Reduced Graphene Oxide Supported Copper Oxide Nanocomposites

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RGO@CuO Nanocomposites From A Renewable Copper Mineral Precursor: A Green Approach For Decarboxylative C(sp3)-H Activation Of Proline Amino Acid To Afford Value-Added Synthons Upasana Gulati, U. Chinna Rajesh, and Diwan S. Rawat ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01376 • Publication Date (Web): 16 Jun 2018 Downloaded from http://pubs.acs.org on June 17, 2018

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ACS Sustainable Chemistry & Engineering

RGO@CuO Nanocomposites From A Renewable Copper Mineral Precursor: A Green Approach For Decarboxylative C(sp3)-H Activation Of Proline Amino Acid To Afford Value-Added Synthons Upasana Gulati,a U. Chinna Rajesh,a,b and Diwan S. Rawata* a

Department of Chemistry, University of Delhi, Delhi-110007, India Fax: 91-11-27667501; Tel: 91-11-

27662683; *E-mail: [email protected] b

Department of Chemistry, Indiana University, 800 E Kirkwood Ave, Bloomington, IN 47405, USA.

ABSTRACT: A green approach for decarboxylative C(sp3)-H activation of proline amino acid was accomplished by coupling with aldehydes and alkynes to afford α-alkynylated N-substituted pyrrolidines as value-added synthons using reduced graphene oxide supported copper oxide (RGO@CuO) nanocatalysts. The RGO@CuO nanocomposites were obtained by the impregnation of micrometer-sized malachite spheres, as a renewable and sustainable copper mineral precursor, on the graphene oxide (GO) sheets followed by calcination at 300 oC to 450 o

C for 5 h. The characterization of as synthesized composites revealed the generation of

monodispersed and uniformly embedded copper oxide (CuO) nanoparticles with size ranges from 10-15 nm on RGO thin sheets via GO as a support as well as indirect template for dissembling and decomposition of micrometer-sized malachite spheres. The RGO@CuO composites were found to be efficient and robust nanocatalysts compared with CuO NPs alone. The present method offers several advantages such as wide substrate scope, avoids the usage of excess equivalent of substrates with minimal waste generation (E-factor = 0.24), high Reaction 1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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mass efficiency (80.7%) and the nanocatalyst was recycled for five times without significant loss in its activity with a negligible leaching of CuO NPs from RGO sheets. KEYWORDS: Decarboxylative tandem coupling, substituted pyrrolidines, copper carbonate mineral, RGO@CuO nanocomposite, Green and sustainable approach

INTRODUCTION Catalyst supports have played significant role in the field of heterogeneous catalysis with several sustainable advantages over homogeneous catalysis such as high catalytic activity associated with large surface area, stability and reusability of catalyst and product selectivity with improved green chemistry metrics.1-3 The catalyst support is also useful to prevent the aggregation of nanostructured catalysts by immobilizing on its surface and thus increases their exposed surface area for the adsorption of reactant substrates for better catalytic activity. Carbon materials such as carbon nanotubes (CNT),4 fullerene,5 activated carbon,6 graphene,7 graphene oxide (GO),8-9 reduced graphene oxide (RGO)10-11 etc. have served as efficient catalyst supports since two decades. However, graphene family has received special attention due to high surface area, thermal stability, prominent chemical inertness, stability and easy electron mobility which makes them ideal catalyst support.12-13 Reduced graphene oxide supported metal oxide (RGO@MO) nanocomposites

such

as

RGO@NiO,19-20 RGO@ZnO,19,

RGO@Co3O4,14-15 21

RGO@α-Fe2O3,16-17

RGO@Fe3O4,17-18

RGO@SnO2,22 RGO@MnO2,23-24 RGO@MoO325 etc. have

found potential applications in various fields. These composites have shown superior properties compared to individual components due to synergistic effect of metal oxide and RGO support. With this inspiration, we reported RGO@ZnO nanocomposites as highly efficient catalysts for the synthesis of wide range of 3-substituted indoles in water.26 The copper based nanomaterials have attracted significant attention as cheap and versatile catalysts to construct C-C, C-N, C-O, 2 ACS Paragon Plus Environment

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C-S bonds in organic synthesis to afford value-added synthons and heterocycles.27-34 However, although, there have been several approaches known for the synthesis of

RGO@CuO

nanocomposites to explore their wide range of applications such as non-enzymatic glucose detection,35-37 photocatalytic degradation of dyes,38 electrodes for lithium ion batteries39 etc., but their catalytic potential for the multicomponent organic reactions have not been investigated. Moreover, still there is a need of search for a sustainable and renewable copper precursor to afford RGO@CuO nanocomposites under green conditions. The utilization of renewable copper ores and minerals as precursors for the preparation of nanocatalysts is one of the considerable approaches to reach sustainable chemistry. The estimated copper content of diffusely distributed copper ores in the first mile depth of mineable portion of the earth's crust is about 3x 1018 metric tons, which is sufficient to supply a copper for a million years' to utilize it in the various applications of science and technology.40 Among the copper minerals, malachite (MC) is a basic copper carbonate with composition of Cu2(OH)2CO3 occurs as a secondary copper mineral in the nature, has attracted unique attention due to its unique properties.41-43 Recently, we reported the catalytic potential of micrometer-sized hierarchically porous sphere-like copper oxide (HS-CuO) from the calcination of commercially available malachite mineral.44-45 In this context, we have chosen malachite as a renewable precursor for the synthesis of RGO@CuO nanocomposites. Multi-component reactions (MCR) employing heterogeneous catalysts for the construction of CC, C-N and C-S bonds under green reaction conditions are considered to be an environmentally friendly approach due to easy work-up, recyclability of catalyst with a negligible chemical waste generation.28,

46

Amino acids are considered to be versatile and naturally occurring building

blocks for various natural products and biomolecules. Their easy access in nature makes them foremost choice of synthetic chemists. Further, the carboxylic group of amino acids opens up the

3 ACS Paragon Plus Environment


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possibility of site specific functionalization via. decarboxylative pathway. Transition metal catalyzed decarboxylative coupling reaction provides eco-friendly pathway for C-C or Cheteroatom bond formation due to in situ formation of organometallic species.46-48 This involves relatively neutral conditions and eliminates expensive and toxic organometallic reagents.49-50 Various decarboxylative Csp3-Csp3, Csp3-Csp2, and Csp3-Csp couplings51-53 have been reported to synthesize a wide range value-added chemicals including propargylamines,54 biaryls,55-56 2alkynylbuta-1,3-dienes,57 1,4-diamino-2-butynes,50 1,2,3-triazoles58 etc. Recently, we reported decarboxylative A3 and KA2 coupling strategies for one-pot multi-component reactions to afford wide range of propargylamines, C1-alkynylation of tetrahydroisoquinolines and N-heterocycles using CuO@Fe2O3 MNPs and Cu@SiO2-NS as recyclable nanocatalysts respectively.59-60 To date, there are very few reports described the decarboxylative tandem coupling among proline, aldehydes and alkynes using Cu-salts as homogeneous catalysts.61-63 There is a need of developing efficient nanocatalysts for these decarboxylative tandem coupling reactions under green and sustainable conditions. In this context, we herein report RGO@CuO nanocatalysed decarboxylative tandem coupling among proline, aldehydes and alkynes with equimolar ratios yielded wide range of substituted pyrrolidines with minimum waste generation and high reaction mass efficiency under green reaction condition.

