Novel amphiphilic poly(2-oxazoline)s bearing l -prolinamide moieties as the pendants: Synthesis, micellization and catalytic activity in aqueous aldol reaction

Polymer 102 (2016) 33e42

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Novel amphiphilic poly(2-oxazoline)s bearing L-prolinamide moieties as the pendants: Synthesis, micellization and catalytic activity in aqueous aldol reaction Fangyu Hu, Ganhong Du, Long Ye, Yuting Zhu, Yao Wang, Liming Jiang* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 June 2016 Received in revised form 15 August 2016 Accepted 25 August 2016 Available online 28 August 2016

Novel amphiphilic poly(2-oxazoline)s bearing L-prolinamide moieties at the alkyl side-chain termini were designed and synthesized by a bottom-up protocol. These polymers formed micelle-type assemblies with a size of 10e30 nm in water, in which the majority of prolinamide active groups was tucked into the hydrophobic inner core. As a nanoreactor, the micellar systems were successfully applied to the direct asymmetric aldol reaction in aqueous media. The results demonstrated that the polymer-bound prolinamide catalysts show a significant improvement in catalytic efficiency when compared with their monomeric counterpart and non-amphiphilic reference polymer, affording the anti-product with a moderate stereoselectivity in the representative aldol addition of cyclohexanone to p-nitrobenzaldehyde. Additionally, the poly(2-oxazoline) derivatives can also promote effectively the reactions of tetrahydro4H-pyran-4-one or cyclopentanone donors to give the corresponding aldol adducts in high yield and with good diastereo- and enantioselectivities. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Asymmetric organocatalysis Poly(2-oxazoline)s Self-assembly

1. Introduction As interest in asymmetric organocatalysis is on the increase, the immobilization of organic catalysts has attracted increasing attention [1]. For the polymer-bound organocatalysts, the polymer matrix seems to integrate itself as a more natural part of the overall catalytic system, influencing the catalyst performance [2]. In this connection, it would be highly desirable to employ amphiphilic polymers as scaffolds, as a result of their unique solution properties to associate in both organic and aqueous media to form a range of nanostructures such as spherical micelles. By covalently attaching the catalytic units into the core-forming block of the copolymer, the self-assembled architectures could promote the catalyst activity by creating a favorable microenvironment around the active sites as is the case for enzymatic catalysis [3]. Recently, O'Reily and co-workers reported the successful incorporation of L-proline into the hydrophobic segment of amphiphilic block copolymers via reversible additionefragmentation chain transfer polymerization giving efficient

* Corresponding author. E-mail address: [email protected] (L. Jiang). http://dx.doi.org/10.1016/j.polymer.2016.08.089 0032-3861/© 2016 Elsevier Ltd. All rights reserved.

micellar catalytic systems for aldol reactions in water [3a]. The nanoreactors showed excellent catalytic activity, which was proposed to be associated with the ability of the nanostructures to effectively concentrate the reagents in the catalytically active micelle core. Thereafter, they developed thermoresponsive polymer-supported proline micelle catalysts for the asymmetric transformation in water with excellent enantioselectivity and a high recyclability [3b]. Once the catalysis was finished, the polymeric catalyst can easily be separated from the solid product by lowering the temperature to below the lower critical solution temperature and then reused for the next round of catalytic reaction. It is well known that amphiphilic block copolymers are often the choice for a wide variety of supramolecular assemblies. However, in the past decade quite a few research was dedicated to the selfassembly behavior of amphiphilic homopolymers with the desire to explore a simplified access to supramolecular structures [4e8]. For example, the group of Thayumanavan proposed a molecular design based on homopolymers in which both the hydrophilic and the hydrophobic moieties are incorporated within each repeating unit [4]. They demonstrated that the as-synthesized polystyrene derivatives are capable of providing both micelle-like and inverse

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micelle-like assemblies depending on the solvent environment. The polymer micelles can act as nanocontainers to carry out organic photoreactions and the selectivity in the reactions are much higher than those obtained with micellar systems resulted from block copolymers or small molecule surfactants [5]. Besides, other types of amphiphilic homopolymers, such as polyacrylamide [9], polyacrylate [10], polynorbornene [11], polyoxanorbornene [12], and more recently, styrene-based cyclic polymer [13] have been shown to self-assemble into various morphologies including micelles and vesicles in selective solvents. It is conceivable that this selfassembly strategy based on homopolymers would provide opportunities not only for structural versatility but also for functional modulation of nanomaterials. In the present work, we synthesized a novel class of amphiphilic homo- and copolymers of 2-oxazoline, in which the side-chain chiral L-prolinamide moiety was connected to the polymer backbone through a five-atom linker (Scheme 1). The prolinamido groups as catalytic sites are envisioned to activate aldol donor via the enamine formation in the aldolisation [14]. Moreover, these polar functionalities would afford both intra- and inter-polymer hydrogen bonds and act as the hydrophilic segment, while the longer alkyl spacer is hydrophobic one. Choosing poly(2-oxazoline) (POX) as the scaffold for anchoring a functional entity is due to its plenty of chemistry that would offer a great deal of flexibility with respect to the molecular design and polymer synthesis [15]. In particular, POXs can be regarded as analogues of polypeptides and also of polypeptoids [15a,16]. Therefore, these amphiphilic polymers based on such a peculiar pseudopeptide skeleton are expected to form a self-assembled architecture in aqueous solution resembling more closely enzymatic system. In order to gain some insight into the specific role of the relative content of the hydrophobic to hydrophilic segments in the self-assembly and compare the possible difference in catalytic properties, a reference polymer PMeOXNHPro (Scheme 1, R1) with only a methylene spacer and the low-molecular-weight counterpart (Scheme 1, M1) were also prepared. To the best of our knowledge, the construction of a biomimetic catalytic system using poly(2-oxazoline)s as scaffolds has remained unexplored, although their potential in biomedical applications is well documented in the literature [15bec].

