Untangling the catalytic importance of the Se oxidation state in organoselenium-mediated oxygen-transfer reactions: the conversion of aniline to nitrobenzene

Abstract

Seleninic acids and their precursors are well-known oxygen-transfer agents that can catalyze several oxidations with H₂O₂ as the final oxidant. Until very recently, the Se(IV) “peroxyseleninic” acid species has been considered the only plausible catalytic oxidant. Conversely, in 2020, the involvement of Se(VI) “peroxyselenonic” acid has been proposed for the selenium mediated epoxidation of alkenes. In this work, we theoretically probe different mechanisms of H₂O₂ activation and of Se(IV) to Se(VI) interconversion. In addition, we investigate through a combined theoretical (DFT) and experimental approach the mechanistic steps leading to the oxidation of aniline to nitrobenzene, when Se(IV) seleninic acid or Se(VI) selenonic acids are used as catalysts and H₂O₂ as the oxidant. This process encompasses three subsequent organoselenium mediated oxidations by H₂O₂. These results provide a mechanistic explanation of the advantages and disadvantages of both oxidation states (IV and VI) in the different stages of catalytic oxygen-transfer reactions: hydrogen peroxide activation and actual substrate oxidation. While the Se(VI) “peroxyselenonic” acid is found to be a better oxidant, the privileged role of “peroxyseleninic” acid as the main active species is assessed and its origin is identified in the lower catalyst-distortion that seleninic acid undergoes when activating H₂O₂. Conversely, the higher catalyst-distortion that characterizes the reaction of selenonic acid with H₂O₂ supports an inactivating role of Se(IV) to Se(VI) interconversion.