Decarboxylative Coupling Strategy To Afford N-Heterocycles Driven

Research Article pubs.acs.org/journal/ascecg

Decarboxylative Coupling Strategy To Afford N‑Heterocycles Driven by Silica-Nanosphere-Embedded Copper Oxide (Cu@SiO2‑NS) Upasana Gulati,†,‡ U. Chinna Rajesh,†,‡,∇ Naveen Bunekar,§ and Diwan S. Rawat*,† †

Department of Chemistry, University of Delhi, Delhi-110007, India Department of Chemistry, Chung-Yuan Christian University, Chung-Li, Taiwan, Republic of China

§

S Supporting Information *

ABSTRACT: An environmentally benign and surfactant-free method was developed for the preparation of porous silica nanospheres (SiO2 NS) from the hydrolysis of tetraethoxyorthosilicate (TEOS), using corn starch as a sacrificial template. The stabilization of malachite on SiO2 NS, followed by calcination at 600 °C, afforded silica-nanosphere-embedded copper oxide (Cu@SiO2-NS). The obtained Cu@SiO2-NS was found to be a versatile nanocatalyst for the decarboxylative coupling strategies to afford aromatic or aliphatic N-heterocycles such as aminoindolozines, pyrrolo[1,2-a]quinolones, and substituted pyrrolidine. The Cu@SiO2-NS was successfully recycled for six times without significant loss in its catalytic efficiency. The present method shows several advantages, such as being an environmentally benign approach for the catalyst preparation, being easy to handle, and having a wide substrate scope for decarboxylative couplings with excellent yields in short reaction time, and showed excellent green chemistry metrics, such as E-factor, process mass intensity (PMI), reaction mass efficiency (RME), carbon efficiency (CE), and comparatively high turnover number/turnover frequency (TON/TOF) values. KEYWORDS: Environmentally benign, Silica nanospheres, Copper oxide, Decarboxylative couplings, N-heterocycles

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INTRODUCTION Nanoporous silica has found widespread applications in diverse areas, including drug delivery,1,2 imaging,3,4 sensors,5,6 and catalysis.7 Large pores with accessible internal volume resulting from interconnected pores are desired factors to utilize silica as an efficient supporting material in the field of catalysis.8−10 Several methods have been developed to achieve the porous silica using nonionic or ionic surfactants.11−13 Environmentally benign, biopolymer template-based methods that avoid the use of toxic surfactants and a template, which could be easily removed by either chemical etching or calcination, are considered to be greener.14,15 Limited study has been focused on the utilization of biopolymers such as starch as a template to synthesize the porous inorganic materials with structural hierarchy.16,17 To this end, we designed an environmentally benign and surfactant-free method, using corn starch as a sacrificial template for the preparation of porous silica nanospheres (SiO2 NS). Furthermore, the impregnation of malachite in SiO2 NS, followed by calcination at 600 °C, afforded the silica nanospheres embedded copper oxide (Cu@ SiO2-NS). Silica-embedded transition-metal oxides are chosen as efficient nanocatalysts to drive a variety of chemical reactions, including cross couplings, amination, oxidation, hydrogenation, and esterification.9,18−24 However, there are very limited reports on silica-embedded copper oxide (CuO@SiO2)© 2017 American Chemical Society

catalyzed multicomponent coupling reactions to afford N/Oheterocycles.25 Indolozine, pyrrolo[1,2-a]quinolone are such heterocycles present in various bioactive natural products, which possess a wide range of biological activities.26−28 To date, various methodologies have been reported for the synthesis of aminoindolozines and pyrrolo[1,2-a]quinolines such as [3 + 2] cyclization of quinolines with alkenyldiazoacetates,29 oxidative cross-coupling of 2-(pyridin-2-yl)acetate and alkenes,30 and 1,3dipolar cycloaddition of pyridinium ylides with electrondeficient ynamides,31 etc. Recently, we reported nanocatalysts such as Cu(II)-hydromagnesite,32 Cu@hematite,33 and CuI/ CSP34 for A3 coupling strategy to afford aminoindolozines or pyrrolo[1,2-a]quinolines (see panel (a) in Scheme 1). Decarboxylative coupling strategies have attracted significant attention in the area of green and sustainable organic synthesis, which avoids the usage of toxic organometallic reagents.35−37 These strategies have been employed by several researchers to afford heterocycles and value-added chemicals such as imidazo[1,2-a]pyridines,38 propargylamines,39,40 α-alkynylated pyrrolidineoxyindoles,41 3-amino-1,4-enynes,42 oxazolidin-2-ones,43 1,4-diamino-2-butynes,44 biaryls,45 and 3,6-dihydro-1H-pyridin-2-ones.46 We reported preliminary results on nanocatalyzed Received: December 31, 2016 Revised: April 25, 2017 Published: May 15, 2017 4672

DOI: 10.1021/acssuschemeng.6b03209 ACS Sustainable Chem. Eng. 2017, 5, 4672−4682

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis of Indolozines, Pyrrolo[1,2-a]quinolines, and Substituted Pyrrolidine

Figure 1. Schematic representation for the synthesis of Cu@SiO2-NS.

