Bioinspired manganese and iron complexes with tetradentate n ligands for the asymmetric epoxidation of olefins

Article abstract of DOI:10.1002/cctc.201300102

The manganese and iron complexes with the tetradentate N ligand (1R,2R)-N,N'-dimethyl-N,N'-bis(1-methyl-2-benzimidazolylmethyl)cyclohexane-1,2-diamine have been synthesized and characterized. The crystal structure of the manganese complex demonstrates a cis-α configuration. Both the manganese and iron complexes are active catalysts for the asymmetric epoxidation of olefins with H₂O₂ as an oxidant and acetic acid as an additive. Up to 96% ee was observed for the epoxidation of α,β-unsaturated ketones at -20°C.

Full text of DOI:10.1002/cctc.201300102

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X. Wang, C. Miao, S. Wang, C. Xia,
W. Sun*
&& – &&
Bioinspired Manganese and Iron
Complexes with Tetradentate N
Ligands for the Asymmetric
Epoxidation of Olefins
Asymmetric barrier overtaken! The
manganese and iron complexes with
the tetradentate N ligand (1R,2R)-N,N’-
dimethyl-N,N’-bis(1-methyl-2-benzimida-
zolylmethyl)cyclohexane-1,2-diamine
have been synthesized and character-
ized. These complexes are active cata-
lysts for the asymmetric epoxidation of
olefins, which resulted in up to 96% ee.
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DOI: 10.1002/cctc.201300102
Bioinspired Manganese and Iron Complexes with
Tetradentate N Ligands for the Asymmetric Epoxidation of
Olefins
[a, b]
[a]
[a]
[a]
[a]
Xiaoe Wang,
Chengxia Miao, Shoufeng Wang, Chungu Xia, and Wei Sun*
The manganese and iron complexes with the tetradentate N
ligand (1R,2R)-N,N’-dimethyl-N,N’-bis(1-methyl-2-benzimidazo-
lylmethyl)cyclohexane-1,2-diamine have been synthesized and
characterized. The crystal structure of the manganese complex
demonstrates a cis-a configuration. Both the manganese and
iron complexes are active catalysts for the asymmetric epoxida-
tion of olefins with H O as an oxidant and acetic acid as an
2
2
additive. Up to 96% ee was observed for the epoxidation of
a,b-unsaturated ketones at ꢀ208C.
Introduction
[
7]
The use of ligands based on a chiral vicinal diamine backbone
is an established strategy for chiral induction in metal-cata-
modest. In 2009, a series of manganese complexes with li-
gands were designed and synthesized by our group by intro-
ducing aromatic groups at the 2-pyridylmethyl positions of
R,R-MCP and relatively high enantioselectivities (up to 89% ee)
were achieved in the epoxidation of a,b-enones with 1 mol%
[
1]
lyzed oxidations. Of the oxidation reactions, the asymmetric
epoxidation of olefins is one of the most important transfor-
mations in organic synthesis because the resulting enantiomer-
ically enriched epoxides are highly useful intermediates and
[
8]
catalyst loading. Then, Bryliakov et al. reported a series of
chiral nonheme aminopyridinylmanganese(II) complexes for ef-
ficiently catalyzing enantioselective olefin oxidation to the cor-
responding epoxides with high yields and enantioselectivities
[2]
building blocks in chemical or pharmaceutical industry. The
manganese and iron centers supported by the nonheme li-
gands have attracted intense interests because of their high ef-
[
3,4]
[9]
ficiency in various oxidation reactions. Pioneering studies by
(up to 93% ee). However, an example referring to chiral N4
Jacobsen and co-workers demonstrated that the iron(II)-N,N’-
iron complexes for the asymmetric epoxidation was rare. We
and Bryliakov et al. reported the corresponding iron complexes
dimethyl-N,N’-bis(2-pyridylmethyl)ethylene-1,2-diamine
com-
plex could efficiently promote the epoxidation of various ali-
using different ligands for the asymmetric epoxidation that re-
[
3a]
[9c,10]
phatic olefins. Then, Stack and co-workers described that the
manganese complex of the MCP (MCP=N,N’-dimethyl-N,N’-
bis(2-pyridylmethyl)cyclohexane-1,2-diamine) ligand could act
as an efficient catalyst for the epoxidation of olefins with per-
sulted in 87 and 86% ee, respectively.
