A homogeneous chiral manganese(III)-salphe catalyst for enantioselective epoxidation of olefins

Article abstract of DOI:10.1080/00397911.2012.656785

A new chiral Mn(III)-Salphe catalyst was synthesized from natural amino acid (R)-phenylalanine and 3,5-di-tert-butyl-hydroxybenzaldehyde and applied to the asymmetric epoxidations of unfunctionalized olefins in ionic liquids. Satisfactory enantioselectivities (79% < ee < 93%) and good yields were achieved when NaClO was used as oxidant. We found that both the pH value (11.3) and reaction temperature (15 °C) were crucial for the epoxidation reactions. In our reaction system, NH₄O Ac was unnecessary. We proposed that alcoholic hydroxyls in the Mn(III)-Salphe compound played the role of axial ligand. However, the reaction time was longer than when using Jacobsen's catalyst because of the structure of the Mn(III)-Salphe compound, in which coordination geometries by the two alcoholic hydroxyls with certain angles affected the substrate approaching the Mn(V) = 0 center. The chiral ligand was characterized by the combination of infrared, ultraviolet, and visible spectra and ¹H NMR. Copyright

Full text of DOI:10.1080/00397911.2012.656785

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A Homogeneous Chiral
Manganese(III)–Salphe Catalyst for
Enantioselective Epoxidation of Olefins
Saili Wei a b , Yuhai Tang a , Ruiqin Zhao a , Xiaoqian Xu a , Guojin Xu
a , Yong Yu c & Yuansuo Zheng a
a Institute of Analytical Sciences, School of Science , Xi'an Jiaotong
University , Xi'an , China
b State Key Laboratory of Loess and Quaternary Geology, IEE, CAS ,
Xi'an , China
c Medical School, Xi'an Jiaotong University , Xi'an , China
Published online: 03 Jun 2013.
To cite this article: Saili Wei , Yuhai Tang , Ruiqin Zhao , Xiaoqian Xu , Guojin Xu , Yong Yu & Yuansuo
Zheng (2013): A Homogeneous Chiral Manganese(III)–Salphe Catalyst for Enantioselective Epoxidation
of Olefins, Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 43:16, 2134-2146
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Synthetic Communications1, 43: 2134–2146, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.656785
A HOMOGENEOUS CHIRAL MANGANESE(III)–SALPHE
CATALYST FOR ENANTIOSELECTIVE EPOXIDATION
OF OLEFINS
Saili Wei,1,2 Yuhai Tang,1 Ruiqin Zhao,1 Xiaoqian Xu,1
Guojin Xu,1 Yong Yu,3 and Yuansuo Zheng1
1Institute of Analytical Sciences, School of Science, Xi’an Jiaotong
University, Xi’an, China
2State Key Laboratory of Loess and Quaternary Geology, IEE,
CAS, Xi’an, China
3Medical School, Xi’an Jiaotong University, Xi’an, China
GRAPHICAL ABSTRACT
Abstract A new chiral Mn(III)–Salphe catalyst was synthesized from natural amino acid
(R)-phenylalanine and 3,5-di-tert-butyl-hydroxybenzaldehyde and applied to the asymmet-
ric epoxidations of unfunctionalized olefins in ionic liquids. Satisfactory enantioselectivities
(79% < ee < 93%) and good yields were achieved when NaClO was used as oxidant. We
found that both the pH value (11.3) and reaction temperature (15 ꢀC) were crucial for
the epoxidation reactions. In our reaction system, NH4OAc was unnecessary. We proposed
that alcoholic hydroxyls in the Mn(III)–Salphe compound played the role of axial ligand.
However, the reaction time was longer than when using Jacobsen’s catalyst because of the
structure of the Mn(III)–Salphe compound, in which coordination geometries by the two
alcoholic hydroxyls with certain angles affected the substrate approaching the Mn(V)¼ 0
Received September 18, 2011.
Address correspondence to Yuhai Tang or Saili Wei, Institute of Analytical Sciences, School of
Science, Xi’an Jiaotong University, Xi’an, 710061, China. E-mail: weipaopao2012@sina.cn; weissaili.
1983@stu.xjtu.edu.cn
2134

