Reactions of Dioxygen

As in the above section, the reader is referred to Part III where details of the biologically oriented reactions of dioxygen are described. Several reviews - have been published relating to the inorganic and bio-inorganic aspects of oxygenation and redox reactions. Recent studies indicate that the primary electron-transfer step for the reduction of O2 is a one-electron process which is followed under acidic conditions by chemical disproportionation to yield HgOa (an overall 2e process) and under basic conditions by production of OH (a 4e process overall). The primary reaction reduction potential is —0.33 V and is independent of pH. 

The reaction of di-iM-oxo-bis(oxo L-(+)-cysteinato molybdate(v)], [(Mo 2-(cys)2], with dioxygen has been investigated in phosphate and ammonia buffers. At pH 6.5 (HP04 --H2P04-) the reaction proceeds (O in excess) according to the scheme 

At least one phosphate group is considered bound to the molbydenum(v). H2O2 reacts about 20 times faster than O2 with [Mo 2(cys)2] and addition of peroxide to Mo i yields the same product as that of the dioxygen reaction. If the metal complex is present in excess, however, there is evidence for adduct formation  

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One of the most important reactions of dioxygen is that with the protein haemoglobin which forms the basis of oxygen transport in blood (p. 1099). Other coordination... 

The hexamethylbenzene complex could be similarly deprotonated [54, 57] but the red complex obtained is then stable and its x-ray crystal structure could be recorded [55], showing a dihedral angle of 32° between the cyclohexadienyl plane and the exocyclic double bond (Fig. 5). This complex can also be cleanly obtained by the reaction of dioxygen with the 19e wostructural complex Fe CpfCgMeg) as shown in the preceding section. 

Dioxygen and its ions can bind in mononuclear and dinuclear structures in a number of ways,962 as illustrated in Scheme 1. The typical reaction of dioxygen with Co compounds involves a number of these binding forms, outlined in Scheme 2. Mononuclear Co111—peroxo complexes are relatively rare, but yellow trigonal bipyramidal complexes [Co(02)L2]+ (L = chelating phosphines dppe or dppp) have been characterized structurally where the 022 is bonded to the Co in the side-on r]2 form (Co—O 1.858(7) 1.881(4) A), with O—O stretching frequencies ( 870 cm-1) consistent with Coin-peroxo speciation.963... 

As a part of their ongoing investigations into the reaction of dioxygen with copper(I) complexes to identify copper-superoxo/peroxo intermediate species, Schindler and co-workers51-53 have also provided examples of a number of mononuclear copper(II) complexes (Table 1) (as well as copper(I) Section 6.6.4.2.1). 

Bimolecular Reaction of Dioxygen with the C H Bond of the Hydrocarbon... 

Bimolecular and Trimolecular Reactions of Dioxygen with the Double Bond of Olefin... 

Radicals produced from the initiator either directly attack the organic compound RH (for instance, this is the case during the decomposition of peroxides) or first react with dioxygen, and then, already as peroxyl radicals, attack RH (for instance, this is the case of decomposition of azo-compounds). RH gives rise to alkyl radicals when attacked by these radicals. The reaction of dioxygen addition to an alkyl radical,... 

The recent quantum-chemical analysis of the reaction of dioxygen with ethyl radical in the gas phase provided evidence for two pathways of interaction [83] ... 

The effect of multidipole interaction was observed in the reactions of dioxygen with C—H bonds of polyatomic esters. The strong influence of some polar groups on the rate constant of this reaction was observed (AG = — RT n(kin kff) [16,17]. 

Rate Constants of Radical Generation by Trimolecular Reaction of Dioxygen with Ethers [68]... 

The trimolecular reaction of dioxygen with the weakest C—H bonds of two nonsaturated esters. 

The trimolecular reaction of dioxygen with double bonds of two molecules of non-saturated esters. As in the case of a similar bimolecular reaction, this reaction seems to be preceded by CTC formation. 

Free radical formation in oxidized organic compounds occurs through a few reactions of oxygen bimolecular and trimolecular reactions with the weakest C—H bond and double bond (see Chapter 4). The study of free radical generation in polymers (PE, PP) proved that free radicals are produced by the reaction with dioxigen. The rate of initiation was found to be proportional to the partial pressure of oxygen [6,97]. This rate in a polymer solution is proportional to the product [PH] x [02]. The values of the apparent rate constants (/ti0) of free radical formation by the reaction of dioxygen (v 0 = k 0[PH][O2]) are collected in Table 13.8. 

The chain generation in polymers in the solid phase and solution is by several orders magnitude higher than that in pattern hydrocarbon. Hence, other fast reactions of dioxygen generate the chains more rapidly than that in hydrocarbons. The comparison of the ki0 values in PE with the percentage of ash showed the correlation (T = 391 K, LDPE [6]). 

This means that the main source of free radicals are reactions of dioxygen with impurities and residues of the technological catalyst. 

Hence, the copper surface catalyzes the following reactions (a) decomposition of hydroperoxide to free radicals, (b) generation of free radicals by dioxygen, (c) reaction of hydroperoxide with amine, and (d) heterogeneous reaction of dioxygen with amine with free radical formation. All these reactions occur homolytically [13]. The products of amines oxidation additionally retard the oxidation of hydrocarbons after induction period. The kinetic characteristics of these reactions (T-6, T = 398 K, [13]) are presented below. 

Most known reactions of dioxygen species involve inner-sphere pathways, or adduct formation. Our studies of the... 

Similarly, peroxo Rh(III) and ethene-peroxo Ir(III) have been prepared by reaction of dioxygen and reduced metal precursor". ... 

Fueled by the biological relevance, the initial steps of Cu-based O2 activation have attracted much interest in the last decades. Various, very different Cux/02 species that result from the reaction of dioxygen with Cu complexes have meanwhile been identified (Fig. 2) [70-90], where mononuclear species A and B as well as dinuclear type E and F species (and some tricopper systems as in, for example, laccase) are considered the most relevant in nature. 

The reaction of dioxygen with transition metal complexes has stimulated the curiosity of scientists for decades. Seminal studies by Vaska in the 1960s revealed that dioxygen coordinates reversibly to the the Ir center in (Ph3P)2lr(CO)Cl, 14 (Eq. 15) [101-104], Following this report, numerous groups reported dioxygen adducts of other late transition metals [105],... 

Relationship Between Reactions of Dioxygen and Alkenes (Including Benzoquinone) with Palladium(O)... 

Once the oxidative-addition reaction of dioxygen to metal d -ions has occurred, the essentially electrophihc dioxygen becomes a nucleophilic peroxide ligand. Since the oxidation of substrates is associated with electron transfer from the substrate to the oxidant, i.e. in this case the dioxygen adduct, effective oxygenations require a further activation to transform the nucleophihc peroxide into an electrophihc species prior to the oxygen transfer. 

The reactions of dioxygen have been amply documented The reduction can occur by a one-, two- or four-electron transfer reaction, the first of which is energetically unfavorable. The one-electron reduction of hydrogen peroxide yields the extremely reactive hydroxyl radical (Scheme 1). 

For the reaction of dioxygen with the manganese(II) phosphine complexes see equation (56). 

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