Although significant
progress has been made using iron or manganese complexes
with chiral N ligands, the attainment of high enantioselectivity
4
(>90% ee) remains a formidable challenge in the asymmetric
[
5]
acetic acid as an oxidant. Costas et al. prepared a novel
family of manganese complexes with chiral pinene-appended
tetradentate ligands derived from R,R-MCP and the complexes
demonstrated only up to 46% ee in the epoxidation of select-
epoxidation of olefins, especially for the iron-catalyzed asym-
metric epoxidation.
2
Generally, this type of N4 ligands is composed of sp
N
3
donors of pyridine and two chiral sp N donors of the diamine
[
6]
2
3
ed alkenes with peracetic acid as an oxidant. Pfaltz and co-
workers synthesized two classes of tetradentate chiral diamino-
bisoxazoline ligands and studied the catalytic activity of the
corresponding manganese complexes in the enantioselective
epoxidation; however, the enantioselectivities were still
backbone. The rational tuning of the sp N donors or chiral sp
N donors has shown an increase both in the reactivity and in
[3,6,8]
the selectivity for the bioinspired oxidation (Figure 1).
For
example, the use of ligands (S,S-BPBP and S,S,R-BPBPP) derived
from the bipyrrolidine backbone increases the regioselectivity
or enantioselectivity of the catalysts in the aliphatic CꢀH oxida-
tion, asymmetric cis-dihydroxylation, and asymmetric epoxida-
[
a] X. 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
[3e–h,9b,c,11]
II
tion.
Our group recently reported the synthesis of Fe
II
or Mn complexes with the C -symmetric tetradentate N ligand
1
Lanzhou 730000 (PR China)
Fax: (+86)931-827-7088
E-mail: wsun@licp.cas.cn
(S-PEB), which consisted of more rigid chiral diamine template
derived from l-proline and two benzimidazole donors, and the
asymmetric epoxidation of various olefins that resulted in up
[
b] X. Wang
[12]
to 98% or 95% ee.
Graduate School of the Chinese Academy of Sciences
We synthesized an easily available ligand, (1R,2R)-N,N’-di-
methyl-N,N’-bis(1-methyl-2-benzimidazolylmethyl)cyclohexane-
1,2-diamine (R,R-MCMB), and described the new nonheme
Beijing 100039 (PR China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300102.
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4
Figure 1. Selected N ligands for the bioinspired oxidation.
manganese and iron complexes (Figure 2), which proved to be
highly active and enantioselective catalyst for the epoxidation
Figure 3. X-ray structure of C1 (hydrogen atoms are omitted for clarity).
of olefins with H O2 as an oxidant and acetic acid as an
2
additive.
tate ligand coordinates to the metal in a cis-a conformation, in
which the two benzimidazole rings are trans to each other and
the two triflate anions are cis to each other (Figure 3). The
MnꢀN (benzimidazole) bond lengths of C1 are 2.221(3) and
2
.244(3) ꢁ, respectively. These lengths are longer than those of
II
MnꢀN (benzimidazole) bonds in the Mn -S-PEB(OTf) complex
2
[12b]
II
(
2.202(4) ꢁ).
Compared to the Mn -MCP(OTf) complex [Mnꢀ
2
N (pyridine): 2.216(4) and 2.199(4) ꢁ], these MnꢀN bond
[5a]
lengths of C1 are longer.
