Stable chiral salen Mn(III) complexes with built-in phase-transfer capability for the asymmetric epoxidation of unfunctionalized olefins using NaOCl as an oxidant

Article abstract of DOI:10.1016/j.jcat.2011.12.016

A series of novel chiral salen Mn(III) complexes with inherent phase-transfer capability were prepared by covalent linkage of the limidazolium IL moieties containing various PEG chains with chiral salen ligand at two sides of 5,5′-position. Technologies of characterization well suggested the presence of polyether chain, the IL linker, and the intact active sites in the complexes. The amphipathic nature of the PEG chain allowed the PEG-based catalysts to undergo built-in phase transfer, which in turn increased the reaction rates in water/organic biphasic systems. Enantioselective epoxidation of styrene, α-methylstyrene, indene, 1,2-dihydronaphthalene, 6-cyano-2,2-dimethylchromene, and 6-nitro-2,2-dimethylchromene catalyzed by the complexes with NaOCl gave >99% conversions within 60 min. The enantiomeric excess (ee) for the epoxides was in the range of 68-93%, except for styrene (ee, 35%) and α-methylstyrene (ee, 42%). Furthermore, the PEG-based complexes were stable and could be separated from the reaction mixture by control of the solvent.

Full text of DOI:10.1016/j.jcat.2011.12.016

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Journal of Catalysis 287 (2012) 170–177
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Stable chiral salen Mn(III) complexes with built-in phase-transfer capability for
the asymmetric epoxidation of unfunctionalized olefins using NaOCl as an oxidant
a
a
a
a,b,
Rongchang Luo a, Rong Tan a, , Zhigang Peng , Weiguo Zheng , Yu Kong , Donghong Yin
⇑
⇑
a Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education), Hunan Normal University, Changsha, Hunan 410081, China
b Technology Center, China Tobacco Hunan Industrial Corporation, No. 426 Laodong Road, Changsha, Hunan 410014, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel chiral salen Mn(III) complexes with inherent phase-transfer capability were prepared by
covalent linkage of the limidazolium IL moieties containing various PEG chains with chiral salen ligand at
two sides of 5,50-position. Technologies of characterization well suggested the presence of polyether
chain, the IL linker, and the intact active sites in the complexes. The amphipathic nature of the PEG chain
allowed the PEG-based catalysts to undergo built-in phase transfer, which in turn increased the reaction
Received 28 October 2011
Revised 16 December 2011
Accepted 21 December 2011
Available online 16 January 2012
rates in water/organic biphasic systems. Enantioselective epoxidation of styrene,
indene, 1,2-dihydronaphthalene, 6-cyano-2,2-dimethylchromene, and 6-nitro-2,2-dimethylchromene
catalyzed by the complexes with NaOCl gave >99% conversions within 60 min. The enantiomeric excess
(ee) for the epoxides was in the range of 68–93%, except for styrene (ee, 35%) and
a-methylstyrene,
Keywords:
Chiral salen Mn(III) complex
Phase-transfer catalysis
Ionic liquid
a-methylstyrene
(ee, 42%). Furthermore, the PEG-based complexes were stable and could be separated from the reaction
Aqueous NaOCl
mixture by control of the solvent.
Enantioselective epoxidation
Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction
or mobility of the components across the phase boundary or
increases the concentration of the catalyst to the interface [19].
