Oxidation of halides

When 6/3-aminopenicillanic acid (6-APA) is diazotized in the presence of chloride ion, the principal product obtained is 6a-chloropenicillanic acid (38) (62JOC2668), presumably by way of the diazo intermediate (39 Scheme 29) (72JCS(P1)895). If the diazotization is carried out in the presence of excess bromide instead of chloride, significant amounts of the 6,6-dibromo derivative are obtained, and in the case of excess iodide the 6,6-diiodopenicillanic acid becomes the predominant product (69JCS(C)2123). The 6,6-dihalo products presumably arise from nitrous acid oxidation of halide to halogen, which then reacts with (39). 

Such solutions are necessarily contaminated with halide ions and with the products of any subsequent decomposition of the hypohalite anions themselves. Alternative routes are the electrochemical oxidation of halides in cold dilute solutions or the chemical oxidation of bromides and iodides ... 

Myeloperoxidase (MPO) is a heme containing protein that catalyzes the two electron oxidation of halides (Cl-, Br- and I-) and pseudohalide (SCN-) to the corresponding hypohalous acid (Eq. (51)) in a process that is dependent upon [X-], [H202] and [H+] (135). MPO is found in high concentrations in neutrophiles and plays important roles in immune response and inflammation (136). 

VHOCs was also observed in response to grazing pressure in Ascophyllum nodosum beds (Nightingale et al. 1995). Their biogenesis involves vanadium haloperoxi-dases (vHPO), which catalyse the oxidation of halides (X ) and generate X+ to yield hypohalous acid (XIO) and other forms of oxidized halides (X2, X2 ). Marine organisms and especially seaweeds have been known for a long time to concentrate halides from their environment (for a recent review, see Leblanc et al. 2006). 

Other inorganic reactions shown to be photo-induced at colloidal semiconducting metal oxide surfaces include the synthesis of ammonia from water and nitrogen (19) and the oxidation of halide ions 1 ,... 

The validity of reaction Scheme 1 is not limited to heterolyses of covalent bonds to carbon. It covers also cases where both X and Y are hetero-atoms (e.g. oxidation of halides and pseudohalides by OH, cf. Ref [40] or where X is a hetero atom and Y is a metal (cf. Ref [41])... 

At a platinum electrode, highly purified FLINAK has a voltammetric window extending from about +1.5 to -2.0 V vs. the nickel reference electrode [7]. The positive limits of the alkali halide melts discussed herein arise from the oxidation of halide ions, whereas the negative limits are due to reduction of the alkali metal ions. Because chloride ion is substantially easier to oxidize than fluoride ion, the potential window of the LiCl-KCl melt is approximately 1.5 V smaller than that for FLINAK. 

In addition to providing new synthetic tools, it has been suggested that semiconducting powders may be of use in the natural decontamination of polluted waters. Solar irradiation of natural water supplies that have been treated with semiconducting powders could be used to oxidatively degrade pollutants. For example, the oxidation of cyanide to isocyanate, which decomposes into nitrogen and carbon dioxide, has been demonstrated. The oxidation of halides to atomic halogen and the oxidation of hydroxide to OH0 has also been observed. These species can be employed as biocides to provide potable water. 

Electrochemical reactions at metal electrodes can occur at their redox potential if the reaction system is reversible. In cases of semiconductor electrodes, however, different situations are often observed. For example, oxidation reactions at an illuminated n-type semiconductor electrode commence to occur at around the flat-band potential Ef j irrespective of the redox potential of the reaction Ergdox Efb is negative of Ere 0 (1 2,3). Therefore, it is difficult to control the selectivity of the electrochemical reaction by controlling the electrode potential, and more than one kind of electrochemical reactions often occur competitively. The present study was conducted to investigate factors which affect the competition of the anodic oxidation of halide ions X on illuminated ZnO electrodes and the anodic decomposition of the electrode itself. These reactions are given by Eqs 1 and 2, respectively ... 

With estimated ip(X ), the percentage of oxidation of halide ions (X ) can be determined as a ratio of iq to the total disk photocurrent measured ... 

The importance of the first factor, the concentration, is clear because the anodic photocurrent due to oxidation of halide ions should be proportional to the product of the concentration of positive holes at the electrode surface and that of halide ions in solution. When ip becomes large, the supply of halide ions to the electrode surface by diffusion becomes unable to follow ip, resulting in a decrease of (X ), which depends on the concentration of X". 

Another model for giving an explanation of the pH dependence of the reactivity of halide ions may be that surface cations serve as effective sites for adsorption of reaction intermediates which are produced in the course of the anodic oxidation of halide ions. Usually, the anodic oxidation of halide ions is believed to... 

VCPOs catalyze the two-electron oxidation of halides (X- = Cl-, Br-, I ) using activated peroxide through a Lewis acid-promoted mechanism (in contrast to possible redox cycling at the V center). Peroxide is bound to V in r/ -fashion after release of the apical oxygen (Littlechild, 2002) this release is catalyzed through the... 

Initial studies on functional mimics of V-BrPO were driven by the lack of spectroscopic techniques capable of observation of the vanadium(V) site in the enzyme. Early on it was found that acidic solutions of cw-dioxovanadium(V) (cis-V02+) catalyzed the oxidation of halides by dihydrogen peroxide, resulting in halogenation of an organic substrate and halide-assisted disproportionation of dihydrogen peroxide [73],... 

The haloperoxidases are a class of enzymes that catalyze the oxidation of halides via a reactive peroxometal active site. These enzymes are named according to the most electronegative halide they are able to oxidize. Hence, a bromoperoxidase can oxidize bromide and iodide but not chloride, whereas a chloroperoxidase can oxidize all three. Haloperoxidases are found in most living organisms and predominately fall into two classes the iron heme-based and vanadium-dependent enzymes. Of these, heme-based enzymes are found in mammals, where they provide a vital... 

