Oxidation of hydroquinones to benzoquinones with hydrogen peroxide using catalytic amount of silver oxide under batch and continuous-flow conditions

Article abstract of DOI:10.1016/j.jcat.2010.02.015

Silver oxide (Ag₂O) can be utilized as heterogeneous catalyst in the oxidation of hydroquinones to the corresponding benzoquinones with 30% aq. H₂O₂. The catalytic properties of Ag₂O have been explored in the model reaction of methylhydroquinone with H₂O₂ performed under batch conditions. Results show that the reaction can be carried out in high yield and selectivity at room temperature under environmentally friendly conditions. The truly heterogeneous catalyst can be recovered by filtration and reused for at least five times, giving the same excellent results (～95% yield, ～98% selectivity). The catalyst can be packed in a tubular reactor and utilized under continuous-flow conditions giving similar good results.

Full text of DOI:10.1016/j.jcat.2010.02.015

Journal of Catalysis 271 (2010) 99–103
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Oxidation of hydroquinones to benzoquinones with hydrogen peroxide using
catalytic amount of silver oxide under batch and continuous-flow conditions
a,b
a
a
a
a,*
Fatemeh Derikvand , Franca Bigi , Raimondo Maggi , Calogero Giancarlo Piscopo , Giovanni Sartori
a
Clean Synthetic Methodologies Group, Dipartimento di Chimica Organica e Industriale dell’Università, and Consorzio Interuniversitario ‘La Chimica per l’Ambiente’ (INCA),
UdR PR2, Viale G.P. Usberti 17A, I-43124 Parma, Italy
b
Department of Chemistry, School of Science, Azzahra University, Vanak, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
2
Silver oxide (Ag O) can be utilized as heterogeneous catalyst in the oxidation of hydroquinones to the
Received 14 December 2009
Revised 8 February 2010
Accepted 9 February 2010
corresponding benzoquinones with 30% aq. H
in the model reaction of methylhydroquinone with H
2
O
2
. The catalytic properties of Ag
2
O have been explored
2 2
O
performed under batch conditions. Results show
that the reaction can be carried out in high yield and selectivity at room temperature under environmen-
tally friendly conditions. The truly heterogeneous catalyst can be recovered by filtration and reused for at
least five times, giving the same excellent results (ꢀ95% yield, ꢀ98% selectivity). The catalyst can be
packed in a tubular reactor and utilized under continuous-flow conditions giving similar good results.
Ó 2010 Elsevier Inc. All rights reserved.
Keywords:
Oxidation
Hydrogen peroxide
Benzoquinones
Hydroquinones
Continuous-flow process
Silver oxide
1
. Introduction
Quinones [1] and the parent p-quinols [2] are found as struc-
but most of these reagents suffer from the drawback of producing
large amounts of noxious waste.
Phenols and naphthols could be directly converted into qui-
nones via one-pot hydroxylation–oxidation process in the presence
of different oxidizing agents such as ammonium persulfate on wet
silica [6], hydrogen peroxide–methyltrioxorhenium in ionic liquids
[7]. The dye-sensitized photooxygenation of 1,5-dihydroxynaph-
thalene to Juglone with oxygen in the presence of moderately con-
centrated sunlight (and solid-supported sensitizers) represents an
interesting example of environmentally friendly alternative to
common thermal processes [8].
The use of immobilized catalysts for selective oxidation in li-
quid phase represents a great synthetic improvement [9]. As an
example, hydroquinone was efficiently oxidized to p-benzoqui-
none with hydrogen peroxide in the presence of acrylic resin-sup-
ported Cu(II) catalyst [10]. Moreover, phenols containing
oxidizable groups could be directly and selectively oxidized to
p-benzoquinones with hydrogen peroxide using a mesoporous
titanium silicate catalyst [11]. Furthermore, a catalyst based on
platinum nanoclusters incarcerated into a styrene polymer was,
more recently, reported for the aerobic oxidation of hydroquinones
to quinones [12].
tural units in a great variety of bioactive natural compounds show-
ing antifungal, antibacterial, antiviral, antigerminative, and
anticancer activities. They are consequently useful intermediates
for organic synthesis with particular interest into the production
of biologically active compounds [3]. For example, trimethyl-1,4-
benzoquinone is a key reagent in the synthesis of vitamin E and
2
-methyl-1,4-naphthoquinone (vitamin K3), an important additive
in animal feed, is also utilized for construction of vitamin K1 [4].
