Metal induced dehalogenation

Abstract Recent advances in the metal-catalyzed one-electron reduction reactions are described in this chapter. One-electron reduction induced by redox of early transition metals including titanium, vanadium, and lanthanide metals provides a variety of synthetic methods for carbon-carbon bond formation via radical species, as observed in the pinacol coupling, dehalogenation, and related radical-like reactions. The reversible catalytic cycle is achieved by a multi-component catalytic system in combination with a co-reductant and additives, which serve for the recycling, activation, and liberation of the real catalyst and the facilitation of the reaction steps. In the catalytic reductive transformations, the high stereoselectivity is attained by the design of the multi-component catalytic system. This article focuses mostly on the pinacol coupling reaction. 

H from C0, the commonest probably being 1,2-dehalogenations and, in particular, 1,2-debromination. This can be induced by a number of different species including iodide ion, I , metals such as zinc, and some metal ions, e.g. Fe2. The reaction with I in acetone is found to follow the rate law (after allowance has been made for the I complexed by the I2 produced in the reaction),... 

Br and Cl are much less effective at inducing 1,2-dehalogenation than I , but metals—particularly Zn—have long been used. Reaction takes place heterogeneously at the surface of the metal, the solvent... 

It seems unlikely that the influence of the metal is of an electrophilic nature, inducing a carbonium-ion mechanism, as the products of substitution and dehydrohalogenation are never found simultaneously. More plausibly the metal donates electrons, and carbanion or concerted processes with carbanion character rather than radical processes are usually favoured. Radical additions occur very easily to olefinic double bonds and l-bromo-l,2-diphenylethane undergoes radical bromination with N-bromosuccinimide without accompanying dehalogenation. ... 

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