Greener Biogenic Approach for the Synthesis of Palladium Nanoparticles Using Papaya Peel: An Eco-Friendly Catalyst for C-C Coupling Reaction.

The development of a green and sustainable synthetic methodology still remains a challenge across the globe. Encouraging the prevailing challenge, herein, we have synthesized Pd nanoparticles (Pd NPs) in a green and environmentally viable route, using the extract of waste papaya peel without the assistance of any reducing agents, high-temperature calcination, and reduction procedures. The biomolecules present in the waste papaya peel extract reduced Pd(II) to nanosize Pd(0) in a one-pot green and sustainable process. As a catalyst, the new Pd NPs offer a simple and efficient methodology in direct Suzuki-Miyaura and Sonogashira coupling with excellent yields under mild reaction conditions.

Introduction

Transition metals, on account of their unique bonding properties and reaction mechanism, provide newer opportunities for synthetic application as a catalyst.[1,2] In this regard, palladium continues to attract great interest because of its versatility as a catalyst that mediates various C–C bond-forming reactions.[3−11] Among them, Suzuki–Miyaura[12−15] and Sonogashira[16−20] cross-coupling reactions are reliable and powerful methods for the synthesis of biaryl[21−24] and acetylenyl[25−27] derivatives which are ubiquitous features in numerous biologically active compounds. They have a vast application potential as a key synthetic precursor for the construction of pharmaceuticals, fine chemicals, agrochemicals, natural products, and smart engineering materials, including conducting polymers and molecular wires.[28−32] Consequently, there is a need to design a highly efficient, cost-effective, simple, commercially viable, and environmentally friendly methods for the synthesis of biaryl and acetylenyl derivatives. Nowadays, metal nanoparticles (NPs) have received particular interest in a wide range of research fields because of their fabulous ability as a nanocatalyst and connecting the homogeneous and heterogeneous catalysis.[33]

These features invariably attract the researchers to use NPs in catalysis. Usually metal NPs are prepared by different chemical, physical, and biological methods.[34] Conventional NP synthesis methods involve the usage of toxic chemicals, formation of hazardous by-products, and contamination from precursor chemicals. Therefore, development of clean, nontoxic, and environment-friendly procedures for NP synthesis is needed. In this aspect, biological techniques have several advantages over physical and chemical methods. It involves the use of environment-friendly green chemistry-based approach that employs microorganisms (bacteria, fungi, algae, and yeast) and plants.[35] In comparison to the rate of reduction of metal ions with microorganisms, plant extract is found to be more effective.[36] However, the use of plant resources for NP synthesis brings up significant drawback, leading to the destruction of ecologically important plants and plant parts. As such, in order to avoid this and to serve the purpose of pollution mitigation, use of agrowaste is amenable, which is otherwise a significant source of pollution and creates waste management issues. The use of various agrowastes for the synthesis of NPs is an attractive platform in last few decades. Moreover, nowadays, use of agricultural waste such as peel extract for the production of cellulose nanofibers is a widely accepted technology.[37,38]

Papaya (Carica papaya L, family: Caricaceae) is one of the most common fruits disseminated throughout the world. Papaya juice has a wide range of purported medicinal properties for the treatment of various diseases. In industry, the processing of this fruit, as well as its fresh consumption, results about 20–25% of waste, such as peels and seeds which are abundant natural waste materials across the world.[39] Waste utilization from food processing industries is highly essential and challenging task all around the globe.[37,38] As such, new aspects toward recyclability of the waste by-products in organic synthesis should be encouraged by the researchers from the point of environmental issues. Keeping this in mind, herein, we have synthesized Pd NPs in an eco-friendly manner by using the water extract of fresh peel of papaya fruits at room temperature without using any reducing agent (Figure ). Use of papaya peel waste (vegetable peel) without conventional treatments fulfilling special requirement of pH, temperature, and rigorous experimental setup is a new route for the Pd NP synthesis. This paper was emphasized on the generation of Pd NPs utilizing waste papaya peels, their detail characterization, and application in ligand-free Suzuki–Miyaura and copper- and amine-free Sonogashira reactions. Traditionally, both the reactions are performed in volatile organic solvents. Therefore, “greening-up” these reactions are important aspect in synthesis. Looking upon this, we present here an effectively green protocol by carrying out these cross-coupling reactions using this Pd NPs as catalysts in the greener solvent as it is environmentally and economically doable.

