Efficient epoxidation reaction of terminal olefins with hydrogen peroxide catalyzed by an iron (II) complex

Article abstract of DOI:10.1016/j.tetlet.2018.05.031

A highly active iron (II) complex that catalyzed epoxidation of terminal olefins with hydrogen peroxide was described. The catalytic system displayed excellent catalytic ability for the selective oxidation of terminal olefins to epoxides with high selecti

Full text of DOI:10.1016/j.tetlet.2018.05.031

Accepted Manuscript
Efficient epoxidation reaction of terminal olefins with hydrogen peroxide cat-
alyzed by an iron (II) complex
Lu Li, Hong-Jin Song, Xiang-Guang Meng, Ren-Qiang Yang, Ni Zhang
PII:
S0040-4039(18)30630-0
https://doi.org/10.1016/j.tetlet.2018.05.031
TETL 49978
DOI:
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
5 March 2018
7 May 2018
14 May 2018
Please cite this article as: Li, L., Song, H-J., Meng, X-G., Yang, R-Q., Zhang, N., Efficient epoxidation reaction of
terminal olefins with hydrogen peroxide catalyzed by an iron (II) complex, Tetrahedron Letters (2018), doi: https://
doi.org/10.1016/j.tetlet.2018.05.031
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Efficient epoxidation reaction of terminal olefins with hydrogen
peroxide catalyzed by an iron (II) complex
Lu Li, Hong-Jin Song, Xiang-Guang Meng*, Ren-Qiang Yang, Ni Zhang
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry,
Sichuan University, Chengdu 610064, P. R. China
Abstract：A highly active iron (II) complex that catalyzed epoxidation of terminal
olefins with hydrogen peroxide was described. The catalytic system displayed
excellent catalytic ability for the selective oxidation of terminal olefins to epoxides
with high selectivity (up to 97.8%) in CH CN at 25 °C. The catalytic activity of three
3
similarly structural iron (II) complexes was comparatively studied. The effect of
various auxiliary ligands on epoxidation was investigated in detail.
Keywords: Iron complex; Alkene; Epoxidation; Hydrogen peroxide
1
. Introduction
Epoxide is a very significant intermediate that has numerous applications in organic
1
synthesis and the manufacture of fine chemicals. Many factors including oxidant,
catalyst, solvent and reaction conditions will cause important influence on oxidation
2
3
4
5, 6
7
of olefins. Variety of oxidants such as PhIO, m-CPBA, H O , NaIO , Oxone and
2
2
4
8
,9
molecular oxygen have been reported to use in the oxidation of olefins. Among
these oxidants, H O has attracted much attention in recent years due to its
2
2
environmental friendliness. Recently, various catalysts such as metal complex,
1
0
11,12
13,14
nanometal, metallic oxide, molecular sieve,
hydrotalcite
and heteropoly
1
5
acid have been reported for the epoxidation of olefins by H O . Among these
2
2
catalysts, metal complexes are one of the most efficient catalysts for the epoxidation
16
17-20
21-25
26-30
of olefins, such as rhenium, ruthenium,
iron
and manganese
complexes.
However, some incidental problems of side-reaction, degradation of H O and toxicity
2
2
bring a challenging task for the design of metal complex. Thus, the development of
iron complex should provide us desired advantages of biological compatibility,
*
Corresponding author. Fax: +86 28 85412291; e-mail address: mengxgchem@163.com (X.-G.
Meng).

