Asymmetric epoxidation of unfunctionalized olefins with C2-symmetrical diphenol-derived axially coordinated homogeneous chiral bi-Mn(III) salen complexes

Article abstract of DOI:10.1016/j.inoche.2019.01.010

A novel type of C₂-symmetrical diphenol-derived and axially coordinated homogeneous chiral bi-Mn(III) salen complexes are synthesized and their catalytic effects in asymmetric epoxidation of unfunctionalized olefins are investigated in details. The results show that excellent enantioselectivities and high activities are achieved (enantioselectivities up to >99% in <10 min, and conversions up to >99.9%) in the absence of expensive NMO. Compared with Jacobsen's catalyst, the configuration of C₂-symmetrical homogeneous chiral bi-Mn(III) salen complex contribute to the catalytic reactivity and stability. Furthermore, these new homogeneous catalysts could be easily recovered and reused for 5 cycles without significant loss of their properties.

Full text of DOI:10.1016/j.inoche.2019.01.010

Accepted Manuscript
Asymmetric epoxidation of unfunctionalized olefins with
C2-symmetrical
diphenol-derived
axially
coordinated
homogeneous chiral bi-Mn(III) salen complexes
Tianyi Dong, Xiaoju Wu, Jiali Cai, Jing Huang
PII:
S1387-7003(18)31128-6
DOI:
Reference:
https://doi.org/10.1016/j.inoche.2019.01.010
INOCHE 7226
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
21 December 2018
4 January 2019
8 January 2019
Please cite this article as: Tianyi Dong, Xiaoju Wu, Jiali Cai, Jing Huang , Asymmetric
epoxidation of unfunctionalized olefins with C2-symmetrical diphenol-derived axially
coordinated homogeneous chiral bi-Mn(III) salen complexes. Inoche (2018),
https://doi.org/10.1016/j.inoche.2019.01.010
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ACCEPTED MANUSCRIPT
Asymmetric Epoxidation of unfunctionalized olefins with
C2-Symmetrical Diphenol-derived Axially Coordinated
Homogeneous Chiral Bi-Mn(III) Salen Complexes
Tianyi Dongb,
Xiaoju Wuc,
Jiali Cai*,d, Jing Huang*,a
a College of biotechnology, Southwest University, Chongqing 400715, PR China.
b Westa College, Southwest University, Chongqing 400715, PR China.
c College of Chemistry and Chemical Engineering, Southwest University, Chongqing 400715, PR
China.
d College of Rongchang, Southwest University, Chongqing 402460, PR China.
E-mail:hj41012@163.com
Abstract: A novel type of C2-symmetrical diphenol-derived and axially coordinated homogeneous
chiral bi-Mn(III) salen complexes are synthesized and their catalytic effects in asymmetric
epoxidation of unfunctionalized olefins are investigated in details. The results show that excellent
enantioselectivities and high activities are achieved (enantioselectivities up to >99% in less than
10 min, and conversions up to >99.9%) in the absence of expensive NMO. Compared with
Jacobsen’s catalyst, the configuration of C2-symmetrical homogeneous chiral bi-Mn(III) salen
complex contribute to the catalytic reactivity and stability. Furthermore, these new homogeneous
catalysts could be easily recovered and reused for 5 cycles without significant loss of their
properties.
Keywords: Bi-Mn(III) salen Complexe; O-coordinating axial bases; Jacobsen's catalyst; Recycle;
Asymmetric epoxidation

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Introduction
Asymmetric epoxidation of unfunctionalized olefins holds a considerable interest in organic
synthesis because the resulting enantiomerically pure epoxides are versatile intermediates in the
preparation of several classes of compounds [1-5]. For industrial purposes, the chiral Mn(III) salen
complexes are preferred since manganese itself is a relatively nontoxic metal, and manganese
complexes are superior to iron complexes for the selective epoxidation of olefins by virtue of
fewer side reactions over manganese complexes[6]. Homogeneous catalysis often provides the
superior results in achieving high levels of enantioselectivity, but the recycle of the expensive
homogeneous catalysts is so difficult that may limit the practical applications of chiral Mn(III)
salen catalysts in both synthetic chemistry and industrial processes[7]. However, heterogeneous
catalyst could offer the advantages of simplified product purification and the potential for
recycling the catalyst. For the purpose of recycling, a number of supported catalysts with big
molecular weights have been synthesized and studied. Among them, the type of Jacobsen’s
complexes are immobilized onto different types of supports such as MCM-41, SBA-15, ZPS-IPPA,
ZPS-PVPA, ZSPP [8-11]. Unfortunately, some of the heterogeneous complexes are generally
confronted with intricate synthesis procedure, high costs, poor activity, lower conversion, poor
recyclability, as well as leaching of the active species [12]. In fact, precipitating the catalyst by
means of solvent is still the most convenient and economical method for both product isolation
and catalyst recovery. A new recyclable oligomeric or dimeric homochiral Mn(III) salen complex
are synthesized and empolyed as catalysts by Janssen [13], Yao[14], Song[15], Kureshy [16], and
Tang [17] for asymmetric epoxidation of unfunctionalized olefins and superior properties have
also been achieved. Furthermore, these oligomeric or dimeric homochiral Mn(III) salen catalysts
indicate different solubilities in different solvents, which provide a reasonable explanation for its
easy recovery and reuse several times without any loss of enantioselectivity.
It is also interesting to note that axial bases play an important role in asymmetric epoxidation
reaction. It is reported that the use of excess and expensive axial bases could stabilize the active
intermediate species Mn(V)-oxo, such as 4-phenyl pyridine N-oxide (PPNO)[18], 4-phenylpropyl

