Biomass-Derived Activated Carbon-Supported Copper Catalyst: An Efficient Heterogeneous Magnetic Catalyst for Base-Free Chan-Lam Coupling and Oxidations.

Development of heterogeneous catalysts from biomass-derived activated carbon is a challenging task. Biomass-derived activated carbon possesses a large specific surface area, highly porous structure, and good thermal/chemical stability. Magnetic copper catalysts based on biomass-derived activated carbon exhibited good catalytic activity in base-free Chan-Lam coupling and oxidations. Herein, biomass-derived activated carbon was prepared by the carbonization of neem dead leaves (abundant waste biomass) followed by chemical activation with KOH. Such a porous carbon material was used as a low cost and highly efficient support material for the preparation of inexpensive and environmentally benign magnetic catalysts [Cu@KF-C/MFe2O4, M = Co, Cu, Ni, and Zn]. In addition, KF modification was done to impart basic character to the catalyst that can perform C-N coupling under base-free conditions. Initially, Brunauer-Emmett-Teller (BET) analysis of the synthesized catalysts was carried out, which indicated that Cu@KF-C/CoFe2O4 possess more surface area as well as pore volume, and so accounting for the highest activity among the other synthesized catalysts. Further, X-ray photoelectron spectroscopy (XPS) analysis was performed, which inferred that Cu@KF-C/CoFe2O4 contains most of the copper in reduced form, i.e., Cu(0), which is the active species responsible for better catalytic activity toward Chan-Lam coupling reactions as well as oxidation of alcohols and hydrocarbons. The physiochemical properties of the most active catalyst, Cu@KF-C/CoFe2O4, was examined by BET, XPS, Fourier transform infrared Spectroscopy (FTIR), thermogravimetric analysis (TGA), field emission gun scanning electron microscopy (FEG-SEM), high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray (EDX) mapping, energy dispersive X-ray (EDX), inductively coupled plasma atomic emission spectroscopy (ICP-AES), powder X-ray diffraction (XRD), and vibrating sample magnetometry (VSM). Moreover, Cu@KF-C/CoFe2O4 shows excellent stability as well as reusability and could be easily separated with the help of an external magnet.

Introduction

Worldwide, large volumes of waste are produced per day from agricultural, food, and chemical industries. More often, serious health problems occur by burning of such waste materials due to the release of large amounts of ash and hazardous gases. It becomes necessary to move a step toward the management of such waste materials by converting them into useful materials from where highly active catalysts can be generated.[1] Recently, biomass has attracted enormous interest for the preparation of different types of carbon-based materials as they possess tunable pore/surface properties, can be easily modified to generate active sites, and have relatively low cost,[2,3] which make them efficient to be used in the field of catalysis. Among the various kinds of biomass, neem dead leaves (Azadirachta indica) are one of the most common materials from the agriculture sector. They can serve as good precursors for the preparation of activated carbons (ACs) due to their low cost and abundance in nature, with comparatively high fixed carbon, presence of porous structure, etc. It is worth mentioning that neem dead leaves have been shown to serve as precursors for making ACs,[4,5] while there has not been any report on the application of neem dead leaves for making magnetic activated carbons (MACs). Moreover, no reports have been available where MACs prepared from neem dead leaves are used as supports for the preparation of heterogeneous nanometal catalysts.

Activated carbons derived from biomass feedstocks not only are eco-friendly and cost-effective but also possess high porosity, good electrical conductivity, surface oxygen functional groups, and heteroatoms, which are desirable as catalyst supports for applications in sensors,[6,7] catalysis,[8,9] and energy storage devices.[6,10,11] They can be prepared by chemical activation of carbonized materials using different reagents such as ZnCl2, H3PO4, KOH, or NaOH. Among the various activators used in chemical activation, KOH finds an increasing interest due to its lower activation temperature, more yield, and well-developed porous structure.[12] Further, the separation of the catalyst from the reaction mixture is laborious and time-consuming even in the case of heterogeneous catalysts. To make the separation process easy, magnetic nanoparticles were impregnated onto activated carbon. In this context, spinel ferrites were used for the preparation of MACs. Spinel ferrites are a class of metal oxide materials that exhibit a wide range of applications.[13,14] In addition, to make the support material basic, modification with KF is the best option since KF is extensively used as important potassium precursors for preparing solid base catalysts.[15−17] In a few decades, a number of reports have been available in the literature for C–N bond formation, which include the use of a polymer-immobilized copper complex,[18] copper(I)-exchanged zeolites,[19] and copper(II) oxide nanoparticles.[20] However, the reported methods have suffered from various limitations such as longer reaction times (8–24 h), high catalyst loading, high reaction temperatures, and a large amount of base, which limit their use. Therefore, in this paper, utilizing neem dead leaves, biomass-derived activated carbon-supported copper catalysts were prepared to carry out the Chan–Lam coupling in the absence of a base as well as expensive ligands. Moreover, the products were obtained in good yield in less time, thereby confirming that KF modification imparts a sufficient basic character to the catalyst.

The present study first demonstrates the synthesis of carbon materials by one-step pyrolysis of neem dead leaves (abundant waste biomass) at a low temperature. Here, the carbon materials derived from neem dead leaves have a low surface area and poor porosity, so in order to make the porous carbon material, chemical activation was done by treating with KOH. Afterward, four different types of carbon-supported metal ferrites, namely, C/CuFe2O4, C/CoFe2O4, C/NiFe2O4, and C/ZnFe2O4, were prepared in situ to facilitate the easier separation of the catalyst. Further, modification of carbon-supported metal ferrites was carried out with KF. It results in the introduction of a basic character to the carbon-supported metal ferrites due to the trapping of KF in voids present in carbon and metal ferrite. Finally, copper(0) nanoparticles were immobilized on KF-modified carbon-supported metal ferrites to get the desired catalysts, Cu@KF-C/MFe2O4 (M = Co, Cu, Ni, and Zn) and their catalytic activity was explored for Chan–Lam coupling and oxidation of alcohols and hydrocarbons.