Figure 1

Preparation of Pd NPs from waste papaya peels.

Results and Discussion

Preparation of Pd NPs Using Waste Papaya Peel

Our first aim was to synthesize Pd NPs by simple replacement of synthetic reagents with natural ones via complete utilization of the residual sources, instead of wasteful dumping. The Pd NP was synthesized by direct mixing of Pd(OAc)2 with aqueous extract of papaya peel at room temperature (as provided in the Experimental Section). After 2 days, the color of the reaction mixture became black from brown (which indicates the formation of Pd NPs). The flavin mononucleotide and flavin adenine dinucleotide present in the papaya peel may act as a reductant for the reduction of Pd(OAc)2.[40]Figure is a pictorial presentation of this color change. It is noteworthy to mention that we have followed the green chemistry protocol to synthesize these Pd NPs without using any base, reducing agent, toxic or hazardous reagent, and any organic solvent.

Figure 2

UV–vis absorption spectra of (a) papaya extract, (b) papaya extract + Pd(OAc)2, and (c) Pd NPs.

Characterization of Pd NPs

The Pd NPs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Brunaur–Emmett–Teller (BET), and UV–visible spectroscopy and used as a catalyst for C–C coupling reactions.

The UV–visible absorption spectroscopy is a commonly used technique to characterize various metal NPs.[41] The light wavelength between 200 and 600 nm is generally used for the characterization of Pd NPs.[42] The UV spectrum (Figure a) shows a prominent band at 269 nm of the papaya peel extract. Again, the absorption bands of the Pd(OAc)2 solution above 270 nm were assigned for ligand-to-metal charge-transfer transition.[43,44] Addition of papaya peel extract into Pd2+ ion solution shows a gradual color change to black over time (Figure a–c). A subsequent color change to black indicates Pd2+ reduction to zero-valent Pd as proved by the UV–visible analysis. Figure b shows the absorption spectrum of Pd(OAc)2 solution after the addition of papaya peel extract, which indicates a distinctive overlapping peak approximately in the same region as that of the extract. The absorption spectrum of the suspension of Pd NPs after bioreduction results in the appearance of a broad absorption in the region 320–450 nm after 2 days which may be due to the scattering effect of solid Pd particles (Figure c).

Figure a shows the powder XRD (PXRD) pattern of Pd NPs which demarcate the formation of the face-centered cubic (fcc) lattice system of the palladium nanostructure (JCPDS card no. 89-4897). The observed peaks of Pd at 2θ = 39.9, 46.4, and 67.1° correspond to (111), (200), and (220) reflections, respectively, along with two additional diffraction peaks of PdO (JCPDS card no. 75-0200) at 2θ = 29.8 and 55° for (100) and (112) reflections, respectively. Figure b shows the energy-dispersive X-ray (EDX) pattern of the sample which confirmed the existence of Pd and O. Thus, XRD and EDX analyses confirm the formation of Pd/PdO. Pd(0) initially formed might have transformed to PdO via aerial oxidation as reported earlier.[45]

Figure 3

(a) PXRD pattern, (b) EDX pattern, and (c,d) SEM images of Pd NPs.

SEM was employed to characterize the morphology and surface topography of the Pd NPs. It is observed that small aggregates of Pd NPs are self-oriented in forming bigger particles with some porous features. In Figure d, the SEM image of Pd NPs is encircled to demarcate the porous features which were further confirmed by pore size distribution analyses (by the Barrett–Joyner–Halenda method). Further TEM studies were performed to understand the exposed crystal planes with definite interplaner spacing. Figure a presents the global TEM image of prepared Pd NPs. It could be observed that highly crystalline spherical Pd NPs are which ranges 1–5 nm in size. The distribution of the synthesized Pd NPs (from Figure a) was analyzed using Gaussian fits and presented in Figure b in the form of histogram.

