Competition between electron transfer and base-induced elimination mechanisms in the gas-phase reactions of superoxide with alkyl hydroperoxides

Abstract

Understanding the mechanism responsible for peroxides decomposition is essential to explain several biochemical processes. The mechanisms of the intrinsic reactions between the superoxide radical anion (O₂˙⁻) and methyl, ethyl, and tert-butyl hydroperoxides (ROOH, with R = Me, Et, and t-Bu) have been characterized to understand the mechanism responsible for peroxides decomposition. The reaction energy diagrams suggest a competition between the spin-allowed and spin-forbidden electron transfer (ET), and base-induced elimination (E_CO2) mechanisms. In all cases, the spin-allowed ET mechanism describes formation of the ozonide anion radical (O₃˙⁻), either complexed with an alcohol molecule or separated. For the O₂˙⁻/MeOOH(EtOOH) reactions, HCO₂⁻ (MeCO₂⁻) + H₂O + HO˙ and OH⁻ + CH₂O(MeCHO) + HO₂˙ products are associated with the spin-forbidden ET and E_CO2 channels, respectively. On the other hand, for the reaction between O₂˙⁻ and t-BuOOH, the spin-forbidden ET route describes formation of the MeCOCH₂⁻ enolate (either separated or hydrated) along with the methyl peroxyl (MeO₂˙) radical. In addition, the regeneration of O₂˙⁻via spin-forbidden ET and E_CO2 channels was also characterized from the decomposition of ROOH, yielding diols (CH₂(OH)₂ and MeCH(OH)₂), aldehydes (CH₂O and MeCHO), and oxirane (cyc-CH₂CMe₂O).