Figure 2. Chiral metal (M=Mn, Fe) complexes of (1R,2R)-N,N’-dimethyl-N,N’-
bis(1-methyl-2-benzimidazolylmethyl)cyclohexane-1,2-diamine.
Catalytic activity testing
Results and Discussion
The exploratory experiments were started by testing this pro-
tocol, and the results of the screening and optimizing condi-
tions, with chalcone as the model substrate and the manga-
nese complex C1 as the catalyst are summarized in Table 1. A
Synthesis of ligand and manganese and iron complexes
The R,R-MCMB ligand can be prepared easily in good yield
through the direct alkylation of the (1R,2R)-N,N-dimethylcyclo-
hexane-1,2-diamine with 2-chloromethyl-1-methyl-benzimida-
zole (Scheme 1). The [Mn(R,R-MCMB)(OTf) ] (C1) complex was
2
Table 1. Optimization of the reaction conditions for the asymmetric ep-
oxidation of chalcone catalyzed by C1.
prepared by mixing MeCN solutions of R,R-MCMB and
Mn(OTf) . The Fe(R,R-MCMB)(OTf) (C2) complex resulted from
[a]
2
2
the reaction of Fe(R,R-MCMB)Cl with silver trifluoromethane-
2
II
II
sulfonate. Both the Mn and Fe products were characterized
by elemental analysis and ESIMS. The complex C1 has also
been characterized by X-ray crystal structure determination
Entry
Catalyst
T
[8C]
HOAc
[equiv.]
H
2
O
2
Yield
[%]
ee
[%]
[
b]
[c]
[
mol%]
[equiv.]
[
13]
1
2
3
4
5
6
7
1
1
RT
5
5
5
5
5
6
2
2
2
1.5
2
71
89
93
62
61
94
94
82
89
91
83
90
90
90
(CCDC 898685). In the crystal structure of C1, the tetraden-
ꢀ20
ꢀ20
ꢀ20
ꢀ20
ꢀ30
ꢀ20
0.5
0.2
0.5
0.5
0.5
5
14
2
[
a] Reaction conditions: 2 equiv. of 50% H
2
O
2
diluted with 0.5 mL of
CH
3
CN was added with a syringe pump for an hour to a stirred solution
ꢀ
3
of the catalyst C1 (1.25ꢂ10 mmol, 0.5 mol%), HOAc (1.25 mmol), and
substrate (0.25 mmol) in 1.0 mL of CH
CN at ꢀ208C under argon, and
then the mixture was stirred for an additional hour; [b] Isolated yield;
c] Determined from HPLC analysis (see the Supporting Information for
details).
3
[
Scheme 1. Synthesis of the (1R,2R)-N,N’-dimethyl-N,N’-bis(1-methyl-2-benzi-
midazolylmethyl)cyclohexane-1,2-diamine ligand.
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total of 82% ee and 71% yield could be obtained by using
using 2 equiv. of 50% H O (entry 2). Then, the catalyst loading
2
2
5
equiv. of acetic acid, 6 equiv. of 50% H O with respect to
was investigated. The asymmetric induction of epoxidation
achieved 91% ee with 0.5 mol% catalyst C1 at ꢀ208C (entry 3).
Further lowering of the catalyst loading led to the decrease in
enantioselectivity and yield (entry 4). In addition, the yield
would evidently reduce but with similar enantioselectivity
when the dosage of H O was reduced from 2 to 1.5 equiv.
2
2
the substrate, and 1 mol% manganese complex as the catalyst
at room temperature (Table 1, entry 1), and 89% ee could be
obtained by lowering the temperature to ꢀ208C even by
2
2
[
a]
Table 2. Enantioselective epoxidation of olefins catalyzed by C1.
(entry 3 vs. entry 5). The same results were obtained on con-
ducting the reaction at ꢀ308C (entry 3 vs. entry 6). There is
clearly no effect of the increase in acetic acid from 5 to
1
4 equiv. on the ee and yield (entry 3 vs. entry 7).
