Novel bis-Salen Mn(III) chiral complexes of rigid structure axially coordinated with bis-diphenolate and bis-diamine for epoxidation of unfunctionalized olefins

Article abstract of DOI:10.1016/j.tetlet.2013.05.087

A series of bis-Salen Mn(III) chiral complexes of rigid structure axially coordinated with bis-diphenolate and bis-diamine ligands were synthesized and their catalytic performances in asymmetric epoxidation of indene and α-methylstyrene were investigated in detail. Compared with Jacobsen's catalyst and the corresponding monomer salen Mn(III) catalyst, bis-Salen Mn complex showed higher activity and enantioselectivity under the same reaction conditions. A point worth emphasizing was that the bis-diphenolate axially coordinated bis-Salen Mn catalysts afforded remarkable increases of ee values in the absence of axial base NMO for the asymmetric epoxidation of olefins, especially for the epoxidation of indene (ee: from 55% to 100%), however, the axial base is indispensable for the bis-diamine axially coordinated bis-Salen Mn(III) catalysts, these performance might attribute to the rigid difference of the structure. Furthermore, the catalyst could be conveniently recovered and reused at least five times without significant losses of both activity and enantioselectivity. Specially, it could also be efficiently used in large-scale reactions with superior catalytic disposition being maintained at the same level, which possessed the potentiality for application in industry.

Full text of DOI:10.1016/j.tetlet.2013.05.087

Tetrahedron Letters 54 (2013) 4041–4044
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Novel bis-Salen Mn(III) chiral complexes of rigid structure axially
coordinated with bis-diphenolate and bis-diamine for epoxidation
of unfunctionalized olefins
⇑
Xuemei Huang, Xiangkai Fu , Xiaoju Wu, Ziyong Jia
College of Chemistry and Chemical Engineering Southwest University, Research Institute of Applied Chemistry Southwest University, The Key Laboratory of Applied Chemistry of
Chongqing Municipality, The Key Laboratory of Eco-environments in Three Gorges Reservoir Region Ministry of Education, Chongqing 400715, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of bis-Salen Mn(III) chiral complexes of rigid structure axially coordinated with bis-diphenolate
and bis-diamine ligands were synthesized and their catalytic performances in asymmetric epoxidation of
indene and a-methylstyrene were investigated in detail. Compared with Jacobsen’s catalyst and the cor-
Received 7 December 2012
Revised 13 May 2013
Accepted 20 May 2013
Available online 24 May 2013
responding monomer salen Mn(III) catalyst, bis-Salen Mn complex showed higher activity and enantiose-
lectivity under the same reaction conditions. A point worth emphasizing was that the bis-diphenolate
axially coordinated bis-Salen Mn catalysts afforded remarkable increases of ee values in the absence of
axial base NMO for the asymmetric epoxidation of olefins, especially for the epoxidation of indene (ee:
from 55% to 100%), however, the axial base is indispensable for the bis-diamine axially coordinated
bis-Salen Mn(III) catalysts, these performance might attribute to the rigid difference of the structure. Fur-
thermore, the catalyst could be conveniently recovered and reused at least five times without significant
losses of both activity and enantioselectivity. Specially, it could also be efficiently used in large-scale
reactions with superior catalytic disposition being maintained at the same level, which possessed the
potentiality for application in industry.
Keywords:
Bis-Salen Mn(III) chiral complex
O-coordinating axial base
Rigid structure
Asymmetric epoxidation
Recycle
Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
Introduction
development of a novel class of catalyst with high activity, enanti-
oselectivity, and easily separation from the reaction mixture is
Asymmetric epoxidation of unfunctional olefins catalyzed by
chiral salen Mn(III) complex has proved to be one of the most
useful reactions in organic synthesis.1,2 Great success has been
achieved for the asymmetric epoxidation using monomeric Jacob-
sen chiral salen Mn(III) complex as catalysts.3–7 However, a major
problem associated with the monomeric catalyst system is the
separation and recycling of the chiral catalyst. To address this is-
sue, significant effort has been made to ‘heterogenize’ the Jacobsen
chiral salen Mn(III) complex, such as grafting Chiral Mn(salen)Cl
complex monomeric catalyst systems onto organic polymers;8
inorganic supports clays,9 MCM-41,10 SBA-15,11 activated silica,12
zeolites;13 organic polymer-inorganic hybrid materials.14–17 Unfor-
tunately, although such modified catalysts show the advantages of
easy separation and reuse, significant gaps exist in the scope of
these methodologies. First, the leaching of salen Mn(III) complexes
during reaction is often troublesome. Second, most of modified
catalysts are less efficient than their corresponding unmodified
catalysts, partly, because of the inaccessibility of the reagents to
the active centers in the heterogeneous reaction.18 Thus, the
highly desirable. As we know, appropriately increased molecular
weight of the catalysts could lower their solubility in certain sol-
vents, thus opening an opportunity for product isolation and cata-
lyst recovery.19 Additionally, increasing the catalytically active
metal centers of the catalysts could enhance the catalytic activities
and recover the catalyst easily.20,18 Recently, a series of homoge-
nous recyclable oligomeric or dimeric chiral salen Mn(III)
complexes with chemical bond connection have been synthe-
sized.21–25 These kind of catalysts obtained comparable or higher
conversion and ee values to the Jacobsen’s catalyst in asymmetric
epoxidation of unfunctionalized olefins in the presence of axial
bases, and part of the di-salen Mn(III) or oligo-salen Mn(III) were
recyclable. However, in most of the dimeric or oligomeric chiral
salen Mn(III) complexes, only one or several carbon atoms con-
nected the two chiral salen Mn(III) complex.
Also, axial bases have a pronounced effect on the reactivity and
selectivity of the enantioselective epoxidation reaction. However,
the axial base is gradually degraded during the epoxidation proce-
dure.26 The exceptional phenomenon has been reported by our
group recently. It was reported that remarkably increased conver-
sions and ee values were obtained without the axial base NMO in
the asymmetric epoxidation of unfunctionalized olefins catalyzed
⇑
Corresponding author.
E-mail address: fxk@swu.edu.cn (X. Fu).
0040-4039/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.05.087

