Polyhalo-alkanes

The compounds CCI4, BrCCla, ICF3, and similar simple polyhalo alkanes add to alkenes in good yield. These are free-radical additions and require initiation, for... 

This type of polyhalo alkane adds to halogenated alkenes in the presence of AICI3 by an electrophilic mechanism. This is called the Prins reaction (not to be confused with the other Prins reaction, 16-53). ... 

Traditionally, this radical addition reaction has been used for the preparation of new poly halo organic compounds by reaction of simple polyhalo alkanes with alkenes689 698. The products from these reactions have often been used as key intermediates for the synthesis of heterocyclic compounds690. In some cases it has been found that metal complexes significantly speed up reactions and/or increase yields. 

Sasson and Webster119 reported some examples of monoprotic carbon acids, for example alkylmalonic esters and phenylacetylene, that are directly halogenated in high yield by polyhalo alkanes under mild conditions when catalyzed by tetrabutylammonium fluoride (TBAF) (equation 76). They do not discuss the halogenation mechanism other than to say that the reaction is apparently taking place via nucleophilic attack on the tetra-halomethane (equation 77). It is reasonable to believe that it is the carbanion of the sub-... 

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. 

Simple polyhalo alkanes, such as CCI4, BrCCl3, ICF3 and related molecules, add... 

This mechanism does not take Into account the possibility of the formation of charge-transfer complexes between the polyhalo-alkanes and the arylamlnes. Such complexes are very well known In the literature for aliphatic amines and polyhaloalkanes. They could play a very Important role In these photochemical imaging systems (147-150). 

Polyhalo Alkanes 1990. During investigation [103] of the reactions of poly-haloalkanes with magnesium in the presence of a third reactant, using THF as the solvent, a variety of interesting products was obtained. Relevant to this monograph was the observation that benzyUdene chloride reacted with magnesium in the presence of benzophenone in a rather predictable way (see [98] p. 57). 

Haloalkenes are versatile synthons for the synthesis of functionalized olefins and dienes, and are often prepared by the dehydrohalogenation of polyhalo-alkanes. The method often suffers from poor yields or low conversion, but Chin-Hsien has demonstrated that phase-transfer conditions allow the reaction to proceed cleanly, in high yield (79— 90%), ... 

The catalytic activity of cytochrome P450 is not restricted to oxidation. Under certain conditions, especially anaerobic conditions or with certain substrates, it can function as a reductase. For example, P450 can catalyze the reductive removal of halide from polyhalo-genated alkanes such as hexachloroethane or halothane (8,9). 

Table 16.3 Names, abbreviations, pseudo-first-order rate constants, and half-lives of polyhalo-genated alkanes in Fe(II)/goethite suspension. Experimental conditions 25 m L" goethite, pH 7.2, tgq>24 h. Fe(II) = 1 mM. b Standard deviation, c number of replicates, d t =5 h. Reprinted with permission from Pecher K, Haderline SB, Schwarzenbach RP (2002) Reduction of polyhalo-genated methanes by surface-bound Fe(II) in aqueous suspensions of iron oxides. Environ Sci Technol 36 1734-1741. Copyright 2002 American Chemical Society...

Side-chain halogenation, like halogenation of alkanes, may yield polyhalo-genated products even when reaction is limited to monohalogenation, it may yield a mixture of isomers. 

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