An Efficient Synthesis of Bis(indolyl)methanes Catalyzed by Recycled Acidic Ionic Liquid

 

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Abstract

Green, mild and efficient electrophilic substitution ­reactions of indoles with various aldehydes were carried out using task-specific ionic liquids to afford the corresponding bis(indolyl)methanes in excellent yields.

References

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Representative Experimental Procedure.
A mixture of indole (0.117 g, 0.50 mmol), 4-(benzo[d]ox-azol-2-yl)benzaldehyde (0.115 g, 0.25 mmol) and [hmim]HSO₄ (0.0264g, 0.50 mmol × 5% mol) in EtOH (4.0 mL) was stirred at r.t. After completion of the reaction as indicated by TLC, the mixture was concentrated in vacuo to remove the solvent. The resulting solution was extracted with Et₂O (5 × 10 mL). The organic layer was dried (Na₂SO₄), concentrated under vacuum and purified by column chromatography to afford the pure product. 3,3′-Bis-indolyl-4-(benzo[d]oxazol-2-yl)phenylmethane (2g): pale powder; mp 234-236 °C. IR (KBr): ν = 3421, 3053 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 5.99 (s, 1 H), 6.72 (s, 2 H), 7.03 (dd, J ₁ = 7.6 Hz, J ₂ = 7.2 Hz, 2 H), 7.19 (t, J = 7.2 Hz, 2 H), 7.35 (s, 2 H), 7.40 (dd, J ₁ = 7.6 Hz, J ₂ = 8.0 Hz, 4 H), 7.77 (s, 1 H), 8.02 (s, 2 H), 8.18 (d, J = 8.0 Hz, 2 H). HRMS: m/z calcd for C₂₃H₂₀N₂S: [M] 439.1685; found: 439.1666 [M⁺].
3,3′-Bis-(5-methyl) indolyl-3-thienylmethane (3c): pink powder; mp 217-219 °C. IR (KBr): ν = 3410, 2909, 2848 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 2.38 (s, 6 H), 5.90 (s, 1 H), 6.69 (s, 2 H), 6.94 (s, 1 H), 7.00-7.06 (m, 4 H), 7.24 (d, J = 5.2 Hz, 4 H), 7.83 (s, 2 H). HRMS: m/z calcd for C₂₃H₂₀N₂S: [M] 356.1347; found: 356.1334 [M⁺].