Mechanism vanadium catalyzed peroxide

Hydrogen peroxide can be replaced by molecular oxygen in vanadium-catalyzed hydroxylations of arenes, as first reported by Mukaiyama (Eq. 3) [24]. Improved versions were later described by Battistel (VCl3/ascorbate) [25] and Shul pin ((n-Bu)4NV03/pyrazine-2-carboxylic acid/ascorbic acid) [26], It is noteworthy that a molecular oxygen/sacrificial reductant (crotonaldehyde, zinc, or ascorbate) combination was used (cf. Udenfriend s reagent above) and that the mechanism is most likely to involve hydroxyl radicals. 

Metalloenzymes contain a bound metal ion as part of their structure. This ion can either partidpate directly in the catalysis, or stabilize the active conformation of the enzyme. In Lewis acid catalysis (typically with zinc, vanadium, and magnesium), the M"+ ion is used instead of H+. Many oxidoreductases use metal centers such as V, Mo, Co, and Fe in much the same way as homogeneous catalysis uses ligand-metal complexes. Figure 5.7 shows a simplified mechanism for the halide oxidation readion catalyzed by vanadium chloroperoxidase. The vanadium atom ads as a Lewis add, activating the bound peroxide [30]. 

Clague, M.J. and A. Butler. 1995. On the mechanism of cw-dioxovanadium(V)-catalyzed oxidation of bromide by hydrogen peroxide Evidence for a reactive, binu-clear vanadium(V) peroxo complex. J. Am. Chem. Soc. 117 3475-3484. 

Mechanistic work on the aforementioned ruthenium and vanadium systems has been more limited. Katsuki has proposed a possible mechanism for the (nitro-so)Ru-salen catalyzed oxidation. Photolytic dissociation of NO enables coordination of alcohol. Presumably, 02 then performs two sequential one-electron oxidations to yield the ketone and hydrogen peroxide. This sequence is based primarily on the performance of certain alcohols in the oxidation [10]. 

Several enzymes have been isolated that do not have heme or vanadium as co-factors, yet are able to catalyze haloper-oxidase-type reactions. The enzymes have a Ser-His-Asp catalytic triad that is characteristic of esterases, hydrolases, and serine proteases, and in the presence of short chain carboxylic acids (e.g., acetic acid), it will halogenate monochlorodimedone. Biochemical, genetic, and structural studies have revealed that the mechanism of the enzyme involves the peroxidation of the carboxylic acid substrate and the resulting peracid reacts spontaneously with halide to generate hypohalous acid, which in turn halogenates monochlorodimedone (Scheme 45.6)." " ... 

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