Enantioselective Friedel-Crafts reaction between indoles and α,β-unsaturated aldehydes catalyzed by recyclable α,α-diarylprolinol derived chiral ionic liquids

Abstract

Imidazolium-based chiral ionic liquid containing α,α-diaryl-(S)-prolinol trimethylsilyl ether acts as an efficient and reusable organocatalyst for the enantioselective Friedel-Crafts reaction between indoles and α, β-unsaturated aldehydes. 3-Alkylated indoles were obtained in 62–89 % yields with 47–88 % enantiomeric excesses using α,α-diphenyl-(S)-prolinol trimethylsilyl ether having hexafluorophosphate anion [PF6]− (20 mol%) and triethylamine (40 mol%) as a base additive in 1,4‑dioxane at 25 °C. The α,α-diphenyl-(S)-prolinol trimethylsilyl ether having hexafluorophosphate anion [PF6]− was recycled and reused for FC reaction up to seven recycles with consistent enantioselectivity, but a considerable loss in yield of the product was observed after the third recycling. The energies of possible conformers (iminium intermediates) were calculated by DFT.