Homochiral Porous Framework as a Platform for Durability

Article pubs.acs.org/cm

Homochiral Porous Framework as a Platform for Durability Enhancement of Molecular Catalysts Qi Sun, Zhifeng Dai, Xiangju Meng, and Feng-Shou Xiao* Key Laboratory of Applied Chemistry of Zhejiang Province and Department of Chemistry, Zhejiang University, Hangzhou 310028, P.R. China S Supporting Information *

ABSTRACT: Self-quenching and vulnerability of active sites are major issues posed for practical applications of highly efficient chiral organometallic catalysts. Here, we demonstrate an effective strategy to address these challenges by constructing them into homochiral porous frameworks, which renders them with extraordinary resistance against deactivation yet fully retains the intrinsic catalytic activities and selectivities under heterogeneous systems. Representatively, after partial metalation of the porous chiral phosphoramidite ligand-based frameworks (Phos-HPFs) with Rh species, the afforded catalysts exhibit dramatically enhanced durability while maintaining the activity and selectivity of the homogeneous counterparts in asymmetric hydrogenation of olefins. The rigid framework of Phos-HPFs can isolate the active sites, thus preventing the self-quenching from forming coordinatively saturated complexes, while the active sites surrounded by dense free chiral ligands in Phos-HPFs can inhibit them from decomposing into metallic particles. Our work thereby highlights the advantages of HPFs for the deployment of catalysts, which offers an opportunity for enhancing the utilization efficiency rather than merely having the benefits of easy separation and recycling of the chiral catalysts.

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INTRODUCTION Chirality is an essential attribute of nature.1−6 The need for enantiomerically pure insecticides and pharmaceuticals drives the search for new catalysts that can perform asymmetric reactions with high efficiency and superb enantiomeric selectivity.7−18 Homogeneous asymmetric catalysis has the advantage of higher selectivity and is less affected by the limitations of the slow transport of reactants and products relative to the heterogeneous catalysis. However, besides the high cost of chiral catalysts, their utilization in the chemical industry has been mainly impeded by the low utilization efficiency as a result of low stability.19,20 Numerous organometallic catalysts have suffered from deactivation caused by two pathways. First, part of the metal species is locked up in a catalytically inactive coordinatively saturated form, due to the propensity of heteroleptic complexes to homoleptic complexes by irreversible ligand redistribution.21 Second, these metal species are usually vulnerable under the catalytic conditions, which makes them thus prone to decomposing into metal colloids.22−25 Therefore, avoidance of these unwanted transformations is essential toward the realization of highly efficient asymmetric catalysis and is of particular economic and academic importance. To tackle the first challenge with molecular catalysts, there is a method of confining them into heterogeneous systems to spatially isolate the catalytic species, thereby prohibiting their self-quenching.26−29 The solution to the second issue is a “self© 2017 American Chemical Society

healing” strategy using free ligands for reconstitution of the damaged catalyst, which is favorable to prevent the decomposition of metal sites into metal colloids.22,30,31 Nevertheless, the catalytic system which features non-selfquenching and enhanced resistance against decomposition has not been carefully explored thus far. We envision that the construction of the chiral ligands into rigid porous solid materials and then partially postsynthetic metalated with catalytic species is supposed to address the aforementioned concerns simultaneously, considering that the rigid framework can effectively isolate the metal species while closely and densely surrounded chiral ligands are beneficial for improving the resistance to decomposition of the complexes in HPFs, as proposed in Figure 1. Phosphoramidites constitute one of the most useful moieties in the current scenario of asymmetric catalysis, because of their availability and exceptional levels of stereocontrol as well as possibilities for the fine-tuning of the steric and electronic properties.32−36 Nonetheless, they also suffer from the aforementioned deactivation pathways.33 Herein, to test our hypotheses, we have synthesized a series of homochiral phosphoramidite based porous frameworks (Phos-HPFs, Figure 2) via polymerization of vinyl functionalized ligands Received: May 7, 2017 Revised: June 19, 2017 Published: June 20, 2017 5720