atmosphere. Scandium triflate [Sc(OTf)3] was prepared as the reported method [17] and used after drying in vacuum at 200  C for 48 h. Other reagents were used as received. 2.2. Instrumentation 1

H NMR spectra were recorded on a Bruker Avance DMX-400 spectrometer, using CDCl3 or D2O as solvents. Chemical shift values are reported in parts per million (d) relative to tetramethylsilane (TMS). Circular dichroism (CD) spectra were recorded on a Biologic MOS-450 CD spectropolarimeter (France) at ambient temperature. D-line specific optical rotations (½a20 D ) were measured in methanol at 20  C using a Perkin-Elmer 341 LC polarimeter. Gel permeation chromatography (GPC) analyses were carried out on a Waters150C apparatus equipped with two PLgel 5 mm MIXED-C 300  7.5 mm columns and a differential refractometer detector using tetrahydrofuran (THF) as the eluent (flow rate 1 mL min1, 40  C). The number-average molecular weight (Mn) and polydispersity index (PDI) of the polymers were calculated on the basis of a polystyrene calibration. Thin-layer chromatography (TLC) was performed on aluminum sheets 60 F254 Merck silica gel, and the compounds were visualized by irradiation with UV light. Highperformance liquid chromatography (HPLC) analyses were performed on a Chromeleon® apparatus equipped with chiral columns (Daicel Chiralpak AD-H, 4.6 mm  250 mm) employing n-hexane/iPrOH (90:10, v/v) as an eluent (flow rate: 0.8 mL min1) and UV detection (l ¼ 254 nm). Dynamic light scattering (DLS) was performed on a Zetasizer Nano Series (Malvern Instruments, UK) at a wavelength of 657 nm and a detection angle of 173 . The polymer solution was used for particle size measurements without filtering. The results of the size measurements showed as a volume-based particle size distribution. Transmission electron microscopy (TEM) observations were conducted on a Hitachi HT7700 electron microscope at an acceleration voltage of 120 kV. A drop of the sample solution was placed on a copper grid coated with Formvar. After drying, the samples were stained with phosphotungstic acid. Steady-state fluorescence spectra were recorded on a Spectra MaxM2 ELIASA (Molecular Devices Company, America) in the rightangle geometry (90 collecting optics). The excitation spectra were monitored at 372 nm.

2. Experimental section 2.1. Materials

2.3. Synthesis of 2-[5-(N-Boc-pentyl)]-2-oxazoline (PenOXNHBoc, OX1, Scheme 2)

Aminocaproic acid, chloroethylamine hydrochloride, N-(tertbutoxycarbonyl)-L-proline (Boc-L-Pro-OH), tert-butoxycarbonyl anhydride, and O-(1H-benzotriazol-1-yl)-N,N,N0 ,N'-tetramethyluronium tetrafluoroborate (TBTU) were purchased from Acros. Dichloromethane (DCM), trimethylamine, acetonitrile, and piperidine were refluxed over CaH2 and stored under dry nitrogen

100 mL NaOH aqueous solution (5.5 M) and aminocaproic acid (32.8 g, 0.25 mol) were placed in a 250 mL flask, followed by tertbutoxycarbonyl anhydride (65.5 g, 0.30 mol), and the resulting mixture was stirred overnight at ambient temperature. The reaction mixture was acidified with 1 N HCl to pH ~4 and then extracted with ethyl acetate (40 mL  3). The collected organic phase was

Scheme 1. Structures of POX-bound L-prolinamide catalysts, the reference polymer PMeOXNHPro (R1), and their monomeric counterpart (M1) in this study.

