Substituent effects on stability of oxiranes, oxirenes, and dioxiranes

Homodesmotic reactions were designed to study substituent effects on stabil ity of oxiranes, oxirenes, and dioxiranes. Good or fair correlation between their homodesmotic stabilization energies and Taft's dual-substituent-para meters has been found. Oxiranes are stabilized by sigma-donating and pi-don ating substituents but destabilized by sigma-accepting and pi-accepting sub stituents. The pi-effects on oxiranes are comparable to sigma-effects for p i-acceptor substituents and are much stronger than sigma-effects for pi-don or substituents. Oxirenes are stabilized by sigma-donating, pi-donating, an d pi-accepting substituents but destabilized by sigma-accepting substituent s. The pi-effects on oxirenes are stronger than sigma-effects. Oxirenes wit h strong pi-donor substituents such as F, OH, and NH2, or strong sigma-dono r substituents such as Li and Na are neither real molecules nor transition states. Dioxiranes are stabilized by sigma-donating and pi-donating substit uents but destabilized by sigma-accepting and pi-accepting substituents. Th e pi-effects on dioxiranes are stronger than sigma-effects for pi-donor sub stituents, but weaker than sigma-effects for pi-acceptor substituents.