Hexagonal mesoporous silica supported ultra-small copper oxides for

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Hexagonal mesoporous silica supported ultra-small copper oxides for oxidative amidation of carboxylic acids Ravishankar Kadam, Martin Petr, Radek Zbo#il, Radha V. Jayaram, and Manoj B. Gawande ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02247 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 23, 2018

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ACS Sustainable Chemistry & Engineering

Hexagonal mesoporous silica supported ultra-small copper oxides for oxidative amidation of carboxylic acids Ravishankar G. Kadam,a,b Martin Petr,b Radek Zbořil, b Manoj B. Gawande*b and Radha V. Jayaram*a a

Department of Chemistry, Institute of Chemical Technology, Matunga, Mumbai 400019, India. Email: [email protected] b

Regional Centre of Advanced Technologies and Materials, Department of Physical

Chemistry, Faculty of Science, Palacký University Olomouc, Šlechtitelů 27, 783 71 Olomouc, Czech Republic. Email: [email protected]

ABSTRACT: Hexagonal mesoporous silica (HMS) supported copper oxide nanoparticles were prepared by a one-pot sol-gel synthesis and their physicochemical properties were determined by XRD, ICP-MS, SEM, TEM, and N2 physisorption, HR-TEM and elemental mapping. A series of CuO/HMS catalysts was prepared with different Cu loadings (1, 1.5,2, 2.5, 3, and 5 wt%). The catalytic performance of the prepared catalysts was investigated for the oxidative amidation of carboxylic acids. Notably, under mild reaction conditions the CuO/HMS catalysts exhibited excellent activity in terms of conversions (70–95%), and selectivity (80-86%). The improved catalytic performance of CuO/HMS catalysts may be attributed to the homogeneous dispersion and uniformity of the active copper species on the HMS support with large surface area. Having investigated the functional group tolerance, the developed protocol can also be extended for the synthesis of insect repellent molecules such as N,N-diethyl phenylacetamide (DEPA), N,N-diethylbenzamide (DEB), and N,N-diethyl mtoluamide (DEET) in good to excellent conversions with better TON and E factor. The

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reaction protocol is sustainable due to its mild reaction conditions, high conversions and selectivities superior to reported protocols. Importantly, the catalysts can be separated by simple centrifugation and can be recycled for several reaction cycles. Keywords: Mesoporous silica, copper oxide, Oxidative amidation, heterogeneous catalysis. Introduction Primary amides and their derivatives have a wide range of applications as intermediates in organic synthesis, starting materials, and lubricants. 1,2 The transformation of carbonyl compounds including aldehydes, ketones, and oximes is a widely used protocol for the synthesis of amides. 3,4 Conventionally, amides are synthesized using carboxylic acid derivatives with an amine in the presence of suitable reagents.5-10 In general, syntheses of N,N-dimethylamides need a prior activation of the carboxylic acid, 5 including the use of transition metal catalysts for DMF-promoted carboxylation of aryl halides. 11-16 Several catalytic protocols have been reported, such as oxidative amidation of alcohols or aldehydes with N,N-dimethylamine.17-20 Further, utilization of benzyl alcohol with DMF in the presence of TBHP as an oxidant and several homogeneous catalysts systems like tetrabutylammonium iodide, iodine, and NaOH have been examined. 21,22 The amide synthesis from carboxylic acids and DMF using Cu(II) salts or complexes in the presence of TBHP is one of the commonly used pathways.23-25 Additionally, some reports for decarboxylative coupling of cinnamic acid with toluene in the presence of CuO and Di-tert-butyl peroxide (DTBP) are also described in literature (Fig. 1). 26-28 Also, very recently

heterogeneous catalytical

systems such as Cu-Fe HT29 Fe2O3@SiO2-acacsil-Cu(II)30 and Co-C-N MOF have been explored for oxidative amidation for carboxylic groups.31 Heterogeneous sustainable catalytic systems with no use of stoichiometric toxic reagents are always preferred to achieve the goal of environmentally benign and eco-


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friendly procedures in the various organic transformations. 32 In order to address these issues, various transition metal oxides and porous solid materials have been developed due to their versatile applications in catalytic processes. 33,34 Generally, mesoporous materials possess several unique properties: pore volume, distinct pore arrangement, high thermal as well as mechanical stability 35 and large surface area (generally >1000 m2/g). Additionally, pore size of this material can be tuned by adjusting the length of the surfactant chains. 36 For example, hexagonal (MCM-41), cubic (MCM-48), lamellar (MCM-50), and hexagonal mesoporous silica (HMS) with different pore sizes and their different variants have attracted a lot of attention in many catalytic transformations. 37-44 Furthermore, several non-noble transition metal oxides supported on a surface of mesoporous silica and mesoporous polymer have achieved significant applications in different fields like adsorption, separation, drug delivery as well as in catalysis. 45-47 Additionally, the use of inexpensive transition copper metal received considerable attention among researcher as it is a naturally abundant and environmentally friendly element.48



