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首页> 美国政府科技报告 >Alkyl Bromides as Mechanistic Probes of Reductive Dehalogenation: Reactions of Vicinal Dibromide Stereoisomers with Zero-Valent Metals
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Alkyl Bromides as Mechanistic Probes of Reductive Dehalogenation: Reactions of Vicinal Dibromide Stereoisomers with Zero-Valent Metals

机译：烷基溴作为还原脱卤作用的机理探针：邻二溴化物立体异构体与零价金属的反应

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The mechanism through which zero-valent metals (most notably iron and zinc) reduce alkyl polyhalides in aqueous solution at room temperature was investigated using several stereoisomers of vicinal dibromides as probe compounds: 2R-3S-2,3-dibromopentane (DBP) (and its stereoisomeric enantiomer, 2S-3R-2,3-dibromopentane), 2R-3R-2,3-dibromopentane (and its stereoisomeric enantiomer, 2S-3S-2 ,3-dibromopentane), and 1R-2R-1,2-dibromo- 1,2-diphenylethane (+/-)-SBr2. All of the probes react with zero-valent metals to give approximately the same E:Z ratio of olefins as observed during dehalogenation by iodide (a classic nucleophilic reductant). Reductions promoted by Cr(II) (a one- electron reductant) yield distinctly different proportions of E- and Z- olefins. Despite these differences, it cannot be definitively concluded that zero-valent metals reduce vicinal dibromides via nucleophilic two-electron pathways rather than via two sequential single electron transfer (SET) steps. This is because the reactions are heterogenous in nature, occurring at a surface which may possess locally elevated concentrations of reducing equivalents. Transfer of one electron may thus be rapidly followed by a second SET step. Estimates of energy barriers for C-C bond rotation in intermediate radicals suggest that if the zero-valent metals studied reduce the vicinal dihalide probe compounds via SET, the intermediate radicals must be reduced to the final olefin products with rate constants of about 10(exp 8) to 10(exp 10)/sec. Steady-state concentrations of free radicals in these systems would be extremely low, and reactions such as coupling of free radicals would therefore be unlikely to compete with reduction to olefins.

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