Electron transfer alkyl bromides

Bromination of 3,7-dibromo-lO-alkyl-phenothiazines, e.g. (78), in acetic acid gives purple radical-cations, e.g. (80). On subsequent heating in the same solvent, these radical cations undergo electron transfer with bromide ions to form the parent phenothiazines, e.g. (78), or are irreversibly dealkylated to yield (79) and, hence, 1,3,7,9-tetrabromophenothiazine, by the action of molecular bromine produced in the previous redox reaction. 

The reduction ofsec-, and /-butyl bromide, of tnins-1,2-dibromocyclohexane and other vicinal dibromides by low oxidation state iron porphyrins has been used as a mechanistic probe for investigating specific details of electron transfer I .v. 5n2 mechanisms, redox catalysis v.v chemical catalysis and inner sphere v.v outer sphere electron transfer processes7 The reaction of reduced iron porphyrins with alkyl-containing supporting electrolytes used in electrochemistry has also been observed, in which the electrolyte (tetraalkyl ammonium ions) can act as the source of the R group in electrogenerated Fe(Por)R. ... 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

Quite similar results have been found recently in the reaction of the cobalt(i) form of vitamin B,2 (Bus) with alkyl halides with n-butyl iodide, bromide and chloride, ethyl bromide and benzyl chloride the representative data point of vitamin B s falls several orders of magnitude above the outer sphere dissociative electron-transfer line (Walder, 1989). 

The interaction of alkyl halides with mercaptans or alkaline mercaptides prodnces thioalkyl derivatives. This is a typical nncleophilic substitution reaction, and one cannot tell by the nature of products whether or not it proceeds through the ion-radical stage. However, the version of the reaction between 5-bromo-5-nitro-l,3-dioxan and sodium ethylmercaptide can be explained only by the intermediate stage involving electron transfer. As found (Zorin et al. 1983), this reaction in DMSO leads to diethyldisulfide (yield 95%), sodium bromide (quantitative yield), and 5,5 -bis(5-nitro-l,3-dioxanyl) (yield 90%). UV irradiation markedly accelerates this reaction, whereas benzene nitro derivatives decelerate it. The result obtained shows that the process begins with the formation of ethylthiyl radicals and anion-radical of the substrate. Ethylthiyl radicals dimerize (diethyldisulfide is obtained), and anion-radicals of the substrate decompose monomolecularly to give 5-nitro-l,3-dioxa-5-cyclohexyl radicals. The latter radicals recombine and form the final dioxanyl (Scheme 4.4). 

It is known that the oxidation of alkyl-substituted aromatic hydrocarbons in acetic acid on metal bromide catalysis follows the one-electron transfer mechanism (Sheldon and Kochi 1981). The rate-determining stage is the one-electron transfer from the substrate to the metal ion in the highest oxidation state (Digurov et al. 1986). As a result, an unstable cation-radical is formed that... 

Organolithium reagents in which the carbanion is delocalized are less subject to competing electron-transfer processes. Allyllithium and benzyllithium reagents can be alkylated by secondary alkyl bromides, and a high degree of inversion of configuration is observed.65... 

In aprotic solvents, the radical anion, RX , for aryl halides has been detected as intermediate. In cyclic voltammetry of aryl halides, though an irreversible two-electron reduction occurs at low scan rate, a reversible one-electron reduction occurs at high scan rate. Thus, it is possible to get the values of the standard potential ( °) for the RX/RX couple and the rate constant (k) for RX -> R (therefore, the lifetime of RX ). In Fig. 8.18, the relation between ° and log k for aryl bromides in DMF is linear with a slope of 0.5 [5If], It is apparent that the lifetime of RX , obtained by 1/k, increases with the positive shift of E0. In contrast, the existence of RX for alkyl monohalides has never been confirmed. With these compounds, it is difficult to say whether the two processes, i.e. electron transfer and bond cleavage, are step-wise or concerted (RX+e -> R +X ). According to Sa-veant [5le], the smaller the bond dissociation energy, the larger the tendency for the concerted mechanism to prevail over the step-wise mechanism. 

Alkyl bromides, on the other hand, undergo halogen-metal exchange via single electron transfer processes, and cyclisations of alkyl bromide-derived organolithiums may proceed with significant contribution from radical pathways.117... 

It was reported by Rozhkov and Chaplina130 that under mild conditions perfluorinated r-alkyl bromides (r-RfBr) in nonpolar solvents can be added across the n bond of terminal alkenes, alkynes and butadiene. Slow addition to alkenes at 20 °C is accelerated in proton-donating solvents and is catalyzed by readily oxidizable nucleophiles. Bromination of the it bond and formation of reduction products of t-RfBr, according to Rozhkov and Chaplina, suggest a radical-chain mechanism initiated by electron transfer to the t-RfBr molecule. Based on their results they proposed a scheme invoking nucleophilic catalysis for the addition of r-RfBr across the n bond. The first step of the reaction consists of electron transfer from the nucleophilic anion of the catalyst (Bu4N+Br , Na+N02, K+SCN , Na+N3 ) to r-RfBr with formation of an anion-radical (f-RfBr) Dissociation of this anion radical produces a perfluorocarbanion and Br, and the latter adds to the n bond thereby initiating a radical-chain process (equation 91). 

In mechanistic matters, it has been demonstrated that co-alkenyl iodides undergo cyclization onto the vinyl function upon treatment with Me2CuLi, in competition with direct substitution. This, as well as the generation of trityl radical in the reaction of Me2CuLi with trityl chloride, constitutes evidence for single electron transfer in reactions of cuprates with iodides (and, to a lesser extent, bromides)16. The intermediacy of alkyl radicals in the substitution process (equation 12) is likely the source of the aforementioned racemization in reactions of secondary iodides4. 

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