The epoxidation of an alkene with peracid to give an oxirane. The commercially available mCPBA is a widely used reagent for this conversion, while magnesium mono-perphthalate and peracetic acid are also employed.
Mechanism of the Prilezhaev Reaction
Peracids tend to adopt an intramolecularly hydrogen-bonded conformation in solution, and the high degree of polarisation results in an electrophilic oxygen atom that is able to add to alkenes.
The transition state, in which oxygen is added and the proton is shifted simultaneously, resembles a butterfly and is known as the "Butterfly Mechanism":