Figure 4

(a) TEM image, (b) Pd NP distribution, and (c,d) HRTEM images of Pd NPs.

As shown in Figure b, ca. 80% of the Pd NPs size falls in 1–3 nm with a mean size of 2.4 nm. Figure c,d confirms the presence of crystal planes with the lattice spacings of 0.22 and 0.19 nm which are consistent with interplaner spacing {111} and {200} lattice planes of fcc Pd NPs. The corresponding selected-area electron diffraction (SAED) pattern of Pd NPs is presented in Figure c (inset) which possesses five well-resolved rings corresponding to fcc. The ring originated is ascribed to the crystallographic planes (111), (200), (220), (311) and (222) of fcc Pd NPs. There is a good match between the calculated d-values and the d-values obtained from standard JCPDS data. Thus, XRD and high-resolution transmission electron microscopy (HRTEM) (including SAED) studies evidently establish the crystalline nature of Pd NPs. The specific surface area was determined by N2 physisorption using the BET method and is presented in Figure along with pore size distribution. The isotherm produced is of type IV isotherm with H3 hysteresis loop with upper closure point P/Po ≥ 0.9 which are demarcated as a characteristic of the mesoporous material. Additionally, the specific surface area as calculated by the BET equation was about 18.2 m2/g with a pore diameter of 3.7 nm. Moreover, the pore size distribution curve (inset) also indicates the mesoporosity of the Pd NPs.

Figure 5

(a) N2 adsorption/desorption isotherm and inset and (b) pore size distribution of Pd NPs.

Catalytic Activity of Pd NPs

The efficiency of these Pd NPs is initially assessed in the reaction of 4-methoxybromobenzene (1 mmol) and phenylboronic acid (1.2 mmol) using different bases in H2O at room temperature (Table ), and K2CO3 was found to be the most effective base (Table , entries 1–8). In addition, we have also performed the reaction in different solvents such as EtOH, i-PrOH, CH3CN, and H2O/EtOH (1:1). Interestingly, the use of H2O/EtOH (1:1) as a solvent gave the desired product within shorter reaction time (Table , entry 13). As illustrated in Table , 0.0009 mmol catalyst was sufficient to obtain 95% yield of the product in both H2O and aqueous ethanol (Table , entries 12 and 13) and the reaction did not proceed without catalyst (Table , entry 14).

Table 1

Screening the Amount of Catalyst and Base for Suzuki Coupling Reactiona

entry	Pd NPs (mmol)	base	solvent (mL)	time	yield (%)b
1	0.001	K2CO3	H2O	2 h	95
2	0.001	Na2CO3	H2O	2 h	95
3	0.001	Cs2CO3	H2O	2 h	95
4	0.001	Na3PO4·12H2O	H2O	2 h	90
5	0.001	NaOH	H2O	3 h	60
6	0.001	KOH	H2O	3 h	65
7	0.001	Et3N	H2O	3 h	40
8	0.001		H2O	12 h
9	0.001	K2CO3	EtOH	8 h	70
10	0.001	K2CO3	i-PrOH	8 h	80
10	0.001	K2CO3	CH3CN	8 h	70
12	0.0009	K2CO3	H2O	2 h	95
13	0.0009	K2CO3	H2O/EtOH (1:1)	30 min	95
14		K2CO3	H2O:EtOH (1:1)	12 h

Reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic acid (0.6 mmol), base (1.5 mmol), RT (25 °C) in air unless otherwise noted.

Isolated yields.

