Asymmetric epoxidation of olefins using novel chiral dinuclear Mn(III)-Salen complexes with inherent phase-transfer capability in ionic liquids

Article abstract of DOI:10.1002/chir.20864

Two new chiral dinuclear Mn(III)-Salen complexes with inherent phase-transfer capability have been synthesized, which serve as catalysts in the asymmetric epoxidation of nonfunctionalized alkenes. Experimental results show these complexes are effective catalysts for the asymmetric epoxidation of some cyclic alkenes and the catalysts have certain inherent phase-transfer capability during the epoxidation because of their weak water solubility. In general, good enantioselectivity and acceptable yields were achieved when NaClO was used as oxidant under three different reaction systems. Among these alkenes, the catalyst 6a gave the highest ee (94%) for 6-chloro-2,2-dimethylchromene in the presence of ionic liquid 2. Additionally, the recovery and recycling of one dimeric Mn(III)-Salen complex were tested to investigate atom efficiency of the catalyst in different reaction systems on the alkenes epoxidations. The catalyst 6a could be recovered and recycled for six times without losing activity and selectivity.

Full text of DOI:10.1002/chir.20864

CHIRALITY 23:69–75 (2011)
Asymmetric Epoxidation of Olefins Using Novel Chiral Dinuclear
Mn(III)-Salen Complexes with Inherent Phase-Transfer
Capability in Ionic Liquids
LONGHAI CHEN,1 FEIXIANG CHENG,2 LEI JIA,1 AIJIANG ZHANG,1 JINCAI WU,1* AND NING TANG
1
*
1College of Chemistry and Chemical Engineering and State Key Laboratory of Applied Organic Chemistry,
Lanzhou University, Lanzhou, People’s Republic of China
2College of Chemistry and Chemical Engineering, Qujing Normal University, Qujing, People’s Republic of China
ABSTRACT
Two new chiral dinuclear Mn(III)-Salen complexes with inherent
phase-transfer capability have been synthesized, which serve as catalysts in the asym-
metric epoxidation of nonfunctionalized alkenes. Experimental results show these com-
plexes are effective catalysts for the asymmetric epoxidation of some cyclic alkenes and
the catalysts have certain inherent phase-transfer capability during the epoxidation
because of their weak water solubility. In general, good enantioselectivity and accepta-
ble yields were achieved when NaClO was used as oxidant under three different reac-
tion systems. Among these alkenes, the catalyst 6a gave the highest ee (94%) for 6-
chloro-2,2-dimethylchromene in the presence of ionic liquid 2. Additionally, the recov-
ery and recycling of one dimeric Mn(III)-Salen complex were tested to investigate atom
efficiency of the catalyst in different reaction systems on the alkenes epoxidations. The
catalyst 6a could be recovered and recycled for six times without losing activity and
C
VV
selectivity. Chirality 23:69–75, 2011.
2010 Wiley-Liss, Inc.
KEY WORDS: dinuclear Mn(III)-salen; asymmetric catalysis; ionic liquids; epoxidation;
phase-transfer; recyclable
INTRODUCTION
centers of the catalysts could enhance the catalytic activ-
ities and catalyst recovery.20,21 Many attempts have been
made to use ionic liquids in asymmetric epoxidation of
alkenes because ionic liquids as green reaction media can
improve the separation, activity, and recycling of the cata-
lysts.22–26 On the basis of these views, we have synthe-
sized two new chiral dinuclear Mn(III)-Salen complexes
with two catalytically active metal centers and investigated
their activities in asymmetric epoxidation of alkenes in the
presence of ionic liquids. Since the transportation of HOCl
from water to oil phase is required for salen Mn(III) com-
plexes catalyzed epoxidation, the design of the water solu-
ble salen Mn(III) complexes become an attractive target.27
To enhance the solubility of dimeric salen Mn(III) com-
plexes in water, we used N,N-dibutylamine to substitute
other groups of salicylaldehyde at the 5,50positions.28,29
Herein, we have extended the application of chiral dimeric
salen Mn(III) complexes in the asymmetric epoxidation