Mn(III)–SALPHE CATALYST FOR EPOXIDATION OF OLEFINS
2135
center. The chiral ligand was characterized by the combination of infrared, ultraviolet, and
visible spectra and 1H NMR.
Supplemental materials are available for this article. Go to the publisher’s online edition
of Synthetic Communications1 to view the free supplemental file.
Keywords Chiral Mn(III)–salphe; enantioselective epoxidation; ionic liquid; olefins;
(R)-phenylalanine
INTRODUCTION
Chiral epoxides are valuable fine chemicals and important synthetic intermedi-
ates because of their high regio- and stereoselective ring-opening reactions.[1–7] Since
Jacobsen’s work in 1990 (Scheme 1), chiral Mn(III)–Salen compounds had emerged
as highly efficient catalysts for the enantioselective epoxidation of unfunctionalized
olefins, giving rise to optically active epoxides. Then, the research topics about these
chiral Mn(III)–Salen species have attracted many chemists’ attention to design
and synthesis of new salen ligands and the rationale of the asymmetric induction
from steric and electronic effects of the catalyst structure.[8–10] In these reports,
almost of chiral Mn(III)–Salen catalysts used the chiral diamines (such as 1,2-diami-
nocyclohexane and 1,2-diphenylethane-1,2-diamine) as the chiral source. However,
the chiral diamines are usually expensive and unavailable.
At the molecular level nature is chiral. So, naturally occurring compounds
almost always exist in two enantiomeric forms. Thus, our studies had focused on
the natural amino acids, which are optically pure and commercially available. In this
work, natural amino acids were designed as the source of chiral factors in Salen-type
systems. The chiral moiety of the Mn(III)–Salen catalyst played a significant role in
the asymmetric epoxidation reactions. Catalysts with different chiral fragments
showed different enantioselectivity and enantiofacial selection, resulting in reversed
enantioselectivity in some cases. The present work aimed to synthesize a new chiral
Mn(III)–Salen-like-type ligand by incorporating the transition metal manganese
into the Schiff base compound derived from salicylaldehyde and amino acid. To
Scheme 1. Structure of Jacobsen catalyst.

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S. WEI ET AL.
mimic the porphyrin ring of natural enzymes, (R)-phenylalanine and (R)-tyrosine
containing benzene groups were selected as the chiral precursor of ligands. However,
(R)-tyrosine containing a phenolic hydroxyl has little acidity, which may affect the
acid-sensitive epoxides in asymmetric synthesis. Meanwhile, the easily avaliable
and natural characteristics of (R)-phenylalanine make it ideal as a chiral precursor
introduced into the molecular structures of the catalysts, so (R)-phenylalanine was
chosen as the starting material. The new ligand was designated as Salphe, and its
metal compound as Mn(III)-Salphe.
So far only a few reports about the synthesis and applications of Schiff
base ligands derived from chiral amino acids have been publishied. Mao et al.[11]
synthesized a series of novel Schiff base complexs from arginine having two kinds
of asymmetric –NH2, which have been proven to be efficient catalysts for the
asymmetirc oxidation of b-isophorone. Ando et al.[12] reported a amino acid–Schiff
base complex of oxovanadium(IV) and applied it to the oxidation of sulfide, but it
furnished the sulfoxide in poor enantiomeric excess (5–20%).
In our research, we introduced the chiral (R)-phenylalanine building blocks
into the catalyst molecules by forming Schiff base between amino group (–NH2)
of the phenylalanine and aldehyde (–CHO) of salicyaldehyde (Scheme 2) in the hope
of obtaining chiral catalysts with generally good activity and enantioselectivity.
Herein, we report the preparation of Mn(III)–Salphe complex, its physicochemical
1
properties such as infrared (IR) ultraviolet (UV), and H NMR spectra, and its
application in the epoxidation of unfunctional alkenes.
Scheme 2. Route for preparation of chiral Mn(III)–Salphe catalyst. (Figure is provided in color online.)