The most general method to obtain quinones is the oxidation of
monohydroxyaromatics and dihydroxyaromatics. The oxidation of
substituted hydroquinones to the corresponding quinones has
been achieved by a variety of oxidants [5]. For example, the Fre-
my’s salt (potassium nitrodisulfonate) [5a,b], sodium dichromate/
sulfuric acid mixture [5c,d], benzyltrimethyl-ammonium tribro-
mide [5e], iodine or hydroiodic acid/hydrogen peroxide mixture
[
5f], diphenyl diselenide/hydrogen peroxide mixture [5g], tert-bu-
tyl hydroperoxide/ceric ammonium nitrate mixture [5h], phenyl
iodosoacetate on alumina under microwave irradiation [5i], and
different transition metal salt derivatives [5j] have been exploited,
Well-known and established reagents such as silver oxide and
silver carbonate show synthetic utility at the laboratory level for
these oxidative reactions [3a,13] but they are evidently not practi-
cal for large-scale preparation due to their high cost.
*
Corresponding author. Fax: +39 0521 905472.
E-mail address: giovanni.sartori@unipr.it (G. Sartori).
0
021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2010.02.015

1
00
F. Derikvand et al. / Journal of Catalysis 271 (2010) 99–103
New green legislations call for the lowering of waste production
The solution of premixed reagents methylhydroquinone (4.5 g,
36 mmol) and 30% aq. H (8.65 mL, 86.4 mmol) in methanol
and use of more friendly alternative reagents and catalysts. This
goal can be achieved through the application of the principles of
green chemistry that provide a framework for designing more
eco-compatible routes to fine chemical production [14]. According
to this strategy, herein we wish to report results of a study con-
cerning the use of silver(I) oxide as solid catalyst in the oxidation
of hydroquinones to quinones by hydrogen peroxide [15] at room
temperature under batch and continuous-flow conditions. To our
knowledge, silver(I) oxide has been previously utilized only as re-
agent in stoichiometric amount in the same reaction [3a,13].
2 2
O
(50 mL) was passed at room temperature with the same flow rate
(0.7 mL/min) with a modest pressure (ꢀ0.2 bar); the time for pas-
sage of the entire solution was about 85 min. The column reactor
was finally rinsed by passing methanol (20 mL) and the eluates
were combined. After removal of methanol and filtration to re-
move traces of silica, the product 3a was crystallized by toluene/
hexane mixture (4.12 g, 94% yield).
The products prepared by both methods were sufficiently pure
(>97%) for possible synthetic use. For analytical purposes, they
were further purified by flash chromatography over silica gel col-
umns with hexane/ethyl acetate mixtures as eluent.
2
. Experimental
3
4
3
3
3
4
3
a. Anal. Calcd for C
.80; mp. 70 °C (Lit. [17] 69 °C).
b. Anal. Calcd for C : C, 66.67, H, 3.73. Found: C, 66.50, H,
.82; mp. 113–115 °C (Lit. [18] 112–114 °C).
c. Anal. Calcd for C : C, 60.87, H, 4.38. Found: C, 60.95, H,
.45; mp. 144–146 °C (Lit. [19] 145–146 °C).
d. Anal. Calcd for C ClO : C, 50.56, H, 2.12. Found: C, 50.72,
7 6 2
H O : C, 68.85, H, 4.95. Found: C, 68.98, H,
2
.1. Materials
6 4 2
H O
2 2 5 2 2
All catalysts, namely Ag, Ag O, V O , CrO (Magtrieve™), TiO ,
, AgNO , and solvents were purchased from Aldrich and were
3 3
MoO
used without previous purification, except for MeOH which was
distilled before use. Hydrogen peroxide 30 wt.% in H O was also
purchased from Aldrich and was used from freshly opened con-
tainers. The following reagents were utilized without further puri-
fication: hydroquinone P99% (Aldrich), methylhydroquinone 99%
7
6 3
H O
2
H
6 3
2
H, 2.30; mp. 53–56 °C (Lit. [17] 55–56 °C).
3e. Anal. Calcd for C
6.78; mp. 31–33 °C (Lit. [18] 31–32 °C).
