Recycling chiral imidazolidin-4-one catalyst for asymmetric Diels-Alder reactions: Screening of various ionic liquids

Various imidazolium ionic liquids such as [Bmim]PF6, [Bmim]SbF6, [Bmim]OTf and [Bmim]BF4 were screened for recycling an organic catalyst [(5S)-5-benzyl-2,2,3-trimethylimidazolidin4-one (1)] for asymmetric Diels-Alder reactions. Good yields and enantioselectivies (up to 85% yield and 93% ee) were obtained from reactions in [Bmim]PF6 or [Bmim]SbF6. However, reactions in [Bmim]OTf or [Bmim]BF4 gave racemic products in low yields. Isolation of the products by simple extraction using diethyl ether allowed recycling of the ionic liquid containing the immobilized catalyst in subsequent reactions without significant decrease of yields and enantioselectivities.