Radical-type oxidation

The reaction pattern includes the formation of PO, its consecutive isomerization to propanal, acetone and ally alcohol on acidic sites and combustion [43aj. Propanal and acrolein are also primary products. The formation of lower alkanes, alkenes, acetaldehyde and methanol results from cracking and oxidative C—C bond cleavage of propene and products. Additional side-reactions may occur in the gas phase, including radical-type oxidation of propene to acrolein, hexadiene and other byproducts. Alkyl dioxanes and alkyl dioxolanes may form via dimerization reactions of PO on acidic catalysts. Indeed, major by-products are heavy compounds that... 

The situation changes drastically if long-chain oxidants are appMed. One example of a successful regioselective oxidation of a nonactivated carbon atom comes from the use of long-chain tertiary amines as catalysts. They are first oxidized to an aminoxide with hydrogen peroxide in the presence of iron(II) salts. These radical-type oxidants to form molecular complexes with long-chain alcohols... 

The chemical (Gif system) and the electrochemical conversion (Gif-Orsay system) have been compared in the oxidation of six saturated hydrocarbons (cyclohexane, 3-ethylpentane, methylcyclopentane, cis- and traus-decalin and adamantane). The results obtained for pyridine, acetone and pyridine-acetone were similar for both systems. Total or partial replacement of pyridine for acetone affects the selectivity for the secondary position and lowers the ratio ketone secondary alcohol. The formation of the same ratio of cis- and traws-decal-9-ol from either cis- or trans-deca in indicates that tertiary alcohols result from a mechanism essentially radical in nature. The C /C ratio between 6.5 and 32.7 rules out a radical mechanism for the formation of ketones and secondary alcohols. Ratios of 0.14 and 0.4 were reported for radical-type oxidations of adamantane and cis-decalin. Partial replacement of pyridine by methanol, ethanol or f-propanol results in diminished yields and a lower selectivity. Acetone gives comparable yields however, the C /C ratio drops to 0.2-10.7. 

Another type of chemical change is initiated by light, which may trigger autolytic, that is, free radical (Type I) or singlet oxygen (Type II) reactions. These changes are routinely classified as oxidation. Rancidity in cosmetics, especially those containing unsaturated Hpids, is commonly prevented by use of antioxidants (qv). 

It was first assumed that the oxidant was H2O2. However, since traces of Fe-cations are present, a Fenton s type oxidation pathway, based on OH radicals, is more likely taking place. 

These data appeared to be very useful for the estimation of the relative O H bond dissociation energies in hydroperoxides formed from peroxyl radicals of oxidized ethers. All reactions of the type R02 + RH (RH is hydrocarbon) are reactions of the same class (see Chapter 6). All these reactions are divided into three groups RO + R (alkane, parameter bre = 13.62 (kJ moC1)172, R02 + R2H (olefin, bre = 15.21 (kJ mob1)1 2, and R02 + R3H (akylaromatic hydrocarbon), hrc 14.32 (kJ mol )12 [71], Only one factor, namely reaction enthalpy, determines the activation energy of the reaction inside one group of reactions. Also,... 

Materials. The radical-type photopolymerizable formulation consisted of a mixture of hexanediol diacrylate (HDDA from UCB) and a polyurethane-diacrylate (Actilane 20 from Arkros). A bis-acylphosphine oxide (BAPO from Ciba) was used as photoinitiator at a typical concentration of 1 wt %. The cationic type photopolymerizable resin consisted of a mixture of the divinylether of triethyleneglycol (RapiCure DVE-3 from ISP) and a divinylether derivative of bis-phenol A (DVE-BPA). The cationic photoinitiator (Cyracure UVI-6990 from Union Carbide) had a composition of 50 wt % of mixed triarylsulfonium hexafluorophosphate salts and 50 wt % of propylene carbonate. The BAPO initiator... 

While many important details of the iron- and manganese-catalyzed reactions are yet to be explored, the common features of the corresponding mechanisms are well established and also applicable in the presence of other catalysts. Thus, the formation of the SO5, SO4 and HSO5 intermediates was reported in all of the free-radical type reactions. These species are very reactive oxidants and this explains the apparent... 

HO2, was considered as a reactive intermediate in both cases. The addition of radical scavengers strongly retarded the oxidation of the phosphinate ion confirming the radical type mechanism. It was also demonstrated that the reaction ceased when the catalyst was masked with EDTA. 