3f. Anal. Calcd for C11 : C, 74.98, H, 6.86. Found: C, 74.90,
9 10 2
H O : C, 71.98, H, 6.71. Found: C, 72.10, H,
(
2
Aldrich), methoxyhydroquinone 98% (Aldrich), 1,4-dihydroxy-
,6-dimethoxybenzene 97% (Aldrich), 1,4-dihydroxynaphthalene
12 2
H O
H, 6.77; mp. 100–103 °C (Lit. [20] 102 °C).
P90% (Aldrich), chlorohydroquinone ꢀ85% (Fluka). Isopentenyl
hydroquinone was synthesized as reported in the literature [16].
Elemental analyses were performed with a Carlo Erba CHNS-O
EA1108 Elemental Analyzer.
The purity of commercially available hydroquinones was care-
fully controlled by GC or HPLC analyses due to their well-known
instability during time.
3g. Anal. Calcd for C
4.76; mp. 255–258 °C (Lit. [21] 254–256 °C).
3h. Anal. Calcd for C10 : C, 75.94, H, 3.82. Found: C, 75.81, H,
3.72; mp. 120–122 °C (Lit. [17] 121–122 °C).
8 8 4
H O : C, 57.14, H, 4.80. Found: C, 57.01, H,
6 2
H O
Conversions and yields were determined by GC analysis, per-
formed on a TraceGC ThermoFinnigan instrument (fused silica cap-
illary column SPB-20 from Supelco (30 m ꢁ 0.25 mm) was used
with helium as carrier) adding 1,4-dimethoxybenzene as internal
standard.
3
. Results and discussion
Our experiments to achieve the optimum reaction conditions
began with the oxidation of methylhydroquinone 1a selected as
model reagent (1 mmol) with 30% aq. H 2 (2 mmol) catalyzed
by some classic, commercially available oxidation catalysts
0.1 mmol). The experiments were carried out at room tempera-
ture in methanol, acetone, water or their mixtures, and 30% aq.
diluted in the selected solvent was added dropwise during
0 min. The reaction mixture was stirred for further 20 min at
2 2
O
2.2. Catalytic testing
(
As examples, we report here the optimized methods. The cata-
lytic tests have been performed by similar procedures with the
amounts of reagents reported in the notes of tables and figures.
2 2
H O
1
room temperature.
Table 1 shows the yields and selectivities for all catalysts and
solvents.
2
.2.1. Typical procedure: oxidation of methylhydroquinone (batch
experiment)
To a mixture of methylhydroquinone (0.12 g, 1 mmol), Ag
0.02 g, 0.1 mmol), and MeOH (2 mL), stirred for 5 min, a solution
of 30% aq. H (0.24 mL, 2.4 mmol) in MeOH (10 mL) was added
To our pleasure, some of the tested metal oxides showed satis-
2
O
factory catalytic activity giving product 3a in good yields: V
MoO , and Ag O afforded 3a in 82%, 78%, and 84% yield, respec-
tively (Table 1, entries 2, 5, and 6).
The best selectivity with respect 3a (99%) was achieved with
O (Table 1, entry 6), whereas with V and MoO , the selectiv-
2 5
O ,
(
3
2
2 2
O
dropwise under stirring in 10 min at room temperature. Then,
the mixture was continuously stirred for 20 min at room tempera-
ture. Subsequently, the reaction mixture was diluted with water
Ag
2
O
2 5
3
ity was lower due to the competitive production of substituted
biaryls (ꢀ8–10%) via oxidative homocoupling of 1a (Table 1, en-
tries 2 and 5). Silver nitrate was less active than silver(I) oxide (Ta-
ble 1, entry 7). Results from Table 1 also suggest that methanol was
the solvent of choice in terms of yield, selectivity, and solubility of
both starting materials and products (Table 1, entry 6). Use of dif-
ferent polar solvents resulted in comparable high yield but lower
selectivity (Table 1, entries 8 and 11).
(
10 mL) and then extracted with diethyl ether (2 ꢁ 15 mL). The
combined organic layers were washed with water (2 ꢁ 10 mL)
and dried over Na SO . The solvents were evaporated under re-
duced pressure affording product 3a (0.11 g, 92% yield).
