Homogeneous catalytic oxidation of styrene and styrene derivatives with hydrogen peroxide in the presence of transition metal-substituted polyoxotungstates

Article abstract of DOI:10.1039/c4cy00702f

The tetrabutylammonium (TBA) salts of the Keggin-type polyoxotungstates with general formula [XW₁₁M(H₂O)O₃₉]^(n-m)-, where X = P, B or Si and M = Mn, Fe or Co, were evaluated as catalysts in the oxidation of styrene, α-methylstyrene, p-methylstyrene, α,p-dimethylstyrene, p-chlorostyrene, p-nitrostyrene, and p-methoxystyrene under mild conditions, using aqueous H₂O₂ as an eco-sustainable oxidant. In this study, the influence of the catalysts and of the different styrene substituents on the oxidation reaction profile was evaluated in terms of conversion and selectivity. For all the performed catalytic studies, the main product results from the oxidative cleavage of the vinyl double bond, except in the case of the oxidation of p-methoxystyrene catalysed by BW₁₁Mn, for which p-methoxyphenol is the main product. The catalysts BW₁₁Mn and SiW₁₁Co give rise to 100% conversion for almost all of the substrates, excluding p-methoxystyrene and p-nitrostyrene for both catalysts and α,p-dimethylstyrene only in the case of BW₁₁Mn. The selectivity for C=C cleavage products resulting from the oxidative cleavage of the vinyl double bond can be as high as 98%, reaching 98% conversion for p-nitrostyrene when SiW₁₁Co was used as a catalyst. Possible pathways are discussed and the oxidation of a few presumed intermediates was carried out. The systematic study of several substituted styrene derivatives suggests a possible reactivity order for these compounds in the catalytic system considered.

Full text of DOI:10.1039/c4cy00702f

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Homogeneous catalytic oxidation of styrene and
styrene derivatives with hydrogen peroxide in the
presence of transition metal-substituted
polyoxotungstates
Cite this: DOI: 10.1039/c4cy00702f
a
b
a
a
*
*
Tiago A. G. Duarte, Ana C. Estrada, Mário M. Q. Simões, Isabel C. M. S. Santos,
Ana M. V. Cavaleiro,b M. Graça P. M. S. Nevesa and José A. S. Cavaleiroa
The tetrabutylammonium (TBA) salts of the Keggin-type polyoxotungstates with general formula
[XW11M(H2O)O39
(n−m)−, where X = P, B or Si and M = Mn, Fe or Co, were evaluated as catalysts in the
]
oxidation of styrene, α-methylstyrene, p-methylstyrene, α,p-dimethylstyrene, p-chlorostyrene,
p-nitrostyrene, and p-methoxystyrene under mild conditions, using aqueous H2O2 as an eco-sustainable
oxidant. In this study, the influence of the catalysts and of the different styrene substituents on the oxida-
tion reaction profile was evaluated in terms of conversion and selectivity. For all the performed catalytic
studies, the main product results from the oxidative cleavage of the vinyl double bond, except in the case
of the oxidation of p-methoxystyrene catalysed by BW11Mn, for which p-methoxyphenol is the main
product. The catalysts BW11Mn and SiW11Co give rise to 100% conversion for almost all of the substrates,
excluding p-methoxystyrene and p-nitrostyrene for both catalysts and α,p-dimethylstyrene only in the case
of BW11Mn. The selectivity for CC cleavage products resulting from the oxidative cleavage of the vinyl
double bond can be as high as 98%, reaching 98% conversion for p-nitrostyrene when SiW11Co was used
as a catalyst. Possible pathways are discussed and the oxidation of a few presumed intermediates was
carried out. The systematic study of several substituted styrene derivatives suggests a possible reactivity
order for these compounds in the catalytic system considered.
