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Synlett 2009(4): 569-572  
DOI: 10.1055/s-0028-1087911
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

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

Kiumars Bahrami*a,b, Mohammad Mehdi Khodaei*a,b, Fardin Naalia
a Department of Chemistry, Razi University, Kermanshah, 67149 Iran
b Nanoscience and Nanotechnology Research Center (NNRC), Razi University, Kermanshah, 67149 Iran
Fax: +98(831)4274559; e-Mail: kbahrami2@hotmail.com; e-Mail: mmkhoda@razi.ac.ir;
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Publication History

Received 17 August 2008
Publication Date:
16 February 2009 (online)

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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.

Key words

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

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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.