Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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A Multifunctional Metal−Organic Framework for Oxidative C−O Coupling Involving Direct C−H Activation and Synthesis of Quinolines Vivekanand Sharma, Dinesh De, and Parimal K. Bharadwaj* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, Uttar Pradesh, India
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S Supporting Information *
ABSTRACT: A robust paddle-wheel Cu(II)-based metal−organic framework (MOF) 1, having dual functionalities, namely, Lewis acid and basic sites, has been explored as a heterogeneous catalyst. This MOF, because of its large void volume (10298 Å3, 67.6%), large surface area (1480 m2/g), and high thermal stability, encouraged us to see its applicability in two catalytic reactions, namely, oxidative C−O coupling (cross-dehydrogentaive coupling reaction) involving direct C−H activation and Friedländer reaction under solvent free and ambient conditions. This study demonstrates the green aspect of MOFs in coupling reactions because of the simplified recovery, shorter reaction time, minimum waste, and smooth activation of the C−H bond, which is very challenging in synthetic chemistry.
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INTRODUCTION Today, it is highly desirable to obtain a catalyst that can exhibit multifunctional activity toward various catalytic transformations. Among these, metal−organic frameworks (MOFs) have captured considerable attention within the scientific community across a range of different laboratories. The past several years have witnessed vigorous efforts to develop and apply MOFs in gas storage, CO2 sequestration, gas separation, sensors, optics, biomedical devices, and catalysis.1−3 However, the utilization of MOFs in catalysis has not been explored that much compared to their possible applications in gas storage and capture. The nanoporosity of a MOF coupled with a replete number of transition metal sites in MOF can be applicable as catalytic sites unless the free coordination positions are not utilized in the construction of MOFs. It has been reported that many MOFs were employed for transition metal-catalyzed reactions, which are routinely used in the synthesis of a number of valuable compounds.4−7 Thus, the MOF has been examined as a heterogeneous catalyst. Some MOFs are more prone to heat and chemicals, and the lack of availability of a free coordination position retards their application in catalysis. In the green chemistry context, it is imperative to develop more environmentally friendly and versatile catalytic systems; we are hopeful we can examine these aspects of MOFs, which are vitally important because of simplified recovery and reusability of a catalyst. Transition metal-catalyzed C−C and C−heteroatom crosscoupling reactions have attracted significant attention for the synthesis of a variety of valuable chemicals and important intermediates.8 Despite the great achievement in classical C−C bond formation methods, generally they use prefunctionalized © XXXX American Chemical Society
starting materials that require additional steps and produce severe environmental hazards.9−11 Also, in the transition metalcatalyzed C−heteroatom coupling reaction, C−H bond activation is a highly challenging process because of the inertness. Thus, the direct activation of the C−H bond is a possible way to solve this problem.12,13 To overcome these synthetic difficulties, we aim to develop such a catalyst that could offer a sustainable and green perspective in terms of such cross-coupling reactions with improved synthetic protocols. To illustrate the aforementioned arguments, we have chosen the cross-dehydrogenative coupling reaction for C−H activation. Such coupling reactions offer short and efficient synthetic schemes, which are key aspects for the next generation of C−C or C−heteroatom bond formations.14 In addition, considering the robustness and Lewis acidity of MOF 1, quinoline synthesis has been performed via Friedländer reaction to probe its further applicability in catalysis. Quinoline and its derivatives are well-known compounds that display a broad range of biological significance.15−18
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EXPERIMENTAL SECTION