R

CHO + N H

1.4 equiv

COOH

CuI catal. Ref. 46

+ Ar

1.5 equiv

Ar, Toluene, 130 oC

1 equiv

N RGO@CuO catal.

R

o

R

CHO +

1 equiv

N H 1 equiv

COOH

+ Ar 1 equiv

Ar, PEG, 100 C

+ CO2

Ar H 2O

Present method: Greener and Robust Minimum waste (Low E-factor) High reaction mass efficiency

Scheme 1: Catalysis for the synthesis of α-alkynylated N-substituted pyrrolidines 4 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION Synthesis and characterization of RGO@CuO nanocomposites The present approach involves the utilization of a renewable copper carbonate mineral as a precursor to afford the RGO@CuO nanocomposites. The first step involves the impregnation of various weight ratios of malachite (MC) mineral with respect to graphene oxide (GO) followed by the calcination to afford corresponding RGO@CuO-1, RGO@CuO-2, RGO@CuO-3 and RGO@CuO-4 as depicted in Scheme 2.

Scheme 2: Schematic representation for the preparation of RGO@CuO nanocomposites

The lattice of malachite crystal structure consist of one type CO32- ion and two crystallographically different Cu2+ and OH- ions to arrange as an elongated octahedron (4 + 2) coordination that is connected by edge-sharing.41 The impregnation of MC mineral on GO sheets (GO@MC composites) was achieved due to weak H-bond interactions and coordination of CO32-, -

OH and Cu2+ species of MC mineral towards epoxide, carboxylate and hydroxide functionalities 5 ACS Paragon Plus Environment


of GO sheets respectively. The calcination of GO@MC composites were performed at 300 ºC and 450 oC for 5 h lead to the generation of corresponding RGO@CuO nanocomposites with their corresponding weight percentages of copper oxide. During the calcination process, the copper carbonate mineral and GO sheets were decomposed in a controlled manner by dehydration, dehydroxylation, decarboxylation to afford CuO NPs embedded in the newly generated defects of RGO sheets (RGO@CuO) as described in Scheme 2. The surface, internal morphologies and weight percentage of copper in GO@MC and RGO@CuO nanocomposites were characterized by scanning electronic microscopy (SEM), EDAX elemental mapping and analysis, transmission electron microscopy (TEM) as shown in Figures 1-3.

Figure 1: SEM, TEM and EDX elemental analysis/ mapping of (a, d, g) GO@MC-3; (b, e, h) GO@MC-2 and (c, f, i, j, k,l) GO@MC-1 respectively 6 ACS Paragon Plus Environment

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The SEM and TEM results of GO@MC composites revealed the impregnation and dissembling of micrometer-sized MC into nano-sized particles on the wrinkled and rippled GO sheets as shown in Figures (1a-1f) (Figure S1, ESI, morphologies of GO and MC). The EDX elemental analysis of GO@MC-3 and GO@MC-2 composites showed the presence of 10 wt% and 15 wt% of copper with the remaining 90 wt% and 85 wt% were of the corresponding carbon, oxygen of GO and MC respectively. The EDX elemental mapping of GO@MC-3 showed the presence of copper, oxygen and carbon as 25 wt%, 25 wt% and 50 wt% respectively (Figure 1i, j, k, l). The GO@MC-3, GO@MC-2 and GO@MC-1composites were calcined at 300 oC and 450 oC for 5h to afford the corresponding RGO@CuO-3, RGO@CuO-2, RGO@CuO-2a and RGO@CuO-1 as shown in Figure 2 and Figure 3.

Figure 2: SEM, TEM and EDX elemental analysis of (a, d, g) RGO@CuO-3 at 300 oC; (b, e, h) RGO@CuO-2a at 450 oC and (c, f, i, j, k,l) RGO@CuO-1 at 300 oC respectively



The SEM, TEM results revealed the wrinkled and rippled surface morphologies of RGO sheets with moderate to densely decorated CuO NPs and the internal morphologies showed the presence of uniformly embedded CuO NPs on RGO sheets (Figure 2). The EDX elemental analysis showed the presence of 20 wt%, 30 wt% and 50 wt% of CuO NPs on RGO sheets correspond to RGO@CuO-3, RGO@CuO-2a and RGO@CuO-1 respectively. The internal and surface morphologies, elemental analysis of RGO@CuO-2 nanocomposites obtained by the calcination temperature at 300 oC for 5h were studied by TEM and SEM-EDX elemental mapping respectively as shown in Figure 3. The TEM images revealed the homogeneous and uniform deposition of CuO nanoparticles on RGO thin nanosheets without any aggregation (Figure 3a and 3b). The SEM-EDX showed the surface morphology as characteristic wrinkled and rippled morphology of RGO sheets with densely decorated CuO NPs were found to be about 30 wt% (15 wt% of Cu) along with 65 wt% of carbon (Figure 3c, 3d, 3e, 3f).

Figure 3: (a, b) TEM and (c, d, e, f) SEM-EDX elemental mapping of GO@MC-2 obtained at 300 oC, 5 h


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The comparison of functional groups in RGO@CuO-1, RGO@CuO-2, RGO@CuO-2a and RGO@CuO-3 nanocomposites were characterized by FT-IR spectroscopy as shown in Figure 4. The broad peak at 3400 cm-1 corresponds to stretching vibration of –OH group. In figure 4a, the peaks at 1727 cm-1 and 1620 cm-1 correspond to characteristic bands of carbonyl of GO, and the gradual decrease and disappearance was observed after calcination.

GO

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1727 1620

(a)

1052

(b) RGO@CuO-1

1490 1390

(c)

RGO@CuO-2

(d)

RGO@CuO-3

876

(e)

RGO@CuO-2a

1052

o 450 C

4000

3500

1116

3000

2500

2000

1500

600 523 413

1000

500

Wavenumber (cm-1) Figure 4: FT-IR spectra of (a) GO and (b, c, d and e) RGO@CuO nanocomposites obtained at 300 oC and 450 oC respectively.