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RESULTS AND DISCUSSION Synthesis and Characterization of Silica-NanosphereEmbedded Copper Oxide (Cu@SiO2-NS). Porous silica nanospheres (SiO2 NS) were synthesized using an environmentally benign approach in three steps as follows. Initially, the hydrolysis of corn starch was achieved in the presence of glucoamylase and α-amylase enzymes, using a buffer solution at pH ∼4.7 for 24 h. As a result, a homogeneous emulsion was observed from the mixture of starch-hydrolyzed products such as oligomers and sugars with the enzymes in the aqueous medium. The second step involved the controlled hydrolysis of tetraethoxyorthosilicate (TEOS) by treating with a preformed

decarboxylative strategies for A3 and KA2 coupling reactions.32,47 This opens up new challenging perspectives to designing efficient nanocatalysts for decarboxylative methodologies to afford biologically significant heterocycles. With this background, we herein report a green and sustainable approach for the synthesis of Cu@SiO2-NS, which is found to be a highly efficient nanocatalyst for the decarboxylative coupling strategies to afford N-heterocycles such as aminoindolozines, pyrrolo[1,2a]quinolines, and substituted pyrrolidine (see panel (b) in Scheme 1). 4673


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The surface morphology of SiO2 NS and Cu@SiO2-NS was characterized from SEM, as shown in Figure 3. The results showed that the formation of nonuniform spheres of SiO2 with sizes that vary over a range of 2.4−5.5 μm (Figure 3a). The stabilization of CuO on the porous SiO2 spheres caused changes in the surface morphology as distorted and aggregated spheres with sizes that vary from 0.5 μm to 5 μm (Figure 3b). Moreover, some of CuO nanoparticles (NPs) were also stabilized on the surface of the broken spheres. The internal morphology of SiO2 and Cu@SiO2-NS was characterized from transmission electron microscopy (TEM), as shown in Figure 4. The results revealed that the formation of uniform hexagonal nanochannels in SiO2 spheres (Figure 4a, b). Copper oxide nanoparticles were embedded on the porous silica, as confirmed by the disappearance of pores in the highmagnification TEM images of Cu@SiO2-NS (see Figures 4c and 4d). The energy-dispersive X-ray analysis (EDAX) elemental analysis revealed the presence of 5.8 wt % or 1.90 at. % of copper in Cu@SiO2-NS, as depicted in Figure 5. The remaining 94.2 wt % corresponds to silicon atoms (∼43.03 wt %) and oxygen atoms (∼51.17 wt %) (see Figure 5). The presence of copper oxide in the Cu@SiO2-NS was further confirmed from X-ray photoelectron spectroscopy (XPS) analysis, as shown in Figure 6. The high-resolution XPS spectrum of the Cu 2p region showed two intense peaks with binding energies (BEs) of ∼934.91 and 954.84 eV, corresponding to Cu 2p3/2 and Cu 2p1/2, respectively. The presence of satellite peaks with BE = 943.66 and 962.93 eV suggests that copper exists in a divalent oxidation state as CuO (see Figure 6). The higher BE value of Cu 2p3/2 (934.91 eV), rlative to that of pure CuO (BE = 933.8 eV) is may be due to the coordination of copper(II) atoms of CuO with the surrounding silicon atoms, which is consistent with the previous results.48 The functional groups of Cu@SiO2-NS were characterized using the FT-IR technique, as shown in Figure 7. The broad peak at 3450 cm−1 is assigned to the −OH stretching vibration of adsorbed water. The sharp peak at 1650 cm−1 corresponds to the Si−OH deformation of adsorbed molecules.48 The peak at 1078 cm−1 with a shoulder at 1230 cm−1 and a sharp peak at 802 cm−1 correspond to internal and external asymmetric Si− O−Si stretching modes. The peaks at 478 and 456 cm−1 are assigned to the Si−O−Si bending mode and the tetrahedral Si− O bending mode, respectively.48 The peak at 464 cm−1 with a shoulder at 580 cm−1 corresponds to the Cu−O bond vibration of CuO (see Figure 7).

