Enantioselective epoxidation of olefins catalyzed by two novel chiral poly-salen-Mn(III) complexes

Article abstract of DOI:10.1016/S0040-4039(00)01843-8

Two novel chiral poly-salen-Mn(III) complexes 1 and 2, derived from (R,R)-1,2-diaminocyclohexane and the disalicylaldehydes 5 and 6 were synthesized and employed in the enantioselective epoxidation of olefins. A range of 30-92% ee and 75-97% yield was ach

Full text of DOI:10.1016/S0040-4039(00)01843-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 10267–10271
Enantioselective epoxidation of olefins catalyzed by two
novel chiral poly-salen–Mn(III) complexes
Xiaoquan Yao, Huilin Chen, Weiran L u¨ , Guizhi Pan, Xinquan Hu and Zhuo Zheng*
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
Received 4 July 2000; revised 9 October 2000; accepted 19 October 2000
Abstract
Two novel chiral poly-salen–Mn(III) complexes 1 and 2, derived from (R,R)-1,2-diaminocyclohexane
and the disalicylaldehydes 5 and 6 were synthesized and employed in the enantioselective epoxidation of
olefins. A range of 30–92% ee and 75–97% yield was achieved in NaClO/4-PPNO and m-CPBA/NMO
oxidant systems when substituted styrenes and substituted 2,2-dimethylchromenes were used as substrates.
Furthermore, the poly-salen–Mn(III) complexes could be recovered easily and recycled efficiently several
times by a simple catalysis/separation method. After five reactions, an 82% ee and 78% yield of epoxide
were obtained using 2,2-dimethylchromene as substrate. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: enantioselective epoxidation; unfunctionalized olefins; chiral poly-salen–Mn(III) complexes.
Enantioselective epoxidation of unfunctionalized olefins catalyzed by chiral salen–Mn(III)
1
complexes has proved to be one of the most useful reactions discovered in the last decade. The
classic salen–Mn(III) catalysts show high activities and enantioselectivities for the reaction, but
the separation and recycling of the catalyst are still major problems. The interest in the reaction
has encouraged some authors to develop supported salen–Mn(III) catalysts, including polymer-
2
–5
6–9
bound catalysts and inorganic-supported catalysts, so as to minimize the degradation of the
catalysts allowing recovery of the catalysts and use for a large number of cycles. However, the
utilization of insoluble polymer-bound catalysts solved the separation problem but usually
suffered from lowered catalytic activity and/or stereoselectivity. Herein, we report two new
poly-salen–Mn(III) complexes 1 and 2 (Scheme 1) for the enantioselective epoxidation of
unfunctionalized olefins. Using a simple catalysis/separation method, moderate to good catalytic
activities and enantioselectivities were achieved, and the catalysts could be recycled effectively
several times.
*
Corresponding author. Tel: +86+411-4671991-712; fax: +86+411-4684746; e-mail: zhengz@ms.dicp.ac.cn
0
040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01843-8

10268
Scheme 1. Two novel poly-salen–Mn(III) complexes
In contrast to most reports on organic polymer-bound chiral salen–Mn(III) catalysts in which
the monomer of a chiral salen–Mn(III) complex bearing two vinyl groups is copolymerized with
2
–5
other suitable monomers,
condensation of a slight excess of (R,R)-1,2-diaminocyclohexane with the corresponding disali-
the chiral poly-salen ligands 3 and 4 were obtained by the
1
0
cylaldehydes 5 and 6 in ethanol, and the precipitated Schiff bases were then collected and
washed with water, ethanol and hexanes in sequence to remove the unreacted diamine and the
low molecular weight Schiff bases. The structures of the chiral poly-salen ligands 3 and 4 were
1
13
confirmed by H NMR and C NMR, no free aldehyde proton being observed. The average
molecular weight of the two ligands was around 5400 (M =ꢀ5400, n=ꢀ12) based on VPO
n
analysis. The poly-salen–Mn(III) complexes 1, 2 were synthesized by inserting the manganic ion
11,12
according to literature procedures (Scheme 2).
It was found that the two poly-salen–Mn(III)
complexes, 1 and 2, were very soluble in dichloromethane, but almost insoluble in hexanes and
diethyl ether.
Scheme 2. The synthesis of poly-salen ligands 3 and 4
The epoxidation reactions catalyzed by the poly-salen–Mn(III) complexes 1 and 2 were
carried out in dichloromethane, styrene being used initially as a typical substrate. When sodium
hypochlorite (NaClO) was used as oxidant in an aqueous-oil biphasic system without additive,
the reaction time for total conversion was so long, usually more than 24 hours, that most of the
poly-salen–Mn(III) complexes 1 and 2 were degraded, and the enantioselectivity of styrene oxide
13
formation was low (entries 1 and 2 in Table 1). When 0.2 equivalents of 4-phenylpyridine-N-

