Heterogeneous Prolinamide-Catalyzed Atom-Economical Synthesis of

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Article Cite This: ACS Omega 2019, 4, 8588−8597

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Heterogeneous Prolinamide-Catalyzed Atom-Economical Synthesis of β‑Thioketones from Bio-Based Enones Xiang Zhou, Fusheng Xu, Zhibing Wu, Hu Li,* and Song Yang* State Key Laboratory Breeding Base of Green Pesticide & Agricultural Bioengineering, Key Laboratory of Green Pesticide & Agricultural Bioengineering, Ministry of Education, State-Local Joint Engineering Lab for Comprehensive Utilization of Biomass, Center for R&D of Fine Chemicals, Guizhou University, Guiyang 550025, China

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S Supporting Information *

ABSTRACT: Sulfur-containing compounds are a class of important motifs extensively applied in pharmaceutical and pesticidal industries as well as in electrochemistry, toxicant separation, and organic syntheses. Herein, we describe a novel and efficient metal-free catalytic strategy for the rapid synthesis of β-thioketones from sustainable enone derivatives and thiols via thia-Michael addition enabled by heterogeneous prolinamide. At room temperature, up to 98% yield of β-thioketones could be obtained over the solid UiO-66-NH-proline catalyst facilely prepared by the covalent immobilization of proline onto UiO66-NH2 (a well-known metal−organic framework) via a stable amido linkage. A cooperative effect of proline (amino group) and UiO-66-NH2 (in situ-derived amide species) was observed to play a promotional role in the proceeding of thia-Michael addition, resulting in a high TOF value of 1124.3 h−1. A threecomponent “iminium” intermediate was illustrated to the key species approaching the product β-thioketone. Moreover, the UiO-66-NH-proline could be easily recovered from the reaction mixture and recycled for at least five times with a slight loss of activity.

1. INTRODUCTION To alleviate the reliance of industrial chemicals and fuels derived from fossil resources, one of the strategies is to synthesize renewable chemicals from the catalytic valorization of lignocellulosic biomass derivatives.1,2 Nowadays, many commodity chemicals and pharmaceuticals bear one or more heteroatoms. However, it is quite challenging to efficiently introduce heteroatom elements regioselectively into hydrocarbons. Most of the current technologies put pressure on our environment, which typically involves the formation of chemical waste and high energy consumption. Therefore, it would be promising to develop alternative strategies for the sustainable production of value-added chemicals containing heteroatom moieties to meet the requirements for more green chemical industry.2−6 Some simpler organosulfur molecules can be found in various biological systems and present in nature.7 Likewise, the sulfur atom is also an important species employed in the chemical industry.8,9 For example, some of those sulfur-containing compounds (e.g., thiophene, fucoidan, and lignosulfonate) can be applied in the production of surfactants and biomedicines.10−13 Considering that the synthesis of sulfur-containing chemicals from natural resources is widely unexplored, and the problems such as high energy consumption and low catalyst reusability are generally encountered,10−12 the development of milder and more efficient methods for C−S coupling of renewable resources © 2019 American Chemical Society

to synthesize new sulfur-containing compounds is highly necessary.2 Among various methodologies for the establishment of C−S bond, thia-Michael addition is one of the general strategies for site-specific reactions at CC bonds to promote the formation of C−S bond, which has attracted great attention in synthetic chemistry and constitutes a key reaction in various biosynthetic processes.9,14−18 In addition, this reaction also plays a critical role in the material chemistry and the synthesis of bioactive compounds especially those possessing antibacterial, antifungal, and anticancer activities.19−23 Although some methodologies on thia-Michael addition were reported in a lot of literature (Scheme 1),24−27 there are various limitations such as high temperature, long reaction time, the requirement of unique techniques to prepare catalysts, and the use of expensive and toxic metals.28 Thus, the further development of sustainable routes for the synthesis of renewable and feasible chemicals from biomass needs to circumvent those problems. Enones, especially chalcones also named (E)-1,3-diphenylpropene-1-ones, are widely distributed in the plant kingdom, which can be used as privileged and simple chemical scaffolds of many natural α,β-unsaturated carbonyl compounds.29−31 Received: February 7, 2019 Accepted: April 30, 2019 Published: May 16, 2019 8588

