Allyl anions electrophiles

Oompound 1, 2-(hydroxymethyl )a11y1trimethylsilane, represents a conjunctive reagent which can be considered as the equivalent of zwitterion 2, possessing a nucleophilic allyl anion synthon and an electrophilic allyl... 

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

The above examples represent Jl-heteroaromatic annulation involving the reaction of allyl anions whose double bond is a part of the heterocyclic ring system (Scheme 1). The corresponding a-oxoketene dithioacetals (1,3-electrophilic component) were generally derived from nonheterocyclic carbonyl precursors. Alternatively the Jl-heteroaromatic annulation can also be employed to a-oxoketene dithioacetals derived from heterocyclic ketones (1,3-bielectrophile) and hetero/nonheteroallyl anions (1,3-binucleophile). These reactions are described below. 

Recent advances include alkyl iodides as substrates that can be activated by metal complexation. Also Jt-allyl "anions", when co-ordinated to palladium, are activated toward attack by nucleophiles. This is very similar to the activation of co-ordinated alkenes and it shows the very high electrophilicity of palladium. The valence state of palladium, and/or the charge on palladium, and therefore also the ligands attached to it are very important ... 

Notice that the final result is a 1,3 charge affinity inversion Umpolung) of an allylic derivative via a FGI of a functional group of type E by a group of type A, followed by a [2,3]-sigmatropic rearrangement. If the intermediate allyl anion reacts with a carbonyl compound as the electrophile the result is then a 1,4-D system, such as ... 

Whereas an allylsilane can serve as an allyl anion synthon, the reaction of 1,3-bis(silyl)propene with electrophiles can afford the 1,3-disubstituted propene. Thus, treatment of a mixture of (If)- and (Z)-2-aryl-l,3-bis(trimethylsilyl)propenes 5 with 2 equivalents of NBS at —78 °C stereoselectively yields the corresponding (Z)-2-aryl-l,3-dibromopropene 6. When 1 equivalent of NBS is employed, the monobromo product 7 is obtained (equation 5). The reactions apparently proceed via the pattern of sequential displacement of allylsilane moieties40,41. 

Should an allylic anion be formed as a result of an initial ionisation, attack of an electrophile on the anion could take place either at the (original) a carbon atom or at the (original) y carbon atom, the latter leading to substitution with rearrangement. An illustration for the case of a crotyl compound follows, viz. 

Deprotonation of BSMA imines followed by trapping with an electrophile of the intermediate anion has been described (see Section IV.C.4). However, when these imines derive from conjugated carbonyl compounds, attack of the base takes place at the end of the conjugated system to give delocalized bis(allyl) anion that could be alkylated or silylated in the (3-position from the nitrogen.227... 

The functionalized silyl enol ethers 156 are useful synthetic intermediates since electrophiles can now be introduced either directly in the P-position by known methodology 55) or in the opposition after deprotonation with LDA to an allyl anion (Eq. 70)61>. Both pathways should enormously widen the scope of specifically substituted y-oxoesters and their derivatives obtained via siloxycyclopropanes. 

Where is the electron density in the allyl anion it system The answer is slightly more complicated than that for the allyl cation because now we have two full molecular orbitals and the electron density comes from a sum of both orbitals. This means there is electron density on all three carbon atoms. However, the HOMO for the anion is now the nonbonding molecular orbital. It is this orbital that contains the electrons highest in energy and so most reactive. In this orbital there is no electron density on the middle carbon it is all on the end carbons. Hence it will be the end carbons that will react with electrophiles. This is conveniently represented by curly arrows. 

X-Substituted Allyl Anions. Allyl anions with alkyl substituents almost always react with carbonyl electrophiles at the more substituted a position, as in the reaction of the prenyl Grignard reagent with aldehydes to give the product 4.39, presumably because the metal attaches itself preferentially to the less-substituted end of the allyl system and then delivers the electrophile in a six-membered transition structure 4.38. In contrast, alkylation of a similar anion with an alkyl halide gives mainly the product 4.40 of y attack, which is normal for an X-substituted allyl anion when a cyclic transition structure is not involved. 

Z-Substituted Allyl Anions—Dienolate Ions. Z-Substituted allyl anions 4.45a are typified by dienolate ions 4.45b, which is how they are best drawn. They almost always react faster at the a carbon than at the y carbon, both with soft and hard electrophiles. 

In organic syntheses allylsilanes and allylstannanes have been used extensively as allyl anion equivalents during the last two decades [187-190]. The regioselective attack of electrophiles, which finally yields products with allylic inversion (Scheme 43), has been explained by the hyperconju-gative stabilization of carbenium centers by the carbon-silicon or carbon-tin bond in the j3-position [191-196], which has initially been derived from solvolytic experiments [197-199]. 

Electrophilic selenenylation and seleninylation (RSe+) of allyl anion equivalents (allyl silanes, stannanes). 

Treatment of the anion generated from the diene cyclobutadiene-colbalt complex 239 and MeLi with racemic oxaziridine 33 gave hydroxyl products 240 and 241 in 44% yield <2004JOC2516>. These products were obtained as a 1 1 mixture of diastereomers arising from exo-facc addition of the oxygen electrophile to the Jt-allyl anion. 

There are two types of reactions of allylsilanes with electrophiles (1) Lewis acid-mediated reactions, and (2) reactions involving allyl anions. 

In accordance with this model one finds diastereoselectively anti products on reaction of aldehydes with ( )-allyl compounds, whereas allyl systems with the (Z)-configuration give mainly syn products and it is even possible to effect asymmetric induction. As the double bond of the product can be oxidatively cleaved to a CW3 group, the reaction can be regarded as a stereoselective aldol reaction, an aspect which explains the widespread interest in this type of reaction. With heterosubstituted allylic anions it is sometimes possible to effect predominantly y-attack with different electrophiles by the choice of the heteroatom.2 For instance it is well known that with sulfur substituents like —SR, —SOR or —SOjR the a-attack dominates, but doubly lithiated allenethiol possesses high y-reactivity and can be used as a homoenolate anion equivalent in reaction with electrophiles such as alkyl halides (Scheme 7). ... 

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 purpose of doing so was twofold in the first place, it did show that we get the same answer by considering the frontier orbitals as we do from the product-development argument, and secondly it showed how the allyl anion and allyl cation are nucleophilic and electrophilic respectively at both C-l and C-3 without our having to draw canonical structures. 

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