Multivariate Chiral Covalent Organic Frameworks ... - ACS Publications

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Multivariate Chiral Covalent Organic Frameworks with Controlled Crystallinity and Stability for Asymmetric Catalysis Jie Zhang,† Xing Han,† Xiaowei Wu,† Yan Liu,*,† and Yong Cui*,†,‡ †

State Key Laboratory of Metal Matrix Composites, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ‡ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China S Supporting Information *

ABSTRACT: The modular construction of covalent organic frameworks (COFs) provides a convenient platform for designing high-performance functional materials, but the synthetic control over their chirality has been relatively barely studied. Here we report a multivariate strategy to prepare chiral COFs (CCOFs) with controlled crystallinity and stability for asymmetric catalysis. By crystallizing mixtures of triamines with and without chiral organocatalysts and with a dialdehyde, a family of two- and three-component 2D porous CCOFs that adopt two different stacking modes is prepared. The organocatalysts are periodically appended on the channel walls, and their contents, which can be synthetically tuned using a three-component condensation system, greatly affect the chemical stability and crystallinity of CCOFs. Specially, the ternary CCOFs displayed relatively high crystallinity and stability compared with the binary CCOFs. Under harsh conditions, the ternary CCOFs can serve as efficient heterogeneous catalysts for an asymmetric aminooxylation reaction, an aldol reaction, and the Diels−Alder reaction, with the stereoselectivity and diastereoselectivity rivaling or surpassing the homogeneous analogues. This work not only opens up a new synthetic route toward CCOFs, but also provides tunable control of COF crystallintity and stability and, in turn, the properties.

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INTRODUCTION The search for new heterogeneous catalysts is of significant importance because a wide range of interesting organic reactions require the use of such species.1 Heterogeneous catalysts offer obvious advantages regarding their reuse and recyclability, but they typically exhibit lower activity and selectivity than their homogeneous counterparts.2 Porous solids are used to enhance the number of active sites accessible to the substrates.2 Among the recognized porous materials, covalent organic frameworks (COFs) are a fascinating class of crystalline porous polymers3−5with significant prospects for addressing current energy and environmental challenges, including gas storage,6 separations,7 catalysis,8,9 smart sensors,5b,10 optoelectronics,11 and energy storage.12 In particular, COFs have shown great promise as heterogeneous catalysts because of their diverse catalysis-friendly features such as high permanent porosity, well-defined channels, and chemical and composition tunability.2,8,9 Rational choices of building blocks may lead to chiral COFs (CCOFs) with uniform active sites that hold great potential in asymmetric catalysis, separation, and fundamental aspects of chirality.7d,9a,d However, by far, there are only three reports that reported the direct synthesis of CCOFs with uniform active sites for asymmetric catalysis and separation by using organic linkers derived from L-tartaric acid and Spyrrolidine.7d,9a,d Postmodification of achiral COFs with Spyrrolidine has also been exploited for preparing single-site © 2017 American Chemical Society

solid catalysts, but it results in an uneven distribution of catalytic sites.9b,c Obviously, the synthesis of CCOFs for enantioselective processes is still complicated because it has to simultaneously achieve the balance and conflict of asymmetry and crystallinity.7d,9a,d Among various types of 2D COFs, the imine-linked COFs are stable in a broad range of solvents and aqueous solutions over a wide pH range, thereby providing distinctive nanoreactors to perform catalysis under harsh conditions.9c,13 However, attaching bulky functional groups onto the pore walls generally lead to an obvious decrease in the COFs’ crystallinity and chemical stability, limiting their use in practical processes.14 In this work, we showed that this drawback can be overcome by a multivariate strategy that offers construction of a family of asymmetric organocatalyst-containing 2D-CCOFs that adopt two different stacking structures with controlled crystallinity and stability. Under harsh conditions, the CCOFs can be used as efficient heterogeneous asymmetric catalysts for an aminooxylation reaction, an aldol reaction, and the Diels− Alder (DA) reaction, with stereoselectivity and diastereoselectivity rivaling or even surpassing the homogeneous counterparts. It should be noted that the multivariate approach has Received: April 4, 2017 Published: May 24, 2017 8277

