Science of Synthesis

Rück-Braun, K.; Freysoldt, T., Science of Synthesis, (2007) 35, 515.

The most common phosphorus-containing reagent used for the ring opening of epoxides ‌53‌ in the synthesis of bromo alcohols ‌54‌ and ‌55‌ is bromotriphenylphosphonium bromide (‌Scheme 21‌).[‌56‌–‌60‌] Similarly to hydrogen bromide, it can be used as an atom-efficient reagent with mild reaction conditions. It is also able to deliver a precise amount of hydrogen bromide, and is thereby chemoselective. Furthermore, it is an inexpensive and easily handled reagent simply prepared from triphenylphosphine and bromine. Bromotriphenylphosphonium bromide is crystalline and readily soluble in chloroform and dichloromethane. In agreement with a suggested borderline SN2 mechanism, alkenes can be observed in some cases as byproducts. This demands a careful choice of the reaction conditions. The reagent can be used in the ring-opening reaction of cyclic as well as acyclic epoxides and has been used in the synthesis of complex[‌56‌,‌60‌] and steroidal bromo alcohols.[‌58‌,‌60‌] N-Bromosuccinimide and 2,4,4,6-tetrabromocyclohexa-2,5-dien-1-one are also useful bromide sources.[‌59‌] These reagents are solid and therefore easier to handle than the toxic bromine used in the preparation of the phosphonium reagent, and provide similar yields; however, the reaction times are notably prolonged.

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References