Enantiopure epoxides are very useful synthetic intermediates
for the synthesis of fine chemicals and many pharmaceuticals,
which can be further converted into many important chiral build-
ing blocks via stereoselective ring-opening or functional group
transformations [1,2]. The asymmetric epoxidation of unfunction-
alized olefins catalyzed by chiral salen Mn(III) complex is one of
the most efficient strategies for obtaining enantiopure epoxides
[3–15]. Sodium hypochlorite (NaOCl) used as an oxygen source
has been drawn much attention in the asymmetric epoxidation
of non-functionalized olefins, since it is reasonably stable, readily
available, environment friendly, and inexpensive. Kureshy et al.
have reported the catalytic asymmetric epoxidation of non-func-
tionalized alkenes catalyzed by dimeric or polymeric chiral salen
Mn (III) complexes using NaOCl as an oxidant. Longer reaction time
(2.5–10 h) is required due to the limited solubility of the catalysts
in the aqueous NaOCl solution, although high epoxide conversions
and enantioselectivities were obtained [16,17]. The reaction rates
can be increased by the addition of surfactants [18]. Polyethylene
glycol (PEG) is commonly used, which enhances mutual solubility
The phase-transfer capability of PEG is dependent upon the amphi-
pathic nature of the polymer chain with its hydrophobic methy-
lene groups interspersed with ether groups which can
compliment the hydrogen bonded structure of water. The remark-
able enhancement of reaction rates with the addition of PEG is ex-
plained by the assumption of the formation of microcapsule in the
water/oil biphasic condition. The microcapsule may act as
‘‘microreactors’’, and thus adjusts the catalyst-substrate orienta-
tion similar to that featured in homogenous system.
Furthermore, the chiral salen Mn(III) complexes with built-in
phase-transfer capability have been developed for the biphasic
condition by introducing the hydrophilic substituents, such as
the tertiary amino alkyl group [16,21–24] or methyl (triphenyl-
phosphonium chloride) group [20] into the frame of the salen
ligand. This type of complexes showed high epoxidation activity
under biphasic condition, but the separation of the catalysts is of-
ten unfavorable. We have earlier found that ionic liquids (ILs) with
the tunable miscibility can be used as phase tags to facilitate recy-
cling and reuse of the chiral salen Mn(III) catalyst in the asymmet-
ric epoxidation of unfunctionalized olefins [25,26]. Furthermore,
the special ‘ionophilicity’ and polarity of the ILs played positive ef-
fects on stabilizing the complex. The microcapsule-forming con-
cept of the PEG and the reusability of the IL-functionalized
catalyst encourage us to develop IL containing PEG chain grafted
chiral salen Mn(III) complexes, which provides the lipophilic metal
⇑
Corresponding authors. Address: Key Laboratory of Chemical Biology and
Traditional Chinese Medicine Research (Ministry of Education), Hunan Normal
University, Changsha, Hunan 410081, China. Fax: +86 731 8872531.
E-mail addresses: yindh@hunnu.edu.cn (R. Tan), yiyangtanrong@126.com
(D. Yin).
0021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.12.016

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R. Luo et al. / Journal of Catalysis 287 (2012) 170–177
171
center, the amphiphilic polyether-long chain, and the hydrophilic
IL linker. We envisage that the amphipathic nature of the PEG
chain can allow PEG-based catalysts to undergo inherent phase
transfer, which is in turn expected to increase the reaction rates
in two-phase systems. Furthermore, the solubility properties of
the PEG, that is, it is soluble in CH2Cl2, but can be precipitated with
n-hexane, potentially endow the novel complexes with the feature
of solvent-regulated separation.
ion spray voltage, 8 psi for curtain gas, 8 psi for drying gas heated
to 100 °C, 7 psi for nebulizing gas, respectively. The optical rotation
of catalysts was measured in dichloromethane on a WZZ-2A Auto-
matic Polarimeter. The conversions and the ee values were mea-
sured by a 6890 N gas chromatograph (Agilent Co.) equipped
with the chiral capillary column (HP19091G-B213, 30 m ꢁ
0.32 mm ꢁ 0.25
lm) and the FID detector.
Herein, the N-polyether-substituted imidazol was synthesized
and successfully introduced into the two sides of 5,50 positions in
the salen ligand of (R,R)-{N-(3-tert-butyl-5-chloromethyl-salicyli-
dine)-N0-(30-tert-butyl-50-chloromethyl-salicylidine)}-1,2-cyclo-
hexanediamine, which provided the imidazolium IL containing
polyether chain functionalized chiral salen Mn(III) complex (PICC).
