Alkyl bromides as mechanistic probes of reductive dehalogenation: reactions of vicinal dibromide stereoisomers with zerovalent metals.

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 stereo-specificity 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.

[Niemann-Pick disease type B: An enzymatically confirmed case with unexpected retinal involvement (author's transl)]. / Morbus Niemann-Pick Typ B - enzymatisch gesichert - mit unerwarteter retinaler Beteiligung.

In a 6-year-old girl with normal to outstanding intelligence Niemann-Pick disease was diagnosed by the demonstration of an about 90% deficient sphingomyelinase activity. Abnormalities of the eye fundi are described which are comparable to but quantitatively deviate from the classic cherry-red spot as known from neurolipidoses. The brownish-red foveola was surrounded by a relatively thin opaque ring around which punctate white deposits (lipids?) could be detected. The absence of any other neurologic symptom was in contrast iwth an extreme hepatosplenomegaly, foam cells in the bone marrow, lung infiltration, underweight, and undergrowth. Therefore, the case was classed with the type B of Niemann-Pick disease, although the common definition of this type excludes cerebral as well as oculoneural involvement. In the literature only one comparable case could be found which, however, had not been enzymatically confirmed. In the future the definition of type B of Niemann-Pick disease should include the possibility of oculoneural involvement.