Entry
Substrate
Yield
ee
[%]
[
b]
[c]
[%]
The scope of the reaction was studied under the optimized
conditions (Table 1, entry 3). As shown in Table 2, the epoxida-
tion of various chalcone derivatives with aqueous H O and
2
2
1
2
93
75
91
91
acetic acid gave the corresponding a,b-epoxyketone in good
[14]
yields and enantioselectivities.
For the substrates bearing
either electron-donating or electron-withdrawing groups on
the phenyl ring of the olefin side, more than 90% ee and 90%
yields were obtained (Table 2, entries 3–5); however, for the
substrate with the p-Me group at the phenyl ring, only a 75%
yield was obtained (entry 2). In contrast, only substitution by
the p-Me group on the phenyl ring of the carbonyl side led to
3
4
5
6
7
8
9
96
96
91
97
83
91
95
94
96
95
84
79
90
96
90% ee and 91% yield (entry 8), and other chalcones with elec-
tron-withdrawing groups on the phenyl ring of the carbonyl
side transformed to the desired epoxides in approximately
80% ee (entries 6 and 7). The electronic property of the sub-
stituents on both sides has a significant effect on the stereo-
control and yield for the present C -symmetric manganese
2
complex. The substrate with the p-Cl group on both phenyl
rings gave 95% yield and 96% ee (entry 9). The outcome is
II
[12b]
better than that for the Mn -S-PEB(OTf) complex.
However,
2
for the p-Me- and p-Cl-substituted substrates, only a 37% yield
was obtained (entry 10). Presumably, the diversity in the reac-
tivity and enantioselectivity for the C -symmetric Mn(R,R-
2
II
MCMB)OTf and C -symmetric Mn -S-PEB(OTf) complexes attri-
2
1
2
[12b]
butes to the nature of the complexes.
With trisubstituted
a,b-unsaturated ketones as substrates, only moderate yields
and 54–84% ee were obtained (entries 11 and 12). In addition,
isopropyl cinnamate resulted in 79% yield and 71% ee
1
0
37
75
(
entry 13). For the epoxidation of styrene, a good yield was
achieved but with a 39% ee (entry 14). Unfortunately, low ee
values were generated by using aliphatic olefins such as 1-
octene and vinylcyclohexane as the substrates, although they
were completely transformed into epoxides (see the Support-
ing Information).
1
1
61
54
54
84
12
Subsequently, the activity of the corresponding iron complex
C2 was also tested (Table 3). A total of 89% ee and 72% yield
were observed with the 2 mol% catalyst C2, 5 equiv. of acetic
acid, and 1.2 equiv. of H O at ꢀ208C (Table 3, entry 1). The ee
1
1
3
4
79
91
71
39
2
2
[
d]
and yield could be increased to 93 and 87%, respectively, by
increasing the amount of H O to 1.5 equiv. (entry 2), and an
2
2
[
a] Reaction conditions: 2 equiv. of 50% H
CH CN was added with a syringe pump for an hour to a stirred solution
of the catalyst C1 (1.25ꢂ10 mmol, 0.5 mol%), HOAc (1.25 mmol), and
substrate (0.25 mmol) in 1.0 mL of CH
CN at ꢀ208C under argon, and
then the mixture was stirred for an additional hour; [b] Isolated yield;
c] Determined from HPLC analysis (see the Supporting Information for
details); [d] The yield and ee were determined from GC analysis.
2 2
O diluted with 0.5 mL of
increase in the H O amount to 2 equiv. led to an increase in
2
2
3
the yield to 94% (entry 3). The reaction proceeded uneventful-
ly in the presence of 3 equiv. of acetic acid, even though with
slightly lower levels of enantioselectivity and yield (entry 3 vs.
entry 4). Overall, the present iron system based on the R,R-
ꢀ
3
3
[
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Table 3. Optimization of the reaction conditions for the asymmetric ep-
oxidation of chalcone catalyzed by C2.
