Preparation of optically active 2-bromo-3-methylbutane from 3-methylbutan-2-ol without rearrangement

Abstract

Bromo-3-rnethylbutane, hitherto an inaccessible compound, has been prepared in the pure state. A number of other attempted methods of preparation are described, each of which involves rearrangement, possibly by hydride shift. Stratford, London E.15THE conversion of a secondary alcohol into the corresponding halide is usually accompanied by some rearrangement, the extent of which depends on the structure of the alcohol. Secondary alcohols are most prone to rearrangement if chain branching occurs at the carbon atom next to that which carries the hydroxygroup (e.g. 3-methylbutan-2-01) .l* Accumulated data 2 9 3 indicate that the interaction of hydrogen halides (HBr, HCI, or HI), thionyl chloride,2f or phosphorus trihalides (PCl,2f and PBr, 2e) with 3methylbutan-2-01 produces 2-halogeno-2-methylbutanes. In some instances the presence of 2-halogeno-3-methylbutane was merely suspected but it was not actually isolated.2 The only method previously found to yield an appreciable amount of 2-bromo-3-methylbutane was the peroxide-catalysed addition of hydrogen bromide to Z-meth~lbut-2-ene.~ Products were previously characterised by refractive index and in terms of stability towards hydrolysis 2' 3 (except ref. 2e). The availability of g.1.c. makes possible a much more effective examination of the reactions of 3-methylbutan-2-01 today.Pure secondary halides have been prepared from the alcohol via the toluene-P-sulphonate 4 3 5 or the ophenylene pho~phite,~ with displacement of the intermediate group by halide ion.6 Methods using halogenophosphorus compounds such as Ph,P (CN)Br, ' Ph,PBr2,8 (PhOj,PBr,,8 and (Ph, have also been shown to convert alcohols into the corresponding bromides in almost quantitative yield and without any rearrangement.The results of reactions between 3-methylbutan-2-01 and hydrogen bromide, phosphorus tribromide, and thionyl bromide are summarised in the Table . At room temperature and below, the alcohol and hydrogen bromide give solely the tertiary bromide, but at and above 100" a small amount of the secondary bromide (2-bromo-3-methylbutane) is also obtained (Scheme 1).

2,4,4,6-Tetrabromocyclohexa-2,5-dienone in the presence of triphenylphosphine as a regiospecific and stereoselective reagent for the nucleophilic substitution of bromine for hydroxyl

“…5 It should be emphasised that in the case studied, the yield of the olefin is only 13% and no rearrangement products are formed. (Literature data for the PPh 3 •CBr 4 complex for an analogous structure, 3-methylbutan-2-ol: 7 the reaction yield is 30%, 8% of which is an isomerised product). Even the reaction with neopentyl alcohol 4 occurs without isomerisation of the carbon skeleton under very mild conditions (0 °C) in a quantitative yield and with high isomeric purity (99%) (for the PPh 3 •CBr 4 complex, the yield of neopentyl bromide is 42% 8 ).…”