Fig. 1 Previous and present reports on the synthesis of N-substitute amide. It is well known that the development of green and sustainable methodologies using heterogeneous catalysts under benign reaction conditions has become a broadly accepted strategy in chemical industry. 49,50 In view of their synthetic utility, we herein report CuO nanoparticles supported on HMS (CuO/HMS) as catalysts for the oxidative amidation of carboxylic acids in benign reaction conditions to achieve good to excellent conversions (70–97%). The combination of CuO and HMS displayed a


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good catalytic performance while TBHP served as a radical initiator in this process. The CuO/HMS catalyst is prepared by a soft-templating method as illustrated in Fig. 2. N,N'-Diethyl-3-methylbenzamide (DEET) is widely used as an active ingredient in insect repellents. The conventional method of DEET is based on the conversion of the m-toluic acid to the corresponding m-toluoyl chloride by consuming an excess of diethylamine. Thionyl chloride is the most commonly used reagent for the formation of the intermediate m-toluoyl chloride51 using oxalyl chloride or triphosgene 52,53 and amino magnesium bromide reagent. 54 Recently, Bannwart reported the synthesis of DEET by using propyl phosphonic anhydride in the presence of equivalents of HCl. 55 As far as the catalytic reaction is concerned, in comparison with the previous reports on the construction of N,Ndimethyl amides, the current protocol is easy and simple and can be carried out without any

ligands, or additives. Moreover, the catalyst can be recycled several

times, thus making the methodology more sustainable and environmentally benign. The scope of work is extended to synthesize the insect repellents like N,N-diethyl methyl

benzamides

(DEET),

N,N-diethyl-m-toluamide,

N,N-Diethyl-2-

phenylacetamide (DEPA), and N,N-Diethylbenzamide (DEB) compounds (Fig. 3).

Experimental Catalyst preparation In a typical synthesis, 6.4 g of dodecyl amine (DDA) was dissolved in a mixture of 50 mL of ethanol and 70 mL of deionized water under vigorous stirring. A dilute aqueous solution of 0.7 g of Cu(NO3)2·3H2O was then added to the solution, followed by dropwise addition of a stoichiometric amount of tetraethylorthosilicate (TEOS)



(relative to the amount of copper) in ethanol and isopropyl alcohol (IPA); molar composition of this solution being 1:7:1 (TEOS: EtOH: IPA). The solution was then stirred at 323 K for 4 h and the gel was aged for 48 h at 313 K. The gel was then collected by centrifugation, washed thoroughly with deionized water and absolute ethanol, and dried in air for 24 h at 373 K, followed by calcination in air for 5 h at 723 K. Various loadings of copper metal on HMS have been obtained by this one-pot synthesis (OPS) method. Catalyst characterization Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker AXS diffractometer using Cu-Kα radiation (λ = 1.540562 A0). Energy dispersive X-ray spectra (EDS) were recorded with an Oxford instrument where 10 kV beam intensity was kept high to get a sufficient detector response. N 2-adsorption isotherms were performed at 77 K on Micrometrics ASAP 2020 instrument. The samples were degassed at 573 K for 3 h under vacuum (10 –3 torr) before the measurement. Specific surface areas were determined by Brunauer–Emmett–Teller (BET) method in the relative pressure range of 0.4–0.9. Pore volumes were computed from the volume of nitrogen adsorbed at the relative pressure. High-resolution transmission electron microscopy (HRTEM) experiments were performed on FEI Model: Tecnai G2, F30 Resolution point: 2.0 Angstrom Line: 1.0 a Magnification: 58× to 1,000,000× at potential 300 kV. The reaction products were analyzed by gas chromatography (Model: Agilent 6820) equipped with an Agilent DB-5 capillary column (30 m × 0.32 mm, 0.5 m) under the operation parameters: inlet temperature 473 K, flame ionization detector (FID) temperature 523 K, oven temperature 523 K with a ramp rate 10 degrees min–1 from 373 K, further Identification of the products was carried out using a Shimadzu gas chromatograph−mass spectrometer equipped with a QP2010 mass


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spectrometer. 1HNMR spectra of amides produced from the catalytic reactions were recorded on 600 MHz NMR Varian spectrometer (Varian, Santa Clara, CA, USA) CDCl 3/DMSO-d6 as a solvent and TMS as an internal standard.

