Hypohalous acids, addition with alkenes

Yet another example of an electrophilic addition is the reaction of alkenes with the hypohalous acids HO—Cl or HO-Br to yield 1,2-halo alcohols, called halohydrins. Halohydrin formation doesn t take place by direct reaction of an alkene with HOBr or HOC1, however. Rather, the addition is done indirectly by reaction of the alkene with either Br2 or Cl2 in the presence of water. 

Cleavage of epoxides with hydrogen halides Addition of hypohalous acids to alkenes... 

Limited data are available on hypofluorous acid, which is a very reactive, explosive reagent.145 Exceptionally among hypohalous acids, it is polarized in the sense H08+-F8. It converts alkenes into fluoroalkanols with regioselectivites opposite to other hypohalous acids [Eq. (6.24)] the large amount of HF present may affect the actual mechanism of addition ... 

The diastereoselectivity of the addition of hypohalous acids and esters to alkenes is potentially even more susceptible to polar effects than peracid epoxidations124 they correlate to the electronegativity of the halogen and of the oxygen substituent. The derived epoxide normally has the opposite configuration to that resulting from the polar effect on peracid epoxidation, e.g., formation of the epoxide 2 with various acyl hypohalites38. 

As shown in Figure 4.1, there are additional reactive sites in phospholipids that potentially may be halogenated in the presence of peroxidase-derived hypohalous acids. In 1998, studies by Winterbourne et al. showed both hypochlorous acid and hypobromous acids react with both unsaturated aliphatic residues and the ethanol-amine of phosphatidylethanolamine (Carr et al. 1998). These studies showed that the amine was more reactive than alkenes and that the alkenes formed halohydrins only after the amine was consumed by the hypohalous acid. This was similar to the findings with plasmalogens which showed the chlorohydrins were only produced once the vinyl ether bond was consumed. Additionally these earlier studies demonstrated that bromoamine was reactive in producing bromohydrins, and this activity was greater than that observed with chloramines (Carr et al. 1998). Others have also shown the reactivity of hypohalous acids with ethanolamine (Jaskolla et al. 2009), but it is unclear whether these compounds are stable in cells or in vivo. 

Scheme 6.22. A representation of the path by which hypohalous acids in general and hypo-bromous acid (HOBr) in particular reacts with alkenes in general and cyclopentene (cyclo-CsHs) in particular. Note that the addition is antarafadal.

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