homogeneous emulsion to afford the silica−template composite as a white solid. The third step involved the removal of template and other organic impurities upon calcination at 600 °C for 4 h to afford pure porous SiO2 NS, as shown in Figure 1. The silica-embedded copper oxide nanospheres (Cu@SiO2NS) were achieved by the impregnation of malachite, followed by calcination at 600 °C for 4 h (Figure 1). The synthesized Cu@SiO2-NS were characterized by various techniques, such as powder X-ray diffraction (PXRD), scanning electronic microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDAX) elemental analysis, X-ray photoelectron spectroscopy (XPS), Brunauer−Emmett−Teller (BET) surface area, and Fourier transform infrared (FTIR) spectroscopy. The PXRD pattern of SiO2 NS confirmed the presence of a hexagonal mesostructure from the phases such as (100), (110), and (200) at a 2θ range of ∼0.7−3.0° (see Figure S1 in the Supporting Information (SI)). The XRD pattern of Cu@SiO2NS could be indexed as a monoclinic structure of CuO with space group C2/c, and the refined lattice parameters of a = 4.6837 Å, b = 3.4226 Å, c = 5.1288 Å, with β = 99.54° (JCPDS File No. 80-1916). The phases such as (002), (111), (202̅), (020), (202), (113̅), (3̅11̅), (220), (311), and (222̅) at diffraction angles of 2θ = 35.4°, 38.6°, 48.7°, 53.3°, 58.1°, 61.4°, 66.2°, 67.8°, 72.3°, and 74.9°, respectively, correspond to CuO. The broad peak in the 2θ range of 10°−30° corresponds to the amorphous silica (Figure 2). The particle size of Cu@ SiO2-NS was found to be 15.3 nm, calculated from the most intense peak using the Scherrer formula.

Figure 2. Powder X-ray diffraction pattern of Cu@SiO2-NS.

Figure 3. SEM images of (a) SiO2 and (b) Cu@SiO2-NS. 4674


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Figure 4. TEM images of (a, b) SiO2 and (c, d) Cu@SiO2-NS.

Figure 5. EDAX elemental analysis of Cu@SiO2-NS.

Figure 7. FT-IR spectrum of Cu@SiO2-NS.

Next, we studied the nitrogen adsorption/desorption isotherm and pore diameter of Cu@SiO2-NS, as shown in Figure 8. It is ascribed to a type IV isotherm with a type H1 hysteresis loop, which is characteristic of a typical mesoporous material (Figure 8a). The calcined spherical Cu@SiO2-NS has a pore size of 5.5 nm (calculated from the adsorption branch by the Barrett−Joyner−Halenda (BJH) model), a Brunauer− Emmett−Teller (BET) surface area of 307 m2 g−1, pore diameter of 5.5 nm and a pore volume of 0.33 cm3 g−1 (see Figure 8). In order to understand whether the CuO NPs are located inside or outside of silica pores, we have studied the changes in the pore size of silica before and after stabilization of the

Figure 6. High-resolution XPS spectrum of Cu 2p region in Cu@ SiO2-NS. 4675


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Figure 8. (a) Nitrogen adsorption−desorption isotherms; (b) pore diameter of Cu@SiO2-NS.

Table 1. Optimization Study for Cu@SiO2-NS-Catalyzed Decarboxylative A3 Coupling Strategy To Afford Aminoindolozine (4a)a

Conversion (%)b entry

catalysis (mg)

solvent

temperature (°C)

time (h)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

Cu@SiO2-NS (10) Cu@SiO2-NS (10) Cu@SiO2-NS (10) Cu@SiO2-NS (10) Cu@SiO2-NS (10) Cu@SiO2-NS (10) Cu@SiO2-NS (10) Cu@SiO2-NS (5) Cu@SiO2-NS (15) Cu@SiO2-NS (10) CuO@Fe2O3 NPs (10) Cu/HM NPs (10) CuI (30) SiO2 NS (10) no catalyst

neat DMF DMSO water PEG DEG EG EG EG EG EG EG EG EG EG

100 100 100 100 100 100 100 100 100 80 100 100 100 100 100

4 4 4 4 4 3 0.5 4 4 5 4 2 2 24 24

4a

70 40 85 96 85 92 83c 85d 75

5a

60

25

a

Reaction conditions: pyridin-2-carboxaldehyde 1a (1 mmol), morpholine 2a (1 mmol), phenylpropiolic acid 3a (1 mmol), catalyst, solvent (3 mL) were stirred at appropriate temperature. bConversions were calculated from the 1H NMR of crude reaction mixtures. cIsolated yields of 80%. d Isolated yields of 82%.

Cu@SiO2-NS-Catalyzed Decarboxylative Coupling Strategies To Afford N-Heterocycles. The catalytic potential of Cu@SiO2-NS was studied for decarboxylative A3 coupling strategy to afford N-heterocycles, such as aminoindolozines and pyrrolo[1,2-a]quinolones with wide substrate scope. Initially, a model reaction was performed among pyridin2-carboxaldehyde (1a), morpholine (2a), and phenyl propiolic acid (3), using 10 mg of Cu@SiO2-NS catalyst by varying the solvents at 100 °C, as shown in Table 1. There was no progress in the reaction either under neat conditions or in polar organic solvents, such as DMF, DMSO (entries 1−3, Table 1). The product (4a) was obtained in 70% conversion selectively in the

malachite precursor, followed by calcination. The pore sizes of the silica support before and after copper stabilization are almost the same (see Figure S4 in the SI), which was found to be ∼5.5 nm. Moreover, the SEM image of malachite precursor showed that the size is in the range of 2−7 μm (see Figure S4). It is impossible to diffuse these micrometer-sized malachite particles inside the 5-nm-sized pores of the silica support. The malachite Cu2CO3(OH)2 has stabilized on the surface of silica and the calcined product Cu@SiO2-NS has shown the particle size to be ∼15 nm, as calculated from PXRD. These results clearly showed that the CuO NPs are located outside of the silica pores. 4676


