Answers to the Exercises "SN/E Competition"
In this case, LHMDS is the only one of the given bases that leads to a chemoselective E2 elimination. As it is a good nucleophile, the ethoxide anion attacks the primary epoxide in an reaction with the subsequent elimination of water, yielding an allyl ether. With DBU, no substitution occurs, because DBU is a very bulky, sterically-demanding and non-nucleophilic base. In addition, with DBU, an E2 elimination does virtually not proceed, in this case, as DBU is not basic enough to generate a sufficiently large driving force for this reaction. By applying LHMDS, an extremely bulky, sterically-demanding and strong base, a substitution is prevented. On the other hand, sufficient driving force is generated in order to make the E2 elimination at an epoxide with the intermediate high-energy alkoxide anion possible.