Synthesis of Bis(indolyl)methanes Catalyzed by Acidic Ionic Liquid Immobilized on Silica (ILIS)

Abstract

A new organocatalytic procedure for the synthesis of bis(indolyl)methanes with acidic ionic liquid immobilized on silica gel (ILIS-SO₂Cl) has been developed. The reaction condition is ­applicable to a variety of aryl and aliphatic aldehydes and indoles. The mild nature of the reaction conditions showed that the acetoxy or TBDMS group in the aldehyde was maintained intact. The catalyst was reused four times in 93% average yield.

Key words

catalysis - condensation - indoles - ionic liquids - supported catalysis

References and Notes

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• 12b

  According to preliminary MOPAC-AM1 calculation, LUMO energy of methanesulfonyl chloride was 4.1 eV lower than that of BF₃, though the shape of the LUMO was not localized.

11

Typical Experimental Procedure: A suspension of the benzaldehyde (1) (30 mL, 0.3 mmol), indole(2) (60 mg, 0.5 mmol) and ILIS-SO₂Cl(4) (143 mg, loading of sulfonyl chloride: 0.35 mmol/g, 0.05 mmol) in MeCN (3 mL) was stirred gently at ambient temperature under nitrogen atmosphere for 5.5 h. The product was triturated with Et₂O and the combined organic layer was evaporated to dryness. Medium pressure LC of the residue (eluent: EtOAc-
n-hexane, 3:1) afforded bis(indolyl)methane(3) (79 mg, 97% based on 2).