Chiral epoxides have two stereocenters and can be con-
verted into a variety of compounds by means of regioselec-
tive ring opening, so epoxides are useful synthetic inter-
mediates for the synthesis of chiral pharmaceuticals and
fine chemicals.1–5 It is still a challenge to get excellent
enantioselectivity epoxides in high yield. Among several
previously reported catalytic systems, chiral Mn(III)-Salen
complexes have received considerable attention in recent
years in asymmetric epoxidation of alkenes due to their
availability and wide usefulness. Since Jacobsen and Ka-
tsuki first reported these key findings, much attention has
been devoted to expand this system. It is demonstrated
that cooperation of steric factors, electronic effects, cata-
lytic centers, and cocatalysts play an important role in the
asymmetric catalysis.6,7 Consequently, the design of novel
chiral catalysts constitutes an important strategy in asym-
metric catalysis.8,9 To enhance atom efficiency and de-
velop environmentally friendly technology, many efforts
have been devoted to investigate chiral salen complexes in
the asymmetric catalysis,10–14 an important strategy is the
recycle of the salen metal complexes due to the high cost
of chiral salen ligands. Therefore, the immobilizations of
Salen catalysts on polymer, organic or inorganic, and ionic
liquids have been developed.15–19 In these recycling sys-
tems, it is presumed that increasing the molecular weight
of the catalysts would lead to recycle the catalysts more
Contract grant sponsor: National Natural Science Foundation of China;
Contract grant number: 20601011.
Contract grant sponsor: Fundamental Research Funds.
*Correspondence to: Jincai Wu or Ning Tang, College of Chemistry and
Chemical Engineering and State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, Lanzhou 730000, People’s Republic of
China. E-mail: wujc@lzu.edu.cn or tangn@lzu.edu.cn
Received for publication 10 June 2009; Accepted 10 March 2010
DOI: 10.1002/chir.20864
Published online 14 May 2010 in Wiley Online Library
easily. Additionally, increasing catalytically active metal (wileyonlinelibrary.com).
C
VV
2010 Wiley-Liss, Inc.

70
CHEN ET AL.
using NaOCl as an terminal oxidant with various reaction NMR (400 MHz, CDCl3): d 13.72 (1H, s, OH); 8.39 (1H, d,
solvents. The catalysts showed high chiral induction in the CHN); 7.28–7.15 (10H, m, ArH); 7.06 (1H, s, ArH); 6.94 (1H,
epoxidation of bulkier olefins with short reaction time, and s, ArH); 4.73 (1H, s, CH); 4.37 (1H, d-d, CH); 3.74–3.69 (2H,
the dimeric salen Mn(III) complex 6a can be effectively m, NH2); 3.48 (2H, s, CH2); 2.40–2.33 (4H, m, CH2); 1.46
recycled for six times. To the best of our knowledge, this (4H, s, CH2); 1.41 (9H, s, C(CH3)3); 1.38–1.20 (4H, m, CH2);
is the first report of dimeric Mn(III)-salen complexes with 0.89–0.82 (6H, m, CH3). LC-MS: m/z 514.0 [M 1 H]1. Anal.
inherent phase-transfer capability as recyclable catalysts Calcd for C34H47N3O: C, 79.49; H, 9.22; N, 8.18. Found: C,
for the asymmetric epoxidation of nonfunctionalized 79.54; H, 9.16; N, 8.16.
alkenes in the presence of ionic liquids.
Preparation of Two Chiral Dinucleating Ligands and
Corresponding Mn(III) Complexes
EXPERIMENTAL SECTION
General Remarks
Synthesis of 5,5-methylenedi-[(R,R)-{N-(3-tert-butyl-
salicylidine)-N0-(30-tert-butyl-50-(N,N-dibutylmethylene)
1H NMR spectra were recorded on a Mercury Plus 400
spectrometer with TMS as internal standard. IR spectra
were obtained on a Nicolet 170SX FTIR spectrophotome-
ter as KBr discs. ESI-HRMS were performed on a Bruker
Daltonics APEXII47e mass spectrometer. LC-MS were
performed on a Bruker Daltonics Esquire6000 mass spec-
trometer. Elemental analyses were taken using a Perkin
Elmer 240C analytical instrument. All reactions were moni-
tored by TLC. TLC was performed on glass plates coated
with silica gel 60F254. The crude products were purified
by flash chromatography. High-performance liquid chro-
matograph with Daicel Chiralcel OD-H chiral column was
used for the measurements (hexane:i-PrOH 5 99:1).
