The First Example of ortho-Arylation of Benzamides over Pd

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The First Example of Ortho-Arylation of Benzamides over Pd / Mesoporous Silica: A Novel Approach for Direct sp C-H Bond Activation 2

T. Parsharamulu, D. Venkanna, M. Lakshmi Kantam, Suresh Kumar Bhargava, and Pavuluri Srinivasu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie503576d • Publication Date (Web): 01 Dec 2014 Downloaded from http://pubs.acs.org on December 7, 2014

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Industrial & Engineering Chemistry Research

The First Example of Ortho-Arylation of Benzamides over Pd /Mesoporous Silica: A Novel Approach for Direct sp2 C−H Bond Activation T. Parsharamulu,† D. Venkanna,† M. Lakshmi Kantam,† Suresh K. Bhargava,‡ and Pavuluri Srinivasu*,† †

Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad-500007, India

‡

Advanced Materials and Industrial Chemistry Group, School of Applied Sciences, RMIT University, Melbourne-3001, Australia

ABSTRACT: Three-dimensional mesoporous silica with highly dispersed palladium nanoparticles composite (PS-3) catalyst has been prepared and characterized by XRD, XPS, FT-IR, nitrogen sorption and HR-TEM. The PS-3 catalyst is utilized for the first time as efficient and reusable catalyst in the ortho-arylation of benzamides by aryl iodides has been demonstrated with the simplest amide CONH2 as a directing group. It can be successfully applied to various benzamides and aryl iodides with both electron donating and withdrawing groups. The catalyst can be simply recovered and reused several times without significant loss in catalytic activity.

1. INTRODUCTION One of the most important challenges in industrial catalytic systems is development of new processes that reduce both energy consumption and minimize evironmental pollution in chemical synthesis.1 Heterogenization of homogeneous catalytic methods is a key to green chemistry, since its ease of separation of reactants, and reusability of catalysts. In recent years, mesoporous structures have been paid considerable attention as new green catalytic materials, because of their unique high specific surface area, large pore volume, in conjuction with tunable pore size.1 In recent years, various mesoporous materials with controlled structures prepared by using cationic, anionic and neutral surfactants as structuredirecting agents, have become a prominent research due to their potential applications in fuel cells, solar cells and catalysis.1-3 Thus, SiO2-based mesophase configurations such as MCM-414 (2D hexagonal, p6mm structure), MCM-485 (cubic, Ia3d), SBA-16 (cubic Pm3n), SBA-27 (3D hexagonal, p63/mmc), SBA-158 (p6mm) and SBA-169 (Im3̄m) have been prepared. Interestingly, 3D porous networks with large pores and high surface areas is expected to have advantages over a 2D oriented porous structures as they can present very attractive features as suports or hosts for active species, especially for diffusion of reactants and catalytic transformation of bulky molecules. However, 3D porous materials have not been explored much for catalytic applications. Recently, mesoporous materials and metal nanoparticles combination has been emerged as rapidly growing research theme as the core research in catalysis is concentrated towards the exploration of metal nano-particles. In particular, Palladium nanoparticles have been involved in various processes, such as Suzuki-Miyaura,10 HeckMizoroki,11 Sonogashira,12 Stille,13 Ullmann,14 Fukuyama,15 Negishi16 and Kumada17 coupling reactions using organoboronic acid, alkene, terminal alkyne, organotin compound, arylhalide, organozinc halide, organozinc

compound and Grignard reagent respectively used as nucleophilic partner to form C-C bond. On the other hand synthesis of biarlys through the cleavage of sp2 C−H bond to construct C−C bonds has received great attention. Interestingly, biarlys and their derivatives have potential applications in natural products, polymers, advanced materials, liquid crystals, ligands and molecules of medicinal pyridyl,19 acylamino,20 oxazolyl,21 interest.18 Recently, carboxyl,22 hydroxyl,23 and oxime,24 were commonly employed as directing gropus for the construction of C-C bond via intermolecular arylation to facilitate C–H activation. However, the traditional approach for synthesis of biaryals invovles homogeneous catalysts such as Pd(PPh3)4,25,26 Pd(OTf)2.2H2O,27 Pd2(dba)3,28 [PdCl(-C3H5)]2,25 and transThese Dichloridobis(triisopropylphosphine-κP)palladium(II)25. strategies suffer from the following drawbacks: (1) complicated synthesis procedure and preactivation of coupling partner, (2) moisture sensitivity, toxicity and poor functional group tolerance of organometallic reagents, (3) high-polarity solvents and high temperature required, (4) environmental issues, higher cost and wastage of large quantities of reactants, (5) homogeneous catalysts have separation, regeneration, and reuse problems. In the direction of heterogenization and with the development of more sustainale chemistry, the directing group assisted one step synthesis of biaryls from aromatic compounds has recently encounted great interest. Herein, we demonstrate the first exaple of ortho-arylation of benzamides using aryl iodides with simple CONH2 as a directing group over 3D mesoporous silica with Pd nanoparticles composite. The problems associated with the homogeneous catalysts in coupling reaction are covered using mesoporous silica and Pd nanoparticles composite as green method for organic synthesis have gained siginificant impetus.