Mn(III)–SALPHE CATALYST FOR EPOXIDATION OF OLEFINS
2137
RESULTS AND DISCUSSION
To obtain more details on the structure of the Mn(III)-Salphe, the FT-IR
and UV-Vis spectra of the Mn(III) complex had been measured and compared with
the chiral ligand and the known Jacobsen catalyst.
FT-IR Spectra
=
The vibration of the imine group (C N) of the ligand A (Fig. 1a) was observed
at 1627 cmꢁ1, while it was shifted to a lower frequency at 1615 cmꢁ1 in the complex B
(Fig. 1b). The result indicated that the nitrogen atom of the imine group had already
coordinated to the manganese ion. The new band at 460 cmꢁ1 of the complex B was
associated with the Mn–N bond. In the FT-IR spectrum of the ligand A, the band at
1198 cmꢁ1 was ascribed to the phenolic C–O stretching vibration. Meanwhile, a
similar band was found at 1175 cmꢁ1 in the spectra of the complex B. The (C–O) fre-
quency shift toward lower values was attributed to the coordination action between
oxygen and manganese ion.
UV-Vis Spectra
The UV-vis electronic spectra of the neat complex B (b) and Jacobsen catalyst
(a) are shown in Fig. 2. It could be seen that the chiral complex B demonstrated broad
peaks near 433 nm and at around 508 nm, which were identical with those of Jacobsen
catalyst. It confirmed that the Mn(III)–Salphe complex was formed. The peak
of chiral Mn(III)–Salphe at 433 nm was assigned to charge-transfer transitions
Figure 1. FT-IR spectra of the ligand A(a) and the complex B(b). (Figure is provided in color online.)

2138
S. WEI ET AL.
Figure 2. UV-vis spectra of the Jacobsen catalyst (a) and chiral Mn(III)–Salphe (b). (Figure is provided in
color online.)
between the metal and ligand. Meanwhile the other characteristic absorption peak
at 508 nm was assigned to the d–d transitions of manganese in the Mn(III)–Salphe
compound.[13]
Optimization of pH and Reaction Temperature
According to the literature, the epoxidation conditions were not identical when
the different Mn(III)–Salen catalysts were used.[14–20] To clarify the effect of this new
Mn(III)–Salphe catalyst, our preliminary experiments were run under the following
conditions: 2 mmol of styrene, 10 mol% of Mn(III)–Salphe catalyst, 20 mol% of
NH4OAc, NaOCl=Na2HPO4 buffer solution (pH 11.3), and 4 ml of BMImPF6, at
0 ꢀC. To get the best results, different pH values (10.0, 10.5, 11.3, 12.0, and 12.5)
Figure 3. Effect of pH for ee value (black line, ꢂ79%) and conversion rate (blue line, ꢂ72%). (Figure is
provided in color online.)