The use of transition-metal-substituted aluminophosphates (AIPOs) as catalysts for free-radical auto-oxidations of hydrocarbons, a type IIaRH reaction, has been explored by several groups [60-62]. In a series of elegant contributions, Thomas... 

These results, as well as rate studies " and kinetic isotope effects ", support a concerted, 5ptra-structured oxenoid-type transition state for the CH oxidations". The original oxygen-rebound mechanism has been discounted (see the computational work in Section I.D). Recently, however, the stepwise radical mechanism was revived in terms of the so-called molecule-induced homolysis , but such radical-type reactivity was severely criticized on the basis of experimental" and theoretical grounds. 

The fates of the G(-H) radicals in DNA are mostly determined by reactions with other substrates. Here, we consider the reactions of the G(-H) radicals with types of free radicals that are generated in vivo under conditions of oxidative stress. One of these radicals is the nitrogen dioxide radical, NO2. This radical can be generated in vivo by the oxidation of nitrite, N02, a process that can be mediated by myeloperoxidase [111, 112] as well as by other cellular oxidants [113, 114]. An alternative pathway of the generation of NO2 is the homolysis of peroxynitrite [102, 115] or nitrosoperoxycarbonate formed by the reaction of peroxynitrite with carbon dioxide [99-101]. The redox potential, E°( NO2/NO2")=1.04 V vs NHE [116] is less than that of guanine, E7[G(-H)7G] = 1.29 V vs NHE [8]. Pulse radiolysis [117] and laser flash photolysis [109] experiments have shown that, in agreement with these redox potentials, N02 radicals do not react with intact DNA. However, N02 radicals can oxidize 8-oxo-dG that has a lower redox potential ( 7=0.74 vs NHE [56]) than any of the normal nucleobases [109]. 

A reduction and activation of HjOj by other one-electron donors, like semiquinones, has also to be considered. This follows from a study of the ethylene production from methionine in the presence of pyridoxal phosphate, a reaction characteristic for OH radicals or for Fenton-type oxidants. The ethylene production in the presence of dioxygen, anthraquinone-2-sulfonate, and an NADPH-generating system in phosphate buffer pH 7.6 was inhibited by SOD and by catalase, but stimulated by scavengers of OH radicals, like 0.1 mM mannitol, a-tocopherol, and formiate... 

Oxidation to Phenols. Direct hydroxylation of benzene to phenol can be achieved in a free-radical process with H202 or 02 as oxidants.739-744 Metal ions [Fe(II), Cu(II), Ti(HI)] may be used to catalyze oxidation with H202. Of these reactions, the so-called Fenton-type oxidation is the most widely studied process.742 Oxidation in the presence of iron(II) sulfate was reported in early studies to yield phenol. Since phenol exhibits higher reactivity than benzene, varying amounts of isomeric dihydroxybenzenes were also formed. 

A very large number of these systems with ring junction heteroatoms exists, and this number is constantly increasing. Only illustrative examples of the preparation of such systems can be given here. The synthetic methods for the formation of this type of heterocycle can be usefully classified as follows (i) various cyclocondensations between the corresponding heterocyclic derivatives and bifunctional units, (ii) intramolecular cyclizations of electrophilic, nucleophilic or (still rare) radical type, (iii) cycloadditions, (iv) intramolecular oxidative coupling, (v) intramolecular insertions, (vi) cyclization of open-chained predecessors, (vii) various reactions (quite often unusual) which are specific for each type of system. Examples given below illustrate all these cases. 

It will be noted that Q does vary among the radicals in Table 29.2. Even more interesting is the fact that the molecules that form anion radicals upon reduction and cation radicals upon oxidation usually display larger proton splittings in the cationic species. Colpa and Bolton [9] proposed a more sophisticated equation based on the fact that negative charge in the p-type orbital will... 

According to this hypothesis, the results are modified from what would be expected from classical radical reactions. The interest in this hypothesis is that, with the sole exception of saturated hydrocarbons, it could apply to all the compounds that can be coordinated at the Tiiv center, such as alkenes, aromatics, alcohols, and sulfides. According to this hypothesis, the weak Lewis acidity of Tilv would help to bring the reactant into its coordination sphere. The initial coordination of the reactant would explain the oxidation of methyl-substituted aromatics in the aromatic ring and not in the side chain, even with a radical-type mechanism. 

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