Hollow Mesoporous Organic Polymeric Nano-Bowls and Nano-Spheres

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Kinetics, Catalysis, and Reaction Engineering

Hollow Mesoporous Organic Polymeric Nano-Bowls and Nano-Spheres: Shell Thickness and Mesopore-Dependent Catalytic Performance in Sulfonation, Immobilization of Organocatalyst and Enantioselective Organocascade Guangxin Xie, Shuai Wei, Li Zhang, and Xuebing Ma Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05931 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Industrial & Engineering Chemistry Research

Hollow Mesoporous Organic Polymeric NanoBowls and Nano-Spheres: Shell Thickness and Mesopore-Dependent Catalytic Performance in Sulfonation, Immobilization of Organocatalyst and Enantioselective Organocascade Guangxin Xie, Shuai Wei, Li Zhang and Xuebing Ma* College of Chemistry and Chemical Engineering, Southwest University, Chongqing, 400715, P. R. China Corresponding Author's email: [email protected]

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ABSTRACT. Heterogeneous asymmetric multicomponent/multicatalytic organocascade faces the enormous challenges of tedious immobilization of catalysts, mass transfer and stereoselective control. In this paper, the mesopore-abundant and well-shaped hollow mesoporous organic polymers, nano-bowls (HMOPBs) and nano-spheres (HMOPSs), are fabricated via emulsion polymerization of styrene on PS core and then removal of PS, accompanied by the adsorption of Co2+ ions, transformation of Co2+ into Co(OH)2 and final removal of Co(OH)2. Among them, the nano-bowl HMOPBs(a) with hollow interior, mesoporous shell and thin shell thickness (16 nm) possesses the largest surface area (185.7 m2 g-1) and displays the highest reaction kinetics in the sulfonation (2.85 mmol H+ g-1) and then immobilization of

9-amino(9-deoxy)epi-quinine

(QNNH2, 1.35 mmol g-1). For the 2, 4-substituted bulky reactants in asymmetric double-Michael cascade, the as-fabricated functional nano-bowl can provide a suitable microenvironment to meet the requirement of the good to excellent double-Michael organocascades, which originates from its thin shell thickness and mesopore-dependent shell.

KEYWORDS: heterogeneous organocatalysis, hollow mesoporous organic polymer, nano-bowl, nanosphere, sulfonation, 9-amino(9-deoxy)epi-cinchona alkaloid, mass transfer


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1. INTRODUCTION Homogeneous organocatalysis has now become a thriving catalytic methodology paralleling metal catalysis and widespread applications in synthetic organic chemistry.1–5 However, most of organocatalytic processes require a high loading of catalyst and remain a major drawback with the tedious separation of expensive organocatalyst from the reaction mixture. From the viewpoint of green chemistry, polymers,6–11 inorganic materials,12–16 metal-organic frameworks (MOFs),17–21 covalent-organic frameworks (COFs) 21–25 and ionic liquids26, 27-covalently and noncovalently supported28–31 heterogeneous organocatalysts have been developed to achieve the reusability of expensive organocatalyst and ensure low environmental impacts. Nevertheless, these heterogeneous organocatalysts have at least one of following disadvantages: (i) less effectiveness than homogeneous counterpart, (ii) mass transfer limitation of reactants in catalyst carrier, and (iii) multiple synthetic manipulation to anchor organocatalyst. Briefly, the less effectiveness of heterogeneous organocatalyst than homogeneous counterpart is mainly ascribed to the poorer accessibility of reactants to embedded catalytic site and products out of catalyst carrier. Theoretically, the mass transport processes in a porous slab and a porous sphere, coupled with convection and reaction kinetics, were thoroughly discussed.32–36 For the first order kinetics, the effectiveness factor (η), introduced to take account of the influence of pore diffusion on reaction rate, is given as a function of Thiele modulus for spherical particle in the case of constant concentration at the surface,37–39

𝜂=

3 1 𝜙𝑠(𝑡𝑎𝑛 ℎ𝜙𝑠

ϕs = R

1

- 𝜙𝑠)

𝐾1 𝐷𝑒

(1) (2)


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where ϕs is Thiele modulus for sphere, k1 is first-order kinetic constant (s-1), R is sphere radius (cm), De is effective diffusion coefficient ( cm2 s-1). Based on equation (1) and (2), the more effective diffusion coefficient (De) and shorter sphere radius (R) are favorable for achieving better effectiveness factor (η). Fick's law-based effective diffusion coefficient (De) is influenced by geometrical pore properties including void fraction (θ) and tortuosity factor (τ). Moreover, a suitable hydrophilic/hydrophobic balance, which drives the transportation process of reactants and products in the framework of catalyst carrier, indeed strongly influences catalytic activity and selectivity of catalytic reaction owing to ventilate ability.40–42 In this regard, the considerable efforts have been devoted to inexpensive polystyrene-based polymeric catalyst carriers in the field of organocatalysis, persuing the profits on superior compatibility with organic substrates to inorganic frameworks such as mesoporous zeolites.43 Furthermore, the hollow structure of catalyst carrier can provide a concentration difference in catalytic reaction, which is better for reactants to access to the catalytic sites in the backbone of catalyst carrier. In the past two decades, the size-,44–46 morphology-47 and hollow structure-48–54 dependent catalytic performances, closely related to effectiveness factor (η), mainly focus on metal catalysis owing to the spectacular advances in the well-controlled syntheses of nano-sized metals and oxides. However, the investigation on po-lymer-based heterogeneous organocatalyst with well-controlled size, pore structure and morpho-logy is in its infancy due to the difficulty in well-controlled synthesis.55, 56 With those as the motivations, there is an overwhelming desire for the synthesis of nano-sized, mesopore-abundant and well-shaped polymer-supported organocatalyst with high void fraction (θ) and short sphere radius (R) to improve the effectiveness factor (η) of organocatalyst, especially to satisfy the spat-ial demand for severe mass transfer of bulky reactants in complex heterogeneous multicompon-ent/multicatalytic enantioselective organocascade.