DOI: 10.1021/acs.chemmater.7b01878 Chem. Mater. 2017, 29, 5720−5726

Article

Chemistry of Materials

(isobutyronitrile) (AIBN) afforded the homochiral porous frameworks of phosphoramidite ligands as white solids (Figures S4 and S5). The resultant frameworks are stable in air and insoluble in water and common organic solvents. X-ray diffraction studies indicate that these polymers are amorphous in nature (Figure S6). As a representative sample among the synthesized HPFs based on phosphoramidite, the preparation of the HPF constructed by N,N-dimethylphosphoramidite (MonoPhos-HPF) is illustrated thoroughly. Thermogravimetric analysis (TGA, Figure S7) conducted in N2 atmosphere shows the decomposition of MonoPhos-HPF starts from 200 °C, indicating its good thermal stability. The local chemical structure of the MonoPhos-HPF was characterized by 13C and 31P MAS NMR spectroscopy (Figure 3A,B). The 13C MAS NMR spectrum of MonoPhos-HPF

Figure 1. Schematic illustration of enhanced durability of active sites in the HPFs, while self-quenching and decomposing of active sites in the homogeneous catalysts.

Figure 2. Structures of vinyl-functionalized phosphoramidite monomers and textural parameters of the corresponding homochiral framework.

(Figures S1 and S2). The porous organic polymer platform was chosen for this study due to its amenability to design, porosity, and chemical stability.37−57 Initial studies reveal that the empolyment of Phos-HPFs as modification ligands has a profound effect on the catalyst durability and provides additional recyclability. While the homogeneous analogues exhibit poor durability for asymmetric hydrogenation of olefins, the Rh metalated Phos-HPFs demonstrate excellent durability due to the inhibition of deactivation pathways. Our work thus underscores the advantages of utility homochiral porous frameworks as a promising platform for the deployment of heterogeneous chiral catalysts and shows great potential for allowing molecular chiral catalysts to develop from novel discoveries into practical applications.

Figure 3. (A) 13C MAS NMR, (B) 31P MAS NMR, (C) SEM image, (D) TEM image, (E) N2 sorption isotherms collected at 77 K of MonoPhos-HPF, and (F) CD spectra of the MonoPhos-HPF synthesized from opposite enantiomers of BINOL.

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RESULTS AND DISCUSSION Materials Preparation, Physiochemical Characterization, and Local Structure Analysis. The BINOL-derived phosphoramidite monomers with different substitutes can be easily obtained by treating vinyl-functionalized BINOL with corresponding amine in one or two steps as shown in Figure S2. Treatment of the vinyl-functionalized BINOL with hexamethylphosphorous triamide gave rise to the methyl substituted phosphoramidite ligand directly. Phosphorylation of phosphorus trichloride (PCl3) with vinyl-functionalized BINOL followed by reacting with secondary amines led to the corresponding substituted phosphoramidites. Dipropylamine and diisopropylamine were chosen in this work to demonstrate the proof-of-concept; but in principle, various types of secondary amines can be introduced. All of the new ligands were characterized by NMR spectroscopy (Figure S3). Under solvothermal conditions in THF at 100 °C, the polymerization of the monomers in the presence of azobis-

shows four connected peaks ranging from 126.3 to 153.7 ppm and a peak at 35.3 ppm, which are assigned to the aromatic carbon and N connected methyl groups, respectively. The successful transformation from the vinyl-functionalized phosphoramidite monomers into highly polymerized polymer is verified by the disappearance of peaks in the range of 110 to 120 ppm, which are related to vinyl groups, and the concomitant emergence of a strong peak at 42.1 ppm, which is attributed to the polymerized vinyl groups.58 The detailed assignments for the 13C MAS NMR signals are presented in Figure S8. The 31P MAS NMR spectrum gives a single peak at 151.1 ppm (Figure 3B) in good agreement with that of the monomer (Figure S3). These results demonstrate that the phosphoramidite moieties are stable during the polymerization process. Morphology of the MonoPhos-HPF was examined by SEM and TEM techniques (Figures 3C,D). Particles on the order of hundreds of nanometers in size display rather rough 5721