DOI: 10.1021/jacs.7b03352 J. Am. Chem. Soc. 2017, 139, 8277−8285

Article

Journal of the American Chemical Society Scheme 1. Synthesis of the CCOFs

afforded amorphous DMTA−TPB2′-4′. The four ternary CCOFs DMTA−TPB1/2−1/5 were prepared by co-condensation of TPB1 (0.05 mol), TPBn (0.05 mmol), and DMTA (0.15 mmol) in a mixed solvent of DCB and n-BuOH or 1,4dioxane and mesitylene in the presence of 9 M acetic acid at 100 °C for 3 days. Similarly, the ternary CCOFs were converted to CCOFs DMTA−TPB1/2′-5′ by treating with TMSOTf or LiOH (aq) and TMSOTf. In all cases, the asprepared powders were collected by centrifugation or filtration and washed with DCM, water, THF, and ethanol. They were then dried under vacuum at 100 °C to give a yellow-colored powder. The synthetic reactions are performed under mild conditions and are easily reproducible. However, attempts to directly crystallize the binary and ternary CCOFs from the TPBn′ monomers with unprotected proline or imidazolidine using solvothermal synthesis failed, and in each case, only amorphous powder was obtained. All seven CCOFs were characterized by Fourier transform infrared spectroscopy (FTIR). The spectra of these CCOFs show the nearly complete disappearance of the characteristic aldehyde and amino stretching bands of the starting materials. The strong stretching vibration bands at 1617, 1617, 1616, 1615, 1617, 1616, and 1617 cm−1 were detected for DMTA− TPB2−4 and DMTA−TPB1/2′-1/5′, respectively, suggesting

been used to expand structural complexity and advance the electronic functions of COFs.15

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RESULTS AND DISCUSSION Synthesis and Characterization. As shown in Scheme 1, we used four L-proline- and L-imidazolidine-based TPB derivatives as chiral knots and DMTA as linkers for the preparation of three binary and four ternary CCOFs. The C3symmetric 1,3,5-tris(4-aminophenyl) benzene (TPB1) was chosen as the basic backbone because the rigid triaryl benzene group is ideal for making COFs with large open channels.16 The highly crystalline 2D-COF DMTA−TPB1 has been reported previously and displays outstanding chemical robustness.9c,16 On the basis of this backbone, the proline and imidazolidine groups were attached at the central aromatic ring to afford the chiral scaffolds TPBn and TPBn′ (n = 2−5, Scheme 1). The three binary CCOFs DMTA−TPB2−4 were prepared by condensation of TPBn (0.06 mmol) and 2,5dimethoxyterephthalaldehyde (DMTA) (0.09 mmol) in a mixed solvent of DMA and n-BuOH in the presence of a 9 M acetic acid catalyst at 100 °C for 3 days. However, we failed to crystallize DMTA−TPB5 following the similar procedure. Treating the binary CCOFs with TMSOTf or LiOH (aq) and TMSOTf led to the removal of the Boc and Me groups, but it 8278


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Journal of the American Chemical Society Table 1. Composition and Structure of the CCOFs CCOFs

a

DMTA/TPB1/TPBna stacking mode

DMTA−TPB2

3:0:2

AA

DMTA−TPB3

3:0:2

AA

DMTA−TPB4

3:0:2

ABC

DMTA−TPB1/2′

3:1.03:1

AA

DMTA−TPB1/3′

3:0.99:1

AA

DMTA−TPB1/4′

3:1.01:1

AA

DMTA−TPB1/5′

3:0.99:1

AA

Rwp, Rp

unit-cell parameters P1 a = 37.3008 Å, b = 37.3052 Å, c = 3.5300 β = 89.99°, γ = 120.02° P1 a = 37.7150 Å, b = 37.6910 Å, c = 3.5500 β = 89.98°, γ = 120.01° P1 a = 37.1718 Å, b = 37.1602 Å, c = 6.7131 β = 89.27°, γ = 120.02° P1 a = 37.6199 Å, b = 37.6063 Å, c = 3.5100 β = 89.94°, γ = 120.05° P1 a = 37.3311 Å, b = 37.3087 Å, c = 3.5200 β = 89.99°, γ = 119.96° P1 a = 37.2093 Å, b = 37.2016 Å, c = 3.5400 β = 89.98°, γ = 120.05° P1 a = 37.5012 Å, b = 37.4998 Å, c = 3.5400 β = 90.06°, γ = 120.01°

