Multiphase oxidation of alcohols and sulfides with hydrogen peroxide catalyzed by heteropolyacids

Article abstract of DOI:10.1016/j.catcom.2010.06.015

This work describes the application of a multiphase system for the oxidation of alcohols to ketones or aldehydes, and the selective conversion of sulfides to sulfoxides or sulfones using Keggin-type heteropolyacids and hydrogen peroxide. Benzylic and secondary alcohols were oxidized to ketones or aldehydes at 70 °C in good yield and selectivity. Similarly, sulfides were converted to sulfoxides or sulfones at room temperature with high yields and selectivity.

Full text of DOI:10.1016/j.catcom.2010.06.015

Catalysis Communications 11 (2010) 1181–1184
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Multiphase oxidation of alcohols and sulfides with hydrogen peroxide catalyzed
by heteropolyacids
c
c
a,b
Pietro Tundo a,b, , Gustavo P. Romanelli , Patricia G. Vázquez , Fabio Aricò
⁎
a
Interuniversity Consortium “Chemistry for the Environment”, Via della Industrie 21/8, 30175 Marghera, Venice, Italy
Cà Foscari University, Dept. of Environmental Science, Dorsoduro 2137, 30123 Venice, Italy
Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge Ronco” (CINDECA), Universidad Nacional de La Plata, CONICET, Calle 47 N 257 (B1900AJK), La Plata, Argentina
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 May 2010
Received in revised form 23 June 2010
Accepted 28 June 2010
Available online 4 July 2010
This work describes the application of a multiphase system for the oxidation of alcohols to ketones or
aldehydes, and the selective conversion of sulfides to sulfoxides or sulfones using Keggin-type
heteropolyacids and hydrogen peroxide. Benzylic and secondary alcohols were oxidized to ketones or
aldehydes at 70 °C in good yield and selectivity. Similarly, sulfides were converted to sulfoxides or sulfones at
room temperature with high yields and selectivity.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Multiphase system
Keggin polyoxometalate
Hydrogen peroxide
Alcohols
Sulfides
1. Introduction
organic substrates are insoluble in aqueous hydrogen peroxide, which
can subsequently lower the selectivity and efficiency of the oxidation
The selective oxidation of organic substrates is an important
transformation in organic synthesis, as the corresponding products
are essential intermediates for many drugs, vitamins and fragrances
[1,2]. In particular, the oxidation of alcohols to aldehydes or ketones is
a fundamental preparative reaction, which has been widely investi-
gated [1,2]. Several common oxidation procedures use heavy metal
complexes that are often toxic, corrosive and expensive. Furthermore,
they are employed in large amounts ranging from stoichiometric to
excess. Additionally these methods include harsh conditions such as
high pressure or temperature and they generally require the presence
of a strong acid [3].
Similarly, the selective catalytic oxidation of sulfides to sulfoxides
has been thoroughly examined for many years due to the importance
of sulfoxides as intermediates in organic synthesis [4]. For the
synthesis of such compounds in industrial processes and laboratory
experimentations, the use of aqueous hydrogen peroxide as an
oxidant is very attractive, since it is environmentally friendly and
easy to handle [5].
reaction. This problem can be avoided with the use of a phase transfer
catalyst (PTC). For example, the use of a co-catalyst [11], such as
sodium tungstate [12] with a PTC has shown to increase the rate of
oxidation in the presence of hydrogen peroxide. Besides, selective
catalytic oxidation of sulfides to sulfoxides with hydrogen peroxide
has been studied with catalysts such as Jacobsen (salen) Mn(III)
complexes [13], vanadium bromoperoxidase and cetyl trimethyl
ammonium tribromide [14].
Herein for the first time, an alternative approach for the oxidation
of such organic substrates involving the use of a triphasic system is
reported. In the triphasic system the homogeneous catalyst is
solubilized in one phase (e.g., Aliquat 336) where the reaction takes
place; the substrate and products are taken up in the organic phase;
the third phase is the aqueous phase. Thus, the reduction product can
be obtained by a simple separation.
