Manganese catalysts with C1-symmetric N4 ligand for enantioselective epoxidation of olefins

Article abstract of DOI:10.1002/chem.201103802

Bioinspired manganese complexes based on N₄ ligands, with a more rigid, chiral diamine derived from proline and two benzimidazoles, were synthesized and applied to epoxidize olefins with hydrogen peroxide as a clean oxidant. Notably, 60-99 % isolated yields and excellent ee values (up to 95 %) were obtained by using low catalyst loadings (0.01-0.2 mol %; see scheme; F green, S yellow). Copyright

Full text of DOI:10.1002/chem.201103802

COMMUNICATION
DOI: 10.1002/chem.201103802
Manganese Catalysts with C1-Symmetric N4 Ligand for Enantioselective
Epoxidation of Olefins
Bin Wang,[a, b] Chengxia Miao,[a] Shoufeng Wang,[a] Chungu Xia,[a] and Wei Sun*[a]
The attainment of both high catalytic efficiency and high
enantioselectivity remains a formidable challenge in asym-
metric epoxidation of olefins, which is a very important or-
ganic transformation, because the resulting enantiomerically
pure epoxides are highly useful intermediates and building
blocks.[1] Initially, naturally occurring metalloenzymes that
use molecular oxygen as the oxidant, such as cytochrome
P450 enzymes, opened the way for these catalytic process-
es.[2] After the pioneering work of Sharpless on the asym-
metric epoxidation (AE) of allylic alcohols,[3] Jacobsen
et al.[4] and Katsuki et al.[5] independently developed the
manganese–salen complex systems for the AE of unfunc-
tionalized alkenes.[6] Even though important advances in the
discovery of asymmetric epoxidation has been demonstrat-
ed, most of the reported catalytic AE systems still suffer
from relatively high catalyst loadings (generally >1 mol%).
However, the turnover numbers (TONs) of asymmetric hy-
drogenation developed by Noyori reach up to 2400000,
while the turnover frequency (TOF) at 30% conversion is
228 000 hÀ1.[7] In this regard, it is highly desirable to develop
an efficient catalyst system to achieve the asymmetric epoxi-
dation rapidly with low catalyst loadings.
Figure 1. Top: Mn complexes used as catalyst herein. Bottom: X-ray
Asymmetric catalysis is an integrated chemical approach
in which the maximum chiral efficiency can be obtained by
a combination of suitable molecular design with proper re-
structure of MnII complex C1. Hydrogen atoms are omitted for clarity.
action conditions. Recently, biologically inspired tetraden-
tate nitrogen (N4)-based ligands and their metal complexes
have drawn more attention.[1f,8,9] Several nonheme iron[9a,h]
and manganese complexes[10] have shown promising epoxi-
dation reactivities. In this respect, manganese complexes of
N4 ligands has provided an operationally simple and rapid
way to epoxidize a wide scope of olefins with low catalyst
loadings.[10,11] And Mn complexes of chiral N4 ligands have
also made substantial advancement in asymmetric epoxida-
tion.[12–15] We herein report a series of manganese complexes
of N4 ligands, incorporating a more rigid chiral diamine de-
rived from proline and two benzimidazoles (Figure 1, C1
and C2). To our delight, the new, bioinspired, manganese
complexes with the designed more rigid ligands could effi-
ciently catalyze asymmetric epoxidation of olefins with 60–
99% isolated yield and up to 95% ee with 0.01–0.2 mol%
catalyst loadings. In addition, the catalyst system was also
suitable for gram-scale production without any loss in enan-
tioselectivity.
The exploratory experiments were started by testing the
activities of the Mn complexes and screening the reaction
conditions by using chalcone as the model substrate
(Table 1). A yield of 89% and 86% ee were obtained at the
initial test by adding 6 equivalents of H2O2 (50%) through
a syringe pump during 60 min into an acetonitrile solution
of the catalyst C1 (1.0 mol%), HOAc (5 equiv) as additive
and the substrate at room temperature (Table 1, entry 1).
