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of 14
MIRZAEE ET AL.
that can precede the epoxidation of cyclooctene in
C H Cl . Other oxygen sources such as hydrogen perox-
(0.065 mmol/g). In addition, the lower reactivity of
Fe@B‐N‐V than Fe@B‐N‐Mo, in contrast to its higher
metal loading, shows a more catalyst activity of the
molybdenum complexes in the title reaction. This was
also confirmed by the previous reports that the molybde-
num complexes were more active than the vanadium
2
4
2
ide and urea‐hydrogen peroxide in C H Cl , CCl or
2
4
2
4
CH CN do not show any activity (Table 1, entries 44–
3
4
9, 53–58, and 62–67). We also used sodium periodate
as the oxidant but even in the presence of 0.01 g of
tetrabutylamonium bromide as the phase‐transfer
reagent, it did not show any reactivity (Table 1, entries
[
4,6]
complexes.
Moreover, the heterogenic character of these catalysts
was checked by the hot filtration test. For this purpose,
each catalyst was removed using a magnet in the middle
of the optimized process, and the procedure was contin-
ued to reach an optimum time of reaction completion.
The results obtained showed that when the catalyst was
removed from the reaction medium, the catalytic proce-
dure stopped and did not continue anymore. This test
proves that the catalytic reactions are held on heteroge-
neous conditions, and the catalysts are not leached to
the reaction medium.
5
0–52, 59–61, and 68–70). In addition, the epoxidation
of cyclooctene by TBHP did not proceed in the absence
of a catalyst. The detailed procedure for the reaction con-
ditions are summarized in Table 1.
The solvent was the first parameter that was investi-
gated, and some solvents with different coordinating abil-
ities were used in the epoxidation of cyclooctene with
TBHP. The observed trend in Table 1 (entries 1–7, 15–
2
1, and 29–35) is the same as the literature reports on
homogeneous alkene epoxidation by the Mo (CO) cata-
6
[13]
lyst.
The coordinating solvents such as CH CN and
We also investigated the reaction progress of the
related homogenous catalysts. The results obtained
showed that by using VO (SO ) or Mo (CO) as the cata-
3
CH OH compete with TBHP to occupy the coordination
3
sites on a transition metal catalyst. Therefore, in the pres-
ence of these solvents, the observed yields are too low.
4
6
lyst in the same conditions of heterogeneous Fe@B‐N‐
Mo, Fe@B‐N‐V, and Fe@B‐Im‐V counterparts, the yields
reduced to 85, 78, and 87%, respectively. The greater reac-
tivity of the heterogeneous catalysts may be related to the
homogenous dispersion of catalytic active sites onto the
surface of Fe@B. This efficiently increased the effective
collision of regents to the metal sites in comparison
with the homogeneous counterparts, and consequently
increased the catalytic properties of the cyclooctene
epoxidation process.
Among other solvents, CCl and 1,2‐dichloroethane with
4
9
1–96% yield have the highest yield of epoxide product
for all catalysts. This could be attributed to their higher
boiling points in comparison with others. However,
because of the economic and environmental concerns,
1
,2‐dichloroethane was chosen as the optimum solvent
for the epoxidation of cyclooctene with TBHP.
As another parameter, the amount of catalyst was
changed in the title reaction. The results in Table 1
(
entries 7–10, 21–24, and 35–38) show that increasing
These new catalysts (Fe@B‐N‐Mo, Fe@B‐N‐V, and
Fe@B‐Im‐V) can also be used for the epoxidation
of a wide range of substituted alkenes (Table 2). Blank
experiments, performed without catalysts or with
Fe O , boehmite, Fe@B, Fe@B‐N, Fe@B‐Cl or Fe@B‐
the amount of different catalysts up to 20 mg increases
the yield of cyclooctene epoxide and then becomes
constant.
The ratio of oxidant to substrate is one of the most
crucial parameters involved in the catalytic epoxidation
processes. The results in Table 1 show that the optimum
ratio of oxidant to substrate is 2 for Fe@B‐N‐Mo, Fe@B‐
N‐V, and Fe@B‐Im‐V (entries 7, 11–14, 21, 25–28, 35,
and 39–42). Decreasing the ratio from 2, decreased the
yields considerably but its increase did not change the
yield.
Figure 6 shows the effect of reaction time on epoxida-
tion of cyclooctene with TBHP. Comparing the reactiv-
ities of the catalysts revealed that Fe@B‐N‐V had the
lowest activity and required 4.5 hr to give an 89% yield
but Fe@B‐Im‐V had the highest activity and reached a
3
4
Im, showed a very low conversion even after five h reflux
but in the presence of these catalysts, a wide range
of both the cyclic and linear alkenes could efficiently
and selectively be convert to epoxides. Based on the
[
4–7]
epoxidation mechanism suggested earlier,
the higher
electron‐donating ability of olefin double bond is
expected to show a more epoxidation reactivity. There-
fore, cyclooctene and cyclohexene with inner double
bonds should exhibit more activities in comparison with
1‐octene and 1‐hexene, which contain terminal double
bonds. In addition, cyclooctene is more reactive than
cyclohexene due to some cyclooctene conformations that
favor the formation of reaction intermediates that
9
5% yield in only 45 min. Fe@B‐N‐Mo stands in the
[
7]
middle and gave a 95% yield in 2.5 hr. These significant
differences could be attributed to the higher metal
loading on Fe@B‐Im‐V (0.56 mmol/g) in comparison
with Fe@B‐N‐V (0.101 mmol/g) and Fe@B‐N‐Mo
decrease the activation energy. In contrast, 1‐octene is
oxidized slower than 1‐hexene, and this observation
could be attributed to the more steric hindrance of
the hexyl group connected to the double‐bond present
Mirzaee, Mahdi