Impurities are a concern in ionic liquids electrochemistry. Whereas even considerable amounts of impurities, like different metal ions, water or organic impurities, might not disturb a technical process (e.g. extractive distillation, organic synthesis) the wide electrochemical windows of an ionic liquid ( 3 V vs. NHE) allow the electrodeposition of even reactive metals like lithium and potassium, as well as the oxidation of halides to the respective gases. In the best case this codeposition only leads to a low level of impurities, in the worst case fundamental physicochemical studies are made impossible as the impurities are adsorbed onto the electrode surface and subsequently reduced. Furthermore, passivation or activation effects at the counter electrode have to be expected. 

The potential window can be limited by the decomposition potential of a solute, not just a solvent. In particular, reactions of anodic oxidation of halides (Cl-, Br, and I-) on diamond are highly irreversible and have much higher overvoltage (for Cl, by 1 V) than on platinum or graphite electrodes [97, 123, 124], In all probability this is due to poor adsorption of intermediates, that is, Cl, Br, and I atoms, on the diamond electrode surface. We recall that the outer-sphere reactions discussed in Section 6.1 generally do not involve adsorption of intermediates and thus are not... 

Photogenerated holes can also be simultaneously utilized to oxidize substrates at the semiconductor surface [43,100-102]. The holes trapped at the Ti02 surface can survive for a duration of microseconds to milliseconds and can react with OH- to generate OH radicals in aqueous medium. These OH radicals can further induce secondary oxidation. Oxidation of halide ions (X-) has been investigated in various colloidal semiconductor suspensions by laser flash photolysis [103,104]. 

Haloperoxidases catalyze the hydroperoxide-dependent oxidation of Cl-, Br- and/or I to electrophilic halogenating species that halogenate organic substrates (equation 1). As predicted by the relative ease of oxidation of halide ions (I- > Br- > Cl" >> F-), chloroper-oxidases oxidize Cl-, Br- and I-, bromoperoxidases oxidize Br- and I-, iodoperoxidase oxidize only I- while no peroxidase can oxidize F-14. 

Direct, two-electron oxidations are rare for most peroxidase enzymes. The one broad exception is the oxidation of halide and pseudohalide ions, specifically F, Br , Cl-, and NCS . Fluoride ion, in contrast, is not known to be oxidized by these enzymes. The oxidation of F and NCS" is common for the peroxidases, whereas that of Br is widespread but is usually less effective, and that of Cl , among the conventional peroxidases, is only important in the case of MPO [46, 83]. The halogenation activities of the mammalian peroxidases are compared in Table 5.3. As the table shows, chloride ion is oxidized by MPO, particularly at pH 5, but it is a very poor substrate for EPO and LPO. Br , F, and SCN" are readily oxidized by all three enzymes, but most efficiently by EPO at pH 5 [84—86]. 

Table 5.3 Apparent second order rate constants for the oxidation of halides and thiocyanate at pH 7 and 4.5 by Compound I of MPO, EPO, and LPO...

Despite that the MPO-catalyzed oxidation of halides seems to employ the same mechanisms active in primary production of the respective hypohalous acids, the final products of peptide and protein oxidation vary, depending on the halide ion employed iodides and bromides when used as substrates yield stable bromo-and iodotyrosine derivatives, whereas direct chlorination of the available free amino moieties and semistable chloramines formation predominate when Cl is oxidized as the substrate (S54, Z3). The chlorination is a unique function of polymorphonuclear neutrophilic leukocytes (W6, Z2). 

Generation of positive-like halogen species in situ has been realized by the oxidation of halide salts with m-chloropeibenzcnc acid (M BA). The procedure can be used for haloetherification and lactoni-zation (equation 28). Oxidation of potassium bromide with MCPBA in the presence of 18-crown-6 (10 mol %) produces m-chlorobenzoylhypobromite, which adds across the double bond to furnish trans-, 2-bromocarboxylates (55 equation 29). ... 

Semiconductor-mediated photoelectrochemical oxidation of halide salts provides a procedure for the halogenation of alkenes via excitated halide species. For example, bromination of cyclohexene has been performed in a Ti02/Bu4NBr (or PhaPMeBr) /O2 system. The reactive bromine species probably arise from a one-electron oxidation of adsorbed bromide ions on the semiconductor by photoin ation, which produces surface-bound bromine atoms (equation 40). 

Known as the Komblum oxidation, this is peihaps die most widely used and best-known mediod for die oxidation of halides. It works best with activated halides such as benzyl halides, a-halocarbonyl... 

This is one of the oldest methods for the oxidation of halides, and has been used quite widely for die preparation of benzaldehydes and heteroaromadc aldehydes from the halomediyl compounds. Unactivated aliphatic halides give reduced yields. 

The cationic species formed by the anodic oxidation of halide anions add to alkenes in the presence of suitable nucleopMles (equation 69). ... 

Haiogenation of aromatic nuclei may also be achieved by halogen or positively charged species of halogen formed in solution by anodic oxidation of halide anions (equation 70). ... 

MPO (Myeloperoxidase) [EC 1.11.1.7] catalyzes the hydrogen peroxide dependent two-electron oxidation of halides (CD, I, Br ) and thiocyanate to the corresponding hypohalous acids and hypothiocyanate which are cytotoxic to invading pathogens. In addition, MPO is capable of the singleelectron oxidation of a wide variety of aromatic alcohols and amines. " ... 

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