Received 30th May 2014,
Accepted 22nd August 2014
DOI: 10.1039/c4cy00702f
www.rsc.org/catalysis
happens in ozonolysis.7,8 The particular case of styrene
oxidation is of considerable interest for academia research
1. Introduction
The oxidation of alkenes is an important transformation in
the production of fine chemicals. One of the common routes
involves a strong oxidant in excess (e.g. peroxyacids, manga-
nese dioxide, chromic acid, potassium dichromate or sele-
nium dioxide), producing large amounts of undesired and/or
toxic wastes, most significantly when applied to industrial
processes.1,2 The strategies to overcome these drawbacks have
led to the use of environmentally benign oxidants in line with
the development of appropriate catalysts.2–6
It is known that the metal-catalysed oxidation of alkenes
can give rise to a whole variety of organic products, namely,
epoxides and diols. In addition, depending on the catalyst,
alkenes can be converted into aldehydes and/or ketones by
oxidative cleavage of the CC double bond, similarly to what
and also for industry due to the important products that
can be obtained such as styrene oxide, benzaldehyde or
phenylacetaldehyde.9–12
Several homogeneous or heterogeneous systems have been
reported for the oxidation of styrene and its derivatives in the
presence of different types of polyoxometalates (POMs) using
mainly hydrogen peroxide (H2O2),13–33 tert-butylhydroperoxide
(TBHP),34–37 and molecular oxygen (O2),26,38–40 among other
oxidants.34,35,41 The major products usually obtained from
styrene in the presence of the reported POMs are styrene
oxide and phenylacetaldehyde (apparently resulting from the
isomerization of the epoxidation product), along with benzal-
dehyde and benzoic acid.19,20,22,25,27,30–33,37
Among the several types of POMs, the transition metal mono-
substituted Keggin-type polyoxotungstates [XW11M(H2O)O39](n−m)−
(X = P, Si, etc.), in which a transition metal cation, Mm+
,
a QOPNA, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro,
Portugal. E-mail: ivieira@ua.pt, msimoes@ua.pt; Fax: +351 234 370 084;
Tel: +351 234 370 200; Tel: +351 234 370 713
is coordinated to the binding sites of a lacunary poly-
oxotungstate anion [XW11O39
n−, are an extraordinarily versa-
]
b CICECO, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro,
Portugal
tile class of complexes with high catalytic activity in a variety
of organic reactions. Moreover, these POMs are able to
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activate hydrogen peroxide, turning it reactive enough to pro-
mote important reactions such as hydroxylation, epoxidation,
oxidative dehydrogenation and oxidative cleavage processes,
as widely demonstrated in our earlier work.27,32,42–52 The use
of aqueous H2O2 in the oxidation of organic substrates is very
attractive from the point of view of synthetic organic chemis-
try, since aqueous H2O2 is an environmentally clean and easy
to handle reagent.3,53–55 To our knowledge, studies on the
oxidation of styrene and its derivatives using aqueous H2O2
in the presence of transition metal-substituted Keggin-,
Dawson-, or sandwich-type polyoxotungstates are still rather
limited in number.13,23,25,27,31,33 Following our interest in
the use of polyoxotungstates as catalysts for the oxidative
transformations of organic compounds,27,32,42–52 we report
here the oxidation of styrene and of some derivatives
(Scheme 1) with H2O2, in acetonitrile, in the presence of
corresponding to a substrate/catalyst (S/C) molar ratio of 333,
30% (w/w) aqueous H2O2 (408 μl, 4 mmol) and 3.0 mL of
acetonitrile were stirred in a closed vessel at 80 °C for 6 h.
The conversion values presented (SD ≤10) are the result of at
least two concordant assays. Blank reactions were performed
for all the substrates, confirming that no oxidation or almost
no oxidation occurs unless both the catalyst and H2O2 are
present.
2.3. Instruments and methods
The GC-FID analyses were performed on a Varian Star 3900
chromatograph equipped with a fused silica capillary DB-5
type column (30 m × 0.25 mm i.d.; 0.25 μm film thickness)
using helium as the carrier gas (30 cm s−1). The chromato-
graphic conditions for all the substrates and their derivatives
were as follows: initial temperature 90 °C; first temperature
rate 5 °C min−1 until 140 °C; second temperature rate 50 °C min−1
until 250 °C; injector temperature 250 °C; detector tempera-
ture 270 °C. Aliquots were taken from the reaction mixtures,
at regular intervals, using a 10 μl syringe. All the reaction prod-
ucts were identified by GC/MS analysis and by co-injection of
commercially available standards. GC/MS was performed
using a Finnigan Trace GC/MS (ThermoQuest CE Instru-
ments) using helium as the carrier gas (35 cm s−1) and a
fused silica capillary DB-5 type column (30 m × 0.25 mm i.d.;
0.25 μm film thickness). The percentages of each compound
in the reaction mixtures were estimated from the correspond-
ing chromatographic peak areas, using cyclohexylmethanol
as the internal standard (added after the reaction).
III
(n−m)−
tetrabutylammonium (TBA) salts of [XW
M
(H2O)O39
]
,
11
X = P, Si, B and M = MnIII, FeIII or CoII (abbreviated
as XW11M). The oxidation of some of the reaction products
and/or possible intermediates was also studied in order to
define the possible reaction routes.
2. Experimental section
2.1. Reagents and synthetic procedures
Acetonitrile, 30% (w/w) aqueous hydrogen peroxide,
styrene, α-methylstyrene, p-methylstyrene, α,p-dimethylstyrene,
p-chlorostyrene, p-nitrostyrene, p-methoxystyrene, styrene oxide
and all other reagents and solvents, obtained from commercial
sources, were used as received or distilled and dried using
standard procedures. 1-Phenylethane-1,2-diol was obtained
by acidification (6 M HCl) of styrene oxide followed by
liquid–liquid extraction with dichloromethane.