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2. EXPERIMENTAL SECTION 2.1 Materails. F127, hydrochoric acid, tetraethyl orthosilicate, palladium acetate and other chemicals and solvents were purchased from Sigma-Aldrich. All the products were purified by using silica gel (60-120 Mesh). 2.2 Synthesis Procedure for PS-3 Materials. In a typical synthesis, 2.9 g of F127 is completely dissolved in 140 g of HCl solution. The solution is stirred for 5 hr and then 14 g of TEOS is added. The mixture was further stirred for 24 hr followed by aging at 100 °C for 24 hr under static condition for hydrothermal treatment. Then, the solution was filtered and dried at 80 °C. Finally, mesoporous silica was obtained after complete removal of the template by calcination at 550 °C for 5 hr in presence of air. The mesoporous silica material (200 mg) and THF solution (25 ml) of Pd (OAc)2 (42.2 mg) are stirred at room temperature for 6 hr. The solid product obtained after complete reomval of the solvent can be further reduced with hydrogen at 200 °C for 4 hr. The final product was denoted as PS-3. The Pd metal content of the catalyst is found to be 8.2%, as measured by ICP-OES analysis. 2.3 Characterization. X-ray diffraction patterns were recorded on Rigaku Ultima – IV with Cu Kα radiation (1.5406A0) at 40KV and 30mA with the scan speed of 10/min. The N2 adsorption/desorption isotherms were performed at 77 K on Quadrasorb SI instrument. Before the analysis, samples were evacuated at 150 °C for 12h. The specific surface areas were calculated using BET method over the relative pressure range of 0.05 to 0.30. Pore sizes was calculated based on BJH method from the desorption branch. The pore volume was taken by a single point method at p/p0 = 0.99. XPS spectras were recorded on a KRATOS AXIS 165 equipped with Mg Kα radiation (1253.6 eV) at 75 W apparatus using Mg Kα anode and a hemi spherical analyser. The FT-IR spectra of all samples recorded on Perkin Elmer-Spectrum GX Spectrometer in the range of 400-4000 cm-1 by using KBr pellets having 1 wt% of the sample. 2.4 Typical Procedure for Synthesis of Biaryals. The ortho-arylation reaction using 0.5 mmol of benzamide, 1 mmol of Iodobenzene and 1.2 mmol of Silver acetate along with 15 mg of PS-3 catalyst was carried out under reflux conditions of solvent at optimum time duration so as to get the maximum yield of Biphenyl-2-carboxamide and their derivatives. After completion of the reaction (as monitored by the thin layer chromatographic studies), the reaction mixture was diluted with 50 mL of distilled water further the catalyst was filtered, washed with ethylacetate and compound extracted with ethylacetate. The resulted obtained product after concentration under reduced pressure was purified by column chromatography on silica gel. The products were identified by 1H NMR, 13C NMR and high resolution mass spectral analysis (HRMS). 2.5 Analytical Data. [1,1'-biphenyl]-2-carboxamide (Table 2, 3a); White solid; 1H NMR (300 MHz, CDCl3) δ 7.80 (d, J = 6.4 Hz, 1H), 7.54-7.49(m, 1H), 7.47−7.40 (m, 6H), 7.37 (d, J = 7.55 Hz, 1H), 5.49 (1H, brs), 5.24 (1H, brs). 13C NMR (75 MHz, CDCl3) δ 171.35, 139.80, 131.92, 130.50, 130.37, 129.00, 128.73, 128.66, 127.89, 127.57, 127.29; ESI-MS: (M+H) + = 198; HRMS (ESI) calculated for C13H12NO (M+H) + :198.09134; found: 198.09073.

