Halohydrins halides

Base promoted cyclization of vicinal halohydrms (Section 16 10) This reaction is an intramolecu lar version of the Williamson ether synthesis The alcohol function of a vicinal halohydrin is con verted to its conjugate base which then displa ces halide from the adjacent carbon to give an epoxide... 

With Acyl Halides, Hydrogen Halides, and Metallic Halides. Ethylene oxide reacts with acetyl chloride at slightly elevated temperatures in the presence of hydrogen chloride to give the acetate of ethylene chlorohydrin (70). Hydrogen haUdes react to form the corresponding halohydrins (71). Aqueous solutions of ethylene oxide and a metallic haUde can result in the precipitation of the metal hydroxide (72,73). The haUdes of aluminum, chromium, iron, thorium, and zinc in dilute solution react with ethylene oxide to form sols or gels of the metal oxide hydrates and ethylene halohydrin (74). 

The primary product of hypohalite photolysis is a 1,5-halohydrin, the stability of which increases on passing from iodo- to bromo- to chloro-hydrins. If spontaneous elimination of hydrogen halide does not occur, the halohydrins can be converted into tetrahydrofurans by base treatment. In several instances it has been possible to isolate the intermediate... 

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. 

Another method for the synthesis of epoxides is through the use of halo-hydrins, prepared by electrophilic addition of HO—X to alkenes (Section 7.3). When halohydrins are treated with base, HX is eliminated and an epoxide is produced by an intramolecular Williamson ether synthesis. That is, the nucleophilic alkoxide ion and the electrophilic alkyl halide are in the same molecule. 

Other examples of nucleophilic attack on the oxirane ring include the formation of (3-halohydrins with silica-gel supported lithium halides <96TL1845>, the addition of amines catalyzed by lithium triflate, an ersatz for lithium perchlorate <96TL7715>, and the addition of pyrroles, indoles and imidazoles under high pressure i.e., 91 —> 93) <96JOC984>. 

Regioselective anbd Chemoselective Synthesis of Halohydrins by Cleavage of Oxiranes with Metal Halides," Bonini. C. Righi, G. Synthesis, 1994, 225... 

Nakamura T, T Nagasawa, F Yu, I Watanabe, H Yamada (1994a) Characterization of a novel enantioselective halohydrin hydrogen-halide lyase. Appl Environ Microbiol 60 1297-1301. 

Silica gel-based catalytic systems have been described as efficient promoters for a number of organic reactions.28 Illustrative examples include the oxidative cleavage of double bonds catalyzed by silica-supported KM11O4,29 reaction of epoxides with lithium halides to give /i-halohydrins performed on silica gel,30 selective deprotection of terf-butyldimethylsilyl ethers catalyzed by silica gel-supported phosphomolybdic acid (PMA),31 and synthesis of cyclic carbonates from epoxides and carbon dioxide over silica-supported quaternary ammonium salts.32... 

Halohydrin dehalogenase activity was determined by monitoring halide liberation at 30 °C in tris-S04 buffer (50 mM, pH 8.0) containing 5 mM 1,3-dichloropropanol or 1,3-dibromopropanol as the substrate. All buffers used for activity assay were prepared with bidest water. From the incubation mixture, 0.5 ml samples were taken and mixed with 1.6 ml of H2O, 0.2 ml or halide reagent 1 and 0.2 ml of halide reagent II. Absorbances were read at 460 nm. A calibration curve of 0-1 mM of chloride or bromide was used to calculate the concentration of halide. The extinctions at 460 nm should be below 0.4 (for chloride) or 0.8 (for bromide). 

Tang, L., Torres Pazmino, D.E., Fraaije, M.W., de Jong, R.M., Dijkstra, B.W. and Janssen, D.B., Improved catalytic properties of halohydrin dehalogenase hy modification of the halide-binding site. Biochemistry, 2005, 44, 6609. 

Peroxidases, in particular haloperoxidases, catalyze the oxidation of olefins in the presence or absence of halide ions. In the latter case, epoxides are produced directly (Fig. 6, route A), while in the former process halohydrins are formed (Fig. 6, route B), which may be transformed to epoxides either enzymatically by halohydrin epoxidase [128] or chemically by base treatment. 

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. 

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 ... 

Halides (confined at present to silver halides) can be deposited by hydrolyzing a water-soluble halogeno-alcohol (halohydrin) to slowly form halide ions in the presence of Ag ions ... 

Silver halides, in particular Agl and AgBr, have been deposited by hydrolysis of halogenoalcohols (halohydrins) to free halide ions in a solution of Ag under... 

Haloalcohols (halohydrins) which are more water soluble than alkyl halides, have been used to generate chloride ions [29], and are therefore more suitable for aqueous CD. The hydrolysis of haloalcohols has been used to deposit films of AgBr and Agl, e.g.. 

Alkenes are converted to halohydrins by the treatment of halides and water. When halohydrins are treated with a strong base (NaOH), an intramolecular cyclization occurs and epoxides are formed. For example, 1-butene can be converted to butylene oxide via butylene chlorohydrin. 

Addition of halides and water to alkenes preparation of halohydrins... 

Addition of chlorine or bromine in the presence of water can yield compounds containing halide 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. 

Alkene Halogen Water Halohydrin Hydrogen halide... 

Nakamura T, Nagasawa T et al (1991) A new catalytic function of halohydrin hydrogen-halide-lyase, synthesis of P-hydroxynitriles from epoxides and cyanide. Biochem Biophys Res Commun 180 124—130... 

Several recent publications describe cleavage of 1,3,2-dioxathiolane. S -dioxides (cyclic sulfates) by halide nucleophiles that furnish halohydrines, which can be used as synthetic intermediates, primarily for preparation of corresponding epoxides or for further reactions with nucleophiles (Table 6). Similar reactions with chloride have been studied for 1,3,2-dioxathiolane J-oxides (cyclic sulfites) <1996ACS832>. 

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