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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 SN2 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.

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