Radicals mechanistic pathways involving

Excluding free radical bromination processes, a schematic picture of the mechanistic pathways involved in olefin bromination is shown in Scheme 1. 

An interesting rationalization of the mechanistic pathways involved in the duorination of aromatic compounds by high-valency metal fluorides has been provided. The main features of the mechanism [outlined for the conversion of fluorobenzene into /)-difluorobenzene by cobalt(in) fluoride in Scheme 2] are oxidation of the aromatic system to a radical-cation (1) and its subsequent reaction with a fluorine atom from the metal fluoride to give the type of Wheland intermediate [or benzenium ion (c/. p. 362)] (2) which would be formed in a conventional electrophilic process. Such a scheme implies that the relative stabilities of Wheland intermediates might be used to rationalize orientation in this type of reaction in much the same way as for more traditional processes, and some examples of such applications have been provided. Alternative routes from the radical-cation (1) are discussed and possible... 

In the recent study on the anodic oxidation of enaminones which possess an unsaturated chain susceptible to react intramolecularly with an electrogenerated radical cation, the evidence for an intramolecular reaction was provided on the basis of the dEp/dlogv slope of 30 mV and one-electron behavior of the voltametric wave [48], The reaction could involve similar mechanistic pathways as shown in Scheme 4 (e-c-dimerization and following chemical reactions). However, the authors were not able to isolate the products after preparative oxidation in order to confirm the possible mechanism. 

The kinetic and activation parameters for the decomposition of dimethylphenylsilyl hydrotrioxide involve large negative activation entropies, a significant substituent effect on the decomposition in ethyl acetate, dependence of the decomposition rate on the solvent polarity (acetone-rfe > methyl acetate > dimethyl ether) and no measurable effect of the radical inhibitor on the rate of decomposition. These features indicate the importance of polar decomposition pathways. Some of the mechanistic possibilities involving solvated dimeric 71 and/or polymeric hydrogen-bonded forms of the hydrotrioxide are shown in Scheme 18. 

The precise mechanism by which NO causes glutamase neurotoxicity is unknown. Calcium must be required because of the requirement for NMDA- and glutamate-induced NO formation in brain tissue (Garthwaite etal., 1988). Although both NMDA-receptor agonists and sodium nitroprusside induce specific neurotoxicity as well as cyclic GMP formation in brain tissue (Dawson et al., 1991), it is unlikely that cyclic GMP is the ultimate cause of the neurotoxicity. Instead, NO is most likely involved in producing target cell death. One possible mechanistic pathway is that locally synthesized NO and superoxide anion react with each other to yield peroxynitrite anion (Beckman et al., 1990), which can destroy cell membranes either directly via interaction with cellular thiols (Radi et al., 1991) or indirectly via decomposition to hydroxyl and other free radicals (Beckman et al., 1990). 

A further significant mechanistic pathway for aromatic nitration can involve a single electron-transfer reaction to an initial radical ion intermediate ... 

The reactivity difference of the silyllithium reagents raises some questions concerning the mechanism. To clarify the mechanistic pathway, it was examined by semiempirical calculations using hydrosilanes instead of silyllithium compounds (Table 5)25. As deduced from the HOMO energies of Table 5, electron-releasing substituents favor the formation of adducts 35 and 36, which indicate that their formation should involve a radical reaction proceeding via electron transfer from the silyllithium to Cgo. 

Reactions of 1,4-dimethoxynaphthalene and its 2-chloro, 2-bromo, and 2-(1,3-dioxolanyl) derivatives with BAIB/TMSX (X = C1, Br) combinations in dichloromethane result in acetoxylation, monohalogenation, or dihalogenation of the more activated ring (Scheme 26) [79]. Specific outcomes depend on the naphthalene derivative and reaction conditions. It is interesting that the 2-(1,3-dioxolanyl) derivative undergoes ipso-bromination with BAIB/TMSBr, and that this mode of reactivity was not observed with 2-(l,3-dioxolanyl)-l,4-di-methoxybenzene. These reactions are mechanistically diverse. Evidence was presented that bromination occurs after the formation of molecular bromine, and that chlorination probably follows a radical pathway involving the homo-... 

