Carbanions ambident nucleophilicity

Nucleophilic substitution, aliphatic, 31, 45, 77-100 Ag catalysis, 97 allyl halides, 85 ambident nucleophiles, 97 benzyl halides, 84, 91 bridgehead halides, 86 bromomethane, 78 2-bromopropanoate, 94 1-bromotriptycene, 87 carbanions in, 100,288... 

Ambident nucleophiles are considered to be bidentate molecules whose nucleophilic centres have direct chemical interaction with each other, such as a ketone with its enol-carbanion tautomers. 

The lithium salt of diethyl 2-propenylphosphonate behaves as an ambident nucleophile in reactions with a variety of electrophilic reagents. In reaction with methyl methacrylate, the intermediate formed in the lirsl step (conjugate addition at the y-carbon of the phosphonate) has a carbanionic center developed at C-5. The second step involves intermolecular conjugate addition to a second molecule of methyl methacrylate, yielding an intermediate capable of the final 1,6-cyclization to produce a phosphonylated polyfunctionalized derivative of cyclohexane in good overall yield (60%, Scheme 8.63). ... 

Since the carbanion-enolates are ambident ions with two different nucleophilic sites, they can be alkylated at C or at O. 

The synthesis of 4-alkyl thioketones is possible by exploiting the stabilizing effect of a sulfur atom upon an adjacent carbanionic center. Ambident allylic anions react so that conjugate addition proceeds exclusively with the a-carbon of the nucleophile,129 243 244 as illustrated in equation (S3) 245 arylsulfinyl and arylsulfonyl groups normally246 behave similarly.247-249 Sulfur-stabilized vinylic carbanions can be prepared and function as Michael donors in difunctionalization sequences.250... 

Enolates and aza-enolates are so-called ambident or ambifimctional nucleophiles. This term describes nucleophiles with two nucleophilic centers that are in conjugation with each other. In principle, enolates and aza-enolates can react with electrophiles either at the heteroatom or at the carbanionic C atom. Ambidoselectivity occurs if one of these alternative modes of reaction dominates. 

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