The applicability of thianthrene 5-oxide as a mechanistic probe for distinguishing between the electrophilic and the nucleophilic character in oxygen-transfer processes from a series of Mo(V1) and W(V1) peroxo complexes has been investigated. In almost all the cases examined, the predominant formation of the corresponding sulfone has been observed. This would indicate a nucleophilic nature of the oxidants, which is, however, at odd with known chemistry of such species. In fact, competitive experiments performed by employing pchlorophenyl methyl sulfide and phenyl methyl sulfoxide together in equivalent amounts as substrates revealed the preferred oxidation of the former over the latter as expected for an electrophilic oxygen transfer. These apparently contrasting findings are interpreted in terms of an incursion of radical pathways in the oxidation reactions of thianthrene 5-oxide by these peroxo complexes.

New asymmetric (salen)Mn III and UO 2 complexes containing a calix [4]arene unit in the ligand framework were synthesized. The UO 2 complexes were characterized by 1 H-, 13 C-, 2D TOCSY and T-ROESY NMR spectroscopy. Furthermore, the structure of one UO 2 complex was determined by singlecrystal X-ray analysis. The data showed that UO 2 complexes, which can be considered in first approximation models of the Mn=O oxidant active species, possess a chiral pocket and

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 M0O5HMPT 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., &2(cis-S0S0):&2(fr'cms-S0S0):&2(SS02) = l.0:4.0:1.0; [ds-S0S0L:[£rcms-S0S0U:[SS02]~= 1.0:4.5:1.2, are in good agreement. The frans-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.

No abstract

No abstract