Remarkable binuclear Schiff-based complex catalyze the epoxidation of alkenes: effects of substituent group

Article abstract of DOI:10.1002/aoc.5608

A series of dimolybdenum metal catalysts with different ligand have been successfully synthesized and used to catalyze the epoxidation of alkenes to generate epoxides by using tert-butyl-hydroperoxide (TBHP) as oxidant. The reaction condition was optimized by the adjustment of some key parameters, such as, temperature, to target high catalytic performance. The oxidation of cyclooctene gave 95.00% conversion and almost 100.00% selectivity. Kinetic study of the oxidation of cyclooctene under different temperature was taken out, indicating the reaction have good catalytic performance. Furthermore, it is calculated from the Arrhenius equation that different functional groups affect the activation energy of the reaction. The electron donor group substituent on the liagnd increases the catalytic activity by reducing the activation energy, and vice versa. Finally, a possible catalytic mechanism has been proposed by measuring the electronic absorption spectrum of the reaction.

Full text of DOI:10.1002/aoc.5608

Received: 2 December 2019
DOI: 10.1002/aoc.5608
Revised: 12 January 2020
Accepted: 25 January 2020
F U L L P A P E R
Remarkable binuclear Schiff-based complex catalyze the
epoxidation of alkenes: effects of substituent group
Yingxiong Guo | Pan Li | Wenhong Zou | Yongle Zhang | Yaoqi Pan |
Renjie Ruan | Longqiang Xiao | Linxi Hou
College of Chemical Engineering, Fuzhou
A series of dimolybdenum metal catalysts with different ligand have been suc-
cessfully synthesized and used to catalyze the epoxidation of alkenes to gener-
ate epoxides by using tert-butyl-hydroperoxide (TBHP) as oxidant. The
reaction condition was optimized by the adjustment of some key parameters,
such as, temperature, to target high catalytic performance. The oxidation of
cyclooctene gave 95.00% conversion and almost 100.00% selectivity. Kinetic
study of the oxidation of cyclooctene under different temperature was taken
out, indicating the reaction have good catalytic performance. Furthermore, it
is calculated from the Arrhenius equation that different functional groups
affect the activation energy of the reaction. The electron donor group substitu-
ent on the liagnd increases the catalytic activity by reducing the activation
energy, and vice versa. Finally, a possible catalytic mechanism has been pro-
posed by measuring the electronic absorption spectrum of the reaction.
University, Xueyuan, Road No. 2, Fuzhou,
50116, China
3
Correspondence
Longqiang Xiao and Linxi Hou, College of
Chemical Engineering, Fuzhou
University, Xueyuan Road No. 2. Fuzhou
350116, China.
Email: xiaolq@fzu.edu.cn; lxhou@fzu.
edu.cn
Funding information
Fuzhou University Testing Fund of
precious apparatus, Grant/Award
Number: 2019T023; National Natural
Science Foundation of China, Grant/
Award Number: 21676057; talent program
of Fuzhou University, Grant/Award
Number: GXRC-18041
K E Y W O R D S
binuclear Schiff-base, Dimolybdenum complex, dynamics, olefin epoxidation catalyst
1
| INTRODUCTION
developed towards the environment friendliness, green
[
8–11]
chemistry.
The catalytic epoxidation of olefins is one
In recent decades, the epoxidation of alkenes has
attracted extensive attention in both academia and indus-
of the effective methods to produce epoxy products
[
12–14]
corresponding epoxy products.
There are two key
[1,2]
try.
Epoxides are essential intermediate products in
parameters for the design of efficient catalyst for olefine
epoxidation: first, the type and charge of a central metal
organic chemistry and important raw materials for indus-
try of fine chemicals, such as petrochemicals and
[
15–17]
ion,
and second, the type of the coordinated ligand
[3–7]
[18]
pharmaceuticals.
and its coordinated center.
In general, epoxides were extensively produced in the
industry by using organic per-acids (for example, chloro
peroxybenzoic acid) to transfer oxygen atom to
alkene. However, these methods need strict reaction
condition and cause environment pollution. Nowadays,
the production of epoxy products is required to be
High oxidized metals are widely used in many cata-
lytic oxidation reactions. Therefore, complexes possess
high oxidation central metals are the most suitable and
[
19]
most popular catalysts in catalytic oxidation.
