Chiral salen Cr(III) complexes encapsulated in thermo-responsive polymer nanoreactors for asymmetric epoxidation of alkenes in water

Weiying Wang , Chaoping Li , Yibing Pi , Jiajun Wang , Rong Tan * and Donghong Yin
National & Local Joint Engineering Laboratory for New Petro-chemical Materials and Fine Utilization of Resources, Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education), Key Laboratory of the Assembly and Application of Organic Functional Molecules of Hunan Province, Hunan Normal University, Changsha 410081, P. R. China. E-mail: yiyangtanrong@126.com; Fax: +86 731 8872531; Tel: +86 731 8872576

Received 15th July 2019 , Accepted 3rd September 2019

First published on 4th September 2019


Chiral salen Cr(III) (Cr(salen)) complexes were encapsulated in thermo-responsive polymer nanoreactors through folding an amphiphilic random copolymer of poly(N-isopropylacrylamide-co-IL/Cr(salen)) (poly(NIPAAM-co-IL/Cr(salen)) around Cr(salen) in water. The resulting catalytic nanoreactors exhibited several advantages over the traditional Cr(salen) system for asymmetric epoxidation of alkenes in water. First, they were dispersed in water, behaving as a quasi-homogeneous catalyst for the aqueous asymmetric epoxidation. Second, they effectively sequestered substrates from the surrounding environment, creating a highly concentrated environment for efficient catalysis. Third, water was excluded from the nanoreactor, minimizing the undesired hydrolysis of epoxides. As a result, the compartmentalized catalysts mediated aqueous asymmetric epoxidation with unprecedented yields (92–95%) and enantioselectivities (ee, 92–99%), whereas the traditional Cr(salen) catalyst was far less efficient (4–12% yield and 29–44% ee). Moreover, the catalytic nanoreactor could be facilely recovered for reuse by thermo-controlled separation. This work highlighted the potential of using folded polymers as a platform for developing highly efficient catalytic nanoreactors for a number of important organic transformations in water.


Introduction

Asymmetric epoxidation of unfunctionalized alkenes is of great importance in organic synthesis, since the obtained enantiopure epoxides are valuable chiral building blocks for the synthesis of fine chemicals.1,2 Various catalysts, especially chiral metallosalen complexes, have been developed for this transformation.3–9 For example, Jacobsen and Katsuki pioneered the catalytic asymmetric epoxidation of unfunctionalized alkenes based on chiral salen Mn(III) complexes.3,4 Although highly efficient, the Mn(salen) systems often exhibit low ee values due to free C–C bond rotation around the C–O bond in the radical intermediate. Gilheany et al. realized highly enantioselective epoxidation of unfunctionalized alkenes by using chiral salen Cr(III) complexes as catalysts in acetonitrile.10,11 Different from Mn(salen) systems, the Cr(salen) catalyzed asymmetric epoxidation via a ring-type metallaoxetane intermediate, which limited the epimerization of chiral epoxides (free rotation about the C–C bond before C–O ring closing), and thus enhanced the enantioselectivity.11 Unfortunately, industrial application was also limited to the use of acetonitrile as a solvent which often results in environmental problems. Chemical conversion in water is of growing interest to chemists because water is inexpensive, safe and environmentally benign,12 while to the best of our knowledge, few reports have focused on efficiently performing the Cr(salen)-catalyzed asymmetric epoxidation in water because of the insolubility of the substrate and the catalyst. Furthermore, hydrolysis of the produced epoxides in the presence of Cr(salen) was inevitable in aqueous systems,10,13 which diminished the selectivity to chiral epoxides. The problems should be solved by trapping the chiral salen Cr(III) complex in the compartmentalized hydrophobic cavity which was protected by a hydrophilic shell. The catalytic compartment may behave as a catalytic nanoreactor, concentrating reagents from the surrounding environment through hydrophobic affinity for efficient catalysis, while the hydrophilic shell made the catalytic nanoreactor compatible in water.14,15 Furthermore, water should be readily excreted off the catalytic, hydrophobic cavity, which minimized the undesired Cr(salen)-catalyzed hydrolysis of epoxides.

Folding an amphiphilic random copolymer around the catalytic site provided a feasible approach to construct the structured inner compartment with a catalytic, hydrophobic core surrounded by a hydrophilic shell.16–22 Intramolecular hydrophobic interaction is one of the most important driven forces to trigger the self-folding of amphiphilic random copolymers into a well-defined compartmentalized architecture in water. We thus decided to incorporate the chiral salen Cr(III) complex as the hydrophobic block of an amphiphilic random copolymer. The incorporated Cr(salen) complexes do not only impart the desired catalytic function to the polymeric architecture but additionally operate as structure-forming elements, inducing the single-chain folding of the amphiphilic random copolymer precursor in water. The polymeric backbone can thus form polymeric pockets around the catalytic Cr(salen) centers, resulting in the structured inner compartment with a hydrophobic Cr(salen) core surrounded by a hydrophilic shell. The catalytic compartment may act as the catalytic nanoreactor to accommodate the asymmetric epoxidation of alkenes in water. However, the bulky polymer matrix inevitably increased local steric hindrance around Cr(salen), which prevented all complexes from adopting their preferred “stepped” conformation (Chart 1) in the confined space.11,23 An imidazolium-based ionic liquid (IL) was thus introduced between Cr(salen) and the polymer backbone. In addition to ensuring the necessary conformational freedom of Cr(salen), such an IL moiety encapsulated in the compartment along with Cr(salen) also stabilized the formed metallosalen active intermediates, thereby further enhancing the catalytic efficiency of the chiral salen Cr(III) catalyst. Unfortunately, the water-soluble compartment is often difficult to recover from water for reuse. If the compartment is thermo-responsive, being able to undergo a hydrophilic-to-hydrophobic switch at its lower critical solution temperature (LCST),24 facile recovery of the catalytic nanoreactor could be realized.


image file: c9cy01398a-c1.tif
Chart 1 The structure and “stepped” conformation of neat-C.

