Chiral DHIP- and Pyrrolidine-Based Covalent Organic Frameworks for

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Chiral DHIP- and Pyrrolidine-Based Covalent Organic Frameworks for Asymmetric Catalysis Zhang Jie, Xing Han, Xiaowei Wu, Yan Liu, and Yong Cui ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05887 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 7, 2019

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ACS Sustainable Chemistry & Engineering

Chiral DHIP- and Pyrrolidine-Based Covalent Organic Frameworks for Asymmetric Catalysis Jie Zhang,† Xing Han,† Xiaowei Wu,† Yan Liu,*,† and Yong Cui*,†,‡ †School

of Chemistry and Chemical Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 800 Dongchuan Road, Minhang District, Shanghai 200240, China ‡Collaborative Innovation Center of Chemical Science and Engineering, 92 Weijin Road, Naikai District, Tianjin 300072, China Corresponding authors, E-mail addresses: [email protected] (Yan Liu). [email protected] (Yong Cui) Supporting Information

ABSTRACT: Covalent organic frameworks (COFs) are a new class of light weight crystalline organic porous materials, which are covalently constructed from organic building block. However, it is very difficult to construct the functional COFs. In particularly, it is very complicated to construct chiral covalent organic frameworks (CCOFs). Because asymmetry, porosity and crystallinity have to be contemporaneously taken into account. Here we prepared two CCOFs, termed TPB2-COF and Tfp2-COF, by directly employing chiral organo-catalysts as building units. Due to the precisely one dimensional pore channels with periodically appended of chiral organo-catalytic sites as well as the ability to accurately control the environment around chiral organo-catalytic sites, these obtained CCOFs could serve as effective heterogeneous catalysts for an asymmetric Steglich rearrangement and Asymmetric Michael addition, with the stereoselectivity and diastereoselectivity up to 84% ee value and 86% ee value, 17:1 d.r. value for asymmetric Steglich rearrangement and Asymmetric Michael addition respectively, rivaling or surpassing the homogeneous and amorphous analogues. Our strategy may use to synthetize a series of functional Covalent Organic Frameworks for variety applications.

KEYWORDS: chiral, COFs, asymmetric, catalysis, heterogeneous.

INTRODUCTION Asymmetric organocatalysis has gained remarkable interest over the last several decades.1 Due to its many advantages such as the avoidance of expensive or toxic metals and being environmentally benign, it is a particularly useful tool for the chemist.2 However, there are often still some limitations such as the high cost and the high catalyst loadings often restricted the potential for industrial and sustainable applications.3 Furthermore, some asymmetric organocatalyst are more serious pollution emissions and environmental pollution due to the homogeneous asymmetric organocatalyst are difficult to recycle. Therefore, an effective asymmetric organocatalyst recycling strategy might overcome these problems. If this strategy can work, the asymmetric catalysts can be easily recycled by using the centrifugation method, which can not only reduce environmental pollution, but also save the cost of catalyst. Basically, There are two strategies can be used for constructing heterogeneous asymmetric organocatalysts: first, involves the post-synthetic integration of asymmetric organocatalysts into the supports.4 However, this strategy often leads to less loading and uneven distribution of asymmetric organocatalysts; second, construction heterogeneous asymmetric organocatalysts directly from the functional building blocks.3 As known, the stability, porosity and crystalline play the important role in the construction of highly efficient heterogeneous asymmetric organocatalysts. Although as one of popular porous crystalline materials, The Metal organic frameworks (MOFs) have been widely employed as hosts for the construction of heterogeneous asymmetric organocatalysts.5-8 However, the necessary expensive or toxic metals and tedious lab