F. Hu et al. / Polymer 102 (2016) 33e42

washed with brine (30 mL  2), dried over anhydrous MgSO4, filtered, and concentrated under vacuum to give 1a in 88% yield (51.0 g). Compound 1a (46.2 g, 0.20 mol), chloroethylamine hydrochloride (25.5 g, 0.22 mol), and TBTU (70.6 g, 0.22 mol) were dissolved in dry DCM (250 mL) sequentially. To the solution triethylamine (55.6 mL, 0.4 mol) was added dropwise over a period of 1 h at 0  C. The reaction mixture was allowed to warm up to room temperature and was stirred overnight before 100 mL of saturated aqueous NaHCO3 was added. The organic phase was washed twice with water and dried over anhydrous MgSO4. After removal of the solvent, the residue was distilled under reduced pressure to afford 1b as a pale-yellow oil (48.7 g, yield 83.2%). The ring closure of 1b (27.8 g, 0.10 mol) was carried out in a saturated solution of NaOH in methanol. After stirring for 12 h at room temperature, the solvent was removed under reduced pressure. The residue was dissolved in DCM (50 mL), washed twice with water and dried over anhydrous MgSO4. The desired product PenOXNHBoc was obtained as a colorless viscous liquid by reduced pressure distillation (18.2 g, yield 75.2%). 1H NMR (400 MHz, CDCl3): d ¼ 1.33e1.50 (m, C(CH3)3 and NCH2CH2(CH2)2CH2, 13H), 1.65 (m, NCH2CH2(CH2)2CH2), 2.27 (t, NCH2CH2(CH2)2CH2, 2H), 3.12 (t, NCH2CH2(CH2)2CH2, 2H), 3.82 (t, NCH2CH2O, 2H), 4.22 (t, NCH2CH2O, 2H), 4.71 ppm (s, BocNH, 1H); 13 C NMR (100 MHz; CD Cl3): d ¼ 168.4, 156.0, 79.0, 67.2, 54.3, 40.3, 29.6, 28.4, 27.8, 26.3, and 25.4 ppm; MS (ESIþ): m/z (%) ¼ 254.2 (23.2) [MþH]þ, 279.2 (32.6) [MþNa]þ, 535.3 (100) [2MþNa]þ.

2.4. Synthesis of 2-Pentyl-2-oxazoline (PenOX, OX2) The monomer was prepared as a colorless liquid by a similar procedure to that of OX1 except for the use of caproid acid as a starting material. Yield: 71%. 1H NMR (400 MHz, CDCl3): d ¼ 0.90 (t, CH3CH2, 3H), 1.32 (m, CH3CH2CH2, 4H), 1.63 (penta, CH3(CH2)2CH2, 2H), 2.26 (t, CH3(CH2)3CH2, 2H), 3.82 (t, NCH2CH2O, 2H), 4.22 ppm (t, NCH2CH2O, 2H). 13C NMR (100 MHz, CDCl3): d ¼ 13.9, 22.5, 25.7, 28.0, 31.4, 54.4, 67.2 and 168.8 ppm; MS (ESIþ): m/z (%) ¼ 142.2

35

(100) [MþH] þ.

2.5. Synthesis of the polymeric catalysts The synthesis began with acid hydrolysis of protecting groups from the parent polymers followed by amide coupling between the intermediate product with free amino groups and N-Boc-L-proline then deprotection of the proline ring. Taking the preparation of PPenOXNHPro (Scheme 2, P1c) as an example, a typical procedure is as follows. To a solution of PPenOXNHBoc (Scheme 2, P1a, 1.0 g) in DCM (10 mL) trifluoroacetic acid (10 mL) was added, and the mixture was stirred for 24 h at room temperature. After removing the solvent in vacuum, the residue was dissolved in methanol. Ether was added to the mixture, and the precipitate was filtered and dried under vacuum overnight (1.04 g, yield 99%). The intermediate (1 g) was dissolved in DMSO (~10 mL) and to the resulting solution was added Boc-L-Pro-OH (0.88 g, 4 mmol), TBTU (1.3 g, 4 mmol), and Et3N (0.97 mL, 6.67 mmol) in turn. The reaction mixture was stirred for 24 h at room temperature before water (~100 mL) was added. The mixture was extracted with CH2Cl2 (3  20 mL), and the collected organic phase was dried (MgSO4), filtered, and concentrated. The residue was dissolved in methanol (2 mL) and poured into an excess amount of ether to precipitate out P1b (1.15 g, yield 84.5%). Deprotection of P1b was conducted in a similar fashion to P1a as described above. Once the process was finished, the crude product was obtained by evaporating solvent. The solid was suspended in distilled water, and saturated aqueous NaHCO3 was added dropwise until pH ~8. The solution was dialyzed in distilled water for two days, followed by freezing and lyophilization to afford the desired polymer catalyst P1c (0.76 g, yield 92.0%). In a similar way to that reported for the synthesis of P1c, the copolymer catalysts and the reference polymer R1 were prepared; the parent polymers P2aP4a and P5a (shown in Table 1) were obtained via a statistical and sequential polymerization route, respectively. The relevant spectral data are provided in the

Scheme 2. Synthesis of the 2-oxazoline monomers and the model compound M1.