Herein, NIPAAm, which underwent hydrophilic-to-hydrophobic switching at its LCST,25,26 was employed as the switching block to copolymerize with vinyl-modified IL/Cr(salen) via reversible addition–fragmentation chain transfer polymerization (RAFT).27 The resulting thermo-responsive random copolymers of poly(NIPAAm-co-IL/Cr(salen)) exhibited LCST behaviors and underwent thermo-switchable single-chain folding in water. At room temperature, they were amphiphilic and folded around the Cr(salen) units in water through intramolecular hydrophobic interactions, forming water-soluble single-chain polymeric nanoparticles (SCPNs) with an IL-modified hydrophobic Cr(salen) interior and a hydrophilic NIPAAm shell. The SCPNs provided a hydrophobic compartmentalization to encapsulate Cr(salen) species and concentrate reactants in water, dramatically accelerating the aqueous asymmetric epoxidation of alkene in water. Unprecedented yields (92–95%) to corresponding (R,R)-epoxides with excellent ee values (92–99%) were observed for a wide range of alkenes, whereas the neat complex was far less efficient (4–12% yield and 29–44% ee value). The catalytic efficiency, especially selectivity, represented the best results so far in asymmetric epoxidation based on metallosalen systems. Upon heating above their corresponding LCST, the catalytic nanoreactor became hydrophobic and could be facilely recovered from the aqueous system for steady reuse.

Experimental

Materials and reagents

Racemic cyclohexanediamine and L-(+) tartaric acid were purchased from MACKLIN. Vinylimidazole and chromium dichloride were obtained from Alfa Aesar. AIBN and NIPAAm were purchased from Aldrich. β-Methylstyrene, 1,2-dihydronaphthalene, trans-stilbene, styrene, indene and 1,3-cyclohexadiene were bought from J&K. Other commercially available chemicals were purchased from local suppliers. 3-tert-Butyl-5-chloromethyl-2-hydroxybenzaldehyde and the chiral salen ligand of (R,R′)-N-(3,5-di-tert-butylsalicylidene)-N′-[3-tert-butyl-5-vinyl]-1,2-cyclohexanediamine (CL) were synthesized as described earlier.28 (R,R')-N-(3,5-Di-tert-butylsalicylidene)-1,2-cyclo-hexanediamine hydrochloride was synthesized according to a reported procedure.29 The traditional chiral ((R,R′)-salen)CrIIICl complex (denoted as neat-C, as shown in Chart 1) was synthesized according to previous literature.10

Methods

FT-IR spectra were obtained as potassium bromide pellets with a resolution of 4 cm −1 and 64 scans in the range 400–4000 cm −1 using an AVATAR 370 Thermo Nicolet spectrophotometer. NMR spectra of products were recorded on a BRUKER AVANCE−500 spectrometer with TMS as an internal standard. TEM images were obtained on a Tecnai F20 microscope at an accelerating voltage of 200 kV. Samples were prepared by depositing the aqueous solution (0.5 mg mL−1) onto an ultrathin carbon-coated copper grid, followed by removal of excess solution by blotting the grid with filter paper. The samples were dried for 72 h at room temperature in desiccators and were negatively stained by phosphotungstic acid. The molecular weight and molecular-weight distribution of the copolymers were obtained by gel permeation chromatography (GPC). Analyses were performed on an Alltech Instrument (Alltech, America) using THF as the solvent eluting at a flow of 1 mL min−1 through a Jordi GPC 10000 A column (300 mm × 7.8 mm) equipped with an Alltech ELSD 800 detector. The system was calibrated with standard polystyrene. The detection temperature is 40 °C and the column temperature is 30 °C. DLS was performed using a ZS90 laser particle size analyzer (Malvern, UK). The aqueous solution of samples (0.5 M) was filtered through a 0.45 μm disposable polyamide membrane to free it from dust particles before measurement. The chromium contents in the catalysts were determined by inductively coupled plasma mass spectrometry (ICP-MS) on a NexION 300X analyzer (Perkin-Elmer Corp.). Elemental analyses of C, H and N were carried out on a Vario EL III Elemental analyser made in Germany. The optical rotation of samples was measured in dichloromethane on a WZZ-2A Automatic Polarimeter. The LCST of copolymers in water was determined by measuring the transmittance of the corresponding aqueous solution (0.5 M) using a UV-visible photometer.

Preparation of poly(NIPAAm-co-IL/Cr(salen)) (denoted as PNx(IL-C)y, x = 60, y = 8; x = 80, y = 5; x = 84, y = 3; x = 90, y = 2, where x and y represent the numbers of NIPAAm and IL/Cr(salen) units, respectively)