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Scheme 2. Cu@SiO2-NS Catalyzed Synthesis of Aminoindolozines and Pyrrolo[1,2-a]quinolines via Decarboxylative A3 Coupling Strategy

almost all screened substrates underwent smooth decarboxylative A3 coupling to afford the corresponding N-heterocycles such as aminoindolozines 4a−4j (entries 1−10 in Table 2) and pyrrolo[1,2-a]quinolones 4k−4t (entries 11−20 in Table 2) in yields of 83%−95% in short reaction time (30 min). Functionalization of the α-Csp3−H bond of nitrogencontaining cyclic amino acids has attracted great attention, because of their significance as versatile building blocks in the synthesis of N-heterocyclic natural products and biomolecules.35,49,50 In 2009, Liang et al. reported the functionalization of α-amino acids by one-pot three-component tandem decarboxylative coupling strategy, using CuI as a homogeneous catalyst.51 In order to check the versatility of the Cu@SiO2-NS nanocatalyst, we further extended the scope of the present method for decarboxylative tandem coupling among proline (6), 3-nitrobenzaldehyde (1c), and phenylacetylene (3) or phenylpropiolic acid (3a) to afford aliphatic N-heterocycle such as 1-benzyl-2-(phenylethynyl)pyrrolidine in excellent yield, as shown in Scheme 3. The results showed that PEG solvent was found to be superior to EG in the presence of 10 mg of Cu@ SiO2-NS catalyst to afford product (7) in 95% yield in 2 h (Scheme 3). The plausible mechanism for Cu(II)-based nanocatalyzed decarboxylative A3 coupling reaction to afford propagylamines or preliminary study to obtain the pyrrolo[1,2-a]quinolones is reported in our previous work.32,47 In order to show the role of EG solvent and porous SiO2 support of Cu@SiO2-NS catalytic system, we described the plausible mechanism for the synthesis of aminoindolozines and pyrrolo[1,2-a]quinolines, as shown in Figure 9. The silica support has the ability to form a hydrogen bond with EG solvent, which may enhance the adsorption of organic substrate intermediates on the copper active sites of the Cu@

presence of water as a solvent (entry 4 in Table 1). However, a complete conversion of starting substrates was achieved in the presence of PEG as a solvent, but a mixture of product (4a) and chalcone (5a) was observed in 40% and 60% conversions, respectively (entry 5, Table 1). Interestingly, EG was found to be a superior solvent to DEG to afford product (4a) in 96% conversion in 30 min (see entries 6 and 7 in Table 1). The progress in reaction was slow upon either decrease (5 mg) or increase (15 mg) of catalyst loading to obtain the product (4a) in 85% or 92% conversion, respectively, in 4 h (entries 8 and 9 in Table 1). No product (4a) was obtained upon decreased temperature to 80 °C (entry 10 in Table 1). It may indicate that a temperature of ∼100 °C is suitable for the thermal activation of Cu@SiO2-NS nanocatalyst to drive decarboxylative A3 coupling in the presence of EG solvent. The catalytic activity of Cu@SiO2-NS was compared with other nanosupported copper oxides, such as CuO/Fe2O3, Cu/HM NPs, and homogeneous catalyst CuI under optimized reaction conditions, as shown in Table 1 (entries 11−13 in Table 1). The results showed that the present catalytic system is superior to other catalysts. The formation of the product (4a) was not observed in the presence of either SiO2 NS alone as a catalyst or in the absence of catalyst upon a prolonged reaction time of 24 h (see entries 14 and 15 in Table 1). Thus, the optimized reaction conditions were determined to be the usage of 10 mg of Cu@SiO2-NS catalyst in the presence of EG solvent at 100 °C for 30 min (entry 7 in Table 1). The generality of optimized reaction condition was studied with a wide range of substrate scope, using various secondary amines bearing electron-withdrawing and electron-donating substituents (2a−2l) and phenylpropiolic acids (3a/3b) with either pyridin-2-carboxaldehyde or quinolin-2-carboxaldehyde to afford aminoindolozines or pyrrolo[1,2-a]quinolones, respectively, as shown in Scheme 2. The results showed that 4677


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dried in an oven at 100 °C. The obtained organic residue was collected by dissolving in ethanol, followed by evaporation to afford a crude product. The recovered catalyst and solvent were reused for five more cycles with gradually slight decreases in the conversions of product 4a (Figure 10), as determined by analyzing the crude reaction mixture using 1H NMR technique. The quantification of green chemistry metrics for the present catalytic system and comparison with our previous methods has been included in the Supporting Information (SI) (see the SI for details). The results showed that the present method is superior to reported methods. Next, we studied the heterogeneity of the Cu@SiO2-NS nanocatalyst by performing hot filtration experiment for a model reaction to afford aminoindolozine (4a) under optimized reaction conditions. The reaction was stopped at ∼50% conversion of the starting materials (i.e., 15 min of reaction time and filtered out the catalyst from the reaction mixture). The filtrate was continued further for 30 min at 100 °C, the results showed that there was no improvement in the progress of reaction. Moreover, the ICP-AES analysis of filtrate showed that there was no detectable amount copper ion in the solution, which confirms that there was no leaching of copper metal ions in the reaction mixture.