However, the ee was only 78% with our previously designed
catalyst C3 under the same conditions (entry 2).[14] With C4
as the catalyst, both the yield and the ee value were only
moderate compared with that of C1, that is, when pyridine
[a] B. Wang, Dr. C. Miao, S. Wang, Prof. Dr. C. Xia, Prof. Dr. W. Sun
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences, Lanzhou, 730000 (P.R. China)
Fax : (+86) 931-827-7088
E-mail: wsun@licp.cas.cn
[b] B. Wang
Graduate School of the Chinese Academy of Sciences
Beijing, 100039 (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103802.
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Table 1. Screening the reaction conditions and the manganese complexe-
s.[a]
Table 2. Enantioselective epoxidation of a,b-unsaturated ketones and
other olefins.[a]
Entry
Cat.
x
T
H2O2
[equiv]
Yield
[%][b]
ee
[oC]
N
[%][c]
1
2
3
4
5
6
7
8
C1
C3
C4
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C1
C2
1
1
1
1
1
1
0.2
0.1
0.2
0.2
0.2
0.2
0.2
0.2
0.2
RT
RT
RT
0
6
6
6
6
6
6
6
6
89
91
73
95
93
99
94
86
93
78
75
85
27
78
93
86
78
68
89
91
92
92
91
92
90
88
91
86
92
88
À20
À40
À20
À20
À20
À20
À20
À20
À20
À20
À20
9
2
Entry
Substrate, R1, R2
Yield [%][b]
ee [%][c]
10
11
12[d]
13[e]
14[f]
15
1.5
1.2
2
2
2
1
2
3
4
5
6
7
8
2a, Ph, Ph
93
99
80
84
83
85
80
80
88
90
96
60
95
92
92
98
83
80
80
75[d]
93
68[d]
92
93
91
93
87
94
87
88
93
91
88
89
91
90
93
95
80
79
72
74[e]
74
39[e]
2b, p-Cl-C6H4, Ph
2c, o-Cl-C6H4, Ph
2d, p-F-C6H4, Ph
2e, o-Br-C6H4, Ph
2 f, p-Me-C6H4, Ph
2g, Ph, p-Cl-C6H4
2h, Ph, p-F-C6H4
2i, Ph, p-Me-C6H4
2j, Ph, p-MeO-C6H4
2k, Ph, m-MeO-C6H4
2l, Ph, b-Nap
2m, p-Cl-C6H4, p-Cl-C6H4
2n, p-Me-C6H4, p-Cl-C6H4
2o, p-Cl-C6H4, p-Me-C6H4
2p, p-F-C6H4, p-Me-C6H4
2q
2
[a] Unless otherwise mentioned, all reactions were performed in CH3CN
(1.0 mL) with substrates (0.25 mmol) in the presence HOAc (5 equiv),
H2O2 (50% in 0.5 mL of CH3CN) was added dropwise through a syringe
pump over 60 min, and an additional 60 min of stirring was allowed.
[b] Isolated yield. [c] Determined by HPLC with a Daicel chiral OD-H
column. [d] 3 equivalents HOAc. [e] 0.5 equivalents HOAc. [f] 30%
H2O2.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
was used instead of benzimidazole (entry 3). Preliminary re-
sults indicate that the reaction temperature had certain ef-
fects on the yield and ee value (entries 1 and 4–6). The ee
value could be promoted up to 91% when the temperature
was decreased to À208C, with complex C1 as catalyst
(entry 5). The ee value almost remained the same by further
lowering the temperature to À408C (entry 6). Subsequently,
the catalyst loading was examined and good performance
was shown with 0.2 mol% of C1 (entries 5, 7, and 8). In ad-
dition, the dosage of H2O2 and HOAc were also investigated
(entries 7, 9–13). After a rough examination, the optimal
conditions are: 0.2 mol% C1, À208C, 5 equivalents of
HOAc, and 2 equivalents of H2O2; this gives 93% yield and
92% ee. Furthermore, the yield decreases to 78% and the ee
value is kept the same by using 30% H2O2 instead of 50%
H2O2 (entries 9 and 14). Finally, the activities of C1 and C2
were compared under the uniform conditions (entries 9 and
15) and the ee value slightly decreases if the ethyl group on
the benzimidazole is replaced by benzyl group. Consequent-
ly, C1 was used as the model catalyst for further investiga-
tions.