DOI: 10.1021/acsomega.9b00358 ACS Omega 2019, 4, 8588−8597

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of chemistry because of its unique and outstanding properties.45−47 In the present study, proline was simply immobilized onto a highly stable porous material UiO-66(Zr)-NH2 with covalent linkage to prepare an organocatalyst (UiO-66-NHproline). UiO-66-NH-proline bearing cooperative active sites (−NH2 and −CONH−) in combination with the suitable choice of organic solvent (DMSO) was found to exhibit pronounced activity in the synthesis of β-thioketones from enone derivatives and thiols via thia-Michael addition (Scheme 1), giving a high TOF value of up to 1124.3 h−1. Because of the heterogeneous nature and good stability, the catalyst could be recycled for several consecutive cycles with no decline in the reactivity. Importantly, the developed catalytic system was also applicable to a variety of substrates to synthesize corresponding β-thioketones in good yields. Notably, almost 100% atomefficiency was realized via the thia-Michael addition reaction process.

Scheme 1. Schematic Illustration of Catalytic Strategies Used for Thia-Michael Addition

Importantly, these compounds exert a great deal of biological and pharmacological activities.29,32,33 The application of ecofriendly synthetic methods for the synthesis of valued-added chalcones has aroused great interest among organic chemists. Currently, the strategy used for catalytic upgrading of chalcone was focused on the reduction of the α,β-unsaturated system by microorganisms, although this method has some obvious disadvantages including slow reaction, low yield, and poor selectivity.34 Moreover, the method of utilizing microwave and ultrasound irradiation for the transformation of chalcones into heterocyclic compounds has still controversy about its “greenness”. Instead, organocatalysis is especially advantageous when metal-free productions or processes are of primary concern, which thus performs a key principle of green chemistry.31 However, few studies have been reported about the synthesis of biomass-derived enones (e.g., chalcones) using an organocatalytic approach.35−37 Proline and its analogs play a crucial role in the flourishing period of organocatalysis because they are typically inexpensive, nontoxic, and environmentally friendly.38−44 Meanwhile, metal−organic frameworks (MOFs) was applied in the various fields including energy technologies and the application

2. RESULTS AND DISCUSSION 2.1. Catalyst Characterization. The prepared UiO-66NH2 and UiO-66-NH-proline were characterized by FT-IR, XRD, N2 adsorption−desorption, STEM-HAADF, UV−Vis Instruments, TGA techniques, and TEM, and the results are shown in Figures 1, 2, 7, and 9, Figures S1 and S2, and Table S1. FT-IR spectra of UiO-66-NH2 and UiO-66-NH-proline are collected in Figure 1a. Typical characteristics of the UiO-66NH2 were observed, including symmetric N−H stretching bands (3384 cm−1), N−H bending (scissoring) vibration at 1626 cm−1, and the strong C−N stretching absorption distinctive of aromatic amines at 1356 cm−1. The primary aromatic amino group displayed two medium absorptions of asymmetric N−H stretching bands (3507 cm−1).49−52 In comparison with UiO-66-NH2, UiO-66-NH2-proline had several additional weaker peaks at 3507 and 3384 cm−1, and the strong peak at 3196 cm−1 was due to the formation of amide between the NH2 group of the UiO-66-NH2 and the carboxyl group of proline. These results indicate that free

Figure 1. Pyridine-adsorbed FT-IR spectra (a), XRD patterns (b), N2 adsorption isotherms (c), and TG curves (d) of UiO-66-NH2 and UiO-66NH-proline. 8589


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Figure 2. STEM-HAADF images and elemental mappings of (a) UiO-66-NH2 and (b) UiO-66-NH-proline.

Figure 5. Reaction kinetics of UiO-66-NH-proline in the 1,4-thiaMichael addition of enone (A1) and 1-propanethiol to 1,3-diphenyl3-(propylthio)propan-1-one (B1). Reaction conditions: 0.50 mmol enone (A1), 0.55 mmol 1-propanethiol, 0.37 mol % UiO-66-NHproline, 5 mL DMSO, T = 25 °C, t = 12 h. TOF = (mole of product) / (mole of prolinamide × time).