Among
them, molybdenum-based complexes have attracted a lot
of attention and show the highest catalytic activity and
[
20–22]
selectivity.
Besides, molybdenum complexes offer
significant advantages such as lower cost, environmental
protection and commercialization. One typical example
Yingxiong Guo and Pan Li, contributed equally to this work and should
be regarded as co-first authors.
Appl Organometal Chem. 2020;e5608.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 11
https://doi.org/10.1002/aoc.5608

2
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GUO ET AL.
is that Mo based catalyst can react with tert-butyl-
hydroperoxide to accelerate the break of the O-O bond
resulting in the rapidly transfer the oxygen atom to the
collected with a Nicolet iS50 Fourier transform infrared
(FT-IR) spectrometer and the sample was prepared by
KBr-disk method. H NMR spectra were performed on a
1
[23,24]
olefin to produce epoxides.
Bruker Avance III. The mass spectra were performed on
Agilent LC-QTOF-MS (G6520B) spectrometer. The con-
version and selectivity of the products were calculated
by gas chromatography (GC ShimadzuGC-2010).
The content of the elements in the product were mea-
sured by elemental analysis (Vario EL cube). The results
of X-ray photoelectron spectroscopy were obtained on
ESCALAB 250.
[25,26]
Schiff base ligands
usually contain aromatic
ring, are ideal candidates to create complex since they
possess N and O donor atoms that can coordinate with
[27]
Mo.
Moreover, Schiff base could be modified by intro-
ducing substituents on the aromatic rings to
enhance/stabilize the catalytic action of the Mo complex.
In the previous work, our group have designed and syn-
thesized dimolybdenum Schiff-base complex (DMSBC)
which has high-efficiency catalytic performance and can
[28]
be used for multiple times.
Herein, a series of DMSBC
2.2 | Synthesis of binuclear Schiff-Base
with different substituents are designed and synthesized
to investigate the effect of functional group on the cata-
lytic activity. The electron-donating substituents on the
aromatic rings can increase the catalytic activity of the
catalyst by reducing the activation energy, and vice versa.
ligands (H L)
2
2.2.1 | Synthesis of O,N-bidentate Schiff-
base ligand (H L )
2
1
The reaction of the amino group and the aldehyde group
is the traditional method to prepare Schiff base, which
[
29]
2
2
| MATERIALS AND METHODS
.1 | Synthesis materials
was shown in Scheme 1.
Salicylaldehyde (5 mmol),
and 3,3-diaminobenzidine (1 mmol) were dissolved
in MeOH by ultrasonic shaking (40 ml) for 10 min,
respectively. Then, the methanol solution of
3,3-diaminobenzidine (1 mmol) was slowly dripped into
the methanol solution of salicylaldehyde under nitrogen
The solvents and chemicals used in this work were pur-
chased from in Aladdin. The infrared (IR) spectra are
SCHEME 1 Procedure for the synthesis of a:
binuclear salophen ligand and b dimolybdenum
Schiff-base complex (DMSBC)

GUO ET AL.
3 of 11
ꢀ
1
atmosphere continuous stirring at 80 C for 24 hr. After
the reaction, the crude product was obtained by centrifu-
gation. Then, the product was recrystallized from metha-
S12 HNMR (400 MHz, d -DMSO) = δ 7.77 (s, 4H), 7.72
6
(s, 4H), 7.52 (s, 4H), 7.01 (s, 2H), 6.99–6.99 (m, 2H), 6.96
(s, 2H), 4.34 (s, 4H), 3.32 (s, 37H), 3.17 (d, J = 3.6 Hz, 36H).
