Radical-based mechanisms

Lumped mechanisms are based on the grouping of chemical compounds into classes of similar stmcture and reactivity. For example, all alkanes might be lumped into a single class, the reaction rates and products of which are based on a weighted average of the properties of all the alkanes present. For example, as shown in Table 1, the various alkanes, CH2 2 > react with OH in a similar manner to form alkyl radicals,. When expressed... 

The proposed mechanism is based on the basis of the fact that ylides (Scheme 23 and Scheme 24) undergo bond fission between the phosphorus atom and the phenyl group in TPPY as reported by Nagao et al. [51] and between the sulfur atom and the phenyl group in POSY as observed in triphenylsulfonium salts [52-55] when they are irradiated by a high-pressure mercury lamp. The phenyl radicals thus produced participate in the initiation of polymerization. 

HPP or (R)-l,l-difTuoroHPP is one involving abstraction of the C2 hydrogen instead [109]. Consequently, the above findings not only support a radical- based mechanism but they also indicate that different reaction pathways are used for the individual enantiomers. 

Doubts against a radical mechanism were based mainly on the observation that no polymers of the alkene used for the arylation were found for a long time. It was not until the 1970s that Kopylova et al. (1971, 1972) and Ganushchak et al. (1972) obtained telomers with the general formula Ar(CH2CHZ)nCl in low yields under conditions of high vinyl monomer concentration. 

The radical-based functionalization of silicon surfaces is a growing area because of the potential practical applications. Although further knowledge is needed, the scope, limitations, and mechanism of these reachons are sufficiently well understood that they can be used predictably and reliably in the modification of hydrogen-terminated silicon surfaces. The radical chemistry of (TMSlsSiH has frequently served as a model in reactions of both hydrogen-terminated porous and flat silicon surfaces. We trust that the survey presented here will serve as a platform to expand silicon radical chemistry with new and exciting discoveries. 

Methylmalonyl-CoA mutase (MCM) catalyzes a radical-based transformation of methylmalonyl-CoA (MCA) to succinyl-CoA. The cofactor adenosylcobalamin (AdoCbl) serves as a radical reservoir that generates the S -deoxyadenosine radical (dAdo ) via homolysis of the Co—C5 bond [67], The mechanisms by which the enzyme stabilizes the homolysis products and achieve an observed 1012-fold rate acceleration are yet not fully understood. Co—C bond homolysis is directly kineti-cally coupled to the proceeding hydrogen atom transfer step and the products of the bond homolysis step have therefore not been experimentally characterized. 

Another mechanism for alkanone-sensitized photodehydrochlorination comprises Norrish type I scission of the ketone, followed by ground-state reactions of radicals (19). However, the evidence for such a mechanism is based on experiments that were carried out in the vapor phase (19). Initiation of the photodegradation of PVC by hexachloroacetone has been suggested to involve the abstraction of hydrogen from the polymer by radicals resulting from the photolysis of the ketone s carbon-chlorine bonds (22). 

This mechanism is based on initiation by electron-transfer which leads to a styrene radical anion, which couples rapidly due to its high concentration, and forms a dimeric styrene dianion that is capable of further propagation by anionic attack on styrene monomer. 

The reaction is considered to proceed via a silyl anion mechanism, although the possibility of a radical-based mechanism has also been discussed.115,125 In order to clarify the mechanism, coupling experiments on a 1 1 mixture of chlorotrimethylsilane, 27 (reduction potential <—3.0 V),126 and chlorotriphenylsilane, 28 (reduction potential vs. standard calomel electrode (SCE) < —3.0 V),120 were performed, in which the mixed coupling product 1,1,1-trimethyl-2,2,2-triphenyldisilane, 29, and the homocoupling product hexaphenyldisilane, 30, only, were found,125 as indicated in Scheme 15. 

Recently, some attempts were nndertaken to uncover the intimate mechanism of cation-radical deprotonation. Thns, the reaction of the 9-methyl-lO-phenylanthracene cation-radical with 2,6-Intidine (a base) was stndied (Ln et al. 2001). The reaction proceeds through two steps that involve the intermediary formation of a cation-radical/base complex before unimolecular proton transfer and separation of prodncts. Based on the value of the kinetic isotope effect observed, it was concluded that extensive proton tnnneling is involved in the proton-transfer reaction. The assumed structure of the intermediate complex involves n bonding between the unshared electron pair on nitrogen of the Intidine base with the electron-deficient n system of the cation-radical. Nonclassical cation-radicals wonld also be interesting reactants for snch a reaction. The cation-radical of the nonclassical natnre are known see Ikeda et al. (2005) and references cited therein. 

The evidence for this mechanism is based on mass spectroscopy of the gas-phase radiolysis of isobutylene, which may not be applicable to the typical liquid-phase polymerization system. Initiation in condensed systems may follow the same course as electroinitiation— coupling of radical-cations to form dicarbocations. 

In vitro tests, used in evaluation of antioxidant properties make use of the ability of antioxidants to quench free radicals. Based on this mechanism, the methods are divided into two groups SET - single electron transfer, and HAT - hydrogen atom transfer. Reactions with antioxidants in assays with the DPPH radical, ABTS and the Folin-Ciocalteu reagent both operate according to the SET and HAT mechanism. Due to the kinetics of the reaction, they are included in the... 