In intramolecular arylations, a new bond is created between two aromatic moieties of the same molecule or between an aromatic nucleus and an atom of a side-chain. Many intramolecular arylation reactions of homocyclic and heterocyclic aromatic halides have been studied mainly in view of their synthetic applications, and it is not always clear which mechanistic pathway is followed. The reaction may start with homolytic or heterolytic dissociation of the carbon-halogen bond and proceed by attack of the aryl radical or aryl cation on another part of the molecule. Electrocyclization followed by elimination of hydrogen halide is another possibility. Especially when heteroatoms such as nitrogen, sulphur or phosphorus are involved, the initial step may be a nucleophilic attack on the carbon atom bearing the halide atom. 

Although Curran s rate data for the reduction of radicals to organosamar-iums allow for an element of predictablity,2 problems can arise when multifunctional substrates are involved. For example, in the attempted intramolecular Barbier reaction of alkyl iodide 13, treatment with Sml2 results in the formation of side product 15 in addition to the expected product cyclohexanol 14 (Scheme 3.7).8 In this case, the p-keto amide motif in 13 is reduced at a rate competitive with alkyl iodide reduction, indicating that there are likely two mechanistic pathways through which the reaction proceeds a thermodynamic pathway initiated by reduction of the R I bond providing the... 

CH Activation is sometimes used rather too loosely to cover a wide variety of situations in which CH bonds are broken. As Sames has most recently pointed out, the term was first adopted to make a distinction between organic reactions in which CH bonds are broken by classical mechanistic pathways, and the class of reactions involving transition metals that avoid these pathways and their consequences in terms of reaction selectivity. For example, radicals such as RO- and -OH readily abstract an H atom from alkanes, RH, to give the alkyl radical R. Also in this class, are some of the metal catalyzed oxidations, such as the Gif reaction and Fenton chemistry see Oxidation Catalysis by Transition Metal Complexes). Since this reaction tends to occur at the weakest CH bond, the most highly substituted R tends to be formed, for example, iPr-and not nPn from propane. Likewise, electrophilic reagents such as superacids see Superacid), readily abstract a H ion from an alkane. The selectivity is even more strongly in favor of the more substituted carbonium ion product such as iPr+ and not nPr+ from propane. The result is that in any subsequent fimctionalization, the branched product is obtained, for example, iPrX and not nPrX (Scheme 1). 

Polyynes have served as starting materials for the synthesis of a wide variety of heterocyclic ring systems. The reactions used involve addition to triple bonds, and any of the common mechanistic pathways may be followed, i.e. nucleophilic, electrophilic or free radical attack as well as concerted cycloadditions. Although the evidence does not permit unequivocal classification of many of the reactions into one of these categories, the ones considered here are those which most likely involve nucleophilic attack at some stage. In a formal sense the reactions amount to successive additions of a divalent nucleophile to two triple bonds the first involves intermolecular and the second intramolecular attack, as illustrated in equation (19) for the addition of HoS to a diyne. 

The exact mechanistic pathway of the classical J-L olefination is unknown. Deuterium-labeling studies showed that the nature of the reducing agent (sodium amalgam or Smb) determines what type of intermediate (vinyl radical or secondary alkyl radical) is involved. Both intermediates are able to equilibrate to the more stable isomer before conversion to the product. The high ( )-selectivity of the Kocienski-modified reaction is the result of kinetically controlled irreversible diastereoselective addition of metalated PT-sulfones to nonconjugated aldehydes to yield anti-P-alkoxysulfones which stereospecifically decompose to the ( )-alkenes. 

Y may be a nucleophile, an electrophile or a free radical. These terms may be used to distinguish the different mechanistic pathways that are characterised by the involvement of these differing attacking species. 

Mechanistic pathways that involve radical intermediates may accompany SN2-type polar oxidative additions. Under certain conditions, such pathways may... 

The second step of the arylation process involves a coupling reaction between two of the ligands linked to the bismuth atom. Various mechanistic pathways were considered to be possible. Study of the relative migratory aptitudes of aryl groups indicated that the C-phenylation reaction with either a phenol or a p-dicarbonyl does not follow an ionic pathway. Although these migratory aptitudes were of the same order as a free-radical type, the relative ratios are more consistent with a non-synchronous concerted mechanism.26... 

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