Transition states of epoxidations: Diradical character, spiro geometries, transition state flexibility, and the origins of stereoselectivity

The transition structures for the epoxidations of ethylene by performic acid, dioxirane, oxaziridine, and peroxynitrous acid have been located with density functional theory methods using the Becke3LYP functional and 6-31G* basis set. All of the epoxidations have spiro transition states; those with performic acid and dioxirane are early and involve synchronous oxygen transfer, while those with oxaziridine and peroxynitrous acid are later with asynchronous oxygen transfer. The results from Becke3LYP/6-31G* theory are compared with MP2/6-31G* literature values. Substitution on ethylene by methoxy, methyl, vinyl, and cyano groups changes the transition state geometries toward asynchronous spiro structures. The activation energies are lowered by all substituents except the cyano group in reactions of performic acid and dioxirane. Experimental stereoselectivities are rationalized by using transition structure models based upon these transition structures.