Peroxovanadium complexes oxidation reactions

Other Complexes. Before we turn to a discussion of other complexes, it is worth making a few general comments about the biomimetic systems studied to date. The model systems are much slower ( 105-fold) than the enzyme (33). All peroxovanadium complexes, whether competent to catalyze bromide oxidation reactions or not, contain tj2-coordinated peroxide (4). Little is known about the binding of peroxide in the enzyme (see above), but one wonders whether the enhanced reactivity is derived from an alternative binding mode, such as end-on peroxide or hydroperoxide. The rapid enzymatic rate could also arise from the nature or configuration of the ligands to the vanadium ion. 

Despite the fact that synthetic peroxovanadium complexes do not model satisfactorily the active centre of the VHPOs in the sense outlined above, most of them are effective oxidants under mild conditions, or even catalysts for, or active intermediates in the catalysis of, oxygenation reactions using hydrogen peroxide as the oxo transfer agent. Examples of 0X0 transfer reactions to organic substrates carried out via peroxovanadium complexes -the hydroxylation of alkanes, alkenes and aromatic compounds by the picolinato complex 22 (Figure 4.21) - are depicted in Scheme 4.8. 

Reports on the ring hydroxylation of electron-poor aromatic componnds are relatively scarce. Chlorobenzene, nitrobenzene, benzonitrile, benzaldehyde, and benzoic acid are inert toward hydroxylation with over TS-1 [81]. The hydroxylation of benzene derivatives XC j (X=F, Cl, Br, NOj) by and in the presenee of eatalytie amonnts of [V0(02)(pie)(H20)2] prodnced corresponding phenols in moderate yields [94]. The reaction was proved to be a radical chain process where initiation prodnces a radical anion of the peroxovanadium complex as the actnal oxidizing species [34]. 

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