Figure 3. Activity of various catalysts in the 1,4-thia-Michael addition of enone (A1) with 1-propanethiol to 1,3-diphenyl-3-(propylthio)propan-1-one (B1). Reaction conditions: 0.10 mmol enone A1, 0.11 mmol 1-propanethiol, 0.37 mol % UiO-66-NH-proline, 1 mL DMSO, 25 °C for 12 h.

Figure 6. Recycling studies of UiO-66-NH-proline in the 1,4-thiaMichael addition of enone (A1) and 1-propanethiol to 1,3-diphenyl3-(propylthio)propan-1-one (B1). Reaction conditions: 0.50 mmol enone (A1), 0.55 mmol 1-propanethiol, 0.37 mol % UiO-66-NHproline, 5 mL DMSO, T = 25 °C, t = 12 h.

Figure 4. Effect of (a) solvent type and (b) UiO-66-NH-proline dosage in the 1,4-thia-Michael addition of enone (A1) with 1propanethiol to 1,3-diphenyl-3-(propylthio)propan-1-one (B1). Reaction conditions: 0.10 mmol enone A1, 0.11 mmol 1-propanethiol, 0.37 mol % UiO-66-NH-proline, 1 mL DMSO, 25 °C for 12 h.

core where the triangular faces of the Zr6-octahedron are alternatively capped by μ3-O and μ3-OH groups.53 This result indicates that the presence of the proline did not play any influence on the structure of the MOF. The N2 adsorption− desorption isotherms of UiO-66-NH2 and UiO-66-NH-proline are shown in Figure 1c, and the BET surface areas of UiO-66NH2 and UiO-66-NH2-proline are 603.3 and 88.8 m2 g−1, respectively. The decline in the surface area after amido functionalization could be due to the significantly decreased micropores of UiO-66-NH2 (Figure S1). These results, on the

proline was successfully immobilized onto the UiO-66-NH2 via the stable amido linkage.51 The XRD spectra of UiO-66-NH2 and UiO-66-NH-proline are shown in Figure 1b. It can be clearly seen that the pattern of UiO-66-NH2 was consistent with that of UiO-66-NHproline, which is typically consisted of an inner Zr6O4(OH)4 8590