A survey of the literature revealed that there are only few reports with excellent efficiencies on the application of pure H2O as a solvent in Suzuki–Miyaura reaction.[46] Therefore, considering the optimized conditions as discussed in Table , we have performed a comparative study for electronically diverse aryl bromides and arylboronic acids in H2O (method A) and 1:1 ratio of H2O and EtOH (method B) (Table ). The reactions of electron-rich aryl bromides are more efficient than (Table , entries 4–7 & 9–12) that of electron-poor aryl bromides (Table , entries 13–22) in H2O (method A). Nitro, formyl, and acetyl substituent aryl bromides require extended reaction time in H2O. Insolubility of these substrates in H2O may delay the reaction process which can be overcome by using H2O/EtOH (1:1) as a reaction medium (method B) (Table , entries 16, 19–22). Interestingly, increase of reaction temperature to 80 °C gave better result within shorter reaction time in H2O (method A) (Table , entries 14 & 15). Again, sterically demanding substrates such as 2-methoxyarylbromide affects the C–X activation and affords significantly low yield even with longer reaction time in comparison to 4-methoxyarylbromides (Table , entries 4 vs 8). It can be observed from Table that there are no such significant differences in yield and duration for diverse range of arylboronic acids. However, only poor yield of the cross-coupling product was obtained with aryl chlorides as a coupling partner even with prolong reaction time and high temperature. Even use of different solvents such as dimethylformamide (DMF) and acetonitrile did not improve the yield (Table , entries 23–25).

Table 2

Substrate Scope for Pd NP-Catalyzed Suzuki–Miyaura Reactiona

				method A		method B
entry	X	R1	R2	time (h)	yield (%)c	time (min)	yield (%)c
1	Br	H	H	1	98	15	98
2	Br	H	OCH3	1	95	15	98
3	Br	H	Cl	3	95	20	98
4	Br	4-OCH3	H	2	95	30	95
5	Br	4-OCH3	OCH3	1	95	30	95
6	Br	4-OCH3	Cl	1	90	30	95
7	Br	4-OCH3	3-CH3	2	90	30	95
8	Br	2-OCH3	H	8	80	60	80
9	Br	4-CH3	H	2	95	20	98
10	Br	4-CH3	OCH3	2	95	20	98
11	Br	4-CH3	t-butyl	2	95	20	96
12	Br	4-CH3	Cl	3	92	20	98
13	Br	4-NO2	t-butyl	8	95	20	96
14	Br	4-NO2	H	7, (2)b	98, (98)b	20	98
15	Br	4-NO2	OCH3	8, (2)b	90, (92)b	20	95
16	Br	4-NO2	Cl	8	50	20	98
17	Br	4-CHO	H	8	90	30	98
18	Br	4-CHO	OCH3	8	85	30	95
19	Br	4-CHO	Cl	8	50	30	96
20	Br	4-COCH3	H	12	60	20	95
21	Br	4-COCH3	OCH3	12	50	30	92
22	Br	4-COCH3	Cl	12	50	30	95
23	Cl	4-OCH3	H	12	20	12	50b
24	Cl	4-CH3	H	12	40	12	50b
25	Cl	4-OCH3	H	24	20d	24 h	nre

Reaction conditions: aryl bromide (1 mmol), arylboronic acid (1.2 mmol), Pd NPs (0.0009 mmol), K2CO3 (1.5 mmol), H2O (4 mL), RT (25 °C).

80 °C in air.

Isolated yield.

90 °C in DMF.

Reflux in CH3CN.

Next, these prepared Pd NPs were employed as a catalyst for Sonogashira reaction.[47−59] Generally, Sonogashira cross-coupling reaction is carried out with a Pd source as a catalyst in the presence of Cu source as a cocatalyst using amine as the reaction medium. Usually, the presence of copper salt led to the formation of Glaser-type coupling. Therefore, we began our experiment using these Pd NPs as a catalyst without using any ligand and copper salt.

For that purpose, 4-nitro-iodobenzene and phenylacetylene were chosen as the model substrates. The reaction conditions were standardized by varying catalyst amount, temperature, and using different bases and solvents (Table ). Initially, we were able to isolate very little amount of cross-coupling product in H2O at ambient temperature even with higher catalyst loading (Table , entries 1 and 2). However, the use of alcoholic solvents such as ethanol and isopropanol gave better result (Table , entries 3 and 4). Using H2O as a cosolvent (ethanol/H2O 1:1), we obtained a reduced amount of the cross-coupled product (Table , entry 5). Other solvents such as toluene, dioxane, THF, and 1,2-dimethoxyethane (DME) did not show any positive impact (Table , entries 6, 7, and 8). The catalyst showed best performance in ethanol at 60 °C (Table , entry 9). The influence of unlike bases was also examined in ethanol at 60 °C. Hydroxide bases NaOH and KOH and organic base Et3N were found to be less effective. In contrast, carbonate bases such as Na2CO3, K2CO3, and Cs2CO3 provide excellent yield (Table , entries 9–11). By varying the catalyst loading, we found that 0.02 mmol Pd NPs was sufficient for this reaction (Table , entry 16).

