Displacements involving unsaturated carbon

The general course of aliphatic displacement at unsaturated carbon involves multistage mechanisms, which consist of a nucleophilic addition to a tt system followed by an elimination step (reaction 11). 

Nucleophilic displacement reactions are often competitive with other processes promoted by a nucleophile, such as addition-elimination, or proton abstraction and base-induced elimination in which the nucleophile acts as a strong base. This particular situation is especially true in reactions that also involve attack at an unsaturated carbon centre. The delicate interplay between these different mechanisms is in itself a matter of great interest, and as yet it has defied attempts to rationalize it on a quantitative basis. 

In unsaturated aliphatic systems the most important reactions are those of czirbonyl compounds of the type —COX, in which X is a good leaving group such as halogen. In general, most displacement reactions of anionic nucleophiles on the carbonyl carbon atom of acyl halides involve an addition-elimination mechanism " (e.g. equation 16). In such reactions bond-formation is in advance of bond-rupture... 

Selenoxides are even more reactive than amine oxides. In fact, many types decompose spontaneously on generation at room temperature. Synthetic procedures based on selenoxide eliminations usually involve synthesis of the corresponding selenide followed by oxidation and elimination. We have already discussed examples of these procedures in Chapter 4 where the synthesis of a,/ -unsaturated carbonyl compounds (Section 4.7) was considered. In addition to the electrophilic addition of selenenyl halides and related compounds to alkenes and enolate selenenation, which was discussed in Section 4.5, selenides can be readily formed by nucleophilic displacement on halides, tosylates, or epoxides.Selenium is also capable of stabilizing an adjacent carbanion so that a-selenenylcarbanions are accessible carbon nucleophiles. One versatile procedure involves conversion of ketone to fe/5-selenoketals which are then cleaved by butyllithium. These carbanions... 

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