Bromination reactions, hydroxyl group conversion

The non-aqueous oxidation of various ketones and aldehydes to hydroxyaldehydes and hydroxy ketones has been carried out in two steps the first step is a bromination and the second a hydrolysis, or replacement of the bromine atoms by hydroxyl groups. Fischer and Landsteiner " brominated acetal in the presence of calcium carbonate (reaction 20). The bromoaldehyde Avas then treated with cold barium hydroxide solution (reaction 21) the resulting glycolic aldehyde was identified by conversion to glyoxal phenylosazone (reaction 22) and by the formation of calcium glycolate, after oxidation with bromine (reaction 23). 

We Studied the reaction of alkenes with chlorine or bromine in water to form halohydrins in Section 6.3E and saw that it is both regioselective and stereoselective (for an alkene that shows cis-trans isomerism, it is also stereospecific). Conversion of a halohydrin to an epoxide with base is stereoselective as well and can be viewed as an internal Sj. 2 reaction. Hydroxide ion or another base abstracts a proton from the halohydrin hydroxyl group to form an alkoxide ion, a good nucleophile, which then displaces halogen on the adjacent carbon. As with all S 2 reactions, attack of the nucleophile is from the backside of the C—X bond and causes inversion of configuration at the site of substitution. 

Nitro-compounds are also prepared by the replacement of benzylic hydroxyl by 2-nitropropyl groups," the replacement of activated bromine with silver nitrate in the presence of triphenylphosphine," and the Michael addition of nitroethane to a,/3-ethylenesulphoximines under phase-transfer conditions." The preparation and reactions of cyclic a-nitroketones have been reviewed." The conversion of primary and secondary nitroalkanes into the corresponding carbonyl compounds, i.e. the Nef reaction, is a useful synthetic transformation. Three new methods" " to effect this important reaction offer a variety of oxidative conditions, and thus enable the minimization of side reactions (Scheme 36). 

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