Functional models of vanadium haloperoxidases

In the living system enzymes are found that work as catalyst. The catalytic ability of an enzyme is found in the so called active site, which is located in a cavity or cleft in the enzyme. This active site differs among enzymes, not only in size but also due to the presence of different catalytically active amino acids. Because of this, enzymes can stabilize different transition states and catalyse different reactions. Enzymes are highly chemoselective, regioselective and sterioselective. In spite of extensive research activity in this area the exact role of vanadium compounds in biological systems is far from being fully understood. Because of the complexity inherent to biochemical processes, chemists often concentrate their attention on model systems. Studies on the interaction of vanadates with biologically important ligands in order to find structural and/or functional models of haloperoxidases are numerous and have been the subject of several reviews. For better understanding of working mechanism and role of metal, numerous vanadium complexes have been studied [91]. cis-Dioxovanadium(V) (VO2+) in acidic aqueous solution is first reported functional mimic of VBrPO [92]. This has been shown to catalyse the bromination of 1,3,5-trimethoxybenzene (TMB) as well as the bromide mediated disproportionation of H2O2. In a first step, (Scheme 1.1) H2O2 is coordinated to vanadium giving red oxoperoxo [VO(O2)+] and yellow oxodiperoxo [VO(O2)2–] complexes [93]. The ratio between these two species depends on the H2O2 concentration and the pH. In a second step these complexes combine, yielding dioxotriperoxodivanadium(V) [(VO)2(O2)3], which is considered to be the actual oxidant. cis-Dioxovanadium(V) functions at acidic condition (~ pH 2 or less), because at lower acid concentration the amount of monoperoxovanadate is insufficient for dimerisation to [(VO)2(O2)3]. When suitable nucliophile is not available, second equivalent of hydrogen peroxide reduces the brominium ion to bromide ion and singlet oxygen [94]. The formation of Br+ is rate determining and results in singlet oxygen and brominated products.

Oxidative bromination of 1,3,5-trimethoxybenzene (TMB) catalysed by [VO(OMe)(MeOH)(sal-oap)] using H2O2 as oxidant has been reported by Butler et al. [95]. The brominated product 2-bromo-1,3,5-trimethoxybenzene was obtained in good yield. The bromination of 1,3,5-trimethoxybenzene to some extent has also been catalysed by [VO(hybeb)]2− and [VO(bhybeb)]– (H2hybeb = 1,2-bis(2-hydroxybenzylamido)benzene, ) [96]. Oxidative bromination of salicylaldehyde to 5-bromosalicylaldehyde and 3,5-dibromosalicylaldehyde catalysed by [VO2(sal-inh)]− (H2sal-inh = Schiff base derived from salicylaldehyde and isonicotinic acid hydrazide) encapsulated in the cavity of zeolite-Y has been reported by Maurya et al. [97]. The encapsulation of [VO(salen)] (H2salen = bis(salicylaldehyde) ethylenediimine) in zeolite Na-Y via the flexible ligand method is described. NaY-[VO(salen)] was found to be an effective catalyst for the room temperature epoxidation of cyclohexene using t-butylhydroperoxide (t-BuOOH) as the oxidant [98].

Other vanadium(V) complexes of multidentate ligands, e.g. salicylideneaminoacids, iminodiacetic acid, nitrilotriacetic acid, citric acid, tripodal ligands etc. are some common functional models for vanadate-bromo peroxidsase. Scheme 1.2 presents the widely accepted mechanism after carrying out many spectroscopic studies [99].