Fig. 2 Schematic representation of the soft-templating method for the one-pot synthesis of CuO/HMS catalyst.

Fig. 3 Some biologically active molecules as insect repellents.

ICP-AES analysis was carried out with ARCOS M/s spectrum, German with R. F. generator at a maximum of 1.6 kW, 27.12 MHz and wavelength range 130 nm to 770 nm.



Microscopic TEM images were obtained by HRTEM TITAN 60-300 with X-FEG type emission gun, operating at 80 kV. This microscope is equipped with Cs image corrector and a STEM high-angle annular dark-field detector (HAADF). The point resolution is 0.06 nm in TEM mode. The elemental mappings were obtained by STEM-energy dispersive X-ray spectroscopy (EDS) with acquisition time of 20 min. For HRTEM analysis, the powder samples were dispersed in ethanol and sonicated for 5 min before drop-casting on the grid. Activity test In a typical reaction, benzoic acid (1 mmol), dimethylformamide (DMF) (8 equiv.), TBHP (3equiv. in 70% water) and 2.5 wt% CuO/HMS (20 mg) were taken in a reaction vessel and stirred at 80 °C for the required time. The progress of the reaction was monitored by GC (Perkin Elmer Clarus 580). After completion of the reaction, the catalyst was separated via centrifugation and the reaction mass was extracted with ethyl acetate. The organic phase was dried over sodium sulfate and the solvent was removed under reduced pressure. The crude product was purified by column chromatography (silica 230–400; n-hexane/ethyl acetate mixture) to attain the desired product. Results and discussion The synthesized CuO/HMS catalysts were examined for their structural and chemical properties by XRD, N2-adsorption isotherms, pore volume analysis, TEM, ICP-MS, and HRTEM-elemental mapping analysis. To identify the diffraction pattern of the prepared material, a wide-angle XRD technique was used. XRD patterns of HMS and CuO/HMS samples showed typical reflections of the corresponding silica structure (Fig. 4). XRD pattern of copper oxide with a different loading (2.5 wt% to 5 wt%) displayed single broad diffraction peak at 2θ = 23°, which can be attributed to the amorphous silica. 56 The XRD pattern of copper oxide loading (1.0, 1.5 and 2.0 wt%), low angle XRD of 2.5wt% CuO/HMS and reused XRD of 2.5wt% CuO/HMS is included in Figure S1, S2 and S3. Notably, CuO is


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difficult to identify in XRD patterns of the sample below 2.5 wt% due to the very low content; though small peaks at 2θ = 35.5° and 38.7° clearly indicate the presence of monoclinic CuO (JCPDS 048-1548). X-ray photoelectron spectroscopy (XPS) was employed for the quantitative and qualitative surface analysis of the sample. Survey XPS spectrum with the quantification of sample 2.5 wt% is presented in Fig. S4. The sample consists of oxygen (58.5 wt%), silicon (38.1 wt%), carbon (2.4 wt%), and copper (1.0 wt%). Fig. 5 shows the high-resolution XPS spectrum of Cu 2p peak. The shape of the spectrum and two peaks at 933.50 eV and 953.30 eV, correspond to Cu 2p3/2 peak of Cu2+ in CuO and to Cu 2p1/2 of Cu2+ in CuO, respectively.

Fig. 4 Powder XRD patterns of CuO/HMS (2.5, 3, and 5 wt%) samples.



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Fig. 5 High-resolution Cu 2p XPS spectra of 2.5 wt% CuO/HMS. Table 1. Surface property of different CuO/HMS catalysts. BET surface

Cua loading

Pore diameter

Pore volume

area (m2g–1)

(wt %)

(nm)

(cm3g–1)

HMS

919

0

2.8

0.73

1% CuO/HMS

904

0.8

2.8

0.68

1.5% CuO/HMS

879

1.4

3.0

0.62

2% CuO/HMS

875

1.9

3.0

0.66

2.5% CuO/HMS

870

2.3

2.8

0.67

3% CuO/HMS

838

2.9

3.1

0.65

5% CuO/HMS

792

4.8

3.2

0.60

Catalysts

a

Total Cu loading determined by ICP-MS.