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pyridine N-oxide (PPPNO) [19], methylmorphline-Noxide (NMO)[20] and imidazole compounds
[21]. On account of the axial bases in the epoxidation systems catalyzed by Mn(III) salen, the
presence of NMO in the m-CPBA oxidant system contributes to high enantioselectivity and rate
acceleration. And then, NMO is expensive and degraded gradually during the epoxidation
procedure [20]. However, chiral Jacobsen’s catalyst immobilized on diphenol-modified zirconium
poly (syrene-phenylvinylphosphonate)-phosphate (ZPS-PVPA) indicates superior conversions and
ee values in the absence of NMO for asymmetric epoxidation of unfunctionalized olefins [22]. We
have deduced that the phenomenon of NMO is ascribed to not the support ZPS-PVPA, but the
diphenol axial coordinating group. Moreover, List’s group have also revealed that adding NMO as
the axial base to alkene epoxidation systems with Mn(III) salen phosphate complexes as catalysts
could lead to a dramatic decrease of enantioselectivity and reactivity[23]. The Mn (III) salen
phosphate complexes could be formed quickly when the corresponding phosphoric acids and the
Mn(III) salen chloride complex are mixed together in the presence of NaOH. Unfortunately, these
catalysts not only couldn’t be recycled, but also could not apply to acidic oxidant systems.
With the purpose of having clearer insight into the performance of bi-catalytic center, we
have developed a novel type of C2-symmetrical diphenol-derived axially coordinated
homogeneous chiral bi-Mn(III) salen complexes under mild conditions. The catalytic properties of
the catalysts and the reusability as well as the factors such as the amount of catalyst, the reaction
time, are also investigated in details.
Scheme 1
The C2-symmetrical diphenol-derived homogeneous chiral bi-Mn (III) salen catalysts (3a-d) are
characterized by FT-IR, DR UV-Vis. The amount of grafted chiral Jacobsen’s catalyst is 1.109,
1.106, 1.109, 1.090 mmol/g, respectively by means of atomic absorption spectroscopy (AAS).
This confirms that 87.4, 87.2, 95.7, 95.3% of the total diphenol-derived is axially coordinated by
chiral Jacobsen's catalyst, respectively.
On account of UV-Vis spectra, the absorption spectrum of homogeneous Jacobsen’s catalyst
indicates a broad peak near 440 nm, due to the d-d transition of Mn(III). Whereas, chiral bi-Mn(III)
salen catalysts (3a-d) show a peak near 415 nm, which confirms the successful grafting of the
Jacobsen’s catalyst (Fig. S1).

