Postfunctionalized Porous Polymeric Aromatic Frameworks with an

Research Article pubs.acs.org/journal/ascecg

Postfunctionalized Porous Polymeric Aromatic Frameworks with an Organocatalyst and a Transition Metal Catalyst for Tandem Condensation−Hydrogenation Reactions Ester Verde-Sesto,†,⊥ Estíbaliz Merino,‡,○ Elizabeth Rangel-Rangel,§ Avelino Corma,∥ Marta Iglesias,*,§ and Félix Sánchez*,‡ †

Instituto Instituto § Instituto ∥ Instituto ‡

de de de de

Ciencia y Tecnología de Polímeros, ICTP-CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain Química Orgánica, IQOG-CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain Ciencia de Materiales de Madrid, ICMM-CSIC, Sor Juana Inés de la Cruz, 3, Cantoblanco, 28049 Madrid, Spain Tecnología Química, ITQ-CSIC. Av. Los Naranjos s/n, 46022 Valencia, Spain

S Supporting Information *

ABSTRACT: Novel heterogenized bifunctional catalysts were prepared combining a rhodium complex with a pyrrolidine on porous polymeric aromatic frameworks (PAFs) based on tetraphenyladamantane and tetraphenylmethane subunits. These new systems catalyze efficiently tandem Knoevenagel condensation and hydrogenation of the resulting olefin. These tandem reactions represent an easy and selective methodology for monoalkylation of methylene active compounds from aldehydes. The obtained bifunctional PAFs exhibit high activity and excellent stability in the cascade reactions and can be recycled up to ten times in a productive process. KEYWORDS: Porous aromatic frameworks, Bifunctional solid, Organocatalysis, One-pot tandem reactions

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INTRODUCTION

Several research groups are working on the development of new multifunctional inorganic and hybrid organic solid catalysts for one-pot multistep reactions.17−28 The zeolitic hybrid organic−inorganic materials with acid sites located in zeolitic counterpart and base centers in the organic component used as bifunctional catalysts in cascade reactions are representative examples of these heterogeneous catalysts.11 Recently, Baba’s group has described the synthesis of a silica-supported Pd complex and tertiary amine system forming the cooperative surface catalysis and its application in Tsuji−Trost reaction.29 Normally, the multisite catalysts are synthesized using ordered micro- and mesoporous organic−inorganic hybrid materials such as PMOs30−34 (periodic mesoporous organosilicas) or MOFs35−37 (metal organic frameworks). In the last years, the porous organic polymers (POPs) have attracted considerable attention due to their potential applications in storage,38 separation,39 and catalysis.37,40−43 The functionalization of POPs has been scarcely explored.44,45 A plausible strategy would be to integrate two components with different catalytic functions into a single nanostructure (for example, polymeric aromatic frameworks (PAFs) with acid and base groups,46 MOFs and noble metal nanoparticles,47−49 etc.) to implement the high-efficient cascade reactions. Previously,50,51 we

Multisite catalysts have been actively used in the last few years in organic synthesis. Examples showing the combination of different transition metals for one catalytic process,1−3 biocatalysis or enzyme catalysis with metal catalysis,4,5 and organocatalysis with transition metal catalysis6 have been recently reported. The development of multifunctional catalysts allows carrying out multistep cascade reactions in a similar way as nature’s strategy is for the synthesis of complex and bioactive organic molecules in living systems. Normally, multistep chemical processes involve different catalysts for each step. From the energy saving point of view, the design and use of solids with groups of different characteristics on their structures (acid and base groups, different metals, etc.) is very exciting since it combines catalytic and green chemistry. The catalyst material with well-defined multisites would allow to carry out consecutive reaction steps in one pot avoiding the deactivation of groups, which would be incompatible in homogeneous conditions, excluding costly intermediate separations and purification processes. Heterogenization of soluble catalysts is an attractive concept to realize multistep cascade reactions.7−13 Heterogeneous multifunctional catalysts can provide a continuous range of functional groups and offer advantages, such as an enhancement of reactivity and stability of antagonist functional groups.14−16 © 2016 American Chemical Society

