The Carbanion Chemistry

Some important reactions of silyl anions are as follows and they have emerged on the basis of the carbanion chemistry. 

Non-enzymic Enolisation. The carbanion chemistry of aldoses and ketoses themselves is masked by the ring opening step and mechanistic work is limited. However, a detailed examination has been made of the enolisation of L-glyceraldehyde 3-phosphate, which cannot form monomeric rings. The use of the unnatural (l) enantiomer enabled any racemisation to the natural... 

The Carbanion Chemistry of Tertiary a-Chiral Primary Amine Synthesis.140... 

THE CARBANION CHEMISTRY OF TERTIARY a-CHIRAL PRIMARY AMINE SYNTHESIS... 

The reaction of toluene with 1 could conceivably lead to four different trans-isomeric products by attack at the ortho, meta, para, or alpha positions. The carbanion chemistry of toluene would predict attack at the methyl group but, as in other transition metal arene activation studies (8,9,10), this is not found. The NMR spectrum of the toluene adducts displays two singlets attributable to trans-tolyl adducts in an intensity ratio of 1 1.6 in addition to resonances of cis-tolyl adducts. Selective mono-deuteration experiments show that the weaker downfield resonance is attributable to the para isomer while the stronger one is attributable to the meta isomer no ortho isomer is observed. If one corrects for statistical effects, the preference for para, meta, and ortho isomers is 1 0.8 0. We attribute this distribution of isomers primarily to... 

The cation pool method is very powerful tool for the mechanic study of the cationic reaction because the cation intermediate can be detected by the spectroscopies. In contrast to the carbanion chemistry, it is generally difficult to get insight into the mechanism of carbocation chemistry. Further application of the cation pool method to explore the mechanistic investigation is also expected. 

For the carbanion chemistry just described, the sulfonyl group is definitely an important control center as far as regioselectivity and stereoselectivity are concerned, but one should also not underrate the leaving group properties of this moiety. 

D. II. O Brien in The Nuclear Magnetic Resonance of Carbanions m Comprehensive Carbanion Chemistry. Part A, E. Buncel, T. Durst, Eds., Vol. 5 A, Chapter 6, pp 271-322, Elsevier. Amsterdam 1980. 

See the discussion in Cram, D.J. Fundamentals of Carbanion Chemistry Academic Press NY, 1965, p. 85. 

Considering the long saga of hydrocarbon chemistry, it is surprising that two new classes of hydrocarbon - ionically dissociative hydrocarbons and hydrocarbon salts - have been discovered in the last decade. The syntheses of authentic samples as analytically pure solids have revealed the very existence of such novel hydrocarbons in an unquestionable way, but the investigation of their basic features is just in the inchoate stage. The search for such novel hydrocarbons depends primarily on the synthesis and examination of highly stabilized hydrocarbon cations and anions. As mentioned above, until now such elaboration has been concentrated on the carbocation side, and examination of the carbanion moiety has only just started. 

In this chapter, decarboxylation of disubstituted malonic acid derivatives and application of the transketolases in organic syntheses are summarized. Although decarboxylation may be seen as a simple C-C bond breaking reaction, it can be regarded as a carbaniongenerating reaction. As the future directions of this field, expansion of some unique decarboxylation reactions is proposed. In relation of carbanion chemistry, promiscuity of enolase superfamily is discussed. 

The decarboxylation reaction usually proceeds from the dissociated form of a carboxyl group. As a result, the primary reaction intermediate is more or less a carbanion-like species. In one case, the carbanion is stabilized by the adjacent carbonyl group to form an enolate intermediate as seen in the case of decarboxylation of malonic acid and tropic acid derivatives. In the other case, the anion is stabilized by the aid of the thiazolium ring of TPP. This is the case of transketolases. The formation of carbanion equivalents is essentially important in the synthetic chemistry no matter what methods one takes, i.e., enzymatic or ordinary chemical. They undergo C—C bond-forming reactions with carbonyl compounds as well as a number of reactions with electrophiles, such as protonation, Michael-type addition, substitution with pyrophosphate and halides and so on. In this context,... 

A major advantage of the sequence presented here is that the aldehyde group is protected at the siloxycyclopropane stage, which allows convenient storage of this stable intermediate. Of equal importance is the valuable carbanion chemistry that can be carried out a to the ester function. Efficient substitution can be achieved by deprotonation with LDA and subsequent reaction with electrophiles.12-13-6 This process makes several a-substituted [1-formyl esters available. Other ring opening variants of siloxycyclopropanes - mostly as one-pot-procedures - are contained in Scheme I. They underscore the high versatility of these intermediates for the synthesis of valuable compounds.6 Chiral formyl esters (see Table, entries 2-5) are of special... 

