Allyl anions heteroatom substituted

Cathodic substitution stands for C,C bond or C, heteroatom bond formation with cathodically generated anions. The question of regioselectivity is encountered in the reaction of such anions with allyl halides (path a) or in the reaction of allyl anions generated in an ECE process from allyl halides (path b). Cathodic reductive silylation of an allyl halide proceeds regioselectively at the less substituted position (Fig. 15) [91]. From the reduction potentials of the halides it is proposed that the reaction follows path b. 

Heteroatom-substituted allylic anions as homoenolate anion equivalents 862... 

On the other hand the inherent a-selectivity of allylsulfur carbanions can sometimes be transformed to a y-reactivity by a sigmatropic rearrangement, earning in addition the stereoselectivity typical for such reactions (see Section 4.S.2.3). Due to their significance in synthesis a lot of work has been done on reactions of heteroatom-substituted allyl anions with special emphasis on their use as homoenolate anion equivalents. The more recent developments, with the possibility of introducing diastereoselectivity, will be discussed later in Section 4.S.3.2.I. 

There are two main synthetic applications where the reaction of an allyl system with electrophiles is accompanied by an allylic rearrangement. One consists of the use of heteroatom-substituted allylic anions as homoenolate anion equivalents and the other represents a synthetically valuable alternative to the aldol reaction by addition of allyl metal compounds to aldehydes. 

Heteroatom-substituted allylic anions can serve as homoenolate anion equivalents in reaction with electrophiles, when y-attack can be realized and the formed vinyl heterocompound can be hydrolyzed to an aldehyde (Scheme 76). ... 

The addition of heteroatom-substituted allylic anions to aldehydes has found particular interest as it represents a homoaldoi reaction. Metalated allyl carbamates seem to be the most valuable reagents developed for this purpose as they are prepared easily and exhibit high y-selectivity and dia-stereoselectivity, at best in their titanated form (Scheme 79). ... 

A more general solution to the problem is to use an allyl anion substituted on both ends with a heteroatom 136. The anion 137 may then react regiospecifically with electrophiles to give products 138 with a masked carbonyl group (vinyl Y in 138) and a leaving group (X in 138) so that hydrolysis gives the enone 139. 

The Offensive Strategy Heteroatom-Substituted Allyl Anions... 

Heteroatom-substituted allylic anions as homoenolate anion equivalents 4.5.322 Addition of ally Imetal compounds to aldehydes synthesis of homoallylic alcohols... 

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. 

For the deprotonation of less acidic precursors, which do not lead to mesomerically stabilized anions, butyllithium/TMEDA in THF or diethyl ether, or the more reactive, but more expensive,. seobutyllithium under these conditions usually are the most promising bases. Het-eroatomic substitution on the allylic substrate, which docs not contribute to the mesomeric or inductive stabilization often facilitates lithiation dramatically 58. In lithiations, in contrast to most other metalations, the kinetic acidity, caused by complexing heteroatom substituents, may override the thermodynamic acidity, which is estimated from the stabilization of the competing anions. These directed lithiations59 should be performed in the least polar solvent possible, e.g.. diethyl ether, toluene, or even hexane. 

Another type of substitution reaction is increasing in popularity—the use of an allylic substrate, such as an allyl acetate where the nucleophile is introduced with stereochemical control in the presence of a palladium catalyst and a chiral ligand. Reactions where a chiral anion, be it derived from a chiral heteroatom group, such as a sulfoxide, or an auxiliary, such as Evans s oxazolidinones, are not included in this chapter because the alkyl halide is usually relatively simple and the stereochemical selectivity is derived from the system itself. 

Z Heteroatom lone pairs as nucleophiles RS- r CN- RO- Softer anions are more nucleophilic. If same atom, more basic, more Nu Substitutions Additions. Nucleophile vs. base decision See allylic sources... 

Catalytic reactions of allylic electrophiles with carbon or heteroatom nucleophiles to form the products of formal S 2 or S 2 substitutions (Equation 20.1) are called "catalytic allylic substitution reactions." Tliese reactions have become classic processes catalyzed by transition metal complexes and are often conducted in an asymmetric fashion. The aUylic electrophile is typically an allylic chloride, acetate, carbonate, or other t)q e of ester derived from an allylic alcohol. The nucleophile is most commonly a so-called soft nucleophile, such as the anion of a p-dicarbonyl compound, or it is a heteroatom nucleophile, such as an amine or the anion of an imide. The reactions with carbon nucleophiles are often called allylic alkylations. 

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