Benzylic carbanion

The solid-state structures of several benzylic carbanion salts have been elucidated by X-ray analysis9 depending on the nature of the benzylic part, the cation, and the additives, the structures range from er-bonded organometallic compounds to delocalized ion pairs, from monomeric to dimeric and polymeric aggregates. Some compounds are listed together with leading references ... 

Although benzylic carbanions are more stable than the simple alkyl type, they have not proved stable enough for isolation so far. The benzyl carbanion has been formed and studied in submicrosecond times Bockrath, B. Dorfman, L.M. J. Am. Chem. Soc., 1974, 96, 5708. 

For polymerizations of butadiene in toluene at 50°C with the Ba-Li catalyst, we have observed a reduction in molecular weight and the incorporation of benzyl groups in chains of polybutadiene. We conclude from this result that proton abstraction from toluene occurs to give benzyl carbanions which are capable of forming new polymer molecules in a chain transfer reaction. 

The benzyl carbanion has been found to be most stable because of the extensive delocalization of the negative charge over the various resonating structures. 

The normal secondary /3-deuterium KIE of 1.11 0.03 found for the formation of the benzyl carbanion from ethylbenzene labelled with three deuteriums at the /3-carbon was thought to be indicative of a transition state with substantial carbanionic character. However, the KIE was rationalized in... 

Thus ethyl groups may be added to a-carbons as long as benzylic hydrogens are available for replacement. The mechanism which has been proposed by Pines et al. 19) for the reaction consists of the addition of the benzylic carbanion formed by reaction of the aromatic and the catalyst with the olefin followed by a transmetalation reaction with more of the aromatic [Reaction (4)]. 

This addition is energetically favored because a more stable primary carb-anion is formed from a less stable tertiary carbanion. The addition step in the side-chain alkylation reaction is probably not energetically favored because a primary carbanion is formed from a resonance-stabilized benzylic carbanion. However, as the rapid and energetically favored transmetalation reaction following it restores the benzylic carbanion, the over-all process takes place readily. An alternative radical combination mechanism has also been proposed by Morton and Ward 39) for the alkylation reaction. 

A hydrogen-transfer reaction involving the olefin and the addition product of the benzylic carbanion and olefin may accompany the side-chain alkylation reaction. The result is that alkanes and arylalkenes are produced 19). This hydride-tiansfer reaction may take place by elimination of a hydride ion from the carbanion adduct followed by addition of the hydride ion to the olefin [Reaction (6)]. The amount of this side reaction probably depends largely on the severity of reaction conditions used. 

This indicates that the benzylic carbanion adds to the olefin to yield a primary carbanion rather than a secondaiy or tertiary carbanion. 

The alkylation of benzene by alkylpotassium compounds has been reported by Bryce-Smith (S9) and is probably due to the increased base strength of organopotassium compounds over organosodium compounds. The potassium hydride eliminated in the cyclization reaction may add to ethylene to form ethylpotassium, which then may react with the aromatic to yield ethane and a benzylic carbanion [Reactions (16) and (17)]. 

The side-chain alkylation reaction of aromatic hydrocarbons has also been studied using unsaturated aromatic olefins, especially styrene. Pines and Wunderlich 43) found that phenylethylated aromatics resulted from the reaction of styrenes with arylalkanes at 80-125° in the presence of sodium with a promoter. The mechanism of reaction is similar to that suggested for monoolefins, but addition does not take place to yield a primary carbanion a resonance stabilized benzylic carbanion is formed [Reaction (23a, b)j. 

The mechanism of this addition is similar to that for styrenes only an allylic carbanion is formed instead of a benzylic carbanion [Reaction (26)]. 

The primary allylic carbanion apparently predominates and reacts with aromatic to yield the alkenylbenzene and regenerate the benzylic carbanion [Reaction (27)]. 