Results and Discussion

Characterization

In the present work, nanosized, water-dispersible, and highly efficient magnetically recoverable heterogeneous biomass-derived activated carbon-based magnetic copper catalysts, Cu@KF-C/MFe2O4, were successfully synthesized. The general method for the preparation of Cu@KF-C/MFe2O4 is shown in Scheme . Here, neem dead leaves as waste biomass were used as a cheap raw material for the preparation of activated carbon, which acts as a support material for the preparation of magnetic copper catalysts owing to its large specific surface area, highly porous structure, and good thermal/chemical stability. First of all, activated carbon was prepared by the carbonization of neem dead leaves under a nitrogen atmosphere followed by activation with KOH so as to make it porous. Then, such activated carbon was used as a cost-effective support material for the in situ synthesis of carbon-supported metal ferrites, C/MFe2O4 (M = Cu, Co, Ni, and Zn), to make the catalysts magnetic in nature so that they can be easily separated with the help of an external magnet. Further, to introduce a basic character to C/MFe2O4 composites, KF modification was done by a wet impregnation method, which resulted in the formation of KF-C/MFe2O4. KF modification imparts a sufficient basic character to the catalyst that helps to carry out Chan–Lam coupling under base-free conditions. Finally, to improve the catalytic activity, metal nanoparticles were immobilized onto KF-C/MFe2O4 using copper acetate followed by reduction with aqueous sodium borohydride solution to produce Cu@KF-C/MFe2O4. The catalytic activity of the developed catalysts was explored for Chan–Lam coupling and oxidation of alcohols and hydrocarbons. Further, the four novel catalysts, Cu@KF-C/CuFe2O4, Cu@KF-C/CoFe2O4, Cu@KF-C/NiFe2O4, and Cu@KF-C/ZnFe2O4, were characterized by Brunauer–Emmett–Teller (BET) and X-ray photoelectron spectroscopy (XPS).

Scheme 1

General Scheme for the Synthesis of Cu@KF-C/MFe2O4

First of all, textural properties of the catalysts were studied by N2 adsorption–desorption measurements. The BET surface areas of Cu@KF-C/CuFe2O4, Cu@KF-C/CoFe2O4, Cu@KF-C/NiFe2O4, and Cu@KF-C/ZnFe2O4 were found to be 19.4, 34.8, 11.2, and 29.3 m2g–1 with pore volumes of 0.0329, 0.1905, 0.00922, and 0.0337 cm3g–1, respectively. Based on this data, it can be concluded that among the four synthesized catalysts, Cu@KF-C/CoFe2O4 possess a more porous structure and thus showed better catalytic activity. Thus, Cu@KF-C/CoFe2O4 was chosen as the most active heterogeneous catalyst. The BET isotherm of Cu@KF-C/CoFe2O4 presented in Figure is of type IV with a hysteresis loop of type H3 at high relative pressures, demonstrating the mesoporous nature of the catalysts.

Figure 1

Nitrogen adsorption–desorption isotherm of Cu@KF-C/CoFe2O4.

An XPS study was carried out to examine the elemental composition and chemical states of the various species existing in the synthesized catalysts. Figure A shows the C 1s XPS spectra, which revealed the presence of three peaks. The main peak present at 284.5 eV is assigned to a C=C bond, whereas the peaks at 286.05 and 288.2 eV are ascribed to C=O and O–C=O bonds, respectively.[21]Figure B,C and Figure B show the core-level XPS spectra of Ni 2p, Zn 2p, and Co 2p of the prepared catalysts (Cu@KF-C/NiFe2O4, Cu@KF-C/ZnFe2O4, and Cu@KF-C/CoFe2O4). Figure B presents Ni 2p spectra with peaks at binding energies of 855.7, 861.7, 873.7, and 880.8 eV. The characteristic peak at 855.7 eV with its satellite peak at 861.7 eV is assigned to Ni 2p3/2, whereas the peaks at 873.7 and 880.8 eV correspond to Ni 2p1/2, thus confirming the presence of Ni2+ in Cu@KF-C/NiFe2O4.[22] The presence of Zn in Cu@KF-C/ZnFe2O4 is confirmed by two peaks at 1021.2 and 1044.2 eV, corresponding to the binding energies of Zn 2p3/2 and Zn 2p1/2, respectively, which are related to the Zn2+ state in the catalyst (Figure C).[23] The Co 2p spectra (Figure B(a)) indicate the presence of two main peaks at 781 and 797.1 eV with the two satellite peaks identified at around 786.3 and 803.2 eV, respectively. The peak at 781 eV corresponds to Co 2p3/2, while the peak at 786.3 eV corresponds to its shakeup satellite. Moreover, the peaks at 797.1 and 803.2 eV correspond to Co 2p1/2 and its shakeup satellites, respectively. From the above observation, it can be concluded that Co is present in the Co2+ state in Cu@KF-C/CoFe2O4.[24]

Figure 2

(A) High-resolution X-ray photoelectron spectra of C 1s. (B) High-resolution X-ray photoelectron spectra of Ni 2p. (C) High-resolution X-ray photoelectron spectra of Zn 2p. (D) X-ray photoelectron spectra depicting the comparison of the Cu 2p region: (a) Cu@KF-C/CoFe2O4, (b) Cu@KF-C/NiFe2O4, (c) Cu@KF-C/ZnFe2O4, and (d) Cu@KF-C/CuFe2O4.

Figure 3

(A) X-ray photoelectron spectra showing the comparison of the Cu 2p region: (a) fresh Cu@KF-C/CoFe2O4, (b) reused Cu@KF-C/CoFe2O4 in the case of C–N coupling, and (c) reused Cu@KF-C/CoFe2O4 in the case of oxidation. (B) X-ray photoelectron spectra showing the comparison of the Co 2p region: (a) fresh Cu@KF-C/CoFe2O4, (b) reused Cu@KF-C/CoFe2O4 in the case of C–N coupling, and (c) reused Cu@KF-C/CoFe2O4 in the case of oxidation.