10269
Table 1
a
The chiral poly-salen–Mn(III) complexes 1 and 2 catalyzed enantioselective epoxidation with NaClO/4-PPNO
b
and m-CPBA/NMO as oxidants
c
d
Entry
Substrate
Catalyst
Oxidant
Temp. (°C)
Ee (%)
Yield (%)
e
1
2
Styrene 8
1
2
1
2
1
1
2
2
NaClO
NaClO
Rt
Rt
0
25
28
31
36
43
45
41
42
61
64
84
75
96
91
91
89
e
3
4
5
6
7
8
NaClO/4-PPNO
NaClO/4-PPNO
m-CPBA/NMO
m-CPBA/NMO
m-CPBA/NMO
m-CPBA/NMO
0
0
−78
0
−78
9
cis-b-Methyl-styrene 9
1
2
1
2
NaClO/4-PPNO
NaClO/4-PPNO
m-CPBA/NMO
m-CPBA/NMO
0
0
0
0
56
58
70
68
74
69
85
86
1
1
1
0
1
2
13
14
15
16
2,2-Dimethyl-chromene 10
1
2
1
2
NaClO/4-PPNO
NaClO/4-PPNO
m-CPBA/NMO
m-CPBA/NMO
0
0
0
0
91
92
91
91
82
81
87
85
17
6-Cyano-2,2-dimethyl-chromene
1
NaClO/4-PPNO
0
86
95
1
1
1
1
8
9
2
1
NaClO/4-PPNO
NaClO/4-PPNO
0
0
89
85
96
96
6-Nitro-2,2-dimethyl-chromene
1
2
2
2
0
1
2
1
NaClO/4-PPNO
NaClO/4-PPNO
0
0
88
88
97
95
6-Bromo-2,2-dimethyl-chromene
1
3
22
2
NaClO/4-PPNO
0
91
94
a
The enantioselective epoxidation reactions were carried out at 0°C in dichloromethane (2 mL) with 1 mmol of
olefin substrate, using TLC to detect the total conversion, usually in 2–3 h. The mole ratio of substrate/poly-salen–
Mn(III) complex was from 1:0.02 to 1:0.04.
b
The enantioselective epoxidation reactions were carried out in dichloromethane (5 mL) with 1 mmol of olefin
substrate, using TLC to detect the total conversion, usually in 10 min at 0°C and 0.5–1 h at −78°C. The mole ratio
of substrate/poly-salen–Mn(III) complex was 1:0.04.
c
The ee’s for the epoxidation product was determined by GC using a chiral capillary column (cyclodex-b,2,3,6-
methylated, 30 m×0.25 mm (i.d.)).
d
Isolated yield.
e
The mole ratio of substrate/poly-salen–Mn(III) complex was 1:0.05, NaClO used as oxidant only. The total
conversion time was more than 24 hours.
oxide (4-PPNO) was added to the catalytic system as the additive, the reaction was complete in
a period of 2–3 hours. Both the yields and the enantioselectivities for the formation of styrene
14
oxide were obviously improved (entries 3 and 4 versus entries 1 and 2, respectively). Under the
condition of NaClO as the oxidant with 4-PPNO as the additive, ee’s of 56–58% were obtained
with cis-b-methylstyrene as substrate (entries 9 and 10). Promisingly good results with ee’s of
8
2
5–92% and yields of 81–97% for epoxide formation were achieved by using substituted
,2-dimethylchromenes as substrates (entries 13 and 14, 17–22).