A recent theoretical study by Takeuchi et al. [140] has examined the mechanism for the reaction of both alkenes and alkynes with the H-terminated silicon surface using periodic DFT calculations, and the results are in good agreement with the proposed radical-based mechanism [137]. In particular, the calculations show that the reaction occurs through a carbon-based radical intermediate which must be sufficiently stabilized to proceed by abstraction of a surface hydrogen (as in the case of styrene) if the intermediate is not stable enough, it will preferentially desorb (as in the case of ethylene). The calculations also show that reaction with terminal alkynes should proceed faster and lead to more stable products than with terminal alkenes [140]. 

Mechanism of Radiation-Accelerated Creep. Apparently the radiation-accelerated creep under stress and the radiation expansion under no stress are interrelated and may, in fact, result from the same cause. Thus, the mechanism of accelerated creep may be elucidated by better understanding the radiation expansion under no stress. Any hypothesis advanced to explain the mechanism of increasing creep rate during irradiation must also explain the reversible nature of the phenomenon— i.e., the creep rate returns to a low value when the beam is turned off. If the mechanism is based on chemical changes, a chemical species must exist during irradiation which does not exist before or long afterward. The only reasonable species of this type with adequate lifetimes are the free radicals, ions, or electrons, and gases formed when polymers are irradiated. 

Racemization of chiral benzylic and aliphatic primary, secondary, and tertiary amines was recently reported by Gastaldi et al. using sulfur-based catalysts operating through a radical-based mechanism. These authors have also reported enzyme DKR of chiral amines [13]. 

The mechanism of base-catalysed deprotonation of the a-CH of 4-methoxybenzyl alcohol radical cations in water has been examined. There is no direct attack of HO- at the a-CH as was believed, but reaction occurs via deprotonation of the OH to produce the benzyloxy radical, which then forms the carbon-centred radical by a 1,2-hydrogen... 

In short, the proposed mechanism was based on the formation of R radicals (CH was detected by ESR (54) by capture of photoproduced holes (RCOJ + h R° + CO2), while O2 was considered to intervene in electron capture, and Pt in the formation of H2 from H° as in the reactions previously discussed in this text. 

The biochemistry of coenzyme B12 generally revolves around either mutase enzyme activity, involving functional group migration, notably by stereospecific 1,2-shifts (Scheme 2.8), or methylation by methionine synthetase. The general mechanism for the mutase activity is a radical-based one and has been established by EPR spectroscopy to be of the general form shown in Scheme 2.9. 

The spontaneous decomposition of A -nitrosomelatonin (NOMel) is accelerated by acidification, presence of oxygen, and TEMPO. In the reduction of NOMel with ascorbic acid, the reactive species is melatonin radical. Based on kinetic data and DFT calculations, a mechanism for the denitrosation of NOMel has been suggested 310... 

Second, any CIDNP based assignments concerning the sign of hfcs are valid only if the radical pair mechanism (RPM) [93-96] is operative they become invalid if the alternative triplet-Overhauser mechanism (TOM), based on electron nuclear cross relaxation [97-100] is the source of the observed effects. For effects induced via the TOM the signal directions depend on the mechanism of cross relaxation and the polarization intensities are proportional to the square of the hfc. Thus, they do not contain any information related to the signs of the hfcs. However, the TOM requires the precise timing of four consecutive reactions and, thus, is not very likely. In fact, this mechanism has been positively established in only two systems [98-100]. 

In more general terms, a review of product yields and distributions in radical hydroxylations shows that a great variety of yields and selectivities can be achieved by careful manipulation of experimental conditions. Occasionally, formation of biphenyl, which is often regarded as indicating the presence of a radical-based process, can be completely suppressed. Therefore, it is very risky to make statements about the mechanism of hydroxylation based solely on product distributions and without thorough mechanistic investigations [11]. 

A concerted, spiro-structured, oxenoid-type transition state has been proposed for C-H oxidation by dioxiranes (Scheme 5). This mechanism is based mainly on the stereoselective retention of configuration at the oxidized C-H bond [20-22], but also kinetic studies [29], kinetic isotopic effects [24], and high-level computational work support the spiro-configured transition structure [30-32], The originally proposed oxygen-rebound mechanism [24, 33] was recently revived in the form of so-called molecule-induced homolysis [34, 35] however, such a radical-type process has been experimentally [36] and theoretically [30] rigorously discounted. 

Although there is still debate as to whether hydroxyl radicals or ferryl species are the key oxidants in Fenton systems, most literature reports on the mechanisms of degradation of organic compounds invoke the hydroxyl radical. Based on the reports discussed above, it seems likely that hydroxyl radical is a major oxidant during Fenton degradations. Although ferryl ions or other highly oxidized forms of iron may occur, either to a limited extent or more abundantly under specific conditions, this section will deal with documented reaction pathways and kinetics for hydroxyl radical or species assumed to be hydroxyl radical. The reader should keep in mind that ferryl pathways may need to be considered under certain conditions. 

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