Table 4. Enantioselective epoxidation of chalcone derivatives catalyzed
by C2.
[
a]
[a]
Entry
Catalyst
mol%]
T
[8C]
HOAc
[equiv.]
H
2
O
2
Yield
[%]
ee
[%]
[
b]
[c]
[
[equiv.]
1
2
3
4
2
2
2
2
ꢀ20
ꢀ20
ꢀ20
ꢀ20
5
5
5
3
1.2
1.5
2
72
87
94
83
89
93
93
90
Entry
Substrate
Yield
ee
[%]
[b]
[c]
[
%]
2
1
2
3
4
94
93
87
95
91
[a] Reaction conditions: 2 equiv. of 50% H
2
O
2
diluted with 0.5 mL of
CH
3
CN was added with a syringe pump for an hour to a stirred solution
ꢀ
3
of the catalyst C2 (1.25ꢂ10 mmol, 0.5 mol%), HOAc (1.25 mmol), and
substrate (0.25 mmol) in 1.0 mL of CH
CN at ꢀ208C under argon, and
then the mixture was stirred for an additional hour; [b] Isolated yield;
c] Determined from HPLC analysis (see the Supporting Information for
71
76
93
3
[
details).
MCMB ligand demonstrates reactivity and enantioselectivity
II
[12a]
comparable with those of the Fe -S-PEB(OTf) complex.
2
To demonstrate the substrate scope of the nonheme iron-
catalyzed epoxidation, a series of chalcone derivatives as well
as trisubstituted cyclic enones were evaluated under optimized
conditions. In most cases, the reaction gave the corresponding
a,b-epoxyketones in good yields and enantioselectivities
5
81
93
6
7
8
85
77
40
87
91
75
(
Table 4). For the trisubstituted cyclic enones, only the sub-
strate without any substituent gave 77% yield and 91% ee
entry 7). With the p-Me–Ph-substituted substrate, the corre-
(
sponding epoxides were isolated in poor yields (entry 8). For
this type of substrate, the steric bulk represented by this p-
Me–Ph group could potentially hinder the oxidation catalyzed
[
2 2
a] Reaction conditions: 2 equiv. of 50% H O diluted with 0.5 mL of
by the Fe(R,R-MCMB)OTf catalyst. However, this substrate is
2
CH
3
CN was added with a syringe pump over an hour to a stirred solution
a good candidate for forming the a,b-epoxyketones promoted
ꢀ3
of the catalyst C2 (5ꢂ10 mmol, 2 mol%), HOAc (1.25 mmol), and sub-
II
[12a]
by the C -symmetric Fe -S-PEB(OTf) complex.
strate (0.25 mmol) in 1.0 mL of CH CN at ꢀ208C under argon, and then
1
2
3
the mixture was stirred for an additional hour; [b] Isolated yield; [c] Deter-
The oxidation mechanisms catalyzed by manganese or iron
complexes with N4 ligands have recently made significant
progress using acetic acid as an additive. On the basis of the
electron paramagnetic resonance spectroscopy and enantiose-
lectivity studies on the epoxidation in the presence of various
carboxylic acids, Bryliakov et al. proposed a reasonable mecha-
mined from HPLC analysis (see the Supporting Information for details).
asymmetric epoxidation of chalcone was performed in the
18
18
presence of excess of H2 O, the O labeling epoxide products
were observed in both manganese and iron complex catalytic
[
9c]
nism, which is shown in Scheme 2. For the present catalytic
system for the epoxidation, acetic acid is essential to achieve
epoxidation activity. As shown in Scheme 2, the presence of
acetic acid would facilitate the heterolysis of the OꢀO bond of
[
10,12]
systems.
These results indicate that high-valent M=O spe-
cies are involved as the active oxidant in the catalytic
[3d,15,16]
process.