Table 2. Cu@SiO2-NS Catalyzed Decarboxylative A3 Coupling Strategy To Afford Aminoindolozines or Pyrrolo[1,2-a]quinolonesa entry aldehyde, 1 amine, 2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

1a 1a 1a 1a 1a 1a 1a 1a 1a 1a 1b 1b 1b 1b 1b 1b 1b 1b 1b 1b

2a 2b 2c 2a 2d 2e 2f 2f 2g 2e 2a 2b 2c 2d 2e 2h 2i 2j 2k 2l

propiolic acid, 3

product, 4

yield of 4b (%)

3a 3a 3a 3b 3a 3a 3a 3b 3a 3b 3a 3a 3b 3a 3a 3b 3a 3a 3a 3a

4a 4b 4c 4d 4e 4f 4g 4h 4i 4j 4k 4l 4m 4n 4o 4p 4q 4r 4s 4t

95 91 83 88 87 90 89 88 90 89 95 93 92 90 91 90 92 94 90 93

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CONCLUSION In summary, we described an environmentally benign approach for the preparation of porous silica nanospheres (SiO2 NS) and copper oxide stabilized on the porous silica nanospheres (Cu@ SiO2-NS). The Cu@SiO2-NS was found to be a versatile nanocatalyst for decarboxylative coupling strategies to afford Nheterocycles, such as aminoindolozines, pyrrolo[1,2-a]quinolones, and substituted pyrrolidine in quantitative yields. The method involved ethylene glycol as a recoverable solvent, which makes the method green. The recoverability and reusability of catalyst up to six cycles without significant loss in its activity further reduced the E-factor of the present methodology. The preliminary results on decarboxylative tandem coupling strategy to functionalize the proline would show the wide scope available to explore the catalytic potential of Cu@SiO2-NS nanocatalyst for the synthesis of natural amino-acid-based heterocycles.

a

Reaction conditions: pyridin-2-carboxaldehyde or quinolin-2-carboxaldehyde 1 (1 mmol), amines 2 (1 mmol), phenylpropiolic acid 3 (1 mmol), Cu@SiO2-NS catalyst (10 mg), EG (3 mL) at 100 °C, 30 min. b Isolated yield.

SiO2-NS catalyst surface. Moreover, the EG solvent has tendency to form intramolecular and intermolecular hydrogen bonds between two adjacent hydroxyl groups, unlike other polar solvents.52 The hydrogen atom of EG is more acidic to activate the carbonyl oxygen of aldehyde (1) and one of the oxygen atoms of EG is relatively basic to abstract the proton of secondary amines (2) or the COOH group of substrates (3) to afford intermediates (III and I, respectively) by releasing one mole of water as a byproduct. The carboxylate anion coordinates with the CuO of the Cu@SiO2-NS catalyst, followed by the formation of copper acetylide intermediate (II) via a decarboxylative manner. The phenylacetylide attacks the preformed iminium intermediate (III) to afford propargyl amine (IV), followed by cycloisomerization to afford Nheterocycles (4), as shown in Figure 9. The recyclability of the Cu@SiO2-NS catalyst was studied for the synthesis of aminoindolozine (4a) under optimized reaction conditions, as shown in Figure 10. After the completion of the first cycle, EG and catalyst was recovered via the addition of excess water, followed by centrifugation and filtration. The recovered catalyst was washed with ethanol and

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EXPERIMENTAL SECTION

Typical Procedure for the Synthesis of SiO2 NS. Corn starch (6 g) was immersed into 50 mL of buffer solution (pH ∼4.7) with stirring to form a starch suspension solution. After being added with an enzyme mixture of 48 mg of glucoamylase and 16 mg of α-amylase (3:1, w/w), the suspension solution in a beaker was placed in a water bath and stirred for 24 h at 40 °C. When the mixture turned became a homogeneous emulsion, 2 mL of TEOS containing 25 mL of ethanol was added dropwise under vigorous stirring to get a final gel. The solutions were allowed to proceed further 6 h at room temperature;

Scheme 3. Cu@SiO2-NS-Catalyzed Synthesis of Substituted Pyrrolidine via Decarboxylative Coupling Strategy