Mn(III)–SALPHE CATALYST FOR EPOXIDATION OF OLEFINS
2139
Figure 4. Effect of reaction temperature for ee value (black line, ꢂ85%) and conversion rate (blue line,
ꢂ80%). (Figure is provided in color online.)
and temperature (ꢁ9 ꢀC, ꢁ4 ꢀC, 0 ꢀC, 15 ꢀC, and 25 ꢀC) were investigated respectively.
The results are shown in Figs. 3 and 4.
In Fig. 3, it is obvious that the pH value of NaOCl=Na2HPO4 buffer greatly
impacted both the conversion rate and the ee value. When the pH value of NaOCl=
Na2HPO4 buffer solution was 11.3, the greatest ee value and conversion rate of epox-
ides were obtained. Because the oxidation reaction first occured in the aqueous phase
and produced HOCl as a weak acid (pKa¼ 7.54[21]), HOCl was transferred to the
organic phase to participate in the epoxidation reaction. If the pH value was too high,
less formation of HOCl and low conversion rate of substrate occurred. On the other
hand, if the pH value was too low, a large quantity of HOCl received, which resulted
in poor yield and poor enantioselective of olefin epoxides. So, pH 11.30 was the optimal
value. Furthermore, the choice of reaction temperature was also very important. In
Fig. 4, it is clear that the greatest ee value and conversion rate of epoxides were given
at 15 ꢀC. When the temperature was increased to 25ꢀC, the enantioselectivity decreased.
The reason was that the temperature caused the olefins to approach the catalyst.[22]
Effect of Ammonium Acetate
In the study of catalytic mechanism of olefin epoxidation by the Mn(III)-Salen
catalysts, it was found that adding axial ligands (such as the pyridine nitrogen oxides
and salt ammonium acetate) could increase the reaction rate and enantioselectivity,
because the axial ligand trans to the oxygen atom of Mn(V) ¼ O compound reduced
the Lewis acidity of Mn(III)–Salen compound in order to inhibit the acid-sensitive
epoxides decomposition.[23] In view of the difference of our new catalyst, whether
NH4OAc was important for the asymetric epoxidation of styrene was investigated.
Overall, all reactions proceeded smoothly within 6 h. Ee valus of epoxides in three
cycles were respectively 87%, 85%, and 81% in the presence of NH4OAc and respect-
ively 86%, 83%, and 79% in the absence of NH4OAc. The results showed that the
NH4OAc was not necessary. We proposed that the oxgen atom of the alcoholic

2140
S. WEI ET AL.
hydroxyl group in the Mn(III)–Salphe complex could also coordinate to the manganese
center so as to stabilize the Mn(V)oxo compound in the epoxidation process. The
alcoholic hydroxyl group played a role of axial ligand instead of the NH4OAc. From
the economic standpoint, our new catalyst is more attractive than others.
Catalytic Performances
In preliminary experiments, enantioselective epoxidation reactions were
performed according to our established procedure[24] (Scheme 3). When the reaction
was finished, the reaction mixture was extracted with n-hexane for obtaining epox-
ides. After separation, ionic liquid BMImPF6, including catalysts, could be recycled.
To evaluate our new Mn(III)–Salphe catalyst, Jacobsen catalyst was also used for
the same reactions. Table 1 summarized the catalytic performance of our Mn(III)–
Salphe catalyst and Jacobsen catalyst in the asymetric epoxidation of styrene,
a-methylstyrene, and indene.[25]
Under the optimized reaction conditions, the Mn(III)–Salphe catalyst showed
excellent reaction activities and enantioselectivities in the asymetric epoxidation of
various unfunctionalized olefins (Table 1, entries 2, 4, and 6). The chiral induction
effects of this new Mn(III)–Salphe catalyst are similar to those of the Jacobsen cata-
lysts. For styrene, a-methylstyrene, and indene, the epoxidation reactions proceeded
smoothly and gave the desired products in 89–92% yields with good to excellent enan-
tioselectivities (86% to 93%) respectively (cycle 1). To check the recyclability of the
catalytic system, it was separated from the reaction mixture by extraction of n-hexane
and reused for further epoxidation reactions. All of those substrates were examined
and gave good yields and acceptable ee values (cycles 2, and 3). The results showed
that the catalytic system is still active but with little detriment to yield and ee value
after being recycled three times. Then, the n-hexane extractant was detected by FT-IR
and its spectroscopy showed the characteristic absorption peak of the Mn(III)–Salphe
catalyst in the second and third cycles. It means that a little bit of Mn(III)–Salphe
catalyst was lost during the extraction. However, it was evident that the reaction time
using Mn(III)–Salphe as catalyst was longer than with the Jacobsen catalyst. This was
related to the hindered structure of the Mn(III)–Salphe compound, which affected the
substrate approached to Mn(III) ¼ O center. By computational chemistry methods,
the optimal conformation of Mn(III)–Salphe was obtained by Gaussian03 in Fig. 5,
which showed that manganese atom retained coordination geometries in which one
side of ligand chains was coordinated with a phenolate and an imine nitrogen in
the equatorial plane, and the other side was coordinated in the oblique plane. Mean-
while, the two hydroxyls of animo acid were coordinated in the axial position of each
plane. In other words, there was a big stereospecific blockade close to the manganese
Scheme 3. Enantioselective epoxidation of olefins in BMImPF6.