Multiphase systems have been used for a variety of important
reduction processes which include: hydrodehalogenation of poly-
chlorinated benzenes [15,16], hydrodechlorination of chlorophenoxy-
pesticides such as dieldrin and DDT [17,18], selective removal of
halogens from functionalized aryl ketones [19,20], and enantioselec-
tive hydrogenation of acetophenone [21].
Heteropolyacids are attractive as catalysts for oxidation processes
due to their low toxicity and high acidity. Heteropolyacids have been
used in a variety of acid-catalyzed reactions such as esterification,
etherification, hydration of olefins and dehydration of alcohols [22].
Additionally, Keggin-type polyoxoanions have been widely employed
A variety of different catalytic systems for hydrogen peroxide
promoted oxidation of alcohols have been developed e.g. tungsten-
based polyoxometalate catalysts [6–8], Lewis acid iron aluminium
chloride [9] and dinuclear Mn (IV) complexes [10]. However, some
⁎
Corresponding author. Department of Environmental Sciences, Ca' Foscari
University of Venice, Italy. Tel.: +39 041 2346601; fax: +39 041 2346650.
E-mail address: tundop@unive.it (P. Tundo).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.06.015

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P. Tundo et al. / Catalysis Communications 11 (2010) 1181–1184
as homogeneous and heterogeneous catalysts for the oxidation of
organic compounds [23–29]. They have been used in the presence of
hydrogen peroxide for the epoxidation of olefins and allylic alcohols,
oxidation of alcohols and diols to ketones, cleavage of 1,2-diols [9] and
selective oxidation of benzyl alcohol [16].
Recently, we have applied the use of Keggin heteropolyacids in a
range of processes i.e. preparation of heterocycles [29,30], protection/
deprotection of organic functional groups [30,31], and oxidation
processes, as well as, conversion of 2,6-dimethylphenol to 2,6-
dimethyl-1,4-benzoquinone [32], and selective oxidation of sulfides
to sulfoxides with hydrogen peroxide [33].
Scheme 1. Oxidation of 1-phenylethanol in a multiphase system.
3. Results and discussion
This work describes the first application of a multiphase system
for the oxidation of alcohols to ketones or aldehydes, and the
selective conversion of sulfides to sulfoxides or sulfones with
hydrogen peroxide in the presence of Keggin-type heteropolyacids.
The oxidation reactions were carried out using a system comprising
of a hydrocarbon solvent (isooctane), an aqueous phase and a
phase-transfer (PT) agent (Aliquat 336 (C8H17)3(CH3)NCl) in the
presence of hydrogen peroxide and a Keggin heteropolyacid (HPA)
catalyst. Employing this system, benzylic and secondary alcohols
were oxidized to ketones or aldehydes at 70 °C with high yield and
excellent selectivity.
Similarly, sulfides were converted to sulfoxides or sulfones
(depending on the concentration of hydrogen peroxide) at room
temperature with high yields and selectivity. Some preliminary
investigations were conducted on the oxidation of 1-phenylethanol
in the multiphase system using various catalysts (Scheme 1). The
results obtained are summarized in Table 1. When Aliquat was
used without catalyst (entry 2), the conversion of 1-phenylethanol
into acetophenone was lower in homogeneous conditions (1%)
compared with the multiphase conditions (22%). However, in the
absence of a catalyst and Aliquat (entry 1), the yields were low in
both cases (3 and 12%, respectively). These lower yields are due to
the absence of an interphase between the substrate and the
catalyst.
2. Experimental
The commercially available heteropolyacids H3PMo12O40 and
H3SiMo12O40 were purchased from Aldrich and H4PW12O40 and
H4SiW12O40 from Fluka. The heteropolyacids containing vanadium,
aluminium and Al–V in the Keggin primary structure (H6PMo11AlO40
,
H4PMo11VO40 and H5PMo11Al0.5V0.5O40) were prepared and fully
characterized according to the procedure already published in the
literature [22]. Thus, starting from these heteropolyacids, different
salts with pyridinium as cation were obtained (Py3PMo12O40
,
H3PyPMo11VO40, HPy3PMo11VO40 and Py4PMo11VO40). For compar-
ative purposes, the Py5PMo10V2O40 catalyst was tested in homoge-
neous and multiphase conditions. GC analyses on the reaction
mixture were performed using a Varian GC 3400 using a fused silica
capillary column “Chromapack CP Sil 8 CB” (film thickness 0.25 μm).