Materials and General Methods. All the required chemicals for the synthesis of MOF and catalytic reactions were purchased from Sigma-Aldrich or S. D. Fine Chemicals. The various spectroscopic data were collected for characterization of the compounds as described in our previous report.19 Field emission scanning electron microscopy (FE-SEM) images were collected on a CARL ZEISS EVO Received: March 14, 2018
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DOI: 10.1021/acs.inorgchem.8b00683 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
(BET) surface area of this MOF was found to be 1480 m2 g−1, with a pore volume of 0.88 cm3 g−1. From a topological point of view, it has a 3,3,4,4-connected net with mfj topology and can be considered as a close packing of channels, shown in Figure 1c. The framework was endowed with large spherical cages (Figure 1b) with a diameter of ∼11 Å (atom−atom distances excluding van der Waals radii). The Lewis acidity of these cages increased because of the vacant coordination positions (Figure 1d) on copper, and hence, the compound was explored as a nanomolecular vessel for C−O bond and C− C bond coupling reactions, viz., cross-dehydrogentaive coupling (CDC) and Friedländer reactions, respectively. Cross-Dehydrogenative Coupling Reactions. Direct C−C and C−heteroatom bond formation through C−H bond activation has emerged as a versatile tool for introducing complexities into molecules from simple precursors in synthetic organic chemistry.20,21 In particular, cross-dehydrogenative coupling reactions under oxidative conditions are atom economical and prevent prefunctionalization of the coupling partner and hence offer a superior pathway for coupling reactions.22−26 We have explored the cross-dehydrogenative coupling reactions by direct C−H activation of 1,4-dioxane with 2carbonyl-substituted phenols (Table 1) under solvent free
50 instrument, at an acceleration voltage of 15.00 kV with 200000× magnification. Synthesis of {[Cu6(L)3(H2O)6]·(14DMF)(9H2O)}n (1). The synthesis of 1 was achieved following our previous report.19 General Procedure for Cross-Dehydrogenative Coupling Reactions. A solution of 2-hydroxybenzaldehyde (1.0 mmol) in 1,4dioxane (4 mL, 50 mmol) was placed in a 10 mL round-bottom flask preloaded with catalyst 1′ (1 wt %). Then, tert-butyl hydroperoxide (70 wt % in water, 0.436 mL, 3.0 mmol) was added to the resulting mixture. The mixture was stirred at 100 °C for 1 h. To monitor the reaction, thin layer chromatography (TLC) (using a 2:1 n-hexane/ ethyl acetate mixture as the eluent) was performed. Then, the reaction mixture was filtered and washed successively with CH2Cl2 to recover the catalyst. The filtrate was passed through anhydrous Na2SO4, and the solvent was evaporated. The crude product was purified by silica gel column chromatography (eluent being 10% ethyl acetate in nhexane) to give the product. The recovered catalyst was washed thoroughly and reused after reactivation at 130 °C under vacuum for 6 h. General Procedure for Friedländer Reaction. Acetylacetone (5 mmol), 1′ (1 wt %), and 2-aminoarylketone (1 mmol) were added to the 25 mL round-bottom flask, and the mixture was heated at 80 °C under solvent free conditions until the total consumption of the reactants was detected by TLC, which took ∼8 h. After cooling, the reaction mixture was filtered and washed thoroughly with CH2Cl2 to remove any possible adhered products on the catalyst surface. The filtrate was passed through Na2SO4, and the solvent was evaporated. The crude product was purified by silica gel column chromatography using an n-hexane/ethyl acetate mixture (19:1 ratio) as the eluent to give the desired product. The recovered catalyst was washed thoroughly and reused after reactivation at 130 °C under vacuum for 6 h.
Table 1. CDC Reactions under Solvent Free Conditions at 100 °Ca
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RESULTS AND DISCUSSION In our previous work,19 we had synthesized the thermally robust MOF {[Cu6(L)3(H2O)6]·(14DMF)(9H2O)}n (1) from a bent amino-functionalized tetracarboxylate ligand, H4L (Figure 1a), and Cu(NO3)2 under solvothermal conditions.
entry
catalyst
R1
R2
yield (%)b
1 2 3 4 5 6 7 8
1′ 1′ 1′ 1′ 1′ Cu(OAc)2 Cu(NO3)2 no catalyst
H H Cl NO2 unsubstituted phenol H H H
H OMe H H − H H H
96 92 84 65 10 73 71 no reaction
a Reaction conditions: hydroxybenzaldehyde (1.0 mmol), 1′ (1 wt %), 1,4-dioxane (4 mL, 50 mmol), tert-butyl hydroperoxide (70 wt % in water, 0.436 mL, 3.0 mmol). bYield of the isolated product.