To find out the most active catalyst among the four synthesized catalysts, we have compared the catalytic activity of these catalysts toward C–N bond formation, which has been explained in detail in Section . It has been found that Cu@KF-C/CoFe2O4 showed better catalytic activity in comparison to other catalysts. So, to determine why Cu@KF-C/CoFe2O4 possess better catalytic activity, comparative analysis of high-resolution Cu 2p spectra of each catalyst was carried out and interesting results were obtained (Figure D(a–d)). Figure D(a) shows Cu 2p spectra of Cu@KF-C/CoFe2O4, where peaks centered at binding energies of 932.4 and 952.2 eV correspond to Cu 2p3/2 and Cu 2p1/2 levels, which are assigned to Cu(0) or Cu(+), respectively.[25−28] The peak at 934.5 eV corresponds to Cu 2p3/2, whereas the peak at 954 eV is assigned to Cu 2p1/2, which are related to the Cu2+ state in the catalyst. In addition, the peak at 943.8 eV corresponds to the shakeup satellite of Cu2+. It is normally difficult to distinguish Cu(0) from Cu(+) because the difference between the binding energy is only 0.1 eV. So, the X-ray-induced Cu LMM Auger electron spectrum (XAES) of Cu@KF-C/CoFe2O4 was recorded to distinguish Cu(0) from Cu(+). The Cu LMM XAES exhibited a stronger peak at 568 eV and a weaker peak at 570.4 eV, which corresponds to Cu(0) and Cu(+), respectively.[29,30] A relatively stronger peak at 568 eV and a weaker peak at 570.4 eV in the Cu LMM Auger spectrum indicated that there is a larger proportion of Cu(0) than Cu(+) in Cu@KF-C/CoFe2O4 (Figure S41). As shown in Figure D(a), there is an increase in the intensity of Cu(0) as indicated by a BE value at 932.4 eV along with an appreciable decrease in the intensity of Cu2+ (934.5 eV), which thus reveals the presence of more Cu(0) in Cu@KF-C/CoFe2O4. In the Cu 2p spectra of Cu@KF-C/NiFe2O4 and Cu@KF-C/ZnFe2O4 (Figure D(b,c)), majority of Cu is present in the +2 oxidation state with less amount of Cu(0), whereas in Cu@KF-C/CuFe2O4, no Cu(0) is present (Figure D(d)). Based on the above observations, it can be inferred that this might be due to the synergism or better stabilization of metallic copper by KF-C/CoFe2O4, whereas other supports based on ferrite might not be able to stabilize metallic copper.

As analyzed by XPS, the reason behind the enhanced catalytic activity of Cu@KF-C/CoFe2O4 might be due to the presence of more Cu(0) species, which are responsible for its better catalytic activity toward C–N coupling and oxidation of alcohols and hydrocarbons.

Moreover, based on XPS analysis, the atomic percentage of various elements present in all of the four catalysts is shown in Table S1. Further, to confirm the presence of more Cu(0) in Cu@KF-C/CoFe2O4, the atomic ratio of Cu(0) in all the four catalysts has been calculated from XPS analysis. From XPS results, the atomic ratio of Cu(0) present in Cu@KF-C/CoFe2O4, Cu@KF-C/NiFe2O4, Cu@KF-C/ZnFe2O4, and Cu@KF-C/CuFe2O4 was around 0.41:0.23:0.20:traces.

Further, the percentage elemental composition of Cu@KF-C/CoFe2O4 determined semiquantitatively by SEM–EDX analysis was compared with the surface elemental composition by XPS analysis and the results are presented in Table S2. Both SEM–EDX and XPS analysis indicate the presence of C, O, K, F, Fe, Co, and Cu in Cu@KF-C/CoFe2O4. The percentage elemental composition of Cu@KF-C/CoFe2O4 determined by XPS analysis was found to be more in comparison to the atomic percentage determined by SEM–EDX analysis. This could be due to the fact that XPS analysis explores the near-surface region chemical composition, whereas SEM–EDX analysis gives the bulk concentration of elements.

Cu@KF-C/CoFe2O4, the most active magnetic heterogeneous catalyst, was further characterized by various techniques, namely, Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), field emission gun scanning electron microscopy (FEG-SEM), high-resolution transmission electron microscopy (HR-TEM), energy dispersive X-ray (EDX) mapping, EDX analysis, inductively coupled plasma atomic emission spectroscopy (ICP-AES), powder X-ray diffraction (XRD), and vibrating sample magnetometry (VSM).

FTIR is a useful technique used for the investigation of functional groups present in the catalyst. Figure shows the FTIR spectra of carbon (derived from neem dead leaves) (a), activated carbon (b), C/CoFe2O4 (c), KF-C/CoFe2O4 (d), and Cu@KF-C/CoFe2O4 (e). The FTIR spectra of carbon showed a broad band at 3289 cm–1 due to the stretching vibration of O–H in the oxygen-containing functional groups, while peaks at 2921 and 2846 cm–1 correspond to the asymmetric and symmetric CH2 stretching vibrations. The peaks at 1616 and 1546 cm–1 indicated the presence of C=O and C=C stretching vibrations of the aromatic ring in the carbon derived from dead neem leaves. The characteristic absorption bands at 1440, 1380, and 1230 cm–1 were due to O–H and C–H bending and C–O stretching vibrations. The FTIR spectrum of activated carbon (Figure b) shows less functional groups with reduced intensities in comparison to carbon derived from dead neem leaves. From this, it is clear that many oxygen-containing functional groups got reduced significantly on the surface of carbon during the activation process. Moreover, there is an increase in the intensity of the band at 1072 cm–1, which corresponds to the presence of an aliphatic C–O–C group. Figure c displays an additional band at 580 cm–1 that is associated with Co–O and Fe–O vibrations, thus indicating the successful incorporation of CoFe2O4 nanoparticles into the activated carbon.[31] The band at 1072 cm–1 in the FTIR spectrum of activated carbon shifts toward 883 cm–1 in the FTIR spectrum of C/CoFe2O4. Figure d indicates no significant absorption bands in the FTIR spectrum of KF-C/CoFe2O4, but a slight shifting of bands takes place on modification with KF. Finally, the presence of Cu(0) nanoparticles immobilized on the surface of KF-C/CoFe2O4 was confirmed by the decreased intensity of the characteristic absorption band associated with Co–O and Fe–O vibrations (Figure e).

Figure 4

FTIR spectra of (a) carbon (derived from neem dead leaves), (b) activated carbon, (c) C/CoFe2O4, (d) KF-C/CoFe2O4, and (e) Cu@KF-C/CoFe2O4.