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4'-methyl-[1,1'-biphenyl]-2-carboxamide (Table2, 3b); White solid; 1H NMR (500 MHz, CDCl3) δ 7.81−7.78 (m, 1H), 7.49 (td, J = 7.5, 1.5 Hz, 1H), 7.41 (td, J = 7.5, 1.5 Hz, 1H), 7.36−7.33 (m, 3H), 7.26−7.23 (m, 2H), 5.55 (brs, 1H), 5.27 (brs,1H), 2.40 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 171.49, 139.81,137.75, 137.16, 134.18, 130.47, 130.37, 129.38, 129.03, 128.61, 127.35, 21.16; ESI-MS: (M+H) + = 212; HRMS (ESI) calculated for C14H14NO (M+H)+: 212.10699; found: 212.10642. 4'-methoxy-[1,1'-biphenyl]-2-carboxamide (Table2, 3c); White solid; 1H NMR (300 MHz, CDCl3) δ 7.82-7.79 (m, 1H), 7.50-7.45 (m, 2H), 7.43-7.38 (m, 2H), 7.37-7.35 (m, 1H), 6.97 (m, J = 8.4, 2H), 5.64 (brs, 1H), 5.30 (brs, 1H), 3.85 (s, 3H); 13 C NMR (100 MHz, CDCl3) δ 171.57, 159.44, 139.46, 134.19, 132.36, 130.49, 130.36, 129.92, 129.04, 127.29, 114.11, 55.27; ESI-MS: (M+H)+ = 228; HRMS (ESI) calculated for C14H14NO2 (M+H)+: 228.10191; found: 228.10266. 4'-acetyl-[1,1'-biphenyl]-2-carboxamide (Table2, 3d); Yellow solid; 1H NMR (500 MHz, CDCl3) δ 8.02 (d, J = 7.9 Hz, 2H), 7.83−7.71 (m, 1H), 7.63−7.51 (m, 3H), 7.50−7.43 (m, 1H), 7.39 (d, J = 7.5 Hz, 1H), 5.64 (brs, 1H), 5.37 (brs, 1H), 2.64 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 197.68, 171.38, 144.93, 138.67, 136.10, 134.72, 130.49, 130.18, 128.85, 128.49, 128.33, 128.10, 26.58; ESI-MS: (M+H)+ = 240; HRMS (ESI) calculated for C15H14NO2 (M+H)+: 240.10191; found: 240.10110. 3'-methyl-[1,1'-biphenyl]-2-carboxamide (Table2, 3e); Brown solid; 1H NMR (500 MHz, CDCl3) δ 7.82−7.77 (m, 1H), 7.57−7.37 (m, 3H), 7.37-7.28 (m, 2H), 7.25−7.18 (m, 2H), 5.85 (brs, 1H), 5.31 (brs,1H), 2.39 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 171.48, 138.30,134.12, 131.85, 130.41, 130.34, 129.35, 128.98, 128.62, 128.50, 127.43, 127.27, 125.78, 21.37; ESI-MS: (M+H)+ = 212; HRMS (ESI) calculated for C14H14NO (M+H)+: 212.10699; found: 212.10645. 5-bromo-[1,1'-biphenyl]-2-carboxamide (Table2, 3f); White solid; 1H NMR (300 MHz, CDCl3) δ 7.69 (d, J = 8.3 Hz, 1H), 7.57 (dd, J = 8.3, 1.5 Hz, 1H), 7.53 (d, J = 2.2 Hz, 1H), 7.46−7.41 (m, 5H), 5.48 (brs, 1H), 5.19 (brs, 1H); 13C NMR (75 MHz, CDCl3) δ 170.15, 141.67, 138.73, 133.20, 132.98, 130.79, 130.70, 128.85, 128.63, 128.49, 124.83; ESIMS: (M+H)+ = 276; HRMS (ESI) calculated for C13H11NO Br (M+H)+: 276.00185; found: 276.00275. 3-chloro-[1,1'-biphenyl]-2-carboxamide (Table2, 3g); 1H NMR (500 MHz, CDCl3) δ 7.51−7.46 (m, 2H), 7.44−7.36 (m, 5H), 7.31−7.28 (m, 1H), 5.62 (brs, 1H), 5.40 (brs, 1H); 13C NMR (100 MHz, CDCl3) δ 168.85, 141.13, 138.98, 134.95, 131.10, 130.04, 128.48, 128.40, 128.04 ; ESI-MS: (M+H) + = 232; HRMS (ESI) calculated for C13H11NOCl (M+H)+: 232.05237; found: 232.05186. 5-methyl-[1,1'-biphenyl]-2-carboxamide (Table2, 3h); White solid; mp 143−144 °C; 1H NMR (500 MHz, CDCl3) δ 7.73 (d, J = 7.5 Hz, 1H), 7.46−7.40 (m, 4H), 7.32−7.21 (m, 2H), 7.16 (s, 1H), 5.62 (brs, 1H), 5.22 (brs, 1H), 2.42 (s, 3H); 13 C NMR (75 MHz, CDCl3) δ 171.36, 140.76, 140.31, 139.90, 131.26, 131.09, 129.26, 128.70, 128.59, 128.24,127.80, 21.27; ESI-MS: (M+H) + = 212; HRMS (ESI) calculated for C14H14NO (M+H)+: 212.10699; found: 212.10623.