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Figure 9. Plausible mechanism for Cu@SiO2-NS-catalyzed decarboxylative coupling strategy. evaporated to obtain the crude products, followed by chromatography to afford pure products. All the synthesized compounds were characterized by FT-IR, 1H and 13C NMR, and mass spectral data. General Procedure for the Synthesis of Substituted Pyrrolidine (7). A mixture of proline 6 (1 mmol), 3-nitrobenzaldehyde 1c (1 mmol), and phenylacetylene (3) or phenylpropiolic acid (3a) (1.5 mmol) and Cu@SiO2-NS catalyst (10 mg) were stirred in PEG solvent (2 mL) at 100 °C for 2 h. After completion of the reaction, PEG was recovered by the addition of water, followed by centrifugation and filtration. The organic residue was dissolved in ethanol and evaporated to obtain the crude products, followed by chromatography, to afford pure products. Spectral Data of Unknown Compounds. 3-Phenyl-1-(4pyridin-2-yl-piperazin-1-yl)-indolizine (4f). Yellow liquid; IR (νmax/ cm−1, CHCl3): 2924, 2851, 1595, 1480, 1436, 1308, 1244, 1156, 1023, 944, 772, 700; 1H NMR (400 MHz, C6D6) δ = 8.14−8.15 (m, 1H), 7.71 (d, J = 7.6 Hz, 2H), 7.29 (d, J = 9.2 Hz, 1H), 7.14 (d, J = 7.6 Hz, 2H), 6.98−7.00 (m, 1H), 6.85−6.89 (m, 1H), 6.44 (s, 1H), 6.17−6.21 (m, 2H), 6.14 (d, J = 8.4 Hz, 2H), 5.83 (t, J = 6.9 Hz, 1H), 3.48 (t, J = 4.6 Hz, 4H), 2.78 (t, J = 4.6 Hz, 4H) ppm; 13C NMR (100 MHz, C6D6) δ = 148.56, 137.14, 133.03, 130.46, 129.12, 126.93, 122.93, 121.78, 118.22, 114.97, 113.16, 111.07, 106.93, 106.64, 54.64, 45.89 ppm; HRMS (ESI) calcd for C23H23N4 [M + H]+: 355.1923, found: 355.1942. 1-[4-(4-Nitro-phenyl)-piperazin-1-yl]-3-phenyl-indolizine (4g). Yellow liquid; IR (νmax/cm−1, CHCl3): 2923, 2851, 1597, 1507, 1321, 1240, 1115, 1020, 753, 699; 1H NMR (400 MHz, C6D6) δ = 7.81 (d, J = 8.4 Hz, 2H), 7.63 (d, J = 6.9 Hz, 1H), 7.22 (d, J = 8.4 Hz, 1H), 7.09 (d, J = 7.6 Hz, 2H), 6.92 (t, J = 8.4 Hz, 2H), 6.81 (t, J = 7.6 Hz, 1H), 6.39 (s, 1H), 6.11−6.15 (m, 1H), 5.90 (d, J = 8.4 Hz, 2H), 5.77 (t, J = 6.9 Hz, 1H), 2.59−2.62 (m, 4H), 2.48−2.51 (m, 4H) ppm; 13 C NMR (100 MHz, C6D6) δ = 154.76, 139.23, 132.79, 129.24, 127.25, 126.34, 125.77, 121.98, 117.79, 115.43, 112.89, 111.25, 106.49, 53.57, 47.42 ppm; HRMS (ESI) calcd for C24H23N4O2 [M + H]+: 399.1821, found: 399.1845. 1-[4-(4-Nitro-phenyl)-piperazin-1-yl]-3-p-tolyl-indolizine (4h). Yellow liquid; IR (νmax/cm−1, CHCl3): 2923, 2852, 1596, 1321, 1240, 1114, 1083, 1021, 825; 1H NMR (400 MHz, C6D6) δ = 7.81 (d, J = 7.6 Hz, 2H), 7.68 (d, J = 7.6 Hz, 1H),7.23 (d, J = 9.2 Hz, 1H), 7.05 (d, J = 6.9 Hz, 2H), 6.76 (d, J = 6.9 Hz, 2H), 6.42 (s, 1H), 6.11−6.15 (m, 1H), 5.89 (d, J = 7.6 Hz, 2H), 5.79 (t, J = 6.9 Hz, 1H), 2.59−2.60 (m, 4H) 2.50−2.52 (m, 4H), 1.87 (s, 3H) ppm; 13C NMR (100 MHz,

Figure 10. Recyclability of Cu@SiO2-NS catalyst for the synthesis of 4a. the resultant precipitate then was filtered, washed with Millipore water, and dried overnight at 60 °C in a vacuum oven. Finally, the assynthesized powder sample was calcined at 600 °C for 4 h (heating rate = 1 °C min−1) in a muffle oven to afford SiO2 as a white powder. Typical Procedure for the Synthesis of Cu@SiO2-NS. The stabilization of malachite on the porous SiO2 NS was achieved by the addition of 100 mg of malachite to the aqueous suspension obtained from the dispersion of 500 mg of SiO2 NS in 30 mL of water. The mixture was stirred overnight at room temperature. The obtained solid was filtered and washed with acetone, followed by drying in an oven at 80 °C for 6 h. The solid was calcined at 600 °C for 4 h (heating rate = 1 °C min−1) to afford silica-nanosphere-embedded copper oxide (Cu@SiO2-NS). General Procedure for the Synthesis of Aminoindolozines and Pyrrolo[1,2-a]quinolones (4). A mixture of pyridin-2-carboxaldehyde or quinolin-2-carboxaldehyde 1 (1 mmol), secondary amines 2 (1 mmol), phenylpropiolic acid 3 (1 mmol), and Cu@SiO2-NS catalyst (10 mg) were stirred in EG solvent (2 mL) at 100 °C for 30 min. After completion of the reaction, EG was recovered by the addition of water followed by centrifugation and filtration. The recovered catalyst was washed with ethanol and dried at 80 °C in a vacuum oven and reused for further cycles. The organic residue was dissolved in ethanol and 4679



