Bases, ambident strong

Treatment of an ethylidene malonic ester such as (a) with strong bases results in loss of a proton from the allylic position to produce the ambident ion fb). Alkylation of such carban-ions usually occurs at the carbon bearing the carbonyl groups, resulting in the establishment of a quaternary center and deconjugation of the double bond fc). 

The common method involves deprotonation of a thiocarbonyl compound and reaction of the intermediate enethiolate with an allyl halide (Scheme 9.8). This actually relies on two noticeable features of the sulfur series. (1) The proton located a to a thiocarbonyl group is much more acidic, by 7-10 pKa units, than the one of a carbonyl moiety [39, 41]. This may be related to the strong ability of the sulfur atom (polarizability) to stabilize the negative charge of the enethiolate. Presently, the preferred conditions involve LDA as a base for optimum deprotonation [42-45]. (2) The resulting anionic species are soft ambident nucleophiles. The... 

In the presence of strong bases, NH-imidazoles are alkylated via the imidazolyl anion, for example, Na-imidazolide (prepared from imidazole and NaOH) undergoes 1-alkylation (->8, R = alkyl) with halogenoalkanes and dialkyl sulfates [378]. Because of the ambident character of unsymmetrical imidazolyl anions (e.g., 9), base-induced alkylation of 4- or 5-substituted imidazoles affords mixtures of 1,4- and 1,5-disubstituted products, for example ... 

The base-catalysed rearrangements of cz-halo ketones are classical examples of the reactions of ambident enolate anions in solution. The extent of each of the two reactions shown in Equations [11] and [12] is principally a function of the type of solvent used. A protic solvent solvates more strongly at the oxygen centre of the ambident anion and thus reaction proceeds through the carbanion centre to yield the Favorskii species as the major product (Eqn [11]). In marked contrast, the Favorskii rearrangement does not occur in the gas phase. Here,... 

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