Carbanions delocalized

Delocalized carbanions are soft and can undergo conjugate additions readily with unsaturated carbonyl compounds in water. For nonstabi-lized carbanions, the change of counter-cation can significantly affect... 

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... 

The independence of CH- carbon acidities from the solvent can be attributed to the fact that only highly delocalized carbanions have been considered. When aggregation or specific solvent ion-pairing effects are operative, deviations from this behaviour are expected. 

A carbon labelling study has elucidated the rearrangement mechanism for formation of chalcone (97) which accompanies formation of (91) by the expected vicinyl elimination of trimethylsilyl and benzotriazolyl groups from 2-benzotriazolyl-2-aryl-3-ketopropylsilanes, on reaction with fluoride ion in DMF. ° Thus, it has been possible to distinguish between the two alternative mechanisms depicted in Scheme 11 (via intermediates (93) or (95), respectively, by determining the fate of the labelled quaternary carbon of substrate (89). The results are consistent with the formation of a cyclopropane intermediate (95) which subsequently ring opens, with relief of strain, to form delocalized carbanion (96), from which the chalcone (97) is obtained (labelled... 

The jT-type interaction to a lithium cation was proposed in the 1960s for delocalized carbanions. The basis for this was the observation of high-held Li chemical shifts ° As mentioned earlier, (Sections LB.2.a and II.B.2), the chemical shift is strongly affected by the ring current of delocalized carbanions. 

The distances shown in the schematic drawing 20 display little bond alternation and support the notion of a delocalized carbanion. 

Olefins are susceptible to isomerization. The API ivermectin undergoes isomerization under basic conditions due to the weak acidity of the allylic proton (which is also alpha to a carbonyl) on C2. The two resulting degradants can be derived from the delocalized carbanion formed from dissociation of the proton at C2. Reprotonation at C2 generates the epimeric product and reprotonation at C4 generates the structural isomer (Fig. 103) (147). 

There is no perfectly linear correlation between the basicity and nucleophilicity of carbanions [66], but higher basicity usually also implies higher nucleophilicity. Carbanions in which the negative charge is highly delocalized (e.g. diethylmalonate) will usually react more slowly with electrophiles than less extensively delocalized carbanions of similar basicity (e.g. malodinitrile) [66],... 

Reactions of the Lochmann base (a mixture of butyllithium with potassium re/T.-butoxide in pentane [195]), with conjugated polyenes yield fully delocalized carbanions in high yield at a high rate [196]... 

They can be used to initiate the polymerization of monomers on several centres simultaneously, i. e. for the preparation of star polymers. Delocalized carbanions of a different type are formed by the deprotonation of acetylene copolymers [198]... 

Mono- and dianions can, of course, be used for the initiation of linear chain propagation. Gordon and Loftus observed interesting differences in styrene polymerization when initiated with BuLi and with delocalized carbanions. In the latter case, the anions have various counter-ions (Li+ and K + ) at each of these, propagation proceeds at a different rate. When the initiator contains a greater number of delocalized carbanions, insolubility of the product and non-uniformity of charge on the anions cause some problems [199],... 

Buter and Kellogg proceeded on the assumption that cesium salts of carboxylic acids due to their big ionic radii and the high polarizability of the cesium cation are present in DMF as weakly solvatized tight ion pairs [51]. This assumption was based on measurements, which showed that cesium salts with soft anions as e.g. delocalized carbanions in THF exist as contact ion pairs... 

The condensation reactions occur in two steps first the growing fatty acyl chain is transferred from the phosphopantetheine group of an AGP onto a cysteine of the KS. Subsequently, the KS binds and decarboxylates a malonyl-AGP, thereby generating a resonance-delocalized carbanion. The formation and stability of this nucleophilic species could be promoted by electrostatic or... 