In Figure 4b, the peaks at 1490 and 1390 cm-1 correspond to C-O stretching mode of carbonate functional groups resulted from incomplete decomposition of MC of RGO@CuO-1 sample at 300 oC. This may be due to excess loading (about 50 wt%) and over saturation of MC spheres on GO sheets. However, the further decrease in MC loading gradually (about 30 to 20 wt%) lead to disappearance of carbonate vibrations in RGO@CuO-2 and RGO@CuO-3 (Figure 4c, d). Moreover in case of RGO@CuO-2a, the raise in calcination temperature to 450 oC leads to



sharpen the intensity of alkoxy C–O stretching vibration of RGO sheets with disappear the trace bands at 1490 and 1390 cm-1. The peaks at 1502 and 1116 cm-1 correspond to the stretching vibration of C–O of various oxygen containing functional groups such as epoxy and alkoxy etc. of RGO support. The peak at 523 cm-1 corresponds to Cu-O stretching vibration characteristic of monoclinic CuO nanoparticles (Figure 4). Next, we further characterized the RGO@CuO-2 composites from powder X-ray diffraction (PXRD), X-ray photoelectron spectra (XPS) and Raman spectroscopic techniques (Figures 5, 6, 7). The powder X-ray diffraction phases of GO@MC-2 composite reveal the presence of pure malachite (copper carbonate mineral) and the data is well matched with the literature (JCPDS file no. 01-076-0660) as shown in Figure 5. The data refers to monoclinic space group P21/a with lattice parameters a = 9.402 Å, b = 11.864 Å and c = 3.240 Å and β = 98.75° (Z = 4) as shown in Figure 5b. The crystallite size of GO@MC-2 was found to be 35.6 nm as obtained from Scherrer equation.

(002) (111)

(a) RGO/CuO

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(-311) (-113) (220) (-202) (202) (020)

(110)

(b) GO/Cu2CO3(OH)2

10

20

30

40

50

60

70

80

90

Two theta (degree) Figure 5: PXRD of (a) RGO@CuO-2 and (b) GO@MC-2 nanocomposites


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The powder X-ray diffraction pattern of RGO@CuO-2 nanocomposite was well matched with the monoclinic phase CuO (JCPDS card no. 80–1916) with a space group C2/c and lattice parameters, a = 4.6837 Å, b = 3.4226 Å, c = 5.1288 Å, β= 99.54º, Z = 4 (Figure 5a). The broadening of peaks revealed the formation of nanocrystalline copper oxide. The peak at 2ߠ = 23.5 corresponds to d-spacing of 0.38 nm along the (002) orientation which is due to higher degree of expoliation of RGO sheets. This spacing can be explained by removal of intercalated water molecules and oxygen functional groups. The surface elemental analysis and oxidation state of copper in RGO@CuO-2 nanocomposite were studied by X-ray photoelectron spectra (XPS) as shown in Figure 6. The survey spectra of RGO@CuO-2 revealed the presence of Cu 2p, O 1s, C 1s, Cu 3s and Cu 3p photoelectrons with binding energies 931, 528, 284, 126 and 77 eV respectively as shown in Figure 6a.47,48

Figure 6: (a) Survey X-ray photoelectron spectra of RGO@CuO-2; (b), (c), (d) High resolution XPS of Cu 2p , O 1s, C 1s respectively.



The d9 electronic state of copper is a characteristic to +2 oxidation state, which was confirmed by presence of Cu 2p1/2 and Cu 2p3/2 peaks with binding energies 952.6 and 932.5 eV along with their satellite peaks at 960.9 and 940.6 eV respectively as shown in high resolution spectra of Cu 2p region (Figure 6b).48 The O 1s excitation at binding energy 528 eV corresponds to oxide as shown in Figure 6c. The binding energy peaks of C 1s at 284.5 and 281.8 eV were attributed to sp2-C and C-C of RGO sheets respectively (Figure 6d). The crystal structural features of RGO@CuO-2 nanocomposites were studied by Raman spectroscopy as depicted in Figure 7. The Raman spectrum of GO showed two characteristic peaks with almost equal intensities at 1609 cm-1 and 1330 cm-1 correspond to G and D bands respectively (Figure 7a). D Band

(a)

1330

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(b)

G Band 1609

1308

1591

800

1200

1600

2000

-1 Raman shift (cm )

Figure 7: Raman spectra of (a) GO and (b) RGO@CuO-2 nanocomposites The G band corresponds to E2g mode of graphite related to the vibration of the sp2-bonded carbon atoms in a two-dimensional hexagonal lattice, while D band corresponds to the defects and disorder in the hexagonal graphitic layers.49,50 The two characteristic peaks at 1308 and 1591


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cm-1 corresponding to the D and G bands respectively of RGO@CuO-2 nanocomposites (Figure 7b). The Raman ratio ID/IG (where ID and IG are intensity of D and G band) corresponds to the density of defects in graphene-based materials.51 The ID/IG ratio for RGO@CuO-2 was found to be 1.56 which is greater than that for GO (1.03) which indicates the presence of more defects in RGO@CuO-2 nanocomposites. RGO@CuO nanocomposites catalyzed decarboxylative tandem coupling reaction The catalytic potential of RGO@CuO nanocomposites was studied for decarboxylative C(sp3)-H activation of proline (2) for the synthesis of α-alkynylated N-substituted pyrrolidines (4) from wide range of aldehydes (1) and alkynes (3) by releasing H2O and CO2 as only byproducts in the presence of PEG600 as recoverable green solvent at 110 ºC as shown in scheme 3.

Scheme 3: RGO@CuO nanocomposites catalyzed decarboxylative tandem coupling reaction 13 ACS Paragon Plus Environment


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Table 1: Optimization study for RGO@CuO nanocatalyzed decarboxylative Tandem coupling among 3-nitrobenzaldehyde (1a), proline (2) and phenylacetylene (3a).a

S No.

Catalyst ( mole% )

Temp ( ºC )

Solvent

Time ( h ) Yield of 4aa ( % )b

1

RGO@CuO-2 (3.7)

110

Toluene

12

90

2

RGO@CuO-2 (3.7)

110

DMF

12

83

3

RGO@CuO-2 (3.7)

110

DMSO

12

60

4

RGO@CuO-2 (3.7)

110

Neat

12

30

5

RGO@CuO-2 (3.7)

100

Water

12

NR

6

RGO@CuO-2 (3.7)

110

EG

12

Trace

7

RGO@CuO-2 (3.7)

110

DEG

12

50

8

RGO@CuO-2 (3.7)

110

PEG600

1

97

9

RGO@CuO-2 (2.6)

110

PEG600

3

85

10

RGO@CuO-2 (1.5)

110

PEG600

3

70

11

RGO@CuO-2 (3.7)c

110

PEG600

3

85

12

RGO@CuO-1 (3.7)

110

PEG600

3

75

13

RGO@CuO-3 (3.7)

110

PEG600

3

86

14

RGO@CuO-2a (3.7)

110

PEG600

1

97

15

HS-CuOd (3.7)

110

PEG600

3

65

16

RGO + HS-CuOe (3.7)

110

PEG600

3

65

17

RGO (7 mg)

110

PEG600

12

-

18

No catal.