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Table 1 Results on cationic ring-opening polymerization of 2-oxazolines initiated with Sc(OTf)3 and post-modification reactions of the corresponding polymers.a Parent polymers

POXNHProBoc

Polymeric catalysts

Code

OX1/OX2

OX1b (%)

Mnc (103)

PDIc

Code

Mnc (103)

PDIc

Code

Dgd (%)

Fe

f ½a20 D

P1a P2a P3a P4a P5ag

100:0 75:25 50:50 25:75 50:50

100 73 51 27 56

11.74 5.36 6.26 4.50 6.29

1.06 1.15 1.12 1.19 1.08

P1b P2b P3b P4b P5b

9.62 3.87 5.99 5.02 9.03

1.08 1.21 1.17 1.19 1.10

P1c P2c P3c P4c P5c

96 95 99 95 99

0.96 0.39 0.50 0.26 0.55

44.5 34.1 22.1 15.0 22.5

a b c d e f g

Polymerization conditions: [I]:[M]total ¼ 1:100 (molar ratio), [M] ¼ 2 mol L1, 90  C, 3 h, in acetonitrile. The content of OX1 in polymers, determined by 1H NMR spectroscopy. Determined by GPC based on calibration with PS standards in THF. The grafting degree of NHPro was measured by 1H NMR, see the Supporting Information. The hydrophilic to hydrophobic ratio is arbitrarily defined as the product of OX1 content in the polymers and the grafting degree of NHPro (Dg). c ¼ 10 mg mL1, MeOH. Block copolymer.

Scheme 3. Synthetic route of L-prolinamide-functionalized poly(2-oxazoline)s.

Supporting Information (Fig. S16eS24).

2.6. Synthesis of N-(5-diethylamido)pentyl-L-prolinamide (M1, Scheme 2) 1a (7.63 g, 33 mmol) and diethylamine (10.3 mL, 10 mmol) were dissolved in DCM (250 mL), followed by slowly addition of TBTU (11.7 g, 37 mmol). The mixture was stirred overnight and then washed with water (50 mL  3). The organic phase was dried over anhydrous MgSO4, filtered, and concentrated under reduced pressure to afford 4 (8.05 g, yield 85%). Then the intermediate was subjected to the chemical modifications in a fashion as above applied for synthesizing polymer catalysts. M1 was obtained as a viscous orange liquid after removal of volatiles in vacuum (7.0 g, 1 1  total yield 86.0%, Scheme 2). ½a20 D ¼ 34 (10 mg mL , MeOH); H NMR (400 MHz; CDCl3): d ¼ 1.08e1.18 (t, CH2CH3, 6H), 1.36 (m, OCCH2CH2CH2CH2CH2, 2H), 1.54 (m, OCCH2CH2CH2CH2CH2, 2H), 1.65 (penta, OCCH2CH2CH2CH2CH2, 2H), 1.77 (m, NCH2CH2CH2CH, 2H), 1.96 and 2.18 (m, NCH2CH2CH2CH, 2H), 2.29 (t,

OCCH2CH2CH2CH2CH2, 2H), 3.00 and 3.08 (m, OCCH2CH2CH2CH2CH2, 2H), 3.27e3.37 (m, CH2CH3 NCH2CH2CH2CH and CH2NHCH2, 7H), 3.92 (tetra, OCCHNH, 1H), 7.8 ppm (s, CH2NHCO, 1H); 13C NMR (100 MHz; CDCl3): d ¼ 173.7, 172.0, 60.4, 47.1, 41.9, 40.1, 38.8, 32.9, 29.3, 26.7, 25.9, 24.9, 14.3 and 13.1 ppm. MS (ESIþ): m/z (%) ¼ 284.3 (56.9) [MþH]þ, 567.4 (100) [2MþH]þ.

2.7. General polymerization procedure All polymerizations were carried out in a Schlenk tube under a nitrogen atmosphere using freshly distilled and dried solvents. Taking the synthesis of PPenOXNHBoc (P1a) as an example, a typical procedure is as follows. PenOXNHBoc (2.56 g, 10 mmol) was added to a solution of Sc(OTf)3 (49.2 mg, 0.1 mmol) in acetonitrile (2.67 mL). The mixture was heated up to 90  C and stirred for 3 h. The polymerization was terminated with piperidine (0.2 mL, 12 mmol) at room temperature for 4 h. The resultant mixture was poured into diethyl ether to precipitate out the crude product. To completely remove the catalyst residue and unreacted monomer, the collected

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(3.6 mL) was added slowly via a syringe under stirring. After addition of all the water, the solution was dialyzed against pure water using a dialysis membrane with a 1 kDa molecular cut-off. Aqueous micelle solutions with concentrations of ca. 1.0 mg mL1 were obtained. Before each DLS measurement, the polymer solution was usually allowed to stand at ambient temperature overnight with the view of insuring complete equilibrium of the micellar solution. 3. Results and discussion 3.1. Polymer synthesis and characterization

Fig. 1. 1H NMR spectra (CDCl3, 400 MHz, T ¼ 20  C) of (a) the parent polymer PPenOXNHBoc (P1a), (b) before (P1b) and (c) after (P1c) deprotection of the L-prolinamide functionalities.