conditions using tert-butyl hydroperoxide (TBHP) as an oxidant. In a typical experiment, a mixture of substituted 2hydroxybenzaldehyde (1.0 mmol) and 1,4-dioxane (4 mL, 50 mmol) was added to a flask containing catalyst 1′ (1 wt %). Then, TBHP (70 wt % in water, 0.436 mL, 3.0 mmol) was added. The reaction mixture was stirred at 100 °C for 1 h, and its progress was observed by TLC. A look at the results indicated that the unsubstituted 2hydroxybenzaldehyde (Table 1, entry 1) and electron-donating group on 2-hydroxybenzaldehyde (Table 1, entry 2) afforded comparable yields, while the presence of electron-withdrawing groups at the para position to the phenolic -OH group (Table 1, entries 3 and 4) led to a decrease in the yield of the desired product. Phenol was almost unreactive with only 10% conversion after 4 h (Table 1, entry 5). In the absence of catalyst 1′, the reaction showed no progress (Table 1, entry 8), which indicated the catalytic nature of 1′. The identity of the product in each case was confirmed by electrospray ionization mass spectrometry (ESI-MS), 1H nuclear magnetic resonance
Figure 1. (a) Ligand H4L. (b) Nanospherical cages in 1. (c) mfj topology in 1 (closely packed by nanospherical cages). (d) Replete number of open metal sites (polyhedra).
The framework consisted of paddle-wheel Cu 2 (CO 2 ) 4 secondary building units (SBUs). Water molecules occupying the axial positions of the SBU could be easily removed by heating, leading to the formation of highly porous (67.6% void volume) 1′ in which the metal ions became coordinatively unsaturated with enough coordination space available for heterogeneous catalysis. The Brunauer−Emmett−Teller B
DOI: 10.1021/acs.inorgchem.8b00683 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry (NMR), and 13C NMR (Figures S1−S12) after silica gel column chromatography. Catalyst 1′ exhibited profound catalytic activity compared to that of other paddle-wheel Cu MOFs such as Cu3(BTC)2, Cu(BDC), and Cu(BPDC) and catalytic activity superior to that of Cu2(BPDC)2(BPY) as reported by the Phan group.27 Also, 1′ offers advantages over homogeneous copper salts such as Cu(OAc)2 and Cu(NO3)2 (Table 1, entries 6 and 7) in terms of both catalytic activity and catalyst recyclability. The recyclability of a catalyst makes it viable for commercial applications; hence, catalyst 1′ was investigated for its recoverability and recyclability in the coupling reaction. After the coupling reaction was complete, the catalyst was separated by simple filtration, washed with methanol followed by DCM to remove the physisorbed product, and then activated by being heated at 130 °C for 6 h under vacuum to reactivate the catalyst. The recovered catalyst was then reused in the successive reactions under the same conditions that were used for the first run. In fact, an 85% yield was still achieved in the fifth run (Figure S13); nonetheless, crystallinity was maintained during the course of the reaction, which was evident after assessment via powder X-ray diffraction (PXRD) (Figure S14). The heterogeneity of catalyst 1′ was justified by a hot filtration test, following the standard protocol.28 The reaction mixture was filtered after 15 min (∼38% yield) to separate the catalyst. The resulting reaction mixture did not afford the further progress, establishing the heterogeneity (Figure S15). A simple and straightforward probable mechanism for the CDC reaction is given in Scheme 1. The open Cu(II) site first
Friedländer Reactions. The Friedländer reaction offers an efficient and straightforward synthetic protocol for the synthesis of various nitrogen heterocycles and quinoline derivatives that are of paramount importance in various biological functions.15−18 In addition to their biological activity, they are also useful in the synthesis of nanostructured material with enhanced optoelectronic properties.29−31 Because of their importance, several methods have been reported using a diverse range of catalysts such as Al2O3,32 H2SO4/ SiO2,33 NaHSO4/SiO2,34 MCM-41,35 etc., but none of these methods are free from harsh reaction conditions or longer reaction times. However, only a few studies have reported using MOF as a catalyst.36−40 Therefore, it is straightforward and logical to develop a catalyst for better catalytic efficiency. Here, the synthesis of quinoline derivatives via the Friedländer heteroannulation reactions between aminoaryl ketones and acetylacetone were attempted using 1′ (1 wt %) as the catalyst (Table 2). Initially, the reaction of acetylacetone with 2-
Scheme 1. Plausible Mechanism for the CDC Reaction
a
forms a chelate complex with hydroxyaldehyde (step A). At the same time, TBHP generates hydroxyl and tert-butoxyl radicals. The produced tert-butoxyl radical abstracts hydrogen from carbon adjacent to the oxygen atom of dioxane and forms a radical dioxane species. The hydroxyl radical takes hydrogen from the chelate complex and leaves a water molecule (step B). The formation of the desired product takes place during the reaction of the dioxane radical with the proposed chelate complex.