2
4
2.2.2. Typical procedure: oxidation of methylhydroquinone
(
continuous-flow experiment)
Silica-KG-60 (Merck) for column chromatography sieved to
Parallel leaching studies were carried out by catalyst filtration
at 25 °C after 10 min reaction time when the yield of 3a was 55%
[22]. No yield increase was observed by keeping the filtrate at
8
0 mesh (1.85 g) and Ag
ment were mixed and packed with methanol in a column reactor
10 cm length, 0.8 cm inner diameter) equipped with a syringe
2
O (0.093 g) without any previous treat-
(
25 °C for further 20 min when Ag
the contrary, amounts of active species were leached into solution
when the reaction was performed in the presence of V and
2
O was utilized as catalyst. On
pump. Methanol was passed at room temperature for 15 min to
achieve column reactor conditioning with a 0.7 mL/min flow rate.
2 5
O

F. Derikvand et al. / Journal of Catalysis 271 (2010) 99–103
101
Table 1
With the goal of achieving the optimum efficiency, the effect of
the amount of H on the yield of product 3a was also explored.
To this end, yield and selectivity of product 3a were determined in
Reaction of methylhydroquinone (1a) with aq. H
2
O
2
(2) in the presence of different
2 2
O
solid catalysts and solvents.a
OH
O
the model reaction performed in methanol, at room temperature,
catalyst
for 10 + 20 min in the presence of 10% mol of Ag
increasing amounts of 30% aqueous H
Results reported in Fig. 2 confirm that Ag
2
O and with
+
2 H O
2
+
H O
2
2
2 2
O .
solvent
2
O alone converts
OH
O
3a
methylhydroquinone 1a into the product 3a without addition of
hydrogen peroxide. However, product 3a was obtained in 20% yield
instead of the expected 10% based on the stoichiometry of the reac-
tion. The higher yield was attributed to the presence of atmo-
spheric oxygen that could be involved in the oxidative process.
Indeed, by performing the reaction under nitrogen atmosphere in
a degassed solvent, product 3a was obtained in 8% yield [23]. These
results represent a quite interesting example of green oxidation
process that is under investigation in our group.
1
a
2
3a yield (select.)b (%)
No.
Catalyst
Solvent
1
2
3
4
5
6
7
8
9
–
V
CrO
TiO
MoO
Ag
AgNO
Ag
Ag
Ag
Ag
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH/H
Acetone
ꢀ3
2
O
5
82 (88)
38 (95)
15 (99)
78 (90)
84 (99)
30 (75)
80 (95)
82 (99)
82 (99)
92 (92)
2
(Magtrieve)
2
3
2
O
3
2
O 20/80
O 50/50
Fig. 2 also shows that the increase of the H
to 2.5 H /1a molar ratio) gave a linear increase of the yield of
product 3a from 20% to 96%. The selectivity was constant around
8%; and only when larger amounts of H were utilized, the
2 2
O amount (from 0
2
2
2
2
O
O
O
O
2
H O
2 2
O
1
1
0
1
MeOH/H
MeCN
2
9
2 2
O
a
Catalyst 0.1 mmol, methylhydroquinone (1a) 1 mmol, solvent 2 mL, 30% aq.
2 2
H O 2 mmol in solvent 10 mL, reaction temperature 25 °C, reaction time 10 min
selectivity decreased due to the mentioned overoxidative
processes.
Concerning the catalyst recycling, we have observed that in the
2
model reaction of Table 1, the catalyst Ag O, simply recovered by
addition and 20 min reaction.
b
Determined by GC analysis.
filtration, washed with hot methanol could be reused at least five
times, without further activation, giving product 3a with un-
changed high yield and selectivity (Fig. 3).
The sole drawback we observed was the increasing crumbling
of the catalyst particles due to the mechanical stress produced by
stirring that made more difficult the catalyst recovery by filtration.
The scope and generality of this process with respect to various
hydroquinones is illustrated in Table 2.
3 2
MoO . For this reason, Ag O was utilized as truly heterogeneous
catalyst in successive studies.
2
The effect of catalyst amount (expressed as Ag O/1a molar ra-
tio) was then evaluated in the model reaction of Table 1 while
keeping the molar ratio of 1a:2 at 1:2, room temperature, reaction
time 10 + 20 min and methanol as solvent. The yield and selectivity
of product 3a are plotted against the amount of catalyst in Fig. 1.
A variety of functionalized quinones could be prepared in good
yield and excellent selectivity. The reaction tolerated both elec-
tronwithdrawing and electrondonating groups (Table 2, entries 3
and 4).