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thioketones via the formation of the C−S bond. Thus, the 1,4thia-Michael addition of enone A1 with 1-propanethiol to 1,3diphenyl-3-(propylthio)propan-1-one (B1) in DMSO at 25 °C for 12 h was chosen as the model reaction in the presence of 20 mol % of different control catalysts, and the obtained results are shown in Figure 3. Almost no reaction occurred in the absence of any catalyst or even in the presence of Cat. 1 (tertbutyl 2-(phenylcarbamoyl)pyrrolidine-1-carboxylate), Cat. 2 (methyl L-prolinate hydrochloride), Boc-L-proline, and UiO66-NH2-proline-Boc. In contrast, UiO-66-NH2 (containing ca. 0.037 mmol proline unit per gram, as determined by elemental analysis) and proline could afford B1 in yields of 18 and 69%, respectively (Figure 3). These results indicate the promotional effect of amino species and the synergistic role between amino and carboxyl groups in the 1,4-thia-Michael addition. Gratifyingly, Cat. 3 (N-phenylpyrrolidine-2-carboxamide) was able to give a satisfactory B1 yield of 87%, implying the crucial role of the prolinamide fragment in the reaction.54,55 More interestingly, an excellent B1 yield (98%) was obtained as UiO-66NH-proline (0.37 mol %) and used as the catalyst. This unprecedented result demonstrates the cooperative effect of the solid support (UiO-66) and prolinamide, which could be promoted by the in situ formed iminium between the carbonyl group of the substrate and the amino species of the catalyst. 2.2.2. Effect of Solvent Type, Temperature, and Catalyst Dosage. The effect of different solvents was further examined to optimize the reaction conditions for the 1,4-thia-Michael addition of enone with 1-propanethiol to produce B1, and the results are summarized in Figure 4a. In the presence of 0.37 mol % UiO-66-NH-proline at 25 °C, the use of hexane, tertbutanol, dichloromethane (DCM), 1,4-dioxane, methanol, and DMSO as solvents could give the product B1 with the yield of 7.0, 9.0, 10, 12, 42, and 98%, respectively. Clearly, DMSO was the best solvent for the reaction, possibly because of its high polarity (Figure 4a) and the good solubility toward the reactants. Then, the effect of temperature on the reaction was examined, for example, at the temperature of 25 °C, 92% yield of A1 was achieved after 12 h. When the reaction temperature was gradually raised to 40, 60, and 80 °C, the comparable yield of A1 was achieved after 8, 6, and 5 h, respectively. In most cases, the reaction temperature of 25 °C was chosen, which is highly applicable to a wide range of substrates, showing great potential in the efficient synthesis of various β-thioketones via C−S bond formation under benign conditions. Considering the pronounced catalytic activity in the 1,4-thiaMichael addition of enone (A1) with 1-propanethiol, the dosage of UiO-66-NH-proline was further examined to study the catalyst efficiency (Figure 4b) at 25 °C in DMSO. It was clearly observed that the B1 yield significantly increased from 64, 88, 95, and 98% to 99% with the increase of catalyst dosage from 1.0 mg (0.037 mol %), 5.0 mg (0.19 mol %), 10.0 mg (0.37 mol %), and 20.0 mg (0.74 mol %) to 30.0 mg (1.11 mol %), respectively. It was worth noting that using 10, 20, or 30 mg of UiO-66-NH-proline resulted in almost quantitative yields of B1. From the economic point of view, 10 mg was selected as the optimal catalyst dosage, which was thus employed for the following investigations. 2.2.3. Reaction Kinetic Studies. The reaction time is another key factor affecting the efficiency of the catalytic processes. At the reaction temperature of 25 °C, the TOF value of the reaction could reach 1124.3 h−1 in 1 min for the 1,4-thia-Michael addition reaction (Figure 5). This reaction rate is much higher than those achieved in many other similar

Figure 7. XRD spectra of fresh and reused (after five cycles) UiO-66NH-proline catalysts.

Figure 8. Time-yield plots for the synthesis of 1,3-diphenyl-3(propylthio)propan-1-one (B1) from enone (A1) and 1-propanethiol in the presence of UiO-66-NH-proline (black line) or without UiO66-NH-proline by removing after 5 min (red line). Reaction conditions: 0.50 mmol enone (A1), 0.55 mmol 1-propanethiol, 0.37 mol % UiO-66-NH-proline, 5 mL DMSO, T = 25 °C, t = 12 h.

other hand, confirmed the immobilization of proline onto the surface and pores of UiO-66-NH2. To investigate the thermal stability of UiO-66-NH2 and UiO-66-NH-proline, TG analysis was implemented, and the results are illustrated in Figure 1d. From ambient temperature to 600 °C, the first weight loss at below 100 °C could be attributed to the loss of residual solvent from of materials. The second weight loss of 5−7% was observed at up to 400 °C, which is possibly because of the framework decomposition and removal of water molecules. However, an additional weight loss event was observed in the temperature range of 100−400 °C in the case of UiO-66-NH-proline, which could be ascribed to the thermal decomposition of the proline group. From 400 to 600 °C, the residual weight of UiO-66-NH2 was lower than UiO-66-NH-proline, indicating that UiO-66-NH-proline contained a certain amount of organic matter residue. HAADF-STEM with elemental mappings was performed to study the elemental dispersion and morphology of UiO-66NH2 and UiO-66-NH-proline (Figure 2a,b). The elements of C, N, O, and Zr were evenly distributed in both UiO-66-NH2 and UiO-66-NH-proline. In addition, the particle size of UiO66-NH-proline (average diameter: ca. 74.3 nm) was found to be larger than UiO-66-NH2 (average diameter: ca. 45.9 nm), as disclosed by TEM images (Figure S2a,b). 2.2. Optimizing Reaction Conditions of 1,4-ThiaMichael Addition. 2.2.1. Effect of Various Catalysts. ThiaMichael addition of enones is a typical reaction to produce β8591


