Thianthrene 5-oxide as a probe for the electronic character of oxygen-transfer reactions: Re-interpretation of experiments required

The electronic character of oxidants, i.e., whether they attack substrates in an electrophilic or nucleophilic way, has extensively been investigated using thianthrene g-oxide (SSO) as probe. The SSO molecule has a sulfide group, which is attacked by electrophilic oxidants, and a sulfoxide moiety, which is oxidized by nucleophilic oxidants. This density-functional study has been carried out in order to gain insight into the origin of the chemo- and stereoselectivity of SSO oxidation. It has been found that the endo and exo stereoisomers of the thianthrene oxides interconvert via ring-inversion with moderate energy barriers. Thus, the stereoselectivity of SSO oxidation has to be interpreted with caution. Furthermore, a topological electron-density analysis of thianthrene 5-oxide reveals that there is an area of charge depletion at the sulfoxide group. The location of this area indicates that the attack of nucleophilic oxidants on SSO is sterically hindered. Therefore, the SSO probe makes oxidants such as dioxiranes appear to be more electrophilic than they actually are.