To analyze the surface properties and porosity of developed catalysts, the adsorption– desorption isotherms were measured (Table 1). The hysteresis curves obtained with relative pressures between 0.4 and 0.9 p/p0, which is typical of mesoporous solid materials (Fig. 6a), revealed that the catalyst displayed a typical type IV N2 adsorption–desorption isotherm with


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a H1 hysteresis loop indicative of mesoporous structure. 57 Pore size distribution curve (Fig. 6b) confirmed that it contained distinct types of pores centered in between 10–30 nm. The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of the 2.5% CuO/HMS catalyst are shown in Fig. 7. The TEM images of synthesized CuO/HMS show round shape HMS support (Fig. 7a and b). The HRTEM images revealed that CuO species are distributed evenly on the HMS surface (Fig. 7c and d). Further, the uniform distribution of CuO ultra small nanoparticles can be seen in STEM/EDX elemental mapping of the CuO/HMS sample (Fig. 8). The transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of the 2.5% CuO/HMS catalyst are shown in Fig. 7. The TEM images of synthesized CuO/HMS show round shape HMS support (Fig. 7a and b). The HRTEM images revealed that CuO species are distributed evenly on the HMS surface (Fig. 7c and d). Further, the uniform distribution of CuO ultra small nanoparticles can be seen in STEM/EDX elemental mapping of the CuO/HMS sample (Fig. 8). To evaluate the catalytic activity of the CuO/HMS first, the benzoic acid (1a) was selected as a model substrate to optimize the reaction parameters. After testing various loadings of CuO/HMS catalysts, it was noted that 2.5 wt% CuO supported on HMS showed the best catalytic performance (Table 2, entries 4 to 9), significantly higher than unsupported CuO and Cu powders (Table 2, entries 3 and 10) providing with 95% conversion at 80 °C within 6h. Furthermore, blank runs (without a catalyst) conducted no activity under optimized conditions (Table 2, entry 1). When the reaction was subjected to a physical mixture of CuO and HMS, it produced similar conversion of the product like pure CuO (Table 2, entry 11) this is, however, a much lower amount compared to 2.5% CuO/HMS sample. We believe that the synergetic effect between HMS and CuO nanoparticles could arise from the prominence



of the porous nature of HMS, in addition to the high dispersion of catalytic species, it will contribute to the improved catalytic performance. 58 The catalyst weight was examined further by using 2.5 wt% CuO/HMS while 20 mg catalyst was found to be enough for the 95% conversion (Table 2, entries 14 and 15) and excellent selectivity with good TON and TOF values of 138 and 23h–1, respectively (Table 2, entry 14). So as to study the effect of oxidants on the oxidative amidation of benzoic acid with DMF, at first, the reaction was carried out in the absence of an oxidant (Table 2, entry 16). Subsequently, we also performed the reactions with a few other oxidizing agents. It is worth noting that no amides formation was detected either during the absence or presence of other oxidants such as H2O2 and O2, (Table 2, entries 17–18), clearly indicating the significance of TBHP as an oxidant.

Fig. 6 a) N2 physisorption isotherms of 2.5 wt% CuO/HMS; b) pore size distribution.


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Fig. 7 a), b) TEM and c), d) HRTEM images of 2.5 wt% CuO/HMS.



Fig. 8 a) HAADF image, b)–e) STEM elemental mapping of 2.5 wt% CuO/HMS showing Cu, O, and Si.


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Table 2 Optimization of the reaction conditions.a

Conversionb Entry

Catalysts

Oxidant

TON/TOF (h–1)

(%) 1

-

TBHP

0

NA

2

HMS(20mg)

TBHP

"

NA

3

CuO

TBHP

25

NA

4

1% CuO/HMS(20mg)

TBHP

30

95/15

5

1.5%CuO/HMS(20mg)

TBHP

50

105/17

6

2% CuO/HMS(20mg)

TBHP

64

101/16

7

2.5% CuO/HMS(20mg)

TBHP

95

131/21

8

3% CuO/HMS(20mg)

TBHP

100

105/17

5% CuO/HMS(20mg)

TBHP

100

63/10

10

Cu powder

TBHP

20

NA

TBHP

32

NA

11

Physical mix of HMS and CuO powder

12

CoO/HMS

TBHP

6

NA

13

NiO/HMS

TBHP

4

NA

14

2.5% CuO/HMS (15 mg)

TBHP

75

138/23

15

2.5% CuO/HMS (30 mg)

TBHP

100

92/15

16

2.5% CuO/HMS (20mg)

No oxidant

0

NA

17

2.5% CuO/HMS (20mg)