work for MOFs severely limit their sustainable applications. Therefore, to develop a green and efficient heterogeneous asymmetric organocatalysts is still a high challenge. Covalent organic frameworks (COFs) are a new class of light weight crystalline organic porous materials, which are constructed from organic building block by covalent bonds.9-12 This crystalline organic porous materials possess well-defined crystalline structures with predictable 2D or 3D ordered porous architectures.10 COFs exhibit diverse potential applications that include gas storage13-16 and separation,17-23 catalysis,24-38 sensing39-42 and optoelectronics.43-45 Recently, chiral covalent organic frameworks (CCOFs) have emerged as an interesting research area toward the hopeful applications in asymmetric catalysis33-38 and enantioselective separation.22-23 Due to the precisely pore channel that make the organo-catalytic sites easily to access substrates, and the ability to accurately control the environment around the active sites.22-23, 33, 37-38 CCOFs hold a great deal of potential as asymmetric heterogeneous catalysts, However, the reports of stereoselective COFs catalysts are very few. Notably, it is still very hard to construct CCOFs because asymmetry, crystallinity and porosity have to be contemporaneously taken into account. Here, we synthesized two CCOFs termed TPB2-COF and Tfp2-COF, by directly employing chiral organo-catalysts as building units. Due to the precisely 1D pore channels with periodically appended of chiral organo-catalytic sites as well as the ability to accurately control the environment around chiral organo-catalytic sites, these obtained CCOFs could serve as effective heterogeneous catalysts for an asymmetric Steglich rearrangement and Asymmetric Michael addition, the stereoselectivity and diastereoselectivity of asymmetric Steglich

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ACS Sustainable Chemistry & Engineering rearrangement and asymmetric Michael addition could rival or surpass the homogeneous and amorphous analogues.

General Procedure for asymmetric steglich rearrangement and asymmetric michael addition reactions. A typical asymmetric steglich rearrangement procedure can be described as follows: In a dry two-necked flask with TPB2-COF and the substrate at 0 °C was added acetone as solvent. The mixture was stirred at 0 °C for 4 days, after the reaction was completed (monitored by TLC), the mixture was centrifugated and the catalyst was washed with Et2O (2 × 3 mL). then the combined organic phase was combined and washed with 0.1 M HCl, then dried over Na2SO4 and evaporated. The crude product was purified by flash chromatography on silica gel. The yield was the isolated yield. The enantiomeric excess was determined by HPLC. The recovered catalyst was washed with dichloromethane (2 × 6 mL), MeOH (with 2% of Et3N) (2× 6 mL), H2O (2 × 6 mL), THF (2 × 6 mL), and Et2O (2 × 6 mL). The resulting powder was dried at room temperature, and then used for the next cycle. For asymmetric michael addition reactions: In two-necked flask with nitrostyrene (0.1 mmol), cyclohexanone (2 mmol), CH3COOH (0.025 mmol) and Tfp2-COF (0.01 mmol) were added water (0.3 ml) and EtOH (0.9 ml) as solvent. The mixture was stirred at room

EXPERIMENTAL SECTION Synthesis of Chiral COFs. The chiral COFs were prepared by solvothermal treatment. Briefly, a mixture of 1,3,5-tris(4-formylphenyl)benzene (Tfp1) (19.5 mg, 0.05 mmol) and TPB2 (24.4 mg, 0.05 mmol) for TPB2-COF or Tfp2 (29.3 mg, 0.05 mmol) and 1,3,5-tris(4-aminophenyl)benzene (TPB1) (17.5 mg, 0.05 mmol) for Tfp2-COF, in presence of 3 M acetic acid (0.15 mL) using 1,4-dioxane (0.5 mL) and mesitylene (1.5 mL) as solvent. The mixture was sonicated until get a homogenous dispersion. Then the tube was flame sealed off and heated at 120 °C for 3 days. After the reaction the obtained powder were filtered out, washed with tetrahydrofuran, ether and dried at 120 °C under vacuum for 12 hours to give yellow colored powder with 86% yield for TPB2-COF. For Tfp2-COF, the obtained powder was furtherly treated with piperidine (0.5 ml) in ethanol (3 ml) at room temperature for 3 h to get the Tfp2-COF with 80% yield.

a)

NH2

CHO

N

CHO H2N

OHC O

N

N

N

N

TPB2-COF

N

Tfp1

N

NH2

O

TPB1

O N H

+

O

N

N

N

+

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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piperidine/EtOH

NH2

CHO

Tfp2-COF

N H

N

N

N N

N

H 2N

O N

O N

NH2 OHC

b)