III
[3d]
the intermediate (M -OOH). Costas et al. trapped the active
oxoiron(V) species in a related catalyst system based on the
Conclusions
[
15]
iron complex with N ligands. In our recent work, when the
4
In summary, we synthesized a chiral C -symmetric tetradentate
2
N ligands from the easily available cyclohexane-1,2-diamine
and 2-chloromethyl-1-methyl-benzimidazole. The correspond-
ing manganese and iron complexes could serve as efficient
and enantioselective catalysts in the epoxidation with H O as
2
2
an oxidant. Notably, when the pyridine moieties of the N,N’-di-
methyl-N,N’-bis(2-pyridylmethyl)cyclohexane-1,2-diamine ligand
are replaced with benzimidazoles, the resulting R,R-MCMB
ligand demonstrates much higher enantioselectivity (up to
96% ee) with its manganese and iron complexes catalyzing the
Scheme 2. Proposed mechanism for the epoxidation catalyzed by the non-
heme manganese or iron complex.
asymmetric epoxidation of olefins. Further studies on the ap-
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II
plication of this N4 ligand in other new reactions are in
progress.
[Mn (R,R-MCMB)(CF
3
SO
)
3
2
]:
HRMS
(ESI):
m/z:
calcd
for
+
C H F MnN O S: 634.1740 [MꢀOTf] ; found: 634.1759; elemental
27 34
3
6
3
analysis calcd (%) for C H F MnN O S ·H O: C 41.95, H 4.53, N
28
34
6
6
6
2
2
1
0.48; found: C 41.67, H 4.40, N 10.34.
Experimental Section
Under argon, FeCl ·4H O (49.4 mg, 0.25 mmol, 1 equiv.) was added
2
2
to a stirred solution of the chiral ligand R,R-MCMB (108 mg,
.25 mmol, 1 equiv.) in acetonitrile (3 mL) at RT. The reaction mix-
General remarks
0
1
13
The H and C NMR spectra were recorded by using a Bruker
Avance III 400 MHz NMR spectrometer. The GC–MS spectra were
recorded by using an Agilent 6890/5973 GC–MS spectrometer. The
HRMS (ESI) spectra were determined by using a Bruker Daltonics
micrOTOF-QII mass spectrometer. X-ray crystallographic data were
collected by using a Bruker SMART CCD 1000 diffractometer with
graphite-monochromated MoKa radiation (l=0.71073 ꢁ) at
ture was stirred for 24 h, and diethyl ether was added to the solu-
tion to completely precipitate out the bright orange solid. The sol-
vent was decanted out of the flask with a pipette, and the solids
were washed thoroughly with ether thrice and dried under
vacuum to yield the Fe(R,R-MCMB)(Cl)2 complex. A flame-dried
1
0 mL flask was charged with Fe(R,R-MCMB)(Cl) (0.21 mmol) sus-
2
pended in acetonitrile (4 mL) under argon. Silver trifluoromethane-
sulfonate (106 mg, 0.42 mmol, 2 equiv.) was weighed under argon
and then added to the vigorously stirred heterogeneous mixture.
The flask was covered with the aluminum foil to protect the silver
salts from light. After 24 h, the reaction mixture was filtered
through a 0.2 mm LC PVDF filter (HPLC certified) twice to ensure no
silver salts remained; the solvent was removed under vacuum to
yield the iron complex C2.
