Abstraction of a halogen atom

Free radicals can also be formed in situ by the abstraction of a halogen atom from an organic halide (RX) as in the sequence of reactions ... 

The catalytic hydrogenation of alkyl halides (RX) probably also proceeds via the intermediate formation of free radicals, which are formed in this case by the abstraction of a halogen atom (see Section III,B). 

At the oxidation of halogen-containing polymers, the release of halogen atoms which are in P-position to a radical site of chloroalkyl radicals should be considered as well. Worth of noting here is the fact that abstraction of a halogen atom may occur also in a-position [90],... 

The first type of mechanism involves a redox chain process. As shown in Eqs. (1-3), it begins with the abstraction of a halogen atom from a polyhalo-alkane reagent by the metal complex. This generates a radical species that further adds to an olefin. A chain-transfer reaction ensues and yields back the reduced metal species, hence the acronym ATRA, for the sequence. 

Atom transfer radical additions and cyclisations have been used successfully in organic chemistry for the preparation of 1 1 adducts from alkyl halides, RX, and alkenes, CH2=CHY (Scheme 9.2) Under such conditions, the required catalytic amount of transition metal, Mt (e.g. CuCl, FeBr2, RuC12 in the presence of corresponding ligand) is used to provide a low stationary concentration of radicals, R (and of oxidised transition metal Mtw+1X, e.g. CuCl2), which subsequently react with an alkene by abstraction of a halogen atom from the oxidised form of the catalyst to produce the final product, R-CH2-CHY-X. 

An important problem that needs to be faced when using radical reactions to stereoselectively form asymmetric centers is that most carbon-centered radicals are planar or nearly so. Once a carbon radical has been created it usually adopts a trigonal planar shape, or nearly so, and both faces of the radical are identical. For example, following abstraction of a halogen atom (by the BujSn radical) from the... 

Binuclear oxidative additions, because they involve le rather than 2e changes at the metals, often go via radicals. One of the best known examples is shown in Eq. 6.21. The rate determining step is abstraction of a halogen atom from RX by the d Co(II) the resulting R combines with a second Co(II) center ... 

Atom-transfer radical polymerization (ATRP) is another example of a CRP technique. The general mechanism for ATRP is shown in Scheme 3. The radicals, or the active species, are generated through a reversible redox process catalyzed by a transition metal complex (Mt"-Y/ligand, where Y may be another ligand or the cotmterion), which undergoes a one electron oxidation followed by abstraction of a halogen atom, X, from a dormant species, R-X. Several transition metals can catalyze the process, but Cu(I) complexes have received the most attention [141]. 

The rate-determining step is net abstraction of a halogen atom from RX by the odd-electron d Co(II) forms R- that subsequently combines with a second Co(II). 

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