3. Results and discussion
The oxidation of styrene (1), α-methylstyrene (2), p-methylstyrene
(3), α,p-dimethylstyrene (4), p-chlorostyrene (5), p-nitrostyrene
(6), or p-methoxystyrene (7) by hydrogen peroxide was carried
out in acetonitrile at 80 °C, in the presence of the TBA salts
The
tetrabutylammonium
(TBA)
salts
TBA4Hn−m−4[XW11M(H2O)O39]·xH2O, with X = B, P or Si,
M = MnIII, FeIII or CoII, n−m = charge of the anion, and x = 0–4,
were prepared and characterized according to previously
described procedures.42,56–59 The obtained compounds were
characterized by elemental analysis (C, H, N, W, Fe, Mn, P,
Si), thermogravimetry and infrared spectroscopy, and the
results were in agreement with previously published
values.42,58–60
of [XW11M(H2O)O39]
(n−m)−, X = P, Si or B; M = FeIII, MnIII or
CoII. Except when reaction time is referred, the results
discussed here were obtained after 6 h of reaction. Substrate
oxidation is negligible or not observed in the absence of a
catalyst, indicating that hydrogen peroxide alone is unable to
oxidize these substrates.
2.2. Typical procedure for the oxidation reactions
3.1. Oxidation of styrene (1)
Based on the experimental conditions developed in previous
studies,27,32 the reactions were typically carried out as fol-
lows: the substrate (1.0 mmol), the catalyst (3 μmol),
The results presented in Table 1 show that styrene (1) in the
presence of the selected POMs is, in general, efficiently
oxidized with hydrogen peroxide. The catalytic activity based
on the percentage of conversion of styrene follows the order:
BW11Mn (100% after 5 h) ~ SiW11Co (100%) > BW11Fe (80%) >
BW11Co (78%) > PW11Co (60%) > SiW11Fe (37%) > PW11Fe
(32%) > SiW11Mn (11%) ~ PW11Mn (10%).
The analysis of the products obtained shows that the
oxidation of 1 occurs only at the vinyl double bond and no
aromatic functionalization is detected. With all the catalysts,
the main product is benzaldehyde (9), reaching selectivities
Scheme 1 Substrates studied in the oxidation with H2O2 in the
presence of transition metal-substituted polyoxotungstates.
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Table 1 Oxidation of styrene (1) with H2O2 catalysed by Mn-, Co-, and Fe-substituted polyoxotungstates after 6 h of reactiona
Selectivityc (%)
Conv.b,c
(%)
CC cleavage
(8 + 9 + 12)
(12)
(13)
Catalyst
(9)
(10)
(11)
(14)
(8)
BW11Mn 100(5)
PW11Mn 10
SiW11Mn 11
—
—
—
—
—
—
3
—
—
—
58
100
82
71
74
64
65
78
87
2
5
22
—
—
12
7
15
8
5
7
6
80
100
82
83
81
79
76
83
87
—
18
6
6
6
2
3
4
—
—
—
6
6
5
3
4
9
—
—
—
5
7
6
—
—
—
—
—
—
—
—
4
BW11Co
PW11Co
SiW11Co
BW11Fe
PW11Fe
SiW11Fe
No
78
60
100
80
32
37
6
9
10
—
—
—
—
100
100
catalyst
a
Reaction conditions: substrate (1.0 mmol), catalyst (3 μmol) and 30% (w/w) aqueous H2O2 (4.0 mmol) were stirred in 3.0 mL of CH3CN at
80 °C. b Numbers in brackets are the hours when the reaction reaches 100%, if different from 6 h. Determined by GC-FID.
c
between 58% and 100%. The highest yield for this compound
was 64%, obtained with SiW11Co, and all BW11M anions orig-
inated yields above 50%. Considering the combined amounts
of benzaldehyde (9) and benzoic acid (12) resulting from
its oxidation (combined selectivity between 73% and
100%), it is apparent that the principal occurrence is the
cleavage of the vinyl carbon–carbon double bond. From the
point of view of the mechanism for this oxidation and taking
into account also the other products obtained, namely,
phenylacetaldehyde (10) (0–18%), acetophenone (11) (0–9%),
2-hydroxy-1-phenylethanone (13) (0–7%) and 1-phenylethane-
1,2-diol (14) (0–10%), the corresponding styrene epoxide (15)
was considered a putative intermediate, as represented in
Scheme 2. In order to clarify this assumption, the oxidation
of styrene oxide (15) was carried out in the presence of the
most efficient catalysts BW11Mn and SiW11Co under the
same conditions used in the oxidation of styrene (1). The
results obtained (Table 2) show that the major product of this
reaction is the corresponding diol (14) for BW11Mn (47%
selectivity and 54% conversion after 6 h) and phenylacet-
aldehyde (10) for SiW11Co (59% selectivity and 29% conver-
sion after 6 h). Benzaldehyde (9) was obtained with 20% and
8% selectivity in the presence of BW11Mn and SiW11Co,
respectively. Considering the amount of diol (14) obtained in
these reactions, namely, 47% for BW11Mn and 33% for
SiW11Co, its oxidation was carried out under the same condi-
tions in the presence of SiW11Co. After 6 h of reaction (27%
conversion, Table 2), the major products detected were
hydroxyketone (13) with 51% selectivity and benzaldehyde (9)
with 44% selectivity.