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C6D6) δ = 153.18, 137.63, 135.31, 128.29, 127.89, 124.55, 124.19, 122.17, 120.26, 116.09, 113.80, 111.32, 109.59, 104.69, 52.11, 45.88, 19.56 ppm; HRMS (ESI) calcd for C25H25N4O2 [M + H]+: 413.1978, found: 413.1997. 1-[4-(4-Methyl-benzyl)-piperazin-1-yl]-3-phenyl-indolizine (4i). Yellow liquid, IR (νmax/cm−1, CHCl3): 2925, 2855, 1727, 1599, 1450, 1018, 763; 1H NMR (400 MHz, C6D6) δ = 7.87 (d, J = 7.6 Hz, 1H), 7.47 (d, J = 9.2 Hz, 1H),7.29 (d, J = 7.6 Hz, 4H), 7.13−7.15 (m, 2H), 7.01−7.03 (m, 3H), 6.69 (s,1H), 6.27−6.31 (m, 1H), 5.98 (t, J = 6.9 Hz, 1H), 3.42 (s, 2H), 3.05 (brs, 4H), 2.55 (brs, 4H), 2.12 (s, 3H) ppm; 13C NMR (100 MHz, C6D6) δ = 135.35, 134.80, 131.89, 129.56, 125.59, 124.87, 121.64, 120.52, 117.12, 113.45, 109.73, 105.38, 62.00, 53.05, 52.73, 19.88 ppm; HRMS (ESI) calcd for C26H28N3 [M + H]+: 382.2283, found: 382.2304. 1-(4-Pyridin-2-yl-piperazin-1-yl)-3-p-tolyl-indolizine (4j). Yellow liquid; IR (νmax/cm−1, CHCl3): 2923, 2849, 1596, 1480, 1434, 1307, 1244, 1022, 782, 738; 1H NMR (400 MHz, C6D6) δ = 7.95 (t, J = 7.6 Hz, 2H), 7.51 (d, J = 9.2 Hz, 1H), 7.30 (d, J = 7.6 Hz, 2H), 7.02 (d, J = 7.6 Hz, 2H), 6.84 (d, J = 8.0 Hz, 1H), 6.67 (s, 1H), 6.37−6.41 (m, 1H), 6.35 (d, J = 8.4 Hz, 2H), 6.06 (t, J = 7.6 Hz, 1H), 3.68 (t, J = 5.3 Hz, 4H), 3.00 (t, J = 5.3 Hz, 4H) 2.15 (s, 3H) ppm; 13C NMR (100 MHz, C6D6) δ = 153.78, 135.72, 128.69, 124.59, 121.05, 116.86, 113.83, 111.73, 110.00, 105.10, 52.33, 46.44, 19.97 ppm; HRMS (ESI) calcd for C24H25N4 [M + H]+: 369.2079, found: 369.2103. 3-(4-Methyl-piperidin-1-yl)-1-phenyl-pyrrolo[1,2-a]quinoline (4l). Yellow liquid; IR (νmax/cm−1, CHCl3): 2922, 2796, 1600, 1559, 1493, 1450, 1379, 1320, 1188, 790, 699; 1H NMR (400 MHz, C6D6) δ = 7.27 (t, J = 10.7 Hz, 2H), 7.02−7.07 (m, 3H), 6.79−6.84 (m, 3H), 6.63 (t, J = 7.6 Hz, 1H), 6.48 (t, J = 7.6 Hz, 1H), 6.40 (d, J = 9.2 Hz, 1H), 6.29 (s, 1H), 2.29 (d, J = 11.4 Hz, 2H), 2.40 (t, J = 11.4 Hz, 2H), 1.15−1.31 (m, 5H), 0.64 (d, J = 6.9 Hz, 3H) ppm; 13C NMR (100 MHz, C6D6) δ = 136.37, 134.70, 133.90, 129.49, 128.69, 128.57, 127.50, 126.55, 126.25, 124.79, 123.38, 118.06, 116.77, 109.36, 55.14, 35.28, 31.03, 22.29 ppm; HRMS (ESI) calcd for C24H25N2 [M + H]+: 341.2018, found: 341.2042. 1-Phenyl-3-(4-phenyl-piperazin-1-yl)-pyrrolo[1,2-a]quinoline (4n). Yellow liquid; IR (νmax/cm−1, CHCl3): 2925, 2862, 1664, 1605, 1506, 1459, 1378, 1227, 1022, 760; 1H NMR (400 MHz, C6D6) δ = 7.31 (d, J = 8.4 Hz, 1H), 7.23 (d, J = 9.2 Hz, 1H), 7.04−7.07 (m, 4H), 6.97 (t, J = 7.6 Hz, 3H), 6.60−6.67 (m, 3H), 6.57 (d, J = 8.4 Hz, 2H), 6.47−6.51 (m, 1H), 6.45 (d, J = 9.2 Hz, 1H), 6.28 (s, 1H), 2.84−2.86 (m, 4H), 2.74−2.77 (m, 4H) ppm; 13C NMR (100 MHz, C6D6) δ = 150.70, 134.81, 133.26, 131.42, 128.14, 127.98, 125.08, 123.59, 122.18, 118.49, 116.72, 116.37, 115.84, 115.14, 52.91, 48.45 ppm; HRMS (ESI) calcd for C28H26N3 [M + H]+: 404.2127, found: 404.2152. 