O

N

N TPB2

N

O

N

CHO

F FF

O

Tfp2

N H

c)

4.01 Å

4.02 Å

d)

e) 100

TPB2-COF

110200 210

Experimental Pawley Refined Simulated Difference

100

Tfp2-COF

110 200 210

001

Experimental Pawley refined Simulated Difference

001

5 10 15 20 25 30 15 20 25 30   degree  2 2 degree Figure 1. Synthesis and structure of TPB2-COF and Tfp2-COF. (a) Synthesis of TPB2-COF and Tfp2-COF. (b, c) Top and side views of TPB2-COF 5

10

2

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and Tfp2-COF. (d, e) PXRD profiles of TPB2-COF and Tfp2-COF. Experimentally observed, Pawley refined, simulated powder X-ray diffraction patterns (AA stacking) and their difference.

temperature for 8 hours. The mixture was centrifugated and the catalyst was washed with ethyl acetate (5 ml twice) then the solvents were combined and washed with brine, then dried over Na2SO4 and evaporated. The d.r. was determined by 1H NMR spectroscopy. The enantiomeric excess was determined using HPLC. The yield was the isolated yield. The recovered catalyst was washed with dichloromethane (2 × 3 mL), MeOH (with 2% of Et3N) (2× 3 mL), H2O (2 × 3 mL), THF (2 × 3 mL), and Et2O (2 × 3 mL). The resulting powder was dried at room temperature, and then used for the next cycle.

TPB2-COF Tfp2-COF

400 -1

)

RESULTS AND DISCUSSION

0.99, respectively. The pore size distribution of both CCOFs was estimated using nonlocal density functional theory (NLDFT), provided a uniform pore size of 1.63 nm for TPB2-COF and 1.67 nm for Tfp2-COF, respectively. They are very close to the theoretical values of 1.56 and 1.62 nm estimated by Zeo++ software for TPB2-COF and Tfp2-COF, respectively.46-48

3

200

Differential Pore Volume (cm3g-1)

Characterization. The synthesized chiral COFs were characterized by FT-IR spectroscopy, 13C CP-MAS NMR spectra, thermal gravimetric analysis (TGA), powder X-ray diffraction (PXRD) analysis, scanning electron microscopy (SEM) and Brunauer−Emmett−Teller (BET). The FT-IR spectra of the activated COFs show vibrational bands at 1610 and 1624 cm-1 for TPB2-COF and Tfp2-COF. which indicated the formation of the imine bond. Meanwhile, The IR spectrum of TPB2-COF and Tfp2-COF showed the amino and aldehyde bands of these COFs disappeared after the condensation reaction, which further demonstrated the Schiff base linkage formed from the condensation of primary amine and aldehyde. The 13C CP/MAS NMR spectra of TPB2-COF and Tfp2-COF exhibited the signals at 152 and 153 cm-1, respectively, which are assigned to the C=N bonds. Which further demonstrated the formation of the C=N bonds. Scanning electron microscopy (SEM) revealed only one morphologically crystalline phase. Thermal gravimetric analysis (TGA) showed that no obvious weight loss of both CCOFs were observed until 370 ℃ . Both CCOFs are not soluble in most organic solvents and water and stable in ordinary solvents and water. Powder X-ray diffraction (PXRD) measurements was performed on activated COFs to determine their crystalline structure. PXRD of TPB2-COF exhibited strong PXRD peaks at 4.09o, 6.85o, 7.92o and 10.48o, which corresponded to the (100), (110), (200) and (210) planes, respectively. The Tfp2-COF has a similar PXRD patterns, so the Tfp2-COF has a similar crystal structure with TPB2-COF. The observed reflections agree well with the calculated patterns, which obtained from structural simulations performed for a partially slipped AA layer stacking (Figure S1 and S2). The Pawley refinement confirmed the peak assignments (Figure 1, red curve), as evident by their small differences (Rwp = 3.72% and Rp = 3.16% for TPB2-COF; Rwp = 3.27% and Rp = 2.47% for Tfp2-COF) and yielded the unit-cell parameters (a =26.19 Å, b = 25.86 Å, c = 4.02 Å, α = 86.34°, β = 86.16°, γ = 119.87° for TPB2-COF; a = 26.16 Å, b = 25.86 Å, c = 4.01 Å, α = 86.41°, β = 86.14°, γ = 119.96° for Tfp2-COF). Such AA layer stacking structure results in open one-dimensional channels of 17 and 17 Å for TPB2-COF and Tfp2-COF, respectively. In the PXRD patterns, the TPB2-COF and Tfp2-COF revealed their 001 plane reflections at 29.06° and 29.00°, respectively, which are correlated to the interlayer distances (4.02 and 4.01 Å). However, the staggered AB stacking modes did not match well with the experimental PXRD patterns on the relative peak intensities. The porosity of the CCOFs was studied via N2 sorption experiment at 77 K. The sorption curves of TPB2-COF and Tfp2-COF exhibited a reversible type I isotherm, which is indicative of a mesoporous material (Figure 2). The Brunauer– Emmett–Teller (BET) surface areas of them were calculated to be 418 and 362 m2g-1, while the pore volumes of TPB2-COF and Tfp2-COF were calculated as 0.63 and 0.49 cm3g-1 at P/P0 =

