Enantioselective Friedel‐Crafts Alkylation of Indoles with α, β‐Unsaturated Aldehydes Catalyzed by Recyclable Second‐generation MacMillan Catalysts

Abstract

AbstractImidazolium ionic liquids containing second‐generation MacMillan catalysts were synthesized and evaluated as organocatalysts (10 mol%) for enantioselective Friedel‐Crafts reaction between N‐benzylindole and crotonaldehyde using trifluoroacetic acid as a co‐catalyst (10 mol%) at −60 °C, the corresponding product was obtained in 87 % yield with 89 % ee. The scope and limitation of organocatalyst were studied using substituted indoles and different α, β‐unsaturated aldehydes, and enantioselective alkylated indoles were obtained in 43–95 % yields with 58–90 % ee's. The second‐generation modified MacMillan catalyst 6 was successfully recovered and reused up to four cycles and a significant drop in yields and ee was observed after the third recycle.