Table 3

Optimization of Catalyst and Solvent in Sonogashira Cross-Coupling Reactiona

entry	catalyst (mmol)	base	solvent	temperature (°C)	time (h)	yield (%)b
1	0.001	K2CO3	H2O	25	12	20
2	0.03	K2CO3	H2O	25	12	40
3	0.03	K2CO3	i-PrOH	25	12	60
4	0.03	K2CO3	EtOH	25	12	60
5	0.03	K2CO3	EtOH/H2O (1:1)	25	12	40
6	0.03	K2CO3	dioxane	25	12	30
7	0.03	K2CO3	THF	25	12	35
8	0.03	K2CO3	DME	25	12	35
9	0.03	K2CO3	EtOH	60	4	98
10	0.03	Na2CO3	EtOH	60	5	98
11	0.03	Cs2CO3	EtOH	60	4	98
12	0.03	Na3PO4·12H2O	EtOH	60	5	90
13	0.03	NaOH	EtOH	60	12	70
14	0.03	KOH	EtOH	60	12	70
15	0.03	NEt3	EtOH	60	12	40
16	0.02	K2CO3	EtOH	60	4	98
17	0.01	K2CO3	EtOH	60	12	70

Reaction conditions: 4-nitro-iodobenzene (0.5 mmol), phenylacetylene (0.75 mmol), solvent (4 mL), base (1.5 mmol).

Isolated yields.

Electronically diverse aryl halides and terminal alkynes can be used as substrates under this optimized reaction conditions (Table ). Iodobenzene reacts with terminal alkynes, delivering good to excellent yields of the products. Even less reactive substrates such as 1-hexyne gave excellent yield of the product (Table , entry 3). Additionally, both electron-poor and electron-rich p-substituted aryl iodides afforded the coupling products in excellent yields (Table entries 5, 8, and 10). However, it is observed that with 4-iodotoluene, the reaction completed within shorter reaction time compared to 4-iodonitrobenzene (Table , entry 5 vs 8).

Table 4

Pd NP-Catalyzed Sonogashira Cross-Coupling Reaction of Aryl Halides with Terminal Alkynea

entry	R1	R2	X	time (h)	yield (%)c
1	H	C6H5	I	5	98
2	H	dodecyl	I	8	85
3	H	hexyl	I	4	95
4	H	cyclohexyl	I	5	90
5	4-NO2	C6H5	I	4	98
6	3-NO2	C6H5	I	6	85
7	2-NO2	C6H5	I	8	40
8	4-CH3	C6H5	I	1	98
9	3-CH3	C6H5	I	6	90
10	4-CH3	4-CH3·C6H5	I	2	96
11	4-CH3	hexyl	I	8	80
12	3-CH3	hexyl	I	8	60
13	4-CH3	cyclohexyl	I	8	85
14	4-CH3	dodecyl	I	8	60
15	4-OCH3	C6H5	I	8	70
16	4-NH2	C6H5	I	8	50
b17	H	C6H5	Br	8	90
b18	4-CH3	4-CH3·C6H5	Br	8	75
b19	4-CH3	C6H5	Br	8	85
b20	4-NO2	C6H5	Br	8	60

Reaction conditions: aryl halide (0.5 mmol), terminal alkyne (0.75 mmol), Pd NPs (0.02 mmol), EtOH (4 mL), base (1.5 mmol).

Reaction done at 80 °C.

Isolated yields.

