The relative reactivity of thioethers and sulfoxides toward oxygen transfer reagents: the oxidation of thianthrene 5-oxide and related compounds by MO5HMPT

The oxidation of thianthrene 5-oxide (SSO) by M0O5HMPT has been studied in 1,2-dichloroethane at 40 °C. Under conditions of excess substrate over the oxidant, three products are formed, i.e., two isomeric cis and trans bissulfoxides (SOSO) and sulfide-sulfone (SSO2) which quantitatively account for the active oxygen of MoO5HMPT consumed. The rates of appearance of the products at different reactant concentrations have been measured. A second-order rate law has been established. The ratios of the rate constants and of the final concentrations of the three products, i.e., k2(cis-SOSO):k2(trans-SOSO):k2(SSO2) = 1.0:4.0:1.0; [cis-SOSO]:[trans-SOSO]∞:[SSO2]∞ = 1.0:4.5:1.2, are in good agreement. The trans-SOSO-forming reaction is only 4-fold faster than that leading to SSO2. cis-SOSO and SSO2 are produced at almost the same rate. Evidence is presented that all the oxidation reactions are electrophilic processes taking place via a simple bimolecular mechanism not involving the coordination of the substrate to the metal. The low selectivity is due to the scarce reactivity of the thioether center in thianthrene 5-oxide. The investigation of the oxidative behavior of structurally related compounds reveals that such a low reactivity results from a combination of stereoelectronic effects. These findings provide a rationale to some ambiguous results obtained when thianthrene 5-oxide is employed as a mechanistic probe of the electronic character of the oxidants.