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Encouraged by the above result, we next investigated the effect of the free ligands on the catalytic performance by adjusting the ratio of ligand to Rh in MonoPhos-HPF. As shown in Figure S11, activity increased with the decrement of the ratio of MonoPhos in MonoPhos-HPF to Rh from 10 to 5, affording the methyl 2-acetamidoacrylate conversion from 48.7% to higher than 99.5%. Similar catalytic activities were reached, when the ratio of the ligand to Rh was in the range of 5 to 2, but further lowering this ratio from 2 to 1, a slightly decreased conversion was observed (from >99.5% to 92.1%). During these reactions, we found that the catalysts with ligand/ Rh at 2 and 1 turned to black from light yellow, signaling the formation of Rh colloids. ICP-OES results also reveal that over 10 wt % Rh species had leached into the supernatant for both of the catalysts tested. These results indicate the high concentration of the ligands around the metal complexes is necessary to prevent them from decomposing and leaching. To further demonstrate the importance of the existence of dense free ligands in the vicinity of active sites on boosting the catalyst stability and to exclude the possibility of deactivation as a result of crowded active sites in the HPFs-based catalysts with ligand/Rh at 2 and 1, we synthesized a porous polymer with low MonoPhos moieties concentration by copolymerization of divinylbenzene (DVB) with vinyl-functionalized MonoPhos in a mass ratio of 4/1 to afford PDVB-1/4MonoPhos (BET: 521 m2/g). After coordinating with Rh(COD)2BF4, the resultant catalyst (Rh/PDVB-1/4MonoPhos, the ratio of MonoPhos moieties to Rh is about 1) showed an obvious catalyst decomposition in the asymmetric hydrogenation of methyl 2acetamidoacrylate, as demonstrated by the fact that the catalyst turned from yellow to black. Taking into account that Rh/ MonoPhos-HPF (the ratio of MonoPhos moieties to Rh is about 5) and Rh/PDVB-1/4MonoPhos have similar surface areas, as well as type and density of active sites, the significant difference in the stability should be ascribed to the existence of dense free ligands arranged around the catalytic species. These ligands proximate to active sites are expected to continuously repair broken metal−ligand bonds during catalysis and thereby enhance the durability of the catalyst.62 On the basis of the above results, the catalyst with the ratio of the ligand to Rh at 5 was chosen for further detailed studies (hereafter denoted Rh/MonoPhos-HPF). At first, we investigated the recyclability of the catalyst, which is a crucial performance metric for cost-effective industrial processes. Analysis of the reaction solution after each cycle by ICP-OES showed that the Rh species leaching was below the detection limit of 0.1 ppm. In addition, the supernatant from hydrogenation of methyl 2-acetamidoacrylate after centrifugation did not afford any additional product, highlighting the heterogeneous nature of the catalytic process. Impressively, the catalyst could be recycled at least 5 times without a drop in product yield and enantioselectivity (Figure 4A). No Rh colloids were observed in the recycled catalyst, confirming its excellent stability (Figure S12). With respect to the homogeneous controls, when the ratio of the MonoPhos to Rh was 2 or 1, both of the catalytic systems afforded full methyl 2-acetamidopropanoate conversion as well as 98.4% ee; nevertheless, obvious catalyst decomposition was observed as black precipitates formed during the reaction. When we increased this ratio to 3 or higher, the reactions completely ceased. The first scenario can be attributed to the excellent acceptor properties of phoshoramidites that renders them with strong binding ability to Rh species and thus makes

