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D. Ramakrishna et al. / C. R. Chimie 17 (2014) 1071–1074
Table 1
advantage of this system is that both the peroxide and the
catalyst are soluble in the ionic liquid. This gives an
oxidation solution that is completely homogeneous.
Epoxidation of styrenea in [Emim]PF6–H2O2 system.
Entry
Reaction time (min)
% Conversion
1
60
360
360
120
120
99.0
<10
NRd
29.9
72.6
2b
3c
4e
5f
a
Unless otherwise indicated, all reactions were carried out with
1 mmol of styrene with 2.4 mmol of catalyst, 2 mmol of H2O2.
P
b
The reaction was carried out under oxygen instead of H2O2.
c
Co
O
Reaction without catalyst.
Cl
NH
d
NR: no reaction.
e
1.5 mmol of catalyst.
N
f
1 mmol of H2O2.
N
3. Results and discussion
The epoxidation of styrene with hydrogen peroxide
catalyzed by 1 was carried out in 2, affording, under
stirring in the presence of hydrogen peroxide, styrene
epoxide (3) and phenylacetaldehyde (4) in 78.8% and 21.2%
yields, respectively. The oxidation of cyclohexene (5) with
hydrogen peroxide and 1 in 2 gave epoxycyclohexane (6),
2-cyclohexenol (7) and 2-cyclohexenone (8) in 87.6, 7.1,
5.3% yields, respectively (Scheme 1), but cyclooctene gave
only epoxycyclooctane in 86% yield.
1
-
N
N
PF6
2
Styrene was used for the optimization of the reaction
conditions in ionic liquid (Table 1). The initial study was
carried out using styrene as the substrate and 2 mmol of
aqueous H2O2 at room temperature in the presence of
2.4 mmol of catalyst. The oxidation proceeded smoothly,
and a 90% isolated yield of benzaldehyde was obtained
with 99% conversion and 79% selectivity after 1-h stirring.
The oxidation occurred only in poor yield by simply
bubbling molecular oxygen through the reaction mixture
under similar reaction conditions (Table 1, entry 2). To
evaluate the catalytic effect of the catalyst, the oxidation of
styrene was carried out under similar reaction conditions
in the absence of catalyst and no conversion was observed
(Table 1, entry 3). The reaction was complete after 60 min
at room temperature (Table 1, entry 1). Furthermore, when
the oxidation was carried out using a lesser amount of the
catalyst, the yield was low (Table 1, entry 5). Also the
reaction was carried out by varying the oxidant’s
2. Experimental
2.1. General procedure for the catalytic reaction
In a typical reaction, the catalyst (8 mg, 2.4 mmol) was
dissolved in [Emim]PF6 (1 cm3). After addition of the
substrate (1 mmol), hydrogen peroxide (30% in water)
(2 mmol) was added. The reaction mixture was stirred at
room temperature for about 1 h. The completion of the
reaction was monitored by GC–MS until the reactants have
disappeared. After completion of the reaction, the reaction
mixture was extracted with diethyl ether (3 Â 3 cm3). The
ether layer was collected and then a definite amount of
toluene was added as an internal standard for GC analysis.
The ionic liquid phase including the catalyst was washed
with diethyl ether and dried prior to recycling. The
epoxidation of the other olefins was carried out similarly
with a little variation in reaction time (Table 2).
O
CHO
Co-Salen-PPh3/H2O2
[Emim]PF6
+
3
4
OH
O
Co-Salen-PPh3/H2O2
[Emim]PF6
O
+
+
6
8
5
7
Scheme 1. Epoxidation of alkenes by hydrogen peroxide catalyzed by 1 immobilized in 2.