More Alkenes as Nucleophiles

We've continued to look at alkenes as nucleophiles but we have made the electrophiles more complicated than simple HX. Our examples of electrophiles include X2, Hg(OAc)2 and BH3. In those cases, we use available electrons to swing back down to the developing carbocation and eliminate the positive charge on the carbon. In the case of BH3, this is relatively straight forward and takes us to a product in which we have added H-BH2 across a C=C. In the other two cases we make a three-membered ring with a positive charge and we must open the ring by reaction with a nucleophile. Note that the nucleophile must attack the three-membered ring from exactly the opposite side of the leaving group (Br or Hg), so there can be stereochemical implications.