Bisguanidinium-Catalyzed Epoxidation of Allylic and Homoallylic Amines under Phase Transfer Conditions

A highly enantioselective epoxidation reaction of allylic and homoallylic amines has been disclosed using an ion pair catalyst, which consists of chiral cationic bisguanidinium [BG]²⁺ and an achiral tetraperoxyditungstate anion [W₂O₂(μ-O)­(O₂)₄]^2–. The terminal oxidant is a stoichiometric amount of aqueous hydrogen peroxide, an environmentally benign reagent. Up to 96% enantiomeric excess and 99% yields were achieved for 1,1′-disubstituted and 1,2-disubstituted allylic protected amines and 1,2-disubstituted homoallylic protected amines. The identity of the ion pair catalyst was uncovered using X-ray crystallography and revealed that the achiral tetraperoxyditungstate anion species [W₂O₂(μ-O)­(O₂)₄]^2– is nudged nicely into the central cavity of the chiral dication. The ion pair catalyst was also characterized using infrared (IR) and Raman spectroscopies. The synthesis of (−)-venlafaxine was achieved via this reported methodology to demonstrate its usefulness.