4042
X. Huang et al. / Tetrahedron Letters 54 (2013) 4041–4044
by phenoxy-modified ZPS-PVPA immobilized chiral salen Mn(III)
complex.14 At the same period, List reported that adding such a li-
gand (NMO) to the reaction mixture resulted in a dramatically re-
duced enantioselectivity and reactivity in the alkene epoxidation
catalyzed by salen Mn(III) phosphate complexes.27 Zhang et al.
has reported that axial coordination of the catalyst could improve
the enantioselectivity in the asymmetric epoxidation of unfunc-
tionalized olefins.28 Based on these, we herein designed and syn-
thesized a series of rigid structure bis-Salen Mn chiral complexes
which were axially coordinated with bis-diphenolate and bis-dia-
mine substituted 1,4-bis(brommethyl)-2,3,5,6-tetra-methylben-
zene derivatives in this Letter (Scheme 1). The prepared catalysts
displayed superior catalytic activities and enantioselectivities to
indene (entry 3 vs 7, ee%, 100 vs 82; conv%, and 97 vs 85) and
-methylstyrene (entry 16 vs 20, ee%, 97 vs. 66; conv%, 100 vs
a
91). The results of catalysts 3a–3g were also better than the re-
ported ethyldiamine-bridged bi-Mn(salen) complexes which only
got 33% yield and have no enantiomeric excess values.29 This is
due to the special structure of the bis-Salen Mn catalysts (Fig. 1)
that increased the local concentration of the active sites and the
cooperative interaction between the two metal centers present in
the catalysts that are not working in isolation;24,19 Moreover, bis-
diphenolate and bis-diamine ligands in bis-Salen Mn complexes
could facilitate the electron transfer between the Mn(salen) com-
plexes.30 Meanwhile, the monomeric catalyst 4b also showed high-
er ee values than the Jacobsen’s catalyst for indene (ee%, 82–65)
the Jacobsen’s catalyst in the asymmetric epoxidation of
a
-methyl-
and a-methylstyrene (ee%, 66–52). According to this, it was indi-
styrene and indene with m-CPBA as oxidant. Specially, remarkably
increased conversions and ee values were obtained without NMO
in the asymmetric epoxidation of olefins catalyzed by the bis-
diphenolate axially coordinated bis-Salen Mn catalysts, however,
the bis-diamine axially coordinated bis-Salen Mn catalysts do not
have this phenomenon.
cated that the rigid caused by the steric effect of the four methyl
in special structure of tetramethylbenzene with aromatic bis-
diphenolate or bis-diamine linkers was devoted to the increase of
ee values by a decrease of the free rotation of two Mn(salen)
complexes.
It has been observed previously in homogeneous systems that
addition of donor ligands such as pyridine N-oxide or N-methylim-
idazole induces a conformational change on the skeleton of the
salen Mn(III) complex, resulting in enhancement of reaction rate
and an improvement in enantioselectivity. It was interesting to
find out here that a remarkable increase of conversion and ee val-
ues were obtained for the epoxidations catalyzed by diphenolate
axially coordinated salen Mn catalysts in the absence of the axial
base, such as the ee values for the epoxides of indene typically in-
Results and discussion
The catalytic activities and enantioselectivities of catalysts 3a–
3g were explored for the asymmetric epoxidation of indene and a-
methylstyrene using m-CPBA as an oxidant system. Jacobsen’s cat-
alyst and monomer chiral salen Mn(III) catalysts 4a–4b as outlined
in Scheme 2 were also examined for comparison purposes, and the
results are summarized in Table 1. All reactions proceeded
smoothly, and the bis-Salen Mn chiral catalysts (3a–3g) showed
higher ee values compared with Jacobsen’s catalyst and the mono-
mer chiral salen Mn(III) catalysts. As described in Table 1, the bis-
Salen Mn chiral catalyst 3c showed higher yield and ee values than
the monomeric catalyst 4b in the asymmetric epoxidation of
creased from 55% to 100% and for a-methylstyrene increased from
90% to 97% without the addition of NMO (Table 1 entries 7 vs 8 and
20 vs 21). However, the axial base was vital in the epoxidations
catalyzed by the bis-diamine axially coordinated salen Mn cata-
lysts, such as for indene the ee values decreased from 92% to 87%
and for
a-methylstyrene decreased from 97% to 69% without the
Scheme 1. Synthesis route of the bis-Salen Mn(III) chiral catalysts.