Received: September 24, 2015 Revised: January 18, 2016 Published: January 21, 2016 1078

DOI: 10.1021/acssuschemeng.5b01147 ACS Sustainable Chem. Eng. 2016, 4, 1078−1084

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ACS Sustainable Chemistry & Engineering

(PAFC) subunits, their postfunctionalization by incorporating proline units (PAFAd-NPro and PAFC-NPro) and rhodium (PAFAd-NPro-Rh and PAFC-NPro-Rh). The behavior of these bifunctional catalysts was tested in one-pot Knoevenagel/ hydrogenation cascade reactions. A comparison between these materials with those obtained previously from a prefunctionalized monomer (PAFPre-NPro-Rh) will be made. These bifunctional base−metal complex porous materials are stable and robust, allowing several recycles without significant loss of catalytic activity.

described acid−base bifunctional PAFs based on tetraphenylmethane or 9,9′-spiro-bifluorene subunits and a mixture of acid− and base−PAFs. These materials showed high efficiency in the catalysis of one-pot cascade reactions (Figure 1). Also,

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RESULTS AND DISCUSSION

The preparation of the PAFs (Scheme 1) was carried out by Suzuki coupling of adequate tetraiodo monomer 1 (obtained from 1,3,5,7-tetraphenyladamantane 53 or tetraphenylmethane50) with 1,4-benzenediboronic acid 2 using microwave heating.51−53 The reactions were quantitative in all the cases. At least ten batches for the preparation of each material were carried out in scale of 200 mg, and the reaction crudes were mixed and subjected to purification together. The PAFs were characterized by solid-state 13C NMR (Figure S2), FT-IR (Figure S6), and elemental analysis54 (the carbon and nitrogen content determined by elemental analysis did not adjust to the theoretical calculations, being lower than expected; similar behavior was previously observed in conjugated porous polymers, where too low carbon content and too low carbon−hydrogen ratio were observed55,56). Bifunctionalized PAF materials with base and metal sites were obtained by postfunctionalization of the porous materials PAFc and PAFAd in only five steps (Scheme 2). In the first step, the treatment of PAF with a nitrictrifluoroacetic acid mixture resulted in the nitro-functionalized PAF. The incorporation of the nitro groups was confirmed by the presence of their characteristic bands at 1527 and 1345 cm−1 in the IR spectrum. The calculation of the nitrogen content was based on the elemental analysis (∼2.72 mmol·g−1 for PAFAd-NO2). Next, the reduction of nitro groups with SnCl2·2H2O in THF or methanol yielded PAFAd-NH2 (Scheme 2). The course of the reaction was followed by FTIR and the reaction was considered finished when the bands corresponding to nitro groups disappeared. The elemental analysis of the material indicated a nitrogen contents of 2.60 mmol·g−1, which is in line with the nitrogen content of the precursor PAFAdNO2. PAFAd-NH2 was treated with N-tBoc-L-proline, isopropyl chloroformate, and triethylamine to introduce the proline derivative in the material forming PAFAd-NProBoc (Scheme 2). The elimination of Boc group with trifluoroacetic acid

Figure 1.

we described the synthesis of a prefunctionalized PAF (PAFcpre) starting from nitro tetraiodophenylmethane as building block, which was postfunctionalized with prolinamide groups and their corresponding rhodium and copper complexes. These materials were successfully used as recyclable catalysts for cyclopropanation and hydrogenation reactions.52 Herein, we report the synthesis of PAFs from 1,3,5,7tetraphenyladamantane (PAFAd) and tetraphenylmethane Scheme 1. Preparation of Polymeric Supports

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ACS Sustainable Chemistry & Engineering Scheme 2. Synthesis of Bifunctionalized Base−Metal PAF Materialsa

