Alkenes halohydrin formation from

A MECHANISM FOR THE REACTION ] Halohydrin Formation from an Alkene 365... 

The more highly substituted carbon atom bears a greater positive charge because it resembles a more stable tertiary carbocation. [Notice how this reaction (and its explanation) resembles that given for halohydrin formation from unsymmetrical alkenes in Section 8.14 and attack on mercurinium ions.]... 

From the following alkene precursors, show products of halohydrin formation. 

In a reaction resembling halohydrin formation (Section 6.17), vicinal haloethers are prepared from alkenes by reaction with an alcohol in the presence of halogens— usually bromine or iodine. This haleotherification proceeds through a cyclic halonium ion, which reacts with the alcohol. 1-Methylcyclohexene undergoes iodoetherification with ethanol in the presence of iodine to give ran -l-ethoxy-2-iodo-l-methylcyclohexane. 

Carbonyl Compounds by Oxidation of Alcohols and Aldehydes. Salts of palladium, in particular PdCl2 in the presence of a base, catalyze the CCI4 oxidation of alcohols to aldehydes and ketones. Allylic alcohols carrying a terminal double bond are transformed to 4,4,4-trichloro ketones at 110 °C, but yield halo-hydrins at 40 °C. These can be transformed to the corresponding trichloro ketones under catalysis of palladium acetate (eq 56). The latter transformation could be useful for the formation of ketones from internal alkenes provided the halohydrin formation is regioselective. 

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

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. 

LiBr solubilized by Bu"3PO (or HMPA) was found to catalyze the formation of 2-oxazolidones from organic isocyanates and terminal alkene epoxides. The epoxide substituent appears at the 5-position of the product as shown in equation (122). This outcome is in keeping with rapid trapping by the isocyanate of the halohydrin salt formed by attack of bromide at the primary center. This interpretation requires that oxazolidone formation be faster than epoxide rearrangement data are not available to confirm this point. 

Haloperoxidases have been shown to transform alkenes by a formal addition of hypohalous acid to produce halohydrins. The reaction mechanism of enzymatic halogenatitHi has been debated for some time and it is now accepted that it proceeds via a halonium intermediate [1770, 1771], similar to the chemical formation of halohydrins (Scheme 2.228). The former species is derived from hypohalous acid or molecular halogen, which is in turn produced by the enzyme via oxidation of halide [1772]. In support of this, a HOCl-adduct of Fe -protoporphyrin IX was identified as a direct enzyme-halogen intermediate involved in chloroperoxidase-catalyzed halogenaticHi [1773]. 

Synthesis of Epoxides from Haiohydrins (Section 11.8C) Formation of the halohydrin and the following intramolecular Sf 2 reaction are both stereoselective (the configuration of the alkene is retained in the epoxide) and stereospecific (for alkenes that show ds, trans isomerism, the configuration of the epoxide depends on the configuration of the alkene). 

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