Science of Synthesis

Rück-Braun, K.; Freysoldt, T., Science of Synthesis, (2007) 35, 514.

Tetrabutylammonium bromide[‌52‌–‌54‌] as well as unsubstituted ammonium bromide[‌52‌,‌55‌] in the presence of a Lewis acid can be used to open epoxides ‌50‌ to yield bromo alcohols ‌51‌ and ‌52‌ (‌Scheme 20‌). Suitable Lewis acids include titanium(IV) isopropoxide[‌55‌] and ammonium cerium(IV) nitrate[‌52‌] as well as the less expensive and easier to handle magnesium nitrate.[‌54‌] Iron(III) chloride supported on silica gel is inexpensive and easy to prepare,[‌53‌] offering the possibility for solid-phase catalysis with the advantage of an easy separation procedure. A wide range of primary bromo alcohols are accessible via ammonium bromide mediated ring opening of epoxides, with most reactions leading regioselectively to β-bromo alcohols ‌51‌ in good to excellent yields. In contrast, the opening of 2-phenyloxirane (‌50‌, R1 = Ph; R2 = H) with ammonium cerium(IV) nitrate or iron(III) chloride produces the corresponding α-bromo alcohol ‌52‌ (R1 = Ph; R2 = H) regioselectively,[‌52‌,‌53‌] while the use of magnesium nitrate leads to a mixture of the regioisomers [(‌51‌/‌52‌) 17:83].[‌54‌] All reactions proceed under very mild conditions. The reaction with magnesium nitrate is especially tolerant to a wide range of functional groups including ethers, esters, cyano groups, and even cyclic ethers such as oxetane, all of which remain unaffected under the reaction conditions.[‌54‌]

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