Redox mechanism hydroperoxides reactions

Hydroperoxides are more widely used as initiators in low temperature appHcations (at or below room temperature) where transition-metal (M) salts are employed as activators. The activation reaction involves electron-transfer (redox) mechanisms ... 

Organic peroxide-aromatic tertiary amine system is a well-known organic redox system 1]. The typical examples are benzoyl peroxide(BPO)-N,N-dimethylani-line(DMA) and BPO-DMT(N,N-dimethyl-p-toluidine) systems. The binary initiation system has been used in vinyl polymerization in dental acrylic resins and composite resins [2] and in bone cement [3]. Many papers have reported the initiation reaction of these systems for several decades, but the initiation mechanism is still not unified and in controversy [4,5]. Another kind of organic redox system consists of organic hydroperoxide and an aromatic tertiary amine system such as cumene hydroperoxide(CHP)-DMT is used in anaerobic adhesives [6]. Much less attention has been paid to this redox system and its initiation mechanism. A water-soluble peroxide such as persulfate and amine systems have been used in industrial aqueous solution and emulsion polymerization [7-10], yet the initiation mechanism has not been proposed in detail until recently [5]. In order to clarify the structural effect of peroxides and amines including functional monomers containing an amino group, a polymerizable amine, on the redox-initiated polymerization of vinyl monomers and its initiation mechanism, a series of studies have been carried out in our laboratory. 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

In accord with this mechanism, free peroxyl radical of the reaction product hydroperoxide activates the inactive ferrous form of enzyme (Reaction (1)). Then, active ferric enzyme oxidizes substrate to form a bound substrate radical, which reacts with dioxygen (Reaction (4)). The bound peroxyl radical may again oxidize ferrous enzyme, completing redox cycling, or dissociate and abstract a hydrogen atom from substrate (Reaction (6)). 

Various transition metals have been used in redox processes. For example, tandem sequences of cyclization have been initiated from malonate enolates by electron-transfer-induced oxidation with ferricenium ion Cp2pe+ (51) followed by cyclization and either radical or cationic termination (Scheme 41). ° Titanium, in the form of Cp2TiPh, has been used to initiate reductive radical cyclizations to give y- and 5-cyano esters in a 5- or 6-exo manner, respectively (Scheme 42). The Ti(III) reagent coordinates both to the C=0 and CN groups and cyclization proceeds irreversibly without formation of iminyl radical intermediates.The oxidation of benzylic and allylic alcohols in a two-phase system in the presence of r-butyl hydroperoxide, a copper catalyst, and a phase-transfer catalyst has been examined. The reactions were shown to proceed via a heterolytic mechanism however, the oxidations of related active methylene compounds (without the alcohol functionality) were determined to be free-radical processes. 

The effects of heteroatoms on autoxidation reactions are reviewed and discussed in terms of six phenomena (1) the effect on reactivity of a-hydrogens in the hydroperoxide chain mechanism in terms of electron supply and withdrawal (2) the effect on a-hydrogen acidity in base-catalyzed oxidation (3) the effect on radical ion stability in base-catalyzed redox chains (4) the possibility of heteroatom hydrogen bond attack and subsequent reactions of the resulting heteroradical (5) the possibility of radical attack on higher row elements via valence expansion (6) the possibility of radical addition to electron-deficient II and III group... 

The hydroperoxides formed are relatively stable at temperatures below 100 °C, but may be decomposed by almost any agent A, which can complex with it and give a redox reaction, reminiscent of the Haber-Weiss mechanism... 

However, no convincing evidence has been presented to support such mechanisms. The contrary notwithstanding, these reactions almost certainly involve initiation via the usual redox decomposition of trace hydroperoxides. 

Figure 6 presents a postulated mechanism for these two reactions. In the peroxidase reaction, a hydroperoxide substrate such as PGG2 converts PPIXFe(III) + to PPIXFe(V)0 +4 (intermediate I) in a two-electron redox step to give PGH2. Intermediate I then removes a hydrogen atom from a nearby tyrosine residue to give PPIXFe(IV)0 -(TYR-) (intermediate II). The tyrosyl radical of intermediate II links... 

The mechanisms and rates of metal-catalyzed initiation operative in individual reaction systems are determined by a complex mixture of factors the metal and type of complexes it forms (inner sphere or outer sphere), the chelator or complex-ing agent, redox potential of the metal and its complexes, solvents, phase localization of the metal, and availability of oxygen or preformed hydroperoxides. The reactions outlined below show the multiplicity of mechanisms possible. 

The generation of free radicals at such low temperatures can be conveniently accomplished by redox reaction between organic hydroperoxides and sulfur dioxide. This is an effective method for initiation at temperatures as low as -80 C (2, 2)- A plausible mechanism for radical formation is given below (4) ... 

Another explanation was the following. The organomontmorillonite used was a natural montmorillonite that contained iron. Chemical analysis of the clay confirmed the presence of a low amount of iron. It was recalled that iron and, in more general terms, metals are likely to induce the photochemical degradation of polymers. Iron at low concentration had a prooxidant effect that was due to the metal ion of iron that can initiate the oxidation of the polymer by the well-known redox reactions with hydroperoxides [93]. It was concluded that the transition metal ions, such as Fe, displayed a strong catalytic effect by redox catalysis of hydroperoxide decomposition, which was probably the most usual mechanism of filler accelerating effect on polymer oxidation. A characteristic of such catalytic effect was that it did not influence the steady-state oxidation rate, but it shortened the induction time. 

Substituted aromatic molecules can be oxidized under mild conditions using Ti or V substituted molecular sieves. The nature and the selectivities of the products formed strongly depend on the oxidant used in the catalytic reaction. H2O2 favours the hydroxylation of the aromatic ring whereas tert-butyl hydroperoxide is very selective in the side-chain oxidation. For V-substituted ole ar sieves, we have proposed a mechanism which involves the redox system... 

Post-heating of a shaped polyethylene which induces its crosslinking may sometimes lead to an undesired deformation of the product. To overcome this difficulty, two-component redox-initiating systems producing free radicals at lower temperatures have been designed [108]. The reaction system involved cumyl hydroperoxide and a transition metal ion, whose higher and lower oxidation states differ by one electron. The decomposition of hydroperoxide proceeds by an electron transfer mechanism an electron is transferred from the metallic ion (e.g. Co ) to the peroxidic bond which splits into two fragments. 

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