Mn(III)–SALPHE CATALYST FOR EPOXIDATION OF OLEFINS
2141
Table 1. Epoxidations catalyzed by Jacobsen’s catalyst and Mn(III)–Salphe catalyst with the ionic liquids
as reaction mediaa
eed(%) (yielde(%), timef (h))
Entryb
Substratec
Styrene
Catalyst
Jacobsen
Cycle 1
Cycle 2
Cycle 3
1
2
3
4
5
6
89 (93, 2)
86 (89, 6)
91 (94, 2)
87 (90, 6)
94 (95, 2)
93 (92, 6)
88 (90, 2)
83 (86, 6)
91 (92, 2)
86 (87, 6)
92 (93, 2)
90 (89, 6)
85 (86, 2)
79 (80, 6)
89 (88, 2)
83 (83, 6)
89 (90, 2)
86 (85, 6)
Mn(III)–Salphe
Jacobsen
a-Methylstyrene
Mn(III)–Salphe
Jacobsen
Mn(III)–Salphe
Indene
aReaction conditions: substrate (2 mmol), catalyst (0.2 mmol, 10 mol%), NaClO (5.8 mmol).
bAll entries in this article were numbered consecutively in spite of the table numbers.
cA, styrene; B, a-methylstyrene; C, indene.
dDetermined by HPLC over a chiral OD–H column after comparing the retention times with those of
three racemic epoxide samples.
eYield of the isolated epoxide.
fMonitored by TLC.
Figure 5. Optimal configuration of catalyst. (Figure is provided in color online.)

2142
S. WEI ET AL.
atom, so it should be more difficult for the substrate to approach the Mn(III) ¼ O
center, which resulted in the low rate of reaction and long reaction time for the
enantioselective epoxidation. However, after the oxidant NaClO interacted with
the manganese atom, the substrate from the direction of the chiral amino acid
approached the Mn(III) ¼ O center to form the reaction intermediate, and the close
proximity was a benefit for the asymmetric selectivity.
CONCLUSIONS
In this work, Mn(III)–Salphe catalyst was synthesized by introducing chiral
(R)-phenylalanine into the catalyst molecules via forming imino group and applied
to the asymmetric epoxidations of olefins. The optimization reaction conditions were
that pH was 11.3 and reaction temperature was 15 ꢀC. Meanwhile, the NH4OAc was
unnecessary in the epoxidation reactions because alcoholic hydroxyl in the Mn(III)–
Salphe complex stabilized the Mn(V)oxo intermediate instead of it. The activity and
chiral induction effect of the Mn(III)–Salphe catalyst were satisfactory except for the
longer reaction time. It was related to the structure of Mn(III)–Salphe with coordi-
nation geometries by the two ligands with certain angles, which affected the substrate
approaching to Mn(V) ¼ O center from the direction of the chiral amino acid. The
experiment provided a theoretical basis for further studying the catalytic mechanism
of the Mn(III)-Salen compound.
EXPERIMENTAL
Materials and Methods
1,2-Diaminocyclohexane, 3,5-di-tert-butyl-hydroxybenzaldehyde, and titanium
tetrachloride were obtained from Alfa Aesar. (R)-Phenylalanine, L(þ)-tartaric
acid, manganese acetate, and lithium chloride were laboratory-grade reagents from
local suppliers. Jacobsen’s catalyst was synthesized according to the literature
procedure.[26] All other chemicals were reagent grade and used as received.
The UV-vis and FT-IR spectra of some samples were recorded from 200 to
800 nm on a UV-3310 spectrophotometer and from 400 to 4000 cmꢁ1 on a Nicolet
Avatar 330 FT-IR instrument (using KBr pellets), respectively. Elemental analyses
were performed on an Elementar VarioEL instrument in the Instrumental Analysis
and Research Center of Xi’an Jiaotong University. High-resolution mass spectrometry
(HRMS) operating conditions were as follows: ion source: electric impact ionization
(EI), positive; electric energy: 55 eV; source temperature: 250 ꢀC; detection mode:
ion detector (ID); accelerative voltage: 5000 V; and mass resolution: 10000, tuned by
FC43. 1H NMR spectra of the samples were recorded with a Bruker-400 spectrometer
using tetramethylsilane (TMS) as an internal reference. The chiral stationary phase
column was a Daicel Chiralcel OD-H manufactured by Daicel Chemical Industries
Ltd., and the associated high-performance liquid chromatographic (HPLC) analysis
was carried out on a Shimadzu Instrument (system controller: LC-10AT VP; UV-vis
detector: SPD-10A VP). The reaction products were analyzed by HPLC, containing
UV detection wavelength: 254 nm; eluent agent: n-hexane=i-PrOH, 99=1, v=v; flow
rate: 0.8 ml=min; column pressure: 2.0–3.0 MPa, and column temperature: 25 ꢀC).