salicyladhyde)}-1,2-cyclohexanediamine]
(5a). The
ethanol (10 ml) solution of (1R,2R)-N-(2-hydroxyl-3-tert-
butyl-5-(N,N-dibutylmethylene)-1-amino-2-cyclohexaneimine
(4a) (2 mmol) was added slowly to the ethanol (10 ml) so-
lution of 5,50-methylene-di-3-tert-butylsalicyladehyde (1
mmol). The mixture was stirred under refluxing for 8 h,
and then solvent was removed under reduced pressure to
afford the crude product. The crude product was purified
by flash chromatography (SiO2, petroleum ether/EtOAc 5
1
10:1) to generate a yellow oil (92%). H NMR (400 MHz,
CDCl3): d 13.74 (2H, s, OH); 13.70 (2H, s, OH); 8.28 (2H, s,
CHN); 8.22 (2H, s, CHN); 7.21 (2H, s, ArH); 7.05 (2H, s,
ArH); 6.93 (2H, s, ArH); 6.74 (2H, s, ArH); 3.67 (2H, s,
CH2); 3.39 (4H, s, CH2); 3.33 (4H, s, CH); 2.36 (8H, s,
CH2); 1.88 (8H, s, CH2); 1.70 (8H, s, CH2); 1.60 (8H, s,
CH2); 1.42 (18H, s, C(CH3)3); 1.39 (18H, s, C(CH3)3); 1.28–
1.23 (8H, m, CH2); 0.96–0.78 (12H, m, CH3). ESI-HRMS:
m/z 1163.8974 [M 1 H]1. Anal. Calcd for C75H114N6O4: C,
77.41; H, 9.87; N, 7.22. Found: C, 77.49; H, 9.81; N, 7.31.
FTIR (KBr): 3345, 2929, 2862, 1631, 1441, 1385, 1266, 1205,
Preparation of Two Chiral Half-Unit Ligands (4a and 4b)
Synthesis of (1R,2R)-N-[2-hydroxyl-3-tert-butyl-5-
(N,N-dibutylmethylene)]-1-amino-2-cyclohexaneimine
(4a). The CHCl3 (10 ml), solution of 2-hydroxyl-3-tert-
butyl-5-(N,N-dibutylmethylene)benzaldehde3 (1 mmol)
was added slowly to the CHCl3 (10 ml) solution of
(1R,2R)-(2)-diaminocyclohexane (1 mmol) with vigorous
stirring at 08C. The reaction mixture was stirred for 48 h
before stirring was discontinued, then the solvent was
removed under reduced pressure affording the crude
product. The crude product was purified by flash chroma-
tography (SiO2, petroleum ether/EtOAc 5 4:1) to gener-
ate a yellow oil (68%). 1H NMR (400 MHz, CDCl3): d 13.82
(1H, s, OH); 8.35 (1H, d, CHN); 7.06 (1H, s, ArH); 6.92
(1H, s, ArH); 4.12 (1H, d-d, CH); 3.38 (1H, s, CH); 3.47
(2H, s, CH2); 2.92–2.74 (2H, m, NH2); 2.40–2.36 (4H, m,
CH2); 2.06–1.60 (8H, m, CH2); 1.43 (4H, s, CH2); 1.39 (9H,
s, C(CH3)3); 1.30–1.24 (4H, m, CH2); 0.90–0.82 (6H, m,
CH3). LC-MS: m/z 416.1 [M 1 H]1. Anal. Calcd for
C26H45N3O: C, 75.13; H, 10.91; N, 10.11. Found: C, 75.21;
H, 10.87; N, 10.93.
1181, 1094, 1023, 963, 750, 689 cm21
.
Synthesis of 5,5-methylenedi-[(R,R)-{N-(3-tert-butyl-
salicylidine)-N0-(30-tert-butyl-50-(N,N-dibutylmethylene)
salicyladhyde)}-1,2-diphenylethylenediamine] (5b).