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Herein, we report an controllable strategy for the synthesis of low-cost hollow mesoporous organic polymeric nano-bowls (HMOPBs) and nano-spheres (HMOPSs) with hollow interior, mesoporous shell and thin shell thickness via the adsorption of Co2+ ions in the outer shell of polystyrene (PS), in-situ transformation of Co2+ into Co(OH)2 clusters and then removal of PS and Co(OH)2 templates. The as-obtained mesopore-abundant void-free HMOPBs, double-shelled HMOPBs and HMOPSs with different shell thicknesses, surface areas and pore volumes provide essential source materials to systematically investigate the mass transport of reactants in sulfonation, immobilization of organocatalyst and heterogeneous organocatalysis. Among them, doubleshelled HMOPBs(a) (Figure 1) is particularly well suited for achieving the more effective diffusion coefficient (De) of reactants and high effectiveness factor (η) of heterogeneous organocatalyst owing to its distinctive advantages such as hollow interior, mesoporous shell and thin shell thickness.

Figure 1. Structural model for double-shelled hollow mesoporous organic polymeric nano-bowls (HMOPBs) in heterogeneous organocatalysis.

2. EXPERIMENTAL SECTION 2.1. Materials. Styrene was purified by distillation under reduced pressure before use. The preparation of poly(styrene/acrylic acid) (PS) core with a mean diameter of 142  9 nm (n = 52)


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was shown in ESI†. 9-Amino(9-deoxy)epi-quinine (QNNH2) was synthesized according to the reference.57 The other chemicals were of analytical grade and used as received without any further purification. 2.2. Characterization. 1H, 13C NMR spectra were conducted on a Bruker av-600 NMR instrument, in which all chemical shifts were reported down-field in ppm relative to the hydrogen and carbon resonances of TMS. Elemental analysis was obtained using a vario Micro cube elemental analyzer. N2 adsorption–desorption isotherm was carried out at 77.4 K on an Autosorb-1 apparatus (Quantachrome), in which samples were degassed at 105 °C for 12 h prior to the measurements. The BET surface areas were calculated from the adsorption data in the relative pressures P/P0 (0.02–0.3). The pore diameter distributions and pore volumes were obtained from the adsorption branches using BJH method. TEM and HR-TEM were performed on JEM-1200EX and TF20-Jeol 2100F to observe the morphologies of samples, respectively at the acceleration voltage of 100 kV and 200 kV. SEM was undertaken on a SU8010 scanning electron microscope operated at 10 kV using secondary electrons to form the images, where the sample was coated with a thin layer of gold before testing. FT-IR spectra were collected by a Perkin-Elmer Model GX spectrometer (KBr pellet) in the range of 400–4000 cm-1. The Dr (trans /cis) values of cyclohexanones were determined by the peak area ratio of CHNO2 in the 1H NMR spectra, and the enantiomeric excesses (%ee trans) of products were detected by HPLC on a Chiralpak AD-H column (λ = 220 nm, 20 °C) eluted with n-hexane/i-PrOH (v/v = 80 : 20) at a flow rate with 0.75 ml min-1. 2.3. Synthesis of Various HMOPBs and HMOPSs. PS core (100 mg) in aqueous polyvinyl alcohol (PVA, 0.5 wt%)-contained ethanol solution (5 mL, vethanol/vwater = 1/10) was well-dispersed under magnetic stirring at room temperature for 8 h. The resulting opalescent suspension


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was added by a syringe to a N2-filled flask (100 mL) containing aqueous PVA (0.5 wt%)-emulsified acrylamide (AA, 142 mg, 2.0 mmol) and Co(OAc)2 (597 mg, 2.4 mmol) solution (15 mL) and well-dispersed. Subsequently, ethyleneglycol dimethacrylate (EGDMA) ethanol solution (6 mL, 119.0 mg, 0.6 mmol), styrene (208 mg, 2.0 mmol) and aqueous potassium persulfate solution (KPS, 4.0 mL, 27.0 mg, 0.1 mmol) were injected sequentially by syringe, stirred for 4 h and irradiated by sonication for 5 min. The resulting translucent emulsion was heated to 80 °C for 24 h. During the process, 3 mL of reaction mixture was regularly sampled at 6 h, 12 h, 15 h, 18 h and 24 h, respectively. The sampled reaction mixture was isolated by centrifugation and washed with water (5 mL × 3) to afford the core-shelled nanospheres PS@poly(St-co-AA)/Co2+. The obtained PS@poly(St-co-AA)/Co2+ was alkalized in NaOH aqueous solution (20 mL, 5 × 10-4 mol L-1) at room temperature for 24 h, where the adsorbed Co2+ ions in the outer shell were transformed into Co(OH)2. The as-prepared PS@poly(St-co-AA)/Co(OH)2 was stirred in HCl solution (30 mL, 1.0 mol L-1) for 24 h to remove Co(OH)2 in the outer shell. The centrifugal solids were further etched in THF (20 mL × 3) for 12 h to remove PS cores, washed with ethanol (5 mL × 2) and dried naturally to afford various hollow and mesoporous samples, hereafter denoted as void-free HMOPBs (12 h), double-shelled HMOPBs(a) (15 h), double-shelled HMOPBs(b) (18 h) and HMOPSs (24 h). According to the same procedure as double-shelled HMOPBs(a), mesopore-deficient double-shelled hollow organic polymeric nano-bowls HOPBs(a) as a contrast sample was prepared in the absence of Co(OAc)2. 2.4. Sulfonation of HMOPBs and HMOPSs. Various mesopore-abundant HMOPBs, HMOPSs and mesopore-deficient HOPBs(a) (100 mg) were degassed at 50 °C for 2 h under vacuum, allowed to cool down to 25 °C and filled with N2 atmosphere. A dioxane solution (5.0 mL)


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containing chlorosulfonic acid (1.0 mL, 15 mmol) was injected by syringe and stirred at 25 °C for 5 h. The light grey sulfonated void-free HMOPBs/SO3H, double-shelled HMOPBs(a)/ SO3H, double-shelled HMOPBs(b)/SO3H, HMOPSs/SO3H and HOPBs(a)/SO3H were obtained by centrifugal separation, copiously washing with deionized water (8 mL × 2), ethanol (8 mL × 2) and natural air-drying. 2.5. General Immobilization of QNNH2. A dioxane solution (0.5 mL) containing HMOPBs(a)/SO3H (81 mg) and QNNH2 (70.4 mg, 0.24 mmol) was stirred at 25 °C for 6 h. After being separated by centrifugation, washed by ethanol (8 mL × 3) and dried naturally, QNNH2functionalized hollow mesoporous organic polymers HMOPBs(a)/SO3H-QNNH2 (118 mg) was obtained via acid-base interaction between -SO3H in HMOPBs(a)/SO3H and –NH2 in QNNH2. 2.6. Heterogeneous Enantioselective Double-Michael Cascade Reaction. A water/toluene mixture (2.0 mL, v/v = 1:1) of α, β-unsaturated ketones (0.22 mmol), aromatic nitroalkenes (0.20 mmol) and HMOPBs(a)/SO3H-QNNH2 (57.6 mg, 30 mol% QNNH2) was stirred at 20 °C till aromatic nitroalkene was completely consumed. To the reaction mixture, ethyl acetate (3.0 mL) was added to extract the products. The catalyst HMOPBs(a)/SO3H-QNNH2 was recovered by centrifugal separation and directly reused in the following catalytic cycles. The isolated organic phase was dried over anhydrous Na2SO4 and evaporated under reduced pressure. The crude products were purified by gradient column chromategraphy on silica gel eluted with petroleum ether/diethyl ether (v/v = 30/1 → 5/1) to give the pure products.

