Allyl anion halides

In the last decade, a new aspect of nickel-catalyzed reactions has been disclosed, where nickel serves to selectively activate dienes as either an al-lyl anion species or a homoallyl anion species (Scheme 1). These anionic species are very important reactive intermediates for the construction of desired molecules. Traditionally they have been prepared in a stoichiometric manner from the corresponding halides and typical metals, e.g., Li, Mg. In this context, the catalytic generation method of allyl anions and homoallyl anions disclosed here might greatly contribute to synthetic organic chemistry and organotransition metal chemistry. 

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. 

The carbon atom next to a carbon-carbon double bond. The term is used in naming compounds, such as an allylic halide, or in referring to reactive intermediates, such as an allylic cation, an allylic radical, or an allylic anion, (p. 673)... 

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. 

Unimolecular solvolyses of allylic secondary halides and similar derivatives are often favoured by the capacity of the double bond to stabilise the intermediate carbonium ion as the mesomeric "allylic cation 4). This extra stability of the intermediate is reflected in the energy of the transition state for its formation whenever the path of departure of the anion... 

According to Bard and Merz [108], in MeCN containing TBAP, allyl bromide and allyl iodide interact chemically with a mercury electrode to form allylmercury halides. These allylmercury halides undergo reduction to yield diallylmercury, which is itself electroactive. Allyl bromide and allyl iodide are reduced at platinum in MeCN in a two-electron process to give the allyl anion, and the allyl radical is not involved as an intermediate. Reduction of allyl halides at platinum in DMF containing TEAOTs and in the presence of trimethylchlorosilane results in silylated compounds [100]. 

Based on these results and results for reduction of other combinations of allyl halides and activated alkenes, it has been suggested that when the allyl halide is more easily reduced than the alkene, the allylic anion (2-F reduction) adds to the activated double bond of the alkene, giving predominantly the terminal alkene [Eq. (32)]. In contrast, initial formation of the radical anion of the (di)activated alkene may lead to an S>j2 reaction between the radical anion and the allyl halide followed by further reduction of the intermediate radical and final protonation [Eq. (33)] [190,191]. However, electron transfer between the alkene radical anion and especially allyl iodide followed by coupling of the allyl radical and a radical anion cannot be ruled out. 

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). ... 

The reactivity of alkenes toward dichlorocarbene also decreases if there is a halogen in an allylic position. In this case side reactions, such as alkylation of trichloromethyl anion, halide exchange in the case of allylic bromides etc., take place. Although the yields of the products from these side reactions are low, they impede the isolation of the desired product, e.g. formation of 3, 4, and... 

Reactions alkylations, reactions with epoxides and aldehydes, conjugate additions Heterocyclic synthesis with allyl silanes Reactions with Co-stabilised cations An Allyl Dianion The Role of Tin in Anion Formation Halide Exchange with Chelation Indium Allyls Allyl Anions by Deprotonation The synthesis ofall-trans dienes The synthesis ofall-trans retinol... 

The most useful of all allyl anion equivalents are the allyl silanes.20 This is because it is easy to make them regioselectively, because they do not undergo allylic rearrangement (silicon does not do a [1,3] shift) and because their reactions with electrophiles are very well controlled addition always occurring at the opposite end to the silicon atom. Symmetrical allyl silanes can be made from allyl-lithiums or Grignards by displacement of chloride from silicon. A useful variant is to mix the halide with a metal, e.g. sodium, and Me3SiCl in the same reaction, rather after the style of the silicon acyloin reaction,21 as in the synthesis of the acetal 80. 

From this point on, the regioselectivity of substituted allyl anions is much less regular, and somewhat less explicable. For a start, X-substituted allyl anions react with carbonyl electrophiles with a selectivity. This is explicable, but it is determined by the site of coordination by the metal, not by the frontier orbitals. We can contrast the reaction of the oxygen-substituted lithium anion 4.57 with an alkyl halide, which is y selective, as usual, and the reaction of the zinc anion 4.58 with a ketone, which is a selective.304 The oxygen substituent coordinates to the zinc cr-bound at the y position, and the aldehyde is then delivered to the a position in a six-membered cyclic transition structure 4.59. The same reaction with the lithium reagent 4.57 gives a 50 50 mixture of a and y products, and so lithium is not so obviously coordinated in the way that the zinc is. This type of reaction is often brought under control in the sense 4.59 for synthetic purposes by... 

Allylic sulfonyl carbanions react with electrophiles such as alkyl halides and aldehydes at the a-position. Although relatively strong bases like Bu"Li and LDA are usually used for deprotonation of allylic sulfur compounds, including sulfones, a catalytic two-phase system that consists of a concentrated aqueous NaOH solution and a quaternary ammonium salt can be used to generate allylic sulfonyl carbanions. 1,1-Dilithiated allyl phenyl sulfone (equation 20) reacts with excess benzaldehyde to afford the 1,3-( )-diadduct, while l,ort/io-dilithiated allyl phenyl sulfone gives the l,ort/io-diadduct predominantly. Other examples of sulfur-substituted allylic anions are summarized in Table 1. 

When deprotonation is effected in a 2-allyl-1,3-oxathiane with 5cc-butyllithium, the expected allylic anion is generated. Interestingly, reaction with alkyl halides leads to substitution at the a-terminus (52), whereas reaction with carbonyl compounds gives products of substitution at the y-terminus (53) (Scheme 18) <92TL250l>. In contrast, when the anion derived from 2-allyl-1,3-dithiane... 

Various lanthanide halides (ScCR, YCI3, LaCl3,Ndl3) react with K2[3-(C3H3SiMe3-l)2SiMe2] to form a/ 5a-bis(allyl) complexes. The lanthanum derivative has been structurally authenticated as a coordination polymer (14), in which allyl anions interleave alternating lanthanum and potassium ions the latter display K- -MeSi contacts of 2.79(3) and 2.96(4) kd... 

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