TGA was employed to measure the change in the mass of the materials under thermal treatment. The TGA thermograms of carbon (derived from neem dead leaves), activated carbon, C/CoFe2O4, KF-C/CoFe2O4, and Cu@KF-C/CoFe2O4 are shown in Figure . From TGA thermograms, it is found that activated carbon showed better thermal stability than carbon (derived from neem dead leaves). Further, immobilization of cobalt ferrite nanoparticles onto activated carbon leads to an overall increase in thermal stability. However, immobilization of Cu(0) nanoparticles onto KF-C/CoFe2O4 decreased the overall thermal stability due to the thermal degradation that arises from the catalytic behavior of the nanoparticles, which enhances the depolymerization and lowers the activation energy during the thermal process.[32−34] The TGA thermogram of Cu@KF-C/CoFe2O4 exhibited two specific weight losses. The first weight loss was observed below 300 °C, which can be ascribed to the elimination of water molecules and various oxygen functionalities from the catalyst surface. The second weight loss in a range of 300–500 °C was responsible for the decomposition of carbonaceous materials. Further, no significant weight loss was observed above 500 °C, thus confirming that the catalyst is stable up to 300 °C. These results illustrate that the synthesized catalyst can be efficiently used for carrying out organic transformations up to 300 °C without any substantial loss in catalytic activity.

Figure 5

TGA thermograms of carbon (derived from neem dead leaves), activated carbon, C/CoFe2O4, KF-C/CoFe2O4, and Cu@KF-C/CoFe2O4.

Typical FEG-SEM images of Cu@KF-C/CoFe2O4 are shown in Figure . The SEM image (Figure a) showed that the catalyst has a composite-type nature with an aggregated spherical morphology (Figure b). The average particle size was found to be 17.5 nm that almost matches with the crystallite size obtained from XRD data (Figure b). Further, in Figure c, some shaded areas are also observed due to the presence of voids, indicating that the catalyst is porous in nature (Figure d).

Figure 6

FEG-SEM images of Cu@KF-C/CoFe2O4: (a) composite nature and (b) spherical morphology of the catalyst; (c, d) porous nature of the catalyst.

The morphology of KF-C/CoFe2O4 and particle size and distribution of copper(0) nanoparticles over KF-C/CoFe2O4 were studied by HR-TEM. In the HR-TEM micrograph shown in Figure a, a porous network morphology of activated carbon is clearly visible. From HR-TEM micrographs (Figure b,c), black-colored spots corresponding to the presence of Cu(0) nanoparticles over KF-C/CoFe2O4 are clearly seen. The average particle size of copper nanoparticles was found to be 4–5 nm (Figure d). The SAED pattern represented in Figure e shows concentric rings and spots, indicating the polycrystalline nature of nanoparticles. Further, the HR-TEM image shown in Figure f represents some spherical particles that correspond to the cobalt ferrite nanoparticles distributed over the activated carbon and the average particle size of cobalt ferrite was estimated to be 14–17 nm. Moreover, the d-spacing values calculated approximately from the HR-TEM image of Cu@KF-C/CoFe2O4 (Figure S42a,b) are found to be 0.21, 0.29, and 0.48 nm. These d-spacing values correspond to the (400), (220), and (111) planes of the cubic CoFe2O4 phase, respectively.[35,36] Meanwhile, SEM–EDX mapping of Cu@KF-C/CoFe2O4 was carried out to confirm the uniform distribution of copper nanoparticles as well as the distribution of other various elements in the catalyst (Figure ). The mapping confirms the proper immobilization of copper nanoparticles in Cu@KF-C/CoFe2O4.

Figure 7

HR-TEM micrographs of Cu@KF-C/CoFe2O4: (a–c) distribution of Cu(0) nanoparticles, (d) average particle size of Cu(0) nanoparticles, (e) SAED pattern of Cu@KF-C/CoFe2O4, and (f) average particle size of cobalt ferrite nanoparticles.

Figure 8

SEM–EDX mapping of Cu@KF-C/CoFe2O4.

The elemental composition in the synthesized catalyst was confirmed by EDX spectroscopy, which shows that the synthesized catalyst is composed of C (49.07%), O (38.81%), F (1.68%), K (1.31%), Fe (4.37%), Co (2.43%), and Cu (2.33%), indicating the successful incorporation of Cu nanoparticles over the KF-modified carbon-supported cobalt ferrite composite (Figure ). Further, ICP-AES analysis of all the four catalysts was carried out to determine the content of copper immobilized onto the support materials (KF-C/MFe2O4). The ICP-AES analysis indicated that Cu@KF-C/CoFe2O4 contains 1.41 wt % Cu; Cu@KF-C/NiFe2O4, 1.38 wt % Cu; Cu@KF-C/ZnFe2O4, 1.28 wt % Cu; and Cu@KF-C/CuFe2O4, 1.24 wt % Cu in 0.1 g of the respective support material.

Figure 9

EDX spectrum of Cu@KF-C/CoFe2O4.

XRD was employed to analyze the crystalline phases of KF-C/CoFe2O4 and Cu@KF-C/CoFe2O4 (Figure ). The XRD pattern of CuKF-C/CoFe2O4 exhibited five main characteristic diffraction peaks at 2θ = 30.1, 35.5, 43.1, 57.0, and 62.5° that correspond to (220), (311), (400), (511), and (440) planes of the face-centered cubic inverse spinel structure of CoFe2O4 (JCPDS 22-1086), thus suggesting the successful grafting of cobalt ferrite over activated carbon.[37] Also, apart from the peaks corresponding to cobalt ferrite phases, the XRD pattern of Cu@KF-C/CoFe2O4 showed a distinct peak with less intensity at 2θ = 74.3°, which matches with the (220) crystal face of metallic copper.[38] Their lower intensity indicates the well dispersion and amorphous nature of copper nanoparticles. Further, no diffraction peak of amorphous carbon was observed in the XRD pattern of Cu@KF-C/CoFe2O4 and KF-C/CoFe2O4, indicating the low crystallinity of amorphous carbon. Moreover, on comparing the XRD pattern of Cu@KF-C/CoFe2O4 with KF-C/CoFe2O4, it was observed that the immobilization of copper(0) nanoparticles onto KF-C/CoFe2O4 causes no observable change in the crystallinity of KF-C/CoFe2O4. The average crystallite size of cobalt ferrite supported on activated carbon calculated using the Scherrer equation[39] was found to be 18.84 nm, which almost matches with the particle size obtained from HR-TEM studies.

Figure 10

XRD spectra of KF-C/CoFe2O4 and Cu@KF-C/CoFe2O4.

