Carbanions allylic heteroatom-stabilized

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 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. 

Comparison of rates of Brook rearrangement of jS-substituted a-silyl allyl alcohols as a means of estimating the a-carbanion stabilizing ability of heteroatom substituents suggests that PhS is much more stabilizing than MesSi. ... 

The nucleophilicity of sulfur and its ability to stabilize a-carbanions provide sulfur compounds with unique opportunities for sigmatropic processes consecutive rearrangements are no exception. The formation of salt (140) via Sn2 alkylation of ( )-2-butenyl bromide (139) followed by deprotonation leads to the intermediate allyl vinyl ether (141) which, under the conditions of the deprotonation, undergoes a thio-Claisen rearrangement to afford thioamide (143 Scheme 10). Thermolysis of (143) at elevated temperature affords the Cope product (142) in addition to some of its deconjugated isomer. Several unique characteristics of the thio-Claisen sequence should be noted first, the heteroatom-allyl bond is made in the alkylation step, this connection teing not notrtudly practised in the parent Claisen reaction ... 

A large number of Pd-catalyzed reactions of allylic substrates involve the attack of a nucleophile to the 7 -allyl moiety in a Pd complex. C-nucleophiles such as malonates and other stabilized carbanions form the body of the so-called allylic alkylation reactions. Heteroatom-containing nucleophiles are also commonly used. They have been applied in selective allylic trasformations leading to functionalized compounds as well as in total synthesis. 

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