powder matter was redissolved in CH3CN and then precipitated again from Et2O. This process was repeated three times to give the desired parent polymer P1a. Yield: 2.0 g (80%). 2.8. General procedure for aldol reaction The catalyst loading was kept fixed at 10 mol% (based on the active unit) relative to p-nitrobenzaldehyde after a preliminary survey. To an aqueous solution of polymer (0.5e1 mL), the aldehyde (0.27 mmol) and an appropriate amount of cyclohexanone were added. The resultant mixture was stirred at room temperature for a certain time. After the specified time elapsed, a saturated aqueous NH4Cl (~2 mL) was added, and the mixture was extracted with dichloromethane (5 mL  3). The collected organic layer was dried over anhydrous MgSO4, filtered, and concentrated under reduced pressure. The residue was purified by flash column chromatography on silica (n-hexane/EtOAc ¼ 4:1) to obtain the aldol product as mixtures of anti and syn diastereomers. The diastereomeric ratio was determined by 1H NMR spectroscopy on the crude samples. The enantiomeric excess was determined by HPLC on a chiral stationary phase. 2.9. Micellization of amphiphilic polymers The polymer sample (4 mg) was dissolved in methanol (0.4 mL) at room temperature and then to the solution deionized water

As illustrated in Scheme 3, the employed protocol involves synthesis of parent polymers (P1aP5a) with a Boc-protected amine appendage by cationic ring-opening polymerization of the corresponding 2-oxazoline monomers followed by subsequent deprotection and modification of the released amino groups with Lproline. The monomers derived from Boc-aminocaproic acid and caproic acid, PenOXNHBoc (OX1) and PenOX (OX2), were obtained readily in high yield according to a general adapted method [18]. The structure of monomers was confirmed by NMR and mass spectral analyses (see the Supporting Information). On the basis of our previous work, the polymerization was allowed to perform in CH3CN using Sc(OTf)3 as initiator [19]. The data summarized in Table 1 show that the polymerization produced the polymers with molar masses in the range 4500 g mol1 < Mn < 11 700 g mol1 and low polydispersity indices (PDI ¼ 1.06e1.19) in yield more than 80%. In the case of copolymerization, increasing the OX2/OX1 ratio of monomer feed composition resulted in a decrease in Mn's of the polymers, and the statistical copolymers (P2aP4a) showing slightly high PDIs compared to the homopolymer P1a and the block copolymer P5a. The copolymer compositions, which were determined by 1H NMR spectroscopy, are in good agreement with the feed ratio. As evidenced by the linear first-order kinetics together with the unimodel molar mass distribution for the resulted polymers with PDI values below 1.20, a good control of polymerization was achieved with this system (see: Fig.S4eS7 in the Supporting Information). The post-modification process of the parent polymers can be monitored by 1H NMR spectroscopy. By way of example, Fig. 1aec shows 1H NMR spectra of P1a and its prolinamide derivative (P1b) and the target product PPenOXNHPro (referred to P1c). In the spectrum of P1b, the methine protons assignable to the pyrrolidine unit were observed at 4.20 ppm (b), and the signal of amide proton labeled “a” has now shifted from d 5.3 to 7.0 ppm due to L-prolyl replacing Boc of P1a (Fig. 1a vs b). From the spectrum of P1c (Fig. 1c), we can see that the characteristic signals of the Boc protons (~1.48 ppm) vanished completely; the peak “b” showed a slight highfield shift (to ca. 3.75 ppm) and the distinctive signals (c) of the secondary amine around 2.05 ppm concomitantly appeared. Also, the peak “a” appeared at a downfield (7.9 ppm) as a result of the intramolecular hydrogen bonding interactions. These observations indicated that the hydrolytic detachment of Boc groups was finished almost quantitatively. Judged from the relative integral intensity of “b” and “d” peaks, the degree of L-proline grafting was estimated to be 96%. In all cases, the prolinamido group was efficiently introduced into the POX scaffolds, and thus the functionalization level could be tuned by simply changing the relative amounts of OX1 and OX2 used in the copolymerization (Table 1). The presence of a plenty of polar functionalities in the side chains enables these polymer catalysts to be easily dissolved in the most polar solvents including water. As shown in Table 1, the specific rotation values (½a20 D ) of the polymers were found to be proportional to the content of incorporated NHPro units but almost

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Fig. 2. DLS measurements of (a) P1c with an inset showing the number distribution and (b) P5c at a concentration of 1 mg mL1. TEM images of nanoparticles formed by (c) P1c and (d) P5c, dried from an aqueous solution of 1 mg mL1. The samples were stained with phosphotungstic acid. Scale bar ¼ 100 nm.