Reaction conditions: (a) CF3CO2H, HNO3, rt, 4 h; (b) SnCl2·H2O, THF, reflux, 48 h; (c) Et3N, ClCO2Et, N-Boc-L-proline, THF, reflux, 48 h; (d) 30% TFA/CH2Cl2, rt, 24 h; (e) [Rh(cod)Cl]2, AgPF6, THF, rt, 3 h. a

provided PAFAd-NPro (3.0 mmol·g−1 of nitrogen). Treatment of this proline-functionalized material with [Rh(cod) (THF)2]BF4 gave the corresponding heterogenized complex PAFAdNProRh (Scheme 2) with a rhodium loading of 0.19 mmol/g determined by inductively coupled plasma analysis (ICP). The metal covered only a fraction of the proline groups, remaining the rest as base sites at the catalyst surface. The synthesis of PAFC-NProRh (0.36 mmol/g of Rh) was accomplished following a similar route.50 For detailed experimental procedures and characterization of the intermediates and final materials, see the Supporting Information. The estimated surface area for parent PAFAd was over 514 m2·g−1, and the estimated pore volume was 0.59 cm3·g−1 (Table 1). It was stable up to 535 °C as measured by TGA. Nitrogen

we could confirm that this group was introduced in the network of the polymer. FTIR spectroscopy showed the typical bands of aromatic compound around 3027−2853 and 1603, 1485, and 816 cm−1 due to C−H stretching and aromatic CC double bonds, respectively (Figure S6−S8, S9a−11a). The thermal stability was evaluated by thermogravimetric analysis (TGA) in air atmosphere (Figures S14−S15). Both functionalized polymers showed high thermal stability, with decomposition at ∼400 °C (TGA of PAF-NO2 showed a small step, which corresponded to the loss of the −NO2 groups). SEM images of functionalized polymers were similar to the precursors, showing the typical porous polymer morphology (Figure S16). Catalytic Activity. Sometimes, the synthesis of organic molecules with interesting properties involves several primary and consecutive steps. Each step is carried out with different reagents and catalysts. Normally, it is necessary to purify the product from the mixture of reagents and subproducts of the reaction after each step. In many cases, the catalysts used in different steps in the synthetic route are incompatible (in homogeneous conditions) and their use in a one-pot reaction becomes impossible. For these reasons, it is very interesting to be able to control the functionalization of the solid support with different groups and carrying out the consecutive steps in a one-pot system. In this way, the isolation and purification of the intermediates is avoided, resulting in a simpler process with reduction of waste. To prove this concept, PAFs-NProRh (with 8−14% of rhodium content and base sites) were evaluated as bifunctional catalysts in a model cascade reaction (Scheme 3). The chosen reaction involved two steps: a base-catalyzed Knoevenagel condensation of benzaldehyde 1 and malononitrile to yield 2benzylidenemalononitrile 2 and subsequent Rh-catalyzed hydrogenation of the resulting olefin bond as shown in Scheme 3. In the preliminary experiments, the catalytic activity of PAFsNProRh was evaluated only for Knoevenagel reaction showing the catalytic behavior of the base sites in the catalyst i.e. uncoordinated proline groups (NPro). The condensation product was formed quantitatively in the presence of the three materials and only small differences on activity were observed (Table 2). With the condensation product in our

Table 1. Surface Area Calculated Using the Brunauer− Emmett−Teller (BET) Theory and Total Pore Volume Using the Barrett−Joyner−Halenda (BJH) Method material

BET surface area (m2·g−1)

pore volume (BJH) (cm3·g−1)

PAFAd PAFAd-NH2 PAFAdNProRh PAFC PAFC-NH2 PAFC-NProRh PAFCpreNProRh

514 413 98

0.59 0.48 0.27

393 248 130 158

0.58 0.32 0.25 0.22

adsorption isotherms for PAFAd-NH2 and PAFAd-NProRh are shown in Figure S12a. The introduction of amino groups in the material decreased the calculated surface area to 413 m2·g−1, and this value diminished up to 98 m2·g−1 when the prolinamide ligand and the metal were introduced (PAFAdNProRh) (Table 1). The solid-state 13C NMR of the material PAFAd (Figure S2, S3a−5a) showed a peak at 39 ppm due to the aliphatic carbons of adamantine nucleus, one intense peak at 127 ppm assigned to the aromatic carbons, and two peaks at 139 and 148 ppm corresponding to quaternary aromatic carbons. The peaks corresponding to proline and t-butoxycarbonyl group were observed in the 13C NMR of PAFAd-NProBoc (Figure S3a), so 1080