3. RESULTS AND DISCUSSION


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3.1. Shell Thickness and Morphology Control. The typical process for the fabrication of hollow mesoporous organic polymeric nano-bowls (HMOPBs) and nano-spheres (HMOPSs) is schematically illustrated in Figure 2a. First, the outer shell thickness of core-shelled PS@poly (St-co-AA)/Co2+ nano-spheres is precisely modulated by simply controlling the coating time of EGDMA-crosslinked polystyrene on the surface of PS core, companied by the adsorption of Co2+ ions via the coordination of directing agent AA. The outer shell thickness can be approximately estimated from SEM images according to the difference in the particle diameter between core-shelled PS@poly(St-co-AA)/Co2+ and PS core. With the increase in the coating time from 6 h to 24 h, the outer shell thicknesses are gradually thickened from 6.5 nm at 6 h to 10 nm at 12 h , 16 nm at 15 h, 22 nm at 18 h and 33.5 nm at 24 h (Figure S3). Second, the adsorbed Co2+ ions in the outer shell are converted to Co(OH)2 in aqueous NaOH solution, where the in-situ generated Co(OH)2 clusters can swell the compact shell and act as porogen templates. Afterwards, both the hard Co(OH)2 porogen in outer shell and PS core are etched by HCl and THF, respectively. Owing to the different support forces provided by different outer shell thicknesses, a variety of wellshaped hollow mesoporous organic polymers, including nano-bowls [void-free HMOPBs (12 h), double-shelled HMOPBs(a) with a high deformation degree (15 h) and HMOPBs(b) with a low deformation degree (18 h)] and nano-spheres [HMOPSs (24 h)], are successively fabricated under the combined actions of centrifugal force and osmotic pressure.


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Figure 2. Schematic illustration (a), SEM images (b) and TEM images (c) of various hollow mesoporous organic polymeric nano-bowls HMOPBs and nano-spheres HMOPSs.

After Co(OH)2 and PS templates are etched to form the hollow and mesoporous structure, it is observed from SEM (Figure 2b) and TEM (Figure 2c) images that the deformation degrees decrease with the increased outer shell thicknesses from 6.5 nm to 33.5 nm. Owing to the lack of sufficient support force, the outer shell with the thinnest thickness of 6.5 nm crumbles into the shapeless fragments. Fortunately, the thicker outer shell (10 nm) can provide a stronger support force and deforms into well-shaped void-free HMOPBs due to the overlapping of two semishells. With the further increase in the outer shell thickness from 10 nm to 16 nm and 22 nm, the support forces of the shells are enhanced, and the collapse of two semi-shells into double-shelled


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HMOPBs(a) with a high deformation degree and double-shelled HMOPBs(b) with a low deformation degree is observed. Unlike the above-mentioned collapse of the outer shells, the thickest outer shell (33.5 nm) can balance the combined actions of centrifugal force and osmotic pressure and produce well-shaped hollow mesoporous organic polymeric nano-spheres HMOPSs with no deformation of the outer shell. 3.2. Mesoporous Shell. Exploring economical and straightforward methods for the synthesis of polystyrene-based mesoporous organic polymers is still full of challenges.57–59 In this paper, the mesopore-abundant shells with the pore diameters of 2-15 nm in void-free HMOPBs, doubleshelled HMOPBs and HMOPSs (Figure S4) are fabricated via the Co2+ ion-adsorbed, Co(OH)2 porogen-generated and porogen-removed procedure. However, the corresponding HOPBs(a) with lower surface area and pore volume (Table S1), prepared according to the similar procedure as double-shelled HMOPBs(a) in the absence of Co(OAc)2, is proven to be mesopore-deficient by its pore size distribution (Figure 3). The adsorption of Co2+ ions, in-situ transformation of Co2+ ions into Co(OH)2 porogen and removal of Co(OH)2 in the shell are evidenced by the emerged or disappeared characteristic XRD peaks of Co(OH)2 (JCPDS card no. 30-0443) at 2θ = 32.5° {100} and 38.0° {101} (Figure S2). Due to the formation of mesoporous shell, the mesopore-abundant HMOPBs and HMOPSs exhibit the higher specific surface areas (116.6–185.7 m2 g-1), average pore diameters (14.9–16.3 nm) and pore volumes (0.24–0.28 cc g-1) than mesoporedeficient HOPBs(a) (55.3 m2 g-1, 12.3 nm and 0.17 cc g-1) (Table S1). It is worthwhile to note that void-free HMOPBs, double-shelled HMOPBs(a), double-shelled HMOPBs(b) and HMOPSs, sampled at different coating times in the same synthetic process, possess the mesoporous shells with the similar pore volumes (0.24–0.28 cc g-1), and the difference among them is shell thickness. It is well-known that bowl-like particles exclude the unnecessary void space and can be cl-


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osely packed within a given amount of space, compared to the spheres with the same diameter.60 As expected, it is found that double-shelled HMOPBs(a) and HMOPBs(b) possess the larger specific surface areas than HMOPSs. Among them, double-shelled HMOPBs(a) with a high deformation degree has the highest specific surface area (185.7 m2 g-1). Unfortunately, void-free HMOPBs exhibits the lowest specific surface area (116.6 m2 g-1) and pore volume (0.24 cc g-1) owing to the lack of hollow interior resulted from the overlapping of two semi-shells. Moreover, the HR-TEM image (Figure 3) shows that the mesopore-abundant nano-bowl HMOPBs(a) appears honeycomb-like convexoconcave surface, which verifies that the porous structure is fabricated by using Co(OH)2 as porogen template.