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Figure 9. Results for the catalytic 1,4-thia-Michael addition of enones (A) and thiols to β-thioketones (B, C, or D) with UiO-66-NH-proline at 25 or 60 °C. Values along with the compounds are B, C, or D isolated yields. Reaction conditions: 0.15 mmol enones (A), 0.16 mmol thiols, 15.0 mg UiO-66-NH-proline, 1.5 mL DMSO, T = 25 or 60 °C, t = 0.3−24 h.

coupling reactions (e.g., TOF = 54.6 h−1 for the aza-Michael reaction using MOF-199, and TOF = 2.75 h−1 for aldol reaction using MOPMs).56,57 However, the reaction gradually became slow with further prolonging the reaction time to 5 min, which was probably due to the coverage of the active sites (prolinamide) by the reactant or product. These results, on the other hand, indicate the formation of relatively stable intermediates like iminium species that can be fast formed in the very early stage, whereas the lack of active sites caused by saturated adhesion with organics in the following period may limit its further conversion. Therefore, a relatively long reaction time seems to be necessary to achieve satisfactory product yields. 2.2.4. Catalyst Recycling Studies. In the view of green chemistry and potential practical application, it would be much

promising if the solid catalyst can be recovered and reused for consecutive recycles. In this regard, the recoverability and reusability of the UiO-66-NH-proline catalyst were thus investigated for the thiol-Michael reaction of enone A1 with 1-propanethiol for five successive reaction runs under optimized reaction conditions of using DMSO as solvent at 25 °C for 12 h. After each cycle of the reaction, the resulting mixture was filtered, whereas the recovered solid catalyst was thoroughly washed with ethanol, dried in an oven at 70 °C for 4 h, and directly used for the next cycle. As shown in Figure 6, the UiO-66-NH-proline catalyst exhibited almost constant catalytic activity in five consecutive cycles with the product yield slightly decreasing by 9%, which is much superior to previously reported solid catalysts.58 The leaching content of Zr species from the UiO-66-NH-proline catalyst was 8592


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derivatives A1−A25 and thiols C1−C4 under optimized reaction conditions (Figure 8). As expected, good isolated yields of product B were obtained using DMSO as a solvent at 25 °C. When no substituent or electron-donating group such as 2-methoxyl was present in the ortho- or para-position of the benzene ring, moderate to excellent yields of products were obtained (B11−B17, B23, B24: 70−98% yields). Whereas moderate yields were achieved (B2−B6, B18−B22: 61−81% yields) as R2 was heterocyclic or electron-withdrawing group located on the benzene ring. Both the electron-donating (e.g., R1 = 4-hydroxyphenyl) and electron-withdrawing group (e.g., R2 = 3-chlorophenyl) could attain moderate product yields (B7−B10: 70−79% yield). Not only enones derived from nature (e.g., A1) but also the ones (e.g., A21) synthesized from bio-based molecules 2-acetyl-5-methylfuran and furfural could be used as substrates to afford moderate isolated yields of corresponding products. In addition, substrates also have a moderate tolerance for different cyclic enones to give C1−C2 in 60−76% isolated yields, and for various thiols to give D1− D4 in 69−88% isolated yields under the same reaction conditions. When the reaction temperature was increased to 60 °C, the enones with a sterical and electronical methyl substituent in β-position could give B26 and 2-cyclohexen-1one C2 in moderate yields. To further expand the scope of the thia-Michael addition reaction, a value-added compound D5, which is an allosteric PIF-Pocket ligand of phosphoinositidedependent protein kinase 1(PDK1),59−61 was also produced with a good yield of 87% in only 20 min. In contrast, as high as 70% yield of D5 was obtained over triethylamine in a previous report, and much longer time (2 h) was required.61 These results explicitly indicate that our developed catalytic system is highly applicable to a wide range of enone derivatives, showing great potential in the efficient synthesis of various valuable βthioketones via C−S bond formation under benign reaction conditions. Furthermore, the enantioselectivity (ee) values of some representative products (e.g., B1) were determined, and no more than 5% ee values could be obtained (Figures S3, S4, and S5). The low ee values, on the other hand, can be an evidence that the solid UiO-66-NH-proline catalyst was prepared by the covalent immobilization of proline onto UiO-66-NH2 via a stable amido linkage. 2.4. Proposed Reaction Mechanism. Before discussing the reaction mechanism, enone (A1) with different molar ratios (0.1−1.0 M) of tetrahydropyrrole was mixed and elucidated by 1H NMR spectra. However, no evident change in intensity and chemical shift assigned to enone was observed. In turn, during the experimental processes, the reaction color was changed by mixing pyrrolidine with 1-propanethiol (Figure S6). Accordingly, UV analysis was adopted to further verify the possible reaction mechanism by dissolving 20 μM A1 or A21, 22 μM 1-propanethiol, and/or 20 μM pyrrolidine into DMSO. Solutions consisted of different reaction mixtures were analyzed using a UV spectrophotometer (TU-1900). As expected, three components were highly needed to work together to afford the target products in both cases (Figure 10). As illustrated in Figure 3, proline and Cat. 3 can afford B1 in moderate yields of 69−87%, indicating the promotional effect of amino species especially the prolinamide fragment in the 1,4-thia-Michael addition. Notably, the unprecedented result obtained with 0.37 mol % UiO-66-NH-proline (98% B1 yield) demonstrates the cooperative effect of the solid support (UiO66) and prolinamide, which can be facilitated by the in situ

