Phosphine-Free Palladium-Catalyzed Direct C-3 Arylation of 2-Phenylimidazo[1,2-<i>a</i>]pyridine Using Silver(I) Carboxylate

Kona, Sridevi; Ravi, Rama Suresh; Chakravarty, Manab; Chava, Venkata N. R.

doi:https://doi.org/10.1155/2013/305934

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Research Article | Open Access

Volume 2013 | Article ID 305934 | https://doi.org/10.1155/2013/305934

Phosphine-Free Palladium-Catalyzed Direct C-3 Arylation of 2-Phenylimidazo[1,2-a]pyridine Using Silver(I) Carboxylate

Sridevi Kona,¹Rama Suresh Ravi,¹Manab Chakravarty,²and Venkata N. R. Chava³

Academic Editor: Samir Messaoudi

Received24 Jun 2012

Accepted05 Aug 2012

Published16 Sept 2012

Abstract

Phosphine-free palladium-catalyzed direct arylation of 2-phenyl-imidazo[1,2-a]pyridine has been developed with the concept of using silver(I) carboxylate. This protocol efficiently catalyzes the C-H arylation of 2-phenyl-imidazo[1,2-a]pyridine with aryl iodides to afford the corresponding 2-phenyl-3-aryl-imidazo[1,2-a]pyridines in moderate to-good yields.

1. Introduction

Syntheses of several heterocycles via palladium-catalyzed direct C-H bond activation using aryl halides are of great importance in recent years along with time-honored coupling reactions where expensive and toxic phosphines are widely used [1–6]. Hence, there are significant efforts to perform reactions without using any phosphines for the same reasons. In this aspect, phosphine-free palladium-catalyzed synthesis of free (N-H) as well as substituted indole and pyrroles are reported in the literature [7–9]. In 2008, Igor Larosa reported the intrinsic idea of using the several different types of Ag(I) carboxylate salts (generated in situ by the combination of Ag₂O/differently substituted benzoic acids) along with Pd(OAc)₂ to undergo C-H bond activation under very mild conditions to afford C-2 arylated indoles with different functionalities [9]. This paper prompted us to use the same approach for the highly demanding arylation for the C3 position of imidazo[1,2-a]pyridine, a heterocycle with outstanding biological activities [10–15]. The most well known drugs like zolimidine (A, antiulcer), alpidem (B, anxiolytic), and zolpidem (C, hypnotic) [14] (Figure 1) also bear this imidazopyridine skeleton.

It is to be noted that, in 2007, the research group of Dalibor reported the phosphine-free Pd(OAc)₂ catalyzed direct C-arylation of free (–NH) indoles and pyrroles in the presence of CsOAC [8]. Recently there are few reports on the Pd-catalyzed direct C-3 arylation of imidazopyridines using phosphines as ligands [16–19] along with the traditional coupling reactions where the starting material should be 3-haloimidazopyridines [20–23]. During the preparation of our paper, Fu and collaborators published the phosphine-free Pd-catalyzed direct arylation of imidazo [1,2-a]pyridine using KOAc as base [24]. However, prior to the work published by Fu et al. to the best of our knowledge, there was no report on the phosphine-free direct arylation of pyridine using palladium as catalyst. In this present work, we describe a new convenient methodology for the phosphine-free direct C3 alkylation of 2-phenylimidazopyridine by the reaction of aryl iodides in the presence of Pd(OAc)₂ as catalyst with silver(I) carboxylate which is assumed to increase the rate of the palladation step in the catalytic cycle along with its basic nature needed at the reductive elimination step [9]. This methodology also offer a new route for the direct arylation of 2-phenylimidazopyridine with aryl iodides.

