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

The oxidation of thianthrene 5-oxide (SSO) by MoOSHMPT 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 (SS02) which quantitatively account for the active oxygen of MoOSHMPT 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., kz(cis-SOSO):kz(trans-SOSO):k~(SSO=2 1) .0:4.0:1.0; [czs-SOSOl,:[truns-SOSOl,:[SSO=~l , 1.0:4.5:1.2, are in good agreement. The trans-SOSO-forming reaction is only 4-fold faster than that leading to SS02. cis-SOSO and SS02 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.