independent on their molecular weights, implicating nonexistence of a secondary or higher-order structure of polymer chains in solution. This speculation was also supported by CD spectroscopic measurements, in which the polymer revealed similar Cotton effects as those of the small molecular counterpart except for the peak maxima being slightly shifted to lower wavelengths (Fig. S25). Moreover, it is worth pointing out that the high hydrophilicity led to difficulties in GPC analyses for the polymers due to the strong interaction with column fillers. Accordingly, the molecular weights of polymeric catalysts were indirectly characterized in the Bocprotected form, i.e., POXNHProBoc (see: Table 1 and the Supporting information). 3.2. Self-association behavior of polymer catalysts in water With the exception of P4c, these L-prolinamido-functionalized poly(2-oxazoline)s were homogenously soluble in water (~10 wt.%) and resultant aqueous solution exhibited remarkable 00 Tyndall effect00 . Thus, their self-assembly behavior was investigated by dynamic light scattering (DLS) (see: Table S1) and transmission electron microscopy (TEM). As shown in Fig. 2a, a bimodal distribution was found with the total PDI valve of 0.53 for the aqueous solution of P1c, indicating the presence of scattering centers presenting two different sizes. The observed peak intensity for particles with a diameter of 188 nm was stronger than that of the peak for particles with the diameter of ~11 nm, presumably due to the higher scattering of larger particles compared to smaller ones [20]. Nevertheless, the size distribution by number showed that the

larger particles are numerically much less relevant (inset in Fig. 2a and Fig. S28). Similar DLS profiles were also observed for the random copolymers P2c and P3c (Fig. S29). An interesting output of the DLS measurements is the self-assemblies formed from the block copolymer P5c showed a smaller size (~14 nm), and it was characterized by a monomodal distribution with a low PDI of 0.20 (Fig. 2b). TEM observations demonstrated that the polymer particles present a spherical morphology (Fig. 2c, d), and the particle sizes (10e30 nm) were consistent with the results of the DLS measurements. Next, a fluorescence probe experiment using pyrene was performed to identify whether the water-soluble aggregates have hydrophobic domains as often observed with micellar assembles [21,22]. Taking P1c and P5c as an example, Fig. 3 plots the changes in the fluorescence intensity ratio of pyrene at 333 and 339 nm (I333/I339) in the presence of different polymer concentration in water. It can be seen that at lower polymer concentrations the I333/I339 ratio varied slightly as the concentration was increased and then dropped sharply upon increasing the concentration to a critical point. This indicates that the polymers did indeed form hydrophobic domains in aqueous solution and that pyrene was solubilized into the interior. From the steady-state fluorescence excitation spectra for pyrene probe, the apparent critical aggregation concentration (CAC) [23] of P1c was estimated to be 0.08 mg mL1, while the value in P5c is 27 times smaller (0.003 mg mL1). Similarly, the comparison of random copolymers P2c and P3c also shows a significant difference in the CAC value: the former has

F. Hu et al. / Polymer 102 (2016) 33e42

39

Fig. 3. Intensity ratio I333/I339 obtained from the fluorescence excitation spectra of pyrene plotted versus the concentrations of the polymers (a) P1c and (b) P5c (1  106e1.0 mg mL1).

a CAC of 0.04 mg mL1 and the latter 0.009 mg mL1 (see Fig. S30). Clearly, the variation of CAC values is well in accordance with the hydrophilic to hydrophobic ratios of polymers (see F values in Table 1). That is, the lower value of F the more hydrophobic the polymer and the smaller CAC was observed. However, the block copolymer P5c is an exception, which showed the lowest CAC although it has a similar hydrophilic/hydrophobic ratio as P3c. The fact indicates that the blocky distribution of hydrophobic segments in a polymer chain exhibits a more strong association tendency in aqueous solution. To obtain more information about the structure of the polymeric nanoparticles, 1H NMR spectroscopic studies were conducted using CDCl3 and D2O. As anticipated, the characteristic NMR signals of P5c were detected in CDCl3 (Fig. 4a). However, these peaks almost completely disappeared in D2O (Fig. 4b), suggesting that all of the

protons is in a restricted state as in solid. More interestingly, the same phenomena were also observed in the NMR measurements for both the homopolymer P1c (Fig. 4ced) and the random copolymers (Fig. S31). These results demonstrated that the polymeric aggregates seem to be not a typical coreshell structure, which is similar to that observed by Akashi et al. [24] in the unimer micellar systems formed by hydrophobically modified poly(g-glutamic acid). It is worth commenting that neither the reference polymer PMeOXNHPro (R1) nor the monomeric counterpart (M1) assembles into any detectable nanoscale particles in the aqueous solution, as confirmed by DLS and TEM. Based on the aforementioned observations, we speculate that besides the hydrophobic driving forces from the main-chain and alkyl linkers, the intra- and intermolecular hydrogen bonding between the tertiary polyamide backbone

Fig. 4. 1H NMR spectra of P5c and P1c in CDCl3 (a, c) and D2O (b, d), respectively. Sample concentration ¼ ~5 mg mL1.