GC–MS analyses were performed using a HP 5971 mass detector
coupled to a HP gas chromatograph fitted with a 30 m×0.25 nm DB5
capillary column.
2.1. General procedure for homogeneous oxidation
In all the reactions, a 25 mL three-necked round-bottomed flask
was used, heated at the reaction temperature and connected with a
water-jacketed condenser. The reactor was loaded with a mixture of
0.7 mmol of substrate, 5 mL of CH3CN, 1 mL of hydrogen peroxide 35%
(w/v), n-decane as internal standard (0.056 g, 0.39 mmol) and 3% of
the heteropolyacid (mmol) with respect to the amount of substrate.
The mixture was stirred at 700 rpm.
Besides, it is interesting to note that commercially available
heteropolyacids containing phosphorous as the heteroatom i.e.
H3PMo12O40 (entry 3) and H3PW12O40 (entry 4), were more active
than those containing silicon H4SiMo12O40 (entry 5) and
H4SiW12O40 (entry 6) within the Keggin primary structure. In
2.2. General procedure for multiphase oxidation
Table 1
Comparison of oxidation of 1-phenylethanol to acetophenone in homogeneous and
multiphase system.
In all the reactions, a 25 mL three-necked round-bottomed flask
was employed, heated at the reaction temperature and connected
with a water-jacketed condenser. The reactor was loaded with a
mixture of aqueous phase (1 mL of aq. hydrogen peroxide 35% (w/v)
and 4 mL of water), Aliquat 336 (tricaprylmethylammonium chloride,
0.232 g, 0.59 mmol), 0.7 mmol of substrate, n-decane as the internal
standard (0.056 g, 0.39 mmol) and 3% of heteropolyacid with respect
to the employed substrate. The organic phase was adjusted to 10 mL
by adding isooctane. The reaction mixture was magnetically stirred at
700 rpm.
Entry
Catalyst
Time (h)
Hom. cond.a (%)
Mult. cond.b (%)
1
2
3
4
5
6
7
8
Nonec
20
20
7
7
7
7
7
7
7
5
7
7
7
3
1
5
4
3
2
4
–
12
22
93
76
78
76
99
70
81
100
73
78
79
68
Noned
H3PMo12O40
H3PW12O40
H4SiMo12O40
H4SiW12O40
H6PMo11AlO40
H4PMo11VO40
H5PMo11Al0.5V0.5O40
Py3PMo12O40
H3PyPMo11VO40
HPy3PMo11VO40
Py4PMo11VO40
Py5PMo10V2O40
9
57(95)e
7
–
10
11
12
13
14
2.3. Sampling procedure and GC analyses
–
–
7
–
The samples were periodically collected from the organic phase.
Approximately 20 μL of the reaction mixture was removed and then
diluted with 1 mL isooctane or acetonitrile to a volume of 1–2 mL. The
sample was then mixed with silica and filtered with cotton wool to
eliminate any impurities of Aliquat 336. Determination of the
concentration for the reaction components was performed using the
internal standard technique.
a
Homogeneous conditions: catalyst (3% in mmol), solvent acetonitrile, H2O2 35% p/v
(1 mL), 1-phenylethanol (0.7 mmol).
b
Multiphase conditions: catalyst (3% in mmol), solvent isooctane–water, H2O2 35%
p/v (1 mL), 1-phenylethanol (0.7 mmol), Aliquat 336 (0.59 mmol).
c
Without Aliquat.
With Aliquat.
Further addition of two aliquots of 35% p/v H2O2 at 1 and 2 h.
d
e

P. Tundo et al. / Catalysis Communications 11 (2010) 1181–1184
1183
Table 2
Oxidation of alcohols with hydrogen peroxide catalyzed by different heteropolyacids at 70 °C in multiphase conditions.