relatively inactive to H O and less side reaction. In previous studies, we used Cu (II)
2
2
complexes to selectively catalyze olefins to aldehydes, ketones and alcohols,
3
1-32
respectively.
Herein, we prepared a new iron complex and used it to catalyze the
epoxidation of olefins. The catalytic system displayed good activity and selectivity for
epoxidation of terminal olefins.
2
2
. Results and discussion
.1 Fe (II) complexes catalyzed epoxidation of styrene
Figure 1. Iron complexes used in this study
Some metalloporphyrins have been reported as efficient catalysts for oxidations of
3
3
alkenes. Metalloporphyrin is composed of four pyrrole fragments coordinated with a
metal atom. In this work, we prepared an iron (II) complex consisting of four N atoms
coordinated with an iron ion which is similar in structure to iron porphyrin. In order to
explore the effect of the complex, three similar ligands were chosen to coordinate
with Fe (II) to form iron (II) complexes (Figure 1). In the presence of
pyridine-2-carboxylic acid, three iron (II) complexes displayed markedly different
catalytic ability for the oxidation of styrene. From table 1, it can be seen that reaction
did not occur in the absence of iron (II) complex (Table 1, entry 1).This indicates that
the metal complex plays an important role in the catalytic oxidation of olefins. When
2
,2'-bipyridine was employed as ligand, the iron (II) complex showed low catalytic
activity (Table 1, entry 3). Only 8.3% of styrene was oxidized and the product was
benzaldehyde (100%). When symmetrical bioxazole was used as coordination groups,
the iron (II) complex showed moderate catalytic activity (Table 1, entry 4).
Unexpectedly, the heterozygous ligand of 2-(pyridin-2-yl)-4,5-dihydrooxazole
coordinated iron (II) complex exhibited excellent catalytic ability for the epoxidation
of styrene and resulted in 97.5% conversion of styrene and 89.8% selectivity of

1
styrene oxide (Table 1, entry 2). It is possible that for L the atom O on oxazole ring
would induce the electronic effect of atom N, and the appropriate coordination of N
2
+
with Fe would result in the appropriate interaction between Fe complex and
substrates H O and olefin. Therefore the electronic effect of O atom of oxazole ring
2
2
on N atom is an essential factor for the oxidation of styrene by H O .
2
2
Table 1. Fe (II) complexes catalyzed epoxidation of styrene by H O *
2
2
Selectivity (%)
Entry
Iron complex
No
Conversion (%)
a
b
1
**
—
—
—
1
2
FeL
97.5
8.3
89.8
—
8.5
100
20.7
2
3
FeL
3
4
FeL
49.2
76.8
*
Conversion was determined by GC analysis. —represent for no reaction. ** No iron complex.
2
.2 Effect of auxiliary ligands on the catalytic epoxidation of styrene
Many cytochrome P450 enzymes were reported to show good catalytic capacity for
3
4
the epoxidation of olefins. It was confirmed that the control of the catalytic activity
of these heme enzymes, in terms of oxidation activation or oxidation rate, is not only
3
5
due to the iron itself or the tetrapyrrole but also the axial ligand. In this work, we
also found the important influence of the auxiliary ligand (axial ligand) on the
oxidation reaction. Herein, various auxiliary ligands were employed to investigate the
influence on the oxidation of styrene by H O . The results were listed in table 2. It can
2
2
be seen that the reaction was slow in the absence of auxiliary ligand (Table 2, entry 1).
The conversion of styrene was only 28% at reaction 30 min and the sole product was
benzaldehyde. When 2,6-dicarboxy pyridine was used as the auxiliary ligand, the
oxidation was completely inhibited and no reaction was observed (Table 2, entry 3).

Imidazole, 3-carboxy pyridine, 2-aldehyde pyridine, pyridine, and benzoic acid as the
auxiliary ligand gave low conversions and generated a major product of benzaldehyde
(Table 2, entries 4-8). When pyridin-2-ylmethanol, DL-pipecolinic acid, DL-proline,
and glycine were employed as the auxiliary ligand, the conversions of styrene
increased and the major product was styrene oxide (s>76.5%)(Table 2, entries 9-12).
Interestingly, if pyridine-2-carboxylic acid was employed as the auxiliary ligand, a
remarkable increase in conversion of styrene and selectivity of the desired epoxide
was observed (Table 2, entry 2). Further, it can be observed that the carboxylic acid
group at 2-position showed a positive effect on the epoxidation reaction. This
experimental data supported a carboxylic acid assisted mechanism proposed by Que
3
6, 37
and Wang.
Table 2. Effect of various auxiliary ligands on the catalytic epoxidation of styrene using H O *
2
2
Entry
Auxiliary ligand
No
Conversion (%)
28
Selectivity (%)
a
b
1
**
24.1
89.8
—
73.9
2
97.5
—
8.5
—
3
4
21.7
19.5
23.9
25.4
32.5
20.7
30.4
35.7
66.2
78.2
68.4
63.2
5
6
7