Similar results were observed with metasubstituted aryl iodides (Table , entries 6 and 9), with 3-iodotoluene being more competent than 3-nitro-iodobenzene. However, a steric effect as in 2-nitro-iodobenzene relatively lowered the reaction yield (Table , entry 7). Notably, coupling of electron-donating 4-iodoanisole and 4-iodoaniline with phenylacetylene gives slightly lower yields (Table , entries 15 and 16). Next, we checked the effectiveness of this reaction process with electronically varied aromatic and aliphatic alkynes (Table , entries 2–4 and 10–14). Comparable yield was isolated with phenylacetylene and 4-tolylphenyl-acetylene (Table , entry 10). Moderate yields were obtained when aliphatic acetylenes were used as coupling partners. Iodobenzene shows better reactivity with the range of aliphatic alkynes (Table , entries 2–4). However, in the reaction between aryl bromides and terminal acetylene, very low product formation was observed. Hence, we performed the reaction at 80 °C keeping the other parameters same, and to our delight, excellent yields were observed. In case of electron-rich aryl bromide gave better (Table , entries 18 and 19) compared to electron-poor aryl bromide. For example, the reaction between 4-bromonitrobenzene and phenylacetylene failed to complete, rendering only 60% yield (Table , entry 20).

The recyclability of a catalyst is an important aspect from the green chemistry point of view. The recycling test was done for both Suzuki and Sonogashira reactions (Figure ), and we have observed that the catalyst can be reused up to four cycles for both the reactions.

Figure 6

Reusability of Pd NPs for coupling reactions.

In our previous work,[60] we have reported the use of water extract of waste papaya bark ash and ethanol for the in situ generation of Pd NPs. The water extract of waste papaya bark ash was basic in nature, and we were able to carry out the cross-coupling reaction without using additional base. The size of the Pd(0) NP was 10–20 nm compared to 2–4 nm found under the current reaction protocol. The current Pd(0) NPs show the advantage in terms of reaction time and yield compared to the previous one.[60] A comparison of the effectiveness of the current Pd(0) NP and some reported catalysts for Suzuki and Sonogashira cross-coupling reactions is listed in Table . The present catalyst has the advantage of milder reaction conditions, aqueous solvent, ligand- and additive-free conditions, and use of natural feedstock.

Table 5

Comparison of Pd NPs with Some Reported Literature

entry	catalyst	reaction	reaction condition	yield (%)
1[61]	in situ Pd NPs formed from Pd(II) complexes of chalcogenated Schiff bases	Suzuki coupling	DMF/H2O (3:1), K2CO3, 90 °C, 2–12 h	62–98
		Sonogashira coupling	DMF, K2CO3, 90–110 °C, 1–24 h	56–99
2[62]	Pd(II) acyclic diaminocarbene complexes@polystyrene	Suzuki coupling	DMF/H2O, Et3N/K2CO3, 65–70 °C, 1–2 h	65–98
		Sonogashira coupling	DMF, Et3N, CuI, 16–70 °C, 5–180 min	93–99
3[63]	SiO2@Fe3O4–Pd	Suzuki coupling	K2PO4, MeOH, 60–110 °C, 1.5–10 h	26–99
		Sonogashira coupling	DMF, K2CO3, 100 °C, 6 h	71–97
4[64]	silica@Pd NPs	Suzuki coupling	DMF, K2CO3, 110 °C, 3–10 h	88–95
		Sonogashira coupling	DMF, K2CO3, 110 °C, 4–9 h	78–95
5[65]	ImmPd(0)–MNPs	Suzuki coupling	H2O, TBAB, K2CO3, rt, 0.3–3.5 h	78–98
		Sonogashira coupling	H2O, TBAB, K2CO3, rt/80 °C, 0.3–3.5 h	73–94
6	biogenic Pd NPs (present work)	Suzuki coupling	H2O, K2CO3, rt, 3–12 h	88–98
			50% aq EtOH, K2CO3, rt, 15–30 min	92–98
		Sonogashira coupling	EtOH, K2CO3, 60 °C, 4–8 h	40–98

Conclusions

A green and economical method for the synthesis of Pd NPs using the waste papaya peel extract which prioritize the utilization of waste is developed. We have synthesized this Pd NPs from inexpensive starting materials without using any reducing and capping reagents in H2O at room temperature. This catalytic system was suitable for Suzuki–Miyaura reaction at ambient temperature in H2O and for ligand-, copper-, and amine-free Sonogashira reaction. This method is a green and economical alternative for the synthesis of biaryl and acetylenyl derivatives because the transformation can be affected under an aerobic atmosphere without the need for any ligand. The current method is associated with reduced waste, materials, hazards, risk, energy, and cost over many of the previously published procedures.

