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  0   Introduction

  Asymmetric epoxidation of styrene catalyzed by homogeneous chiral Mn^Ⅲ(Salen)^[1] is an important area of research, because of the versatility of chiral styrene epoxides as useful intermediates in the preparation of a diverse array of functional molecules^[2-3]. Immobiliza-tion of Mn(salen) complexes onto solid supports such as organic polymers^[4-6] and porous inorganic supports^[7-10] are often of greater interest than homogeneous catalysts, since heterogeneous Mn^Ⅲ(Salen), which are recyclable and readily separable from the reaction system, offer the opportunity to reduce the impact on environment and increase industrial interest for the liquid phase oxidation catalytic process. Although significant advances have been made in recent years towards the new ligand-based Mn^Ⅲ(Salen)^[11] and the new supports^[12-14], reports paying attention to the effect of linker group and inorganic-organic hybrid support on the activity of the catalyst have been scarce. It is important to consider the nature of the linker component connecting the support and Mn^Ⅲ(Salen).

  Zhang^[15] reported that the rigidity of the linkage was found to be a key factor affecting the catalytic performance of immobilized catalysts. The immobilized catalyst with a rigid linkage exhibited comparable chemical selectivity, enantioselectivity and cis/trans ratio of product formation to that obtained with homogeneous Jacobsen catalysts. In contrast, the immobilized catalyst with a flexible linkage gave remarkably lower chemical selectivity, enantioselectivity and inverted cis/trans ratio compared with the results obtained with the homogeneous Jacobsen catalyst and the immobilized catalyst with rigid linkage. Thus, for immobilized Mn(salen) catalysts, a rigid linkage connecting active centers to the support is essential to obtain activity and enantioselectivity as high as those obtained in homogeneous systems. Li et al^{[8, 16]} reported that the length of the linkage was found to influence the enantioselectivity in heterogeneous asymmetric epoxida -tion. Linkages (-CH₂-Ph-SO₃-, -CH₂CH₂-Ph-SO₃-, -CH₂CH₂CH₂Ph-SO₃-) with different length were used in the immobilization of Mn^Ⅲ(Salen) for the epoxidation of styrene. They found that longer linkage can result in higher conversion of styrene, higher chemical sele-ctivity and higher enantioselectivity towards epoxides. The similar trend was obtained by our group^[17-25], where higher levels of catalytic activity were observed in the epoxidation of unfunctionalized olefins using a Mn^Ⅲ(Salen) catalyst immobilized onto ZSPP, ZPS-IPPA, ZPS-PVPA or ZnPS-PVPA with higher linkage length. Furthermore, the different catalytic results were obtained with Mn^Ⅲ(Salen) catalyst immobilized on the same support ZPS-PVPA by different linkage (-NH-C₆H₄-NH- vs -O-C₆H₄-O-). It was observed that the remarkably increases of conversion and ee values were obtained in the absence of expensive O-coordin-ating axial bases for the asymmetric epoxidation with the heterogeneous catalyst bearing the -O-C₆H₄-O- linker group^[23], while, there is a sharply decrease in in the absence of expensive O-coordinating axial bases for the asymmetric epoxidation with the heterogeneous catalyst bearing the -NH-C₆H₄-NH- linker group^[22].These results indicate that there is a correlation between the nature of the linker group and the activity of the catalyst.

  With the purpose of having clearer insight into the nature of the linker component, especially for rigid phenoxyl linkers. In this work, we further explored the effect of the phenoxyl linkage of immobilized Mn^Ⅲ(Salen) catalysts bearing different substituents at the o-positions of the phenoxyl group on their catalytic acitivities in epoxidation of styrene. We have prepared several inorganic-organic hybrid support ZPS-IPPA supported Mn^Ⅲ(Salen) catalysts (Cat₁ ~Cat₃), for comparison, we have also prepared a series of non-supported analogues (Cat₅~Cat₇), and both of these series were evaluated as catalyst for epoxidation of styrene. The influences of the parameters such as the axial additive NMO and the solvents on the epoxidation of styrene were also investigated in detail. Furthermore, we have evaluated the recyclability of these catalytic systems and found that they continued to show superior levels of enantioselectivity and catalytic activity for at least five runs.

