THIANTHRENE 5-OXIDE AS A MECHANISTIC PROBE FOR ASSESSING THE ELECTRONIC CHARACTER OF OXYGEN-TRANSFER AGENTS

Thianthrene 5-oxide (SSO) was employed to assess the electronic nature of oxygen-transfer reagents: Those oxidants that attack preferentially the sulfide "S" site to give the bis(sulfoxide) SOSO are electrophilic in their reactivity; those that predominantly react at the sulfoxide "SO" site to give the sulfone SSO2 are nucleophilic. The X(SO) parameter was introduced, defined as the mole fraction of SSO2 product (SO attack), for which strongly electrophilic oxygen-transfer agents (typically acidified hydroperoxides and hypochlorite) take near-zero values and strongly nucleophilic ones (typically basified hydroperoxides and superoxide) near-unity. On the X(SO) scale, ozone and peroxy acids are as expected electrophilic oxidants and dioxiranes are significantly more nucleophilic but more electrophilic than carbonyl oxides. The latter exhibit pronounced nucleophilic reactivity toward SSO, which is in agreement with their observed reactivity. Free radicals, e.g., t-BuOO., display very high electrophilicity in their oxygen-transfer propensity by reacting essentially exclusively at the S site. Control experiments have established that such radicals do not act through electron transfer to afford the SSO.+ radical cation, although the latter, generated either by photosensitized or chemical oxidation, behaves toward dioxygen strongly electrophilic. While the SSO probe provides a realistic measure of the electronic nature of oxygen-transfer agents, caution should be excercised when preferential complexation of the reagent at the S or SO site of SSO takes place or when electron transfer is involved with SSO to produce the SSO.+ or SSO.- radical ions. Also, during the in situ generation of transient oxidants, several species of different electronic character might act simultaneously and the composite X(SO) value erroneously express the reactivity of the oxidant in question. In such suspicious cases, control experiments are obligatory to acquire meaningful X(SO) data with SSO.