Reduction of allylic halides

A classic paper by Baizer and Chruma [35] describes synthetic applications based on the reduction of allyl halides. Electrolyses of 4-bromo- and 4-chloro-2-butene can be employed for the allylation of acetone and benzaldehyde [36], and the reduction of allyl halides in the presence of trimethylchlorosilane affords silylated compounds [34]. 

Transmetallation of allyltributyltin with organolithium species has been used for the generation of allyllithium solutions free of the coupling byproducts which often result from reduction of allylic halides with lithium metal. These solutions may then be used directly for the preparation of Gilman reagents and other reactive modifications of the parent allyllithium. 

For reduction of allylic halides with double bond transposition, see page 229, Section 4. 

According to Bard and Merz [108], in MeCN containing TBAP, allyl bromide and allyl iodide interact chemically with a mercury electrode to form allylmercury halides. These allylmercury halides undergo reduction to yield diallylmercury, which is itself electroactive. Allyl bromide and allyl iodide are reduced at platinum in MeCN in a two-electron process to give the allyl anion, and the allyl radical is not involved as an intermediate. Reduction of allyl halides at platinum in DMF containing TEAOTs and in the presence of trimethylchlorosilane results in silylated compounds [100]. 

Organotin compounds may be synthesized by the cathodic reduction of organic compounds in the presence of tin halides. For example, the reduction of allylic halides in the presence of chlorostannanes gives the corresponding allylstannanes in good yields [69]. Combination of this reaction with in situ palladium-catalyzed reaction with allylic halides leads to effective formation of the head-to-tail homo coupling products as shown in Eq. (17). 

While the parent allylcobalt complex (R = R = H) was stable in an aqueous alkaline solution, the butenylcobalt complex (R = CH3, R = H) gradually evolved an approximately equimolar mixture of butenes and butadiene, suggesting a disproportionation involving the intermediate formation of hydrido complex (Reactions 3 and 4) (20, 22). Since apparently the same butenylcobalt complex is formed by adding hydrido complex to butadiene, the reduction of allylic halides is discussed with the intimately related hydrogenation of dienes later in the text. 

The reduction of allylic halides (in the cepham family) by SmI2/THF in the presence of some water gave a very fast transformation into exomethylene ce-phams [113]. The authors made the hypothesis that an intramolecular protonation occurs in a a-allylsamarium by H20 coordinated to samarium. 

As mentioned in Section 7.1.2, the electrochemical reduction of allylic halides in the presence of chlorotrimethylsilane can be achieved using a microflow cell and the desired allylic silanes are obtained (Table 7.2). 

A pathway for the catalytic reduction of allyl halides is formulated as involving homolytic cleavage of the substrate by pentacyanocobaltate(II), with subsequent cleavage of the allyl complex so formed by hybrido complex ... 

The reduction of allylic halides or acetates, as well as benzyl halides, can be achieved by the Pd-catalyzed reaction with formate ions in the biphasic system of toluene-water or heptane-water in the presence of hydrophilic phosphine ligands TPPMS, sodium 3-(diphenylphosphino)benzoate, and PEG-modilied trialkylphosphine. The process is accelerated by the addition of PEGJ ... 

The bipyridyl complexes of Co(ID showed electrocatalytic activity for reduction of allyl halides to 1,5-hexadiene in micellar media. Catalysis lowered the overpotential for reduction of allyl chloride in 0.1 M SDS and CTAB by 1.4 V compared to direct reduction. Yields of about 60% of l,5 hexadiene were obtained from electrolyses at carbon felt electrodes. Small micellar enhancements of reaction rates were found for tris 2,2 bipyridyl)cobalt(II) (Table 1). Catalytic efficiency followed the order CTAB > SDS = acetonitrile. Preliminary work with the long chain derivative bis(2,2 -bipyridyl)(4,4 -hexadecyl-2,2 -bipyridyl)cobalt(II)... 

The practical use of chromium(II) chloride in organic synthesis was begun by Hiyama and Nozaki in 1976. They used anhydrous chromium(II) chloride for the reduction of allylic halides to get allylic chromium reagents [47]. Since then, useful C-C bond formation reactions between organic halides and carbonyl compounds, which were mediated by chromium(II) salt have been developed. The most important features of these reactions were chemoselectivity and stereoselectivity. In these transformations, treatment of organic halides with chromium(II) salt was considered to afford the intermediary organochromium compounds although these compounds have not been isolated. As described in the previous section, reduction of gem-dihalides with chromium(II) salt may afford gem-dichromium species. 

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