Carbanions allyl anion

For carbanionic addition, the relative negative charge and the electron densities in the 1- and 3-position in the HOMO of the ambident allylic anion determine, in addition to steric effects, the regioselectivity of the hydroxyalkylation. According to the allopolarization principle13 the following generalizations can be made ... 

Extensive theoretical studies have been carried out to probe the nature of the allyl anion. These studies supplement and extend the experimental results. Allyl anion is of special interest because it is the simplest 7r-delocalized carbanion with 4 electrons and 3 Pjr-centers. Much recent theoretical discussion has concerned the role of resonance in the stabilization of such conjugated systems, a stabilization defined as the enthalpy difference between the localized double-bonded system and its conjugated state. The stabilization of allyl anion has generally been attributed to the delocalization of charge associated... 

Even the allyl anion can be seen as an example of resonance-enhanced coordination. As shown in Section 4.9.2, r -CsHs- complexation is accompanied by a shift toward the localized H2C —CH=CH2 resonance structure that places maximum anionic character at the metal-coordinated carbon atom. In effect, the carbanionic lone pair nc is shared between intramolecular nc 7icc (allylic resonance) and intermolecular nc—>-n M (metal coordination) delocalizations, and the former can be diminished to promote the latter, if greater overall stabilization of the metal-ligand complex is achieved thereby. 

The synthesis of 4-alkyl thioketones is possible by exploiting the stabilizing effect of a sulfur atom upon an adjacent carbanionic center. Ambident allylic anions react so that conjugate addition proceeds exclusively with the a-carbon of the nucleophile,129 243 244 as illustrated in equation (S3) 245 arylsulfinyl and arylsulfonyl groups normally246 behave similarly.247-249 Sulfur-stabilized vinylic carbanions can be prepared and function as Michael donors in difunctionalization sequences.250... 

The electronic spectra of carbanionic polymers have been exploited for many years as a means of determining the concentration of the absorbing species. More recently, careful study has shown that other information may be obtained — notably concerning the conformation of allylic anions. Even more powerful in this respect has been the application of NMR techniques, greatly helped by the development of instrumentation capable of examining nuclei (such as 13C) at the natural abundance level. 

The protonation leads specifically to the trans-decalin system, though reduction could apparently give rise to two stereoisomeric products. The guiding principle appears to be that protonation of the intermediate allylic anion 12 takes place axially, orthogonal to the plane of the double bond, and to the most stable conformation of the carbanion which allows the best sp3-orbital overlap on the /Tcarbon with the -orbital system of the double bond. 

If the donor is a sulfur atom, its lone pair is practically nonbonding (the electronegativities of sulfur and carbon are similar), above 2 and scheme c is no longer valid. If the sulfur is modeled by a carbanion, the sulfur-substituted diene and the dienophile are represented by the pentadienyl and the allyl anions, respectively. Their HOMOs will both be nonbonding. Hence FO theory predicts that placing the sulfur on the dienophile and the attractor on the diene may be slightly more favorable than the opposite substitution pattern. 

In isomerization reactions, an alkene is deprotonated to form an allyl anion, which is reprotonated to give the more stable alkene (double-bond migration). The most simple example is the isomerization of 1-butene producing a mixture of cis- and trans-2-butene (Scheme 3). Because the stability of the cis-allyl anion formed as an intermediate is greater than for the trans form, a high cis/trans ratio is observed for base-catalyzed reactions whereas for acid-catalyzed reactions the ratio is close to unity. Thus, the cis/trans ratio of the products has frequently been used as an indication of base-catalyzed reaction mechanisms. The carbanions formed in the course of such superbase reactions are not freely mobile in solution,... 

The coefficients in the HOMO can be estimated using the simple arguments developed earlier [see (Section 2.1.2.3) page 64], as in Fig. 4.3. The HOMO of an X-substituted allyl anion will have some of the character of ijj2 of the allyl anion, which is symmetrical, but it will also have some of the character of a carbanion conjugated to an allyl anion, in other words i/ 3 of butadiene, which has the larger coefficient on C-4, corresponding to the y position. 

Pentadienyl carbanions are analogous to allyl anions with an extended delocalization of charge. Reaction of 1,3- or 1,4-pentadienes and alkali metals in THF in the presence of a base, such as NMes or TMEDA, affords crystalline pentadienylalkali metal complexes. A contact ion pair structure is predicted for these compounds by theoretical calculations and is consistent with solution structural data obtained by NMR. The pentadienyl anion usually interacts with the cation as an rj - or ) -ligand depending on the structural orientation of the backbone carbon atoms of the pentadienyl anion (W-, S-, or U-shaped skeletal structures). A contact ion pair structure having a W-shaped pentadienyl ligand is shown (16). 2,4-Disubstituted... 

There are many examples of BF3-OEt2 promoted openings of (1) by carbanions, including sulfone-stabilized anions, vinylic anions, allylic anions, and phosphonate-stabilized anions. For example, the lithium anion of trans-1,2-Bis(tributylstannyl)ethylene opens (5)-(l) in the presence of BF3-OEt2 in THF at —78°C, affording fra/w-l-(tributylstannyl)-5-tosyl-4-hydroxypent-l-ene in 50% yield the latter is converted into oxirane (3) in 76% yield on treatment with powdered Sodium Hydroxide in monoglyme. ... 

A high degree of asymmetric induction has been realized in the carbanion-accelerated Claisen rearrangement of phosphorus-stabilized anions. Treatment of 1,3,2-oxazaphosphorinane (166) with freshly prepared lithium dimsylate led to a 95 5 ratio of a-methyl ketones (167) and (168) (Scheme 33). Li coordination combined with steric interactions provide the necessary control elements for stabilization of the highly organized allyl anion conformation (169). 

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. 

The rotational barriers of allylpotassium 23c (16.7 kcal/mol) and allylcesium 23d (18.0 kcal/mol) are much higher than those of 23 a and 23b. This observation and the question ofwhether aggregates are involved in the rotational process, clearly emphasize once more the important influence of the gegenion in carbanion reactions. The experimental results with 23a-d thus lead only to a lower limit (18.0 kcal/mol) for the rotational barrier of the allyl anion in solution. Incidently, this value comes close to the lowest calculated (MP2/4-31 + G//4-31 + G) value of the allyl anion rotational barrier (22.2 kcal/mol)33). 

In previous chapters the influence of the gegenion and the solvent on rearrangements of carbanions mostly concerned the rate of a certain reaction (and thus kinetic aspects), as, e.g., in the case of the cyclopropyl-allyl anion rearrangement (Sect. 2.1), the ally anion isomerization (Sect. 2.2), the cyclization of 5-hexenyl alkali metal compounds (Sect. 4) or the configurational isomerization of a-substituted vinyl-lithium compounds (Sect. 5). 

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