2. Experimental

2.1. General

All the chemicals used were purchased from Aldrich Chemical Co. and were used without further purification. Freshly distilled solvents were used. For TLC, aluminum plates coated with silica gel containing F254 indicator were used and the spots were visualized by UV light and/or by exposing to iodine. Column chromatography was performed on silica gel 100–200 mesh, using EtOAc and hexanes mixture as eluent. The ¹H and ¹³C NMR spectra were recorded using 5 mm tubes on a Bruker 500 MHz NMR spectrometer (field strengths: 500 and 125 MHz, resp.,) or 400 MHz NMR spectrometer (field strengths: 400 and 100 MHz resp.,) in CDCl₃ solution (unless specified otherwise) with shifts referenced to SiMe₄ (¹H, ¹³C: ). All J values were in Hz. IR spectra were recorded on a JASCO FT/IR 5300 spectrometer. Elemental (C, H, N) analysis was done using Perkin-Elmer 240C CHN FLASH EA analyzer. Melting points were determined by using a SUPERFIT hot-stage melting point apparatus and are uncorrected.

2.2. General Procedure for the Synthesis of 2-Phenylimidazo[1,2-a]pyridine(1)

To the solution of 2-aminopyridine (5 mmol) in ethanol (30 mL), 2-bromo acetophenone (5 mmol) was added at room temperature and the reaction mixture was heated under reflux for 3 h. The resulting white solid was filtered off, dried, and used without further purification [25, 26].

2.3. General Procedure for the Direct Arylation of 2-Phenylimidazo[1,2-a]pyridine

A mixture of 2-phenylimidazopyridine (194 mg, 1.0 mmol), iodobenzene (224 µL, 2.0 mmol), Pd(OAc)₂ (11.2 mg, 5 mol%), Ag₂O (174 mg, 0.75 mmol), and 2-nitrobenzoic acid (251 mg, 1.5 mmol) was stirred at 120°C for 12 h under nitrogen in DMF. The reaction mixture was filtered through a plug of silica gel and then evaporated to dryness under reduced pressure. The crude product was purified by column chromatography (Hexane : EtOAc 80 : 20) to afford 2,3-diphenylimidazopyridine. For the synthesis of compounds 3b-k: similar molar quantities of the respective aryl iodides were used with 1.0 mmol of 2-phenylimidazo[pyridine.

2,3-Diphenylimidazo[1, 2-a]pyridine ( 3a). Colorless solid, Yield 82%, 0.22 g, Mp 150–152°C; ¹H NMR (400 MHz, CDCl₃): 6.75 (t, ³J(H-H) = 6.4 Hz, 1H, Ar-H), 7.20–7.31 (m, 4H, Ar-H), 7.46–7.56 (m, 5H, Ar-H), 7.68–7.71 (m, 3H, Ar-H), 7.97 (d, ³J(H-H) = 6.8 Hz, 1H, Ar-H). ¹³C NMR (100 MHz, CDCl₃): . 112.4, 117.5, 120.0, 123.3, 124.8, 127.6, 128.1, 128.3, 129.0, 129.6, 129.8, 130.8, 134.0, 137.4, 144.7. IR (, cm⁻¹): 2924, 1506, 1346, 754, 698. LC/MS, m/z 271 [M + 1]⁺; Anal. Calcd. for C₁₉H₁₄N₂: C, 84.42; H, 5.22; N, 10.36%. Found: C, 84.31; H, 5.18; N, 10.26%.

2-Phenyl-3-p-tolylimidazo[1, 2-a]pyridine ( 3b). Colorless solid, Yield 81%, 0.23 g, Mp 126–128°C; ¹H NMR (400 MHz, CDCl₃): 2.47 (s, 3H, CH₃), 6.72 (t, ³J(H-H) = 6.0 Hz, 1H, Ar-H), 7.17–7.34 (m, 8H, Ar-H), 7.67–7.71 (m, 3H, Ar-H), 7.95 (d, ³J(H-H) = 6.8 Hz, 1H, Ar-H). ¹³C NMR (100 MHz, CDCl₃): δ_C. 21.4, 112.1, 117.5, 121.1, 123.3, 124.5, 126.8, 127.4, 128.1, 128.3, 130.3, 130.5, 134.3, 138.9, 142.2, 144.8. IR (, cm⁻¹): 2924, 1506, 1346, 754, 698. LC/MS, m/z 285 [M + 1]⁺; Anal. Calcd. for C₂₀H₁₆N₂: C, 84.48; H, 5.67; N, 9.85%. Found: C, 84.36; H, 5.61; N, 9.96%.