C NMR (101 MHz, d -DMSO) = δ 161.18 (s),
160.97–160.54 (m), 136.87 (s), 135.63–135.53 (m),
135.50–135.38 (m), 132.98–132.74 (m), 129.79 (s), 122.77
(s), 119.96 (s), 119.63–119.49 (m), 117.70 (s), 117.58–117.47
(m), 117.22–117.12 (m), 40.04 (dp, J = 42.0, 21.0 Hz). MS
(ESI): m/z = 1138.74 [M + CH COOH]. C H N O
56 66 8 4
13
nol and dried in vacuum (Yield: 90.00%).Fig S1 Fig S4 Fig
S11 H NMR (400 MHz, d -DMSO) δ 12.99 (s, 4H), 9.09 (s,
H), 9.03 (s, 2H), 7.71 (dd, J = 4.7, 2.5 Hz, 4H), 7.69 (d,
J = 1.8 Hz, 2H), 7.63–7.61 (m, 2H), 7.43 (ddd, J = 4.90,
.80, 1.7 Hz, 6H), 6.99 (s, 4H), 6.98 (d, J = 2.9 Hz, 4H).
C NMR (101 MHz, DMSO) δ 164.18 (s), 160.95 (d,
6
1
6
2
2
13
3
J = 3.7 Hz), 143.54 (s), 138.96 (s), 134.00 (s), 133.01 (d,
(1078.73): calcd. Anal calcd for C H N O : C, 80.11; H,
72 94 4 4
J = 13.7 Hz), 126.28 (s), 120.61 (s), 120.06 (s), 119.99 (s),
8.78; N, 5.19. Found: C,78.98; H, 8.57; N, 5.17.
1
19.56 (s), 118.49 (s), 117.18 (s). MS (ESI): m/z = 629.21
[
M-H]. C H N O (630.21): calcd. Anal calcd for
56
66 8 4
C H N O : C, 76.18; H, 4.79; N, 8.88. Found: C,
2.3 | Synthesis of symmetrical
dimolybdenum Schiff-base catalyst
40
30 4 4
7
5.08; H, 4.92; N, 8.82.
(DMSBC)
2
.2.2 | Synthesis of O,N-bidentate Schiff-
2.3.1 | Synthesis of symmetrical
dimolybdenum Schiff-base catalyst
base ligand (H L )
2
2
(
DMSBC-1)
3
3
,5-Dichloro-2-hydroxybenzaldehyde (5 mmol), and
,3-diaminobenzidine (1 mmol) were dissolved in
MoO (acac)2 (2.2 mmol) was subjected to ultrasonic
2
MeOH (50 ml) by ultrasonic shaking for 20 minutes,
respectively. Then, the methanol solution of
shaking in MeOH (40 ml) for 10 min and binuclear
Schiff-base ligand (H L ) (1 mmol) were subjected to
2
1
3
,3-diaminobenzidine (1 mmol) was slowly dripped into
ultrasonic shaking in chloroform (40 ml) for 10 min
the methanol solution of 3,5-dichloro-2-hydroxyben
zaldehyde (5 mmol) under nitrogen atmosphere with
continuous stirring at 80 C for 48 hr. After the reaction,
respectively. Then, MoO (acac)2 was added into H2L1
2
and the mixture was stirred under nitrogen atmosphere
ꢀ
ꢀ
(Scheme 1b) at 80 C for 24 hr. The crimson product was
the crude product was obtained by centrifugation.
Finally, the product was recrystallized from methanol
isolated by removing the solvent under reduced pressure.
Finally, the crimson product was washed by MeOH for
[30]
1
and dried in vacuum (Yield: 74.00%). Fig S2 H NMR
500 MHz, d -DMSO) δ 10.20 (s, 4H), 9.23–9.06 (m, 1H),
several times and dried in vacuum (Yield: 85.00%). Fig S6
1
(
Fig S9 H NMR (400 MHz, d -DMSO) δ 9.09–9.02 (m,
6
6
8
.22 (d, J = 2.3 Hz, 4H), 8.04–7.98 (m, 2H), 7.83–7.82 (m,
4H), 7.71 (dd, J = 4.9, 2.5 Hz, 4H), 7.69 (s, 2H), 7.63–7.60
13
2
H), 7.72 (t, J = 2.7 Hz, 6H). Anal calcd for
(m, 2H), 7.46–7.39 (m, 6H), 7.00–6.97 (m, 8H). C NMR
(101 MHz, d6-DMSO) 164.19 (s), 160.97 (s),
143.64–143.54 (m), 138.93–138.71 (m), 133.99 (s), 133.08
s), 126.67–126.42 (m), 120.60 (s), 120.26–120.15 (m),
20.05 (s), 119.51–119.41 (m), 117.70 (s), 116.95 (s). Anal
Calcd for C H Mo N O : C, 54.44; H, 2.97; N, 6.35.