surfaces and appear to be aggregates of much smaller particles with dimensions of around tens of nanometers. Nitrogen sorption measurements collected at 77 K (Figure 3E) indicate that the MonoPhos-HPF is highly porous with a BET surface area and pore volume of 499 m2/g and 0.66 cm3/g, respectively. Pore size distribution shown in Figure S9 suggests that the majority of the porosity is in the micropore region. A small percentage of the porosity is in the mesopore and macropore region, likely coming from the interparticle spacing in the MonoPhos-HPF. The chirality of the MonoPhos-HPF was assessed by examination of their circular dichroism spectra (CD spectra) in the solid state that had been converted to KBr pellets. Figure 3F reveals that CD spectra of the MonoPhos-HPF made from opposite enantiomers of BINOL are mirror images of each other, in support of their enantiomeric nature. Furthermore, related absorptions are also observed in the solid state UV−vis spectrum of MonoPhos-HPF, confirming that these signals are not artificial (Figure S10). These results disclose the stability of the chiral configuration toward the polymerization conditions.59,60 Catalytic Performance Investigation. To evaluate the performance of Phos-HPFs as solid ligands in the enantioselective catalysis, the Rh-catalyzed asymmetric hydrogenation of prochiral olefins as models was chosen, as it is a significant type of reaction for the production of fine chemicals.61 The catalysts were prepared in situ by combining a catalyst precursor Rh(COD)2BF4 and the polymeric ligands, followed by stirring under N2 for 2 h. For an initial screening, we turned our attention to the hydrogenation of dehydroamino acid esters with a focus on the benchmark substrate methyl 2acetamidoacrylate. Initial hydrogenation experiments were performed at 1 bar of H2 pressure in CH2Cl2, using 1 mol % of catalyst based on Rh with a ratio of ligand to Rh at 5.0. We observed that employing MonoPhos-HPF as the modification ligand gave a desired product yield higher than 99.5% with an enantioselectivity of 97.7% after 3 h (Table 1, entry 1). Table 1. Asymmetric Hydrogenation of Methyl 2Acetamidoacrylate over Various Catalystsa

entry

catalyst

time (h)

conv. (%)

ee (%)

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

Rh/MonPhos-HPF Rh/MonPhos-HPF Rh/MonPhos Rh/MonPhos-HPF Rh/MonPhos Rh/PrPhos-HPF Rh/PrPhos Rh/PrPhos-HPF Rh/PrPhos Rh/i-PrPhos-HPF Rh/i-PrPhos Rh/i-PrPhos-HPF Rh/i-PrPhos

3 3 3 24 24 8 8 60 60 12 12 72 72

>99.5 98.5 >99.5 >99.5 32.3 >99.5 >99.5 >99.5 41.3 >99.5 >99.5 >99.5 47.5

97.7 97.7 98.4 97.7 98.0 94.3 94.5 94.2 94.1 94.2 94.5 84.8 84.6

a

Reaction conditions: methyl 2-acetamidoacrylate (143 mg, 1 mmol), CH2Cl2 (5 mL), H2 (1 bar), RT, Rh(COD)2BF4 (4.1 mg, 1 mol %). b Recycles for 5 times. cMethyl 2-acetamidoacrylate (1.43 g, 10 mmol), Rh(COD)2BF4 (2.0 mg, 0.05 mol %), CH2Cl2 (10 mL), H2 (10 bar), and RT. 5722


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Figure 4. . Catalytic performance comparison of Rh/MonoPhos-HPF and Rh/MonoPhos. (A) Plots of conversion and ee values of asymmetric hydrogenation of methyl 2-acetamidoacrylate in the recycle of Rh/MonoPhos-HPF under the conditions of Table 1, entry 1. (B and C) Plots of GC conversion vs time using Rh/MonoPhos-HPF and Rh/MonoPhos catalysts with S/C ratio at 500 and 2000, respectively. (B) Reaction conditions: methyl 2-acetamidoacrylate (2.5 mmol), CH2Cl2 (5 mL), H2 (1 bar), RT, Rh(COD)2BF4 (2.0 mg), and MonoPhos-HPF (14 mg) or MonoPhos (3.5 mg). (C) Reaction conditions: methyl 2-acetamidoacrylate (10 mmol), CH2Cl2 (10 mL), H2 (10 bar), RT, Rh(COD)2BF4 (2.0 mg), and MonoPhos-HPF (14 mg) or MonoPhos (3.5 mg).