The ethanol (10 ml) solution of (1R,2R)-N-(2-hydroxyl-3-
tert-butyl-5-(N,N-dibutylmethylene)-amino-1,2-diphenyletha-
neimine (4b) (2 mmol) was added slowly to the ethanol
(10 ml) solution of 5,50-methylene-di-3-tert-butylsalicylade-
hyde (1 mmol) with vigorous stirring at room temperature
The reaction mixture was stirred for 16 h, then solvent was
removed under reduced pressure affording the crude prod-
uct. The crude product was purified by flash chromatogra-
phy (SiO2, petroleum ether/EtOAc 5 8:1) to generate a
1
yellow oil (91%). H NMR (400 MHz, CDCl3): d 13.63 (2H,
Synthesis of (1R,2R)-N-(2-hydroxyl-3-tert-butyl-5- s, OH); 13.59 (2H, s, OH); 8.33 (2H, s, CHN); 8.25 (2H, s,
(N,N-dibutylmethylene)]-1-amino-1,2-diphenylethanei- CHN); 7.23–7.09 (20H, m, ArH); 7.08 (2H, s, ArH); 7.04
mine (4b). The ethanol (10 ml) solution of 2-hydroxyl-3- (2H, s, ArH); 6.97 (2H, s, ArH); 6.70 (2H, s, ArH); 4.74–
tert-butyl-5-(N,N-dibutylmethylene)benzaldehde3 (1 mmol) 4.67 (4H, m, CH); 3.67 (2H, s, CH2); 3.48 (4H, s, CH2);
was added slowly to the ethanol (10 ml) solution of (1R,2R)- 2.40 (8H, s, CH2); 1.58 (8H, s, CH2); 1.42 (18H, s,
(1)-diphenyldiamine (1 mmol) with vigorous stirring at 08C. C(CH3)3); 1.37 (18H, s, C(CH3)3); 1.29–1.23 (8H, m, CH2);
The reaction mixture was stirred for 48 h before stirring 0.98–0.82 (12H, m, CH3). ESI-HRMS: m/z 1359.9287 [M 1
was discontinued, then the solvent was removed under H]1. Anal. Calcd for C91H118N6O4: C, 80.37; H, 8.75; N,
reduced pressure affording the crude product. The crude 6.18. Found: C, 80.46; H, 8.86; N, 6.11. FTIR (KBr): 3318,
product was purified by flash chromatography (SiO2, petro- 2955, 2865, 1627, 1440, 1384, 1264, 1208, 1159, 1094, 1062,
1
leum ether/EtOAc 5 2:1) to generate a yellow oil (86%). H 908, 734, 700 cm21
.
Chirality DOI 10.1002/chir

ASYMMETRIC EPOXIDATION OF OLEFINS
71
Synthesis of 5,5-methylenedi-[(R,R)-{N-(3-tert- was chloromethylated30 and reacted with N,N-dibutyl-
butylsalicylidine)-N0-(30-tert-butyl-50-(N,N-dibutylme- amine giving 2-hydroxy-3-t-butyl-5-(N,N-dibutylmethylene)-
thylene)salicyladhyde)}-1,2-cyclohexanediaminato(2-) salicyladhyde 3 with high yield.31 Compound 3 reacted
manganese(III)chloride] (6a). The ethanol solution of with (1R,2R)-(2)-diaminocyclohexane and (1R,2R)-(1)-
5a (1 mmol) was stirred under reflux with the ethanol so- diphenyldiamine, respectively, in a 1:1 molar ratio afford-
lution of Mn(OAc)2ꢀ4H2O (3 mmol) under nitrogen atmos- ing half-unit ligands 4a and 4b. The synthetic procedure
phere for 6 h. The reaction mixture was cooled to room of 4b was carried out with some modification following
temperature. Lithium chloride (6 mmol) was added and the reported method.32 Dinucleating ligands (5a and 5b)
the resulting mixture was refluxed for additional 2 h while were synthesized by condensation of 5,50-methylene-di-3-t-
exposed to air. Then solvent was removed under reduced tert-butylsalicylaldehyde with 4a and 4b, respectively.33
pressure and the residue was extracted with dichlorome- The synthetic produce of dinucleating ligands have differ-
thane (3 3 10 ml). The extract was washed with water (2 ent reaction conditions because the ligand 4b reacted with
3 10 ml) and brine and dried over anhydrous Na2SO4, 5,50-methylene-di-3-t-tert-butylsalicylaldehyde can rear-