Å, α = 90.01°,

Rwp = 2.69%, Rp = 5.1%

Å, α = 90.01°,

Rwp = 2.75%, Rp = 6.5%

Å, α = 90.12°,

Rwp = 2.71%, Rp = 5.3%

Å, α = 90.01°,

Rwp = 3.12%, Rp = 6.3%

Å, α = 90.04°,

Rwp = 3.35%, Rp = 5.53%

Å, α = 90.06°,

Rwp = 3.17%, Rp = 5.73%

Å, α = 90.01°,

Rwp = 3.07%, Rp = 5.53%

The ratios of different monomers were determined by 1H NMR analysis of the digested CCOFs

Figure 1. PXRD patterns of the CCOFs (a) DMTA−TPB2, (b) DMTA−TPB4, and (c) DMTA−TPB1/2′ with the experimental profiles in black, Pawley-refined profiles in red, calculated profiles in blue, and the differences between the experimental and refined PXRD patterns in dark green (Inset: views from the b-axis). Views of space-filling models of (a) DMTA−TPB2, (b) DMTA−TPB4, and (c) DMTA- TPB1/2′ are along the c-axis.

∼157 ppm for all of these CCOFs. The aldehyde carbon peaks were barely observed. The chemical shifts of other fragments,

the formation of a CN linkage. The 13C CP-MAS NMR spectra showed the characteristic signal for the CN group at 8279


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Journal of the American Chemical Society

cell parameters for DMTA−TPB1/2′-1/5′ were given in Table 1. These modeled structures possess a 1D open channel of 27.6, 26.8, 26.5, and 26.3 Å with the interlayer separations of 3.51, 3.52, 3.54, and 3.54 Å, respectively. In the PXRD patterns, peaks at 25.35°, 25.28°, 25.14°, and 25.14°, correlating to the values of the interlayer distances were observed. The porosity of the CCOFs was studied by measuring N2 adsorption−desorption at 77 K on the activated samples. The adsorption curves of DMTA−TPB2, DMTA−TPB3, and DMTA−TPB1/2′-1/5′ exhibited the classic type IV isotherm (Figure 2), which is characteristic of mesoporous materials.

including chiral groups, are in good agreement with those of the monomers. In addition, no resonance peaks of the protecting groups were observed, which is indicative of the successful removal of the protecting groups (Figure S16). Thermal gravimetric analysis (TGA) revealed these CCOFs to be highly thermostable. In particular, significant weight loss was observed for DMTA−TPB2−4 and DMTA−TPB1/2−1/5 at ∼250 °C, as a consequence of the expulsion of the Boc group. No significant weight loss was observed for DMTA−TPB1/2′-1/5′ at ∼250 °C, which is indicative of the successful removal of the Boc groups. Scanning electron microscopy (SEM) showed that these CCOFs have a rod-like shape or irregular morphology. In addition, the transmission electron microscopy (TEM) images showed long-ordered channels in DMTA−TPB1/n (n = 2−5). Circular dichroism (CD) spectra of these CCOFs made from (R)- and (S)-enantiomers of the monomers are mirror images of each other, which is indicative of their enantiomeric nature (Figure S11). To quantitatively determine the ratios of the two linkers in the DMTA−TPB1/n (n = 2−5), we hydrolyzed the DMTA− TPB1/n (n = 2−5) samples and measured their 1H NMR spectra. Resonances with the predicted coupling patterns were observed in the expected regions for each of the linkers’ unique protons. By integrating the resonance peak intensities, the TPB1 and TPBn linkers were present in ratios of approximately 1:1 for DMTA−TPB1/n (Table 1, Table S1 and Figure S15). These proton integrations quantitatively confirmed the lattice components of DMTA−TPB1/n. Crystal Structure. The crystalline structures of the CCOFs were determined by powder X-ray diffraction (PXRD) analysis with Cu Kα radiation. DMTA−TPB2 exhibited strong PXRD peaks at 2.78°, 4.90°, 5.64°, and 25.21°, corresponding to the (100), (110), (200), and (001) facets, respectively. Similar PXRD patterns were observed for the DMTA−TPB3, indicating that they have a similar crystal structure (Figure 1). The experimental PXRD patterns were consistent with the simulated patterns for the most-stable slipped AA stacking mode. The refinement results yielded unit-cell parameters for DMTA−TPB2 and DMTA−TPB3 (Table 1). Such grids stack along the c axis leading to 1D open channels of 25.1 and 23.3 Å for them, respectively, with interlayer separations of 3.53 and 3.55 Å. In the PXRD patterns, the presence of the (001) facet at 25.21° for DMTA−TPB2 and 25.06° for DMTA−TPB3 corresponds to a interlayer distance of 3.53 and 3.55 Å, respectively, indicating that the structural ordering extends along the stacking direction perpendicular to the 2D layers. DMTA−TPB4 exhibited five peaks at 4.82°, 9.65°, 12.27°, 12.94°, and 14.48° corresponding to the (110), (220), (230), (001), and (021) facets, respectively. The slipped ABC stacking mode was the most stable structure. The afforded unit-cell parameters for DMTA−TPB4 were listed in Table 1. The modeled structure has 1D open channels of 9.6 Å, with the interlayer separation of 3.35 Å. In the PXRD pattern, the peak at 12.94° correlating to the value of the interlayer distance of 3.35 Å was observed. The ABC stacking mode of this CCOF led to small pores covered by adjacent layers (Figure 1b). The family of CCOFs DMTA−TPB1/2−1/5 and DMTA− TPB1/2′-1/5′ exhibited a similar PXRD pattern, indicative of their similar crystal structures. Typically, DMTA−TPB1/2′ gave six peaks at 2.79°, 4.88°, 5.63°, 7.43°, 9.74°, and 25.35°, corresponding to the (100), (110), (200), (210), (220), and (001) facets, respectively. The Pawley-refined PXRD profiles matched the experimental patterns very well. The yielded unit-

