Mechanistic Insight into the Oxidation of Organic Phenylselenides by H₂ O₂

The oxidation of organic phenylselenides by H₂ O₂ is investigated in model compounds, namely, n-butyl phenyl selenide (PhSe(nBu)), bis(phenylselanyl)methane (PhSeMeSePh), diphenyl diselenide (PhSeSePh), and 1,2-bis(phenylselanyl)ethane (PhSeEtSePh). Through a combined experimental (¹ H and ⁷⁷ Se NMR) and computational approach, we characterize the direct oxidation of monoselenide to selenoxide, the stepwise double oxidation of PhSeMeSePh that leads to different diastereomeric diselenoxides, the complete oxidation of the diphenyldiselenide that leads to selenium-selenium bond cleavage, and the subsequent formation of the phenylseleninic product. The oxidation of PhSeEtSePh also results in the formation of phenylseleninic acid along with 1-(vinylseleninyl)benzene, which is derived from a side elimination reaction. The evidence of a direct mechanism, in addition to an autocatalytic mechanism that emerges from kinetic studies, is discussed. By considering our observations of diselenides with chalcogen atoms that are separated by alkyl spacers of different length, a rationale for the advantage of diselenide versus monoselenide catalysts is presented.