  1   Experimental

  1.1   Materials and methods

  (1S, 2S)-1, 2-Diaminocyclohexane, n-nonane, NMO, chloromethy methyl ether (toxic compound) and m-CPBA were supplied by Alfa Aesar (Tianjin, China) and used as received without further purification. All of the other reagents chemicals used in this study were purchased as the laboratory grade from local suppliers. Styrene was distilled before use. Homogen-eous Mn^Ⅲ(Salen) catalyst was synthesized according to the standard literature procedures^[1]. The preparation and characterization of chloromethyl-zirconium poly (styrene-isopropenyl pho-sphonate)-phosphate (ZCMPS-IPPA) followed reported procedures^[21-22].

  FT-IR spectra were recorded on a Bruker RFS100/S spectrophotometer (Karlsruhe, Germany) as KBr disks. Diffuse reflectance UV-Vis spectra of the solid samples were recorded on a Bruker RFS100/S spectrophotometer with an integrating sphere using BaSO₄ as a reference standard. Number- and weight-average molecular weights (M_n and M_w) and polydis-persity values (M_n and M_w) were estimated using a Waters 1515 gel permeation chromatograph (GPC) system (Massachusetts, USA) against polystyrene standards using Tetrahydrofuran (THF) as an eluent (1.0 mL·min^-1) at 35 ℃. TG analysis was performed on a SBTQ600 Thermal Analyzer with a heating rate of 20 ℃ min^-1 from room temperature to 1 000 ℃ under flowing compressed N₂ (100 mL·min^-1). The Mn content of the catalyst was determined by TAS-986G (Beijing, China). Elemental analyses were conducted on an EATM1112 automatic elemental analyzer (Massachusetts, USA). X-ray photoelectron spectra (XPS) were recorded on an ESCALAB250 system (Massachusetts, USA). Scanning electron microscopy (SEM) analysis was performed on a KYKY EM3200 system (Beijing, China). Transmission electron micros-copy (TEM) analysis was conducted on a Philips TECNAI10 system (Amsterdam, Holland). Atomic force microspcopy (AFM) analysis was conducted on a NanoScope Quadrex system (New York, USA).

  1.2   Preparation of the supports

  1.3   Preparation of the supported catalysts and non-supported analogues

  1.4   Asymmetric epoxidation of styrene

  The racemic epoxides were prepared by the epoxidation of the styrene with m-CPBA in CH₂Cl₂, and their structures were confirmed by NMR analysis on a Bruker AV-300 system. The GC was calibrated using n-nonane, as well as several olefins and the corresponding racemic epoxides. A solution of styrene (0.500 mmol), NMO (338 mg, 2.50 mmol, if necessary), nonane (internal standard, 56.0 mL, 0.500 mmol), and the immobilized Mn^Ⅲ(Salen) complex (0.025 0 mmol, 5%, based on the Mn amount of the catalyst) in CH₂Cl₂ (3 mL) was cooled to the desired temperature. Solid m-CPBA (138 mg, 0.80 mmol) was added in four portions over 2 min, and the resulting mixture was stirred at ambient temperature. Upon completion of the reaction, as indicated by TLC, the mixture was washed sequentially with saturated sodium hydroxide and brine to remove any residual m-CPBA and the corresponding acid and dried over anhydrous Na₂SO₄. The conversion and ee values were determined by GC using nonane as an internal standard. After the first run of the epoxidation reaction, the catalyst was separated by centrifugation and washed thoroughly with dichloromethane before being dried, and subjected to another cycle with fresh reactants under similar epoxidation conditions. The conversion (with n-nonane as an internal standard) and the ee values were determined by gas chromatography (GC) using a Shimadzu GC2014 (Japan) instrument equipped with a chiral column (HP19091G-B213, 30 m×0.32 mm×0.25 μm) and FID detector.