#
Substrate
Product
Catalyst
Time (h)
PhCOCH3 (%)
Other products (%)
1
2
3
4
5
6
7
8
1-Phenylethanol
1-Phenylethanol
1-Phenylethanol
Benzhydrol
Acetophenone
Acetophenone
Acetophenone
Benzophenone
Benzophenone
Benzaldehyde
Benzaldehyde
Benzaldehyde
4-Chloro benzaldehyde
4-Chloro benzaldehyde
4-Methyl benzaldehyde
4-Methyl benzaldehyde
4-Methoxy benzaldehyde
4-Methoxy benzaldehyde
2-Octanone
2-Octanone
2-Octanone
2-Decanone
2-Decanone
H3PMo12O40
Py3PMo12O40
H6PMo11AlO40
H3PMo12O40
Py3PMo12O40
H3PMo12O40
Py3PMo12O40
H6PMo11AlO40
H3PMo12O40
H6PMo11AlO40
H3PMo12O40
Py3PMo12O40
H3PMo12O40
Py3PMo12O40
H3PMo12O40
Py3PMo12O40
H6PMo11AlO40
H3PMo12O40
H6PMo11AlO40
H3PMo12O40
Py3PMo12O40
H6PMo11AlO40
9
5
7
5
4
5
3
4
5
3
4
3
3
2
7
6
6
7
6
7
7
7
95 (5)a
100 (6)a
99
None
None
None
None
99 (8)a
99
Benzhydrol
None
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
4-Chloro-benzyl alcohol
4-Chloro-benzyl alcohol
4-Methyl-benzyl alcohol
4-Methyl-benzyl alcohol
4-Methoxy-benzyl alcohol
4-Methoxy-benzyl alcohol
2-Octanol
2-Octanol
2-Octanol
2-Decanol
2-Decanol
1-Decanol
1-Decanol
1-Decanol
84 (12)a
89 (5)a
85
(Benzoic acid) (4)
(Benzoic acid) (2)
(Benzoic acid) (2)
(4-Chlorobenzoic acid) (1)
(4-Chlorobenzoic acid) (2)
(4-Methylbenzoic acid) (3)
(4-Methylbenzoic acid) (3)
(4-Methoxybenzoic acid) (6)
(4-Methoxybenzoic acid) (5)
None
None
None
None
None
None
(Decanoic acid) (1)
None
9
93 (10)a
95
10
11
12
13
14
15
16
17
18
19
20
21
22
96 (7)a
96
90
91
68
71 (3)a
68
64 (3)a
69
2-Decanal
2-Decanal
2-Decanal
20
22 (1)a
19
a
In homogeneous system corresponding to the numbers in the parentheses.
addition, among the HPA examined, those containing molybdenum
were more reactive than those containing tungsten (entries 3 and
4). This is attributed to the differing redox properties of tungsten
and molybdenum heteropolycompounds and is consistent with
the fact that molybdenum is a better oxidant [31–35].
The substitution of a molybdenum atom by an aluminium atom
within the Keggin primary structure (entry 7) leads to achieve a
quantitative yield of acetophenone in multiphase conditions. Similar
results were obtained with H3PMo12O40 (93%, entry 3). On the other
hand, substituting one vanadium atom in place of the aluminium
atom led to a lower yield (70%) (entry 8).
Further experiments conducted using a Keggin structure where
a molybdenum atom was substituted by Al–V atoms (H5PMo11-
Al0.5V0.5O40) (entry 9), led to 57 and 81% yield of acetophenone
for homogeneous and multiphase conditions, respectively. In the
homogeneous conditions, it was possible to further increase the
yield of acetophenone to 95% (entry 9, note e) by a supplementary
addition of hydrogen peroxide that, in these reaction conditions,
tends to decompose very quickly. Overall, the results showed that
H6PMo11AlO40 was the most active catalyst among those incorpo-
rating 11 Mo atoms in the Keggin structure. Besides, when Al and
V atoms are both present in the Keggin primary structure the yield
of acetophenone increased in comparison to the result obtained
with a Keggin structure that incorporates only a V atom. In this
case, the V atom stabilizes the Keggin structure [29–32] and this
avoids the formation of catalytic compounds in the interphase. In
addition, when Py3PMo12O40 was used as catalyst it was possible
to obtain acetophenone in quantitative yield after 5 h of reaction
(Table 1, entry 10). For comparative purposes, the pyridinium salts
H3PyPMo11VO40
, HPy3PMo11VO40 and Py4PMo11VO40, obtained
from H4PMo11VO40, were also employed as catalyst (Table 1,
entries 11–14). These salts gave higher yields compared to those
obtained when no pyridinium cations were present (entry 9). It is
evident that an increasing number of pyridinium cations leads to
higher yield (entries 12–14). The presence of two V atoms in the
primary structure decreased the oxidative properties of the Keggin
heteropolyacid. In fact when Py5PMo10V2O40 was used (entry 14)
the yield decreased (68%).