The synthesis of PNx(IL-C)y (x = 60, y = 8; x = 80, y = 5; x = 84, y = 3; x = 90, y = 2) is outlined in Scheme 1.
image file: c9cy01398a-s1.tif
Scheme 1 Schematic representation of the synthesis and self-folding of PNx(IL-C)y (x = 60, y = 8; x = 80, y = 5; x = 84, y = 3; x = 90, y = 2).
Synthesis of IL-modified salicylaldehyde (A). Vinyl imidazole (5.0 mmol, 0.47 g) in toluene (25 mL) was mixed with 3-tert-butyl-5-chloromethyl-2-hydroxy-benzaldehyde (5.0 mmol, 1.13 g) in toluene (25 mL) under an argon atmosphere. The mixture was refluxed for 48 h and then concentrated in vacuum. The crude product was washed with ethyl acetate (15 × 3 mL) to remove the unreacted 1-vinyl imidazole and 3-tert-butyl-5-chloromethyl-2-hydroxy-benzaldehyde. After being dried in vacuo, compound A was obtained as a yellow oily liquid (1.45 g, yield: 90%). Calc. for (C17H24N2O2Cl): C: 61.83, H: 7.46, N: 9.01%. Found: C: 61.85, H: 7.47, N: 9.00%. FT-IR (KBr): [small nu, Greek, tilde] = 3030, 2954, 2797, 1651, 1542, 1364, 1189, 1068, 959, 844, 620, 598 cm−1. 1H NMR (500 MHz, CDCl3): δ = 9.87 (s, 1 H, CHO), 7.80–7.58 (s, 2 H, ArH), 5.93 (m, 1 H, CH2[double bond, length as m-dash]CH), 5.71 (s, 2 H, N–CH2–N), 5.36 (m, 2 H, CH2[double bond, length as m-dash]CH), 5.19 (s, 1 H, Ar–OH), 4.45 (s, 2 H, Ar–CH2–N), 3.57 (m, 4 H, N–CH2–CH2–N), 1.24 ppm (m, 9 H, CH3).
Synthesis of IL/Cr(salen). The obtained A (5 mmol, 1.5 g) dissolved in ethanol (25 mL) was added dropwise to a solution of (R,R')-N-(3,5-di-tert-butylsalicylidene)-1,2-cyclo-hexanediamine hydrochloride (5 mmol, 1.8 g) in ethanol (25 mL) followed by the addition of triethylamine (9 mmol, 1.3 mL). The mixture was stirred at room temperature for 4 h. After the removal of the solvent in vacuo, the residue was dissolved in tetrahydrofuran (25 mL). A solution of CrCl2 (5 mmol, 0.61 g) in tetrahydrofuran (25 mL) was added dropwise. The mixture was stirred for 24 h at room temperature under an argon atmosphere and then was further oxidized under air for 24 h. The solution was poured into dichloromethane (25 mL). The mixture was washed with a saturated aqueous solution of NH4Cl (15 × 3 mL) and a saturated aqueous solution of NaCl (15 × 3 mL) to remove the unreacted CrCl2. The organic layer was dried over anhydrous Na2SO4 and concentrated. After being dried in vacuo, IL/Cr(salen) was obtained as a brown solid (3.13 g, yield: 87%). Calc. for (C38H54N4O2CrCl2): C: 63.23, H: 7.54, N: 7.76%. Found: C: 63.24, H: 7.53, N: 7.78%. FT-IR (KBr): [small nu, Greek, tilde] = 3441, 2924, 2856, 1622, 1531, 1461, 1319, 1254, 1167, 1136, 1093, 1032, 920, 843, 812, 790, 627, 624, 551 cm−1.
Synthesis of PNx(IL-C)y. A certain feed molar ratio of IL/Cr(salen) and NIPAAm (10 mmol of NIPAAm and 1.43 mmol of IL/Cr(salen) for PN60(IL-C)8, 10 mmol of NIPAAm and 0.67 mmol of IL/Cr(salen) for PN80(IL-C)5, 10 mmol of NIPAAm and 0.4 mmol of IL/Cr(salen) for PN84(IL-C)3, 10 mmol of NIPAAm and 0.29 mmol of IL/Cr(salen) for PN90(IL-C)2) were dissolved in anhydrous methanol (15 mL) in a Schlenk tube. 2-Aminoethanethiol hydrochloride (DABE, 1 mmol, 0.196 g) was used as the chain transfer reagent for this copolymerization. N,N′-Azobis(isobutyronitrile) (AIBN, 0.5 mmol, 0.082 g) as the radical initiator was then added into the solutions. After being degassed by bubbling with argon, the reaction was carried out at 60 °C for 24 h under an argon atmosphere. The solution was cooled with liquid nitrogen to stop the reaction and then was concentrated in vacuo. The crude products were dissolved in tetrahydrofuran (5 mL) and subsequently precipitated three times in 100 mL of diethyl ether. After being dried in vacuo, the copolymers were obtained as a yellowish-brown powder, which were denoted as PNx(IL-C)y (x and y represent the number of NIPAAm and IL/Cr(salen) units, respectively, which were calculated from their Mn and the corresponding chromium contents). PN60(IL-C)8 (1.76 g, yield: 75%): FT-IR (KBr): [small nu, Greek, tilde] = 3411, 3274, 3057, 2966, 2940, 2868, 1630, 1538, 1453, 1375, 1361, 1244, 1165, 1126, 1087, 1021, 910, 835, 806, 785, 628, 621, 549 cm−1. GPC (THF): Mn = 12[thin space (1/6-em)]782, Mw = 15[thin space (1/6-em)]678, PDI (Mw/Mn) = 1.23. αD25 = −13.4 (C = 0.005 g mL−1, CH2Cl2). Chromium content: 0.64 mmol g−1. PN80(IL-C)5 (1.53 g, yield: 85%): FT-IR (KBr): [small nu, Greek, tilde] = 3425, 3287, 3064, 2966, 2926, 2875, 1643, 1538, 1453, 1394, 1348, 1237, 1152, 1119, 1080, 1041, 929, 831, 804, 765, 648, 621, 556 cm−1. GPC (THF): Mn = 12[thin space (1/6-em)]317, Mw = 13[thin space (1/6-em)]480, PDI (Mw/Mn) = 1.09. αD25 = −14.5 (C = 0.005 g mL−1, CH2Cl2). Chromium content: 0.39 mmol g−1. PN84(IL-C)3 (1.36 g, yield: 84%): FT-IR (KBr): [small nu, Greek, tilde] = 3429, 3294, 3075, 2979, 2933, 2881, 1643, 1538, 1453, 1375, 1322, 1263, 1165, 1126, 1093, 1053, 929, 835, 811, 772, 654, 620, 567 cm−1. GPC (THF): Mn = 12[thin space (1/6-em)]113, Mw = 12[thin space (1/6-em)]759, PDI (Mw/Mn) = 1.05. αD25 = −14.7 (C = 0.005 g mL−1, CH2Cl2). Chromium content: 0.25 mmol.g−1. PN90(IL-C)2 (1.27 g, yield: 82%): FT-IR (KBr): [small nu, Greek, tilde] = 3425, 3306, 3071, 2972, 2933, 2875, 1649, 1531, 1453, 1394, 1354, 1244, 1158, 1119, 1099, 1027, 922, 844, 818, 779, 634, 621, 562 cm−1. GPC (THF): Mn = 11[thin space (1/6-em)]926, Mw = 12[thin space (1/6-em)]518, PDI (Mw/Mn) = 1.05. αD25 = −12.6 (C = 0.005 g mL−1, CH2Cl2). Chromium content: 0.17 mmol g−1.

Preparation of PN84C3

To evaluate the function of the IL linker, a random copolymer analog of the IL-free counterpart of PN84C3 in which the Cr(salen) unit was directly anchored on the polymer backbone, was prepared as a control catalyst. The preparation procedure was similar to that of PN84(IL-C)3, except for using vinyl-Cr(salen) instead of IL/Cr(salen) to copolymerize with NIPAAm, as shown in Scheme 2. PN84C3 was obtained as a yellowish-brown powder (where the repeating number of NIPAAm units was 84, and the repeating number of Cr(salen) units was 3, as calculated from its Mn and chromium content (1.27 g, yield: 81%). FT-IR (KBr): [small nu, Greek, tilde] = 3429, 3294, 3075, 2960, 2940, 2861, 1641, 1525, 1460, 1375, 1329, 1237, 1172, 1132, 1060, 968, 922, 877, 842, 792, 707, 667, 567 cm−1. GPC (THF): Mn = 12[thin space (1/6-em)]887, Mw = 14[thin space (1/6-em)]858, PDI (Mw/Mn) = 1.15. αD25 = −11.5 (C = 0.005 g mL−1, CH2Cl2). Chromium content: 0.31 mmol g−1.
image file: c9cy01398a-s2.tif
Scheme 2 Synthesis of PN84C3.