the formation of benzaldehyde (Scheme 2). The oxidative cleav-
age of 1,2-diols is well known from the literature, namely, in
the presence of hydrogen peroxide and polyoxometalates.61–63
Benzaldehyde can be formed by C–C cleavage of the diol
(after hydrolysis of the epoxide) and by epoxide ring opening
by direct action of H2O2, which results in the formation of
methanal and benzaldehyde (9). Methanal (formaldehyde)
was detected both in the reactor gas phase and in the
solution by using GC-FID. In fact, it was possible to detect
methanal in solution after cooling down the reaction mixture
at −20 °C. Moreover, since the reactions were performed in
closed vessels, it was also possible to analyse directly the
headspace of the reactor and detect a small amount of
methanal by GC. The first step leading to the formation of
the epoxide is usually described in the literature23,64,65 as the
formation of an intermediate peroxide or hydroperoxide
complex with the catalyst, in which H2O2 is coordinated to a
metal centre of the POM. The oxygen transfer from the
bonded peroxide to styrene may occur by different mecha-
nisms,65,66 namely, through the formation of a 5-membered
cyclic intermediate involving a metal centre (M = W, Fe, Mn,
Co). We propose that this intermediate could also react with
a H2O2 molecule, resulting in the direct formation of benzal-
dehyde (Scheme 2). This third pathway for the CC cleavage
is suggested since, based on the results in Table 2, the epoxide
and the diol may not be responsible for all the benzaldehyde
formed.
Phenol (8) was also detected in a very small amount and
only in the presence of BW11Fe. The formation of phenols is
well established in the Baeyer–Villiger (B.V.) oxidation of
aromatic aldehydes.67–71 Thus, benzaldehyde can be oxidized
to the corresponding formate ester, which may be hydrolyzed
to the corresponding phenol (Scheme 2). Considering also
From this study, it can be concluded that styrene oxide
(15) (although not detected in the oxidation of styrene
reported here) and diol (14) are potential intermediates in
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Scheme 2 General reaction pathways for the oxidation of styrene (1) with H2O2 catalysed by polyoxometalates (POMs).
the formation of phenol (8) as resulting from benzaldehyde
(9) Baeyer–Villiger oxidation, the selectivity for the CC
cleavage rises to 76% in the case of BW11Fe.
3.2. Oxidation of α-methylstyrene (2), p-methylstyrene (3),
α,p-dimethylstyrene (4), p-chlorostyrene (5), p-nitrostyrene (6),
and p-methoxystyrene (7)
The results obtained in the oxidation of styrene with H2O2
in the presence of tert-butanol, a hydroxyl (HO˙) radical scav-
enger, indicate that the reactions do not involve radicals. In
fact, the reaction profiles with tert-butanol are exactly the
same as without tert-butanol, after 6 h, maintaining all other
reaction conditions.
The reports found in the literature on the oxidation of sty-
rene with H2O2 in the presence of some of the POMs used in
this work23,27,33 or related metal-substituted anions24,25,31,32
indicate different results, depending on the conditions used.
Cobalt was used in some studies23,25,31,33 and the results
obtained here with SiW11Co at 6 h compare well with others
previously described.33 Other catalysts seem to yield interest-
ing results, namely, the borotungstates, which, to our best
knowledge, have not been considered in any previous study.
The results obtained in the oxidation of the substituted
styrenes 2 to 7 with H2O2, in the presence of the mono-
substituted polyoxotungstates, under the same conditions
described for styrene, are shown in Tables 3–8, respectively.
Generally, the oxidation of the styrene derivatives seems to
follow a course similar to that of styrene, as depicted in
Scheme 2. Considering the data in Tables 3–8, the analysis of
the products obtained from each substrate shows some regu-
larity, as described next.
The highest conversions are obtained in the presence
of BW11Mn and SiW11Co, frequently reaching 100%; other
POMs also show excellent efficiency (namely, BW11Fe, BW11Co,
and PW11Co), depending on the substrate. The major product
is either a substituted benzaldehyde or acetophenone (and
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Table 2 Oxidation of styrene oxide (15) and of 1-phenylethane-1,2-diol (14) with H2O2 catalysed by substituted polyoxotungstates after 6 h of
reactiona
Catalyst
Substrate
Conv.b (%)
Selectivityb (%)
(15)
(9)
(10)
(12)
(13)
(14)
BW11Mn
SiW11Co
54
29
20
8
16
59
3
—
14
—
47
33
(14)
(9)
(10)
(11)
(13)
SiW11Co
27
44
1
4
51
a
Reaction conditions: substrate (1.0 mmol), catalyst (3 μmol) and 30% (w/w) aqueous H2O2 (4.0 mmol) were stirred in 3.0 mL of CH3CN at
80 °C. b Determined by GC-FID.