Mn(III)–SALPHE CATALYST FOR EPOXIDATION OF OLEFINS
2143
Preparation of Catalysts
A 1000-ml, three-necked flask equipped with a mechanical stirrer, a reflux
condensor, and an addition funnel was charged with 166 mmol NaBH4, 55 mmol
(R)-phenylalanine, and 220 ml THF. The resulting mixture was cooled to 0 ꢀC,
and 60 mmol TiCl4 was added in a steady stream over 30 min under stirring. Mean-
while the reaction produced hydrogen and continued overnight at room tempera-
ture. The stirred solution was heated to reflux for 3 h until no gas evolved and
was cooled to room temperature. Then 200 ml ether was added. In the ice bath,
the reaction was terminated by adding an appropriate amount of water. NaOH
150 ml, was added and stirred for 30 min. The organic layer was separated, dried
over Na2SO4, and concentrated under reduced pressure. The final product, (R)-2-
amino-3-phenyl-1-propanol, was recrystallized from toluene (7.14, yield: 86%) (mp
90–91 ꢀC). FT-IR (KBr) v: 3366, 3299, 3123, 3081, 2941, 2920, 2878, 2842, 2825,
1579, 1495, 1463, 1338, 1122, 1092, 1070, 994, 975, 965, 906, 837, 704 665 cmꢁ1
.
1H NMR (CDCl3, 400 MHz): d ppm 2.52 (d, 1H, 3-H), 2.79(d, 1H, 3-H),
3.08–3.14 (m, 1H, 2-H) 3.38 (d, 1H, 1-H), 3.63 (d, 1H, 1-H), 7.18–7.32 (m, 5H,
PhH). Anal. calcd. for C9H13NO: C, 71.52; H, 8.61; N, 9.27, Found: C, 71.69; H,
8.48; N, 9.23.
A mixture of 3,5-di-tert-butylsalicylaldehyde (1.17 g, 5 mmol) and (R)-2-
amino-3-phenyl-1-propanol (0.585 g, 5 mmol) in EtOH (10 ml) was stirred for 24 h
at the room temperature. Evaporation of the solvent afforded a residue that was
purified on column chromatography (5% hexane=ethyl acetate) to give compound A,
f2,4-di-tert-butyl-6-((1-hydroxy-3-phenylpropan-2-ylimino)methyl)phenolg (1.53 g,
yield: 92%) as yellow liquid. (bp159–161 ꢀC). FT-IR (KBr): v: 3680, 3670, 3120,
1
2910, 1660, 1627, 1510, 1280, 710, 680, 330 cmꢁ1; H NMR (CDCl3, 400 MHz): d
ppm 8.18 (s, 1 H), 7.28 (s, 1 H), 7.21 (d, 2 H), 7.18 (s, 1 H), 7.12 (d, 2 H), 7.08 (s,
1 H), 3.72 (d, 2H), 3.60 (m, 1 H), 2.99 (d, 2H), 1.45 (q, 18 H). Anal. calcd. for
C24H33NO2: C, 78.47; H, 8.99; N, 3.81. Found: C,78.56; H, 8.75; N, 3.74.
The preparation of catalyst is outlined in Scheme 2. Under vigorous stirring,
15 ml of dry ethanol containing 5 mmol of manganese acetate was added dropwise
to 15 ml of dry ethanol solution containing 8 mmol of chiral compound A under
nitrogen protection. The mixture was refluxed for 5 h, and then 10 ml of ethanol
containing 24 mmol of lithium chloride was added to the mixture under stirring
for 3 h. After heating and nitrogen addition were discontinued, the crude product
was collected by evaporation of the solvent under reduced pressure, redissolved in
dichloromethane (50 ml), washed with deionized water (2 ꢃ 15 ml) and brine
(10 ml), and then filtered. The catalyst (compound B) was obtained and dried
under vacuum (0.77 g, yield: 85%(mp 135–136 ꢀC). FT-IR (KBr) v: 3520, 3120,
3040, 2610, 1615, 1540, 1490, 1280, 1175, 990, 460, 310 cmꢁ1; UV–vis (CH2Cl2):
508, 433, 327 nm. Anal. calcd. for C48H64N2O4MnCl: C, 70.03; H, 7.78; N, 3.40.
Found: C, 70.14; H, 7.58; N, 3.37. Meanwhile the HRMS analysis of compound
B was made. Anal. calcd. for C48H64N2O4MnCl: 820.3779. Found: 820.3783.
The results of HRMS and EA analysis for the Mncomplex were almost the
same, which proved the structure of complex. A goal to prepare single crystals
of the Mn(III)–Salphe compound for x-ray diffraction measurements has been
unsuccessful.

