Investigations on the Stereoisomerism of Unsaturated Compounds. V. A Mechanism for the Formation of Butenes from 2,3-Dibromobutanes by the Action of Iodide Ion

Winstein, S.; Pressman, David; Young, William G.

doi:10.1021/ja01876a005

Cited by 40 publications

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“…This "two electron" 20 reaction may be envisioned as a nucleophilic "X-philic" attack of the reductant on one of the 21 bromines, rather than on carbon as in a typical S N 2 reaction. Reductive elimination of vicinal 22 dihalides by the strong nucleophile iodide has commonly been attributed to just such a concerted 23 nucleophilic mechanism (18,20,21).…”

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“…Some of the reactivity parameters are listed in Table 2. 18 Reductions promoted by Cr(II). Cr(II) was used as a model reducing agent because it has been 19 demonstrated to act as a one-electron reductant of vicinal dibromides, at least in mixed 20 dimethylsulfoxide/water solution (20)(21)(22)32). For both the DBPs and (±)-SBr 2 , the rate of 21 reduction by Cr(II) was fast relative to hydrolysis, and disappearance exhibited pseudo-first order 22 kinetics.…”

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“…10 Reductions promoted by iodide. The stereospecificity encountered in reduction of the dibromide 11 probe molecules by iodide was explored because prior work has shown that this species serves as 12 a nucleophilic (two-electron) "X-philic" reductant, at least in organic solvents (20)(21)(22)32).…”

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Alkyl Bromides as Mechanistic Probes of Reductive Dehalogenation: Reactions of Vicinal Dibromide Stereoisomers with Zerovalent Metals

Totten

Jans

Roberts

2001

Environ. Sci. Technol.

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Whether reductive dehalogenation proceeds via a one-or a two-electron mechanism has been suggested to affect product distributions, hence potentially influencing the success of engineered treatment systems. In this work, we explore vicinal dibromide stereoisomers as "probes" of the concertedness of electron transfer in reduction by aqueous suspensions of iron and zinc metal. Dibromides consisted of 2,3-dibromopentane (diBP) stereoisomers and (()-1,2-dibromo-1,2-diphenylethane. All dibromides reacted with metals to give the same E:Z ratio of olefins observed during dehalogenation by iodide (a two-electron reductant). Reduction by Cr(II) (a one-electron reductant) yielded distinctly different proportions of E and Z olefins. Although this might be construed as evidence that metals function as two-electron reductants, high stereospecificity was also obtained for reduction of diBPs by Fe(II) adsorbed to goethite, a presumed one-electron reductant; this can be explained by two single-electron transfers in rapid succession, facilitated by the locally elevated concentration of reducing equivalents at the oxide-water interface. The results suggest that reduction of alkyl halides by metals is not likely to produce free radicals that persist long enough to undergo radical-radical coupling or hydrogen-atom abstraction from minor dissolved constituents. Apparent free-radical coupling products are more likely to result from (possibly surface-bound) organometallic intermediates.

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Alkyl Bromides as Mechanistic Probes of Reductive Dehalogenation: Reactions of Vicinal Dibromide Stereoisomers with Zerovalent Metals

Totten

Jans

Roberts

2001

Environ. Sci. Technol.

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“…This trend was also observed by Young (69) and Winstein (67) in studies of the rates of elimination of the erythro and threo isomers of 2,3-dibromobutane ; 3,4-dibromopentane; 3,4-dibromohexane; and 4,5-dibromooctane ( Table 9). An erythro to threo rate ratio of approximately two is.…”

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confidence: 81%

Studies in stereochemistry

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“…The stereospecific trans additions of hydrogen iodide to tiglic and angelic acids, which were interpreted (122) as consistent with the fact that the acids were geometric isomers, are illustrated in chart I. They were also used as methods of synthesis for the pure isomeric 2-butenes (189,201). These reactions were used (198) to relate the configurations of tiglic and angelic acids to those of the 2-butenes.…”

Section: Stereochemistrymentioning

confidence: 92%