Halogenation halohydrins

Hydroxylation Halogenation Halohydrin foimation Cycloadditions Introduction of fimctional groups... 

Dehydro- halogenation Halohydrin Epoxide Synthesis of epoxides resolution of halohydrins and epoxides Halohydrin dehalogenase... 

When treated with bromine or chlorine in aqueous solution alkenes are con verted to vicinal halohydrins A haloni um ion IS an intermediate The halogen adds to the carbon that has the greater number of hydrogens Addition is anti... 

Halohydrin (Section 6 17) A compound that contains both a halogen atom and a hydroxyl group The term is most often used for compounds in which the halogen and the hydroxyl group are on adjacent atoms vicinal halohydrins) The most commonly encountered halohydrins are chlorohydnns and brornohydrins... 

Addition of chlorine or bromine in the presence of water can yield compounds containing haUde and hydroxyl on adjacent carbon atoms (haloalcohols or halohydrins). The same products can be obtained in the presence of methanol (13) or acetic acid (14). As expected from the halonium ion intermediate, the addition is anti. As expected from Markovnikov s rule, the positive halogen goes to the same carbon that the hydrogen of a protic reagent would. 

The success of the halo ketone route depends on the stereo- and regio-selectivity in the halo ketone synthesis, as well as on the stereochemistry of reduction of the bromo ketone. Lithium aluminum hydride or sodium borohydride are commonly used to reduce halo ketones to the /mm-halohydrins. However, carefully controlled reaction conditions or alternate reducing reagents, e.g., lithium borohydride, are often required to avoid reductive elimination of the halogen. 

In aqueous solution chlorine and bromine react with alkenes to fonn vicinal halohydrins, compounds that have a halogen and a hydroxyl group on adjacent carbons. 

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

Bromohydrin (Section 6.17) A halohydrin in which the halogen is bromine (see halohydrin). 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

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 activities of both haloalkanol dehalogenase (halohydrin hydrogen lyase) that catalyzes the formation of epoxides from alkanes with vicinal hydroxyl and halogen groups, and epoxide hydrolase that brings about hydrolysis of epoxyalkanes to diols are involved in a number of degradations that involve their sequential operation. 

However, when dissolved in butyl ether and refluxed with concentrated solutions of the halogen acids, dieldrin reacts typically to yield the halohydrin. 

Hydratropaldehyde has been prepared by hydrolysis of phenylmethylglycidic ester,2 3 4 by chromyl chloride oxidation of cumene,5 by the elimination of halogen acid or water from halohydrins or glycols,5 8 and by the distillation at ordinary pressure of methylphenylethylene oxide.9,10... 

Treating ally lie and homoallylic epoxy alcohols with an equivalent amount of halogen (Br2, I2) in the presence of a stoichiometric amount of Ti(OPr )4 provides halohydrins under mild conditions with a high degree of generality and with good regioselectivity (Scheme 4-12).26... 

Haloperoxidases act as halide-transfer reagents in the presence of halide ions and hydrogen peroxide. In the first step, the halide ion is oxidized to a halonium-ion carrier, from which the positive halogen species is then transferred to the double bond. In an aqueous medium, the intermediary carbocation is trapped and racemic halohydrins are formed (Eq. 7). Selective examples of CPO-cata-lyzed formation of halohydrins are given in Table 9. In CPO-catalyzed reaction. 

Peroxidases catalyze not only the formation of halohydrines, but also the halo-genation of 1,3-dicarbonyl compounds. In CPO-catalyzed halogenation of eno-lizable substrates, the halonium ion is trapped by the enolate to afford the corresponding mono- and dihalogenated products (Eq. 9, Table 10). 

The mechanism and stereochemistry of halogenation, physical methods / 273 Dihalogenation / 276 The use of A-halo compounds / 280 The use of other reagents / 282 The oxidation of halohydrins / 283 Vinylogous a-halo ketones / 284... 

Halohydrins of type 370 (X = halogen) with piperidine or morpholine give compound 371.821 Compound 372 (obtained by tetracyanoalkyl-ation of dihydroresorcinol) is ring contracted by 10% NaOH to com-... 

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