H₂O₂/Fe(NO₃)₃-Promoted Synthesis of 2-Arylbenzimidazoles and 2-Arylbenzothiazoles

Abstract

A new, convenient synthesis of 2-substituted benzimidazoles and benzothiazoles is described. Short reaction time, easy and quick isolation of the products, environmentally friendly procedure, excellent chemoselectivity and excellent yields are main advantages of this procedure.

2-arylbenzimidazole - 2-arylbenzothiazole - hydrogen peroxide - chemoselectivity - aryl aldehydes - solvent-free

References and Notes

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24

Ferric nitrate, hydrogen peroxide (30%) as well as all the aromatic aldehydes, 1,2-phenylenediamine derivatives and 2-aminothiophenol employed as substrates are commer-cially available and were used without further purification. The concentration of the commercial 30% H₂O₂ solution was checked iodometrically prior to use.
General procedure for the Synthesis of Benzimidazoles and Benzothiazoles: A mixture of 1,2-phenylenediamine (1 mmol), aryl aldehyde (1 mmol), H₂O₂ (30%, 4 mmol, 0.4 mL) and Fe(NO₃)₃˙9 H₂O (0.1 mmol, 0.04 g) was heated at 50 ˚C for the appropriate time as mentioned in Table 2. After completion of the reaction, the reaction mixture was dissolved in EtOH (10 mL) and then poured into ice-water (30 mL). The pure solid product was filtered, washed with ice-water and subsequently dried. An identical procedure was employed using 2-aminothiophenol (1 mmol), aryl aldehyde (1 mmol) in the presence of 30% H₂O₂ (4 mmol) and Fe(NO₃)₃.9 H₂O (0.1 mmol) for the synthesis of benzothiazoles (Table 4).
Spectroscopic and Physical Data for Selected Compounds:
2-Phenyl-1 H -benzimidazole: mp 290-291 ˚C (lit. [¹8] 292 ˚C). ^¹H NMR (200 MHz, DMSO-d ₆): δ = 7.19-7.23 (m, 2 H), 7.51-7.61 (m, 5 H), 8.16-8.21 (m, 2 H), 12.91 (br, 1 H, NH). ^¹³C NMR (50 MHz, DMSO-d ₆): δ = 120.1, 123.0, 127.0, 127.3, 129.8, 130.8, 131.0, 152.1.
2-(2-Chlorophenyl)-1 H -benzimidazole: mp 232-234 ˚C (lit. [¹8] 234 ˚C). ^¹H NMR (200 MHz, DMSO-d ₆): δ = 7.26-7.32 (m, 2 H), 7.56-7.73 (m, 5 H), 7.93-7.96 (m, 1 H), 12.78 (br, 1 H, NH). ^¹³C NMR (50 MHz, DMSO-d ₆): δ = 112.1, 119.4, 122.1, 123.1, 127.8, 130.3, 130.7, 131.6, 132.4, 149.4.
2-(4-Hydroxyphenyl)-1 H -benzimidazole: mp 276-278 ˚C (lit. [²5] 279 ˚C). ^¹H NMR (200 MHz, DMSO-d ₆): δ = 5.04 (br, 1 H, OH), 6.91 (d, J = 8.7 Hz, 2 H), 7.12-7.21 (m, 2 H), 7.48-7.57 (m, 2 H), 7.97 (d, J = 8.7 Hz, 2 H), 10.02 (br, 1 H, NH). ^¹³C NMR (50 MHz, DMSO-d ₆): δ = 120.1, 121.4, 125.9, 127.6, 133.9, 144.2, 157.2, 165.
2-(3-Bromophenyl)benzothiazole: mp 91-93 ˚C. ^¹H NMR (200 MHz, DMSO-d ₆): δ = 7.34-7.66 (m, 4 H), 7.91-8.12 (m, 3 H), 8.29-8.31 (m, 1 H). ^¹³C NMR (50 MHz, DMSO-d ₆): δ = 122.1, 123.5, 123.8, 126, 126.6, 126.9, 130.6, 130.9, 134.2, 135.4, 135.8, 154.3, 166.5. Anal. Calcd for C₁₃H₈NSBr: C, 53.81; H, 2.70; N, 4.83; S, 11.05. Found: C, 54.00; H, 2.70; N, 5.20; S, 11.10.
2-(4-Formylphenyl)benzothiazole: mp 135-136 ˚C. ^¹H NMR (200 MHz, DMSO-d ₆): δ = 7.39-7.56 (m, 2 H), 7.90-8.26 (m, 6 H), 10.06 (s, 1 H).^¹³C NMR (50 MHz, DMSO-d ₆): δ = 122.2, 124.1, 126.0, 126.3, 127.1, 128.4, 130.7, 135.8, 138.0, 139.1, 154.5, 191.9. Anal. Calcd for C₁₄H₉NSO: C, 70.27; H, 3.79; N, 5.85; S, 13.41. Found: C, 70.00; H, 3.70; N, 6.10; S, 13.20.
2-(4-Methylphenyl)benzothiazole: mp 85-86 ˚C (lit. [²6] 85-86 ˚C). ^¹H NMR (200 MHz, DMSO-d ₆): δ = 2.41 (s, 3 H), 7.24-7.48 (m, 4 H), 7.86-8.04 (m, 4 H). ^¹³C NMR (50 MHz, DMSO-d ₆): δ = 22.0, 122.0, 123.5, 125.5, 126.7, 127.9, 130.2, 131.3, 135.4, 141.9, 154.6, 168.7.

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