C H Cl N O : C, 53.01; H, 2.45; N, 6.18. Found: C,
5
δ
40
22
8 4 4
3.78; H, 2.82; N, 6.58.
(
1
2
.2.3 | Synthesis of O,N-bidentate Schiff-
40
26
2 4 8
base ligand (H L )
Found: C, 54.70; H, 3.25; N, 6.60.
2
3
3
3
,5-Di-tert-butylsalicylaldehyde
,3-diaminobenzidine (1 mmol) were dissolved in
(5
mmol),
and
2.3.2 | Synthesis of symmetrical
dimolybdenum Schiff-base catalyst
(DMSBC-2)
MeOH(40 ml) by ultrasonic shaking for 10 min, respec-
tively. Then, the methanol solution of 3,3-diaminob
enzidine (1 mmol) was slowly dripped into the methanol
solution of 3,5-di-tert-butylsalicylaldehyde (5 mmol)
MoO (acac)2 (2.2 mmol) was subjected to ultrasonic
2
under nitrogen atmosphere with continuous stirring at
shaking in MeOH (50 ml) for 10 min and binuclear
Schiff-base ligand (H L ) (1 mmol) were subjected to
ꢀ
8
0 C for 48 hr. The crude product was isolated by remov-
2
2
ing the solvents under reduced pressure. After the reac-
tion, the crude product was obtained by centrifugation.
Finally, the product was recrystallized from acetone and
ultrasonic shaking in N,N-Dimethylformamide (DMF,
50 ml) for 10 min respectively. Then, MoO (acac)
2
2
was added into H L and the mixture was stirred under
2
2
[31]
ꢀ
dried in vacuum (Yield: 76.00%).
Fig S3 Fig S5 Fig
nitrogen atmosphere (Scheme 1b) at 80 C for 24 hr. The

4
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GUO ET AL.
crimson product was isolated by removing the solvent
under reduced pressure. Finally, the crimson product
was washed by MeOH for several times and dried in vac-
125.88–125.35 (m), 120.46–119.69 (m), 119.03–118.61 (m),
118.56–118.14 (m), 35.14 (s), 34.19 (s), 31.47 (s), 29.47 (s).
Anal Calcd for C H Mo N O : C, 64.95; H, 6.81; N,
72
90
2 4 8
1
uum (Yield: 78.00%).Fig S7 H NMR (500 MHz, d6-
4.21. Found: C, 63.25; H, 6.45; N, 4.70.
DMSO) δ 9.19–9.14 (m, 4H), 7.92 (d, J = 2.0 Hz, 2H), 7.82
(d, J = 2.9 Hz, 4H), 7.79 (dd, J = 5.7, 2.7 Hz, 6H), 7.73 (s,
2
1
H). Anal Calcd for C H Mo N O : C, 41.48; H,
.57; N, 4.84. Found: C, 42.41; H, 1.46; N, 4.93.
3 | RESULTS AND DISCUSSION
40
18
2 4 8
3
.1 | The characterizations of H L and
2
catalysts with Fourier-transform infrared
2
.3.3 | Synthesis of symmetrical
dimolybdenum Schiff-base catalyst
DMSBC-3)
The FT-IR spectra of ligands and catalysts were recorded
(samples prepared with KBr pellets). The result were
showed in Figure 1. The spectra of catalysts show pat-
terns similar to those of ligands. Those samples also
showed in-plane ring deformation bands at
(
MoO (acac) (2.2 mmol) was subjected to ultrasonic shak-
ing in MeOH (50 ml) for 10 min and binuclear Schiff-base
ligand (H L ) (1 mmol) was subjected to ultrasonic shak-
ing in chloroform (50 ml) for 10 min, respectively. Then,
MoO (acac) was added into H L and the mixture was
stirred under nitrogen atmosphere (Scheme 1b) at 80 C
for 12 hr. The crimson product was isolated by removing
solvent under reduced pressure. Finally, the crimson
product was washed by cold MeOH for several times and
2
2
−
1
500–700 cm . In addition, catalysts showed strong
absorption bands at 900–920 cm− assigned to symmetric
and asymmetric stretching vibrations of O = Mo = O
indicating that oxygen atom were coordinated to the
2
3
1
2
2
2 3
ꢀ
[
32,33]
molybdenum metal.