X. Huang et al. / Tetrahedron Letters 54 (2013) 4041–4044
4043
Scheme 2. Synthesis route of corresponding monomer chiral salen Mn(III) catalysts.
addition of NMO (Table 1 entries 11 vs 12 and 24 vs 25). Recently,
it was reported by our group that the alkoxy-modified AlSPP/salen
Mn(III), diphenolate-modified ZnPS-PVPA/salen Mn(III), and
diphenolate-modified ZPS-PVPA/salen Mn(III) also exhibited this
phenomenon in asymmetric epoxidations,14,31,32 namely a sharp
increase of conversion and ee values in the absence of the axial
base. Whereas, ethylenediamine-modified AlSPP or ZPS-PVPA/
salen Mn(III) did not show this phenomenon. In short, dipheno-
late–manganese, and alkoxy–manganese (RO–Mn) salen com-
plexes could efficiently catalyze the asymmetric epoxidation with
higher conversion and ee values in the absence of NMO, whereas
amine-manganese (RN–Mn) salen complexes could not. It was pre-
sumed that (1) the formation of Mn–O ionic bond may alter the
bond length of Mn(V)@O, and there are steric repulsion between
cyclohexanediamine moiety and the axial ligands coming close to
the Mn center which brought the axial ligands to one asymmetric
direction and then put down the salicylidene rings33 and could
make the active intermediate unstable. (2) Jacobsen had presented
the evidence that the N-oxides behaved as axial ligands, since the
catalyst with a strapped N-oxide ligand was more active than the
standard catalyst and is no longer affected by added N-oxide li-
gands.34 According to the unusual phenomenon, it was thought
that –OR sections whose structures were similar with N-oxide li-
gand behaved as axial ligands. Thus, there was a steric hindrance
when N-oxide ligand was added and the optimal geometric config-
uration of the reactive intermediate or transition states of salen
Mn(V)@O was altered, accompanied with the decreased chiral rec-
ognition of the catalytic system and the lower enantioselectivity.
(3) The balance between Mn(V)@O complex and Mn(III) complex
when N-oxide additive bound to the coordinatively unsaturated
Mn(III) complex35 is interfered when Mn(III) is bound to the
diphenolate or alkoxy-modified support or linker, which results
in the increasing of Mn(III) concentration and decreasing of
Mn(V)@O concentration and the optimal configuration of
Table 1
Asymmetric epoxidation of indene and a-methylstyrene catalyzed by ACbi-Salen Mn
catalyst 3a–3g and monomeric catalyst 4a–4b with m-CPBA as oxidanta
Entry
Substrateb
Catalyst
Oxidant
ee%c
conv%
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
A
A
A
A
A
A
A
A
A
A
A
A
A
B
B
B
B
B
B
B
B
B
B
B
B
B
Jacobsen
4a
4b
4b
3a
3b
3c
3c
3d
3e
3f
3f
3g
Jacobsen
4a
4b
4b
3a
3b
3c
3c
3d
3e
3f
3f
3g
m-CPBA/NMO
m-CPBA
m-CPBA
m-CPBA/NMO
m-CPBA
m-CPBA
m-CPBA
m-CPBA/NMO
m-CPBA
m-CPBA/NMO
m-CPBA/NMO
m-CPBA
m-CPBA/NMO
m-CPBA/NMO
m-CPBA
m-CPBA
m-CPBA/NMO
m-CPBA
m-CPBA
m-CPBA
m-CPBA/NMO
m-CPBA
m-CPBA/NMO
m-CPBA/NMO
m-CPBA
65
93
82
78
94
93
100
55
98
94
92
87
94
52
64
66
69
95
78
97
90
96
98
97
69
99
100
93
85
56
94
97
97
97
100
97
93
86
97
100
90
91
33
100
100
100
91
100
94
96
84
98
m-CPBA/NMO
a
Reactions were carried out at desired temperature in CH2Cl2 (4 mL) with alkene
(1.0 mmol), m-CPBA (2.0 mmol), NMO (5.0 mmol, if necessary), nonane (internal
standard, 1.0 mmol), and catalysts (2.5 mmol %) at 0 °C for 1 h. The conversion and
the ee value were determined by GC with chiral capillary columns (HP19091G-
B233, 30 m  25
l
m  0.25
lm).
b
A = indene, B =
(S)-Form.
a
-methylstyrene.
c
Figure 1. Schematic illustration of asymmetric epoxidation of alkene in the bis-Salen Mn catalysts.

