Dehalogenation, of alkyl halides

Cr(II) has been used to bring about dehalogenation of alkyl halides involving the production of alkyl radicals, and details have been provided in a substantive review (Castro 1998). The ease of reduction is generally iodides > bromides > chlorides, while tertiary halides are the most reactive and primary halides the least (Castro and Kray 1963, 1966). 

Alkenes are obtained by the transformation of various functional groups, e.g. dehydration of alcohols (see Section 5.4.3), dehalogenation of alkyl halides (see Section 5.4.5) and dehalogenation or reduction of alkyl dihalides (see Section 5.4.5). These reactions are known as elimination reactions. An elimination reaction results when a proton and a leaving group are removed from adjacent carbon atoms, giving rise to a tt bond between the two carbon atoms. 

Lithium triethylborohydride (Super-Hydride) is a much more powerful reducing agent than lithium aluminium hydride. It is useful for the reductive dehalogenation of alkyl halides, but unlike lithium aluminium hydride does not affect aryl halides. It is available as solution in tetrahydrofuran in sealed containers under nitrogen. The solutions are flammable and moisture sensitive and should be handled with the same precautions as are taken with other organometallic reagents (see Section 4.2.47, p. 442). 

Mechanism of the radical chain dehalogenation of alkyl halides by tributyltin hydride. The various termination steps are not shown. 

For a way of overcoming this problem, let s go back to the reaction we looked at a few pages ago, the dehalogenation of alkyl halides by Bu3SnH. The mechanism involves formation of an alkyl (carbon-centred) radical by abstraction of Br by Bu3Sn. This alkyl radical then just abstracted H from... 

Red-Al [sodium bis(2-methoxyethoxy)aluminium hydride] reduces aliphatic halides and aromatic halides to hydrocarbons. Reductive dehalogenation of alkyl halides is most commonly carried out with super hydride. Epoxide ring can also be opened by super hydride. 

A currently employed method for dehalogenation of alkyl halides is to use a Li- or Na-alcohol reagent system. This method is effective not only for simple alkyl halides but also for the reduction of a halogen atom attached to a bridgehead. Carbon-carbon unsaturated bonds are not affected under the conditions, as shown in Scheme 5. ... 

Scheme 5.23. Dehalogenation of alkyl halides over bismuth-based catalysts.

Since it is unlike that hydride anions are involved under acidic conditions, the most reasonable mechanism for the debromination is the C-Br homolytic bond breaking followed by hydrogen abstraction from the medium of the resulting radical. Related precedents for a radical mechanism for the dehalogenation of alkyl halides are well documented in the chemical literature [13],... 

Catalytic hydrogenolysis of alkyl halides is one of the classical methods of alkane syntheses. Among other catalysts, Raney Ni has proved to be quite effective for the hydrogenolysis of iodides (equation 39) . A relatively recent discovery is the reductive dehalogenation of alkyl halides under the conditions of ionic hydrogenation. 

Dissolved alkali metals have been traditionally used for the dehalogenation of alkyl halides. The zinc-acetic acid system and metal-ammonia solutions were the oldest methods used for this purpose. They still constitute an efficient medium for dehalogenations and several convenient procedures have been introduced. Sodium in... 

Chiral amines have been conveniently prepared also by asymmetric reductive amination of ketones using iridium catalysts and intriguing results with up to 96% ee have been obtained by Zhang and co-workers employing a catalytic system based on Ir./-binaphane in the presence of Ti(OPr )4 and iodine (Scheme 61). Water-soluble aquo complexes [Cp lr(H20)3](0Tf)2 494, [CpP Ir(H20)2](0Tf)2 504, and [Cp Ir(bpy)(H20)](0Tf)2 505 have been used to catalyze the reductive amination of hydrosoluble aldehydes and ketones as well as the dehalogenation of alkyl halides. The activity is markedly pH dependent and inactivation of the catalyst takes place reversibly on increasing the solution basicity due to Ir(H20), deprotonation and formation of mono- or dinuclear hydroxo complexes which are catalytically inactive. The structure of one of these compounds, [Cp Ir(bpy)(OH)]OTf 506, which reversibly forms from 494 around pH 6.6, is presented in Figure 42. 

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