2-Phenyl-3-m-tolylimidazo[1, 2-a]pyridine ( 3c). Colorless solid, Yield 78%, 0.22 g, Mp 156-158°C; ¹H NMR (400 MHz, CDCl₃): 2.41 (s, 3H, ), 6.73 (t, ³J(H-H) = 6.4 Hz, 1H, Ar-H), 7.18–7.44 (m, 8H, Ar-H), 7.67–7.71 (m, 3H, Ar-H), 7.95 (d, ³J(H-H) = 6.4 Hz, 1H, Ar-H). ¹³C NMR (100 MHz, CDCl₃): . 21.5, 112.2, 117.5, 121.3, 123.4, 124.6, 127.4, 127.9, 128.0, 128.3, 129.5, 129.7, 129.8, 131.2, 134.3, 139.3, 142.2, 144.7. IR (, cm⁻¹): 2924, 1604, 1505, 1385, 1342, 754, 704. LC/MS, m/z 285 [M + 1]⁺; Anal. Calcd. for C₂₀H₁₆N₂: C, 84.48; H, 5.67; N, 9.85%. Found: C, 84.38; H, 5.63; N, 9.79%. X-ray analysis was done for this sample.

3-(3-Nitrophenyl)-2-Phenylimidazo[1, 2-a]pyridine ( 3d). Yellow colored solid, Yield 90%, 0.28 g, Mp 158–160°C; ¹H NMR (500 MHz, CDCl₃): 6.68 (td, ³J(H-H) = 7.0 Hz, 1.5 Hz, 1H, Ar-H), 7.28–7.33 (m, 4H, Ar-H), 7.60–7.80 (m, 5H, Ar-H), 8.02 (d, ³J(H-H) = 6.5 Hz, 1H, Ar-H), 8.34–8.36 (m, 1H, Ar-H), 8.42 (t, ³J(H-H) = 2.0 Hz, 1H, Ar-H). ¹³C NMR (125 MHz, CDCl₃): δ_C. 112.2, 117.1, 117.8, 121.8, 123.7, 124.6, 127.2, 127.5, 127.6, 130.1, 132.5, 135.8, 143.6, 144.8, 146.5. IR (, cm⁻¹): 2924, 1506, 1346, 754, 698. LC/MS, m/z 316 [M + 1]⁺; Anal. Calcd. for C₁₉H₁₃N₃O₂: C, 72.37; H, 4.16; N, 13.33%. Found: C, 72.45; H, 4.08; N, 13.45%.

1-(4-(2-Phenyllimidazo[1, 2-a]pyridine-3-yl)phenyl)ethanone ( 3e). Colorless solid, Yield 85%, 0.26 g, Mp 122–124°C; ¹H NMR (400 MHz, CDCl₃): 2.67 (s, 3H, CH₃), 6.79 (t, ³J(H-H) = 6.8 Hz, 1H, Ar-H), 7.22–7.30 (m, 4H, Ar-H), 7.56-7.63 (m, 4H, Ar-H), 7.71 (d, ³J(H-H) = 1.6 Hz, 1H, Ar-H), 8.04 (d, ³J(H-H) = 6.8 Hz, 1H, Ar-H), 8.09 (d, ³J(H-H) = 8.0 Hz, 2H, Ar-H). ¹³C NMR (100 MHz, CDCl₃): . 26.7, 112.7, 117.9, 119.9, 123.1, 125.2, 127.9, 128.4, 129.5, 130.6, 133.8, 134.8, 137.0, 143.6, 145.3. IR (, cm⁻¹): 3053, 1684, 1605, 1267, 958, 698 LC/MS, m/z 313 [M + 1]⁺; Calcd. for C₂₁H₁₆N₂O: C, 80.75; H, 5.16; N, 8.97%. Found: C, 80.65; H, 5.21; N, 8.86%.