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ACS Sustainable Chemistry & Engineering Scheme 3. PAF-NProRh-Catalyzed Tandem Reaction between Benzaldehyde 1 and Malononitrile (Knoevenagel Condensation + Hydrogenation)

Table 3. PAFC-NProRh-Catalyzed C−C Coupling− Hydrogenation Reactiona

a

C−C conditions: aldehyde (0.36 mmol), malononitrile (0.43 mmol), catalyst (2 mol %, based on Rh), toluene (2 mL), 90 °C, overnight. Hydrogenation conditions: Autoclave Engineers, toluene/ethanol 1/2 (40 mL), 60 °C (tint), 5 bar H2. bTime for hydrogenation reaction in parentheses.

Table 2. PAF-Catalyzed Tandem Knoevenagel− Hydrogenation Reactiona

place (Table 4). Surprisingly, the hydrogenation of double bond was not observed under used conditions even with longer conv (%)

yield GC (%) (h)

entry

catalyst

1

2d

3e

1 2 3 4 5 6 7 8

PAFAd-NProRh PAFAd-NProb NProRhc PAFC-NProRh PAFC-NPro PAFC-NPro + NProRh PAFCpre-NProRh PAFC

100 100

80 92

100 100 100 100 0

94 90 90 97 0

100 (6) 0 100 (4) 100 (6)

Table 4. PAFC-NProRh-Catalyzed C−C Coupling− Hydrogenation Reactiona

100 (5) 100 (8)

a

Reaction conditions: (i) benzaldehyde (0.36 mmol), malononitrile (0.43 mmol), catalyst (2 mol %, based on Rh), toluene (2 mL), 90 °C; (ii) hydrogenation conditions Autoclave Engineers, toluene/ethanol 2/1 (10 mL), 60 °C (tint), 5 bar H2. bOnly condensation reaction. c Only hydrogenation reaction. dCondensation step took place overnight. eTime for the hydrogenation step in parentheses.

a

C−C conditions: aldehyde (0.36 mmol), malononitrile (0.43 mmol), catalyst (2 mol %, based on Rh), toluene (2 mL), 90 °C, overnight. Hydrogenation conditions: Autoclave Engineers, toluene/ethanol 1/2 (40 mL), 60 °C (tint), 5 bar H2. bTime for hydrogenation reaction in parentheses.

hands, we carried out the hydrogenation of the double bond. The bifunctional materials made it able to catalyze the tandem one-pot reaction with the product 3 obtained only (Table 2). The previously reported rhodium complex NProRh52 (Figure 1) was used for comparative purposes. We chose PAFC-NProRh as catalyst due to the easier availability of parent material to evaluate the scope of the reaction. PAFC-NProRh-catalyzed Knoevenagel condensation between several aldehydes and malononitrile or ethyl cyanoacetate gave the corresponding benzyliden-derivatives (2); subsequent hydrogenation of the intermediate product (2) lead in good yield to the benzyl compound (3) (benzaldehyde (entries a, b), 4-F-benzaldehyde (entry c)); the reaction did not work with 4-methoxybenzaldehyde (d) neither between benzaldehyde and ethyl nitroacetate (Table 3). In the special case of 4-nitrobenzaldehyde, Knoevenagel condensation with malononitrile or ethyl cyanoacetate yielded quantitatively in the corresponding condensation product mediated by supported pyrrolidine and selective Rh-catalyzed reduction of −NO2 to 2-(4-aminobenzylidene)-product took

reaction times. This behavior allows an easy preparation of 2(4-aminobenzylidene) compounds, key intermediates in the synthesis of dyes and antihypertensive drugs.57,58 Similar results have been reported with Pd@IRMOF-3 catalyst.49 The final product 2 (GC yield = 80%) was isolated in 70% yield after workup and purification by flash chromatography when the reaction was scaled-up to 4 mmol of benzaldehyde. Another key parameter to evaluate the catalytic performance is the stability of the catalyst. Figure 2 demonstrates the conversion of benzaldehyde and the hydrogenation of intermediate product over ten successive cycles for PAFCNProRh. The heterogeneity of PAFC-NProRh was determined by separation of the material from the reaction media. The mother liquor was put in a new experiment, observing no further conversion (the concentration of Rh in the solution was less than 0.01 ppm determined by ICP). These results indicated that no significant metal leaching occurred during the reaction as no homogeneous reaction could be detected in the filtrate. 1081