determined by inductively coupled plasma optical emission mass spectrometry (ICP-MS) with the aid of acid digests. The contents of Zr in the fresh and used (after 5 cycles) catalysts were found to be 30.0 and 29.7 wt %, respectively, indicating little Zr leaching into the reaction mixture (ca. 0.3 wt %). Likewise, elemental analysis disclosed that ca. 0.2 wt % nitrogen content (fresh: 4.5 wt %, used: 4.3 wt %) was leached in the five consecutive cycles. The XRD spectra of fresh and reused (after five cycles) UiO-66-NH-proline catalysts exhibit no obvious change between each other (Figure 7), indicating that the loss of catalytic activity could be ascribed to the organic adsorption. These results illustrate that the UiO-66NH-proline catalyst can maintain its crystalline structure during the reaction process under the tested conditions. 2.2.5. Heterogeneous Catalytic Behavior. To examine whether the solid UiO-66-NH-proline catalyst behaves as heterogeneous catalysis in the 1,4-thia-Michael addition reaction, the effeciency of the leached Zr and N species were investigated under the identical reaction conditions (Table 1). Table 1. Catalytic Performance of Homogeneous Zr and N Species in the 1,4-Thia-Michael Addition of Enone (A1) and 1-Propanethiol to 1,3-Diphenyl-3-(Propylthio)Propan1-One (B1)a

entry

catalyst

yield

1 2 3 4 5 6 6 8

0.06 wt % ZrCl4 0.3 wt % ZrCl4 0.04 wt % proline 0.2 wt % proline 0.04 wt % 2-aminoterephthalic acid 0.2 wt % 2-aminoterephthalic acid 0.04 wt % proline/0.06 wt % ZrCl4 0.04 wt % 2-aminoterephthalic acid/0.06 wt % ZrCl4