It has proved that the new catalysts could achieve the process of
phase-transfer catalysis (PTC) in the asymmetric epoxidation of
unfunctionalized olefins using NaOCl as an oxidant and could be
facilely separated for reuse by control of the solvent. Thus, the
problems associated with the phase-transfer limitation in the bi-
phasic reaction and the separation of the catalysts can be well re-
solved. In addition, the total length of polyether chain significantly
affected the catalytic performances of the novel complexes.
2.3. Preparation of PICC-n (n = 5, 15, 25)
The preparation of PICC-n (n = 5, 15, 25) was outlined in
Scheme 1.
2.3.1. Synthesis of N-polyoxyethyleneimidazole A
Frozen imidazole (1.7 g, 25 mmol) and n-hexane (15 mL) were
added into an autoclave (100 mL). Different amount of the frozen
ethylene oxide liquid (EO, 10 mL for n = 5; 20 mL for n = 15;
30 mL for n = 25) was subsequently added into the autoclave with
a magnetic stirrer. The resulted mixture was kept at room temper-
ature under N2 overnight, and then stirred for 5 h at 60 °C. After
removing solvent, the mixture was dried under vacuum to afford
the compound A as a dark brown viscous liquid. The average num-
bers of ethylene oxide unit of the polyether chain were calculated
from the weight increase of products [33]. Compound A (n = 15):
1HNMR (D2O, 500 MHz) dH, ppm: 7.65 (s, 1H, ring NCHN), 7.15
(s, 1H, ring C@NCH), 6.96 (s, 1H, ring NAC@CH), 4.15 (t, 2H,
J = 10 Hz, OACH2ACH2ANring), 3.55–3.63 (m, 58H, H(OAC2H4AO)14
2. Experimental
2.1. Materials and reagents
( )-1,2-diaminocyclohexane and 3-chloroperoxybenzoic acid
(m-CPBA) were purchased from Alfa Aesar. Indene and 1,2-dihy-
dronaphthalene were obtained by TCI. Pyridine-N-oxide (PyNO)
was bought from Aldrich. Other commercially available chemicals
were laboratory grade reagents from local suppliers. All of the sol-
vents were purified by standard procedures [27]. Styrene and in-
dene were passed through a pad of neutral alumina before use.
6-cyano-2,2-dimethylchromene and 6-nitro-2,2-dimethylchrom-
ene were synthesized according to the literature procedures [28].
[(R,R0)-(N,N0-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanedi-
aminato] manganese(III) chloride (the neat complex) was prepared
according to the procedures described in Ref. [29]. 3-tert-butyl-2-
hydroxybenzaldehyde and 3-tert-butyl-5-chloromethyl-2-hydroxy
benzaldehyde were prepared according to the procedures de-
scribed in Refs. [30,31], respectively.
CH2). FT-IR (KBr): c
max/cmꢀ1 3369, 2871, 1655, 1510, 1455, 1351,
1291, 1249, 1107, 947, 887, 833, 747, 668, 626.
2.3.2. Synthesis of modified salicylaldehyde B
N-polyoxyethylene imidazole A (10 mmol) in dry benzene
(20 mL) was added dropwise into the benzene solution of 3-tert-
butyl-5-chloromethylsalicylaldehyde (10 mmol, 2.27 g) under stir-
ring. The obtained mixture was refluxed for 48 h. After the comple-
tion of the reaction, the lower viscous liquid was washed three
times with dry benzene. The solvent was removed to obtain the
compound B as the dark brown viscous liquids. Compound B
(n = 15): 1HNMR (D2O, 500 MHz) dH, ppm: 9.76 (s, 1H, CH@O),
7.48–7.60 (m, 5H, ring NCH and ArH), 5.33 (s, 2H, ArACH2ANring)),
4.34 (t, 2H, J = 9.8 Hz, OACH2ACH2ANring), 3.48–3.82 (m, 58H,
H(OAC2H4AO)14CH2), 1.11–1.28 (s, 9H, t-butyl). FT-IR (KBr): cmax
/
2.2. Methods
cmꢀ1 3378, 2948, 2873, 1647, 1560, 1440, 1351, 1390, 1323,
1272, 1238, 1213, 1188, 1106, 946, 887, 835, 775, 705, 528.