In the absence of Ag
However, the 3a yield was dramatically increased to 71% when
only 0.05 molar ratio Ag O/1a was utilized. The maximum perfor-
mance was achieved when the Ag O/1a molar ratio was 0.1, being
product 3a obtained in 86% yield. By further increasing Ag O/1a
molar ratio, the yield and, in particular, the selectivity of product
a underwent a little, but continuous lowering. This is due to the
special oxidizing property of Ag O that is known to give competi-
tive oxidative coupling of phenols including hydroquinones [3].
2
O, product 3a was obtained in traces (ꢀ3%).
2
2
0
Due to the mild oxidizing conditions, 1,4-dihydroxy-2-(3 -
2
0
methylbut-2 -enyl)benzene can be efficiently converted into the
corresponding quinone without over-oxidation or epoxide produc-
tion also after prolonged reaction time (Table 2, entry 6) [24]. In
some cases, the reaction required longer reaction times (Table 2,
entries 2, 3, 4 and 7).
3
2
The good synthetic results and, in particular, the effective cata-
lyst recyclability achieved in the batch experiments prompted us
to develop a reaction method based on a continuous-flow catalytic
100
90
80
70
60
50
40
30
20
10
0
1
00
9
8
7
6
0
0
0
0
yield
50
40
yield
selectivity
selectivity
3
2
1
0
0
0
0
0
0.05
0.1
0.15
0.2
0
0.5
1
1.5
2
2.5
3
molar ratio Ag O/methylhydroquinone
molar ratio H O /methylhydroquinone
2
2
2
Fig. 1. Effect of the amount of catalyst (Ag
selectivity. Reaction conditions: methylhydroquinone (1a) 1 mmol, MeOH 2 mL,
2
O) on the product 3a yield and
Fig. 2. Effect of molar ratio H
selectivity. Reaction conditions: Ag
2
O
2
/methylhydroquinone on the product 3a yield and
O 0.1 mmol, methylhydroquinone (1a) 1 mmol,
2
3
0% aq. H
2
O
2
2 mmol in MeOH 10 mL, reaction temperature 25 °C, reaction time:
MeOH 2 mL, reaction temperature 25 °C, reaction time: 10 min addition and 20 min
1
0 min addition and 20 min reaction.
reaction.

1
02
F. Derikvand et al. / Journal of Catalysis 271 (2010) 99–103
100
one-day intervals giving product 3a with the following yields:
st 98%, 2nd 97%, 3rd 98%, 4th 99%, 5th 96%, and 6th 98%. Given
1
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
the effectiveness of the present column reactor and experimental
ease of its assembling and use in the oxidation of hydroquinones
to quinones, such system could hold promise for the future lar-
ger-scale application.
We have not investigated the reaction mechanism; however,
the experimental results achieved in this study may allow us to
make some general mechanistic comments. It is well known that
different metals may catalyze oxidation of phenolic compounds
affording numerous products by a variety of ways. The mechanistic
rationalizations of these reactions are frequently difficult and give
conflicting results even in reactions performed under homoge-
neous conditions. We have initially assumed, for the Ag
2
2 2
O/H O
combination, a pathway similar to that for Cu(I)/H
2 2
O system
involving generation of hydroxyl radicals [27]. However, when
the model reaction was performed in the presence of a stoichiom-
etric amount of the radical scavenger 2,6-di-tert-butyl-4-methyl-
phenol with respect reagent 1a, product 3a was obtained in 86%
yield showing only a modest deactivation. In addition, no para-
magnetic species could be recognized by a preliminary EPR analy-
sis of the reaction mixture.
These results seem to be in better agreement with a heterolytic
catalysis process where free radicals are not crucial intermediates.
Furthermore, all critical steps of the entire oxidative reaction seem
to occur on the surface of the catalyst and no active species are lea-
ched into solution. This is confirmed by both the filtration test al-
ready described in this study [22] and the ‘‘pre exposure” test
recommended by Jacobs and coworkers [28] that confirmed the
1
st cycle
2nd cycle
3rd cycle
4th cycle
5th cycle
Fig. 3. Relative activity of Ag
washing with hot methanol (h yield of 3a; j selectivity of 3a).
2
O in five successive cycles of the model reaction after
Table 2
Reaction of various hydroquinones (1) with aq. H
2
O
2
(2).a
OH
O
R
R
Ag O
2
+
H O
2
2
R1
R2
MeOH
R1
R2
OH
O
3
1
2
2
great stability of Ag O under the reaction conditions and further
demonstrates that every key species is produced and reacts on
the surface of the catalyst. Quite interesting on this subject are re-
sults achieved by performing the model reaction under homoge-
neous conditions (Table 1, entry 7). Indeed, the reaction
No.