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(a)

(b) Pd(111)

Intensity (a.u.)

Intensity (a.u.)

PS-3 MPS

2 (c)

4

6

Amorphous silica

Pd(200) Pd(220)

20

8

2θθ (degree) (

40

60

80

2θθ (degree) (

(d)

D v(d )/ cm 3 .(g.nm ) -1 )

0.10

Amount Adsorbed (cm 3.g-1)

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


PS-3 MPS

0.0

0.2

0.4

0.6

0.8

PS-3 MPS

0.08 0.06 0.04 0.02

1.0

Relative Pressure (P/P0)

0.00 2

4

6

8

10

Pore Diameter (nm) Figure 1. (a) Powder XRD patterns of MPS and PS-3 materials; (b) wide-angle XRD patterns; (c) Nitrogen adsorption-desorption isotherms of MPS and PS-3 materials, and (d) the corresponding pore size distribution. .

4-methoxy-[1,1'-biphenyl]-2-carboxamide (Table2, 3i); White solid; mp 161−162 °C; 1H NMR (300 MHz, CDCl3) δ 7.43−7.36 (m, 5H), 7.34 (d, J = 3.0 Hz, 1H), 7.29 (s, 1H), 7.05 (dd, J = 8.3, 2.2 Hz, 1H), 5.58 (brs, 1H), 5.26 (brs, 1H), 3.88 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 171.03, 158.94, 139.86, 135.14, 132.26, 131.68, 128.86, 128.63, 127.56, 117.10, 113.45, 55.52; ESI-MS: (M+H)+ = 228; HRMS (ESI) calculated for C14H14NO2 (M+H)+: 228.10191; found: 228.10133. 3',4-dimethoxy-[1,1'-biphenyl]-2-carboxamide (Table2, 3j); White solid; 1H NMR (300 MHz, CDCl3) δ 7.35 (s, 1H), .34−7.32 (m, 2H), 7.30−7.28 (m, 1H), 7.04 (dd, J= 8.4, 2.8 Hz, 1H), 6.97−6.88 (m, 2H), 5.57 (brs, 1H), 5.31 (brs,1H), 3.88 (s, 3H), 3.83 (s, 3H); 13C NMR (75 MHz, CDCl3)δ 171.11, 159.77, 159.01, 135.18, 131.54,129.52, 121.25, 119.17 , 118.17, 116.96, 114.37, 112.54, 55.50, 55.22; ESI-MS: (M+H)+ = 258; HRMS (ESI) calculated for C15H16NO3 (M+H)+: 258.11247; found: 258.11225.

4',5-dimethyl-[1,1'-biphenyl]-2-carboxamide (Table2, 3k); White solid; 1H NMR (300 MHz, CDCl3) δ 7.73 (d, J = 7.5 Hz, 1H), 7.32 (d, J = 8.3 Hz, 2H), 7.26−7.20 (m, 3H), 7.14 (m, 1H), 5.55 (brs, 1H), 5.24 (brs, 1H), 2.41 (s, 3H), 2.40 (s, 3H); 13 C NMR (75 MHz, CDCl3) δ 171.29, 140.75, 139.91, 137.67, 137.38, 131.24, 131.11,129.34, 128.63, 128.07, 21.29, 21.16; ESI-MS: (M+H)+ = 226; HRMS (ESI) calculated for C15H16NO (M+H)+: 226.12264; found: 226.12320. 4-methoxy-4'-methyl-[1,1'-biphenyl]-2-carboxamide (Table2, 3l); White solid; 1H NMR (500 MHz, CDCl3) δ 7.34 (d, J = 2.7 Hz, 1H), 7.32−7.28 (m, 2H), 7.26−7.24 (m, 1H), 7.23−7.20 (m, 2H), 7.03 (dd, J = 8.5, 2.7 Hz, 1H), 5.50 (brs, 1H), 5.26 (brs, 1H), 3.87 (s, 3H), 2.39 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 171.04, 158.84, 137.42, 136.94, 134.96, 132.33, 131.68, 129.36, 128.76, 117.16, 113.49, 55.52, 21.13; ESI-MS: (M+H)+ = 242; HRMS (ESI) calculated for C15H16NO2 (M+H)+: 242.11756; found: 242.11696. 5-chloro-[1,1'-biphenyl]-2-carboxamide (Table2, 3m); White solid; 1H NMR (500 MHz, CDCl3) δ 7.76 (d, J = 8.2