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(hydrogen accepting) and the prolinamide moiety at the side-chain termini (hydrogen donating) probably may play a crucial role in the self-assembly of the amphiphilic poly(2-oxazoline)s. By the multiple hydrogen-bonding interactions, the majority of hydrophilic NHPro units (catalytic active moieties) should be tucked into the micellar interior composed of the alkyl linker and the main chain. 3.3. Evaluation of the catalytic activity As a proof of concept, the potential of these polymeric micelles to be catalytic nanoreactors for L-prolinamide catalyzed asymmetric reaction was investigated. The aldol reaction of cyclohexanone (CH) with p-nitrobenzaldehyde (PNBA) was chosen the benchmark reaction, as it has been extensively studied using other supported systems [1e,25]. Table 2 reveals that in water without any additive the polymercatalyzed aldolisation afforded the aldol products with good diastereoselectivity (anti/syn ¼ 7183%) but with moderate to poor enantioselectivity (16e58% ee) for the anti product. On closer inspection, the outcomes of this reaction were found to be strongly dependent on both the structural variance of the polymer catalysts and the reaction conditions. For example, at a [CH]/[PNBA] ratio of 7, 14, or 28 (entries 1e16), the homopolymer P1c always gave a higher enantiomeric excess (ee) and a slightly lower yield compared to the copolymers. Meanwhile, the random copolymer

P2c exhibited somewhat better enantioselectivity than its analogue P3c (4e8% ee higher), while the lowest ee value was observed for P5c under the identical conditions. On the other hand, increasing the catalyst concentration via reducing the water content in the reaction media led to a higher conversion and identical or better ee values than those achieved at lower catalyst concentrations (entries 9e12 vs. 5e8). Note also that as the reactant ratio varies from 7 to 28 the enantioselectivity increased and a best ee value of up to 58% was achieved with P1c, at the cost of a drop in the yield (entries 1, 5, and 13). One possible reason for the decreased yield (entry 13) is that the overloaded ketone would significantly dilute the aldehyde substrate within the nanoreactor, in this way enabling the transformation to slow down. The above results might be explained in terms of the hydrophobic/hydrophilic balance in the nanoreactor, which itself is closely related to the chemical composition and microstructure of the micelle-forming polymers. Intuitively, we can infer that the micellar interior formed by the block copolymer P5c is more hydrophobic than that of homopolymer P1c; therefore, the former would concentrate more effectively the hydrophobic reagents and thereby enhancing the reaction rate. In contrast, P1c has a high hydrophobic/hydrophilic ratio (F ¼ 0.96), and the resultant nanoparticles should have a relatively dense catalytic site, which providing a more favorable asymmetric induction environment for the organic conversion to take place enantioselectively. Based on

Table 2 Aldol addition of cyclohexanone to p-nitrobenzaldehyde catalyzed by various catalysts in water.a

Entry

Cat.

Water (mL)

TFAb (equiv)

[CH]/[PNBA]c

Time (h)

Yieldd (%)

syn/antie

eef[%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

P1c P2c P3c P5c P1c P2c P3c P5c P1c P2c P3c P5c P1c P2c P3c P5c R1g R1 R1 M1 M1 M1 P1c P1c P1c P1c P1c

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 1.0 1.0 0.5 1.0 1.0 0.5 0.5 0.5 0.5 0.5 0.5

e e e e e e e e e e e e e e e e e e e e e e 0.25 0.5 0.75 0.5 0.5

7 7 7 7 14 14 14 14 14 14 14 14 28 28 28 28 7 14 28 7 14 28 28 28 28 14 7

12 12 12 12 24 24 24 24 12 12 12 12 12 12 12 12 12 24 12 12 24 12 24 48 48 48 48

86 94 91 92 85 87 86 91 91 95 94 96 54 78 72 74 <5 19 <5 27 30 54 53 50 <5 42 47

22:78 21:79 21:79 20:80 17:83 20:80 17:83 29:71 27:73 23:77 22:78 25:75 25:75 24:76 22:78 26:74 e 24:76 e 30:70 31:69 31:69 36:64 25:75 e 21:79 26:74

37 31 24 16 49 40 32 18 48 44 42 37 58 34 30 23 e 35 e 45 41 50 61 75 e 52 43

a b c d e f g

Reaction conditions: PNBA (40.7 mg, 0.27 mmol), the catalyst loading was 10 mol% relative to the aldehyde in NHPro units of the polymer, 20  C. Relative to the catalytic group. [CH]/[PNBA] molar ratio. Combined yield of the isolated diastereomers. Determined by 1H NMR spectroscopic analysis of the crude product. Diastereoisomer (anti) determined by HPLC using a chiral column. Mn ¼ 8750, PDI ¼ 1.05. The absolute configurations of the products were deduced by comparing the HPLC retention times with reported values [26] (also see: Fig. S33e34).