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groups supported onto SBA-15 mesoporous silica catalyze a similar three-step cascade reactions in one-pot (deacetylation, Henry, and hydrogenation reactions).59

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CONCLUSIONS In summary, the bifunctional PAF-NProRh catalysts can be readily prepared; these catalyze a cascade reaction in an efficient way. Knoevenagel condensation of an aldehyde and malononitrile or ethyl cyanoacetate allowed the formation of a carbon− carbon coupling product mediated by pyrrolidine groups, with subsequent hydrogenation of this intermediate assisted by a heterogenized Rh-complex. The used methodology represents a very simple and general procedure for the selective monobenzylation of methylene-active compounds. Furthermore, these porous polymer hybrid catalytic materials obtained are easily recoverable and can be recycled without loss of catalytic activity in the repetitive reuse cycles. Their combination with an acidic PAF allows carrying out one-pot three step reactions where the acid, base, and metallic sites selectively act in the three consecutive reaction steps. The rational design of functionalized porous polymers opens up opportunities for the production of other high-performance multifunctional catalysts.

Figure 2. Recycling experiments with PAFC-NProRh in one-pot Knoevenagel-hydrogenation reaction starting from benzaldehyde and malononitrile.

Recently, we reported that the bifunctional catalyst PAFSO3H-NH2, incorporating acid and base centers, was an effective catalyst for deacetalization−Knoevenagel cascade reaction.51 This catalytic system only showed a slight catalytic activity improvement respect to a physical mixture of acid and base functionalized materials (PAF-SO3H+PAF-NH2)50 and we did not observe a significant cooperative effect for these reactions. Thus, with a similar point of view, PAFC-SO3H could be combined with the bifunctional base-transition metal complex PAFC-NProRh in order to perform the deacetalization−Knoevenagel−hydrogenation one-pot cascade reaction as depicted in Scheme 4. When the physical mixture of PAFC-

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EXPERIMENTAL DETAILS

Detailed experimental procedures and characterization of the parent PAFs, intermediates, and final materials are described in the Supporting Information. General procedure for the synthesis of PAF-NProRh: In a first step, [Rh(cod)(L)]BF4 (1 equiv) was prepared by reaction between [RhCl(cod)]2 (0.5 equiv) and a stoichiometric amount of solid AgBF4 (1 equiv) in anhydrous THF (10 mL) under nitrogen atmosphere for 1 h at room temperature. The resulting AgCl precipitate was filtered with Celite. In the second step, a suspension of PAF-NPro (50 mg)54 in anhydrous THF (10 mL) was stirred vigorously for 15 min under nitrogen atmosphere. Then, the solution of [Rh(cod)(L)]BF4 was added to the mixture and stirred for 24 h at room temperature. The solid was centrifuged and washed with THF (3 × 10 mL), and diethyl ether (10 mL). Finally, the solid was dried under vacuum at 100 °C overnight to yield the corresponding rhodium heterogeneous catalyst. PAFAd-NPro-Rh: Anal. Found: %C = 60.8, %H = 5.4, %N = 4.0, Rh = 0.19 mmol/g. PAFC-NPro-Rh: Anal. Found: %C = 60.3; %H = 5.4; %N = 4.3; Rh = 0.36 mmol/g. Evaluation of Catalytic Performances for Cascade Reactions. The catalytic performance of different PAFs catalysts was evaluated by using two- and three-cascade reactions: Knoevenagel condensation joined with subsequent hydrogenation process and deacetalization− Knoevenagel condensation−olefin hydrogenation reaction. Knoevenagel Condensation−Olefin Hydrogenation. In a typical procedure: PAF-NProRh (15 mg) was dispersed in toluene (2 mL) under argon in a micro reaction vessel (5 mL), followed by adding of aldehyde (0.4 mmol), malononitrile (0.45 mmol), and dodecane (19 μL). Next, the mixture was stirred at 100 °C and subsequently was transferred to a Stainless Autoclave Engineers with 30 mL of a mixture of toluene/ethanol (1/2). The reactor was purged with H2 for 3 times and the H2 pressure was set to 5 bar and the temperature at 60 °C (tint) for the hydrogenation process and the mixture was stirred (1000 rpm). When the reaction was finished, the catalyst was separated by filtration and washed with ethanol and then reused for the above onepot tandem reaction. The samples were analyzed by GC-MS chromatography. Deacetalization−Knoevenagel Condensation−Olefin Hydrogenation Reaction. In a typical catalytic reaction, a 5 mL micro reaction vessel was charged with 1.5 mL of toluene, benzaldehyde dimethyl acetal (0.19 mmol), dodecane (0.12 mmol), water (1.6 μL),