Figure 2. (a) N2 sorption isotherms (77 K) and (b) pore-size distribution profiles of DMTA−TPB2 (purple), DMTA−TPB3 (magenta), DMTA−TPB4 (olive), DMTA−TPB1/2′ (wine), DMTA−TPB1/3′ (blue), DMTA−TPB1/4′ (black), DMTA− TPB1/5′ (red).

Their Brunauer−Emmett−Teller (BET) surface areas were found to be 1218, 662, 1947, 1689, 1772, and 1903 m2 g−1, respectively, and the total pore volumes were calculated to be 0.73, 0.49, 1.44, 1.47, 1.53, and 1.41 cm3 g−1 at P/P0 = 0.99. The nonlocal density functional theory (NLDFT) gave rise to a narrow pore-size distribution with an average pore width of 2.5, 2.3, 2.7, 2.7, 2.7, and 2.7 nm for them, corresponding to their simulated structures, respectively. In contrast, DMTA−TPB4 exhibited the type I isotherm, which is characteristic of microporous materials. The BET surface area was 118 m2 g−1, and its total pore volume was 0.44 cm3 g−1 at P/P0 = 0.99, with an average pore width of 1.35 nm, nearly corresponding to the ABC stacking structure. Compared with DMTA−TPB2 and DMTA−TPB3, DMTA-TBP4 gave a decreased BET surface area, consistent with the introduction of bulky chiral groups, the different stacking models, and less crystallinity.8c Crystallinity and Chemical Stability. The ternary CCOFs displayed enhanced chemical stability and crystallinity compared with the binary solids, as shown in Figure 3. After removal of the protected groups, the ternary DMTA−TPB1/n′ 8280


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Figure 3. PXRD patterns of (a) DMTA−TPB2, (b) DMTA−TPB4, (c) DMTA−TPB1/2′, (d) DMTA−TPB10.75/2′0.25, and (e) DMTA−TPB10.25/ 2′0.75 upon treatment in different solvents for 1 week. (f) N2 sorption curves of the DMTA−TPB1/2′ upon treatment in different solvents.