F. Hu et al. / Polymer 102 (2016) 33e42

this point of view, a small difference in enantiomeric excess mentioned above between the random copolymers is also rationalized to be due to the difference in NHPro content in the two micelles (see: F factor in Table 1). Additionally, the enantioselectivity of P3c was found to be slightly higher than that of P5c (e.g., entries 7e8), although both polymers possess similar F values, indicating the microstructure of polymer chain seems to have a considerable degree of influence on the stereochemistry of the micellar catalysis. Most likely, P3c is more inclined to the intramolecular association in aqueous solution relative to its blocky counterpart, which would enable the catalytic units to be more packed into the micellar interior just like seen in the case of homopolymer P1c. As can be seen from Table 2, either R1 or M1 exhibited poorer outcomes with respect to stereoselectivity, especially to the reaction rate as compared to P1c system (Table 2, entries 17e22). This illustrates the importance of having the normally hydrophilic catalyst tethered to the micelle scaffold for efficient catalysis of hydrophobic substrates in aqueous media. When 0.5 equivalents of TFA as additive were added, a good enantioselectivity (75% ee) could be obtained with a moderate yield (50%; entry 24). We found that the polymer is still able to form micelles in the acidic aqueous solution, but the particle size is slightly larger than the ones in pure water (Fig. S32). By fixing the addition amount of TFA (0.5 equiv) and changing the reactant ratio the reaction resulted in some decrease in the ee value and yield (entries 26 and 27 vs. 24). Finally, we explored the applicability of the micellar catalyst P1c by the choice of cyclic ketones to react with aromatic aldehydes in water without the use of acidic additives (Table 3). The data show that cyclohexanone underwent reaction with benzaldehydes carrying strong electron-withdrawing substituents such as nitro and cyano groups to produce the corresponding b-hydroxy ketone in moderate yields (46e68%) and enantioselectivities (~60% ee) at room temperature (entry 1e3). The poor results were observed in cases where benzaldehyde, p-anisaldehyde or methyl 4formylbenzoate was served as an aldol acceptor, giving low yields despite the longer reaction time (entries 4e6), which similar to the

41

previously reported [27]. Also noted that when tetrahydro-4H-pyran-4-one was used as a pronucleophile to react with p-nitrobenzaldehyde, P1c afforded a high ee value of up to 85% for the anti-aldol product (entry 7). In contrast, the aldol reaction using a cyclopentanone donor led to the preferential formation of syn-adducts in excellent yields (entry 10). Remarkably, the polymer P1c proved to be more efficient than the corresponding non-micellar catalysts (R1, M1) both in terms of yield and of stereoselectivity (Table 3, entries 8, 9, 11 and 12). 4. Conclusions In summary, we report herein a class of amphiphilic poly(2oxazoline) derivatives containing L-prolinamide functionality in the side chains. Upon direct dissolution in water, the polymers selfassembled into micelle-type aggregates with a particle diameter of 10e30 nm, as confirmed by DLS measurements and TEM images. 1H NMR studies demonstrated that the nanoparticles do not have a definite core-shell structure as often observed with block copolymer micelles. It is likely that the interior of the nanoparticles are composed of both the hydrophobic segments and the polar prolinamide moieties. The catalytic efficiency of the assembled nanoparticles was investigated using the aldol addition between cyclohexanone and p-nitrobenzaldehyde as a model reaction in aqueous media. The micelle system formed by homopolymer P1c exhibited best catalytic performance, resulting in much higher activity compared to its monomeric counterpart and the reference polymer. The reaction afforded the anti-aldol products with an 83:17 anti/syn ratio and ~60% ee in good yield, and a higher enantioselectivity (75% ee) was achieved in the presence of TFA (0.5 equiv) as an additive. Moreover, it has been proved that the polymer microstructure have a certain effect on the catalytic properties of the resultant nanoreactors. Although these polymeric micelles present some limitations and more work remains to be done to improve their stereoselectivity as well as to expand the substrate scope in the micellar catalysis, the present study provides a novel route for the biomimetic design of organocatalytic systems to carry

Table 3 Substrate scope of direct asymmetric aldol reactions catalyzed by P1c in water.a

Entry

Cata.

X

R

Time (h)

Yieldb (%)

syn/antic

eed (%)

1 2 3 4 5 6 7 8 9 10 11 12

P1c P1c P1c P1c P1c P1c P1c R1 M1 P1c R1 M1

eCH2CH2e eCH2CH2e eCH2CH2e eCH2CH2e eCH2CH2e eCH2CH2e eCH2Oe eCH2Oe eCH2Oe eCH2e eCH2e eCH2e

2-NO2 3-NO2 4-CN H 4-OCH3 4-CO2CH3 4-NO2 4-NO2 4-NO2 4-NO2 4-NO2 4-NO2

12 12 12 24 24 12 12 12 12 12 12 12

46 68 52 <5 <5 14 97 68 89 95 42 69

16:84 17:83 25:75 21:79 20:80 14:86 20:80e 20:80e 27:73e 76:24e 74:26e 70:30e

60 61 58 e e 46 85 79 75 65f 59f 56f

a b c d e f

The reactions were carried out with 0.27 mmol of aldehyde and 7.56 mmol of ketone in water (0.5 mL), 20  C. Combined yield of the isolated diastereoisomers. Determined by 1H NMR spectroscopic analysis of the crude product. Determined by HPLC using a chiral column. The absolute configurations of the products were deduced by comparing the HPLC retention times with reported values [28] (also see: Fig. S35e36). Determined by chiral HPLC analysis of the syn product.

42

F. Hu et al. / Polymer 102 (2016) 33e42

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