The relative amounts of these produced at various temperatures are shown in Table VII. The formation of these products may be explained using carbanionic mechanisms. The cyclic material may form by addition of an allylic carbanion to a molecule of the styrene, followed by a cyclization to yield a benzylic carbanion [Reaction (33a, b, c)]. 

Further data from the polarography and cyclic voltammetry in dimethylformamide are given in Table 5.1 for a series of overall two-electron processes leading to cleavage of a benzyl-heteroatom bond. The first electron transfer step is of the dissociative electron transfer type leading to a benzyl radical. This radical is reduced firrther, at the working potential, to the benzyl carbanion. The carbanion fi om benzyl chlorides, esters, ethers, sulphides, sulphones and quaternary ammonium salts can be trapped by carbon dioxide to form phenylacetic acid [2]. Reac-... 

The diastereomers 251/ewf-251 and 252/ent-252 could be separated and were decom-plexed separately. From the fraction of 251/ewt-251,253 was obtained with 85% ee (e.r. = 92.5 7.5), and the fraction of 252/ent-252 yielded ewt-253 with 88% ee (e.r. = 6 94). A similar situation results from the reaction with tributyltin chloride or alkylation reagents, but the diastereomeric ratio is strongly dependent on the electrophile. The following conclusion is drawn from these and further experiments The enantiomeric ratio is determined by a selection of the chiral base between the diastereotopic methylene groups, since the benzylic carbanionic centres are labile, whereas the diastereomeric ratio results from the relative rate of the electrophile approach syn or anti with respect to the A-methyl group. One question remains—why are opposite d.r. values formed in the alkylation by methyl iodide and ethyl iodide ... 

AUyl transfer reactions, 73, 1 Allylic alcohols, synthesis from epoxides, 29, 3 by Wittig rearrangement, 46, 2 Allylic and benzylic carbanions, heteroatom-substituted, 27, 1 Allylic hydroperoxides, in... 

Interestingly, if the tribromo compound is treated with five equivalents of n-BuLi, then tetralithiation occurs, as was shown by the isolation of an a-butyl-2,4,5-trimethylthio derivative after reaction with excess dimethyl-disulfide [87JCS(P1)1453]. The a-butyl group in the product is derived from reaction of the a-benzyl carbanion with the n-butyl bromide produced by the initial bromine-lithium exchange reaction (Scheme 59). However,... 

A common intermediate is required in this Favorskii rearrangement to set the same product. Removal of an aH by OH- is followed by an SN2 displacement of Cl- to give a cyclopropanone ring. Ring opening occurs to give the more stable benzylic carbanion. 

Some protonation of the benzyl carbanion by the starting ether (selfprotonation reaction) and other side reactions, such as hydrolysis caused by in situ generation of OH (through protonation of the benzyl anion by traces of water), can be avoided by addition of a suitable acid. Under these conditions, electrolysis leads to an effective conversion of the ether into toluene and phenoxide ion with an electron consumption of 2 F/mol. 

For a review of allylic and benzylic carbanions substituted by hetero atoms, see Biellmann Duccp Org. React. 1982, 27. 1-344. 

The nature of the catalyst and certain experimental observations, such as product distribution [Eq. (5.61)], indicate a carbanionic reaction mechanism. A benzylic carbanion formed through proton abstraction by an organoalkali metal compound... 

Eq. (5.63)] initiates the transformation.235 The next step, the attack of anion 36 on the alkene [Eq. (5.64)], is the rate-determining step since it results in the transformation of a resonance-stabilized anion (36) into anions (37 and 38) that are not stabilized. Transmetalation of 37 and 38 forms the end products and restores the benzylic carbanion [Eqs. (5.65) and (5.66)] ... 

The structures of benzylic carbanions have been extensively studied and a summary of the more important recent results is presented in Table 15. The H-NMR spectrum of benzyl potassium 202) in THF is upfield of that of benzyllithium 203) as expected on the basis of the relative polarizing powers of the two cations. The spectrum of oligomeric poly(styryl)lithium which is qualitatively similar, is shifted upfield on... 

---