Model peroxy complex

Related V complexes show activity toward the oxidative decomposition of pinacol with C—C bond cleavage and aerobic oxidation of 4-methoxybenzylalcohol and other lignin model compounds." Other oxidovanadium(V) complexes with c 5-2,6-bis-(methanolate)-piperidine ligands of the type depicted on Scheme 3 were appHed as catalysts to convert prochiral alkenols into 2-(tetrahydrofiiran-2-yl)-2-propanols, 2-(tetrahydropyran-2-yl)-2-propanols, oxepan-3-ols and epoxides, upon oxidative alkenol cyclization with TBHP as oxidant (Scheme 3)." These catalysts are rather stable and possess improved chemoselectivity, e.g., epoxidation of geraniol occurs enantioselectively. It was ruled out the vanadium(V) ieri-butyl peroxy complex formation is a key step to activate peroxides. 

The mechanism shown in Scheme 5 postulates the formation of a Fe(II)-semi-quinone intermediate. The attack of 02 on the substrate generates a peroxy radical which is reduced by the Fe(II) center to produce the Fe(III) peroxide complex. The semi-quinone character of the [FeL(DTBC)] complexes is clearly determined by the covalency of the iron(III)-catechol bond which is enhanced by increasing the Lewis acidity of the metal center. Thus, ultimately the non-participating ligand controls the extent of the Fe(II) - semi-quinone formation and the rate of the reaction provided that the rate-determining step is the reaction of 02 with the semiquinone intermediate. In the final stage, the substrate is oxygenated simultaneously with the release of the FemL complex. An alternative model, in which 02 attacks the Fe(II) center instead of the semi-quinone, cannot be excluded either. 

The peroxy intermediate derived from 02 attack on the activated substrate is proposed to act as a tridentate ligand to the iron(III) center as shown in Figure 15. Molecular modeling of this adduct using the structure of the PCD ES complex as a starting point shows that such a structure is reasonable [133], This coordination mode is precedented in the structure of the 02 adduct of [Ir(III)(tri-phos)(dbc)]+ [150,151],... 

The performance of ZnDTP as an antioxidant is a complex interaction pattern involving hydroperoxides and peroxy radicals. The performance matrix is additionally influenced by other additives present in industrial or engine oil formations. In a model system comprising cumene hydroperoxide and diverse ZnDTPs, it was demonstrated that the antioxidant mechanism proceeds by an acid-catalysed ionic decomposition of the hydroperoxide. The catalyst species is (9,0 -dialkyl-hydrogendithiophosphate, (RO)2PS2H, derived from the ZnDTP, Reaction (4.57) ... 

In principle, the computational approach to the kinetics of the complex photooxidation process can give meaningful insight into the effects of outdoor weathering of hydrocarbon polymers. For clear amorphous linear polyethylene, the model suggests that the optimum stabilizer would be a molecularly dispersed additive in very low concentration which could trap peroxy radicals. An additive which decomposes hydroperoxides would also be effective but would require higher concentrations. The useful lifetime of unstabilized polyethylene is predicted to vary from a few months in hot weather (100°F) to almost two years in cool weather (45°F), which correlates well with experimental results and general experience. 

The titanium-tartrate complex was proven to have a dimeric structure by X-ray analysis [30,31 ]. The complex has been considered to maintain the dimeric structure also in solution. Based on the X-ray structure. Sharpless has proposed the transition state model 7 for the present epoxidation (Fig. 1). The coordination of the distal oxygen (O ) in the loaded TBHP activates the peroxy bond and facilitates the nucleophihc attack of the double bond as discussed in the beginning of this chapter. Both stereochemistry and substrate reactivity in this reaction can be reasonably explained with the model 7 [3,4,6,30,31]. The loaded substrate takes a conformation having a small dihedral angle (0-C-C=C, ca. 30°) to deliver its olefinic moiety in an appropriate space for the epoxidation. The -substituent... 

T. thermophilus MnSOD, and the data showed that there were three phases to this reaction a fast burst phase of quick HO2 dismutation, a slower second phase, and a final fast phase (78). A dead-end form of the enzyme was implicated to account for the slow phase (78). This has been formulated as a side-on-bound Mn -peroxo species based on spectroscopic similarities to manganese model complexes with side-on-bound peroxy groups (83). Such phases are not observed for FeSOD (84). 

Model reactions in homogeneous solution indicate that the bleach reaction itself is a multistep process [66]. The reaction is first order with respect to both the active oxidant and the stain, and depends on the ionization state of the reactants one has to be dissociated, and the other one should be in undissociated form. A transition complex is formed, in which one hydrogen atom is involved, and finally the oxygen is transferred to the substrate to complete the reaction. The reaction is governed by the pH of the solution, and maximum bleach rates are achieved at a pH that is the mean of pKa (stain) and pKa (peroxy acid) [67]. 

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