General procedure for asymmetric epoxidation of alkenes in water

The selected catalyst (0.5 mol% of substrate based on the chromium content) and PhI(OAc)2 (0.625 mmol, 0.20 g) were stirred in water (1.0 mL) for 30 min at 25 °C. The solution was initially dark green, and then became orange-yellow. Unfunctionalized alkene (0.25 mmol) was then added. The resulting mixture was stirred at room temperature until the reaction was judged to be complete based on TLC analysis. The reaction mixture was then heated to above 54 °C and held at this temperature for 5 min to make the catalyst completely precipitate from the aqueous system. The separated catalyst was washed with n-hexane (3 × 5.0 mL) and dried at room temperature in vacuum. The reaction solution was extracted with dichloromethane (3 × 0.5 mL). Notably, this extraction process was expected in large-scale industrial processes, in which the oily product phase could be directly separated from water. The combined organic phase was dried with anhydrous Na2SO4. After the evaporation of the solvent, the residue was further purified by column chromatography on silica gel (Acros, 40–60 μm, 60 Å, eluent: petroleum ether/ethyl acetate = 5/1 (v/v)) to afford the pure chiral epoxides. All products were identified by 1H NMR spectroscopy. Conversions and ee values of corresponding chiral epoxides were determined using a 6890 N gas chromatograph (Agilent Co.) equipped with a capillary column (HP19091G-B213, 30 m × 0.32 mm × 0.25 μm) and a FID detector using n-decane as an internal standard. Detailed NMR spectroscopy and GC analysis for corresponding chiral epoxides are available in the ESI.
β-Methylstyrene epoxide. Yield: 92%; ee value: 94%, determined by GC using nitrogen as the carrier gas at a gas flow rate of 30 mL min−1, an injector temperature and a detector temperature of 250 °C, and a column temperature of 100 °C. Major enantiomer: tR,R = 13.4 min, minor enantiomer tS,S = 12.8 min. 1H NMR (500 MHz, CDCl3): δ = 7.2 (s, 5 H, ArH), 5.3 (m, 1 H, Ar–CH), 4.1 (m, 1 H, CH–CH–CH3), 2.3 ppm (m, 3 H, CH–CH3).
trans-Stilbene epoxide. Yield: 93%; ee value: 92%, determined by GC using nitrogen as the carrier gas at a gas flow rate of 30 mL min−1, an injector temperature and a detector temperature of 250 °C, and a column temperature of 100 °C. Major enantiomer: tR,R = 26.1 min, minor enantiomer tS,S = 25.0 min. 1H NMR (500 MHz, CDCl3): δ = 7.3 (m, 10 H, ArH), 3.9 ppm (s, 2 H, Ar–CH–CH–Ar).
Styrene epoxide. Yield: 93%; ee value: 99%, determined by GC using nitrogen as the carrier gas at a gas flow rate of 30 mL min−1, an injector temperature and a detector temperature of 250 °C, and a column temperature of 100 °C. Major enantiomer: tR,R = 8.2 min, minor enantiomer: tS,S = 7.9 min. 1H NMR (500 MHz, CDCl3): δ = 7.3 (m, 5 H, ArH), 3.9 (m, 1 H, Ar–CH), 3.1 (m, 1 H, CH–CH2), 2.8 ppm (m, 1 H, CH–CH2).
Indene epoxide. Yield: 94%; ee value: 94%, determined by GC using nitrogen as the carrier gas at a gas flow rate of 30 mL min−1, an injector temperature and a detector temperature of 250 °C. The column temperature was programmed from 80 to 180 °C at a rate of 5 °C min−1. Major enantiomer: tR,R = 12.9 min, minor enantiomer: tS,S = 12.2 min. 1H NMR (500 MHz, CDCl3): δ = 7.2 (m, 4 H, ArH), 4.0 (m, 2 H, Ar–CH–CH–CH2), 2.3 ppm (m, 2 H, Ar–CH2).
1,2-Dihydronaphthalene epoxide. Yield: 95%; ee value: 93%, determined by GC using nitrogen as the carrier gas at a gas flow rate of 30 mL min−1, an injector temperature and a detector temperature of 250 °C. The column temperature was programmed from 80 to 180 °C at a rate of 5 °C min−1. Major enantiomer: tR,R = 24.3 min, minor enantiomer: tS,S = 23.4 min. 1H NMR (500 MHz, CDCl3): δ = 7.1 (m, 4 H, ArH), 4.1 (m, 1 H, Ar–CH–CH), 3.7 (m, 1 H, CH–CH–CH2), 2.8 (m, 2 H, Ar–CH2), 2.3 ppm (m, 2 H, CH2–CH2–CH).

Asymmetric epoxidation of β-methylstyrene for kinetic measurement

The catalyst (0.5 mol% of substrate based on chromium content) and PhI(OAc)2 (0.625 mmol, 0.20 g) were stirred in water (1 mL) for 30 min at 25 °C. β-Methylstyrene (0.25 mmol, 0.03 g) was then added. Aliquots were taken from the reaction mixture every 5 min. The reaction solution was extracted with CH2Cl2 (3 × 0.5 mL). The combined organic phase was dried with anhydrous Na2SO4. The products were purified by silica gel column chromatography (petroleum ether/ethyl acetate = 5/1 (v/v)) and analyzed by GC.