2-Phenyl-3-(3,4,5-trimethoxyphenyl)imidazo[1, 2-a]pyridine ( 3f). Colorless solid, Yield 86%, 0.31 g, Mp 122–124°C; ¹H NMR (400 MHz, CDCl₃): 3.80 (s, 6H, (OCH₃)₂), 3.97 (s, 3H, OCH₃), 6.66 (s, 2H, Ar-H), 6.75–6.78 (m, 1H, Ar-H), 7.19–7.34 (m, 4H, Ar-H), 7.67–7.75 (m, 3H, Ar-H), 7.98 (dd, ³J(H-H) = 0.8 Hz, 6.8 Hz, 1H, Ar-H). ¹³C NMR (100 MHz, CDCl₃): . 56.3, 61.1, 107.7, 112.3, 117.6, 121.1, 123.5, 124.6, 125.2, 127.5, 127.9, 128.3, 134.1, 138.5, 142.1, 144.7, 154.2. IR (, cm⁻¹): 2930, 1579, 1238, 1120, 736, 698. LC/MS, m/z 361 [M + 1]⁺; Anal. Calcd. for C₂₂H₂₀N₂O₃: C, 73.32; H, 5.59; N, 7.77%. Found: C, 73.45; H, 5.52; N, 7.85%.

2-Phenyl-3-o-tolylimidazo[1, 2-a]pyridine ( 3g). Colorless solid, Yield 51%, 0.14 g, Mp 124–126°C; ¹H NMR (500 MHz, CDCl₃): 2.48 (s, 3H, CH₃), 6.73 (td, ³J(H-H) = 7.0 Hz, 1.0 Hz, 1H, Ar-H), 7.19–7.36 (m, 8H, Ar-H), 7.69–7.72 (m, 3H, Ar-H), 7.96 (d, ³J(H-H) = 7.0 Hz, 1H, Ar-H). ¹³C NMR (100 MHz, CDCl₃): . 21.5, 112.2, 117.5, 121.3, 123.4, 124.7, 127.5, 127.9, 128.0, 128.3, 129.5, 129.7, 131.2, 134.1, 139.3, 142.1, 144.7. IR (, cm⁻¹): 3021, 1523, 1378, 1351, 752, 612. LC/MS, m/z 285 [M + 1]⁺; Anal. Calcd. for C₂₀H₁₆N₂: C, 84.48; H, 5.67; N, 9.85%. Found: C, 84.58; H, 5.61; N, 9.75%.

3-(4-Nitrophenyl)-2-phenylimidazo[1, 2-a]pyridine ( 3h). Yellow colored solid, Yield 89%, 0.28 g, Mp 150–152°C; ¹H NMR (500 MHz, CDCl₃): 6.86–6.88 (m, 1H, Ar-H), 7.28–7.36 (m, 4H, Ar-H), 7.59–7.60 (m, 2H, Ar-H), 7.68 (d, ³J(H-H) = 9.0 Hz, 2H, Ar-H), 7.75 (d, ³J(H-H) = 9.0 Hz, 1H, Ar-H), 8.11 (d, ³J(H-H) = 7.0 Hz, 1H, Ar-H), 8.38 (d, ³J(H-H) = 8.5 Hz, 2H, Ar-H). ¹³C NMR (125 MHz, CDCl₃): . 113.2, 118.1, 118.8, 122.8, 124.7, 125.6, 128.2, 128.5, 128.6, 131.1, 133.5, 136.7, 144.5, 145.7, 147.5. IR (, cm⁻¹): 3063, 1599, 1566, 1105, 854, 736. LC/MS, m/z 316 [M + 1]⁺; Anal. Calcd. for C₁₉H₁₃N₃O₂: C, 72.37; H, 4.16; N, 13.33%. Found: C, 72.34; H, 4.23; N, 13.21%.