1
50(2) K. HPLC analysis was performed by using a Waters Breeze
system with a 2487 Dual l absorbance detector and a 1525 Binary
HPLC pump. Chiralpak OD, AD, OJ, AS, and OB columns were pur-
chased from Daicel Chemical Industries, Ltd. GC analysis was per-
formed by using an Agilent 7890A GC with a CP-Chirasil-Dex CB
column. Column chromatography was performed on silica gel
(
200–300 mesh), and TLC inspections were performed on silica gel
GF254 plates. All reactions were performed under argon in dried
glassware. All chemicals and solvents were used as received unless
otherwise stated. Diethyl ether (Na/benzophenone) and acetonitrile
II
[
Fe (R,R-MCMB)(CF SO ) ]:
HRMS
(ESI):
m/z:
calcd
for
3
3 2
+
C H F FeN O S: 635.1709 [MꢀOTf] ; found: 635.1715; elemental
27
34
3
6
3
analysis calcd (%) for C H F FeN O S ·1.5 H O: C 41.44, H 4.60, N
28
34
6
6
6
2
2
(
CaH ) were distilled under argon before use. 2-Chloromethyl-ben-
2
1
0.35; found C 41.45, H 4.73, N 10.18.
zimidazole was synthesized by using the modified literature
[17]
method.
Representative method for the manganese-catalyzed asym-
metric epoxidation of olefins
General method for the synthesis of the R,R-MCMB ligand
-Chloromethyl-1-methyl-benzimidazole (2 mmol), (1R,2R)-N,N-di-
ꢀ
3
2
Under argon, the catalyst C1 (1.0 mL, 1.25ꢂ10 mmol,
ꢀ
1
methylcyclohexane-1,2-diamine (1 mmol), and anhydrous acetoni-
0.979 mgmL
in MeCN), substrate (0.25 mmol), and AcOH
trile (10 mL) were mixed in a 25 mL flask. Then, anhydrous Na CO3
(1.25 mmol, 75 mg, 5 equiv.) were added to a 10 mL flask and the
solution was stirred for 2 min at RT. Then the mixture was cooled
2
(
0.87 g) and tetrabutylammonium bromide (0.04 g) were added di-
rectly as solids and the resulting mixture was heated to reflux for
4 h under argon. After cooling to RT, the mixture was filtered and
the filter cake was washed with CH Cl . The combined filtrates
to ꢀ208C; 50% H
O (0.5 mmol, diluted with 0.5 mL MeCN,
2 2
2
2 equiv.) was added dropwise with a syringe pump for an hour;
and the mixture was stirred at ꢀ208C for an hour. The crude prod-
uct was purified by using chromatography on silica gel (PET/EtOAc
50:1) to afford the epoxide product.
2
2
were evaporated under reduced pressure. To the resulting residue,
NaOH (1m, 8.75 mL) was added and the mixture was extracted
with CH Cl (4ꢂ15 mL). The combined organic layer was washed
2
2
successively with saturated aqueous solutions of NaHCO , NaCl,
3
and finally H O. The organic layers were dried over anhydrous Acknowledgements
2
Na SO , and the solvent was removed under reduced pressure; the
2
4
crude product was purified by using chromatography on silica gel
We are grateful for the financial support from the Chinese Acade-
my of Sciences and the National Natural Science Foundation of
China (21133011 and 21073210).
(
PET/EtOAc 2:1) to afford the desired ligand R,R-MCMB.
2
D
0
1
8
(
1% yield; yellow solid; ½aꢁ = +12.8 (c=0.5 in CHCl3); H NMR
400 MHz, CDCl , TMS): d=7.74–7.72 (m, 2H), 7.27–7.25 (m, 6H),
3
3
1
.94 (s, 4H), 3.79 (m, 6H), 2.68 (s, 2H), 2.14 (s, 6H), 2.02 (d, 2H, J=
0.9 Hz), 1.79 (d, J=6.7 Hz, 2H), 1.27–1.15 ppm (m, 4H); C NMR
Keywords: asymmetric epoxidation · iron · manganese ·
13
N ligand · olefin
4
(
100 MHz, CDCl , TMS): d=152.5, 142.3, 136.4, 122.4, 121.8, 119.6,
3
1
09.1, 62.7, 51.7, 35.8, 29.9, 25.6, 24.0 ppm; HRMS (ESI): m/z: calcd
+
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ꢀ
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: February 6, 2013
Published online on && &&, 0000
ꢀ
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 7
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ÞÞ
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