Crotyl double bond geometry

One of the major advantages of 2-alkenylpotassium derivatives over the appropriate lithium and magnesium analogs is the possibility of controlling the double-bond configuration in the allylie anion. For instance, the geometry of ( )-2-butene is retained in the e.vu-2-butenyl anion below — 50 0C, but equilibration above - 25 C leads to the endo-crotyl anion with >97% selectivity12. 

---