  Figure Scheme 2.  Synthetic route to the supported and non-supported analogues catalysts

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  Figure Scheme 3.  Synthetic route to the non-supported analogues catalysts

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  1.2.2   Synthesis of supports (S₁, S₂, and S₃)

  The appropriate amounts of L₁, L₂, and L₃ were mixed with ZCMPS-IPPA (2.5 g), K₂CO₃ (4.50 g, 30.0 mmol), and ethanol (20.0 mL) (the molar ratio of diphenol to elemental chlorine in ZCMPS-IPPA was set at 5:1), respectively, and the resulting mixtures were held at 80 ℃ for 24 h under N₂. The resulting resin like products were then filtered and washed sequentially with large volumes of deionized water and ethanol to remove any residual diphenol before being dried under vacuum at 80 ℃ to give the correspon-ding products in yields of 90.0%, 83.0%, and 80.0%, respectively. Following their reaction with sodium phenoxide, the residual Cl contents of the supports were obtained by titration against AgNO₃. The results of these titrations allowed for the number of -CH₂Cl groups replaced by phenoxyl groups to be conveniently calculated.

  1.2.1   Synthesis of zirconium poly (styrene-phenylvinylphosphonate)-phosphate (ZCMPS-IPPA)

  The synthesis and characterization of ZCMPS-IPPA have previously been reported by our group^[21].

  1.3.1   Preparation of supported catalysts: phenoxyl modified ZCMPS-IPPA-Mn^Ⅲ(Salen) (Cat₁~Cat₃)

  A solution of chiral Mn^Ⅲ(Salen) (2.73 g, 4.30 mmol), phenoxyl modified ZCMPS-IPPA (S₁~S₃), (0.50 g), and an appropriate amount of sodium hydroxide in tetrahydrofuran (50.0 mL) was stirred vigorously for 24 h under reflux. The reaction mixture was then cooled to ambient temperature and filtered to give a dark brown powder, which was washed sequentially with ethanol, CH₂Cl₂, and demonized water before being dried under vacuum at 80 ℃. The CH₂Cl₂ filtrate was analyzed by UV-Vis spectroscopy until no peaks could be detected (with CH₂Cl₂ as reference). The products (Cat₁~Cat₃) were isolated in yields of 90.0%, 88.3%, and 80.8%, respectively. The loading of the Mn^Ⅲ(Salen) complex in the heterogenized catalyst, based on the amount of elemental Mn, was determined to be in the range of 0.36~0.45 mmol·g^-1 by AAS.

  Figure Scheme 1.  Synthetic route to the supports

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  1.3.2   Preparation of non-supported analogues catalysts (Cat₅~Cat₇)^[19]

  Mn^Ⅲ(Salen) (1.56 g, 2.56 mmol) and the appro-priate amount of the linker (L₅~L₇, 1.2 equiv.) were added to ethanol (60.0 mL), and the resulting mixture was refluxed for 5 h at 80 ℃. The reaction was then cooled to room temperature and evaporated to dryness, The resulting residue was dissolved in CH₂Cl₂ and washed sequentially with deionized water and brine before being dried over anhydrous Na₂SO₄. The solvent was then removed under vacuum to gives the desired product as a brown-dark solid (Cat₅~Cat₇). Cat₄: Yield 80.0%; Anal. Calcd. for Cat₅ (C₄₂H₅₇MnN₂O₃): C 72.81%, H 8.28%, N 4.05%; Found: C 74.15%, H 8.42%, N 4.17%. Cat₅: Yield 76.3%, Anal. Calcd. for Cat₆ (C₄₃H₅₉MnN₂O₃): C 73.06%, H 8.41%, N 3.96%; Found: C 75.26%, H 8.63%, N 4.21%. Cat₆: Yield 62.7%; Anal. Calcd. for Cat₇ (C₄₆H₆₅MnN₂O₃): C 73.77%, H 8.75%, N 3.74%; Found: C 74.88%, H 8.98%, N 3.93%.

  2   Results and discussion

  2.1   FT-IR spectroscopy

  The immobilized catalysts, as well as the homog-eneous Mn^Ⅲ(Salen) for comparison, were characterized by FT-IR spectroscopy (Fig. 1). The FT-IR spectrum of Mn^Ⅲ(Salen) showed characteristic vibration bands around 1 622, 1 532, and 568 cm^-1, which were attributed to the stretching vibration modes of C=N, C-O, and Mn-O, respectively. These bands were still present in the FT-IR spectra of the immobilized catalysts, which suggested that the heterogeneous catalysts had similar structures to that of neat chiral Mn^Ⅲ(Salen) and the structure of the catalytic active site remained intact. The stretching vibration observed at 1 030 cm^-1, which was assigned to the characteristic vibrations of phosphonate and phosphate in the support, was clearly weakened as a consequence of changes in the electronic structure of the host-guest interaction. These observations suggested that the chiral Mn^Ⅲ(Salen) complex has been successfully immobilized onto the tunable phenoxyl group modified ZPS-IPPA.