2144
S. WEI ET AL.
Enantioselective Epoxidation of Unfunctionalized Olefins
Enantioselective epoxidation reactions were performed according to the
procedure established by our group[24] as follows. First, 2 mmol of unfunctionalized
olefins (styrene, a-methylstyrene, and indene) and 0.4 mmol of axial base NH4OAc
(20 mol% based on the mol of the olefins) were added to 4 ml of reaction
medium (1-n-butyl-3-methylimidazolium hexafluorophosphate, BMImPF6) contain-
ing 0.2 mmol of catalyst (10 mol% based on the mol of the olefins) under stirring.
A solution of Na2HPO4 (0.05 mol=L) was added to a precooled solution of aqueous
NaClO (0.58 mol=L) as an oxidant, and then pH value of the mixed solution was
accurately adjusted to a desirable amount by slow instillation of HCl (1 mol=L)
and NaOH (1 mol=L) solutions. The mixture was kept at the appointed temperature.
After the reaction was finished, 20 ml of n-hexane was directly added to the reaction
mixture, and then a triphase system was formed: organic phase (n-hexane phase,
upper layer), water phase (middle layer), and ionic liquid phase (the bottom). The
n-hexane phase was separated and concentrated under reduced pressure, and then
the residue was purified by column chromatography (SiO2, petroleum ether=CH2Cl2,
2=1, v=v). On the other hand, the ionic liquid including catalyst was also separated
for recycling.
SUPPLEMENTARY MATERIAL
A list of HPLC chromatograms of the epoxide products of styrene,
a-methylstyrene, and indene in Table 1 is available online.
ACKNOWLEDGMENTS
The authors are grateful to the chairperson Foundation of Xi’an Jiaotong
University (No. 01380011) for financial support. At the same time, the authors
gratefully acknowledge Langchong He for helpful discussions in chiral HPLC
separations.
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