Besides the chiral ketones, chiral amine based catalysts 13 and 14 have been explored for the epoxidation of unfunctionalized alkenes. For example, chiral pyrrolidine 15 has been used for the α,β -unsaturated aldehydes with excellent enantioselectivity in the presence of 35% H₂O₂ (Scheme 13). α,β -Unsaturated aldehydes containing an aromatic substituent at the β -position are good substrates affording the epoxides with high diastereo- and enantioselectivities.

Proposed Mechanism

The proposed mechanism states that the reaction takes place through the Weitz-Scheffer mechanism (Scheme 14). The addition of hydrogen peroxide to the β -carbon atom of the electrophilic iminium ion is reversible and the attack on the electrophilic oxygen atom by the nucleophilic enamine determines the product stereochemistry.

While chiral N -spiro ammonium salt 14 bearing an axially chiral binaphthyl unit functions as phase transfer catalyst for the epoxidation of enones with high enantioselectivity (Scheme 14). The hydroxyl groups are appropriately bonded to recognize and activate the enone substrate by hydrogen bonding.

Scheme 14

Scheme 15