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Hz, 1H), 7.47−7.39 (m, 6H), 7.36 (d, J = 1.9 Hz, 1H), 5.54 (brs, 1H), 5.19 (brs, 1H); 13C NMR (75 MHz, CDCl3) δ 170.15, 141.53, 138.82, 136.43, 132.53, 130.64, 130.29, 128.82, 128.59, 128.45, 127.70; ESI-MS: (M+H) + = 232; HRMS (ESI) calculated for C13H11ONCl (M+H)+: 232.05237; found: 232.05196. 4-methoxy-3'-methyl-[1,1'-biphenyl]-2-carboxamide (Table 2, 3n); White solid; 1H NMR (500 MHz, CDCl3) δ 7.36 (d, J = 2.7 Hz, 1H), 7.32−7.28 (m, 1H), 7.27−7.25 (m, 1H), 7.23−7.17 (m, 3H), 7.04 (dd, J = 8.3, 2.7 Hz, 1H), 5.52 (brs, 1H), 5.26 (brs, 1H), 3.88 (s, 3H), 2.39 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 171.03, 158.90, 139.89, 138.32, 134.94, 132.50, 131.66, 129.56, 128.53, 128.34, 125.97, 117.11, 113.50, 55.50, 21.38; ESI-MS: (M+H) + = 242; HRMS (ESI) calculated for C15H16NO2 (M+H)+: 242.11756; found: 242.11697. 3',5-dimethyl-[1,1'-biphenyl]-2-carboxamide (Table 2, 3o); White solid; 1H NMR (500 MHz, CDCl3) δ 7.75 (d, J = 7.9 Hz, 1H), 7.31 (t, J = 7.4 Hz, 1H), 7.25−7.19 (m, 4H), 7.157.14 (m, 1H), 5.45 (brs, 1H), 5.22 (brs, 1H), 2.41 (s, 3H), 2.40 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 171.15, 140.73, 140.36, 140.07, 138.30, 131.10, 131.07, 129.38, 128.58, 128.50, 128.17, 125.82, 21.38, 21.26; ESI-MS: (M+Na)+ = 248; HRMS (ESI) calculated for C15H16NO (M+H)+: 226.12264; found: 226.12211. 3. RESULTS AND DISCUSSION 3.1 Characterization of PS-3 catalyst. The structural characterization by X-ray diffraction of PS-3 catalyst together with mesoporous silica (MPS) support is shown in Figure 1a, which governs the order of mesoporosity and lattice parameter values. The Figure 1a illustrates well resolved characteristic planes of (111), (200) and (220) signifying the 3D facecentered cubic Fm3n symmetry. In addition, the intensity of PS-3 material is well maintained even after dispersion of palladium nanoparticles reveals that high degree of mesostructural regularity. Interestingly, the diffraction peak shifts to lower 2θ values indicates that the unit cell parameter (a0) of 17.3 nm for MPS material decreases to 16.8 nm for PS3 material. The results confirm that the contraction in unit cell size is pronounced from the reduction in pore diameter of PS-3 material. The wide-angle XRD pattern of PS-3 material (Figure 1b) showed the appearance of metallic palladium crystallites, which is evidenced by the presence of the Pd(111) and (200) reflections at 40.20 and 46.50 respectively. In general, PdO possesses five distinctive XRD diffraction peaks at 2θ = 33.8, 42.0, 54.8, 60.7 and 71.4, corresponding to (101), (110), (112), (103) and (211), planes respectively (JCPDS file no: 41-1107). We surmise that the two relatively low intense wide angle XRD peaks of PS-3 material are due to unexposed minor amount of PdO remained after reductive treatment at 240 °C for 4 hrs. According to Scherrer formula, the average palladium crystallite size is estimated to be 7.4 nm. To assess the porosity, pore network structure and texture of PS-3 material, systematic sorption studies are performed. Nitrogen adsoption-desorption isotherm of PS-3 and NPS materials demonstrate type IV isotherm with H2 type hysterisis (Figure 1c). The steep capically condensation step in these materials is indicative of nitrogen condensation within mesopores and high

Figure 2. (a) X-ray photoelectron Survey spectrum of PS-3 catalyst (b) High resolution X-ray photoelectron spectroscopy (XPS) scan of Pd 3d bands.