Figure 3. Pore size distributions (1) of (a) nano-bowl HMOPBs(a), (b) nano-bowl HOPBs(a) by a BJH model and HR-TEM image (2) of nano-bowl HMOPBs(a).


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3.3. Mesopore and Shell Thickness-Dependent Sulfonation. The benzene moieties, covalently attached to the mesoporous shells of various well-shaped HMOPBs and HMOPSs, can be easily sulfonated by ClSO3H in dioxane at 25 °C for 5 h to afford corresponding void-free HMOPBs/SO3H, double-shelled HMOPBs(a)/SO3H, HMOPBs(b)/SO3H, HMOPSs/SO3H and HOPBs(a)/SO3H with the similar well-shaped morphologies (Figure 4) and mesoporous shells (Figure S6) as their own precursors. The successful sulfonation of benzene moieties in the mesoporous shells is clearly seen from the emerged typical bands of sulfonic acid groups in the FT-IR spectra (Figure S5). Typically, the absorption peaks at 1250 cm−1, 1050 cm−1 and 573 cm−1 are assigned to the asymmetric, symmetric stretching vibrations of S−O bond in sulfonic acid groups and S–C stretching vibration, respectively.61–63 Furthermore, the successful attachment of SO3H groups onto benzene moieties is also evidenced by the decreased surface areas, average pore diameters and pore volumes (Table S3).

Figure 4. The ordinal SEM (a) and TEM (b) images of void-free HMOPBs/SO3H, double-shelled HMOPBs(a)/SO3H, double-shelled HMOPBs(b)/SO3H and HMOPSs/SO3H.


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Sulfonation kinetics during the whole sulfonation process is monitored by elemental analysis of sulfur contents (Table S2). Sulfonation reaction rate (r) can be estimated by the formula: (3)

r =η·kr·Csn

where kr is rate constant, Cs is the concentration on particles surface, n is reaction order, and η is effectiveness factor. It is well-known that the diffusion of reactants in the pores with a broad distribution of pore diameter will proceed by both Knudsen and bulk diffusion. The effective diffusion coefficient (De) in the transition regime can be expressed as the following equation:64 De = Dθ/τ =

1 ― 3ῦl(1-𝑒

2𝑟𝑝 𝑙

) θ/τ

𝑟𝑝

(0.1˂ 𝑙 ˂10)

(4)

where τ is tortuosity factor, θ is void fraction, rp is pore radius, l is mean free path of species, ῦ is average molecular motion rate. Based on the size ratio of rp to l, the effective diffusion coefficient (De) can be rewritten as Eq.(5) and Eq.(6), D e = DK =

2rp 8kBT 3 πm 0.707

De = DB = 3𝜋𝜎2𝑐

𝑇

𝑟𝑝

( 𝑙 ˂0.1, Knudsen diffusion)

8𝑘𝐵𝑇 𝜋𝑚

rp

( l ˃10, bulk diffusion)

(5) (6)

where CT is total component concentration, σ and m are mean diameter and mass of component. From Figure 5b, when the ClSO3H concentrations are doubled from 1.5 mmol L-1 to 3.0 mmol L-1 in the sulfonations of both mesopore-abundant HMOPBs(a)/SO3H and mesopore-deficient HOPBs(a)/SO3H, the very small differences in the sulfonation reaction rates (r) are observed, which sheds light on the significantly decreased effectiveness factors (η) of both mesopore-abundant HMOPBs(a) and mesopore-deficient HOPBs(a) according to Eq. (3). Based on the relationship of effectiveness factors (η) and diffusion coefficients (De) shown in Eq.(1) and Eq.(2), the decreased effective diffusion coefficients (De) of ClSO3H in HMOPBs(a) and HOPBs(a) is logically inferred as a consequence of the doubled concentrations of ClSO3H. In view of concen-


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tration-dependent De, the mass transfer of ClSO3H, whether in mesopore-abundant HMOPBs(a) or in mesopore-deficient HOPBs(a), is convinced to be controlled by bulk diffusion. Moreover, the significant difference in sulfonation rates (r) between mesopore-abundant HMOPBs(a) and mesopore-deficient HOPBs(a) clearly illustrates that the Co(OH)2-imprinted mesopores are essentially beneficial to the improved mass transfer of ClSO3H in sulfonation and result in the higher effectiveness factor (η) of HMOPBs(a) than that of HOPBs(a) .

Figure 5. Sulfonation kinetics: (a) sulfur contents versus reaction times in 3.0 mmol L-1 of ClSO3H [void-free HMOPBs/SO3H (1), HMOPBs(a)/SO3H (2), HMOPBs(b)/SO3H (3), HMOPSs/SO3H (4) and HOPBs(a)/SO3H (5)] and (b) sulfur contents of HMOPBs (a)/SO3H (2) and HOPBs(a)/SO3H (5) in ClSO3H (3.0 mmol L-1 and 1.5 mmol L-1) versus reaction times.


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More importantly, due to the adequately exposed benzene moieties resulted from the asfabricated mesopores in the shell, the mesopore-abundant HMOPBs and HMOPSs with the high specific surface areas (116.6–185.7 m2 g-1), are easily sulfonated by ClSO3H at 25 °C for 5 h to afford mesopore-abundant HMOPBs/SO3H and HMOPSs/SO3H with the sulfur contents in the 2.47–2.85 mmol g-1 range of (Table S2). Among them, double-shelled HMOPBs(a) exhibits the fastest sulfonation reaction rate (Figure 4a) and achieves the highest sulfur content (2.85 mmol g-1), which is mainly attributed to the thinnest shell thickness (16 nm) and the highest surface area (185.7 m2 g-1). With the increase in shell thickness from 16 nm to 22 and 33.5 nm, the contents of sulfur in double-shelled HMOPBs(b)/SO3H and HMOPSs/SO3H decrease to 2.61 mmol g-1 and 2.59 mmol g-1, respectively. Owing to the thickened shell thickness (20 nm) and obsolescent internal and external concentration difference of ClSO3H in the sulfonation, void-free HMOPBs with the overlapping of two semi-shells contains the lowest sulfur contents (2.47 mmol g-1). Especially, the key difference between HOPBs(a) and HMOPBs(a) is whether the mesoporous shells are involved. Due to the lack of mesoporous shell, HOPBs(a) exhibits a significantly poorer mass transfer of ClSO3H, and possesses the lowest sulfur content (0.95 mmol g-1). Furthermore, the acid exchange capacities, determined by acid-base titration (Figure S7 and Table S4), are consistent with the sequence of sulfur content obtained from elemental analysis (Table S2). It is worthwhile to note that the gap (0.28-0.33 H+ mmol g-1) between sulfur contents and acid exchange capacities is originated from the inadequate exchange of acid sites by NaCl solution resulted from the poorer swelling capacity of nano-bowls and nano-spheres in aqueous ethanol solution. In aqueous dioxane solution, the disparity determined by elemental analysis and acid-base titration is below 0.08 H+ mmol g-1.