8
9
16.4
33.6
70.2
65.4
39.7
20.3
76.5
86.3
83.1
79.6
78.5
21.7
10.8
14.7
18.3
10
11
12
*
Conversion was determined by GC analysis. —represent for no reaction. ** No auxiliary ligand
2
.3 Geometry configuration of Fe (II) complex
Figure 2. Optimized geometry configuration of Fe (II) complex
In order to understand the coordination of Fe with N and/or O on the atom level in
the iron (II) complex, we calculated the geometry configuration of the iron (II)
complex by the density functional theory (DFT) method at the B3LYP/ (6-31G (d) for
C,H,O,N; LANL2DZ for Fe) level using Gaussian 09 program. The six possible
binding forms were illustrated in Figure S1 in supporting information. The optimized
geometry was illustrated in Figure 2. The calculated result was listed in Table 3. The
computed results indicated the coordination form of iron (II) with five N atoms and
one O atom was the lowest energy state (Figure 2), which is lower by 50.92, 112.47,

-1
5
8.84, 113.21, and 170.22 kJ ▪mol than states b, c, d, e, and f (supporting
information, Table S1), respectively. This calculation revealed that O atom on oxazole
group would not participate in coordination with iron (II) owing to the limitation of
high energy barrier. From Figure 2, it can be seen that the iron is coordinated with five
N atoms and one O atom, where four N atoms are originated from pyrox and the other
two atoms from pyridine-2-carboxylic acid, to construct a distorted octahedral
geometry.
Table 3. The calculated bond distances (Å) and bond angles (°) for Fe (II) complex
Fe (II) complex
Bond
Distance / Å
Bonds
Angle / °
79.98
Fe-N17
Fe-N6
1.993
N6-Fe-N17
N23-Fe-N34
N41-Fe-O48
2.061
80.49
Fe-N23
Fe-N34
Fe-N41
Fe-O48
2.047
83.26
2.015
2.007
1.941
2
.4 Effects of solvent and oxidant on the epoxidation of styrene
3
8
Solvent displays crucial influence on many oxidations of olefin. In this work, we
selected styrene as the substrate to carry out the oxidation reaction by using 2
equivalents of H O as oxidant in six different solvents N,N-dimethylformamide
2
2
(
DMF), tetrahydrofuran (THF), acetone, acetonitrile, dichloromethane (CH Cl ) and
2 2
tert-butyl alcohol (t-BuOH), respectively. It can be seen that no reaction could be
detected for the oxidation of styrene when DMF, THF and CH Cl were used as
2
2
solvent (Table 4, entries 4-6). t-BuOH showed relatively good solvent effect (Table 4,
entry 2). Acetonitrile was the best solvent system to afford the desired product styrene
oxide with 89.8% selectivity and 97.5% conversion at reaction 30 min (Table 4, entry
1
).
In order to investigate the effect of various oxidants on the oxidation reaction, five

kinds of oxidants 3-chloroperoxybenzoicacid (m-CPBA), NaIO , the urea hydrogen
4
peroxide adduct (UHP), tert-butyl hydroperoxide (TBHP) and 1,4-benzoquinone (BQ)
were chosen to carry out the oxidation of styrene. It was found that the oxidation of
styrene didn’t occur when BQ, UHP and NaIO were used as oxidant, respectively
4
(
Table 4, entries 9-11). m-CPBA showed good oxidation ability, and cheap H O2
2
showed the best conversion and selectivity of epoxide.
a
Table 4. Effects of solvent and oxidant on the catalytic epoxidation of styrene
Selectivity of epoxide
%)
Entry
Solvent
Oxidant
H O
Conversion (%)
(
1
2
3
4
5
6
7
8
9
CH CN
97.5
53.2
15.2
—
89.8
80.9
—
3
2
2
2
2
2
2
2
t-BuOH
Acetone
DMF
H O
2
H O
2
H O
2
—
THF
H O
2
—
—
CH Cl2
H O
2
—
—
2
CH CN
m-CPBA
TBHP
BQ
89.1
19.7
—
84.5
83
3
CH CN
3
CH CN
—
3
1
0
1
CH CN
UHP
—
—
3
1
CH CN
NaIO4
—
—
3
a
1
Reaction conditions: styrene (0.1 mmol), FeL (5.0 mol %), pyridine-2-carboxylic acid (5.0
mol %), oxidant (2 equiv), CH CN (5 ml), 30 min, 25 °C. Conversion was determined by GC
3
analysis. —represent for no reaction.
1
2
.5 Epoxidation of various olefins catalyzed by complex FeL
Under the optimized conditions, the catalytic oxidations of various substituted
aromatic olefins, aliphatic olefins, and cyclic olefin were investigated. The results
were listed in Table 5. It can be found that the terminal olefins 1a–1h can be oxidized
into the corresponding epoxide product in a relatively short time with good selectivity.
For the aryl-substituted terminal olefins 1a–1f, the substituted group exhibited
different influence on the oxidation reaction. Electron-rich styrene derivatives 1b and