Experimental Section

General Information

In this experiment, the chemicals were used without further drying or purification. The UV–visible spectra were observed by a UV–visible spectrophotometer, (Shimadzu Corporation, UV-2550). The XRD patterns were measured with the help of a Rigaku MultiFlex instrument using a nickel-filtered Cu Kα (0.15418 nm) radiation source. The SEM analyses were recorded on using a “JEOL (JSM model 6390 LV”) scanning electron microscope, operating at an accelerating voltage of 15 kV. The elemental composition of the catalyst was confirmed through EDX analyses (the same instrument attached with a scanning electron microscope). The particle size distribution was characterized by a TEM instrument (model: JEOL JEM-2010).[66] The surface area and pore size distribution were analyzed by BET analysis. 1H and 13C spectra were recorded in CDCl3 using tetramethylsilane as an internal standard on a JEOL, JNM ECS NMR spectrometer operating at 400 MHz. Reaction products were confirmed by comparing the 1H and 13C NMR spectra (in the Supporting Information).[12]

Catalyst Preparation

Preparation of Pd NPs

Papaya peel (10 g) was washed with distilled H2O, finely chopped, and then mixed with 100 mL of distilled H2O. The extract was filtered through a sintered glass crucible. In a 5 mL round-bottom flask, 2 mL of the aqueous extract was mixed with 0.1 g of Pd(OAc)2 and stirred for 48 h at room temperature under nitrogen atmosphere. The resulting Pd NPs were separated through centrifugation, and the black products were dried under vacuum.

General Information about Catalytic Experiments

Suzuki–Miyaura and Sonogashira reactions were carried out under aerobic condition, and the progress of the reactions was monitored by aluminum-coated TLC plates (Merck silica gel 60F254) and visualized under a UV lamp. The isolation of desired products was achieved by a column chromatographic technique using a silica gel (60–120 mesh). The isolated products were identified by comparing their 1H and 13C NMR spectra as providing in the Supporting Information.

Typical Procedure for Suzuki–Miyaura Reaction of Aryl Halides Using Pd NPs

Aryl halide (0.5 mmol), arylboronic acid (0.6 mmol), K2CO3 (1.5 mmol), catalyst (0.0009 mmol), and distilled H2O (3 mL) were taken in a 25 mL round-bottom flask. The reactants were stirred at room temperature for the required time. After completion (vide TLC), the catalyst was separated from the reaction mixture by centrifugation and the crude reaction mixture was extracted with ethyl acetate (3 × 10 mL). The resultant organic phases was washed with brine (2 × 10 mL) and dried over anhydrous Na2SO4, filtered, and evaporated under reduced pressure, and purified by column chromatography using ethyl acetate and hexane as an eluent. The products were characterized by 1H and 13C NMR spectroscopic analyses.

For recycling experiments, the residue catalyst after centrifugation was washed four times with excess water and ethyl acetate in sequence. The resultant catalyst was dried under vacuum and subjected to consequent run.

Typical Procedure for Sonogashira Cross-Coupling Reactions Using Pd NPs

Aryl halide (1 mmol), terminal acetylene (1.5 mmol), base (1.5 mmol), catalyst (0.02 mmol), and ethanol (3 mL) were taken in a 25 mL round-bottom flask. The reactants were stirred at 60 °C under aerobic condition. After completion (vide TLC), the catalyst was separated from the reaction mixture by centrifugation and the crude reaction mixture was extracted with ethyl acetate (3 × 10 mL) and dried over anhydrous Na2SO4. The products were purified by column chromatography using hexane and ethyl acetate as an eluent. The products were identified by 1H and 13C NMR spectroscopic analyses.

For recycling experiments of Sonogashira reaction, a procedure similar to that of Suzuki–Miyaura cross-coupling reaction was followed.