To examine the utility and generality of the catalytic
system for the epoxidation, we applied the present catalytic
system to a series of alkenes. Interestingly, almost all a,b-un-
saturated ketones were converted into epoxides with 80%
yield and nearly 90% ee (Table 2, entries 1–16).[16] These re-
sults indicate that the steric hindrance and the electronic
effect have almost no significant effect on the enantioselec-
tivity, except for 3-(2-bromophenyl)-1-phenylprop-2-en-1-
one, but have a certain influence on the yield. By employing
2r
2s
2t
2u
2v
[a] Unless otherwise mentioned, all reactions were performed in CH3CN
(1.0 mL) with substrates (0.25 mmol), catalyst (0.2 mol%), and HOAc
(5 equiv) at À208C and H2O2 (2 equiv., 50% diluted with 0.5 mL
CH3CN) was added dropwise through a syringe pump over 60 min, and
an additional 60 min of stirring was allowed. [b] Isolated yield. [c] Deter-
mined by HPLC with Daicel chiral columns. [d] GC yield. [e] Deter-
mined by GC with a CP-Chirasil-Dex CB column.
substrates with different substituents at the phenyl ring of
the olefin side, 87–94% ee and 80–99% yield were obtained
(entries 2–6). In addition, substituents with different steric
and electronic characters at the phenyl ring of the carbonyl
side were also investigated and most of the yields were still
less than that of chalcone (entries 7–9 and 12). Substitution
by p-OCH3 led to comparable ee values and 90% yield
(entry 10). A yield of 96% and 88% ee were obtained by
employing substrate with an m-OCH3 group (entry 11). Fur-
thermore, the more hindered 2-naphthyl derivative gives ee
values up to 89%, but acquires a relatively low yield (60%,
entry 12). Additionally, a,b-unsaturated ketones with sub-
stituents on both sides gave excellent ee values and yields
(92–98% yield and 90–95% ee, entries 13–16). When a tri-
substituted a,b-unsaturated ketone was selected as the sub-
strate, 80% ee was observed (entry 17). Notably, this system
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Manganese-Catalyzed Epoxidation of Olefins
COMMUNICATION
is also suitable for the epoxidation of unfunctionalized ole-
fins, such as chromene and a-methylstyrene, providing the
products in good yields and ee values (entries 18–20). The
present catalyst system is not suitable for the enantioselec-
tive epoxidation of simple enones, such as 3-methyl-cyclo-
hexanone, only 39% ee was observed (entry 22).
To demonstrate the efficiency of the newly developed cat-
alyst system, asymmetric epoxidation of alkenes were car-
ried out with low catalyst loadings. For example, when a mix-
ture of 6-cyano-2,2-dimethylchromene (0.25 mmol), C1
(0.01 mol%), and HOAc (5 equiv) in acetonitrile (1.0 mL)
was stirred for an hour at À208C after adding H2O2
(2 equivalents in 0.5 mL of acetonitrile) dropwise through
a syringe pump over an hour, the epoxide product was ob-
tained with 96% yield and 72% ee. Under such conditions,
the TON reached as high as 9600 [Eq. (1)].
other hand, experiments with peracetic acid as the oxidant
by adding H218O (50 equiv) showed an 8.1% incorporation
of oxygen atoms from water into epoxide (see the Support-
ing Information). The high level of H218O incorporation into
products might be induced by large amounts of HOAc in
peracetic acid. These observations suggest that high-valent
manganese oxo species may be involved in the catalytic
process.[11b]
Epoxomicin, a peptide epoxyketone isolated from the ac-
tinomycete strain No.Q996-17, possesses potent in vivo anti-
tumor and anti-inflammatory activities.[19a] Carfilzomib is
a new epoxyketone-based irreversible proteasome inhibi-
tor.[19b] Both important compounds contain epoxyketone
structures. Although the key intermediate epoxyketone
could be synthesized according to the literature method
(Scheme 1),[19a] the corresponding methodology for both the
When H2O2 was added rapidly, the reaction proceeded with
a 59% yield and a slightly low ee value, and the TOF was
59000 hÀ1 or 16.4 sÀ1 [Eq. (1)]. These results may be attrib-
uted to the higher stability and the lifetime of the manga-
nese catalyst under the oxidizing conditions. The crystal
structure of C1 with the ligand coordinated to the manga-
nese center in a cis-a topology[17,18] revealed that the Mn N
À
bond length is 2.202 ꢁ (see the Supporting Information),
which is shorter than the pyridine analogue C4 or other cat-
alysts reported previously.[11a,12,14] The present epoxidation
synthetic protocol can be efficiently carried out on a gram
scale. Chalcone gave the corresponding epoxide without any
loss of enantioselectivity with 0.1 mol% catalyst loading
(still up to 93% ee). Additionally, 68% yield and above
99% ee were obtained after recrystallization of the product
[Eq. (2)].
