Substituent effects on stability of oxiranes, oxirenes, and dioxiranes

Homodesmotic reactions were designed to study substituent effects on stability of oxiranes, oxirenes, and dioxiranes. Good or fair correlation between their homodesmotic stabilization energies and Taft's dual-substituent-parameters has been found. Oxiranes are stabilized by σ-donating and π-donating substituents but destabilized by σ-accepting and π-accepting substituents. The π-effects on oxiranes are comparable to σ- effects for π-acceptor substituents and are much stronger than σ-effects for π-donor substituents. Oxirenes are stabilized by σ-donating, π-donating, and π-accepting substituents but destabilized by σ-accepting substituents. The π-effects on oxirenes are stronger than σ-effects. Oxirenes with strong π- donor substituents such as F, OH, and NH₂, or strong σ-donor substituents such as L and Na are neither real molecules nor transition states. Dioxiranes are stabilized by σ-donating and π-donating substituents but destabilized by σ-accepting and π- accepting substituents. The π-effects on dioxiranes are stronger than σ-effects for π-donor substituents, but weaker than σ- effects for π-acceptor substituents.