Table 3 Oxidation of α-methylstyrene (2) with H2O2 catalysed by Mn-, Co- and Fe-substituted polyoxotungstates after 6 h of reactiona
Selectivityc (%)
CC cleavage
(8 + 11)
Catalyst
Conv.b,c (%)
(8)
(11)
(16)
(17)
BW11Mn
BW11Mn
BW11Mn
PW11Mn
SiW11Mn
BW11Co
PW11Co
SiW11Co
BW11Fe
100(5)
76d
78e
23
22
87
80
100(3)
65
2
1
5
—
—
4
—
1
64
63
57
84
100
81
87
84
81
6
5
27
30
26
—
—
5
5
6
8
66
64
62
84
100
85
87
85
85
10
16
—
10
8
9
7
4
PW11Fe
SiW11Fe
No catalyst
30
69
8
—
—
—
88
88
100
2
12
—
10
—
—
88
88
100
a
Reaction conditions: substrate (1.0 mmol), catalyst (3 μmol) and 30% (w/w) aqueous H2O2 (4.0 mmol) were stirred in 3.0 mL of CH3CN at
b
c
d
80 °C. Numbers in brackets are the hours when the reaction reaches 100%, if different from 6 h. Determined by GC-FID. Reaction runs
with 2.0 mmol of H2O2. e Reaction runs at 60 °C.
substituted acetophenone), resulting from the cleavage of the
vinyl double bond. The only exception was observed in the
oxidation of p-methoxystyrene (7) in the presence of BW11Mn
that yielded p-methoxyphenol (37) as the major product
(Table 8). Phenols were obtained mainly in the oxidation of
2, 4 and 7. With the best catalysts (BW11Mn, SiW11Co,
BW11Fe, BW11Co, and PW11Co), the selectivity for substituted
acetophenone) ranges (with a few exceptions) between 60%
and 89%. PW11Mn and SiW11Mn were generally poor cata-
lysts, yielding the lowest conversions, with one exception
(PW11Mn, with 60% conversion in the oxidation of 4). Yet,
the reactions could be highly selective, like in the oxidation
of α-methylstyrene (2) (SiW11Mn, 22% conversion, 100%
acetophenone, Table 3) or of p-methylstyrene (3) (PW11Mn,
11% conversion, 100% p-methylbenzaldehyde, Table 4).
benzaldehyde
or
acetophenone
(and
substituted
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Table 4 Oxidation of p-methylstyrene (3) with H2O2 catalysed by Mn-, Co- and Fe-substituted polyoxotungstates after 6 h of reactiona
Selectivityc (%)
CC cleavage
(18 + 21)
Catalyst
Conv.b,c (%)
(18)
(19)
(20)
(21)
(22)
(23)
BW11Mn
PW11Mn
SiW11Mn
BW11Co
PW11Co
SiW11Co
BW11Fe
PW11Fe
SiW11Fe
No catalyst
100(3)
11
4
76
74
100(2)
100
51
59
5
59
100
100
72
66
65
59
65
73
100
7
5
19
—
—
12
10
18
19
6
10
—
—
6
8
9
8
9
5
—
—
—
—
—
3
—
—
—
—
—
78
100
100
84
76
83
78
71
79
100
—
—
8
13
4
10
15
14
—
—
—
2
—
4
4
5
2
—
6
—
a
Reaction conditions: substrate (1.0 mmol), catalyst (3 μmol) and 30% (w/w) aqueous H2O2 (4.0 mmol) were stirred in 3.0 mL of CH3CN at
80 °C. b Numbers in brackets are the hours when the reaction reaches 100%, if different from 6 h. Determined by GC-FID.
c
Table 5 Oxidation of α,p-dimethylstyrene (4) with H2O2 catalysed by Mn-, Co- and Fe-substituted polyoxotungstates after 6 h of reactiona
Selectivityc (%)
CC cleavage
(20 + 24)
Catalyst
Conv.b,c (%)
(24)
(20)
(25)
(26)
BW11Mn
PW11Mn
SiW11Mn
BW11Co
PW11Co
SiW11Co
BW11Fe
PW11Fe
SiW11Fe
No catalyst
96
60
25
76
88
100(2)
65
28
80
17
3
—
11
—
7
5
—
7
—
51
86
90
63
89
60
86
80
76
18
11
9
17
8
16
9
20
12
—
5
—
1
4
68
89
90
74
89
67
91
80
83
3
10
—
—
2
4
100
—
100
a
Reaction conditions: substrate (1.0 mmol), catalyst (3 μmol) and 30% (w/w) aqueous H2O2 (4.0 mmol) were stirred in 3.0 mL of CH3CN at
80 °C. b Numbers in brackets are the hours when the reaction reaches 100%, if different from 6 h. Determined by GC-FID.