R
R
1
R
2
3
Time
3 yield
(
min)
(select.)b (%)
1
2
3
4
5
6
7
8
Me
H
OMe
Cl
Me
CH
H
H
H
H
Me
H
OMe
H
H
H
H
Me
H
3a
3b
3c
3d
3e
3f
20
40
40
40
20
20
40
20
96 (98)
94 (99)
70 (95)
90 (97)
92 (98)
98 (98)
97 (98)
99 (97)
3
performed in the presence of AgNO in water–methanol mixture
as solvent gave lower conversion of reagent 1a and modest selec-
tivity, being product 3a accompanied by different amounts of
CAC and CAO linked biaryl by-products due to the competitive
oxidative coupling of the hydroquinone [29].
2 2
CH@CMe
H
OMe
H
3g
3h
CH@CHACH@CH
a
A possible essential catalytic cycle is described in Fig. 4 on the
basis of experimental and literature information. It is, indeed,
2 2 2
Ag O 0.1 mmol, hydroquinone (1) 1 mmol, MeOH 2 mL, 30% aq. H O 2.4 mmol
in MeOH 10 mL, reaction temperature 25 °C, reaction time 10 min addition and 20
(
or 40) min reaction.
Determined by GC analysis.
known from literature and confirmed in this study that Ag
dizes hydroquinones to benzoquinones in stoichiometric ratio
being reduced to metal silver that can be reoxidized to Ag O and
other silver oxides by H as very early reported in the literature
[30]. Furthermore, in the blank experiment, we observed that the
commercially available silver powder (<250 m, 99.99%) gives
2
O oxi-
b
2
system for performing the model reaction. Such a system, once
optimized, can allow for a rapid, easy, and efficient synthesis and
purification of the target compounds [25]. Indeed, the continu-
ous-flow process is, in short, an integrated reaction and separation
approach that can be regarded as one of the most efficient contri-
butions to the process intensification [26].
2 2
O
l
product 3a in 25% yield when utilized as catalyst in the model
To this end, a simple apparatus was constructed using a stan-
dard laboratory small chromatographic column (10 cm
OH
length ꢁ 0.8 cm inner diameter) packed with a mixture of Ag
2
O
H O
Ag O
2
2
(
0.093 g) and KG-60 silica for column chromatography (1.85 g)
sieved to 80 mesh and equipped with a syringe pump to charge
the reaction mixture and to carefully control the flow rate. The col-
umn reactor was previously washed by flowing MeOH (10 mL) for
OH
1
a
1
5 min at 0.7 mL/min flow rate at room temperature; then, a solu-
tion of methylhydroquinone 1a (36 mmol) and 30% aq. H
86.4 mmol) in MeOH (50 mL) was passed at room temperature
2
O
2
O
(
with the same flow rate (0.7 mL/min) and a minimum pressure ap-
plied (ꢀ0.2 bar). The column reactor was then rinsed with addi-
tional MeOH (10 mL), and the combined eluates were analyzed
by GC giving product 3a in 98% yield.
To evaluate the stability and efficiency during time of the col-
umn reactor, the same experiment was repeated six times at
H O
2 Ag
2
2
+ H O
2
O
3
a
Fig. 4. Proposed general mechanism.

F. Derikvand et al. / Journal of Catalysis 271 (2010) 99–103
103
reaction. Finally, the possible intervention of the alcoholic solvent
MeOH as a reagent able to give the corresponding hydroperoxide
intermediate by primary reaction with H O [31] cannot be ex-
2 2
(b) S. Itoh, M. Ogino, S. Haranou, T. Terasaka, T. Ando, M. Komatsu, Y. Ohshiro,
S. Fukuzumi, K. Kano, K. Takagi, T. Ideka, J. Am. Chem. Soc. 17 (1995) 1485;
(
c) F.B. Vliet, in: H. Gilman, A.H. Blatt (Eds.), Organic Syntheses, Collective Vol.
I, New York, 1958, p. 482;
cluded even if product 3a has been also obtained in 90% yield by
performing the model reaction in acetonitrile.
