Dibromocarbene addition to strained alkenes

The addition of dibromocarbene to prochiral olefins, when conducted in the presence of a chiral ]3-hydroxyethyltrialkylammonium salt leads to products which possess small enantiomeric excesses. Thus styrene and j3-methylstyrene undergo chiral dibro-mocyclopropanation in 73% and 60% yields respectively, but their optical rotations (at the sodium D-line) are less than one degree [5]. 

Addition of dibromocarbene to strained carbon-carbon double bonds can lead to rearranged products. These products result from ionization of the initial 1,1-dibromo-cyclopropane adduct with ring opening to yield an allylic carbonium ion/bromide anion pair. Alkyl group shifts followed by ion pair collapse can then lead to rearranged products (see Sect. 2.6). Three examples of this phenomenon are illustrated as equations 4.2—4.4 [9, 12, 13]. For related examples, see equations 2.17—2.21. 

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