H2O2

0

NA

18

2.5% CuO/HMS (20mg)

O2

0

NA

19e

2.5% CuO/HMS (20mg)

TBHP

0

NA

20f

2.5% CuO/HMS (20mg)

TBHP

45

62/10

21g

2.5% CuO/HMS (20mg)

TBHP

95

131/21

c

9 d

a

Reaction conditions: Benzoic acid 1a (1 mmol), DMF 2a (8 equiv.), TBHP (3 equiv. in 70% water)

and catalyst 2.5wt% CuO/HMS (20 mg) (0.73 mol%), at for 6 h, bThe conversion were determined by GC,

c,d

amount of catalysts taken as with equal copper content to our best catalyst, e at 30°C,f 60 °C

and g100 °C temperature, CoO/HMS and NiO/HMS catalysts prepared by standard procedure from reported literature (For the preparation of NiO/HMS, nickel chloride salt was used). 59



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Table 3 The scope of the oxidative amidation of various carboxylic acids. a Entry

Substrate

Product

Conversionb (sel.) (%)

1

95 (86)

2

74 (86)

3

78 (83)

4

80 (86)

78 (85)

5

6

76 (84)

7

82 (86)


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8

77 (86)

9

82 (80)

10

74 (86)

11

78(81)

12

72 (82)

13

82 (85)

14

74 (83)

15

70 (86)



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16

90 (85)

17

78 (86)

18

80 (86)

19

86 (86)

20

92 (85)

21

88 (84)

22

83 (82)

23 87 (86)

80 (86)

24


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a

Reaction conditions: Benzoic acid 1a (1 mmol), DMF 2a (8 equiv.), TBHP (3 equiv. in 70% water) and catalyst 2.5% CuO/HMS (20 mg), at 80 °C for 6 h. Conversions were determined by GC. To check the comparative effect of other transition metal oxides in that point of view, we prepared several other transition metal oxides supported on HMS (Table 2, entries 12 and 13) and tested them for the reaction. CuO/HMS turned out to be more effective and it exhibited a better catalytic activity than the others. To check the effect of temperature, we performed the reaction at different temperatures ranging from 60 °C to 100 °C and found out that 80 °C is the optimum temperature to get an excellent yield (Table 2, entries 7, 19, to 21). With the optimal reaction conditions in hand, we then explored the substrate scope of the catalytic system for oxidative amidation of a variety of substituted benzoic acids using the 2.5 wt% CuO/HMS. Notably, the selected catalytic system exhibited a good activity towards the whole range of substrate (Table 3); benzoic acids substituted with electron-rich group including (-CH3, -OCH3) (Table 3, entries 2–5) gave considerably higher yields compared to the electron-withdrawing groups such as halogens to ortho, meta, and para (Cl, Br, F) (Table 3, entries 6–11). Meanwhile, halogen substituted benzoic acids also provided the desired products in moderate to good yields. Further, the scope for sterically hindered ortho-substituted benzoic acids such as (2,6-dichlorobenzene acid was investigated to find out that the reactions underwent efficiently and gave a good yield (Table 3, entries 12). Additionally, heterocyclic acids such as furolic acid, 2-thiophenecarboxylic acid, and nicotinic acid converted into the desired products in good yields (Table 3, entries 17–19). When the phenyl acetic acid derivative of α, β-unsaturated carboxylic acids were subjected to the reaction, the corresponding amide products were obtained in good yields (Table 3, entries 20–24). Importantly, the protocols are better or comparable to reported protocols in terms of E-factor 4 (Tables S1, ESI†). Often, the



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reported processes took several hours (5–20 h) to complete the reaction (while the present method gets completed within 6 h) and most of them involved relatively unstable and nonrecyclable homogeneous catalysts. Moreover, some of them contained environmentally hazardous chlorinated solvents (Tables S1, ESI†). In order to check the efficiency as well as the practical applicability of the catalyst (CuO/HMS), we further subjected it to react with 10 mmol of benzoic acid 1a, with equivalent amount of DMF 2a, and 70% TBHP in water with the equivalent amount of catalyst at 80 °C for 6 h. After completion of the reaction, the catalyst was separated and the reaction mixture was purified using column chromatography to obtain the corresponding amide product 3a in 80% yield. To investigate the scope of this protocol towards the synthesis of insect repellent molecules, we employed the reaction of benzoic acids bearing either electron-donating or heteroaromatic ring with N, N-diethylformamide under similar conditions (Table 4). We observed that the molecules were synthesized in excellent conversions up to 92% with good TON and TOF. Table 4 The scope of the various insect repellent molecules.a