FT-IR spectra were obtained as potassium bromide pellets with
a resolution of 4 cmꢀ1 and 32 scans in the range 400–4000 cmꢀ1
using an AVATAR 370 Thermo Nicolet spectrophotometer. The
ultraviolet–visible light (UV–vis) spectra were recorded on a
UV–vis Agilent 8453 spectrophotometer. The solution of samples
in dichloromethane (ca. 1.0 mM) was poured into a 1 cm quartz
cell for UV–vis adsorption with dichloromethane as the reference.
The thermogravimetric and differential thermogravimetric
(TG–DTG) curves were obtained on a NETZSCH STA 449C thermal
analyzer. Samples were heated from room temperature up to
700 °C under flowing air using alumina sample holders. The sample
weight was ca. 10 mg and the heating rate was 10 K/min. 1HNMR
spectra of samples were recorded on a Varian-500 spectrometer
with TMS as an internal standard. Thin layer chromatography
2.3.3. Synthesis of the PEG-IL-functionalized chiral salen ligand (PICL)
(1R, 2R)-(ꢀ)-1, 2-diaminocyclohexane mono-(+)-tartrate salt
(2.5 mmol, 0.660 g) was treated with saturated potassium hydrox-
ide (5.0 mmol, 0.280 g) in 20 mL dichloromethane under vigor-
ously stirring at room temperature. The resulted cloudy mixture
was stirred for another 2 h. The liberated diamine was dried over
anhydrous Na2SO4. After removal of the solvent in vacuum, the
(1R, 2R)-(ꢀ)-1, 2-diaminocyclohexane was dissolved in anhydrous
ethanol and added dropwise into the solution of PEG-IL modified
salicylaldehyde B (5 mmol) in anhydrous ethanol (10 mL) at room
temperature. The resulting mixture was refluxed for another 8 h to
synthesize the PEG-IL-functionalized chiral salen ligand (PICL).
PICL (n = 15): 1H NMR (D2O, 500 MHz): dH, ppm: 8.35 (s, 2H,
CH@N), 7.02–7.57 (s, 10H, ring NCH and ArH), 5.42 (s, 4H,
ArACH2ANring), 4.60 (m, 4H, OACH2ACH2ANring), 3.66–3.91
(m, 116H, H(OAC2H4AO)14CH2), 3.60–3.62 (m, 2 H, C@NCH),
1.44–1.83 (m, 8H, cyclohexyl-H), 1.20–1.28 (s, 18H, t-butyl). FT-
(TLC) was conducted on glass plates coated with silica gel GF254
.
Mn ion contents were measured by the method of complexometry
with ethylenediamine tetraacetic acid (EDTA) according to Ref.
[32]. Mass spectrometry analyses were performed using an API
3000 tandem mass spectrometer (Applied Biosystems, USA) with
electrospray interface (ESI-MS/MS in MeOH). The operating condi-
tions in positive ionization mode were optimized using a +5500 V
IR (KBr):
1211, 1105, 942, 842, 776, 649.
c
max/cmꢀ1 3341, 2941, 2868, 1629, 1445, 1353, 1246,

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172
R. Luo et al. / Journal of Catalysis 287 (2012) 170–177
CHO
CHO
OH
t-Bu
N
O
ClH2C
OH
N
N
Cl
N
n
H
t-Bu
HN
O
n-hexane, 60 oC
N
n
H
toluene, reflux, 48 h
O
A
B
N
N
OH
HO
Cl
N
N
Cl
H2N
NH2
t-Bu
t-Bu
N
n
N
EtOH, reflux, 8 h
H
H
n
O
O
PICL
N
N
Mn
O
O
Cl
.