The C-N vibration was found at
−
1
−1
1260–1270 cm .The sharp band at 1600–1610 cm
assigned to C=N vibration in the free ligands proving the
1
[34]
dried in vacuum (Yield: 72.00%). Fig S8 Fig S10 H NMR
ammonia-aldehyde condensation occurred.
(
400 MHz, CDCl ) δ 8.75 (dd, J = 10.1, 5.8 Hz, 4H),
3
7
.55–7.50 (m, 2H), 7.46 (d, J = 1.9 Hz, 4H), 7.44 (d,
J = 3.1 Hz, 2H), 7.26 (d, J = 2.8 Hz, 2H), 7.25–7.22 (m,
3.2 | The X-ray photoelectron
spectroscopy (XPS) of catalysts
13
4
H), 1.45 (s, 12H), 1.32 (s, 12H). C NMR (101 MHz,
CDCl ) δ 164.68–164.09 (m), 158.64 (s), 143.72–142.64
3
(
1
m), 140.84–140.12 (m), 139.46–138.93 (m), 137.28 (s),
37.26–137.19 (m), 128.63–127.95 (m), 126.87 (s),
Surface analyses of those catalysts were carried out using
X-ray photoelectron spectroscopy (XPS). As showed in
FIGURE 1 FT-IR spectra of (a) H2L1
(
(
b) DMSBC-1 (c) H2L2 (d) DMSBC-2 (e) H2L3
f) DMSBC-3 (g) DMSBC-2 after six runs

GUO ET AL.
5 of 11
FIGURE 2 XPS survey spectra of (a) DMSBC-1 (b) DMSBC-1-Core level spectra of Mo3d,and (c) DMSBC-1-Core level spectra of O1s
d) DMSBC-2 (e) DMSBC-2-Core level spectra of Mo3d,and (f) DMSBC-2-Core level spectra of O1s(g) DMSBC-3 (h) DMSBC-3-Core level
(
spectra of Mo3d,and (i) DMSBC-3-Core level spectra of O1s
Figure 2, the peaks of 232.45 eV, 284.8 eV, 399.24 eV,
3.3 | Study of the catalytic activity of
catalyst in the epoxidation of cyclooctene
[
35]
5
32.07 eV are attributed to Mo3d, C1s, N1s, O1s,
respectively, while Cl (200.51 eV) is only observed for
DMSBC-2 (Figure 2d). Furthermore, the wide scan spec-
tra which curve-fitting into major two peaks with the
binding energy values of 232.60 and 235.7 eV for the
3.3.1 | The influence of solvent on
catalytic activity of catalysts
Mo3d level, were attributed to Mo 3d5/2 and Mo 3d3/2
,
Exploring the effect of solvents on epoxy reaction, epoxi-
dation of cyclooctene were investigated in N,N-
respectively (Figure 2b, 2e, 2h) and proved that the Mo is
[36,37]
the 6+ oxidation state (Figure 2b, 2e, 2h).
The coor-
dimethylformamide (DMF), acetonitrile (CH CN), etha-
3
dination environment of oxygen was validated by the O
spectrum (Figure 2c, 2f, 2i), in which peaks have been
divided into major two peaks at 532.8 eV (peroxo-oxygen)
nol
(C H OH),
chloroform
(CHCl3)
and
2
5
1,2-dichloroethane (C H Cl ). As can be known from the
2
4
2
results, the highest activity of catalysts was obtained in
C H Cl , attaining more than 90.00% (Table 1, Enter
[38]
and 530.8 eV (Mo = O).
Higher-electron-binding-
2
4
2
energy oxygen (peroxo-oxygen) was attributed to the oxy-
gen in molybdenum oxide.