and then concentrated to give the crude product. The range to monomeric salen ligand without any dinucleating
crude product was recrystallized with petroleum ether ligand 5b when the mixture were heated to reflux. Finally,
affording the desired complex 6a (yield 85%) as dark ligands 5a and 5b reacted with manganese yielding 5,5-
brown powder. LC-MS: m/z 1304.3 [M 2 Cl]1, 635.9 [M methylenedi-[(R,R)-{N-(3-tert-butylsalicylidine)-N0-(30-tert-
2 2Cl]21. Anal. Calcd for C75H110Cl2Mn2N6O4: C, 67.20; butyl-50-(N,N-dibutylmethylene)salicyladhyde)}-1,2-cyclo-
H, 8.27; N, 6.27. Found: C, 67.31; H, 8.34; N, 6.19. FTIR hexanediaminato(2-)manganese(III)chloride] 6a and
(KBr): 3400, 2953, 2866, 2362, 1613, 1541, 1435, 1387, 5,5-methylenedi-[(R,R)-{N-(3-tert-butylsalicylidine)-N0-(30-tert-
1342, 1311, 1169, 1028, 830 cm21
.
butyl-50-(N,N-dibutylmethylene)salicyladhyde)}-1,2-diphe-
nylethylenediaminato (2-)manganese(III)chloride] 6b,
respectively. Chiral dinuclear Mn(III)-Salen complexes
6a and 6b were characterized by LC-MS, FTIR, and Ele-
mental analyses.
Synthesis of 5,5-methylenedi-[(R,R)-{N-(3-tert-butyl-
salicylidine)-N0-(30-tert-butyl-50-(N,N-dibutylmethylene)
salicyladhyde)}-1,2-diphenylethylenediaminato(2-)man-
ganese(III)chloride] (6b). Complex 6b was prepared as
a brown solid (yield 87%) according to the same procedure
The Catalysis of the Novel Chiral Dimeric
Mn(III) Complexes
as 6a. LC-MS: m/z 1500.5 [M 2 Cl]1, 732.7 [M 2 2Cl]21
.
Anal. Calcd for C91H114Cl2Mn2N6O4: C, 71.12; H, 7.48; N,
5.47. Found: C, 71.20; H, 7.43; N, 5.52. FTIR (KBr): 3409,
2954, 2867, 2364, 1610, 1539, 1420, 1386, 1343, 1308, 1169,
The asymmetric epoxide reactions catalyzed by the two
novel dimeric catalysts were systematically investigated
with the substrates of styrene (A), a-methylstyrene (B),
trans-stilbene (C), indene (D), 1,2-dihydronaphthalene
(E), 2,2-dimethylchromene (F), and 6-chloro-2,2-dimethyl-
chromene (G). As is known to all, the organic cocatalyst
plays an important role in the Jacobsen epoxidation
because it coordinates to the unsaturated Mn(III) species
emerging form the first catalytic cycle and prevents this
reacting with active Mn(V) oxo species to produce catalyti-
cally inactive l-oxo-Mn(IV) dimmer.34,35 Based on this
point, Pyridine N-oxide was used as cocatalyst since it has
remarkable effects on both the activity and enantioselectiv-
ity of the enantioselcetive epoxidation. To apprehend the
effects of the solvent factors on alkenes epoxidation, the
asymmetric epoxidation experiments were preformed in
pure CH2Cl2, mixed solvent of CH2Cl2/ionic liquid 1, and
mixed solvent of CH2Cl2/ionic liquid 2, respectively, at
08C using buffer NaOCl as terminal oxidant with pyridine
N-oxide as cocatalyst. As expected, reaction solvent plays
a crucial role in asymmetric epoxidation of alkenes. After
the completion of the reaction, products in ionic liquids
system can be easily separated with the extraction by hex-
ane than in pure CH2Cl2. It was also observed that the
reaction time of ionic liquids system was shorter than the
system without ionic liquid, since ionic liquids can stabi-
lize the catalyst or ionic intermediates and enhance the ac-
tivity of the catalyst.36,37 The results data were summar-
1006, 828 cm21
.
