Halohydrin dehydrohalogenation

Oxidative dehydrohalogenation is a common metabolic pathway for many halogenated hydrocarbons (25,29,30,31). The CYP450-catalyzed oxidation generates the transient gem-halohydrin (analogous to alkane hydroxylation) that can eliminate the hydrohalic acid to form carbonyl derivatives (aldehydes, ketones, acyl halides, and carbonyl halides) (Fig. 10.7). This reaction... 

Halohydrins are easily prepared and dehydrohalogenation occurs readily at low temperatures. Another way epoxides can be formed is by catalytic vapor-phase oxidation of alkenes to form oxiranes. 

Epoxides may also be formed by oxidation of alkenes with other reagents. For a summary, see Hudlicky, M. Oxidations in Organic Chemistry, ACS Monograph No. 186 American Chemical Society Washington, DC, 1990. Dehydrohalogenation of halohydrins is another path to epoxides. 

The mechanism of dehydrohalogenation under basic conditions of trons-fused bicyclo[4,n,0]alkane halohydrins (563)—(565) has been studied. Three reaction types are noted (i) epoxide formation, (ii) ketone formation, and (iii) ring contraction. trans-Diaxial chlorohydrins corresponding to (563)—(565) gave epoxides (566)—(568) with relative rates (derived from bimolecular rate constants) of 1 3 17. This rate sequence was rationalized in terms of deformation of the cyclohexane ring brought about by the nature of the fused ring. In particular, deformation is probably towards the half-chair conformation favoured by the cyclohexane epoxide which is formed in the slow step. trans-Diequatorial chlorohydrins represented by (563)—(565)... 

In the chlorohydrin route, generally preferred in the epoxidation of C3-C4 olefins, stoichiometric amounts of sodium or calcium chlorides are produced by the dehydrohalogenation of intermediate halohydrins. Chlorinated organic by-products, such as halogen ethers and dichlorides, are formed as well in the process, fur er increasing the quantity of wastes. 

Electrophilic addition of bromodimethylsulphonium bromide to alkenes gives, after dehydrobromination, variable yields of vinylsulphonium salts. Hydrolysis of the vinyl thioether derivatives gives a formal conversion of an alkene into a carbonyl compound with some degree of control over regioselect-ivity. Halohydrins (derived from alkenes) can be converted into ketones by dehydrohalogenation with potassium carbonate, catalysed by palladous acetate. ... 

Epoxides of higher olefins are available as specialty chemicals. They can be prepared either by dehydrohalogenation of the corresponding halohydrin or by epoxidation of the corresponding olefin with an organic peracid such as peracetic acid or perbenzoic acid (17,18). A large number of these epoxides have been prepared (19) and are described in the literature. 

Oxides are usually formed in the laboratory by the peroxidation of olefins with H Oj, peracids, or by the dehydrohalogenation of halohydrins. 

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