Halohydrins from Alkenes

The formation of vicinal halohydrins from alkenes was described in Section 6.17. Halohydrins are readily converted to epoxides on treatment with base ... 

This problem is similar to the one in part d) in that it requires the preparation of a halohydrin from an aUcyl halide. The strategy is the same. Convert the alkyl halide to an alkene, and then form the halohydrin by treatment with the appropriate halogen in aqueous solution. 

FIGURE 10.17 A test of the regiochemistry of the addition of Brg/HgO to alkenes. Halohydrin A would result from water attacking the more substituted carbon of the bromonium ion and halohydrin B would result from attack at the less substituted carbon. 

The following section describes the preparation of epoxides by the base-promoted ring closure of vicinal halohydrins. Because vicinal halohydrins are customarily prepared from alkenes (Section 6.17), both methods—epoxidation using peroxy acids and ring closure of halohydrins—are based on alkenes as the starting materials for preparing epoxides. 

See Ref. 150, p. 136, for reagents that produce alkenes from p-halo ethers and esters, and from halohydrins. 

Ketones from halohydrins. Palladium acetate complexed with a triarylphos-phine, particularly tri-o-tolylphosphine, converts halohydrins into ketones in the presence of K2C03. Yields are about 70-85% for substrates in which the halogen is secondary or tertiary, but less than 50% when the halogen is primary because of epoxide formation. The reaction is useful for conversion of alkenes to ketones in those instances in which halohydrins are formed regioselectively. 

They can also be obtained from alkenes in a two-step process (Fig. A). The first step involves electrophilic addition of a halogen in aqueous solution to form a halohydrin. Treatment of the halohydrin with base then ionises the alcohol group, that can then act as a nucleophile. The oxygen uses a lone pair of electrons to form a bond to the neighbouring electrophilic carbon, thus displacing the halogen by an intramolecular SN2 reaction. 

The reaction of an alkene with a halogen like bromine and chlorine generally gives a vicinal dihalide. However, if the reaction is carried out in water as solvent, the product obtained is a halohydrin where the halogen adds to one end of the double bond and a hydroxyl group from water adds to the other. 

Halohydrins are /J-halogenated alcohols. They can be obtained in H20-containing solvents from alkenes and reagents, which transfer Hal ions. N-Broniosuccinnuide (transfers Br Figures 3.43 and 3.44 as well as 3.47), chloramine-T (transfers Cl Figure 3.46), and elemental iodine (transfers I Figure 3.47) have this ability. Bromonium and chloronium ions react with H20 via an SN2 mechanism. This furnishes the protonated bromo- or chlorohydrins, which are subsequently deprotonated. 

Using Hydrohalic Acids When an epoxide reacts with a hydrohalic acid (HC1, HBr, or HI), a halide ion attacks the protonated epoxide. This reaction is analogous to the cleavage of ethers by HBr or HI. The halohydrin initially formed reacts further with HX to give a 1,2-dihalide. This is rarely a useful synthetic reaction, because the 1,2-dihalide can be made directly from the alkene by electrophilic addition of X2. 

Sec. 7.8, 18.5) from alkenes by treatment with a peroxyacid (Sec. 18.5) from halohydrins by treatment with base... 

Several related procedures for syn hydroxylation of alkenes involve a halohydrin ester (32) as the key intermediate. In Woodward s procedure an alkene in glacial acetic acid is treated with iodine and silver acetate. Acetyl hypoiodite, MeCOjI, formed by reaction of the latter two reagents attacks the alkene, R R 0=CR R in an electrophilic manner, from the less hindered side to give, by overall anti addition. 

When an organic compound contains both a hydroxy group and a halogen atom on adjacent carbon atoms, an intramolecular version of this reaction forms an epoxide. The starting material for this two-step sequence, a halohydrin, is prepared from an alkene, as we will leam in Chapter 10. 

Oxiranes are usually prepared from halohydrins. A general method of obtaining chloroalcohols from olefins is by the addition of hypochlorous acid formed in situ form -chloramides or fert-butyl hypochlorite. Bromoalcohols are produced from alkenes with Ai-bromosuccinimide or iV-bromoacetamide, while iodoalcohols are prepared with iodine in the presence of oxidants (iodic acid, oxygen, and nitrite). 

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