The enhanced activities of the heterogeneous catalysts relative to their homogeneous analogues were also observed over Rh/PrPhos-HPF and Rh/iPrPhos-HPF. However, they gave lower activities and ee values under identical reaction conditions in comparison with that of Rh/MonoPhos-HPF catalyst. Changing from dimethyl- to di-n-propylamine led to a small decrease in ee value (Table 1, entries 6−9), whereas the use of the di-i-propylamine-derived ligand led to a clear drop in selectivity (Table 1, entries 10−13). Interestingly, when aromatic enamide was employed as a reagent, an opposite ee trend was observed. Specifically, employing i-PrPhos-HPF gave rise to the acetylated amine ee value at up to 98.0%, while using MonoPhos-HPF and PrPhos-HPF afforded ee values of 86.3% and 94.4%, respectively. These results are well comparable with their homogeneous counterparts (Table 2). Given the modular

them more prone to self-quenching via multimolecular deactivation pathways forming coordinatively saturated complexes. Meanwhile, the yielded low-ligated or naked Rh species are inclined to be decomposed and agglomerated into metallic Rh nanoparticles. While, in the second case, in the presence of an excess of phoshoramidites, catalytically inactive tris- and tetraphoshoramidite Rh species might be predominant.33 Consequently, it did not come as a surprise that the reaction ceased when the ratio of ligand to Rh was increased to higher than 3 and the decomposition of the catalysts occurs when the ratio of ligand to Rh ratio was lower than 2 in the homogeneous hydrogenation. In contrast, when the MonoPhos-HPF was employed as the modification ligand, the considerable strain of polymer chains as a result of the high cross-linking levels can not only inhibit the formation of the coordinatively saturated complexes, even at high ratios of ligand to Rh, but also eliminate self-quenching pathways by site isolation. More importantly, the presence of free ligands in the Rh/MonoPhos-HPF, which closely and densely surround the active sites in the homochiral porous framework, in turn enhances the resistance of active sites against decomposition, thereby preserving high activity for the catalyst over a long duration of reaction time. To further illustrate the benefit of the use of homochiral porous frameworks as modification ligands, we compared the activities of Rh/MonoPhos-HPF and Rh/MonoPhos at high S/ C ratios (the molar ratio of reagent to Rh). The difference in catalyst durability between Rh/MonoPhos-HPF and Rh/ MonoPhos was clearly manifested in the reaction kinetics. Time-dependent GC conversion curves indicate that, in spite of the required mass diffusion of reactants in polymer channels, Rh/MonoPhos-HPF showed comparable activity to that of Rh/ MonoPhos (MonoPhos/Rh = 2) at low conversion, while it afforded superior activity at high conversion. Specifically, at 0.2 mol % catalyst loading, the hydrogenation of methyl 2acetamidoacrylate was completed in 10 and 18 h for Rh/ MonoPhos-HPF and Rh/MonoPhos, respectively (Figure 4B). Notably, the difference became larger at higher S/C ratios. For instance, with S/C ratio at 2000, Rh/MonoPhos-HPF is at least 5 times as active as the homogeneous control. The conversion steadily increased along with the time increase in the presence of Rh/MonoPhos-HPF. In striking contrast, Rh/MonoPhos was deactivated within 8 h, and prolonging the reaction time did not lead to any fruitful product formation (Figure 4C and Table 1, entries 4 and 5).

Table 2. Asymmetric Hydrogenation of N-(1Phenylvinyl)acetamide over Various Catalystsa

entry

catalyst

time (h)

conv. (%)

ee (%)

1 2 3 4 5 6

Rh/MonPhos-HPF Rh/MonPhos Rh/PrPhos-HPF Rh/PrPhos Rh/i-PrPhos-HPF Rh/i-PrPhos

4 4 4 4 5 5

>99.5 >99.5 >99.5 >99.5 >99.5 >99.5

86.3 86.7 94.4 94.4 98.0 98.2

a

Reaction conditions: The catalysts used in all reactions were in situ prepared, the ratio of ligand to the Rh for homogeneous and heterogeneous is the same to that given in Table 1, N-(1phenylvinyl)acetamide (161 mg, 1 mmol), Rh(COD)2BF4 (4.1 mg, 1 mol %), THF (5 mL), H2 (10 bar), and RT.

nature of phoshoramidites and the de novo synthesis of homochiral frameworks, we can thus readily accommodate the ligands in HPFs for specific substrates, similar to molecular ones, while greatly extending their durability as well as the additional benefit of recyclability.