110

PEG600

24

-

19

RGO@CuO-2 (3.7)

rt

PEG600

24

-

a

Reaction conditions: 3-nitrobenzaldehyde 1a (1 mmol), proline 2 (1 mmol), phenylacetylene 3a (1 mmol), RGO@CuO catalysts ( mole%), solvent (3 mL) were stirred at appropriate temperature.b isolated yield.c Without Ar atmosphere.d,eHS-CuO alone and its physical mixture with RGO (7 mg); rt room temperature, (-) no product


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The model reactions were performed among 3-nitrobenzaldehyde (1a), proline (2) and phenylacetylene (3a) using 3.7 mole% of RGO@CuO-2 catalyst in various solvents and neat conditions as summarized in Table 1. In the presence of organic solvents such as toluene, DMF, DMSO, the reactions proceeded smoothly to afford the product (4aa) in quantitative yields (Table 1, Entries 1-3). In order to find a green and sustainable reaction conditions, the model reaction was performed under solvent free and in green solvents such as water, EG, DEG, PEG600 etc. as shown in Table 1 (Entries 4-8). The results showed that the progress of reaction was slow to afford product (4aa) in 30% yield under neat condition in 12 h (Table 1, Entry 4). The reaction did not proceed in the presence of water as a solvent (Table 1, Entry 5). The reason may be due to high solubility of proline (2) in water versus poor solubility of other substrates and also hydration of carboxylate ion by water molecules may slow down the decarboxylation step. Next, EG was also not found to be an appropriate solvent to afford a trace amount of product (4aa) and DEG showed moderate performance to afford 4aa in 50% yield in 12 h (Table 1, Entries 6, 7). Interestingly, PEG600 was found to be a suitable green solvent to afford product 4aa in 97% yield (Table 1, Entry 8). The yield of product (4aa) was dropped gradual from 97% to 70% upon decreasing the catalyst loading from 3.7 to 1.5 mol% respectively (Entries 9, 10). In the absence of Ar atmosphere, the yield of product (4aa) was dropped to 85% (Entry 11). The reason may be due to hygroscopic nature of PEG600 solvent which absorbs the water from air to slow down the progress of the reaction. Therefore, the optimized reaction condition was found to be usage of 3.7- 4 mole% of RGO@CuO nanocomposites as catalysts in PEG600 solvent at 110 ºC for 1 h under Ar atmosphere (Table 1, Entry 8).



Table 2: RGO@CuO-2 catalysed decarboxylative Tandem coupling among proline, aldehydes and alkynes.a

a

Reaction conditions: aldehydes 1 (1 mmol), proline 2 (1 mmol), alkynes 3 (1 mmol), RGO@CuO-2 catalyst

(3.7 mole%), PEG600 (3 mL) were stirred at 110 ºC, 1 h under Ar atmosphere.b reaction time 12 h.


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Next, we performed the controlled experiments by comparing the catalytic activity of RGO@CuO-2 nanocatalyst with RGO@CuO-1, RGO@CuO-2a, RGO@CuO-3 and also the physical mixture and individual case of HS-CuO and RGO nanomaterials as shown in Table 2 (Entries, 12-17). These results revealed the superior catalytic performance of RGO@CuO nanocomposites compared with HS-CuO alone and its physical mixture with RGO sheets. The RGO@CuO-2a showed the same performance as RGO@CuO-2 to afford the product (4aa) in 97% yield (Entries 8, 14). The reason may be due to the presence same weight percentage of Cu and almost similar morphologies of these both catalysts. However, in case of RGO@CuO-3 and RGO@CuO-1, the yield of 4aa was dropped to 75% and 86% respectively (Entries 12, 13). Moreover, there was no progress in the reaction, in case of RGO alone as a catalyst (Entry 17) and in the absence of catalysts under optimization conditions at 100 oC (Entry 18) or at room temperature in the presence of RGO@CuO-2 catalyst (Entry 19). This indicates the requirement of both copper active sites and thermal activation energy to trigger the coupling reactions. The high catalytic activity of these nanocomposites may be due to the presence of uniformly embedded nano-sized CuO with exposed (002), (111) facets on RGO thin sheets.

The generality of RGO@CuO-2 nanocatalyst was investigated by screening various aromatic aldehydes and alkynes bearing electron donating and withdrawing groups as summarised in Table 2. The desired products were obtained in excellent yields in case of aldehydes bearing electron withdrawing groups such as 3-nitro, 4-nitro, 4-bromo, 3,4-fluoro and 2,4-CF3 and hetero-aromatic aldehydes. Whereas, in case of aldehydes bearing electron donating groups such as 4-tert-butyl, 4-propyl, 4-butyl, 4-methyl, 3,4,5-tri-methoxy, 2-methoxy etc, the corresponding products were obtained in relatively low yield (Table 2). The reason may be due to less electrophilic nature of aldehydes bearing electron donating groups than the withdrawing groups 17 ACS Paragon Plus Environment


to trigger the decarboxylative resonance of negative charge on the both α-carbon of N-atom of proline moiety as shown in Figure 7. Moreover, this was further supported by usage of aliphatic aldehydes such as phenylacetaldehyde to afford product (4la) in 40% yield. However, the cyclohexanecarboxaldehyde was failed to generate corresponding product even after prolonged reaction time (Table 2). On the basis of experimental observation and the reported literature,46-48 the plausible mechanism of RGO@CuO catalysed Tandem coupling reaction is as depicted in Figure 7.

Figure 7: Plausible mechanism for RGO@CuO catalysed Tandem coupling reaction


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The first step involves the formation of iminium carboxylate intermediate (I) from the reaction of proline (2) and aldehyde (1) by releasing one mole of water molecule. The intermediate (I) undergoes decarboxylation via migration of negative charge from carboxylate ion to α-carbons of N-atom to generate the resonance structures (IIa) and (IIb) as intermediates by delocalization of negative charge. The previously reported theoretical study showed the energy of (IIb) is ca. 15.28 kJ/mol lower than that of (IIa) (where R1 = Ph).46 Thus, the copper acetylide on RGO@CuO catalyst surface (IV) from the activation of alkyne (3) undergoes nucleophilic attack on electron deficient α-carbon of iminium (IIb), is a relatively stable intermediate, to yield the regioselective product (4) as shown Figure 7. The catalyst was regenerated in the catalytic process and easily recoverable to reuse in the next cycles. The recyclability of RGO@CuO-2 nanocatalyst was examined for a model reaction of 3 mmol scale using 30 mg (3.7 mole%) of catalyst to afford substituted pyrrolidine (4aa) in 97% yield under optimized reaction condition as shown in Figure 8. Recycled RGO@CuO catalysts fresh RGO@CuOcatalyst

100 Yield of 4aa (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


97

95

95

94

94

2

3

4

5

90

80 60 40 20 0 1

6

No. of cycles

Figure 8: Recyclability of RGO@CuO-2 nanocatalyst for synthesis of (4aa)