In the presence of potassium cyanide the N-methyl-C-phenylnitrone (112) takes part in yet a further cycloaddition reaction to give l-methyl-4,5-diphenylimidazole. With lithium metal the aldimine (113) forms a delocalized carbanion which will cyclize with aryl cyanides (Scheme 61) (80AHC(27)24l). See also Section 4.08.1.1.3. 

Zoltewicz and Helmick (665) found that, whereas pyrazine reacts with potassium amide to form the anionic o complex (20), methylpyrazine undergoes deprotonation to give the delocalized carbanion (21) and the p.m.r. spectrum shows that pyrazine with phenyllithium at —45° is converted completely into a dihydro adduct analogous to (20) (720a). 

In another paper Abraham provided a thermodynamic analysis that placed f-Bu Cl and the separated ions at about the same free energy, 14.5 kcal/mol above the reactant. This implies a AG of about 5 kcal/mol for collapse of the contact ion pair back to reactants, although the uncertainties in Abraham s analysis are at least 5 kcal/mol. Few related data are available, except some dynamic NMR results for the collapse of trityl and tropylium chloride ion pairs. " In both the NMR work and Abraham s analysis, solvent-separated ion pairs are not included as distinct entities. However, the interconversion of contact and solvent-separated ion pairs has been observed in ultraviolet studies of delocalized carbanions in THF. ... 

Conjugate addition of a nucleophile to an activated olefin is generally referred to as a Michael addition reaction (1). Of particular interest is the addition of delocalized carbanions to unsaturated acceptors, a process resulting in the construction of a carbon-carbon bond, often stereoselectively (2). However, the addition of metal enolates to unsaturated acceptors is not completely general and two major modifications have been developed wherein covalently bound enolate equivalents are added to unsaturated acceptors. 

Metal enolates used in Michael reactions are generated from mono- or bifunctional compounds. For the sake of simplicity, bifunctional reagents will be classified under the heading corresponding to the functionality that bears the chiral auxiliary. The reactions of delocalized carbanions will also be considered in this chapter. As already discussed for aldol reactions, one or two new stereocenters can be created in the reactions of these reagents [161],... 

Dicarboration of conjugated dienes can occur in either a 1,2- or a 1,4-addition mode. Several examples of stepwise stereoselective c -l,4-dicarborations of open-chain and cyclic dienes, via palladium-allyl complexes, which use stoichiometric amounts of palladium salts and soft, delocalized carbanions have been reported65 67. Catalytic dicarboration of 1,3-butadiene, stereoselectively leading to ( )-alkenes with up to 70% yield, is achieved using aryl iodides and delocalized carbanions of the malonate type in the presence of palladium(II) chloride and l,2-bis(diphenylphosphino)ethane68. A Tt-allyl-palladium intermediate is thought to determine the E geometry of the product. 

Butadiene polymerization studies with HMTT/CH2Li catalysts have given results which are directly contrary to those expected from the Hay mechanism. Activity at 0.5 HMTT/CH2Li was double that at 1 1 ratio whereas the reverse should have been obtained if the tetramine solvated lithium compound were the active species. Our lithium catalyst studies suggest that all of the known TMED complexes are active for butadiene polymerization with activity increasing roughly in the order (RLi)4 TMED < (RLi)2 TMED < RLi TMED << RLi(TMED)2. The equilibrium to form RLi(TMED )2 is believed to be unfavorable except when R" is a highly delocalized carbanion. 

The chemical reactivities of the alkali metal organometallic compounds (RM) vary widely depending on metal M, basicity of the solvent systems used, and steric and electronic properties of the organic group R. In many reactions an important factor is the stabilization resulting from formation of a delocalized carbanion system as in the polymerization of dienes or aromatic substituted ethylenes, and in Reactions 3, 4, 5, and 10 in Table I. It is primarily with these delocalized carbanion systems that this review is concerned although saturated organolithium compounds are discussed briefly. 

There have been numerous attempts to elucidate both the nature of the ion-pair interactions and the delocalized carbanion geometries of chelated alkali metal organic systems in solution (18). There is now apparent agreement that this equilibrium ... 

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