Ytterbium Triflate Promoted Synthesis of Benzimidazole Derivatives

Abstract

Differently substituted benzimidazoles have been synthesised in very good yields in solvent-free conditions from o-phenylenediamine and aldehydes in the presence of Yb(OTf)₃ as catalyst. The method is applicable to aromatic, unsaturated and ­aliphatic aldehydes and to substituted o-phenylenediamines without significant differences.

Key words

aldehydes - benzimidazoles - catalysis - lanthanides

References

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• 13 Typical Procedure. A mixture of o-phenylenediamine (1 mmol) and aldehyde (1.1 mmol) was well stirred with Yb(OTf)₃ (0.05 mmol) at r.t. for 30 min. To this solution, CH₂Cl₂ (2 mL) was added to get Yb(OTf)₃ crystallized; the catalyst was removed by filtration under reduced pressure and the residue was purified by SiO₂ gel column chromatography using CH₂Cl₂-MeOH 99:1 as eluent. Compounds 1, 2, 3, 5, 6, 7, 8, 9 and 10 were identified by comparison with commercial samples. 2-(2,6-dimethylhept-5-enyl)-1 H -benzimidazole ( 4). White solid; mp 93-94 °C. ¹H NMR (200 MHz, CDCl₃): δ = 1.01 (d, 6 H, J = 6.6 Hz), 1.27-2.27 (m, 5 H), 2.67-3.10 (m, 2 H), 5.01-5.21 (m, 1 H), 6.75 (s, 1 H), 7.15-7.65 (m, 4 H). ¹³C NMR (50 MHz, CDCl₃): δ = 18.9, 19.5, 25.5, 25.6, 32.9, 36.9, 37.0, 116.7, 120.2, 122.1, 124.2, 136.3, 142.6, 146.6, 154.2. GC-MS: m/z = 242 [M⁺]. Anal. Calcd for C₁₆H₂₂N₂: C, 79.29; H, 9.15; N, 11.56. Found: C, 79.31; H, 9.16; N, 11.54
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14

The reaction to yield compound 1 has been repeated three times, through the catalyst washed with CH₂Cl₂ and dried for 2 h at 70 °C with the following yields: 94%, 91%, and 91%.