The magnetic properties of C/CoFe2O4 and Cu@KF-C/CoFe2O4 were investigated at room temperature, and their corresponding magnetic hysteresis loops are displayed in Figure . The magnetization curve of C/CoFe2O4 revealed a ferromagnetic behavior with a saturation magnetization of 12.3 emu/g. After coating of KF and copper nanoparticles, the magnetic saturation value of Cu@KF-C/CoFe2O4 was found to be 10.9 emu/g. The reason for this small decrease in saturation value could be ascribed to the incorporation of KF onto C/CoFe2O4 as well as their coating with the metal nanoparticles. It is pertinent to mention that even with this reduction in the saturation magnetization, the synthesized catalyst is magnetic enough to be separated with the help of an external magnet.

Figure 11

VSM spectra of C/Co-Fe2O4 and Cu@KF-C/Co-Fe2O4.

Catalytic Activity for the C–N Bond Formation

The copper-catalyzed C–N bond formation reported by Chan and Lam is one of the most important synthetic methodologies, which involve the cross-coupling of arylboronic acids and nitrogen-based nucleophiles for the preparation of functionalized amines.[40] A number of reports have been available in the literature where the Chan–Lam coupling reaction has been catalyzed using an external base. For example, Reddy et al. have developed two catalytic systems based on copper for the chemoselective N-arylation of 3-aminophenols and 4-aminophenols. Selective N-arylation of 3-aminophenols has been carried out with Cu(OAc)2 in the presence of AgOAc as a base in MeOH at room temperature, whereas the chemoselective N-arylation of 4-aminophenols was done with Cu(OAc)2 using Cs2CO3 as a base and benzoic acid as an additive.[41] Antilla and Buchwald have demonstrated N-arylation of amines by using stoichiometric quantities of Cu(OAc)2 as a catalyst in the presence of 2,6-lutidine and myristic acid as a base and additive, respectively.[42] Sawant et al. have developed a bimetallic Cu–Mn catalyst to synthesize N-arylated amines using K2CO3 as a base in an aqueous medium under ligand-free conditions.[43] In most of the reports, C–N bond formation has been carried out in the presence of an external base. Thus, to avoid the use of a base in the reaction, we have developed [Cu@KF-C/MFe2O4, M = Co, Cu, Ni, and Zn] as efficient solid base catalysts since KF modification imparts basic character to the catalysts that could perform the C–N bond formation under base-free conditions.[44] Further, it is more advantageous to use solid base catalysts over liquid bases since they are easier to remove, making the recovery of products and solvents simple, and also they are noncorrosive in nature. The aim of this study was to develop an efficient catalyst for the preparation of functionalized amines via cross-coupling of arylboronic acids and nitrogen-based nucleophiles. The detailed study was made for the cross-coupling of 4-methoxyaniline with phenylboronic acid under mild reaction conditions. Moreover, the application of the best catalyst was also extended to other aromatic amines. The reaction conditions were optimized by evaluating various influencing parameters. The details are systematically described below.

Influence of the Catalyst

The catalytic activity of the synthesized catalysts, Cu@KF-C/CuFe2O4, Cu@KF-C/CoFe2O4, Cu@KF-C/NiFe2O4, and Cu@KF-C/ZnFe2O4, and their precursors was compared for the C–N coupling reaction. In order to find out the best catalyst, the reaction of 4-methoxyaniline with phenylboronic acid in ethanol at 90 °C was selected as a test reaction. First, the model reaction was performed using KF-C/MFe2O4 (precursor of Cu@KF-C/MFe2O4) to see the effect of metal ferrite. It was found that KF-C/CuFe2O4 catalyzes the C–N bond formation with low conversion, whereas no reaction takes place in the presence of other support materials KF-C/CoFe2O4, KF-C/NiFe2O4, and KF-C/ZnFe2O4 (Table ). Moreover, to improve the catalytic activity, copper nanoparticles were incorporated in the support materials to produce Cu@KF-C/MFe2O4. Cu@KF-C/CuFe2O4 exhibited improved activity when compared to KF-C/CuFe2O4 (Table , entry 5), which demonstrated that Cu NPs are the active species in catalyzing the C–N bond formation. With Cu@KF-C/NiFe2O4, catalytic activity was increased (Table , entry 7). However, with Cu@KF-C/CoFe2O4, better results were obtained (entry 6, Table ). This observation was also consistent with XPS results where we found that majority of copper is present as Cu(0) in Cu@KF-C/CoFe2O4, whereas in the remaining three catalysts, a large proportion of Cu is present in the +2 oxidation state. Therefore, Cu@KF-C/CoFe2O4 was selected as the best catalyst.

Table 1

Comparison of Catalytic Activity of Cu@KF-C/MFe2O4 with Support Materials for the C–N Cross-coupling between 4-Methoxyaniline and Phenylboronic Acida

entry	catalyst	time (h)	yield (%)b
1	KF-C/CuFe2O4	2	25
2	KF-C/CoFe2O4	2	N.R.
3	KF-C/NiFe2O4	2	N.R.
4	KF-C/ZnFe2O4	2	N.R.
5	Cu@KF-C/CuFe2O4	2	40
6	Cu@KF-C/CoFe2O4	2	85
7	Cu@KF-C/NiFe2O4	2	60
8	Cu@KF-C/ZnFe2O4	2	50

Reaction conditions: 4-methoxyaniline (1 mmol), phenylboronic acid (1 mmol), and catalyst (0.1 g) in ethanol (5 mL) at 90 °C.

Column chromatography yield.

Influence of the Solvent and Temperature

The influence of the solvent was studied by performing a test reaction with Cu@KF-C/CoFe2O4 in different solvents, namely, water, ethanol, acetonitrile, and toluene, keeping the reaction time and temperature constant (2 h and 90 °C, respectively). With acetonitrile and toluene, lower yields were obtained, whereas in the case of water, the yield was improved to 70% but good results were obtained with ethanol, so ethanol was chosen as the best solvent for carrying out C–N bond formation reactions (Figure a). After the solvent optimization, the test reaction was also investigated at different temperatures such as 70, 80, 90, and 100 °C in ethanol. It was found that the yield of the desired product was increased from 75 to 85% as the reaction temperature increased from 70 to 90 °C. However, a similar yield was obtained by a further increase in temperature to 100 °C. Therefore, 90 °C was selected as the most favorable reaction temperature since it provided the maximum yield of the product (Figure b). Therefore, after performing different experiments, the optimized reaction conditions for C–N cross-coupling are Cu@KF-C/CoFe2O4 as the catalyst, ethanol as the reaction medium, and 90 °C as the reaction temperature under base-free conditions.