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3.4. Mesopore and Shell Thickness-Dependent Immobilization of QNNH2. The successful immobilization of versatile chiral QNNH2 organocatalyst into the sulfonated shells to afford QNNH2-functionalized nano-bowls and nano-spheres is confirmed by the IR spectra with the enhanced C–H stretching vibrations at 3044 cm-1 and 2963 cm-1, C–H bending vibrations at 1458 cm-1 and 1392 cm-1 and characteristic absorption peaks of aromatic ring at 1600 cm-1, 1507 cm-1 and 785 cm-1 (Figure S11). The narrowed stretching vibration of S=O in the range of 1350–1150 cm−1 illustrates the formation of amine salt between QNNH2 and SO3H via acid-base reaction (Figure 6a). Meanwhile, the decreased surface area, pore volume and pore diameter provide further evidences for the successful immobilization of QNNH2 into the sulfonated shells (Table S6). Moreover, it is observed from the SEM, TEM images (Figure S8) and pore size distributions (Figure S9) that QNNH2-functionalized nano-bowls and nano-spheres remain their original wellshaped morphologies and mesoporous shells. The Co(OH)2-templated mesopore in the shell of HMOPBs/SO3H displays a key role in the immobilization kinetics of QNNH2 via fast acid-base interaction. Figure 6c provides the model fits showing the consumption and loading capacity of QNNH2 as a function of reaction time for mesopore-abundant HMOPBs(a)/SO3H and mesopore-deficient HOPBs(a)/SO3H with the same shell thickness (16 nm). According to the consumption of QNNH2 as a function of reaction time, the relationship between concentration and reaction time in 48 mmol L-1 of QNNH2 can be expressed as follow. HMOPBs(a)/SO3H: c1 = 0.16/t + 0.26 (R2 = 0.983), HOPBs(a)/SO3H: c2 = 0.02/t + 0.39 (R2 = 0.978).


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Figure 6. Structural representation (a), loading capacities of QNNH2 versus reaction times in 48 mmol L-1 of QNNH2 (b) [void-free HMOPBs/SO3H-QNNH2 (1), HMOPBs(a)/SO3H-QNNH2 (2), HMOPBs(b)/SO3H-QNNH2 (3), HMOPSs/SO3H-QNNH2 (4) and HOPBs(a)/SO3H-QNNH2 (5)] and concentrations, loading capacities of QNNH2 versus reaction times in 48 mmol L-1 and 24 mmol L-1 of QNNH2 (c) [HMOPBs(a)/SO3H-QNNH2 (2), HOPBs(a)/SO3H-QNNH2 (5)].


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Then, d[c1]/dt = 0.16/t2, and d[c2]/dt = 0.02/t2, respectively. The initial adsorption rates of QNNH2 in 48 mmol L-1 of QNNH2 at 1 h were calculated to be 0.16 mol L-1 h-1 and 0.02 mol L-1 h-1, respectively for mesopore-abundant HMOPBs(a)/SO3H and mesopore-deficient HOPBs(a)/ SO3H, which indicates that the Co(OH)2-templated mesopores can significantly enhance the adsorption rates of QNNH2 into the sulfonated shells. When the concentration of QNNH2 is halved to 24.0 mmol L-1, the concentration-independent loading capacities of QNNH2 are observed for both mesopore-abundant HMOPBs(a)/SO3H and mesopore-deficient HOPBs(a)/SO3H during the whole process. Based on Eq. (3), it is deduced that the effectiveness factor (η) significantly decrease with the increased concentration of QNNH2. In view of the relationships among effectiveness factor (η), diffusion coefficient (De) and concentration, the mass transfer of QNNH2 in both mesopore-abundant HMOPBs(a)/SO3H and mesopore-deficient HOPBs(a)/SO3H is also controlled by bulk diffusion, similar as ClSO3H in the sulfonation of HMOPBs(a) and HOPBs(a). On the one hand, the shell thickness is another key factor influencing the immobilization kinetic of QNNH2. Due to the fast acid-base interaction between QNNH2 and attached SO3H, the loading capacity of QNNH2 mainly depends on the adsorptive flux (or reactive flux JR) of QNNH2 in the shells. According to the ever-reported theoretical research, 32, 65 the reactive flux and adsorption rate constant are given as 𝐽𝑅 = 𝑘𝑎𝑑𝑠𝐶𝑠𝜃𝑣𝑎𝑐 𝑘𝑎𝑑𝑠 =

𝑘𝑏𝑇 𝑝𝑜𝑛𝐴𝑐𝑎𝑡 2𝜋𝑚 𝑁𝑐𝑎𝑡

(7) (8)

Combining Eq. (7) and Eq. (8) leads to 𝐽𝑅 = 𝐶𝑠

𝑘𝑏𝑇 𝑝𝑜𝑛𝐴𝑐𝑎𝑡𝜃𝑣𝑎𝑐 2𝜋𝑚 𝑁𝑐𝑎𝑡

(9)

Where m is the mass of component, kbT is energy, pon is the probability that a striking particle sticks, Acat is the total area of catalyst, Ncat is the number of catalytic sites, and θvac is the frac-


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tional vacancy of catalytic sites at a steady state. In the immobilization of QNNH2 via fast acidbase reaction, the number of total acid site (SO3H) (Ncat) is obtained by the content of sulfur from elemental analysis. The ratio of acid exchange capacity (Table S4) to sulfur content (Table S2) can be roughly regarded as the fractional vacancy (θvac) of acid site. Therefore, the reactive fluxes of QNNH2 in HMOPBs(a)/SO3H, HMOPBs(b)/SO3H, HMOPSs/SO3H and HMOPBs/SO3H in 48.0 mmol L-1 of QNNH2 are calculated to be 2.55 × 106 106

