Structure and reactivity of amphoteric oxygen species

The structure and chemical properties of 1,3-dipolar peroxidic species (X+- O-O-) such as ozone (X = O), nitroso oxides or nitrenium peroxides (X = RN) , carbonyl oxides or carbenium peroxides (X = RIG), and persulfoxides or su lfonium peroxides (X = R2S) have been attracting much attention from synthe tic, biological, and theoretical standpoints. These active oxygen species c an be classified into two types depending on whether the X+ is an onium (ca rbenium, nitrenium, silylenium etc.) ion or an onium (ammonium, oxonium, ph osphonium, sulfonium etc.) ion, and their reactivities are quite different depending on the nature of X. The individual features as well as the overvi ews of structure and reactivities of these X-O-O species are reviewed. Ozon e and nitroso oxides have an electrophilic character while carbonyl oxides usually act as a nucleophilic oxygen transfer agent, their reactivities bei ng controlled by substituents. It is important to see whether or not XOO sp ecies can isomerize to the cyclic isomers. Although cyclic O-3 is thermodyn amically unstable, dioxiranes, the cyclic form of carbonyl oxides, have bee n isolated and fully characterized. It has been suggested that nitroso oxid es isomerize to the cyclic form yielding the corresponding nitro compounds unimolecularly. In the case of persulfoxides (X = R2S), the dipolar structu re (X+-O-O-) is important, their characteristic reaction being nucleophilic O-transfer to sulfoxides, while 3 lambda(5)-dioxaphosphirane (X = R3P) can exist only in the cyclic form, showing electrophilic reactivity. Effects o f X's on structure and reactivity of these oxides were systematically exami ned by the density functional BLYP/6-31G* calculations. The inductive effec t through sigma-framework has been found to dominate the thermodynamic stab ilities of carbonyl oxides, while the pi-donating property of substituents governs the activation energy for the cyclization to dioxiranes.