General Procedure in Epoxidation Reactions
Substrate (2 mmol), 0.3 mmol PyNO, and ionic liquids
were added to the dichloromethane (2 ml) solution of
dinuclear Mn(III)-Salen complex 6a/6b (0.1 mmol). The
mixture was stirred vigorously at 08C, then NaOCl which
was combined with Na2HPO4 (0.05 mol/l) as oxidant was
added to the solution. The pH of the oxidant was adjusted
to 11.3 by the instillation of 1 mol/l HCl and 1 mol/l
NaOH solutions. After the reaction was totally finished,
the mixture was diluted with CH2Cl2 (2 3 10 ml). The or-
ganic layer was separated, washed with water and brine,
and then dried with MgSO4. The solvent was removed
under reduced pressure affording the crude product,
which was purified by flash chromatography (SiO2, petro-
leum ether/CH2Cl2 5 2:1) to yield the epoxide. Addition-
ally, the dinuclear catalysts can be precipitated by adding
n-hexane after partial concentration of the solvent for
recycling.
RESULTS AND DISCUSSION
The Synthesis of the Novel Chiral Dimeric
Mn(III) Complexes
According to the previous studies, two chiral dinucleat- ized in Table 1. According to these data, in pure CH2Cl2
ing ligands (5a and 5b) and corresponding Mn(III) com- system dimeric salen Mn(III) complex 6a gives high
plexes (6a and 6b) were prepared (as show in Scheme 1). entantioselectivity and chemical yield of alkenes (yield 76–
Additionally, in a stepwise manner, 3-t-butylsalicyladhyde 89%, ee 21–93%) (Table 1, entries 1, 3, 5, 7, 9, 11, and 13).
Chirality DOI 10.1002/chir

72
CHEN ET AL.
Scheme 1. Reagents and conditions: (i) paraformaldehyde, conc. HCl, tetrabutylammonium bromide, rt, 48 h, 93%; (ii) N,N-dibutylamine, toluene,
Et3N, reflux, 8 h, 84%; (iii) (1R,2R)-(1)-diphenyldiamine, EtOH, 08C, 48 h, 86%; (iv) (1R,2R)-(2)-diaminocyclohexane, CH3Cl, 08C, 48 h, 68%; (v) 5,50-
methylene-di-3-tert-butylsalicyladehyde, EtOH, rt, 16 h, 91%; (vi) 5,50-methylene-di-3-tert-butylsalicyladehyde, EtOH, reflux,
8 h, 92%; (vii (1)
Mn(OAC)2ꢀ4H2O, EtOH, reflux, 6 h, (2) LiClꢀH2O, EtOH, reflux, 2 h, 87%; and (viii) (1) Mn(OAC)2ꢀ4H2O, EtOH, reflux, 6 h, (2) LiClꢀH2O, EtOH, reflux,
2 h, 85%.
The use of 6b instead of 6a led to slightly reduced entan-
In the three different solvent systems, in most cases,
tioselectivity and chemical yield (yield 53–80%, ee 15–78%) both catalysts gave the highest ee and the highest chemi-
(Table 1, entries 2, 4, 6, 8, 10, 12, and 14). In general, the cal yields in the presence of ionic liquid 2, especially for 6-
salen-Mn(III) complex derived from chiral diaminocyclo- chloro-2,2-dimethylchromene (yield 93%, ee 94%) (Table 1,
hexane (6a) afforded higher entantioselectivity on alkenes entries 1–14). For instance, the reaction of 6a in ionic liq-
epoxidation than its analogue catalyst from chiral diphenyl- uid 2 gave better enantioselectivity and chemical yield
diamine (6b). This feature may be attributed to the built than in ionic liquid 1 (yield 72–93%, ee 29–94%) (Table 1,
in phase-transfer capability in the catalytic system.