N2 uptake (cm g

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0

0.0

0.2

0.4

0.03 0.02 0.01 0.00 0

2

4 6 8 10 12 14 Pore width (nm)

0.6

0.8

1.0

Relative Pressure (P/P0)

Figure 2. N2 adsorption/desorption isotherms (77K) and the pore size distribution profiles (insert) of TPB2-COF (red) and Tfp2-COF (blue).

Asymmetric Catalysis. Asymmetric organocatalysis is a very important synthesis tool. Due to its many advantages such as being environmentally, the avoidance of metals, and being sustainable, it has gained significant interest over the last several decades. For example, DHIP and imidazolidine derivatives are excellent asymmetric organocatalysts for many very useful organic transformations, such as DHIP for asymmetric steglich rearrangement49 and pyrrolidine for asymmetric michael addition reactions,36 asymmetri aldol reactions37 and However, the high catalyst loadings and the high cost often restricted the potential of asymmetric organocatalysis for industrial and sustainable applications. The organocatalyst recycling maybe an effective strategy to overcome these shortcomings. As there are 1D periodic channels in the 2D COFs, the 2D CCOFs not only could facilitate the transport, but also could provide efficient access to active sites. So the 2D CCOFs often could give a high activity and selectivity in asymmetric catalytic reactions. The CCOFs may be a highly sustainable, efficient and reusable catalysts. So we studied the catalytic performances of TPB2-COF and Tfp2-COF. Asymmetric Steglich rearrangement is one of a few ways for producing the quaternary stereocenter, the products of the asymmetric Steglich rearrangement process represent useful building blocks for synthetic organic chemistry.49 Zhang’s chiral DHIP catalysts49 served as a efficient catalyst for asymmetric Steglich rearrangement. However, Zhang’s chiral DHIP catalysts still has some limitations for sustainable and industrial applications, such as the high catalyst loadings and expensive reagents. The DHIP based heterogeneous catalysts might solve these problems, as it can be easily recycled. The TPB2-COF maybe an effective and sustainable heterogeneous catalyst for asymmetric steglich rearrangement. As shown in Table 1, the TPB2-COF could catalyze the asymmetric Steglich rearrangement. At 10 mol% loading, the Steglich rearrangement reaction proceeded cleanly and smoothly with conversion and enantioselectivity of 85% and 80%, respectively. We investigated the scope of the reactions utilizing different reactants to illustrate the catalyst’s generality. We change the group of R1 or Ar for the asymmetric Steglich rearrangement. 3

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ACS Sustainable Chemistry & Engineering Without the electron-donating −OMe group, the substrates also could asymmetrically rearrange to corresponding products at 0 °C with good conversion and enantioselectivity. However, when R1 is the Bn, the reaction with good conversion but lower enantioselectivity. The reason may be the larger stereohindrance of the substrates. For comparison, a model catalyst, TB1 was synthesized as the homogeneous counterpart of TBP2-COF. Then we investigated its catalytic activity under the same conditions. The result indicates that TBP2-COF (Table1, entry 5) showed comparable enantioselectivities to TB1 (Table1, entry 6). To the best of our knowledge, the use of heterogeneous catalysts derived from DHIP for asymmetric to catalyze steglich rearrangement is the first reports until now.