that best reflect the quality of the COF crystals were used for comparison. As shown in Figure 4, the crystallinity of the family

retained high crystallinity, but the binary DMTA−TPBn lost its crystallinity and became amorphous, as revealed by PXRD and N2 adsorption. After a one-week treatment in boiling water, 2 M HCl (aq) and 6 M NaOH (aq), the ternary DMTA−TPB1/ n and DMTA−TPB1/n′ still displayed high crystallinity. For example, the as-treated samples of DMTA−TPB1/2′ have almost unchanged BET surface areas of 1943, 1588, and 1828 m2 g−1, respectively, further confirming the framework stability and permanent porosity. In contrast, DMTA−TPB2 nearly lost its crystallinity and became amorphous after similar treatments, even in 1 M HCl (aq) and 3 M NaOH (aq). It is worth noting that all seven CCOFs are stable in common organic solvents and water at r.t. (Figure S9). To further study the influence of multiple components on the chemical stability and crystallinity, we prepared the ternary CCOFs DMTA−TPB10.75/20.25 and DMTA−TPB10.25/20.75 by varying the molar ratios of TPB1 to TPB2 under solvothermal synthetic conditions. PXRD and N2 adsorption indicated that both of them were isostructural with DMTA−TPB1/2. After removal of the protected groups of organocatalysts, isostructural CCOFs DMTA−TPB10.75/2′0.25 and DMTA−TPB10.25/ 2′0.75 were obtained. DMTA−TPB10.75/2′0.25 displayed a similar chemical stability to DMTA−TPB19c and DMTA−TPB1/2 (Figure 3d). In contrast, DMTA−TPB10.25/2′0.75 exhibited a low chemical stability. This is indicated by the crystalline sample, which was only stable in boiling water and 3 M MaOH (aq) and not stable in 2 M HCl (aq) (Figure 3e). Taken together, the above results revealed the three-component CCOFs DMTA−TPB11−x/2x possessed a higher chemical stability than the two-component CCOF DMTA−TPB2. Tuning of the chiral TPBn content can also lead to a systematic change in the crystallinity, as indicated by PXRD. It should be noted that the PXRD patterns were used strictly for a qualitative comparison, and attempts were made to employ the same sample morphology, thickness, and other conditions. To clarify this point, the patterns were normalized at the 100 facet. The full-width at half-maximum (fwhm) values of the 100 facet

Figure 4. (a) PXRD patterns of the COFs DMTA−TPB11−x/2x. (b) Normalized 100 facets (the colors represent the same COFs in two cases).

of COFs DMTA−TPB11‑n/2n increased as the concentration of TPB2 was decreased, as revealed by the decreased fwhm values of the 100 facets, which were 0.462°, 0.434°, 0.398°, 0.335°, and 0.279° for DMTA−TPB2, DMTA−TPB1 0.25 /2 0.75 , DMTA−TPB1/2, DMTA−TPB1 0.75 /2 0.25 , and DMTA− TPB1, respectively. 8281