Mn(OAc)2 4H2O /Ethanol
Cl
N
N
Cl
t-Bu
t-Bu
N
N
LiCl /Ethanol
H
H
n
PICC
O
O
n
PICC-n
(n=5, 15, 25)
Scheme 1. Synthesis of the catalysts of PICC-n (n = 5, 15, 25).
2.3.4. Synthesis of PICC-n (n = 5, 15, 25)
2.4. Preparation of the IL-complex
Under nitrogen protection and stirring at 50 °C, a solution of
Mn(OAc)2ꢂ4H2O (5 mmol, 1.225 g) in anhydrous ethanol (20 mL)
was added dropwise into the above-obtained PICL solution. The
resulting brown mixture was refluxed for 12 h, then cooled to
room temperature, followed by addition of solid LiCl (12 mmol,
0.504 g), and stirred for additional 5 h. The mixture was bubbled
with a gentle stream of air for another 2 h, and then exposed to
air overnight. After removal of the solvent in vacuum, the mixture
was washed with ether for several times. The gummy residue was
dissolved in dichloromethane (100 mL). The organic phase was
washed with saturated sodium chloride and dried over anhydrous
Na2SO4, and then dried at 40 °C in vacuum to obtain the sticky-
brown products PICC-n (where n is the average numbers of ethyl-
ene oxide unit in the PEG chains, n = 5, 15, 25). The PICC containing
the average numbers of ethylene oxide unit in the PEG chains of 5,
15, and 25 were denoted as PICC-5, PICC-15, and PICC-25, respec-
For comparison, the 1-methyl-3-methyleneimidazolium chlo-
rine-functionalized chiral salen Mn(III) complex (denoted as IL-
complex) was also prepared by the similar preparation procedure
of PICC-n (n = 5, 15, 25). During the procedure, N-methylimidazole
was used instead of the material of N-polyoxyethyleneimidazole.
The structures of the IL-complex and the neat complex were shown
in Chart 1.
2.5. Catalyst testing
The catalysts of the PICC-n (n = 5, 15, 25) (4 mol%), unfunction-
alized alkene (0.25 mmol), and PyNO (0.05 mmol, 0.0005 g) were
added into 1 mL dichloromethane under stirring. The buffered
NaOCl as an oxidant (0.5 mmol, pH = 11.5) was added in four equal
portions at 0 °C. The progress of the epoxidation reaction was mon-
itored on GC. After the reaction, the reaction mixture was sepa-
rated by separatory funnel. The aqueous phase was extracted
with CH2Cl2 for several times. The extract was combined with
the organic phase, and dried over anhydrous sodium sulfate. After
the evaporation of volatile solvents, the catalysts of PICC-n (n = 5,
15, 25) could be separated from the mixture by the addition of n-
hexane. The upper organic phase with the epoxides was separated
from the lower polyether-based catalyst by simple decantation.
Further purification of the n-hexane layer by flash column chroma-
tography afforded the epoxides. The conversions and ee values
were measured by a 6890N gas chromatograph (Agilent Co.)