𝑘𝑏𝑇 2𝜋𝑚

pon and 1.96 × 106



pon, 2.47 × 106


pon, 2.33 ×

pon, respectively. Fortunately, the calculated sequence of

reactive fluxes (JR) is consistent with the actual loading capacities of QNNH2 in 48.0 mmol L-1 QNNH2 (Figure 6b). Among the influencing factors, the surface area (Acat) is most relevant to the reactive flux (JR) according to the obtained data. It is well known that bowl-like particles exclude the unnecessary void space and can be closely packed within a given amount of space compared to hollow spheres with the same diameter.60 The HMOPBs(a)/SO3H with the largest surface area (171.1 m2 g-1) and the shortest shell thickness (16 nm) exhibits the highest loading capacity (1.35 mmol g-1) of QNNH2. With the increase of shell thickness from 16 nm to 22 nm and 33.5 nm, the loading capacities of QNNH2 successively decrease to 1.12 mmol g-1 and 1.03 mmol g-1, respectively for bowl-like HMOPBs(b)/SO3H and spherical HMOPSs/SO3H. In particular, although void-free HMOPBs/SO3H possesses a relatively shorter shell thickness (20 nm), the lowest loaded QNNH2 (0.96 mmol g-1) is observed, which is attributed to the lack of internal and external concentration difference as a result of no hollow interior. Therefore, it is concluded that the Co(OH)2-templated mesopore, PS-templated hollow interior and thin shell thickness play important roles in the immobilization kinetics of QNNH2.66 3.5. Mesopore and Shell Thickness-Dependent Catalytic Performance. It is well known that a combination of 9-amino(9-deoxy)epi-cinchona alkaloid and Brønsted acid can provides a


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powerful strategy to stereochemically construct complex products with multiple stereocenters in highly optical purities.1, 2, 67 As a typical representative, the asymmetric double-Michael addition of 4-phenyl-α, β-unsaturated butanone to β-nitrostyrene to furnish 3, 5-diphenyl-4-nitrocyclohexanone is selected as the benchmark reaction to investigate mesopore and shell thickness-dependent catalytic performances.68 The whole processes of the double-Michael reactions, promoted by

HMOPBs/SO3H-QNNH2,

HMOPBs(a)/SO3H-QNNH2,

HMOPBs(b)/SO3H-QNNH2,

HMOPSs/SO3H-QNNH2 and HOPBs(a)/SO3H-QNNH2 (30 mol% QNNH2) at 20 °C for 48 h, are monitored by chiral HPLC. The concentrations of β-nitrostyrene and yields, stereoselectivities of 3, 5-di-phenyl-4-nitrocyclohexanone are shown in Figure 7 and listed in Table S7–S9. From Figure 7, it is found that the mesopore and shell thickness of heterogeneous organocatalyst play important roles in the kinetics of double-Michael organocascade. First, although HOPBs(a)/SO3H-QNNH2 and HMOPBs(a)/SO3H-QNNH2 possess the same shell thickness (16 nm), mesopore-deficient HOPBs(a)/SO3H-QNNH2 promotes the double-Michael organocascade in the significantly lower conversions of β-nitrostyrene and yields of cyclohexanone than mesopore-abundant HMOPBs(a)/SO3H-QNNH2 during the whole process owing to the lack of mesopore in the shell (Figure 7a). Based on the relationship between concentration and reaction time, their initial reaction rates at 6 h are calculated to be 10.3 × 10-3 mol L-1 h-1 and 2.6 ×10-3 mol L-1 h-1, respectively. Furthermore, the mesopore-abundant HMOPBs(a)/SO3H-QNNH2, HMOPBs(b) /SO3H-QNNH2 and HMOPSs/SO3H-QNNH2 with same pore volume (~0.19 cc g-1) and hollow interior exhibit the differential catalytic activities. It is observed that the thinner shell thickness shows the higher conversion of β-nitrostyrene and yield of cyclohexanone, which is attributed to the bowl-like deformation resulting in larger surface area. Their initial rates at 6 h are calculated to be 10.3 × 10-3 mol L-1 h-1, 5.8 × 10-3 mol L-1 h-1 and 4.6 × 10-3 mol L-1 h-1, respectively. More-


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over, owing to the lack of internal and external difference in the concentration of reactant, mesopore-abundant void-free HMOPBs/SO3H-QNNH2 with a thin shell thickness (20 nm) promotes the double-Michael organocascade with a very low initial rate (3.6 × 10-3 mol L-1 h-1). Compared with aluminium phosphonate-supported 9-amino(9-deoxy)epi-cinchona alkaloid (73%, 96 h),69 the well-shaped HMOPBs(a)/SO3H-QNNH2 with a higher surface area, thinner transmission distance and hollow interior produces cyclohexanone in a higher yield (78 %, 48 h).

Figure 7. The concentrations of β-nitrostyrene and yields (a), enantioselectivity (b), diaste-reoselectivity (c) of 3, 5-diphenyl-4-nitrocyclohexanone [HMOPBs/SO3H-QNNH2 (1), HMOPBs(a)/SO3H-QNNH2 (2), HMOPBs(b)/SO3H-QNNH2 (3), HMOPSs/SO3H-QNNH2 (4) and HOPBs(a)/SO3H-QNNH2 (5)].


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On the one hand, the stereoselectivity of 3, 5-diphenyl-4-nitrocyclohexanone in the progress of the asymmetric double-Michael reaction displays the regular pattern of mesopore and shell thickness-dependent diastereoselectivities and enantioselectivities. From Figure 7b, c, HOPBs(a) /SO3H-QNNH2 produces the cyclohexanone with the lowest diastereoselectivities and enantioselectivities owing to the lack of mesoporous shell. Among the mesopore-abundant HMOPBs(a)/ SO3H-QNNH2, HMOPBs(b)/SO3H-QNNH2 and HMOPSs/SO3H-QNNH2 with a hollow interior, the thinner shell thickness affords the higher diastereoselectivities and enantioselectivities (Table S8). However, void-free HMOPBs/SO3H-QNNH2 promotes double-Michael organocascade with the relatively lower diastereoselectivities and enantioselectivities than the other mesopore-abundant organocatalysts. The reason for the mesopore and shell thickness-dependent diastereoselectivity and enantioselectivity is related to the competition between SO3H/QNNH2-synergistically promoted and SO3H-individually promoted double-Michael organocascades. When the SO3H/ QNNH2-promoted reaction rate is weakened, the SO3H-individually promoted double-Michael organocascade out of the stereoselective control of QNNH2 is relatively enhanced and results in the decreased stereoselectivity of 3, 5-diphenyl-4-nitrocyclohexanone, which is also evidenced by the ever-reported multifunctional heterogeneous organocatalyst with a slow catalytic rate (trans/cis = 5.7, 86 %ee, 96 h).69 3.6. Cooperative Catalysis of Attached SO3H and QNNH2. The cooperative catalysis of attached-SO3H and QNNH2, where neither SO3H nor QNNH2 are insufficient to individually promote double-Michael organocascade reaction, works synergistically to achieve a desired catalytic performance in heterogeneous organocatalysis. 70 To clarify the relationship of cooperative ca-talysis of SO3H and QNNH2 to their own molar ratio, various HMOPBs(a)/SO3H-QNNH2 with the different molar ratios of SO3H to QNNH2 (2.55, 1.55, 1.01 and 0.87) are prepared by