3-(3,5-Dimethylphenyl)-2-diphenylimidazo[1, 2-a]pyridine ( 3i). Colorless solid, Yield 84%, 0.25 g, Mp 158–160°C; ¹H NMR (400 MHz, CDCl₃): 2.39 (s, 6H, )₂), 6.74 (td, ³J(H-H) = 6.4 Hz, 1.2 Hz, 1H, Ar-H), 7.08 (s, 2H, Ar-H), 7.14 (s, 1H, Ar-H), 7.18–7.33 (m, 4H, Ar-H), 7.67–7.74 (m, 3H, Ar-H), 7.94 (d, ³J(H-H) = 6.8 Hz, 1H, Ar-H). ¹³C NMR (100 MHz, CDCl₃): . 21.4, 112.1, 117.4, 121.4, 123.5, 124.5, 127.4, 127.9, 128.2, 128.3, 129.7, 130.7, 134.3, 139.2, 142.0, 144.7. IR (, cm⁻¹): 2920, 1601, 1444, 1342, 752, 702. LC/MS, m/z 299 [M + 1]⁺; Anal. Calcd. for C₂₁H₁₈N₂: C, 84.53; H, 6.08; N, 9.39%. Found: C, 84.45; H, 6.15; N, 9.45%.

3-(2-Nitrophenyl)-2-phenylimidazo[1, 2-a]pyridine ( 3j). Yellow colored solid, Yield 76%, 0.24 g, Mp 154–156°C; NMR (400 MHz, ): 6.83–6.87 (m, 1H, Ar-H), 7.27–7.35 (m, 4H, Ar-H), 7.56–7.67 (m, 4H, Ar-H), 7.73(d, = 8.8 Hz, 1H, Ar-H), 8.09 (d, = 6.8 Hz, 1H, Ar-H), 8.38 (d, = 8.5 Hz, 2H, Ar-H). NMR (100 MHz, ): δC. 113.2, 118.1, 118.8, 122.8, 124.7, 125.6, 126.1, 128.2, 128.5, 128.6, 131.0, 133.4, 136.7, 139.4, 145.7, 147.5. IR (, cm⁻¹): 2924, 1528, 1352, 851, 750. LC/MS, m/z 316 ; Anal. Calcd. for : C,72.37; H, 4.16; N, 13.33%. Found: C, 72.41; H, 4.26; N, 13.26%.

2-Phenyl-3-(thiophen-2-yl)imidazo[1, 2-a]pyridine ( 3k). Yellow colored solid, Yield 66%, 0.18 g, Mp 118–120°C; ¹H NMR (400 MHz, CDCl₃): 6.80 (d, ³J(H-H) = 6.8 Hz, 1H, Ar-H), 7.23–7.33 (m, 6H, Ar-H), 7.60 (d, ³J(H-H) = 4.8 Hz, 1H, Ar-H), 7.68–7.76 (m, 2H, Ar-H), 7.83(d, ³J(H-H) = 7.2 Hz, 1H, Ar-H), 8.00 (d, ³J(H-H) = 6.8 Hz, 1H, Ar-H). ¹³C NMR (100 MHz, CDCl₃): δ_C 112.6, 113.4, 117.4, 123.9, 124.3, 125.3, 127.8, 128.0, 128.3, 128.4, 129.9, 130.3, 133.7, 144.4, 145.3. IR (, cm⁻¹): 2957, 2854, 1462, 1377, 1261, 1097, 1020, 804. LC/MS, m/z 277 [M + 1]⁺; Anal. Calcd. for C₁₇H₁₂N₂S: C, 73.88; H, 4.38; N, 10.14%. Found: C, 73.80; H, 4.42; N, 10.12.

3. Results and Discussion

Using Pd(OAc)₂ as catalyst and several different types of silver(I) acetates or carboxylates, we have begun phosphine-free reactions of 2-phenylimidazo[1,2-a]pyridines (1) with phenyl iodide to afford the direct C-3 arylated product 2,3-diphenyl imidazo[1,2-a]pyridine (2a) (Scheme 1). We have taken this reaction as a model to optimize the yield by changing different combinations of Ag(I) carboxylate (generated in situ from Ag₂O and different carboxylic acids) along with other bases and solvents (Table 1). Indeed, compound 1 is a very stable starting material that could be synthesized in a convenient way by the reaction of cheap commercially available 2-aminopyridine and 2-bromoacetophenone. Moreover, 2-phenylimidazopyridine derivatives are well established as potent and selective ligands for peripheral benzodiazepine receptor [10].