Figure 3. HRTEM image of PS-3 catalyst.

ordering of mesopores even after dispersion of palladium nanoparticles. The specific surface area is calcualted according to the Brunauer-Emmett-Teller method and pore volume obtained from from the total nitrogen volume adsorbed are 612 m2 g-1 and 0.38 cm3 g-1 for MPS material, which changes to 574 m2 g-1 and 0.31 cm3 g-1 for PS-3 materail. Interstingly


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Transmittance (%)

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(b)

Si-O-H

(a)

Si-O-Si 3450 1650

4000

3000

1085

2000

960 800 465

1000 -1

Wavenumber (cm ) Figure 4. FT-IR spectra of (a) MPS, and (b) PS-3 materials.

Table 1. Optimization of reaction conditions for the orthoarylation of benzamide reaction catalyzed by silica-palladium catalysta entry

additive

1 2 3 4 5 6 7 8 9b 10c

AgOAc AgOAc AgOAc AgOAc AgOAc AgNO3 Ag2SO4 Cu(OAc)2 AgOAc AgOAc

solvent Dioxane DCE DMF Toluene AcOH AcOH AcOH AcOH AcOH AcOH

yield(%) NR NR trace trace 68 NR NR NR 42 NR

a

Unless specified, all reactions were carried out with 0.5 mmol of benzamide, 1.0 mmol of Iodobenzene, 15 mg of PS-3 catalyst (2.3 mol% of palladium) and 1.2 mmol of additive were stirred in the given solvent(4 mL) at its reflux temperature for 15 h. b reaction refluxed for 36 h. c reaction carried out without PS-3 catalyst. NR: No Reaction.

the capillary condensation step of MPS material shifts to lower relative pressure for PS-3, indicating that the pore size decreases from 4.4 to 4.1 nm upon dispersion of palladium nanoparticles (Figure 1d). The above results demonstrate that systematic decrease in textural properties of MPS material is more pronounce the dispersion of nanoparticle active phases on high surface and large pore size support. XPS spectra of PS-3 material demonstrates the evidence of two palladium metallic species with binding energies of two prominent bands at 335.4 eV and 341.0 eV, which can readily be assigned to Pd3d5/2 and Pd3d3/2 bands,29 respectively (Figure 2). In general, the samples exposed to the atmosphere have a detectable quantity of adventitious carbon contamination, which may be removed by argon sputtering. Moreover, it is

generally believed that hydrocarbons are commonly encountered impurities in XPS system. This adventitious carbon contamination is commonly used as a charge reference for XPS spectra.30c Further, to investigate palladium nanoparticles dispesion in mesochannels, PS-3 material is examined by high resolution transimission electorn microscopy (HR-TEM). Figure 3 shows ordered mesoporosity with linear array of pores and pore channels is confirmed. The palldium nanoparticles are well dispersed on mesoporous matrix with particle size of 7-8 nm., which confirms that efficient formation of nanoparticles on mesoporous channels. The HRTEM observations are in good agreement with aforementioned nitrogen-sorption and XRD results. To determine the progressive consumption of surface silonol groups after dispersion of palladium nanoparticles, FTIR was used to monior the coverage of silica surface. Figure 4 shows FTIR spectra of MPS and PS-3 materials with bands mainly in two regions (a) the bands below 1300 cm-1, (b) a stong broad band around 3450 cm-1. The three bands centered at 465 cm-1, 800 cm-1, and 1085 cm-1 in both the materials are attributed to the rocking, bending and asymmetric streching virations of the intertetrahedral oxygen atoms in the SiO2 structure respectively. In addition the band at 960 cm-1 correspond to SiO-H teriminal streching of silonol groups. It is found that the ordering of silicate framework at pore surface is resulted at IR band of 1084 cm-1 with shoulder peak between 1100-1300 cm-1 are correspond to Si-O-Si strenching vibration. In particular, the intensity of weak IR band at 1650 cm-1 and the broad band at 3450 cm-1 can be attributed to physically adsorbed water bending and water hydroxyl streching vibrations respectively. The reduction in IR bands of PS-3 material at 960 cm-1 and 3450 cm-1 is implying that the Si-OH groups are consumed, which clearly signify the surface modification of MPS material after dispersion of palladium nanoparticles.30 3.2. Catalytic studies. In order to heterogenize the orthoarylation of benzamides, PS-3 catalyst has been effectively used with benzamide (0.5 mmol) and iodobenzene (1.0 mmol) as model substrates under reflux conditions of different solvents for 15 h. The obtained results are summarized in Table 1. Surprisingly, no product is formed when 1,4-dioxane, and dichloroethane (DCE) solvents are used (Table 1, entries 1 and 2). On the other hand dimethylformamide (DMF), and toluene are employed as solovent for the same reaction showed poor yield (Table 1, entries 3 and 4). Among the solvents tested acetic acid (AcOH) was found to be the best with 68% yield of corresponding [1,1'-biphenyl]-2carboxamide (Table 1, entry 5) over PS-3 catalyst. Thus, AcOH is used as a solvent for further opimization of reaction conditions. In addition, the effect of additives such as AgNO3, Ag2SO4, Cu(OAc)2 and AgOAc is investigated to obtain high yield of carboxamide from ortho-arylation of benzamides. When AgNO3, Ag2SO4, Cu(OAc)2 additives are used, the reaction did not proceed (Table 1, entry 5). Remarkably, prolonged reaction time produced unwanted byproducts, thereby decreasing the product yield (Table 1, entry 9). However, AgOAc has shown high yield of carboxamide under optimized reaction conditions over PS-3 (Table 1, entry 10). It is worth while to note that, the reaction resulted without any product when the reaction is carried out without PS-3 catalyst, suggests the role of catalyst (Table 1, entry 10). The above



