Arylation of Aldehydes with Indoles Using 1,3-Diisopropylimidazolinium Tetrafluoroborate as a Recyclable Catalyst for the Synthesis of Bis(indolyl)methanes in Water

Abstract

Bis(indolyl)methanes have attracted considerable attention for their interesting biological activities. Green organic-catalytic synthesis has promoted the development of green chemistry, especially in aqueous-phase reaction systems. We have developed a green and efficient catalytic arylation of aldehydes with indoles using low-cost 1,3-diisopropylimidazolinium tetrafluoroborate as a recyclable and reusable catalyst for the synthesis of bis(indolyl)methanes in water.

Key words

organocatalysis - bisindolylmethanes - green chemistry - aqueous-phase reaction

• References and Notes

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• 16 IiPrim·HBF₄ NH₄BF₄ (624 mg, 6 mmol) was added to a stirred solution of N,N′-diisopropylethylenediamine (719 mg, 5 mmol) and triethyl orthoformate (740 mg, 5 mmol) in EtOH (20 mL), and the mixture was refluxed overnight. When the reaction was complete (TLC), the mixture was concentrated in vacuo. H₂O (100 mL) was added to the residue and the mixture was extracted with CH₂Cl₂ (50 mL). The organic layer was dried and concentrated in vacuo to give a colorless solid. Purity (HPLC): >97%. ¹H NMR (400 MHz, DMSO-d ₆): δ = 8.41 (s, 1 H), 3.82–3.88 (m, 6 H), 1.25 (d, J = 8.0 Hz, 12 H). BIMs; General Procedure IiPrim·HBF₄ (0.1 mmol) was added to a stirred solution of the appropriate indole or N-methylindole (0.5 mmol) and aldehyde (0.3 mmol) in H₂O (2 mL), and the mixture was stirred at 80 °C for 1–3 h. When the reaction was complete (TLC), the mixture was diluted with EtOAc (5 mL) and the organic layer was dried and concentrated in vacuo. The residue was purified by column chromatography. 3,3′-(Phenylmethylene)bis-1H-indole (1a) White solid; yield: 91%. ¹H NMR (400 MHz, CDCl₃): δ = 7.79 (s, 2 H), 7.44 (d, J = 7.4 Hz, 2 H), 7.40 (d, J = 7.4 Hz, 2 H), 7.30–7.36 (m, 4 H), 7.24–7.27 (m, 3 H), 7.07 (t, J = 7.8 Hz, 2 H), 6.63 (s, 2 H), 5.92 (s, 1 H). ¹³C NMR (100 MHz, CDCl₃): δ = 144.13, 136.69, 128.73, 128.25, 127.06, 126.02, 123.10, 121.33, 119.87, 119.73, 119.22, 110.87, 40.19.