Chapter 3: Activating Hydroperoxides by Vanadium(V) Compounds
Published:05 Nov 2020
Compounds of vanadium(v) activate hydroperoxides via two border mechanisms. The first mechanism operates in hydroperoxide-mediated oxidations catalysed by protonated orthovanadates in protic solvents, as encountered in marine bromoperoxidases. The substrate to be oxidized by bromoperoxidases is bromide, requiring hydrogen peroxide to be converted into an electrophile. A scale allowing estimating peroxide electrophilicity uses σ*(O,O)-orbital energies. Anionic vanadium(v) peroxides σ*(O,O) energies are above limit, defined by the energy of the corresponding orbital in hydrogen peroxide – a substrate that in pH-neutral environment is inert toward bromide. Adequate peroxide electrophilicity in the bromoperoxidase reaction is attainable from the side-on-conformer of orthovanadium(v) peroxoic, transferring with virtually no activation energy the proximal peroxide oxygen to bromide from the octahydrate. The second border mechanism operates when donating substituents at orthovanadate(v) and hydroperoxide compensate withdrawing orbital effects of the metal in peroxy compounds of vanadium(v), for example in trialkyl orthovanadate-catalysed oxidations by TBHP in aprotic solvents. Side-on conformation in such cases does not stabilize σ(O,O) and σ*(O,O), but assists heterolytically breaking the peroxide bond, and the bond between vanadium and the distal peroxide oxygen for being transferred to a reductant.