equipped with the chiral capillary column (HP19091G-B213,
tively. PICC-5: FT-IR (KBr):
1545, 1519, 1457, 1388, 1348, 1307, 1246, 1205, 1108, 1035,
c
max/cmꢀ1 3390, 2918, 2867, 1616,
946, 834, 570, 481, 420; UV–vis (CH2Cl2): kmax/nm
(emax/
L molꢀ1 cmꢀ1) 506 (1594), 410 (4681), 322 (13,218); Mn ion con-
tent: 1.02 mmol/g; aD25 ¼ þ960 (C = 0.02, CH2Cl2); ESI-MS: m/
z = 488 (M-2(CH2 + IL))+. PICC-15: FT-IR (KBr):
c
max/cmꢀ1 3390,
2918, 2867, 1616, 1545, 1519, 1457, 1388, 1348, 1307, 1246,
1205, 1108, 1035, 946, 834, 570, 481, 420; UV–vis (CH2Cl2): kmax
/
nm (e
max/L molꢀ1 cmꢀ1) 506 (1589), 410 (3929), 322 (12,930);
Mn ion content: 0.48 mmol/g; a2D5 ¼ þ955 (C = 0.02, CH2Cl2); ESI-
MS: m/z = 488 (M-2(CH2 + IL))+. PICC-25: FT-IR (KBr): max/cmꢀ1
c
3390, 2918, 2867, 1616, 1545, 1519, 1457, 1388, 1348, 1307,
1246, 1205, 1108, 1035, 946, 834, 570, 481, 420; UV–vis (CH2Cl2):
30 m ꢁ 0.32 mm ꢁ 0.25
times of the corresponding epoxides are as follows: (a) styrene
epoxide: T = 90 °C, tR = 15.2 min, tS = 15.7 min; (b) -methylstyrene
epoxide: T = 80 °C, tS = 16.3 min, tR = 16.5 min; (c) indene epoxide:
lm) and the FID detector. The retention
kmax/nm (e
max/L molꢀ1 cmꢀ1) 506 (2015), 410 (6563), 322 (13,756);
Mn ion content: 0.34 mmol/g; a2D5 ¼ þ950 (C = 0.02, CH2Cl2);
a
ESI-MS: m/z = 488 (M-2(CH2 + IL))+.

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R. Luo et al. / Journal of Catalysis 287 (2012) 170–177
173
N
N
Mn
Cl
N
N
Mn
Cl
O
O
t-Bu
O
O
t-Bu
Cl
N
N
Cl
t-Bu
t-Bu
t-Bu
t-Bu
N
N
Neat complex
IL-complex
Chart 1. The structures of the IL-complex and the neat complex.
T = 80–180 °C, tSR = 11.5 min, tRS = 11.9 min; (d) 1,2-dihydronaph-
thalene epoxide: T = 80–180 °C, tSR = 13.4 min, tRS = 13.6 min; (e)
3.2. Characterization of samples
3.2.1. FT-IR spectra
6-cyano-2,20-dimethylchromene epoxide: T = 80–200 °C, tSS
=
24.0 min, tRR = 24.3 min; (f) 6-nitro-2,20-dimethylchromene epox-
ide: T = 80–200 °C, tSS = 30.3 min, tRR = 30.8 min. The lower catalyst
could be recycled by adding fresh solvents and reaction substrates.
The synthesized PICC-n (n = 5, 15, 25), as well as the neat chiral
salen Mn(III) complex for comparison, were characterized by FT-IR
spectra (Fig. 1). The FT-IR spectra of the neat complex show char-
acteristic vibration bands at around 1613, 1535, 569, and
413 cmꢀ1, which are associated with the stretching vibration
modes of C@N, CAO, MnAO, and MnAN, respectively (Fig. 1a)
[25,26,34], while the characteristic bands in the FT-IR spectra of
3. Results and discussion
PICC-n (n = 5, 15, 25) appear as red shift from 1613 cmꢀ1
,
,
3.1. Preparation and characterization of PICC-n (n = 5, 15, 25)
1535 cmꢀ1, 569 cmꢀ1, and 413 cmꢀ1 to 1616 cmꢀ1, 1545 cmꢀ1
570 cmꢀ1, and 420 cmꢀ1, respectively, compared with those of neat
complex (Fig. 1b–d vs. a). It is mainly due to the election-deficient
substitutes of the imidazolium cations at the 5,50-positions in the
We have found that the ILs derived from N,N-dialkylimidazoli-
um can stabilize the chiral salen Mn(III) complex during the pro-
cess of the epoxidation or can act as a modifier that accelerates
the reaction and enhances the selectivity [25,26]. Thus, we chose
IL containing imidazolium moiety as the linker to combine the chi-
ral salen Mn(III) complex with the polyether chain. A strategy that
we have designed here is to append covalently the limidazolium IL
moieties containing PEG chain at two sides of 5,50 position in the
salen ligand to make the PEG-based IL-functionalized chiral ligand
(PICL) resemble PEG.