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control-ling the reaction times (1 h, 2 h, 3 h and 5 h) in the immobilization of QNNH2. Their catalytic performances including yields, diastereoselectivities and enantioselectivities are shown in Figure 8 and listed in Table S11.

Figure 8. Yields (a) and stereoselectivities (b) of (3S, 5S)-3, 5-diphenyl-4-nitrocyclohexanone catalyzed by HMOPBs(a)/SO3H-QNNH2 (30 mol% QNNH2) with the different molar ratios of SO3H to QNNH2: (1) 0.87, (2) 1.01, (3) 1.55, (4) 2.55.

From Figure 8, two key points about the reaction kinetics and stereoselectivities including enantioselectivities and diastereoselectivities deserve special attention. First, the molar ratios of SO3H to QNNH2 have significant influences on the initial reaction rates. At 6 h, under the same used amount of QNNH2 (30 mol%), the molar ratios of SO3H to QNNH2 at 1.55 and 2.55 display


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the highest initial rate (1.7 × 10-3 mol L-1 h-1). With the decrease in the molar ratio of SO3H to QNNH2 from 1.55 to 1.01 and 0.87, the initial rates of the reactions are gradually reduced to 1.3 × 10-3 mol L-1 h-1 and 8.3 × 10-4 mol L-1 h-1, respectively. Although various HMOPBs(a)/SO3HQNNH2 with different molar ratios display the obvious differences in the initial rates, the same yield of the product can be achieved after 24 h. The key reason is that the greater molar ratios of SO3H to QNNH2, the faster formation of synergistic catalysis between SO3H acid and QNNH2 organocatalyst. Second, in the progress of the reactions, the enantioselectivities and diastereoselectivities are getting better and intimately correlated with the molar ratios of SO3H to QNNH2. Among them, HMOPBs(a)/SO3H-QNNH2 with the molar ratios of SO3H/QNNH2 = 0.85–1.01 exhibits the best diastereoselectivity (trans/cis = 13.4) and enantioselectivity (98 %ee) after 48 h. The worse disastereoselectivities and enantioselectivities for the molar ratios of SO3H/QNNH2 at 1.55 and 2.55 are attributed to the enhanced competition of SO3H-individually promoted doubleMichael organocascade out of stereoselective control of chiral QNNH2, which ultimately results in the decreased stereoselectivity of 3, 5-diphenyl-4-nitrocyclohexanone. Based the above-mentioned results, it is concluded that a suitable molar ratio is better for the cooperative catalysis of SO3H acid and QNNH2 organocatalyst to achieve a desired excellent catalytic performance in heterogeneous organocatalysis. 3.7. The Scope of Mesopore and Shell Thickness-Dependent Double-Michael cascade. Encouraged by the excellent catalytic performances of HMOPBs(a)/SO3H-QNNH2 with the molar ratio of SO3H/QNNH2 = 1.01 in the double-Michael organocascade, the substituents of 4phenyl-α, β-unsaturated butanone and β-nitrostyrenes are extended to electron-withdrawing or electron-donating groups at the 2, 4-positions, and double-shelled HMOPBs(a)/SO3H-QNNH2, spherical HMOPSs/SO3H-QNNH2, mesopore-deficient HOPBs(a)/SO3H-QNNH2 and homoge-


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neous 2-F-C6H4CO2H/QNNH2 with the similar molar ratios of SO3H to QNNH2 (1.01–1.05) are selected as examples to investigate their mesopore and shell thickness-dependent catalytic performances. From the catalytic results shown in Table S12, several points are fixed on the mesopore and shell thickness-dependent catalytic performances. First, whether in homogeneous or heterogeneous organocascade, both phenyl-α, β-unsaturated butanone and β-nitrostyrene with electron-withdrawing groups (R1 = R2 = Cl) at the 2, 4-positions produce cyclohexanones in higher yields within shorter time (24 h) than those with electron-donating groups (R1 = R2 = OCH3). Meanwhile, one of electron-withdrawing group (R1 or R2 = Cl) instead of electron-donating groups can improve the reaction rates of double-Michael organocascade. Second, owing to the spatial requirement of the bulky reactants, the mesopore and shell thickness play key roles in achieving the excellent catalytic activities. According to the mesopore-abundant HMOPBs(a)/ SO3H-QNNH2 and meso-pore-deficient HOPBs(a)/SO3H-QNNH2-promoted yields, the mesopores in HMOPBs(a)/SO3H-QNNH2 can significantly accelerate the mass transfer of reactants in the shell and are responsible for the higher yields with the 17–33% increments. For HMOPBs(a) /SO3H-QNNH2 and HMOPSs/SO3H-QNNH2 with similar mesoporous structure and pore volume, the bowl-like HMOPBs(a)/SO3H-QNNH2 with a thinner shell thickness promotes the doubleMichael organocascade to produce the cyclohexanones in the higher yields with the 4–13% decrements than the spherical HMOPSs/SO3H-QNNH2 owing to its larger surface area and shorter seepage distance of reactants. Third, the mesopore and shell thickness also play important roles in achieving good to excellent stereoselectivity including diastereoselectivity and enantioselectivity. When the substituents (R1 = R2 = Cl and OCH3) are attached to the 4-positions, only mesopore-abundant HMOPBs(a)/SO3H-QNNH2 with a thin shell thickness can achieve the same excellent enantioselectivities (92–98% trans) and diastereoselectivities (trans/cis = 93/7–96/4) as a


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combination of homogeneous 2-FC6H4CO2H/QNNH2. For the bulky reactants with the substituents (R1, R2 = Cl, OCH3) in the 2-positions, there are the 2–7 % decrements in enantioselectivities due to the vicinal effect of the substituents. Unfortunately, the spherical HMOPSs/SO3H-QNNH2 with a thicker shell-thickness and mesopore-deficient HOPBs(a)/SO3H-QNNH2 exhibit the very disappointing stereoselectivities including diastereoselectivities and enantioselectivities. In conclusion, in the complex heterogeneous double-Michael organocascade of bulky reactants, the catalytic performances including activity and stereoselectivity strongly depend on the mesopores in the shell and shell thickness of catalyst carrier to achieve the fast mass transfer and short seepage distance of reactants.