  Figure 1.  FT-IR spectra of (a) Jacobsen′s catalyst (b) Cat₁; (c) Cat₂; (d) Cat₃

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  2.2   DR UV-Vis spectroscopy

  The diffuse reflectance UV-Vis spectra of the Mn^Ⅲ(Salen) complex and immobilized catalysts are shown in Fig. 2. These spectra provided further evidence in support of the successful immobilization of the Mn^Ⅲ(Salen) complex, based on the fact that the charact-eristic bands for Mn^Ⅲ(Salen) at 335, 433, 510 nm had been blue-shifted to 330, 410, and 505 nm, respecti-vely, for the immobilized catalysts, which indicated the occurrence of an interaction between the Mn^Ⅲ(Salen) complex and the support through the tunable phenoxyl linkers. The characteristic bands at 262 and 330 nm were attributed to the charge transfer transition of the Salen ligand, whereas the bands at 410 and 505 nm were attributed to the ligand-to-metal charge transfer transition and d-d transition of the Mn^Ⅲ(Salen) complex^[9].

  Figure 2.  DR UV-Vis spectra of(a) Cat₁; (b) Cat₂; (c) Cat₃; (d) Jacobsen′s catalyst

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  2.3   Microscopic analysis

  SEM, TEM and AFM images of the immobilized Mn^Ⅲ(Salen) catalyst (Cat₃) are shown in Fig. 3. The SEM image of the particles of the Cat₃ catalyst indicated that they were submicron in diameter. The SEM image also showed that Cat₃ was in a loose, amorphous state. Numerous cavities, holes, and pores of different sizes and shapes also existed on the surfaces of the particles. The TEMand AFM images revealed that the particles of the Cat₃ catalyst consisted of much smaller particles. The images indicated that the surface properties of the catalyst would lead to an increase in the surface area of the catalyst and allow the substrates better access to the catalytically active sites.

  Figure 3.  SEM (a), AFM (b) and TEM (c) photographs of immobilized catalyst Cat₃

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  2.4   X-ray photoelectron spectroscopy

  XPS has been used extensively to obtain detailed information about the state of metal species immobilized on the surfaces of catalyst in terms of allowing for the electronic properties of these species to be effectively investigated. Fig. 4 shows the Mn2p_3/2 XPS spectra of the Mn^Ⅲ(Salen) complex and the immobilized catalyst (Cat₃). The shift in the binding energy of the major peak from 642.1 to 642.4 eV was attributed to Mn2p_3/2 following the immobilization process, which is in agreement with the values previously reported for Mn^Ⅲ(Salen) and porphyinic ligands^{[21, 26]}. The observed increase of 0.3 eV in the chemical shift for the immobilized salen complex comparing with Mn^Ⅲ(Salen) was attributed to differen-ces in the coordination environment of Mn.

  Figure 4.  XPS spectra of Cat₃

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  2.5   Thermal analysis

  The TG curve of Cat₃ was shown in Fig. 5. In the TG curve of Cat₃ three stepwise weight losses are observed. The first step with a weight loss of (5.3%) at A small mass loss(6.3%) at 20~150 ℃ is attribute to the loss of the surface or interstitial water. In the temperature range of 150~500 ℃, a weight loss of 46.5% is corresponded to the decomposition of the organic group. The last weight loss is attribute to the loss of the water from Zr(HPO₄)₂ to ZrP₂O₇.

  Figure 5.  TG curve of the Cat₃

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  2.6   Catalytic properties of the immobilized catalyst

  2.6.1   Asymmetric epoxidation of styrene

  The catalytic activities and enantioselectivities of the catalysts (Cat₁~Cat₇) were evaluated for the asymmetric epoxidation of styrene in CH₂Cl₂ using m-CPBA as an oxidant. Table 1 shows that the Jacobsen′s catalyst and the non-supported analogues (Cat₅~Cat₇) were active and enantioselective for the epoxidation of styrene. After anchoring the Mn^Ⅲ(Salen) catalyst onto the ZPS-IPPA using phenoxyl linkers of varying steric hindrance, there was a clear reduction in the activities of these heterogeneous catalysts (Cat₁~Cat₃) for the epoxidation of model substrate under the same conditions. However, when the reaction temperature was lowed from 0 to -78 ℃, the enantioselectivity of the Cat₃ towards the epoxidation of styrene improved significantly from 35.6% to 55.8% ee, which were 8.8% higher than those achieved with Mn^Ⅲ(Salen).

