Elcb Eliminations in protecting group chemistry

It was indicated already in Section 4.1.5 that Het /HeE-eliminations offer possibilities for regio- and stereocontrol in the synthesis of alkenes that are quite different from those of H/Het-eliminations. It was also indicated that the value of these reactions for alkene synthesis depends on, among other things, how laborious it is to obtain the Het -and Het2-contain-ing substrates. Accordingly, in the next sections we will discuss such eliminations and the preparation of the elimination precursors as well. 

A /1-elimination of this type—often also referred to as fragmentation—takes place in the initiation of a Grignard reaction using ethylene bromide  

The purpose of this operation is not to release ethylene but to etch the Mg shavings. 

A second example of this type of elimination is the generation of dichloroketene from trichloroacetyl chloride and zinc  

You should remember that fi-heterosubstituted saturated organometallic compounds hardly ever exist. On the contrary, they generally fragment through M /Het -elimination to an alkene  

---