Catalysis of iodide oxidation

Another important example of catalytic oxidation of inorganic compounds by peroxidases is the catalysis of iodide oxidation by TPO. TPO is involved in the biosynthesis of thyroid hormone and catalyzes the reactions of iodination and coupling in the thyroid gland. Magnusson et al. [215] considered two possible pathways of iodination the formation of enzyme-bound hypoiodite and the formation of free hypoiodide (Reactions (17) and (18)) ... 

Ozone is also a better-oxygen-atom-transfer reagent in the atmosphere because UV light results in the formation of 02 and O atoms, which react with halides. The role of metals in the catalysis of iodide oxidation is also more likely in the atmosphere because of the enrichment of iodide relative to the other halides in rainwater and aerosols (13). 

Bromide ndIodide. The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaHy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colorless leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to cataly2e ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidi2ed to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

The mechanism of reaction (Scheme 5) involves the proton equilibrium of vanadium(v), followed by reaction with the hydrated form of the a-keto-acid. The values K=3A (35 °C) and 2.0 (30 °C) have been derived kinetically. Initial oxidation of glyoxylic acid to oxalic acid is ruled out in view of the complex reaction of the latter with V. A study has been made, however, of the oxalic acid catalysis of the oxidation of iodide by vanadium(v). ... 

Specific catalysis by sodium ions is also found in the oxidation of iodide ion by octacyanomolybdate(V) which otherwise shows simple, second-order kinet-ics with E = 5.68 2.57 kcal.mole" and A5 = —39 8.5 eu and /c2 = 3.52+0.13 l.mole sec , at 25.7 °C and fi = 0.1 M. Mo(IV), which is produced stoichiometrically exerts slight retardation upon the reaction. An outer-sphere one-electron transfer is proposed. 

Reaction (9) generates methyl iodide for the oxidative addition, and reaction (10) converts the reductive elimination product acetyl iodide into the product and it regenerates hydrogen iodide. There are, however, a few distinct differences [2,9] between the two processes. The thermodynamics of the acetic anhydride formation are less favourable and the process is operated much closer to equilibrium. (Thus, before studying the catalysis of carbonylations and carboxylations it is always worthwhile to look up the thermodynamic data ) Under standard conditions the AG values are approximately ... 

A UVV spectrophotometric method for the specific detection of peroxycarboxylic acids in the presence of H2O2 is based on direct oxidation of ABTS (103) by the analyte in an acidic medium. The spectrum of the resulting green free radical presents five absorption maxima where measurements can be made 406, 415 (the most intense), 649, 732 and 815 nm. Full color development may be accelerated by traces of iodide. The LOD is 1 p,M of AcOOH, with linearity in the 2.5 to 100 p,M range. Note that reaction of 103 with H2O2 in Section in.B.2.a requires peroxidase catalysis. 

Two other Ni(CO)4 substitutes, Ni(CO)3PPh3 and Ni(COD)2/dppe, prove to be appropriate for the catalysis of tandem metallo-ene/carbonylation reactions of allylic iodides (Scheme 7)399. This process features initial oxidative addition to the alkyl iodide, followed by a metallo-ene reaction with an appropriately substituted double or triple bond, affording an alkyl or vinyl nickel species. This organonickel species may then either alkoxycar-bonylate or carbonylate and undergo a second cyclization on the pendant alkene to give 51, which then alkoxycarbonylates. The choice of nickel catalyst and use of diene versus enyne influences whether mono- or biscyclization predominates (equations 200 and 201). 

Several further experimental and theoretical studies of the oxidative addition of methyl iodide to Rh(I) complexes have been reported, in part because of its importance in the rhodium-catalyzed carbonylation of methanol (see Carbonylation Processes by Homogeneous Catalysis) to... 

Extensive studies have been made of the oxidations of all the halides by hydrogen peroxide. Mellor in 1904 was already able to cite fourteen investigations of kinetics of the hydrogen peroxide-hydrogen iodide reaction including studies of the temperature dependence of the rates and the kinetic form of catalysis by salts of molybdenum and iron. Since the processes (1) and (2)... 

Metal Catalysis with 02 as Oxidant. Oxidation of sulfide can be enhanced by trace metals (27). However, iodide oxidation is not likely to be catalyzed by such a process. Luther (25) indicated that sulfide transfers electrons to oxygen through the metal. The main requirement appears to be a... 

When the surfactant was sodium dodecyl sulfate, some butanol had to be present to achieve the microemulsion. A typical run used 2 mmol aryl iodide, 0.5 g surfactant, 1 mL butanol, 10 mL water, and 2 mmol base. The butanol was unnecessary with nonionic surfactants, such as the one derived from 1-dodecanol with 23 eq of ethylene oxide. The advantages of such a system include the following (a) No organic solvent is needed. The substrate is the oil phase, (b) The microemulsions form without the need for vigorous agitation, (c) No excess base is needed, in contrast with some reactions in which phase-transfer catalysis is used, (d) The surfactants can be recovered and recycled. They are inexpensive and biodegradable. 

As was mentioned in the introduction and is discussed more fully under catalysis by iron salts, it is possible for hydrogen peroxide to react by single electron transfers thus giving rise to the free radicals HO- and HO2. Mechanisms for the reactions of the halides and halogens based on these ideas have been proposed by Weiss (26) and by Abel (27). The oxidation of iodide may be formulated ... 

Catalysis by cobalt(ii) has been observed in the oxidation of iodide by per-oxodisulphate, the mechanism involving formation of the reactant complex C0I+. It is perhaps of interest that this is the first paper concerned with a redox mechanism that the Reporter has seen from this source. 

Copper(ii)-ion catalysis has been shown to take place in the oxidation of iodide by peroxodiphosphate. The results conform to the rate law... 

For the development of sustainable, more efficient, and selective catalysis, the use of comparatively nontoxic Fe-based catalysts is particularly interesting owing to their Lewis acidic character and the facile change of their oxidation state [29]. There have been several reports on using a Fe/Cu catalytic dyad to couple aryl iodides and terminal alkynes in good yields. Such catalytic systems typically employed Fe(II) or Fe(III) salts and Cul in combination with CS2CO3 as the base in polar... 

Homogeneous catalysis often involves changes in oxidation number of the ions involved in catalysis. For example, small amounts of iodide ions catalyse the decomposition of hydrogen peroxide. In the catalysed reaction, iodide ions, H, are first oxidised to iodate(I) ions, 10. The IQ- ions then react with further molecules of hydrogen peroxide and are reduced back to iodide ions. 

Oxidation. Hydrogen peroxide is a strong oxidant. Most of its uses and those of its derivatives depend on this property. Hydrogen peroxide oxidizes a wide variety of organic and inorganic compounds, ranging from iodide ions to the various color bodies of unknown stmcture in ceUulosic fibers. The rate of these reactions may be quite slow or so fast that the reaction occurs on a reactive shock wave. The mechanisms of these reactions are varied and dependent on the reductive substrate, the reaction environment, and catalysis. Specific reactions are discussed in a number of general and other references (4,5,32—35). 

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