5, 10, 15). Furthermore, a similar positive effect of sol-
vents such as EtOH and CHCl , which resulting a
3

6
of 11
GUO ET AL.
a
TABLE 1 The influence of solvent on catalytic activity of catalysts (cyclooctene as substrate)
ꢀ
Entry
Catalyst
solvent
T ( C)
80
Time(h)
Conversion(%)
6.67
Selectivity(%)
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
100.00
1
2
3
4
5
6
7
8
9
DMSBC-1
DMSBC-1
DMSBC-1
DMSBC-1
DMSBC-1
DMSBC-2
DMSBC-2
DMSBC-2
DMSBC-2
DMSBC-2
DMSBC-3
DMSBC-3
DMSBC-3
DMSBC-3
DMSBC-3
DMF
5
CH
EtOH
CHCl
Cl
DMF
CH CN
EtOH
CHCl
Cl
DMF
CH CN
EtOH
CHCl
Cl
3
CN
80
5
45.03
66.13
72.86
95.07
7.20
80
5
3
80
5
C
2
H
4
2
80
5
80
12
12
12
12
12
10
10
10
3
3
80
87.00
84.00
88.00
87.50
4.36
80
3
80
10
11
12
13
14
15
C
2
H
4
2
80
80
3
80
45.45
88.73
94.50
90.33
80
3
80
C
2
H
4
2
80
3
a
Reaction conditions: catalyst (0.04 mmol), cyclooctene (10 mmol), solvent (10 ml), TBHP(30 mmol) and reflux. Selectivity and yields were determined by GC,
Selectivity toward the corresponding epoxide.
[
32,40,41]
suitable conversion of the reaction (Table 1, Enter 3, 4,
O-O-tBu has been proposed (Scheme 2).
The elec-
9
, 10, 13, 14). On the contrary, the DMF may interfere
tronic absorption experiment was used to monitor the
reaction process (Figure 3) in C H Cl solution between
catalyst and TBHP causing the detrimental effect of the
reaction, giving less than 7% conversion in 5 hr for
DMSBC-1, 8% conversion in 12 hr for DMSBC-2 and
2
4
2
200–700 nm. The peaks in the range of 230–245 nm could
[
34,42]
be attributed to the ligand absorption peaks.
The
4
1
.36% conversion at 10 hr for DMSBC-3 (Table 1, Enter
, 6, 11). Same results were achieved when the reaction
absorption band at 270–400 nm was attributed to charge
[
43]
transfer from ligand to Mo.
After adding TBHP, the
was taken out in CH CN. (Table 1, Enter 2, 7, 12). The
absorption peak of 270–400 nm decreased sharply. This
phenomenon can be explained that the TBHP interact
with Mo, which weakens the electron transfer of Mo to
ligand. Although the intermediate has not been success-
fully separated, it has been reported that the formation of
Mo (VI) complexes interacts with TBHP to form Mo-O-O-
3
results indicated that the conversion is independent of
the polar and dielectric properties of the solvents. Fur-
thermore, the coordination ability of the solvent with the
catalyst strongly affect the epoxidation. Competitive coor-
dination solvents for example CH CN and DMF, compete
3
[
44]
with TBHP to interact with Mo-active intermediate, lead-
ing to the remarkable lowered conversion of the reaction.
On the other hand, the non-competitive solvents, such as
C H Cl and CHCl , generate quite higher conversion
tBu.
Moreover, there is no obvious change in the elec-
tronic absorption spectrum after adding cyclooctene.
Based on electronic absorption spectroscopy and previous
reports, a possible mechanism has been proposed. In the
first step, TBHP approaches the catalyst and transmits
protons on the terminal oxygen atom of molybdenum,
resulting in tert-butyl peroxide coordinated with molyb-
denum. The next step, cyclooctene, as nucleophilic
reagent, coordinates to metal center and oxygen inserted
into the cyclooctene. Then, the epoxy cyclooctanane was
produced and tert-butylperoxide anion was converted
into the tert-butoxide anion. Finally, tert-butylalcohol
and epoxy cyclooctanane were released and the next
cycle was started. This mechanism can easily explain the
result that the amount of TBHP has a significant effect
on the reaction (Table 2).
2
4
2
3
[32,39]
and selectivity within short time.