3.8. Stability and Reusability. The stability and reusability is an important parameter to evaluate the potential application of heterogeneous catalyst. After the completion of double-Michael organocascade (R1 = R2 = 4-OCH3), HMOPBs(a)/SO3H-QNNH2 can be facilely recovered by centrifugation and directly reused in the following catalytic cycles. From Table S13, the catalyst can maintain its original excellent diastereoselectivities (trans/cis = 94/6–92/8). However, the yields and enantioselectivities of products decrease from 68% and 97 %ee to 51 % and 93 %ee, respectively in the fifth cycle. To elucidate the reason for the sharply decreased yields, the fresh and 5th-reused HMOPBs(a)/SO3H-QNNH2 are characterized contrastively by SEM, N2 adsorption-desorption and elemental analy-sis. From the SEM images (Figure S15c, d), some doubleshelled nano-bowls HMOPBs(a) collapse into the void-free nano-bowls HMOPBs owing to the overlapping of two semi-shells, which results in no driving force of internal and external concentration difference. N2 adsorption-desorption indicates that the surface area and pore volume decrease from 144 m2 g-1 and 0.19 cc g-1 to 36 m2 g-1 and 0.03 cc g-1, and the original mesopores in the range of 3.0–10 nm disappear due to the adsorption of reactants or products (Figure S15b).


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The above-mentioned changes in the mesopore structure and morphology are unfavorable for the mass transfer of reactants. Moreover, a slight decrease in the nitrogen contents from 5.69% to 5.47% is determined by elemental analysis. It is concluded that the collapsed morphology and mesopore blockage are responsible for the decreased yields of cyclohexanone.

4. CONCLUSIONS In summary, the well-shaped mesopore-abundant hollow mesoporous organic polymeric nanobowls (HMOPBs) and nano-spheres (HMOPSs) are fabricated via Co(OH)2-templated method involving the adsorption of Co2+ ions in the shell, in-situ transformation to Co(OH)2 and removal of Co(OH)2. The as-fabricated well-shaped HMOPBs(a) with hollow interior, mesoporous shell and thin shell thickness exhibits the highest reaction kinetics in the sulfonation by ClSO3H and succedent immobilization of chiral organocatalyst. Furthermore, the multifunctional HMOPBs(a) /SO3H-QNNH2 displays superior catalytic performances to other nano-bowls, nano-spheres and similar heterogeneous organocatalyst, and achieves the similar excellent catalytic performances as a combination of homogeneous 2-FC6H4CO2H/QNNH2 in the double-Michael organocascade. Especially, the mesoporous shell of HMOPBs(a)/SO3H-QNNH2 provides the enough space for the spatial requirement of enantioselective double-Michael organocascade to produce bulky optically pure cyclohexanones in good yields with excellent stereoselectivities including diastereoselectivities and enantioselectivities. Although it is found that the catalytic activity and enantioselectivity of HMOPBs(a)/SO3H-QNNH2 reduce gradually along the number of cycles owing to the collapsed morphology and mesopore blockage, it is believed that the pioneering shell thickness and mesopore-dependent strategy in this paper for improving the mass transfer of reactants


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and stereoselectivities of bulky products has a brilliant application future in the field of complex multicomponent/multicatalysed cascade/domino reactions.

■ ASSOCIATED CONTENT

●Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Characterization of various sulfonated, QNNH2-functionalized hollow mesoporous organic polymeric nano-bowls and nano-spheres by SEM, N2 adsorption–desorption isotherms, IR and elemental analysis; Catalytic performances including yields, diastereoselectivities and enantioselectivities determined by 1H NMR and HPLC; Process monitoring of cascade reaction by HPLC.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Xuebing Ma: 0000-0002-6585-7246 Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS


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Financial support of this work was provided by Basic Science and Advanced Technology Research Project of Chongqing Science&Technology Commission (cstc2017jcyjBX0027), P. R. China.

■ ABBREVIATIONS QNNH2, 9-amino(9-deoxy)epi-quinine; PS, poly(styrene/acrylic acid); AA, acrylamide; EGDMA, ethyleneglycol dimethacrylate; HMOPBs, hollow mesoporous organic polymeric bowls; HMOPSs, hollow mesoporous organic polymeric spheres; HOPBs(a), hollow organic polymeric bowls; HMOPBs/SO3H, sulfonated hollow mesoporous organic polymeric nanobowls; HMOPSs/SO3H, sulfonated hollow mesoporous organic polymeric nano-spheres; HOPBs(a)/SO3H, sulfonated hollow organic polymeric bowls; HMOPBs/SO3H-QNNH2, QNNH2-functionalized HMOPBs/SO3H; HMOPSs/SO3H-QNNH2, QNNH2- functionalized HMOPSs/SO3H; HOPBs(a)/SO3H-QNNH2, QNNH2-functionalized HOPBs(a)/SO3H.

■ REFERENCES (1) Qin, Y.; Zhu, L. H.; Luo, S. Z. Organocatalysis in Inert C−H Bond Functionalization. Chem. Rev. 2017, 117, 9433−9520. (2) Zhan, G., Du, W.; Chen Y.-C. Switchable Divergent Asymmetric Synthesis via Organocatalysis. Chem. Soc. Rev. 2017, 46, 1675−1692. (3) Borie, C.; Ackermann, L.; Nechab, M. Enantioselective Syntheses of Indanes: from Organocatalysis to C-H Functionalization. Chem. Soc. Rev. 2016, 45, 1368−1386.


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Table of Contents (TOC)

The as-fabricated hollow mesoporous organic polymeric nano-bowls and nano-spheres display the shell thickness and mesopore-dependent catalytic performances in sulfonation of benzene ring, immobilization of organocatalyst and enantioselective organocascade of bulky reactants.


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Hollow Mesoporous Organic Polymeric Nano-Bowls and Nano-Spheres

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