Alkene dehalogenation

Preparation of alkenes Dehalogenation of vicinal-dihalides with Nal in acetone produces alkene via E2 reactions. 

General Preparations.—Alkenes. Dehalogenation of vicinal dihalides is easily carried out using sodium in liquid ammonia, e.g. 1,2-dichlorocyclo-octane gave ds-cyclo-octene in 95% yield.The scope of the synthesis of ap-unsaturated carbonyl compounds by syn-elimination from 2-phenylselenoxides has been studied only low yields of the desired ketone were obtained from 2-phenylseleno-cycloheptanone and cyclo-octanone because of the competing Pummerer rearrangements, but improved yields could be obtained by prior conversion of the ketones into ketals. The reaction worked well for 2-methoxycarbonyl cyclic ketones, e.g. 2-methoxycarbonylcyclo-oct-2-en-l-one was prepared in 93% yield. ... 

The dehalogenation of organic halides by organotin hydrides takes place in most cases with a free-radical mechanism [1, 84, 85], The stereospecific reduction of 1,1-dibromo-l-alkenes with Bu3SnH discovered by Uenishi and coworkers [86-89], however, did not occur in the absence of palladium complexes and did not involve radicals. For the synthesis of (Z)-l-bromo-l-alkenes, [(PPh3)4Pd] proved to be the most effective catalyst which could also be generated in situ. The reaction in Eq. (7) proceeded at room temperature and a wide range of solvents could be used. 

The reductive dehalogenation of vic-dibromides to give the alkenes, using sodium sulphide [32] or sodium trithiocarbonate [33] is aided by the addition of quaternary ammonium salts. Anri-elimination normally occurs is good yield, but is susceptible to steric factors [34], Other functional groups are not reduced by the sulphide. 

Fig. 24. Reductive dehalogenation of a chlorinated hydrocarbon in the presence of a metal forming an alkyl radical, showing (Pathway (I)) the alkyl radical scavenging a hydrogen atom, and (Pathway (II)) the alkyl radical losing a second halogen to form an alkene...

Another group of compounds that have a twisted double bond are the bicyclic compounds with bridgehead double bonds such as 1,2-norbomene (9) and 1,7-norbornene (10). " It has been found that many compounds, such as 11, which is based on trawi-cyclooctene, may be isolated whereas those based on smaller trauj -cycloalkenes are usually quite unstable. Some evidence for the formation of 9 has been obtained by trapping the product of the dehalogenation of 1,2-diha-lonorbornanes." Here, the simplest view is that the two p orbitals that form the double bond in 9 and 10 are roughly perpendicular to each other. However, pyr-amidalization and rehybridization also are involved. One indication is the reduced localized 7i-orbital population found in the NBO analysis. Whereas normal alkenes have 71 populations of 1.96 e, for 9 with OS = 57 kcal/mol, it is 1.921, and for 10 with OS = 86 kcal/mol, it is 1.896. With 9, the deviations of the a and n orbitals from the line of centers are 24° and 19°, respectively, and with 10, the deviations are 34° and 29°. 

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. 

Sodium metal in ether has also been used to dehalogenate cyclic 1,2-dihalo-alkenes to cyclic allenes with some cyclic alkyne as a by-product [13]. 

Certain allyl cations react quite efficiently with nonactivated alkenes to give cyclobutanes. Such cations can be generated by Lewis acid catalyzed dehalogenation of allyl halides, protonation of conjugated dienes and Lewis acid eomplexation of conjugated carbonyl derivatives. For example, 2-chloro-2,4-dimethylpent-3-ene ( ) reacts with alkenes in the presence of zinc(II) chloride to give the corresponding cyclobulanes.1 Alkyl substitution of the allyl cation at the 2-position results in [3 + 2]-cycloaddition products. 

Halo-alkenes are common pollutants. Therefore, there is an ongoing study on plausible approaches to the dehalogenation of halo-alkanes. One of these approaches involves their electrocatalytic reduction. NinL2 + (L = a tetraaza macrocyclic ligand) complexes were proposed as plausible electrocatalysts (150). A pulse radiolytic study on the mechanism and kinetics of the reaction ... 

Fluorinated alkynes are usually prepared from the corresponding alkenes, but a twofold didechlorination of the corresponding alkane has also been applied. Alkynes bearing a fluorine atom on the triple bond are extremely reactive and hence special precautions have to be taken in their preparation under ordinary conditions, this fluorine atom is lost to afford a non-halogenated triple bond. Perfluoroalkynes with an internal triple bond can even be prepared using zinc. Examples of dehalogenations of fluorinated compounds to provide fluoroalkynes are listed in Table 3. 

Dehalogenation a-Bromo or a-chloro ketones are reduced by nickel boride in DMF to ketones in 70-95% yield. Wc-Dibromides are reduced to alkenes in 80-90% yield. 

A typical example is the dehalogenation of 1,2-dihalides. The elimination of 1,2-dibromides occurs stereospecifically in the form of an anh -elimination. Thus, the /neso-dibromide yields the (E)- and the d, /-dibromide yields the (Z)-alkene selectively [37] ... 

Dihalocydopropanes readily undergo reductive dehalogenation under a variety of conditions. Suitable choice of reagents and reaction conditions will allow the synthesis of monohalocyclopropanes or the parent cyclopropanes.19 " The ease of reduction follows the expected order I > Br > Cl > F. In general, complete reduction of dibromo and dichloro compounds is accomplished by alkali metal in alcohol,99-102 liquid ammonia103 or tetrahydrofuran (equations 28 and 29).104 The dihalocydopropanes can be reduced conveniently with LAH (equation 30).105 LAH reduction is particularly suited for difluoro compounds which are resistant to dissolving metal reductions.19 106 It is noteworthy that the sequence of dihalocar-bene addition to an alkene followed by the reduction of the dihalocyclopropyl compounds (equation 31) provides a convenient and powerful alternative to Simmons-Smith cyclopropanation, which is not always reliable. 

Radical cyclization of 6,6-dihaloalkenes promoted by LAH in THF is a useful route to halocyclopentanes (equation 145)535,536. A small quantity of the dehalogenated alkene is also produced. 

---