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After completion of reaction, the PEG was recovered by addition of water and the catalyst was recovered by addition of ethanol followed by centrifugation. The catalyst was washed several times with ethanol and dried at 85 oC in vacuum oven for 8 h. The ethanol solvent fractions were combined and evaporated to afford the crude product (4aa). The recovered RGO@CuO catalyst was reused in a model reaction to afford the product (4aa) in 95% yield, and the procedure was repeated for 4 more times. These results showed that there was no considerable loss in its catalytic activity, however the decrease in the yield of product (4aa) from 97% to 90% was due to physical loss of catalyst quantity during recovery and washings. The morphology and crystalline structure of recycled RGO@CuO nanocatalyst was characterized from TEM and PXRD (ESI, Figure S1 & S2). The percentage of copper in the recovered RGO@CuO-2 catalyst was analysed by SEM-EDX and ICP-AES elemental analysis, found to be about 15% copper same as fresh catalyst (Figure S4). Next, the hot filtration experiment was performed to study the heterogeneity of RGO@CuO nanocatalyst for a model reaction among 3-nitrobenzaldehyde (1a), proline (2) and phenylacetylene (3a) under optimized condition to afford product (4aa). The reaction was stopped after 30 min i.e. around at ∼50% conversion of the starting materials, and filtered out the catalyst from the reaction mixture. There was no considerable improvement in the progress of reaction upon further continuing the filtrate for next 30 min. The ICP-AES analysis of filtrate did not show the detectable amount of copper ion, which confirms the heterogeneity of catalyst with its negligible leaching into the reaction mixture. The present method was found to be a green and sustainable approach with minimum waste generation and high reaction mass efficiency than the reported method (Table S1, SI). The green chemistry metrics calculations were found to be close to ideal values with low E-factor is equal to


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0.24, low process mass intensity (PMI = 1.24), high reaction mass efficiency (RME = 80.7%), high atom economy (AE = 83%) (see ESI for calculations and comparison table).

CONCLUSION In summary, we developed a sustainable approach for the synthesis of RGO@CuO nanocomposites from a renewable malachite mineral precursor. The RGO@CuO nanocatalysts were found to be robust catalysts for decarboxylative C(sp3)-H activation of proline amino acid via tandem coupling with wide range of aldehydes and alkynes to afford α-alkynylated Nsubstituted pyrrolidines as value-added synthons under green reaction conditions. The present method is advantageous due to facile synthetic procedure for catalyst preparation, wide substrate scope with excellent yields, involves green solvent and avoids the usage of excess equivalents of substrates and extra reagents with low waste generation (E-factor = 0.24) and high reaction mass efficiency (80.7%) and recycled for five times with no considerable loss in its activity.

EXPERIMENTAL SECTION Preparation of graphene oxide (GO) from graphite: The graphene oxide (GO) sheets were prepared by modified Hummer’s method.64 The procedure involves the sequential addition of 10 g of graphite flakes and sodium nitrate (88.2 mmol, 7.5 g) in to 1 liter round bottom flask containing 300 mL of concentrated sulfuric acid (98%) under constant stirring at room temperature. To this mixture, potassium permanganate (253 mmol, 40 g) was added slowly portion wise for 1 h and left the mixture for 3 days under constant stirring at room temperature. One liter of hydrogen peroxide (1% in water) solution was then slowly into the mixture, followed 21 ACS Paragon Plus Environment


by filtered and washed several times with deionized water until pH 7 to afford the black cake. The obtained black cake was purified by dialysis for one week to remove the residual salts and acids to obtain a brown suspension. The GO as a black solid was obtained by sonicating the suspension in water for 1 h followed by centrifugation and dried at 40 oC in vacuum oven. Preparation of micrometer-sized malachite spheres (MC): The hierarchical sphere-like malachite was prepared by modified reported method.65 The titration of a 0.1 M CuSO4 aqueous solution with 0.1 M Na2CO3 aqueous solution under vigorous stirring at 50 oC for 30 min and transferred the mixture in to a Teflon-lined stainless steel autoclave and heated at 90 oC for 5 h. The autoclave was then cooled to room temperature naturally to collect the green precipitates by filtration and washed with deionized water and the final product was dried at 50 °C for 5 h. Preparation of GO@MC and RGO@CuO: The GO@MC nanocomposites were synthesized by the addition of various weight ratios of malachite (0.5 g, 0.3 g and 0.2 g) were dispersed in three separate aqueous dispersion solutions of GO (1g in 300 mL water) and stirred the resulted mixtures at RT for 30 h. The obtained dark solid dispersions were separated by centrifugation at 5000 rpm for 5 min by washed with water and acetone followed by dried in an oven at 100 oC for 4 h. The obtained dark solid materials were named as GO@MC-1, GO@MC-2 and GO@MC-3 respectively. These composites were calcined at 300 oC or 450 oC for 5 h to afford the corresponding RGO@CuO-1, RGO@CuO-2 and RGO@CuO-3 or RGO@CuO-2a respectively.


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General procedure for the synthesis of substituted pyrrolidines (4): A mixture of proline 2 (1 mmol), aldehyde 1 (1 mmol) and alkyne (3) (1 mmol) and RGO@CuO catalyst (10 mg) were added in 10 mL flask containing PEG (2 mL) and mixture was stirred at 110 ºC for 1 h. After completion of the reaction, catalyst was recovered by the addition of ethanol followed by centrifugation and filtration. The organic residue was dissolved in ethanol and evaporated to obtain the crude products, followed by column chromatography to afford the pure products. Spectral data of unknown compounds 1-(4-Bromo-benzyl)-2-phenylethynyl-pyrrolidine (4ca): Yellow liquid; IR (ϑmax/cm-1, CHCl3): 3744, 2921, 2809, 1595, 1486, 1070, 1011, 803, 755, 691; 1H NMR (400 MHz, CDCl3) δ = 7.427.45 (m, 5H), 7.27-7.32 (m, 4H), 3.98 (d, J = 12.9 Hz, 1H), 3.54-3.59 (m, 2H), 2.72-2.78 (m, 1H), 2.49-2.56 (m, 1H), 2.12-2.21 (m, 1H), 1.75-2.06 (m, 3H) ppm;