3
.3.2 | Proposed mechanism of the
epoxidation of cyclooctene in the presence
of catalyst
The mechanism of olefine epoxidation is still under con-
troversial. However, based on the experimental conclu-
sions and the theoretical reports of others, a possible
reaction mechanism that the addition of TBHP interact
with Mo6+ leading to the production of Mo-OH and Mo-

GUO ET AL.
7 of 11
SCHEME 2 The mechanism of the epoxidation of cyclooctene
3.3.3 | Epoxidation of alkenes with
catalysts
According to the results of the experiment, these catalysts
were suitable for a wide range of cyclic and terminal
alkenes (Table 3). Cyclic alkenes could be oxidized to
mainly the corresponding epoxides with high yield and
selectivity. The oxidation of cyclooctene gave high 95.00%
conversion for DMSBC-1 (Table 3, entry 3), 87.50% con-
version for DMSBC-2 (Table 3, entry 9), 90.33% conver-
sion for DMSBC-3 (Table 3, entry 15), indicating that
these catalysts supply enough energy to break the C=C
bond under the selected conditions. The epoxidation of
norbornene proceeded efficiently and produced
corresponding product quantitatively (Table 3, entry
7, 13, 19). Cyclohexene was moderately transformed into
FIGURE 3 UV–vis spectra of mixtures: (a) DMSBC-3
b) DMSBC-3 + TBHP, (c) DMSBC-3 + TBHP+ cyclooctene
cyclohexene oxide with DMSCB-1, DMSBC-2, and
DMSBC-3 (Table 3, entry 5, 11, 17). On the contrary,
(

8
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GUO ET AL.
a
TABLE 2 The result of the epodxidation of cyclooctene by DMSBC-3 with different amount of TBHP
Entry
Cat.(mmol)
0.04
solvent
TBHP (mM)
Conversion(%)
Selectivity(%)
82.00
1
2
3
4
C
C
C
C
2
2
2
2
H
H
H
H
4
4
4
4
Cl
Cl
Cl
Cl
2
2
2
2
10
20
30
40
42.00
81.00
90.4
0.04
85.00
0.04
100.00
0.04
93.3
100.00
a
Reaction conditions: catalyst (0.04 mmol), 10 mmol cyclooctene, 1,2-dichloroethane (10 ml), time (3 hr) and reflux. Selectivity and yields were determined by
GC, Selectivity toward the corresponding epoxide.
TABLE 3 Epoxidation of different alkenes in the presence of catalyst
ꢀ
Entry
Cat.(mg)
none
Time(h)
Alkenes
cyclooctene
styrene
T ( C)
Conversion(%)
3.90
Selectivity(%)
85.00
1
2
3
4
5
6
7
8
9
10
5
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
DMSBC-1
DMSBC-1
DMSBC-1
DMSBC-1
DMSBC-1
DMSBC-1
DMSBC-2
DMSBC-2
DMSBC-2
DMSBC-2
DMSBC-2
DMSBC-2
DMSBC-3
DMSBC-3
DMSBC-3
DMSBC-3
DMSBC-3
DMSBC-3
45.00
62.00
5
cyclooctene
octylene
95.07
100.00
85.00
5
61.00
5
cyclohexene
hexene
58.82
100.00
60.50
5
69.98
5(12)
12
12
12
8
Norbornene
styrene
57.66(87.14*)
81.98
100.00
36.50
cyclooctene
octylene
87.50
100.00
90.00
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
71.30
cyclohexene
hexene
84.42
100.00
14.00
12
12
10
3
99.30
Norbornene
styrene
93.30
91.00
25.20
40.93
cyclooctene
octylene
90.33
100.00
55.81
10
5
58.50
cyclohexene
hexene
65.50
100.00
61.20
10
10
42.80
Norbornene
99.50
100
a
Reaction conditions: catalyst (0.04 mmol), 10 mmol substrate, 1,2-dichloroethane (10 ml), time (3 hr) and reflux. Selectivity and yields were determined by
GC, Selectivity toward the corresponding epoxide.
terminal alkenes were more difficult to be oxidized, and
shown poor selectivity. DMSCB-1, DMSBC-2, and
DMSBC-3 also catalyzed the epoxidation of hexene, giv-
ing hexene oxide as the main product along with uni-
dentified byproducts. (Table 3, entry 6, 12, 18). The
epoxidation of octylene presented better result than that
of hexane. The octylene transformed into octylene oxide
with almost 60.00% conversion and 90.00% selectivity
indicated that the catalytic activity increased with the
chain length of alkene. However, the epoxidation of sty-
rene showed lowed conversion and selectivity which was
induced by the over-oxidation of styrene oxide to
hyacinthin (Table 3, entry 2, 8, 14).
