Oxidation of halohydrins

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... 

Electrochemical Process. Applying an electrical current to a brine solution containing propylene results in oxidation of propylene to propylene oxide. The chemistry is essentially the same as for the halohydrin process. AH of the chemistry takes place in one reactor. Most of the reported work uses sodium or potassium bromide as the electrolyte. Bromine, generated from bromide ions at the anode, reacts with propylene and water to form propylene bromohydrin. Hydroxide generated at the cathode then reacts with the bromohydrin to yield propylene oxide (217—219). The net reaction involves transfer of two electrons ... 

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... 

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 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 instances69, 75, 88)115 it has been possible to isolate the intermediate halohydrin. Oxidation of this intermediate followed by solvolysis leads to the corresponding hydroxy ketones or in the presence of base to cyclopropyl ketones.5,57... 

Cleavage of epoxides. As with simpler haloboranes, these reagents cleave epoxides of cycloalkenes to give, after nonoxidative workup, halohydrins in 65-90% yield. When carried out at -78 to -100°, the cleavage can show high en-antioselectivity. Thus the halodiisopinocampheylboranes derived from (+ )-pinene react with the oxide of cyclohexene or of cyclopentene to furnish (1R,2R) halo-... 

Only products which can be attributed to bromohydrin (salt) intermediates are formed when epoxides are treated with LiBr (solubilizer) in refluxing benzene. Any apparent exceptions to this generalization are believed to be due to interconversion of halohydrin stereoisomers, as discussed below. The bromohydrin is formed by traditional anti opening (antiperiplanar, probably nearly exclusively trans diaxial). It has been established that a model epoxide (1-methylcyclohexene oxide) reacts by a process that is kinetically first order in a 1 1 complex of LiBr-HMPA, but the state of aggregation [n in the formula (LiBr HMPA)n] is not known.The nature of the epoxide-(LiBr-HMPA)n interaction that allows bromide to attack (exclusively) in the anti mode is not known, but must require fairly extensive ion pair (aggregate) reorganization. 

The epoxides were prepared from the corresponding olefins either by oxidation with various peracids such as peracetic acid or m-chloroper-benzoic acid or in a two-step procedure via the halohydrin (SCHEME IV). The direct epoxidation procedure is preferred due to the generally low yields of halohydrins obtained in the second method. The halohydrins are active herbicides and will be discussed in a future publication (9). 

The major biotransformation pathway involves an oxidative step with introdnction of an oxygen atom and subsequent formation of halohydrin. The unstable halohy-drin loses hydrobromic acid to yield trifluoroacetyl chloride, which in turn is hydrolyzed to trifluoroacetic acid. This final metabolite is fonnd in the nrine. °... 

Reactions of Oxirans with Organometallic Compounds. - A general method for the preparation of halohydrin esters via the reaction of organic halides with CO (20 atm) in the presence of PhPd(PPh3)2l has been described.In the particular case of benzyl chloride and cyclohexene oxide, (228) (57.5%) was obtained. A possible mechanism for this reaction is shown in Scheme 14. 

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