Thianthrene 5-oxide as a mechanistic probe in oxygen transfer reactions: The case of carbonyl oxides versus dioxiranes revisited

Thianthrene 5-oxide (SSO) constitutes a useful mechanistic tool for the assessment of the electronic character of oxygen transfer agents by means of their X(SO) values, which reflect the extent of nucleophilic oxidation at the SO site in SSO. Treatment of dioxiranes 1 a-d with the SSO probe confirms that these are electrophilic oxidants (X(SO) < 0.15). Dioxirane sulfoxidation is sensitive to protic solvents and acids, which implies a polar mechanism (S(N)2) with nucleophilic attack of the sulfide electron pair on the dioxirane peroxide bond. In contrast, the carbonyl oxides 2a and b, diethyl persulfoxide (3), and adamantylideneadamantane perepoxide (4) are nucleophilic oxidants (X(SO) > 0.85). However, the cyclopentadienone carbonyl oxides 2c and d show low X(SO) values typical for electrophilic so oxygen transfer agents. For these carbonyl oxides, photoisomerization to the respective dioxiranes is proposed under the photooxidation conditions of the diazoalkanes. Additionally, the trans/cis ratio of bissulfoxides (SOSO) formed provides valuable information on the stereochemical course of the oxygen-transfer process due to the steric requirements of the oxidant. Thus, the electrophilic dioxiranes preferentially attack the axial lone pair of SSO because of repulsion by the peri hydrogen atoms to afford trans-SO-SO.