  Table 1.  Asymmetric epoxidation of styrene catalyzed by Cat₁~Cat₇ with m-CPBA as oxidants^a

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  Entry	Catalyst	Oxidant	Conversionb/%	Selectivitb/%	eeb/%
  1	Jaco	m-CPBA/NMO	99.9	98.7	47.0c
  2	Cat5	m-CPBA/NMO	96.5	99.1	46.3c
  3	Cat6	m-CPBA/NMO	93.8	98.2	46.1c
  4	Cat7	m-CPBA/NMO	93.6	98.3	46.8c
  5	Cat1	m-CPBA/NMO	27.6	53.5	6.3c
  6	Cat1	m-CPBA	92.7	80.6	35.6c
  7	Cat2	m-CPBA	83.5.5	82.3	36.3c
  8	Cat3	m-CPBA	81.8	80.6	42.3c
  9	Cat3	m-CPBA	60.2	86.5	55.8c
  10	Cat4	m-CPBA/NMO	76.5	66.3	18.5c
  11	Cat4	m-CPBA	26.1	12.3	4.3c
  a Reactions were carried out at 0 ℃ for 60 min in CH2Cl2 (3 mL) with styrene(0.5 mmol) NMO (337.5 mg, 2.5 mmol, if necessary), nonane (internal standard, 56 μL, 0.5 mmol), and immobilized MnⅢ(Salen) complexes (0.025 mmol, 5.0 mol%). Determined by GC by integration of product peaks against an internal quantitative standard (nonane), correcting for response factors. b Determined by GC with a chiral capillary column (HP 19091G-B233, 30 m×0.32 mm×0.25 μm). c Epoxide configuration S.

  Table 1.  Asymmetric epoxidation of styrene catalyzed by Cat₁~Cat₇ with m-CPBA as oxidants^a

  2.6.5   Steric hindrance effect originated in the tunable phenoxyl linkers

  Table 1 shows a comparison of the supported and non-supported catalysts in terms of their epoxidation of styrene under similar conditions. When the styrene was used as substrate, the ee values of the resulting epoxides were found to be equal level in the range of 46.1%~46.8%, with conversions in the range of 93.6%~96.5%. These values were obtained after the Cl atoms were replaced with a series of different phenoxyl linker groups, which suggested that the steric hindrance effects of the phenoxyl linker were not having a discernible impact on the homogeneous reaction results. However, the use of the heterogen-eous catalyst Cat₃ led to much higher ee values (up to 42.3%), which were 6.7% and 6.0% higher than those achieved with Cat₁ and Cat₂, respectively. With regard to the conversions, there was a slightly decrease going from 92.7% to 81.8%. These results demonstrated that the substituents at the o-positions of the tunable phenoxyl linkers have a critical impact on the catalytic activity, in that the o-t-Bu group provided higher levels of enantioselectivity than the o-H and o-CH₃ groups of the phenoxyl linker.

  2.6.4   Effect of axial ligands NMO

  It is clear from the results reported previously in asymmetric epoxidation of unfunctionalized olefins that the addition of an axial additive such as NMO induces a conformational change in the skeleton of the Mn^Ⅲ(Salen) complex, resulting in an increase in the reaction rate and an improvement in the enantiosele-ctivity^[27-28]. However, an increases in conversions and ee values were observed in the current study for the heterogeneous epoxidations of styrene in absence of axial bases of NMO catalyzed by Mn^Ⅲ(Salen) bearing axially coordinated phenoxyl linkers of varying steric hindrance. This phenomenon has been reported previously by our group, where it was attributed to the organic polymer-inorganic hybrid support ZPS-PVPPA and the axially coordinating phenoxide group^[23]. In the current study, this novel catalytic result was induced by another important factor, namely the steric hindrance effect derived from the ortho substituents of the tunable phenoxyl axial linkers. It was envisaged that the use of tunable phenoxyl axial linkers with substituents bearing structures similar to those of the N-oxide ligand would promote the catalytic reactions.

