Halide ions, reaction with epoxides

At somewhat higher pH, direct nucleophilic attack of halide ion on diol epoxide 81 (fci[X-]) becomes important, and a rate plateau is reached in which this term is the main one. If the pH is sufficiently low, the pH-dependent equilibrium between halohydrin 131 and diol epoxide 81 (shown in Scheme 41) favors halohydrin, which reacts via an SnI reaction (k3) to form tetrols 129. As the pH is increased, however, the pH-dependent equilibrium between halohydrin 131 and diol epoxide 81 shifts to favor diol epoxide, and there is a resulting rate decrease that gives an inflection point in the pH-rate profile at intermediate pH that resembles those in the profiles in Figs 4 and 5. Rate and product observations are rationalized by the mechanism of Scheme 40, and comparable mechanisms can be expected for reactions of other epoxides susceptible to reaction with nucleophiles. 

The reaction of chlorodifluorotnethane with alkoxide ions generated in low concentration from halide ions and epoxides [28, 29] is an interesting, higher temperature method that gives good to excellent yields of gem-difluorocyclo-propanes from just moderately nucleophilic olefins (equation 9)... 

Substitution of an additional nitrogen atom onto the three-carbon side chain also serves to suppress tranquilizing activity at the expense of antispasmodic activity. Reaction of phenothia zine with epichlorohydrin by means of sodium hydride gives the epoxide 121. It should be noted that, even if initial attack in this reaction is on the epoxide, the alkoxide ion that would result from this nucleophilic addition can readily displace the adjacent chlorine to give the observed product. Opening of the oxirane with dimethylamine proceeds at the terminal position to afford the amino alcohol, 122. The amino alcohol is then converted to the halide (123). A displacement reaction with dimethylamine gives aminopromazine (124). ... 

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. 

Halohydrins. The reaction of a slight excess of 1, Br, or Cl, and of P(C6H,)j in CH2CI2 with an epoxide results in a halohydrin in generally high yield. The orientation depends in part on the bulkiness of the halide ion, but the halogen ion generally attacks the less substituted carbon atom. 

The reaction of an aryl diazonium halide with an aliphatic unsaturated compound to yield an a-halo-P-phenyl alkene and alkanes. The reaction is performed in the presence of cupric ious. The presence of an electron-withdrawing group is useful in promoting the reactivity of the alkene. See Kochi, J.K., The Meerwein reaction. Catalysis by cuprous chloride, J. Am. Chem. Soc. 11, 5090, 1955 Morales, L.A. and Eberlin, M.N., The gas-phase Meerwein reaction, Chemistry 6, 897-905, 2000 Riter, L.S., Meurer, E.C., Handberg, E.S. et al, lon/molecule reactions performed in a miniature cylindrical ion trap mass spectrometer. Analyst 128, 1112-1118, 2003 Meurer, E.C., Chen, H., Riter, E.S. et al., Meerwein reaction of phosphonium ions with epoxides and thioepoxides in the gas phase, J. Am. Soc. Mass Spectrom. 15, 398 05, 2004 Meurer, E.C. and Eberlin, M.N., The atmospheric pressure Meerwein reaction, J. Mass Spectrom. 41, 470-476, 2006. 

Similar SN 2 mechanisms occur in the uncatalyzed ring-opening reactions of ethylene oxide and other epoxides with nucleophiles such as halide ions or amines [151, 154]. The dependence of the rate coefficient kx on the attacking nucleophile is the same as in SN 2 reactions of alkyl halides. It follows the Swain—Scott [155] and Edwards [156] relationships. 

Scheme 40 shows the mechanism of reaction of diol epoxide 81 in solutions containing chloride, bromide or iodide salts.113 At sufficiently low pH, fcH[H + ] is the dominant term of the rate equation, and 81 reacts with H + to form carbocation 83. Carbocation 83 partitions between reaction with water (ks) and capture by halide ion (A 2[X-]). Iodide ion reacts with 83 at or near the diffusion-controlled limit, and is 3-4 times more reactive than bromide ion and 28 times more reactive than chloride... 

The 1,3-dioxolans (234 R = CH2CI, Me, or Et) have been prepared under neutral conditions by the reactions of the corresponding epoxides with benz-aldehyde. The reactions are catalysed by halide ion and provide a mixture of cis- and rra s-2,4-disubstituted compounds. Using Lewis acid catalysts, the ds-isomer was preferentially formed whereas catalysis by lithium halides favoured the rrans-isomer. 

Although the reaction with ethylene oxide is quite successful, substituted epoxides often lead to mixtures. In addition to ring opening by the halide ion. isomerization of the epoxide to an isomeric aldehyde or ketone is catalyzed by the l.ewis-acidic species of the Grignard reagent 6.1471. The... 

Epoxides, like other ethers, react with hydrogen halides. In the first step of the reaction, the nucleophilic oxygen is protonated by the acid. The protonated epoxide is then attacked by the halide ion. Because epoxides are so much more reactive than ethers, the reaction takes place readily at room temperature. (Recall that the reaction of an ether with a hydrogen halide requires heat.)... 

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