Results and discussion

Preparation of catalysts

In nature, enzymes carry out highly enantioselective reactions in aqueous bioenvironments within small hydrophobic pockets.30 Numerous synthetic systems have been designed to mimic the natural ones, from single-molecule catalytic species31–33 to compartment-forming ones,34,35 in which catalytic functionalities have been introduced to yield highly efficient nanoreactors. An enhancement in catalytic activity is often observed in these systems as a result of the concentrated organic substrates within the hydrophobic cavity. The fascinating features inspired us to encapsulate the chiral salen Cr(III) complex in a compartmentalized hydrophobic cavity to yield a catalytic nanoreactor for confined catalysis of asymmetric epoxidation of alkenes in water. Folding a Cr(salen)-containing amphiphilic random copolymer around the catalytic Cr(salen) site provided a feasible approach to construct the structured inner compartment with a hydrophobic Cr(salen) core surrounded by a hydrophilic shell.36–39 We thus decided to copolymerize the hydrophobic Cr(salen) monomer with NIPAAm to prepare the Cr(salen)-containing amphiphilic random copolymer for this fabrication. The hydrophobic Cr(salen) moiety not only imparted the desired catalytic function to the polymeric architecture but also operated as a structure-forming element for the single-chain folding. Thermo-sensitive PNIPAAm was used as the switching block to control the self-folding behavior of the copolymers in water and ensured the recovery of the catalytic nanoreactors in a temperature-controllable way.

PN x (IL-C) y copolymers were synthesized according to the process illustrated in Scheme 1. Vinylimidazole IL-modified salicylaldehyde (compound A) was synthesized through N-alkylation of vinylimidazole with the benzyl chloride group in 3-tert-butyl-5-chloromethyl-2-hydroxybenzaldehyde. Condensation of compound A with (R,R′)-N-(3,5-di-tert-butylsalicylidene)-1,2-cyclo-hexanediamine hydrochloride together with the following CrCl2 treatment gave a vinylimidazolium IL-modified chiral salen Cr(III) complex (IL/Cr(salen)). The IL/Cr(salen), featuring a vinylimidazolium IL on the 5′-position of the chiral salen Cr(III) complex, was copolymerized with NIPAAm through RAFT by using AIBN as the radical initiator and DABE as the chain transfer agent, providing thermo-responsive, random copolymers of PNx(IL-C)y. The x and y represent the repeating unit numbers of NIPAAm and IL/Cr(salen), respectively, which could be conveniently controlled by adjusting the feed ratios of the monomers during the copolymerization.

For comparison, an IL-free counterpart of PN84C3, where multiple chiral salen Cr(III) complexes were directly appended on the polymer backbone, was prepared as a control catalyst. (R,R′)-N-(3,5-Di-tert-butylsalicylidene)-N′-[3-tert-butyl-5-vinyl]-1,2-cyclohexanediamine (CL) was treated with CrCl2, giving vinyl-Cr(salen). Copolymerization of the hydrophobic vinyl-Cr(salen) monomer with thermo-sensitive NIPAAm through RAFT provided the thermo-responsive, random copolymer of PN84C3, as shown in Scheme 2.

The number-average molecular weight (Mn) and weight-average molecular weight (Mw) of the obtained copolymers were determined by GPC (Table 1). Low polydispersity index (PDI) values that ranged from 1.05 to 1.50 suggested good control of the molecular weight distribution. The polymerization degree of copolymers was calculated from their Mn and the corresponding chromium content determined by ICP-MS (Table 1).

Table 1 The compositions of synthesized copolymersa
Samples Chromium contenta(mmol g−1) M n M w PDIc
a Determined by ICP-MS. b Determined by GPC. c PDI was calculated by Mw/Mn.
PN 60 (IL-C) 8 0.64 12[thin space (1/6-em)]782 15[thin space (1/6-em)]678 1.23
PN 80 (IL-C) 5 0.39 12[thin space (1/6-em)]317 13[thin space (1/6-em)]480 1.09
PN 84 (IL-C) 3 0.25 12[thin space (1/6-em)]113 12[thin space (1/6-em)]759 1.05
PN 90 (IL-C) 2 0.17 11[thin space (1/6-em)]926 12[thin space (1/6-em)]518 1.05
PN 84 C 3 0.31 12[thin space (1/6-em)]887 14[thin space (1/6-em)]858 1.50


As expected, the thermo-responsive PNIPAAm block endowed the PNx(IL-C)y with switchable temperature-dependent water solubility. The copolymers were soluble in water at room temperature, giving clear aqueous solutions (Fig. 1c–f), although neat-C was practically water-insoluble (Fig. 1a). As the local temperature was increased, the aqueous solutions became turbid, as shown by the typical PN84(IL-C)3 (Fig. 1e′). This suggested a transformation from the amphiphilic to the hydrophobic state at this temperature range. Indeed, the PNx(IL-C)y copolymers were finally completely precipitated out of water, as shown by the typical PN84(IL-C)3 (Fig. 1e′′). The IL moiety did not affect the water solubility of PNx(IL-C)y since PN84C3 also dissolved in water (Fig. 1b).


image file: c9cy01398a-f1.tif
Fig. 1 Photographs of neat-C (a), PN84C3 (b), PN60(IL-C)8 (c), PN80(IL-C)5 (d), PN84(IL-C)3 (e) and PN90(IL-C)2 (f) at 25 °C and PN84(IL-C)3 in water at 50 °C (e′) and 54 °C (e′′).

