MECHANISTIC ASPECTS OF VANADIUM-CATALYZED OXYGEN-TRANSFER REACTIONS

Vanadium-catalysed oxidations in organic media have been investigated using mainly tert-butyl hydroperoxide, TBHP, as the terminal oxidant a nd vanadyl(IV) acetylacetonate, OV(IV)(acac)(2), vanadyl(IV) benzoylca mphorate, OV(IV)(camp), or vanadyl(IV) 5,10,15,20-tetraphenylporphyrin , OV(tpp), as the catalysts. Kinetic studies of the initial reaction b etween OV(IV)(acac)(2) and TBHP were performed using EPR and UV spectr oscopy. The experimental data suggest a second-order reaction, first o rder in OV(IV)(acac)(2) and in TBHP. IR spectroscopic studies of the r eaction show that the V=O functionality is intact after the oxidation has taken place and, furthermore, UV, IR and H-1 NMR spectroscopic inv estigations all indicate that the acetylacetonate ligands remain bound to the vanadium atom during the oxidation of OV(IV)(acac)(2) to a OV( V)(acac)(2) complex. The oxidation of the vanadium(IV)- to the vanadiu m(V)-complexes is very dependent on the organic ligand attached to the metal. Studies of the oxidation of thianthrene 5-oxide using differen t vanadium catalysts and TBHP as the terminal oxidant show that the el ectronic nature of the transferred oxygen atom is electrophilic. The i nitial oxygen-transfer step in the epoxidation of(Z)-stilbene is propo sed to be a reversible process that occurs via a non-concerted pathway . The vanadium-catalysed oxidation of conjugated dienes has also been discussed and the results are discussed in relation to the oxidation t aking place on a vanadyl pyrophosphate surface.