reactions of cyclohexanone with β-nitrostyrene, 4-chloro-β-nitrostyrene and 3-chloro-β-nitrostyrene proceeding very smoothly, cleanly and giving the products in 86, 86 and 85% ee, 92, 95 and 93% yield, and 17:1, 17:1 and 17:1 anti/syn ratio, respectively. Therefor the Tfp2-COF have significantly catalytic activity. Again, a model catalyst (TB2) as the homogeneous counterpart of the Tfp2-COF was prepared to catalyze Michael addition under the same reaction conditions. The result could indicate that the Tfp2-COF showed comparable enantioselectivities and higher distereoselectivities to the homogeneous control. Table 2. Asymmetric Michael addition reactions catalyzed by Tfp2-COF and related catalysts a

Table 1. Asymmetric Steglich rearrangement catalyzed by TPB2-COF and related catalysts a O BnO

O

R

N O

O

10 mol% Cat.

Ar

acetone, 0 oC

N

Ar

O

10 mol% Cat. CH3COOH

O R

OBn R

+

NO2

O

entrya

ee (%)c

catalyst

ee (%)d

92

17:1

86

95

17:1

86

96

11:1

85

4-Cl

95

13:1

83

3-Cl

93

17:1

85

Ar

R

yield (%)b

1

Tfp2-COF

H

1

TPB2-COF

4-MeOPh

Me

85

80

2

Tfp2-COF

4-Cl

2

TPB2-COF

4-MeOPh

CH2CH2SCH3

88

83

3

TB2

4-Cl

3

TPB2-COF

4-MeOPh

Bn

93

61

4

Amorphous 2

4

TPB2-COF

Ph

Me

83

81

5

Tfp2-COF

5

TPB2-COF

Ph

CH2CH2SCH3

86

84

6

TB1

Ph

CH2CH2SCH3

87

81

7

Amorphous 1

Ph

CH2CH2SCH3

86

80

8

TPB2-COF

Ph

Bn

95

61

Conditions: 0.1 M substrates, 10 mol % Tfp2-COF, 10 mol % CH3COOH, 1.0 mL of solvent at r.t. for 12 h. b Isolated yield, c d.r. value were calculated from 1H NMR. d ee value were determined by HPLC analysis

Reaction conditions: 0.1 M substrates, 10 mol % TBP2-COF, 1.0 mL acetone at 0 °C for 4 days. bIsolated yield. c ee are determined by chiral HPLC.

Multiple experiments proved that TPB2-COF and Tfp2-COF are recyclable and heterogeneous catalysts. First, the CCOFs catalyst (10 mol% loading) could be easily recycled and reused by using the centrifugation method. As shown in Figure 3, the TPB2-COF and Tfp2-COF catalysts could retaine their activity, enantioselectivity and diastereoselectivity (>81% yield and 83%, 82%, 81%, 81%, and 80% ee for TPB2-COF and >90% yield, 81-86% ee and 13:1-17:1 d.r. for Tfp2-COF, respectively). Second, multiple experiments revealed that the CCOFs catalyst after five cycles retained their porosity and crystallinity (Figure 3). Third, after removing the CCOFs catalyst from the reaction system, the yield of the reaction did not increase, indicating COFs catalyst exhibited a characteristic heterogeneous nature.

The Michael addition is a elegant and atom-economic organocatalytic reaction which is one of the most versatile C–C bond formation reactions. The Michael addition also provides a powerful synthetic tool for the formation of many useful natural and biologically active products.50 As we all know, pyrrolidine derivatives are a good organocatalysts for Michael addition reactions.51 We constructed chiral pyrrolidine functional Tfp2-COF to overcome the drawbacks such as the high catalyst loading and laborious separation processes. Then we studied the catalytic performances of Tfp2-COF. As shown in Table 2, A 10 mol % loading of Tfp2-COF catalyzed the Michael addition

c)

e)

TPB2-COF

80

Adsorption Desorption

200

N2 uptake (cm3 g-1)

Yield and ee value (%)

Yield ee

TPB2-COF

yield (%)b

a

a

100

R

d.r.c

R

catalyst

a)

NO2

O

EtOH/H2O, r.t.