  2.6.3   Effect of support between ZPS-IPPA and ZPS-PVPA

  It was found that the use of different support can also affect the catalytic behaviors of immobilized Mn^Ⅲ(Salen). Two heterogeneous Mn^Ⅲ(Salen) catalysts with same linker -O-C₆H₄-O- but different support were tested in epoxidation of styrene. The Mn^Ⅲ(Salen) immobilized on ZPS-IPPA gave a 92.7% conversion with a 35.6% ee, which was higher 5.4% conversion and 22.7% ee than obtained by Mn^Ⅲ(Salen) immo-bilized on ZPS-PVPA^[19]. The increase in conversion and ee was mainly attributed to the support effect. The ZPS-IPPA is formed by the reaction poly(styrene-isopropenylphosphonate) and hydrated zirconyl chloride, while, the ZPS-PVPA is formed by more steric hindrance poly(styrene-phenylvinylphosphonate) and hydrated zirconyl chloride. The steric hindrance existed in ZPS-PVPA had a negative effect in catalytic efficiency. Furthermore, the amount of styrene unites in ZPS-PVPA wasmore than styrene unites in ZPS-IPPA, thus, the degree of chloromethyl for ZPS-PVPA is higher than ZPS-IPPA, correspondingly, the more Mn^Ⅲ(Salen) wasimmobilized on ZPS-PVPA. However, the more Mn^Ⅲ(Salen) immobilized on ZPS-PVPA had a chance to form μ-oxo-Mn^Ⅵ which was negative in epoxidatin of ole-fins reported earlier.

  2.6.7   Recycling of the supported chiral Mn^Ⅲ(Salen)catalyst

  The recyclability of the immobilized catalyst Cat₃ was evaluated using the epoxidation of styrene as a model reaction. After the first run of the epoxidation reaction, the catalyst was separated by centrifugation. The separated catalyst was then collected and washed thoroughly with dichloromethane before being dried and subjected to another cycle with fresh reactants under similar epoxidation conditions (i.e., the same ratio of the substrate-to-catalyst and solvent-to-catalyst). The recovery of the catalyst after each run was found to be in the range of 90.6%~95.0%. As shown in Fig. 7, there were no significant changes in the catalytic activity and enantioselectivity exhibited by the catalyst after five runs, and no leaching of Mn into the filtrate was observed by AAS, which indicated that the chiral Mn^Ⅲ(Salen) complex was strongly bonded to the tunable phenoxyl modified ZPS-IPPA support.

  Figure 7.  Recycles of Cat₃ for the epoxidation of styrene (0.5mmol) using m-CPBA (0.80 mmol) in CH₂Cl₂(3 mL) catalyzed by Cat₄ (5.0%(molar ratio) based on Mn) at 0 ℃

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  2.6.2   Effect of linker group

  It was found that the length of linkage can affect the catalytic behaviors of immobilized Mn^Ⅲ(Salen). To maximally reduce the differences in linkage length, two linkages (-NH-C₆H₄-NH-, -O-C₆H₄-O-) with almost similar length were compared in this study. Table 1 shows Cat₁ with -O-C₆H₄-O- linkage gave a conversion of the substrate (> 90%) with a 35.6% ee value in the asymmetric epoxidation of styrene at 0 ℃ for 60 min. In contrast, only 26.1% conversion with a 4.3% ee value was observed over the Cat₄ with -NH-C₆H₄-NH- linkage under identical conditions. Similar trend for epoxide selectivity can also observed in table 1. The remarkable enhancement of the catalytic efficiency might be due to the positive effect of the -O-C₆H₄-O- based linker, which is similar to those of the N-oxide ligand.

  2.6.6   Effect of solvents on the styrene

  As shown in Fig. 6, the choice of solvents is crucial in the enantioselective epoxidation of styrene. In this study, a systematic of the solvent nature was performed in different solvents. The results revealed that dichloromethane was the optimal solvent for the reaction, providing a conversion of 85.3% and an ee of 36.6%. In contrast, the use of hexane and alcohol resulted in conversions of 26.2% and 26.3% and ee values of 6.6% and 12.5%, respectively. These differe-nces among the solvents were attributed to the fact that Cat₄ swelling to a much greater extent in dichloro-methane than any other solvent. The enantioselectivity was lower in ethyl acetate and acetone than it was in dichloromethane, indicating that solvents containing oxygen atoms bearing lone pairs of electrons could be coordinating to the metal center of the Mn^Ⅲ(Salen) complex and suppressing the formation of the active oxygen transfer species (Mn-V-oxo) required for the epoxidation of styrene.