Figure 12

(a) Effect of different solvents at 90 °C. (b) Influence of variation of temperature in ethanol (5 mL), keeping the other reaction conditions fixed [4-methoxyaniline (1 mmol), phenylboronic acid (1 mmol), Cu@KF-C/CoFe2O4 (0.1 g), and time (2 h)].

Influence of the Substrate

Knowing the optimized conditions, the scope of the present catalytic system was explored for C–N cross-coupling between phenylboronic acid and a variety of aromatic amines containing electron-withdrawing and electron-donating groups and good results were obtained (Table ). It has been found that the presence of electron-donating groups on aromatic amines enhances the formation of N-arylated products as compared to unsubstituted amine. However, the presence of electron-withdrawing groups decreases the rate of the reaction. The reaction of 4-fluoroaniline with phenylboronic acid occurs smoothly to give the corresponding N-arylated product with improved yield (entry 3e, Table ). In the case of 4-bromoaniline, the yield of the C–N product was decreased due to the formation of a side product (C–C coupling product) (entry 3g, Table ). It is pertinent to mention that when Chan–Lam cross-coupling was carried out with 4-methoxyaniline, the desired product was obtained at a faster rate with good yield in comparison to 3-methoxyaniline (entries 3b and 3c, Table ). However, the reaction proceeds slowly in case of 2-methoxyaniline with low yield (entry 3d, Table ). Thus, it can be concluded that para- and meta-substituted anilines were more reactive, while ortho-substituted aniline was less reactive due to steric hindrance. Further, the reaction was also tried with n-butyl amine but the expected coupling product was not formed. Even for indole and 5-methoxyindole, no product was formed even after a long reaction time. However, in the case of imidazole, an N-arylated product was obtained in good yield (entry 3j, Table ).

Table 2

Cu@KF-C/CoFe2O4-Catalyzed C–N Cross-coupling Reaction of Aryl Amines or Imidazole with Arylboronic Acid in Ethanola

Reaction conditions: aryl amine or imidazole (1 mmol), phenylboronic acid (1 mmol), and Cu@KF-C/CoFe2O4 (0.1 g, Cu = 2.22 mol %) in ethanol (5 mL) at 90 °C.

Column chromatography yield.

TOF.

Isolated yield in the case of 3j.

Catalytic Activity for Oxidation Reactions

In the case of oxidation reactions, the catalytic activity of these four synthesized catalysts as well as their precursors was evaluated for oxidation of 4-methoxybenzyl alcohol and again Cu@KF-C/CoFe2O4 was found to be the most active catalyst. The catalytic activity of the most active catalyst, Cu@KF-C/CoFe2O4, was further explored for the oxidation of primary and secondary alcohols and hydrocarbons. The influence of other reaction parameters was also studied using 4-methoxybenzyl alcohol as a model substrate.

Influence of the Solvent, Oxidant, and Temperature

First, the effect of different solvents, namely, water, ethanol, and acetonitrile on the model reaction was studied, keeping the time fixed at 2 h and temperature at 80 °C using TBHP as an oxidant. With acetonitrile, the reaction did not go well and only a 40% yield was obtained. However, the percentage conversion was improved to 68 and 82% in water and ethanol, respectively, so ethanol was selected as the best solvent (Figure a). Further, the effect of different oxidants (air, O2, H2O2, and TBHP) on the model reaction was investigated in ethanol at 80 °C. It was found that the reaction did not proceed in air even after a long reaction time (8 h). However, a somewhat lower yield was obtained when the same reaction was carried out using O2. With H2O2 and TBHP, the reaction proceeded well and the yield increased to 73 and 82%. Thus, TBHP was selected as the oxidant (Figure b). Next, we investigated the effect of elevated temperatures on the model reaction. It was found that similar results were obtained when the reaction temperature increased from 80 to 100 °C, so 80 °C was selected as the optimum reaction temperature (Figure c). Hence, the optimized reaction conditions for the oxidation reactions are ethanol as the solvent, TBHP as the oxidant, and 80 °C as the reaction temperature.

Figure 13

(a) Effect of different solvents and (b) oxidants at 80 °C. (c) Influence of variation of temperature in ethanol (5 mL), keeping the other reaction conditions fixed [4-methoxybenzylalcohol (1 mmol), TBHP, Cu@KF-C/CoFe2O4 (0.1 g), and time (2 h)].

Influence of the Substrate

After optimization of the reaction conditions, the scope of the protocol was explored for a series of primary alcohols having both electron-donating and electron-withdrawing groups, secondary alcohols, and hydrocarbons. As is evident from Table , 4-methoxybenzyl alcohol and 4-chlorobenzyl alcohol undergo oxidation smoothly in less time with an improved yield in comparison to meta- and ortho-substituted primary alcohols. Further, the oxidation of a variety of primary alcohols did not give any over-oxidized product such as carboxylic acid. The oxidation of secondary alcohol to the ketone using Cu@KF-C/CoFe2O4 takes place smoothly with a moderate yield (entry 5h, Table ). With hydrocarbons such as fluorene and diphenylmethane, promising results were obtained (entry 5i–5j, Table ).

Table 3

Cu@KF-C/CoFe2O4-Catalyzed Oxidation of Alcohols in Ethanola

Reaction conditions: alcohol or hydrocarbon (1 mmol), TBHP (1 mmol), Cu@KF-C/CoFe2O4 (0.1 g, Cu = 2.22 mol %), and ethanol (5 mL) at 80 °C.

Column chromatography yield.

TOF.

The catalytic activity of Cu@KF-C/CoFe2O4 was compared with the reported heterogeneous catalytic systems for both the reactions. Comparative catalytic activity data suggest that the present catalyst demonstrated better activity (TOF comparison) than various reported catalysts (Tables S3 and S4).[18,44−49] In the case of the oxidation reaction, some catalytic systems have high TOF, but the catalyst cannot be recovered from the reaction mixture and longer reaction times are required. Further, in the case of C–N coupling between 4-methoxyaniline and phenylboronic acid, the present catalytic system affords good to excellent yields in shorter reaction times under base-free conditions.