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reactivity and enantioselectivity than the latter. In contrast, DMTA−TPB1/2′ could not promote the reactions under identical conditions. Typically, at 10 mol % loading, the CCOF catalyzed the addition of nitrosobenzene to propanal to give the α-aminooxylation product in a 76% isolated yield and 94% ee in DMF at 23 °C for 24 h. In this reaction, no undesired αhydroxyamino aldehyde byproduct was observed. Other linear chain aldehydes such as n-butanal, i-pentanal, and 3-phenylpropanal also reacted with nitrosobenzene smoothly, affording α-aminooxy aldehydes in 75−77% isolated yields with up to a 95% ee (Table 2, entries 4, 6, and 8). For comparison, a model catalyst, PTOP was prepared as the homogeneous counterpart of the CCOF to catalyze aminooxylation of aldehydes under the same conditions. The result indicates that the DMTA−TPB 1/3′ showed comparable enantioselectivities to the homogeneous control (entries 4 vs 5 and 6 vs 7). To the best of our knowledge, there are no reports on the use of solid catalysts derived from proline for aminooxylation of aldehydes until now.21 Multiple experiments suggested that DMTA−TPB1/3′ is a heterogeneous and reusable catalyst. First, the COF catalyst (20 mol % loading) could be readily recycled and reused for at least five times without an obvious loss of activity and enantioselectivity (>68% yield and 94%, 94%, 93%, 91%, and 90% ee for 1−5 runs). Second, the crystallinity of the solid catalyst recovered after five runs was still maintained as the PXRD of the recovered catalyst remained the same as the freshly prepared COF. Third, the progress of the reaction was stopped by removing the CCOF catalyst from the reaction mixture, suggesting that the supernatant is inactive in catalyzing the α-aminooxylation reactions. Fourth, the porosity of the recovered catalyst after five runs was maintained (BET surface area = 1466 m2 g−1, Figure S14q). DMTA−TPB1/2′-1/4′ can also catalyze the aldol reaction, which is one of the most powerful carbon−carbon-bondforming reactions in organic synthesis.19 As shown in Table 3, DMTA−TPB1/4′ displayed higher reactivity, diastereo-, and enantioselectivity than the other two CCOFs. Trifluoroacetic acid was added to the reaction mixture in order to protonate proline to generate the actual catalytic species.23 A 30 mol % loading of DMTA−TPB1/4′ catalyzed the reactions of cyclohexanone with 4-nitrobenzaldehyde and 3-nitrobenzaldehyde, proceeding smoothly and affording the products in 92 and 86% ee, 95 and 94% yield, and 90:10 and 90:10 anti/syn ratio, respectively. Again, the CCOF catalyst (entries 3 and 5) displayed comparable enantio- and distereoselectivities to the homogeneous analogue PTMP (entries 4 and 6). Several tests also demonstrated the heterogeneous nature and the ability to reuse this solid catalyst in asymmetric aldol reactions (>87% yield and 91%, 90%, 90%, 89%, and 89% ee for 1−5 runs, respectively). After five cycles, the CCOF exhibited a PXRD pattern and BET surface area (1564 m2 g−1, Figure S14r), similar to the pristine sample, suggesting the robustness of the framework. The imidazolidinone-containing DMTA−TPB1/5′ could catalyze the asymmetric Diels−Alder cycloaddition reaction, which is one of the most powerful methods for the synthesis of cyclic molecules.22 Specifically, in the presence of CF3CO2H as the cocatalyst, 20 mol % loading of this CCOF promoted the reaction of cyclopentadiene with (E)-cinnamaldehyde to afford the cycloadduct in an 83% isolated yield and excellent selectivity (13:1 endo/exo, 90% ee for the endo isomer). Variation in the steric contribution of the olefin substituent (R

The low chemical stability and crystallinity of the binary CCOFs DMTA−TPB2−5 might be caused by the steric hindrance of the high number of organocatalysts protected by bulky groups, which are capable of affecting interlayer π−π interactions.9c,17 The amorphous nature of DMTA−TPB2′-5′ might be caused by the strong supramolecular interaction ability of a large number of the deprotected organocatalysts, which significantly weakened the interlayered interactions by interacting with the methoxy and/or imine groups of the 2D layers.18 When the third building block (TPB1) was introduced, the number of organocatalysts protected by bulky groups decreased, and the interlayer repulsion was weakened. Moreover, in the DMTA building blocks, the two lone pairs from the oxygen atoms softens the interlayer charge repulsion and stabilizes the COF, aiding in its crystallization.8c Thus, it is likely that the chemical stability and crystallinity of the CCOFs can be controlled by carefully tuning the density of the chiral functionalities appended on the channel walls. Further studies on preparing more related CCOFs and understanding their crystallization behaviors and crystal structures are underway. Asymmetric Catalysis. Pyrrolidine and imidazolidine (the MacMillan catalysts) derivatives have been found to be excellent asymmetric organocatalysts for a variety of important organic transformations, such as pyrrolidine for asymmetric aldol reactions,19 Mannich reactions,20 aminooxylation reactions,21 and imidazolidine for Diels−Alder reactions.22 The straight crystalline channels within the 2D-CCOFs could provide efficient access to uniformly distributed organocatalyst sites and facilitate the transport of reactants and products, which may give rise to high activity and enantioselectivity in catalytic reactions. Considering that the three-component CCOFs are more stable under acidic conditions, we selected DMTA−TPB1/2′-1/5′ to study their catalytic performances. α-Aminooxylation reactions offer direct access to optically active α-hydroxy carbonyl moieties that are commonly found in a diverse array of important natural products.21 As shown in Table 2, both DMTA−TPB1/3′, and DMTA−TPB 1/4′ were capable of catalyzing α-aminooxylation reactions between aldehydes and nitrosobenzene, and the former gave a higher Table 2. Asymmetric α-Αminooxylation of Aldehydes Catalyzed by CCOFs and Related Catalystsa