Scheme 4. PAFC-SO3H+PAFC-NProRh-Catalyzed Cascade Deacetalization−Knoevenagel−Hydrogenation Reaction

SO3H+PAFC-NProRh was used as catalyst the first acetal hydrolysis reaction took place in 1.5 h, similar to previously described for PAFC-SO3H50 (1 h), the Knoevenagel condensation product was obtained after 12 h and hydrogenation reaction needed 6 additional hours, to yield final product 3 quantitatively. The reusability of PAFC-SO3H/PAFC-NProRh mixture for the three-step cascade reactions was checked by separation of the material via centrifugation. Then the material was washed with THF, buffered with AcONa/AcOH to recover some of acid centers on the catalyst, and washed with water until neutral pH was achieved (in the same way that the acid−base mixture). The catalyst activity did not altered after four uses. Recently, a solid catalyst containing acid, base, and palladium nanoparticle 1082


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ACS Sustainable Chemistry & Engineering malononitrile (29 mg), and catalyst (41 mg, 26 mg of PAF-SO3H, and 15 mg of PAF-NHProRh). The reaction mixture was stirred at 90 °C under argon for 1.5 h; then the mixture was placed into an autoclave at 5 bar H2 pressure (60 °C). The progress of the reaction was monitored by GC-MS chromatography. After the completion of the reaction, the catalytic mixture was separated by filtration and washed with toluene, a buffer AcONa/AcOH, water, THF, and Et2O. It was then reused for the above one-pot tandem reaction at least three times.