1-Phenyl-3-(4-pyridin-2-yl-piperazin-1-yl)-pyrrolo[1,2-a]quinoline (4o). Yellow liquid; IR (νmax/cm−1, CHCl3): 2925, 2854, 1726, 1599, 1483, 1440, 1375, 1244, 763, 701; 1H NMR (400 MHz, C6D6) δ = 8.05−8.07 (m, 1H), 7.29 (d, J = 8.4 Hz, 1H), 7.19 (d, J = 9.2 Hz, 1H), 7.03−7.06 (m, 4H), 6.79−6.83 (m, 3H), 6.64 (t, J = 7.6 Hz, 1H), 6.49 (t, J = 8.4 Hz, 1H), 6.43 (d, J = 9.2 Hz, 1H), 6.18 (s, 1H), 6.09−6.13 (m, 1H), 6.05 (d, J = 8.4 Hz, 1H), 3.38 (t, J = 5.3 Hz, 4H), 2.69 (t, J = 5.3 Hz, 4H) ppm; 13C NMR (100 MHz, C6D6) δ = 158.53, 147.18, 135.79, 134.82, 133.26, 131.45, 128.13, 125.05, 123.62, 122.16, 116.71, 116.39, 115.73, 111.77, 107.94, 105.59, 52.69, 44.51 ppm; HRMS (ESI) calcd for C27H25N4 [M + H]+: 405.2079, found: 405.2105. 3-(3,5-Dimethyl-piperidin-1-yl)-1-phenyl-pyrrolo[1,2-a]quinoline (4q). Yellow liquid; IR (νmax/cm−1, CHCl3): 2950, 2923, 1601, 1493, 1450, 1378, 1319, 1061, 789, 699; 1H NMR (400 MHz, C6D6) δ = 7.57 (t, J = 8.4 Hz, 3H), 7.35 (d, J = 6.1 Hz, 3H), 7.29 (d, J = 7.6 Hz, 1H), 6.90 (t, J = 7.6 Hz, 1H), 6.76 (t, J = 7.6 Hz, 1H), 6.68 (d, J = 9.2 Hz, 2H), 6.55 (s, 1H), 3.21−3.24 (m, 2H), 2.21 (t, J = 11.4 Hz, 2H), 1.87−1.95 (m, 1H), 1.60−1.63 (m, 1H), 1.25−1.34 (m, 2H), 0.79 (d, J = 6.9 Hz, 6H) ppm; 13C NMR (100 MHz, C6D6) δ = 136.40, 134.66, 129.56, 128.69, 126.56, 126.29, 124.91, 123.32, 118.06, 116.74, 109.66, 62.61, 42.32, 31.97, 30.04, 19.42 ppm; HRMS (ESI) calcd for C25H27N2 [M + H]+: 355.2174, found: 355.2192.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b03209. Characterization of fresh SiO2 NS, and recycled Cu@ SiO2 NS catalyst using PXRD, SEM; E-factor and turnover number (TON) calculations; 1H NMR and 13 C NMR spectral data of known compounds and spectra of all synthesized compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 91-11-27662683. Fax: 91-11-27667501. E-mail: [email protected]. ORCID

Diwan S. Rawat: 0000-0002-5473-7476 Present Address ∇

Department of Chemistry, Indiana University, Bloomington, IN 47405, USA.

Author Contributions ‡

These authors contributed equally to the work.

Author Contributions

D.S.R. conceived the project, and U.G. and U.C.R. were involved in the preparation of Cu@SiO2 NS catalyst and its catalysis for decarboxylative coupling strategies at DU. The characterization of organic compounds were carried out at DU. N.B. was involved in the synthesis of porous SiO2 NS. The characterization of SiO2 and Cu@SiO2 NS nanomaterials was carried out at CYCU. All authors have discussed the results and analyzed the data. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.S.R. acknowledges R&D grant, University of Delhi, India for financial support. U.G. acknowledges CSIR for the award of junior research fellowship. We thank USIC−CIF, University of Delhi, for their assistance in acquiring analytical data.

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REFERENCES

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Decarboxylative Coupling Strategy To Afford N-Heterocycles Driven

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