entrya

catalyst

R

yield (%)b

ee (%)c

1 2 3 4 5 6 7 8 9

DMTA−TPB1/2′ DMTA−TPB1/4′ DMTA−TPB1/3′ DMTA−TPB1/3′ PTOPd DMTA−TPB1/3′ PTOPd DMTA−TPB1/3′ amorphous1/3′

iPr iPr iPr Me Me Et Et CH2Ph Et

trace 73 75 76 77 77 80 75 62

n.d. 75 94 94 94 95 98 81 85

a

Reaction conditions: aldehyde (0.3 mmol), nitrosobenzene (0.1 mmol), CCOFs (0.01 mmol), and DMF (1.0 mL) at r.t. for 24 h. b Isolated yield. cDetermined by chiral HPLC. dCatalyzed by 10 mol % (2S,4S)-4-((5′-phenyl-[1,1′:3′,1″-terphenyl]-4′-yl)oxy)pyrrolidine-2carboxylic acid [PTOP, Section 2.1 in Supporting Information (SI)], which was synthesized as the homogeneous counterpart of DMTA− TPB1/3′. 8282


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Journal of the American Chemical Society Table 3. Asymmetric Aldol Reactions Catalyzed by CCOFs and Related Catalystsa

entrya

catalyst

R

yield (%)b

anti/sync

ee (%)d

1 2 3 4 5 6 7 8 9 10 11 12 13

DMTA−TPB1/2′ DMTA−TPB1/3′ DMTA−TPB1/4′ PTMPe DMTA−TPB1/4′ PTMPe HHBs-B8T2-Prof Zn-MOF-Prog CMIL-1-Proh Cd-MOF-Proi UIO-68-Proj Cd-MOF-Proi amorphous1/4′

4-NO2 4-NO2 4-NO2 4-NO2 3-NO2 3-NO2 4-NO2 4-NO2 4-NO2 4-NO2 4-NO2 3-NO2 4-NO2

85 90 95 97 94 93 98 100 81 97 97 77 78

1:1 1:1 9:1 9:1 9:1 9:1 9:1 3:1 4:1 1:1 1:7 1:1 9:1

68 82 92 88 86 87 96 14 66 58 76 61 87

Table 4. Asymmetric Diels−Alder Reactions Catalyzed by CCOFs and Related Catalystsa

entrya

catalyst

Ar

yield (%)b

endo/ exoc,d

major isomer ee (%)c,d

1

DMTA−TBP 1/5′ PTMIe DMTA−TBP 1/5′ DMTA−TBP 1/5′ PTMIe

Ph

83

13:1

90

Ph 4-BrPh

83c,d 82

1.4:1 1:5

90 95

4MeOPh 4MeOPh 4-NO2Ph

76

1:7

95

75

1:1.6

97

85

1:22

90

4-NO2Ph Ph Ph 4-BrPh 4-NO2Ph 4-NO2Ph

89 94 58 84 83 73

1:3 1:1.1 1:1.1 1:1 1:1.2 1:3

91 86 89 90 92 88

2 3 4 5 6 7 8 9 10 11 12

a

Reaction conditions: aldehyde (0.10 mmol), ketone (0.2 mL), CCOFs (0.03 mmol), TFA (0.03 mmol), H2O (0.2 mL), and DMF (0.6 mL) at r.t. for 4 days. bIsolated yield. cDetermined by 1H NMR. d Determined by chiral HPLC. eCatalyzed by 30 mol % (2S,4R)-4-((5′phenyl-[1,1′:3′,1″-terphenyl]-4′-yl)methoxy)pyrrolidine-2-carboxylic acid (PTMP, Section 2.1 in SI), which was synthesized as the homogeneous counterpart of DMTA−TPB1/4′. f-jData were taken from references 25a−25e, respectively.