c
In particular, the oxidation of α-methylstyrene (2) affords
acetophenone (11) as the major product with 8 (0–4%), 16 (0–16%)
and 17 (0–27%) as minor products (Table 3). SiW11Co and
BW11Mn catalyse this oxidation with 100% conversion after 3
and 5 h, respectively. With almost all catalysts, the reaction
is very much selective for acetophenone (11) (81–100% selec-
tivity), since few secondary products are formed. This may be
due to the existence of the α-methyl group, which may ham-
per the formation of the hydroxyketone or the isomerisation
of the epoxide at the benzylic position. Moreover, unlike
benzaldehydes, which may be oxidized to the equivalent
benzoic acids, acetophenone is not easily over-oxidized under
the present conditions. The best compromise between
conversion and selectivity is observed for SiW11Co (100%
conversion and 84% selectivity for acetophenone after 3 h of
reaction).
The
oxidation
of
p-methylstyrene
(3)
yields
p-methylbenzaldehyde (18) (the major product, with selec-
tivity above 59%), p-methylbenzoic acid (21) and a few other
minor products (Table 4). The best catalysts were SiW11Co
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Table 6 Oxidation of p-chlorostyrene (5) with H2O2 catalysed by Mn-, Co- and Fe-substituted polyoxotungstates after 6 h of reactiona
Selectivityc (%)
CC cleavage
(27 + 30)
Catalyst
Conv.b,c (%)
(27)
(28)
(32)
(29)
(30)
(31)
BW11Mn
PW11Mn
SiW11Mn
BW11Co
PW11Co
SiW11Co
BW11Fe
PW11Fe
SiW11Fe
No catalyst
100(4)
6
6
95
82
100(5)
93
62
56
4
62
100
100
66
73
69
70
72
84
100
5
4
22
—
—
22
13
18
15
10
6
3
4
84
100
100
88
86
87
85
82
90
100
—
—
6
4
5
6
7
5
—
—
—
6
5
8
8
11
5
—
—
—
—
—
—
—
—
—
—
—
—
5
—
1
—
—
—
—
—
a
Reaction conditions: substrate (1.0 mmol), catalyst (3 μmol) and 30% (w/w) aqueous H2O2 (4.0 mmol) were stirred in 3.0 mL of CH3CN at
80 °C. b Numbers in brackets are the hours when the reaction reaches 100%, if different from 6 h. Determined by GC-FID.
c
Table 7 Oxidation of p-nitrostyrene (6) with H2O2 catalysed by Mn-, Co- and Fe-substituted polyoxotungstates after 6 h of reactiona
Selectivityb (%)
CC cleavage
(33 + 36)
Catalyst
Conv.b (%)
(33)
(34)
(35)
(36)
BW11Mn
PW11Mn
SiW11Mn
BW11Co
PW11Co
SiW11Co
BW11Fe
PW11Fe
SiW11Fe
No catalyst
93
0
4
52
51
98
86
57
62
0
54
—
1
7
38
—
—
15
28
30
29
16
20
—
92
—
—
—
—
—
—
2
—
—
—
—
—
17
7
2
5
12
6
—
100
68
65
68
64
72
74
—
100
83
93
98
93
88
94
—
a
Reaction conditions: substrate (1.0 mmol), catalyst (3 μmol) and 30% (w/w) aqueous H2O2 (4.0 mmol) were stirred in 3.0 mL of CH3CN at
80 °C. b Determined by GC-FID.
(100% conversion and 65% selectivity for 18 after 2 h) and
BW11Mn (100% conversion after 3 h). Considering the amount
of p-methylbenzaldehyde (18) together with p-methylbenzoic
acid (21), the cleavage of the vinyl double bond is responsible
for an overall selectivity of 78% (BW11Mn and BW11Fe) and
83% (SiW11Co).
between 51% (BW11Mn) and 90% (SiW11Mn), and three other
products (Table 5). PW11Co seems to be the most promising
catalyst since it gives rise to 88% conversion concomitantly
to 89% selectivity for p-methylacetophenone (20).
The results obtained in the oxidation of p-chlorostyrene
(5) show that the catalysts BW11Mn and SiW11Co are able to
totally convert the substrate after 4 and 5 h of reaction, respec-
tively (Table 6). p-Chlorobenzaldehyde (27) was obtained as
The cleavage of the vinyl double bond in α,p-dimethylstyrene
(4) affords p-methylacetophenone (20), with selectivity values
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Table 8 Oxidation of p-methoxystyrene (7) with H2O2 catalysed by Mn-, Co- and Fe-substituted polyoxotungstates after 6 h of reactiona
Selectivityb (%)
CC cleavage
(37 + 38 + 41)
Catalyst
Conv.b (%)
(37)
(38)
(39)
(40)
(41)
(42)
(43)
BW11Mn
PW11Mn
SiW11Mn
BW11Co
PW11Co
SiW11Co
BW11Fe
PW11Fe
SiW11Fe
No catalyst
75
20
12
39
86
95
78
38
56
4
34
—
—
11
10
16
—
18
8
22
61
78
62
56
42
57
38
53
100
17
20
22
18
10
24
18
21
16
—
—
—
—
—
5
4
2
—
4
—
—
—
5
10
9
6
—
6
—
—
8
—
—
—
2
—
—
3
16
11
—
4
9
2
17
18
9
56
61
78
78
76
67
63
56
67
—
—
—
—
100
a
Reaction conditions: substrate (1.0 mmol), catalyst (3 μmol) and 30% (w/w) aqueous H2O2 (4.0 mmol) were stirred in 3.0 mL of CH3CN at
80 °C. b Determined by GC-FID.