entries 1, 3, 5, 7, 9, 11, and 13). While 6b had better enan-
Chirality DOI 10.1002/chir

ASYMMETRIC EPOXIDATION OF OLEFINS
73
TABLE 1. Asymmetric epoxidation of alkenes using complexes 6a and 6b as catalyst in the presence of ionic liquidsa
eec (%) (yieldd (%), timee(h))
Entry
Alkeneb
Catalyst
PyNO
Ionic liquid 1g
Ionic liquid 2h
Configurationf
1
2
3
4
5
6
7
8
A
A
B
B
C
C
D
D
E
E
F
6a
6b
6a
6b
6a
6b
6a
6b
6a
6b
6a
6b
6a
6b
48 (80, 3.5)
33 (63, 3.5)
31 (66, 2.5)
18 (63, 2.5)
21 (84, 4.0)
15 (80, 4.0)
75 (83, 3.0)
64 (74, 3.0)
54 (60, 3.0)
35 (53, 3.0)
90 (78, 2.0)
78 (77, 2.0)
93 (89, 2.5)
78 (80, 2.5)
32 (85, 3.0)
34 (71, 3.0)
27 (76, 2.0)
21 (74, 2.0)
21 (88, 3.0)
11 (83, 3.0)
68 (79, 2.0)
72 (76, 2.0)
38 (68, 2.5)
33 (53, 2.5)
83 (80, 1.0)
87 (80, 1.0)
87 (91, 1.5)
84 (85, 1.5)
49 (88, 2.5)
36 (73, 2.5)
30 (82, 2.0)
25 (69, 2.0)
29 (89, 2.0)
16 (86, 2.0)
76 (84, 1.5)
62 (80, 1.5)
55 (72, 2.0)
36 (58, 2.0)
91(89, 1.0)
77 (84, 1.0)
94 (93, 1.0)
79 (86, 1.0)
R-(1)
R-(1)
R-(1)
R-(1)
1R,2R-(1)
1R,2R-(1)
1R,2S-(2)
1R,2S-(2)
1S,2R-(1)
1S,2R-(1)
3R,4R-(1)
3R,4R-(1)
3R,4R-(1)
3R,4R-(1)
9
10
11
12
13
14
F
G
G
aReaction conditions: substrate (2 mmol), catalyst (5 mol %), NaOCl (4 mmol), PyNO (15 mol %).
bA, styrene; B, a-methylstyrene; C, trans-stilbene; D, indene; E, 1,2-dihydronaphthalene; F, 2,2-dimethylchromene; G, 6-chloro-2,2-dimethylchromene.
cDetermined by HPLC on chiral OD-H column.
dIsolated yield.
eMonitored by TLC every other 20 min.
fAbsolute configuration was determined by comparison of the sign of [a]D with the literature value.
gIonic liquid 1: [BMIM]PF6.
hIonic liquid 2: L-1-ethyl-3-(10-hydroxy-20-propanyl)imidazolium bromide [38].
tioselectivity and modest chemical yield in ionic liquid 2 inherent phase-transfer capability of 6a and 6b with meth-
too for the epoxidation of styrene, a-methylstyrene, trans- ylene aminoalkyl groups on their salicylaldehyde moieties.
stilbene, and 1,2-dihydronaphthalene than in ionic liquid 1 The fact of the epoxidation reaction time was shorten than
(ee 16–36%) (Table 1, entries 2, 4, 6, and 10). It is pre- general Jocobsen Salen Mn(III), also demonstrates the in-
sumed that chiral catalyst cooperates with the ionic liquid herent phase-transfer capability of 6a and 6b.13 It was also
2 which contains one chiral carbon in asymmetric epoxi- observed that the reaction time in solvent of CH2Cl2/ionic
dation reaction. When ionic liquid 1 was used in the epoxi- liquid 2 was shorter than in CH2Cl2/ionic liquid 1 system.