13

C NMR (100 MHz,

CDCl3) δ = 137.8, 131.65, 131.24, 130.80, 128.22, 127.96, 123.18, 120.73, 88.39, 85.05, 56.50, 54.35, 51.51, 31.57, 21.98 ppm; HRMS (ESI) calcd for C19H19BrN [M+H]+: 340.0701, found: 340.0703. 1-(3,4-Difluoro-benzyl)-2-phenylethynyl-pyrrolidine (4da): Yellow liquid; IR (ϑmax/cm-1, CHCl3): 3743, 2923, 2814, 1609, 1516, 1436, 1283, 1206, 1112, 817, 755, 692; 1H NMR (400 MHz, CDCl3) δ = 8.04 (d, J = 7.6 Hz, 1H), 7.88 (s, 1H), 7.77 (d, J = 8.4 Hz, 1H), 7.38-7.40 (m, 2H), 7.28-7.29 (m, 3H), 4.21 (d, J = 15.3 Hz, 1H), 3.95 (d, J = 15.3 Hz, 1H), 3.73-3.76 (m, 1H), 2.80-2.86 (m, 1H), 2.54-2.60 (m, 1H), 2.19-2.28 (m, 1H), 1.80-2.11 (m, 3H) ppm; 13C NMR (100 MHz, CDCl3) δ = 151.40 (d, 3JC-F = 12.5 Hz), 150.62 (d, 4JC-F = 12.5 Hz), 148.94 (d, 5JC-F = 12.5 Hz), 148.17 (d, 6JC-F = 12.5 Hz), 136.06, 131.66, 128.25, 128.03, 124.69, 123.16, 117.74 (d, 1JC-F = 17.2 Hz), 116.75 (d, 2JC-F = 17.2 Hz), 88.24, 85.16, 56.14, 54.35, 51.48, 31.62, 22.03 ppm; HRMS (ESI) calcd for C19H18F2N [M+H]+: 298.1407, found: 298.1407. 1-(2,4-Bis-trifluoromethyl-benzyl)-2-phenylethynyl-pyrrolidine (4ea): Yellow liquid; IR (ϑmax/cm-1, CHCl3): 2920, 1342, 1270, 1118, 1052, 909, 851, 754, 679; 1H NMR (400 MHz, CDCl3) δ = 8.03 (d, J = 8.4 Hz, 1H), 7.88 (s, 1H), 7.77 (d, J = 7.6 Hz, 1H), 7.38-7.40 (m, 2H), 7.28-7.29 (m, 3H), 4.23 (d, J = 16.0 Hz, 1H), 3.94 (d, J = 15.3 Hz, 1H), 3.72-3.75 (m, 1H), 2.83 (dd, J = 8.4 Hz, 1H), 2.57 (dd, J = 8.4 Hz, 1H), 2.18-2.27 (m, 1H), 1.79-2.11 (m, 3H) ppm; 13C 23 ACS Paragon Plus Environment


NMR (100 MHz, CDCl3) δ = 143.08, 131.63, 131.11, 129.31, 128.99, 128.46, 128.22, 128.04, 124.98 (d, 1JC-F = 18.2 Hz), 123.03, 122.83, 122.26 (d, 2JC-F = 15.3 Hz), 88.15, 85.11, 55.04, 52.71, 51.85, 31.81, 22.23 ppm; HRMS (ESI) calcd for C21H18F6N [M+H]+: 398.1343, found: 398.1362. 2-(4-Methoxy-phenylethynyl)-1-(3-nitro-benzyl)-pyrrolidine (4ad): Yellow oil; IR (ϑmax/cm-1, CHCl3): 2921, 1605, 1524, 1507, 1346, 1288, 1245, 1106, 1030, 831, 733, 695; 1H NMR (400 MHz, CDCl3) δ = 8.26 (s, 1H), 8.10 (d, J = 8.4 Hz, 1H), 7.72 (d, J = 7.6 Hz, 1H), 7.46 (d, J = 7.6 Hz, 1H), 7.37 (d, J = 8.4 Hz, 2H), 6.83 (d, J = 9.2 Hz, 2H), 4.09 (d, J = 13.7 Hz, 1H), 3.79 (s, 3H), 3.69 (d, J = 12.9 Hz, 1H), 3.58-3.61 (m, 1H), 2.77 (dd, J = 8.4 Hz, 1H), 2.57 (dd, J = 8.4 Hz, 1H), 2.13-2.22 (m, 1H), 1.78-2.06 (m, 3H) ppm; 13C NMR (100 MHz, CDCl3) δ = 159.42, 148.25, 141.41, 135.10, 133.06, 129.04, 123.79, 122.04, 115.14, 113.86, 86.52, 85.12, 56.45, 55.24, 54.60, 51.66, 31.69, 22.08 ppm; HRMS (ESI) calcd for C20H21N2O3 [M+H]+: 337.1552, found: 337.1579. 1-(4-tert-Butyl-benzyl)-2-phenylethynyl-pyrrolidine (4fa): Yellow oil; IR (ϑmax/cm-1, CHCl3): 3743, 2958, 2807, 1362, 1266, 1109, 1021, 831, 752, 690; 1H NMR (400 MHz, CDCl3) δ = 7.437.46 (m, 2H), 7.28-7.34 (m, 7H), 4.01 (d, J = 12.9 Hz, 1H), 3.56-3.61 (m, 2H), 2.76-2.82 (m, 1H), 2.52-2.57 (m, 1H), 2.11-2.19 (m, 1H), 1.74-2.06 (m, 3H), 1.30 (s, 9H) ppm; 13C NMR (100 MHz, CDCl3) δ = 149.78, 135.59, 131.71, 128.89, 128.21, 127.89, 125.07, 123.41, 88.77, 84.97, 56.72, 54.38, 51.55, 34.44, 31.59, 31.39, 21.98 ppm; HRMS (ESI) calcd for C23H28N [M+H]+: 318.2222, found: 318.2256. 1-(4-Butyl-benzyl)-2-phenylethynyl-pyrrolidine (4ga): Yellow oil; IR (ϑmax/cm-1, CHCl3): 2924, 2857, 1599, 1216, 890, 755, 693; 1H NMR (400 MHz, CDCl3) δ = 7.44-7.47 (m, 2H), 7.28-7.33 (m, 5H), 7.13 (d, J = 7.6 Hz, 2H), 4.02 (d, J = 12.9 Hz, 1H), 3.57-3.61 (m, 2H), 2.762.81 (m, 1H), 2.52-2.61 (m, 3H), 2.11-2.20 (m, 1H), 1.75-2.06 (m, 3H), 1.59 (pentet, J = 7.6 Hz, 2H), 1.36 (sextet, 2H), 0.92 (t, J = 7.6 Hz, 3H) ppm; 13C NMR (100 MHz, CDCl3) δ = 141.57, 135.74, 131.70, 129.13, 128.20, 127.88, 126.94, 123.40, 88.76, 84.94, 56.83, 54.34, 51.47, 35.30, 33.65, 31.58, 22.37, 21.97, 13.94 ppm; HRMS (ESI) calcd for C23H28N [M+H]+: 318.2222, found: 318.2248. 2-Phenylethynyl-1-(3,4,5-trimethoxy-benzyl)-pyrrolidine (4ja): Yellow oil; IR (ϑmax/cm-1, CHCl3): 2923, 2849, 1589, 1456, 1231, 1120, 1005, 754, 692; 1H NMR (400 MHz, CDCl3) δ = 24 ACS Paragon Plus Environment