the major product, together with the corresponding acid 30
(selectivity between 0% and 22%). The amount of secondary
products 28–32 is dependent on the catalyst. The products
resulting from the CC cleavage represent always more than
80%. In this case, the diol pathway (Scheme 2) is irrelevant,
and the isomerization seems to be the major secondary path-
way, giving rise to 28 and 29.
The oxidation of p-nitrostyrene (6) follows a similar
profile, affording p-nitrobenzaldehyde (33) as the major
product (Table 7). None of the catalysts was able to totally
convert this substrate after 6 h of reaction. The selectivity for
p-nitrobenzaldehyde (33) varies in the range of 54–74% for
the catalysts that show conversions higher than 50%. Gener-
ally, the second major product is p-nitrobenzoic acid (36),
reaching up to 38% selectivity in the case of BW11Mn and
around 30% for PW11Co, SiW11Co, and BW11Fe. In the case of
BW11Co, the second major product is p-nitrophenylacetaldehyde
(35) with 17% selectivity. The products resulting from CC
bond oxidative cleavage (33 plus 36) can reach 98% overall
selectivity (SiW11Co), being at least 83% in the case of
BW11Co.
SiW11Co was able to catalyse the oxidization of
p-methoxystyrene (7) with a conversion up to 95% after 6 h of
reaction and PW11Co, BW11Fe, and BW11Mn give rise to conver-
sions higher than 75% (Table 8). SiW11Co gives only 42% selec-
tivity for the major product, p-methoxybenzaldehyde (38), being
better than BW11Mn (22%) and PW11Fe (38%) only. In general,
the second major product was p-methoxyphenylacetaldehyde
(39), SiW11Co being the most selective for 39 with 24% selec-
tivity. In the case of BW11Mn, p-methoxyphenol (37) was the
major product (34% selectivity), unlike for the other catalysts
tested. This phenol was also detected in the presence of other
POMs but with selectivity varying from 8% to 18%.
The Baeyer–Villiger (B.V.) oxidation of 38 constitutes a very
important pathway in the oxidation of 7, since for almost all
the catalysts phenol (37) was significant. The formate ester was
not detected by GC, but the formation of phenol constitutes
evidence of this B.V. pathway from p-methoxybenzaldehyde
(38) to p-methoxyphenol (37). The methoxy substituent is
probably responsible for this dissimilar behaviour.68
As can be seen in Fig. 1 and 2, for all the substrates
studied in the presence of SiW11Co and BW11Mn, the prefer-
ential pathway is always the cleavage of the vinyl double
bond, with the combined selectivity for the products in the
range 56–98%. In the cases where the p-substituent is a
chloro or a nitro group, the selectivity for CC cleavage
products is higher in comparison with styrene. If the sub-
stituent is a methoxy group, the selectivity for the CC
cleavage is the lowest. Benzaldehyde, acetophenone and their
derivatives account for about 60–99% of the CC cleavage
products, depending on the substrates and with only one
exception (oxidation of p-methoxystyrene in the presence of
BW11Mn). Similar graphs are obtained for other good cata-
lysts (PW11Co, BW11Co, and BW11Fe) and most of these con-
clusions apply.
The conversion profile vs. time of reaction for each
substrate was evaluated in the presence of BW11Mn and
SiW11Co. The comparison of the conversion for different
substrates oxidized in the presence of SiW11Co as catalyst is
illustrated in Fig. 3. With this catalyst, the reactions are
always faster for all the styrenes substituted with one or two
methyl groups compared with styrene. This can be explained
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Fig. 1 Selectivity for the products resulting from the oxidative cleavage of the vinyl double bond for the reactions catalysed by BW11Mn in the
presence of H2O2 at the end of the reactions.
Fig. 2 Selectivity for CC cleavage products resulting from the oxidative cleavage of the vinyl double bond for the reactions catalysed by
SiW11Co in the presence of H2O2 at the end of the reactions.