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Table 2: One step synthesis of biaryls through ortho-arylation of benzamides over PS-3a catalyst

O

O

NH2

NH2 PS-3 catalyst

H R

a

1

I 2

R1

AgOAc, AcOH, 120 0C 15 - 30 hrs

R

3

R1

Reaction conditions: benzamide (0.5 mmol), iodobenzene(1 mmol), PS-3 catalyst (15mg), AgOAc ( 0.75 mmol) and Acetic acid (5 mL) at 120 °C.


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results suggests that the PS-3 is an efficient catalyst for orthoarylation of benzamides using AcOH as a solvent and AgOAc as an additive. To understand the maximum amount of catalyst needed to get the high yield of the carboxiamido product, different amounts of PS-3 catalyst was investigated as shown in Figure 5. Interesting to note that the yield of the reaction directly selected 15 mg of PS-3 catalyst as standard amount for the ortho-arylation of benzamide. With the optimized reaction conditions in hand, PS-3 catalytic system was employed to study the reactivity of different benzamides with aryliodides as coupling partners for the synthesis of functionalized biphenyl2-carboxamides. The catalytic system is very versatile in nature and is applicable to wide variety of benzamides as both electron rich and electron deficient aromatic benzamides reacted smoothly to furnish the corresponding products in moderate to good yields (Table 2). The reaction of benzamide with aryl iodides containing electron donating groups such as –CH3, –OCH3 gave good yields of the respective products irrespective of their orientation on the phenyl ring (Table 2, 3b, 3c & 3e). On the other hand there was significant decrease in the product yield when electron deficient substrate like 1(4-iodophenyl)ethanone was reacted with benzamide (Table 2, 3d). From the above results, it is clear that the catalyst system employed for the ortho-arylation of benzamides works well with electron rich aryl iodides. In a similar way, substituted benzamides having electron donating groups like methyl and methoxy also reacted smoothly with iodobenzene to afford the corresponding products in good yields (Table 2, 3h & 3i). Notably, halosubstituted benzamides like 4-bromobenzamide and 4-chlorobenzamide tolerated the reaction conditions to afford the corresponding products in moderate yields (Table 2, 3f & 3m). It is quite interesting to note that PS-3 catalytic system was also effective for sterically hindered ortho substrates like o-chlorobenzamide (Table 2; 3g). Significantly, the reaction of electron donating aryliodides with electron donating benzamides also proceeded well. As shown in Table 2, methoxybenzamide reacted with 1-iodo-3methoxybenzene to furnish the 76% of the desired product (Table 2, 3j). Similarly, when 4-methylbenzamide reacted with 1-iodo-4-methylbenzene to produce the desired product in good yields (Table 2, 3k). The reaction of 3methoxybenzamide with 1-iodo-4-methylbenzene and 1-iodo3-methylbenzene afforded around 65% yields (Table 2, 3l and 3n). Moderate yield is obtained in case of the reaction of 4methylbenzamide with 1-iodo-3-methylbenzene (Table 2, 3o). The above substituent effects clearly disclose the electrophilic nature for the C−H activation process, as substrates with electrondonating groups generally gave higher yields when compared to that of with electron-withdrawing groups. Notably, the traditional Pd-catalyzed arylation of benzamides with aryl iodides produces a small amount of diarylated benzamides as byproducts in most cases. However, the selective monoarylation of primary benzamides is challenging except for sterically demanding meta-substituted benzamides.31 It is noteworthy to mention that the yields of biaryls from orthoaryaltion of benzamides using homogeneous Pd(OAc)2 catalysts32 are comparable with the results obtained over PS-3 catalysts. The above results demonstrates that PS-3 catalytic system offered predominantly monoarylation

Figure 5. Effect of PS-3 catalyst amount on the yield of carboxmide (reactions were carried out using benzamide (0.5 mmol), Iodo benzene(1 mmol), AgOAc (0.75 mmol), PS-3 catalyst and Acetic acid (5 ml).