Characterization of samples

LCST. UV-vis spectroscopy was used to further determine the LCST of the corresponding PNx(IL-C)y copolymers (Fig. 2). PN60(IL-C)8 was well soluble in water, giving clear aqueous solution at temperatures below 45 °C (Fig. 2A). When the local temperature was raised to 45 °C, the aqueous solution became turbid, and transmittance decreased dramatically (Fig. 2A). This suggested the hydrophobic state of PN60(IL-C)8 at this temperature range. Therefore, the LCST of PN60(IL-C)8 was determined as ca. 45 °C. Varying the molar ratios of NIPAAm to IL/Cr(salen) changed the hydrophilic–lipophilic balance of PNx(IL-C)y and hence tuned the LCST. Their LCST increased with decreasing the content of hydrophobic IL/Cr(salen) block accordingly. PN80(IL-C)5, PN84(IL-C)3 and PN90(IL-C)2 exhibited LCSTs of about 48, 50 and 54 °C, respectively (Fig. 2A). PN84C3 exhibited an LCST (ca. 29 °C) much lower than those of PNx(IL-C)y probably due to the absence of the highly polar IL moiety (Fig. 2A).40 Notably, the LCSTs for all copolymers were higher than room temperature (25 °C), suggesting that all copolymers were amphiphilic at room temperature. It was desirable for folding these copolymers in water to create catalytic nanoreactors. When the temperature was heated to above their corresponding LCSTs, the PNx(IL-C)y copolymers became hydrophobic and were finally precipitated out of water. The water solubility switch could be reversibly repeated several times by controlling the local temperature, as shown by the typical PN84(IL-C)3 (Fig. 2B).
image file: c9cy01398a-f2.tif
Fig. 2 Transmittance curves of the PNx(IL-C)y and PN84C3 aqueous solutions (concentration: 0.5 M, (A)), and optical transmittance at 450 nm of PN84(IL-C)3 solution observed upon several cycles under heating at 50 °C and then cooling to 25 °C (B). The insets show the photographs of PN60(IL-C)8 at 25 °C and at 45 °C.
TEM. TEM confirmed the self-folding behavior of PNx(IL-C)y in water at room temperature, as shown in Fig. 3. As expected, PNx(IL-C)y coiled into uniform, spherical nanoparticles with an average size of ca. 6.0 nm in water at room temperature (Fig. 3a–d). The ultrasmall size suggested a single-chain folded conformation of PNx(IL-C)y in solution.22 Logically, the hydrophobic Cr(salen) pendants on PNx(IL-C)y avoided coming into contact with water, triggering the self-folding of PNx(IL-C)y around Cr(salen) into SCPNs with a hydrophobic Cr(salen) core surrounded by a PNIPAAm shell,37,41 as shown in Scheme 1. The imidazolium IL moiety in PNx(IL-C)y did not significantly affect their single-chain folding behavior, since PN84(IL-C)3-based SCPN gave a similar morphology to the SCPN based on PN84C3 (Fig. 3cvs. e). Therefore, the self-folding process of PNx(IL-C)y was only triggered by the Cr(salen) group, independent of the IL linker. High dispersion of the SCPNs in water confirmed the presence of the hydrophilic PNIPAAm shell at room temperature (Fig. 3a–d). This made the catalytic nanoreactor a quasi-homogeneous catalyst for the aqueous asymmetric epoxidation and therefore high catalytic efficiency was expected. Notably, the SCPNs tended to gather together in water when the local temperature was raised to their LCST, as shown by the typical PN84(IL-C)3 (Fig. 3cvs. c). Thermo-controlled switching of the NIPAAm corona from the hydrophilic to the hydrophobic state should be responsible for the aggregation of PNx(IL-C)y at the higher temperature. Actually, PNx(IL-C)y could be finally converted into the double-hydrophobic random copolymers and precipitated from water upon heating. The thermo-switchable solubility allowed for efficient catalysis coupled with facile recovery of PNx(IL-C)y in aqueous systems in a thermo-controllable way.
image file: c9cy01398a-f3.tif
Fig. 3 TEM micrographs of self-folded PN60(IL-C)8 (a), PN80(IL-C)5 (b), PN84(IL-C)3 (c), PN90(IL-C)2 (d), and PN84C3 (e) in water at room temperature, and self-folded PN84(IL-C)3 in water at 50 °C (c′) stained with phosphotungstic acid.

Dynamic light scattering (DLS)

DLS was employed to further determine the hydrodynamic diameter (Dh) of PNx(IL-C)y-based SCPNs. Obviously, all of the copolymers showed a single modal DLS intensity distribution, typically shown in Fig. 4. This indicated the homogeneous distribution and uniform sizes of the corresponding SCPNs.42 The Dh of PNx(IL-C)y-based SCPNs was ultra-small, as determined by DLS. PN90(IL-C)2, PN84(IL-C)3, PN80(IL-C)5, and PN60(IL-C)8 gave a Dh of only ca. 18.2, 10.1, 6.5, and 5.7 nm, respectively. The diameter values were consistent with that adopting a single-chain folded conformation in solution. Notably, the size of SCPNs gradually decreased with increasing content of the hydrophobic Cr(salen) block in the copolymer. Reduced sizes were mainly due to the enhanced intramolecular hydrophobic interaction arising from increased Cr(salen), which made the SCPNs more compact. These observations provided further evidence for the self-folding of PNx(IL-C)y around the catalytic Cr(salen) sites, rather than aggregation of multiple polymer chains As a result, the polymeric backbone formed polymeric pockets around the catalytic Cr(salen) centers, resulting in the structured inner compartment with a hydrophobic Cr(salen) core surrounded by a hydrophilic shell.
image file: c9cy01398a-f4.tif
Fig. 4 Size distribution of folded PN60(IL-C)8, PN80(IL-C)5, PN84(IL-C)3 and PN90(IL-C)2 in water at a concentration of 0.5 M at room temperature.