(d) L.F. Fiester, in: H. Gilman, A.H. Blatt (Eds.), Organic Syntheses, Collective
Vol. I, New York, 1958, p. 383;
(
e) S. Kajigaeshi, Y. Morikawa, S. Fujisaka, T. Kakinami, K. Nishihira, Bull.
Chem. Soc. Jpn. 64 (1991) 336;
(
(
(
f) F. Minisci, A. Citterio, E. Vismara, F. Fontana, S.D. Bernardis, J. Org. Chem. 54
1989) 728;
g) D.V. Pratt, F. Ruan, P.B. Hopkins, J. Org. Chem. 52 (1987) 5053;
4
. Conclusion
We have demonstrated that the commercially available silver
(h) K. Krohn, J.J. Vitz, Prakt. Chem. 342 (2000) 825;
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(
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oxide (Ag
2
O), until now utilized as reagent in stoichiometric
j) D.H.R. Barton, J.P. Finet, M. Thomas, Tetrahedron 44 (1988) 6397. and
amount to perform oxidation of hydroquinones to quinones, can
be now exploited as efficient heterogeneous catalyst in the same
reaction with 30% aqueous H
[
[
[
[
2
O
2
. The catalytic properties of Ag
2
O
in the model reaction of methylhydroquinone with H
2 2
O
have been
explored. The results show that the reaction can be performed in
high yield and selectivity at room temperature under environmen-
tally friendly operating conditions. The truly heterogeneous cata-
lyst can be recovered by filtration and reused for, at least, five
times giving the same excellent results.
The catalyst can be dispersed in silica and packed in a tubular
reactor that can be more efficiently utilized to perform the same
oxidation reaction under continuous-flow conditions with a mini-
mum pressure applied. The column reactor can be utilized at least
six times during one week without any lowering of activity.
The catalyst is very active and selective for the oxidation of a
series of hydroquinones to the corresponding quinones; and due
to its soft oxidizing properties, it can be efficiently utilized also
with compounds containing oxygen sensitive groups.
9] L. Alaerts, J. Wahlen, P.A. Jacobs, D.E. De Vos, Chem. Commun. (2008) 1727.
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[
[
[12] H. Miyamura, M. Shiramizer, R. Matsubara, S. Kobayashi, Angew. Chem. Int. Ed.
7 (2008) 8093.
4
[
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Compounds, vol. 1, Part 1, Wiley, London, 1974, p. 111;
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14] P.T. Anastas, M.M. Kirchhoff, Acc. Chem. Res. 35 (2002) 686.
[
[
15] (a) J.M. Campos-Martin, G. Blanco-Brieva, J.L.C. Fierro, Angew. Chem. Int. Ed.
4
5 (2006) 6962;
(b) W.R. Sanderson, Pure Appl. Chem. 72 (2000) 1289.
[16] L. Jurd, K. Stevens, G. Manners, Tetrahedron Lett. 25 (1971) 2275.
[
[
[
[
[
17] T. Sakamoto, H. Yonehara, C. Pac, J. Org. Chem. 62 (1997) 3194.
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Acknowledgments
8
731.
[
22] Only with V
2 5 3
O and MoO the solution obtained after removal of the solid
The authors acknowledge the support of the Ministero dell’Uni-
versità e della Ricerca (MIUR), Italy, and the University of Parma
catalyst by filtration showed catalytic activity H.E.B. Lempers, R.A. Sheldon, J.
Catal. 175 (1988) 62.
23] Attempts to perform the process by bubbling oxygen in the reaction medium
or by using an oxygen pressurized reactor in the presence of 10% mol Ag O
2
gave product 3a with 27% maximum yield.
24] Blank experiment confirmed that allylbenzene was completely recovered
unchanged after treatment under the present reaction conditions.
25] (a) S. France, D. Bernstein, A. Weatherwax, T. Lectka, Org. Lett. 7 (2005) 3009;
(b) A. Kirschning, W. Solodenko, K. Mennecke, Chem. Eur. J. 12 (2006) 5922;
[
(
National Project ‘‘Attivazione ossidativa catalitica e fotocatalitica
per la sintesi organica”). The Centro Interdipartimentale Misure
CIM) is acknowledged for the use of NMR instruments. The
[
[
(
authors are indebted to Prof. Elio Giamello (Università di Torino)
for EPR investigations.
(
c) A.R. Bogdan, B.P. Mason, K.T. Sylvester, D.T. McQuade, Angew. Chem. Int.
Ed. 46 (2007) 1;
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