Figure 6. Recyclability of the PS-3 catalyst.

Scheme 1. Plausible Mechanism Involved in Directed Arylation of Benzamide with AgOAc/ArI over PS-3 Catalyst.



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Figure 7. Structural characterization of recycled PS-3 catalyst: (a) XRD pattern, (b) XPS spectra, and (c) HRTEM image. opportunity for practical application of highly reusable products, and also curbed the formation of diaryl product. Leaching of active palladium species was investigated by filtering the reaction mixture after reaction carried out using PS-3 catalyst. The analysis of the metal content was investigated using ICP analysis, the absence of the palladium metal ion in the solution phase indicated that no leaching of palladium metal from PS-3 catalyst occurred during reaction. To demonstrate the reusability of PS-3 catalyst for the ortho-arylation, the reaction was performed using benzamide and iodobenzene under standard conditions. The spent catalyst was recovered from the reaction mixture by simple filtration. It was washed, air dried and used directly for thenext cycle. It is noteworthy to mention that the catalyst showed consistent activity and selectivity up to 5 cycles (Figure 6). The plausible reaction pathway for the arylation of benzamide is depicted in Scheme 1. Nitrogen in amide functional group is completely non-basic and does not coordinate to metal ions unless it is deprotonated. The strong electron withdrawing nature of the carbonyl group by resonance allows for delocalization of the non-bonding electrons of nitrogen as shown below. So, in amides coordination occurs through the carbonyl oxygen of the amide group. The proposed catalytic cycle involves the following steps: (i) cyclopalladation of benzamide via C-H activation, (ii) oxidative addition of aryl iodide to cyclopalladated intermediate, (iii) reductive elimination affording monoarylated product and PS-3 catalyst. The XRD pattern, XPS spectrum and HR-TEM image of recycled PS-3 catalyst are shown in Figure 7. The wide-angle XRD pattern reveals that metallic palladium active sites are retained even after several cycles of the reaction (Figure 7a). The XPS spectra of recycled PS-3 is shown in Figure 7b demonstrates palladium metallic species at similar binding energies of fresh PS-3 catalyst, which are correspond to the 3d3/2, and 3d5/2 level of Pd(0). In addition, the ordered mesoporous structure with high dispersion of palladium particles without much aggregation of large particles for recycled PS-3 catalyst is confirmed by HRTEM (Figure 7c). The above results clearly evident that the PS-3 catalyst is highly stable and the catalyst structure did not alter even after the reaction. Therefore, the present strategy offers a good

palladium-based catalysts in many coupling reactions. 4. CONCLUSIONS In summary, we have described the ortho-arylation of benzamides with aryl iodides for synthesis of biaryls under ligand-free conditions by using high surface area novel heterogeneous PS-3 catalyst. Furthermore, the catalyst was succefully employed to a wide range of benzamides and aryl iodides with electron-donating and electron-withdrawing groups. The PS-3 catalyst is easy-to-handle and can be recovered and reused, thus making this procedure more environmentally acceptable. The methodology may find widespread use in organic synthesis for the ortho-arylation of benzamides to prepare biphenyl-2-carboxamides. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT T.P.R thanks the U.G.C, India for the award of a research fellowship

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Abstract The First Example of Ortho-Arylation of Benzamides over Pd /Mesoporous Silica: A Novel Approach for Direct sp2 C−H Bond Activation T. Parsharamulu,† D. Venkanna,† M. Lakshmi Kantam,† Suresh K. Bhargava,‡ and Pavuluri Srinivasu *,†

50 nm

Three-dimensional mesoporous silica with highly dispersed palladium nanoparticles composite (PS-3) catalyst has been prepared and characterized by XRD, XPS, FT-IR, nitrogen sorption and HR-TEM. The PS-3 catalyst is utilized for the first time as efficient and reusable catalyst in the ortho-arylation of benzamides by aryl iodides has been demonstrated with the simplest amide CONH2 as a directing group. It can be successfully applied to various benzamides and aryl iodides with both electron donating and withdrawing groups. The catalyst can be simply recovered and reused several times without significant loss in catalytic activity.


The First Example of ortho-Arylation of Benzamides over Pd

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