
4-Nitro benzaldehyde
4-Chlorobenzaldehyde
4-Methylbenzaldehyde
2.5
2.5
2.5
3
8
104
47
9
10
11
204
248
4-Methoxy benzyl alcohol 4-Methoxy benzaldehyde
2.5
4-Hydroxy
4-Hydroxy benzyl alcohol
benzaldehyde
12
3
81
117
280
13
14
15
Cinnamyl alcohol
Fufuryl alcohol
Benzhydrol
Cinnamaldehyde
Furfural
3
3
3
62
87
87
125
161
49
-
-
Benzophenone
238
bIsolated yield.
443

Mardur & Gokavi
products. Generally, over-oxidation of the primary alcohols to
of sodium hexamolybdochromate(III) by hydrogen peroxide.
The violet solution of sodium hexamolybdochromate(III)
was treated with hydrogen peroxide and the resulting green
solution was evaporated to get the crystals of oxidized sodium
hexamolybdochromate(V). The FT-IR spectra of both
unoxidized and oxidized sodium hexamolybdochromate were
recorded as KBr pellet. The comparison of the two spectra
indicated that the polyoxometalate characterstic M-Oc-M peak
at 643 cm-1 splitted into two peaks in the oxidized form due to
the change in the oxidation state of chromium. There was also
a new peak at 829 cm-1 charasterstic of CrV=O in the FT-IR of
the oxidized form of sodium hexamolybdochromate [31]. On
the basis of FT-IR spectral examination of both unoxidized
and oxidized forms of hexamolybdochromate, the formation of
CrV can be shown as in the following equation where the
formed CrV=O acts as an oxidant in the present study.
the corresponding acids occurs with other catalysts, but in the
presence of hexamolybdochromate(III) the formation of acids
was not observed. Although numerous methods have been
reported in the literature for the selective oxidation of alcohols
to corresponding carbonyl compounds, the present method is
compared with some recently [25-30] reported methods (Table
3). The advantage of the present method over the others is that
it does not require any activator [25] like Na2HPO4 or phase
transfer reagent like IRS400 [27] and tetraalkylpyridinium ion
[29] to effect the oxidation. Therefore, the present method
gives selectively good yield in reasonable time.
Mechanistic Insights
In the context of this research, we were also interested in
gaining insight into the mechanism of these reactions. To
establish the electron transfer mechanism of POM in
oxidations, the oxidation of 4-nitrobenzyl alcohol was carried
out using the optimized conditions in the presence of 0.5 ml of
acrylonitrile. We did not notice polymerization of acrylonitrile.
Hence, it is confirmed to be a two-electron transfer reaction.
The two-electron path of the reaction occurs due to the
formation of Chromium(V) in situ as a result of the oxidation
V
CrIII(H2O)
Cr=O
2H2O
H2O2
+
+
Since, under the reaction conditions studied, but in the absence
of POM, the oxidation of alcohols did not take place, we
surmised that the initial activation mechanism worked through
an interaction between the H2O2 and the POM as shown in
Table 3. Comparison of the Reported Methods with the Present Method for Oxidation of Benzyl Alcohol to Benzaldehyde
Sr. No.
Catalyst
Conditions
Yield
(%)
Ref.
Na2WO4/Na2HPO4
Dimethylacetamide,
90 °C, 1 h
Acetone, 25 °C, 3 h
1
2
3
4
5
99
[25]
[26]
[27]
[28]
[29]
8-HydroxyquinolinatoMn(III) complexes
Na2WO4/IRA 400 Resin
H3PW12O40
82
Acetonitrile, reflux,
4-6 h
Butanol, reflux (60-
70 °C), 2.5 h
Water, reflux, 1.7
h
83
78
Tetraalkylpyridinium octamolybdate
93.6
[LMn(O)3MnL]
(L = 1,4,7 trimethyl, 1,4,7
triazacyclononane)
Acetonitrile,
22 °C, 24 h
6
7
43.5
85
[30]
Hexmolybdochromate(III)
Acetonitrile, 50 °C,
3 h
Present work
444

Selective Oxidation of Alcohols to Aldehydes by Hydrogen Peroxide
[CrV(O)Mo6O24]7- + H2O
[CrIIIMo6O24]9- + H2O2
[CrV(O)Mo6O24]7- + R1R2CHOH
[CrIIIMo6O24]9- + R1R2C=O + H2O
Scheme 2.
OH
C
Monograph Ser. (186), American Chemical Society,
Washington, DC, 1990.
R1
R2
V
---------
O
----
---
H
Cr
[2]
[3]
B.M. Trost, I. Fleming, Comprehensive Organic
Synthesis, Pergamon Press, 1st ed., Oxford, 1991.
R.A. Sheldon, J.K. Kochi, Metal-Catalyzed Oxidations
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G. Strukul, Catalytic Oxidations with Hydrogen
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Scheme 3
Scheme 2. The possible transition state of the mechanism
which involves the development of negative charge is shown
in the Scheme 3. Such a transition state is largely stabilized by
electron withdrawing substrates and makes the reaction to
occur faster. Moreover, the 4-nitro and 4-chloro benzyl
alcohols reacted at a faster rate than that of the benzyl alcohol.
[4]
[5]
[6]
[7]
[8]
[9]
CONCLUSIONS
In this new catalytic application of the Anderson type
polyoxometalate, Na3[CrMo6O24H6].8H2O, it has been shown
that the reaction of the polyoxometalate with the H2O2 leads to
the formation of a Cr(V) POM anion which in turn leads to the
formation of carbonyl derivatives as the final products. The
possible synthetic outcome of such a procedure has been noted
for a series of aliphatic and benzylic alcohols and even the
heterocyclic alcohols were converted into carbonyl
compounds without the formation of any side products. The
method is environmentally benign as water is the only
byproduct.
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ACKNOWLEDGEMENTS
We gratefully acknowledge the generous grant received
from University Grants Commission [grant No. F.12-29/2003
(SR)] to carry out this research project.
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