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 gr oup, which is attacked by electrophilic oxidants, and a sulfoxide moiety, w hich is oxidized by nucleophilic oxidants. This density-functional study ha s been carried out in order to gain insight into the origin of the chemo- a nd 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 oxidatio n has to be interpreted with caution. Furthermore, a topological electron-d ensity analysis of thianthrene 5-oxide reveals that there is an area of cha rge depletion at the sulfoxide group. The location of this area indicates t hat the attack of nucleophilic oxidants on SSO is sterically hindered. Ther efore, the SSO probe makes oxidants such as dioxiranes appear to be more el ectrophilic than they actually are.