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CONCLUSION In summary, we have demonstrated an effective strategy to enhance the durability of molecular chiral catalysts via the 5723



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construction of homochiral porous frameworks, as exemplified by the phoshoramidites. The spatial isolation of catalytically active species in the HPFs effectively prevents the selfquenching, and the dense ligand proximate to the metal complexes generates a decomposition resistant system, which therefore leads to superior performance over homogeneous counterparts. The ready tunability of such a modular approach based on chiral ligands and homochiral framework synthesis promises to enable a number of chiral solid catalysts with unique reactivity and useful enantioselective functions, which are not trivially achieved with traditional heterogeneous and homogeneous systems. The strategy presented herein therefore holds a great promise to advance the practical utility of chiral catalysis.

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Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01878.

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Material synthesis; characterization details; XRD, TEM, UV−vis, and NMR; and supporting figures (PDF)

AUTHOR INFORMATION

Corresponding Author

*(F.-S.X.) E-mail: [email protected]. ORCID

Feng-Shou Xiao: 0000-0001-9744-3067 EXPERIMENTAL SECTION

Notes

The authors declare no competing financial interest.

Material Synthesis. Synthesis of MonoPhos-HPF. A total of 1.0 g of A (Figure 2) was dissolved in 10 mL of THF, followed by addition of 25 mg of azobis(isobutyronitrile) (AIBN). The mixture was transferred into an autoclave at 100 °C for 24 h. After being washed with CH2Cl2 and dried under vacuum, the polymer was obtained and is denoted as MonoPhos-HPF. Synthesis of PrPhos-HPF. A total of 1.0 g of B (Figure 2) was dissolved in 10 mL of THF, followed by addition of 25 mg of azobis(isobutyronitrile) (AIBN). The mixture was transferred into an autoclave at 100 °C for 24 h. After being washed with CH2Cl2 and dried under vacuum, the polymer was obtained and is denoted as PrPhos-HPF. Synthesis of i-PrPhos-HPF. A total of 1.0 g of C (Figure 2) was dissolved in 10 mL of THF, followed by addition of 25 mg of azobis(isobutyronitrile) (AIBN). The mixture was transferred into an autoclave at 100 °C for 24 h. After being washed with CH2Cl2 and dried under vacuum, the polymer was obtained and is denoted as iPrPhos-HPF. Synthesis of PDVB-1/4MonoPhos. A total of 0.2 g of A (Figure 2) and 0.8 g of divinylbenzene were dissolved in 10 mL of THF, followed by addition of 25 mg of azobis(isobutyronitrile) (AIBN). The mixture was transferred into an autoclave at 100 °C for 24 h. After being washed with CH2Cl2 and dried under vacuum, the polymer was obtained and is denoted as PDVB-1/4MonoPhos. Catalytic Tests. Asymmetric Hydrogenation over Heterogeneous Catalysts at 1 atm H2 Pressure. In a typical run, the catalyst was prepared in situ by mixing Rh(COD)2BF4 (0.01 mmol, 4.1 mg) and MonoPhos-HPF (28 mg) in CH2Cl2 (5 mL) for 2 h under N2 in a Schlenk tube. The corresponding substrate (1.0 mmol) was introduced, the reactor was charged with hydrogen gas (balloon), and the solution was stirred at room temperature for a predetermined period of time. After the reaction, the catalyst was taken out from the system by centrifugation and the liquid was passed through a short column before analysis by gas chromatography (Kexiao gas chromatography equipped with a flame ionization detector and a Supelco γ-DEX 225 capillary column). For recycling of the catalyst, the catalyst was separated by centrifugation, washed with CH2Cl2 (5 × 5 mL) under N2, and then used directly for the next catalytic reaction without drying. Asymmetric Hydrogenation over Heterogeneous Catalysts under High H2 Pressure. The procedures are similar to those of the reactions operated under atmospheric conditions except that the in situ prepared catalyst was transferred into a 100 mL stainless steel autoclave equipped with a Teflon liner, which contained the corresponding substrate and a magnetic stirring bar. Asymmetric Hydrogenation over Homogeneous Catalysts. The procedures are similar to those of using homochiral porous frameworks as modification ligands except that soluble ligands were used.

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ACKNOWLEDGMENTS We acknowledge National Natural Science Foundation of China (21333009 and 21422306).

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