dation of A, B, C, D, and E, moderate ee values of alkenes This fact may result from high miscibility of ionic liquid 1
with acceptable chemical yields were observed (Table 1, with water and a higher proportion of the catalyst in the
entries 1–10), but significantly improved ee values and aqueous phase, in which the catalyst might be unstable.25
chemical yields were observed in the epoxidation of 2,2-
The Recovery and the Recycling of the Dimeric
dimethylchromene and 6-chloro-2,2-dimethylchromene
(Table 1, entries 11–14). To our surprise, in this system,
Mn(III) Complexes (6a)
catalyst 6b gave better ee values for indene (72%), 2,2-
To further investigate the atom efficiency of dimeric
dimethylchromene (87%), and 6-chloro-2,2-dimethylchro- Mn(III)-Salen complexes with inherent phase-transfer
mene (84%) than other two systems. (Table 1, entries 8, capability, the recycling experiments of catalyst 6a in three
12, and 14). This result is definitively better than that catalytic conditions were examined. Epoxidation of 6-
reported by Kureshy et al.28 The remarkable improvement chloro-2,2-dimethylchromene was carried out with catalyst
in the performance of 6b can be attributed to the coopera- 6a under optimized reaction conditions for reusability
tion of catalyst, PyNO, and ionic liquids. When PyNO was experiments. The catalyst 6a can be easily separated from
used in the asymmetric epoxidation without ionic liquids, the concentrated reaction mixture by the addition of excess
although ee values of epoxides were better than ionic liq- of n-hexane due to its high molecular weight and lower sol-
uid 1, the chemical yields were lower than ionic liquid 1 ubility. The recycled complex was washed with hexane,
in most cases. That because ionic liquids have strong coor- dried in vacuum, and used for the subsequent run without
dination action with the metal center of chiral dinuclear any purification. The recycling data were listed in Table 2.
Mn(III)-Salen, which occupy the most of coordination sites In CH2Cl2 system, 80% yield was achieved in the last reac-
and strongly influence the catalytically active species.20
tion with only minor decrease in the entantioselectivity
After the completion of the asymmetric epoxidation, the compared with the first run (Table 2, entries 1–6). In ionic
water phase converted slightly yellow, which indicates the liquid 1 system, 72% yield was afforded in the last reaction
Chirality DOI 10.1002/chir

74
CHEN ET AL.
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Catalytic cycle
PyNO
Ionic liquid 1
Ionic liquid 2
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1
2
3
4
5
6
93 (89, 2.5)
91 (86, 3.5)
90 (83, 3.5)
88 (83, 4.0)
85 (80, 4.5)
83 (80, 5.5)
87 (91, 1.5)
87 (90, 2.5)
84 (88, 2.5)
83 (84, 3.0)
81 (77, 3.5)
78 (72, 3.5)
94 (93,1.0)
91 (90, 1.5)
90 (87, 2.0)
90 (86, 2.5)
89 (82, 3.0)
83 (76, 3.0)
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with only minor decrease in the entantioselectivity (Table
2, entries 1–6). In ionic liquid 2 system, 83% entantioselec- 10. Gill CS, Venkatasubbaiah K, Phan NTS, Weck M, Jons CW. Enhanced
cooperativity through design: pendant CoIII_salen polymer brush cata-
tivity was obtained in the last reaction with only modest
increasing of the reaction time (Table 2, entries 1–6).
According to these data, it is evident that the catalyst 6a
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alkenes with high atom efficiency. The results of recycling
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CONCLUSIONS
In conclusions, two new chiral dinuclear Mn(III)-Salen
complexes with inherent phase-transfer capability have
been prepared. They are efficient catalysts for the asym-
metric epoxidation of nonfunctionalized alkenes with good-
to-excellent chemical yields and enantioselectivity in the
presence of ionic liquids, especially epoxidation of 6-chloro-
2,2-dimethylchromene (yield 93%, ee 94%) with catalyst 6a.
The chiral catalysts can be recovered easily and reused for
six times and still retain its high atom efficiency. The inher-
ent phase-transfer capability of weakly water soluble cata-
lysts had been revealed through the catalytic effect of their
collaboration in different solvents and the color change of
the upper water at the end of epoxidation. This work
emphasizes a combinational strategy to encourage more
attempts to explore new effective and recoverable oligo-
meric catalysts with strongly water solubility to transport
HOCl from water to oil phase. However, the asymmetric
induction was low in some cases, further optimizations and
studies of the scope and mechanisms of the chiral bimetal-
lic catalyst for asymmetric epoxidation of alkenes with the
ionic liquids are ongoing in our laboratory.
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