Allyl anions ambident nucleophilicity

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... 

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... 

There are other paths that fit the medium, sources and sinks protonation of an anion, p.t., and addition to a polarized multiple bond by the oxygen of the enolate by AdgS or AdN2. There are two sites that can serve as a base or as a nucleophile on this ambident allylic source. We have three choices to evaluate proton transfer to oxygen,... 

A common problem in the manufacture of fine chemicals is that a substrate may have more than one reactive site. A well known, but intractable, problem is the nitration of monosubstituted benzenes, which always leads to a mixture of isomers. Reactions which involve nucleophilic substitution are more amenable to control of regioselectivity by choice of solvent. An excellent review on the reactivity of ambident anions is available, in which this subject is treated [17]. An instructive example is the alkylation of phenol with allyl chloride [18] (equation 12.10). Table 12.13 shows how the properties of O- and C-alkylation are affected by solvent. 

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