  Figure 6.  Effect of different solvents on the epoxidation of styrene (0.5 mmol) using m-CPBA (0.80 mmol) in CH₂Cl₂ (3 mL) catalyzed by Cat₃ (5.0% (molar ratio) based on Mn) at 0 ℃

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  3   Conclusions

  The catalytic results indicated that the substitu-ents at the o-positions of the tunable phenoxyl linkers have a critical impact on the catalytic activity, in that the o-t-Bu group provided higher levels of enantiosele-ctivity than the o-H and o-CH3 groups of the phenoxyl linker. At the same time, two linkages (-NH-C₆H₄-NH-, -O-C₆H₄-O-) with similar length were compared, the results demonstrated that the linkage of -O-C₆H₄-O- have more positive effect than the use of -NH-C₆H₄-NH-in absence of NMO in m-CPBA oxidant systems on the catalytic efficiency. Furthermore, the catalysts could be recycled up to five times without any significant reduction in their catalytic activity or enantioselectivity.

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  Scheme 1  Synthetic route to the supports

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  Scheme 2  Synthetic route to the supported and non-supported analogues catalysts

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  Scheme 3  Synthetic route to the non-supported analogues catalysts

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  Figure 1  FT-IR spectra of (a) Jacobsen′s catalyst (b) Cat₁; (c) Cat₂; (d) Cat₃

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  Figure 2  DR UV-Vis spectra of(a) Cat₁; (b) Cat₂; (c) Cat₃; (d) Jacobsen′s catalyst

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  Figure 3  SEM (a), AFM (b) and TEM (c) photographs of immobilized catalyst Cat₃

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  Figure 4  XPS spectra of Cat₃

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  Figure 5  TG curve of the Cat₃

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  Figure 6  Effect of different solvents on the epoxidation of styrene (0.5 mmol) using m-CPBA (0.80 mmol) in CH₂Cl₂ (3 mL) catalyzed by Cat₃ (5.0% (molar ratio) based on Mn) at 0 ℃

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  Figure 7  Recycles of Cat₃ for the epoxidation of styrene (0.5mmol) using m-CPBA (0.80 mmol) in CH₂Cl₂(3 mL) catalyzed by Cat₄ (5.0%(molar ratio) based on Mn) at 0 ℃

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  Table 1.  Asymmetric epoxidation of styrene catalyzed by Cat₁~Cat₇ with m-CPBA as oxidants^a

  Entry	Catalyst	Oxidant	Conversionb/%	Selectivitb/%	eeb/%
  1	Jaco	m-CPBA/NMO	99.9	98.7	47.0c
  2	Cat5	m-CPBA/NMO	96.5	99.1	46.3c
  3	Cat6	m-CPBA/NMO	93.8	98.2	46.1c
  4	Cat7	m-CPBA/NMO	93.6	98.3	46.8c
  5	Cat1	m-CPBA/NMO	27.6	53.5	6.3c
  6	Cat1	m-CPBA	92.7	80.6	35.6c
  7	Cat2	m-CPBA	83.5.5	82.3	36.3c
  8	Cat3	m-CPBA	81.8	80.6	42.3c
  9	Cat3	m-CPBA	60.2	86.5	55.8c
  10	Cat4	m-CPBA/NMO	76.5	66.3	18.5c
  11	Cat4	m-CPBA	26.1	12.3	4.3c
  a Reactions were carried out at 0 ℃ for 60 min in CH2Cl2 (3 mL) with styrene(0.5 mmol) NMO (337.5 mg, 2.5 mmol, if necessary), nonane (internal standard, 56 μL, 0.5 mmol), and immobilized MnⅢ(Salen) complexes (0.025 mmol, 5.0 mol%). Determined by GC by integration of product peaks against an internal quantitative standard (nonane), correcting for response factors. b Determined by GC with a chiral capillary column (HP 19091G-B233, 30 m×0.32 mm×0.25 μm). c Epoxide configuration S.

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