Allylic anions heteroatom-stabilized

Heteroatom-Stabilized Allylic Anions i, LiNEij MeCOsH... 

Heteroatom-stabilized Carbanions. Heteroatom-stabilized and allylic carbanions serve as homoenolate anions and acyl anion equivalents, e.g. a-anions of protected cyanohydrins of aldehydes and Q ,/3-unsaturated aldehydes are intermediates in general syntheses of ketones and Q ,/3-unsaturated ketones (eq 36). Allylic anions of cyanohydrin ethers may be a-alkylated (eq 37) or, if warmed to —25°C, may undergo 1,3-silyl migration to cyanoenolates which may be trapped with TMSCl. Metalated Q -aminonitriles of aldehydes are used for the synthesis of ketones and enamines (eq 38). Similarly, allylic anions from 2-morpholino-3-alkenenitriles undergo predominantly a-C-alkyl-ation to give, after hydrolysis, a,/3-unsaturated ketones (eq 39). ... 

The addition of metalated sulfones to aldehydes is reversible and in simple cases the reaction displays modest selectivity for the erythro isomer (eq 6). The reverse reaction is favored when the adducts are sterically compressed (e.g. ketone adducts) or when the sulfone anion is stabilized by conjugation (i.e. allylic or ben-zylic sulfones) or proximate heteroatoms. However, in unfavorable cases the position of the equilibrium can be tuned by varying the metal. For example, the lithio sulfone (2) did not give a stable adduct with aldehyde (4) but the ate complex derived from the lithio derivative and BF3 gave the desired adduct (5) (eq 1) ... 

The regioselectivity of the reactions of heteroatom-stabilized allylic anions with electrophiles has been thoroughly reviewed. An unsymmetrically substituted allyl anion is an ambident anion, which can react with electrophiles at two sites. The discussion of site selectivity is a major objective of this article. 

The regioselectivity of the reactions of heteroatom-stabilized allyl anions with electrophiles and carbanion stabilization by adjacent sulfur (wifh particular reference to intrinsic rate constants for proton transfer) have been reviewed. 

Addition of f-BuLi to other carbon-carbon double bonds is not a facile process unless (1) the jr-system is conjugated (in which case anionic polymerization may result), (2) the alkene is at least somewhat strained (eq 27), (3) the resulting anion is stabilized as in vinylsilanes, vinylphosphines, and the like (eq 28), (4) a good leaving group is present at the allylic position, or (5) there is intramolecular assistance provided by a suitably placed heteroatom (eq 29). ... 

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