Fig. 3 Oxidation reaction conversion profile vs. time of reaction for styrene and styrene derivatives in the presence of SiW11Co, with H2O2 at
80 °C in CH3CN.
by the fact that methyl is a weak electron-donating group,
which makes the vinyl double bond more reactive. For
p-chlorostyrene, the reaction occurs at a similar rate as for
styrene. Despite chlorine being an electron-withdrawing
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group, the reactivities of p-chlorostyrene and styrene are
similar. When oxidizing p-methoxy or p-nitro substituted sty-
renes, the oxidation reactions are slower than those of
unsubstituted styrene.
observed. This can be explained assuming an interaction of
the oxygen of the methoxy group with the POM, either by
coordination to a metal or otherwise. In this case, the
unshared pairs of electrons from oxygen will not be available
for the resonance effect that increases the electronic density
at the vinyl group, increasing reactivity. Also, this interaction
would possibly block the access to the metal centres needed
for catalysis, lowering the reactivity.
The reactions catalysed by BW11Mn are faster in the first
hour for all the substrates if compared with styrene itself,
except when the substituent is a methoxy group (Fig. 4). The
nitro group does not seem to affect the reaction rate during
the first hours, but at the end of the reaction, it is clear that
the conversion is not complete, as is the case for almost
all the other substituents (except for p-methoxystyrene).
As expected from the reactions described above, the results
show that α-methylstyrene (2) and p-methylstyrene (3) are
more reactive than styrene, p-chlorostyrene (5) has no marked
difference, and p-nitrostyrene (6) is less reactive. Again,
the substrate with the p-methoxy group is the only one with
The behaviour of p-nitrostyrene is expected in view of the
electron-withdrawing character of the nitro group. According
to the literature,72 the following order was proposed for
the epoxidation reaction of styrenes in the presence of
iron(III) porphyrin (FeTPPCl) and PhIO as an oxidant in CH2Cl2
at room temperature: p-OCH3 > p-CH3 > p-H ≅ p-Cl > p-NO2.
Despite the differences from the current catalytic condi-
tions, the reactivity pattern in the presence of the selected
POMs seems to follow an analogous sequence, excluding
p-methoxystyrene. This order is more clearly observed when
comparing the values of conversion at 1 or 2 h of reaction,
as the values tend to reach 100% at longer reaction times.
The lower reactivity of the vinyl double bond of
p-methoxystyrene (7) seems to be atypical. The electron-
donor effect of the methoxy group should facilitate the
reaction cleavage of the vinyl double bond, contrarily to the
Fig. 4 Oxidation reaction conversion profile vs. time of reaction for styrene and styrene derivatives in the presence of BW11Mn, with H2O2 at
80 °C in CH3CN.
Fig. 5 Styrene and hydrogen peroxide conversion after 6 h of reaction (*after 5 h) at 80 °C in CH3CN in the presence of all the catalysts and in
the absence of a catalyst.
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possibly a non-conventional behaviour under the present con-
ditions, being much less reactive than the other substrates.
The explanation is possibly the same as for SiW11Co. The
results obtained with the two catalysts BW11Mn and SiW11Co
may thus be resumed in the following order of substrate reac-
tivity under the conditions of this study: p-CH3, α-CH3,
α,p-(CH3)2 > p-H, p-Cl > p-NO2 > p-OCH3.
As a final remark, we may note that for the more reactive
substituted styrenes the time of reaction needed to attain
100% conversion lowers significantly in comparison with
styrene. The plots presented in Fig. 3 and 4 illustrate this
point clearly.
after Baeyer–Villiger oxidation of the aldehyde. This last B.V.
pathway was also registered for the acetophenones resulting
from CC oxidative cleavage, since phenols were detected
for α-methyl and for α,p-dimethylstyrene. Merging the results
of H2O2 conversion (Fig. 5) with the radical scavenger reac-
tion results, the unproductive conversion of H2O2 seems to
be mainly dismutation into H2O and O2 and not the H2O2
homolytic cleavage leading to the formation of hydroxyl and
hydroperoxyl radicals.
Acknowledgements
The conversion of hydrogen peroxide at the end of the
styrene oxidation reactions was also evaluated in the pres-
ence of all the catalysts and in the absence of a catalyst.73
The comparison is illustrated in Fig. 5. The unproductive
dismutation of hydrogen peroxide into water and oxygen
seems to be the essential pathway for PW11Mn and SiW11Mn,
whereas for BW11Mn and SiW11Co the high conversion of
hydrogen peroxide is surpassed by the total conversion of
styrene. Opposite to what one might have expected, PW11Fe
and SiW11Fe give rise to relatively low percentages of styrene
conversion, but the dismutation of hydrogen peroxide seems
to be moderate. All the cobalt catalysts associate high to very
high conversions of styrene to high conversions of hydrogen
peroxide.
We would like to thank Fundação para a Ciência e a Tecnologia
(FCT, Portugal), the European Union, QREN, FEDER, and
COMPETE for funding the Organic Chemistry Research Unit
(QOPNA) (PEst-C/QUI/UI0062/2013) and CICECO (PEst-C/CTM/
LA0011/2013).
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