Halohydrins, formation

Propylene oxide has found use in the preparation of polyether polyols from recycled poly(ethylene terephthalate) (264), haUde removal from amine salts via halohydrin formation (265), preparation of flame retardants (266), alkoxylation of amines (267,268), modification of catalysts (269), and preparation of cellulose ethers (270,271). 

Addition of Hypohalous Acids to Alkenes Halohydrin Formation... 

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. 

Figure 7.3 Mechanism of the oxymercuration of an alkene to yield an alcohol. The reaction involves a mercurinium ion intermediate and proceeds by a mechanism similar to that of halohydrin formation. The product of the reaction is the more highly substituted alcohol, corresponding to Markovnikov regiochemistry.

The final product is called a halohydrin (indicating that we have a halogen— Br— and an OH in the same compound). This reaction is commonly called halohydrin formation. 

The profile of halohydrin formation can be summarized in the following chart ... 

Product identification was carried out by NMR analysis using H, and H/ H correlation spectroscopy techniques. Under the given conditions, molar conversion yields were >90 % after 10 min, >90 % after 20 min and >60 % after 30 min for iodo-, bromo-and chloro-halohydrin formation respectively. 

The reagent may require acid activation depending on the type of transformation being attempted. The mechanism parallels that of halohydrin formation using an electrophilic source of halide in an aqueous medium ... 

Halohydrin formation, as depicted in Figure 6.13, is mechanistically related to halogen addition to alkenes. A halonium ion intermediate is formed, which is attacked by water in aqueous solution. 

The diastereoselective halohydrin formation, resulting from the reaction of chiral /V -enoyI -2-oxazoI idinones with Br2/l2 and water, promoted in the presence of silver , in aqueous organic solvents, has been found to occur with high regioselectivity and moderate to good diastereoselectivities. The alkenoyl, cinnamoyl, and electron-deficient cinnamoyl substrates readily produced the bromohydrin in aqueous acetone, but no iodohydrin formation was observed under these conditions. On the other hand, ( ) electron-rich cinnamoyl substrates preferred to afford iodohydrins in aqueous acetone with moderate diastereoselectivity enhanced diastereoselectivity was observed for aqueous THF.31... 

Halohydrin formation, anti addition, Markovnikov s rule... 

Halohydrin Formation Section 11.5 Figure 11.4 Hell-Volhard-Zelinsky Reaction Section 26.4 ... 

Fig. 6.8 Activation of halogens by CPO (a) reactions with 1,3-dicarbonyl compounds (b) halohydrins formation...

Several examples of halohydrin formation from styrene derivatives and saccharides catalyzed by CPO are reviewed by Adam and coworkers [23], Formation of bromohydrin derivatives of some saccharides can be of interest for the preparation of bioactive compounds [72]. 

Stereochemistry of Halohydrin Formation Because the mechanism involves a halonium ion, the stereochemistry of addition is anti, as in halogenation. For example, the addition of bromine water to cyclopentene gives fran.v-2-bromocyclopentanol, the product of anti addition across the double bond. 

Orientation of Halohydrin Formation Even though a halonium ion is involved, rather than a carbocation, the extended version of Markovnikov s rule applies to halohydrin formation. When propene reacts with chlorine water, the major product has the... 

Orientation of halohydrin formation The more substituted carbon of the chloronium ion bears more positive charge than the less substituted carbon. Attack by water occurs on the more substituted carbon to give the Markovnikov product. 

The Markovnikov orientation observed in halohydrin formation is explained by the structure of the halonium ion intermediate. The two carbon atoms bonded to the halogen have partial positive charges, with a larger charge (and a weaker bond to the halogen) on the more substituted carbon atom (Figure 8-5). The nucleophile (water) attacks this more substituted, more electrophilic carbon atom. The result is both anti stereochemistry and Markovnikov orientation. 

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