Peroxides reductive cleavage

Early examples of such branched polysulphides, e.g. Thiokol FA, are believed to possess hydroxyl end groups and are coupled by means of zinc compounds such as the oxide, hydroxide, borate and stearate by a mechanism which is not understood. Later elastomers, e.g. Thiokol ST, have been modified by a restricted reductive cleavage (see below) and this generates thiol (mercaptan) end groups. These may be vulcanised by oxidative coupling as illustrated below with lead peroxide ... 

During attempted reductive cleavage of the ether with aluminium dichloride hydride etherate an explosion occurred. Peroxides may have been present in the susceptible ether. 

A much more precise investigation of the variation of a with the electrode potential has been reported recently.44 It concerns the electrochemical reductive cleavage of the O—O bond in a series of five organic peroxides ... 

Dichloroaluminium hydride in ether or sodium borohydride in TEA can lead to formation of ethers from ozonides by reductive cleavage of the two C—O bonds of the peroxide bridge (Equation (19)) <85JOC275>. 

Iron(n) is known to decompose hydrogen and dialkyl peroxides to free radicals by reductive cleavage of the 0—0 bond and early investigations established the parasite s sensitivity to these species. When treated with radiolabelled C-artemisinin, the hemin-hemozoin fraction of the lysed malaria-infected erythrocytes was shown to contain a radiolabel, though the mechanism of incorporation is not clear. Meshnick and coworkers demonstrated that uninfected cells did not contain radiolabelled proteins whereas six radiolabelled proteins were isolated from cells infected with the Plasmodium falciparum (P. falciparum) strain of the parasite. It was suspected that one of the alkylated proteins was the Histidine Rich Protein (HRP) that was known to bind multiple heme monomers and therefore thought to be instrumental to the parasite s detoxification process. Moreover, iron chelators were found to inhibit the lethal effects of peroxides on the parasite. ... 

They concluded that the parasiticidal action of trioxanes involved reductive cleavage of the peroxide bond by intracellular iron-sulphur redox centres (rather than heme) and subsequent alkylation of the redox centre. This type of redox centre is known to exist in many enzymes and Wu and coworkers proposed that structural differences between those in the parasite and those in mammalian systems could account for the high selective cytotoxicity of artemisinin. 

Reductive cleavage of O—O, O—N, O—S and S—N bonds is also possible. For example, the peroxide bond in the dioxazine (152) is cleaved by treatment with zinc in ethanolic potassium hydroxide, or by catalytic hydrogenolysis (64JOC291). Zinc in acid cleaves the S—N bond in (153) (68LA(715)223). Concomitant reduction of the sulfoxide also occurs, so that subsequent recyclization and aromatization with loss of ammonia leads to the ben-zothiazole (154). The O—S bond in (3) is reductively cleaved by sodium borohydride to give the alcohol (155) (77TL4245). 

Four methods have been developed for enzyme immobilization (1) physical adsorption onto an inert, insoluble, solid support such as a polymer (2) chemical covalent attachment to an insoluble polymeric support (3) encapsulation within a membranous microsphere such as a liposome and (4) entrapment within a gel matrix. The choice of immobilization method is dependent on several factors, including the enzyme used, the process to be carried out, and the reaction conditions. In this experiment, an enzyme, horseradish peroxidase (donor H202 oxidoreductase EC 1.11.1.7), will be imprisoned within a polyacrylamide gel matrix. This method of entrapment has been chosen because it is rapid, inexpensive, and allows kinetic characterization of the immobilized enzyme. Immobilized peroxidase catalyzes a reaction that has commercial potential and interest, the reductive cleavage of hydrogen peroxide, H202, by an electron donor, AH2 ... 

Hydration with boranes proved difficult due to extensive reductive cleavage of the epoxide (Scheme 45). The best results (54%) were achieved with borane in THF. The subsequent oxidation with basic hydrogen peroxide led to the primary alcohol as well as to saponification of the acetate. Selective protection of the primary alcohol 397 as trichloroethyl carbonate was followed by oxidation with RuCl3/NaI04, thereby producing not only the y-lactone but also the cyclic ketone. 

The following discussion will be focused on the effect of structural variations on the kinetics of fragmentation, the cleavage mode, and the stepwise or concerted nature of the process. Similar concepts have also been applied to the reductive cleavage reactions of other series of substrates, i.e. a-substituted acetophenones [246-250], perbenzoates [240, 251], peroxides [242, 252-256], sulfides [244, 257, 258], sulfonium salts [259-263], nitrocumenes [264, 265], arylmethyl aryl ethers and thioethers [266-270]. 

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