aminobenzophenone was chosen as a model reaction that afforded the corresponding product in an excellent yield (95%). The profound catalytic activity of 1′ for Friedländer synthesis could be attributed to the replete number and easy accessibility of the empty Cu(II) binding sites and also accessible amine groups promoting facile substrates and active site interactions. Thereafter, a variety of 2-aminoaryl ketones that afforded the desired products in satisfactory isolated yields were used. With 4-nitro and 4-chloro derivatives of 2aminobenzophenone, the yields were 76 and 90%, respectively (Table 2, entries 3 and 4, respectively). Furthermore, in the absence of 1′, the reaction gave an 11% yield (Table 2, entry 5). In general, 1′ (1 wt %) was added to a mixture of 2aminoarylketone (1 mmol) and acetylacetone (5 mmol), and the mixture was heated at 80 °C under solvent free conditions for 8 h. After the mixture had cooled, the catalyst was removed by filtration and the reaction mixture was washed thoroughly with CH2Cl2 to remove the possible adhered product from the catalyst surface. The desired product was purified by silica gel column chromatography using an n-hexane/ethyl acetate mixture (19:1 ratio) as the eluent. In each case, the catalytic product was characterized by ESI-MS, 1H NMR, and 13C NMR (Figures S16−S27). The recovered catalyst was dried at 130 °C for 6 h under vacuum for its reuse. No significant decrease in catalytic activity or crystallinity was observed after up to four cycles (Figures S28 and S29), and the
Table 2. Friedländer Reactions under Solvent Free Conditions at 80 °Ca
entry
catalyst
R1
R2
yield (%)b
1 2 3 4 5
1′ 1′ 1′ 1′ 1′
Ph CH3 Ph Ph Ph
H H NO2 Cl H
95 89 76 90 11
Reaction conditions: acetylacetone (5.0 mmol), 2-aminoarylketone (1 mmol) reacted at 80 °C in the presence of 1′ (1 wt %). No solvents were taken. bYield of the isolated product.
C
DOI: 10.1021/acs.inorgchem.8b00683 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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CONCLUSIONS In summary, a robust Cu MOF possessing mf j topology is systematically investigated as a molecular vessel for oxidative CDC and Friedländer reactions. In CDC and Friedländer reactions, the catalyst could be easily recovered and recycled without a significant loss of catalytic activity after up to five and four times, respectively. This study provides an ideal platform for further improving catalyst design, and the catalysts can serve as single versatile tools for various catalytic needs.
heterogeneous nature of the catalyst was again established by a hot filtration test (Figure S30). A probable mechanism of Friedländer reaction has also been proposed (Scheme 2) on the basis of the work reported by the Scheme 2. Plausible Mechanism for the Friedländer Reaction
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00683. Several types of spectroscopic data, PXRD patterns, and additional figures (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Parimal K. Bharadwaj: 0000-0003-3347-8791 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support received from the MNRE (New Delhi, India) (to P.K.B. and D.D.). P.K.B. thanks DST for a JCB Fellowship. V.S. thanks the University Grants Commission (UGC) (New Delhi, India).
Cejka group.36 Concerted effects of a pair of two paddle-wheel units here lead to the proximity of reactants to each other and enhance the reaction rate. The proposed mechanism involves first stabilization of both reactants on neighboring open metal sites (A). This aids the formation of intermolecular aldol intermediate B, followed by loss of a water molecule to form C. Subsequently, intramolecular proton transfer generates D, from which the loss of a water molecule takes place, resulting in the desired product quinoline. The crystallinity and morphology of the recovered catalyst were further analyzed by FE-SEM. FE-SEM images in the case of the CDC reaction before and after the fifth run of catalytic cycle were compared (Figure 2). The crystalline nature and
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REFERENCES
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Figure 2. FE-SEM images (a) of the fresh catalyst, (b) after the fifth run of the CDC reaction, and (c) after the fourth run of the Friedländer reaction.
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DOI: 10.1021/acs.inorgchem.8b00683 Inorg. Chem. XXXX, XXX, XXX−XXX