4% 5% 9% 12% 98% purity) and used as received, unless otherwise noted. All compounds were fully characterized by spectroscopic data. The NMR spectra were recorded on Bruker Avance I (1H NMR 400 MHz, 13C NMR 101 MHz) and JOEL ECX-500 M (1H NMR 500 MHz, 13C NMR 125 MHz). Chemical shifts (δ) are expressed in parts per million (ppm), J values are given in hertz, and DMSO-d6 and CDCl3 were used as solvents. The reactions were monitored by a thin-layer chromatography (TLC) using silica gel GF254. HRMS (ESI) analysis was measured on a Q Exactive UHPLC−MS/MS instrument (Thermo Fisher Scientific, Waltham, USA). UV−Vis spectra were performed on a TU-1900 spectrophotometer (Beijing Pgeneral Instrument Co., China). The enantioselectivity (ee) values of samples were determined by HPLC analyses, which were measured on Water systems with a Empower3 system controller, an Alliance column heater, and a 2998 Diode Array Waters 2489 UV−Vis detector. Chiralcel brand chiral columns from Daicel Chemical Industries were used with models ASH, or OD-H in 4.6 × 250 mm size. All chemicals and solvents were used as received without further purification unless otherwise stated. 4.2. Preparation of UiO-66-NH2. The synthesis of UiO66-NH2 was performed according to a previously reported method48 with slight modification. In a typical procedure, 0.24 g of ZrCl4 (1.029 mmol) and 0.186 g of 2-aminoterephthalic acid (1.029 mmol) were dissolved into 60 mL DMF in a stainless steel autoclave. Then, the autoclave was sealed and heated in an oven at 120 °C for 48 h under autogenous pressure. Upon completion, the reactor was automatically cooled down to room temperature, followed by filtration, and washed with DMF (20 mL × 3) and methanol (20 mL × 4). The obtained solid was then dissolved into 60 mL methanol in a 100 mL round-bottom flask, and heated to 60 °C under vigorously stirred for 12 h. After cooling down, the reaction mixture was subjected to filtration, and the resulting precipitate was dried at 70 °C overnight in an oven to give the metal− organic framework UiO-66-NH2 as pale yellow powder. 4.3. Preparation of UiO-66-NH-proline-Boc. The immobilization of Boc-proline onto UiO-66-NH2, denoted as UiO-66-NH-proline-Boc, was achieved as follows. DMF (206 μL) and oxalyl chloride (250) were added into DCM (about 20 mL) in a round bottom flask, which was then placed into an ice bath under magnetic stirring for 10 min, followed by additions of 223 μL pyridine and 595 mg Boc-proline under magnetic stirring for another 1 h at room temperature. Afterward, UiO-66-NH2 was added into the above mixture and stirred overnight. The resulting mixture was filtered and washed twice with DCM and once with water, finally dried at

Figure 10. (a−h) UV absorption spectra of different reaction mixtures in DMSO: (a) 20 μM enone A1, (b) the mixture of 20 μM enone A1 and 20 μM pyrrolidine, (c) the mixture of 20 μM enone A1 and 22 μM 1-propanethiol, (d) the mixture of 20 μM enone A1, 20 μM pyrrolidine, and 22 μM 1-propanethiol, (e) 20 μM enone A21, (f) the mixture of 20 μM enone A21 and 20 μM pyrrolidine, (g) the mixture of 20 μM enone A21 and 22 μM 1propanethiol, (h) the mixture of 20 μM enone A21, 20 μM pyrrolidine, and 22 μM 1-propanethiol.

formed iminium between the carbonyl group of the substrate and the amino species of the catalyst. On the basis of the above discussions, Figure 11 vividly depicts the possible mechanism

Figure 11. Possible catalytic mechanism for the thia-Michael addition of enones (A) with thiol to β-thioketone (B, C, or D).

where three components composed of UiO-66-NH-proline, enones, and thiol are first subjected to form an “iminium ion” intermediate via dehydration, which is similar to that reported in literature,62−66 followed by addition and hydrolysis to give the product β-thioketone.

3. CONCLUSIONS In summary, we developed an efficient C−S coupling approach for the thia-Michael addition at room temperature enabled by heterogeneous prolinamide immobilized on the stable and porous support UiO-66(Zr). A simple method was developed to successfully prepare the novel catalyst UiO-66-NH-proline, which was demonstrated to possess moderate proline loading (ca. 0.037 mmol proline unit per gram, as determined by elemental analysis) and be highly recyclable for at least five cycles with little decrease in catalytic activity. With this catalytic system, a series of sulfur-containing scaffolds were synthesized from enone derivatives, including biomass-derived chalcones. Interestingly, reaction kinetics was further studied, and the resulting TOF value of the reaction was up to 1124.3 h−1. The possible reaction progress was illustrated by ex situ UV−Vis analysis, where the in situ formed “iminium” intermediate was demonstrated to be crucial for efficiently yielding the β-thioketone product. The rapid and green synthesis of β-thioketones from enone derivatives and thiols in the assistance of prolinamide on UiO-66(Zr) in DMSO at 8594