Proposed Mechanisms

C–N Coupling

The comparative analysis of Cu 2p XPS spectra of the four different synthesized catalysts (Figure D(a–d)) revealed a major difference in the proportion of Cu(0) species, which in a way provided sufficient explanation for the better catalytic activity of Cu@KF-C/CoFe2O4 as compared to the other catalysts. In the most active catalyst, majority of Cu is present in a reduced form, whereas in the remaining three catalysts, a large proportion of Cu is present in the +2 oxidation state, which indicated that Cu(0) is the more active species for the C–N coupling reaction. Based on this analysis, a proposed mechanism for the C–N cross-coupling reaction between phenylboronic acid and aromatic amines is presented in Figure . The reaction was proposed to progress in a course of a catalytic cycle comprising three main steps, i.e., oxidative addition, transmetallation, and reductive elimination. The first step involves the oxidative addition of aromatic amine to Cu(0) to give intermediate A, which undergoes transmetallation to give intermediate B. Finally, the reductive elimination of intermediate B gives the desired product C and the catalyst is regenerated. Further, the X-ray photoelectron spectra of reused Cu@KF-C/CoFe2O4 (after five catalytic runs) was recorded and comparative analysis of Cu 2p and Co 2p spectra has been carried out. As shown in Figure A(a,b), B(a,b), Cu 2p and Co 2p spectra of fresh as well as reused catalysts show no noticeable change in the binding energy values in the case of C–N coupling. This suggests good recyclability as well as heterogeneity of the catalyst.

Figure 14

Plausible mechanism for the Cu@KF-C/CoFe2O4-catalyzed C–N coupling reaction.

Oxidation

To formulate the mechanism for oxidation of alcohols and hydrocarbons, and to discover if the catalytic reaction involves the radical process, TEMPO, a radical scavenger, was used to carry out the radical-trapping experiment. For this experiment, the test reaction (oxidation of 4-methoxybenzyl alcohol) was performed first until the conversion was up to 50% (1 h). Now, TEMPO (1 mmol) was added to the reaction mixture and the reaction was further carried out for 2 h (completion time without TEMPO). It was found that no considerable change was observed, which confirms that radical intermediates were involved in the oxidation reaction. Furthermore, on comparing the X-ray photoelectron spectra of fresh and reused catalysts, it has been observed that no change in binding energy values of Co was observed (Figure B(c)). However, in Cu 2p spectra, some noticeable change was observed as majority of copper is present in a oxidised form, which might be due to the presence of TBHP, but still the catalyst is recyclable in nature and shows good catalytic activity. A plausible radical mechanism of oxidation of alcohols and hydrocarbons catalyzed by Cu@KF-C/CoFe2O4 is shown in Figure .

Figure 15

Plausible mechanism for the Cu@KF-C/CoFe2O4-catalyzed oxidation of alcohols and hydrocarbons.

Recyclability

To confirm the recyclable nature of the synthesized catalyst, we have carried out the reaction between 4-methoxyaniline and phenylboronic acid in the presence of Cu@KF-C/CoFe2O4 in ethanol under base-free conditions at 90 °C (entry 3b, Table ). After completion of the reaction, the catalyst was recovered using a magnet followed by washing with deionized water (3 × 10 mL) and ethyl acetate (3 × 10 mL) and drying under a vacuum. Then, a fresh reaction was performed with the same reactants using the catalyst recovered from the previous run. This process was repeated up to five times, and no notable change in the catalytic activity of the catalyst up to five consecutive runs (Figure ) was observed. Similarly, the recyclability of the catalyst was studied for the oxidation reaction (entry 5a, Table ) and results showed that Cu@KF-C/CoFe2O4 is recyclable up to five times with minimum loss of activity.

Figure 16

Recyclability of Cu@KF-C/CoFe2O4 for the Chan–Lam coupling reaction (entry 3b, Table ) and oxidation (entry 5a, Table ) under the optimized reaction conditions.

Conclusions

Here, we have reported the preparation of activated carbon from neem dead leaves (abundant waste biomass). This strategy involves two steps: first, carbonization and then chemical activation with KOH to increase the porosity of the carbon material. Then, such a porous carbon material was used to prepare C/MFe2O4 composites, which, upon KF modification followed by immobilization of Cu(0) nanoparticles, generate four magnetic catalysts [Cu@KF-C/MFe2O4, M = Co, Cu, Ni, and Zn]. Impregnation of KF onto C/MFe2O4 introduces some basic character to the composites and make the catalysts basic in nature that helps to carry out C–N coupling under base-free conditions. Four prepared catalysts were characterized by BET and XPS analysis. From BET analysis, it was inferred that Cu@KF-C/CoFe2O4 has a more porous structure and thus showed the highest activity among the other synthesized catalysts. Moreover, from XPS analysis, it was found that majority of copper is present in a Cu(0) form in Cu@KF-C/CoFe2O4, which is responsible for better catalytic activity. This is an environmentally benign and safe protocol that offers numerous advantages such as mild reaction conditions, excellent yields, and ease of separation using an external magnet with an added benefit of recyclability. The novelty of the work, therefore, is the synthesis of magnetic catalysts from neem dead leaves (abundant waste biomass), which is cheaper and could serve as an alternative source for activated carbons.

Experimental Section

Materials

Neem (A. indica) dead leaves were collected from the Jammu University campus. All the chemical materials utilized were bought from Aldrich Chemical Company and used further without purification.

Synthesis of Activated Carbon

Large quantity of dead neem leaves were collected from the Jammu University campus and thoroughly washed with water to remove dust. These leaves were dried in an oven at 60 °C overnight. The dried leaves became crispy and were crushed in a mixer grinder to get the fine powder, which was carbonized at 350 °C for 16 h under N2 atmosphere. Initially, the temperature was increased slowly (10 °C/min) up to 350 °C and then heating was continued for another 16 h under N2 atmosphere. As a result of continuous heating, light green color of neem leaf powder turned black due to the formation of carbon, which was allowed to cool, and then washed with dilute sulfuric acid to remove the ash content. The obtained carbon was then dried in an oven at 80 °C for 24 h and ground in a mortar pestle to get fine powder. For activation, the obtained carbon (2 g) was mixed with potassium hydroxide (4 g) in 20 mL of distilled water followed by stirring at room temperature for 1 h to make a uniform suspension, which was then dried in an oven at 110 °C for 12 h followed by thermal activation at 400 °C for 4 h under a N2 atmosphere. The activated carbon formed was washed repeatedly with 1 M HCl solution and distilled water (200 mL) to remove chloride ions. Finally, the activated carbon was dried at 110 °C for 12 h in an oven.

