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 performi c acid, dioxirane, oxaziridine, and peroxynitrous acid have been locat ed with density functional theory methods using the Becke3LYP function al 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 peroxyn itrous acid are later with asynchronous oxygen transfer. The results f rom Becke3LYP/6-31G theory are compared with MP2/6-31G* literature va lues. Substitution on ethylene by methoxy, methyl, vinyl, and cyano gr oups changes the transition state geometries toward asynchronous spiro structures. The activation energies are lowered by all substituents e xcept the cyano group in reactions of performic acid and dioxirane. Ex perimental stereoselectivities are rationalized by using transition st ructure models based upon these transition structures.