Eliminations Reactions Not Involving C-H Bonds

The discussion of elimination processes thus far has focused on reactions that involve removal of a proton bound to a 6-carbon, but it is the electrons in the C-H a bond that are essential to the elimination process. Compounds bearing other substituents that can release electrons undergo p-eliminations. Many such reactions are known, and they are frequently stereospecific. 

Vicinal dibromides can be debrominated by certain reducing agents, including iodide ion. The stereochemical course in the case of 1,1,2-tribromocyclohexane was determined using a Br-labeled sample prepared by anti addition of Br2 to bromo-cyclohexene. Exclusive anti elimination gave unlabeled bromocyclohexene, whereas Br-labeled product resulted from syn elimination. Debromination with sodium iodide was found to be cleanly an anti elimination.  

The rate-determining expulsion of bromide ion through a bridged intermediate requires an anti orientation of the two bromides. The nucleophilic attack of iodide at one bromide enhances its nucleophilicity and permits formation of the bridged ion. The stereochemical preference in noncyclic systems is also anti, as indicated by the fact that meso-stilbene dibromide yields frans-stilbene, whereas /,/-stilbene dibromide gives mainly ds-stilbene under these conditions.  

One of the pieces of evidence supporting this mechanism is the fact that the A// for deoxymercuration of trans-2-methoxycyclohexylmercuric iodide is about 8 kcal/mol less than for the cis isomer. Only the trans isomer can undergo elimination by an anti process through a chair conformation. 

There are a number of synthetically valuable 6-elimination processes involving organosilicon and organotin compounds. Treatment of 6-hydroxyalkylsilanes or 6-hydroxyalkylstannanes with acid results in stereospecific anti eliminations that are much more rapid than for compounds lacking the group IV substituent. 

---