DMTA−TBP 1/5′ PTMIe MSNs-Mace PMO-Mace MSNs-Mace MSNs-Mace amorphous 1/5′

a

Reaction conditions: aldehyde (0.10 mmol), cyclopentadiene (0.50 mmol), CCOFs (0.02 mmol), TFA (0.02 mmol), and MeOH/H2O (95:5) (1.0 mL) at r.t. for 24−48 h. bIsolated yield. c,dDetermined by chiral HPLC. eCatalyzed by 20 mol % (2S,5S)-2-(tert-butyl)-3-methyl5-((5′-phenyl -[1,1′:3′,1″-ter phenyl]-4′-yl)methyl)imidazolidin-4-one (PTMI, Section 2.1 in SI), which was synthesized as the homogeneous counterpart of DMTA−TPB1/5′. f,gData were taken from references 26a and 26b, respectively.

= p-BrPh, p-MePh, p-NO2Ph, entries 3, 4, and 6) is possible without a loss in yield and stereoselectivity (76−85% yield, 90− 95% ee for the major exo isomer, 5:1−22:1 exo/endo). The CCOF catalyst afforded the cycloaddition products in 4−7 times greater distereoselectivites and in comparable ee’s to the homogeneous catalyst PTMI for the examined substrates (Table 4, entries 1 vs 2, 4 vs 5, and 6 vs 7). We have also demonstrated the heterogeneous nature and recyclability of the CCOF catalyst (>75% yield and 88%, 86%, 86%, 85%, and 82% exo ee for 1−5 runs, respectively). After five cycles, the recovered catalysts retained high crystallinity and porosity (BET surface area = 1543 m2 g−1, Figure S14s). The crystallinity and porosity of CCOFs play a vital role in determining their catalytic activities and selectivities. Amorphous polymers 1/3′, 1/4′, and 1/5′, analogues to DMTA− TPB1/3′, DMTA−TPB1/4′, and DMTA−TPB1/5′, respectively, exhibited sluggish reactions, requiring a long time to complete (Table 2, entry 9; Table 3, entry 13; Table 4, entry 12) and decreased enantioselectivites. The observed differences are probably a result of the immobilized organocatalysts not being uniformly distributed in the solids and only part of catalytic sites being effective for the reactions. As a class of the most privileged and versatile organocatalysts, both imidazolidinones and proline have been immobilized on diverse supports such as amorphous porous organic polymers, layered solids, and mesoporous silica through the postsynthesis method,24−26 but these catalysts in general suffer from the disadvantages of uneven catalytic sites and low catalyst loading (Table 3). With few exceptions,24,25a the applied materials typically gave unsatisfactory enantioselectivity and/or diastereoselectivity. For example, imidazolidinone-grafted mesoporous silica was prepared and used to promote asymmetric DA reactions.26a,b In comparison with DMTA−TPB1/5′, they gave comparable enantioselectivities, but low diastereoselectivites for

the tested substrates (Table 4, entries 9−11). Direct incorporation of asymmetric organocatalysts into crystalline solids with high activity and selectivity remains underdeveloped.9a,d Several proline-MOFs were prepared and employed in different types of aldol additions (Table 3, entries 8−12).25b−e Among these MOF catalysts, proline-Uio-68 gave the highest enantioselectvity (76% ee) and diastereoselectivity (7:1 anti/syn) for the examined aldol reactions (Table 3, entry 11),25e but the two values were much lower than those observed for the CCOF catalyst DMTA−TPB1/4′.

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CONCLUSIONS

We have designed and synthesized a series of 2D-CCOFs containing chiral pyrrolidine and imidazolidine catalysts with a tunable concentration using a multivariate approach. Chemical stability and crystallinity that are relevant to the density of organocatalysts appended on the channel walls of CCOFs have been observed. Compared with the binary CCOFs, the ternary CCOFs displayed improved hemical stability and crystalinity and demonstrated to be efficient and recyclable heterogeneous asymmetric catalysts for three types of meaningful organic reactions. The multivariate approach offers a valuable toolbox to introduce chiral functionality and control stability and crystallinity for CCOFs, holding great promise for efficiently preparing a wide range of CCOFs that will display interesting chirality properties. 8283


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b03352. Experimental procedures and characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Yong Cui: 0000-0003-1977-0470 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Science Foundation of China (Grants 21371119, 21431004, 21401128, 21522104, and 21620102001), the National Key Basic Research P r o g r a m o f Ch i n a ( G r a n t s 2 0 1 4 CB 9 3 2 1 0 2 a n d 2016YFA0203400), and the Shanghai “Eastern Scholar” Program.

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