Scheme 1. The epoxyketone intermediate of epoxomicin and carfilzomib.
enantiomers is rarely studied. As a further demonstration of
the utility of our newly developed catalyst system, the epoxi-
dation reaction was carried out under our conditions with
C1 as catalyst (Scheme 2). A nearly quantitative yield of ep-
oxyketone was obtained. Based on the NMR spectra and X-
ray structure of the product, epoxyketone-a is the major
product, and the ratio of epoxyketone-a/epoxyketone-b is
about 7.[20] The high diastereoselectivity is clearly deter-
mined by the substrate and the catalyst, as the present
system is not good enough for the simple enone.
In summary, new, bioinspired, manganese complexes
based on ligands with a more rigid, chiral diamine derived
from proline and two benzimidazoles were synthesized and
applied in epoxidation of olefins. Isolated yields of 60–99%
and up to 95% ee were obtained within 2 h with 0.01–0.2
mol% catalyst loading and aqueous H2O2. The TOFs and
TONs reached 59000 hÀ1 and 9600, respectively. In addition,
the catalyst system is also suitable for a gram-scale produc-
tion. This protocol is clean, efficient, and offers a very prac-
tical method of chiral epoxide synthesis. Further studies on
designing efficient catalysts, asymmetric catalysis, and inves-
tigations of the mechanism are in progress in our laboratory.
Furthermore, the gram-scale reaction even proceeded well
with an 89% ee when the catalyst loading was reduced to
0.01 mol% (TON=3700). By comparison, the use of per-
acetic acid as the oxidant to perform the epoxidation of
chalcone under the same conditions only lead to a slight loss
both in yield and ee value (89% yield, 89% ee, see the Sup-
porting Information).[14] Isotopic labeling experiments were
carried out to reveal the reaction mechanism. Under stan-
dard conditions, epoxidation of chalcone with H2O2/HOAc
as the oxidant in the presence of H218O (50 equiv) resulted
in only 0.5% incorporation of 18O into the epoxide. On the
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Scheme 2. Top: The synthesis of epoxyketone catalyzed by C1. Bottom:
X-ray structure of epoxyketone-a.
Acknowledgements
This work was supported by the “Hundred Talents Program” of the Chi-
nese Academy of Sciences and the National Natural Science Foundation
of China (20873166, 21073210, and 21133011).
Keywords: asymmetric epoxidation · bioinspired catalysis ·
manganese · N4 ligand · olefins
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Received: December 3, 2011
Revised: March 1, 2012
Published online: && &&, 2012
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Chem. Eur. J. 0000, 00, 0 – 0
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These are not the final page numbers!

Manganese-Catalyzed Epoxidation of Olefins
COMMUNICATION
Bioinspired manganese complexes
based on N4 ligands, with a more rigid,
chiral diamine derived from proline
and two benzimidazoles, were synthe-
sized and applied to epoxidize olefins
with hydrogen peroxide as a clean oxi-
dant. Notably, 60–99% isolated yields
and excellent ee values (up to 95%)
were obtained by using low catalyst
loadings (0.01–0.2 mol%; see scheme;
F green, S yellow).
Asymmetric Catalysis
B. Wang, C. Miao, S. Wang, C. Xia,
W. Sun* ........................ &&&& — &&&&
Manganese Catalysts with C1-
Symmetric N4 Ligand for Enantioselec-
tive Epoxidation of Olefins
Chem. Eur. J. 2012, 00, 0 – 0
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These are not the final page numbers!