Reductive dehalogenation -one of the earliest reactions described in the organic chemical literature -has achieved special significance since the 1980s when the harmful properties of numerous halogenated (chiefly chlorinated) hydrocarbons became clear. The identification of methods capable of neutralizing or, at least, diminishing these dangers, remains a major challenge for chemistry. It is reasonable to convert stocks of the prohibited chemicals (e.g., polychlorobiphenyls, PCBs; chlorofluorocarbons, CFCs) to valuable products as far as possible. At the same time, the halogen-containing wastes should be detoxified by degradation. During the past two decades, the mainly heterogeneous (but also homogeneous) catalytic dehalogenation provided a major share towards solving these problems. Within this period, substantial progress was also made in the application of these reactions in organic syntheses.Hydrodehalogenation -that is, hydrogenolysis of the carbon-halogen bondinvolves the displacement of a halogen bound to carbon by a hydrogen atom. This chapter is devoted to dehalogenations mediated by transition-metal complexes (Eq. (1)): ÀC j j ÀX reducing agent L n Mj 3 ÀC j j ÀH Y 1 where X = F, Cl, Br, I, and [L n M] = a transition metal complex. The use of a wide variety of reducing agents (H 2 , hydrides of metals and metalloids, organic reductants, etc.) under the most diverse reaction conditions (e.g., one-or two-phase systems) has been reported. Organic halides have also been reduced by electrochemical and photochemical methods in the presence of compounds of the type [L n M]. Reductive transformations of organic halides not relevant to Eq.(1) (e.g., coupling reactions) and dehalogenation of acyl halides will not be included here.
513The Handbook of Homogeneous Hydrogenation. Edited by J. G. de Vries and C. J. ElsevierThe reactivity of the carbon-halogen bond in Eq. (1) depends on several factors: · the nature of the halogen atom; · the environment of the halogen atom in the molecule; and · the reagents and conditions used in Eq. (1) [1].The order of reactivity of the C-X bond (generally: I > Br > Cl > F) is consistent with its strength. For instance, the experimentally found dissociation energies for phenyl halides (D Ph-X ) are 528, 402, 339, and 272 kJ mol -1 at 298 K for X = F, Cl, Br, and I, respectively [2]. Consequently, catalytic defluorination in the literature is comparatively rare. The different reactivity of the C-X bonds renders possible the selective dehalogenation of compounds containing two dissimilar halides, leaving intact the stronger C-X bond.