3.3.4 | Dynamics of the epoxidation of
cyclooctene by catalysts
(Table 3, entry 4, 10, 16). The epoxidation of terminal
olefine indicated that catalysts have the ability to over-
oxidize the terminal double bond. However, the selectiv-
ity of 1-octene was higher than that of 1-hexene, which
To explore the catalytic mechanism and catalytic activity
of catalysts, kinetic analysis is necessary. The initial k at

GUO ET AL.
9 of 11
FIGURE 4 The conversion profiles of cyclooctene obtained at the indicated reaction temperature by using a) DMSBC-1 b) DMSBC-2 c)
DMSBC-3. Arrhenius plot of the epoxidation of cyclooctene with d) DMSBC-1 e) DMSBC-2 f) DMSBC-3
−
1
different temperatures was obtained by the linear fitting
of the time versus conversion curve under the low con-
version. According to the Arrhenius equation of ln k = ln
ligands, such as -diethylamino(95.30 ± 2 kJÁmol and
10 −1 3 −1 [28]
7.90 × 10 mol Ádm Ás ),
−tert-butyl et al, could
increase the catalytic activity of the catalyst by reducing
the activation energy and increasing the collision fre-
quency of the reaction. As shown in the result, 92.00%
conversion and almost 100.00% selectivity within 1 hr
was obtained for DMSBC. 90.33% conversion and almost
100.00% selectivity within 3 hr was obtained for DMSBC-
3. The electron-withdrawing group, such as -Cl, reduced
the catalytic activity of catalyst, leading to the highest
conversion rate. The result indicated that the substituent
group of electron donor group increased the catalytic
activity of the catalyst by reducing the activation energy,
and vice versa. However, the stability of the catalyst will
be increased when the substitution group belongs to
reduce the activity of the catalyst.
A − E /RT, we plotted the relationship between ln k and
a
45]
[
1
/T.
According to the results of Figure 4, ln k and 1/T
have excellent linear relationship. These catalyst medi-
ated olefin epoxidation reactions, which have a typical
Arrhenius dependency. Further, the apparent activation
energy (E ) can be calculated by the Arrhenius equation
a
where R (8.134 J/KÁmol) is the molar gas constant, A is
pre-exponential factor and T is the reaction temperature.
Apparent activation energies determined from Arrhenius
dependences (Figure 4) are close, 105.15 and
1
14.40 kJ/mol for DMSBC-1 and DMSBC-2, respectively.
The Apparent activation energies of DMSBC-3
68.78 kJ/mol) was lower than that of DMSBC-1 and
(
DMSBC-2, which can be explained by the change of func-
tional groups on ligands. The -tBu group increased the
electron cloud density of ligands leading to the increase
of the catalytic activity of the catalyst. Cl atom reduced
the electron cloud density of ligands, leading to the
decrease of the catalytic activity of the catalyst. Conclu-
sion was drawn from the results of Table 3 and Figure 4
that the activity of ligands has a significant effect on the
catalytic ability of those catalysts. The catalytic activity of
the catalyst varies with the change of the substituted
group connected to the benzene ring(−diethylamino>
3.4 | Recycling experiments of DMSBC-2
for epoxidation of cyclooctene under
optimized conditions
Interestingly, DMSBC-2 was a heterogeneous catalyst. In
order to evaluate the stability and reusability of DMSBC-
2, this catalytic activity of the recovered catalyst was
investigated. The epoxidation of cyclooctene was used as
model substrate under optimum conditions and the
results was shown in Figure 5. After six successive runs
−
tert-butyl>-H > -Cl). Substituent group of activated

10 of 11
GUO ET AL.
FIGURE 5 Reusability in the epoxidation
of cyclooctene with TBHP and DMSBC-2
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