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In view of IR spectra (Fig. S2) in the regions of (4000-400 cm-1), the appearance of a strong
band at 1631 cm-1 owing to stretch of C=N (azomethine), and a medium intensity band due to the
band of Mn–N at 515-562 cm-1 confirms the grafting of the Jacobsen’s catalyst on the
diphenol-derived compounds successfully.
Table 1
The catalytic activity and selectivity of catalysts 3a-d are explored for asymmetric epoxidation
of unfunctionalized olefins using m-CPBA as an oxidant system and Jacobsen’s catalyst as the
reference (Table 1). All reactions proceeds smoothly, and the bi-Mn(III) salen catalysts 3a-d
indicate higher ee values than Jacobsen's catalyst (indene, up to 99.7% vs 65, entry 6 vs 1;
α-methylstyrene, up to 97.4% vs 52, entry 15 vs 10) except styrene.
It is also observed that the enantioselectivities increase with the lengths of diphenol-deriveds
axial coordinating group (indene, 93.6 vs 99.7%, entry 2 vs 6; α-methylstyrene, 94.5 vs 97.4%,
entry 11 vs 15; styrene, 29.5 vs 32.1, entry 20 vs 24), which is similar to Li’s group [24]. The
reason might be that olefins toward the Mn-oxo center more expediently with the increase of
linkage. While for the catalyst 3d, there is a slight decrease in ee values (entries 8, 17, 26), owing
to the steric hindrance of 3d which could lead to a twist against each other, shielding and
interfering with their active catalytic centers and further lowering their catalytic performance. In
addition, the performance of the catalyst 3b is poorer than the catalyst 3a, which is due to an
increase of steric hindrance and a weaker electron-donating properties from diphenol-deriveds
axial coordinating group 2b.
Another noteworthy activities of our catalysts is that superior performance also could be
obtained in the absence of additional O-coordinating axial base NMO. In fact, adding such a
ligand NMO to our reaction system could result in a dramatic decrease of enantioselectivity, for
instance indene (ee%: 99.7 vs 54.4; entry 6 vs 7); α-methylstyrene (ee%: 97.4 vs 79.5, entry 15 vs
16) and styrene (ee%, 32.1 vs 27.5, entry 24 vs 25). In our previous work, we have deduced that
the special phenomenon is ascribed to the synergistic effect of organic-inorganic hybrid support
ZPS-PVPA and the phenoxide axial coordinating group [22]. But in this context, this research has
verified that these phenomenon still exist whether diphenol-derived axial coordinating group is

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immobilized onto ZPS-PVPA or not. In a sense, the role of diphenol-derived group in this paper is
similar to that of O-coordinating axial bases such as NMO, owing to similar properties in
electronic structure and steric configuration. The addition of NMO may contribute to the
competition with diphenol-derived axial coordinating group, and further refraining from the
formation of transition-state or intermediates with well-defined configuration. When NMO is
added to our reaction system, chiral recognition of catalysts 3a-d may decrease significantly.
Further studies concerning the mechanism of NMO phenomenon for the C2-bi-Mn (III) salen
complex are currently in progress.
Fig. 1
Fig. 2
We also have explored the relationship of catalytic performance and different time with the
catalyst 3c as the representative (Fig. 1-2). In practice, it is found that most of the catalytic
reactions have been carried out rapidly and indicate superior performance (ee%, 99.2; conv%,
96.5; Fig. 1-2) in less than 10 min. However, the similar result for Jacobsen’s catalyst would last
more than 25 min. On account of the results, it could be deduced that the type of C2- bi-Mn(III)
salen complexes show superior activity and that the two metal active centers of the catalysts are
not working in isolation but indicate synergetic effects.
It is interesting that the diphenol-derived compounds 2a-d are slightly soluble in chloroform
and almost insoluble in other organic solvent. After assembling with the Jacobsen’s catalyst, the
diphenol-derived chiral bi-Mn(III) salen complexes 3a-d are completely dissolved in
dichloromethane but almost insoluble in hexane, cyclohexane, and diethyl ether. In view of these
results, it could be deduced that the catalyst could be easily precipitated and separated from the
reaction system by means of the simple usage of solvents.
Furthermore, the recovered catalyst could be recycled for catalytic reactions without further
purification. The results after recycling with indene as the substrate are shown in Table 2.
Table 2

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After five cycles, the catalyst 3c still indicate favorable reusability (ee%, 83.2; yield%, 90.5),
which confirms the good stability. The activity of the catalyst gradually decrease upon
successively recycling, which indicates some degradation of the catalyst 3c under the conditions
for epoxidation.
In conclusion, a novel type of C2-symmetrical diphenol-derived chiral bi-Mn(III) salen
complexeshave been synthesized and indicate superior properties (ee%, up to >99; conv%, >99.9)
in the absence of NMO in asymmetric epoxidation of unfunctionalized olefins. Moreover, the
performance of the catalyst 3c is so superior that asymmetric epoxidation reactions could be
terminated in less than 10 min. In addition, the catalyst could be conveniently separated from the
reaction system by simple precipitation and be recycled five times without significant loss of
activity. The novel phenomenon of NMO is remarkable, which could prompt the study on the
mechanism and pave the way for the potential application in industry.
Acknowledgements
This work was financially supported by the Fundamental Research Funds for the Central
Universities (XDJK2013C120), the Xihua University Key Projects (Z1223321), Sichuan Province
Applied Basic Research Projects (2013JY0090), the Department of Education of Sichuan Province
Projects (13ZB0030), Chongqing Special Funding for Postdoctoral (Xm2014028), Chongqing
Postdoctoral Daily Fund (Rc201419), Scientific and Technological Research Program of
Chongqing Municipal Education Commission (KJ1501127) and the Ministry of Education
Chunhui project (Z2016161).
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Figures captions:
Scheme 1 Synthesis of C2-symmetrical diphenol-derived axially coordinated chiral bi-Mn(III)
salen catalysts(1a–d)
Fig 1 The enantioselectivity versus reaction time plot for epoxidation of indene catalysed by 1c
with m-CPBA as oxidant system in CH2Cl2 at 0℃
Fig 2 The conversion versus reaction time plot for epoxidation of indene catalysed by 1c with
m-CPBA as oxidant system in CH2Cl2 at 0℃
Table captions:
Table 1 Enantioselective epoxidation of alkenes catalyzed by 1a–1d with m-CPBA as oxidant
Table 2 Enantioselective epoxidation of indene with recycled catalysts 1c with m-CPBA as oxidant