The oxidation of additional alcohols was then investigated
using the HPA catalysts that were found to be the most active in
the preliminary oxidation of 1-phenylethanol (Py3PMo12O40
,
H6PAlMo11O40 and H3PMo12O40 (Table 1)); results shown in
,
Table 2. In most cases, no appreciable amount of by-product was
detected. However, in the case of benzyl alcohols and 4-chloro
benzyl alcohol, additional hydrogen peroxide was twice added as
it decomposes very quickly under homogeneous conditions. This
Table 3
Oxidation of sulfides with hydrogen peroxide (1.1 mmol unless otherwise stated)
catalyzed by different heteropolyacids in multiphase condition, at room temperature.
Entry
Substrate
Catalyst
Time (min)
RR1SO (%)
RR1SO2 (%)
1
2
3
4
5
6
7
8
9
10
PhSMe
H3PMo12O40
H3PMo12O40
H3PMo12O40
H3PMo12O40
Py3PMo12O40
H3PMo12O40
Py3PMo12O40
H3PMo12O40
Py3PMo12O40
H3PMo12O40
60
45
60
95
1
2
99
1
11
2
PhSMea
PhSMe
Scheme 2. Oxidation of sulfides to sulfoxides in a multiphase system.
b
b
64
88
97
98
100
99
100
99
PhSMea
PhSMe
PhSPh
60
120
120
120
60
60
60
1
–
PhSPh
(C4H9)2S
(C4H9)2S
(PhCH2)2S
1
–
1
a
Molar ratio substrate/H2O2 1/20 mmol.
b
In homogeneous system the use of an excess of hydrogen peroxide (1:20
equivalents) afforded the corresponding sulfone.
Scheme 3. Oxidation of sulfides to sulfones in a multiphase system.

1184
P. Tundo et al. / Catalysis Communications 11 (2010) 1181–1184
Table 4
(Aliquat 336) while the majority of H2O2 remains in the aqueous
phase. Besides, oxidation in homogeneous systems leads to the
formation of benzoic acid due to the excess of H2O2 in the reaction
mixture.
Oxidation of sulfides with hydrogen peroxide (20 mmol) catalyzed by different
heteropolyacids in multiphase condition, at 70 °C.
Entry
Substrate
Catalyst
Time (min)
RR1SO (%)
RR1SO2 (%)
1
2
3
4
5
6
7
8
PhSMe
PhSMe
PhSMe
PhSPh
H3PMo12O40
H6PMo11AlO4
Py3PMo12O40
H3PMo12O40
Py3PMo12O40
H3PMo12O40
Py3PMo12O40
H3PMo12O40
15
10
10
20
20
15
15
15
1
–
–
–
–
2
–
1
99
100
100
98
99
97
Acknowledgements
The authors thank the Consorzio Interuniversitario Nazionale La
Chimica per lAmbiente (INCA), Consejo Nacional de Investiga-ciones
Científicas y Técnicas (CONICET) and University of La Plata.
PhSPh
(C4H9)2S
(C4H9)2S
(PhCH2)2S
100
97
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In conclusion, the multiphase system studied was found to be an
excellent medium for the selective oxidation, in high yields, of sulfides
to either sulfoxides or sulfones, and benzylic and secondary alcohols
to their corresponding carbonyl derivatives. In comparison with
oxidation in homogeneous conditions, the multiphase system has a
reduced reaction time and requires less hydrogen peroxide. The
oxidation of alcohols to aldehydes in a multiphase system is an
extremely appealing process since it is very selective (with only traces
of product in a higher oxidation state). This is due to the fact that the
heteropolyacid and H2O2 are in contact only in the third phase