Carbanions 1,2-chlorohydrins

Florio s group published an interesting series of reports detailing the application of 2-(chloromethyl)oxazolines in organic synthesis.Deprotonation of 2-(chloromethyl)-4,4-dimethyloxazoline 392 yields a carbanion 393 that undergoes a wide variety of synthetically useful reactions (Scheme 8.128). Thus, quenching 393 with a carbonyl compound at —100 °C gave chlorohydrins 394 that can be... 

Generation of the sulfinyl carbanion of chloromethylphenyl sulfoxide (Bu"Li/THF, -78 °C), followed by treatment with cyclohexanone, acetone or benzophenone, affords the corresponding chlorohydrin (27) in good yield (68-79%). Treatment of the cyclohexanone or acetone adducts with methanolic KOH provides the a, 3-epoxy sulfoxides in yields of greater than 90% (Scheme 7). Each of the halohydrin products appeared to be a single diastereomer, although the relative stereochemistry was not assigned. 

Similarly, carbanions of a-halonitriles generated at the interface with concentrated aqueous NaOH react with benzaldehyde to produce the corresponding chlorohydrine anions, which, being in the interfacial region, undergo rapid intramolecular substitution giving cyano-oxiranes [107]. This reaction is discussed in more detail on pp. 193-194. 

Cyclization of the intermediate isomeric chlorohydrine anions is slower than reversible addition of the carbanions to the carbonyl group hence, there is an equilibrium between diastereomeric (erythro and threo) chlorohydrine anions. Thus, the ratio of cis and trans stereoisomers is governed by the energies of the isomeric transition states of the cyclization reaction. In the catalytic process, which occurs in the isotropic homogeneous phase, the transition state I is energetically preferred due to smaller steric interaction, leading to the trans isomer. At the interface, additional stabilization of the isomeric transition state II results in preferential formation of the cis isomer ... 

An intriguing report on the alkylation of alkali-metal salts of acetylacetone and methyl acetoacetate with chiral 2-bromobutane has been published the C-alkylated product is found to retain the configuration of the alkyl group, whereas the expected inversion is shown by the O-alkylated product. The reaction of carbanions of jS-ketoesters and /S-diketones with 1,4-dichloro-butan-2-one leads to epoxyannelation and annelated chlorohydrins as a route to epoxyketones, it avoids the possible blocking-deblocking sequence in the normal basic peroxidation of enones (Scheme 86). 

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