4044
X. Huang et al. / Tetrahedron Letters 54 (2013) 4041–4044
Table 2
Conclusion
The recycles of catalyst 3c in the asymmetric epoxidation of indene with m-CPBA as
oxidanta
In summary, a series of bis-Salen Mn(III) chiral complexes were
synthesized and characterized. These bis-Salen Mn chiral catalysts
showed comparable or even higher enantioselectivities than Jacob-
sen’s catalyst and the monomer Salen Mn(III) catalyst under the
same conditions. Specially, the prepared catalysts 3a–3d exhibited
remarkable high catalytic activity and enantioselectivity in the
absence of NMO, but the axial base is indispensable for catalysts
3e–3f in asymmetric epoxidation.
Run
Time (h)
ee%b
conv%
Fresh
0.5
0.5
1
1.5
3
100
99
93
91
83
97
95
96
95
91
2
3
4
5
a
Reactions were carried out at desired temperature in CH2Cl2 (4 mL) with alkene
(1.0 mmol), m-CPBA (2.0 mmol), nonane (internal standard, 1.0 mmol), and cata-
lysts (2.5 mmol %) at 0 °C. The conversion and the ee value were determined by GC
Acknowledgments
with chiral capillary columns (HP19091G-B233, 30 m  25
l
m  0.25
l
m).
b
(S)-Form.
This work was financially supported by National Ministry of
Science and Technology Innovation Fund for High-tech Small and
Medium Enterprise Technology (No. 09C26215112399) and
National Ministry of Human Resources and Social Security Startup
Support Projects for Returned oversea Students to Business, Office
of Human Resources and Social Security Issued 2009 (143).
Table 3
Large-scale asymmetric epoxidation of indene with m-CPBA as oxidant
Entry
Substrate
Time (h)
ee%a
conv%
1b
2c
Indene
Indene
2
2
95
96
97
96
References and notes
a
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The usage amounts of reagents were alkene (100.0 mmol), m-CPBA
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It was found that the bis-Salen Mn chiral catalysts were very
soluble in dichloromethane, but almost insoluble in hexanes and
diethyl ether. Based on the solubility difference of the bis-Salen
Mn chiral catalysts mentioned above, the catalyst was precipitated
in hexane for recovery from the concentrated reaction mixture. The
results with recycled catalyst are shown in Table 2. Obviously, the
yield and the enantioselectivity decreased slightly after recycling
for five times and still gave yield (91%) and enantioselectivity
(83%). The activity of the recycled catalyst gradually decreased
upon successive use possibly due to minor degradation under
epoxidation conditions and/or weight loss during recovery process
of the catalyst. The amount of Mn in catalyst 3c was calculated by
AAS after 5 runs. The result indicated that 37.2% of the Jacobsen
salen Mn was gradually degraded from the catalyst 3c under the
epoxidation conditions.
We further performed different proportions of large-scale
asymmetric epoxidation reactions with n-nonane, indene, and
m-CPBA. The large-scale experiments proceeded smoothly using
the same procedure as for the experimental-scale reactions. As
can be seen from the results summarized in Table 3, delightfully,
the conversion and enantioselectivity maintained at the same le-
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