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(9) Motokura, K.; Tada, M.; Iwasawa, Y. Acid−base bifunctional catalytic surfaces for nucleophilic addition reactions. Chem. - Asian J. 2008, 3, 1230−1236. (10) Santos, L. L.; Serna, P.; Corma, A. Chemoselective synthesis of substituted imines, secondary amines, and β-amino carbonyl compounds from nitroaromatics through cascade reactions on gold catalysts. Chem. - Eur. J. 2009, 15, 8196−8203. (11) Corma, A.; Díaz, U.; García, T.; Sastre, G.; Velty, A. Multifunctional hybrid organic−inorganic catalytic materials with a hierarchical system of well-defined micro- and mesopores. J. Am. Chem. Soc. 2010, 132, 15011−15021. (12) Climent, M. J.; Corma, A.; Iborra, S. Heterogeneous catalysts for the one-pot synthesis of chemicals and fine chemicals. Chem. Rev. 2011, 111, 1072−1133. (13) Arnanz, A.; Pintado-Sierra, M.; Corma, A.; Iglesias, M.; Sánchez, F. Bifunctional metal organic framework catalysts for multistep reactions: MOF-Cu(BTC)-[Pd] catalyst for one-pot heteroannulation of acetylenic compounds. Adv. Synth. Catal. 2012, 354, 1347−1355 and references cited therein.. (14) Lee, J. M.; Na, Y.; Han, H.; Chang, S. Cooperative multi-catalyst systems for one-pot organic transformations. Chem. Soc. Rev. 2004, 33, 302−312. (15) Cozzi, F. Immobilization of organic catalysts: when, why, and how. Adv. Synth. Catal. 2006, 348, 1367−1390. (16) Shylesh, S.; Thiel, W. R. Bifunctional acid−base cooperativity in heterogeneous catalytic reactions: advances in silica supported organic functional groups. ChemCatChem 2011, 3, 278−287. (17) Alauzun, J.; Mehdi, A.; Reyé, C.; Corriu, R. J. P. Mesoporous materials with an acidic framework and basic pores. A successful cohabitation. J. Am. Chem. Soc. 2006, 128, 8718−8719. (18) Zeidan, R. K.; Hwang, S.-J.; Davis, M. E. Multifunctional heterogeneous catalysts: SBA-15-containing primary amines and sulfonic acids. Angew. Chem., Int. Ed. 2006, 45, 6332−6335. (19) Shylesh, S.; Wagener, A.; Seifert, A.; Ernst, S.; Thiel, W. R. Mesoporous organosilicas with acidic frameworks and basic sites in the pores: an approach to cooperative catalytic reactions. Angew. Chem., Int. Ed. 2010, 49, 184−187. (20) Yang, Y.; Liu, X.; Li, X.; Zhao, J.; Bai, S.; Liu, J.; Yang, Q. A Yolk−shell nanoreactor with a basic core and an acidic shell for cascade reactions. Angew. Chem., Int. Ed. 2012, 51, 9164−9168. (21) Dufaud, V.; Davis, M. E. Design of heterogeneous catalysts via multiple active site positioning in organic−inorganic hybrid materials. J. Am. Chem. Soc. 2003, 125, 9403−9413. (22) Huh, S.; Chen, H.-T.; Wiench, J. W.; Pruski, M.; Lin, V. S. Y. Cooperative catalysis by general acid and base bifunctionalized mesoporous silica nanospheres. Angew. Chem., Int. Ed. 2005, 44, 1826−1830. (23) Zeidan, R. K.; Hwang, S.-J.; Davis, M. E. Multifunctional heterogeneous catalysts: SBA-15-containing primary amines and sulfonic acids. Angew. Chem. 2006, 118, 6480−6483. (24) Sharma, K. K.; Asefa, T. Efficient bifunctional nanocatalysts by simple postgrafting of spatially isolated catalytic groups on mesoporous materials. Angew. Chem., Int. Ed. 2007, 46, 2879−2882. (25) Motokura, K.; Tada, M.; Iwasawa, Y. Layered materials with coexisting acidic and basic sites for catalytic one-pot reaction sequences. J. Am. Chem. Soc. 2009, 131, 7944−7945. (26) Shylesh, S.; Wagner, A.; Seifert, A.; Ernst, S.; Thiel, W. R. Cooperative acid−base effects with functionalized mesoporous silica nanoparticles: applications in carbon−carbon bond-formation reactions. Chem. - Eur. J. 2009, 15, 7052−7062. (27) Shiju, N. R.; Alberts, A. H.; Khalid, S.; Brown, D. R.; Rothenberg, G. Mesoporous silica with site-isolated amine and phosphotungstic acid groups: a solid catalyst with tunable antagonistic functions for one-pot tandem reactions. Angew. Chem., Int. Ed. 2011, 50, 9615−9619. (28) Climent, M. J.; Corma, A.; Iborra, S.; Sabater, M. J. Heterogeneous catalysis for tandem reactions. ACS Catal. 2014, 4, 870−891.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01147. Experimental details on the preparation and characterization of the PAF materials; additional tables and figures are included (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.I.) *E-mail: [email protected] (F.S.). Present Addresses ⊥

E.V.-S.: Adolphe Merkle Institute, Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland. ○ E.M.: Department of Chemistry, University of Zürich, Winterthurerstrasse 190, 8057-Zürich, Switzerland. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by MINECO through the followings projects: Consolider Ingenio 2009 (CSD-0050, MULTICAT) and MAT2011-29020-C02-02. MAT2014-52085-C2-2-P. E.M. thanks the JAE program for financial support.

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REFERENCES

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