Synthesis of Carbon-Supported Metal Ferrites (C/MFe2O4)

For the synthesis of C/MFe2O4 composites, activated carbon (0.5 g) was dispersed in distilled water (50 mL) and sonicated for 1 h. Then, a separate solution of Fe(NO3)3·9H2O (2 mmol) and M(OAc)2 (M = Cu, Co, Ni, and Zn) (1 mmol) was prepared in distilled water (10 mL) and then added to the dispersed solution of activated carbon and the reaction mixture was further stirred for 1 h. After this, 8 M NaOH solution was added dropwise to the obtained mixture to maintain pH 10 and stirring was continued for another 1 h. Later, the suspension was removed under reduced pressure to obtain a solid residue, which was heated at 400 °C for 8 h under a N2 atmosphere. The resultant composites were named as C/CuFe2O4, C/CoFe2O4, C/NiFe2O4, and C/ZnFe2O4, which were separated with the help of an external magnet and washed with water followed by drying in an oven at 80 °C for 6 h.

Synthesis of Cu@KF-C/MFe2O4

KF-modified carbon-supported metal ferrites were prepared by a wet impregnation method. C/MFe2O4 was first immersed in anhydrous methanol solution of KF (20 wt %) followed by stirring at 40 °C for 6 h. After this, powder was obtained by removing methanol under reduced pressure using a rotavapor, which was dried in an oven at 100 °C for 8 h. The obtained powder was further heated at 350 °C for 5 h under N2 atmosphere to activate KF, which leads to the formation of KF-C/MFe2O4. For the immobilization of Cu nanoparticles onto KF-C/MFe2O4, a solution of Cu(OAc)2 (0.181 g, 1.0 mmol) in distilled water (5 mL) was added into the solution of KF-C/MFe2O4 (1 g) in ethanol (10 mL). Then, an aqueous solution of NaBH4 (1.2 mmol, 5 mL) was added dropwise with continuous stirring over a period of 1 h. After the complete addition, the mixture was stirred for another 10 h at room temperature. The obtained catalyst [Cu@KF-C/MFe2O4] was separated with the help of an external magnet followed by washing with ethanol (3 × 20 mL) and water (3 × 20 mL) and then dried under vacuum at room temperature overnight.

General Procedure for the Cu@KF-C/MFe2O4-Catalyzed Chan–Lam Cross-coupling Reaction

In a typical procedure, a mixture of amine or imidazole (1 mmol), arylboronic acid (1 mmol), and Cu@KF-C/MFe2O4 (0.1 g) was suspended in ethanol (5 mL) in a round-bottom flask (25 mL). The mixture was stirred at 90 °C for a suitable time, and the progression of the reaction was examined via TLC. After the end of the reaction, the mixture was allowed to cool and the catalyst was recovered using an external magnet, washed with ethyl acetate (3 × 5 mL) and deionized water (2 × 10 mL), and dried under vacuum. Ethanol was removed under vacuum, and the residue was diluted with ethyl acetate (20 mL) and washed with brine solution (3 × 20 mL) followed by drying over anhydrous Na2SO4. The crude product obtained after removal of the solvent under reduced pressure was purified by column chromatography using a silica gel (60–120 mesh) and elution with petroleum ether and ethyl acetate to furnish the desired N-arylated product.

General Procedure for the Cu@KF-C/MFe2O4-Catalyzed Oxidation of Alcohols and Hydrocarbons

For the oxidation, a mixture of alcohol or hydrocarbon (1 mmol), Cu@KF-C/MFe2O4 (0.1 g), and TBHP (1 mmol) was dispersed in ethanol (5 mL) and stirred at 80 °C till the reaction was completed as monitored by TLC. Afterward, the catalyst was collected with the help of an external magnet and washed successively with ethyl acetate (3 × 5 mL) and deionized water (2 × 10 mL) followed by drying under vacuum for use in the next reaction. Ethanol was removed under reduced pressure, and the residue was diluted with ethyl acetate (20 mL), washed with brine solution (3 × 20 mL) followed by deionized water, and dried over anhydrous Na2SO4. Lastly, the crude product obtained after removal of the solvent under reduced pressure was purified by passing through a column of silica gel using petroleum ether and ethyl acetate as eluting solvents.

Material Characterizations

Neem (A. indica) dead leaves were collected from the Jammu University campus. All the chemicals utilized were obtained from Aldrich Chemical Company and used further without purification. The BET specific surface area was determined from N2 adsorption–desorption isotherms at 77 K using a Belsorb Mini-X analyzer. Before N2 adsorption, the samples were pretreated at 200 °C for 4 h under vacuum to dehydrate the catalysts prior to analysis. X-ray photoelectron spectra of the catalysts were recorded on a KRATOS ESCA model AXIS 165 (Resolution). The FTIR spectra of the samples were recorded on a PerkinElmer FTIR spectrophotometer. The samples were made ready by mixing 5 mg of each catalyst with 200 mg of KBr and then pressed into a disk for FTIR measurement. To analyze the thermal resistance of the prepared catalysts, TGA was done on a Perkin Elmer, Diamond TG/DTA with a heating rate of 10 °C min–1. SEM images were recorded using FEG SEM JSM–7600F Scanning Electron Microscope, and HR-TEM images were recorded using FEG, Tecnai G2, F30 Transmission Electron Microscope. EDX mapping was recorded on a JSM-7600F, JEOL, 30 KV. EDX analysis was accomplished using an OXOFORD X–MAX model JSM–7600F, and the quantity of metal loaded in respective support materials was estimated by an ICP-AES study using an ARCOS, Simultaneous ICP spectrometer. Approximately 0.05 g of the sample was taken in a microwave digestion vessel, and conc. HNO3 (4 mL), conc. HCl (2 mL), and conc. HF (2 mL) acid were added to it. After that, the solution was diluted to 30 mL with distilled water and heated at two different temperatures. First, the resulting solution was heated at 130 °C (ramp: 10 mm·ss) and held for 15 mm·ss. In the next step, the solution was heated at 190 °C (ramp: 15 mm·ss) and held for 20 mm·ss. XRD was recorded in a 2θ span of 10–80° on a Bruker AXSD8 Advance X-ray diffractometer deploying Cu Kα radiations. The degree of magnetism (magnetic moment) of the prepared catalysts was estimated with a vibrating sample magnetometer (VSM, bearing model: 7410 series, Lakeshore) at room temperature from −15,000 to +15,000 Oe. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra of the products were recorded in deuterated chloroform on a Bruker Avance III spectrometer using TMS as an internal standard.