Entry Substrate

Product

Conversionb (sel.) (%)

TON/TOF (h–1)

92 (86)

127/21

90 (85)

124/20

1

2


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3 88 (85)

121/20

90 (86)

124/20

80 (85)

110/18

4

5

a

Reaction conditions: Benzoic acid 1a (1mmol), DEF 2a (8equiv.), TBHP (3 equiv. in 70% water) and catalyst 2.5% CuO/HMS (20 mg), at 80 °C for 6 h. b conversion were determined by GC. To be able to test for the heterogeneity of the reaction and stability of CuO/HMS catalysts, we carried out a hot filtration test for on the reaction of benzoic acid (1a) and DMF (2a).

Fig. 9 Catalyst recyclability study. Catalyst was filtered after when the conversion of reactant 1a reached 50%. Further, the filtrate did not afford conversion in the continuous reaction signifying that no significant amount of active CuO species was leached in the reaction mixture. The catalyst can be



separated and reused for six cycles (Fig. 9). After completion of the reaction, the catalyst was recovered by centrifugation, washed with water and methanol, dried at 80 °C, and then reused for the next cycle. The concentration of Cu species leached into the reaction mixture after the 6th cycles was negligible (0.07 mg, of Cu by ICP analysis).

Fig. 10 The proposed mechanism for the reaction A possible catalytic mechanism is depicted in Fig. 10 based on the free radical reaction initiated by transition metals.60,61 At first, tert-butoxy radical is formed by the reaction of Cu(II) catalyst with TBHP via a Cu(I)-mediated catalytic cycle. The tert-butoxy radical, thus


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formed, reacts with both starting materials to yield the final product. Among the possible pathways, pathway A involves the abstraction of hydrogen from carboxylic acid and dimethyl foramide to produce carboxylate radical A and amidyl radical B. Further, these radicals react with each other and form a carbamic anhydride intermediate D, which finally undergoes decarboxylation to give the final product as secondary amide. Using another alternative pathway (pathway B), carboxylic acid reacts with Cu(II) to form carboxylate C. The intermediate C will then react with amidyl radical B by single electron transformation (SET), which in turn reduces the Cu(II) to Cu(I) and forms a carbamic anhydride intermediate D. 62-65 The subsequent decarboxylation allowed the formation of amide product and Cu(I) got reoxidized to Cu(II) in the presence of TBHP and then the catalytic cycle went on. Conclusions A series of copper oxide nanoparticles supported on mesoporous silica (HMS) have been synthesized via a controlled one-pot sol-gel method wherein the CuO particles were found to be finely dispersed onto the surface of HMS. The loading of 2.5 wt% of CuO onto HMS is effective to catalyze oxidative amidation with TBHP affording a variety of amides with good to excellent yields. In this protocol, we have established and environmentally benign conditions without any additives. The catalyst can be recovered by a simple centrifugation separation technique and can be recycled several times without apparent loss of catalytic activity. Further, the by-products were not generated in reaction mixture either, so the process addresses the route of atom economy. The variety of benzoic acid substituted functional groups can smoothly undergo this process. The scope of the reaction was extended for the synthesis of insect repellent bioactive molecules in grams scale. ASSOCIATED CONTENT * Supporting Information



The Supporting Information is available free of charge on the ACS Publications. A relevant XPD, low angle XRD, XPS, TEM images, comparison table, selected GC-MS and proton NMR spectra are included (PDF). AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. *E-mail: [email protected] ORCID Manoj B. Gawande: 0000-0003-1575-094X Radek Zboril: 0000-0002-3147-2196 Conflicts of interest There are no conflicts to declare. Acknowledgements Ravishankar G. Kadam thanks the UGC-UPE Green Technology centre, New Delhi, India for a fellowship. The authors are thankful to Ondřej Tomanec for HRTEM-elemental analysis. The authors gratefully acknowledge support from the Ministry of Education, Youth and Sports of the Czech Republic (project LO1305) and the assistance provided by the Research Infrastructures NanoEnviCz under project LM2015073. The work was also partly funded by the Internal Grant of the Palacký University Olomouc, Czech Republic (Project No. IGA_PrF_2017_007).


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The benign and recyclable CuO/HMS nanocatalysts employed for oxidative amidation of carboxylic acids and synthesis of insect repellent molecules under mild reaction conditions.


Hexagonal mesoporous silica supported ultra-small copper oxides for

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