Alkyl Bromides as Mechanistic Probes of Reductive Dehalogenation: Reactions of Vicinal Dibromide Stereoisomers with Zero-Valent Metals

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 EZ ratio of olefins as observed during dehalogenation by iodide a classic nucleophilic reductant. Reductions promoted by CrII 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 10exp 8 to 10exp 10sec. 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.