Carbanions lead-stabilized

The reaction of aldehydes or ketones with stabilized phosphorus ylides (phosphonate carbanions) leads to olefins with excellent E-selectivity. 

Stabilization of the ylide-like carbanion leads to an E-configuration of the product double bond. 

On the other hand spectroscopic study of the stability of the polystyryl carbanion leading to the propagation of the polymerization of styrene initiated by a C.B. confirmed the living polymer hypothesis. Horeover the rate of disappearence of the carbanion in our conditions, compared to literature data, led us to the conclusion that alKoxide complexes the carbanion which promotes polymerization. 

The Claisen condensation is initiated by deprotonation of an ester molecule by sodium ethanolate to give a carbanion that is stabilized, mostly by resonance, as an enolate. This carbanion makes a nucleophilic attack at the partially positively charged carbon atom of the e.ster group, leading to the formation of a C-C bond and the elimination ofan ethanolate ion, This Claisen condensation only proceeds in strongly basic conditions with a pH of about 14. 

The haloform reaction of unsymmetrical perfluoroalkyl and co-hydroper-fluoroalkyl trifluororaethyl ketones gives the alkane corresponding to the longer alkyl chain [54] (equation 53) If the methyl group contains chlorine, the reaction can take different pathways, leading to loss of chlorine (equation 54), because of the variable stability of the chlorine-substituted methyl carbanions in alkali. 

A cyanide anion as a nucleophile adds to an aldehyde molecule 1, leading to the anionic species 3. The acidity of the aldehydic proton is increased by the adjacent cyano group therefore the tautomeric carbanion species 4 can be formed and then add to another aldehyde molecule. In subsequent steps the product molecule becomes stabilized through loss of the cyanide ion, thus yielding the benzoin 2 ... 

The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

In addition reactions to chiral carbonyl compounds, the stereochemical course taken by resonance-stabilized alkali metals or magnesium benzyl anions resembles that taken by localized carbanions of similar bulk. Thus, conditions can be delineated which lead to either the steric approach or chelation control the following serve as examples. 

A novel chiral dissymmetric chelating Hgand, the non-stabiUzed phosphonium ylide of (R)-BINAP 44, allowed in presence of [Rh(cod)Cl]2 the synthesis of a new type of eight-membered metallacycle, the stable rhodium(I) complex 45, interesting for its potential catalytic properties (Scheme 19) [81]. In contrast to the reactions of stabihzed ylides with cyclooctadienyl palladium or platinum complexes (see Scheme 20), the cyclooctadiene is not attacked by the carbanionic center. Notice that the reactions of ester-stabilized phosphonium ylides of BINAP with rhodium(I) (and also with palladium(II)) complexes lead to the formation of the corresponding chelated compounds but this time with an equilibrium be-... 

The reaction of the stabilized yUde 46 (a-vinyl substituted) with the cycloocta-dienyl Pd(II) allows the synthesis of a novel complex, the (rj -allyl)palladium 47, in which the olefmic double bond participates in the coordination (Scheme 20) [83]. The coordination of the bis-yUde 48 with the same starting Cl2Pd(COD) leads to the formation of another new palladium complex 49 via COD exchange reactions. A C-coordination mode takes place between the carbanionic centers of the bis-ylide and the soft palladium and two stereogenic centers of the same configuration are thus created [83]. In contrast to the example described in Scheme 19, the Cl2M(COD) (M=Pd or Pt), in presence of a slightly different car-... 

It is intriguing to note that this reaction scheme for the reduction of a sulphone to a sulphide leads to the same reaction stoichiometry as proposed originally by Bordwell in 1951. Which of the three reaction pathways predominates will depend on the relative activation barriers for each process in any given molecule. All are known. Process (1) is preferred in somewhat strained cyclic sulphones (equations 22 and 24), process (2) occurs in the strained naphtho[l, 8-hc]thiete 1,1-dioxide, 2, cleavage of which leads to a reasonably stabilized aryl carbanion (equation 29) and process (3) occurs in unstrained sulphones, as outlined in equations (26) to (28). Examples of other nucleophiles attacking strained sulphones are in fact known. For instance, the very strained sulphone, 2, is cleaved by hydride from LAH, by methyllithium in ether at 20°, by sodium hydroxide in refluxing aqueous dioxane, and by lithium anilide in ether/THF at room temperature. In each case, the product resulted from a nucleophilic attack at the sulphonyl sulphur atom. Other examples of this process include the attack of hydroxide ion on highly strained thiirene S, S-dioxides , and an attack on norbornadienyl sulphone by methyllithium in ice-cold THF . ... 

The preferential -configuration of the enol esters, derived from p-dicarbonyl compounds under phase-transfer conditions, contrasts with the formation of the Z-enol esters when the reaction is carried out by classical procedures using alkali metal alkoxides. In the latter case, the U form of the intermediate enolate anion is stabilized by chelation with the alkali metal cation, thereby promoting the exclusive formation of the Z-enol ester (9) (Scheme 3.5), whereas the formation of the ion-pair with the quaternary ammonium cation allows the carbanion to adopt the thermodynamically more stable sickle or W forms, (7) and (8), which lead to the E-enol esters (10) [54],... 

An extensive review appeared on the configurational stability of enantiomeric organolithium reagents and the transfer of the steric information in their reactions. From the point of view of the present chapter an important factor that can be evaluated is the ease by which an inversion of configuration takes place at the metallation site. It happens that H, Li, C and P NMR spectra of diastereotopic species have been central to our understanding of the epimerization mechanism depicted in equation 26, where C and epi-C represent the solvated complex of one chiral species and its epimer, respectively. It has been postulated that inversion of configuration at the Li attachment site takes place when a solvent-separated ion pair is formed. This leads to planarization of the carbanion, its rotation and recombination to form the C—Li bond, as shown in equation 27, where Li+-L is the solvated lithium cation. An alternative route for epimerization is a series of... 

Deprotonation is the most convenient manner for the generation of alkyllithium derivatives. However, most of the potential precursors have insufficient acidity. Notable exceptions are those substrates which lead to dipole-stabilized carbanions , among them O-alkyl iV,A-dialkylcarbamates 15 ° and A-Boc-(cyclo)alkylamines 18 ° (equation 5). A... 

Carbanions of the type [HjCR ], [HCR R ] and [CR R R ] (R = NO and R R = CN, NO, NO2) can be considered to be resonance-stabilized, nonlinear pseudohalides. All experimentally known resonance-stabilized methanides are reported to be planar or nearly planar (Table 1). While the parent ion, the methanide anion HsC, adopts a pyramidal structure [Afipianar-pyramidai = 9.8 kJmol rf(CH) = 1.099 A, <(HCH) = 109.7° cf. rf(CH) = 1.093 A, <(HCH) = 109.6°] due to the lack of delocalization (no resonance for the p-AO-type lone pair possible) , substitution of one H atom by NO results in a planar anion since the empty jr -orbitals of the NO group are perfectly suitable to delocalize the carbon lone pair. Further substitution of the second H atom again results in planar anions, and the same holds for the third substitution in case of R = CN. In case of R = NO and NO2, the third substitution leads either to a propeller-type structure with only a small distortion from planarity or one NO2 group is twisted by 90°, nevertheless leaving the central carbon in an almost trigonal planar environment . ... 

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