150

Pristine

60

100

40 20

TPB2-COF after 5 runs reaction

50

After 5 runs reaction 0

0

1

2

3

4

5

3

Catalytic cycles

b) 100

Yield ee d.r.

Tfp2-COF

6

9

12 15 18 21 2θ (degree)

24

27

0.0

30

d)

f) Tfp2-COF

80

Pristine

60

250

N2 uptake (cm3 g-1)

entrya

Yield ,ee and d.r. value (%)

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 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

0.2 0.4 0.6 0.8 Relative Pressure (P/P0)

1.0

Adsorption Desorption

200 150 100

Tfp2-COF after 5 runs reaction

50

After 5 runs reaction

20

0 0

1

2

3

4

Catalytic cycles

5

3

6

9

12 15 18 21 2θ (degree)

24

27

30

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0.0

0.2

0.4

0.6

0.8

Relative Pressure (P/P0)

1.0

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ACS Sustainable Chemistry & Engineering Figure 3. Recycling tests of the (a) TPB2-COF and (b) Tfp2-COF, respectively; PXRD patterns of the (c) TPB2-COF and (d) Tfp2-COF after 5 runs reaction, respectively; N2 sorption curves of the (e) TPB2-COF and (f) Tfp2-COF after 5 runs reaction, respectively.

In all cases, the porosity and crystallinity of heterogeneous catalysts play a very important role in determining their catalytic activities and selectivities. For comparison, amorphous polymers 1 and 2 were prepared, which were analogues to TPB2-COF and Tfp2-COF, respectively, needing a long time to accomplish (Table 1, entry 7; Table 2, entry 4). In particularly, Amorphous polymers 2 exhibited a lower distereoselectivities. The amorphous polymers can drastically decrease the catalytic activity and selectivities. The reason may be the immobilized active sites distributed uneven in the amorphous polymers catalyst and only partial catalytic sites effectively for the reactions.

CONCLUSIONS A green and sustainable preparation strategy is critical for the industrial applications of heterogeneous catalysts. Here we have direct constructed two chiral functional COFs from chiral organic building blocks by solvothermal method. This strategy completely avoided the using of noble metal catalysts and tedious lab work. The CCOFs which termed TPB2-COF and Tfp2-COF are crystalline metal-free catalysts display activity, enantioselectivity and diastereoselectivity for Asymmetric Steglich rearrangement and Michael addition reactions, respectively. In particularly, the use of solid catalysts derived from DHIP for asymmetric steglich rearrangement was first repotted until now. The TPB2-COF and Tfp2-COF catalyst could be very easily to recover and reuse at least for 5 times without obvious loss of enantioselectivity and diastereoselectivity. We believe that TPB2-COF and Tfp2-COF may act as an efficient, reusable, green, lower cost and sustainable heterogeneous asymmetric organocatalyst to catalyze a variety of asymmetric organic reactions. In particularly, our strategy could use to synthetize a series of functional COFs for high value-added applications and significantly promote their sustainable applications.

ASSOCIATED CONTENT Supporting Information. Detailed synthetic procedures, experimental procedures and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] ORCID Yan Liu: 0000-0002-7560-519X Yong Cui: 0000-0003-1977-0470 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (Grants 21431004, 21620102001, 21875136 and 91856204), the National Key Basic Research Program of China (Grant 2016YFA0203400), Key Project of Basic Research of Shanghai (17JC1403100 and 18JC1413200), and the Shanghai “Eastern Scholar” Program.

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Synopsis: Chiral Covalent Organic Frameworks could act as efficient, reusable, green, lower cost and sustainable heterogeneous asymmetric organocatalysts applicable to a variety of asymmetric organocatalyzed reactions.

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