Hydrogen peroxide, as substrate

The mechanistic details of the nigerythtrin-catalyzed reaction, and in particular the molecular basis for its preference for hydrogen peroxide as substrate, are not yet established. A mechanism has recently been proposed for the peroxidation re-... 

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

In conclusion, the above summary of oxidation methods shows that there is still room for further improvements in the field of selective olefin epoxidation. The development of active and selective catalysts capable of oxidizing a broad range of olefin substrates with aqueous hydrogen peroxide as terminal oxidant in inexpensive and environmentally benign solvents remains a continuing challenge. 

Oxidases catalyze the removal of hydrogen from a substrate using oxygen as a hydrogen acceptor. They form water or hydrogen peroxide as a reaction product (Figure 11-1). 

HYDROPEROXIDASES USE HYDROGEN PEROXIDE OR AN ORGANIC PEROXIDE AS SUBSTRATE... 

The conception, proposed by Haber [17-19], was very close to Traube s hypothesis. Haber considered the formed hydrogen peroxide as an intermediate that can oxidize other substrate for example, the induced oxidation of S02 during the oxidation of As203 was treated according to the following scheme ... 

Two basic methods have been used to grow metal oxide thin films by the SILAR technique (see Table 8.1). The more common of these methods consists of the adsorption of metal hydroxide ions on the substrate surface followed by thermal treatment to convert hydroxide to an oxide. Another way to produce metal oxide films is to use hydrogen peroxide as the anion precursor and then to convert the formed metal peroxide film to an oxide film. Several examples of each approach are discussed in more detail below. 

PB and its derivatives are of interest for a variety of reasons, the most important of which is its electrochromism [93]. In addition, it is an electrocatalyst for several different types of substrates, notably hydrogen peroxide, as will be seen below. Synthesis of nanopartides of Prussian Blue is relatively straightforward. It relies on many of the prindples of colloid chemistry, and produces ionically stabilized colloidal solutions (Figure 4.7). As a consequence, the electrochemical behavior of PB N Ps has been examined by several groups. In this section, we discuss the behavior of P B N Ps immobilized at electrodes. 

Peroxidases (EC 1.11.1.7), which have ferric protoheme prosthetic groups, react non-selectively via free radical mechanisms, using hydrogen peroxide as the electron acceptor. A reactive Fe(IV)-0 species and a radical heme intermediate are formed, and the intermediate then reacts with the reducing substrate to produce the oxidized product, regenerating the Fe(III) ion. 

A biphasic system consisting of the ionic liquid [BMIM]PF6 and water was used for the epoxidation reactions of a, 3-unsaturated carbonyl compounds with hydrogen peroxide as an oxidant at room temperature 202). This biphasic catalytic system compared favorably with the traditional phase transfer catalysts. For example, under similar conditions (15°C and a substrate/NaOH ratio of five), the [BMIM]PF6/H20 biphasic system showed a mesityl oxide conversion of 100% with 98% selectivity to oc, 3-epoxyketone, whereas the phase-transfer catalyst with tet-rabutylammonium bromide in a CH2CI2/H2O biphasic system gave a conversion of only 5% with 85% selectivity. 

Holm KA. Automated determination of microbial peroxidase activity in fermentation samples using hydrogen peroxide as the substrate and 2,2 -azino-bis(3-ethylbenzothiazoUne-6-sulfo-nate) as the electron donor in a flow-injection system. Analyst 1995 120 2101-2105. 

The effect of structural variation and the use of different caboxylate salts as cocatalysts was investigated by Pietikainen . The epoxidation reactions were performed with the chiral Mn(III)-salen complexes 173 depicted in Scheme 93 using H2O2 or urea hydrogen peroxide as oxidants and unfunctionalized alkenes as substrates. With several soluble carboxylate salts as additives, like ammonium acetate, ammonium formate, sodium acetate and sodium benzoate, good yields (62-73%) and moderate enantioselectivities (ee 61-69%) were obtained in the asymmetric epoxidation of 1,2-dihydronaphthalene. The results were better than with Ai-heterocycles like Ai-methylimidazole, ferf-butylpyridine. 

Besides a variety of other methods, phenols can be prepared by metal-catalyzed oxidation of aromatic compounds with hydrogen peroxide. Often, however, the selectivity of this reaction is rather poor since phenol is more reactive toward oxidation than benzene itself, and substantial overoxidation occurs. In 1990/91 Kumar and coworkers reported on the hydroxylation of some aromatic compounds using titanium silicate TS-2 as catalyst and hydrogen peroxide as oxygen donor (equation 72) . Conversions ranged from 54% to 81% with substituted aromatic compounds being mainly transformed into the ortho-and para-products. With benzene as substrate, phenol as the monohydroxylated product... 

Catalytic H2O2 Oxidations. The reactions were carried out in 10-mL tubes equipped with serum cap and a stirring bar. The catalyst, hydrogen peroxide, and substrate were dissolved in of CH3CN. Trimethylacetonitrile was added to the reaction as an internal standard. All reactions were done under argon, each was purged by three freeze-thaw cycles and GC analysis was performed on aliquots withdrawn directly from the reaction mixture. Typically, alkene (1 mmol) was added... 

The discovery of titanium substituted ZSM-5 (TS-1) and ZSM-11 (TS-2) have led to remarkable progress in oxidation catalysis (1,2). These materials catalyze the oxidation of various organic substrates using aqueous hydrogen peroxide as oxidant. For example, TS-1 is now used commercially for the hydroxylation of phenol to hydroquinone and catechol (/). Additionally, TS-1 has also shown activity for the oxidation of alkanes at temperatures below 1()0 °C (3,4). 

Peroxidase (POD) catalyzes oxidation of a wide range of o-diphenolic substrates to o-quinones, using hydrogen peroxide as a co-substrate (3) ... 

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