The Relative Reactivity of Thioethers and Sulfoxides toward Oxygen Transfer Reagents: The Oxidation of Thianthrene 5-Oxide and Related Compounds by MoO5HMPT

Article abstract of DOI:10.1021/jo00119a026

The oxidation of thianthrene 5-oxide (SSO) by MoO5HMPT has been studied in 1,2-dichloroethane at 40 deg 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; _infinite:_infinite:_infinite = 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.