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7.46-7.48 (m, 2H), 7.32-7.33 (m, 3H), 6.66 (s, 1H), 4.01 (d, J = 12.9 Hz, 1H), 3.87 (s, 6H), 3.86 (s, 3H), 3.64-3.67 (m, 1H), 3.58 (d, J = 12.9 Hz, 1H), 2.82 (dd, J = 8.4 Hz, 1H), 2.63 (dd, J = 8.4 Hz, 1H), 2.16-2.25 (m, 1H), 1.83-2.10 (m, 3H) ppm; 13C NMR (100 MHz, CDCl3) δ = 152.95, 136.81, 134.45, 131.58, 128.21, 127.93, 123.26, 105.94, 88.56, 85.12, 60.74, 57.51, 56.01, 54.40, 51.58, 31.55, 29.62, 22.01 ppm; HRMS (ESI) calcd for C22H26NO3 [M+H]+: 352.1913, found: 352.1939. 1-(2-Methoxy-benzyl)-2-phenylethynyl-pyrrolidine (4ka): Yellow oil; IR (ϑmax/cm-1, CHCl3): 2924, 1599, 1492, 1242, 1095, 1029, 819, 753, 691; 1H NMR (400 MHz, CDCl3) δ = 7.41-7.46 (m, 2H), 7.28-7.31 (m, 3H), 7.09 (d, J = 9.2 Hz, 1H), 6.93 (t, J = 7.6 Hz, 1H), 6.87 (d, J = 8.4 Hz, 1H), 6.59 (d, J = 8.4 Hz, 1H), 4.09 (d, J = 13.7 Hz, 1H), 3.82 (s, 3H), 3.74 (d, J = 13.7 Hz, 1H), 3.60-3.63 (m, 1H), 2.87-2.92 (m, 1H), 2.49-2.55 (m, 1H), 2.13-2.22 (m, 1H), 1.73-2.07 (m, 3H) ppm;

13

C NMR (100 MHz, CDCl3) δ = 157.72, 131.71, 130.78, 129.08, 128.15, 128.05,

127.80, 126.74, 123.53, 120.24, 116.19, 110.44, 89.25, 84.50, 55.42, 54.71, 51.74, 50.53, 31.61, 29.67, 22.09 ppm; HRMS (ESI) calcd for C20H22NO [M+H]+: 292.1701, found: 292.1736. 1-(4-tert-Butyl-benzyl)-2-p-tolylethynyl-pyrrolidine (4fc): Yellow oil; IR (ϑmax/cm-1, CHCl3): 2958, 1511, 1462, 1363, 1266, 1110, 1021, 816; 1H NMR (400 MHz, CDCl3) δ = 7.22-7.27 (m, 6H), 7.03 (d, J = 7.6 Hz, 2H), 3.94 (d, J = 12.9 Hz, 1H), 3.48-3.52 (m, 2H), 2.68-2.74 (m, 1H), 2.43-2.49 (m, 1H), 2.26 (s, 3H), 2.02-2.11 (m, 1H), 1.68-1.97 (m, 3H), 1.23 (s, 9H) ppm;

13

C

NMR (100 MHz, CDCl3) δ = 149.72, 137.89, 135.63, 131.57, 128.94, 128.89, 125.29, 125.04, 120.32, 87.98, 85.01, 56.70, 54.39, 51.51, 51.39, 34.42, 31.59, 31.38, 21.96, 21.41 ppm; HRMS (ESI) calcd for C24H30N [M+H]+: 332.2378, found: 332.2402. 2-(4-Fluoro-phenylethynyl)-1-(4-methyl-benzyl)-pyrrolidine (4ib): Yellow oil; IR (ϑmax/cm-1, CHCl3): 2921, 2811, 1601, 1505, 1217, 1154, 834, 809, 753, 667; 1H NMR (400 MHz, CDCl3) δ = 7.32-7.36 (m, 2H), 7.19 (d, J = 8.4 Hz, 2H), 7.05 (d, J = 7.6 Hz, 2H), 6.89-6.94 (m, 2H), 3.93 (d, J = 12.9 Hz, 1H), 3.45-3.51 (m, 2H), 2.67-2.72 (m, 1H), 2.41-2.47 (m, 1H), 2.25 (s, 3H), 2.03-2.12 (m, 1H), 1.67-1.97 (m, 3H) ppm; 13C NMR (100 MHz, CDCl3) δ = 162.26 (d, 1JC-F = 249.2 Hz), 136.53, 135.44, 133.51 (d, 5JC-F = 7.7 Hz), 130.61 (d, 4JC-F = 8.6 Hz), 129.14, 128.87, 119.46, 115.42 (d, 1JC-F = 22.0 Hz), 114.96 (d, 2JC-F = 21.1 Hz), 88.46, 83.81, 56.86, 54.29, 51.47, 31.56, 21.96, 21.08 ppm; HRMS (ESI) calcd for C20H21FN [M+H]+: 294.1658, found: 294.1694. 25 ACS Paragon Plus Environment


2-(4-Fluoro-phenylethynyl)-1-(4-propyl-benzyl)-pyrrolidine (4hb): Yellow oil; IR (ϑmax/cm-1, CHCl3): 2957, 1601, 1505, 1457, 1229, 1110, 1019, 834, 763; 1H NMR (400 MHz, CDCl3) δ = 7.39-7.43 (m, 2H), 7.28 (d, J = 8.4 Hz, 2H), 7.12 (d, J = 8.4 Hz, 2H), 6.97-7.01 (m, 2H), 4.01 (d, J = 12.9 Hz, 1H), 3.55-3.59 (m, 2H), 2.76-2.81 (m, 1H), 2.49-2.58 (m, 3H), 1.78-2.18 (m, 4H), 1.63 (sextet, 2H), 0.93 (d, J = 7.6 Hz, 3H) ppm; 13C NMR (100 MHz, CDCl3) δ = 162.25 (d, 1JCF=

249.2 Hz), 141.38, 135.71, 133.51 (d, 3JC-F = 8.6 Hz), 129.08, 128.45, 128.27, 126.98, 126.42,

115.41 (d, 2JC-F = 22.0 Hz), 88.48, 83.79, 56.92, 54.34, 51.53, 37.71, 31.56, 24.54, 21.97, 13.83 ppm; HRMS (ESI) calcd for C22H25FN [M+H]+: 322.1971, found: 322.1992. 1-(2-Methoxy-benzyl)-2-p-tolylethynyl-pyrrolidine (4kc): Yellow oil; IR (ϑmax/cm-1, CHCl3): 2920, 1600, 1508, 1491, 1240, 1101, 1030, 816, 753; 1H NMR (400 MHz, CDCl3) δ = 7.43 (d, J = 7.6 Hz, 1H), 7.36 (d, J = 8.4 Hz, 2H), 7.24 (t, J = 7.6 Hz, 1H), 7.11 (d, J = 7.2 Hz, 2H), 6.94 (t, J = 7.6 Hz, 1H), 6.88 (d, J = 8.4 Hz, 1H), 4.11 (d, J = 13.7 Hz, 1H), 3.82 (s, 3H), 3.74 (d, J = 13.7 Hz, 1H), 3.61 (t, J = 6.9 Hz, 1H), 2.88-2.93 (m, 1H), 2.49-2.55 (m, 1H), 2.35 (s, 3H), 1.752.22 (m, 4H) ppm;