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The homogeneous oxidation system of m-chloroperbenzoic acid (m-CPBA) and N-methyl-
morpholine-N-oxide (NMO) in dichloromethane, a highly efficient epoxidation system at low
1
5,16
temperature,
catalyzed enantioselective epoxidation. The results of the reaction are also summarized in Table
. When m-CPBA/NMO was used as the oxidant, the enantioselectivities and activities for the
has also been employed in the chiral poly-salen–Mn(III) complex 1 and 2
1
substituted styrene oxides were better than those obtained using NaClO/4-PPNO. At 0°C, an ee
of 43% was obtained for styrene oxide (entry 5). This result is equivalent to that obtained by
15
using Jacobsen’s salen–Mn(III) catalyst at the same temperature. When the reaction tempera-
ture was lowered to −78°C, the catalysts showed high catalytic efficiency. However, no
significant improvement in ee’s was observed in our experiments at −78°C, in contrast to other
15,16
systems in which the ee’s increased remarkably.
We are also interested in recycling the catalysts as well as in the ee’s of the epoxidation. Based
on the solubility of our poly-salen–Mn(III) complexes 1 and 2 mentioned above, the catalysts
can be recovered easily by using hexanes to precipitate the catalyst from the concentrated
reaction mixture. The solid can be conveniently isolated by filtration, and then dissolved in
dichloromethane for another reaction. Any small amount of catalyst remaining in the filtrate
1
7
can be removed through Celite. Either recovered 1 or 2 could be recycled efficiently both in
NaClO/4-PPNO and m-CPBA/NMO oxidation systems. Table 2 lists the typical results for five
cycles using poly-salen–Mn(III) complex 1 as catalyst with 2,2-dimethylchromene as the
substrate and NaClO/4-PPNO as the oxidant. As can be seen, a 78% yield was obtained in the
fifth reaction with only a small decrease in catalytic activity compared to the fresh catalyst (entry
5
versus entry 1 in Table 2). It was discovered that the enantioselectivities decreased from 91%
ee using the fresh catalyst to 87% ee in the first cycle (entry 1 versus entry 2), then to 83% in
the fourth cycle (entries 2–5). Considering that the poly-salen–Mn(III) complexes fall behind the
Jacobsen’s salen–Mn(III) catalysts in term of activity, especially enantioselectivity, we suspected
that some lower molecular weight catalysts, which could provide better ee’s during the fresh
reaction, were being degraded more easily than the higher molecular weight catalysts, which
were being recycled without obvious degradation, several times.
Table 2
The recycling experiment for the enantioselective epoxidation of 2,2-dimethylchromene using NaClO/4-PPNO as
a
oxidant catalyzed by complex 1
b
c
Entry
Cycle
Time (h)
Ee (%)
Yield (%)
1
2
3
4
5
Fresh
3
4
4.5
4.5
4.5
91
87
85
84
83
82
80
80
79
78
1
2
3
4
a
The enantioselective epoxidation reactions were carried out according to the procedure described in Table 1. After
the total conversion, the organic phase was separated, washed with saturated NaCl solution (2×5 mL) and dried
(
Na SO ). The solution was concentrated and hexane (2 mL) was added to precipitate the catalyst. The solid was
2 4
isolated by filtration and washed with hexane (2 mL), and then dissolved in dichloromethane for another reaction.
b
The ee’s for the epoxidation product were determined by GC using a chiral capillary column (cyclodex-b,2,3,6-
methylated, 30 m×0.25 mm (i.d.)).
c
Isolated yield.

10271
Acknowledgements
This work was supported by the National Science Foundation of China (29933050). One of
the authors, Xiaoquan Yao, thanks Professor Zuwei Xi and Dr. Ning Zhou for helpful
discussions.
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