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40 °C overnight to afford UiO-66-NH-proline-Boc as yellow powder. 4.4. Preparation of UiO-66-NH-proline. The synthesis of UiO-66-NH-proline was performed by the following procedures. UiO-66-NH-proline-Boc (395.0 mg) and trifluoroacetic acid (2 mL) were dissolved into DCM (about 20 mL) in a round bottom flask placed in an ice bath. After stirring 6 h at room temperature, 20 mL of water was added into the mixture. The mixture was then basified with a 50% NaOH (about 2 mL) aqueous solution was dripped to pH = 7.5−8.0. The resulting precipitate was filtered off, washed with 10 mL DMSO and 50 mL ethanol, and then dried to give UiO-66NH2-proline as white powder. 4.5. Catalyst Characterization. Scanning transmission electron microscope and high-angle annular dark-field (STEMHAADF) imaging were measured by JEOL 2100 TEM/STEM. FT-IR spectra were recorded on a Nicolet iS 10 FT-IR instrument (KBr disc; Thermofisher, MA) in the range of 400−4000 cm−1. Scanning electron microscopy (SEM) image was performed using a FESEM XL-30 (Philips) electron microscope. X-ray diffraction (XRD) measurements were carried out using a D/Max-3c X-ray diffractometer with a radiation source of Cu Kα, scanning from 5 to 90°, at an operating voltage and current of 40 kV and 30 mA, respectively. BET (Brunauer−Emmett−Teller) surface areas of the porous materials were determined from nitrogen physisorption measurements at liquid nitrogen temperature on a Micromeritics ASAP 2010 instrument. The volume of pores was estimated from the BJH adsorption cumulative volume of pores. Thermogravimetric (TG) analysis was conducted using a Netzsch STA 429 instrument in a temperature range of 25−600 °C with a heating rate of 10 °C min−1 under N2 atmosphere (flow rate: 30 mL min−1). The products obtained by 1,4-thia-Michael addition were dissolved in CDCl3 or DMSO-d6 for NMR analysis. UV−Vis spectra were performed on a TU-1900 spectrophotometer (Beijing Purkinje general Instrument Co., China). Elemental analysis on nitrogen was performed on an Elemental Vario-III CHN analyzer (Hanau, Germany). Zr contents of the fresh and used UiO-66-NH-proline catalysts were determined by ICP-MS on Thermo Scientific iCAP Q ICP-MS with the aid of acid digests. The fresh and used UiO-66-NH-proline samples were digested in 70% nitric and 98% sulfuric acids (1/1, v/v) at 90 °C for 12 h, and the resulting solution was then diluted 20 times with water before ICP analysis. 4.6. Synthesis of Compounds. All enone derivatives were efficiently synthesized via the one-step procedure, and the detailed synthetic procedures and characterization data of corresponding isolated productions could be found in the Supporting Information. The product yields were determined by 1H NMR using 1,3,5-trimethoxybenzene as the internal standard.

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A21; general experimental procedures; spectroscopic data of all the synthesized compounds (experimental protocols) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: +86(851)8829-2171. (H.L.). *E-mail: [email protected] (S.Y.). ORCID

Hu Li: 0000-0003-3604-9271 Song Yang: 0000-0003-1301-3030 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21576059 & 21666008), Key Technologies R&D Program of China (2014BAD23B01), Fok Ying-Tong Education Foundation (161030), and Guizhou Science & Technology Foundation ([2018]1037 & [2017] 5788).

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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/acsomega.9b00358. Pore size distribution; TEM images; textural structure of the UiO-66-NH2 and UiO-66-NH-proline samples; stereochemistry determination of B1, B2, B3; observation of color change in the reaction of enone A1 and 8595


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