The synthesis route for the PICC-n (n = 5, 15, 25) was outlined
in Scheme 1. At first, N-polyoxyethyleneimidazole (A) with differ-
ent numbers of polymerized ethylene oxide unit (n = 5, 15, 25),
which was provided by ethoxylation between imidazol and
ethylene oxide, directly reacted with 3-tert-butyl-5-chloro-
methyl-2-hydroxybenzaldehyde to afford PEG-based IL-substi-
tuted 3-tert-butyl-2-hydroxybenzaldehyde (B). The successive
condensation between the aldehyde (ACHO) group of the com-
pound B and the amino (ANH2) groups of (R,R0)-1,2-cyclohexane-
diamine was used to form the PICL. Treatment of the PICL with
manganese (II) acetate tetrahydrate under nitrogen gave the dian-
ionic complex. The acetic ion was replaced by chloride ion with
lithium chloride at room temperature. The dianionic complex
was readily oxidized by oxygen, affording the PICC-n (n = 5, 15,
25). The catalysts newly synthesized are brown and viscous liq-
uids at room temperature. The viscosity increases with the length
of polyether chain.
The numbers of ethylene oxide unit of the polyether chain were
determined on the basis of the weight increase amount of imidazol
after ethoxylation and the molecular weight of one ethylene oxide
unit [33]. It is interested that all the PICC-n (n = 5, 15, 25) is soluble
in water at room temperature. We speculated that the water-solu-
bility of the complex was related with the total length of polyether
chain. Moreover, it is also found that the PICC-n (n = 5, 15, 25) are
miscible in some organic solvents, e.g., CH2Cl2 and DMF, but can be
precipitated with other organic solvents, e.g., Et2O and n-hexane. It
is suggested that the amphipathic PICC-n (n = 5, 15, 25) should be
an easily recoverable catalyst for asymmetric epoxidation of al-
kenes by simple phase separation techniques via change of
solvents.
salen ligands [10]. The new stretching vibration
m(CAN) of CAN
bond at around 1457 cmꢀ1 further suggests the grafting of imi-
dazolium IL moiety on the salen ligand (Fig. 1b–d). In addition,
the FT-IR spectra of the PICC-n (n = 5, 15, 25) show another new
bands at 1108 cmꢀ1, which is assigned to stretching vibration of
CAOAC groups in the PEG chain [35]. The very broad band cen-
tered at 3390 cmꢀ1 is assigned to the terminal hydroxyl group of
the PEG chains for the PICC-n (n = 5, 15, 25) (Fig. 1b–d). The results
suggest that the neat complex is functionalized by the PEG modi-
fied-imidazolium IL through covalent bonds, and the functionaliza-
tion presented here does not change the structure of catalytic
active sites.
3.2.2. UV–vis spectra
Fig. 2 showed UV–vis spectra of the PICC-n (n = 5, 15, 25) and
neat chiral salen Mn(III) complex. UV–vis spectra give more obvi-
ous evidence for the successful grafting based on the fact that
a
1613
569
b
413
1535
b'
c
d
1616
1545
570
3390
420
1108
1457
4000
3000
2500
2000
1500
1000
3500
500
Wavenumbers (cm-1
)
Fig. 1. FT-IR spectra of the neat complex (a), PICC-5 (b), the recovered PICC-5 after
the 5th reuse in the aqueous NaClO oxidant system at 0 °C (b0), PICC-15 (c), and
PICC-25 (d).