1
c gave the corresponding epoxide product along with faster reaction rate (conversion
100%) and better selectivity (S > 93%)(Table 5, entries 2 and 3). Although
=
electron-deficient styrene derivatives 1e–1f gave lower conversion, the similar
selectivity (S > 84%) could be observed (Table 5, entries 5 and 6). Until now, the
3
9
epoxidation of aliphatic olefins is still a challenging task. Interestingly, this catalytic
system is also suitable for the epoxidation of aliphatic olefins such as allylbenzene (1g)
and 1-octene (1h) (Table 5, entries 7 and 8). As seen in table 5, the conversions of
aryl-substituted terminal olefins were higher than those of aliphatic olefins. Some
internal alkenes trans-4-octene, trans-stilbene and cyclohexene were also employed as
substrates. Unfortunately, the oxidation reaction did not occur (Table 5, entries 9-11).
This is quite different from other reports. Further, we found that the residual amount
of H O was 0.65 equivalent after 30 min reaction of 1c with 2 equivalents of H O in
2
2
2
2
1
the presence of FeL (Table 5, entry 3), which indicates that the prepared iron (II)
1
complex FeL displayed slower decomposition to H O . The property is very different
2
2
from those of Mn complexes which showed obvious activity of the decomposition of
H O . The advantage of effective utilization of H O is significant compared to the
2
2
2
2
40
other metal (Mn, Ru) complexes.
Table 5. Conversion and selectivity of epoxidation of various olefins by H O2
a
2
Entry
Substrate
Product
Conversion (%) Selectivity (%)
97.5
89.8
1
2
3
4
5
100
100
93.5
97.8
87.5
86.2
94.8
80.5

6
7
8
9
74.9
78.5
72.6
—
84.6
86.3
85.5
—
—
—
10
—
—
—
11
—
—
a
1
Reaction conditions: substrate (0.1mmol), FeL (5.0 mol %), pyridine-2-carboxylic acid (5.0
mol %), H O (2 equiv), CH CN (5 ml), 30 min, 25 °C. Conversion was determined by GC
2
2
3
analysis. —represent for no reaction.
3
. Conclusion
1
This study has demonstrated that the FeL complex is an efficient catalyst for
epoxidation of aromatic and aliphatic terminal olefins by H O . By comparing the
2
2
catalytic activity of iron complexes, we found that the electronic effect of the O atom
of the oxazole ring on the N atom is an essential factor for the oxidation of styrene by
H O . The presence of the auxiliary ligand is necessary to obtain efficient catalytic
2
2
activity. Among various auxiliary ligands used in this study, pyridine-2-carboxylic
acid was the most efficient coordination ligand. The quantum chemical calculation
shows that the iron complex cooperates with pyridine-2-carboxylic acid to construct a
distorted octahedral geometry.
Acknowledgements
We gratefully acknowledge financial support from the National Natural Science Foundation of
China (No. 21273156).
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Highlights
1
2
3
. The catalytic system shows high activity and selectivity for
epoxidation of terminal olefins.
. Good yield of epoxide was achieved in short time (30min)
under mild conditions.
. The effect of the auxiliary ligand on epoxidation was
investigated in detail.