Organocuprate reactions in phosphonium-based ionic liquids

Abstract

Ionic liquids (ILs) based on the trihexyl(tetradecyl)phosphonium cation were purified or synthesized in high yields (83.6-93.0 %) via salt metathesis. These were used as solvents for 1,4-congugate additions of lithium dibutylcyanocuprate [Bu₂CuLi∙LiCN] to trans -aphenyl-3-buten-2-one; with and without tetrahydofuran co-solvent. Low conversion of substrate to product (4-phenyloctane-2-one) was achieved in 1 hour (0-59 %) in the IL alone while retaining the organic solvent offers better product formation (65-94 %) as determined by GC-MS analysis. Good stability of the cuprate in the ILs is experienced when the chloride anion is exchanged for bis(trifluoromethylsulfonyl)amide or dicyanimide, greatly reducing undesired alkenes formed via in situ generation of Wittig reagents from the cation. Methods to isolate the product and recycle the IL after reaction require further work. Three ether containing ILs were also synthesized in good yields (66.8-75.9 %); however, their hygroscopic nature proved deleterious to their role in the model reaction.