Metal complexes with peroxides

Martini M, Termini J (1997) Peroxy radical oxidation of thymidine. Chem Res Toxicol 10 234-241 Masarwa M, Cohen H, Meyerstein D, Hickman DL, Bakac A, Espenson JH (1988) Reaction of low-va-lent transition-metal complexes with hydrogen peroxide. Are they,Fenton-like or not 1. The case of Cu+aq and Cr2+aq. J Am Chem Soc 110 4293-4297 Maskos Z, Koppenol WH (1991) Oxyradicals and multivitamin tablets. Free Rad Biol Med 11 609-610... 

There is an extensive literature dealing with metal-catalyzed decompositions of peroxides.67,68 For the purposes of this article we will concentrate primarily on the reactions of metal complexes with hydrogen peroxide, alkyl hydroperoxides, and peracids, since these are the usual peroxidic intermediates in autoxidations. 

Similarly, a recent study141 of the homogeneous oxidation of cyclohexene by various low-valent phosphine complexes of Group VIII transition metals yielded no definite proof for initiation by oxygen activation. Results were consistent with reactions involving chain initiation via the usual redox reactions of the metal complexes with traces of hydroperoxides. Long induction periods were observed with peroxide-free hydrocarbons. 

Rate constants have been determined for the reduction of hydrogen peroxide by iron(II) and a number of iron(II) complexes. These rate constants have been compiled in Table 2. It is immediately clear that there is not much agreement between the results of various groups. However, there is a discernable trend metal complexes with more water-accessible coordination sites react faster. Graf et al. [117] have commented upon the importance of coordinated water molecules for the Fenton reaction. It is also clear that the rate of the Fenton reaction for a chelated complex near neutral pH is much faster than that of aqueous iron(II) at low pH. The use of the low-pH value of 16M ] s l in a recent calculation [118] of the flux of hydroxyl radicals in a cell gives an estimate that is at least two orders of magnitude too low. 

The synthesis of peroxo metal complexes with hydrogen peroxide is easily accomplished, normally by one of two methods. Firstly, using early transition... 

In Mechanism I, which is favored for the SOD enzymes and most redox-active metal complexes with SOD activity, superoxide reduces the metal ion in the first step, and then the reduced metal ion is reoxidized by another superoxide, presumably via a metal-peroxo complex intermediate. In Mechanism II, which is proposed for nonredox metal complexes but may be operating in other situations as well, the metal ion is never reduced, but instead forms a superoxo complex, which is reduced to a peroxo complex by a second superoxide ion. In both mechanisms, the peroxo ligands are protonated and dissoeiate to give hydrogen peroxide. 

Coimnon bleach activators are not very weight- and volume-effective, as they work stoichiometri-cally with peroxide. This drawback is overcome by using bleach catalysts, normally transition metal complexes with nitrogen- or oxygen-containing ligands. 

The dioxygen complexes which have been discussed may be prepared either by reaction of a metal complex with molecular oxygen or with peroxidic species. Both... 

Peroxo complexes of groups V and VI are usually formed by reaction of the parent metal complex with hydrogen peroxide. Hydroperoxides are often present during catalytic oxidation of organic substrates. Since certain of these peroxo complexes are capable of selectively oxidizing unsaturated organic compounds, a brief discussion of methods of formation of group V and VI metal peroxo complexes is in order. 

Like the alkali metal, boron and aluminium hydride compounds, hydroorganosilanes are so polarised that the electron cloud is concentrated on the hydrogen atom, which aids the formation of the hydride anion. Combinations of hydroorganosilanes with organic and inorganic acids or with certain catalysts (the transition metal complexes, the peroxides, etc.), are especially suitable for the reduction of organic compounds. The relative ease of hydride anion formation follows this series [537, 538] ... 

The first reported controlled polymerization based on the OMRP-RT principle appears to have been presented by Minoura in a series of articles starting in 1978, where the redox initiating system BPO/Cr was used for the polymerization of vinyl monomers.Not only were the kinetics different than in free-radical polymerization (very low reaction orders in Cif and BPO), but also the polymerization was observed to continue after all Cr had been converted by the peroxide to Cr and the degree of polymerization was found to increase with monomer conversion at low temperatures (<30 0). These studies included the report of a block copolymer (PMMA-b-PAN). Polydispersity indexes were not reported for these studies. Minoura formulated the mechanistic hypothesis of the formation of a metal complex with the free radical and stated that "the recombination of free radicals formed by the dissociation of the complexed radicals competes with a disproportionation of free radicals". However, these studies did not have a great impact in the polymer community, being cited only a handful of times before 1994. A few subsequent contributions reported the application of similar conditions to other metals but well-controlled polymerizations were not found."- " ... 

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

Chromium (ITT) can be analy2ed to a lower limit of 5 x 10 ° M by luminol—hydrogen peroxide without separating from other metals. Ethylenediaminetetraacetic acid (EDTA) is added to deactivate most interferences. Chromium (ITT) itself is deactivated slowly by complexation with EDTA measurement of the sample after Cr(III) deactivation is complete provides a blank which can be subtracted to eliminate interference from such ions as iron(II), inon(III), and cobalt(II), which are not sufficiently deactivated by EDTA (275). 

Transition metal-catalyzed epoxidations, by peracids or peroxides, are complex and diverse in their reaction mechanisms (Section 5.05.4.2.2) (77MI50300). However, most advantageous conversions are possible using metal complexes. The use of t-butyl hydroperoxide with titanium tetraisopropoxide in the presence of tartrates gave asymmetric epoxides of 90-95% optical purity (80JA5974). 

E++02, etc where E is an element. The metallic atoms are bonded to the oxygen bridge with ionic bonding. For convenience, differentiation is made between simple and complex inorganic peroxide compds. According to VoPnov (Ref 5) simple peroxide compds also... 

The beauty of bromide-mediated oxidations is that they combine mechanistic complexity with practical simplicity and, hence, utility. They involve an intricate array of electron transfer steps in which bromine atoms function as go-betweens in transfering the oxidizing power of peroxidic intermediates, via redox metal ions, to the substrate. Because the finer mechanistic details of these elegant processes have often not been fully appreciated we feel that their full synthetic potential has not yet been realized. Hence, we envision further practical applications in the future. 

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