Carbanions, ambident

Carbon is alkylated ia the form of enolates or as carbanions. The enolates are ambident ia activity and can react at an oxygen or a carbon. For example, refluxing equimolar amounts of dimethyl sulfate and ethyl acetoacetate with potassium carbonate gives a 36% yield of the 0-methylation product, ie, ethyl 3-methoxy-2-butenoate, and 30% of the C-methylation product, ie, ethyl 2-methyl-3-oxobutanoate (26). Generally, only one alkyl group of the sulfate reacts with beta-diketones, beta-ketoesters, or malonates (27). Factors affecting the 0 C alkylation ratio have been extensively studied (28). Reaction ia the presence of soHd Al O results mosdy ia C-alkylation of ethyl acetoacetate (29). 

For carbanionic addition, the relative negative charge and the electron densities in the 1- and 3-position in the HOMO of the ambident allylic anion determine, in addition to steric effects, the regioselectivity of the hydroxyalkylation. According to the allopolarization principle13 the following generalizations can be made ... 

The reaction of the phenylsulphinyl allylic lithium a-carbanion 342 with oxiranes was found by Guittet and Julia to give, after rearrangement and desulphurization, dihydroxy-dienes 343427 (equation 197). Demoute and coworkers have described the alkylation reaction of a very sophisticated 2-alkenyl sulphoxide 344 as a part of the total synthesis of a juvenile hormone 345428 (equation 198). Since the allylic sulphoxide carbanion has an ambident character, the alkylation may occur sometimes also at the y-position. This direction of alkylation is observed in the case of acyclic allylic sulphoxide anions 346, and results in the formation of the corresponding allylic sulphoxide 347 and vinylic sulphoxide 348423 (equation 199). 

The proposed intramolecular solvation is not the only feature differentiating between the polystyryl and polymethyl methacrylate salts. The former are classified as the salts of carbanions, whereas the latter are ambident salts having the character of allylic enolates with the cation interacting with the partially negatively charged carbon and oxygen atoms. The degree to which the one or the other interaction is favored is affected by the size of the cation. 

Chlorodiphenylphosphine 488 reacts with a-sulphinyl carbanions to give a-sulphinylphosphines 489 which undergo ready isomerization to a-sulphenylphosphine oxides 4W (equation 295). The report of Almog and Weissman that a-sulphinyl carbanions react with phosphorochloridates 491 to give a-phosphoryl sulphoxides 492 calls for correction (equation 296). Actually, the phosphorylation occurs at the oxygen atom of the ambident dimsyl anion, and is followed by the Pummerer-type reaction affording diethylphosphoric acid and tetraethyl pyrophosphate among other products . ... 

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... 

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

PhO " is an ambident anion with negative charge on both O and the ortho C atoms of the ring. Attack by O gives the ether attack by the ortho carbanion gives o-allylphenol. 

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. 

The reaction of a-sulfenylated allylic carbanions with electrophiles may give both the a- and y-prod-ucts (equation 7). Regiochemical control of this ambident anion is dependent upon many factors, including substituents, counterions, the solvent system, the type of electrophile and steric effects. 

The 1,4-addition reaction of stabilized allylic carbanions with conjugated enones is normally complicated by the ambident reactivity of the carbanion. Clean 7-selective conjugate addition to cyclic enones, however, has been observed with lithiated allylic sulfoxides, phosphine oxides and phosphonates... 

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). ... 

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