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35.2.1.5.3 Method 3: Substitution of Alcohols with Isomerization

DOI: 10.1055/sos-SD-035-00308

Braun, M.Science of Synthesis, (200735337.

The conversion of alcohols into bromoalkanes is frequently faced with the problem of rearrangements that may be an anticipated and planned step in a synthesis. Nevertheless, rearrangements are in most cases considered as undesired side reactions, particularly if they lead to mixtures of isomeric products. The tendency of rearrangements to take place clearly depends on the structure of the substrate on the one hand and on the particular method that is chosen for the replacement of the hydroxy group by bromine on the other. As a rule, highly branched primary and secondary alcohols have a strong tendency for alkyl group migration (see HoubenWeyl, Vol.5/4, pp361374). The treatment of 2,2-dimethylpropan-1-ol (neopentyl alcohol) with gaseous hydrogen bromide in a sealed tube over several weeks leads to the formation of the rearranged product, 2-bromo-2-methylbutane (20)[‌87‌,‌88‌] (Scheme 22). Even under milder phase-transfer conditions, the formation of the rearranged product cannot be completely suppressed.[‌89‌] In a similar way, products arising from rearrangement or hydride shift, or mixtures of products, are obtained if hydrobromic acid is allowed to react with the alcohols shown in Scheme 22. The tendency of various protocols to bring about rearrangement during the substitution of 2-bromopentane has been studied.[‌90‌] It turns out that the formation of 3-bromopentane is widely suppressed with phosphorus tribromide at 0°C. Pure nonrearranged 2-bromopentane is obtained with the combination of triphenylphosphinebromine or by using the 4-toluenesulfonate method (see Section 35.2.1.5.6.1).[‌90‌]

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