Catalytic performances

The PNx(IL-C)y-based SCPNs provided hydrophobic, catalytic compartments to accommodate asymmetric epoxidation of alkenes in water (Table 2). Only 0.5 mol% PNx(IL-C)y was sufficient to afford a high yield (68–92%) of β-methyl styrene epoxide with remarkable enantioselectivity (92–94%) within 60 min (Table 2, entries 1–4). In contrast, an extremely low yield (25%) with a disappointing ee value (46%) was obtained when the catalytic monomer of IL/Cr(salen) was used as the catalyst (Table 2, entry 6). In particular, many undesired by-products including the diol and benzyl methyl ketone were observed over the neat system due to inevitable hydrolysis and rearrangement of the epoxide catalyzed by the Cr(salen) complex.10,13 Furthermore, neat-C was even less active and selective than the IL/Cr(salen) (Table 2, entry 7 vs. 6), giving only 12% yield of β-methyl styrene epoxide with 29% ee value. These results confirmed the importance of the PNx(IL-C)y-based hydrophobic compartment for the aqueous catalysis. Actually, the hydrophobic pocket endowed PNx(IL-C)y with salient features as follows: (1) effectively sequestering substrates from the surrounding water: this resulted in a high local concentration of substrates around the catalytic sites for efficient catalysis;15 (2) hydrophobic shielding of Cr(salen) from the aqueous environment: this prevented the rearrangement of the ring-type metallaoxetane intermediate to ring-opened cationic intermediate in water (Scheme 3),11,27 which enhanced the ee values; (3) excluding water from the hydrophobic pocket: this minimized the undesired hydrolysis of epoxide and further maximized the selectivity to chiral epoxide.14,15 To further confirm the importance of compartmentalization, we carried out the reaction with PN84(IL-C)3 in acetonitrile, a solvent in which PN84(IL-C)3 failed to self-fold. Apparently, the PN84(IL-C)3 was far less active (conversion of 75%) and enantioselective (89%) under identical conditions (Table 2, entry 8). This result was consistent with our hypothesis that the high local concentration of substrates around catalytic sites was crucial for this system to be active, and this is only achieved in the folded state of PNx(IL-C)y and in the presence of catalytic compartments.
Table 2 Results of asymmetric epoxidation of unfunctionalized alkenes over different chiral salen Cr(III) catalystsa
Entry Catalyst Product Solvent Conv.b (%) Yieldc (%) eeb (%)
a Catalyst (0.5 mol% of substrate, based on chromium content), substrate (0.25 mmol), PhI(OAc)2 (0.625 mmol), solvent (1 mL), 25 °C, 60 min. b Determined by GC. c Isolated yield after column chromatography.
1 PN 60 (IL-C) 8 image file: c9cy01398a-u1.tif H2O 80 68 92
2 PN 80 (IL-C) 5 H2O 86 78 94
3 PN 84 (IL-C) 3 H2O 99 92 94
4 PN 90 (IL-C) 2 H2O 84 74 94
5 PN 84 C 3 H2O 77 64 93
6 IL/Cr(salen) H2O 38 25 46
7 Neat-C H2O 19 12 29
8 PN 84 (IL-C) 3 CH3CN 75 42 89
9 PN 84 (IL-C) 3 image file: c9cy01398a-u2.tif H2O 94 93 92
10 PN 84 C 3 H2O 90 85 91
11 IL/Cr(salen) H2O 25 16 42
12 Neat-C H2O 15 9 31
13 PN 84 (IL-C) 3 CH3CN 72 36 87
14 PN 84 (IL-C) 3 image file: c9cy01398a-u3.tif H2O 99 93 99
15 PN 84 C 3 H2O 81 74 93
16 IL/Cr(salen) H2O 31 16 53
17 Neat-C H2O 18 8 38
18 PN 84 (IL-C) 3 CH3CN 74 38 90
19 PN 84 (IL-C) 3 image file: c9cy01398a-u4.tif H2O 99 94 94
20 PN 84 C 3 H2O 89 83 92
21 IL/Cr(salen) H2O 39 22 56
22 Neat-C H2O 6 4 37
23 PN 84 (IL-C) 3 CH3CN 78 46 89
24 PN 84 (IL-C) 3 image file: c9cy01398a-u5.tif H2O 99 95 93
25 PN 84 C 3 H2O 88 81 93
26 IL/Cr(salen) H2O 43 26 49
27 Neat-C H2O 13 7 44
28 PN 84 (IL-C) 3 CH3CN 81 48 91



image file: c9cy01398a-s3.tif
Scheme 3 Cr(salen)-mediated asymmetric epoxidation of β-methyl styrene through a metallaoxetane intermediate.

Since the asymmetric epoxidation occurred in the Cr(salen)-containing hydrophobic pocket, different sizes of the nanoreactors as well as different Cr(salen) numbers within the catalytic pocket affected the catalytic efficiency. PN84(IL-C)3 with a Dh of 10.1 nm exhibited the highest catalytic efficacy for this aqueous transformation. Almost quantitative conversion (99%) of β-methyl styrene with 94% ee value was achieved when PN84(IL-C)3 was used as a catalyst (Table 2, entry 3). Fewer Cr(salen) units per polymer chain (PN90(IL-C)2) were undesirable for the aqueous reaction due to the lower local concentration of Cr(salen) sites in the catalytic pocket, although the larger sized compartment (Dh of 18.2 nm) favored the substrate diffusion (Table 2, entry 4), while too dense packing of Cr(salen) in PNx(IL-C)y, such as PN80(IL-C)5 (Dh of 6.5 nm) and PN60(IL-C)8 (Dh of 5.7 nm), not only limited the diffusion of substrate into the catalytic pocket but also restricted all Cr(salen) from adopting their preferred “stepped” conformation11,23 in SCPNs, which also resulted in a decline in activity (Table 2, entries 1 and 2).

Apart from the compartmental structures, the catalytic efficiency of PNx(IL-C)y also benefited from the local catalyst microenvironment created by the IL spacer, since the IL moiety was encapsulated in the catalytic compartment along with Cr(salen) units. Indeed, PN84(IL-C)3 was more efficient than the IL-free counterpart of PN84C3 (Table 2, entry 3 vs. 5), although the number of Cr(salen) units in PN84(IL-C)3 was identical with that in PN84C3. In fact, the IL linker played multiple roles in enhancing the catalytic performance. It located the bulky complexes away from the polymer backbone, enabling all active sites to adapt their preferential “stepped” conformation for epoxidation, and it created a compatible IL environment in the hydrophobic pocket, making active catalytic centers more accessible to reagents.43–46 In addition, the proximity of the IL unit to the active site may stabilize the active O[double bond, length as m-dash]CrV(salen) intermediate44 and lower the activation energy for the addition of alkene to the O[double bond, length as m-dash]CrV(salen) model in the asymmetric epoxidation,47 thus increasing the catalytic efficiency.

Kinetics was used to further evaluate the advantage of the hydrophobic catalytic compartment in overall aqueous catalysis as well as the positive effects of the imidazolium-based IL moiety on the catalytic efficiency. The kinetic curves and corresponding rate curves of asymmetric epoxidation of β-methylstyrene over the designed Cr(salen) catalysts are shown in Fig. 5. Obviously, PN84(IL-C)3 was more efficient in water than in acetonitrile (Fig. 5avs. c). In fact, acetonitrile was a good solvent for both blocks of the amphiphilic copolymer, hence self-folding of PN84(IL-C)3 into compartmental SCPNs did not occur. PN84(IL-C)3 was just a conventional “polymer supported” catalyst in the asymmetric epoxidation of β-methylstyrene in acetonitrile. The lower local concentration of substrates and catalytic species in the organic system was insufficient for efficient catalysis. The results were consistent with our hypothesis that the PN84(IL-C)3-based catalytic compartment create a highly concentrated cavity for efficient asymmetric epoxidation of β-methylstyrene in water catalysis. The gradient increase in conversion of β-methylstyrene, together with the parabolic profile of rate constants (kobs), demonstrated the “concentration effect” of PN84(IL-C)3-based SCPN in the aqueous asymmetric epoxidation.48 In particular, the observed kobs initially rose rapidly due to the dramatically increasing concentration of the substrate in the hydrophobic compartments, went through a maximum, and then drastically decreased due to a dilution effect (Fig. 5a). Furthermore, we noticed that the kobs values of PN84(IL-C)3 in water were initially lower than those in acetonitrile (Fig. 5avs. c). The lower initial reaction rate might be probably due to the fact that the compartmentalized structure of SCPNs somewhat hindered the permeation of substrates into the catalytic, hydrophobic pocket. For this reason, it was not surprising that the water-insoluble neat-C gave the lowest kobs in water due to poor accessibility to the substrate (Fig. 5d). The observations provided further evidence for the confined catalysis of asymmetric epoxidation in the catalytic compartment. Moreover, despite the confined catalysis, the kobs over PN84C3 was lower than that over PN84(IL-C)3 in the aqueous epoxidation (Fig. 5bvs. a). The observations did agree with the beneficial effect of the imidazolium-based IL on the overall reaction mechanism.


