Nucleophilic Carbon

NUCLEOPHILIC CARBON

Carbon-centered nucleophiles are those compounds or intermediates which contain an electron-rich carbon atom and thus are capable of donating an electron pair from that carbon atom to an electrophile. The electron pair that is donated is found in a filled orbital in the nucleophilic carbon and the electrons are not tightly bound. Donation to the electrophilic carbon occurs by overlap of the filled orbital of the donor with an unfilled orbital of the acceptor. The most common carbon nucleophiles fall into three main classes:

(a) Organometallic molecules contain a carbon–metal bond which is polarized toward carbon. As a result the carbon is very electron rich and reacts vigorously with electrophiles.

Examples of organometallic compounds most commonly used as carbon nucle-ophiles include Grignard reagents (RMgX), organolithiums (RLi), organocuprates (R₂CuLi), and occasionally organocadmiums (R₂Cd) and organozincs (RZnBr).

(b) Enolates are anionic derivatives of carbonyl compounds formed by proton removal from a position adjacent to the carbonyl group. Resonance delocalization of the negative charge with the carbonyl group stabilizes enolate anions and makes them somewhat less reactive than organometallic compounds.

They are, however, reactive carbon nucleophiles. Examples of enolates include anionic derivatives of aldehydes, ketones, acid derivatives, and dicarbonyl compounds.

Structurally related to enolates are anionic derivatives of imines and nitro compounds. The former are less stable (more reactive) than enolates because nitrogen cannot support a negative charge as well as oxygen and thus resonance stabilization is diminished compared to enolate anions. Nitronate anions are much more stable (less reactive) than enolates because resonance with the nitro group transfers the negative charge to oxygen where it is stabilized by the formal positive charge on nitrogen.

(c) A third type of molecule capable of functioning as a nucleophilic carbon equivalent is one that contains an electron-rich π bond. Such species are usually uncharged and function as carbon nucleophiles because the π electrons are less tightly bound and can consequently be donated to good electron acceptors. Enol derivatives are neutral derivatives of carbonyl compounds which have an oxy-gen or nitrogen substituent attached to a carbon–carbon double bond. They are covalent analogs of enolate ions, and because they neutral, they are much less reactive electron donors than enolate anions. In fact, most enol derivatives are stable compounds which can be isolated. In spite of being neutral, however, reso-nance interaction of the lone pairs of the heteroatom with the π system increases the electron density at the β position of the double bond and the double bond is electron rich. Besides the parent enol, examples of enol derivatives of carbonyl compounds include those shown below.

Besides a heteroatom substituent, which renders a double bond electron rich by resonance interaction of the lone pairs with the π system, other substituents can also result in π bonds being electron rich and thus reacting as electron donors. Attachment of substituents less electronegative than carbon to the double bond increases its π -electron density significantly by an inductive effect. Vinylsilanes and vinyl stannanes can both be considered to have electron-rich π bonds and have been used as π -electron donors. Allylsilanes, by virtue of hyperconjugation between the allylic carbon–silicon bond and the π system, also function as good π-electron donors in many reactions.