There was no progress of this reaction at room temperature. The highly coordinating dimethylformamide (DMF) was the best choice as a solvent for the different combination of Ag(I) salt. The more acidic o-nitro benzoic acid was more effective combination than other acids with Ag₂O for the above reaction to afford compound 2a with high yield in DMF at 120°C under nitrogen. To find out the extent of this reaction, we have used several aryl iodides substituted with both electron withdrawing and donating groups for this reaction. The products and yields are pointed out in Table 2.

It is experiential that the percentages of yields do not vary considerably (entries 2, 4, 5, and 6) with the type of substiutents present (or absent; entry 1) in the phenyl ring of aryl iodides. Both, electron-withdrawing (entries 4, 5, 8, and 10) and electron-donating substitutents (entries 2, 3, 6, 7, and 9) show the same result. As expected, the more hindered ortho-substituted aryl iodides react considerably more slowly than the meta- and para-counterparts to afford slightly lower yields (entries 7 and 10). Further, we also have checked the reactivity of 2-iodothiophene as a heteroaryl iodide under the same reaction condition which affords the compound 3k with 66% yield. All the compounds (3a–k) were characterized by IR, NMR, mass spectroscopy, and elemental analysis. In the ¹H-NMR spectra all aromatic protons resonate in the region 6.66–8.42 Hz. Finally the structure for one of these compounds 3c is proven by X-ray crystallography (Figure 2). ( X-ray data was collected on a Bruker AXS-SMART diffractometer using Mo- ( = 0.71073 Å) radiation. The structure was solved and refined by standard methods. Crystal data for 3c: C₂₀H₁₆N₂, M = 284.35, Monoclinic, Space group P2(1)/n, a = 7.4312(15), b = 11.708(2), c = 17.611(4) Å, α = 90.00°, (3)°, γ = 90.00°, V = 1525.6(5) Å³, Z = 4, μ = 0.073 mm⁻¹, data/restraints/parameters: 2677/0/200, R indices (I > 2(I)): R1 = 0.0513, wR2 (all data) = 0.1208. CCDC no. 885064).

Figure 2

Molecular structure of the compound 3c (entry 3). (X-ray data was collected on a Bruker AXS-SMART diffractometer using Mo-( = 0.71073 Å) radiation. The structure was solved and refined by standard methods. Crystal data for 3c: C₂₀H₁₆N₂, M = 284.35, Monoclinic, Space group P2(1)/n, a = 7.4312(15), b = 11.708(2), c = 17.611(4) Å, α = 90.00°, (3)°, γ = 90.00°, V = 1525.6(5) Å³, Z = 4, μ = 0.073 mm⁻¹, data/restraints/parameters: 2677/0/200, R indices (I > 2(I)): R1 = 0.0513, wR2 (all data) = 0.1208. CCDC no. 885064).

Selected bond lengths (Å) and angles (°): N2-C5 1.323(2); N2-C6 1.369(2); N1 C5 1.395(2); C13 N1 C5 106.90(14); N2 C5 N1 110.93(15).

4. Conclusions

In conclusion, we have applied the catalytic Pd(OAc)₂ and Ag(I) carboxylate combination to develop a successful methodology for this challenging direct C-3 arylation of 2-phenylimidazopyridine with differently substituted aryl iodides without the presence of phosphines or other ligands to give 3-aryl-2-phenyl imidazopyridines with moderate to-high-yield. The structure of one of these compounds has been characterized by X-ray crystallography.

Acknowledgment

S. Kona and R. Suresh thank UGC Networking Resource Centre for carrying out research work at University of Hyderabad. M. Chakravarty thanks DST-FAST Track and BITS-Pilani Hyderabad Campus for support.

Supplementary Materials

Supplementary S1: ¹H and ¹³C NMR spectra for compounds 3a-j.

Supplemental S2: CIF file for compound 3c.

Supplementary Material

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Copyright © 2013 Sridevi Kona et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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