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Table 1. Enantioselective epoxidation of alkenes catalyzed by 3a–3d with m-CPBA as oxidant.[a]
Entry
Substrate
Catalyst
Oxidant
Time(h)
Temp[℃]
ee(%)b
65.0[d]
93.6[d]
35.3[d]
83.8[d]
21.9[d]
99.7[d]
54.4[d]
98.0[d]
35.4[d]
conv(%)c
99.9
1
2
3
4
5
6
7
8
9
Jacobsen m-CPBA/NMO
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
3a
3a
3b
3b
3c
3c
3d
3d
m-CPBA
94.0
m-CPBA/NMO
m-CPBA
96.9
96.7
m-CPBA/NMO
m-CPBA
97.6
96.8
m-CPBA/NMO
m-CPBA
97.3
99.9
m-CPBA/NMO
97.9
10
11
12
13
14
15
16
17
18
Jacobsen m-CPBA/NMO
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
52 .0[e]
94.5[e]
48.0[e]
78.1[e]
40.1[e]
97.4[e]
79.5[e]
96.4[e]
48.5[e]
99.9
99.9
96.0
99.9
99.9
>99.9
91.4
99.9
92.3
3a
3a
3b
3b
3c
3c
3d
3d
m-CPBA
m-CPBA/NMO
m-CPBA
m-CPBA/NMO
m-CPBA
m-CPBA/NMO
m-CPBA
m-CPBA/NMO
19
20
21
22
23
24
25
26
27
Jacobsen m-CPBA/NMO
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
47.0[f]
29.5[f]
15.3[f]
11.7[f]
10.7[f]
32.1[f]
27.5[f]
30.8[f]
21.4[f]
99.9
3a
3a
3b
3b
3c
3c
3d
3d
m-CPBA
>99.9
>99.9
>99.9
97.2
m-CPBA/NMO
m-CPBA
m-CPBA/NMO
m-CPBA
99.4
m-CPBA/NMO
m-CPBA
99.4
>99.9
97.1
m-CPBA/NMO
[a]
Reactions were carried out at desired temperature in CH2Cl2 (3 mL) with alkene (0.5 mmol), NMO (337.5 mg,
2.5 mmol, if necessary), nonane (internal standard, 56 µL, 0.5 mmol) and catalysts (0.025 mmol, 5.0 mol%).
[b]
Conversions were determined by GC, by integration of product peaks against an internal quantitative standard
(nonane), correcting for response factors.
[c] Determined by GC with a chiral capillary column (HP19091G-B233, 30 m×25µm×0.25µm).
[d] Expoxide configuration R.
[e] Epoxide configuration R.
[f] Epoxide configuration 1R, 2S.

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Table 2. Enantioselective epoxidation of indene with recycled catalysts 1ca
Run
fresh
99.9
97.1
0.5
2
3
4
5
eeb (%)
98.7
95.2
0.5
92.9
96.1
1
90.6
95.1
1.5
83.2
90.5
3
Conversionc (%)
Time (h)
a Reactions were carried out at 0 ℃in CH2Cl2 (3 mL) with indene (1 mmol), n-nonane (internal standard, 1 mmol),
m-CPBA (1.6 mmol), the catalyst 1c (5 mol%). The conversions and the ee values were determined by GC with
chiral capillary columns HP19091G-B213, 30 m×0.25 µm×0.25 µm.
b Same as in Table 1.
c Same as in Table 1.

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Highlights
Asymmetric Epoxidation of unfunctionalized olefins with
C2-Symmetrical Diphenol-derived Axially Coordinated Homogeneous
Chiral Bi-Mn(III) Salen Complexes
Tianyi Dong
Xiaoju Wu
Jiali Cai*, Jing Huang*


A novel type of C2-symmetrical diphenol-derived and axially coordinated homogeneous
chiral bi-Mn(III) salen complexes are synthesized
Excellent enantioselectivities and high activities are achieved (enantioselectivities up
to >99% in less than 10 min, and conversions up to >99.9%) in the absence of expensive
NMO

These new homogeneous catalysts could be easily recovered and reused for 5 cycles
without significant loss of their properties.

Figure 1

Figure 2