A certain dualism is observable in carbonium ion-carbanion chemistry, a dualism rather like that of lines and points in projective geometry. The reader may recall that interchanging the words "line and "point in a theorem of projective geometry converts it into a statement that is also a theorem, sometimes the same one. For most carbonium ion reactions a corresponding carbanion reaction is known. The dualism can be used as a method for the invention of new, or at least unobserved, carbanion reactions. The carbanionic reaction corresponding to the carbonium ion rearrangement is of course the internal nucleophilic... 

In the absence of radical traps, the radical R is converted immediately into the carbanion R by an ECE or a DISP mechanism, according to the distance from the electrode where it has been formed. B is a strong base (or nucleophile) that will react with any acid (or electrophile) present. Scheme 2.21 illustrates the case where a proton donor, BH, is present. The overall reduction process then amounts to a hydrogenolysis reaction with concomitant formation of a base. This is a typical example of how singleelectron-transfer electrochemistry may trigger an ionic chemistry rather than a radical chemistry. This is not always the case, and the conditions that drive the reaction in one direction or the other will be the object of a summarizing discussion at the end of this chapter (Section 2.7). 

In this section we shall examine in detail the role of n—a interactions in carbanion chemistry. Carbanion stability as well as relative acidities of organic molecules are important probes of n—a interactions. 

Functionalization of these reactive anionic chain ends can be achieved by a variety of methods all based on the general concepts of carbanion chemistry. For example, reaction with C02 or succinnic anhydride leads to the carboxy terminated derivatives [10], while hydroxy-terminated polymers can be easily obtained by reaction with ethylene oxide (Scheme 3) [11]. In select functionalization reactions, such as alkylation with p-vinyl benzyl chloride, the nucleophilicity of the carbanionic species may be necessary and this can be achieved by reaction of the chain end with 1,1-diphenylethene followed by functionalization [12,13]. 

The opportunity for tandem cyclization was explored. Here, the results can be accommodated by postulating the intermediacy of a vinyl radical [66]. For example, the controlled potential reduction of enol phosphate 246 affords 247 as a mixture of stereoisomers, in addition to a 15% yield of the linearly fused tricyclopentanoid 248. Assuming that the initial reduction cleaves the phosphate unit, then there exists the opportunity for the resulting radical 249 to be further reduced to afford a carbanion, or undergo a 5-exo-trig radical cyclization onto the pendant alkene. Given the nature of the products and the fact that they are inconsistent with the expectations of carbanion chemistry, it seems clear that the latter pathway dominates. 

An example where the presence of a counterion makes a difference between the gas phase and solution phase pathways involves the intriguing carbanion produced on deprotonation of 1,3-dithiane at C-2. In solution, this species, almost invariably produced by reaction of the dithiane with butyllithium, is widely used as an acyl anion equivalent in synthetic chemistry. Its importance for the present work is that this is a configurationally stable lithiated species in solution the carbanion stays sp -hybridized, and the lithium prefers the equatorial position, even to the extent of driving a terr-butyl group on the same acidic C-2 carbanion to the axial position in the lithiocarbon species. The carbanion is thought to be stabilized primarily by orbital overlap with the C-S antibonding orbitals, as opposed to more conventional polar and 7t-resonance stabilization. ... 

These reactions, in as far as the authors of this chapter know, have not been put to any practical use in the chemical or petroleum industry. As time progresses, commercial uses of these catalysts will be developed. In the meantime, the theoretical interest in carbanion chemistry has been... 

Tlte reduction potential for an alkyl or benzyl radical, relative to that of the carbon-halogen bond from which it is derived, is important in determining the isolated products. Products are derived either by radical or by carbanion chemistry. The half-wave potential for the second polarographic wave of alkyl halides is connected with reduction of the radical. Sophisticated methods have been devised for deducing radical reduction potentials in cases where (his second wave is not seen. Values are collected in Table 4.4. 

In organic chemistry this stabilizing effect is well known the stability of carbanions is known to be enhanced by nitro groups. The stability of the cyclopentadienide anion is increased by complexing with a typical Lewis acid so that it becomes less reactive. For example, ferrocene is not ionized in nitromethane solution. Addition of a Lewis acid such as aluminum chloride facilitates the occurrence of intramolecular race-mization (75) a process which is believed to involve ionic intermediates [16). This belief is supported by kinetic evidence and the failure of the reaction to occur in nearly inert solvents like methylene chloride and in those of high donidty. Whereas the former do not support the solvation of the cation formed in the process of ionization, the latter will react preferentially with the Lewis acid, which is then no longer available for the stabilization of the carbanion. 

In another early application to natural product synthesis, Fleming and coworkers utilized this approach in the efficient formation of the gelsemine model (47) from 45 according to Scheme 8. The cyclization step to form the spiro-oxindole (46) proceeded in 85% yield and provided a means of generating the spiro-fused quaternary carbon without the need for carbenium ion or carbanion chemistry. 

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