image file: c9cy01398a-f5.tif
Fig. 5 Fitted kinetic curves (A) and rate curves (B) of asymmetric epoxidation of β-methylstyrene over PN84(IL-C)3 (a), PN84C3 (b) and neat-C (d) in water and over PN84(IL-C)3 in acetonitrile (c).

Benefiting from the IL-modified compartmental structure, PN84(IL-C)3 also exhibited the highest catalytic efficiency for a wide range of unfunctionalized alkenes in water, such as trans-stilbene (Table 2, entry 9 vs. entries 10–12), styrene (Table 2, entry 14 vs. entries 15–17), indene (Table 2, entry 19 vs. entries 20–22) and 1,2-dihydronaphthalene (Table 2, entry 24 vs. entries 25–27). Almost quantitative yields of the corresponding chiral epoxides (92–95%) were obtained over an extremely low amount of PN84(IL-C)3 (0.5 mol%) within 60 min in water (Table 2, entries 9, 14, 19, and 24). In particular, in the case of indene, the yield of chiral indene epoxide over PN84(IL-C)3 increased even up to 24 times as compared with that of neat-C (Table 2, entry 19 vs. 22). Apart from ensuring high efficiency, the hydrophobic, catalytic pocket also made asymmetric epoxidation highly enantioselective, as it limited the further hydrolysis and epimerization of corresponding chiral epoxides.27 Unprecedented chiral induction (92–99%) was indeed obtained for all the alkenes over the PN84(IL-C)3-based SCPN system. The ee values presented the best results so far in the metallosalen system.10,49–51 Moreover, we noticed that the PN84(IL-C)3 in aqueous systems was much more active and selective than in organic systems due to the high local concentration of substrates in the catalytic pocket (Table 2, entry 3 vs. 8, entry 9 vs. 13, entry 14 vs. 18, entry 19 vs. 23, entry 24 vs. 28).

After the reactions, the water-soluble nanoreactors turned hydrophobic upon heating to above their corresponding LCST of PNx(IL-C)y. As a result, they could be facilely recovered from the aqueous system for reuse by thermo-controlled separation. The recovered PNx(IL-C)y could be redissolved in water after cooling the aqueous systems to room temperature. Fig. 6 shows the reusability of PNx(IL-C)y in asymmetric epoxidation of β-methyl styrene in water. To our delight, the catalysts could be reused at least seven times without significant loss in activity and selectivity. This demonstrated the excellent stability and reusability of PNx(IL-C)y in the aqueous asymmetric epoxidation. Evidence of the stability of PNx(IL-C)y was provided by ICP-MS measurement of chromium content in fresh PNx(IL-C)y and that reused seven times. The chromium contents in the recovered catalysts (0.622, 0.386, 0.252, and 0.175 mmol g−1 for PN60(IL-C)8, PN80(IL-C)5, PN84(IL-C)3, and PN90(IL-C)2, respectively) were almost identical to that of the corresponding fresh one (Table 1). Furthermore, less than 1.0 ppb chromium species (determined by ICP) was detected in the reaction medium, confirming the negligible leaching loss of chromium species during oxidation. The excellent stability of catalysts should arise from the shielding of the chiral salen Cr(III) complex in a hydrophobic compartment, which prevented or at least limited the leaching of chromium species to aqueous solution.14,15 Moreover, the imidazolium-based IL linker should also have a positive effect on stabilizing the O[double bond, length as m-dash]CrV(salen) intermediate, which prevented the catalyst degradation.44,45 The reuse of such Cr(salen) catalysts not only eliminated contamination of the chiral epoxides by trace amounts of heavy metals but also reduced processing and waste disposal costs in large-scale production.


image file: c9cy01398a-f6.tif
Fig. 6 Reuse of PN60(IL-C)8 (A), PN80(IL-C)5 (B), PN84(IL-C)3 (C) and PN90(IL-C)2 (D) in asymmetric epoxidation of β-methylstyrene in water.

Conclusions

Thermo-responsive catalytic nanoreactors for asymmetric epoxidation of alkenes in water have been constructed by folding an amphiphilic random copolymer around catalytic Cr(salen) species. The catalytic compartments were composed of the hydrophobic IL/Cr(salen) core and the thermo-responsive PNIPAAm shell. The hydrophobic IL/Cr(salen) core provided a catalytic, hydrophobic pocket in water to concentrate reagents for efficient catalysis and exclude water to minimize the undesired hydrolysis of epoxides with Cr(salen). In addition, the thermo-responsive PNIPAAm shell endowed the catalytic nanoreactor with reversible thermal driven water-solubility switching. As a result, the catalytic nanoreactor could be easy dispersed in water similar to a quasi-homogeneous catalyst under reaction conditions and be facilely recovered for efficient reuse by thermo-controlled separation. The fascinating features made the thermo-sensitive catalytic nanoreactors highly promising for various asymmetric organic reactions in water.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21676078, 21476069), the Natural Science Foundation of Hunan Province for Distinguished Young Scholar (2016JJ1013) and the Key Laboratory of the Assembly and Application of Organic Functional Molecules of Hunan Province.

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Footnote

Electronic supplementary information (ESI) available: Detailed NMR spectra and GC analysis for the chiral epoxides. See DOI: 10.1039/c9cy01398a

This journal is © The Royal Society of Chemistry 2019