13

C NMR (100 MHz, CDCl3) δ = 157.70, 137.78, 131.57, 130.77, 128.89,

127.99, 126.79, 120.45, 120.21, 110.40, 88.55, 84.51, 55.40, 54.73, 51.72, 50.51, 31.61, 22.07, 21.39 ppm; HRMS (ESI) calcd for C21H24NO [M+H]+: 306.1858, found: 306.1879. 1-Phenethyl-2-phenylethynyl-pyrrolidine (4la): Yellow oil; IR (ϑmax/cm-1, CHCl3): 2923, 1693, 1601, 1491, 1448, 1261, 756, 699; 1H NMR (400 MHz, CDCl3) δ = 7.94 (d, J = 6.9 Hz, 1H), 7.17-7.22 (m, 5H), 7.07-7.11 (m, 3H), 5.89 (t, J = 7.6 Hz, 1H), 3.74 (d, J = 14.5 Hz, 1H), 3.69-3.71 (m, 1H), 3.40 (d, J = 13.7 Hz, 1H), 3.31 (d, J = 7.6 Hz, 2H), 2.67-2.73 (m, 1H), 2.562.62 (m, 1H), 1.68-2.09 (m, 4H) ppm; 13C NMR (100 MHz, CDCl3) δ = 141.08, 139.93, 138.73, 131.64, 128.56, 128.37, 128.29, 128.18, 128.07, 127.88, 126.88, 125.82, 88.39, 85.18, 60.26, 54.36, 51.27, 34.96, 31.55, 22.06 ppm; HRMS (ESI) calcd for C20H22N [M+H]+: 276.1752, found: 276.1791. 2-Hex-1-ynyl-1-(3-nitro-benzyl)-pyrrolidine (4ae): Yellow oil; IR (ϑmax/cm-1, CHCl3): 2926, 2857, 1531, 1349, 903, 734; 1H NMR (400 MHz, CDCl3) δ = 8.16 (s, 1H), 8.03 (d, J = 8.4 Hz, 1H), 7.64 (d, J = 7.6 Hz, 1H), 7.40 (t, J = 7.6 Hz, 1H), 3.98 (d, J = 12.9 Hz, 1H), 3.53 (d, J = 12.9 Hz, 1H), 3.28 (brs, 1H), 2.64 (dd, J = 8.4 Hz, 1H), 2.41 (dd, J = 8.4 Hz, 1H), 2.15-2.18 (m, 2H), 1.96-2.06 (m, 1H), 1.77-1.87 (m, 1H), 1.66-1.74 (m, 1H), 1.32-1.48 (m, 4H), 0.86 (t, J = 7.6 Hz, 3H) ppm; 13C NMR (100 MHz, CDCl3) δ = 148.28, 141.46, 135.12, 129.01, 123.84, 122.03, 26 ACS Paragon Plus Environment

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85.55, 78.49, 56.39, 54.32, 51.33, 31.83, 31.06, 29.68, 21.97, 18.41, 13.59 ppm; HRMS (ESI) calcd for C17H23N2O2 [M+H]+: 287.1760, found: 287.1782. 3-((2-(phenylethynyl)pyrrolidin-1-yl)methyl)pyridine (4pa): Yellow Oil; IR (ϑmax/cm-1, CHCl3): 3029, 2812, 1692, 1577, 1486, 1426, 1326, 1106, 1028, 796, 756, 692; 1H NMR (400 MHz, C6D6) δ = 8.81 (s, 1H), 8.49-8.51 (m, 1H), 7.43-7.46 (m, 1H), 6.89-6.99 (m, 3H), 6.746.77 (m, 1H), 3.87 (d, J = 12.8 Hz, 1H), 3.44-3.47 (m, 1H), 3.39 (d, J = 13.3 Hz, 1H), 2.52-2.58 (m, 1H), 2.22-2.28 (m, 1H), 1.84-1.94 (m, 2H), 1.62-1.70 (m, 1H), 1.40-1.49 (m, 1H) ppm;

13

C

NMR (100 MHz, C6D6) δ = 150.66, 148.71, 136.04, 134.75, 131.86, 128.42, 123.70, 123.11, 88.81, 85.55, 54.61, 54.38, 51.37, 31.83, 22.19 ppm; HRMS (ESI) calcd for C18H19N2 [M+H]+: 263.1548, found: 263.1589. 2-((2-(phenylethynyl)pyrrolidin-1-yl)methyl)quinolone (4oa): Yellow Oil; IR (ϑmax/cm-1, CHCl3): 3056, 2954, 2812, 1688, 1599, 1498, 1428, 1314, 1218, 1112, 830, 756, 692; 1H NMR (400 MHz, C6D6) δ = 8.04 (t, J = 7.8 Hz, 2H), 7.72 (d, J = 8.2 Hz, 1H), 7.58-7.64 (m, 2H), 7.44 (t, J = 7.8 Hz, 1H), 7.31-7.34 (m, 2H), 7.19-7.22 (m, 3H), 4.29 (d, J = 13.7 Hz, 1H), 3.92 (d, J = 13.7 Hz, 1H), 3.70-3.74 (m, 1H), 2.78-2.84 (m, 1H), 2.55-2.61 (m, 1H), 2.14-2.26 (m, 1H), 1.862.05 (m, 3H) ppm;

13

C NMR (100 MHz, C6D6) δ = 156.84, 144.37, 131.93, 128.00, 125.83,

125.22, 124.39, 123.57, 121.95, 119.90, 117.36, 85.54, 81.25, 56.36, 51.78, 48.06, 28.13, 18.54 ppm; HRMS (ESI) calcd for C22H21N2 [M+H]+: 313.1705, found: 313.1756. 2-((2-(phenylethynyl)pyrrolidin-1-yl)methyl)pyridine (4na): Yellow Oil; IR (ϑmax/cm-1, CHCl3): 3056, 2955, 2809, 1692, 1361, 1149, 1113, 994, 755, 692; 1H NMR (400 MHz, C6D6) δ = 8.46-8.47 (m, 1H), 7.35.7.44 (m, 3H), 6.89-6.91 (m, 4H), 6.57-6.60 (m, 1H), 4.37 (d, J = 13.7 Hz, 1H), 3.96 (d, J = 14.2 Hz, 1H), 3.58-3.62 (m, 1H), 2.74-2.79 (m, 1H), 2.35-2.41 (m, 1H), 1.86-1.96 (m, 2H), 1.61-1.71 (m, 1H) 1.37-1.47 (m, 1H) ppm;

13

C NMR (100 MHz, C6D6) δ =

160.36, 148.79, 135.98, 132.06, 128.69, 123.97, 122.59, 121.78, 89.53, 85.28, 59.46, 56.61, 51.93, 32.11, 22.56 ppm; HRMS (ESI) calcd for C18H19N2 [M+H]+: 263.1548, found: 263.1587.

■ ASSOCIATED CONTENT Supporting Information



Page 28 of 36

SEM, TEM images of GO, malachite and recycled RGO@CuO-2; EDX elemental analysis and PXRD characterization of recycled RGO@CuO-2 nanocatalyst; Green chemistry metrics calculation of product (4aa) compared with reported method; 1HNMR and

13

C NMR spectral

data and spectra of known and all synthesized compounds respectively (PDF).

ACKNOWLEDGMENTS

D.S.R. acknowledges DST-JSPS grant 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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Page 36 of 36

RGO@CuO was found to be a sustainable nanocatalyst for decarboxylative C(sp3)-H activation of proline to afford α-alkynylated N-substituted pyrrolidines.

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