Carbanions and related synthetic intermediates

In this section, we ll take a break from our survey of reaction mechanisms and focus instead on a class of intermediates, namely, carbanions. We will also discuss carbanion cognates such as enols, enolates, enamines, and ylides. As classic nucleophiles, carbanions react in highly characteristic ways, particularly via 8 2 displacements, as well as via other pathways (e.g., carbonyl addition and conjugate addition) we have discussed above. The material in this section will thus help you flesh out your understanding of what we have discussed so far. 

The best known among unstabilized carbanionic derivatives are the Grignard reagents (organomagnesium compounds) and organolithiums  

Alkylsodiums and alkylpotassiums are considerably more reactive and are less commonly used. Of particular importance are stabilized carbanions derived from deprotonation of C-H bonds adjacent to carbonyl groups such anions are often referred to as enolates because the negative charge is localized more on the carbonyl oxygen than on the carbanionic carbon  

A strong base such as an alkali metal amide (p T 35-40) is typically required to quantitatively convert a simple aldehyde or ketone to an enolate. On the other hand, an alkoxidebase (p/fg 16) is sufficient for deprotonating a )5-diketone or a -ketoester, since the resulting enolate is much more stabilized by resonance  

Stabilized carbanions may also be derived from nitriles  

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